Department of Physics • University of Cologne

Electronic transport measurements guide the discovery and characterization of quantum materials. The Biot-Savart law dictates that the motion of electrons generates a stray magnetic field during such a transport experiment. I will show how imaging these magnetic fields with a scanning SQUID microscope allows us the determine the path that current takes through our devices. I will then discuss the application of this approach to the quantum anomalous Hall regime in magnetically doped topological insulators. Using local information about the current distribution, I will show how bulk conduction processes give rise to a quantized Hall effect. I will then describe our efforts to visualize the breakdown of the quantum anomalous Hall effect under the application of large bias currents.

Due to their intrinsic topology,Weyl semimetals are potential candidates for topological superconductivity, but so far have always been connected with bulk superconductivity, leaving the possibility of intrinsic superconductivity of their topological surface states, the Fermi arcs, practically without attention, even from the theory side. By means of angle-resolved photoemission spectroscopy (ARPES) and ab initio calculations, we identify topological Fermi arcs on two opposing surfaces of the non-centrosymmetric Weyl material trigonal PtBi2. We show these states become superconducting at temperatures 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 showing in addition spatial inhomogeneity of the superconducting gap. In some samples superconducting gaps as large as 20 meV are revealed by STM resembling the phenomenology found in high-Tc superconductors. However bulk probes such as magnetization, transport and thermodynamics do not show superconductivity. 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 platform to host Majorana modes in intrinsically topological superconductor–normal metal–superconductor junctions.

Magnonics aims to employ quanta of spin waves, magnons, to carry, transport and process information, avoiding the dissipation of energy inherent to electronics. Experiments on magnons in regular (ferro)magnets have yielded demonstrations of basic logic devices, albeit macroscopic (mm-scale) in size and operating at GHz frequencies. Recently, the spotlight has shifted towards the use of antiferromagnets, in which neighbouring spins are aligned antiparallel to each other. This alternating order leads to significantly higher spin wave propagation velocities and might enable devices operating at terahertz (trillion of hertz) clock- rates. However, the absence of the net magnetisation also makes antiferromagnets magnetically ‘invisible’: it is very hard to detect and influence the antiferromagnetic order. Yet, in some antiferromagnets strong spin–orbit coupling results in canting of the spins, thereby producing net magnetization. The canted antiferromagnets combine antiferromagnetic order with phenomena typical for ferromagnets and hold a great potential for spintronics and magnonics. In this way they can be identified as closely related to the recently proposed novel class of magnetic materials, called altermagnets. In my talk I will discuss a new functionality of canted antiferromagnets and altermagnets for magnonics and show that these materials facilitate mechanisms allowing to generate, detect and nonlinearly convert propagating magnons at the nanoscale [1-3].

Electron-hole bilayer systems in InAs/(In)GaSb composite quantum wells (CQWs) have been extensively investigated as a host of the quantum spin Hall (QSH) insulating phase [1]. Although the QSH phase is no longer protected by time-reversal symmetry in an external magnetic field, below a critical magnetic field, helical edge channels due to band inversion remain, which evolve into counter-propagating chiral edge channels originating from electron- and hole-like Landau levels in the quantum-Hall (QH) regime [2]. In narrow-gap systems, charge puddles form in the bulk, which affect the helical edge transport [3]. Meanwhile, time-resolved transport measurements performed on chiral edge channels of QH systems have revealed the dynamics of nonequilibrium charge propagating 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. Figure 1 shows the schematic of the device, comprising a band-inverted InAs/InGaSb CQW and experimental setup for time-resolved transport measurements of pulsed EMPs [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 factors net (= e  h) = 2 and 0 for propagation lengths L = 5 and 20 m, respectively. Pulsed EMPs appear only for one field direction at net = 2, indicating the chiral edge transport without counter-propagating modes, while pulsed EMPs measured at net = 0 show the symmetry for the positive and negative B, indicating the helical edge transport. Figure 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 between counter-propagating channels [6]. Our measurement scheme will pave the way for investigations of dynamical properties of topological edge states.

The quantum nature of a physical system often emerges from its fundamental building blocks and demands a profound understanding to harvest its advantages for quantum devices. In this talk, I will introduce single atoms and molecules as building blocks for highly controlled quantum experiments. Utilizing a scanning tunneling microscope (STM) in combination with electron spin resonance (ESR) establishes an atomic “workbench” for 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 the nanoscale. In addition, the high energy resolution also grants access to the hyperfine interaction between the electron and nuclear spin of different atomic species. Lastly, by employing pulsed ESR schemes, a coherent manipulation of the surface spin becomes possible, for instance in Rabi and Hahn echo schemes. This opens up a path towards quantum information processing and quantum sensing using atomic building blocks, including atoms and molecules.

Bose–Einstein condensation is a fundamental physical phenomenon demonstrating fascinating and application-important properties such as spontaneous coherence, superconductivity, and superfluidity. This phenomenon can be achieved either by a decrease in the temperature of ultracold atomic gases and cryogenic liquids or by an increase in the density of particles (quasiparticles). Spin wave quanta—magnons—are bosons, and thus they can form a Bose–Einstein condensate (BEC), which can be created by an external injection of these quasiparticles even at room temperature. In this talk, I will present the main ways of forming and observing magnon BEC in solid-state magnetic samples, outline its place in the 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 touch upon the issue of temporal and spatial coherence of the magnon BEC.

Chirality, which arises from the breaking of mirror symmetries combined with any spatial rotations, 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 states or mesoscopic spin textures. Despite being mathematically well-defined, the term "chirality" has been extensively used, often in confusing ways, in recent years. In steady states, chirality (C) does not change with time-reversal operation, while chirality prime (C) denotes the breaking of time-reversal symmetry in addition to broken all mirror symmetries, combined with any spatial rotations. Various examples of magnetic chirality and chirality prime and their emergent phenomena, such as self-inductance, directional nonreciprocity in magnetic fields, current-induced magnetization, chirality-selective spin-polarized current, Schwinger scattering, magneto-optical Kerr effect, linear magnetoelectricity, and chiral tunneling will be discussed. Many of these phenomena can be understood with one hypothesis on “kinetomagnetism in chiral systems” that I will present. Some of these exotic phenomena have been recently observed, while many others require experimental confirmation in the future.

Reciprocity is a hallmark of thermal equilibrium, but ubiquitously broken in far-from-equilibrium systems on various scales. I will give some insights into how nonreciprocal interactions can fundamentally affect the phases and fluctuations of classical many-body systems. Using a two-dimensional XY model, where spins interact only with neighbours within their 'vision cones' inspired by the effective interactions between animals, we show how nonreciprocity can lead to true long-range order and directional propagation of defects [1]. In binary fluids, nonreciprocal coupling between fluid components can cause the emergence of travelling waves through PT symmetry-breaking phase transitions, as observed for mixtures of catalytic particles or membrane protein dynamics. Using a hydrodynamic model, we find that fluctuations not only inflate, as in equilibrium criticality, but also develop an asymptotically increasing time-reversal asymmetry [2-4] and associated surging entropy production. We can trace the formation of dissipative patterns and the emergence of irreversible fluctuations to the same origin, namely a mode-coupling mechanism near critical exceptional points. [1] Loos, Klapp, Martynec, Long-Range Order and Directional Defect Propagation in the Nonreciprocal XY Model with Vision Cone Interactions, Phys. Rev. Lett. 130, 198301 (2023). [2] Suchanek, Kroy, Loos, Irreversible mesoscale fluctuations herald the emergence of dynamical phases, Phys. Rev. Lett. in press (2023). [3] Suchanek, Kroy, Loos, Time-reversal and parity-time symmetry breaking in non-Hermitian field theories, Phys. Rev. E in press (2023). [4] Suchanek, Kroy, Loos, Entropy production in the nonreciprocal Cahn-Hilliard model, Phys. Rev. E in press (2023).

Controlling ultrafast spin dynamics directly 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 charge and spin transport as well as interactions with spatially extended quasiparticles such as phonons and magnons. These processes generally lead to a nanoscale spatial rearrangement of magnetization, calling for experimental techniques that can directly access the ultrafast evolution of spatially inhomogeneous spin profiles and detect the transfer or accumulation of spins at interfaces. Such observables providing nanometer spatial and femto- to picosecond temporal resolution are particularly required for investigations of complex heterostructures and antiferromagnets, where competing interactions result in a variety of complex spin structures already in equilibrium. In this talk, I will focus on time-resolved resonant soft-X-ray scattering (RSXS) as a unique technique for probing magnetic order in time and space with element-selectivity as well as in buried layers. RSXS combines large spectroscopic and magnetic contrast in the soft-X-ray range with access to reciprocal space in addition to nanometer depth- and lateral spatial resolution. Based on the in-house development of two laser-driven soft-X-ray sources at Max Born Institute, we have recently demonstrated the feasibility of time-resolved RSXS in laboratory experiments as a true alternative to large-scale facilities. I will discuss our recent RSXS results on the dynamics of artificial antiferromagnets, ferromagnetic domains and magnons, as well as all-optical switching in ferrimagnetic alloys.

Designing materials with tailored physical and chemical properties requires a quantitative understanding of interacting quantum systems. In order to provide predictability, a promising route is to create botom-up platforms, where the electronic properties of individual and interacting atoms can be emulated in a tuneable manner. Here, I will present a solid state quantum simulator based solely on paterned Cs atoms on the surface of semiconducting InSb(110), a system characterized 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 realize artificial molecular structures with different orbital symmetries which are subsequently probed. Moreover, I will illustrate newer examples of structures which are described by correlated electron models, as well as discuss new perspectives in magnetic fields and the role of the Quantum Hall Effect.

In this talk, I'll discuss current developments towards the phase structure of the strong interaction with functional methods. With the recent detection of gravitational 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 renormalization group perspective.

Abstract: Two-dimensional (2D) materials such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have recently received lots of attention due to their unique material properties and numerous potential applications. The 2D atomic structure can also facilitate distinct defect formation mechanisms and offer new possibilities for defect engineering. In my talk, I will present the results from layered molybdenum dichalcogenides (MoS2, WSe2, etc.), where vacancy, substitutional, 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 electron microscopy and scanning tunneling microscopy. First-principles calculations are used to provide microscopic insight into the energetics and kinetics of these processes. The gained understanding together with the computationally predicted defect properties can be used to guide future efforts in tailoring the 2D material properties via defect engineering.

Chemical doping is a powerful method of controlling the physical properties of magnetic materials. Unlike the uniform changes brought about by pressure-induced parameter tuning, chemical doping introduces randomness into the system. This randomness can sometimes lead to unintended consequences, but it can also reveal unexpected, unique physics. The first part of the talk will detail how the dilution of magnetic ions can suppress the effective magnetic interactions, leading to a quantum disordered phase that differs from the traditional percolation disorder. The second part of the talk will focus on a field-induced phase transition that arises from the asymmetry of single-ion anisotropy between different ions, and therefore does not occur when the system consists of a single type of magnetic moment. Both cases demonstrate the extent to which we can tune the physical property by substituting magnetic ions, and the emergence of new physical phenomena rooted in the effects of randomness.

