Department of Physics • University of Cologne

Repeated oxidation and reduction of a platinum electrode, which can be compared to the switching ON and OFF of a device (e.g. fuel cell, electrolyzer,…), leads to the roughening of the surface, caused by the nucleation and growth of nano-islands, which first grow only laterally (2D) and then exclusively in height (3D). Surprisingly, this particular nucleation and growth combines several remarkable aspects that all deviate from standard text book- and literature-knowledge. The basic mechanism is rooted in the larger lattice constant of platinum-oxide such that the growth is promoted by the creation of adatoms and vacancies, implying that not only growth processes but also etching processes have to be considered. It is remarkable that the nano-islands arrange in an almost perfect, hexagonal pattern, which clearly deviates from standard random growth. Surprisingly, many other chemical surface systems develop highly ordered nano-islands during cycles of surface reaction and restoration. In the case of platinum the nano-islands growth eventually enhances the dissolution, which makes electrochemical energy applications, like in fuel cells and electrolysers, economically less feasible, as it is scarce and expensive. Preventing nucleation would be most effective for preventing roughening. However, little is known about the atomic details regarding the nucleation; a process almost impossible to observe. I will show that platinum exhibits four different (pre-)oxidation stages that are responsible for the mysterious observations, all leading to very peculiar atomic changes and surface structures, before real bulk oxidation occurs. First the adatoms of the surface-background adatom gas are oxidized, but the equilibrium adatom pressure shoots up simultaneously such that nucleation does not occur during the oxidation, where the atoms are created and pushed on top of the surface. Surprisingly, they nucleate during the reduction. Secondly, analysing the island distance-distribution, we derive a rarely observed non-random nucleation, which can only occur if there is a repulsive interaction between the depositing atoms or the existence of preferential nucleation sites. Fundamentally interesting, as never observed before, we do show that special, preferential nucleation sites that a priori do not exist, develop initially via a precursor and eventually form a semi-ordered Pt-oxide structure: the famous spoke wheels, which are assembled from Pt atoms that exchanged their places with oxygen atoms going subsurface. At low coverages the system is fully reversible and restores upon reduction. We can model the reversible place exchange coverage with a Frumkin isotherm combined with an Arrhenius diffusion term. At high coverages irreversible place exchange atoms are created that are pushed onto the Pt(111) surface, leading eventually to the ordered nano-islands. Interestingly, the pre-oxidation stages are expected to occur also on many other surfaces during oxidation and nano-islands growth has been reported before also on Cu, Ag, and Au. We expect similar precursors and pre-stages to form also during nitride and sulfide formation.

In this presentation, I will mainly focus on the functionalization of graphite, graphene, and transition metal dichalcogenides using molecules, though the concepts can be applied to other 2D materials too. Nanostructuring is at the heart of all functionalization protocols that we develop because it opens new possibilities for control and functionality. A variety of scanning probe microscopy methods are used for visualization, characterization, and manipulation. The first approach is based on molecular self-assembly at the interface between a liquid or air, and graphite or graphene. A second approach is based on the covalent attachment of molecules on 2D materials via covalent chemistry. It will be demonstrated how in addition to bottom-up strategies that provide control on the density and layer thickness, as well as submicron to nanoscale nanostructuring, also top-down scanning probe microscopy and optical lithography can be used to structure such covalently modified surfaces. A third approach does not focus on the functionalization of the surface, but uses the surface as a support for the in-plane covalent stitching of molecules, leading to the formation of on-surface 2D dynamic covalent polymers.

Quantum dot (QD) light-emitting diodes (QLEDs) are considered one of the most promising devices for future full-colour displays due to their high colour purity with narrow emission bandwidth, high brightness, and solution processability. However, fundamental mechanisms, such as charge injection into QDs, exciton recombination, and operational stability, should be understood to realise the QLED displays. Here, we present the recent research progress in our group to unravel the mechanisms of charge carrier and exciton dynamics in QLEDs, and the device design and fabrication processes to achieve high performance QLEDs.

Graphene--the one-atom-thick sheet of carbon--is the most famous of 2D materials due to its unique electronic properties and mechanical strength. However, its chemical inertness makes graphene also an excellent nearly electron-transparent support for other materials and nanostructures. In this presentation, I will give an overview of our recent work enabled by a unique interconnected vacuum system containing an aberration-corrected scanning transmission electron microscope (Nion UltraSTEM 100) with a unique objective area that allows sample cleaning via laser, in situ chemical experiments, and direct vacuum transfer to an atomic force microscope (AFSEM by GeTEC Microscopy), an argon glove box, target chamber with a plasma ion source and evaporators, and long term vacuum sample storage. In this presentation, I will demonstrate that defect-engineering of graphene enables its substitutional heteroatom doping and growth of nanoclusters with a well-defined concentration and a narrow size distribution, as well as the direct correlation of its atomic structure and mechanical properties.

