BEGIN:VCALENDAR PRODID:University of Cologne\, ITP\, Events Server X-WR-TIMEZONE:Europe/Berlin BEGIN:VEVENT SUMMARY:SFB1238 | Hidenori Takagi DTSTART;TZID=Europe/Berlin:20161004T163000 DTEND;TZID=Europe/Berlin:20161004T173000 DTSTAMP:20260527T090434Z UID:0000000921@events.thp.uni-koeln.de DESCRIPTION:Hidenori Takagi\, MPI Stuttgart\n\nOpening Colloquium SFB 1238 \n\n\n\nContact Person: Simon Trebst LOCATION:HS III END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Tusharkanti Dey DTSTART;TZID=Europe/Berlin:20161019T143000 DTEND;TZID=Europe/Berlin:20161019T153000 DTSTAMP:20260527T090434Z UID:0000000949@events.thp.uni-koeln.de DESCRIPTION:Tusharkanti Dey\, University of Augsburg\n\nPossible materials for realization of (i) Heisenberg-Kitaev physics in triangular lattice an d (ii) excitonic magnetism\n\nPresence of a strong spin-orbit coupling (SO C) comparable to Coulomb interaction (U) and crystal field in iridium base d oxides give rise to novel properties. Especially the honeycomb lattice i ridates attracted lot of attention in the context of Heisenberg-Kitaev mod el. Extension of this model to triangular lattice has been studied theoret ically and a rich phase diagram has been proposed [1\, 2]. Possible materi als for the realization of Heisenberg-Kitaev model in a triangular lattice will be discussed in this talk.\n\nMaterials with 5d4 electronic configur ation are believed to be in a nonmagnetic ground state (J=0). An interplay between U and SOC in these materials may lead to novel magnetism called e xcitonic magnetism [3]. In the 2nd part of the talk\, Ba2YIrO6 with 5d4 (I r5+) electronic configuration will be discussed in the context of excitoni c magnetism.\nReferences:\n[1] M. Becker et al.\, Phys. Rev. B 91\, 155135 (2015)\n[2] A. Catuneanu et al.\, Phys. Rev. B 92\, 165108 (2015)\n[3] G. Khaliullin\, Phys. Rev. Lett. 111\, 197201 (2013)\n\nContact Person: T. L orenz LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Bernd Buechner DTSTART;TZID=Europe/Berlin:20161123T143000 DTEND;TZID=Europe/Berlin:20161123T160000 DTSTAMP:20260527T090434Z UID:0000000947@events.thp.uni-koeln.de DESCRIPTION:Bernd Buechner\, IFW Dresden\n\nAnisotropic Magnetism and Spin Gap in alpha-RuCl3\n\nQuantum spin liquids are a central theme in current condensed matter physics as they host emergent topological order and frac tionalized excitations. An important example are interacting Kitaev spins on a honeycomb lattice which are theoretically predicted to exhibit topolo gical and quantum spin liquids. Identifying signatures of Kitaev physics\, however\, is extremely challenging as real materials\, such as the iridat es (Na\,Li)2IrO3\, inevitably entail an isotropic Heisenberg and additiona l spin-exchange interactions\, thereby stabilizing a magnetically ordered state. Hexagonal Ru trichloride α-RuCl3 has been recently reported to be near the Kitaev spin-liquid phase boundary. The layered honeycomb structur e of α-RuCl3 contains layers weakly coupled by van der Waals interaction\ , i.e. it is a correlated 2d-material offering many possibilities for “m aterials engineering”. We have characterized the electronic and magnetic properties of α-RuCl3 by a broad spectrum of experimental techniques. Fr om electron spectroscopy basic parameters determining the insulating state of α-RuCl3 are extracted. Moreover\, strongly anisotropic magnetic prope rties as measured from magnetization\, specific heat\, thermal conductivit y\, and NMR measurements will be presented and discussed. Both\, the beha vior of α-RuCl3 in the paramagnetic phase as well as the properties of th e ordered state strongly differ from that found in conventional two-dimens ional magnets. In particular\, we find a very unusual field-temperature ph ase diagram with evidence for a novel quantum critical point.\n\nContact P erson: Markus Braden LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Titus Neupert DTSTART;TZID=Europe/Berlin:20161207T143000 DTEND;TZID=Europe/Berlin:20161207T153000 DTSTAMP:20260527T090434Z UID:0000000940@events.thp.uni-koeln.de DESCRIPTION:Titus Neupert\, University of Zurich\n\nOne-dimensional edge m odes of three-dimensional topological insulators\n\nI will discuss two ins tances of one-dimensional conducting edge channels that can appear on the boundary of three-dimensional topological crystalline insulators\, one sup ported by an experimental observation and the second one being a theoretic al prediction. For the first part\, I will discuss channels that appear at step edges on the surface of (Pb\,Sn)Se. These conducting channels can be understood as arising from a Berry curvature mismatch between Dirac surfa ce states on either side of the step edge. Experimentally\, they have been found to be remarkably robust against defects\, magnetic fields and eleva ted temperature. Second\, I will introduce the concept of higher-order thr ee-dimensional topological insulators\, which have gapped surfaces\, but s upport topologically protected gapless states on their one-dimensional phy sical edges.\n\nContact Person: Simon Trebst LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Xenophon Zotos DTSTART;TZID=Europe/Berlin:20170118T143000 DTEND;TZID=Europe/Berlin:20170118T153000 DTSTAMP:20260527T090434Z UID:0000000998@events.thp.uni-koeln.de DESCRIPTION:Xenophon Zotos\, University of Crete\n\nNovel view on the dyna mics of a 1D and 2D prototype quantum many body system\n\nI will present r ecent results\, (i) on a novel TBA approach to magneto-thermal transport i n the Heisenberg spin 1/2 chain\, as well as an extension  to far out of equilibrium thermal transport (thermal quench) [1]\, (ii) on the high temp erature dynamics of quantum compass models [2]. \n\n[1] X. Zotos\, arXiv: 1604.08434 \n\n[2] A. Briffa and X. Zotos\, arXiv: 1611.00637\n\n\nContact Person: P. van Loosdrecht LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Matthias Punk DTSTART;TZID=Europe/Berlin:20170125T143000 DTEND;TZID=Europe/Berlin:20170125T153000 DTSTAMP:20260527T090434Z UID:0000000992@events.thp.uni-koeln.de DESCRIPTION:Matthias Punk\, LMU Munich\n\nQuantum critical points in metal s: non-Fermi liquids and their field theoretical description\n\nMetals are found frequently in nature and their properties are usually very well des cribed within Landau’s Fermi liquid theory. Various strongly correlated materials exhibit strange metallic phases which do not fit into the Fermi- liquid framework\, however. The theoretical description of such non-Fermi liquids remains one of the main unsolved problems in condensed matter phys ics. In this talk I will give an introduction to the problem and show how interesting strongly coupled field theories arise in the low energy descri ption of such states\, which are still poorly understood. I will focus on the paradigmatic problem of a metal coupled to fluctuations of a critical Ising order parameter and discuss unexpected scaling properties at finite temperature.\n\n\nContact Person: Simon Trebst LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Juergen Braun DTSTART;TZID=Europe/Berlin:20170222T143000 DTEND;TZID=Europe/Berlin:20170222T153000 DTSTAMP:20260527T090434Z UID:0000001018@events.thp.uni-koeln.de DESCRIPTION:Juergen Braun \, LMU München\n\nCorrelation\, temperature and dynamical aspects: recent developments in the one-step description of ang le-resolved photoemission\n\n\n\nContact Person: P. van Loosdrecht LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Pantelis Bamboulis DTSTART;TZID=Europe/Berlin:20170308T143000 DTEND;TZID=Europe/Berlin:20170308T153000 DTSTAMP:20260527T090434Z UID:0000001017@events.thp.uni-koeln.de DESCRIPTION:Pantelis Bamboulis \, Uni Twente\, NL\n\nStructure and Dynamic s of 2D Material Confined Water\n\n\n\nContact Person: T. Michely LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Dmitry Novoselov DTSTART;TZID=Europe/Berlin:20170315T143000 DTEND;TZID=Europe/Berlin:20170315T153000 DTSTAMP:20260527T090434Z UID:0000001031@events.thp.uni-koeln.de DESCRIPTION:Dmitry Novoselov\n\nCorrelation effects and phonon modes softe ning with doping in Ba1-xKxBiO3\n\n\n\nContact Person: D. Khomskii LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Vladislav Kataev DTSTART;TZID=Europe/Berlin:20170329T143000 DTEND;TZID=Europe/Berlin:20170329T153000 DTSTAMP:20260527T090434Z UID:0000001019@events.thp.uni-koeln.de DESCRIPTION:Vladislav Kataev\, IFW Dresden\n\nSpin-orbit coupling effects in complex iridium oxides as seen by high-field ESR spectroscopy\n\nCompl ex iridium oxides have attracted recently a lot of attention due to an int imate entanglement of spin and orbital degrees of freedom which may give r ise to a novel spin-orbital Mott insulating behavior and exotic quantum sp in liquid phases. Electron spin resonance (ESR) spectroscopy is known to b e an instructive tool for studying the spin-orbit coupling (SOC) effects a s it can directly access the relevant parameters sensitive to SOC\, such a s the g-factor tensor\, magnetic anisotropy gaps and spin dynamics. In thi s talk\, two of our recent results on multi-frequency sub-THz ESR spectros copy on iridium oxides in strong magnetic fields will be discussed:\n(1) s urprising inversion of the orbital states in the prototypical spin-orbital Mott insulator Sr2IrO4 with Ir4+ (5d5\, J = 1/2) due to the long-range cr ystal field effects\;\n\n(2) evidence that the unexpected magnetism in the “non-magnetic” Ir5+ (5d4\, J = 0) double perovskite Ba2YIrO6 may not be due to the “failed dominance of SOC” as proposed in recent literatu re.\n\n\nContact Person: T. Lorenz LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Daniele Fausti DTSTART;TZID=Europe/Berlin:20170412T143000 DTEND;TZID=Europe/Berlin:20170412T153000 DTSTAMP:20260527T090434Z UID:0000001022@events.thp.uni-koeln.de DESCRIPTION:Daniele Fausti\, University Triest Elettra\n\nOptical control and quasiparticle witnessing in strongly correlated electron systems\n\n\n \nContact Person: Paul van Loosdrecht LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Julius Janusonis DTSTART;TZID=Europe/Berlin:20170425T140000 DTEND;TZID=Europe/Berlin:20170425T150000 DTSTAMP:20260527T090434Z UID:0000001074@events.thp.uni-koeln.de DESCRIPTION:Julius Janusonis\n\nMagnetoelastic excitations in ferromagneti c films\n\n\n\nContact Person: Paul van Loosdrecht LOCATION:conference room old Theory END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Jeroen van den Brink DTSTART;TZID=Europe/Berlin:20170503T143000 DTEND;TZID=Europe/Berlin:20170503T153000 DTSTAMP:20260527T090434Z UID:0000001030@events.thp.uni-koeln.de DESCRIPTION:Jeroen van den Brink\, IFW Dresden\n\nJosephson Currents Induc ed by the Witten Effect\n\n\n\nContact Person: M. Grüninger LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Xiao Lin DTSTART;TZID=Europe/Berlin:20170510T143000 DTEND;TZID=Europe/Berlin:20170510T160000 DTSTAMP:20260527T090434Z UID:0000001032@events.thp.uni-koeln.de DESCRIPTION:Xiao Lin\, University of Cologne\n\nUnconventional superconduc tivity and strange metallicity in SrTiO3\n\n\n\nContact Person: Joachim He mberger LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Je-Geun Park DTSTART;TZID=Europe/Berlin:20170517T143000 DTEND;TZID=Europe/Berlin:20170517T160000 DTSTAMP:20260527T090434Z UID:0000001068@events.thp.uni-koeln.de DESCRIPTION:Je-Geun Park \, Seoul National University\n\nOpportunities and challenges of two-dimensional magnetic van der Waals materials: magnetic graphene?\n\n\n\nContact Person: Daniel Khomskii LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Igor Mazin DTSTART;TZID=Europe/Berlin:20170522T153000 DTEND;TZID=Europe/Berlin:20170522T170000 DTSTAMP:20260527T090434Z UID:0000001070@events.thp.uni-koeln.de DESCRIPTION:Igor Mazin\n\nMolecular orbitals vs relativistic orbitals in t 2g honeycomb lattics: SrRu206 as compared to Na2Ir03\, RuC13\, and Li2Ru03 \n\n\n\nContact Person: Daniel Khomskii LOCATION:SR alte Theorie END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Marco Moretti DTSTART;TZID=Europe/Berlin:20170524T143000 DTEND;TZID=Europe/Berlin:20170524T153000 DTSTAMP:20260527T090434Z UID:0000001015@events.thp.uni-koeln.de DESCRIPTION:Marco Moretti\, ESRF\, Grenoble\n\nMagnetic and orbital excita tions studied by x-rays\n\n\n\nContact Person: M. Grüninger LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Nadine Hauptmann DTSTART;TZID=Europe/Berlin:20170531T143000 DTEND;TZID=Europe/Berlin:20170531T153000 DTSTAMP:20260527T090434Z UID:0000001039@events.thp.uni-koeln.de DESCRIPTION:Nadine Hauptmann\, Universität Nijmegen\n\n2D materials and n ano-scale skyrmions investigated by spin-resolved scanning probe microscop ies\n\n\n\nContact Person: T. Michely LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Urs Staub DTSTART;TZID=Europe/Berlin:20170614T143000 DTEND;TZID=Europe/Berlin:20170614T160000 DTSTAMP:20260527T090434Z UID:0000001041@events.thp.uni-koeln.de DESCRIPTION:Urs Staub\, Paul Scherrer Institute\n\nUltrafast manipulation of magnetic\, electronic and crystal structure\n\nThe understanding of ele ctronic and structural phase transitions is of basic importance in condens ed matter physics. In this presentation\, I will concentrate on phase tran sitions that can be triggered externally by electronic excitations and add ress the how we can excite particle modes of lattice excitation\, and expl ore how these are related to the electronic and magnetic properties and th e corresponding phase transitions. The goal is to understand how and how f ast we can trigger electronic transitions and how they might couple to str uctural motions of the atomic lattice. [1-3] These can be done using pump- probe schemes\, with x-rays as a probe down to ultrafast time scales\, as created by an x-ray free electron laser. \nThe presented examples will inc lude the charge and orbital order phase transition in half doped manganite s [1\,3] and multiferroic TbMnO3. I will also present very briefly newest results on the octahedral rotation transitions in simple perovskite SrTiO3 and EuTiO3\, which shows opposite behaviour when triggering this phase tr ansition. \n\n\nReferences\n[1] P. Beaud et al. Nature Materials 13\, 923 (2014) \n[2] Kubacka et al. Science 343\, 1333 (2014)\n[3] V. Esposito et al. Phys. Rev. Lett. (in press) (2017).\n\n\nContact Person: Markus Braden LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Eran Sela DTSTART;TZID=Europe/Berlin:20170628T143000 DTEND;TZID=Europe/Berlin:20170628T154500 DTSTAMP:20260527T090434Z UID:0000001025@events.thp.uni-koeln.de DESCRIPTION:Eran Sela\, Tel Aviv University\n\nMajorana States in the Coul omb Blockade Regime – Signatures and Applications\n\nIn this talk I will discuss recent experimental and theoretical advances combining one dimens ional topological nanowires into small mesoscopic islands\, pushing Majora na physics into the Coulomb blockade regime\, and creating new quantum pha ses governed by correlations. I will elucidate that transport through such islands gives access to a rather unstudied type of quantum impurity physi cs\, where the electrons in the leads combine with the Majorana fermions i n the island to yield effective bosonic charge carriers. This gives rise t o novel non-equilibrium transport behavior. Pushing these islands using a gate volatge into charge degeneracy points presents very interesting regim es allowing to probe strongly correlated Majorana fermions at short time s cales and at high energy scales. In addition\, creating arrays of such is lands may form a highly convenient platform for topological quantum comput ing schemes based on the surface code. Here\, electron transport can be us ed for quantum measurements and manipulation of the stored information in the Majorana qubits.\n\nContact Person: Simon Trebst LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Andrei Bernevig DTSTART;TZID=Europe/Berlin:20170714T163000 DTEND;TZID=Europe/Berlin:20170714T173000 DTSTAMP:20260527T090434Z UID:0000001077@events.thp.uni-koeln.de DESCRIPTION:Andrei Bernevig\, Princeton University\n\nHow to find topologi cal states of matter\n\nThe previous 11 years have seen several breakthrou ghs in our understanding of topological states of matter. New Insulators a nd semimetals\, exhibiting new properties imposed by a variety of symmetr ies (the most well known of which is time reversal) have been theoreticall y predicted and experimentally discovered. These materials are described b y topological indices\, a set of "serial numbers" that characterize the st ate of matter. \n\nIn this talk\, I will review the progress made in these years with a focus on the prediction of new topological indices and on ma terial discovery. I will then show a surprising link between chemistry bon ding and topological states of matter which leads to the prediction of man y new classes of materials as topological. \n\nContact Person: Simon Trebs t LOCATION:Seminar Room 0.03\, ETP END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Udai Raj Singh DTSTART;TZID=Europe/Berlin:20170721T150000 DTEND;TZID=Europe/Berlin:20170721T163000 DTSTAMP:20260527T090434Z UID:0000001088@events.thp.uni-koeln.de DESCRIPTION:Udai Raj Singh\, Institute for Nanostructure and Solid State P hysics\, University of Hamburg\n\nSpectroscopic-Imaging and Spin-Polarized STM Studies of Superconductivity\, Nematicity and Magnetism in 11-Iron Ch alcogenide Superconductors\n\n\n\nContact Person: T. Michely LOCATION:Seminar room THP 'Altbau' END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Markus Kriener DTSTART;TZID=Europe/Berlin:20170726T140000 DTEND;TZID=Europe/Berlin:20170726T150000 DTSTAMP:20260527T090434Z UID:0000001034@events.thp.uni-koeln.de DESCRIPTION:Markus Kriener\, RIKEN Center for Emergent Matter Science\, Ja pan\n\nDoping into the polar semiconductor GeTe: From nanoscale magnetic p hase separation to superconductivity\n\n\n\nContact Person: Thomas Lorenz LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Taner Esat DTSTART;TZID=Europe/Berlin:20170927T143000 DTEND;TZID=Europe/Berlin:20170927T153000 DTSTAMP:20260527T090434Z UID:0000001137@events.thp.uni-koeln.de DESCRIPTION: Taner Esat\, Peter-Gruenberg-Institut Juelich\n\nA chemically driven quantum phase transition in a two-molecule Kondo system\n\nMagneti sm of molecules is in the focus of intense research\, because it could lea d to new spin-based electronic devices. In this context\, two molecules ad sorbed on a metal surface and carrying magnetic moments constitute an impo rtant model system which helps to understand the fundamental properties of molecular magnetism – in particular the interaction between their magne tic moments. Typically the interaction of spins on a metal surface is domi nated and tuned by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction – an indirect exchange interaction mediated by the conduction electrons of the metal.\nHere we show that the non-magnetic\, chemical interaction b etween the molecules can become the decisive effect if the spin-moment-car rying molecular orbitals extend in space and therefore lead to the formati on of bonding and antibonding orbitals due to wave function overlap. We de monstrate that this splitting has a crucial influence on the magnetic prop erties exemplified by two neighbouring metal-molecule (Au-PTCDA) complexes \, i.e. dimers\, on the Au(111) surface. Interestingly\, in this particula r case\, competition between the binding energy gain\, due to the chemical interaction between the moment-carrying orbitals\, and the gain of additi onal hybridization energy\, due to the strong entanglement between the loc al moment and the conduction band of the substrate\, is the driving force of a quantum phase transition (QPT) in the dimer. Small changes in the wav e function overlap\, involving slightly different dimer configurations\, d rive the system through a quantum critical point (QCP) from a partially Ko ndo-screened triplet to a singlet ground state\, with one dimer configurat ion being located extremely close to a quantum critical point.\n\n\nContac t Person: T. Michely LOCATION:Seminar room Institute of physics 2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Jeison A. Fischer DTSTART;TZID=Europe/Berlin:20170928T110000 DTEND;TZID=Europe/Berlin:20170928T120000 DTSTAMP:20260527T090434Z UID:0000001133@events.thp.uni-koeln.de DESCRIPTION:Jeison A. Fischer\, MPI Halle\n\nExchange-driven non-collinear magnetism in nanostructures studied by spin polarized-STM\n\n\n\nContact Person: T. Michely LOCATION:Seminar room of the Institute of Physics 2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Tim Wehling DTSTART;TZID=Europe/Berlin:20171009T130000 DTEND;TZID=Europe/Berlin:20171009T140000 DTSTAMP:20260527T090434Z UID:0000001125@events.thp.uni-koeln.de DESCRIPTION:Tim Wehling\, Universität Bremen\n\nElectronic Interactions i n Two-Dimensional Materials\n\nSince the isolation of monolayer graphene\, the library of two dimensional (2d) materials has been continuously growi ng. Together with recent developments in on-demand stacking of these atomi cally thin crystals\, a whole new class of hybrid materials is coming into reach\, which holds promises from the realization of exotic electronic qu antum phases at elevated temperatures to novel optoelectronic applications . Interactions between electrons are central to all these kinds of promise s. In my talk\, I will discuss why Coulomb interactions in 2d materials ar e very different from their 3d counterparts\, how they influence material properties and how interactions can be manipulated in 2d materials. As exa mple systems I will consider graphene as well as transition metal dichalco genides like MoS2. In these systems interactions renormalize electronic qu asiparticles through pronounced band gap shifts\, lead to the emergence of many-particle modes such as excitons with unusually high binding energies and can finally drive the electron system towards different kinds of inst abilities including superconductivity and charge density wave states.\n\nC ontact Person: T. Michely LOCATION:Seminar room Institute of physics 2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Christian Hess DTSTART;TZID=Europe/Berlin:20171018T143000 DTEND;TZID=Europe/Berlin:20171018T153000 DTSTAMP:20260527T090434Z UID:0000001105@events.thp.uni-koeln.de DESCRIPTION:Christian Hess\, IFW Dresden\n\nResonantly enhanced quasiparti cle interference and superconductivity in LiFeAs: evidence for q~0 bosonic modes\n\nThe identification of the electron-boson interaction is believed to be the key for rationalizing the Cooper pairing mechanism in a superco nductor. In strong- coupling conventional superconductors\, the fingerprin ts of the electronic band renormalization due to the electron-boson intera ction are found in the fine structure of tunneling spectra\, where the ide ntification of this fine structure with the phononic density of states rep resents one of the hallmarks of modern condensed matters physics. In many unconventional superconductors\, the unknown nature of the pairing boson\, absence of momentum resolution\, as well as possible inelastic tunneling effects render the interpretation of the tunneling spectra often cumbersom e and ambiguous. Here we exploit a so far unnoticed boson-assisted resonan t enhancement of the impurity scattering potential and thus the quasiparti cle interference (QPI) amplitude in order to identify the nature of a poss ible pairing boson in the unconventional superconductor LiFeAs. In high-re solution Fourier transform scanning tunneling spectroscopy (FT-STS) measur ements we indeed observe a resonance-like enhancement of the QPI amplitude in both the normal and the superconducting state\, which implies an impor tant coupling of electronic states to corresponding bosonic states at q~0 and energy at about 8meV. We show that these bosonic states seen in the QP I are in perfect agreement with a strong inelastic contribution on the tun neling dI/dU spectra recorded away from impurities which renders the obser ved small-q modes promising candidates for providing the pairing interacti on in LiFeAs.\n\nContact Person: P. van Loosdrecht LOCATION:Seminar Room of the Institute of Physics 2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Peter Milde DTSTART;TZID=Europe/Berlin:20171024T130000 DTEND;TZID=Europe/Berlin:20171024T140000 DTSTAMP:20260527T090434Z UID:0000001131@events.thp.uni-koeln.de DESCRIPTION:Peter Milde\, TU Dresden \n\nScanning Force Microscopy Investi gations of Skyrmions\n\n\n\nContact Person: I. Lindfors-Vrejoiu LOCATION:Seminar Room Kernphysik END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Dirk Sander DTSTART;TZID=Europe/Berlin:20171025T143000 DTEND;TZID=Europe/Berlin:20171025T153000 DTSTAMP:20260527T090434Z UID:0000001050@events.thp.uni-koeln.de DESCRIPTION:Dirk Sander\, Max-Planck-Institut für Mikrostrukturphysik\, H alle\n\nNew insights into nanomagnetism and superconductivity by low-tempe rature scanning tunneling microscopy\n\nSpin-polarized scanning tunneling microscopy at low temperature (8 K) and in high magnetic fields (6 T) is a powerful technique to investigate magnetic properties of individual nanos cale objects ranging in size form single atoms to several thousand atoms. I focus on the magnetization reversal and the spin-dependent electronic pr operties of bilayer Co\, Fe-decorated Co and Fe islands on Cu(111). We fin d a novel noncollinear\, helical magnetic order in the Fe islands\, which is identified by a magnetic stripe contrast with a period of 1.28 nm. The high spatial resolution of the scanning tunneling spectroscopy reveals th e significance of structural and electronic relaxation for the magnetic an isotropy and the spin-dependent transport properties of single islands. In an outlook I present first results from our new STM\, which operates at 0 .3 K in a vector magnetic field\, on superconductivity of Pb monolayers an d on the proximity effect.\n\nContact Person: T. Michely LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Felix Gunkel DTSTART;TZID=Europe/Berlin:20171107T150000 DTEND;TZID=Europe/Berlin:20171107T160000 DTSTAMP:20260527T090434Z UID:0000001171@events.thp.uni-koeln.de DESCRIPTION:Felix Gunkel\, FZ Juelich\n\nThermodynamic processes and defec t concentration profiles at engineered complex oxide interfaces and surfac es \n\nThe properties of thin films and heterostructures of complex and st rongly correlated oxides are one of the most fascinating and demanding top ics in today´s solid state physics research. Oxide heterostructures give rise to novel and unexpected physical phenomena such as metallicity in nom inally non-metallic materials\, magnetism in nominally non-magnetic materi als\, ferroelectricity in nominally non-ferroelectric materials\, or so fa r unobserved topological effects. Many of these phenomena are affected or even more driven by the nanoscale defect structure established during mate rial synthesis and processing\, ultimately limited by the thermodynamics o f the system. Tuning the thermodynamic processes by engineering dedicated heterostructures may be used to tailor their defect structure in a desire d way\, hence\, to deplete or accumulate defects at interfaces\, to spatia lly separate electronic and ionic charge carriers\, or to confine defects to dedicated regions.\nIn this talk\, I discuss the low-dimensional electr on transport observed along complex oxide heterointerfaces\, such as the o ne in LaAlO3/SrTiO3 heterostructures. While the formation of these 2-dimen sional electron gases is attributed to electronic charge transfer triggere d by a built-in electric field\, the ionic defect structure at these inter faces is still being discussed controversially. I address the thermodynami c processes associated with built-in electric fields and derive ionic defe ct concentration profiles established at polar/non-polar oxide interfaces. [1] The specific ionic-electronic defect structure stabilized within such interfacial space charge layers strongly depends on ambient oxygen partia l pressure applied during sample fabrication and on the strength of the bu ilt-in electric field. \nAs will be discussed\, the low-temperature behavi or of these novel electron system is unambiguously correlated to the adjac ent ionic structure typically set at high temperature\, making thermodynam ic considerations indispensable in order to understand novel phenomena occ urring at cryostatic temperatures. Therefore\, a comprehensive study of lo w temperature physics on the one hand and high temperature thermodynamics on the other hand is essential for a unified understanding of the manifold (and sometimes contradictory) phenomena observed in these low-dimensional electron systems. Here\, we explicitly compare the thermodynamic ground s tates obtained for various oxide heterostructure systems [2] and discuss r esulting implications for important measures characterizing the electron g as\, such as electron mobility [3] as well as its magnetic signature [4]\, both controllable by thermodynamic means. The thermodynamic model obtaine d for oxide heterointerfaces is furthermore linked to kinetic space charge formation occurring at complex oxide surfaces [5\, 6]. \n[1] F. Gunkel et al.\, “Defect concentration profiles at complex oxide interfaces”\, P hysical Review B 93\, 245431 (2016) \n[2] F. Gunkel et al.\, “Thermodyna mic ground states of complex oxide heterointerfaces”\, ACS Applied Mater ials & interfaces\, ACS Appl. Mater. Interfaces\, 9 (1)\, 1086 (2017) \n[3 ] C. Xu et al.\, “Disentanglement of growth dynamic and thermodynamic ef fects in LaAlO3/SrTiO3 heterostructures”\, Scientific Reports 6\, 22410 (2016) \n[4] F. Gunkel\, et al.\, “Defect-control of anomalous and conve ntional electron transport in NdGaO3/SrTiO3 heterostructures”\, Physical Review X\, 6\, 031035 (2016) \n[5] R. Meyer et al.\, “Dynamics of the m etal-insulator transition of donor-doped SrTiO3”\, Physical Review B 94\ , 115408 (2016)\n[6] M. Andrae et al.\, “Oxygen partial pressure depende nce of surface space charge formation in donor-doped SrTiO3”\, APL Mater ials 5\, 056106 (2017)\n\n\nContact Person: Paul van Loosdrecht LOCATION:SR Kernphysik END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Marko Kralj DTSTART;TZID=Europe/Berlin:20171113T140000 DTEND;TZID=Europe/Berlin:20171113T153000 DTSTAMP:20260527T090434Z UID:0000001175@events.thp.uni-koeln.de DESCRIPTION:Marko Kralj\, Institute of Physics\, Zagreb\n\nTransferring mi llimeter sized atomically thin layers: atomic-scale defects and structural modifications\n\n\n\nContact Person: Thomas Michely LOCATION:Seminarroom New Theorie 0.03 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Harold Zandvliet DTSTART;TZID=Europe/Berlin:20171122T143000 DTEND;TZID=Europe/Berlin:20171122T153000 DTSTAMP:20260527T090434Z UID:0000001124@events.thp.uni-koeln.de DESCRIPTION:Harold Zandvliet\, University of Twente\n\n1D electron systems studied with scanning tunneling microscopy and spectroscopy\n\nScanning t unneling microscopy is an ideal technique to explore the structural and el ectronic properties of low-dimensional electron system. In this talk I wil l focus on the study of physical properties self-organizing metallic nanow ires on semiconductor surfaces and transport through single molecules. The metallic nanowires show unique physical properties owing to their one-dim ensional nature. Many of these properties are intimately related to electr on-electron interactions\, which play a much more prominent role in one di mension than in two or three dimensions. I will present our work on Pt\, A u and Ir atom chains on Ge(001) surfaces and provide a few examples of the ir unique properties\, such as Peierls instability\, quantum confinement\, suppression of the density of density states near the Fermi level and nan owire length quantization. If time allows I will also elaborate on the tra nsport through single molecules.\n\nContact Person: T. Michely LOCATION:Seminar Room of the Institute of Physics 2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Cornelius Krellner DTSTART;TZID=Europe/Berlin:20171129T143000 DTEND;TZID=Europe/Berlin:20171129T153000 DTSTAMP:20260527T090434Z UID:0000001119@events.thp.uni-koeln.de DESCRIPTION:Cornelius Krellner\, Universität Frankfurt \n\nCorrelated mat ter: Insights from new materials\n\nWithin the field of solid-state physic s\, the discovery of remarkable phases and transitions is often tightly co upled to the design\, growth and characterization of novel materials. Ther efore\, the past several decades of work in the field of correlated electr on physics can be described by a list of materials that have defined new s tates of matter at extreme conditions\, e.g. low temperatures\, high magne tic field or pressure. In most cases\, a thorough understanding of the und erlying physical mechanisms is accessible only\, if high-quality single cr ystals with sufficient sizes are available. \nIn this lecture\, I shall gi ve an overview about my own contributions to this research area in recent years focusing on three very different material classes (i) quantum critic ality and superconductivity in heavy-fermion metals [1-3]\, (ii) crystal g rowth of LnFeAsO iron-pnictide superconductors [4\,5]\, and (iii) spin-liq uid phases in Cu-based frustrated spin systems [6].\n \nReferences\n[1] A . Steppke et al.\, Science 339\, 933 (2013).\n[2] E. Schuberth et al.\, Sc ience 351\, 6272 (2016).\n[3] H. Pfau et al.\, Phys. Rev. Lett. 119\, 1264 02 (2017). \n[4] A. Jesche et al.\, Phys. Rev. B 86\, 020501(R) (2012). \n [5] A. Adamski et al.\, Phys. Rev. B 96\, 100503(R) (2017).\n[6] P. Puphal et al.\, J. Mater. Chem. C 5\, 2629 (2017).\n\n\nContact Person: M. Brade n LOCATION:Seminar Room of the Institute of Physics 2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Philip Hofmann DTSTART;TZID=Europe/Berlin:20171221T110000 DTEND;TZID=Europe/Berlin:20171221T120000 DTSTAMP:20260527T090434Z UID:0000001177@events.thp.uni-koeln.de DESCRIPTION:Philip Hofmann\, University Aarhus\n\nELECTRONIC STRUCTURE AND ELECTRON DYNAMICS IN TWO-DIMENSIONAL DIRAC MATERIALS\n\n\n\nContact Perso n: Thomas Michely LOCATION:SR II. Physik END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Mathias Wickleder DTSTART;TZID=Europe/Berlin:20180117T143000 DTEND;TZID=Europe/Berlin:20180117T153000 DTSTAMP:20260527T090434Z UID:0000001174@events.thp.uni-koeln.de DESCRIPTION:Mathias Wickleder\, Department of Chemistry\, University of Co logne\n\nOxoanionic Compounds Syntheses\, Properties\, Perspectives\n\n\n\ nContact Person: M. Braden LOCATION:Seminar room of the Institute of Physics 2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Istvan Kezsmarki DTSTART;TZID=Europe/Berlin:20180124T143000 DTEND;TZID=Europe/Berlin:20180124T160000 DTSTAMP:20260527T090434Z UID:0000001180@events.thp.uni-koeln.de DESCRIPTION:Istvan Kezsmarki\, Uni Augsburg\n\nUnidirectional light propag ation in multiferroics and multi-antiferroics\n\nMultiferroics permit the magnetic control of the electric polarization and the electric control of the magnetization. These static magnetoelectric (ME) effects are of enormo us interest: The ability to read and write a magnetic state current-free b y an electric voltage would provide a huge technological advantage. Dynami c or optical ME effects are equally interesting\, because they give rise t o unidirectional light propagation as recently observed in several multife rroic compounds [1].\n\nIn conventional media light propagation is recipro cal\, that is counter-propagating beams experience the same refractive ind ex. However\, reciprocity can be violated in multiferroic materials\, wher e the refractive index depends not only on the polarization of light but a lso on the +- k direction of the propagation [1]. Such unidirectional tran smission is the consequence of the dynamic magnetoelectric effect emerging in materials with simultaneously broken time reversal and spatial inversi on symmetries. This phenomenon\, exclusively observed in multiferroic and magnetoelectric materials [2-5]\, may allow the development of optical dio des transmitting unpolarized light in one\, but not in the opposite\, dire ction [5]. Recently\, the emergence such unidirectional light propagation\ , governed the dynamic magnetoelectric effect\, has also been demonstrated in multi-antiferroics [6]\, i.e. in materials with coexisting purely anti ferroelectric and antiferromagnetic orders.\n\n\nReferences\n\n[1] D. Szal ler et al.\, Phys. Rev. B 87\, 014421 (2014)\n[2] I. Kezsmarki et al.\, Ph ys. Rev. Lett. 106\, 057403 (2011)\n[3] S. Bordacs et al.\, Nat. Phys. 8\, 734 (2012)\n[4] I. Kezsmarki et al.\, Nat. Commun. 5\, 3203 (2014)\n[5] I . Kezsmarki et al.\, Phys. Rev. Lett. 115\, 127203 (2015)\n[6] V. Kocsis e t al.\, arXiv:1711.08124 (2017)\n\nContact Person: Joachim Hemberger LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Davide Bossini DTSTART;TZID=Europe/Berlin:20180227T143000 DTEND;TZID=Europe/Berlin:20180227T153000 DTSTAMP:20260527T090434Z UID:0000001201@events.thp.uni-koeln.de DESCRIPTION:Davide Bossini\, TU Dortmund\n\nFemtosecond manipulation of ma gnets via photoconducted magnons at the edge of the brillouin zone\n\n\n\n Contact Person: Paul van Loosdrecht LOCATION:SR II. Physik END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Yuichiro Ando DTSTART;TZID=Europe/Berlin:20180227T153000 DTEND;TZID=Europe/Berlin:20180227T163000 DTSTAMP:20260527T090434Z UID:0000001221@events.thp.uni-koeln.de DESCRIPTION:Yuichiro Ando\, Kyoto University\n\nSpintronic devices based o n topological insulators\n\n\n\nContact Person: Yoichi Ando LOCATION:SR Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Jens Brede DTSTART;TZID=Europe/Berlin:20180321T143000 DTEND;TZID=Europe/Berlin:20180321T153000 DTSTAMP:20260527T090434Z UID:0000001220@events.thp.uni-koeln.de DESCRIPTION:Jens Brede \, Centro de Física de Materiales\, UPV (Spain)\n\ nOn-surface synthesis and magnetic characterization of graphene nanoribbon s on top of a two-dimensional magnetic surface alloy\n\nThe high reactivit y of magnetic substrates toward molecular overlayers has so far inhibited the realization of more sophisticated on-surface reactions\, thereby depri ving these interfaces of a significant class of chemically tailored organi cs such as graphene nanoribbons (GNRs)\, oligonuclear spin-chains\, and me tal−organic networks. \nIn the first part of this talk\, I will present a multi-technique characterization of the polymerization of 4\,4″-dibrom o-p-terphenyl precursors into ordered poly(p-phenylene)[1] arrays on top o f an emerging class of two-dimensional magnetic surface alloys[2\,3]. The chemical reaction was followed by temperature-dependent X-ray photoelectro n spectroscopy (XPS) and the structural characterizations of supramolecula r and polymeric phases were performed by low-energy electron diffraction ( LEED) and scanning tunneling microscopy (STM). Taking advantage of the hig h homogeneity of the samples\, the electronic structure of the valence ban d was determined with angle-resolved photoemission spectroscopy (ARPES). M oreover\, ferromagnetic ordering in the two-dimensional GdAu2 alloy was de monstrated for all phases by X-ray absorption spectroscopy (XAS)[4]. \nIn the second part\, I will briefly show that GdAu2 is not only a suitable t emplate for the surface-confined Ullmann-like coupling reaction but is als o thermally sufficiently stable to sustain a cyclodehydrogenation reaction for the on-surface synthesis of more complex GNRs. Thus\, the system is d emonstrated to close a longstanding experimental gap in the study of GNRs as it enables spin-polarized STM characterizations[5] of e.g. the spin-po larization of the elusive GNR edge-states\, which were theoretically predi cted for chiral GNRs[6] and experimentally\, albeit lacking spin-sensitivi ty\, observed on Au(111)[7]. However\, successive theoretical studies ques tioned [8] and refined [9]\, respectively\, the experimental prerequisites under which magnetic ordering of edge-states in GNRs may be observed\, th ereby highlighting the need for direct experimental access of the spin-pol arization of these states.\nReferences:\n[1] Vasseur\, G.\; Abadía\, M.\; Miccio\, L. A.\; Brede\, J.\; Garcia-Lekue\, A.\; de Oteyza\, D. G.\; Rog ero\, C.\; Lobo-Checa\, J. & Ortega\, J. E. Pi Band Dispersion along Conj ugated Organic Nanowires Synthesized on a Metal Oxide Semiconductor J. Am. Chem. Soc. 138\, 5685-5692 (2016)\n[2] Corso\, M.\; Fernández\, L.\; Sch iller\, F. & Ortega\, J. E. Au(111)-Based Nanotemplates by Gd Alloying ACS Nano 4\, 1603-1611 (2004)\n[3] Ormaza\, M.\; Fernández\, L.\; Lafuente\, S.\; Corso\, M.\; Schiller\, F.\; Xu\, B.\; Diakhate\, M.\; Verstraete\, M. J. & Ortega\, J. E. LaAu2 and CeAu2 surface intermetallic compounds gro wn by high-temperature deposition on Au(111) Phys. Rev. B 88\, 125405 (201 3)\n[4] Abadía\, M.\; Ilyn\, M.\; Piquero-Zulaica\, I.\; Gargiani\, P.\; Rogero\, C.\; Ortega\, J. E. & Brede\, J. Polymerization of Well-Aligned O rganic Nanowires on a Ferromagnetic Rare-Earth Surface Alloy\, ACS Nano 11 \, 12392-12401 (2017)\n[5] Brede\, J. & Wiesendanger\, R. Spin-resolved im aging and spectroscopy of individual molecules with sub-molecular spatial resolution MRS Bulletin 39\, 608-613 (2014)\n[6] Yazyev\, O. V.\; Capaz\, R. B. & Louie\, S. G. Theory of magnetic edge states in chiral graphene na noribbons Phys. Rev. B 84\, 115406 (2011)\n[7] Tao\, C.\; Jiao\, L.\; Yazy ev\, O. V.\; Chen\, Y.-C.\; Feng\, J.\; Zhang\, X.\; Capaz\, R. B.\; Tour\ , J. M.\; Zettl\, A.\; Louie\, S. G.\; Dai\, H. & Crommie\, M. F. Spatiall y resolving edge states of chiral graphene nanoribbons\, Nature Physics 7\ , 616-620 (2011)\n[8] Kunstmann\, J.\; Özdoğan\, C.\; Quandt\, A. & Fehs ke\, H. Stability of edge states and edge magnetism in graphene nanoribbon s Phys. Rev. B\, 83\, 045414 (2011)\n[9] Golor\, M.\; Lang\, T. C. & Wesse l\, S. Quantum Monte Carlo studies of edge magnetism in chiral graphene na noribbons Phys. Rev. B\, 87\, 155441 (2013)\n\n\nContact Person: N. Atodir esei LOCATION:Seminar room Institute of Physics 2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Luca Perfetti DTSTART;TZID=Europe/Berlin:20180328T143000 DTEND;TZID=Europe/Berlin:20180328T153000 DTSTAMP:20260527T090434Z UID:0000001192@events.thp.uni-koeln.de DESCRIPTION:Luca Perfetti\, Ecole Polytechnique\, Paris-Saclay\, France \n \nTime resolved photoelectrons spectroscopy of novel materials for optoele ctronic applications\n\nI will review our recent investigations of semicon ducting materials at the focus of optoelectronic research. We measure the surface of CH3NH3PbI3 single crystals by making use of two-photon photoemi ssion spectroscopy. By these means\, we explicitly discriminate the initia l thermalization of the electrons from slower dynamical processes. The pic osecond localization of excited electrons in degraded CH3NH3PbI3 samples i s consistent with the progressive reduction of photoconversion efficiency in operating devices. In the second part of the seminar I will discuss the dynamics of hot carriers in InSe. The electrons excited by photons of 3.1 eV experience a manifold relaxation. First\, they thermalize to the elect ronic states degenerate with the M valley within 0.35 ps. Subsequently\, t he electronic cooling is solely dictated by Froehlich coupling to phonon o f small momentum transfer. Ab-initio calculations of the electron-phonon c oupling are in excellent agreement with the observed dynamics.\n\nContact Person: P. van Loosdrecht LOCATION:Seminar room 201\, Institute of Physics 2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Bongju Kim DTSTART;TZID=Europe/Berlin:20180418T143000 DTEND;TZID=Europe/Berlin:20180418T153000 DTSTAMP:20260527T090434Z UID:0000001251@events.thp.uni-koeln.de DESCRIPTION:Bongju Kim \n\nDimensional study in ultra-thin 3d/4d oxide fil m.