Inducing superconductivity in topological materials stimulates the formation of novel quantum states of matter. Besides the original prediction in 3D topological insulators, the notion of topological phases has been generalized to different dimensions and extended to the higher-order states. In the last few years, our research has demonstrated the possibility of realizing the topological superconductivity 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 bulk Dirac fermions, two-dimensional Fermi arcs [4], and one-dimensional hinge states [5] or non-Hermitian states [6]. These topological states have different characteristic length scales in electronic transport. We show that the superconducting proximity effect can also be a sensitive probe for distinguishing these states. [1] B. de Ronde, et al., Nanomaterials 10, 794 (2020). [2] Li, C. et al. Nat. Mater. 17, 875 (2018). [3] Wang, A. Q. et al. Phys. Rev. Lett. 121, 237701 (2018). [4] Li, C.-Z. et al. Nat. Communications. 11, 1150 (2020). [5] Li, C.-Z. et al. Phys. Rev. Lett. 124, 156601 (2020). [6] Wang, A.Q. et al. Submitted (2022)

Scanning probe microscopes have been very successful tools for studying individual atoms and molecules as well as complex surface structures. Systems which bear magnetic spin moments can be built with them on surfaces and stabilized in junctions. When such spins interact with each other or with the supporting electron baths, entanglement and correlated many-particle states can emerge, making them ideal prototypical quantum systems. In this talk I will show 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 also for the appearance of novel topological quantum phases [3, 4]. The tip as atomically sharp control element can be used to detail spin-spin correlations [5], to detect “dark” moments [6,7], and even to portray transitions between entirely different quantum phases [3]. 1. J. Martinez, et al., Comm. Mat. 3,57 (2022) 2. Y. Wang et al., Comm. Phys. 4, 103 (2021) 3. S, Arabi, et al., arXiv:2208.10377 (2022) 4. J. Martinez, et al., arXiv:2304.08142 (2023) 5. B. Verlhac et al., Science 366, 623 (2019) 6. M. Muenks et al., Nature Communications8, 14119 (2017) 7. M. Ternes et al. Phys Rev. Lett. 124, 167202 (2020)

The combination of electron spin resonance and scanning tunneling microscopy drastically widened the range of interactions that can be studied on the atomic scale [1]. Overcoming the energy resolution of typical scanning tunneling spectroscopy by 3 orders of magnitude, it has 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 feature that so far eluded experimentalists [5]. The second part of the talk will be devoted to the study of dynamics of atomic spins. We first demonstrate that a DC-pump-probe scheme can be used to study the free evolution of two coupled atomic spins: by controlling their level of entanglement, 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 electron and a nuclear spin mediated via the hyperfine interaction [8]. [1] Baumann et al., Science 350, 6259 (2015) [2] Choi et al., Nat. Nano 12, (2017) [3] Yang et al., Phys. Rev. Lett. 119, 227206 (2017) [4] Willke et al., Science 362 (2018) [5] Farinacci, et al., Nano Letters 22, 8470 (2022) [6] Veldman et al., Science 372, 6545 (2021) [7] Veldman et al., in preparation [8] Veldman et al., in preparation

In my talk, I will focus on two regimes of probing and driving spin dynamics that each allow for unique insights. First, I will introduce the concept of electron paramagnetic resonance realized inside a scanning tunneling microscope [1]. This novel approach allows for probing magnetic interactions at atomic scales with an energy resolution far below the thermal limit using gigahertz drives [2-4]. Second, I will turn to the field of terahertz spintronics [5]. I will highlight recent advances in the field of terahertz emission spectroscopy to reveal fundamental channels of femtosecond spin currents in magnetic thin film heterostructures including their applications in spintronic terahertz emittters [6,7]. [1] Baumann, Susanne, et al. Science 350.6259 (2015): 417-420. [2] Seifert, Tom S., et al. Science advances 6.40 (2020): eabc5511. [3] Seifert, Tom S., et al. Physical Review Research 2.1 (2020): 013032. [4] Kovarik, Stepan, et al. Nano Letters 22.10 (2022): 4176-4181. [5] Walowski, Jakob, and Markus Münzenberg. Journal of Applied Physics 120.14 (2016): 140901. [6] Seifert, Tom, et al. Nature photonics 10.7 (2016): 483-488. [7] Seifert, Tom S., et al. Applied Physics Letters 120.18 (2022): 180401

I will introduce a new type of symmetry-breaking order called "Incommensurate Kekule Spiral (IKS)" order, and argue that it is ubiquitous in the experimentally observed phase diagram of magic-angle graphene

As part of the "Meet The Alumni" lecture series, our former PhD student Timo Knispel gives insights into his recent start at his new employer d-fine. d-fine is a European consulting firm with a focus on quantitative topics and the development of future-proof technological solutions. The consulting approach of d-fine is based on many years of practical experience and around a team with a strong analytical and technological background. A combination that has proven its worth through tailored, efficient and sustainable implementations for more than two hundred clients from all economic sectors. Together with his colleague Thomas Bschorr, Timo will give us exciting insights into the project work and his start at d-fine as well as answer any questions you might have. Afterwards, Timo and Thomas will be available for further questions and in-depth discussions in the get together with drinks and snacks.

The rich phase diagrams of many transition metal oxides (TMOs) is the result of the intricate interplay between electrons, phonons, and magnons. This makes TMOs very susceptible to external parameters such as pressure, doping, magnetic field, and temperature which in turn can be used to finely tune their properties. The same susceptibility makes TMOs the ideal playground to design experiments where the interaction between tailored electromagnetic fields and matter can trigger the onset of new, sometimes exotic, physical properties. This aspect has been explored in time domain studies [1] and has led to the demonstration that ultrashort mid-IR light pulses can “force” the formation of quantum coherent states in matter, disclosing a new regime of physics where thermodynamic limits may be bridged and quantum effects can, in principle, appear at ambient temperatures. In this presentation, I will review our recent results in archetypal strongly correlated cuprate superconductors and demonstrate the 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 spectroscopy going beyond mean photon number observables [5-10] and show that the statistical features of light can provide richer information than standard linear and non-linear optical spectroscopies[11,12]. Finally, I will elaborate on the possibility offered by embedding complex materials into resonant optical cavities[13] and show how properties for the light-matter assembly different from those of its constituents can emerge. [1] Advances in physics 65, 58-238, 2016 [2] Science 331, 189-191 (2011) [3] Phys. Rev. Lett. 122, 067002 (2019) [4] Nature Physics 17, 368–373 (2021) [5] Phys. Rev. Lett. 119, 187403 (2017) [6] New J. Phys. 16 043004 (2014) [7] Nat. Comm. 6, 10249 (2015) [8] Nat. Comm. 13, 2667 (2022) [9] J. of Physics B 53, 145502 (2019) [10] Optics Letters 45, 3498 (2020) [11] Light: Science & Applications, 11, 44 (2022) [12] PNAS March 19, 116 (12) 5383-5386 (2019) [13] Rev. Sci. Inst. 93, 033102 (2022)

Main subject I will talk is ultrafast photocurrent generation and transport in high-quality graphene photodetector. The current generated by femtosecond laser pulse through photo-thermoelectric effect was detected in time domain with the resolution of faster than 1 picosecond by on-chip THz spectroscopy. We could comprehensively understand how incident light is converted to electrical current in graphene, which is crucial information for developing ultrafast graphene optoelectronics. I will also briefly talk about two other subjects in our group: Growth and transport in InAs/InGaSb two-dimensional topological insulator, and Coherent transport of fractional quasiparticle at fractional-integer quantum Hall junction. Reference: Yoshioka, NK et al., Nature Photonics (accepted), arXiv:2203.05752

https://uni-koeln.zoom.us/j/95031559873?pwd=d1gwRlhlaXYrK085S0lqU295ZlNkZz09 Meeting ID: 950 3155 9873 Password: 745603

In 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 effect can be observed even in the presence of magnetic impurities. Our experiments on (Hg,Mn)Te quantum wells with an inverted band structure reveal that the quantum spin Hall quantization is observed only at low temperatures (T < 400 mK), where Kondo screening of the magnetic impurities suppresses backscattering. The second part of the talk will focus on understanding the fluctuations in the quantized spin Hall conductance that has been observed in almost all realizations of the quantum spin Hall effect - including the first paper in 2007. To examine these fluctuations, we fabricate high-quality quantum spin Hall microstructures using a carefully optimized wet-etching process. We perform temperature and gate-dependent measurements in the regime of quantized conductance. The fluctuations in conductance show a characteristic temperature dependence that is related to Kondo interactions of helical edge channels with small puddles. These charge puddles - acting as Kondo correlated quantum dots due to small dimensions - arise due to fluctuations in the potential landscape in narrow gap semiconductors.

Topological edge states are protected states that form between two insulators with different topological properties. The dimensionality of the interface, as well as the type of insulators 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 dimensional, the topological states are Majorana zero-modes localized in the core of a vortex or bound to the end of a nanowire. They are anyons with non-Abelian braiding statistics, but when they are immobile, one cannot demonstrate this by exchanging them in real space and therefore indirect methods are usually needed. I will talk about how to use the chiral motion along the boundary of the superconductor to braid a mobile vortex, either with 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 process, 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 electrical signature of the non-Abelian exchange of Majorana zero-modes. Next, I will discuss how one can deconfine the Majorana zero-modes localized in the vortex cores by applying a spatially oscillating pair potential. In the deconfined phase at zero chemical potential the Majorana fermions form a dispersionless Landau level, protected by chiral symmetry against broadening due to vortex scattering. The Majorana Landau level also has a signature that distinguishes it from a regular Landau level: the coherent superposition of electrons and holes in the Majorana Landau causes a local density of states oscillation with a wave vector set by the Cooper pair momentum as well as the inverse coherence length. This striped density of states pattern also provides a means to measure the chirality of the Majorana fermions. I will conclude by discussing the quantal dynamics in the deconfined phase.