Artificial molecular motors that convert external energy into controlled motion have seen great developments in the last decades [1]. While many studies exist in solution, little is known how such functional molecules behave on a surface. However, such a solid support can be advantageous as it offers fixed points of reference as well as confinement in two dimensions, making it easier to study the directionality of their motion. We have studied so-called Feringa motors on various surfaces by low-temperature scanning tunnelling microscopy (STM). On Cu(111) these molecules adsorb in three different conformations, which are related to the states of the motor cycle in solution. The relative stability of the in-solution motor states is reversed on the surface. It was found that rotations of individual molecules can be induced over rather long distances up to about 100 nm by voltage pulses with the STM tip. Importantly, these rotations show high directionality (clockwise or anticlockwise), which will be discussed in view of their specific chemical structure and adsorption. [1] W. R. Browne and B. L. Feringa, Nat. Nanotech. 1, 25 (2006)

Superconducting nanowire single-photon detectors (SNSPDs) provide sufficient sensitivity to enable laser induced fluorescence (LIF) experiments in the mid-infrared, an exciting technical development for physical chemistry given the importance of vibrational spectroscopy to molecular science. In this talk, I will describe how an SNSPD works and how to use it. I will also present results of experiments on the vibrational dynamics of monolayers and multilayers of solid CO adsorbed at the surface of a NaCl crystal, to show the capabilities of time-resolved infrared LIF spectroscopy and provide observations of astonishing phenomena arising from dipole-dipole coupling between molecules. In these experiments, a laser light pulse excites about half of the CO to its

The talk will cover spectroscopic investigations on the catalytic properties of iron nanoparticle arrays and details on the formation of nanographene in nano-heterostructures. The increased complexity of real-life catalysts as compared to surface science leads to the “materials gap”. To close this gap, studies of adsorption and reaction experiments on nanoparticles supported on 2D materials were conducted under in situ conditions with the particular aim to elucidate the reactivity of such more “realistic” nanoparticles. It is shown that the use of a quantitative spectroscopy allows for gaining a detailed insight even to these complex systems, thereby facilitating a further step into bridging the materials gap from fundamental science to applied sciences. The use of the 2D materials as a chemically “innocent” substrate gives the intriguing possibility to study the system without the additional influence of side processes such as spill over and reverse spill over. Such studies allow for characterization of novel cluster systems towards their reactivity and adsorption behavior, e.g. for iron nanoclusters with the probe molecule CO. Such reactivity studies are also possible for the more complex chemistry of ethylene on Pt nanoclusters where detailed insights to the reaction pathway become available. Thereby the changes when going from single crystals to nanoclusters become evident. As a second topic, the formation of van-der-Waals nano-heterostructures is discussed. Insights from spectroscopic and microscopic studies allowed us to characterize the individual preparation steps and the final nano-heterostructure consisting of graphene / metal / h-BN stack. Experiments with different cluster sizes are shown to obtain insights to the pore filling of the h-BN substrate. This gives a direct measure of the agglomeration of the nanoclusters during preparation. Literature F. Düll, et al. ACS Applied Nano Materials 2 (2019) 7019. F. Düll, et al. Journal of Chemical Physics 152 (2020) 224710. F. Düll, et al. Physical Chemistry Chemical Physics 21 (2019) 21287.

Nonlinear optics is limited by the weak nonlinear response of available materials, a problem that is generally circumvented by relying on macroscopic structures in which light propagates over many optical cycles, thus giving rise to accumulated unity-order nonlinear effects. While this strategy cannot be extended to subwavelength optics, one can alternatively use localized optical resonances with high quality factors to increase light-matter interaction times at the expense of inelastic losses associated with the nonlinear response. In this context, highly-doped graphene has been identified as a promising material platform for nonlinear optics that combines long-lived, electrically-tunable plasmons with an intensely anharmonic response to light originating from its unique electronic band structure. I will discuss recent progress in the exploration of nonlinear graphene plasmonics, starting with a description of the appealing properties of plasmons in graphene nanostructures down to molecular sizes, followed by a discussion of the unprecedented level of intrinsic optical nonlinearity in graphene and its enhancement by resonant coupling to its supported plasmons of high harmonic and Kerr nonlinearities, strong interactions of nonlinear plasmons with quantum emitters or amongst themselves, and the extraordinary thermo-optical capabilities of this material enabling nonlinear optical switching on the single-photon level.

Continuum limit of Dirac solids can be described as a Minkowski spacetime. Certain nonsymmorphic 2D lattices host tilted Dirac fermions. The effective spacetime felt by these electrons is a deformation of Minkowski spacetime where temporal and spatial affairs are mingled. In this talk, first we use irreducible representations of the 8Pmmn borophene to show that not only the resulting spacetime is the tilt deformation of Minkowski spacetime, but also the resulting tilt can be manipulated by a perpendicular electric field. We discuss unusual plasmons and electromagnetic response of 2D tilted-Dirac cone solids.