\n\nLast few decades\, the oxide heterostructure systems were intensivel y studied as a bulk crystal and thin film. Therefore\, we have a deep unde rstanding of the core physics on these systems. But still we can find or d esign a new research playground among these systems. Dimensionality\, arti ficial structure\, interface engineering\, and exploring new device techni que\, these keywords opened the new research opportunities. In this talk\, Firstly\, I would like to introduce dimensional study on ultra-thin manga nite film with element specific scattering/spectroscopy technique. Secondl y\, the Topological Hall Effect in SrRuO3 system will be discussed.\n\nCon tact Person: Ionela Lindfors-Vrejoiu LOCATION:Seminar Room II. Physik END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | DTSTART;TZID=Europe/Berlin:20180502T140000 DTEND;TZID=Europe/Berlin:20180502T160000 DTSTAMP:20260527T090434Z UID:0000001226@events.thp.uni-koeln.de DESCRIPTION:\n\nMember's Assembly\n\n\n\nContact Person: Paul van Loosdrec ht LOCATION:Seminar-Room II. Physics END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Rossitza Pentcheva DTSTART;TZID=Europe/Berlin:20180516T143000 DTEND;TZID=Europe/Berlin:20180516T153000 DTSTAMP:20260527T090434Z UID:0000001212@events.thp.uni-koeln.de DESCRIPTION:Rossitza Pentcheva\, Uni Duisburg\n\nDesigning electronic phas es at oxide interfaces for electronic\, spintronic and energy applications \n\n\n\nContact Person: Ionela Lindfors-Vrejoiu LOCATION:Seminar Room of the Institute Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Vasily Temnov DTSTART;TZID=Europe/Berlin:20180518T110000 DTEND;TZID=Europe/Berlin:20180518T120000 DTSTAMP:20260527T090434Z UID:0000001271@events.thp.uni-koeln.de DESCRIPTION:Vasily Temnov\, Uni LeMans\n\nPhysical mechanisms and interact ions in acousto-magneto-plasmonics\n\nIn functional magnetic nanostructure s the acoustic\, magnetic and plasmonic excitations can co-exist and inter act on the nanometer spatial and ultrafast time scales. Optical spectrosc opy with femtosecond laser pulses highlights a variety of nontrivial spati o-temporal dynamics\, which are not only used to monitor individual excita tions in real time\, but also study interaction mechanisms between them\, often observed in frequency mixing phenomena. Whereas the second harmoni c generation (SHG)\, sum- and different frequency mixing in the optical fr equency range originate from the chi(2)-nonlinearities\, in acoustics and magnetism they are often dominated by parametric resonances\, where system parameters are modulated at frequencies comparable to the natural oscilla tion frequencies\, typically in the MHz-GHz range. \nKeeping in mind the i ntrinsic differences in the physical nature and frequency range of these p henomena\, here we discuss\, in a comparative manner\, two examples of fre quency mixing in magneto-plasmonics [1\,2] and magneto-acoustics [3\,4]. The ability to experimentally tune both systems through Surface Plasmon Re sonance (SPR) and Ferromagnetic Resonance (FMR) as well as to theoreticall y describe these resonant interactions within the framework of phenomenolo gical models based on the Lorentz oscillator\, represent the key idea behi nd this presentation. \nIn nonlinear magneto-plasmonic experiments\, the Kretschmann configuration SPR resonances for different optical wavelengths occur at different angles Fig. (a\,b)\, offering the unique possibility t o match the fundamental and SHG resonances. In magneto-plasmonic Au/Co/Ag/ glass samples the plasmonically assisted SHG also depends on the direction of magnetization M in ferromagnetic cobalt\, which can be reversed with a weak external magnetic field. A simple model utilizing the resonant plasm onic enhancement of the chi(2)-susceptibility confirms the experimental ob servation that magnetic effects are most pronounced between the SHG and fu ndamental SPR resonances [1\,2].\n \nIn the second experiment [3\,4] the m agnetization in a Ni/glass sample is excited by two distinct transient sur face acoustic waves (SAW and SSLW). Magnetic tuning of the FMR frequency i n resonance to their SHG\, sum- and difference frequencies demonstrates th e full variety of frequency mixing phenomena Fig. (c). In contrast to nonl inear optics\, the frequency mixing is dominated by the parametric effect in the externally driven FMR oscillator. An analytical model based on the resonant enhancement of frequency-mixed signals explains the experimental observations [4]. A practical application of these findings to magneto-ela stic switching in bi-stable systems\, i.e. nickel nanomagnets\, is under way [5]. \nWhereas the use of phenomenological\, non-microscopic modeling may fail to capture fine details\, it represents a useful tool for experi mentalists measuring macroscopic quantities such as magnetization dynamics and/or optical nonlinearities. \nThe presented methodology can furt her exploited to design experimental architectures based of coupled resona nces of different nature (acoustic\, SPR\, FMR\, excitons in semiconductor quantum dots\, nanomagnets) providing a playground for future optical met rology and the physics of nano-devices [5\,6]. \nReferences:\n[1] I. Raz dolski et al.\, ACS Photonics 3\, 179 (2016)\n[2] V.V. Temnov et al.\, J. Opt. 18\, 093002 (2016) \n[3] J. Janusonis et al.\, Phys. Rev. B 94\, 0244 15(2016)\n[4] C.L. Chang et al.\, Phys. Rev. B 95\, 060409 (2017)\n[5] V.S . Vlasov et al.\, Multifunctional Materials (to appear in 2018) \n[6] D.A . Kuzmin et al.\, Nanophotonics 7\, 597 (2018) \n\n\nContact Person: Paul van Loosdrecht LOCATION:HS II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Roman Pisarev DTSTART;TZID=Europe/Berlin:20180621T140000 DTEND;TZID=Europe/Berlin:20180621T150000 DTSTAMP:20260527T090434Z UID:0000001296@events.thp.uni-koeln.de DESCRIPTION:Roman Pisarev\, Ioffe Physical-Technical Institute St-Petersbu rg\n\nClose correlations between magnetic properties and optical effects i n the noncentrosymmetric antiferromagnet CuB2O4\n\n\n\nContact Person: Pau l van Loosdrecht LOCATION:Seminar Room of the Theory Institute END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Davide Pincini DTSTART;TZID=Europe/Berlin:20180625T100000 DTEND;TZID=Europe/Berlin:20180625T110000 DTSTAMP:20260527T090434Z UID:0000001272@events.thp.uni-koeln.de DESCRIPTION:Davide Pincini\, Diamond Light Source\n\nOn the hierarchy of m agnetic and electronic energy scales in novel 3d\, 4d and 5d transition me tal oxides probed by x-ray techniques\n\nTransition metal oxides (TMOs) re present the natural playground for the investigation of the exotic phases produced by electronic correlations\, which include high-temperature super conductivity\, giant magnetoresistance and topological insulators. The pec uliar electronic and magnetic properties of TMOs stem from the interplay b etween the electron-electron correlations\, the crystal field and the spin -orbit coupling (SOC) of the TM d valence electrons. The balance of these energy scales significantly depends on TM element considered: moving from the 3d to the 5d row of the periodic table\, the electronic correlations d ecrease due to the larger size of the atomic orbitals\, while SOC increase s as a result of the increased atomic charge.\n\nIn the present talk\, I p resent three separate case studies of 3d\, 4d and 5d TMOs which highlight the impact of the competing interactions just mentioned on the electronic and magnetic properties of the system. Concerning 5d TMOs\, I address the spin-wave spectrum of the electron-doped perovskite iridate (Sr1-xLax)2IrO 4\, where the strong SOC of 5d electrons gives rise to a spin-orbit entang led Mott state with peculiar exchange interactions. I then examine the int ermediate case of the 4d oxide Ca2RuO4: here\, electronic correlations\, S OC and octahedral distortions act on an equal footing to determine the TM ground state. In particular\, I show how the crystal field tuning achieved by La substitution affects the electronic and magnetic properties. Finall y\, I discuss the magnetic ground state of a family of weak ferromagnets\, where the weak SOC is responsible for the appearance of a net magnetizati on in the main antiferromagnetic order.\n\nContact Person: M. Grueninger LOCATION:Seminar room Institute of Physics 2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Markus Grüninger DTSTART;TZID=Europe/Berlin:20180627T143000 DTEND;TZID=Europe/Berlin:20180627T153000 DTSTAMP:20260527T090434Z UID:0000001269@events.thp.uni-koeln.de DESCRIPTION:Markus Grüninger\, Institute of Physics 2\n\nOngoing RIXS exp eriments in spin-orbit coupled matter\n\n\n\nContact Person: M. Grüninger LOCATION:Seminar room Institute of Physics 2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Zhe Wang DTSTART;TZID=Europe/Berlin:20180711T143000 DTEND;TZID=Europe/Berlin:20180711T153000 DTSTAMP:20260527T090434Z UID:0000001295@events.thp.uni-koeln.de DESCRIPTION:Zhe Wang\, Helmholtz-Zentrum Dresden-Rossendorf (HZDR)\n\nQuan tum Spin Dynamics of a Heisenberg-Ising Spin-1/2 Antiferromagnetic Chain"\ n\nQuantum states in low-dimensional spin systems are characterized by the ir characteristic spin dynamics. In this seminar\, I will talk about terah ertz spectroscopy of quantum spin dynamics in a spin-1/2 Heisenberg-Ising antiferromagnetic chain as a function of temperature and magnetic field. A t zero field\, confined spinon excitations are observed below the Néel te mperature [1]. In a longitudinal magnetic field\, high-energy string excit ations are observed as well as low energy spin excitations [2]. In contras t\, in a transverse magnetic field\, emergent fermionic excitations are ob served\, when the confinement is suppressed and an order-disorder phase tr ansition is induced by the transverse field [3]. These experimental result s are understood by comparison to precise calculations of dynamic structur e factors using Bethe ansatz and infinite time evolving block decimation\, respectively\, for the longitudinal and transverse fields [2\,3\,4]. In a ddition\, the results in transverse fields provided strong indication of p ossible existence of an experimentally well-defined quantum critical point of the transverse-field Ising-chain [5]. The existence of such a quantum critical point indeed has been revealed by our recent thermodynamic measur ements [6]\, the quantum criticality of which is consistent with a recent analytical study of the transverse-field Ising-chain [7].\n\n[1] ZW et al. Phys. Rev. B 91\, 140404(R) (2015).\n[2] ZW et al. Nature 554\, 219–223 (2018).\n[3] ZW et al. Phys. Rev. B 94\, 125130 (2016).\n[4] Wang Yang et al. arXiv:1702.01854 (2017)\n[5] S. K. Niesen et al. Phys. Rev. B 87\, 224413 (2013).\n[6] ZW et al. Phys. Rev. Lett. 120\, 207205 (2018).\n[7] J ianda Wu et al. Phys. Rev. B 97\, 245127 (2018).\n\nContact Person: T. Lor enz LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Arun Paramekanti DTSTART;TZID=Europe/Berlin:20180718T143000 DTEND;TZID=Europe/Berlin:20180718T153000 DTSTAMP:20260527T090434Z UID:0000001213@events.thp.uni-koeln.de DESCRIPTION:Arun Paramekanti\, University of Toronto\n\nOxide heterostruct ures: Impact of spin-orbit coupling and strain.\n\nRecent progress in maki ng oxide interfaces and heterostructures has led to an interest in underst anding how one might realize topological phases or flat bands in such corr elated systems. In particular\, this might be accomplished by spin-orbit c oupling or strain effects. In this context\, I will discuss our theory wor k on understanding the "atomic limit" of spin-orbit coupled oxides\, showi ng that one can use resonant inelastic X-ray spectroscopy of such systems to extract microscopic couplings. Using such "lego" building blocks\, I wi ll discuss mixed oxide 2DEGs comprised of local moments coupled to a "cond uction fluid" of electrons with strong spin-orbit coupling\, to show that one can realize high temperature quantum anomalous Hall insulators. I will also discuss a different setting in which strain in Dirac materials acts as a pseudo-gauge field. This provides a way to realize Landau-like levels of Bogoliubov quasiparticles in high Tc d-wave superconducting oxides. Sp in-orbit coupling and strain can thus pave the way to realizing novel topo logical quantum phases in solids.\n\nContact Person: Simon Trebst LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Tohru Suemoto DTSTART;TZID=Europe/Berlin:20180724T140000 DTEND;TZID=Europe/Berlin:20180724T150000 DTSTAMP:20260527T090434Z UID:0000001262@events.thp.uni-koeln.de DESCRIPTION:Tohru Suemoto\, Toyota Physical and Chemical Research Institut e\n\nFemtosecond luminescence from metals and semimetals\n\nLuminescence i s a standard method to study the excited electronic states and their relax ation dynamics in solids\, especially in insulators and semiconductors. Ho wever\, the luminescence from metals and semimetals have rarely been studi ed. Several years ago\, we found femtosecond infrared luminescence from so me semimetals\, such as graphite [1]\, bismuth\, antimony [2] and also fro m metallic states at the surface of topological insulators [3]. We success fully obtained versatile information about the relaxation dynamics in thes e materials. \n In addition\, very recently\, we discovered ultrafast infr ared luminescence in many ordinary metals\, Au\, Ag\, Cu\, Pt\, Al etc.\, when they have surface roughness [4]. In this talk\, I would like to propo se the "femtosecond infrared luminescence" as a conventional tool for inve stigating the electron dynamics in metals\, providing information similar to that could be obtained from time-resolved photoemission spectroscopy.\n \nReferences\n[1] T. Suemoto\, S. Sakaki\, M. Nakajima\, Y. Ishida\, and S . Shin\, Phys. Rev. B87\, 224302 (2013).\n[2] M. Takeda\, S. Maezawa\, H. Watanabe and T. Suemoto\, Spring meeting of Physical Society of Japan (201 5). \n[3] S. Maezawa\, H. Watanabe\, M. Takeda\, K. Kuroda\, T. Someya\, I . Matsuda and T. Suemoto\, Sci. Rep. 5. 16443 (2015).\n[4] T. Suemoto\, N. Sugimoto\, K. Nishikawa\, K. Yamanaka\, S. Inagaki\, The XXI Internationa l Conference on Ultrafast Phenomena (Hamburg\, 2018).\n\nContact Person: P aul van Loosdrecht LOCATION:Seminar Room of the Institute Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Hendrik Weimer DTSTART;TZID=Europe/Berlin:20181017T143000 DTEND;TZID=Europe/Berlin:20181017T153000 DTSTAMP:20260527T090434Z UID:0000001330@events.thp.uni-koeln.de DESCRIPTION:Hendrik Weimer\, Uni Hannover\n\nVariational Principle for Ope n Quantum Many-Body Systems\n\nDissipative quantum many-body systems are e xtremely challenging to analyze\, as most theoretical tools developed for equilibrium systems cannot be applied. I will present a variational princi ple that can be applied to obtain a better understanding of the essential physics within these systems\, such as dissipative phase transitions [1\,2 ]. Within the context of a dissipative Ising model\, I will compare the va riational method to recent results obtained using a tensor network approac h [3]. Finally\, I will demonstrate how a better fundamental understanding of dissipative quantum many-body systems enables the realization of a nov el class of dissipative quantum sensors [4].\n\nReferences\n\n[1] H. Weime r\, Phys. Rev. Lett. 114\, 040402 (2015).\n[2] V. R. Overbeck\, M. F. Magh rebi\, A. V. Gorshkov\, H. Weimer\, Phys. Rev. A 95\, 042133 (2017).\n[3] A. Kshetrimayum\, H. Weimer\, R. Orus\, Nature Commun. 8\, 1291 (2017).\n[ 4] M. Raghunandan\, J. Wrachtrup\, H. Weimer\, Phys. Rev. Lett. 120\, 1505 01 (2018).\n\nContact Person: Sebastian Diehl LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Satoru Nakatsuji DTSTART;TZID=Europe/Berlin:20181031T143000 DTEND;TZID=Europe/Berlin:20181031T153000 DTSTAMP:20260527T090434Z UID:0000001326@events.thp.uni-koeln.de DESCRIPTION:Satoru Nakatsuji\, University of Tokyo\n\nTopological Metallic States in Frustrated Magnets\n\nTopological Weyl magnets are characterize d by a magnetic semimetallic\nstate that harbors Weyl fermions near the Fe rmi energy. As a parent state of such a magnetic Weyl state\, we first int roduce a new class of semimetals with quadratic band touchings\, the “Lu ttinger semimetal” [1\,2]. We demonstrate the case in pyrochlore iridium oxides that exhibit various exotic phenomena\, including a spontaneous Ha ll effect without magnetization in a spin liquid state [3].\nThen\, we dis cuss a new type of frustrated antiferromagnets\, Mn3X (X = Sn and Ge) as t he examples of a topological Weyl magnet [4\,5\,6]. We show that the clust er multipole order on the kagome lattice of Mn moments can be easily contr olled and allows the system to exhibit a variety of new functions\nat room temperature that have never been seen in antiferromagnetic metals. These include the large anomalous Hall and Nernst effects [4\,5\,7]\, large magn etic optical Kerr effect [8] and a novel type of spin Hall effect (magneti c spin Hall effect). Finally\, we show that they should be significantly u seful for designing antiferromagnetic spintronics\, and energy harvesting technology. \n\n[1] T. Kondo et al.\, Nature Communications 6\, 10042 (201 5).\n[2] B. Cheng et al.\, Nature Communications 8\, 2097 (2017).\n[3] Y. Machida\, S. Nakatsuji\, S. Onoda\, T. Tayama\, and T. Sakakibara\,\nNatur e 463\, 210 (2010).\n[4] S. Nakatsuji\, N. Kiyohara and T. Higo\, Nature 5 27\, 212 (2015).\n[5] N. Kiyohara\, T. Tomita\, S. Nakatsuji\, Phys. Rev. Applied 5\, 064009 (2016).\n[6] K. Kuroda\, T. Tomita et al.\, Nature Mate rials 16\, 1090 (2017).\n[7] M. Ikhlas\, T. Tomita et. al.\, Nature Physic s 13\, 1085 (2017).\n[8] T. Higo et al.\, Nature Photonics\, 12\, 73 (2018 ).\n\nContact Person: Simon Trebst LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Hua Wu\, Shanghai DTSTART;TZID=Europe/Berlin:20181102T120000 DTEND;TZID=Europe/Berlin:20181102T130000 DTSTAMP:20260527T090434Z UID:0000001335@events.thp.uni-koeln.de DESCRIPTION:Hua Wu\, Shanghai\n\nUnderstanding of some unusual magnetism f rom spin-orbital physics\n\nTransition-metal compounds have long been a fe rtile field to explore new materials and novel properties. In this talk\, I will present our recent works about unusual magnetism in some transition -metal compounds\, using crystal field level diagrams\, density functional calculations\, and Monte Carlo simulations. We will show how a joint effe ct of special ligand field\, atomic multiplet and spin-orbit coupling prod uces a record breaking giant magnetic anisotropy energy for Co\, Ru\, or O s adatoms on MgO (001) surface [1]. We will explain the surprising high-T c ferromagnetic metallicity in the disordered perovskite SrFe0.5Ni0.5O3 us ing the unusual high-spin and high valence states [2]. Moreover\, we find a quite strong ferromagnetism in the layered insulator LaSrNiRuO4 with a v ery unique charge-spin-orbital state [3\,4].\n[1] X. Ou et al.\, PRL 115\, 257201 (2015).\n[2] F. Fan et al.\, PRB 94\, 214401 (2016).\n[3] S. Zhu e t al.\, EPL 117\, 37005 (2017) (Editor’s choice).\n[4] K. Yang et al.\, JPCC\, in press (2018).\n\nContact Person: Daniel Khomskii/Markus Braden LOCATION:Konferenzraum Alte Theorie END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Lew Bulaevskii DTSTART;TZID=Europe/Berlin:20181113T130000 DTEND;TZID=Europe/Berlin:20181113T140000 DTSTAMP:20260527T090434Z UID:0000001324@events.thp.uni-koeln.de DESCRIPTION:Lew Bulaevskii\, University of Oregon\n\nComparison of phonon and spin wave induced superconductivity\n\n\n\nContact Person: Daniiel Kho mskii LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Mikhail Sadovskii DTSTART;TZID=Europe/Berlin:20181114T143000 DTEND;TZID=Europe/Berlin:20181114T153000 DTSTAMP:20260527T090434Z UID:0000001323@events.thp.uni-koeln.de DESCRIPTION:Mikhail Sadovskii\, Institute of Electrophysics of the Russian Academy of Sciences\n\nSuperconductivity in FeSe monolayers: why Tc is so high?\n\nWe shall briefly review the present day situation with unexpecte dly high-temperature superconductivity\n\nin FeSe monolayers on SrTiO_3 (F eSe/STO)\, the main experimental results\, the electronic structure (ARPES \n\ndata and calculations) and possible mechanisms of Tc enhancement. We s hall discuss the origin of unusual\n\nshallow conduction bands in FeSe/STO with quite small values of Fermi energy\, which we attribute to the incre asing electronic correlations (as compared to the bulk FeSe). We shall bri efly review the idea of Tc enhancement due to FeSe electrons interactions with high - energy optical phonons of STO\, stressing the importance of antiadiabatic effects\, which will be discussed within the Eliashberg - Mc Millan approach.\n\nContact Person: Daniiel Khomskii LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Stephen Winter DTSTART;TZID=Europe/Berlin:20181128T143000 DTEND;TZID=Europe/Berlin:20181128T153000 DTSTAMP:20260527T090434Z UID:0000001342@events.thp.uni-koeln.de DESCRIPTION:Stephen Winter\, Goethe Universität Frankfurt\n\nExcitations and Energy Scales in RuCl3\n\nIntensive study of RuCl3 has been motivated recently by signatures of strongly anisotropic and frustrated interactions reminiscent of the Kitaev honeycomb model. Even though RuCl3 orders antif erromagnetically at zero field\, it exhibits a broad continuum of magnetic excitations\, which coexists with sharp magnons in different regions of k -space. Many works have interpreted the breakdown of magnons at certain wa vevectors as a signature of proximity to the Kitaev spin liquid. A similar interpretation has been suggested for the continuum of excitations arisin g when magnetic order is suppressed either by high temperature\, or in-pla ne magnetic fields. In order to evaluate these proposals\, it is important to understand the underlying interactions and energy scales in RuCl3 in t he context of recent experiments. We have considered the dynamical respons e and stability of magnons at zero and finite field with respect to the fu ll range of realistic magnetic interactions suggested by recent ab-initio calculations. Combining extensive exact diagonalization studies with semic lassical analysis\, we will address the (i) relevant mechanisms for magnon breakdown\, (ii) origin and robustness of the continuum\, (iii) evolution of the spectra under applied field\, and (iv) possible nature of the fiel d-induced state.\n\nContact Person: Ciaran Hickey LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Alberto Crepaldi DTSTART;TZID=Europe/Berlin:20181205T143000 DTEND;TZID=Europe/Berlin:20181205T153000 DTSTAMP:20260527T090434Z UID:0000001343@events.thp.uni-koeln.de DESCRIPTION:Alberto Crepaldi\, École polytechnique fédérale de Lausanne \n\nNovel topological materials investigated by spin-resolved and time-res olved photoelectron spectroscopy.\n\nThe discovery of topological insulato rs (TIs) has fuelled the search for novel topological phases of matter\, b y combining theory\, computational high-throughput screening and experimen ts [1\, 2]. From the experimental point of view\, angle-resolved photoelec tron spectroscopy (ARPES) has played a key role due its direct access to t he material band structure. The advent of efficient spin detector\, and ul trafast light source in the UV energy range\, has further extended our cap ability to map also the vectorial spin texture and the out-of-equilibrium dynamics of the topological states. \nWe have recently exploited these tec hniques to investigate TIs and several topological semimetal. We have trac ked the evolution of the band structure of MoTe2 across the topological ph ase transition between a trivial semimetal and a type-II Weyl semimetal\, as a function of the sample temperature [3]. Our results show that the pre sence of several bulk and surface states hamper a direct determination of the topological phase. However\, the temporal dynamics of the photoexcited electrons reveals clear signatures of the Weyl point formation in the uno ccupied band structure [4]. \nIn the second part of my talk\, I will show that optical excitation can be also used to alter the material band disper sion\, as for the case of the nodal line semimetal ZrSiSe [5]\, which disp lays both spin polarized surface states and a bulk hidden spin polarizatio n.\n\n\n[1] M. Z. Hasan and C. L. Kane\, Reviews of Modern Physics 82\, 30 45 (2010).\n[2] B.-J. Yang and N. Nagaosa\, Nature Communications 5\, 1 (2 014).\n[3] A. Crepaldi\, et al.\, Phys. Rev. B 95\, 041408 (2017).\n[4] A. Crepaldi\, et al.\, Phys. Rev. B 96\, 241408(R) (2017).\n[5] G. Gatti\, e t al.\, in preparation.\n\nContact Person: Paul van Loosdrecht LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Carmine Autieri DTSTART;TZID=Europe/Berlin:20190306T140000 DTEND;TZID=Europe/Berlin:20190306T153000 DTSTAMP:20260527T090434Z UID:0000001371@events.thp.uni-koeln.de DESCRIPTION:Carmine Autieri\, Polish Academy of Sciences\n\nTopological co ntribution to the anomalous Hall effect at the interface\n\nGeometric phas es in condensed matter play a central role in topological transport phenom ena such as the quantum\, spin and anomalous Hall effect (AHE). In contras t to the quantum Hall effect - which is characterized by a topological inv ariant and robust against perturbations - the AHE depends on the Berry cur vature of occupied bands at the Fermi level and is therefore highly sensit ive to subtle changes in the band structure. We demonstrate that the Berry curvature can be manipulated by interface engineering of the correlated i tinerant ferromagnet and antiferromagnets. Using theoretical calculations\ , we show that the tunability of the AHE at interfaces arises from the com petition between two topologically non-trivial bands.\nD. J. Groenendijk\, C. Autieri et al. Berry phase engineering at oxide interfaces\, https://a rxiv.org/abs/1810.05619.\nM. Asa\, C. Autieri et al. Anomalous Hall effect at the antiferromagnetic/non magnetic interface\, in manuscript\n\nContac t Person: I. Vrejoiu LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Katharina Franke DTSTART;TZID=Europe/Berlin:20190318T143000 DTEND;TZID=Europe/Berlin:20190318T160000 DTSTAMP:20260527T090434Z UID:0000001388@events.thp.uni-koeln.de DESCRIPTION:Katharina Franke\, Freie Universität Berlin\n\nYu-Shiba-Rusin ov states of individual magnetic atoms on 3D and quasi-2D superconductors\ n\nMagnetic impurities in conventional superconductors induce a pair-break ing potential\, which leads\nto bound states inside the superconducting en ergy gap. These states are called Yu-Shiba-Rusinov\nstates\, or short: Shi ba states\, and can be probed by scanning tunneling spectroscopy at the at omic\nscale.\nHere\, we explore the Shiba states of individual magnetic at oms on superconducting substrates. The\npatterns of Shiba states of indivi dual Mn atoms on a Pb(100) surface exhibit the shape of the\ncorresponding spin-carrying d-orbital [1]. When the adatoms are brought into sufficient ly close\ndistance\, they hybridize by forming symmetric and anti-symmetri c linear combinations of the Shiba\nwave functions [2].\nNbSe2 is a van de r Waals-layered material with quasi-2D character. In addition to its super conducting\nproperties it hosts a charge-density wave (CDW) at low tempera tures. Magnetic atoms and\nmolecules have been shown to give rise to long- range oscillatory Shiba states [3\,4]. Here\, we show\nthat the CDW affect s the energy and symmetry patterns of Shiba states around individual Fe at oms.\n[1] M. Ruby\, Y. Peng\, F. von Oppen\, B.W. Heinrich\, K.J. Franke\, Phys. Rev. Lett. 117\, 186801 (2016)\n[2] M. Ruby\, B.W. Heinrich\, Y. Pe ng\, F. von Oppen\, K.J. Franke\, Phys. Rev. Lett. 120\, 156803 (2018)\n[3 ] G. Menard\, et al\, Nat. Phys. 11\, 1013 (2015)\n[4] S. Kezilebieke\, M. Dvorak\, T. Ojanen\, P. Liljeroth\, Nano Lett. 18\, 2311 (2018)\n\nContac t Person: Yoichi Ando LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Jonathan Buhot DTSTART;TZID=Europe/Berlin:20190320T143000 DTEND;TZID=Europe/Berlin:20190320T153000 DTSTAMP:20260527T090434Z UID:0000001369@events.thp.uni-koeln.de DESCRIPTION:Jonathan Buhot\, HFML Nijmegen\, NL\n\nExploring exotic quantu m phases by Raman spectroscopy under extreme conditions\n\nIn condensed ma tter physics\, Raman spectroscopy is an extremely powerful probe of the in elastic and dynamic response of many-body systems and helps to identify th e relevant energy scales that drive fundamental physical phenomena. It can also provide information on the symmetry of the relevant order parameter\ , even those exotic states that remain “hidden” to other spectroscopie s\, unveiling the rich microscopic and anisotropic nature of the phase in question\, as well as its possible (complex) interplay with other ordered states. Over the past few years\, I have developed two unique Raman scatte ring experiments\, capable of probing low energy excitations (0.6 meV) in extreme conditions of low temperatures (4.2 K)\, high pressures (17 GPa) ( Materials and Quantum Phenomena Laboratory\, Paris) and high magnetic fiel ds (30-37 T) (High Field Magnet Laboratory\, Nijmegen). To illustrate the scope and potential of these new spectroscopic tools\, studies carried out on heavy fermion systems [1\, 2]\, filled skutterudites [3] and multiferr oics [4] will be presented.\n[1] JB et al. PRL 113 266405 (2014)\n[2] JB e t al. arXiv:1805.11307 (2018)\n[3] JB et al. In preparation\n[4] JB et al. PRL 115 267204 (2015)\n\nContact Person: P. van Loosdrecht LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Martina Esposito DTSTART;TZID=Europe/Berlin:20190322T143000 DTEND;TZID=Europe/Berlin:20190322T160000 DTSTAMP:20260527T090434Z UID:0000001391@events.thp.uni-koeln.de DESCRIPTION:Martina Esposito\, University of Oxford\n\nParametric amplific ation with superconducting circuits. Development of a flux-pumped Josephso n parametric amplifier.\n\nJosephson parametric amplification is a tool of paramount importance in circuit quantum electrodynamics (circuit-QED)\, e specially for the quantum-noise-limited single-shot readout of superconduc ting qubits. I will first present the development and characterization of a flux-pumped Josephson parametric amplifier (JPA) based on a lumped-eleme nt LC resonator\, in which the inductance L is composed by a geometric ind uctance and an array of superconducting quantum interference devices (SQUI Ds) [1]. In addition\, I will present preliminary experiments where the JP A is used as the first stage of amplification for the readout of a superco nducting qubit based on a coaxial architecture recently developed in our l ab in Oxford [2]. Finally\, I will introduce my future scientific directio n based on using JPAs for generation and control of non-classical states i n microwave photons.\n[1] M. Esposito et al. EPJ Web of Conferences 198\, 00008 (2019)\n[2] J. Rahamim et al. Applied Physics Letters 110\, 222602 ( 2017)\n\nContact Person: Yoichi Ando LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Arkady Krasheninnikov DTSTART;TZID=Europe/Berlin:20190327T143000 DTEND;TZID=Europe/Berlin:20190327T153000 DTSTAMP:20260527T090434Z UID:0000001389@events.thp.uni-koeln.de DESCRIPTION:Arkady Krasheninnikov\, Helmholtz Zentrum Dresden Rossensdorf\ n\nDesign of 2D materials through introduction of defects: simulations and experiments\n\nArkady V. Krasheninnikov 1\,2\n1 Helmholtz Zentrum Dresden -Rossendorf\, Institute of Ion Beam Physics and Materials Research\, Germa ny\n2 Department of Applied Physics\, Aalto University School of Science\, Finland\nFollowing isolation of graphene\, many other 2D systems\, e.g.\, single sheets of transition metal dichalcogenides (TMDs) have been manufa ctured. All these materials contain defects and impurities\, which may gov ern their electronic and optical properties. Moreover\, defects can intent ionally be introduced using beams of energetic particles – ions and elec trons. All of these calls upon the studies on defects and mechanisms of th eir formation. In my talk\, I will present the results [1] of our recent t heoretical studies of defects (native and irradiation-induced) in inorgani c 2D systems obtained in close collaboration with several experimental mic roscopy groups. I will further discuss defect- and impurity-mediated engin eering of the electronic structure of TMDs.\n[1] https://users.aalto.fi/~a rk/publist.html\n\nContact Person: T. Michely LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Laura Corman DTSTART;TZID=Europe/Berlin:20190410T143000 DTEND;TZID=Europe/Berlin:20190410T153000 DTSTAMP:20260527T090434Z UID:0000001397@events.thp.uni-koeln.de DESCRIPTION:Laura Corman\, ETH Zurich\n\n"Mesoscopic transport with ultrac old atoms: particles\, spin and heat"\n\nMesoscopic transport with ultraco ld atoms: particles\, spin and heat"\nLaura Corman\, ETH Zürich\n\nThe qu antum behavior of fermionic particles can be revealed when the structure t hrough which they flow is smaller than their coherence length. For electro ns\, this principle is the foundation of the field mesoscopic physics whic h has emerged thanks to the tremendous progress in fabrication techniques. \n\nAnother platform to study mesocopic transport has appeared in atomic p hysics using ultracold gases of fermionic lithium. Although they are order s of magnitude more massive\, more dilute and colder than electron gases i n materials\, it is possible to access similar regimes in both systems. Th is was demonstrated by the measurement of quantized conductance for neutra l matter at an atomic quantum point contact [1].\n\nUltracold atom transpo rt experiments allow some advantages compared to their electronics counter part. First\, structures on the order of the Fermi wavelength can be optic ally imprinted\, which is fast and flexible. Second\, the interactions bet ween particles can be varied in a broad range\, from weakly interacting (a ttractive or repulsive) to the unitary limit which corresponds to saturate d contact interactions.\n\nIn this talk\, I will present how we engineer s uch transport experiments with atoms as well as some recent results of tra nsport through 1D systems. \nFirst\, by engineering the structure in which the atoms flow\, we were able to explore insulating behaviors from a band insulator to a correlated\, Luther-Emery insulator [2]. \nSecond\, we ext ended these structures to control to the effective spin degree of freedom of the atoms\, creating local effective magnetic fields with Zeeman shifts on the order the Fermi energy [3]. \nLast\, we can also study the couplin g between different transported quantities to understand the nature of a s ystem's elementary excitations\, as we did for the unitary Fermi gas by ob serving the breakdown of the Wiedemann-Franz law [4].\n\n[1] Krinner et al .\, Nature 517\, 64-67 (2015).\n[2] Lebrat et al.\, Physical Review X 8\, 011053 (2018).\n[3] Lebrat et al.\, arXiv:1902.05516 (2019)\n[4] Husmann e t al.\, PNAS 115\, 8563-8568 (2018).\n\nContact Person: Sebastian Diehl LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Jeffrey Rau DTSTART;TZID=Europe/Berlin:20190417T143000 DTEND;TZID=Europe/Berlin:20190417T153000 DTSTAMP:20260527T090434Z UID:0000001386@events.thp.uni-koeln.de DESCRIPTION:Jeffrey Rau\, MPI-PKS Dresden\n\nTopological magnons and the n on-Hermitian topology of spontaneous magnon decay\n\nRecently\, magnon exc itations have been explored as a potential route to\nrealizing analogues o f the topological band structures that have become\nubiquitous in electron ic systems. However\, magnons are not necessarily\nstable quasi-particles \, with spontaneous magnon decay a generic feature\nof anisotropic magnets or magnets with non-colinear order. In this talk\nI will discuss two aspe cts of this physics: First\, I will discuss how\nsharp topological magnon bands can exist in spite of the presence\nallowed decays\, as realized in a Kitaev model in a large magnetic field.\nSecond\, I will discuss an alte rnative scenario where decay in fact\ndrives the appearance new kinds of i ntrinsically non-Hermitian\ntopological physics. This can lead to new kind s of spectral features\,\nsuch as exceptional points and bulk "arcs" conne cting them. I will\nillustrate this physics through a concrete example: a honeycomb\nferromagnet with Dzyaloshinskii-Moriya exchange\, comparing int eracting\nspin-wave calculations with an effective non-Hermitian model. Fi nally\, I\ndiscuss more broadly how the zoo of known topological protected magnon\nband structures may serve as a nearly ideal platform for realizin g\nnon-Hermitian physics in solid-state systems.\n\nContact Person: Ciará n Hickey LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Jens Brede DTSTART;TZID=Europe/Berlin:20190424T143000 DTEND;TZID=Europe/Berlin:20190424T153000 DTSTAMP:20260527T090434Z UID:0000001392@events.thp.uni-koeln.de DESCRIPTION:Jens Brede\, Institute of physics 2\n\nNews from the M-STM lab \n\nSoon after the discovery of superconductivity in CuxBi2Se3 [Hor2010]\, this compound has been predicted to be a topological superconductor (TSC) [Fu2010]. Recently\, spontaneous symmetry breaking of the SC state in the bulk as well as on the surface of CuxBi2Se3 was demonstrated experimental ly [Matano2016\, Yonezawa2017\, Tao2018]. The observed nematic SC state is in agreement with the proposed odd-parity two-component superconducting o rder parameter [Fu2010] and confirms the topological nature of the SC stat e.\nIn the first part of this talk\, I will present preliminary results re cently obtained in the M-STM lab on the surface of CuxBi2Se3 crystals grow n in-house. In particular\, scanning tunneling spectroscopy performed at 400 mK shows that the three-fold symmetry of the underlying lattice is bro ken in the nematic SC state. Specifically\, the SC density of states measu red as a function of the azimuthal angle between the external magnetic fie ld (|B|=0.5 T ) direction and the sample surface shows a characteristic tw o-fold symmetry. Taking advantage of the atomic resolution capabilities of the STM we can deduce the nematic axis with respect to the crystal lattic e locally on the sample surface. Going beyond pervious work [Tao2018]\, we were able to find all three symmetry related nematic domains on the same sample thus offering the prospect to study the physics at the boundary of the nematic SC domains in the future.\nIn the second part\, I will discuss possible future research to be performed in the M-STM lab. The long-term goal could be to bridge the gap between clean-room based (nano)device-fab rication and characterization of such devices with the STM on the atomic s cale. In order to achieve this goal several obstacles have to be overcome. Specifically\, I will propose an approach which allows (i) identification (and positioning) of nano-devices on mm sized sample carriers and (ii) re alization of ultra-clean surfaces. \n\n[Hor2010] Hor et al. Phys. Rev. Let t. 104\, 057001 (2010)\n[Fu2010] Fu and Berg\, Phys. Rev. Lett. 105\, 0970 01 (2010)\n[Yonezawa2017] Yonezawa et al. Nat. Phys. 13\, 123 (2017)\n[Tao 2018] Tao et al. Phys. Rev. X 8\, 041024 (2018)\n\nContact Person: Y. Ando LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Ursula Wurstbauer DTSTART;TZID=Europe/Berlin:20190508T143000 DTEND;TZID=Europe/Berlin:20190508T153000 DTSTAMP:20260527T090434Z UID:0000001384@events.thp.uni-koeln.de DESCRIPTION:Ursula Wurstbauer\, Universität Münster\n\nLight matter inte raction in tunable 2D materials and artificial van der Waals solids\n\nAto mically thin two-dimensional layered materials receive great interest beca use of their unique properties. Particularly\, monolayers of semiconductin g transition metal dichalcogenides (SC-TMDs)\, such as MoS2\, excel due to their strong light-matter interaction that is dominated by exciton phenom ena [1-3]. Key to the integration of SC-TDM and related artificial van der Waals solids into circuitries is the possibility to tune and engineer the ir properties on demand and on-chip e.g. by defects\, dielectric environme nt or doping [4-8]. \nWe apply inelastic light scattering together with em ission and absorption to study the manifold coupling mechanism in van der Waal hetero- and hybrid structures. We introduce the influence of the diel ectric environment\, the charge carrier density as well as defects on the optical properties of these atomically thin materials. Moreover\, interlay er excitons (IX) in vdW hetero-bilayers are intriguing systems to explore classical and quantum phases of interacting bosonic ensembles due to their enhanced lifetimes. We observe multiplet IX emission lines for MoSe2/WSe2 and MoS2/WS2 hetero-bilayers that are interpreted in terms of multi-valle y excitons [3\, 9]. All-2D stark effect devices allows for the manipulatio n of the excitons by external electric fields [9]. Our results provide fun damental insights into long-lived interlayer states in van der Waals heter ostructures with possible bosonic many-body interactions. \n\nWe acknowled ge support by the Deutsche Forschungsgemeinschaft (DFG) via excellence clu ster Nanosystems Initiative Munich (NIM)\, e-conversion as well as DFG pro ject WU 637/4-1.\n\n[1] U. Wurstbauer\, et al. J. Phys. D: Appl. Phys. 50\ , 173001 (2017).\n[2] S. Funke\, et al.\, J. Phys.: Condens. Matter 28\, 3 85301 (2016).\n[3] B. Miller\, et al.\, Nano Lett. 17(9)\, 5229–5237 (20 17).\n[4] E. Parzinger\, et al.\, Nature 2D material 1\, 40 (2017).\n[5] S . Diefenbach\, et al.\, J. Phys. Chem. C\, 122 (17)\, 9663–9670 (2018).\ n[6] J. Klein\, et al.\, 2D Materials 5\, 011007 (2018)\, J. Klein et al.\ , arXiv:1901.01042 (2019).\n[7] B. Miller\, et al.\, Appl. Phys. Lett. 106 \, 122103 (2015).\n[8] B. Miller et al. Nature Commun. 10\, 807 (2019).\n[ 9] J. Kiemle et al. arXiv:1901.01042 (2019).\n\nContact Person: Alexander Grüneis LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Gustav Bihlmayer DTSTART;TZID=Europe/Berlin:20190515T143000 DTEND;TZID=Europe/Berlin:20190515T153000 DTSTAMP:20260527T090434Z UID:0000001390@events.thp.uni-koeln.de DESCRIPTION:Gustav Bihlmayer\, FZ Jülich\n\nExploring 2D magnetic topolog ical materials with density functional theory\n\nTopological insulators (T Is) are fascinating materials due to their protected\nedge-states and thei r possibilities for spintronic applications. Spin-orbit\ncoupling and exch ange effects at interfaces give us a possibility to tailor\nthe properties of two-dimensional (2D) topological materials. While magnetism\nis usuall y detrimental to the topological properties\, we show that exchange\ninter actions offer the possibility to realize a variety of new phases depending \non the structural symmetry and the direction of the magnetic (exchange) field.\nUsing density functional theory we explore the quantum spin Hall t o quantum\nanomalous Hall state transition in TIs and topological crystall ine insulators.\nWe propose the existence of new states like mixed nodal-l ine semimetals or\nmixed Weyl semimetals in the phase diagram of these 2D materials\, showing\ninteresting magnetoelectric coupling phenomena as fun ction of the magnetization\ndirection.