Zoom PW: 573506

Motivated 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 geometry realized in the spin S=1/2 system KCu6AlBiO4(SO4)5Cl [1], we discuss the possible microscopic origins of this state, and more broadly highlight this novel geometry as an ideal playground for stabilizing a diverse array of exotic quantum phases [2]. Employing state-of-the-art quantum many-body numerical techniques such as variational Monte Carlo (VMC) with versatile Gutzwiller-projected Jastrow wave functions, unconstrained multi-variable variational Monte Carlo (mVMC), and pseudo-fermion/Majorana functional renormalization group (PF/PM-FRG) methods, we establish the presence of a quantum paramagnetic ground state and investigate its nature, by classifying symmetric and chiral quantum spin liquids, and inspecting their instabilities towards competing valence-bond-crystal (VBC) orders. Our analysis 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 neighbor Heisenberg and/or Dzyaloshinkii-Moriya interactions play a crucial role towards explaining the experimental observations. References: [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, Shigetoshi Sota, Tomohiko Kuwai, Akihiro Koda, Hirotaka Okabe, Hua Lee, Shinichi Itoh, Takafumi Hawai, Takatsugu Masuda, Hajime Sagayama, Akira Matsuo, Koichi Kindo, Seiko Ohira-Kawamura & Kenji Nakajima [2] Phys. Rev. B 104, L220408 (2021), Pinwheel valence-bond-crystal ground state of the spin-1/2 Heisenberg antiferromagnet on the shuriken lattice, Nikita Astrakhantsev, Francesco Ferrari, Nils Niggemann, Tobias Müller, Aishwarya Chauhan, Augustine Kshetrimayum, Pratyay Ghosh, Nicolas Regnault, Ronny Thomale, Johannes Reuther, Titus Neupert, Yasir Iqbal Zoom-Link: https://uni-koeln.zoom.us/j/9524918125?pwd=dWN1RW1mMUpDRWdyMWwzWGtURWVIZz09

Rude colleagues say that solid states chemists follow for their reactions only the principle "shake and bake". Of course, high temperatures help to overcome activation barriers. However, we all know that this is not enough. A multitude of other parameters play an essential role for efficient solid-state reactions. I will present you how we use extremely strong oxidizing agents, such as sulfuric acid or its anhydride SO3, to synthesize silicate analogous materials such as borosulfates. In these, the charge compensating heteropolyanionic subunits are composed of vertex-linked (SO4)- and (BO4)-tetrahedra. In contrast to the immense structural diversity of silicates, the number of borosulfates is yet very limited and the extent of their properties is still unknown. Furthermore, allow the extreme reaction conditions for the stabilization of cationic species with unprecedented properties, like 1D metal behavior in [Au2Cl4](B(S2O7)2](SO3). In an effort to expand the knowledge on oxoanionic networks even further, we have also gone and tested our systems on rhenates, manganates 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 use of a Multianvil press (pmax = 26 GPa) allows for investigations of the afore mentioned reactions in a regime that is still almost unexplored. Using a wide array of analytical methods such as optical spectroscopy, magnetochemistry and photoelectron spectroscopy we try to investigate all samples as detailed as possible and corroborate our findings by quantum chemical calculations. Herein, I will therefore present an overview of our investigations on borosulfates and oxometallates, the effects of the counter 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.

When cold 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. To this end, we discuss the second order electrical conductivity in materials lacking inversion and time-reversal symmetry, which exhibits a mixed axial-gravitational anomaly. We can explain this surprising result in terms of dynamical 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 proposition that quasiparticles behave essentially like quantum cats has powerful implications for all types of quantum transport and may allow to probe synthetic gravitational fields in a bulk condensed matter setting.

The talk will be given in the HS III. You can all attend in presence. As an alternative you can also participate via zoom: https://uni-koeln.zoom.us/j/4225866762

By quenching electron kinetic energy, one can reveal correlated phases in previously weakly correlated materials, such as in twisted bilayer graphene. Disorder typically obscures correlated phases; for instance, it might just lead to localized electron wave functions. Incommensurability-driven phases provide a platform to observe both phase transitions typically associated with disorder (Anderson-like transitions) as well as flat bands and correlated phases. In this talk, I will discuss our work to understand this, particularly within the context of topological band structures. In both two-dimensional semimetals and topological insulators, we see that disorder induces a delocalization eigenstate transition in momentum space. The wave functions exhibit multifractality in this phase, and in the case of two-dimensional semimetals, it occurs at the "magic-angle" as for twisted bilayer graphene. Concomitantly, there is a precise sense in which the "bands" become increasingly flat at these transitions. In the case of the topological insulator, the Berry curvature also becomes more uniform as the wave functions near criticality: clearing the way for potential fractional Chern insulators in these systems.

The search for novel topological phases of matter is on-going, with many recent efforts focusing on study of non-electronic and other analogues of the foundational electronic topological phases of matter of the ten-fold way. We return to study of the original electronic topological phases by introducing the topological skyrmion phases of matter. These are symmetry-protected topological phases defined by skyrmions, or topological spin textures, in momentum-space for Bloch Hamiltonians with more than two bands. These topological phases are protected by a generalized particle-hole symmetry C' present in some centrosymmetric superconductors. We first present a method for constructing various four-band toy models for skyrmion phases 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 correspondence, trapping exotic localized "cross" zero-mode states at defects in the bulk and on the boundary of the system. Finally, we consider a more complex tight-binding model for spin-triplet superconductivity in transition metal oxides, which can realize topological skyrmion phases of matter 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 superconducting gap. The second kind, however, occurs when spin is not conserved and without the closing of direct energy gaps. As this type-II topological phase transition occurs without breaking the protecting symmetry of the topological 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.

2D materials are an exciting host to engineer atomic quantum systems by chemical design rules. In this colloquium, I will give an overview on our efforts to design and probe defect systems in monolayer transition metal dichalcogenides (TMDs) by means of chemical doping, Helium ion beam bombardment and atomic manipulation. By means of low-temperature scanning probe microscopy, we directly resolve the discrete electronic spectrum of single dopants in a charge neutral or ionized state and map out their associated defect orbitals [1-5]. Different types of defects reveal the interplay between chemical impurity states [1-3], multi-valley hydrogenic bound states [4,6], and electron-phonon coupling [1,7] at reduced dimensions. In particular, we will discuss the atomically controlled generation of carbon radical ions (CRIs) by hydrogen depassivation [7] and electrically driven photon emission from individual defects [8]. References: [1] B. Schuler et al., Phys. Rev. Lett. 123, 076801 (2019) [2] S. Barja et al., Nat. Commun. 10, 3382 (2019) [3] B. Schuler et al., ACS Nano 13, 10520 (2019) [4] M. Aghajanian et al., Phys. Rev. B 101, 081201(R) (2020) [5] E. Mitterreiter et al., Nano Lett. 20, 4437 (2020) [6] K. Cochrane et al., 2D Mater. 7, 031003 (2020) [7] K. Cochrane et al., arxiv: 2008.12196 [8] B. Schuler et al., Sci. Adv. 6, eabb5988 (2020)

Everyone 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 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? The event will be with Sherard Robbins, a development consultant, author, and CEO of Visceral Change. He will address the topic of “Racism and Physics” and present us some background information and numbers about it. There will also be space for discussions: How does it affect our work environment and what can we do about it?

The discovery of the quantum Hall effect (QHE) in 1980 marked a turning point in condensed matter physics. The measurement of the Hall conductivity of a two-dimensional electron system showed that electrical conduction can be precisely defined only in terms of fundamental constants. But, what happens to the QHE in three dimensions? At a first glance, the answer seems straightforward and sobering: The quantization of the Hall conductivity gets destroyed. This has led to the general conjecture that quantum-Hall physics is exclusive to two dimensions. However, we show that the Hall effect of genuine three-dimensional (3D) metals is actually deeply related to the QHE and can be understood from quantum Hall physics. We propose that - as a general rule - plateau-like features in the Hall conductivity of 3D electron systems are precisely given by the conductance quantum, scaled by the Fermi wave vector of the electrons along the magnetic field direction.

Only the Speed of Light limits You? - Discrimination and its impact on the academic field of physics Everyone 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 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? The talk will be given by Prof. Tomas Brage. He has a very successful career in physics and is currently a physics professor at the Division of Mathematical Physics of the Department of Physics at Lund University. Besides all his publications in this field, he also does research concerning “Gender and Physics”. He will inform us about numbers, bias, and stereotypes regarding gender, and their influence on physics. After his talk, we can also ask questions and have a discussion about the topic.

Textbook quantum criticality refers to phenomena which originate in states which scale-invariant in both space and time, occurring at continuous zero-temperature phase transitions. Various experimental 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 number of strongly correlated electron compounds. After an extended introduction, I will discuss one of the simplest cases, namely the bandwidth-tuned Mott transition in a single-band setting. Recent transport studies, both theoretical and experimental, near this transition have uncovered - surprisingly - apparent quantum critical scaling of the electrical resistivity at elevated temperatures, despite the fact that the actual low-temperature phase transition is of first order. This raises the question whether there is a hidden Mott quantum critical point. I will argue that the dynamical mean-field theory of the Hubbard model admits, in the low-temperature limit, scale-invariant (i.e. power-law) solutions which had been overlooked before. It is this incoherent power-law regime, corresponding to approximate local quantum criticality, which is continuously connected to and responsible for the apparent quantum critical scaling above the classical 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.

We 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 Kitaev interaction. In this talk, I will discuss the key parameters which have very large impacts on the exchange constant, and the crystal field effect is emphasized as an efficient phase control method driving the magnetically ordered cobaltates into the spin liquid state. In good agreement with the experimental data, our theoretical predications verify the great potential of realizing the Kitaev spin liquid in 3d transition metal compounds.

Magnetooptical observations of thin magnetic cylinders: theory and experiment (K. Richter1*, O. Vahovsky3, A. Thiaville2, R. Varga3 and J. McCord1) 1 University of Kiel, Kaiserstraße 2, 24143 Kiel, Germany 2University Paris Sud XI, 914 00, Orsay, France 3 University of Pavol Jozef Safarik, 040 01 Kosice, Slovakia *e-mail: kornel.richter@gmail.com Thin cylindrical 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 allow for well controllable deposition of magnetic structures with complex geometries [2], a reliable determination of the surface magnetization usually meets several obstacles in case of samples with curved surface. Here, we provide a full analytical calculation of the Magneto-Optical Kerr Effect (MOKE) contrast for cylinders with reduced diameter. It is shown that the cylindrical shape of sample surface gives rise to a spatial distribution of the planes of incidence that are all tilted each to other [3]. Such mutual orientation of incident rays in combination with circumferential dependence of a local angle of incidence gives rise to apparent magneto-optical contrasts [3,4] that cannot be interpreted well without considering the curved surface of a cylinder. Theoretical calculations [3] are tested experimentally on amorphous glass-coated microwires with well-defined cylindrical shape. Finally, we use our framework to study shape of the domain wall in FeSiB microwire characterized by very fast domain wall propagation. The setup is optimized for characteristic optical and magneto-optical features of thin magnetic cylinders. Time-resolved measurements of a magnetic contrast along the circumference of wire are used to reconstruct a three-dimensional image of a surface domain wall shape. Our measurements confirm a tilted shape of domain wall [5, 6], in agreement with previous observations of microwires with positive magnetostriction alloy. The experimental technique represents a new way for direct visualizations of a surface magnetism of thin cylinders in optical range. [1] M. Yan, et al., Phys. Rev. Lett. 104, 057201 (2010). [2] A. Fernández-Pacheco, et al., Sci. Rep. 3, 1492 (2013). [3] K. Richter, et al., Phys. Rev.B 96, 064421 (2017). [4] K. Richter, et al., IEEE Trans. Magn. 50, 2501404 (2014). [5] K. Richter, et al., J. Appl. Phys.127, 193305 (2020). [6] O. Vahovsky, et al., J. Magn. Magn. Mater. 483, 266 (2019).