I will present a consistent scheme of quantization of chiral flows (flows with extensive vorticity) in ideal hydrodynamics in two dimensions. Chiral flows occur in rotating superfluid, rotating turbulence and also in electronic systems in the magnetic field in the regime of a fractional Hall effect. The quantization is based on a geometric relation of chiral flows to two-dimensional quantum gravity and is implemented by the gravitational anomaly. The effect of the gravitational anomaly violates the major property of classical hydrodynamics, the Helmholtz law: vortices are no longer frozen into the flow. Effects of quantization could be cast in the form of quantum stress. I show that the quantum stress generates Virasoro algebra, the centrally extended algebra of holomorphic diffeomorphisms.

Spin plays a central role in a wide range of optoelectonic processes in organic materials and devices. I will introduce electrical and optical detection schemes for electron spin resonance (ESR), which can be used to probe the Optoelectronic properties of organic electronic devices. I will then discuss results of ESR experiments on materials for singlet fission, electrical and optical detection of ESR in organic light emitting diodes, and correlations between ESR and magnetoluminescence and magnetoconductivity experiments in OLEDs.

Almost periodic chains (models for incommensurate phases) and quasiperiodic chains (models for quasicrystals) have a richer topological phase structure than periodic chains, due to the existence of phasons. This effects the bulk boundary correspondence for these models. We investigate in particular quasiperiodic chains where the phason degree of freedom lives in a totally disconnected space. We show how we can nevertheless define a winding number for the boundary resonances and thus obtain an equation which relates the integrated density of states at a gap to a winding number which can be interpreted as the work the phason motion exhibits on the edge states of the system.

Superconducting quantum circuits are artificial structures with a possibility to engineer their key parameters by design and fabrication. Moreover, many of these parameters can be in-situ tuned during experiments. These attractive properties have made superconducting circuits an ideal platform for quantum engineering such as quantum information processing and quantum optics on a chip. In this talk, I will underline the main physical mechanisms of these systems and will focus on benefits of the superconducting devices for waveguide quantum electrodynamics (QED), a regime where one or several quantum emitters are coupled strongly to a continuum of electromagnetic modes in one dimension. I will show that a basic ingredient of waveguide QED, a single qubit in a waveguide, behaves as a mirror with its transparency depending on the frequency and power of the incoming radiation. I will then show that when two qubits are inserted in a waveguide one can observe additional cooperative effects dependent on the distance between the qubits. Finally, I will outline how these non-local cooperative interactions can be used to create non-reciprocal transmission, distribute entanglement and generate new physical regimes of light-matter interaction.

Quantum advantage refers to exploiting the two essential elements of quantum physics, non-locality and coherence, in modern technologies such as high-precision sensing or large-scale computing. The actual operation of a quantum device requires a unitary operation, and the corresponding time evolution needs to be engineered dynamically. The desired unitaries are implemented by time-dependent external electromagnetic fields. Quantum optimal control theory provides a means to derive the shape of these fields. I will discuss example applications of optimal control theory from quantum sensing and quantum computing and I will show how suitably shaped pulses reach maximum fidelity in minimum time, despite the presence of losses and imprecision.

Floquet systems offer a versatile toolbox for engineering quantum phases that are not allowed or at least more difficult to obtain in equilibrium systems. A prominent example are time crystals, which spontaneously break discrete time-translation symmetry. Closely related to time crystals are Floquet topological phases, such as those found in the driven Kitaev chain. I will discuss how to perform braiding of non-Abelian degrees of freedom - the elementary operation of topological quantum computation - in such a driven topological superconductor. Unlike the equilibrium case, where braiding can only be performed in two-dimensional systems (or at least 2d networks of 1d systems), braiding here is realized in a strictly one-dimensional system, thus potentially simplifying experimental demonstration.

Graphene can acquire salient properties by the intercalated nano structures. To functionalize the graphene as designed, understanding the growth kinetics of the nano structures is a prerequisite. In that regards, Kr atoms are selectively intercalated just below the surface graphene layer of HOPG(0001) by the incidence of low energy Kr ions. The growth kinetics of the encapsulated Kr nano structures is investigated by both scanning tunneling microscopy, molecular dynamics simulations, and the first principles calculations. The intercalation proceeds via defect sites, such as surface vacancies. At room temperature, the thermal diffusion of intercalated Kr is almost frustrated despite the low diffusion barriers, which remains as a puzzle. The growth of Kr nano structures proceeds via the transient mobility of both the intercalating Kr atoms and previously intercalated Kr atoms that are mobilized by collision with the incident Kr ions. At the elevated temperatures where thermal diffusion becomes effective, some Kr nano structures disappear, releasing pressurized Kr atoms, while others coalesce to form blisters via the delamination of the adjacent graphene. Some of the larger blisters explode to leave craters of varying depths at the surface. In contrast to growth on the substrate, the growth of each encapsulated nano structure depends significantly on extrinsic variables, such as surface vacancies and local topography around the nano structure, that affect the Kr diffusion and limit the maximal Kr pressure.