\n\nContact Person: Alexander Grüne is LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Michael Scherer DTSTART;TZID=Europe/Berlin:20190520T150000 DTEND;TZID=Europe/Berlin:20190520T160000 DTSTAMP:20260527T090434Z UID:0000001411@events.thp.uni-koeln.de DESCRIPTION:Michael Scherer\, Institute for Theoretical Physics\, Universi ty of Cologne\n\nCorrelated behavior in moiré bands of twisted bilayer gr aphene and transition metal dichalcogenides\n\n\n\nContact Person: Paul va n Loosdrecht LOCATION:SR 0.03 neue Theorie END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Johann Coraux DTSTART;TZID=Europe/Berlin:20190522T143000 DTEND;TZID=Europe/Berlin:20190522T153000 DTSTAMP:20260527T090434Z UID:0000001380@events.thp.uni-koeln.de DESCRIPTION:Johann Coraux\, Institute Neel\, Grenoble\n\nStuffing thin lay ers into the space between two-dimensional materials and their growth subs trates\n\nTwo-dimensional materials have a large fraction of their atoms\, if not all of them\, directly "feeling the world" around them. Their prop erties can thus be strongly influenced by the contact to other materials\, for instance a growth substrate\, and in return\, materials with a large contact-to-volume ratio will be influenced by their interface with the two -dimensional material. Different kinds of interactions and sometimes uncon ventional properties are found depending on the nature of the two-dimensio nal material and of the contacted material. In the past few years\, we hav e addressed single-layer graphene and molybdenum disulphide\, which have v ery different electronic properties -- semimetallic and semiconducting res pectively. We prepare these materials by epitaxial growth on metallic subs trates\, and in between the two materials\, we intercalate very thin layer s of transition or alkali metals. Our first concern is to understand (and hopefully\, control) how the process of intercalation occurs\, whenever po ssible with the help of real-time observations. Is it reversible? Is the s pace between the two-dimensional material and the substrate\, where the in tercalated material grows\, sealed? And how confined growth can occur in s uch a sealed two-dimensional region? Our interest is then on the effects o f the interaction between the two-dimensional material and the intercalate d layer. We have discovered that the magnetic properties of cobalt interca lated underneath graphene are very unusual\, with a strong tendency to per pendicular-to-the-surface magnetisation that is appealing for spintronics applications. Using cesium intercalation\, we observe electron doping in m olybdenum disulphide together with a significant electronic disorder.\n\nC ontact Person: T. Michely LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Michael Knap DTSTART;TZID=Europe/Berlin:20190529T143000 DTEND;TZID=Europe/Berlin:20190529T153000 DTSTAMP:20260527T090434Z UID:0000001393@events.thp.uni-koeln.de DESCRIPTION:Michael Knap\, TU Munich\n\nDynamical Quantum Phases: New rout es to ergodicity breaking in interacting many-body systems.\n\nIt has been the believe that generic quantum many-body systems necessarily approach t hermal equilibrium after a long time evolution. As a consequence any quant um information encoded in the initial state is lost in the course of the d ynamics and the late time dynamics is described by simple hydrodynamics. H owever\, recently exceptions have been found to the rule. For example\, di sordered and interacting many-body systems realize the non-ergodic\, many- body localized phase. In this talk\, we show how constraints can significa ntly alter the quantum dynamics of interacting many-body system. To this e nd\, we will first discuss the far-from-equilibrium dynamics of a two-dime nsional quantum dimer model\, which leads to a rich dynamical phase diagra m with some almost ergodicity breaking regimes [1]. As a second example we show that the combination of charge and dipole conservation\, characteris tic of fractonic quantum matter\, leads to an extensive fragmentation of t he Hilbert space\, which in turn can lead to a breakdown of thermalization [2].\n\n[1] Dynamical Phase Transitions in the Quantum Dimer Model on a S quare Lattice. Johannes Feldmeier\, Frank Pollmann\, Michael Knap [arXiv:1 901.07597]\n[2] Ergodicity-breaking arising from Hilbert space fragmentati on in dipole-conserving Hamiltonians. Pablo Sala\, Tibor Rakovszky\, Ruben Verresen\, Michael Knap\, Frank Pollmann [arXiv:1904.04266]\n\nContact Pe rson: S. Diehl LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Oded Zilberberg DTSTART;TZID=Europe/Berlin:20190605T143000 DTEND;TZID=Europe/Berlin:20190605T153000 DTSTAMP:20260527T090434Z UID:0000001395@events.thp.uni-koeln.de DESCRIPTION:Oded Zilberberg\, ETH Zurich\n\nDissipative phase transitions in light-matter systems\n\nWe study a paradigmatic driven-dissipative mode l\, dubbed interpolating Dicke-Tavis-Cummings (IDTC)\, where a collection of two-level systems interact with both quadratures of a quantum cavity mo de. The closed system exhibits rich physics\, including discrete and conti nuous symmetry-breaking phase transitions meeting at a multicritical point . We show that the closed system's phase diagram is profoundly altered by the presence of dissipation\, eliminating the presence of broken continuou s symmetry transitions\, leading to novel regions of coexistence of phases and the appearance of two tricritical points. Using Keldysh formalism\, w e explore the dynamical response of this fascinating new phase diagram. Su rprisingly\, we find that an additional intriguing dynamical phase transit ion manifests in the frequency response of the system. At the transition\, low-energy particle- and hole-like processes change roles due to a soft m ode gap closing.\n\nContact Person: S. Diehl LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Gianluigi Catelani DTSTART;TZID=Europe/Berlin:20190614T130000 DTEND;TZID=Europe/Berlin:20190614T140000 DTSTAMP:20260527T090434Z UID:0000001422@events.thp.uni-koeln.de DESCRIPTION:Gianluigi Catelani\, FZ Juelich\n\nQuasiparticles and qubits: history and recent developments\n\nOver the years\, the coherence of super conducting qubits has been significantly improved\, so that at present the y reach quality factors in excess of 10^6. Further improvements in coheren ce would help reducing the overhead necessary for implementing quantum err or correction algorithms\; therefore\, despite the past successes\, it is still necessary to investigate decoherence processes and to devise ways to suppress them. Since the early years of superconducting qubits\, quasipar ticles have been recognized as a possible source of decoherence. I will br iefly review the history of quasiparticle effects in qubits\, both in theo ry and experiments. Interestingly\, the qubit sensitivity to quasiparticle s makes it possible to measure their dynamics. I will discuss ways to cont rol this dynamics\, so that the detrimental effects of quasiparticles can be minimized. Finally\, I will discuss some recent findings hinting at pos sible origins of quasiparticles.\n\nContact Person: Y. Ando LOCATION:Seminar Room of the Institute of theoretical Physics (old buildin g) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Arun Paramekanti DTSTART;TZID=Europe/Berlin:20190710T143000 DTEND;TZID=Europe/Berlin:20190710T153000 DTSTAMP:20260527T090434Z UID:0000001382@events.thp.uni-koeln.de DESCRIPTION:Arun Paramekanti\, University of Toronto\n\nA tale of two nick elates\n\nI will discuss two examples of the interplay of distortion and M ott insulator physics. In certain ABO3 perovskite materials including BiNi O3\, it is found that the Mott transition is accompanied by a symmetry cha nge and a colossal volume collapse upon entering the metallic phase with p ressure or temperature. We show that may be viewed as a new type of “hyb ridization-switching” Mott transition. In a different setting\, the spin el compound NiRh2O4 has been argued to possibly form a S=1 spin liquid sta te on the diamond lattice. However\, we show that the physics in the prese nce of distortion and spin-orbit coupling is quite distinct\, leading to a non-magnetic “atomic” ground state\, and we use slave boson theory to discuss its magnetic excitations\, make comparison with current experimen ts\, and predict “dark states” which are invisible to neutrons but may be possibly probed using RIXS experiments.\n\nContact Person: Simon Trebs t LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Max Hirschberger DTSTART;TZID=Europe/Berlin:20190712T140000 DTEND;TZID=Europe/Berlin:20190712T150000 DTSTAMP:20260527T090434Z UID:0000001398@events.thp.uni-koeln.de DESCRIPTION:Max Hirschberger\, Riken\, Japan\n\nSkyrmion formation in cent rosymmetric materials and associated giant Hall and thermoelectric respons es\n\n\n\nContact Person: Achim Rosch LOCATION:Room 0.03\, new theory building END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Debanjan Chowdhury DTSTART;TZID=Europe/Berlin:20190826T140000 DTEND;TZID=Europe/Berlin:20190826T150000 DTSTAMP:20260527T090434Z UID:0000001431@events.thp.uni-koeln.de DESCRIPTION:Debanjan Chowdhury\, MIT\n\nWhat is "strange" about non-Fermi liquid metals?\n\nThe concept of electronic quasiparticles\, as introduced by Landau\, is one of the cornerstones of the theory of quantum many-body systems. However\, a growing number of recent experiments in strongly cor related quantum materials have forced us to confront the existence of quan tum matter for which the concept of electronic quasiparticles does not app ly. Inspired by the rich phenomenology of the parent states of numerous hi gh-temperature superconductors\, I will describe some recent progress in o ur understanding of metallic states that do not admit a quasiparticle desc ription but that nonetheless have a sharply-defined Fermi surface. I will also present some experimental results on unconventional transport propert ies of magic-angle twisted bilayer graphene and comment on their possible connections with the rest of my talk.\n\nContact Person: Ciaran Hickey LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Marin Alexe DTSTART;TZID=Europe/Berlin:20190926T110000 DTEND;TZID=Europe/Berlin:20190926T120000 DTSTAMP:20260527T090434Z UID:0000001456@events.thp.uni-koeln.de DESCRIPTION:Marin Alexe\, University of Warwick\n\nBulk- and Flexo-Photovo ltaic Effects\n\n\n\nContact Person: Dr. Ionela Lindfors-Vrejoiu LOCATION:tba END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Hannu-Pekka Komsa DTSTART;TZID=Europe/Berlin:20191009T143000 DTEND;TZID=Europe/Berlin:20191009T153000 DTSTAMP:20260527T090434Z UID:0000001412@events.thp.uni-koeln.de DESCRIPTION:Hannu-Pekka Komsa\, Aalto University\, Finland\n\nSimulating R aman signatures of defective 2D materials\n\nRaman spectroscopy is a widel y used\, powerful\, and nondestructive tool for studying the vibrational p roperties of bulk and low-dimensional materials. Raman spectra can be simu lated using first-principles methods\, but due to the high computational c ost calculations are usually limited only to fairly small unit cells\, whi ch makes it difficult to carry out simulations for alloys and defects. We recently developed an efficient method for simulating Raman spectra at nea rly first-principles accuracy of (certain) systems which commonly require excessively large supercells. In my talk\, I will introduce the method and demonstrate its applicability in the case of 2D materials when the parent material is modified through alloying\, introduction of defects\, or stac king of layers\, although the approach is not limited to 2D materials and should be applicable to any crystalline solid with defects and impurities. As a result\, we can not only provide simulated spectra that can be direc tly compared to the experimental one\, but also gain atomistic insight on the modifications of the vibrational spectra that are responsible for the observed changes.\n\nContact Person: T. Michely LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Mukul Laad DTSTART;TZID=Europe/Berlin:20191016T143000 DTEND;TZID=Europe/Berlin:20191016T153000 DTSTAMP:20260527T090434Z UID:0000001440@events.thp.uni-koeln.de DESCRIPTION:Mukul Laad\, IMSc\, Chennai\n\nOn quantum criticality at a con tinuous metal-insulator transition\n\nMetal-insulator transitions (MIT)\, driven entirely by strong electron-electron interactions and/or disorder\, continue to underpin many novel phenomena in quantum matter. We study di sorder-driven MITs in the "strong disorder" category using a novel semi-an alytic cluster adaptation of dynamical mean-field theory (CDMFT) for the s implest\, binary-alloy disorder (also known as spinless Falicov-Kimball\, or "simplified" Hubbard) model. Such an (oversimplified) model is a caric ature for a disordered system where mobile electrons interact strongly wit h positionally random\, localized charges.\n\n We uncover anomalous quant um criticality in the full dc conductivity tensor\nacross the MIT which\, remarkably\, is reminiscent of "Mott quantum criticality"\nthat obtains at the low-T end-point of the line of first-order transitions in\nthe Hubbar d model (experimentally best studied for 2D organics). Interestingly\,\ns uch novel criticality is also found to underpin thermal transport (heat co nductivity\, Seebeck co-efficient\, Lorenz number) across the MIT. Moreov er\, we also provide an explicit realization of the famed "universal diele ctric response" (UDR) of Jonscher in optical responses across the MIT. Th e associated stretched-exponential relaxation in the long-time dielectric response strongly\nsuggests anomalous diffusion\, an emergent "electron gl assy" dynamics and weak ergodicity breaking. These findings suggest that a truly novel type of carrier localization obtains in the "strong disorder " limit.\n\nContact Person: Simon Trebst LOCATION:Seminar Room 0.02\, ETP END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Arie van Houselt DTSTART;TZID=Europe/Berlin:20191018T130000 DTEND;TZID=Europe/Berlin:20191018T140000 DTSTAMP:20260527T090434Z UID:0000001425@events.thp.uni-koeln.de DESCRIPTION:Arie van Houselt\, University Twente\n\nThe growth of hBN on I r(111): an in-situ LEEM and LEED study\n\n\n\nContact Person: Thomas Miche ly LOCATION:Seminar Room of the old Theory building END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Wojciech Grochala DTSTART;TZID=Europe/Berlin:20191106T143000 DTEND;TZID=Europe/Berlin:20191106T153000 DTSTAMP:20260527T090434Z UID:0000001419@events.thp.uni-koeln.de DESCRIPTION:Wojciech Grochala\, University of Warsaw\n\nSilverland\, terra (still) incognita of divalent silver (Ag2+)\n\nDivalent silver (Ag2+) is a homologue of its well-known copper sibling\, and it comes with Jahn-Tell er effect\, paramagnetism\, and strong influence of electronic correlation on chemical bonding. I will briefly outline our two-decade lasting effort s to show that many analogies exist between parent compounds of oxocuprate superconductors and fluorides of Ag2+. Strong magnetic superexchange and prospect for achieving superconductivity in the latter will be discussed.\ n\nContact Person: Markus Braden LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Kimberley Modic DTSTART;TZID=Europe/Berlin:20191119T143000 DTEND;TZID=Europe/Berlin:20191119T153000 DTSTAMP:20260527T090434Z UID:0000001383@events.thp.uni-koeln.de DESCRIPTION:Kimberley Modic\, Max-Planck-Institut für Chemische Physik fe ster Stoffe\, Dresden\n\nScale-invariant magnetic anisotropy in RuCl3 - a signature of spin-liquid behavior?\n\nThe continuum of excitations in RuCl 3 have been studied to high temperature—less-well studied are the effect s of high magnetic fields. We measured the magnetotropic coefficient—sec ond derivative of the free energy with respect to field-angle—up to 65 t esla. We find a scale-invariant response with applied temperature and magn etic field\, contrasting with the large exchange energy apparent in the an gle-dependence. This paradox hints at a strongly-correlated magnetic state in the high-field limit of RuCl3.\n\nContact Person: Simon Trebst LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Ruediger Klingeler DTSTART;TZID=Europe/Berlin:20191127T143000 DTEND;TZID=Europe/Berlin:20191127T153000 DTSTAMP:20260527T090434Z UID:0000001437@events.thp.uni-koeln.de DESCRIPTION:Ruediger Klingeler\, Universitaet Heidelberg\n\nMagnetism in γ-Li2FeSiO4\n\nLarge Li2FeSiO4 single crystals featuring the high tempera ture Pmnb-phase (γ-Li2FeSiO4) were grown for the first time by the optica l floating zone method under high Ar-pressure and the single crystal struc ture was solved. The system exhibits tetrahedrally coordinated Fe2+ ions i n a high-spin (i.e.\, \n\nContact Person: Markus Braden LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Andreas Danilewsky DTSTART;TZID=Europe/Berlin:20191211T143000 DTEND;TZID=Europe/Berlin:20191211T153000 DTSTAMP:20260527T090434Z UID:0000001470@events.thp.uni-koeln.de DESCRIPTION:Andreas Danilewsky\, Universität Freiburg\n\nDefect and Strai n Characterisation in Single Crystals by Diffraction Imaging\n\n\n\nContac t Person: Petra Becker-Bohatý LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Sergey Artyukhin DTSTART;TZID=Europe/Berlin:20191217T143000 DTEND;TZID=Europe/Berlin:20191217T153000 DTSTAMP:20260527T090434Z UID:0000001488@events.thp.uni-koeln.de DESCRIPTION:Sergey Artyukhin\, Italian Institute of Technology\, Genoa\, I taly\n\nLocalized vibrations in solids: domain walls\, cluster systems\n\n Topological defects in ferroic materials are low-dimensional systems that may enable promising applications. We study domain wall and vortex-localiz ed vibrations in proper and improper ferroelectrics and their role in diel ectric loss\, conductivity\, acoustic phonon scattering and elastic anomal ies. The lowest energy vibrations correspond to sliding modes of these top ological defects\, are polar and are found at microwave frequencies.\nAn o pposite situation of hard localized vibrations is found in cluster compoun ds\, where transition-metal ions form clusters. In these cases vibrations are hard. Particularly\, in IrTe2 they give rise to a complex phase diagra m with a large number of phases. A simplified model is proposed\, paramete rized from first principles\, and solved for the phase diagram. The influe nce of spin-orbit coupling\, pressure and doping is studied.\n\nContact Pe rson: Daniil Khomskii LOCATION:Room 0.01\, new theory building END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Shahal Ilani DTSTART;TZID=Europe/Berlin:20200108T143000 DTEND;TZID=Europe/Berlin:20200108T153000 DTSTAMP:20260527T090434Z UID:0000001436@events.thp.uni-koeln.de DESCRIPTION:Shahal Ilani\, Weizmann Institute\n\nCascade of Phase Transiti ons and Dirac Revivals in Magic Angle Graphene\n\nTwisted bilayer graphene near the magic angle exhibits remarkably rich electron correlation physic s\, displaying insulating\, magnetic\, and superconducting phases. Here\, using measurements of the local electronic compressibility\, we reveal tha t these phases originate from a high-energy state with an unusual sequence of band populations. As carriers are added to the system\, rather than fi lling all the four spin and valley flavors equally\, we find that the popu lation occurs through a sequence of sharp phase transitions\, which appear as strong asymmetric jumps of the electronic compressibility near integer fillings of the moire lattice. At each transition\, a single spin/valley flavor takes all the carriers from its partially filled peers\, "resetting " them back to the vicinity of the charge neutrality point. As a result\, the Dirac-like character observed near the charge neutrality reappears aft er each integer filling. Measurement of the in-plane magnetic field depend ence of the chemical potential near filling factor one reveals a large spo ntaneous magnetization\, further substantiating this picture of a cascade of symmetry breakings. The sequence of phase transitions and Dirac revival s is observed at temperatures well above the onset of the superconducting and correlated insulating states. This indicates that the state we reveal here\, with its strongly broken electronic flavor symmetry and revived Dir ac-like electronic character\, is a key player in the physics of magic ang le graphene\, forming the parent state out of which the more fragile super conducting and correlated insulating ground states emerge.\n\nContact Pers on: Simon Trebst LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Wolfram Brenig DTSTART;TZID=Europe/Berlin:20200115T143000 DTEND;TZID=Europe/Berlin:20200115T153000 DTSTAMP:20260527T090434Z UID:0000001490@events.thp.uni-koeln.de DESCRIPTION:Wolfram Brenig\, TU Braunschweig\n\nThermal transport in Kitae v magnets\n\nMott insulators with strong spin orbit coupling have become a playground to search for exotic quantum phases\, spin liquids and emergen t Majorana fermions. In this context I will present some of our results fo r the dynamics of Kitaev (-Heisenberg) models\, focusing on magnetic therm al transport\, but also on phonon relaxation. Fractionalization of magneti c moments into mobile Majorana matter and a static Z2 gauge field in the p ure Kitaev limit\, lead to characteristic spectral continua. These continu a are strongly affected by thermal flux excitations which act as an emerge nt disorder. I will discuss the consequences\, using results from several approaches comprising a mean-field theory\, complete summation over all ga uge sectors\, exact diagonalization\, and quantum typicality calculations. \n\nContact Person: P. van Loosdrecht LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Jochen Geck DTSTART;TZID=Europe/Berlin:20200122T143000 DTEND;TZID=Europe/Berlin:20200122T153000 DTSTAMP:20260527T090434Z UID:0000001471@events.thp.uni-koeln.de DESCRIPTION:Jochen Geck\, TU Dresden\n\nManipulating collective quantum st ates in layered 1T-TaS2 and alpha-RuCl3\n\nLayered materials host various collective quantum states such as intertwined electronic spatial orders\, superconductivity or topological spin liquids. I will present two examples from our recent work\, which are concerned with then manipulation of such collective quantum states. The first example is the laser-induced metal-i nsulator transition of 1T-TaS2\, which is currently discussed intensively in relation to a somewhat enigmatic hidden state. Our experiments reveal t he microscopic nature of this hidden state and show that the non-thermal m etal-insulator transition of 1T-TaS2 corresponds to a transition between t wo long-range ordered electronic crystals. The second example is alpha-RuC l3 - a candidate material for Kitaev magnetism and the corresponding spin- liquid ground state. However\, at ambient pressure and low temperatures th is material does not exhibit a spin-liquid phase\, but antiferromagnetic l ong-range order. We discovered a high-symmetry rhombohedral phase of alpha -RuCl3 at non-ambient pressure\, which may indeed support a Kitaev spin-li quid ground state.\n\nContact Person: M. Grüninger LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Hermann Dürr DTSTART;TZID=Europe/Berlin:20200218T153000 DTEND;TZID=Europe/Berlin:20200218T170000 DTSTAMP:20260527T090434Z UID:0000001497@events.thp.uni-koeln.de DESCRIPTION:Hermann Dürr\, Uppsala University\, Sweden\n\nTracking the no n-equilibrium energy flow between electron\, spin and lattice degrees of f reedom\n\nThe idea to probe\, change and control functional materials prop erties with the help of light has long intrigued researchers in materials science. Using femtosecond laser pulses it is now possible to control the magnetic order or even enhance superconductivity. Femtosecond laser excita tion of solid-state systems creates out-of-equilibrium hot electrons that cool down by transferring their energy to other degrees of freedom and ult imately to lattice vibrations of the solid. The understanding of this stro ngly non-equilibrium dynamics in solids is still very limited\, in spite o f its emerging importance from a fundamental and applied science viewpoint . The three-temperature model\, commonly used for magnetic metals\, assume s that the electronic\, spin and lattice subsystems are each in separate e quilibrium at all times and reach global equilibrium by exchanging heat. T he equilibrium concept of “temperature” is so powerful that is is ofte n ignored that it takes time before non-equilibrium dynamics can be descri bed by it. In this lecture I will give an overview how modern ultrafast sp ectroscopy and scattering techniques allow us to determine in a momentum-r esolved way the electron\, spin and lattice excitations in the laser-heate d state. These results demonstrate that we need to view non-equilibrium dy namics by its associated flow of energy between the various degrees of fre edom.\n\nContact Person: Paul van Loosdrecht LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Dennis Meier DTSTART;TZID=Europe/Berlin:20200219T143000 DTEND;TZID=Europe/Berlin:20200219T153000 DTSTAMP:20260527T090434Z UID:0000001492@events.thp.uni-koeln.de DESCRIPTION:Dennis Meier\, Uni Trondheim\n\nTopological defects and domain walls in chiral magnets\n\nDomain walls naturally arise whenever a symmet ry is spontaneously broken. They interconnect regions with different reali zations of the broken symmetry\, promoting structure formation from cosmol ogical length scales to the atomic level. In my talk\, I will present 1D s pin textures and domain walls with intriguing physical properties\, which emerge in spin-spiral multiferroics and chiral magnets and which hold grea t promise for nanoelectronics and spintronics applications. In particular\ , I will discuss that a wide variety of new domain walls occurs in the pre sence of spatially modulated domain states. In contrast to domain walls in conventional ferroics\, such domain walls exhibit a well-defined inner st ructure\, which—analogous to cholesteric liquid crystals—consists of t opological disclination and dislocation defects. Similar to the magnetic s kyrmions\, the domain walls can carry a finite topological charge\, permit ting an efficient coupling to spin currents and contributions to a topolog ical Hall effect. Our studies establish domain walls in chiral magnets as functional nano-objects with non-trivial topology\, opening the door to in novative device concepts in information and communication nanotechnology.\ n\nContact Person: T. Lorenz LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Naoto Nagaosa DTSTART;TZID=Europe/Berlin:20200506T100000 DTEND;TZID=Europe/Berlin:20200506T113000 DTSTAMP:20260527T090434Z UID:0000001525@events.thp.uni-koeln.de DESCRIPTION:Naoto Nagaosa\, University of Tokyo\n\nQuantum Nonlinearity an d Nonreciprocity\n\n\n\nContact Person: A. Rosch LOCATION:https://uni-koeln.zoom.us/j/94945416651 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Henry Legg DTSTART;TZID=Europe/Berlin:20200527T143000 DTEND;TZID=Europe/Berlin:20200527T153000 DTSTAMP:20260527T090434Z UID:0000001535@events.thp.uni-koeln.de DESCRIPTION:Henry Legg\, ITP\, University of Cologne\n\nResistance is futi le: Non-reciprocal transport in a nodal line material\n\npassword is 04284 9\n\nContact Person: Oliver Breunig LOCATION:Zoom ( https://uni-koeln.zoom.us/j/94311729441 ) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Fabian Lux DTSTART;TZID=Europe/Berlin:20200624T140000 DTEND;TZID=Europe/Berlin:20200624T150000 DTSTAMP:20260527T090434Z UID:0000001537@events.thp.uni-koeln.de DESCRIPTION:Fabian Lux\, Forschungszentrum Juelich\n\nJournal Club: Effect ive Seiberg-Witten gauge theory of noncollinear magnetism\n\n\n\nContact P erson: Michael Scherer LOCATION:Zoom ( https://uni-koeln.zoom.us/j/96851691514 ) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Andreas Rost DTSTART;TZID=Europe/Berlin:20200715T143000 DTEND;TZID=Europe/Berlin:20200715T153000 DTSTAMP:20260527T090434Z UID:0000001547@events.thp.uni-koeln.de DESCRIPTION:Andreas Rost\, University of St. Andrews\n\nDevelopments in To pological Semimetals: Creating and Controlling Weyl Fermions and the Quasi particle Dynamics of a Dirac Line Node material\n\n\n\nContact Person: Hen ry Legg LOCATION:Zoom ( https://uni-koeln.zoom.us/j/99456947424 ) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Matthias Roessler DTSTART;TZID=Europe/Berlin:20200722T143000 DTEND;TZID=Europe/Berlin:20200722T153000 DTSTAMP:20260527T090434Z UID:0000001538@events.thp.uni-koeln.de DESCRIPTION:Matthias Roessler\, PH2\n\nJournal Club: Superconductivity in top-down fabricated topological insulator nanowires\n\n\n\nContact Person: Ciaran Hickey LOCATION:Zoom ( https://uni-koeln.zoom.us/j/96851691514 ) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Alessio Chiocchetta DTSTART;TZID=Europe/Berlin:20200916T140000 DTEND;TZID=Europe/Berlin:20200916T150000 DTSTAMP:20260527T090434Z UID:0000001557@events.thp.uni-koeln.de DESCRIPTION:Alessio Chiocchetta\, University of Cologne\n\nJournal Club: C avity-induced Quantum Spin Liquids\n\n\n\nContact Person: Ciaran Hickey LOCATION:Zoom ( https://uni-koeln.zoom.us/j/96851691514 ) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Julia Link DTSTART;TZID=Europe/Berlin:20200925T140000 DTEND;TZID=Europe/Berlin:20200925T150000 DTSTAMP:20260527T090434Z UID:0000001558@events.thp.uni-koeln.de DESCRIPTION:Julia Link\, Simon Fraser University\n\nBogoliubov-Fermi surfa ces in non-centrosymmetric multi-component superconductors\n\nIn this talk \, I show that when the time reversal symmetry is broken in a multi-compon ent\nsuperconducting condensate without inversion symmetry the resulting B ogoliubov quasiparticles\ngenerically exhibit mini Bogoliubov-Fermi (BF) s urfaces\, for small superconducting order parameter.\nThe absence of inver sion symmetry makes the BF surfaces stable with respect to weak perturbati ons.\nWith sufficient increase of the order parameter\, however\, the Bogo liubov Fermi surface may disappear\nthrough a Lifshitz transition\, and th e spectrum this way become fully gapped [1].\nFurthermore\, an explicit ex ample for the emergence of BF surfaces in non-centrosymmetric multicompone nt\nsuperconductors is given by the discussion of d-wave superconductivity in Rarita-\nSchwinger-Weyl semimetals [2]. The three-dimensional Rarita-S chwinger-Weyl semimetals host\npseudospin-3/2 fermions at a fourfold linea r band crossing point and the spin-3/2 nature of the fermions\nallows for the formation of spin-2 Cooper pairs.\n[1] Julia M. Link and Igor F. Herbu t\, Arxiv:2006.10899\n[2] Julia M. Link and Igor F. Herbut\, Phys. Rev. B 101\, 184503 (2020)\n\nContact Person: Michael Scherer\, Achim Rosch LOCATION:Zoom ( URL tba ) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Joerg Schmalian DTSTART;TZID=Europe/Berlin:20201209T143000 DTEND;TZID=Europe/Berlin:20201209T153000 DTSTAMP:20260527T090434Z UID:0000001585@events.thp.uni-koeln.de DESCRIPTION:Joerg Schmalian\, KIT Karlsruhe\n\nNon-local transport and Lev y flights in Dirac systems\n\n\n\nContact Person: Achim Rosch LOCATION:Zoom ( https://uni-koeln.zoom.us/j/95048688263?pwd=WkxXMy8wVE1Jdi svSXJvMiszWUE2QT09 ) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Gertjan Lippertz DTSTART;TZID=Europe/Berlin:20210113T143000 DTEND;TZID=Europe/Berlin:20210113T160000 DTSTAMP:20260527T090434Z UID:0000001587@events.thp.uni-koeln.de DESCRIPTION:Gertjan Lippertz\, Physics Institute II (Ando's group)\n\nJour nal club: Scaling limitations of the quantum anomalous Hall effect in magn etically doped (Bi1-xSbx)2Te3\n\n\n\nContact Person: Oliver Breunig LOCATION:Zoom END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Jimin Wang DTSTART;TZID=Europe/Berlin:20210203T143000 DTEND;TZID=Europe/Berlin:20210203T153000 DTSTAMP:20260527T090434Z UID:0000001588@events.thp.uni-koeln.de DESCRIPTION:Jimin Wang\, University of Regensburg\n\nTwo-Dimensional-Dirac Surface States and Bulk Gap Probed via Quantum Capacitance in a Three-Dim ensional Topological Insulator\n\n\n\nContact Person: Oliver Breunig LOCATION:Zoom: https://uni-koeln.zoom.us/j/94243153241?pwd=WlZDNU1naHdZb0c 4ek93ZTBmQkVWQT09 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | CRC 1238 DTSTART;TZID=Europe/Berlin:20210218T093000 DTEND;TZID=Europe/Berlin:20210218T180000 DTSTAMP:20260527T090434Z UID:0000001607@events.thp.uni-koeln.de DESCRIPTION:CRC 1238\, University of Cologne\n\nKick-off Meeting CRC 1238\ n\n\n\nContact Person: Achim Rosch LOCATION:Zoom END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | CRC 1238 DTSTART;TZID=Europe/Berlin:20210219T093000 DTEND;TZID=Europe/Berlin:20210219T130000 DTSTAMP:20260527T090434Z UID:0000001608@events.thp.uni-koeln.de DESCRIPTION:CRC 1238\, University of Cologne\n\nKick-off Meeting CRC 1238\ n\n\n\nContact Person: Achjim Rosch LOCATION:Zoom END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Kornel Richter DTSTART;TZID=Europe/Berlin:20210224T143000 DTEND;TZID=Europe/Berlin:20210224T153000 DTSTAMP:20260527T090434Z UID:0000001606@events.thp.uni-koeln.de DESCRIPTION:Kornel Richter\, University of Kiel\n\nMagnetooptical observat ions of thin magnetic cylinders: theory and experiment\n\nMagnetooptical o bservations of thin magnetic cylinders: theory and experiment\n(K. Richter 1*\, O. Vahovsky3\, A. Thiaville2\, R. Varga3 and J. McCord1)\n\n1 Univers ity of Kiel\, Kaiserstraße 2\, 24143 Kiel\, Germany\n2University Paris Su d XI\, 914 00\, Orsay\, France\n3 University of Pavol Jozef Safarik\, 040 01 Kosice\, Slovakia\n*e-mail: kornel.richter@gmail.com\n\nThin cylindrica l wires attract considerable attention due to the interesting features of domain wall motion like absence of the Walker breakdown that prevents very high domain wall velocities [1]. While several experimental techniques al low for well controllable deposition of magnetic structures with complex g eometries [2]\, a reliable determination of the surface magnetization usua lly meets several obstacles in case of samples with curved surface. Here\, we provide a full analytical calculation of the Magneto-Optical Kerr Effe ct (MOKE) contrast for cylinders with reduced diameter. It is shown that t he cylindrical shape of sample surface gives rise to a spatial distributio n of the planes of incidence that are all tilted each to other [3]. Such m utual orientation of incident rays in combination with circumferential dep endence of a local angle of incidence gives rise to apparent magneto-optic al contrasts [3\,4] that cannot be interpreted well without considering th e curved surface of a cylinder. Theoretical calculations [3] are tested ex perimentally on amorphous glass-coated microwires with well-defined cylind rical shape. \nFinally\, we use our framework to study shape of the domain wall in FeSiB microwire characterized by very fast domain wall propagatio n. The setup is optimized for characteristic optical and magneto-optical f eatures of thin magnetic cylinders. Time-resolved measurements of a magnet ic contrast along the circumference of wire are used to reconstruct a thre e-dimensional image of a surface domain wall shape. Our measurements confi rm a tilted shape of domain wall [5\, 6]\, in agreement with previous obse rvations of microwires with positive magnetostriction alloy. The experimen tal technique represents a new way for direct visualizations of a surface magnetism of thin cylinders in optical range.\n\n[1] M. Yan\, et al.\, Phy s. Rev. Lett. 104\, 057201 (2010). \n[2] A. Fernández-Pacheco\, et al.\, Sci. Rep. 3\, 1492 (2013). \n[3] K. Richter\, et al.\, Phys. Rev.B 96\, 06 4421 (2017). \n[4] K. Richter\, et al.\, IEEE Trans. Magn. 50\, 2501404 (2 014). \n[5] K. Richter\, et al.\, J. Appl. Phys.127\, 193305 (2020).\n[6] O. Vahovsky\, et al.\, J. Magn. Magn. Mater. 483\, 266 (2019).\n\nContact Person: Ionela Lindfors-Vrejoiu LOCATION:Zoom: https://uni-koeln.zoom.us/j/96710889575?pwd=MjVKdkk1dnZKazB vVkdTdk5TTHZyUT09 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Huimei Liu 刘慧美 DTSTART;TZID=Europe/Berlin:20210505T143000 DTEND;TZID=Europe/Berlin:20210505T153000 DTSTAMP:20260527T090434Z UID:0000001632@events.thp.uni-koeln.de DESCRIPTION:Huimei Liu 刘慧美\, MPI-FKF Stuttgart\n\nTowards Kitaev Spi n Liquids in 3d Transition Metal Compounds\n\nWe have proposed to realize the Kitaev spin liquid state in 3d honeycomb cobaltates\, due to the more localized wave functions of 3d ions compared with that of 4d and 5d ions\, and also the easy tunability of the exchange Hamiltonian in favor of Kita ev interaction. In this talk\, I will discuss the key parameters which hav e very large impacts on the exchange constant\, and the crystal field effe ct is emphasized as an efficient phase control method driving the magnetic ally ordered cobaltates into the spin liquid state. In good agreement with the experimental data\, our theoretical predications verify the great pot ential of realizing the Kitaev spin liquid in 3d transition metal compound s.\n\nContact Person: Simon Trebst LOCATION:Zoom (https://uni-koeln.zoom.us/j/97571937622?pwd=QzUxNTFEN05abmN yWGUxUjRpWTRDdz09) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Matthias Vojta DTSTART;TZID=Europe/Berlin:20210512T143000 DTEND;TZID=Europe/Berlin:20210512T153000 DTSTAMP:20260527T090434Z UID:0000001616@events.thp.uni-koeln.de DESCRIPTION:Matthias Vojta\, TU Dresden\n\nLocal quantum criticality near the Mott transition\n\nTextbook quantum criticality refers to phenomena wh ich originate in states which scale-invariant in both space and time\, occ urring at continuous zero-temperature phase transitions. Various experimen tal findings have raised interest in local forms of quantum criticality\, where scale invariance in space in either absent or irrelevant. Prominent examples include variants of the Mott metal-to-insulator transition in a n umber of strongly correlated electron compounds. After an extended introdu ction\, I will discuss one of the simplest cases\, namely the bandwidth-tu ned Mott transition in a single-band setting. Recent transport studies\, b oth theoretical and experimental\, near this transition have uncovered - s urprisingly - apparent quantum critical scaling of the electrical resistiv ity at elevated temperatures\, despite the fact that the actual low-temper ature phase transition is of first order. This raises the question whether there is a hidden Mott quantum critical point. I will argue that the dyna mical mean-field theory of the Hubbard model admits\, in the low-temperatu re limit\, scale-invariant (i.e. power-law) solutions which had been overl ooked before. It is this incoherent power-law regime\, corresponding to ap proximate local quantum criticality\, which is continuously connected to a nd responsible for the apparent quantum critical scaling above the classic al critical end point. I will discuss the role of magnetic frustration in these phenomena\, non-local corrections\, and broader implications for the phenomenology of bad metals.\n\nContact Person: Achim Rosch LOCATION:Zoom (Meeting-ID: 994 8387 5045 Password: CRC1238 | https://uni-k oeln.zoom.us/j/99483875045?pwd=K1RQbHV1eFdTVldCK2NUOWpTNFhzdz09) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Tomas Brage DTSTART;TZID=Europe/Berlin:20210518T160000 DTEND;TZID=Europe/Berlin:20210518T170000 DTSTAMP:20260527T090434Z UID:0000001630@events.thp.uni-koeln.de DESCRIPTION:Tomas Brage\, Department of Physics\, Lund University\n\nOnly the Speed of Light limits You? - Discrimination and its impact on the acad emic field of physics (1st talk - Gender & Physics)\n\nOnly the Speed of L ight limits You? - Discrimination and its impact on the academic field of physics\n\nEveryone who is good enough in physics can work in the academic field\, no matter what their identity is besides that\, right? We do not have any issues with discrimination\, or do we? Is our study and work envi ronment really benefiting everyone equally? Or are people marginalized her e as well – and if so\, what is going on? And what can we do about it?\n \nThe talk will be given by Prof. Tomas Brage. He has a very successful ca reer in physics and is currently a physics professor at the Division of Ma thematical Physics of the Department of Physics at Lund University. Beside s all his publications in this field\, he also does research concerning “Gender and Physics”. He will inform us about numbers\, bias\, and ste reotypes regarding gender\, and their influence on physics. After his talk \, we can also ask questions and have a discussion about the topic.\n\nCon tact Person: Maria Melamed LOCATION:Zoom (https://uni-koeln.zoom.us/j/96279829895?pwd=Sk1KWFFzZlRBUEF 0cjI2cjM5bjdtQT09) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Amilcar Bedoya Pinto DTSTART;TZID=Europe/Berlin:20210526T143000 DTEND;TZID=Europe/Berlin:20210526T153000 DTSTAMP:20260527T090434Z UID:0000001629@events.thp.uni-koeln.de DESCRIPTION:Amilcar Bedoya Pinto\, Max Planck Institute of Microstructure Physics\n\nRecent developments of magnetism in flatland: Realization of 2 D-XY ferromagnetism in a van der Waals monolayer\n\n\n\nContact Person: Je ison Fischer LOCATION:Zoom (https://uni-koeln.zoom.us/j/98166130629?pwd=RmhiMHRIM0ZiV0F xRWx2WG9ld2dXQT09) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Johannes Gooth DTSTART;TZID=Europe/Berlin:20210609T143000 DTEND;TZID=Europe/Berlin:20210609T153000 DTSTAMP:20260527T090434Z UID:0000001635@events.thp.uni-koeln.de DESCRIPTION:Johannes Gooth\, MPI for Chemical Physics of Solids\, Dresden\ n\nQuantum-Hall physics and beyond in ZrTe5\n\nThe discovery of the quantu m Hall effect (QHE) in 1980 marked a turning point in condensed matter \np hysics. The measurement of the Hall conductivity of a two-dimensional elec tron system showed that \nelectrical conduction can be precisely defi ned only in terms of fundamental constants. But\, what \nhappens t o the QHE in three dimensions? At a first glance\, the answer seems straig htforward and \nsobering: The quantization of the Hall conductivity gets destroyed. This has led to the general \nconjecture that quan tum-Hall physics is exclusive to two dimensions. However\, we show that th e Hall \neffect of genuine three-dimensional (3D) metals is actually d eeply related to the QHE and can be \nunderstood from quantum Hall phy sics. We propose that - as a general rule - plateau-like features in \nthe Hall conductivity of 3D electron systems are precisely given by the condu ctance quantum\, scaled \nby the Fermi wave vector of the electrons along the magnetic field direction.\n\nContact Person: Yoichi Ando LOCATION:Zoom (https://uni-koeln.zoom.us/j/97572764640?pwd=aGhQU0VuUkZsYm1 NbnJ6emJualVCdz09) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Sherard Robbins DTSTART;TZID=Europe/Berlin:20210615T170000 DTEND;TZID=Europe/Berlin:20210615T180000 DTSTAMP:20260527T090434Z UID:0000001631@events.thp.uni-koeln.de DESCRIPTION:Sherard Robbins\, CEO of Visceral Change\n\nOnly the Speed of Light limits You? - Discrimination and its impact on the academic field of physics (2nd talk - Racism & Physics)\n\nEveryone who is good enough in p hysics can work in the academic field\, no matter what their identity is b esides that\, right? We do not have any issues with discrimination\, or do we? Is our study and work environment really benefiting everyone equally? Or are people marginalized here as well – and if so\, what is going on? And what can we do about it?\n\nThe event will be with Sherard Robbins\, a development consultant\, author\, and CEO of Visceral Change. He will ad dress the topic of “Racism and Physics” and present us some background information and numbers about it. There will also be space for discussion s: How does it affect our work environment and what can we do about it?\n\ nContact Person: Maria Melamed LOCATION:Zoom (https://uni-koeln.zoom.us/j/96279829895?pwd=Sk1KWFFzZlRBUEF 0cjI2cjM5bjdtQT09) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Jens Brede DTSTART;TZID=Europe/Berlin:20210623T143000 DTEND;TZID=Europe/Berlin:20210623T153000 DTSTAMP:20260527T090434Z UID:0000001639@events.thp.uni-koeln.de DESCRIPTION:Jens Brede\, II. Physikalisches Institut (UzK)\n\nJournal Club : The Vertical Position of Sr Dopants in the SrxBi2Se3 Superconductor\n\n\ n\nContact Person: Oliver Breunig LOCATION:Zoom ( https://uni-koeln.zoom.us/j/96349253814?pwd=MUNtZFRwRmtMR0 V5VFZMbTdPeDdXZz09) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Bruno Schuler DTSTART;TZID=Europe/Berlin:20210630T143000 DTEND;TZID=Europe/Berlin:20210630T153000 DTSTAMP:20260527T090434Z UID:0000001637@events.thp.uni-koeln.de DESCRIPTION:Bruno Schuler\, Nanotech@surfaces Laboratory - EMPA\, Switzerl and\n\nThe Beauty of Imperfection - An Atomic-Scale Look at Point Defects in 2D Semiconductors\n\n2D materials are an exciting host to engineer atom ic quantum systems by chemical design rules. In this colloquium\, I will g ive an overview on our efforts to design and probe defect systems in monol ayer transition metal dichalcogenides (TMDs) by means of chemical doping\, Helium ion beam bombardment and atomic manipulation. \nBy means of low-te mperature scanning probe microscopy\, we directly resolve the discrete ele ctronic spectrum of single dopants in a charge neutral or ionized state an d map out their associated defect orbitals [1-5]. Different types of defec ts reveal the interplay between chemical impurity states [1-3]\, multi-val ley hydrogenic bound states [4\,6]\, and electron-phonon coupling [1\,7] a t reduced dimensions. In particular\, we will discuss the atomically contr olled generation of carbon radical ions (CRIs) by hydrogen depassivation [ 7] and electrically driven photon emission from individual defects [8].