In this talk, I show that when the time reversal symmetry is broken in a multi-component superconducting condensate without inversion symmetry the resulting Bogoliubov quasiparticles generically exhibit mini Bogoliubov-Fermi (BF) surfaces, for small superconducting order parameter. The absence of inversion symmetry makes the BF surfaces stable with respect to weak perturbations. With sufficient increase of the order parameter, however, the Bogoliubov Fermi surface may disappear through a Lifshitz transition, and the spectrum this way become fully gapped [1]. Furthermore, an explicit example for the emergence of BF surfaces in non-centrosymmetric multicomponent superconductors is given by the discussion of d-wave superconductivity in Rarita- Schwinger-Weyl semimetals [2]. The three-dimensional Rarita-Schwinger-Weyl semimetals host pseudospin-3/2 fermions at a fourfold linear band crossing point and the spin-3/2 nature of the fermions allows for the formation of spin-2 Cooper pairs. [1] Julia M. Link and Igor F. Herbut, Arxiv:2006.10899 [2] Julia M. Link and Igor F. Herbut, Phys. Rev. B 101, 184503 (2020)

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Domain walls naturally arise whenever a symmetry is spontaneously broken. They interconnect regions with different realizations of the broken symmetry, promoting structure formation from cosmological length scales to the atomic level. In my talk, I will present 1D spin textures and domain walls with intriguing physical properties, which emerge in spin-spiral multiferroics and chiral magnets and which hold great promise for nanoelectronics and spintronics applications. In particular, I will discuss that a wide variety of new domain walls occurs in the presence of spatially modulated domain states. In contrast to domain walls in conventional ferroics, such domain walls exhibit a well-defined inner structure, which—analogous to cholesteric liquid crystals—consists of topological disclination and dislocation defects. Similar to the magnetic skyrmions, the domain walls can carry a finite topological charge, permitting an efficient coupling to spin currents and contributions to a topological Hall effect. Our studies establish domain walls in chiral magnets as functional nano-objects with non-trivial topology, opening the door to innovative device concepts in information and communication nanotechnology.

The idea to probe, change and control functional materials properties 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 excitation of solid-state systems creates out-of-equilibrium hot electrons that cool down by transferring their energy to other degrees of freedom and ultimately to lattice vibrations of the solid. The understanding of this strongly non-equilibrium dynamics in solids is still very limited, in spite of its emerging importance from a fundamental and applied science viewpoint. The three-temperature model, commonly used for magnetic metals, assumes that the electronic, spin and lattice subsystems are each in separate equilibrium at all times and reach global equilibrium by exchanging heat. The equilibrium concept of “temperature” is so powerful that is is often ignored that it takes time before non-equilibrium dynamics can be described by it. In this lecture I will give an overview how modern ultrafast spectroscopy and scattering techniques allow us to determine in a momentum-resolved way the electron, spin and lattice excitations in the laser-heated state. These results demonstrate that we need to view non-equilibrium dynamics by its associated flow of energy between the various degrees of freedom.

Layered 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-insulator transition of 1T-TaS2, which is currently discussed intensively in relation to a somewhat enigmatic hidden state. Our experiments reveal the microscopic nature of this hidden state and show that the non-thermal metal-insulator transition of 1T-TaS2 corresponds to a transition between two long-range ordered electronic crystals. The second example is alpha-RuCl3 - a candidate material for Kitaev magnetism and the corresponding spin-liquid ground state. However, at ambient pressure and low temperatures this material does not exhibit a spin-liquid phase, but antiferromagnetic long-range order. We discovered a high-symmetry rhombohedral phase of alpha-RuCl3 at non-ambient pressure, which may indeed support a Kitaev spin-liquid ground state.

Mott insulators with strong spin orbit coupling have become a playground to search for exotic quantum phases, spin liquids and emergent Majorana fermions. In this context I will present some of our results for the dynamics of Kitaev (-Heisenberg) models, focusing on magnetic thermal transport, but also on phonon relaxation. Fractionalization of magnetic moments into mobile Majorana matter and a static Z2 gauge field in the pure Kitaev limit, lead to characteristic spectral continua. These continua are strongly affected by thermal flux excitations which act as an emergent disorder. I will discuss the consequences, using results from several approaches comprising a mean-field theory, complete summation over all gauge sectors, exact diagonalization, and quantum typicality calculations.

Twisted bilayer graphene near the magic angle exhibits remarkably rich electron correlation physics, displaying insulating, magnetic, and superconducting phases. Here, using measurements of the local electronic compressibility, we reveal that these phases originate from a high-energy state with an unusual sequence of band populations. As carriers are added to the system, rather than filling all the four spin and valley flavors equally, we find that the population 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 after each integer filling. Measurement of the in-plane magnetic field dependence of the chemical potential near filling factor one reveals a large spontaneous magnetization, further substantiating this picture of a cascade of symmetry breakings. The sequence of phase transitions and Dirac revivals 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 Dirac-like electronic character, is a key player in the physics of magic angle graphene, forming the parent state out of which the more fragile superconducting and correlated insulating ground states emerge.

Topological defects in ferroic materials are low-dimensional systems that may enable promising applications. We study domain wall and vortex-localized vibrations in proper and improper ferroelectrics and their role in dielectric loss, conductivity, acoustic phonon scattering and elastic anomalies. The lowest energy vibrations correspond to sliding modes of these topological defects, are polar and are found at microwave frequencies. An opposite situation of hard localized vibrations is found in cluster compounds, where transition-metal ions form clusters. In these cases vibrations are hard. Particularly, in IrTe2 they give rise to a complex phase diagram with a large number of phases. A simplified model is proposed, parameterized from first principles, and solved for the phase diagram. The influence of spin-orbit coupling, pressure and doping is studied.

Large Li2FeSiO4 single crystals featuring the high temperature Pmnb-phase (γ-Li2FeSiO4) were grown for the first time by the optical floating zone method under high Ar-pressure and the single crystal structure was solved. The system exhibits tetrahedrally coordinated Fe2+ ions in a high-spin (i.e.,

The continuum of excitations in RuCl3 have been studied to high temperature—less-well studied are the effects of high magnetic fields. We measured the magnetotropic coefficient—second derivative of the free energy with respect to field-angle—up to 65 tesla. We find a scale-invariant response with applied temperature and magnetic field, contrasting with the large exchange energy apparent in the angle-dependence. This paradox hints at a strongly-correlated magnetic state in the high-field limit of RuCl3.

Divalent silver (Ag2+) is a homologue of its well-known copper sibling, and it comes with Jahn-Teller effect, paramagnetism, and strong influence of electronic correlation on chemical bonding. I will briefly outline our two-decade lasting efforts 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.

Metal-insulator transitions (MIT), driven entirely by strong electron-electron interactions and/or disorder, continue to underpin many novel phenomena in quantum matter. We study disorder-driven MITs in the "strong disorder" category using a novel semi-analytic cluster adaptation of dynamical mean-field theory (CDMFT) for the simplest, binary-alloy disorder (also known as spinless Falicov-Kimball, or "simplified" Hubbard) model. Such an (oversimplified) model is a caricature for a disordered system where mobile electrons interact strongly with positionally random, localized charges. We uncover anomalous quantum criticality in the full dc conductivity tensor across the MIT which, remarkably, is reminiscent of "Mott quantum criticality" that obtains at the low-T end-point of the line of first-order transitions in the Hubbard model (experimentally best studied for 2D organics). Interestingly, such novel criticality is also found to underpin thermal transport (heat conductivity, Seebeck co-efficient, Lorenz number) across the MIT. Moreover, we also provide an explicit realization of the famed "universal dielectric response" (UDR) of Jonscher in optical responses across the MIT. The associated stretched-exponential relaxation in the long-time dielectric response strongly suggests anomalous diffusion, an emergent "electron glassy" dynamics and weak ergodicity breaking. These findings suggest that a truly novel type of carrier localization obtains in the "strong disorder" limit.

Raman spectroscopy is a widely used, powerful, and nondestructive tool for studying the vibrational properties of bulk and low-dimensional materials. Raman spectra can be simulated using first-principles methods, but due to the high computational cost calculations are usually limited only to fairly small unit cells, which makes it difficult to carry out simulations for alloys and defects. We recently developed an efficient method for simulating Raman spectra at nearly 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 stacking 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 directly compared to the experimental one, but also gain atomistic insight on the modifications of the vibrational spectra that are responsible for the observed changes.

The 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 correlated quantum materials have forced us to confront the existence of quantum matter for which the concept of electronic quasiparticles does not apply. Inspired by the rich phenomenology of the parent states of numerous high-temperature superconductors, I will describe some recent progress in our understanding of metallic states that do not admit a quasiparticle description but that nonetheless have a sharply-defined Fermi surface. I will also present some experimental results on unconventional transport properties of magic-angle twisted bilayer graphene and comment on their possible connections with the rest of my talk.

I will discuss two examples of the interplay of distortion and Mott insulator physics. In certain ABO3 perovskite materials including BiNiO3, it is found that the Mott transition is accompanied by a symmetry change and a colossal volume collapse upon entering the metallic phase with pressure or temperature. We show that may be viewed as a new type of “hybridization-switching” Mott transition. In a different setting, the spinel compound NiRh2O4 has been argued to possibly form a S=1 spin liquid state on the diamond lattice. However, we show that the physics in the presence 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 experiments, and predict “dark states” which are invisible to neutrons but may be possibly probed using RIXS experiments.

Over the years, the coherence of superconducting qubits has been significantly improved, so that at present they reach quality factors in excess of 10^6. Further improvements in coherence would help reducing the overhead necessary for implementing quantum error 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, quasiparticles have been recognized as a possible source of decoherence. I will briefly review the history of quasiparticle effects in qubits, both in theory and experiments. Interestingly, the qubit sensitivity to quasiparticles makes it possible to measure their dynamics. I will discuss ways to control this dynamics, so that the detrimental effects of quasiparticles can be minimized. Finally, I will discuss some recent findings hinting at possible origins of quasiparticles.

We study a paradigmatic driven-dissipative model, dubbed interpolating Dicke-Tavis-Cummings (IDTC), where a collection of two-level systems interact with both quadratures of a quantum cavity mode. The closed system exhibits rich physics, including discrete and continuous 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 continuous symmetry transitions, leading to novel regions of coexistence of phases and the appearance of two tricritical points. Using Keldysh formalism, we explore the dynamical response of this fascinating new phase diagram. Surprisingly, we find that an additional intriguing dynamical phase transition manifests in the frequency response of the system. At the transition, low-energy particle- and hole-like processes change roles due to a soft mode gap closing.