Motivated by supersymmetry, Batalin and Vilkovisky reformulated the equations governing Lagrangian mechanics in the variational calculus as a Maurer-Cartan equation (vanishing of curvature). This allowed them to arrive at a new understanding of symmetries off-shell (i.e. where the Euler-Lagrange equation does not hold). In this talk, I will show how a modification of their Maurer-Cartan equation can handle the action of the diffeomorphism group on the world-sheet (i.e. general covariance of the theory). Our approach involves the introduction of a curvature to Maurer-Cartan equation. This curvature is central (a scalar multiple of the identity matrix) : this is analogous to the Berry phase in the Hamiltonian approach to quantum theory (though this is really nothing more than a formal analogy).

In recent years, enormous data sets have begun to appear in real-space visualizations (scanning probes) and reciprocal-space visualizations (scattering probes) of electronic quantum matter. Increasing volume and variety of such data present new challenges and opportunities that are ripe for a new approach: machine learning. However, the scientific questions in the field of electronic quantum matter require fundamentally new approaches to data science for two reasons: (1) quantum mechanical imaging of electronic behavior is probabilistic, (2) inference from data should be subject to fundamental laws governing microscopic interactions. In this talk, I will review the aspects of machine learning that are appealing for dealing with quantum complexity and present how we implemented a machine learning approach to analysis of scanning tunneling spectroscopy data.

Quantum computers hold the promise to allow one to solve important problems that cannot be efficiently treated on classical computers. To date, the construction of a fault-tolerant quantum computer remains a fundamental scientific and technological challenge, due the influence of unavoidable noise which affects the fragile quantum states. In our talk, we first introduce basic concepts of topological quantum error-correcting color codes, which allow one to protect quantum information during storage and processing by distributing logical quantum information over quantum many-body spin systems. We then discuss progress on experimental quantum error correction, in particular the realisation of a minimal topological color code with trapped ions, which for the first time demonstrated basic quantum computations on an encoded logical qubit. In the second part, I will focus on fault-tolerant quantum computing in trapped-ion quantum processors, in which scalability can be achieved by shuttling ions between various trapping regions and by a toolbox of ion-crystal reconfiguration techniques. I will present recent theory work of our group on resource-efficient and fault-tolerant protocols to control single and coupled logical qubits of increasing size and robustness.

Antiferromagnets and ferromagnets represent two fundamental forms of magnetism with antiferromagnets being the more abundant of the two. However, it has been notoriously difficult to manipulate and detect antiferromagnetic order by any practical means due to the compensated magnetic moment. This has left antiferromagnets over their hundred-year history poorly explored, in striking contrast to the thousands of years of fascination and utility of ferromagnets. This has changed with the proposal and subsequent discovery of a new relativistic spin-torque phenomenon, the Neél spin-orbit torque, that allow us to efficiently control antiferromagnetic moments in spintronic devices. This allows for antiferromagnets to become active elements in devices. An additional concept that has emerged is that antiferromagnets provide a unifying platform for realizing synergies among three prominent fields of contemporary condensed matter physics: Dirac quasiparticles and topological phases. These recent developments have unlocked a multitude of known and newly identified unique features of this "dormant-giant" class of materials that the community is beginning to explore.

We develop a novel real-time approach to computing the entanglement between spatial regions for Gaussian states in quantum field theory. The entanglement entropy is characterized in terms of local correlation functions on space-like Cauchy hypersurfaces. The framework is applied to explore an expanding light cone geometry in the particular case of the Schwinger model for quantum electrodynamics in 1+1 space-time dimensions. We observe that the entanglement entropy becomes extensive in rapidity at early times and that the corresponding local reduced density matrix is a thermal density matrix for excitations around a coherent field with a time dependent temperature. Since the Schwinger model successfully describes many features of multiparticle production in electron-positron collisions, our results provide an attractive explanation in this framework for the apparent thermal nature of multiparticle production even in the absence of significant final state scattering.