\n\ nReferences:\n[1] B. Schuler et al.\, Phys. Rev. Lett. 123\, 076801 (2019) \n[2] S. Barja et al.\, Nat. Commun. 10\, 3382 (2019)\n[3] B. Schuler et a l.\, ACS Nano 13\, 10520 (2019)\n[4] M. Aghajanian et al.\, Phys. Rev. B 1 01\, 081201(R) (2020)\n[5] E. Mitterreiter et al.\, Nano Lett. 20\, 4437 ( 2020) \n[6] K. Cochrane et al.\, 2D Mater. 7\, 031003 (2020) \n[7] K. Coch rane et al.\, arxiv: 2008.12196\n[8] B. Schuler et al.\, Sci. Adv. 6\, eab b5988 (2020)\n\nContact Person: Wouter Jolie LOCATION:Zoom (https://uni-koeln.zoom.us/j/94292208989?pwd=OFh4Wi9yTW1Sdmp ueGo2aklUTlV1UT09) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Ashley Cook DTSTART;TZID=Europe/Berlin:20210721T143000 DTEND;TZID=Europe/Berlin:20210721T153000 DTSTAMP:20260527T090434Z UID:0000001648@events.thp.uni-koeln.de DESCRIPTION:Ashley Cook\, MPI PKS\n\nTopological skyrmion phases of matter \n\nThe search for novel topological phases of matter is on-going\, with m any recent efforts focusing on study of non-electronic and other analogues of the foundational electronic topological phases of matter of the ten-fo ld way. We return to study of the original electronic topological phases b y introducing the topological skyrmion phases of matter. These are symmetr y-protected topological phases defined by skyrmions\, or topological spin textures\, in momentum-space for Bloch Hamiltonians with more than two ban ds. These topological phases are protected by a generalized particle-hole symmetry C' present in some centrosymmetric superconductors. We first pres ent a method for constructing various four-band toy models for skyrmion ph ases in two-dimensions\, including models describing superconductors\, and discuss some experimental signatures. We then introduce three-dimensional topological skyrmion phases and show that they display a bulk-boundary co rrespondence\, trapping exotic localized "cross" zero-mode states at defec ts in the bulk and on the boundary of the system. Finally\, we consider a more complex tight-binding model for spin-triplet superconductivity in tra nsition metal oxides\, which can realize topological skyrmion phases of ma tter in individual mirror subsectors\, even for a parameter set previously used to model superconducting Sr2RuO4. Importantly\, we find two types of topological phase transitions by which the skyrmion number can change. In a superconductor\, the first kind occurs with a closing of the supercondu cting gap. The second kind\, however\, occurs when spin is not conserved a nd without the closing of direct energy gaps. As this type-II topological phase transition occurs without breaking the protecting symmetry of the to pological phase in an effectively non-interacting system\, it contradicts the "flat band" limit assumption of the ten-fold way classification scheme of topological phases of matter.\n\nContact Person: S. Diehl LOCATION:Zoom ( URL https://uni-koeln.zoom.us/j/99781388489?pwd=U1FKR1ZRY zFTUUEwRzlodjlBbm1CZz09 ) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Justin Wilson DTSTART;TZID=Europe/Berlin:20211020T143000 DTEND;TZID=Europe/Berlin:20211020T153000 DTSTAMP:20260527T090434Z UID:0000001659@events.thp.uni-koeln.de DESCRIPTION:Justin Wilson\, Louisiana State University\n\nIncommensurabili ty-driven phases\, flat bands\, and topology in two-dimensional materials\ n\nBy quenching electron kinetic energy\, one can reveal correlated phases in previously weakly correlated materials\, such as in twisted bilayer gr aphene. Disorder typically obscures correlated phases\; for instance\, it might just lead to localized electron wave functions. Incommensurability-d riven phases provide a platform to observe both phase transitions typicall y associated with disorder (Anderson-like transitions) as well as flat ban ds and correlated phases. In this talk\, I will discuss our work to unders tand this\, particularly within the context of topological band structures . In both two-dimensional semimetals and topological insulators\, we see t hat disorder induces a delocalization eigenstate transition in momentum sp ace. The wave functions exhibit multifractality in this phase\, and in the case of two-dimensional semimetals\, it occurs at the "magic-angle" as fo r twisted bilayer graphene. Concomitantly\, there is a precise sense in wh ich the "bands" become increasingly flat at these transitions. In the case of the topological insulator\, the Berry curvature also becomes more unif orm as the wave functions near criticality: clearing the way for potential fractional Chern insulators in these systems.\n\nContact Person: not spec ified LOCATION:Institute of Physics II Seminarraum END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Prof. Jiadong Zang (New Hampshire/Cologne) DTSTART;TZID=Europe/Berlin:20211027T163000 DTEND;TZID=Europe/Berlin:20211027T173000 DTSTAMP:20260527T090434Z UID:0000001668@events.thp.uni-koeln.de DESCRIPTION:Prof. Jiadong Zang (New Hampshire/Cologne)\n\nTopological spin textures from 2D to 3D\n\nThe talk will be given in the HS III. You can a ll attend in presence. \n\nAs an alternative you can also participate via zoom: https://uni-koeln.zoom.us/j/4225866762\n\nContact Person: Achim Ros ch LOCATION:HS III END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Matthias Rößler DTSTART;TZID=Europe/Berlin:20211110T160000 DTEND;TZID=Europe/Berlin:20211110T170000 DTSTAMP:20260527T090434Z UID:0000001676@events.thp.uni-koeln.de DESCRIPTION:Matthias Rößler\n\nIdeas Lab - Advancing ideas together\n\n\ n\nContact Person: Matthias Rößler LOCATION:HS III END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Alexey Kimel DTSTART;TZID=Europe/Berlin:20211201T160000 DTEND;TZID=Europe/Berlin:20211201T170000 DTSTAMP:20260527T090434Z UID:0000001672@events.thp.uni-koeln.de DESCRIPTION:Alexey Kimel\n\nUltrafast emergence of ferromagnetism in antif erromagnetic FeRh // Zoom link: https://uni-koeln.zoom.us/j/4225866762\n\ n\n\nContact Person: Paul van Loosdrecht LOCATION:HS III END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Tobias Holder DTSTART;TZID=Europe/Berlin:20211215T143000 DTEND;TZID=Europe/Berlin:20211215T153000 DTSTAMP:20260527T090434Z UID:0000001683@events.thp.uni-koeln.de DESCRIPTION:Tobias Holder\, Weizmann Institute\n\nElectrons flow like fall ing cats: Deformations and emergent gravity in quantum transport\n\nWhen c old electrons move in a perfect\, static\, and stable lattice\, according to conventional wisdom\, they move like almost free quasiparticles in flat space. Here\, we present evidence that this is not entirely correct\, and quasiparticles actually move in an emergent curved spacetime.\nTo this en d\, we discuss the second order electrical conductivity in materials lacki ng inversion and time-reversal symmetry\, which exhibits a mixed axial-gra vitational anomaly. We can explain this surprising result in terms of dyna mical deformations of the semiclassical wavepacket as it moves through the periodic lattice potential\, thereby establishing a common framework for the appearance of anomalous terms in many response functions. Our proposit ion that quasiparticles behave essentially like quantum cats has powerful implications for all types of quantum transport and may allow to probe syn thetic gravitational fields in a bulk condensed matter setting.\n\nContact Person: Simon Trebst LOCATION:zoom / https://uni-koeln.zoom.us/j/97104991740?pwd=Y0RhNk9ReVExT0 lpWjdDL1ZjZ29idz09 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Dr. Rejaul SK DTSTART;TZID=Europe/Berlin:20220110T140000 DTEND;TZID=Europe/Berlin:20220110T150000 DTSTAMP:20260527T090434Z UID:0000001689@events.thp.uni-koeln.de DESCRIPTION:Dr. Rejaul SK\n\nElectron and spin-based properties of metal-c oordination chain and network architectures\n\n\n\nContact Person: Jeison Fischer LOCATION:HS I and zoom: https://uni-koeln.zoom.us/j/92573385974?pwd=QjlzcX NUZlkrS0Nxdy9BSDZmeFVoZz09 Meeting ID: 925 7338 5974 Password: 387733 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Joern Bruns DTSTART;TZID=Europe/Berlin:20220202T160000 DTEND;TZID=Europe/Berlin:20220202T173000 DTSTAMP:20260527T090434Z UID:0000001684@events.thp.uni-koeln.de DESCRIPTION:Joern Bruns\, University of Cologne\n\nChemistry under extreme conditions – from polycations with 1D metal behaviour to reactions unde r GPa pressure\n\nRude colleagues say that solid states chemists follow fo r their reactions only the principle "shake and bake". Of course\, high te mperatures help to overcome activation barriers. However\, we all know tha t this is not enough. A multitude of other parameters play an essential ro le for efficient solid-state reactions. I will present you how we use extr emely strong oxidizing agents\, such as sulfuric acid or its anhydride SO3 \, to synthesize silicate analogous materials such as borosulfates. In the se\, the charge compensating heteropolyanionic subunits are composed of ve rtex-linked (SO4)- and (BO4)-tetrahedra. In contrast to the immense struct ural diversity of silicates\, the number of borosulfates is yet very limit ed and the extent of their properties is still unknown. Furthermore\, allo w the extreme reaction conditions for the stabilization of cationic specie s with unprecedented properties\, like 1D metal behavior in [Au2Cl4](B(S2O 7)2](SO3). In an effort to expand the knowledge on oxoanionic networks eve n further\, we have also gone and tested our systems on rhenates\, mangana tes and technetates. In this context\, the handling of technetium and its compounds is another challenge. In addition to the acids\, strong alkaline media can be a perfect medium for the synthesis of oxidic and oxoanionic materials\, and the application of pressure in O2 gas autoclaves or the us e of a Multianvil press (pmax = 26 GPa) allows for investigations of the a fore mentioned reactions in a regime that is still almost unexplored. \nUs ing a wide array of analytical methods such as optical spectroscopy\, magn etochemistry and photoelectron spectroscopy we try to investigate all samp les as detailed as possible and corroborate our findings by quantum chemic al calculations. \nHerein\, I will therefore present an overview of our in vestigations on borosulfates and oxometallates\, the effects of the counte r cations on the chemistry and some high pressure approaches that we have access to at the University of Cologne. It will turn out that “shake and bake” does not always hold true\, and that there is more to solid state chemistry than meets the eye.\n\nContact Person: Markus Braden LOCATION:HS III (zoom: https://uni-koeln.zoom.us/j/4225866762) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Yasir Iqbal DTSTART;TZID=Europe/Berlin:20220216T143000 DTEND;TZID=Europe/Berlin:20220216T153000 DTSTAMP:20260527T090434Z UID:0000001695@events.thp.uni-koeln.de DESCRIPTION:Yasir Iqbal\, IIT Madras\, Chennai\n\nQuantum Spin Liquid Phys ics on a novel square-kagome lattice material KCu6AlBiO4(SO4)5Cl\n\nMotiva ted by a recent reporting of a first of a kind experimental realization of a gapless quantum spin liquid state on the novel square-kagome lattice ge ometry realized in the spin S=1/2 system KCu6AlBiO4(SO4)5Cl [1]\, we discu ss the possible microscopic origins of this state\, and more broadly highl ight this novel geometry as an ideal playground for stabilizing a diverse array of exotic quantum phases [2]. Employing state-of-the-art quantum man y-body numerical techniques such as variational Monte Carlo (VMC) with ver satile Gutzwiller-projected Jastrow wave functions\, unconstrained multi-v ariable variational Monte Carlo (mVMC)\, and pseudo-fermion/Majorana funct ional renormalization group (PF/PM-FRG) methods\, we establish the presenc e of a quantum paramagnetic ground state and investigate its nature\, by c lassifying symmetric and chiral quantum spin liquids\, and inspecting thei r instabilities towards competing valence-bond-crystal (VBC) orders. Our a nalysis reveals that a VBC with a pinwheel structure of spin correlations emerges as the lowest-energy variational ground state\, and it is obtained as an instability of the U(1) Dirac spin liquid. We argue that further ne ighbor Heisenberg and/or Dzyaloshinkii-Moriya interactions play a crucial role towards explaining the experimental observations. \n\nReferences:\n\n [1] Nat. Commun. 11\, 3429 (2020)\, Gapless spin liquid in a square-kagome lattice antiferromagnet\, Masayoshi Fujihala\, Katsuhiro Morita\, Richard Mole\, Setsuo Mitsuda\, Takami Tohyama\, Shin-ichiro Yano\, Dehong Yu\, S higetoshi Sota\, Tomohiko Kuwai\, Akihiro Koda\, Hirotaka Okabe\, Hua Lee\ , Shinichi Itoh\, Takafumi Hawai\, Takatsugu Masuda\, Hajime Sagayama\, Ak ira Matsuo\, Koichi Kindo\, Seiko Ohira-Kawamura & Kenji Nakajima\n\n[2] P hys. Rev. B 104\, L220408 (2021)\, Pinwheel valence-bond-crystal ground st ate of the spin-1/2 Heisenberg antiferromagnet on the shuriken lattice\, N ikita Astrakhantsev\, Francesco Ferrari\, Nils Niggemann\, Tobias Müller\ , Aishwarya Chauhan\, Augustine Kshetrimayum\, Pratyay Ghosh\, Nicolas Reg nault\, Ronny Thomale\, Johannes Reuther\, Titus Neupert\, Yasir Iqbal\n\n Zoom-Link: https://uni-koeln.zoom.us/j/9524918125?pwd=dWN1RW1mMUpDRWdyMWwz WGtURWVIZz09\n\nContact Person: Simon Trebst LOCATION:Seminar Room of the Institute of Physics II\, zoom END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Kevin Jenni DTSTART;TZID=Europe/Berlin:20220427T170000 DTEND;TZID=Europe/Berlin:20220427T183000 DTSTAMP:20260527T090434Z UID:0000001728@events.thp.uni-koeln.de DESCRIPTION:Kevin Jenni\n\nMeet the Alumni - CRC1238 Students council\n\nZ oom PW: 573506\n\nContact Person: not specified LOCATION:Zoom (https://uni-koeln.zoom.us/j/99974200575 ) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Inanc Adagideli DTSTART;TZID=Europe/Berlin:20220504T160000 DTEND;TZID=Europe/Berlin:20220504T173000 DTSTAMP:20260527T090434Z UID:0000001721@events.thp.uni-koeln.de DESCRIPTION:Inanc Adagideli\, Sabanci University\n\nQuantum dynamics and c omputation at the topological edge\n\nTopological edge states are protecte d states that form between two insulators with different topological prope rties. The dimensionality of the interface\, as well as the type of insula tors that form it\, endow the topological edge state further properties. I will focus on situations where one of the materials is a superconductor - -a quasiparticle insulator. In the case where the edge is zero dimensiona l\, the topological states are Majorana zero-modes localized in the core o f a vortex or bound to the end of a nanowire. They are anyons with non-Abe lian braiding statistics\, but when they are immobile\, one cannot demonst rate this by exchanging them in real space and therefore indirect methods are usually needed.\n\nI will talk about how to use the chiral motion alon g the boundary of the superconductor to braid a mobile vortex\, either wit h another mobile vortex in the edge channel or with an immobile vortex in the bulk. When two vortices are fused\, they transfer charge into a metal contact. We calculate the time dependent current profile for the fusion pr ocess\, which consists of ±e/2 charge pulses that flip sign if the world lines of the vortices are braided prior to the fusion. This is an electric al signature of the non-Abelian exchange of Majorana zero-modes. \n\nNe xt\, I will discuss how one can deconfine the Majorana zero-modes localize d in the vortex cores by applying a spatially oscillating pair potential. In the deconfined phase at zero chemical potential the Majorana fermions f orm a dispersionless Landau level\, protected by chiral symmetry against b roadening due to vortex scattering. The Majorana Landau level also has a s ignature that distinguishes it from a regular Landau level: the coherent s uperposition of electrons and holes in the Majorana Landau causes a local density of states oscillation with a wave vector set by the Cooper pair mo mentum as well as the inverse coherence length. This striped density of st ates pattern also provides a means to measure the chirality of the Majoran a fermions. I will conclude by discussing the quantal dynamics in the deco nfined phase.\n\nContact Person: Erwann Boquillon LOCATION:0.03 ETP building END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Saquib Shamin/ University of Würzburg DTSTART;TZID=Europe/Berlin:20220615T160000 DTEND;TZID=Europe/Berlin:20220615T170000 DTSTAMP:20260527T090434Z UID:0000001720@events.thp.uni-koeln.de DESCRIPTION:Saquib Shamin/ University of Würzburg\n\nKondo interactions o f quantum spin Hall edge channels with charge puddles\n\nIn this talk\, I will discuss some of our recent results on the quantum spin Hall effect in HgTe-based two-dimensional topological insulators. In the first part\, I will show that the conductance quantization due to the quantum spin Hall e ffect can be observed even in the presence of magnetic impurities. Our exp eriments on (Hg\,Mn)Te quantum wells with an inverted band structure revea l that the quantum spin Hall quantization is observed only at low temperat ures (T < 400 mK)\, where Kondo screening of the magnetic impurities suppr esses backscattering. The second part of the talk will focus on understand ing the fluctuations in the quantized spin Hall conductance that has been observed in almost all realizations of the quantum spin Hall effect - incl uding the first paper in 2007. To examine these fluctuations\, we fabricat e high-quality quantum spin Hall microstructures using a carefully optimiz ed wet-etching process. We perform temperature and gate-dependent measurem ents in the regime of quantized conductance. The fluctuations in conductan ce show a characteristic temperature dependence that is related to Kondo i nteractions of helical edge channels with small puddles. These charge pudd les - acting as Kondo correlated quantum dots due to small dimensions - ar ise due to fluctuations in the potential landscape in narrow gap semicondu ctors.\n\nContact Person: Erwann Boquillon LOCATION:room 0.03 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Hamoon Hedayat & Michael Buchhold DTSTART;TZID=Europe/Berlin:20220629T143000 DTEND;TZID=Europe/Berlin:20220629T153000 DTSTAMP:20260527T090434Z UID:0000001733@events.thp.uni-koeln.de DESCRIPTION:Hamoon Hedayat & Michael Buchhold\n\nOpen discussion: nontherm al states of matter\n\nhttps://uni-koeln.zoom.us/j/95031559873?pwd=d1gwRlh laXYrK085S0lqU295ZlNkZz09\n\nMeeting ID: 950 3155 9873\n\nPassword: 745603 \n\nContact Person: Hamoon Hedayat LOCATION:Seminar Room of the Institute of Physics II / https://uni-koeln.z oom.us/j/95031559873?pwd=d1gwRlhlaXYrK085S0lqU295ZlNkZz09 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Alexander Grüneis & Wouter Jolie DTSTART;TZID=Europe/Berlin:20220706T160000 DTEND;TZID=Europe/Berlin:20220706T170000 DTSTAMP:20260527T090434Z UID:0000001734@events.thp.uni-koeln.de DESCRIPTION:Alexander Grüneis & Wouter Jolie\n\n2D Materialien\n\n\n\nCon tact Person: Wouter Jolie LOCATION:room 0.03/ https://uni-koeln.zoom.us/j/97355091986?pwd=VDR1ZitSOW VuNm5sQU5GZTZGQm50QT09 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Oliver Breunig & Erwann Bocquillon DTSTART;TZID=Europe/Berlin:20220713T160000 DTEND;TZID=Europe/Berlin:20220713T170000 DTSTAMP:20260527T090434Z UID:0000001736@events.thp.uni-koeln.de DESCRIPTION:Oliver Breunig & Erwann Bocquillon\n\nTopical Discussion: Topo logical Devices\n\n\n\nContact Person: Oliver Breunig LOCATION:room 0.03 / https://uni-koeln.zoom.us/j/95537984043?pwd=WU9QcU42Z UFEZFliejF5SUp0UnR4dz09 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Ciaran Hickey DTSTART;TZID=Europe/Berlin:20220715T140000 DTEND;TZID=Europe/Berlin:20220715T150000 DTSTAMP:20260527T090434Z UID:0000001747@events.thp.uni-koeln.de DESCRIPTION:Ciaran Hickey\n\nBrainstorming Session: Spin-Orbit Coupled Mag nets\n\n\n\nContact Person: Ciaran Hickey LOCATION:Seminar Room 0.01\, ETP and https://uni-koeln.zoom.us/j/978634718 03 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Sergey Artyukhin DTSTART;TZID=Europe/Berlin:20220801T140000 DTEND;TZID=Europe/Berlin:20220801T153000 DTSTAMP:20260527T090434Z UID:0000001750@events.thp.uni-koeln.de DESCRIPTION:Sergey Artyukhin\n\nMagnetoelectric switching in GdMn2O5 and s piral multiferroics\n\n\n\nContact Person: Paul van Loosdrecht LOCATION:room 0.03 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Norio Kumada/ NTT Basic Research Labs Japan DTSTART;TZID=Europe/Berlin:20220914T160000 DTEND;TZID=Europe/Berlin:20220914T170000 DTSTAMP:20260527T090434Z UID:0000001744@events.thp.uni-koeln.de DESCRIPTION:Norio Kumada/ NTT Basic Research Labs Japan\n\nUltrafast optic al-to-electrical conversion in graphene investigated by on-chip THz spectr oscopy\n\nMain subject I will talk is ultrafast photocurrent generation an d transport in high-quality graphene photodetector. The current generated by femtosecond laser pulse through photo-thermoelectric effect was detecte d in time domain with the resolution of faster than 1 picosecond by on-chi p THz spectroscopy. We could comprehensively understand how incident light is converted to electrical current in graphene\, which is crucial informa tion for developing ultrafast graphene optoelectronics.\nI will also brief ly talk about two other subjects in our group: Growth and transport in InA s/InGaSb two-dimensional topological insulator\, and Coherent transport of fractional quasiparticle at fractional-integer quantum Hall junction.\nRe ference: Yoshioka\, NK et al.\, Nature Photonics (accepted)\, arXiv:2203.0 5752\n\nContact Person: Erwann Boquillon LOCATION:room 0.03 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Prof. Daniele Fausti DTSTART;TZID=Europe/Berlin:20221006T133000 DTEND;TZID=Europe/Berlin:20221006T143000 DTSTAMP:20260527T090434Z UID:0000001755@events.thp.uni-koeln.de DESCRIPTION:Prof. Daniele Fausti\n\nQuantum spectroscopies for quantum mat erials\n\nThe rich phase diagrams of many transition metal oxides (TMOs) i s the result of the intricate interplay between electrons\, phonons\, and magnons. This makes TMOs very susceptible to external parameters such as p ressure\, doping\, magnetic field\, and temperature which in turn can be u sed to finely tune their properties. The same susceptibility makes TMOs th e ideal playground to design experiments where the interaction between tai lored electromagnetic fields and matter can trigger the onset of new\, som etimes exotic\, physical properties. This aspect has been explored in time domain studies [1] and has led to the demonstration that ultrashort mid-I R light pulses can “force” the formation of quantum coherent states in matter\, disclosing a new regime of physics where thermodynamic limits ma y be bridged and quantum effects can\, in principle\, appear at ambient te mperatures. \nIn this presentation\, I will review our recent results in archetypal strongly correlated cuprate superconductors and demonstrate th e feasibility of a light-based control of quantum phases in real materials [2\,3\,4]. I will then introduce our new approaches to time domain spectr oscopy going beyond mean photon number observables [5-10] and show that th e statistical features of light can provide richer information than standa rd linear and non-linear optical spectroscopies[11\,12]. Finally\, I will elaborate on the possibility offered by embedding complex materials into r esonant optical cavities[13] and show how properties for the light-matter assembly different from those of its constituents can emerge.\n \n[1] Adva nces in physics 65\, 58-238\, 2016 \n[2] Science 331\, 189-191 (2011)\n[3 ] Phys. Rev. Lett. 122\, 067002 (2019) \n[4] Nature Physics 17\, 368–37 3 (2021)\n[5] Phys. Rev. Lett. 119\, 187403 (2017)\n[6] New J. Phys. 16 0 43004 (2014)\n[7] Nat. Comm. 6\, 10249 (2015) \n[8] Nat. Comm. 13\, 2667 (2022)\n[9] J. of Physics B 53\, 145502 (2019)\n[10] Optics Letters 45\, 3 498 (2020)\n[11] Light: Science & Applications\, 11\, 44 (2022) \n[12] PN AS March 19\, 116 (12) 5383-5386 (2019)\n[13] Rev. Sci. Inst. 93\, 033102 (2022)\n\nContact Person: Paul van Loosdrecht LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Nick Bultinck DTSTART;TZID=Europe/Berlin:20221026T143000 DTEND;TZID=Europe/Berlin:20221026T153000 DTSTAMP:20260527T090434Z UID:0000001754@events.thp.uni-koeln.de DESCRIPTION:Nick Bultinck\, University of Oxford\n\nThe ubiquity of Kekule spiral order in magic-angle graphene\n\nI will introduce a new type of sy mmetry-breaking order called "Incommensurate Kekule Spiral (IKS)" order\, and argue that it is ubiquitous in the experimentally observed phase diagr am of magic-angle graphene\n\nContact Person: Ciarán Hickey LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Timo Knispel DTSTART;TZID=Europe/Berlin:20221026T170000 DTEND;TZID=Europe/Berlin:20221026T183000 DTSTAMP:20260527T090434Z UID:0000001783@events.thp.uni-koeln.de DESCRIPTION:Timo Knispel\n\nMeet the Alumni\n\nAs part of the "Meet The Al umni" lecture series\, our former PhD student\nTimo Knispel gives insights into his recent start at his new employer\nd-fine. d-fine is a European c onsulting firm with a focus on quantitative\ntopics and the development of future-proof technological solutions. The\nconsulting approach of d-fine is based on many years of practical\nexperience and around a team with a s trong analytical and technological\nbackground. A combination that has pro ven its worth through tailored\,\nefficient and sustainable implementation s for more than two hundred\nclients from all economic sectors.\n\nTogethe r with his colleague Thomas Bschorr\, Timo will give us exciting\ninsights into the project work and his start at d-fine as well as answer\nany ques tions you might have. Afterwards\, Timo and Thomas will be\navailable for further questions and in-depth discussions in the get\ntogether with drink s and snacks.\n\nContact Person: Julian Wagner LOCATION:Seminar Room 0.01\, ETP END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Henry Legg DTSTART;TZID=Europe/Berlin:20221102T143000 DTEND;TZID=Europe/Berlin:20221102T153000 DTSTAMP:20260527T090434Z UID:0000001782@events.thp.uni-koeln.de DESCRIPTION:Henry Legg\, University of Basel\n\nTopological superconductin g diodes\n\n\n\nContact Person: Ciarán Hickey LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Felix Hensling DTSTART;TZID=Europe/Berlin:20230201T143000 DTEND;TZID=Europe/Berlin:20230201T153000 DTSTAMP:20260527T090434Z UID:0000001809@events.thp.uni-koeln.de DESCRIPTION:Felix Hensling\, Max-Planck-Institut Stuttgart\n\nEmerging phy sical phenomena at pyrochlore interfaces\n\n\n\nContact Person: Dr. Ionela Lindfors-Vrejoiu LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Prof Dr. Ali Esfandiar from Sharif University of Technol ogy\, Tehran\, Iran DTSTART;TZID=Europe/Berlin:20230208T143000 DTEND;TZID=Europe/Berlin:20230208T153000 DTSTAMP:20260527T090434Z UID:0000001797@events.thp.uni-koeln.de DESCRIPTION:Prof Dr. Ali Esfandiar from Sharif University of Technology\, Tehran\, Iran\n\nQuantum confinement of ions at the interface of two dimen sional materials for selective transport and energy harvesting\n\n\n\nCont act Person: Thomas Michely LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Tom Seifert DTSTART;TZID=Europe/Berlin:20230322T143000 DTEND;TZID=Europe/Berlin:20230322T153000 DTSTAMP:20260527T090434Z UID:0000001824@events.thp.uni-koeln.de DESCRIPTION:Tom Seifert\, FU Berlin\n\nProbing and driving spins at gigahe rtz and terahertz rates: From single magnetic atoms on surfaces towards ef ficient spintronic terahertz emitters\n\nIn my talk\, I will focus on two regimes of probing and driving spin dynamics that each allow for unique in sights. First\, I will introduce the concept of electron paramagnetic reso nance realized inside a scanning tunneling microscope [1]. This novel appr oach allows for probing magnetic interactions at atomic scales with an ene rgy resolution far below the thermal limit using gigahertz drives [2-4]. S econd\, I will turn to the field of terahertz spintronics [5]. I will high light recent advances in the field of terahertz emission spectroscopy to r eveal fundamental channels of femtosecond spin currents in magnetic thin f ilm heterostructures including their applications in spintronic terahertz emittters [6\,7]. \n\n[1] Baumann\, Susanne\, et al. Science 350.6259 (2015): 417-420.\n[2] Seifert\, Tom S.\, et al. Science advances 6.40 (202 0): eabc5511.\n[3] Seifert\, Tom S.\, et al. Physical Review Research 2.1 (2020): 013032.\n[4] Kovarik\, Stepan\, et al. Nano Letters 22.10 (2022): 4176-4181.\n[5] Walowski\, Jakob\, and Markus Münzenberg. Journal of App lied Physics 120.14 (2016): 140901.\n[6] Seifert\, Tom\, et al. Nature pho tonics 10.7 (2016): 483-488.\n[7] Seifert\, Tom S.\, et al. Applied Physic s Letters 120.18 (2022): 180401\n\nContact Person: Jeison Fischer LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Christian Dickel DTSTART;TZID=Europe/Berlin:20230414T140000 DTEND;TZID=Europe/Berlin:20230414T150000 DTSTAMP:20260527T090434Z UID:0000001843@events.thp.uni-koeln.de DESCRIPTION:Christian Dickel\, Institute of Physics II\, UoC\n\nCircuit-QE D & Matter\n\n\n\nContact Person: Achim Rosch LOCATION:0.03 (ETP) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Laetitia Farinacci DTSTART;TZID=Europe/Berlin:20230426T143000 DTEND;TZID=Europe/Berlin:20230426T153000 DTSTAMP:20260527T090434Z UID:0000001829@events.thp.uni-koeln.de DESCRIPTION:Laetitia Farinacci\, FU Berlin\n\nDynamics of atomic spins - f rom the propagation of a spin excitation to dynamics mediated by the hyper fine interaction\n\nThe combination of electron spin resonance and scannin g tunneling microscopy drastically widened the range of interactions that can be studied on the atomic scale [1]. Overcoming the energy resolution o f typical scanning tunneling spectroscopy by 3 orders of magnitude\, it ha s been used to characterize both dipolar and exchange interactions between single spins [2]\, tune their level of entanglement [3] and even resolve the hyperfine interaction of single atomic spins [4]. In this talk I will first discuss how studying the anisotropy of the hyperfine interaction can be used to determine the electronic ground state of TiH/MgO/Ag(100)\, a f eature that so far eluded experimentalists [5].\n\nThe second part of the talk will be devoted to the study of dynamics of atomic spins. We first de monstrate that a DC-pump-probe scheme can be used to study the free evolut ion of two coupled atomic spins: by controlling their level of entanglemen t\, we find a tuning point in which a spin excitation is swapped back and forth between them [6]. This is then used as the basis for a larger study of the free propagation of spin excitations within atomic chains and quasi -2D structures [7]. Finally\, I will discuss our most recent experiment in which the size of the system is reduced to a single atom: using the same technique we intend to gain insight into the dynamics between a single ele ctron and a nuclear spin mediated via the hyperfine interaction [8].\n\n[1 ] Baumann et al.\, Science 350\, 6259 (2015)\n[2] Choi et al.\, Nat. Nano 12\, (2017)\n[3] Yang et al.\, Phys. Rev. Lett. 119\, 227206 (2017)\n[4] W illke et al.\, Science 362 (2018)\n[5] Farinacci\, et al.\, Nano Letters 2 2\, 8470 (2022)\n[6] Veldman et al.\, Science 372\, 6545 (2021)\n[7] Veldm an et al.\, in preparation\n[8] Veldman et al.\, in preparation\n\nContact Person: Erwann Bocquillon LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Markus Ternes DTSTART;TZID=Europe/Berlin:20230517T143000 DTEND;TZID=Europe/Berlin:20230517T153000 DTSTAMP:20260527T090434Z UID:0000001830@events.thp.uni-koeln.de DESCRIPTION:Markus Ternes\n\nFrom excitations of individual atomic and mol ecular spins to highly correlated topological states\n\nScanning probe mic roscopes have been very successful tools for studying individual atoms and molecules as well as complex surface structures. Systems which bear magne tic spin moments can be built with them on surfaces and stabilized in junc tions. When such spins interact with each other or with the supporting ele ctron baths\, entanglement and correlated many-particle states can emerge\ , making them ideal prototypical quantum systems. \nIn this talk I will sh ow that experiments in conjunction with model Hamiltonians can be used as model systems to explore this fascinating quantum world\, where symmetries are crucial not only for electronic [1] and magnetic properties [2] but a lso for the appearance of novel topological quantum phases [3\, 4]. The ti p as atomically sharp control element can be used to detail spin-spin corr elations [5]\, to detect “dark” moments [6\,7]\, and even to portray t ransitions between entirely different quantum phases [3]. \n\n1. J. Martin ez\, et al.\, Comm. Mat. 3\,57 (2022)\n2. Y. Wang et al.\, Comm. Phys. 4\, 103 (2021)\n3. S\, Arabi\, et al.\, arXiv:2208.10377 (2022)\n4. J. Martin ez\, et al.\, arXiv:2304.08142 (2023)\n5. B. Verlhac et al.\, Science 366\ , 623 (2019)\n6. M. Muenks et al.\, Nature Communications8\, 14119 (2017)\ n7. M. Ternes et al. Phys Rev. Lett. 124\, 167202 (2020)\n\nContact Person : Wouter Jolie LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Chuan Li DTSTART;TZID=Europe/Berlin:20230614T143000 DTEND;TZID=Europe/Berlin:20230614T153000 DTSTAMP:20260527T090434Z UID:0000001818@events.thp.uni-koeln.de DESCRIPTION:Chuan Li\, University of Twente\n\nSuperconducting proximity e ffect in topological Dirac materials\n\nInducing superconductivity in topo logical materials stimulates the formation of novel quantum states of matt er. Besides the original prediction in 3D topological insulators\, the not ion of topological phases has been generalized to different dimensions and extended to the higher-order states. \nIn the last few years\, our resear ch has demonstrated the possibility of realizing the topological supercond uctivity in engineered 3D topological insulators [1]\, 3D Dirac semimetals [2\,3]\, and their 1D hinge states. Particularly\, Cd3As2 is predicted to be a higher-order topological semimetal\, possessing three-dimensional bu lk Dirac fermions\, two-dimensional Fermi arcs [4]\, and one-dimensional h inge states [5] or non-Hermitian states [6]. These topological states have different characteristic length scales in electronic transport. We show t hat the superconducting proximity effect can also be a sensitive probe for distinguishing these states.\n\n[1] B. de Ronde\, et al.\, Nanomaterials 10\, 794 (2020).\n[2] Li\, C. et al. Nat. Mater. 17\, 875 (2018).\n[3] Wan g\, A. Q. et al. Phys. Rev. Lett. 121\, 237701 (2018).\n[4] Li\, C.-Z. et al. Nat. Communications. 11\, 1150 (2020).\n[5] Li\, C.-Z. et al. Phys. Re v. Lett. 124\, 156601 (2020).\n[6] Wang\, A.Q. et al. Submitted (2022)\n\n Contact Person: Erwann Bocquillon LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Yoshito Watanabe DTSTART;TZID=Europe/Berlin:20230621T143000 DTEND;TZID=Europe/Berlin:20230621T153000 DTSTAMP:20260527T090434Z UID:0000001851@events.thp.uni-koeln.de DESCRIPTION:Yoshito Watanabe\n\nControl of magnetic interactions and magne tic-field-induced phase transition in doped Melilite compounds\n\nChemical doping is a powerful method of controlling the physical properties of mag netic materials. Unlike the uniform changes brought about by pressure-indu ced parameter tuning\, chemical doping introduces randomness into the syst em. This randomness can sometimes lead to unintended consequences\, but it can also reveal unexpected\, unique physics.\n\nThe first part of the tal k will detail how the dilution of magnetic ions can suppress the effective magnetic interactions\, leading to a quantum disordered phase that differ s from the traditional percolation disorder. The second part of the talk will focus on a field-induced phase transition that arises from the asymme try of single-ion anisotropy between different ions\, and therefore does n ot occur when the system consists of a single type of magnetic moment. Bo th cases demonstrate the extent to which we can tune the physical property by substituting magnetic ions\, and the emergence of new physical phenome na rooted in the effects of randomness.\n\nContact Person: Simon Trebst LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Prof. Hannu-Pekka Komsa\, University of Oulu\, Oulu\, Fi nland DTSTART;TZID=Europe/Berlin:20230802T153000 DTEND;TZID=Europe/Berlin:20230802T163000 DTSTAMP:20260527T090434Z UID:0000001858@events.thp.uni-koeln.de DESCRIPTION:Prof. Hannu-Pekka Komsa\, University of Oulu\, Oulu\, Finland\ n\nEngineering point and extended defects in 2D transition metal dichalcog enides\n\nAbstract:\nTwo-dimensional (2D) materials such as graphene\, hex agonal boron nitride\, and transition metal dichalcogenides have recently received lots of attention due to their unique material properties and num erous potential applications. The 2D atomic structure can also facilitate distinct defect formation mechanisms and offer new possibilities for defec t engineering.\n\nIn my talk\, I will present the results from layered mol ybdenum dichalcogenides (MoS2\, WSe2\, etc.)\, where vacancy\, substitutio nal\, interstitial\, and grain boundary defects are introduced by electron irradiation or by various chemical treatments. Due to the 2D nature\, the defect formation and properties can be characterized using transmission e lectron microscopy and scanning tunneling microscopy. First-principles cal culations are used to provide microscopic insight into the energetics and kinetics of these processes. The gained understanding together with the co mputationally predicted defect properties can be used to guide future effo rts in tailoring the 2D material properties via defect engineering.\n\nCon tact Person: Prof. Thomas Michely LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Nicolas Wink DTSTART;TZID=Europe/Berlin:20231018T143000 DTEND;TZID=Europe/Berlin:20231018T153000 DTSTAMP:20260527T090434Z UID:0000001884@events.thp.uni-koeln.de DESCRIPTION:Nicolas Wink\n\nTowards the phase diagram of QCD\n\nIn this ta lk\, I'll discuss current developments towards the phase structure of the strong interaction with functional methods. With the recent detection of g ravitational waves from Neutron star mergers\, the high density region of the theory has become a particular point of interest. This region shares a striking similarity with the Hubbard model\, when viewed from a renormali zation group perspective.\n\nContact Person: Sebastian Diehl LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Prof. Alex Khajetoorians / University Nijmegen DTSTART;TZID=Europe/Berlin:20231102T133000 DTEND;TZID=Europe/Berlin:20231102T153000 DTSTAMP:20260527T090434Z UID:0000001876@events.thp.uni-koeln.de DESCRIPTION:Prof. Alex Khajetoorians / University Nijmegen\n\nDesigner lat tices on surfaces: from molecular orbitals toward correlated electronic st ructure and Quantum Hall states\n\nDesigning materials with tailored physi cal and chemical properties requires a quantitative understanding of inter acting quantum systems. In order to provide predictability\, a promising r oute is to create botom-up platforms\, where the electronic properties of individual and interacting atoms can be emulated in a tuneable manner. Her e\, I will present a solid state quantum simulator based solely on paterne d Cs atoms on the surface of semiconducting InSb(110)\, a system character ized by dilute two-dimensional electron gas decoupled from the substrate ’s bulk bands. This platform can be used to create electron traps which emulate artificial atoms by precisely positioning Cs atoms using STM-based atom manipulation. These artificial atoms served as building blocks to re alize artificial molecular structures with different orbital symmetries wh ich are subsequently probed. Moreover\, I will illustrate newer examples o f structures which are described by correlated electron models\, as well a s discuss new perspectives in magnetic fields and the role of the Quantum Hall Effect.\n\nContact Person: Prof. Thomas Michely LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Daniel Schick DTSTART;TZID=Europe/Berlin:20231115T143000 DTEND;TZID=Europe/Berlin:20231115T153000 DTSTAMP:20260527T090434Z UID:0000001883@events.thp.uni-koeln.de DESCRIPTION:Daniel Schick\, Max Born Institute Berlin\n\nFollowing complex spin structures in time & space\n\nControlling ultrafast spin dynamics di rectly on a quantum level by femtosecond light pulses promises a dramatic increase in speed\, energy efficiency\, and density of how we transport\, process\, and store information. Recent advances in the field have enabled an understanding of laser-driven spin dynamics from microscopic processes toward macroscopic functionality in magnetic nanostructures\, including c harge and spin transport as well as interactions with spatially extended q uasiparticles such as phonons and magnons. These processes generally lead to a nanoscale spatial rearrangement of magnetization\, calling for experi mental techniques that can directly access the ultrafast evolution of spat ially inhomogeneous spin profiles and detect the transfer or accumulation of spins at interfaces. Such observables providing nanometer spatial and f emto- to picosecond temporal resolution are particularly required for inve stigations of complex heterostructures and antiferromagnets\, where compet ing interactions result in a variety of complex spin structures already in equilibrium.\nIn this talk\, I will focus on time-resolved resonant soft- X-ray scattering (RSXS) as a unique technique for probing magnetic order i n time and space with element-selectivity as well as in buried layers. RSX S combines large spectroscopic and magnetic contrast in the soft-X-ray ran ge with access to reciprocal space in addition to nanometer depth- and lat eral spatial resolution. Based on the in-house development of two laser-dr iven soft-X-ray sources at Max Born Institute\, we have recently demonstra ted the feasibility of time-resolved RSXS in laboratory experiments as a t rue alternative to large-scale facilities. I will discuss our recent RSXS results on the dynamics of artificial antiferromagnets\, ferromagnetic dom ains and magnons\, as well as all-optical switching in ferrimagnetic alloy s.\n\nContact Person: Ionela Lindfors-Vrejoiu LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Sarah Loos/Cambridge (SFB1238/Kolloquium) DTSTART;TZID=Europe/Berlin:20240110T143000 DTEND;TZID=Europe/Berlin:20240110T153000 DTSTAMP:20260527T090434Z UID:0000001909@events.thp.uni-koeln.de DESCRIPTION:Sarah Loos/Cambridge (SFB1238/Kolloquium)\, Cambridge\n\nPhase transitions and fluctuations of systems with nonreciprocal interactions\n \nReciprocity is a hallmark of thermal equilibrium\, but ubiquitously brok en in far-from-equilibrium systems on various scales. I will give some ins ights into how nonreciprocal interactions can fundamentally affect the pha ses and fluctuations of classical many-body systems. Using a two-dimension al XY model\, where spins interact only with neighbours within their 'visi on cones' inspired by the effective interactions between animals\, we show how nonreciprocity can lead to true long-range order and directional prop agation of defects [1]. In binary fluids\, nonreciprocal coupling between fluid components can cause the emergence of travelling waves through PT sy mmetry-breaking phase transitions\, as observed for mixtures of catalytic particles or membrane protein dynamics. Using a hydrodynamic model\, we fi nd that fluctuations not only inflate\, as in equilibrium criticality\, bu t also develop an asymptotically increasing time-reversal asymmetry [2-4] and associated surging entropy production. We can trace the formation of d issipative patterns and the emergence of irreversible fluctuations to the same origin\, namely a mode-coupling mechanism near critical exceptional p oints.