It has been the believe that generic quantum many-body systems necessarily approach thermal equilibrium after a long time evolution. As a consequence any quantum information encoded in the initial state is lost in the course of the dynamics and the late time dynamics is described by simple hydrodynamics. However, recently exceptions have been found to the rule. For example, disordered and interacting many-body systems realize the non-ergodic, many-body localized phase. In this talk, we show how constraints can significantly alter the quantum dynamics of interacting many-body system. To this end, we will first discuss the far-from-equilibrium dynamics of a two-dimensional quantum dimer model, which leads to a rich dynamical phase diagram with some almost ergodicity breaking regimes [1]. As a second example we show that the combination of charge and dipole conservation, characteristic of fractonic quantum matter, leads to an extensive fragmentation of the Hilbert space, which in turn can lead to a breakdown of thermalization [2]. [1] Dynamical Phase Transitions in the Quantum Dimer Model on a Square Lattice. Johannes Feldmeier, Frank Pollmann, Michael Knap [arXiv:1901.07597] [2] Ergodicity-breaking arising from Hilbert space fragmentation in dipole-conserving Hamiltonians. Pablo Sala, Tibor Rakovszky, Ruben Verresen, Michael Knap, Frank Pollmann [arXiv:1904.04266]

Two-dimensional materials have a large fraction of their atoms, if not all of them, directly "feeling the world" around them. Their properties 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 unconventional properties are found depending on the nature of the two-dimensional material and of the contacted material. In the past few years, we have addressed single-layer graphene and molybdenum disulphide, which have very different electronic properties -- semimetallic and semiconducting respectively. We prepare these materials by epitaxial growth on metallic substrates, and in between the two materials, we intercalate very thin layers of transition or alkali metals. Our first concern is to understand (and hopefully, control) how the process of intercalation occurs, whenever possible with the help of real-time observations. Is it reversible? Is the space between the two-dimensional material and the substrate, where the intercalated material grows, sealed? And how confined growth can occur in such a sealed two-dimensional region? Our interest is then on the effects of the interaction between the two-dimensional material and the intercalated layer. We have discovered that the magnetic properties of cobalt intercalated underneath graphene are very unusual, with a strong tendency to perpendicular-to-the-surface magnetisation that is appealing for spintronics applications. Using cesium intercalation, we observe electron doping in molybdenum disulphide together with a significant electronic disorder.

Topological insulators (TIs) are fascinating materials due to their protected edge-states and their possibilities for spintronic applications. Spin-orbit coupling and exchange effects at interfaces give us a possibility to tailor the properties of two-dimensional (2D) topological materials. While magnetism is usually detrimental to the topological properties, we show that exchange interactions offer the possibility to realize a variety of new phases depending on the structural symmetry and the direction of the magnetic (exchange) field. Using density functional theory we explore the quantum spin Hall to quantum anomalous Hall state transition in TIs and topological crystalline insulators. We propose the existence of new states like mixed nodal-line semimetals or mixed Weyl semimetals in the phase diagram of these 2D materials, showing interesting magnetoelectric coupling phenomena as function of the magnetization direction.

Atomically thin two-dimensional layered materials receive great interest because of their unique properties. Particularly, monolayers of semiconducting transition metal dichalcogenides (SC-TMDs), such as MoS2, excel due to their strong light-matter interaction that is dominated by exciton phenomena [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 their properties on demand and on-chip e.g. by defects, dielectric environment or doping [4-8]. We apply inelastic light scattering together with emission and absorption to study the manifold coupling mechanism in van der Waal hetero- and hybrid structures. We introduce the influence of the dielectric environment, the charge carrier density as well as defects on the optical properties of these atomically thin materials. Moreover, interlayer 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-valley excitons [3, 9]. All-2D stark effect devices allows for the manipulation of the excitons by external electric fields [9]. Our results provide fundamental insights into long-lived interlayer states in van der Waals heterostructures with possible bosonic many-body interactions. We acknowledge support by the Deutsche Forschungsgemeinschaft (DFG) via excellence cluster Nanosystems Initiative Munich (NIM), e-conversion as well as DFG project WU 637/4-1. [1] U. Wurstbauer, et al. J. Phys. D: Appl. Phys. 50, 173001 (2017). [2] S. Funke, et al., J. Phys.: Condens. Matter 28, 385301 (2016). [3] B. Miller, et al., Nano Lett. 17(9), 5229–5237 (2017). [4] E. Parzinger, et al., Nature 2D material 1, 40 (2017). [5] S. Diefenbach, et al., J. Phys. Chem. C, 122 (17), 9663–9670 (2018). [6] J. Klein, et al., 2D Materials 5, 011007 (2018), J. Klein et al., arXiv:1901.01042 (2019). [7] B. Miller, et al., Appl. Phys. Lett. 106, 122103 (2015). [8] B. Miller et al. Nature Commun. 10, 807 (2019). [9] J. Kiemle et al. arXiv:1901.01042 (2019).

Soon 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 experimentally [Matano2016, Yonezawa2017, Tao2018]. The observed nematic SC state is in agreement with the proposed odd-parity two-component superconducting order parameter [Fu2010] and confirms the topological nature of the SC state. In the first part of this talk, I will present preliminary results recently obtained in the M-STM lab on the surface of CuxBi2Se3 crystals grown in-house. In particular, scanning tunneling spectroscopy performed at 400 mK shows that the three-fold symmetry of the underlying lattice is broken in the nematic SC state. Specifically, the SC density of states measured as a function of the azimuthal angle between the external magnetic field (|B|=0.5 T ) direction and the sample surface shows a characteristic two-fold symmetry. Taking advantage of the atomic resolution capabilities of the STM we can deduce the nematic axis with respect to the crystal lattice 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. In 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-fabrication and characterization of such devices with the STM on the atomic scale. 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) realization of ultra-clean surfaces. [Hor2010] Hor et al. Phys. Rev. Lett. 104, 057001 (2010) [Fu2010] Fu and Berg, Phys. Rev. Lett. 105, 097001 (2010) [Yonezawa2017] Yonezawa et al. Nat. Phys. 13, 123 (2017) [Tao2018] Tao et al. Phys. Rev. X 8, 041024 (2018)

Recently, magnon excitations have been explored as a potential route to realizing analogues of the topological band structures that have become ubiquitous in electronic systems. However, magnons are not necessarily stable quasi-particles, with spontaneous magnon decay a generic feature of anisotropic magnets or magnets with non-colinear order. In this talk I will discuss two aspects of this physics: First, I will discuss how sharp topological magnon bands can exist in spite of the presence allowed decays, as realized in a Kitaev model in a large magnetic field. Second, I will discuss an alternative scenario where decay in fact drives the appearance new kinds of intrinsically non-Hermitian topological physics. This can lead to new kinds of spectral features, such as exceptional points and bulk "arcs" connecting them. I will illustrate this physics through a concrete example: a honeycomb ferromagnet with Dzyaloshinskii-Moriya exchange, comparing interacting spin-wave calculations with an effective non-Hermitian model. Finally, I discuss more broadly how the zoo of known topological protected magnon band structures may serve as a nearly ideal platform for realizing non-Hermitian physics in solid-state systems.

Mesoscopic transport with ultracold atoms: particles, spin and heat" Laura Corman, ETH Zürich The quantum behavior of fermionic particles can be revealed when the structure through which they flow is smaller than their coherence length. For electrons, this principle is the foundation of the field mesoscopic physics which has emerged thanks to the tremendous progress in fabrication techniques. Another platform to study mesocopic transport has appeared in atomic physics using ultracold gases of fermionic lithium. Although they are orders of magnitude more massive, more dilute and colder than electron gases in materials, it is possible to access similar regimes in both systems. This was demonstrated by the measurement of quantized conductance for neutral matter at an atomic quantum point contact [1]. Ultracold atom transport experiments allow some advantages compared to their electronics counterpart. First, structures on the order of the Fermi wavelength can be optically imprinted, which is fast and flexible. Second, the interactions between particles can be varied in a broad range, from weakly interacting (attractive or repulsive) to the unitary limit which corresponds to saturated contact interactions. In this talk, I will present how we engineer such transport experiments with atoms as well as some recent results of transport through 1D systems. First, 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]. Second, we extended 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]. Last, we can also study the coupling between different transported quantities to understand the nature of a system's elementary excitations, as we did for the unitary Fermi gas by observing the breakdown of the Wiedemann-Franz law [4]. [1] Krinner et al., Nature 517, 64-67 (2015). [2] Lebrat et al., Physical Review X 8, 011053 (2018). [3] Lebrat et al., arXiv:1902.05516 (2019) [4] Husmann et al., PNAS 115, 8563-8568 (2018).

Arkady V. Krasheninnikov 1,2 1 Helmholtz Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Germany 2 Department of Applied Physics, Aalto University School of Science, Finland Following isolation of graphene, many other 2D systems, e.g., single sheets of transition metal dichalcogenides (TMDs) have been manufactured. All these materials contain defects and impurities, which may govern their electronic and optical properties. Moreover, defects can intentionally be introduced using beams of energetic particles – ions and electrons. All of these calls upon the studies on defects and mechanisms of their formation. In my talk, I will present the results [1] of our recent theoretical studies of defects (native and irradiation-induced) in inorganic 2D systems obtained in close collaboration with several experimental microscopy groups. I will further discuss defect- and impurity-mediated engineering of the electronic structure of TMDs. [1] https://users.aalto.fi/~ark/publist.html

Josephson parametric amplification is a tool of paramount importance in circuit quantum electrodynamics (circuit-QED), especially for the quantum-noise-limited single-shot readout of superconducting qubits. I will first present the development and characterization of a flux-pumped Josephson parametric amplifier (JPA) based on a lumped-element LC resonator, in which the inductance L is composed by a geometric inductance and an array of superconducting quantum interference devices (SQUIDs) [1]. In addition, I will present preliminary experiments where the JPA is used as the first stage of amplification for the readout of a superconducting qubit based on a coaxial architecture recently developed in our lab in Oxford [2]. Finally, I will introduce my future scientific direction based on using JPAs for generation and control of non-classical states in microwave photons. [1] M. Esposito et al. EPJ Web of Conferences 198, 00008 (2019) [2] J. Rahamim et al. Applied Physics Letters 110, 222602 (2017)

In condensed matter physics, Raman spectroscopy is an extremely powerful probe of the inelastic and dynamic response of many-body systems and helps to identify the 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 spectroscopies, 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 scattering 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 fields (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 multiferroics [4] will be presented. [1] JB et al. PRL 113 266405 (2014) [2] JB et al. arXiv:1805.11307 (2018) [3] JB et al. In preparation [4] JB et al. PRL 115 267204 (2015)

Magnetic impurities in conventional superconductors induce a pair-breaking potential, which leads to bound states inside the superconducting energy gap. These states are called Yu-Shiba-Rusinov states, or short: Shiba states, and can be probed by scanning tunneling spectroscopy at the atomic scale. Here, we explore the Shiba states of individual magnetic atoms on superconducting substrates. The patterns of Shiba states of individual Mn atoms on a Pb(100) surface exhibit the shape of the corresponding spin-carrying d-orbital [1]. When the adatoms are brought into sufficiently close distance, they hybridize by forming symmetric and anti-symmetric linear combinations of the Shiba wave functions [2]. NbSe2 is a van der Waals-layered material with quasi-2D character. In addition to its superconducting properties it hosts a charge-density wave (CDW) at low temperatures. Magnetic atoms and molecules have been shown to give rise to long-range oscillatory Shiba states [3,4]. Here, we show that the CDW affects the energy and symmetry patterns of Shiba states around individual Fe atoms. [1] M. Ruby, Y. Peng, F. von Oppen, B.W. Heinrich, K.J. Franke, Phys. Rev. Lett. 117, 186801 (2016) [2] M. Ruby, B.W. Heinrich, Y. Peng, F. von Oppen, K.J. Franke, Phys. Rev. Lett. 120, 156803 (2018) [3] G. Menard, et al, Nat. Phys. 11, 1013 (2015) [4] S. Kezilebieke, M. Dvorak, T. Ojanen, P. Liljeroth, Nano Lett. 18, 2311 (2018)