A revolutionary new direction in the field of superconductivity emerged recently with the synthesis of superconductors with strong inherent spin-orbit coupling of electrons, such as the half-Heusler compounds YPtBi or LuPdBi. Due to band inversion, the low-energy degrees of freedom are electrons at a three-dimensional quadratic band touching point with an effective spin 3/2, which allows for Cooper pairs with spins ranging from 0 to 3. I will illuminate some of the unconventional superconducting properties that arise from this band structure and attractive short-range interaction: (i) At strong coupling, the system features an s-wave superconducting quantum critical point with non-Fermi liquid scaling of fermions and several other unusual scaling properties. (ii) The system may further undergo a transition into a phase with complex tensor order, which is a superconducting state captured by a complex-valued matrix order parameter describing Cooper pairs having spin 2. Here the interplay of both tensorial and complex nature results in a rich and intriguing phenomenology. I will discuss the mean-field phase structure as a function of doping and temperature, and relate our finding to experiments in YPtBi. Further, the critical properties of this new paradigm for superconductivity will be addressed.

State-of-the-art nano-fabrication process allow to control the spatial confinement of electronic carriers, with close to atomic precision. Structures are thus designed to separate spatially oppositely charged electrons and holes, notably bilayer heterostructures. When electrons and hole are separated in such adjacent layers they experience a strong Coulomb attraction pairing them into spatially indirect excitons, i.e. boson-like particles, which exhibit a giant electric dipole of about 500 Debye. Excitons then experience a repulsive dipolar potential stabilizing cold gases against collapse while providing access to a rich variety of collective quantum phases, possibly ranging from superfluidity to supersolidity [1]. Here, first signatures for the superfluid crossover of such dipolar excitons are reported [2]. When confined in a microscopic trap we show that they realize a four-component superfluid at sub-Kelvin temperatures, distributed between two optically active and two optically inactive spin states. By imaging the condensate bright part, we study in-situ the profiles of the exciton density and the phase coherence in the trap. We thus reveal quantum spatial coherence, in a sub-Kelvin regime bound to very dilute densities and probably limited by the strength of dipolar interactions. Also, we evidence quantized vortices, efficiently trapped in the slight electrostatic disorder of our trapping potential [2]. Analyzing the interplay between quasi long-range order, vortex formation, and density profiles across the range of explored parameters it is finally shown that our experimental findings provide a direct evidence for a Berezinskii-Kosterlitz-Thouless crossover [3]. The work presented here results from contributions of S.Dang, R.Anankine, M.Beian, M.Alloing, E.Cambril, A.Lemaitre and M.Holzmann.

In the first part of the talk a brief overview of the research activities in area of fabrication and characterization of hybrid device structures based on amorphous silicon and organic semiconductors will be presented. The second part of the talk will be devoted to the discussion of the dynamic grating technique based on non-steady-state photo-EMF effect and its application for characterization of the photo-physical properties of organic, inorganic and hybrid semiconductors.

Magnetic nanoparticles reveal unique magnetic properties and relaxation phenomena which make them relevant for data storage, electronic and mechanical engineering, and biomedical applications1,2. Whereas the implementation of nanomagnetic properties into technological applications is progressing rapidly, understanding the microscopic origin of phenomena such as magnetization enhancement or decrease, magnetic anisotropy and the related magnetization distribution in individual nanoparticles, and interparticle interactions leading to aggregation or even ordered assemblies of nanoparticles is fundamentally challenging and needs intensive research. Polarized neutrons are an excellent, microscopic probe for spatial and time-resolved studies of magnetism. In this contribution, I will present examples of our recent studies of magnetic nanoparticles on different scales, ranging from atomic magnetic structures to the mesoscale intraparticle magnetization, and interparticle interactions in well-ordered arrays of magnetic nanocubes. References: [1] S. D. Bader, Rev. Mod. Phys. 78, 1 (2006); DOI: 10.1103/RevModPhys.78.1 [2] Q. A. Pankhurst et al., J. Phys. D: Appl. Phys. 36, R167 (2003); DOI: 10.1088/0022-3727/36/13/201

Topological states of quantum matter such as topological insulators and superconductors have been an active field of research in physics for many years. The recent experimental progress on their realization with ultracold atomic gases raises natural questions about the notion of topological quantum matter far from thermal equilibrium. In this talk, we present our theoretical findings in this context. We first discuss the non-equilibrium Hall response of a system initialized in a topologically trivial state before its Hamiltonian is ramped into a Chern insulator phase, comparing the coherent dynamics with effects of dephasing. In the second part of the talk, we discuss non-equilibrium properties of driven systems. In particular, we show how a perfect spin momentum locking, so far only known from edge states of two-dimensional topological insulators can emerge in the stroboscopic dynamics of one-dimensional Floquet lattice systems. Finally, we discuss dynamically defined topological quantum numbers that have no non-equilibrium counterpart, and report on their very recent experimental observation.