\n\n[1] Loos\, Klapp\, Martynec\, Long-Range Order and Directional D efect Propagation in the Nonreciprocal XY Model with Vision Cone Interacti ons\, Phys. Rev. Lett. 130\, 198301 (2023).\n[2] Suchanek\, Kroy\, Loos\, Irreversible mesoscale fluctuations herald the emergence of dynamical phas es\, Phys. Rev. Lett. in press (2023).\n[3] Suchanek\, Kroy\, Loos\, Time- reversal and parity-time symmetry breaking in non-Hermitian field theories \, Phys. Rev. E in press (2023).\n[4] Suchanek\, Kroy\, Loos\, Entropy pro duction in the nonreciprocal Cahn-Hilliard model\, Phys. Rev. E in press ( 2023).\n\nContact Person: Carl Philipp Zelle LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Sang-Wook Cheong (SFB1238/Kolloquium) DTSTART;TZID=Europe/Berlin:20240112T140000 DTEND;TZID=Europe/Berlin:20240112T153000 DTSTAMP:20260527T090434Z UID:0000001919@events.thp.uni-koeln.de DESCRIPTION:Sang-Wook Cheong (SFB1238/Kolloquium)\, Rutgers Center for Eme rgent Materials\n\nChirality and Kinetomagnetism\n\nChirality\, which aris es from the breaking of mirror symmetries combined with any spatial rotati ons\, plays a ubiquitous role in a wide range of phenomena\, from the DNA functionality\, vine climbing to the piezoelectricity of quartz crystals. It's important to note that chirality does not necessarily involve a screw -like twisting\, and magnetic chirality means chirality in spin ordered st ates or mesoscopic spin textures. Despite being mathematically well-define d\, the term "chirality" has been extensively used\, often in confusing wa ys\, in recent years. In steady states\, chirality (C) does not change wit h time-reversal operation\, while chirality prime (C) denotes the break ing of time-reversal symmetry in addition to broken all mirror symmetries\ , combined with any spatial rotations. Various examples of magnetic chiral ity and chirality prime and their emergent phenomena\, such as self-induct ance\, directional nonreciprocity in magnetic fields\, current-induced mag netization\, chirality-selective spin-polarized current\, Schwinger scatte ring\, magneto-optical Kerr effect\, linear magnetoelectricity\, and chira l tunneling will be discussed. Many of these phenomena can be understood w ith one hypothesis on “kinetomagnetism in chiral systems” that I will present. Some of these exotic phenomena have been recently observed\, whil e many others require experimental confirmation in the future.\n\nContact Person: Markus Grüninger LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Dr. Oleksandr Serha (SFB1238/Kolloquium) DTSTART;TZID=Europe/Berlin:20240131T143000 DTEND;TZID=Europe/Berlin:20240131T153000 DTSTAMP:20260527T090434Z UID:0000001922@events.thp.uni-koeln.de DESCRIPTION:Dr. Oleksandr Serha (SFB1238/Kolloquium)\, Rheinland-Pfälzisc he Technische Universität\n\nExperimental studies of Bose-Einstein conden sation of magnons\n\nBose–Einstein condensation is a fundamental physica l phenomenon demonstrating fascinating and application-important propertie s such as spontaneous coherence\, superconductivity\, and superfluidity. T his phenomenon can be achieved either by a decrease in the temperature of ultracold atomic gases and cryogenic liquids or by an increase in the dens ity of particles (quasiparticles). Spin wave quanta—magnons—are bosons \, and thus they can form a Bose–Einstein condensate (BEC)\, which can b e created by an external injection of these quasiparticles even at room te mperature. In this talk\, I will present the main ways of forming and obse rving magnon BEC in solid-state magnetic samples\, outline its place in th e spectrum of dipole-exchange magnons\, describe the time dynamics of the overpopulated magnon gas and magnon condensate in real and momentum spaces \, discuss the relation of this dynamics with magnon supercurrents\, and t ouch upon the issue of temporal and spatial coherence of the magnon BEC.\n \nContact Person: Matteo Cacco (SFB1238/Kolloquium) LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Philip Willke (SFB1238/Kolloquium) DTSTART;TZID=Europe/Berlin:20240207T143000 DTEND;TZID=Europe/Berlin:20240207T153000 DTSTAMP:20260527T090434Z UID:0000001911@events.thp.uni-koeln.de DESCRIPTION:Philip Willke (SFB1238/Kolloquium)\, Karlsruhe Institute of Te chnology\n\nControl and Dynamics of Single Atomic and Molecular Spins on S urfaces\n\nThe quantum nature of a physical system often emerges from its fundamental building blocks and demands a profound understanding to harves t its advantages for quantum devices. In this talk\, I will introduce sing le atoms and molecules as building blocks for highly controlled quantum ex periments. Utilizing a scanning tunneling microscope (STM) in combination with electron spin resonance (ESR) establishes an atomic “workbench” f or coherent control of individual spins on surfaces. This unique technique allows us to address single atoms and molecules with unprecedented energy and spatial resolution and use them for instance as magnetic sensors on t he nanoscale. In addition\, the high energy resolution also grants access to the hyperfine interaction between the electron and nuclear spin of diff erent atomic species. Lastly\, by employing pulsed ESR schemes\, a coheren t manipulation of the surface spin becomes possible\, for instance in Rabi and Hahn echo schemes. This opens up a path towards quantum information p rocessing and quantum sensing using atomic building blocks\, including ato ms and molecules.\n\nContact Person: Wouter Jolie LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Hiroshi Kamata (SFB1238/Kolloquium) DTSTART;TZID=Europe/Berlin:20240223T110000 DTEND;TZID=Europe/Berlin:20240223T120000 DTSTAMP:20260527T090434Z UID:0000001925@events.thp.uni-koeln.de DESCRIPTION:Hiroshi Kamata (SFB1238/Kolloquium)\, NTT Basic Research Labs\ , Atsugi (Japan)\n\nTime-resolved measurement of edge magnetoplasmons in t wo-dimensional topological insulators\n\nElectron-hole bilayer systems in InAs/(In)GaSb composite quantum wells (CQWs) have been extensively investi gated as a host of the quantum spin Hall (QSH) insulating phase [1]. Altho ugh the QSH phase is no longer protected by time-reversal symmetry in an e xternal magnetic field\, below a critical magnetic field\, helical edge ch annels due to band inversion remain\, which evolve into counter-propagatin g chiral edge channels originating from electron- and hole-like Landau lev els in the quantum-Hall (QH) regime [2]. In narrow-gap systems\, charge pu ddles form in the bulk\, which affect the helical edge transport [3]. Mean while\, time-resolved transport measurements performed on chiral edge chan nels of QH systems have revealed the dynamics of nonequilibrium charge pro pagating unidirectionally as edge magnetoplasmon (EMP) modes\, reflecting an influence of charge puddles in the bulk [4\, 5]. Here\, we investigate time-resolved helical edge transport of EMPs in the QH regime of electron- hole bilayer systems in band-inverted InAs/InGaSb CQWs.\nFigure 1 shows th e schematic of the device\, comprising a band-inverted InAs/InGaSb CQW and experimental setup for time-resolved transport measurements of pulsed EMP s [5]. Figures 2(a) and 2(b) show color-scale plots of EMP waveforms as a function of a perpendicular magnetic field B measured at the net filling f actors net (= e  h) = 2 and 0 for propagation lengths L = 5 an d 20 m\, respectively. Pulsed EMPs appear only for one field direction at net = 2\, indicating the chiral edge transport without counter-propa gating modes\, while pulsed EMPs measured at net = 0 show the symmetry for the positive and negative B\, indicating the helical edge transport. F igure 3 shows waveforms meausred at the net = 0 of B = 9 T for various L. The waveforms broaden along the time axis as L increases. The observed dispersive transport can be understood by considering charge transfer betw een counter-propagating channels [6]. Our measurement scheme will pave the way for investigations of dynamical properties of topological edge states .\n\nContact Person: Erwann Bocquillon LOCATION:Seminar Room I. Physik END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Prof. Dr. Rostislav Mikhaylovskiy (SFB1238/Kolloquium) DTSTART;TZID=Europe/Berlin:20240408T123000 DTEND;TZID=Europe/Berlin:20240408T134500 DTSTAMP:20260527T090434Z UID:0000001943@events.thp.uni-koeln.de DESCRIPTION:Prof. Dr. Rostislav Mikhaylovskiy (SFB1238/Kolloquium)\, Lanca ster University\n\nTerahertz coherent magnonics in canted antiferromagnets and altermagnets\n\nMagnonics aims to employ quanta of spin waves\, magno ns\, to carry\, transport and process\ninformation\, avoiding the dissipat ion of energy inherent to electronics. Experiments on\nmagnons in regular (ferro)magnets have yielded demonstrations of basic logic devices\, albeit \nmacroscopic (mm-scale) in size and operating at GHz frequencies. Recentl y\, the spotlight has\nshifted towards the use of antiferromagnets\, in wh ich neighbouring spins are aligned\nantiparallel to each other. This alter nating order leads to significantly higher spin wave\npropagation velociti es and might enable devices operating at terahertz (trillion of hertz) clo ck-\nrates. However\, the absence of the net magnetisation also makes anti ferromagnets\nmagnetically ‘invisible’: it is very hard to detect and influence the antiferromagnetic order.\nYet\, in some antiferromagnets str ong spin–orbit coupling results in canting of the spins\,\nthereby produ cing net magnetization. The canted antiferromagnets combine\nantiferromagn etic order with phenomena typical for ferromagnets and hold a great potent ial\nfor spintronics and magnonics. In this way they can be identified as closely related to the\nrecently proposed novel class of magnetic material s\, called altermagnets. In my talk I will\ndiscuss a new functionality of canted antiferromagnets and altermagnets for magnonics and\nshow that the se materials facilitate mechanisms allowing to generate\, detect and nonli nearly\nconvert propagating magnons at the nanoscale [1-3].\n\nContact Per son: Evgeny Mashkovich LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Bernd Büchner (SFB1238/Kolloquium) DTSTART;TZID=Europe/Berlin:20240418T140000 DTEND;TZID=Europe/Berlin:20240418T153000 DTSTAMP:20260527T090434Z UID:0000001965@events.thp.uni-koeln.de DESCRIPTION:Bernd Büchner (SFB1238/Kolloquium)\, IFW Dresden\n\nEvidence for superconducting topological surface states in PtBi2\n\nDue to their in trinsic topology\,Weyl semimetals are potential candidates for topological superconductivity\, but so far have always been connected with bulk super conductivity\, leaving the possibility of intrinsic superconductivity of t heir topological surface states\, the Fermi arcs\, practically without att ention\, even from the theory side. By means of angle-resolved photoemissi on spectroscopy (ARPES) and ab initio calculations\, we identify topologic al Fermi arcs on two opposing surfaces of the non-centrosymmetric Weyl mat erial trigonal PtBi2. We show these states become superconducting at tempe ratures around 10 K. Remarkably\, the corresponding coherence peaks appear as the strongest and sharpest excitations ever detected by photoemission from solids. Superconductivity is also found in our STM measurements showi ng in addition spatial inhomogeneity of the superconducting gap. In some s amples superconducting gaps as large as 20 meV are revealed by STM resembl ing the phenomenology found in high-Tc superconductors. However bulk probe s such as magnetization\, transport and thermodynamics do not show superco nductivity. This is consistent with the absence of gaps in bulk electronic states measured by ARPES. Our findings indicate that superconductivity in PtBi2 can occur exclusively at the surface\, rendering it a possible plat form to host Majorana modes in intrinsically topological superconductor– normal metal–superconductor junctions.\n\nContact Person: Markus Braden LOCATION:Seminar Room of the Theory Institute (old building) END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Dr. George Matthew Ferguson (SFB1238/Kolloquium) DTSTART;TZID=Europe/Berlin:20240424T143000 DTEND;TZID=Europe/Berlin:20240424T153000 DTSTAMP:20260527T090434Z UID:0000001948@events.thp.uni-koeln.de DESCRIPTION:Dr. George Matthew Ferguson (SFB1238/Kolloquium)\, MPI Dresden \n\nVisualizing electronic transport in the quantum anomalous Hall regime\ n\nElectronic transport measurements guide the discovery and characterizat ion of quantum materials. The Biot-Savart law dictates that the motion of electrons generates a stray magnetic field during such a transport experim ent. I will show how imaging these magnetic fields with a scanning SQUID m icroscope allows us the determine the path that current takes through our devices. I will then discuss the application of this approach to the quant um anomalous Hall regime in magnetically doped topological insulators. Usi ng local information about the current distribution\, I will show how bulk conduction processes give rise to a quantized Hall effect. I will then de scribe our efforts to visualize the breakdown of the quantum anomalous Hal l effect under the application of large bias currents.\n\nContact Person: Torsten Roeper/ Erwann Bocquillon LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Yasir Iqbal DTSTART;TZID=Europe/Berlin:20240424T160000 DTEND;TZID=Europe/Berlin:20240424T170000 DTSTAMP:20260527T090434Z UID:0000001970@events.thp.uni-koeln.de DESCRIPTION:Yasir Iqbal\, Indian Institute of Technology Madras\n\nWhat ma kes 3D quantum spin liquids possible?\n\nFor decades\, frustrated quantum magnets have been a seed for scientific progress and innovation in condens ed matter. As much as the numerical tools for low-dimensional quantum magn etism have thrived and improved in recent years due to breakthroughs inspi red by quantum information and quantum computation\, higher-dimensional qu antum magnetism can be considered as the final frontier\, where strong qua ntum entanglement\, multiple ordering channels\, and manifold ways of para magnetism culminate. At the same time\, efforts in crystal synthesis have induced a significant increase in the number of tangible frustrated magnet s which are generically three-dimensional in nature\, creating an urgent n eed for quantitative theoretical modeling. This talk will present the stat e-of-the-art in pseudo-fermion (PF) and pseudo-Majorana (PM) functional re normalization group (FRG) and their specific ability to address higher-dim ensional frustrated quantum magnetism. First developed more than a decade ago\, the PFFRG interprets a Heisenberg model Hamiltonian in terms of Abri kosov pseudofermions\, which is then treated in a diagrammatic resummation scheme formulated as a renormalization group flow of m-particle pseudofer mion vertices. We will discuss the achievements of PFFRG in successfully p redicting the spectroscopic signatures of candidate quantum spin liquid ma terials based on complex three-dimensional geometries with intricate Hamil tonians. These include the recently studied bi-trillium lattice S=1 K2Ni2( SO4)3 and the hyperhyperkagome lattice S=1/2 PbCuTe2O6 compounds. We also present the success of PMFRG in treating finite-temperature properties of three-dimensional systems by describing phase transitions and the associat ed critical exponents.\n\nContact Person: Simon Trebst LOCATION:Seminar Room 0.03\, ETP END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Nicholas Güsken DTSTART;TZID=Europe/Berlin:20240510T140000 DTEND;TZID=Europe/Berlin:20240510T153000 DTSTAMP:20260527T090434Z UID:0000001982@events.thp.uni-koeln.de DESCRIPTION:Nicholas Güsken\, Stanford\n\nTowards tunable photon-emitter interfaces for on-chip and free space emission control\n\nMastering intera ctions between light and matter in solid-state platforms is at the forefro nt of today's advancements in photonic technologies and beyond. This exten ds across both traditional and emerging applications\, encompassing high-s peed telecommunications\, manufacturing\, medical sensing\, as well as tec hnologies like self-driving cars and AR/VR\, all rooted in scientific brea kthroughs within the field of optics and photonics.\n\nIn recent years\, i t has become increasingly evident that photons are poised to play a pivota l role in the ongoing development of quantum technologies. Unlike other in formation carriers\, photons exhibit remarkable stability at room temperat ure\, making them ideal for conveying quantum information over extended di stances. Photons play a crucial role in distributed quantum computing as t hey allow the formation of interlinks for large-scale quantum networks. He re\, photon-emitter interfaces are the fundamental building blocks in such networks\, facilitating the controlled creation of single photons and act ive light-matter interaction on the atomic scale - essential features for future quantum memories. While great advancements in the field have been a chieved\, the tunability of photon-emitter interfaces remains challenging. \n\nThis talk explores strategies for establishing integrated active emitt er–photon interfaces in the form of i) waveguides and ii) metasurfaces. It commences by introducing narrowband and broadband waveguide environment s. Following this\, it presents a tunable platform that enables active wav efront shaping of emission in free space\, utilizing phase change material s. Lastly\, it outlines a vision for how this research will contribute to scientific discoveries and address current technological challenges in the fields of classical and quantum optical communication.\n\nContact Person: Annika Kurzmann LOCATION:Seminar Room 0.03\, ETP END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Aisha Aqeel DTSTART;TZID=Europe/Berlin:20240604T090000 DTEND;TZID=Europe/Berlin:20240604T103000 DTSTAMP:20260527T090434Z UID:0000001994@events.thp.uni-koeln.de DESCRIPTION:Aisha Aqeel\, University of Augsburg\n\nSpintronics with chira l helimagnetic insulators\n\nNature forms many patterns\, and it's fascina ting to observe how individual components interact to create such complexi ty. In magnets\, beautiful patterns can emerge even without inversion symm etry. The twisting magnetic patterns in chiral helimagnetic materials\, wh ether topologically trivial or not\, can provide collective magnetic excit ations across a wide frequency range from a few GHz to several THz. These unique properties make helimagnets promising candidates for applications i n spintronics and unconventional computing. Realizing these concepts\, how ever\, requires ultraclean magnetic systems with minimal losses and a deep understanding of their magnetization dynamics.\nIn this talk\, I will int roduce the fundamental aspects of chiral helimagnets and explore their mag netization dynamics\, particularly in single crystals. The focus will be o n the chiral helimagnetic insulator Cu₂OSeO₃\, with experiments done p rimarily through broadband magnetic resonance spectroscopy. Our findings r eveal the potential to modify magnetic textures via interface effects.\n\n Contact Person: Erwann / Matteo LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Nathalia Perkins DTSTART;TZID=Europe/Berlin:20240605T143000 DTEND;TZID=Europe/Berlin:20240605T153000 DTSTAMP:20260527T090434Z UID:0000001949@events.thp.uni-koeln.de DESCRIPTION:Nathalia Perkins\, University of Minnesota\n\nExploring Quantu m Spin Liquids: From Theory to Experiment\n\nIdentifying quantum spin liqu ids in experiments is challenging due to several factors. Firstly\, these systems lack long-range magnetic order\, making them difficult to distingu ish from disordered magnetic phases or paramagnetic states. Secondly\, qua ntum spin liquids exhibit emergent phenomena\, such as fractionalized exci tations like Majorana fermions\, which do not have direct analogs in class ical or conventional quantum magnetism. While these novel excitations are challenging to detect and characterize experimentally\, direct comparison of the experimental findings with theoretically computed dynamical respons es in corresponding probes such as neutron scattering\, Raman spectroscopy \, phonon dynamics\, or scanning tunneling microscopy allows for the ident ification of unique features in these responses which allows to distingui sh quantum spin liquid behavior from other phenomena. \n\nIn my talk\, I w ill focus on potential identifications of quantum spin liquid behavior in Kitaev materials. In the exactly solvable Kitaev model\, spin excitations are fractionalized into two types of quasiparticles: itinerant spinon-like excitations described by Majorana fermions and localized gapped Z2 fluxes . Our studies indicate that characteristic signatures of such spin fractio nalization can be observed in phonon dynamics\, such as the temperature an d field evolution of sound attenuation measured in ultrasound experiments. Another example is the analysis of the dynamical response of the site-dil uted Kitaev spin liquid\, which reveals that defect-induced localized exci tations and spin fractionalization can potentially be observed using scann ing tunneling microscopy.\n\nContact Person: Simon Trebst LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Andrey Chubukov DTSTART;TZID=Europe/Berlin:20240607T163000 DTEND;TZID=Europe/Berlin:20240607T180000 DTSTAMP:20260527T090434Z UID:0000001950@events.thp.uni-koeln.de DESCRIPTION:Andrey Chubukov\, University of Minnesota\n\nUnconventional di scontinuous transitions in isospin systems\n\nI will show that two-dimensi onal fermions with k2 and k4 dispersion undergo a first-order Stoner tran sition to a fully spin-polarized state despite that the spin susceptibilit y diverges at the critical point. I will then extend the analysis to syste ms with dispersions k2a and spin and valley isospin and show there is a ca scade of instabilities into fractional-metal states with some electron ba nds fully depleted. I will discuss applications to Bernal bilayer graphene in a displacement field and 2D electron gases. I will also discuss supe rconductivity near an isospin ordering.\n\nContact Person: Simon Trebst LOCATION:Seminar Room 0.03\, ETP END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | François Parmentier DTSTART;TZID=Europe/Berlin:20240610T160000 DTEND;TZID=Europe/Berlin:20240610T170000 DTSTAMP:20260527T090434Z UID:0000001963@events.thp.uni-koeln.de DESCRIPTION:François Parmentier\, Université Paris-Saclay\n\nVanishing b ulk heat flow in the nu=0 quantum Hall ferromagnet in monolayer graphene\n \nUnder high perpendicular magnePc field and at low temperatures\, graphen e develops an\ninsulaPng state at the charge neutrality point. This state\ , dubbed nu=0\, is due to the interplay\nbetween electronic interacPons an d the four-fold spin and valley degeneracies in the flat band\nformed by t he n=0 Landau level. Determining the ground state of nu=0\, including its spin and\nvalley polarizaPon\, has been a theorePcal and experimental unde rtaking for almost two\ndecades. Here\, we present experiments probing the bulk thermal transport properPes of\nmonolayer graphene at nu=0\, which d irectly probe its ground state and collecPve excitaPons.\nWe observe a van ishing bulk thermal transport\, in contradicPon with the expected ground\n state\, predicted to have a finite thermal conductance even at very low te mperature. Our result\nhighlight the need for further invesPgaPons on the nature of nu=0.\n\nContact Person: Erwann Bocquillon LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Patrick Ledwith DTSTART;TZID=Europe/Berlin:20240611T170000 DTEND;TZID=Europe/Berlin:20240611T180000 DTSTAMP:20260527T090434Z UID:0000001987@events.thp.uni-koeln.de DESCRIPTION:Patrick Ledwith\, Harvard University\n\nNonlocal Moments and M ott Semimetal in a model of Twisted Bilayer Graphene\n\nEarly on it was no ticed that twisted bilayer graphene (TBG) has elements in common with two paradigmatic examples of strongly correlated physics: Hubbard physics and quantum Hall physics. On the one hand\, TBG hosts flat topological Landau- level-like bands which realize quantum anomalous Hall states and orbital f erromagnetism under the right conditions. On the other hand\, these bands are characterized by concentrated charge density and show experimental sig ns of fluctuating magnetism\, and unconventional superconductivty. The eme rgence of fluctuating moments is particularly surprising\, as localized Wa nnier states do not exist in topological bands. I will discuss a phenomeno logical model for the flat bands in TBG that centers the concentration of charge density and\, relatedly\, the concentration of Berry flux. The band s obtained have excellent quantitative agreement with the Bistritzer-Macdo nald model for realistic parameters. I will show that\, rather remarkably\ , the model hosts decoupled flavor moments which\, despite being power-law delocalized with infinite localization length\, have parametrically small overlap with each other. These "nonlocal" moments lead to a novel "Mott S emimetal" regime\, with large flavor entropy and mostly\, but not entirely \, frozen charge.\n\nContact Person: Urban Seifert LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Qimiao Si DTSTART;TZID=Europe/Berlin:20240612T143000 DTEND;TZID=Europe/Berlin:20240612T153000 DTSTAMP:20260527T090434Z UID:0000001961@events.thp.uni-koeln.de DESCRIPTION:Qimiao Si\, Rice University\n\nStrange Metals for Correlated T opology and Amplified Entanglement\n\nThe field of correlation physics con tinuously expands its horizons. As a primary objective of the field\, we a im to determine the organization of the many billions of billions of elect rons in the quantum universe of a solid\, taking into account both the ele ctrons’ quantum mechanical nature and their electrostatic repulsive inte raction. In the standard description\, quasiparticle is a central concept. It acts as the adiabatic continuation of a bare electron in the presence of interactions\, and is resilient when the interactions are treated pertu rbatively. \n\nIn this talk\, I will describe how quasiparticles can break apart in the presence of strong correlations\, leading to strange metal b ehavior. As exemplary settings\, heavy fermion metals [1] will be consider ed and flat band systems will be touched upon [2]. I will also discuss how strange metallicity nucleates topology without quasiparticles [3] and amp lifies quantum entanglement [4]. \n\n[1] H. Hu et al.\, arXiv:2210.14183\; S. Paschen & Q. Si\, Nat. Rev. Phys. 3\, 9 (2021)\; S. Kirchner et al\, R ev. Mod. Phys. 92\, 011002 (2020)\; Q. Si et al.\, Nature 413\, 804 (2001) .\n[2] L. Chen et al. arXiv:2307.09431\; L. Chen et al\, Nat. Comm (to app ear)\; H. Hu and Q. Si\, Sci. Adv. 9\, eadg0028 (2023).\n[3] H. Hu et al.\ , arXiv:2110.06182\; L. Chen et al.\, Nat. Phys. 18\, 1341 (2022).\n[4] Y. Fang et al.\, arXiv:2402.18552.\n\nContact Person: Achim Rosch LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Farsane Tabataba-Vakili DTSTART;TZID=Europe/Berlin:20240619T140000 DTEND;TZID=Europe/Berlin:20240619T153000 DTSTAMP:20260527T090434Z UID:0000001981@events.thp.uni-koeln.de DESCRIPTION:Farsane Tabataba-Vakili\, Ludwig-Maximilians-Universität Mün chen\n\nThe rich world of two-dimensional semiconductors: on moiré physic s\, magnetism\, and strong light-matter interaction\n\nSince the discovery of graphene in 2004\, a plethora of two-dimensional (2D) quantum material s\nhas been investigated. Among 2D semiconductors\, transition metal dicha lcogenides have been of\nparticular interest due to their direct band gap in the monolayer\, large exciton binding energies\,\nand strong light-matt er interaction. Within the class of 2D magnets\, air-stable CrSBr stands o ut as\nan antiferromagnetic semiconductor with a high Néel temperature an d excitons coupled to the\nmagnetic order. In this talk\, I will present t hree of my postdoctoral projects. First\, I will introduce\nmoiré physics and show large-area atomic reconstruction in MoSe2-WSe2 heterobilayers [1 ]. The\nsecond part will be about doping-control of excitons and magnetism in few-layer magnetic\nsemiconductor CrSBr [2]. Lastly\, I will show diff erent realizations of polariton microcavities with\nmonolayers of transiti on metal dichalcogenides [3].\nReferences:\n[1] Z. Li\, F. Tabataba-Vakili \, S. Zhao et al.\, Nano Lett. 23\, 4160 (2023).\n[2] F. Tabataba-Vakili e t al.\, Nat. Commun. 15\, 4735 (2024).\n[3] F. Tabataba-Vakili\, L. Krelle et al.\, arXiv:2402.09037 (2024).\n\nContact Person: Erwann/Matteo LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Pascal Puphal DTSTART;TZID=Europe/Berlin:20240625T090000 DTEND;TZID=Europe/Berlin:20240625T103000 DTSTAMP:20260527T090434Z UID:0000001995@events.thp.uni-koeln.de DESCRIPTION:Pascal Puphal\, MPI für Festkörperforschung Stuttgart\n\nCon trol of bulk single crystals from high TC superconductors\, over spin liqu ids to topological magnets\n\nExperimental control of ground states is a h oly grail in physics\, enabling the design of exotic\nstates as supercondu ctivity\, topological magnetism\, quantum spin liquids (QSL) etc. There ar e\nvarious ways to control materials: chemical pressure\, chemical strain\ , chemical doping\, entropy\,\ntopochemistry\, dimensionality/heterostruct ures\, external pressure\, external strain\, optical\npumping etc. In my t alk\, I will discuss how pressure is already used in synthesis to control\ nsystems and next show how far via chemical control combined with external stimuli we can shape\nour desired materials. This will be discussed on al l kinds of materials in the gold standard form of\nmaterials: single cryst als. The talk starts with the class of high TC superconductors with the ne w\nfamily of nickelates\,[1-4]. Here\, the class of infinite layer nickela tes\, show a promising example of\nmaterial design\, where the unique Ni1. 2+ state is reached via topochemistry and optimal doping is\nrealized via epitaxial strain. We could show that superconductivity is bound to thin fi lms [1-3]\, as\nfor bulk doping is reaching a solubility limit. Nickelates realize a second class\, where we found a\nunique stacking pattern [4]\, that allows for heterostructure like design and with the addition of\npres sure as a tuning parameter TC of up to 80 K are reached [4]. Afterwards\, focused on the\ncontrol parameters and their scope low dimensional quantum magnets [5]\, frustrated magnets\,\nwhere we can readily switch between a QSL and order [6] and topological magnets [7] will be\ndiscussed.\n[1] P. Puphal et al.\, APL Mater 11\, 081107 (2023)\n[2] P. Puphal et al.\, Phys . Rev. Materials 7\, 014804 (2023).\n[3] P. Puphal et al.\, Sci. Adv. 7\, eabl8091 (2021)\n[4] P. Puphal et al\,. arXiv:2312.07341 (2023).\n[5] P. P uphal et al.\, PRB 93\, 174121 (2016)\n[6] D. Chatterjee\, P. Puphal et al .\, PRB 107\, 125156 (2023).\n[7] P. Puphal et al.\, PRB 101\, 214416 (202 0)\n\nContact Person: Erwann/Matteo LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Marcel Reutzel DTSTART;TZID=Europe/Berlin:20240625T150000 DTEND;TZID=Europe/Berlin:20240625T163000 DTSTAMP:20260527T090434Z UID:0000001996@events.thp.uni-koeln.de DESCRIPTION:Marcel Reutzel\, Georg-August-Universität Göttingen\n\nUltra fast and coherent phenomena on fundamental length and time scales\n\nIn 2D semiconducting quantum materials\, organic semiconductors and their heter ostructures\, the\nenergy of absorbed light is stored in Coulomb-bound ele ctron-hole pairs\, which are called\nexcitons. For future technological ap plications of these classes of materials\, for instance in\noptoelectronic s and for energy harvesting\, it is crucial to study the initial exciton f ormation and\nalso the subsequent relaxation and dissipation processes at the fundamental level and on the\nrelevant length and time scales.\nIn our research\, we have built a new photoemission-based experiment [1] that is capable of\nstudying excitons at the space-time limit corresponding to na nometers and femtoseconds. In a\nseries of experiments\, we identified cha racteristic signatures in the exciton formation process and\nthe pathways of subsequent energy conversion and thermalization. In addition\, the new\ nexperiment gives us access to the so-called “dark exciton energy landsc ape”. In the first part of\nmy talk\, I will present the ultrafast forma tion dynamics of dark interlayer excitons in twisted WSe2/\nMoS2 heterostr uctures [2-6]. In particular\, I will report on the identification of a ke y signature of\nthe moiré superlattice that is imprinted on the momentum- resolved interlayer exciton\nphotoemission signal [2] and how such photoem ission data can be used to reconstruct the real-\nspace wavefunction of th e non-equilibrium excited state [2\,7]. In the second part\, I will discus s\nour recent efforts to monitor the interlayer exciton formation dynamics with spatiotemporal\nresolution using femtosecond photoelectron dark-fiel d microscopy [6]. By probing the WSe2/\nMoS2 heterostructure with 50 fs ti me- and 500 nm spatial resolution\, we identify an astonishing\ninhomogene ity in the interlayer exciton formation dynamics\, a quantity that so far has not been\naddressed by any other experiment. These first proof-of-prin ciple experiments of femtosecond\nphotoelectron dark-field microscopy can be considered as a door opener for future research\naddressing non-equilib rium many-body interactions on the ultrashort time- and length-scales.\nRe ferences\n[1] Keunecke et al.\, Rev. Sci. Ins. 91\, 063905 (2020)\, Time-r esolved momentum microscopy with a\n1 MHz high-harmonic extreme ultraviole t beamline.\n[2] Schmitt et al.\, Nature 608\, 499 (2022)\, Formation of m oiré interlayer excitons in space and\ntime.\n[3] Bange et al.\, Science Advances 10\, eadi1323 (2024)\, Probing correlations in the exciton\nlands cape of a moiré heterostructure.\n[4] Bange et al.\, 2D Materials 10\, 03 5039 (2023)\, Ultrafast dynamics of bright and dark excitons in\nmonolayer WSe2 and heterobilayer WSe2/MoS2.\n[5] Düvel et al.\, Nano Letters 22\, 4897-4904 (2022)\, Far-from-equilibrium electron-phonon\ninteractions in o ptically-excited graphene.\n[6] Schmitt et al.\, arXiv:2305.18908 (2023)\, Ultrafast nano-imaging of dark excitons.\n[7] Bennecke et al.\, Nat. Comm un. 15\, 1804 (2024)\, Multiorbital exciton formation in an organic\nsemic onductor.\n\nContact Person: Erwann/Matteo LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Hebatalla Elnaggar DTSTART;TZID=Europe/Berlin:20240626T140000 DTEND;TZID=Europe/Berlin:20240626T153000 DTSTAMP:20260527T090434Z UID:0000001978@events.thp.uni-koeln.de DESCRIPTION:Hebatalla Elnaggar\, IMPMC\, Sorbonne Université - CNRS\n\nSh edding New Light on the Puzzles of Iron Oxides\n\nThe role of electronic c orrelations in materials can be captured by the idea that "the whole is\ng reater than the sum of its parts". Correlated materials defy description b y a sum of non-\ninteracting electrons\; instead\, the interactions both a mong the electrons and with their\nsurrounding environment critically dete rmine their electronic and magnetic behaviour. These\ninteractions give ri se to intriguing phenomena such as magnetic order\, Mott (metal-insulator) \ntransitions\, and superconductivity. In this talk\, I will present two s triking examples of this intricate\ninterplay in iron oxides.\nIn the firs t part of my talk\, I will focus on Fe3O4 which undergoes a metal-to-insul ator transition\nwhen it is cooled below 125 K[1]. This transition is acco mpanied by complex long-range charge\nand orbital ordering\, as well as a structural distortion[2]. Despite extensive research since the\ndiscovery of this transition in 1939\, its mechanism has remained elusive. Here\, I will present\nResonant Inelastic X-ray Scattering (RIXS) magnetic dichrois m experiments and theoretical\ncomputations that reveal the existence of s hort-range noncollinear orbital magnetic ordering in\nthe high-temperature phase of Fe3O4[3\,4]. Furthermore\, I will discuss a novel charge reorder ing\noccurring in this phase\, which serves as a descriptor of electronic correlations[5]. These results\nprovide a new perspective on the metal to insulator transition of Fe3O.\nIn the second part\, I will explore the cap ability of RIXS to measure higher-order THz magnetic\nexcitations\, known as multi-magnons. Conventional wisdom suggests that a photon carrying one\ nunit of angular momentum can change the spin angular momentum of a magnet ic system by\none unit. This implies that a two-photon scattering process such as RIXS can change\nthe spin angular momentum of the magnetic system by two units. Here\, I will\ndemonstrate how\, despite this expectation\, RIXS can excite multi-magnons in Fe2O4 with\nchanges in the spin angular m omentum beyond ### due to spin non-conserving\ninteractions[6].\nFinally\, I will briefly present my future research plans\, which aim to understand the role of spin\nnon- conserving interactions in enhancing the efficienc y of exciting THz multi-magnons using\nlight. I will conclude by outlining my long-term vision to translate the understanding of THz multi-\nmagnons \, gained through x-ray spectroscopy\, into novel functionalities for futu re ultrafast spin-\nbased technology.\nReferences:\n[1] E. Verwey\, Nature 144\, 327 (1939).\n[2] M. Senn et al.\, Nature 481\, 173 (2012).\n[3] H. Elnaggar et al.\, ACS Appl. Mater. Interfaces 11\, 36213 (2019).\n[4] H. E lnaggar et al.\, Phys. Rev. Lett. 123\, 207201 (2019).\n[5] H. Elnaggar et al.\, Phys. Rev. Lett. 127\, 186402 (2021).\n[6] H. Elnaggar et al.\, Nat . Commun. 14\, 2749 (2023).\n\nContact Person: Erwann/Matteo LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Luca Banszerus DTSTART;TZID=Europe/Berlin:20240702T090000 DTEND;TZID=Europe/Berlin:20240702T103000 DTSTAMP:20260527T090434Z UID:0000001997@events.thp.uni-koeln.de DESCRIPTION:Luca Banszerus\, Niels Bohr Institute\, Copenhagen\n\nGate def ined quantum dots in bilayer graphene & synthetic flat bands in hybrid Jos ephson junction arrays\n\nI will present results from two different resear ch directions. First\, I will focus on gate defined\nquantum dots in bilay er graphene\, which are promising hosts for spin and valley qubits. I will \ndiscuss the single particle spectrum\, spin and valley relaxation times\ , and how the electron-hole\nsymmetry leads to a protected spin and valley blockade allowing for readout of spin and valley\nstates.\nThe second par t will focus on Josephson junction arrays (JJAs) based on a superconductor /\nsemiconductor hybrid material. Magnetic fields induce frustration\, lea ding to complex ground\nstates such as vortex lattices and liquids\, depen ding on the lattice geometry. I will present our\nefforts to map out the p hase diagram of JJAs with dice lattice geometry\, which are predicted to\n host flat bands when fully frustrated.\n\nContact Person: Erwann/Matteo LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Laëtitia Farinacci DTSTART;TZID=Europe/Berlin:20240702T150000 DTEND;TZID=Europe/Berlin:20240702T163000 DTSTAMP:20260527T090434Z UID:0000001998@events.thp.uni-koeln.de DESCRIPTION:Laëtitia Farinacci\, Uni. Stuttgart\n\nFrom magnetic bound st ates to coherent spin dynamics\n\nSpins are the epitome of the quantum nat ure of matter and are\, as such\, at the root of many fascinating phenomen a. In this talk\, I will discuss the interaction of a spin first with a co ntinuum formed by the Cooper pairs of a superconductor\, and then with ano ther discrete spin\, where quantum coherent dynamics can be tuned and obse rved directly.\nMagnetic impurities on top of superconductors induce bound states inside the superconducting gap\, called Yu-Shiba-Rusinov (YSR) sta tes. These states are promising building blocks for topological supercondu ctivity\, and I will present two experiments that shed light on their fund amental properties. YSR states arise due to exchange coupling between the magnetic impurity and Cooper pairs of the substrate\, leading to two possi ble ground states in which the impurity spin is either free or screened. B y approaching the tip toward the Fe center of an Fe-porphine molecule\, we can tune the exchange coupling strength and thereby drive the system thro ugh a quantum phase transition from a screened to a free-spin ground state [1]. In a second experiment\, we investigate the formation of YSR bands i n a kagome lattice\, using molecular self-assembly to ensure both atomic p recision and long-range order [2].\nIn the second part of my talk\, I will present recent technological advances to access the free coherent dynamic s of atomic spins. By carefully placing two Ti atoms on MgO/Ag(100) next t o each other and adjusting the height of a spin-polarized STM tip above on e of them\, we can tune the level of entanglement between their two spins\ , which we observe by means of ESR-STM measurements. We then investigate t he dynamics of the system using DC pump-probe spectroscopy. We find that a t maximal entanglement\, a spin excitation can be coherently swapped back and forth between the two atomic spins [3]. The frequency and characterist ics of this coherent oscillation are solely dictated by the intrinsic prop erties of the system\, providing us with a unique way to investigate the p roperties of quantum materials. We further expanded our investigations to the study of spin-flip propagation in 1D and quasi-1D networks\, and lastl y\, resolved coherent dynamics between a nuclear and electron spin in a si ngle atom [4].\nThese studies set the foundation for exciting future resea rch in which I will combine the atomic-scale control of scanning tunneling microscopy with coherent driving to dynamically tune the properties of qu antum materials.\n \n[1] Farinacci et al.\, Phys. Rev. Lett. 121\, 196803 (2018)\n[2] Farinacci et al.\, arXiv:2307.09993 (2023)\n[3] Veldman\, Fari nacci et al.\, Science 372\, 964 (2021)\n[4] Veldman\, … Farinacci\, Ott e\, arXiv:2309.03749 (2023)\n\nContact Person: Erwann/Matteo LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Zlata Fedorova DTSTART;TZID=Europe/Berlin:20240703T140000 DTEND;TZID=Europe/Berlin:20240703T153000 DTSTAMP:20260527T090434Z UID:0000001979@events.thp.uni-koeln.de DESCRIPTION:Zlata Fedorova\, Uni. Jena\n\nAdvancing optical control of val ley degree of freedom in 2D semiconductors\n\nValleys\, defined as maxima or minima in the energy-momentum relation where carriers reside\nisolated by momentum mismatch\, present a promising material degree of freedom\, ak in to\nelectron charge or spin. Despite early theoretical interest\, pract ical utilization of valleys has been\nhindered by the challenge of measura ble valley contrast in natural systems. The advent of two-\ndimensional (2 D) materials\, particularly transition metal dichalcogenides (TMDs)\, has\ nrevolutionized valleytronics due to their intrinsic valley contrast. Spec ifically\, monolayer TMDs\nexhibit a direct band gap\, enabling straightfo rward access to valley degrees of freedom via\nphotoluminescence experimen ts and valley-contrasting optical selection rules (see inset in Fig. 1\n(a )). Circularly polarized photons can selectively address each valley type\ , with emitted photons\nbeing circularly polarized as well retaining valle y information. Although significant advances have\nbeen made in addressing \, reading out\, and tuning valley degrees of freedom\, these processes\nr emain challenging\, requiring sophisticated and costly experiments. The op tical addressability of\nvalleys in 2D TMDs holds potential for integratio n with nanophotonics\, such as resonant\nnanoantennas and structured light . This talk will present a comprehensive experimental and\nnumerical study of a hybrid system where valley-polarized photoluminescence from a MoS2\n monolayer is coupled with a plasmonic nanosphere (see Fig. 1 (a))\, aiming to refine simulation\napproaches and enhance understanding of nanoscale l ight-matter interactions. Additionally\, the\ntalk will explore the intera ction of vortex beams with valley excitons\, examining valley\ndepolarizat ion effects\, and the potential for directional exciton transport (see Fig . 