Geometric phases in condensed matter play a central role in topological transport phenomena such as the quantum, spin and anomalous Hall effect (AHE). In contrast to the quantum Hall effect - which is characterized by a topological invariant and robust against perturbations - the AHE depends on the Berry curvature of occupied bands at the Fermi level and is therefore highly sensitive to subtle changes in the band structure. We demonstrate that the Berry curvature can be manipulated by interface engineering of the correlated itinerant ferromagnet and antiferromagnets. Using theoretical calculations, we show that the tunability of the AHE at interfaces arises from the competition between two topologically non-trivial bands. D. J. Groenendijk, C. Autieri et al. Berry phase engineering at oxide interfaces, https://arxiv.org/abs/1810.05619. M. Asa, C. Autieri et al. Anomalous Hall effect at the antiferromagnetic/non magnetic interface, in manuscript

The discovery of topological insulators (TIs) has fuelled the search for novel topological phases of matter, by combining theory, computational high-throughput screening and experiments [1, 2]. From the experimental point of view, angle-resolved photoelectron spectroscopy (ARPES) has played a key role due its direct access to the material band structure. The advent of efficient spin detector, and ultrafast light source in the UV energy range, has further extended our capability to map also the vectorial spin texture and the out-of-equilibrium dynamics of the topological states. We have recently exploited these techniques to investigate TIs and several topological semimetal. We have tracked the evolution of the band structure of MoTe2 across the topological phase transition between a trivial semimetal and a type-II Weyl semimetal, as a function of the sample temperature [3]. Our results show that the presence 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 unoccupied band structure [4]. In the second part of my talk, I will show that optical excitation can be also used to alter the material band dispersion, as for the case of the nodal line semimetal ZrSiSe [5], which displays both spin polarized surface states and a bulk hidden spin polarization. [1] M. Z. Hasan and C. L. Kane, Reviews of Modern Physics 82, 3045 (2010). [2] B.-J. Yang and N. Nagaosa, Nature Communications 5, 1 (2014). [3] A. Crepaldi, et al., Phys. Rev. B 95, 041408 (2017). [4] A. Crepaldi, et al., Phys. Rev. B 96, 241408(R) (2017). [5] G. Gatti, et al., in preparation.

Intensive 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 antiferromagnetically 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 wavevectors as a signature of proximity to the Kitaev spin liquid. A similar interpretation has been suggested for the continuum of excitations arising when magnetic order is suppressed either by high temperature, or in-plane magnetic fields. In order to evaluate these proposals, it is important to understand the underlying interactions and energy scales in RuCl3 in the context of recent experiments. We have considered the dynamical response and stability of magnons at zero and finite field with respect to the full range of realistic magnetic interactions suggested by recent ab-initio calculations. Combining extensive exact diagonalization studies with semiclassical 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 field-induced state.

We shall briefly review the present day situation with unexpectedly high-temperature superconductivity in FeSe monolayers on SrTiO_3 (FeSe/STO), the main experimental results, the electronic structure (ARPES data and calculations) and possible mechanisms of Tc enhancement. We shall discuss the origin of unusual shallow conduction bands in FeSe/STO with quite small values of Fermi energy, which we attribute to the increasing electronic correlations (as compared to the bulk FeSe). We shall briefly 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 - McMillan approach.

Transition-metal compounds have long been a fertile 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 effect of special ligand field, atomic multiplet and spin-orbit coupling produces a record breaking giant magnetic anisotropy energy for Co, Ru, or Os adatoms on MgO (001) surface [1]. We will explain the surprising high-Tc ferromagnetic metallicity in the disordered perovskite SrFe0.5Ni0.5O3 using the unusual high-spin and high valence states [2]. Moreover, we find a quite strong ferromagnetism in the layered insulator LaSrNiRuO4 with a very unique charge-spin-orbital state [3,4]. [1] X. Ou et al., PRL 115, 257201 (2015). [2] F. Fan et al., PRB 94, 214401 (2016). [3] S. Zhu et al., EPL 117, 37005 (2017) (Editor’s choice). [4] K. Yang et al., JPCC, in press (2018).

Topological Weyl magnets are characterized by a magnetic semimetallic state that harbors Weyl fermions near the Fermi energy. As a parent state of such a magnetic Weyl state, we first introduce a new class of semimetals with quadratic band touchings, the “Luttinger semimetal” [1,2]. We demonstrate the case in pyrochlore iridium oxides that exhibit various exotic phenomena, including a spontaneous Hall effect without magnetization in a spin liquid state [3]. Then, we discuss a new type of frustrated antiferromagnets, Mn3X (X = Sn and Ge) as the examples of a topological Weyl magnet [4,5,6]. We show that the cluster multipole order on the kagome lattice of Mn moments can be easily controlled and allows the system to exhibit a variety of new functions at room temperature that have never been seen in antiferromagnetic metals. These include the large anomalous Hall and Nernst effects [4,5,7], large magnetic optical Kerr effect [8] and a novel type of spin Hall effect (magnetic spin Hall effect). Finally, we show that they should be significantly useful for designing antiferromagnetic spintronics, and energy harvesting technology. [1] T. Kondo et al., Nature Communications 6, 10042 (2015). [2] B. Cheng et al., Nature Communications 8, 2097 (2017). [3] Y. Machida, S. Nakatsuji, S. Onoda, T. Tayama, and T. Sakakibara, Nature 463, 210 (2010). [4] S. Nakatsuji, N. Kiyohara and T. Higo, Nature 527, 212 (2015). [5] N. Kiyohara, T. Tomita, S. Nakatsuji, Phys. Rev. Applied 5, 064009 (2016). [6] K. Kuroda, T. Tomita et al., Nature Materials 16, 1090 (2017). [7] M. Ikhlas, T. Tomita et. al., Nature Physics 13, 1085 (2017). [8] T. Higo et al., Nature Photonics, 12, 73 (2018).

Dissipative quantum many-body systems are extremely challenging to analyze, as most theoretical tools developed for equilibrium systems cannot be applied. I will present a variational principle 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 variational method to recent results obtained using a tensor network approach [3]. Finally, I will demonstrate how a better fundamental understanding of dissipative quantum many-body systems enables the realization of a novel class of dissipative quantum sensors [4]. References [1] H. Weimer, Phys. Rev. Lett. 114, 040402 (2015). [2] V. R. Overbeck, M. F. Maghrebi, A. V. Gorshkov, H. Weimer, Phys. Rev. A 95, 042133 (2017). [3] A. Kshetrimayum, H. Weimer, R. Orus, Nature Commun. 8, 1291 (2017). [4] M. Raghunandan, J. Wrachtrup, H. Weimer, Phys. Rev. Lett. 120, 150501 (2018).

Luminescence is a standard method to study the excited electronic states and their relaxation dynamics in solids, especially in insulators and semiconductors. However, the luminescence from metals and semimetals have rarely been studied. Several years ago, we found femtosecond infrared luminescence from some semimetals, such as graphite [1], bismuth, antimony [2] and also from metallic states at the surface of topological insulators [3]. We successfully obtained versatile information about the relaxation dynamics in these materials. In addition, very recently, we discovered ultrafast infrared luminescence in many ordinary metals, Au, Ag, Cu, Pt, Al etc., when they have surface roughness [4]. In this talk, I would like to propose the "femtosecond infrared luminescence" as a conventional tool for investigating the electron dynamics in metals, providing information similar to that could be obtained from time-resolved photoemission spectroscopy. References [1] T. Suemoto, S. Sakaki, M. Nakajima, Y. Ishida, and S. Shin, Phys. Rev. B87, 224302 (2013). [2] M. Takeda, S. Maezawa, H. Watanabe and T. Suemoto, Spring meeting of Physical Society of Japan (2015). [3] S. Maezawa, H. Watanabe, M. Takeda, K. Kuroda, T. Someya, I. Matsuda and T. Suemoto, Sci. Rep. 5. 16443 (2015). [4] T. Suemoto, N. Sugimoto, K. Nishikawa, K. Yamanaka, S. Inagaki, The XXI International Conference on Ultrafast Phenomena (Hamburg, 2018).

Recent progress in making oxide interfaces and heterostructures has led to an interest in understanding how one might realize topological phases or flat bands in such correlated systems. In particular, this might be accomplished by spin-orbit coupling or strain effects. In this context, I will discuss our theory work on understanding the "atomic limit" of spin-orbit coupled oxides, showing that one can use resonant inelastic X-ray spectroscopy of such systems to extract microscopic couplings. Using such "lego" building blocks, I will discuss mixed oxide 2DEGs comprised of local moments coupled to a "conduction 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. Spin-orbit coupling and strain can thus pave the way to realizing novel topological quantum phases in solids.

Quantum states in low-dimensional spin systems are characterized by their characteristic spin dynamics. In this seminar, I will talk about terahertz spectroscopy of quantum spin dynamics in a spin-1/2 Heisenberg-Ising antiferromagnetic chain as a function of temperature and magnetic field. At zero field, confined spinon excitations are observed below the Néel temperature [1]. In a longitudinal magnetic field, high-energy string excitations are observed as well as low energy spin excitations [2]. In contrast, in a transverse magnetic field, emergent fermionic excitations are observed, when the confinement is suppressed and an order-disorder phase transition is induced by the transverse field [3]. These experimental results are understood by comparison to precise calculations of dynamic structure factors using Bethe ansatz and infinite time evolving block decimation, respectively, for the longitudinal and transverse fields [2,3,4]. In addition, the results in transverse fields provided strong indication of possible 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 measurements [6], the quantum criticality of which is consistent with a recent analytical study of the transverse-field Ising-chain [7]. [1] ZW et al. Phys. Rev. B 91, 140404(R) (2015). [2] ZW et al. Nature 554, 219–223 (2018). [3] ZW et al. Phys. Rev. B 94, 125130 (2016). [4] Wang Yang et al. arXiv:1702.01854 (2017) [5] S. K. Niesen et al. Phys. Rev. B 87, 224413 (2013). [6] ZW et al. Phys. Rev. Lett. 120, 207205 (2018). [7] Jianda Wu et al. Phys. Rev. B 97, 245127 (2018).

Transition metal oxides (TMOs) represent the natural playground for the investigation of the exotic phases produced by electronic correlations, which include high-temperature superconductivity, giant magnetoresistance and topological insulators. The peculiar electronic and magnetic properties of TMOs stem from the interplay between 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 decrease due to the larger size of the atomic orbitals, while SOC increases as a result of the increased atomic charge. In the present talk, I present 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)2IrO4, where the strong SOC of 5d electrons gives rise to a spin-orbit entangled Mott state with peculiar exchange interactions. I then examine the intermediate case of the 4d oxide Ca2RuO4: here, electronic correlations, SOC 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. Finally, I discuss the magnetic ground state of a family of weak ferromagnets, where the weak SOC is responsible for the appearance of a net magnetization in the main antiferromagnetic order.