The coupling between the spin of an electron and its momentum is recognized to generate a variety of new phases in condensed matter systems. For example, it has been recently demonstrated that spin-orbit coupling can change the nature of a trivial insulator to endow it with topological properties. Or, in symmetry broken states, spin-orbit coupling permits exotic low energy excitations such as skyrmions in helimagnets. The interplay between superconductivity and spin-orbit effects gives rise to additional surprising features, which I will discuss in my talk. For instance, it stabilizes a condensate of Cooper pairs with finite momentum (a variant of the Fulde-Ferrel-Larkin-Ovchinikov state) up to high magnetic fields. Even more surprisingly, in the presence of spin-orbit coupling the transition temperature into the superconducting state may increase under application of a Zeeman magnetic field. Furthermore, such superconductors can exhibit persistent currents without magnetic fields in doubly connected geometries.

Developments in quantum optics and quantum engineering have brought forth the possibility of studying emergent collective phenomena in hybrid quantum light-matter systems. These platforms, which are intrinsically driven and dissipative, allow to probe fundamental many body physics in uncharted territories. In this talk I will discuss the steady state properties of paradigmatic lattice models of photons coupled to qubits, highlighting those aspects of the physics which are genuine nonequilibrium features and cannot be understood even qualitatively from an effective equilibrium. I will then discuss the role of non-Markovian bath correlations on a prototypical light-matter phase transition, the Dicke superradiance of N qubits coupled to a cavity, and show that ohmic/subohmic environemnts can induce a dissipative phase transition surving even at finite N, smoothly connected to the well studied Caldeira-Leggett/spin-boson one. Finally, I will present our ongoing effort to develop a new approach to driven-dissipative lattice models, based on non-equilibrium dynamical mean field theory.

Many-body localization (MBL) is the quantum phenomenon involving the interplay of disorder and particle interaction, characterized by the nonergodic behaviour of physical observables. It is intensively investigated theoretically within disordered many-body models and experimentally in optical lattices of cold atoms. Within a one-dimensional random-field spin system dynamical density correlations can be used as an indicator for the MBL phase and are closely related to the dynamical spin conductivity and d.c. transport. An analytical approach indicates that one dimension might still be singular with the subdiffusion in the ergodic phase. On the other hand, an analogous simulation of the Hubbard chain shows that a disordered potential does not induce full MBL, but only charge localisation.

In this talk, we will discuss our recent results on scrambling of quantum information and thermalization in the one-dimensional Bose-Hubbard model. Numerically exact results are presented for the out-of-time (OTO) as well as time ordered correlation functions using matrix product operator techniques. I will also give a short account on how to measure such OTO correlators in quantum gas microscopes. If time permits, we will also discuss pre-thermalization and heating dynamics in the periodically driven Bose-Hubbard model and a related O(N) field theory. Relevant references include: [1] Scrambling and thermalization in a diffusive quantum many-body system. A. Bohrdt, et al. [arXiv:1612.02434] [2] Floquet prethermalization and regimes of heating in a periodically driven, interacting quantum system. S. Weidinger, et al. [arXiv:1609.09089] [3] Prethermal Floquet steady-states and instabilities in the periodically-driven, weakly-interacting Bose-Hubbard model. M. Bukov et al. Phys. Rev. Lett. 115, 205301 (2015) [arXiv:1507.01946]

The chiral anomaly is based on a non-conserved chiral charge and can happen in Dirac fermion systems under the influence of external electromagnetic fields. In this case, the spectral flow leads to a transfer of right- to left-moving excitations or vice versa. The corresponding transfer of chiral particles happens in momentum space. We here describe an intriguing way to introduce the chiral anomaly into real space. Our system consists of two quantum dots that are formed at the helical edges of a quantum spin Hall insulator by means of magnetic barriers. Such a setup gives rise to fractional charges which we show to be sharp quantum numbers for large barrier strength. Interestingly, it is possible to map the system onto a quantum spin Hall ring in the presence of a flux pierced through the ring where the relative angle between the magnetization directions of the barriers takes the role of the flux. The chiral anomaly in this system is then directly related to the excess occupation of particles in the two quantum dots. This analogy allows us to predict an observable consequence of the chiral anomaly in real space.

It is demonstrated now both experimentally and theoretically that graphene is usually not flat but covered by ripples resulting from both intrinsic flexural instability of two-dimensional membranes and roughness of substrate. Thus charge carriers are not just Dirac fermions but Dirac fermions moving in a curved space. The effect of the corrugations on the electron spectrum can be described in terms of gauge (pseudo-magnetic) fields which result, in particular, in formation of pseudo-Landau levels predicted theoretically and found experimentally . These gauge fields can be used for "strain engineering", including tunable gap opening, quantum pumping [and creation of valley-polarized current. Ripples can induce puddles, that is, charge inhomogeneities. The scattering by the ripples is also one of the limiting factors restricting the charge carrier mobility in graphene.