1 (b)).\nFigure 1 Optically pumped hybrid system consisting of a gold na noparticle resonantly interacting\nwith photoluminescence emitted by a mon olayer MoS2. Inset: valley-contrasting optical selection\nrules. (b) Circu larly polarized vortex beam incident on a monolayer TMD transfers momentum to\nvalley excitons.\n\nContact Person: Erwann/Matteo LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Jannis Gebhard DTSTART;TZID=Europe/Berlin:20240705T150000 DTEND;TZID=Europe/Berlin:20240705T160000 DTSTAMP:20260527T090434Z UID:0000002015@events.thp.uni-koeln.de DESCRIPTION:Jannis Gebhard\, Heidelberg\n\nSmall System Puzzle – Explori ng High-Momentum Probes in Light Ion Collisions\n\nNuclear collisions allo w the study of the Quark Gluon Plasma - a medium of deconfined quarks and gluons. At particle colliders\, its presence is signaled by modifications of the measured particle spectra. The recent discovery of anisotropically distributed particles („collective flow“) in smaller collision system s while modifications to high-momentum spectra are absent is puzzling sinc e both are associated with the presence of the QCD medium. In this work\, no-medium baseline predictions for high-momentum observables are presented and theoretical limits to observing medium effects are discussed.\n\nCont act Person: Simon Trebst LOCATION:Seminar Room 0.03\, ETP END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Davide Bossini DTSTART;TZID=Europe/Berlin:20240709T090000 DTEND;TZID=Europe/Berlin:20240709T103000 DTSTAMP:20260527T090434Z UID:0000001999@events.thp.uni-koeln.de DESCRIPTION:Davide Bossini\, Uni. Konstanz\n\nOptical coherent control of quantum materials\n\nA major research effort in the area of condensed matt er physics aims to optically manipulate the\nmacroscopic properties of qua ntum materials [1]. Remarkable critical phenomena have been\nreported\, su ch as the laser-induced metal-insulator transition [2]\, metamagnetic phas e\ntransitions [3]\, and even superconductivity [4]. The dynamical control of the order parameter has\nbeen achieved in magnetic materials [5]\, eve n via coherent processes\, which rely on the optical\ngeneration of cohere nt collective eigenmodes [6\,7]. In my talk\, I will introduce the main li ght-\nmatter interaction mechanisms enabling the optical activation of coh erent quasiparticles\, with a\nparticular focus on magnetic excitations. I will discuss in details two aspects. First\, the generation\nof collectiv e spin excitations\, namely spin waves or magnons\, both at the center and at the\nedges of the Brillouin zone\, while avoiding the absorption of li ght by the lattice and the\nelectrons [8-9]. The regime of quantum spin dy namics triggered by the activation of high-energy\nmagnons will be discuss ed as well [9-10]. Second\, I will address the coupling between spins and\ nelectrons at the ultrashort timescale. Mechanisms allowing to drive spin waves both at the\ncenter and at the edges of the Brillouin zone\, via mix ed electronic and magnonic transitions\, will\nbe the topic of discussion [11-12]. I will then report on our very recent successful attempts to\ncou ple coherent THz magnons to charges at the picosecond timescale and our on going efforts\nto establish a manipulation of the spin-orbit coupling\, vi a the excitation of coherent THz\nphonons [13]. In the final part of my ta lk\, I will then present novel concepts to manipulate\nquantum materials\, i.e. the dynamical modification of the spectrum and dispersion relation. I will\nshow an unprecedented scheme to couple zone-edge and zone-centre m odes [14]\, which\nresults in the renormalization of the frequency and amp litude of the zone-centre magnons [15].\nThe observed regime of strongly n onlinear coherent spin dynamics is elusive of the conventional\npicture pr ovided by linear spin wave theory. The development of a quantitative theor etical\nunderstanding of the observations\, in collaboration with theorist s\, will be presented as well. In\nconclusion\, I will briefly show extrem ely novel results concerning an alternative route to modify\nthe magnon sp ectrum\, which does not involve zone-edge magnons at all. The concepts\ndi scussed in my talk provide basic tools for a dynamical engineering of macr oscopic coherent\nstates of quantum materials\, operating at the intrinsic characteristic time scales of the collective\neigenmodes. The approaches presented here can be applied to a wide variety of systems and\ncan be gen eralized to lattice excitations.\n[1] Advances in Physics 65\, 58–238 (2 016).\n[2] Nature 449\, 72–74 (2007).\n[3] Physical Review Letters 116\, 097401 (2016).\n[4] Science 331\, 189–191 (2011).\n[5] Physical Review Letters 99\, 047601 (2007).\n[6] Nature 435\, 655 (2005).\n[7] Nature Phot onics 5\, 31–34 (2010).\n[8] Physical Review B 89\, 060405(R) (2014).\n[ 9] Nature Communications 7\, 10645 (2016).\n[10] Physical Review B 100\, 0 24428 (2019).\n[11] Physical Review Letters 127\, 077202 (2021).\n[12] Nat ure Physics 14\, 370–374 (2018).\n[13] ArXiv (2023) doi:10.48550/arxiv.2 310.08411.\n[14] Laser Photonics Rev. 2301152\, (2024).\n[15] ArXiv (2023) doi: 10.48550/arXiv:2310.19667v1\n\nContact Person: Erwann/Matteo LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Heng Wu DTSTART;TZID=Europe/Berlin:20240710T140000 DTEND;TZID=Europe/Berlin:20240710T153000 DTSTAMP:20260527T090434Z UID:0000001980@events.thp.uni-koeln.de DESCRIPTION:Heng Wu\, TU Delft\n\nNovel phenomena of superconductivity\n\n Superconductivity has attracted much attention in the field\, not only due to its intriguing\nfundamental mechanism\, but also its practical applica tions such as superconducting wires and\ndetectors\, as well as the potent ial applications including superconducting computing and logics.\nThough i t’s been discovered more than a century ago\, the novel phenomena based on\nsuperconductivity are still emerging. In this talk\, I will introduce my research on the intriguing\nphenomena of superconductivity. First\, I w ill discuss the superconducting diode effect\, which\ndescribes the differ ent positive and negative critical currents in a superconductor. We observ ed\nthis effect with and without an external magnetic field in heterostruc tures based on van der Waals\nmaterials. The second phenomenon is the “g lobal critical current effect”\, which means that the\ncritical current of a superconductor is not only dependent on its own properties but can be \ninfluenced or even determined by adjacent superconductors. Inspired by t his observation\, we\ndesigned a superconductor with a special geometry\, in which we transferred the superconducting\ndiode effect to another part of the superconductor that cannot manifest it on its own.\n\nContact Perso n: Erwann/Matteo LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Christoph Kastl DTSTART;TZID=Europe/Berlin:20240716T090000 DTEND;TZID=Europe/Berlin:20240716T103000 DTSTAMP:20260527T090434Z UID:0000002000@events.thp.uni-koeln.de DESCRIPTION:Christoph Kastl\, WSI TU München\n\nBroken symmetries in van der Waals quantum materials\n\nInterfacing van der Waals materials allows manipulating the fundamental symmetries of electron\nstates in solids with unprecedented precision using the individual layers as atomically precise \,\nbottom-up building blocks. The resulting heterostructures have emerged as a versatile\,\nelectronically tunable solid-state platform for studyin g quantum phases with tailored electronic\,\noptical\, and spin properties . Here\, I will discuss recent examples of intentionally and\nunintentiona lly broken symmetries in van der Waals heterointerfaces and their interrog ation by\nnon-linear optoelectronic transport [1].\nIn graphene-based hete rostructures\, we address gate-tunable\, non-linear transport arising from \nproximity-induced spin-orbit coupling and breaking of the relevant cryst al symmetries\, which may\nbe used to efficiently manipulate spin-polarize d carriers by both optical and electrical means. In\nepitaxial graphene/Bi 2Te2Se heterointerfaces with a commensurate lattice alignment\, we find an \nenhanced helicity-dependent photocurrent\, which originates from the pec uliar spin-orbit proximity\nof the commensurate interface [1]. In graphene /WTe2 heterostructures\, we demonstrate a current-\ninduced Kerr rotation related to non-linear polarisation of the Fermi surface. We provide a\nthe oretical model based on the non-linear anomalous Hall effect in the hetero structure\, whose\ndetection is enabled by the reduced symmetry of the inc ommensurate heterointerface [2].\nFinally\, I will outline the use of nano lithography to control the symmetry of van der Waals materials\nin a top-d own approach. In particular\, nanoscale superlattices may allow to enhance electronic\ncorrelations by flat band formation in a broad class of mater ials. Such correlated flat bands have\nnon-trivial quantum geometric and t opological properties that are addressable in suitable\noptoelectronic exp eriments with a resonant optical excitation.\nReferences\n[1] J. Kiemle\, P. Zimmermann\, A.W. Holleitner\, C. Kastl\, Light-field and spin-orbit-dr iven currents in\nvan der Waals materials\, Nanophotonics 2020\, 9\, 2693 - 2708.\n[2] J. Kiemle\, L. Powalla\, K. Polyudov\, L. Gulati\, M. Singh\, A. W. Holleitner\, M. Burghard\, C. Kastl\,\nGate-Tunable Helical Current s in Commensurate Topological Insulator/Graphene\nHeterostructures\, ACS N ano 2022\, 16\, 12338-12344.\n[3] L. Powalla\, J. Kiemle\, E. J. König\, A. P. Schnyder\, J. Knolle\, K. Kern\, A. W. Holleitner\, C. Kastl\,\nM. B urghard\, Berry curvature-induced local spin polarisation in gated graphen e/WTe2\nheterostructures\, Nat. Commun. 2022\, 13\, 3152.\n\nContact Perso n: Erwann/Matteo LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Magnus Vigsø DTSTART;TZID=Europe/Berlin:20240723T140000 DTEND;TZID=Europe/Berlin:20240723T153000 DTSTAMP:20260527T090434Z UID:0000002021@events.thp.uni-koeln.de DESCRIPTION:Magnus Vigsø\, Niels Bohr Institute/University of Copenhagen\ n\nDissipative Circuit Quantum Electrodynamics\n\nClassical non-conservati ve equations of motion are inapplicable in quantum mechanics. This constit utes a major problem in the study of quantum systems\, that can not be reg arded as isolated from their environment. The standard method of implement ing dissipation in quantum mechanics is to include the environment as infi nitely many degrees of freedom\, which are linearly coupled to the system of interest. By applying reduced system descriptions\, where the infinite degrees of freedom in the environment is encapsulated in scalar functions such as the spectral density\, dissipative dynamics can be included in qua ntum descriptions. However\, this is often achieved with assumptions that are justified a posteriori in the phenomonological modelling of dissipativ e quantum mechanics. As an alternative\, it can be shown that dissipative dynamics can be derived from a microscopic model of a quantum system. By a pplying circuit quantum electrodynamics to circuit diagrams\, a spectral d ensity corresponding to Drude dampening is derived from an infinite transm ission line in the continuum limit acting as a dissipative environment. Th e resulting dynamics is captured in a Heisenberg-Langevin equation for the flux of a resonator circuit\, and the correlation function of its stochas tic noise is demonstrated to be an example of the fluctuation-dissipation theorem. Along the way the physical justification of the rotating-wave app roximation is touched upon\, and Poincaré recurrences are demonstrated as a feature of environments with finite degrees of freedom.\n\nContact Pers on: Urban Seifert LOCATION:Seminar Room 0.03\, ETP END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Alexander Wowchik DTSTART;TZID=Europe/Berlin:20240726T140000 DTEND;TZID=Europe/Berlin:20240726T153000 DTSTAMP:20260527T090434Z UID:0000002022@events.thp.uni-koeln.de DESCRIPTION:Alexander Wowchik\, Uni Heidelberg\n\nTowards quantitative sim ulations of cold Bose-gases from the 2PI effective action\n\n\n\nContact P erson: Matteo Cacco LOCATION:Seminar Room 0.03\, ETP END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Haoyang Tian DTSTART;TZID=Europe/Berlin:20240820T140000 DTEND;TZID=Europe/Berlin:20240820T153000 DTSTAMP:20260527T090434Z UID:0000002024@events.thp.uni-koeln.de DESCRIPTION:Haoyang Tian\, FU Berlin\n\nThe effect of interface disorder o n the tunnel conductance across Weyl-semimetal interfaces\n\nThe chiral an omaly in Weyl semimetals is responsible for various\nanomalous transport p henomena. In tunnel junctions between Weyl semimetals\nwith staggered Weyl node projections\, the chiral anomaly leads to a\nmagnetic-field activate d magnetotransport. In this talk\, I discuss the\neffect of interface diso rder on the magnetotransport across such a tunnel\njunction employing a se miclassical Boltzmann approach. Our results show\nthat\, compared to conve ntional transport channels\, the topological\nconnectivity of interface Fe rmi arcs ensures that anomalous\nmagnetotransport exhibits stronger robust ness against disorder.\nAdditionally\, interface disorder enhances magneti c breakdown\, a quantum\ntunneling effect\, between the Fermi arcs.\n\nCon tact Person: Urban Seifert LOCATION:Seminar Room 0.03\, ETP END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Minh Duc Tran DTSTART;TZID=Europe/Berlin:20240823T140000 DTEND;TZID=Europe/Berlin:20240823T150000 DTSTAMP:20260527T090434Z UID:0000002030@events.thp.uni-koeln.de DESCRIPTION:Minh Duc Tran\, Kaiserslautern\n\nQuantum and classical simula tion of quantum systems\n\n\n\nContact Person: Simon Trebst / Xhek Turkesh i LOCATION:Seminar Room 0.03\, ETP END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Fabian Kugler DTSTART;TZID=Europe/Berlin:20240830T130000 DTEND;TZID=Europe/Berlin:20240830T134500 DTSTAMP:20260527T090434Z UID:0000002028@events.thp.uni-koeln.de DESCRIPTION:Fabian Kugler\, Flatiron Institute\n\nTheoretical Spectroscopy of Quantum Materials\n\nno abstract\n\nContact Person: Simon Trebst / Ach im Rosch LOCATION:Seminar Room 0.03\, ETP END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Masayuki Hashisaka DTSTART;TZID=Europe/Berlin:20240913T143000 DTEND;TZID=Europe/Berlin:20240913T153000 DTSTAMP:20260527T090434Z UID:0000002007@events.thp.uni-koeln.de DESCRIPTION:Masayuki Hashisaka\, ISSP\, University of Tokyo\n\nAharonov-B ohm interference of fractionalized electron-spin excitations\n\nTomonaga-L uttinger liquid nature of copropagating spin-up and spin-down quantum Hall \nedge channels splits an electron excitation into fast and slow excitatio ns. Previous works\nidentified the fractionalization process by injecting a spin-up or spin-down excitation into one\nof the copropagating channels and measured the resultant multiple excitations [1]. On the\nother hand\, this work studies the fractionalization of a spin-up and spin-down superpo sition\nstate prepared in the interacting channels [2\,3]. We measured the interference of the\nfractionalized excitations in a Mach-Zehnder interfe rometer (MZI) employing interacting\ncopropagating channels as the interfe rence paths [Fig. 1]. The observed interference visibility\noscillates as a function of the voltage bias applied between the channels\, showing the “lobe\nstructure\,” which indicates the signature of the second-order interference in the one-way MZI.\nThe lobe structure manifests the phase e volutions of the fractionalized excitations different\nfrom each other\, r eflecting the difference between their speeds.\nThis work was supported by KAKENHI Grant No. JP22H00112\, 19H05603\, and 24H00827.\n\nContact Person : Erwann / Matteo LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Steve Johnson DTSTART;TZID=Europe/Berlin:20240913T110000 DTEND;TZID=Europe/Berlin:20240913T120000 DTSTAMP:20260527T090434Z UID:0000002041@events.thp.uni-koeln.de DESCRIPTION:Steve Johnson\, ETH Zurich\n\nX-ray probes of temporal coheren ce in CoF2 and GeTe\n\n\n\nContact Person: Paul van Loosdrecht LOCATION:Seminar room Institute of Physics 2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Tobias Korn DTSTART;TZID=Europe/Berlin:20240920T100000 DTEND;TZID=Europe/Berlin:20240920T110000 DTSTAMP:20260527T090434Z UID:0000002043@events.thp.uni-koeln.de DESCRIPTION:Tobias Korn\, Uni Rostkock\n\nTuning excitons in two-dimension al crystals and van der Waals heterostructures via interlayer alignment\n\ nTransition-metal dichalcogenide (TMD) monolayers are direct-gap semicondu ctors with peculiar spin-valley coupling. Combining two different TMDs can lead to a type-II band alignment and formation of interlayer excitons (IL E). In MoSe2-WSe2 heterobilayers\, these are only optically bright if the layers are aligned in parallel orientation\, or with an interlayer twist o f 60 degrees. Depending on alignment\, ILE transitions are either valley-c onserving or between valleys. This allows us to engineer the ILE emission energy as well as its g factor\, changing its magnitude and even its sign. Additionally\, applied magnetic fields induce a valley polarization of th e ILE\, and its buildup can directly be observed in helicity- and time-res olved photoluminescence (PL)\, with peculiar features due to the dependenc e of ILE optical selection rules on interlayer registry [1].\n\nStacking T MD monolayers with parallel orientation leads to ferroelectric fields at t he interfaces of adjacent layers\, whose orientation can be switched by in -plane sliding\, potentially giving rise to a novel type of nonvolatile da ta storage. While artificial stacking of TMDs always results in finite int erlayer twist\, leading to small\, moiré-like ferroelectric domains\, nat urally grown parallel-stacked TMD crystals can have mesoscopic domains. Re markably\, the ferroelectric order in such crystals can be determined usin g optical spectroscopy\, as it leads to a stacking-dependent energetic spl itting of intralayer exciton transitions [2]. This allows us to map ferroe lectric domains and track their switching in externally applied electric f ields.\n\n[1] J. Holler\, TK et al.\, Phys. Rev. B 105\, 085303 (2022).\n[ 2] S. Deb\, TK et al.\, Nature Comms. 15\, 7595 (2024).\n\nContact Person: Paul van Loosdrecht LOCATION:Seminar room Ph2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Borna Pielic DTSTART;TZID=Europe/Berlin:20241009T143000 DTEND;TZID=Europe/Berlin:20241009T153000 DTSTAMP:20260527T090434Z UID:0000002003@events.thp.uni-koeln.de DESCRIPTION:Borna Pielic\, Universität Siegen\n\nVan der Waals Epitaxy of Transition Metal Disulphide Nanostructures: Tailoring Electronic Structur e by Growth Variations\n\nVan der Waals Epitaxy of Transition Metal Disulp hide Nanostructures: Tailoring Electronic Structure by Growth Variations\n \nSemiconducting transition metal dichalcogenides (TMDs) are fascinating d ue to their exceptional properties at the atomic scale: direct band gap\, strong exciton binding energy\, high charge mobility\, and high thermal co nductivity. These properties make two-dimensional (2D) TMDs excellent cand idates for applications such as photodetectors\, solar cells\, light-emitt ing devices\, field-effect transistors\, and sensing devices.\nWhen studyi ng the intrinsic properties of 2D materials\, it is desirable to grow them on a weakly interacting substrate\, most commonly another 2D material. We have epitaxially grown MoS₂ on graphene and investigated the effects of self-intercalation (intercalation of native atoms of the TMD) using scann ing tunneling microscopy (STM) and low-energy electron diffraction (LEED). Our findings indicate that the interaction between graphene and MoS₂ is tunable\, resulting in lattice strain and dielectric screening\, both of which significantly influence the electronic structure\, recorded by scann ing tunnelling spectroscopy (STS) (1). Additionally\, interlayer charge co rrelations may give rise to plasmonic features in the spectra.\nOur resear ch also explores lateral and vertical TMD heterostructures composed of sem iconducting MoS₂\, WS₂\, and metallic TaS₂. Through band bending mea surements using STS\, we observe atomically sharp pn and Schottky junction s.\n\n(1) Pielić\, B.\, Mužević\, M.\, Novko\, D.\, Jiaqi\, C.\, Bremer ich\, A.\, Ohmann\, R.\, Kralj\, M.\, Šrut Rakić\, I.\, Busse C.\, Probi ng the interplay of interactions\, screening and strain in monolayer MoS2 via self-intercalation. npj 2D Mater Appl 8\, 61 (2024).\n\nContact Person : Thomas Michely / Matteo LOCATION:Seminar Room of the Institute of Physics II END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Henry Legg DTSTART;TZID=Europe/Berlin:20241016T143000 DTEND;TZID=Europe/Berlin:20241016T153000 DTSTAMP:20260527T090434Z UID:0000002060@events.thp.uni-koeln.de DESCRIPTION:Henry Legg\, Basel University\n\nFrom perfect to imperfect to false Majoranas in semiconductor nanowires and minimal Kitaev chains\n\nno abstract\n\nContact Person: Achim Rosch / Matteo LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Szabolcs Csonka DTSTART;TZID=Europe/Berlin:20241113T143000 DTEND;TZID=Europe/Berlin:20241113T153000 DTSTAMP:20260527T090434Z UID:0000002081@events.thp.uni-koeln.de DESCRIPTION:Szabolcs Csonka\, Budapest University\n\nRealization of Hetero atomic Andreev molecule\n\nno abstract\n\nContact Person: Yoichi Ando / Ma tteo LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Claudio Gianetti DTSTART;TZID=Europe/Berlin:20241211T143000 DTEND;TZID=Europe/Berlin:20241211T153000 DTSTAMP:20260527T090434Z UID:0000002074@events.thp.uni-koeln.de DESCRIPTION:Claudio Gianetti\, Università Cattolica del Sacro Cuore\, Bre scia\n\nMott materials: a journey from non-thermal switching to ultrafast coherent control\n\nAchieving the full understanding and control of the in sulator-to-metal transition in Mott materials is key for the next generati on of electronics devices\, with applications ranging from ultrafast trans istors\, volatile and non-volatile memories and artificial neurons for neu romorphic computing. In this work\, we will review the state-of-the-art kn owledge of the Mott transition\, with specific focus on the paradigmatic M ott insulator V2O3. We will emphasize the current attempts in controlling the Mott switching dynamics via the application of external voltage and el ectromagnetic pulses and we will discuss how the recent advances in time- and space-resolved techniques are boosting the comprehension of the firing process and the role of topological defects of the order parameter. The n ature of the voltage/light-induced Mott switching is inherently different from what is attainable by the slower variation of thermodynamic parameter s\, thus offering promising routes to achieving the reversible and ultrafa st coherent control of conductivity in Mott nanodevices.\n\nA. Milloch et al. Mott resistive switching initiated by topological defects. Accepted in Nat. Commun.\nA. Milloch et al. Mott materials: unsuccessful metals with a bright future. npj Spintronics 2\, 49 (2024)\nA. Ronchi et al. Nanoscale self-organization and metastable non-thermal metallicity in Mott insulato rs. Nat. Commun. 13\, 3730 (2022)\n\nContact Person: Paul van Loosdrecht LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Guangyao Miao DTSTART;TZID=Europe/Berlin:20250122T143000 DTEND;TZID=Europe/Berlin:20250122T153000 DTSTAMP:20260527T090434Z UID:0000002101@events.thp.uni-koeln.de DESCRIPTION:Guangyao Miao\, Humboldt-Universität zu Berlin\n\nThe low-dim ensional quantum states and their atomic-scale engineering in elemental ch alcogens and chalcogenides\n\nWhen reduced to lower dimensions\, materials exhibit diverse electronic\, optical\, and magnetic properties. Their tun able physical properties under multiple external fields hold significant p romise for fundamental research and future technological applications. By understanding the microscopic mechanisms that govern these macroscopic pro perties\, we can guide the design and fabrication of materials for functio nal applications. Here\, I will introduce several MBE-grown low-dimensiona l quantum materials and the atomic-scale tailoring of their properties\, i ncluding elemental Te with helical crystal structure\, transition metal te lluride\, and superconducting iron selenide.\nReferences: Phys. Rev. Lett. 133\, 236201 (2024)\n Adv. Electron. Mater.2400720 (2024)\n Nature 635\, 332–336(2024).\n Phys. Rev. B 103\, 235 421(2021)\n Phys. Rev. B 101\, 035407 (2020)\n\nContact Person: Wouter Jolie / Matteo LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Chafic Fawaz DTSTART;TZID=Europe/Berlin:20250124T103000 DTEND;TZID=Europe/Berlin:20250124T113000 DTSTAMP:20260527T090434Z UID:0000002100@events.thp.uni-koeln.de DESCRIPTION:Chafic Fawaz\, Université Grenoble Alpes\n\nWhat is the role of magnetic excitation in the electronic properties of cuprates?\n\nA majo r discovery in condensed matter physics is the discovery of High-Temperatu re Superconductivity in cuprates (copper oxides)\, which still hold the re cord for the highest critical temperature at ambient pressure. They featur e layers of CuO2 planes\, believed to be responsible for their electronic properties\, involving strong electronic correlations. Doping gives rise t o a very complex phase diagram\, going from an antiferromagnetic insulatin g phase to a pseudo-gap\, superconductivity\, and a strange metal phase\, along with coexisting and/or competing orders of charge and spin. However\ , a comprehensive theoretical explanation of the Cooper pairing mechanism in these materials is still a debated subject\, in which magnetic excitati on (paramagnons) are promising candidates. \nThe hole-doped Ca2CuO2Cl2 cop per oxychloride serves as an excellent compound to investigate all these p hases on common ground. Its stable and simple I4/mmm 1-layer structure and strong 2D character make it very suitable for theoretical calculations\, allowing direct comparison with experimental work. \nIn this talk I will d iscuss the magnetic excitation measured by Resonant Inelastic X-ray scatte ring (RIXS) up to the optimal doping. The paramagnon exhibits a similar di spersion with doping\, along the (h\,0) direction\, similar to all cuprate s\, and a softening along the (h\,h) direction\, as also measured in other cuprates. Along the (h\,h) direction\, the bimagnon weakens in the underd oped phase\, while a charge continuum seems to arise at higher doping. Ram an spectroscopy confirm that the bimagnon become weaker with doping. \nThe paramagnon band-with have the same energy as a waterfall feature in the e lectronic bands\, as measured in Angle resolved Photo-Emission Spectroscop y (ARPES)\, suggesting a link between the two phenomena. This is indeed su pported by cluster-DMFT calculations\, which suggest a spin-polaron band e merge at such energy scale.\n\nContact Person: Markus Grüninger / Matteo LOCATION:0.02 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Chris Laumann DTSTART;TZID=Europe/Berlin:20250131T163000 DTEND;TZID=Europe/Berlin:20250131T173000 DTSTAMP:20260527T090434Z UID:0000002045@events.thp.uni-koeln.de DESCRIPTION:Chris Laumann\, Boston University\n\nThe fine structure of qua ntum spin ice\n\nCondensed-matter systems provide alternative “vacua” exhibiting emergent low-energy properties dramatically different from thos e of the standard model. A case in point is the emergent quantum electrody namics (QED) in the family of magnetic materials known as quantum spin ice . The emergent QED possesses many features familiar from our universe\, su ch as charges\, anti-charges and photons\, but also many unfamiliar one\, such as magnetic monopoles. Thus these magnetic insulators provide a labor atory for exploring effective QED in regimes quite inaccessible to traditi onal Maxwell electromagnetism.\n\nIn this talk\, I will review the beautif ul picture of how QED emerges in these frustrated magnets. We will then tu rn to several results regarding its `fine structure’. We will see that t he fine structure constant α -- the dimensionless coupling which controls the interactions between emergent light and charges -- generically takes values ~0.1 in quantum spin ice\, much larger than the α ~ 1/137 of our u niverse [1]. This leads to a variety of predictions regarding the coherent dynamics of the spinons which we expect can be probed by neutron scatteri ng [2]. Finally\, we will consider how true electric fields couple into th is magnetic insulator -- and may permit the indirect observation of the em ergent magnetic monopole and a curious ‘inverted’ Lorentz force [3].\n \n[1] Pace\, Morampudi\, Moessner\, Laumann. Phys. Rev. Lett. 127\, 117205 (2021).\n[2] Morampudi\, Wilczek\, Laumann. Phys. Rev. Lett. 124\, 097204 (2020).\n[3] Laumann\, Moessner. Phys. Rev. B 108\, L220402 (2023)\n\nCon tact Person: Simon Trebst LOCATION:0.01 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Matthias Gohlke DTSTART;TZID=Europe/Berlin:20250203T160000 DTEND;TZID=Europe/Berlin:20250203T171500 DTSTAMP:20260527T090434Z UID:0000002123@events.thp.uni-koeln.de DESCRIPTION:Matthias Gohlke\, Okinawa Institute of Science and Technology\ n\nProximate Tomonaga-Luttinger Liquid with Spinon-like Excitations in an Anisotropic Kitaev-Gamma Model\n\nThe characterization of quantum spin liq uid (QSL) phases in Kitaev materials has been a subject of intensive studi es over the recent years\, both theoretically and experimentally. The Kita ev-Gamma model with bond-dependent diagonal and off-diagonal exchange has been considered as a minimal model in this context. Many theoretical studi es have focused on spatially isotropic coupling strength along the differe nt bonds in an attempt to simplify the problem. However\, Kitaev materials often have a reduced lattice symmetry that allows for different coupling strength along the bonds\, or the lattice symmetry can be reduced by exter nal means.\nHere\, we consider a Kitaev-Gamma model with varying coupling strength along one of the bonds connecting the limit of isolated chains to the two-dimensional model. The chain limit is known to exhibit an emergen t $SU(2)_1$ Tomonaga-Luttinger liquid (TLL) phase [Yang et al. (2020)]. In the isotropic limit the nature of the phase diagram is still under debate . Various phases---including magnetically ordered\, incommensurate\, quant um paramagnetic\, as well as a quantum spin liquid---have been proposed de pending on the applied method and/or underlying assumptions.\nUsing ED on a symmetric cluster and iMPS\, we map out the phase diagram in between the two limits. Our numerical results suggest that the isotropic limit is clo se to a phase transition between a trivial paramagnet and a quantum parama gnetic phase with a strong one-dimensional character\, that can be traced back to the TLL phase in the chain limit. This proximate TLL would be diff erent from the adjacent KSL in that it exhibits spinon-like excitation sim ilar in nature to the 1D Heisenberg AF.\n\nContact Person: Simon Trebst LOCATION:0.03 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Ville Maisi DTSTART;TZID=Europe/Berlin:20250212T143000 DTEND;TZID=Europe/Berlin:20250212T153000 DTSTAMP:20260527T090434Z UID:0000002097@events.thp.uni-koeln.de DESCRIPTION:Ville Maisi\, University of Lund\n\nDetecting single electrons and microwave photons with semiconductor quantum dots\n\nBuilding a detec tor capable to resolve individual microwave photons has attained a strong focus in recent years. Approaches based on superconducting qubits have rea ched single photon resolution with high quantum efficiency. However\, thes e detectors operate non-continuously\, thus requiring precise pulsing and considerations about the timing to perform the detection optimally. I pres ent our efforts to build a microwave photodetector based on counting elect ron tunnelling events in a double quantum dot photon absorber. This detect or operates continuously avoiding the pulsing and thus responds to the inp ut photons as they arrive. In addition to these technological developments \, I'll discuss the relevant physics effects taking place in these detecto rs such as hybridization of the quantum states and the coherence under con tinuous charge readout.\n\nContact Person: Erwann Bobquillon / Matteo LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Andrei Pimenov DTSTART;TZID=Europe/Berlin:20250217T143000 DTEND;TZID=Europe/Berlin:20250217T160000 DTSTAMP:20260527T090434Z UID:0000002117@events.thp.uni-koeln.de DESCRIPTION:Andrei Pimenov\, Uni Vienna\n\nTerahertz Classical and Quantum Physics in 2D Materials\n\nClassical terahertz magneto-spectroscopy of co nducting systems is dominated by cyclotron resonance. Here not only inform ation about the charge carriers but also key insights into the band struct ure of various two-dimensional systems can be extracted.\nQuantum correcti ons to the optical spectra manifest themselves in several forms. For insta nce\, optical analogs of microwave-induced resistance oscillations can be observed or dynamic equivalent of Shubnikov-de Haas oscillations emerges.\ nIn strong magnetic fields\, quantum effects dominate the optical response \, leading to phenomena such as quantized Faraday rotation. This effect is especially pronounced in magnetically-doped two-dimensional topological i nsulators\, where experiments directly reveal the quantum of the rotation angle equal to the fine structure constant\, α ≈ 1/137. This dimensionl ess physical constant provides a profound connection between solid-state p hysics\, cosmology\, and mathematics\, highlighting its universal signific ance.\n\nContact Person: Paul Van Loosdrecht / Matteo LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Joaquin Fernández-Rossier DTSTART;TZID=Europe/Berlin:20250219T143000 DTEND;TZID=Europe/Berlin:20250219T153000 DTSTAMP:20260527T090434Z UID:0000002114@events.thp.uni-koeln.de DESCRIPTION:Joaquin Fernández-Rossier\, INL (Braga)\n\nAn overview on mag netic 2D materials\n\ntba\n\nContact Person: Jeison Fischer / Matteo LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Komalavalli Thirunavukkuarasu DTSTART;TZID=Europe/Berlin:20250221T143000 DTEND;TZID=Europe/Berlin:20250221T153000 DTSTAMP:20260527T090434Z UID:0000002085@events.thp.uni-koeln.de DESCRIPTION:Komalavalli Thirunavukkuarasu\, Department of Physics\, Florid a A & M University\, Tallahassee\, FL\, USA NationalHigh Magnetic Field La boratory\, Tallahassee\, FL\, USA\n\nProbing spin phonon coupling in magne tic materials using magneto-Raman spectroscopy\n\nWe present recent studie s on the nature of spin-phonon exchange interactions indifferent classes o f magnetic materials using magneto-Ramanspectroscopy. Thefirst compound th at will be discussed is SrCu2(BO3)2.It is a quasi-2D orthogonal spin dimer system with a spinsinglet ground state and considered as typical example for the realization of the Shastry-Sutherland model and exhibits a sequen ce of magnetization plateau at magnetic fields higher than 20 T\,i.e.\, hi gh enough to close the low temperature spin gap. The unique behavior ofthi s quantum spin liquidresults from the interplay between geometrical frustr ation andstrong quantumfluctuations. The origin and characteristics of the strongspin-latticecoupling in SrCu2(BO3)2revealed by Ramanscattering stud ies at high magnetic fields up to 45T will bepresented. Theinterpretation of experimental results is supported by atheoretical analysisand predictio n of active Raman modes that include theso-called pantograph mode[1]. The second compound is [(CH3)2NH2]Co(HCOO)3belonging to the family of multifer roic metal organicframeworks (MOF). In theinfrared studies under applied m agnetic fields upto 35T\, itwas concluded thatCo MOF adopts a different me chanism for facilitatingsaturation of magneticstates by involving formate stretching distortions unlikeother complexes inthe family that use the for mate bending mode. We performedRaman spectroscopyon [(CH3)2NH2]Co(HCOO)3at magneticfields up to 31T to probe the magneto-elastic coupling. It wasfou nd that aweak Raman active vibrational mode at about 798 cm-1corresponding to symmetric bending of the formate ion does exhibitmagnetic-field-induced frequency shifts [2]. Lastly\,we will present CoII(acac)2(H2O)2(acac = ace tylacetonate) which is a molecular magnet. We were able to show forthe fi rst timedistinct couplings of g phonons of and its deuterated analogswithz ero-field-split\, excited magnetic/spin levels (Kramersdoublet (KD)) of th e S= 3/2 electronic ground state. The couplings are observed asavoided cro ssingsin magnetic-field-dependent Raman spectra with couplingconstants of 1–2 cm−1[3]. The results of ourinvestigations and itsimplications will be discussed in detail in thispresentation.\n\n \n[1] K.Thirunavukkuarasu \,G. Radtke\, Z. Lu\, M. Lazzeri\, P. C. M. Christianen\, M. V.Ballottin\, H. A.Dabkowska\, B. D. Gaulin\, D. Smirnov\, M. Jaime\, and A. Saúl\,Phy sical Review B 107\,064410 (2023).\n\n[2] K.Thirunavukkuarasu\,R\, Richard son\, Z. Lu\, D. Smirnov\, N. Huang\, N. Combs\, G.Pokharel\, and D.Mandru s\, AIP Advances\, 2021\, 11\, 015040 (2021).\n\n[3] D.Moseley\, S.Stavret is\, K. Thirunavukkuarasu\, M. Ozerov\, Y. Cheng\,L. Daemen\, J.Ludwig\, Z . Lu\, D. Smirnov\, C. Brown\, A. Pandey\, A. J.Ramirez-Cuesta\, A. Lamb\, M. Atanasov\, E. Bill\, F. Neese\, Z. Xue\, Nature Communications\, 2018\, 9\,2572 (2018).\n\nContact Person: Markus Grüninger / Matteo LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Samir Lounis DTSTART;TZID=Europe/Berlin:20250225T143000 DTEND;TZID=Europe/Berlin:20250225T160000 DTSTAMP:20260527T090434Z UID:0000002118@events.thp.uni-koeln.de DESCRIPTION:Samir Lounis\, Martin-Luther-Universität Halle-Wittenberg\n\n Topological magnetism in 2D materials\n\nThe discovery of two-dimensional (2D) van der Waals (vdW) magnetic materials and their heterostructures has revolutionized the exploration of emergent phenomena at the intersection of magnetism\, spintronics\, and topology\, with significant implications for information technology. In this talk\, I will show how the stackabilit y of vdW layers enables the creation and electrical control of novel topol ogical magnetic states\, such as antiferromagnetic meronic textures and fe rromagnetic skyrmions in CrTe2-based heterostructures [1\,2\,3]. Additiona lly\, I introduce 'kagomerization\,' a transformative process that generat es 2D kagome lattices within transition metal monolayers via hexagonal bor on nitride (h-BN) overlayers [4]\, dramatically altering magnetic properti es and stabilizing topological solitons like skyrmions and bimerons. Lastl y\, I explore the interaction of 2D excitons—critical for photonic and o ptoelectronic applications—with topological magnetism\, unveiling new po ssibilities for excitons transport and stabilization.\n\nReferences\n\n[1] N. Abuawwad\, M. dos Santos Dias\, H. Abusara\, S. Lounis\, J. Phys.\; Co nd. Matt. 34\, 454001 (2022)\n[2] N. Abuawwad\, M. dos Santos Dias\, H. Ab usara\, S. Lounis\, Phys. Rev. B 108\, 094409 (2023)\n[3] N. Abuawwad\, M. dos Santos Dias\, H. Abusara\, S. Lounis\, npj Spintronics 2\, 10 (2024)\ n[4] H. Zhou\, M. dos Santos Dias\, Y. Zhang\, W. Zhao\, S. Lounis\, Natur e Communications 15\, 4854 (2024)\n\nContact Person: Jeison Fischer / Matt eo LOCATION:0.03 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Christian Schneider DTSTART;TZID=Europe/Berlin:20250226T143000 DTEND;TZID=Europe/Berlin:20250226T153000 DTSTAMP:20260527T090434Z UID:0000002089@events.thp.uni-koeln.de DESCRIPTION:Christian Schneider\, Uni Oldenburg\n\nMagneto-optical propert ies of Van-der-Waals Exciton-Polaritons\n\nTwo dimensional materials have emerged as a new and interesting platform for studies of tightly bound exc iton in ultimately thin materials. Meanwhile\, various types of 2D- or qua si 2D materials have become available that feature giant light-matter inte ractions\, charge tunability\, and intriguing magnetic and topological pro perties. These features can be exploited for implementing novel photonic d evices\, and for fundamental\, as well as quantum photonic investigations in the framework of cavity quantum electrodynamics [1]. I will discuss the implementation of our open optical cavity in a liquid-helium-free magneto -optical cryostat [2]. It is ideally suited for the study of exciton-polar itons based on van-der-Waals materials\, even in most complex geometries. I will address examples of such experiments\, with a focus on magnetic pro perties [3] of charge-correlated exciton-polaritons in the regime of stron g light-matter interaction in moiré lattices [4] as well as in the van-de r-Waals magnet CrSBr [5].\n\nReferences\n\n[1] C. Schneider et al. Nature Comm. 9\, 2695 (2018)\n[2] J. Drawer\, V. Mitryakhin et al. Nano letters\, 23 8683 (2023)\n[3] B. Han et al. (just accepted in) Physical Review Lett ers (2025)\n[4] J. Scherzer\, L. Lackner et al. Arxiv \, 2405.12698 (2024) \n[5] B. Han et al. Arxiv\, 2501.18233 (2025)\n\nContact Person: Erwann Bo cquillon / Matteo LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Eric Anderson DTSTART;TZID=Europe/Berlin:20250305T143000 DTEND;TZID=Europe/Berlin:20250305T153000 DTSTAMP:20260527T090434Z UID:0000002090@events.thp.uni-koeln.de DESCRIPTION:Eric Anderson\, Centre National de la Recherche Scientifique\n \nDiscovery of the Fractional Quantum Anomalous Hall Effect in Moiré MoTe 2\n\nThe recent discovery of fractional Chern insulators (FCIs) – lattic e analogs of the fractional quantum Hall states – has caused tremendous excitement in the condensed matter community. These topological phases of matter\, which have been found in both homobilayer MoTe2 and rhombohedral graphene-hBN moirés\, hold great promise for the study of robust anyonic excitations at zero magnetic field. Here\, I will discuss experimental wor k at the University of Washington which established\, leveraging both opti cal and transport probes\, the first realization of zero-field fractional Chern insulators in twisted bilayer MoTe2 (tMoTe2).1\,2\,3 More recent mea surements\, exploiting the strong coupling between photonic\, electronic\, and spin degrees of freedom in tMoTe2\, have provided additional evidence for the theoretically predicted zero-field composite Fermi liquid near ha lf filling of the flat Chern band.4 While not yet observed experimentally\ , theory work has predicted FCI phases hosting non-Abelian anyons – cruc ial building blocks for topologically protected quantum computation – to occur in the system. Given the array of optical and transport measurement tools available to probe tMoTe2\, it remains an extremely promising platf orm for exploring and manipulating these exotic phases of matter.\n\n1. An derson\, E. et al. Programming correlated magnetic states with gate-contro lled moiré geometry. Science 381\, 325–330 (2023).\n2. Cai\, J. et al. Signatures of fractional quantum anomalous Hall states in twisted MoTe2. N ature 622\, 63–68 (2023).\n3. Park\, H. et al. Observation of fractional ly quantized anomalous Hall effect. Nature 622\, 74–79 (2023).\n4. Ander son\, E. et. al. Trion sensing of a zero-field composite Fermi liquid. Nat ure 635\, 590-595 (2024).