In functional magnetic nanostructures the acoustic, magnetic and plasmonic excitations can co-exist and interact on the nanometer spatial and ultrafast time scales. Optical spectroscopy with femtosecond laser pulses highlights a variety of nontrivial spatio-temporal dynamics, which are not only used to monitor individual excitations in real time, but also study interaction mechanisms between them, often observed in frequency mixing phenomena. Whereas the second harmonic generation (SHG), sum- and different frequency mixing in the optical frequency 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 oscillation frequencies, typically in the MHz-GHz range. Keeping in mind the intrinsic differences in the physical nature and frequency range of these phenomena, here we discuss, in a comparative manner, two examples of frequency mixing in magneto-plasmonics [1,2] and magneto-acoustics [3,4]. The ability to experimentally tune both systems through Surface Plasmon Resonance (SPR) and Ferromagnetic Resonance (FMR) as well as to theoretically describe these resonant interactions within the framework of phenomenological models based on the Lorentz oscillator, represent the key idea behind this presentation. In nonlinear magneto-plasmonic experiments, the Kretschmann configuration SPR resonances for different optical wavelengths occur at different angles Fig. (a,b), offering the unique possibility to 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 plasmonic enhancement of the chi(2)-susceptibility confirms the experimental observation that magnetic effects are most pronounced between the SHG and fundamental SPR resonances [1,2]. In the second experiment [3,4] the magnetization in a Ni/glass sample is excited by two distinct transient surface acoustic waves (SAW and SSLW). Magnetic tuning of the FMR frequency in resonance to their SHG, sum- and difference frequencies demonstrates the full variety of frequency mixing phenomena Fig. (c). In contrast to nonlinear 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-elastic switching in bi-stable systems, i.e. nickel nanomagnets, is under way [5]. Whereas the use of phenomenological, non-microscopic modeling may fail to capture fine details, it represents a useful tool for experimentalists measuring macroscopic quantities such as magnetization dynamics and/or optical nonlinearities. The presented methodology can further exploited to design experimental architectures based of coupled resonances of different nature (acoustic, SPR, FMR, excitons in semiconductor quantum dots, nanomagnets) providing a playground for future optical metrology and the physics of nano-devices [5,6]. References: [1] I. Razdolski et al., ACS Photonics 3, 179 (2016) [2] V.V. Temnov et al., J. Opt. 18, 093002 (2016) [3] J. Janusonis et al., Phys. Rev. B 94, 024415(2016) [4] C.L. Chang et al., Phys. Rev. B 95, 060409 (2017) [5] V.S. Vlasov et al., Multifunctional Materials (to appear in 2018) [6] D.A. Kuzmin et al., Nanophotonics 7, 597 (2018)

Last few decades, the oxide heterostructure systems were intensively studied as a bulk crystal and thin film. Therefore, we have a deep understanding of the core physics on these systems. But still we can find or design a new research playground among these systems. Dimensionality, artificial structure, interface engineering, and exploring new device technique, these keywords opened the new research opportunities. In this talk, Firstly, I would like to introduce dimensional study on ultra-thin manganite film with element specific scattering/spectroscopy technique. Secondly, the Topological Hall Effect in SrRuO3 system will be discussed.

I will review our recent investigations of semiconducting materials at the focus of optoelectronic research. We measure the surface of CH3NH3PbI3 single crystals by making use of two-photon photoemission spectroscopy. By these means, we explicitly discriminate the initial thermalization of the electrons from slower dynamical processes. The picosecond localization of excited electrons in degraded CH3NH3PbI3 samples is 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 electronic states degenerate with the M valley within 0.35 ps. Subsequently, the electronic cooling is solely dictated by Froehlich coupling to phonon of small momentum transfer. Ab-initio calculations of the electron-phonon coupling are in excellent agreement with the observed dynamics.

The high reactivity of magnetic substrates toward molecular overlayers has so far inhibited the realization of more sophisticated on-surface reactions, thereby depriving these interfaces of a significant class of chemically tailored organics such as graphene nanoribbons (GNRs), oligonuclear spin-chains, and metal−organic networks. In the first part of this talk, I will present a multi-technique characterization of the polymerization of 4,4″-dibromo-p-terphenyl precursors into ordered poly(p-phenylene)[1] arrays on top of an emerging class of two-dimensional magnetic surface alloys[2,3]. The chemical reaction was followed by temperature-dependent X-ray photoelectron spectroscopy (XPS) and the structural characterizations of supramolecular and polymeric phases were performed by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Taking advantage of the high homogeneity of the samples, the electronic structure of the valence band was determined with angle-resolved photoemission spectroscopy (ARPES). Moreover, ferromagnetic ordering in the two-dimensional GdAu2 alloy was demonstrated for all phases by X-ray absorption spectroscopy (XAS)[4]. In the second part, I will briefly show that GdAu2 is not only a suitable template for the surface-confined Ullmann-like coupling reaction but is also thermally sufficiently stable to sustain a cyclodehydrogenation reaction for the on-surface synthesis of more complex GNRs. Thus, the system is demonstrated to close a longstanding experimental gap in the study of GNRs as it enables spin-polarized STM characterizations[5] of e.g. the spin-polarization of the elusive GNR edge-states, which were theoretically predicted for chiral GNRs[6] and experimentally, albeit lacking spin-sensitivity, observed on Au(111)[7]. However, successive theoretical studies questioned [8] and refined [9], respectively, the experimental prerequisites under which magnetic ordering of edge-states in GNRs may be observed, thereby highlighting the need for direct experimental access of the spin-polarization of these states. References: [1] Vasseur, G.; Abadía, M.; Miccio, L. A.; Brede, J.; Garcia-Lekue, A.; de Oteyza, D. G.; Rogero, C.; Lobo-Checa, J. & Ortega, J. E. Pi Band Dispersion along Conjugated Organic Nanowires Synthesized on a Metal Oxide Semiconductor J. Am. Chem. Soc. 138, 5685-5692 (2016) [2] Corso, M.; Fernández, L.; Schiller, F. & Ortega, J. E. Au(111)-Based Nanotemplates by Gd Alloying ACS Nano 4, 1603-1611 (2004) [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 grown by high-temperature deposition on Au(111) Phys. Rev. B 88, 125405 (2013) [4] Abadía, M.; Ilyn, M.; Piquero-Zulaica, I.; Gargiani, P.; Rogero, C.; Ortega, J. E. & Brede, J. Polymerization of Well-Aligned Organic Nanowires on a Ferromagnetic Rare-Earth Surface Alloy, ACS Nano 11, 12392-12401 (2017) [5] Brede, J. & Wiesendanger, R. Spin-resolved imaging and spectroscopy of individual molecules with sub-molecular spatial resolution MRS Bulletin 39, 608-613 (2014) [6] Yazyev, O. V.; Capaz, R. B. & Louie, S. G. Theory of magnetic edge states in chiral graphene nanoribbons Phys. Rev. B 84, 115406 (2011) [7] Tao, C.; Jiao, L.; Yazyev, 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. Spatially resolving edge states of chiral graphene nanoribbons, Nature Physics 7, 616-620 (2011) [8] Kunstmann, J.; Özdoğan, C.; Quandt, A. & Fehske, H. Stability of edge states and edge magnetism in graphene nanoribbons Phys. Rev. B, 83, 045414 (2011) [9] Golor, M.; Lang, T. C. & Wessel, S. Quantum Monte Carlo studies of edge magnetism in chiral graphene nanoribbons Phys. Rev. B, 87, 155441 (2013)

Multiferroics permit the magnetic control of the electric polarization and the electric control of the magnetization. These static magnetoelectric (ME) effects are of enormous interest: The ability to read and write a magnetic state current-free by an electric voltage would provide a huge technological advantage. Dynamic or optical ME effects are equally interesting, because they give rise to unidirectional light propagation as recently observed in several multiferroic compounds [1]. In conventional media light propagation is reciprocal, that is counter-propagating beams experience the same refractive index. However, reciprocity can be violated in multiferroic materials, where the refractive index depends not only on the polarization of light but also on the +- k direction of the propagation [1]. Such unidirectional transmission is the consequence of the dynamic magnetoelectric effect emerging in materials with simultaneously broken time reversal and spatial inversion symmetries. This phenomenon, exclusively observed in multiferroic and magnetoelectric materials [2-5], may allow the development of optical diodes transmitting unpolarized light in one, but not in the opposite, direction [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 antiferroelectric and antiferromagnetic orders. References [1] D. Szaller et al., Phys. Rev. B 87, 014421 (2014) [2] I. Kezsmarki et al., Phys. Rev. Lett. 106, 057403 (2011) [3] S. Bordacs et al., Nat. Phys. 8, 734 (2012) [4] I. Kezsmarki et al., Nat. Commun. 5, 3203 (2014) [5] I. Kezsmarki et al., Phys. Rev. Lett. 115, 127203 (2015) [6] V. Kocsis et al., arXiv:1711.08124 (2017)

Within the field of solid-state physics, the discovery of remarkable phases and transitions is often tightly coupled to the design, growth and characterization of novel materials. Therefore, the past several decades of work in the field of correlated electron physics can be described by a list of materials that have defined new states of matter at extreme conditions, e.g. low temperatures, high magnetic field or pressure. In most cases, a thorough understanding of the underlying physical mechanisms is accessible only, if high-quality single crystals with sufficient sizes are available. In this lecture, I shall give an overview about my own contributions to this research area in recent years focusing on three very different material classes (i) quantum criticality and superconductivity in heavy-fermion metals [1-3], (ii) crystal growth of LnFeAsO iron-pnictide superconductors [4,5], and (iii) spin-liquid phases in Cu-based frustrated spin systems [6]. References [1] A. Steppke et al., Science 339, 933 (2013). [2] E. Schuberth et al., Science 351, 6272 (2016). [3] H. Pfau et al., Phys. Rev. Lett. 119, 126402 (2017). [4] A. Jesche et al., Phys. Rev. B 86, 020501(R) (2012). [5] A. Adamski et al., Phys. Rev. B 96, 100503(R) (2017). [6] P. Puphal et al., J. Mater. Chem. C 5, 2629 (2017).

Scanning tunneling microscopy is an ideal technique to explore the structural and electronic properties of low-dimensional electron system. In this talk I will focus on the study of physical properties self-organizing metallic nanowires on semiconductor surfaces and transport through single molecules. The metallic nanowires show unique physical properties owing to their one-dimensional nature. Many of these properties are intimately related to electron-electron interactions, which play a much more prominent role in one dimension than in two or three dimensions. I will present our work on Pt, Au and Ir atom chains on Ge(001) surfaces and provide a few examples of their unique properties, such as Peierls instability, quantum confinement, suppression of the density of density states near the Fermi level and nanowire length quantization. If time allows I will also elaborate on the transport through single molecules.