Let D be a domain in some surface M. In fact, M could have any dimension. We would like to construct global quantum states on M which are localized in D and completely fill up D without much spillover outside D. We answer the question by making the quantum states eigenstates of a certain Hamiltonian defined so that D is the energy set H < E. The projection onto the corresponding eigenspaces defines a partial Bergman kernel and when normalized properly, it tends to 1 on D and zero outside D as Planck's constant tends to zero. There is a universal transition from 0 to 1.

Antiferromagnets are often used only for their resilience to external magnetic fields and appear ill-suited for use as active elements in spintronic devices. But when shrunk to nanometer dimensions antiferromagnets gain in functionality. We use low-temperature scanning tunneling microscopy to create small antiferromagnets of our own design atom by atom. At this extreme scale of individual atoms we can begin to harness quantum mechanical effects. In this way we can build antiferromagnetic nanostructures that show two stable magnetic states with no net magnetic moment or structures that have a unique ground state but feature rich magnetic dynamics of their excited states. I will introduce how inter-atomic exchange bias gives control over the spin ground state and excitation dynamics of antiferromagnets and demonstrate how coupling to the dissipative environment can be used to stabilize spins.

The quantum mechanical wave equation for a single photon is conspicuously missing from textbooks on quantum mechanics. Opinions range from the claim that this notion is meaningless to suggestions that Maxwell's electromagnetic field equations supply it in disguise. After reviewing the arguments pro and con, we will propose a Lorentz covariant wave equation for a photon and demonstrate that it possesses the desirable features. This is joint work with A. Shadi Tahvildar-Zadeh.

Since the discovery of quantum Hall effect, two-dimensional electron system under magnetic field has provided lots of intriguing physics topics both experimentally and theoretically, including integer/fractional quantized Hall conductance, anyon statistics, and non-Abelian particles. Electron-electron interactions play crucial roles in such quantum Hall problems. Even richer phenomena take place for two parallel two-dimensional sheets. Bilayer systems, or more generally multicomponent systems, have additional degrees of freedom, and they allow interlayer paired states at even-denominator filling fraction in each layer, which exhibits the Fermi sea of composite fermions without interactions. I will discuss the interlayer paired state in the presence of the Coulomb interaction and its pairing symmetry, and will make a comment on a possible topological order of the ground state.

Functional molecules on surfaces and their assembly into pre-defined architectures are key challenges in nanotechnology and of interest in various fields from molecular electronics over novel materials to molecular machines. Various examples of functional molecules, studied by scanning tunneling microscopy (STM) under ultrahigh vacuum conditions, will be discussed. Specifically designed molecular building blocks are connected to two-dimensional networks or one-dimensional chains, which can act as molecular wires. On the other hand, chemical processes within individual molecular can be controlled via their environment. This was observed for molecular switches, where the atomic-scale surroundings cause drastic changes in their switching probability. It could be shown that the rate of an intramolecular hydrogen transfer reaction can be tuned up and down by single atoms in the vicinity of the molecule. Single atoms were also found to modify the appearance of adsorbed molecules in STM images during their diffusion underneath the molecules. A combination of the two approaches will be discussed, potentially leading to heterogeneous molecular nanostructures that contain functional molecules.

Nonsymmorphic symmetry is crystal symmetry realized as combination of a point group operation and a non-primitive lattice translation. Many crystals have such nonsymmorphic crystal symmetry, but it effects on topological phases just begin to be explored very recently. It has been discovered that nonsymmorphic symmetry provides intrinsically new topological phases in insulators and superconductors. For example, the new topological phases may be characterized by new Z2 and Z4 topological indices, hosting exotic Mobius twisted surface states. In addition, nonsymmorphic symmetry also provides new gapless structure in nodal superconductors. In this talk, I will explain our recent results on nonsymmorphic topological phases.

Many-body systems with both coherent dynamics and dissipation constitute a rich class of models which are nevertheless much less explored than their dissipationless counterparts. The advent of numerous experimental platforms that simulate such dynamics poses an immediate challenge to systematically understand and classify these models. In particular, nontrivial many-body states emerge as steady states under non-equilibrium dynamics. In this talk, I use a systematic approach to study nonequilibrium phases and phase transitions and the dynamics in such models. I show that an effective thermal behavior generically emerges as a result of dissipation. Finally I use this equivalence to make quantitative predictions about the properties of the steady-state its dynamical properties.

Heterostructures with ultrathin layers are an exciting playground for materials science, since quantum confinement and interfacial properties often lead to new functionalities. In this talk a survey is given of the magnetic and magnetotransport properties of SrRuO3/manganite superlattices and trilayers. The itinerant ferromagnet SrRuO3 has a comparatively high conductivity and large magnetocrystalline anisotropy and can be grown in excellent quality in these all-perovskite heterostructures. The properties of the superlattices are rather complex showing a transition from orthorhombic to tetragonal crystalline symmetry of the SrRuO3 layers coupled with large changes in the magnetocrystalline and magnetoresistance anisotropy. Embedding of La0.7Ba0.3MnO3 within SrRuO3 layers leads to the stabilization of ferromagnetism above 150 K in single unit cell thick manganite layers. This appears to be related to an interfacial anomalous Hall effect observed in the superlattices.