\n\nContact Person: Torsten Röper / Matteo LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Inti Sodemann DTSTART;TZID=Europe/Berlin:20250306T143000 DTEND;TZID=Europe/Berlin:20250306T160000 DTSTAMP:20260527T090434Z UID:0000002135@events.thp.uni-koeln.de DESCRIPTION:Inti Sodemann\, Uni Leipzig\n\nNon-equilibrium quantum liquids of periodically driven fermions\n\nNon-equilibrium quantum liquids of per iodically driven fermions\n \nWhen fermions are in equilibrium with a heat bath they occupy states according to the well-known Fermi-Dirac distribut ion. When the bath is very cold\, this distribution displays a discontinuo us jump defining the location of the Fermi surface. But how should fermion s occupy states when they are driven away from equilibrium by a time depen dent periodic force? would their occupation still have sharp jumps? or wou ld the periodic drive simply heat them up and smear their fermi surfaces?\ n \nRecent investigations have revealed that the non-equilibrium steady st ates of periodically driven fermions can retain sharp fermi surfaces and r emain much more quantum than previously anticipated. Interestingly\, the n on-equilibrium steady states of fermions can be very different in a grand- canonical setting where the system exchanges particles and energy with the bath (i.e. fermions coupled to a fermionic bath) and a canonical setting where the system only exchanges energy with the bath (i.e. fermions couple d to a bosonic bath). In the grand-canonical setting there is a non-equili brium fermi-liquid-like steady state with an occupation that displays mult iple jumps resembling a staircase shape\, and therefore features multiple non-equilibrium fermi surfaces. In contrast\, in the canonical setting the re is a non-equilibrium non-fermi-liquid steady state where the occupation does not have jumps but rather multiple sharp kinks\, which\, remarkably\ , remain sharp even when the bath is at finite temperature. Some of the pl atforms and regimes to realize experimentally these states are readily acc essible\, and include ultra-clean and cold two-dimensional metallic system s such as Gallium arsenide hetero-structures or graphene irradiated with m icrowaves.\n\nContact Person: Urban Seifert / Matteo LOCATION:0.02 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Herbert Spohn DTSTART;TZID=Europe/Berlin:20250312T143000 DTEND;TZID=Europe/Berlin:20250312T160000 DTSTAMP:20260527T090434Z UID:0000002113@events.thp.uni-koeln.de DESCRIPTION:Herbert Spohn\, TUM\n\nUniversality classes for coupled Kardar -Parisi-Zhang equations\n\nConsidered is the KPZ equation with two compone nts\, constrained to be symmetric\nunder exchange. This model is of intere st in the context of exciton-polaritons condensates.\nA central issue is t o understand the phase diagram. We argue that\, as for the single componen t\,\nthe static scaling exponent is 1/2 and the dynamic scaling exponent i s 3/2. More surprisingly\,\nthe three-dimensional parameter space is folia ted into two-dimensional universality classes.\nThis is joint work with A. Dhar\, M. Kulkarni\, Dipankar Roy\, ICTS Bangalore.\n\nContact Person: Se bastian Diehl / Matteo LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Johanna Eichhorn DTSTART;TZID=Europe/Berlin:20250407T100000 DTEND;TZID=Europe/Berlin:20250407T113000 DTSTAMP:20260527T090434Z UID:0000002162@events.thp.uni-koeln.de DESCRIPTION:Johanna Eichhorn\, TUM\n\nDefects and Nanoscale Interfaces in Solar Energy Materials\n\nTransition metal oxide and nitride semiconductor s show promise for application in solar energy conversion. However\, their efficiencies and stabilities are often controlled by the presence of defe cts as well as nanoscale chemical and physical heterogeneities. These mate rial properties must be understood and controlled to enable their implemen tation in functional systems. Here\, recent experimental advances in under standing nanoscale defect and interface properties will be presented\, whi ch are key to optimizing material properties\, corrosion protection\, and catalyst layers for solar energy conversion.\n\nEconomically viable and sc alable photosystems often use polycrystalline thin films with complex stru ctures\, where grain boundaries\, facet orientations\, and composition var iations affect their photoelectrochemical properties. For copper vanadate thin films\, we correlated local structural\, chemical\, and optoelectroni c properties with their photoelectrochemical performance to link nanoscale and macroscale characteristics for designing durable and efficient photos ystems. [1] \n\nFurthermore\, we synthesized Ta3N5 thin film photoelectrod es with differing degrees of structural disorder and defect concentrations to reveal the relationship between atomic-scale point defects and macrosc ale photoelectrochemical stability. Specifically\, we showed that shallow oxygen donors can kinetically stabilize the interface\, whereas deep-level defects facilitate rapid photocarrier trapping and surface oxidation. [2] \n\nTo stabilize and activate their surface\, we interfaced photoelectrod es with conformal and nanoscale layers that maintain chemical stability\, enable efficient interfacial charge transport\, and minimize parasitic lig ht absorption. In particular\, we leveraged unsaturated oxidation conditio ns in plasma-enhanced atomic layer deposition to synthesize cobalt oxide l ayers with tailored catalytic activities and chemical stabilities. [3] \n\ nReferences\n[1] J. Eichhorn et al.\, ACS Appl. Mater. Interfaces 13\, 235 75 (2021).\n[2] L. M. Wolz et al.\, Adv. Funct. Mater. 34\, 2405532 (2024) .\n[3] M. Kuhl et al.\, Adv. Mater. Inter. 9\, 2200582 (2022).\n\nContact Person: Erwann Bocquillon / Matteo Cacco LOCATION:OTHER END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Aifei Zhang DTSTART;TZID=Europe/Berlin:20250409T143000 DTEND;TZID=Europe/Berlin:20250409T153000 DTSTAMP:20260527T090434Z UID:0000002164@events.thp.uni-koeln.de DESCRIPTION:Aifei Zhang\, University of Paris\n\nDisorder and quantization in symmetry broken quantum Hall states in graphene\n\nThe Quantum Hall ef fect (QHE) is known to be stabilized by disorder\, thanks to the emergence of bulk localized states that do not participate in quantized transport a long the edge. However\, disorder also induces the loss of QHE quantizatio n: if a large enough energy (called hopping energy) is provided\, charge c arriers trapped in a localized state will have the possibility to tunnel f rom state to state\, making the bulk conducting. This phenomenon occurs ev en at temperatures much lower than the Landau level (LL) spacing\, is usua lly described in the framework of variable range hopping (VRH) [1]\, and c orresponds to a rich physics governed by universal scaling [2\, 3]. The co mpetition between these two antagonistic effects of disorder is set by the interplay between two energy scales (LL spacing and disorder broadening) [4]. To explore this interplay\, we have probed the temperature dependence of QH states in graphene in a Corbino geometry [5]\, over three orders of magnitude. The fourfold spin and valley symmetry of graphene is lifted at high magnetic field\, providing two well-separated scales for the energy level spacing [6]: cyclotron gap for a fully filled LL\, and spin/valley g aps for the symmetry broken states. Tuning the carrier density with an ele ctrostatic gate allows us to probe states with vastly different gaps at ot herwise fixed magnetic length and disorder.\nWe observe a difference of tw o orders of magnitude in the extracted hopping energies and thus in the lo calization lengths between cyclotron gap states and symmetry broken states . To understand such discrepancy\, we propose a scenario of localization l ength saturation based on the overlap of localized states from different L L\, preserving the plateau quantization at T=0 but rendering it more fragi le at finite temperature. Finally\, our measurements also suggest that the re may be a contribution to disorder that is spin- and valley-polarization dependent.\n\nReferences\n\n[1] B.I. Shklovskii\, A.L. Efros\, Springer S eries in Solid-State Sciences\, vol 45. Springer\, Berlin\, Heidelberg\n(1 984).\n[2] K. Bennaceur et al.\, Phys. Rev. B 86\, 085433 (2012).\n[3] S. Kaur et al.\, Nat. Commun. 15\, 1 (2024).\n[4] Robert Joynt and R. E. Pran ge\, Phys. Rev. B 29\, 3303 (1984).\n[5] Y. Zeng et al.\, Phys. Rev. Lett. 122\, 137701 (2019).\n[6] A. Young\, et al.\, Nature Phys 8\, 550–556 ( 2012).\n\nContact Person: Erwann Bocquillon / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Zhe Wang DTSTART;TZID=Europe/Berlin:20250423T143000 DTEND;TZID=Europe/Berlin:20250423T153000 DTSTAMP:20260527T090434Z UID:0000002138@events.thp.uni-koeln.de DESCRIPTION:Zhe Wang\, TU Dortmund\n\nCharacteristic dynamics for emergent symmetry in transverse field Ising chains\n\nOne-dimensional quantum spin models are textbook examples for illustrating basic physical concepts. In this talk\, I will present our recent studies of Ising-like quantum spin- chain ferromagnetic and antiferromagnetic materials by using high-resoluti on terahertz spectroscopy and other complementary techniques in high magne tic fields [1-4]. In particular\, I will focus on the observation of chara cteristic low-energy quantum spin excitations\, possibly evidencing for em ergent E8 symmetry [5]. \n \n\n[1] K. Amelin et al\, J. Phys. A: Math. The or. 55 484005 (2022).\n[2] Z. Zhang\, K. Amelin\, et al\, Phys. Rev. B 101 \, 220411(R) (2020).\n[3] K. Amelin et al\, Phys. Rev. B 102\, 104431 (202 0).\n[4] ZW et al. Phys. Rev. Lett. 120\, 207205 (2018).\n[5] A. B. Zamolo dchikov\, Int. J. Mod. Phys. A 4\, 4235 (1989).\n\nContact Person: Paul Va n Loosdrecht / Matteo LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Karin Everschor-Sitte DTSTART;TZID=Europe/Berlin:20250430T143000 DTEND;TZID=Europe/Berlin:20250430T153000 DTSTAMP:20260527T090434Z UID:0000002121@events.thp.uni-koeln.de DESCRIPTION:Karin Everschor-Sitte\, Universität Duisburg Essen\n\nRethink ing Linking: The Power of Connections\n\ntba\n\nContact Person: Achim Rosc h / Matteo LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Gerbold Ménard DTSTART;TZID=Europe/Berlin:20250505T160000 DTEND;TZID=Europe/Berlin:20250505T180000 DTSTAMP:20260527T090434Z UID:0000002156@events.thp.uni-koeln.de DESCRIPTION:Gerbold Ménard\, CNRS - ENS Paris\n\nTime-resolved sensing of electromagnetic fields with single-electron interferometry\n\nThe quantum coherence of single electronic states in mesoscopic system is usually ver y fragile. This fragility can be harnessed to create hyper-sensitive quant um sensors such that a single electron can detect a few photons excitation . In this work\, we demonstrate a quantum sensor that exploits the variati ons of the phase of a single electron wavefunction upon its interaction wi th a classical time-dependent electric field. We use a Fabry-Perot interf erometer in the quantum Hall regime to extract the phase of the wave packe t as a function of time. Such device ensures a fast and sensitive detectio n as are able to detect a signal equivalent to a few microwave photons wit h a resolution of 50 ps. Moreover\, by measuring both the phase and contra st of the interferometer\, it is possible to obtain information on both th e amplitude of the electromagnetic field and its fluctuations. This opens possibilities for on-chip detection of non-classical radiation such as squ eezed states or Fock states.\n\n[1] Bartolomei\, Hugo\, et al. "Time-resol ved sensing of electromagnetic fields with single-electron interferometry. " Nature Nanotechnologies https://rdcu.be/edRw8 (2025)\n\nContact Person: Erwann Bocquillon / Matteo Cacco LOCATION:OTHER END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Mahdi Ghorbani-Asl DTSTART;TZID=Europe/Berlin:20250516T160000 DTEND;TZID=Europe/Berlin:20250516T173000 DTSTAMP:20260527T090434Z UID:0000002161@events.thp.uni-koeln.de DESCRIPTION:Mahdi Ghorbani-Asl\, Helmholtz-Zentrum Dresden-Rossendorf\n\nE xploring Novel 2D Materials: New Frontiers in Energy Storage and Magnetism \n\nThe intercalation of atomic and molecular species into layered materia ls has emerged as an effective strategy for producing novel two-dimensiona l (2D) materials with tunable electronic\, magnetic\, and energy storage p roperties. Recent in situ transmission electron microscopy (TEM) studies h ave revealed the presence of dense lithium multilayers confined between gr aphene sheets\, as well as the intercalation of double layers of alkali me tals (K\, Rb\, Cs) within bilayer graphene. [1\,2] Here\, we investigated the intercalation of alkali metals (AMs) into bilayer graphene using first -principles calculations. We explored the energetics and structural config urations of single-layer and multilayer AM intercalates\, as well as the i nfluence of atomic-scale defects and impurities on their stability and for mation. [3\,4] \nBeyond AM intercalation\, we demonstrated the formation o f confined 2D metal chloride phases with distinct stoichiometries inside b ilayer graphene. [5-7] High-resolution TEM imaging revealed the emergence of various polymorphs of aluminum chloride\, iron chloride\, and molybdenu m chloride\, along with phase transformations induced by electron beam exp osure. Our electronic structure calculations indicate that these 2D interc alated systems exhibit diverse electronic properties\, ranging from insula ting to semimetallic behavior depending on their composition and phase\, w hile also serving as a promising platform for studying magnetism in reduce d-dimensional environments. [8] \nWe also studied the structural and elect ronic properties of high-quality Janus monolayers synthesized on a gold su bstrate. In particular\, we characterized the effect of the substrate on t he electronic structure of Janus monolayer and spin-orbit splitting of the valence band by using our experimental and theoretical techniques. [9]\n\ n\nReferences\n[1] M. Kühne\, F. Börrnert\, S. Fecher\, M. Ghorbani-Asl\ , et al.\, Nature 564\, 234 (2018).\n[2] YC. Lin\, R. Matsumoto\, Q. Liu\, et al.\, Nat. Commun. 15\, 6445 (2024).\n[3] I. Chepkasov\, M. Ghorbani-A sl\, et al.\, Nano Energy 75\, 104927 (2020).\n[4] X. Zhang\, M. Ghorbani- Asl\, et al.\, Mater. Today Energy 34\, 101293 (2023). \n[5] Y. Li\, F. B örrnert\, M. Ghorbani-Asl\, et al.\, Adv. Funct. Mater. 30\, 2406034 (202 4).\n[6] YC. Lin\, S. Kretschmer\, M. Ghorbani-Asl\, et al.\, Adv. Mater. 33\, 2170415 (2021).\n[7] Q. Liu\, S. Kretschmer\, M. Ghorbani-Asl\, et al .\, ACS Nano 17\, 23659 (2023).\n[8] Q. Liu\, H. Sun\, Y. Lin\, M. Ghorban i-Asl\, et al.\, ACS Nano 19\, 4845 (2025).\n[9] J. Picker\, M. Ghorbani-A sl\, et al.\, Nano Lett. 25\, 3330 (2025).\n\nContact Person: Thomas Miche ly / Matteo Cacco LOCATION:215 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Kai Phillip Schmidt DTSTART;TZID=Europe/Berlin:20250521T143000 DTEND;TZID=Europe/Berlin:20250521T153000 DTSTAMP:20260527T090434Z UID:0000002125@events.thp.uni-koeln.de DESCRIPTION:Kai Phillip Schmidt\, FAU Erlangen\n\nQuantum Science of Corre lated Light and Matter\n\nThe collective behaviour of matter as well as it s interplay with light is one of the most important topics of modern scien ce. Understanding it is crucial in basic research\, as it holds the key to a variety of correlated quantum many-body phenomena like spin liquids or superconductvity. At the same time\, this understanding forms the basis fo r many (quantum) technological applications which define the modern era. I n this talk I mainly concentrate on light-matter systems\, where strong ma tter-matter and strong light-matter interactions are present simultaneousl y. From a condensed matter perspective\, one might expect to tune the prop erties of quantum materials by quantum light and from a quantum optics per spective one might engineer interesting novel facets of quantum light orig inating from such entangled light-matter systems. The main focus is to in vestigate the quantum properties of such correlated light-matter systems a nd the physical consequences of the induced long-range interactions. Speci fically\, I will discuss the pysical properties of the paradigmatic Dicke- Ising model being the sum of a matter-matter Ising interaction and a quant um Rabi (Dicke) Hamiltonian on unfrustrated and frustrated geometries as w ell as extensions to algebraically decaying long-range interactions. Final ly\, I will further shortly discuss topologically ordered systems by intro ducing a novel XY toric code which is exactly solvable and displays topolo gical and fractonic phases in two dimensions simultaneously.\n\nContact Pe rson: Simon Trebst LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Namrata Bansal DTSTART;TZID=Europe/Berlin:20250521T130000 DTEND;TZID=Europe/Berlin:20250521T140000 DTSTAMP:20260527T090434Z UID:0000002203@events.thp.uni-koeln.de DESCRIPTION:Namrata Bansal\, University of Karlsruhe\n\nMagnetic 2d materi als\n\ntba\n\nContact Person: Wouter Jolie / Matteo LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Gustav Bihlmayer DTSTART;TZID=Europe/Berlin:20250528T143000 DTEND;TZID=Europe/Berlin:20250528T153000 DTSTAMP:20260527T090434Z UID:0000002189@events.thp.uni-koeln.de DESCRIPTION:Gustav Bihlmayer\, Forschungszentrum Jülich\n\n2D magnetism o f lanthanide layers on surfaces: some insights from DFT calculations\n\nAt omically thin layers of lanthanide materials are ideal systems to study th e stability\nof magnetic order in two dimensions. Open shell atoms like Dy or Ho show large magnetic\nanisotropies even in small crystal fields. Spe cies with quenched orbital moments (Gd or\nEu) can be adsorbed on or inter calated between graphene and magnetic substrates like\nCo or Ni\, also sta cking several layers is possible. We analysed the exchange interaction\nbe tween the 4f layers and the 3d substrates and their modification by the gr aphene layers\nusing density functional theory (DFT). Comparison with expe rimental data shows good\nagreement where available. Remarkably\, the inte raction between the 4f states and graphene\nleads to spin-selective modifi cations of the π band that is observed both in angle resolved\nphotoemiss ion spectroscopy experiments and the calculated band structures. We invest igate\nthe origin of this effect and consequences for graphene's transport properties.\n\nContact Person: Jeison Fischer / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Christian Hess DTSTART;TZID=Europe/Berlin:20250604T143000 DTEND;TZID=Europe/Berlin:20250604T153000 DTSTAMP:20260527T090434Z UID:0000002194@events.thp.uni-koeln.de DESCRIPTION:Christian Hess\, University of Wuppertal\n\nSuperconductivity and topology in the Weyl semimetal PtBi2\n\nMaterials combining topologica lly non-trivial behavior and superconductivity offer a potential route for quantum computation. However\, the set of available materials intrinsical ly realizing these properties are scarce. Recently\, superconductivity has been reported in PtBi2 in its trigonal phase and an inherent Weyl semimet al phase has been predicted. We present scanning tunneling microscopy and spectroscopy data of trigonal PtBi2 which reveal surface superconductivity at elevated temperatures (5 K). The gap magnitude is elusive: it is spati ally inhomogeneous and spans from 0 to 20 meV. In particular\, the large g ap value and the shape of the quasiparticle excitation spectrum resemble t he phenomenology of high-Tc superconductors. Furthermore\, we reveal the s ignatures of topological Fermiarcs in the normal state patterns of the qua siparticle interference. We show that the scattering between Fermi arcs do minates the interference spectra\, providing conclusive evidence for the r elevance of Weyl fermiology for the surface electronic properties of trigo nal PtBi2.\n\nContact Person: Markus Grüninger / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Antonio Manesco DTSTART;TZID=Europe/Berlin:20250618T153000 DTEND;TZID=Europe/Berlin:20250618T163000 DTSTAMP:20260527T090434Z UID:0000002210@events.thp.uni-koeln.de DESCRIPTION:Antonio Manesco\, Delft University of Technology\n\nValley phe nomena in gate-defined devices\n\nCoherent control and detection of the va lley degree of freedom is a cornerstone for valleytronics and valley-based quantum computation. However\, access to valley-coherent phenomena requir es samples without short-range scattering. Recent fabrication advances on gate-defined bilayer graphene devices minimize the effects of intervalley scattering. Consequently\, a series of recent experiments reported control over valley-polarized states. As an example of this progress\, I will int roduce our recent efforts to generate valley-polarized electric currents i n bilayer graphene. I will first show the suppression of intervalley scatt ering in gate-defined quantum point contacts via electron focusing experim ents. Then\, I will present a magnetically controlled source of valley-pol arized currents using Fermi surface warping. In the second part of my talk \, I focus on theoretical proposals for the next generation of experiments . Namely\, I will present how to use gate-defined valley-helical states to probe and engineer correlated phases of matter.\n\nContact Person: Achim Rosch / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Ciarán Hickey DTSTART;TZID=Europe/Berlin:20250625T143000 DTEND;TZID=Europe/Berlin:20250625T153000 DTSTAMP:20260527T090434Z UID:0000002220@events.thp.uni-koeln.de DESCRIPTION:Ciarán Hickey\, University College Dublin.\n\nProbing Quantum Magnets via Non-Linear Spectroscopy\n\ntba\n\nContact Person: Achim Rosch / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Maurits Haverkort DTSTART;TZID=Europe/Berlin:20250729T100000 DTEND;TZID=Europe/Berlin:20250729T113000 DTSTAMP:20260527T090434Z UID:0000002226@events.thp.uni-koeln.de DESCRIPTION:Maurits Haverkort\, Heidelberg University\n\nNonperturbative S eries Expansion of n-point Green’s Functions: Effective operators in Res onant Inelastic X-ray Scattering\n\n\n\nContact Person: Markus Grüninger / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Senthil Todadri DTSTART;TZID=Europe/Berlin:20250829T140000 DTEND;TZID=Europe/Berlin:20250829T153000 DTSTAMP:20260527T090434Z UID:0000002232@events.thp.uni-koeln.de DESCRIPTION:Senthil Todadri\, MIT\n\nSuperconductivity and the Fractional Quantum Anomalous Hall Effect\n\n\n\nContact Person: Achim Rosch LOCATION:0.03 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Femke Witmans DTSTART;TZID=Europe/Berlin:20251001T143000 DTEND;TZID=Europe/Berlin:20251001T153000 DTSTAMP:20260527T090434Z UID:0000002254@events.thp.uni-koeln.de DESCRIPTION:Femke Witmans\, University of Twente\n\nElectronic transport m easurements in SnTe nanowires\n\ntba\n\nContact Person: Erwann Bocquillon / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Thomas Schäpers DTSTART;TZID=Europe/Berlin:20251008T143000 DTEND;TZID=Europe/Berlin:20251008T153000 DTSTAMP:20260527T090434Z UID:0000002251@events.thp.uni-koeln.de DESCRIPTION:Thomas Schäpers\, FZJ\n\nQuantum Transport in GaAs/InAs Core/ Shell Nanowire Based Structures\n\nQubit systems based on high quality hyb rid superconducting quantum devices are often\nrealized by using semicondu ctor nanowires. In core/shell GaAs/InAs nanowires\,\ncontaining a tubular conductor in the InAs shell wrapped around an insulating GaAs core\,\nthe strong confinement near the surface ensures good coupling to a superconduc tor. We\npresent magnetotransport measurements on zincblende phase-pure Ga As/InAs\ncore/shell nanowires\, where the reduced disorder compared to pol ycrystalline nanowires\nis expected to result in improved transport proper ties. When an axial magnetic field is\npenetrating the tubular conductor\, h/e-periodic Aharonov-Bohm type oscillations are\nobserved in the magneto conductance [1]. In addition\, phase-rigid h/2e-periodic\noscillations cor responding to Altshuler-Aronov-Spivak oscillations are observed. By\ntempe rature-dependent measurements\, we identify a quasi-ballistic transport re gime\nwith few scattering centers in the conducting shell\, which neverthe less leads to an\nAltshuler-Aronov-Spivak correction. In addition\, Joseph son junctions comprising a\ncore/shell GaAs/InAs nanowire as a weak link w ere investigated [2\,3]. Here\, we observed\nan oscillation of the switchi ng current in an axial magnetic field with a period of h/2e.\nWork done in collaboration with: F. Basaric\, A. Pawlis\, V. Brajovic\, K. Moors\, W. Schaarman\, A.\nManolescu\, R. Deacon\, K. Ishibashi\, P. Zellekens\, R. J uluri\, A. M. Sanchez\, J. H. Bae\, H. Lüth\, D.\nGrützmacher\, M. D. Ra ndle\, P. Perla\, M. I. Lepsa\, G. Behner\, E. Zimmermann\, B. Bennemann\, C.\nKrause\n[1] Farah Basaric\, Vladan Brajovic\, Gerrit Behner\, Kristof Moors\, William Schaarman\, Andrei\nManolescu\, Raghavendra Juluri\, Ana M. Sanchez\, Jin Hee Bae\, Hans Lüth\, Detlev\nGrützmacher\, Alexander P awlis\, Thomas Schäpers\, Aharonov-Bohm and Altshuler-Aronov-\nSpivak osc illations in the quasiballistic regime in phase-pure GaAs/InAs core/shell\ nnanowires\, Phys. Rev. B. 112\, 075301 (2025).\n[2] Patrick Zellekens\, R ussell Deacon\, Pujitha Perla\, Detlev Grützmacher\, Mihail Ion Lepsa\,\n Thomas Schäpers\, Koji Ishibashi\, Microwave spectroscopy of Andreev stat es in InAs\nnanowire-based hybrid junctions using a flip-chip layout\, Com munications Physics 5\, 267\n(2022).\n[3] Patrick Zellekens\, Russell S. D eacon\, Farah Basaric\, Raghavendra Juluri\, Michael D Randle\,\nBenjamin Bennemann\, Christoph Krause\, Erik Zimmermann\, Ana M. Sanchez\, Detlev\n Grützmacher\, Alexander Pawlis\, Koji Ishibashi\, Thomas Schäpers\, Flux -periodic supercurrent\noscillations in an Aharonov-Bohm-type nanowire Jos ephson junction\, Communications\nPhysics 8\, 363 (2025).\n\nContact Perso n: Alina Rupp / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Joachim Deisenhofer DTSTART;TZID=Europe/Berlin:20251015T143000 DTEND;TZID=Europe/Berlin:20251015T153000 DTSTAMP:20260527T090434Z UID:0000002213@events.thp.uni-koeln.de DESCRIPTION:Joachim Deisenhofer\, Uni Augsburg\n\nAll you can eat in a ma gnetic insulator: feasting on the delicacies of the polar antiferromagnet s Fe2Mo3O8and Co2Mo3O8\n\nTheir crystallographic parity-broken symmetry an d collinear antiferromagnetic order makes Fe2Mo3O8 and Co2Mo3O8 the perfec t basis for a rich buffet for hungry condensed-matter scientists: The low- energy THz excitations of Fe2Mo3O8 reportedly exhibit a hybrid taste of ch iral phonon [1] and magnon-polaron [2] states when probed via magneto-Rama n measurements or neutron scattering\, respectively. However\, we could de scribe their THz absorption spectra and optical magnetoelectric effects su ch as the non-reciprocal directional dichroism [3] by using an advanced si ngle-ion approach to the Fe2+ multiplet states [4]. The rich physics of Fe 2Mo3O8 can be additionally spiced up by a combination of slight Zn doping and magnetic field [5\, 6]\, where a metastable ferrimagnetic phase can be created with a peculiar magnetization reversal [7]. In addition to warmin g up some of these tasty but already published snacks\, I will serve our r ecent findings on\nstrong Faraday effects of the THz-excitations in Fe2Mo3 O8 and on our comparison of spin-group and magnetic-point group descriptio n of phonon selection rules in Co2Mo3O8 as an altermagnetic candidate [8]. Enjoy the meal\n\n1] F. Wu\, S. Bao\, J. Zhou\, Y. Wang\, J. Sun\, J. Wen \, Y. Wan\, and Q. Zhang\, Fluctuation-enhanced phonon magnetic moments in a polar antiferromagnet\, Nature Physics https://doi.org/10.1038/s41567-0 23-02210-4 (2023).\n[2] S. Bao\, Z.-L. Gu\, Y. Shangguan\, Z. Huang\, J. L iao\, X. Zhao\, B. Zhang\, Z.-Y. Dong\, W. Wang\, R. Kajimoto\, M. Nakamur a\, T. Fennell\, S.-L. Yu\, J.-X. Li\, and J. Wen\, Direct observation of topological magnon polarons in a multiferroic material\, Nature Communicat ions 14\, https://doi.org/10.1038/s41467-023-41791-9 (2023).\n[3] S. Resch ke\, D. G. Farkas\, A. Strini´c\, S. Ghara\, K. Guratinder\, O. Zaharko\, L. Prodan\, V. Tsurkan\, D. Szaller\, S. Bord´acs\, J. Deisenhofer\, and I. K´ezsm´arki\, Confirming the trilinear form of the optical magnetoel ectric effect in the polar honeycomb antiferromagnet Co2Mo3O8\,\nnpj Quant um Mater. 7\, 1 (2022).\n[4] K. V. Vasin\, A. Strini´c\, F. Schilberth\, S. Reschke\, L. Prodan\, V. Tsurkan\, A. R. Nurmukhametov\, M. V. Eremin\, I. K´ezsm´arki\, and J. Deisenhofer\, Optical magnetoelectric effect in the polar honeycomb antiferromagnet Fe2Mo3O8\, Physical Review B.\n[5] T. Kurumaji\, S. Ishiwata\, and Y. Tokura\, Doping-tunable ferrimagnetic pha se with large linear magnetoelectric effect in a polar magnet Fe2Mo3O8\, P hys. Rev. X 5\, 031034 (2015).\n[6] Q. Chen\, A. Miyake\, T. Kurumaji\, K. Matsuura\, F. Kagawa\, S. Miyahara\, Y. Tokura\, and M. Tokunaga\, Multip le Magnetoelectric Plateaux in Polar Magnet Fe2Mo3O8 https://doi.org/10.48 550/arXiv.2309.10583 (2023).\n[7] S. Ghara\, E. Barts\, K. Vasin\, D. Kame nskyi\, L. Prodan\, V. Tsurkan\, I. K´ezsm´arki\, M. Mostovoy\, and J. D eisenhofer\, Magnetization reversal through an antiferromagnetic state\, N ature Communications 14\, 10.1038/s41467-023-40722-y (2023).\n[8] F. Schil berth\, M. Kond´akor\, D. Ukolov\, A. Pawbake\, K. Vasin\, O. Ercem\, L. Prodan\, V. Tsurkan\, A. A. Tsirlin\, C. Faugeras\, P. Lemmens\, K. Penc\, I. K´ezsm´arki\, S. Bord´acs\, and J. Deisenhofer\, Optical phonons as a testing ground for spin group symmetries https://doi.org/10.48550/arXiv .2508.09793 (2025).\n\nContact Person: Markus Grüninger / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Maxim Mostovoy DTSTART;TZID=Europe/Berlin:20251022T143000 DTEND;TZID=Europe/Berlin:20251022T153000 DTSTAMP:20260527T090434Z UID:0000002209@events.thp.uni-koeln.de DESCRIPTION:Maxim Mostovoy\, University of Groningen\n\nPedestrian approac h to altermagnets\n\nAltermagnets have recently emerged as a new class of magnetic materials sharing properties of both antiferromagnets and ferroma gnets. Despite very small net magnetization\, they exhibit phenomena typic ally associated with ferromagnetism\, such as the Faraday\, Kerr and Anoma lous Hall effects\, resulting from the relativistic spin-orbit coupling\, as well as the spin splitting of electron bands and Spin Hall Effect of no n-relativistic origin. Spin space groups and magnetic multipoles are used to explain the properties of altermagnets. I will provide a pedagogical i ntroduction to a symmetry analysis of altermagnets using an antiferromagne tic order parameter [1]. This time-honored approach applies to all physica l phenomena observed in these materials. In particular\, I will discuss sy mmetry of altermagnets with non-collinear and non-coplanar spin orders.\n\ nContact Person: Paul van Loosdrecht LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Anna S. Semisalova DTSTART;TZID=Europe/Berlin:20251029T143000 DTEND;TZID=Europe/Berlin:20251029T153000 DTSTAMP:20260527T090434Z UID:0000002260@events.thp.uni-koeln.de DESCRIPTION:Anna S. Semisalova\, University of Duisburg-Essen\n\nInertial effects in spin dynamics in ferromagnets: a quantitative framework for sub -THz magnonics\n\nThe phenomenon of additional high-frequency (sub-THz/THz ) spin dynamics resonance in ferromagnets arising due to the inertia of ma gnetization has been recently demonstrated both theoretically [1] and subs equently by experimental observation in permalloy\, CoFeB and cobalt [2\,3 ]. Non-resonant excitation of magnetic nutation using ultrashort laser pul ses was also shown [4]. The existence of a sub-THz resonant frequency in f erromagnetic materials\, together with the capability to excite magnetizat ion nutation coupled to its precession\, broadens the potential range of a pplications of ferromagnets in ultrafast magnetism and sub-THz magnonics.\ nIn this talk I will show that in addition to a direct detection of nutati on resonance\, the inertial effects can be resolved using ferromagnetic re sonance [5\,6]. We derive the full tensor of magnetic susceptibility analy tically and address the impact of material parameters\, including the Gilb ert damping constant\, the inertial relaxation time\, and the applied magn etic field\, on the intensity and linewidth of coupled precession and nuta tion resonances. The present work provides a guideline for identifying mat erials that allow the detection of inertial effects in ferromagnets using time or frequency domain experiments. \n\n[1] M.-C. Ciornei\, J. Rubi\, J. -E. Wegrowe\, Physical Review B\, Vol. 83\, 020410 (2011)\n[2] K. Neeraj\, N. Awari\, S. Kovalev et al.\, Nature Physics\, Vol. 17\, 245 (2021) \n[3 ] V. Unikandanunni\, R. Medapalli\, M. Asa et al.\, Physical Review Letter s\, Vol. 129\, 237201 (2022)\n[4] A. De\, J. Schlegel\, A. Lentfert et al. \, Physical Review B\, Vol. 111\, 014432 (2025)\n[5] M. Cherkasskii\, I. B arsukov\, R. Mondal et al.\, Physical Review B\, Vol. 106\, 054428 (2022)\ n[6] J. Wiemeler\, M. Farle\, A. Semisalova\, Physical Review B\, Vol. 112 \, 094433 (2025)\n\nContact Person: Evgeny Mashkovich LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Sandheep Ravishankar DTSTART;TZID=Europe/Berlin:20251105T143000 DTEND;TZID=Europe/Berlin:20251105T153000 DTSTAMP:20260527T090434Z UID:0000002241@events.thp.uni-koeln.de DESCRIPTION:Sandheep Ravishankar\, FZJ\n\nInsights into the Physics of Ope ration of Perovskite Solar Cells\n\nHalide perovskite solar cells have eme rged as a significant photovoltaic technology for commercial applications\ , due to their high power conversion efficiencies and relative ease of fab rication. However\, further improvement in their performance is limited ma inly by non-radiative recombination losses due to defect densities in the bulk and interfaces of the solar cell\, in addition to charge extraction l osses related to transporting electrons and holes from the perovskite laye r to the electrodes\, via the low-mobility transport layers.\nIn this talk \, I will explain how to characterize these loss mechanisms using a combin ation of luminescence and optoelectronic measurements\, both in the time a nd frequency domain. Through the development of analytical/semi-analytical models in combination with drift-diffusion simulations\, we develop data analysis methods that allow determination of characteristic parameters rel ated to these loss mechanisms. Such a strategy allows for a physics-inform ed optimisation of these solar cells for improved performance\, while prov iding a deeper understanding of their physics of operation\n\nContact Pers on: Markus Grüninger / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Tomasz Smolenski DTSTART;TZID=Europe/Berlin:20251119T143000 DTEND;TZID=Europe/Berlin:20251119T153000 DTSTAMP:20260527T090434Z UID:0000002257@events.thp.uni-koeln.de DESCRIPTION:Tomasz Smolenski\, Uni Basel\n\nOptical control over many-body electronic phases in layered materials\n\nUnderstanding and controlling s trongly correlated many-body systems is one of the central challenges in m odern condensed matter physics. Among the most promising experimental plat forms for exploring this frontier are van der Waals (vdW) heterostructures based on transition metal dichalcogenide (TMD) monolayers. Owing to their gate tunability\, weak dielectric screening\, and low carrier effective m asses\, TMD monolayers readily host correlated electronic phases such as W igner crystals. The stability of such phases can be further enhanced when two TMD monolayers are combined into a twisted bilayer\, where the electro nic kinetic energy is further suppressed due to band folding in the period ic moire potential. In all of these structures\, the excellent optical pro perties of TMD monolayers\, along with their spin-valley-selective selecti on rules\, provide a unique optical interface to both the charge and spin state of the many-body electron system.\n \nIn this talk\, I will review o ur recent ultra-low-temperature magneto-optical investigations of collecti ve electronic phases in TMD-based vdW structures. In the first part\, I wi ll demonstrate the formation of novel optical excitations in a TMD monolay er hosting a robust Wigner crystal. These excitations coexist with previou sly explored exciton-umklapp resonances [1] but\, unlike them\, arise due to strong attractive exciton-electron interactions that dress the excitons with particle-hole excitations across the many-body Wigner crystal gap. I n the second part\, I will focus on AA-stacked MoTe2 homobilayers\, where strong interlayer hybridization of hole orbitals gives rise to flat topolo gical valence bands supporting robust ferromagnetic metals as well as frac tional and integer Chern insulators at various moire filling factors. I wi ll show that the spin state of all these topological ferromagnets can be d ynamically reversed by resonantly driving exciton-polaron resonances in th e optical absorption spectrum with circularly polarized light [2]. This in cludes both integer and fractional Chern insulating phases\, for which the spin orientation is equivalent to all-optical switching of their many-bod y Chern number. I will demonstrate that by illuminating the sample with di ffraction-limited spot\, it is possible to induce such a Chern number flip in selected sample area\, which paves the way for optical generation of p rogrammable topological circuits.\n \n[1] T. Smoleński et al.\, Nature 59 5\, 53–57 (2021)\n[2] O. Huber\, (…)\, T. Smoleński\, arXiv:2508.1906 3 (2025)\n\nContact Person: Urban Seifert LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Berit Goodge DTSTART;TZID=Europe/Berlin:20251126T143000 DTEND;TZID=Europe/Berlin:20251126T153000 DTSTAMP:20260527T090434Z UID:0000002249@events.thp.uni-koeln.de DESCRIPTION:Berit Goodge\, Max Planck Institue for Chemical Physics of Sol ids\, Dresden\n\nDirect visualization of lattice and electronic structures in quantum materials by advanced microscopy and spectroscopy\n\nThe rich properties of strongly correlated and quantum materials derive from comple x interplay between atomic lattice\, charge\, spin\, and orbital interacti ons. Direct experimental measurements of these order parameters in real sp ace are therefore important tools for understanding many emergent phenomen a including charge order\, superconductivity\, and magnetism\, especially in systems with heterogeneity or phase coexistence/competition. Locally\, electron energy loss spectroscopy (EELS) in the scanning transmission elec tron microscope (STEM) is a powerful platform to probe the elemental and e lectronic configuration of new materials with spatial resolution down to t he atomic scale for quantifying valence states and charge evolution\, espe cially in atomically-designed heterostructures or across buried interfaces [1\,2\,3]. Alternatively\, harnessing beyond-dipole spectroscopy via s-or bital nonresonant inelastic x-ray scattering (sNIXS) enables direct\, real -space mapping of orbital configurations without input from atomistic or f irst-principles modeling [4]. Here\, I will highlight these advanced spect roscopic methods through their application across the families of strongly correlated nickel oxide compounds which exhibit rich physical properties ranging from superconductivity to metal-insulator transitions. Local EELS measurements in infinite-layer nickelate superconductors demonstrate key d istinctions from the seemingly analogous high-Tc cuprates and reveal the i mportance of the film-substrate interface on their macroscopic properties. We use sNIXS to directly measure spin configurations and effective crysta l field splitting more broadly across formal valences from 1+ to 3+ [5]\, demonstrating the potential of this technique for more exotic compounds in the future [6]. \n\n\n1. Goodge et al. PNAS 118(2)\, e2007683118 (2021). \n2. Goodge et al. Nat. Mater. 22\, 466–473 (2023).\n3. Husremovic et al . Nat. Comm. 16\, 1208 (2025).\n4. Yavaș et al. Nat. Phys. 15\, 559-562 ( 2019).\n5. Abarca Morales et al. in preparation.\n6. Ko et al. Nature 638\ , 935–940 (2025)\; Bhatt et al. arXiv:2501.08204 (2025).\n\nContact Pers on: Markus Grüninger / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Melanie Müller DTSTART;TZID=Europe/Berlin:20251203T143000 DTEND;TZID=Europe/Berlin:20251203T153000 DTSTAMP:20260527T090434Z UID:0000002250@events.thp.uni-koeln.de DESCRIPTION:Melanie Müller\, FHI Berlin MPG\n\nProbing light-matter inter actions and ultrafast dynamics at the angstrom scale\n\nRecent advances in coupling ultrafast optical and THz pulses to low-temperature scanning tun nelling microscopes have opened up new possibilities for probing light-dri ven processes with simultaneous femtosecond temporal and nanometre-to-angs trom spatial resolution. In this talk\, I introduce two approaches to expl oring ultrafast surface dynamics with high spatial resolution: photon-assi sted STM (phSTM) and THz-lightwave-driven STM (THz-STM) [1]. \n\nFirst\, I will demonstrate coherent phonon spectroscopy in ultrathin ZnO/Ag(111) fi lms with nanometer resolution using phSTM [2]. Here\, the excitation of lo calized surface plasmons inside the STM enables the local excitation of co herent phonons and their efficient detection through resonant plasmon-assi sted tunneling. \n\nSecond\, I will present THz-STM measurements on the d ynamical evolution of the commensurate charge density wave (CDW) phase in the layered material 1T-TaS₂ [3]. We track the ultrafast dynamics of coh erent collective modes\, including the CDW amplitude mode\, in real space and time with a precision down to individual CDW superlattice sites. We fu rther identify a low-frequency mode at 1.3 THz that emerges near a local C DW irregularity and whose frequency coincides with that predicted for inte rlayer phonon modes. In addition to these ultrafast responses\, we find th at tip-enhanced THz pulses can induce long-lived modifications of the loca l CDW order and local density of states\, which we attribute to THz-driven changes in the stacking configuration.\n\nTogether\, these results highli ght the power of ultrafast STM to resolve and control non-equilibrium phen omena with unprecedented spatial resolution. This technique provides direc t access to local excited-state physics and opens the door to ultrafast ma nipulation of emergent phases at the atomic scale. If time permits\, I wil l conclude with an outlook\, including first theoretical steps toward prob ing the local properties of light-induced Floquet states in graphene using ultrafast STM [4].\n \n[1] M. Müller\, Prog. Surf. Sci. 99\, 1 (2024)\n[ 2] S. Liu et al.\, Sci. Adv. 8\, 42 (2022)\n[3] L. E. Parra López et al.\ , arXiv:2505.20541 (2025)\n[4] N. Jacobsen et la.\, in preparation\n\nCont act Person: Achim Rosch / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Kobus Kuipers DTSTART;TZID=Europe/Berlin:20251204T153000 DTEND;TZID=Europe/Berlin:20251204T163000 DTSTAMP:20260527T090434Z UID:0000002285@events.thp.uni-koeln.de DESCRIPTION:Kobus Kuipers\, FZJ\n\nTopological structures of and for light -surprises\, tragedy and usefulness-\n\nTopology is an invaluable tool t o elucidate the properties of physical systems. Typically\, it refers to i nstances where an entity has so‑called topological protection\, which me ans that the entity cannot disappear if small changes are made to the syst em. In this lecture I will address two illustrations of topology relevant to light fields.\n\nThe first relates to optical singularities\, which can have a topological charge\, and a topological feature of guided light cal led transverse optical spin. I will experimentally show that the charge of both phase‑ and polarization singularities plays a role in how they are distributed in space and also in the way they are created and annihilated . Transverse optical spin can be exploited to affect quantum emission enab ling the creation of a valley-photon interface. The second relates to the behavior of light in photonic nanostructures of which the properties have a non‑trivial topology. We experimentally investigated the propagation o f edge states between topologically non‑trivial photonic crystals. I wil l show how the propagation direction is related to the far‑field optical spin of the modes\, but also the breaking of helicity-momentum locking in the near-field. Time allowing I will discuss the creation of photonic Lan dau levels in strained photonic crystals.