The properties of thin films and heterostructures of complex and strongly correlated oxides are one of the most fascinating and demanding topics in today´s solid state physics research. Oxide heterostructures give rise to novel and unexpected physical phenomena such as metallicity in nominally non-metallic materials, magnetism in nominally non-magnetic materials, ferroelectricity in nominally non-ferroelectric materials, or so far unobserved topological effects. Many of these phenomena are affected or even more driven by the nanoscale defect structure established during material synthesis and processing, ultimately limited by the thermodynamics of the system. Tuning the thermodynamic processes by engineering dedicated heterostructures may be used to tailor their defect structure in a desired way, hence, to deplete or accumulate defects at interfaces, to spatially separate electronic and ionic charge carriers, or to confine defects to dedicated regions. In this talk, I discuss the low-dimensional electron transport observed along complex oxide heterointerfaces, such as the one in LaAlO3/SrTiO3 heterostructures. While the formation of these 2-dimensional electron gases is attributed to electronic charge transfer triggered by a built-in electric field, the ionic defect structure at these interfaces is still being discussed controversially. I address the thermodynamic processes associated with built-in electric fields and derive ionic defect 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 partial pressure applied during sample fabrication and on the strength of the built-in electric field. As will be discussed, the low-temperature behavior of these novel electron system is unambiguously correlated to the adjacent ionic structure typically set at high temperature, making thermodynamic considerations indispensable in order to understand novel phenomena occurring at cryostatic temperatures. Therefore, a comprehensive study of low 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 states obtained for various oxide heterostructure systems [2] and discuss resulting implications for important measures characterizing the electron gas, such as electron mobility [3] as well as its magnetic signature [4], both controllable by thermodynamic means. The thermodynamic model obtained for oxide heterointerfaces is furthermore linked to kinetic space charge formation occurring at complex oxide surfaces [5, 6]. [1] F. Gunkel et al., “Defect concentration profiles at complex oxide interfaces”, Physical Review B 93, 245431 (2016) [2] F. Gunkel et al., “Thermodynamic ground states of complex oxide heterointerfaces”, ACS Applied Materials & interfaces, ACS Appl. Mater. Interfaces, 9 (1), 1086 (2017) [3] C. Xu et al., “Disentanglement of growth dynamic and thermodynamic effects in LaAlO3/SrTiO3 heterostructures”, Scientific Reports 6, 22410 (2016) [4] F. Gunkel, et al., “Defect-control of anomalous and conventional electron transport in NdGaO3/SrTiO3 heterostructures”, Physical Review X, 6, 031035 (2016) [5] R. Meyer et al., “Dynamics of the metal-insulator transition of donor-doped SrTiO3”, Physical Review B 94, 115408 (2016) [6] M. Andrae et al., “Oxygen partial pressure dependence of surface space charge formation in donor-doped SrTiO3”, APL Materials 5, 056106 (2017)

Spin-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 nanoscale objects ranging in size form single atoms to several thousand atoms. I focus on the magnetization reversal and the spin-dependent electronic properties of bilayer Co, Fe-decorated Co and Fe islands on Cu(111). We find 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 the significance of structural and electronic relaxation for the magnetic anisotropy 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 and on the proximity effect.

The identification of the electron-boson interaction is believed to be the key for rationalizing the Cooper pairing mechanism in a superconductor. In strong- coupling conventional superconductors, the fingerprints of the electronic band renormalization due to the electron-boson interaction are found in the fine structure of tunneling spectra, where the identification of this fine structure with the phononic density of states represents 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 cumbersome and ambiguous. Here we exploit a so far unnoticed boson-assisted resonant enhancement of the impurity scattering potential and thus the quasiparticle interference (QPI) amplitude in order to identify the nature of a possible pairing boson in the unconventional superconductor LiFeAs. In high-resolution Fourier transform scanning tunneling spectroscopy (FT-STS) measurements we indeed observe a resonance-like enhancement of the QPI amplitude in both the normal and the superconducting state, which implies an important 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 QPI are in perfect agreement with a strong inelastic contribution on the tunneling dI/dU spectra recorded away from impurities which renders the observed small-q modes promising candidates for providing the pairing interaction in LiFeAs.

Since the isolation of monolayer graphene, the library of two dimensional (2d) materials has been continuously growing. Together with recent developments in on-demand stacking of these atomically thin crystals, a whole new class of hybrid materials is coming into reach, which holds promises from the realization of exotic electronic quantum phases at elevated temperatures to novel optoelectronic applications. Interactions between electrons are central to all these kinds of promises. In my talk, I will discuss why Coulomb interactions in 2d materials are very different from their 3d counterparts, how they influence material properties and how interactions can be manipulated in 2d materials. As example systems I will consider graphene as well as transition metal dichalcogenides like MoS2. In these systems interactions renormalize electronic quasiparticles 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 instabilities including superconductivity and charge density wave states.

Magnetism of molecules is in the focus of intense research, because it could lead to new spin-based electronic devices. In this context, two molecules adsorbed on a metal surface and carrying magnetic moments constitute an important model system which helps to understand the fundamental properties of molecular magnetism – in particular the interaction between their magnetic moments. Typically the interaction of spins on a metal surface is dominated and tuned by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction – an indirect exchange interaction mediated by the conduction electrons of the metal. Here we show that the non-magnetic, chemical interaction between the molecules can become the decisive effect if the spin-moment-carrying molecular orbitals extend in space and therefore lead to the formation of bonding and antibonding orbitals due to wave function overlap. We demonstrate that this splitting has a crucial influence on the magnetic properties exemplified by two neighbouring metal-molecule (Au-PTCDA) complexes, i.e. dimers, on the Au(111) surface. Interestingly, in this particular case, competition between the binding energy gain, due to the chemical interaction between the moment-carrying orbitals, and the gain of additional hybridization energy, due to the strong entanglement between the local 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 wave function overlap, involving slightly different dimer configurations, drive the system through a quantum critical point (QCP) from a partially Kondo-screened triplet to a singlet ground state, with one dimer configuration being located extremely close to a quantum critical point.

The previous 11 years have seen several breakthroughs in our understanding of topological states of matter. New Insulators and semimetals, exhibiting new properties imposed by a variety of symmetries (the most well known of which is time reversal) have been theoretically predicted and experimentally discovered. These materials are described by topological indices, a set of "serial numbers" that characterize the state of matter. In this talk, I will review the progress made in these years with a focus on the prediction of new topological indices and on material discovery. I will then show a surprising link between chemistry bonding and topological states of matter which leads to the prediction of many new classes of materials as topological.

In this talk I will discuss recent experimental and theoretical advances combining one dimensional topological nanowires into small mesoscopic islands, pushing Majorana physics into the Coulomb blockade regime, and creating new quantum phases governed by correlations. I will elucidate that transport through such islands gives access to a rather unstudied type of quantum impurity physics, where the electrons in the leads combine with the Majorana fermions in the island to yield effective bosonic charge carriers. This gives rise to novel non-equilibrium transport behavior. Pushing these islands using a gate volatge into charge degeneracy points presents very interesting regimes allowing to probe strongly correlated Majorana fermions at short time scales and at high energy scales. In addition, creating arrays of such islands may form a highly convenient platform for topological quantum computing schemes based on the surface code. Here, electron transport can be used for quantum measurements and manipulation of the stored information in the Majorana qubits.

The understanding of electronic and structural phase transitions is of basic importance in condensed matter physics. In this presentation, I will concentrate on phase transitions that can be triggered externally by electronic excitations and address the how we can excite particle modes of lattice excitation, and explore how these are related to the electronic and magnetic properties and the corresponding phase transitions. The goal is to understand how and how fast we can trigger electronic transitions and how they might couple to structural 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. The presented examples will include the charge and orbital order phase transition in half doped manganites [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 transition. References [1] P. Beaud et al. Nature Materials 13, 923 (2014) [2] Kubacka et al. Science 343, 1333 (2014) [3] V. Esposito et al. Phys. Rev. Lett. (in press) (2017).

Complex iridium oxides have attracted recently a lot of attention due to an intimate entanglement of spin and orbital degrees of freedom which may give rise to a novel spin-orbital Mott insulating behavior and exotic quantum spin liquid phases. Electron spin resonance (ESR) spectroscopy is known to be an instructive tool for studying the spin-orbit coupling (SOC) effects as it can directly access the relevant parameters sensitive to SOC, such as the g-factor tensor, magnetic anisotropy gaps and spin dynamics. In this talk, two of our recent results on multi-frequency sub-THz ESR spectroscopy on iridium oxides in strong magnetic fields will be discussed: (1) surprising inversion of the orbital states in the prototypical spin-orbital Mott insulator Sr2IrO4 with Ir4+ (5d5, J = 1/2) due to the long-range crystal field effects; (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 literature.

Metals are found frequently in nature and their properties are usually very well described 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 physics. In this talk I will give an introduction to the problem and show how interesting strongly coupled field theories arise in the low energy description 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.

I will present recent results, (i) on a novel TBA approach to magneto-thermal transport in 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 temperature dynamics of quantum compass models [2]. [1] X. Zotos, arXiv: 1604.08434 [2] A. Briffa and X. Zotos, arXiv: 1611.00637

I will discuss two instances of one-dimensional conducting edge channels that can appear on the boundary of three-dimensional topological crystalline insulators, one supported by an experimental observation and the second one being a theoretical 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 surface states on either side of the step edge. Experimentally, they have been found to be remarkably robust against defects, magnetic fields and elevated temperature. Second, I will introduce the concept of higher-order three-dimensional topological insulators, which have gapped surfaces, but support topologically protected gapless states on their one-dimensional physical edges.

Quantum spin liquids are a central theme in current condensed matter physics as they host emergent topological order and fractionalized excitations. An important example are interacting Kitaev spins on a honeycomb lattice which are theoretically predicted to exhibit topological and quantum spin liquids. Identifying signatures of Kitaev physics, however, is extremely challenging as real materials, such as the iridates (Na,Li)2IrO3, inevitably entail an isotropic Heisenberg and additional 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 structure of α-RuCl3 contains layers weakly coupled by van der Waals interaction, i.e. it is a correlated 2d-material offering many possibilities for “materials engineering”. We have characterized the electronic and magnetic properties of α-RuCl3 by a broad spectrum of experimental techniques. From electron spectroscopy basic parameters determining the insulating state of α-RuCl3 are extracted. Moreover, strongly anisotropic magnetic properties as measured from magnetization, specific heat, thermal conductivity, and NMR measurements will be presented and discussed. Both, the behavior of α-RuCl3 in the paramagnetic phase as well as the properties of the ordered state strongly differ from that found in conventional two-dimensional magnets. In particular, we find a very unusual field-temperature phase diagram with evidence for a novel quantum critical point.

Presence of a strong spin-orbit coupling (SOC) comparable to Coulomb interaction (U) and crystal field in iridium based oxides give rise to novel properties. Especially the honeycomb lattice iridates attracted lot of attention in the context of Heisenberg-Kitaev model. Extension of this model to triangular lattice has been studied theoretically and a rich phase diagram has been proposed [1, 2]. Possible materials for the realization of Heisenberg-Kitaev model in a triangular lattice will be discussed in this talk. Materials with 5d4 electronic configuration 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 excitonic magnetism [3]. In the 2nd part of the talk, Ba2YIrO6 with 5d4 (Ir5+) electronic configuration will be discussed in the context of excitonic magnetism. References: [1] M. Becker et al., Phys. Rev. B 91, 155135 (2015) [2] A. Catuneanu et al., Phys. Rev. B 92, 165108 (2015) [3] G. Khaliullin, Phys. Rev. Lett. 111, 197201 (2013)