\n\nContact Person: Paul van Loos drecht LOCATION:H3 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Kai Chang DTSTART;TZID=Europe/Berlin:20251208T140000 DTEND;TZID=Europe/Berlin:20251208T153000 DTSTAMP:20260527T090434Z UID:0000002304@events.thp.uni-koeln.de DESCRIPTION:Kai Chang\, Beijing Academy of Quantum Information Sciences\n\ n2D IV-VI ferroelectric semiconductors and ultra-narrow lateral heterostru ctures\n\n2D ferroelectric and ferromagnetic materials discovered in the r ecent decade have opened a new era for the construction and tuning of hete rostructures for electronic and computing applications. Semiconducting 2D ferroic materials are especially interesting as their electronic structure s deeply intertwine with the spontaneously broken symmetry\, thus new free doms like spin\, orbital and electronic valleys are generated. In this tal k\, I will mainly focus on the development of group-IV monochalcogenide 2D ferroelectric semiconductors\, from the discovery\, ferroelectric mechani sm\, spin-valley correlation in the electronic structures\, to the recentl y realized 2D lateral heterostructures and superlattices. The most interes ting advancement is the design and in situ molecular beam epitaxial growth of a new type of valley-controlled ferroelectric sandwich heterostructure that is analogous to the classical spin valve\, in which the transmission probability of electronic states is determined by the alignment of the po larization of two ferroic layers separated by a thin barrier. The mechanis m of this ferroelectric valley valve relies on the polarization-tuned hole valleys in group-IV monochalcogenide ferroelectric semiconductors. The cr eation of such device is enabled by our ability of precisely controlling t he growth mode of these materials\, which eventually generates SnTe-PbTe m onolayer superlattices with 2-nm wide material section\, the narrowest eve r 2D lateral superlattices to the best of our knowledge. Based on such str uctures\, we plan to further develop novel non-volatile logic and storage devices\, as well as topological qubits.\n\nContact Person: Jeison Fischer LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Markus Garst DTSTART;TZID=Europe/Berlin:20251210T143000 DTEND;TZID=Europe/Berlin:20251210T153000 DTSTAMP:20260527T090434Z UID:0000002287@events.thp.uni-koeln.de DESCRIPTION:Markus Garst\, Karlsruhe Institute of Technology\n\nDynamics o f magnetic skyrmions\n\nWe give an overview of our recent work on the dyna mics of magnetic skyrmions. In bulk chiral magnets two-dimensional skyrmio n textures extend in the third direction forming strings and skyrmion stri ng lattices. These lattices exhibit various excitations modes. Magneto-opt ic and magneto-elastic couplings allow to access excitation modes with mul tipole character that are inaccessible with magnetic resonance spectroscop y. Brillouin light scattering is able to resolve a quadrupole mode [1]. Th e magneto-elastic coupling leads to a hybridization of ultrasound with var ious excitation modes and\, in addition\, it leads to a phason gap due to a distortion of the atomic crystal [2]. Going beyond linear dynamics\, we demonstrate that a single skyrmion string supports solitary waves similar to vortex filaments in fluids [3]. Finally\, we discuss the quantum dynami cs of skyrmions and show that it is governed by a dipole conservation law associated with the topological charge. As a result\, the mobility of isol ated quantum skyrmions is constrained resembling the phenomenology of frac ton theories [4].\n\n[1] P. Che\, R. Ciola\, M. Garst\, V. Kravchuk\, P.R. Baral\, A. Magrez\, H. Berger\, T. Schöneberger\, H. M. \nRønnow\, and D. Grundler\, Communication Materials 6\, 139 (2025).\n[2] R. Ciola\, PhD Thesis\, Karlsruhe Institute of Technology.\n[3] P. Kravchuk\, U. Rößler \, J. van den Brink\, and M. Garst\, Phys. Rev. B 102\, 220408(R) (2020).\ n[4] S. Sorn\, J. Schmalian\, and M. Garst\, arXiv:2412.16284\, Physical R eview X in press.\n\nContact Person: Achim Rosch / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | James Analytis DTSTART;TZID=Europe/Berlin:20251217T130000 DTEND;TZID=Europe/Berlin:20251217T140000 DTSTAMP:20260527T090434Z UID:0000002303@events.thp.uni-koeln.de DESCRIPTION:James Analytis\, UC Berkeley\n\nEntanglement Randomness in a t riangular lattice spin liquid candidate\n\n\n\nContact Person: Simon Trebs t LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Mario di Luca DTSTART;TZID=Europe/Berlin:20251218T160000 DTEND;TZID=Europe/Berlin:20251218T170000 DTSTAMP:20260527T090434Z UID:0000002310@events.thp.uni-koeln.de DESCRIPTION:Mario di Luca\, EPFL\n\nQuantum Hall Antidots in Bilayer Graph ene as fractional Coulombmeters\n\n\n\nContact Person: Erwann Bocquillon / Matteo Cacco LOCATION:0.02 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Alexey Chernikov DTSTART;TZID=Europe/Berlin:20260114T143000 DTEND;TZID=Europe/Berlin:20260114T153000 DTSTAMP:20260527T090434Z UID:0000002306@events.thp.uni-koeln.de DESCRIPTION:Alexey Chernikov\, TU Dresden\n\nOptically detected exciton tr ansport in 2D semiconductors\n\nTransport of optical excitations in semico nducting solids plays a central role from both fundamental and technologic al perspectives. In systems with strong Coulomb interaction the propagatio n of optically injected carriers is dominated by excitons instead of free electrons or holes. These correlations can affect both the overall energy landscape and the interactions with vibrational modes\, with a strong impa ct on the mobility of the excitations. In two-dimensional semiconductors\, the electron-hole correlations present a particularly interesting case co mbining the properties of Wannier-Mott excitations in inorganic quantum we ll systems with high exciton binding energies\, strong-light matter intera ction\, and spin-valley degrees of freedom. In addition\, the possibility to artificially create heterostructures featuring a manifold of excitonic states as well as the recent demonstration of excitons in two-dimensional magnets offer a rich playground for research and\, possibly\, technologica l applications. In this talk\, I will discuss optical access to exciton tr ansport via transient microscopy\, linear and non-linear diffusion\, and p resent indications of quantum transport phenomena. An additional focus wil l be placed on the coupling of exciton propagation to magnetic order and s pin fluctuations in two-dimensional semiconducting magnets.\n\nContact Per son: Paul van Loosdrecht LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Matthias Vojta DTSTART;TZID=Europe/Berlin:20260121T143000 DTEND;TZID=Europe/Berlin:20260121T153000 DTSTAMP:20260527T090434Z UID:0000002302@events.thp.uni-koeln.de DESCRIPTION:Matthias Vojta\, TU Dresden\n\nFun with spin liquids: Of Bose surfaces and fractionalized superconductivity\n\n\n\nContact Person: Urban Seifert LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Giulio Monaco DTSTART;TZID=Europe/Berlin:20260128T143000 DTEND;TZID=Europe/Berlin:20260128T153000 DTSTAMP:20260527T090434Z UID:0000002308@events.thp.uni-koeln.de DESCRIPTION:Giulio Monaco\, Universita' degli studi di Padova\n\nShaping G lasses with X-Rays\n\nIlluminating an insulating or semiconducting glass u sing X-rays changes the properties of the glass\, no matter what dose and dose-rates are used. The reason behind this is the complexity of the energ y landscape of glasses\, characterized by a huge number of different minim a: during x-ray illumination the glass becomes an active system\, powered by the absorbed energy\, and explores this landscape. An x-ray illuminated glass becomes then the playground to study\, under controlled conditions\ , phenomena which also appear in other contexts.\nOn the basis of x-ray sc attering combined with nano-calorimetry experiments\, I will discuss how x -ray irradiation can be used to prepare rejuvenated\, high-enthalpy glasse s [1\,2]. In particular\, while enthalpy increases with irradiation in a f irst stage\, a sufficiently long irradiation results in a stationary state \, independent of the initial state of the glass. The transition to this s tationary state corresponds to the yielding transition\, usually encounter ed in glasses under active dynamics as well as in stress-strain experiment s [3]. In that context\, a solid loaded beyond the yield stress loses its elastic properties and becomes plastic. Here\, the yield point is reached by increasing the density of plastic regions by generation of point defect s during x-ray irradiation. As the density of defects increases\, the mech anical response of the glass changes from elastic to more and more plastic \, until reaching the limit where it becomes the one characteristic of a f lowing system\, which signals that the yield point is reached. \n\nReferen ces\n[1] J. Baglioni et al.\, Rep. Prog. Phys. 87\, 120503 (2024).\n[2] J. Baglioni et al.\, Newton 2\, 100338 (2026).\n[3] A. Martinelli et al.\, P hys. Rev. X 13\, 041031 (2023).\n\nContact Person: Markus Grüninger / Mat teo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Sajna Hameed DTSTART;TZID=Europe/Berlin:20260223T133000 DTEND;TZID=Europe/Berlin:20260223T170000 DTSTAMP:20260527T090434Z UID:0000002342@events.thp.uni-koeln.de DESCRIPTION:Sajna Hameed\, Max Planck Institute for Solid State - Stuttgar t\n\nUniaxial Strain-Induced Emergent Phases in Quantum Materials\n\nEmerg ent phases in quantum materials arise from a delicate interplay between la ttice\, spin\, orbital\, and electronic degrees of freedom. A central chal lenge is to control this balance cleanly and reversibly\, without introduc ing chemical disorder. In this talk\, I show that uniaxial strain acts as a powerful symmetry-breaking tuning parameter that stabilizes emergent pha ses across a wide range of systems\, from spin-orbit-coupled magnets to qu antum paraelectrics. I first discuss strain control of magnetism in the la yered spin-orbit-coupled Mott insulator Sr2IrO4\, where less than one perc ent uniaxial strain drives multiple magnetic phase transitions. In situ Ra man and resonant x-ray scattering under controlled strain reveal how latti ce distortions reshape spin-orbital wavefunctions and interlayer exchange interactions\, underscoring the key role of spin-lattice coupling in stabi lizing distinct magnetic ground states. I then turn to the incipient ferro electric and dilute superconductor SrTiO3\, where uniaxial strain tunes a paraelectric-ferroelectric quantum phase transition and enhances the super conducting transition temperature. Combining strain with Raman and neutron scattering\, we demonstrate access to quantum ferroelectric fluctuations and directly reveal the connection between incipient ferroelectricity and superconductivity in SrTiO3 [1\,2].\nTogether\, these case studies establi sh uniaxial strain as a universal\, disorder-free knob for engineering and reversibly controlling emergent phases in quantum materials.\n\n[1] S. Ha meed*\, D. Pelc* et al.\, Nat. Materials 21\, 54 (2022)\n[2] I. Khayr*\, N . Somun*\, S. Hameed* et al.\, arXiv:2511.10623 (2025)\n\nContact Person: Erwann Bocquillon / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Lisa Marie-Kern DTSTART;TZID=Europe/Berlin:20260302T090000 DTEND;TZID=Europe/Berlin:20260302T123000 DTSTAMP:20260527T090434Z UID:0000002343@events.thp.uni-koeln.de DESCRIPTION:Lisa Marie-Kern\, Massachusetts Institute of Technology\n\nCon trolling Spin Textures in Magnetic Systems\n\nControlling magnetic propert ies with precision is a central goal in magnetism and spintronics\,\noffer ing insights into fundamental magnetic interactions as well as pathways to high-density\ndata storage and low-power processing. In particular\, shap ing magnetic energy landscapes has\nrecently gained momentum as a powerful strategy to spatially guide spin textures and control\ntheir dynamics.\nI will present our work on the controlled manipulation of topologically non -trivial spin textures\,\nincluding skyrmions and higher-order variants\, using tailored magnetic anisotropy landscapes\ncreated via local ion irrad iation. Anisotropy engineering proves effective for magnetic field-\,\nspi n torque- and laser-driven control of spin textures\, and therefore consti tutes a versatile\nplatform bridging emergent magnetic phenomena and spint ronic functionality. Using high-\nresolution x-ray imaging\, we demonstrat ed nanometer spatial control over skyrmion nucleation\nsites\, motion path s and complex magnetization dynamics. Recently\, we also realized skyrmion -\nskyrmion interactions in controlled collisions as well as in collective ensembles within higher-\norder skyrmion bags.\nSuch real-space experimen ts allow us to uncover fundamental aspects of topological charge\nand expl ore the potential of topological textures for logic operations and unconve ntional\ncomputing. The achieved level of control is readily transferable across material classes\, offering\nexciting opportunities to explore emer gent magnetic phenomena on ultrasmall length and\nultrafast time scales.\n \nContact Person: Erwann Bocquillon / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Wouter Jolie DTSTART;TZID=Europe/Berlin:20260302T133000 DTEND;TZID=Europe/Berlin:20260302T170000 DTSTAMP:20260527T090434Z UID:0000002344@events.thp.uni-koeln.de DESCRIPTION:Wouter Jolie\, Universität zu Köln\n\nImaging strongly corre lated quasiparticles in 2D semiconductors\n\nElectrons are prone to strong correlations when confined into one-dimensional (1D) or 0D cavities. Many exotic ground states can emerge\, depending on the type of interactions a t play. Examples are Peierls transitions\, Tomonaga-Luttinger liquids or A nderson impurities.\nAn ideal experimental testbed for the observation of correlated electronic behaviour are point defects and metallic mirror twin boundaries (MTBs) in two-dimensional semiconductors such as MoS2. These q uantum objects have well-defined structural and electronic properties\, ar e only weakly coupled to the environment and accessible to spatially resol ved spectroscopic techniques such as scanning tunnelling microscopy (STM). In the first part of my talk\, I will show that the confined quasipartic les within finite MoS2 MTBs transform into spin and charge excitations as described by the Tomonaga-Luttinger liquid theory of strongly interacting 1D electrons. In addition\, a Kondo resonance emerges when the highest occ upied state is filled by a single electron\, in quantitative agreement wit h the Anderson impurity model. In the second part\, I will focus on magnet ic point defects\, which can be created and manipulated on the atomic scal e using STM. After quantifying the magnetic properties of isolated and cou pled spins\, I will show how one can study their spin dynamics using elect ron spin resonance STM (ESR-STM).\n\nContact Person: Erwann Bocquillon / M atteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Paulina Majchrzak DTSTART;TZID=Europe/Berlin:20260303T090000 DTEND;TZID=Europe/Berlin:20260303T123000 DTSTAMP:20260527T090434Z UID:0000002345@events.thp.uni-koeln.de DESCRIPTION:Paulina Majchrzak\, Stanford University\n\nEngineering electro nic structures in van der Waals materials\n\nThe electrical transport and optical properties of van der Waals materials are shaped by the complex in teractions between charge\, lattice\, and spin\, all encoded in their surf ace electronic structure. By engineering two-dimensional heterostructures\ , and applying external perturbations such as electric fields\, this delic ate balance can be continuously tuned within a single material platform\, opening up a route towards quantum phenomena “by design”.\nIn this tal k\, I will highlight angle-resolved photoemission spectroscopy (ARPES) as a powerful tool for revealing dominant many-body effects and informing str ategies to enhance electronic correlations. Using model systems such as tw isted bilayer graphene and twisted transition metal dichalcogenides\, I wi ll show how nanoscale ARPES bridges the gap between spectroscopy and trans port\, providing insight into the underlying design principles for enginee ring electronic properties. Together\, these results establish ARPES not o nly as a diagnostic of emergent behaviour\, but as a guide for controlling it.\n\nContact Person: Erwann Bocquillon / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Banabir Pal DTSTART;TZID=Europe/Berlin:20260303T133000 DTEND;TZID=Europe/Berlin:20260303T170000 DTSTAMP:20260527T090434Z UID:0000002346@events.thp.uni-koeln.de DESCRIPTION:Banabir Pal\, Max Planck Institute of Microstructure Physics - Halle\n\nBeyond ferromagnetism: new quantum materials for next-generation spintronics\n\nSpintronics aims to go beyond the charge-based paradigm of silicon microelectronics by\nutilizing the spin degree of freedom. While the field has achieved significant success\, the\nreliance on ferromagnets (FMs) introduces bottlenecks regarding thermal stability\, switching\nspe ed\, and energy efficiency. My research addresses these limitations by exp loring the frontier\nof 'post-ferromagnetic' spintronics\, specifically fo cusing on non-collinear antiferromagnets\n(NCAFs) [1].\nNCAFs uniquely com bine the advantages of FMs (efficient reading and writing) with those of\n antiferromagnets (negligible stray fields and ultrafast dynamics). In this talk\, I will present\nrecent advances in this field\, with a special foc us on the topological semimetal Mn3Sn. Key\ntopics include the electrical manipulation [2] and efficient reading [3] of the NCAF order\, as\nwell as the generation of unconventional spin-orbit torques [4\, 5]. Finally\, I show that although\nkey components of spintronic devices based on NCAFs ha ve been demonstrated\, a wide range\nof open questions remain to be answer ed. Thus\, the field of NCAFs is a vibrant and exciting\nsubfield of spint ronics with much potential for next-generation memory and computing\ntechn ologies.\n\nReferences:\n\n[1] B. H. Rimmler\, B. Pal\, and S. S. P. Parki n\, Nat. Rev. Mater. 10\, 109 (2025)\n[2] B. Pal et al.\, Sci. Adv. 8\, ea bo5930 (2022)\n[3] B. H. Rimmler\, B. K. Hazra\, B. Pal et al.\, Adv. Mate r. 35\, 2209616 (2023)\n[4] A. Pandey\, …\, B. Pal et al.\, Phys. Rev. R esearch 7\, L042058 (2025)\n[5] B. K. Hazra\, B. Pal et al.\, Nat. Commun. 14\, 4549 (2023)\n\nContact Person: Erwann Bocquillon / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Tobia Nova DTSTART;TZID=Europe/Berlin:20260304T090000 DTEND;TZID=Europe/Berlin:20260304T123000 DTSTAMP:20260527T090434Z UID:0000002347@events.thp.uni-koeln.de DESCRIPTION:Tobia Nova\, University of Geneva\n\nWeaving light into quantu m matter: mastering control from strong fields to vacuum hybrids\n\nQuantu m materials are solids in which quantum effects persist across broad tempe rature and length scales\, giving rise to interacting networks of electron ic\, structural\, and magnetic degrees of freedom. This complex choreogra phy produces rich energy landscapes where quantum phases compete\, coexist \, and intertwine.\nThe ability to rationally design and manipulate these emergent phenomena is a modern frontier of condensed matter physics\, prom ising both fundamental discoveries and transformative technologies. My res earch addresses this challenge by exploring light as a versatile tuning kn ob\, spanning from classical light fields to quantum vacuum fluctuations.\ nIn the classical regime\, I show how intense\, ultrafast terahertz (THz) pulses\, matched to the energy scales of collective modes\, can selectivel y drive lattice vibrations and steer materials across phase boundaries—e nhancing\, stabilizing\, and even inducing phases inaccessible at equilibr ium.\nBut what if we could transcend external driving altogether? What if the quantum vacuum itself\, teeming with electromagnetic field fluctuation s\, could become a permanent control parameter?\nIn the quantum regime\, I investigate this possibility by embedding materials in cavities smaller t han the wavelength of light. By dramatically enhancing their coupling to v acuum fields\, we create hybrid light–matter states that can reshape mat erial properties. Here\, the boundary between light and matter dissolves\, opening pathways to designer quantum phases.\nThis talk will traverse bot h frontiers\, establishing light as a universal tool for sculpting quantum matter\, with a particular focus on my experimental platform for THz spec troscopy and control of two-dimensional van der Waals heterostructures.\n\ nContact Person: Erwann Bocquillon / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Taner Esat DTSTART;TZID=Europe/Berlin:20260304T133000 DTEND;TZID=Europe/Berlin:20260304T170000 DTSTAMP:20260527T090434Z UID:0000002348@events.thp.uni-koeln.de DESCRIPTION:Taner Esat\, Forschungszentrum Jülich\n\nFrom Correlated Elec trons to Atomic-Scale Quantum Sensing\n\nScanning tunneling microscopy (ST M) enables the fabrication and investigation of nanostructures with atomic precision and provides access to correlated electron phenomena as well as a route toward atomic-scale quantum devices. In this talk\, I demonstrate how artificial molecular nanostructures serve both as model systems for K ondo correlations in the weak-coupling regime and as single-molecule quant um sensors. We fabricate these nanostructures by positioning planar aromat ic molecules in an upright geometry on a pedestal of two metal atoms on a surface [1] or at the STM tip apex\, where they host a well-isolated spin- 1/2 that exhibits Kondo physics [2].\nIn the first part of the talk\, I fo cus on the correlations and the Kondo scale of these structures on the sur face. Performing tunneling spectroscopy at temperatures between 1 K and 30 mK in variable magnetic fields\, and comparing the results with numerical renormalization group calculations\, we show that the intrinsic Kondo sca le lies 6-11 orders of magnitude below the experimental temperature. The r esults highlight the asymptotic freedom in the Kondo problem: the exchange coupling decreases only logarithmically at high energies\, allowing Kondo signatures to persist far above the intrinsic Kondo scale.\nIn the second part\, I demonstrate how the spatial resolution of quantum sensors can be advanced to the atomic length scale\, which remains elusive for existing techniques. Using the same decoupling strategy\, we fabricate a single-mol ecule quantum sensor at the STM tip. The weakly coupled spin-1/2 forms a t wo-level quantum system in a magnetic field that can be addressed by elect ron spin resonance (ESR)\, enabling the detection of magnetic and electric dipole fields from individual atoms with sub-angstrom spatial resolution [3].\n\n[1] Esat\, et al. Nature 558\, 573-576 (2018).\n[2] Esat\, et al. Physical Review Research 5\, 033200 (2023).\n[3] Esat\, et al. Nature Nano technology 19\, 1466-1471 (2024).\n\nContact Person: Erwann Bocquillon / M atteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Steef Smit DTSTART;TZID=Europe/Berlin:20260305T090000 DTEND;TZID=Europe/Berlin:20260305T123000 DTSTAMP:20260527T090434Z UID:0000002349@events.thp.uni-koeln.de DESCRIPTION:Steef Smit\, The University of British Columbia\n\nSuperconduc tivity and density waves in multilayered cuprates and nickelates\n\nUnconv entional superconductivity and its interplay with various ordered phases h as long been a central topic in condensed matter research. Multilayered cu prates and nickelates in particular have garnered lots of attention as the se cuprates hold the record for highest critical temperature at ambient pr essure\, where the nickelates have only very recently been discovered to h ost superconductivity at high pressures or strain. I will discuss two rece nt papers [1][2] showing photoemission signatures of a layer-selective cha rge density wave in the trilayer cuprate Bi2223[1]\, and of an incommensur ate spin density wave in the multilayered nickelate La2Ni3O7[2]. We propos e a novel mechanism where interfacing superconducting layers of distinct d oping optimizes both pairing and phase coherence in the combined structure \, thereby maximizing TC. In the nickelates\, we additionally use ARPES di chroism to show that the first electron removal states have a symmetry rem iniscent of the Zhang-Rice-singlets and three-spin-polarons found in the c uprates\, suggesting a deep connection between the low energy electronic s tructures of the two material families.\n\n[1] S. Smit*\, M. Bluschke* et al\, Coherence and pairing enhancement in a trilayer cuprate with layer-se lective charge order (2025) arXiv:2506.01448 \n\n[2] CC. Au-Yeung*\, X. C hen*\, S.Smit*\, M. Bluschke* et al\, Universal electronic structure of mu lti-layered nickelates via oxygen-centered planar orbitals\, Nature Physic s (2026) arXiv:2502.20450\n\nContact Person: Erwann Bocquillon / Matteo Ca cco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Maciej Dendzik DTSTART;TZID=Europe/Berlin:20260305T133000 DTEND;TZID=Europe/Berlin:20260305T170000 DTSTAMP:20260527T090434Z UID:0000002350@events.thp.uni-koeln.de DESCRIPTION:Maciej Dendzik\, KTH (Royal Institute of Technology of Stockho lm)\n\nUltrafast Photoemission Spectroscopy of Van der Waals Quantum Mater ials\n\nVan der Waals (vdW) materials host a wide range of quantum phenome na\, offering exciting opportunities for both fundamental research and nex t-generation electronic and optoelectronic technologies. In this presentat ion\, I will highlight our recent efforts to explore vdW systems using ang le-resolved photoemission spectroscopy (ARPES) and its advanced extensions \, including spin-resolved\, time-resolved\, and momentum microscopy techn iques. These methods enable direct insight into electronic band structure\ , spin and orbital textures\, and ultrafast carrier dynamics.\nI will focu s on key results in transition metal dichalcogenides\, black phosphorus\, and cuprate superconductors. In particular\, I will present recent time-re solved momentum microscopy experiments at a free-electron laser\, revealin g the ultrafast dynamics of the excitonic Mott transition in WSe_2. I will also discuss emerging studies of black phosphorus\, where optical excitat ion induces valley-dependent carrier relaxation and drives light-induced t ransport phenomena analogous to the nonlinear Hall effect occuring on the femtosecond timescale.\nFinally\, I will briefly highlight our ongoing dev elopments in high-resolution time-resolved ARPES instrumentation at KTH Ro yal Institute of Technology\, along with novel in situ exfoliation methods that enable the preparation of pristine two-dimensional materials under u ltra-high vacuum conditions.\n\nContact Person: erwann Bocquillon / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Marco Valentini DTSTART;TZID=Europe/Berlin:20260306T090000 DTEND;TZID=Europe/Berlin:20260306T123000 DTSTAMP:20260527T090434Z UID:0000002351@events.thp.uni-koeln.de DESCRIPTION:Marco Valentini\, University of California Santa Barbara (UCSB )\n\n2D Materials: Platform for Creating and Manipulating Correlations and Topology\n\nBeing able to create and govern new phases of matter is one o f the most fascinating aspects of condensed matter physics. In particular\ , it has been proposed that by leveraging correlations and topology\, new types of quasiparticles can emerge that follow entirely new rules—namely \, non-Abelian anyons\, which might serve as building blocks for fault-tol erant qubits.\nI will first show how correlations and topology can be comb ined in rhombohedral multilayer graphene aligned with hexagonal boron nitr ide. We have shown that this system can be tuned into either a superconduc ting (SC) or a quantum anomalous Hall (QAH) phase simply by applying an el ectric field\, opening a pathway toward topological devices based on SC– QAH hybrid platforms [1].\nThen\, by leveraging atomic force microscope– based\, ultra-clean and ultra-precise nanofabrication on van der Waals mat erials\, I will show how to control and confine single quasiparticles in t he integer quantum Hall regime of monolayer graphene\, and how to study fr actional quantum Hall dots by investigating different edge–quantum-dot c oupling regimes\, opening new opportunities for the controlled detection a nd manipulation of Abelian even-denominator quantum Hall states.\nFinally\ , I will outline how correlations and topology can be harnessed to enginee r novel nanoelectronic devices—beyond topological qubits—including qua ntum Hall–protected qubits and high-impedance\, magnetic-field-resilient quantum couplers.\n\n1 Y. Choi*\, Y. Choi*\, MV* et al. Nature 342 Vol 6 39 (2025).\n\nContact Person: Erwann Bocquillon / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Tom Seifert DTSTART;TZID=Europe/Berlin:20260306T133000 DTEND;TZID=Europe/Berlin:20260306T170000 DTSTAMP:20260527T090434Z UID:0000002352@events.thp.uni-koeln.de DESCRIPTION:Tom Seifert\, Freie Universität Berlin\n\nTerahertz Nanoscopy of Quantum Materials: From Ultrafast Dynamics to Nanoscale Control\n\nThe native time and length scales of interactions between electrons\, spins\, and the lattice in solids reside typically in the nanometer and femtoseco nd domain. To resolve the resulting non-equilibrium dynamics in quantum ma terials\, ultrafast optical probes must overcome the diffraction limit imp osed by far-field optics. In this talk\, I will present terahertz (THz) na noscopy as a powerful tool that enables simultaneous femtosecond temporal and nanometer spatial resolution.\nI will first discuss THz-emission spect roscopy\, showing how this sensitive far-field probe provides novel insigh ts into femtosecond transport dynamics in spinorbitronic heterostructures (including 2D materials)\, the multiferroic BiFeO3\, and the topological i nsulator Bi2Se3 [1-3]. Then\, I will demonstrate how the combination of TH z-emission spectroscopy with a near-field probe allows for the direct film ing of ultrafast spintronic transport at the nanoscale.\nI will conclude b y outlining a promising path to extend THz nanoscopy toward atomic resolut ion. This approach aims to enable local coherent control and read-out of q uantum matter [4-6] on femtosecond time scales\, bridging the gap between macroscopic transport and atomic-scale dynamics. \n\nReferences\n[1] T. S. Seifert et al.\, Nature Photonics 10\, 483 (2016)\n[2] T. S. Seifert et al.\, Nature Nanotechnology 18\, 1132 (2023)\n[3] C. In\, D. Jain\, T. S. Seifert\, …\, T. Kampfrath\, ArXiv preprint\, arXiv:2601.01030 (2026)\n [4] T. S. Seifert et al.\, Science Advances 6\, eabc5511 (2020)\n[5] V. Ra i\, J. Sim\, F. Faaber\, N. Bogdanoff\, S. Trishin\, P. Wiechers\, T. S. S eifert\, …\, K. J. Franke\, Science Advances 11\, eadz6549 (2025)\n[6] T . Siday\, …\, R. Huber\, Nature 629\, 329 (2024)\n\nContact Person: Erwa nn Bocquillon / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Francesco Rosa DTSTART;TZID=Europe/Berlin:20260317T110000 DTEND;TZID=Europe/Berlin:20260317T123000 DTSTAMP:20260527T090434Z UID:0000002360@events.thp.uni-koeln.de DESCRIPTION:Francesco Rosa\, Politecnico di Milano\n\nMagnetic and orbital properties of infinite-layer nickelate and cuprate superconductors\n\nInf inite-layer (IL) nickelates recently gained strong interest thanks to thei r apparent analogies with cuprates\, including a square lattice-based stru cture dominated by superexchange antiferromagnetic interaction and the eme rgence of superconductivity upon hole doping. X-ray spectroscopies with sy nchrotron light are a very powerful tool for the investigation of such sys tems\, providing complementary information to the more traditional inelast ic neutron scattering. In this seminar\, we discuss some of the recent res ults obtained by our group using both Resonant Inelastic X-ray Scattering (RIXS) and X-ray Magnetic Circular Dichroism (XMCD)\, mainly focusing on m agnetism. RIXS results concern the analysis of dynamic spin excitations (m agnons/paramagnons)\, investigating the effect of hole-doping on them and providing an estimate for the magnetic exchange coupling constants. Result s on IL nickelates will be compared to analogous ones on cuprates\, highli ghting analogies and differences. XMCD analysis is devoted to the static s pin order in IL nickelates\, reporting the presence of a field induced out -of-plane Ni1+ spin moment. Based on magnetic field- and temperature-depen dent measurements\, we justify our observations with an out-of-plane canti ng of in-plane anti-ferromagnetically correlated Ni1+ spins\, tentatively attributed to a symmetry lowering of the NiO2 planes.\n\nContact Person: M arkus Grüninger / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Mathias S. Scheurer DTSTART;TZID=Europe/Berlin:20260318T143000 DTEND;TZID=Europe/Berlin:20260318T153000 DTSTAMP:20260527T090434Z UID:0000002357@events.thp.uni-koeln.de DESCRIPTION:Mathias S. Scheurer\, University of Stuttgart\n\nQuantum geome try\, broken time-reversal symmetry\, and pairing\n\nAnalyzing the consequ ences of the quantum geometry induced by the momentum dependence of Bloch states has emerged as a very rich and active field in condensed matter phy sics. For instance\, for the superfluid stiffness or the pairing mechanism \, these geometric aspects can play an important role. In the first part o f the talk\, we will discuss that quantum geometry can also be essential f or the disorder sensitivity of a superconductor\, in particular when time- reversal symmetry is broken in the normal-state Bloch Hamiltonian. More sp ecifically\, we will identify "quantum geometric pair breaking"\, where an y superconductor becomes susceptible to microscopically non-magnetic impur ities\, and formally relate it to the maximum possible localization of two -particle Wannier states. Further\, in the presence of kinetic pair breaki ng\, impurities can also enhance pairing\, leading to an overall more comp lex\, non-monotonic behavior of Tc with impurity concentration. We will il lustrate the findings using rhombohedral graphene and superconducting alte rmagnets as examples. In the second part of the talk\, we will discuss und er which conditions on the underlying magnetic order parameter non-recipro cal superconductivity can be stabilized without net magnetization. We will then scrutinize a specific heterostructure where such a magnetization-fre e superconducting diode is expected\, in line with recent experiments.\n\n Contact Person: Urban Seifert LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Sebastian Loth DTSTART;TZID=Europe/Berlin:20260415T143000 DTEND;TZID=Europe/Berlin:20260415T153000 DTSTAMP:20260527T090434Z UID:0000002309@events.thp.uni-koeln.de DESCRIPTION:Sebastian Loth\, Universität Stuttgart\n\nAtomic-scale spin d ynamics at the quantum to classical crossover\n\nMagnetic atoms on surface s provide a precisely controllable platform for studying spin dynamics acr oss a wide range of regimes from individual quantum spins to nanoscale cla ssical magnets. Using scanning tunneling microscopy (STM) combined with at omic force microscopy (AFM) at millikelvin temperatures\, we investigate a tomic-scale spin dynamics at the boundary between quantum and classical be havior.\nAntiferromagnetic Fe spin chains on Cu₂N/Cu(100) serve as a mod el system to resolve the quantum-to-classical crossover in magnetization r eversal. By driving fast magnetic field ramps at kT/s through avoided leve l crossings\, we identify Landau-Zener dynamics in antiferromagnetic spin chains. Complementary measurements with magnetic exchange force microscopy on individual paramagnetic Fe atoms on MgO/Ag(100) give direct access to the forces exerted by a dynamically switching quantum spin on a macroscopi c cantilever. This coupling between atomic spin dynamics and mechanical de grees of freedom ultimately enables a single-atom electric motor\, in whic h current-induced spin switching drives free mechanical oscillations of th e cantilever demonstrating direct conversion of quantum spin excitations i nto classical mechanical work with high efficiency.\n\nContact Person: Tho mas Michely / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Andrea Damascelli DTSTART;TZID=Europe/Berlin:20260428T140000 DTEND;TZID=Europe/Berlin:20260428T150000 DTSTAMP:20260527T090434Z UID:0000002374@events.thp.uni-koeln.de DESCRIPTION:Andrea Damascelli\, University of British Columbia\n\nProbing condensates' coherence by time-resolved ARPES\n\nWith its direct correspon dence to the electronic structure\, angle-resolved photoemission spectrosc opy (ARPES) is a ubiquitous tool for the study of quantum materials. When extended to the temporal domain\, time-resolved ARPES offers the potential to move beyond equilibrium properties\, exploring both the unoccupied ele ctronic structure as well as its dynamical response under ultrafast pertur bation [1]. In this talk\, I will discuss how time-resolved ARPES can prob e the coherence of many-body condensates\, from high-temperature supercond uctivity [2\,3]\, to spin-correlation-driven pseudogap [4]\, and excitonic insulating behavior [5].\n\n[1] F. Boschini\, M. Zonno\, A. Damascelli\, Rev. Mod. Phys. 96\, 015003 (2024).\n[2] F. Boschini et al.\, Nature Mater ials 17\, 416 (2018).\n[3] M. Zonno\, F. Boschini\, A. Damascelli\, JESRP 251\, 147091 (2021).\n[4] F. Boschini et al.\, npj Quantum Materials 5\, 6 (2020).\n[5] D. Golez et al.\, Phys. Rev. B 106\, L121106 (2022).\n\nCont act Person: Paul Van Loosdrecht LOCATION:0.03 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Kirsten von Bergmann DTSTART;TZID=Europe/Berlin:20260429T143000 DTEND;TZID=Europe/Berlin:20260429T153000 DTSTAMP:20260527T090434Z UID:0000002311@events.thp.uni-koeln.de DESCRIPTION:Kirsten von Bergmann\, Universität Hamburg\n\nTopology of Spi n Textures and Magnet-Superconductor-Hybrids\n\nTopology is a useful conce pt to describe material properties. In magnetism topology can arise when t he spins in a material are pointing in different real-space directions\, l eading to non-coplanar magnetic order. Such complex magnetic states featur e a scalar spin chirality\, which can manifest in an additional\, topologi cal\, contribution to the Hall effect and an orbital magnetization\, even in the absence of spin-orbit coupling. In superconductors the topological properties are governed by the band structure in reciprocal space\, and ed ge modes arise at boundaries to sample regions with different topology.\nI n this colloquium I will present spin-polarized scanning tunneling microsc opy and spectroscopy experiments [1] of different topological systems. I w ill discuss examples of complex topological magnetic domain walls\, such a s meron-antimeron domain walls which transition into chains of skyrmions i n applied magnetic fields [2]\, or non-coplanar superposition domain wall junctions in an antiferromagnet [3]. When magnetic films are in contact wi th superconductors topological superconductivity can emerge. Such magnet-s uperconductor-hybrid systems can show low-energy edge modes at the boundar y to the pristine superconductor [4]. Also at boundaries between two diffe rent magnet-superconductor-hybrids with the same magnetic state chiral edg e modes can arise\, and experiments demonstrate that they are spin-polariz ed [5].\n[1] K. von Bergmann et al.\, J. Phys.: Condens. Matter 26\, 39400 2 (2014).\n[2] R. Brüning et al.\, Phys. Rev. X 15\, 021041 (2025).\n[3] V. Saxena et al.\, Nature Commun. 16\, 10808 (2025).\n[4] R. Brüning et a l.\, ACS Nano 19\, 36215 (2025).\n[5] F. Zahner et al.\, Nature Commun. 17 \, 3457 (2026).\n\nContact Person: Thomas Michely / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Laura Guasco DTSTART;TZID=Europe/Berlin:20260506T143000 DTEND;TZID=Europe/Berlin:20260506T153000 DTSTAMP:20260527T090434Z UID:0000002356@events.thp.uni-koeln.de DESCRIPTION:Laura Guasco\, MPI-Stuttgart\n\nReversal mechanism in antiferr omagnetically coupled Ru-doped manganite/nickelate multilayers studied by polarized neutron reflectometry\n\nAll-oxide synthetic antiferromagnets (S AFs) were realized in multilayer structures composed of Ru-doped manganite and nickelate spacer layers. By employing an odd number of ferromagnetic layers\, the design enables manipulation of the SAF state at relatively lo w magnetic fields due to the presence of uncompensated magnetic moments. F or practical implementation in spintronic devices\, it is crucial to deter mine the stability of these SAF states and their sensitivity to stray magn etic fields. Magnetization reversal in such multilayers is non trivial\, arising from the interplay between antiferromagnetic interlayer exchange c oupling\, tilted magnetic anisotropy\, and structurally favored domain for mation. This leads to strongly direction-dependent hysteretic behavior. Po larized neutron reflectometry provides a powerful approach to probe the in -plane magnetization components versus external field\, offering key insig hts into the underlying reversal mechanisms and SAF robustness.\n\nContact Person: Ionela Vrejoiu / Matteo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Martin Hefel DTSTART;TZID=Europe/Berlin:20260520T143000 DTEND;TZID=Europe/Berlin:20260520T153000 DTSTAMP:20260527T090434Z UID:0000002375@events.thp.uni-koeln.de DESCRIPTION:Martin Hefel\, TU München\n\nEmergent U(1) × U(1) Gauge Theo ry in the Classical Honeycomb Three Color Model\n\nStrongly correlated sys tems provide a fertile ground for emergent phenomena that cannot be unders tood in terms of independent particles. Prominent examples include fractio nalization and emergent gauge fields in systems such as spin ice and quant um dimer models\, which motivate the study of similarly constrained lattic e models. We study a classical model of hardcore bosons on the honeycomb l attice\, where the Hamiltonian enforces ground states with exactly $m$ par ticles per plaquette. At the commensurate filling fractions $f_m = m/6$ wi th $m \\in \\{1\,2\,3\\}$\, the system hosts an extensive ground-state deg eneracy. Upon doping\, a single boson fractionalizes into three individual ly colored but collectively color-neutral quasiparticles. Using a Monte Ca rlo worm algorithm\, we compute real-space two-point correlation functions of the bosons and various quasiparticle configurations. The bosonic corre lations decay algebraically\, and the associated structure factor exhibits pinch points in momentum space\, identifying the phase as a classical Cou lomb liquid. The quasiparticle correlations reveal a color and charge-depe ndent interaction structure. These observations are supported by an effect ive field theory obtained by mapping the model onto three coupled spin-ice models. This mapping yields three coupled divergence-free fields\, with t he key result that the low-energy physics is governed by two independent e mergent $U(1)\\times U(1)$ gauge fields. Within this framework\, the quasi particles appear as composite objects carrying gauge charges under both fi elds. Further\, we briefly discuss the quantum extension of the model. In the strong-coupling limit\, the system maps onto a Rokhsar-Kivelson-type H amiltonian\, with quantum fluctuations arising from ring-exchange processe s within the constrained manifold.\n\nContact Person: Urban Seifert / Matt eo Cacco LOCATION:PH2 END:VEVENT BEGIN:VEVENT SUMMARY:SFB1238 | Harrison LaBollita DTSTART;TZID=Europe/Berlin:20260715T143000 DTEND;TZID=Europe/Berlin:20260715T153000 DTSTAMP:20260527T090434Z UID:0000002408@events.thp.uni-koeln.de DESCRIPTION:Harrison LaBollita\, Flatiron Institute\n\nTo be announced\n\n tba\n\nContact Person: Fabian Kugler LOCATION:PH2 END:VEVENT END:VCALENDAR