Unconventional Superconductivity (USC) in layered Sr$_{2}$RuO$_{4}$ is of long-standing interest because it is long thought to be a superconducting analogue of $^{3}He$. However, resurgence of recent data points toward a much more involved pairing symmetry, where the interplay between multi-band character, sizable multi-band electronic correlations and strong spin-orbit coupling conspires to select an apparently rather unusual, hitherto unknown, USC pair symmetry. This mandates detailed revisiting of the normal state and, in particular, of the $T$-dependent incoherence-coherence crossover. Here, using a modern first-principles correlated view, we study this issue in the real structure of Sr$_{2}$RuO$_{4}$ in detail and present a unified and quantitative description of a range of unusual physical responses in the normal state across the crossover. Armed with these strengths, we propose that a new and important element, that of dominant (quasi-one-dimensional $xz,yz$) interband charge fluctuations in a ``Hund'' metal, may be a primary pair glue for USC in this system. We will emphasize internal consistency of our proposed scenario vis-a-vis a wide range of constraints imposed by extant data.

Conjugated organic molecules and polymers enable the demonstration of flexible semiconductor devices that can be processed at low temperature onto different substrates with variable form factors. Over the past decades, device performance has improved greatly as a large number of new compounds were synthesized and tested in various device geometries. While molecular-level models have been successful in providing a rational design of molecules and polymers for higher performance, models to predict the morphology of thin films and consequently macroscopic physical properties, such as charge mobility, remain elusive. The lack of such models has resulted in a trial and error approach and limits our ability to predict ultimate performance. Furthermore, organic solids exhibit photophysical properties that are unique and different from those of conventional inorganic semiconductors. The question then arises whether semiconductor equations developed for inorganic semiconductor devices can be applied to describe the physical properties of organic electronic devices. In this talk, we will highlight some fundamental differences in the physical properties between organic and inorganic semiconductors, but show that general models such as the Shockley diode equation can be applied to organic photodiodes and solar cells. In particular, we will emphasize the importance to understand and mitigate parasitic effects such as shunt resistance effects in these devices as they often overshadow the intrinsic physical properties and can lead to different interpretations of results.

Processes involving electrons and spins often take place on sub-picosecond timescales. This is particularly true for electronic phase transitions, ultrafast demagnetization, or local redox processes during resistive switching. The rapid development in femtosecond laser sources enables new experimental avenues which open the access to the electronic, spin, and lattice response on such ultrashort timescales. With JuSPARC we plan to establish a unique experimental environment to explore ultrafast processes in condensed matter by means of spectroscopy, microscopy and scattering techniques.

The Kardar-Parisi-Zhang (KPZ) equation provides one of the simplest non trivial examples of a Non-Equilibrium Steady State. Its universal features capture a very wide range of physical phenomena ranging from varied types of interfaces to turbulent hydrodynamic flows or thermodynamics of polymers in noisy environments. Enormous progress has been made in the last 10 years thanks to an exact derivation of the statistics of the KPZ dynamics in the case of one-dimensional systems driven by a stochastic white noise. The cases of higher dimensions and/or correlated noise remain however unsolved. In this presentation I discuss the physics of a one-dimensional interface that is subjected to a noise with smooth spatio-temporal correlations. This problem was previously studied numerically as well as with the Replica Trick in a Gaussian variational approach.It was found that the small scale features depend on the details of the microscopic noise while (up to the overall amplitude factors) the exact solution with white noise governs the large scales. In the present work, Functional Renormalisation Group (FRG) methods are employed in order to resolve the non-perturbative features of KPZ dynamics. The FRG makes it possible to follow the renormalisation group flow from its initial conditions all the way down to its fixed point, that is from microscopic dynamics to the large distance properties. I show that the exact solution emerges on large scales independently of the details of the noise correlations. Moreover the small scale features (and their dependence on the particular choice of the noise correlations) are resolved and compared to direct numerical simulations.

Oxide-based memristive devices, which exhibit two or more resistive states under electrical biasing, are promising candidates for future non-volatile memories as well as for active elements for neuromorphic computing. It has become widely accepted that memristive switching in oxides is in most cases connected with a voltage-driven oxygen vacancy movement and a resulting metal-to-insulator transition. However, the current knowledge of the microscopic details is very limited so far. One of the obstacles for its further elucidation has been that the net changes of the atomic and electronic structure during memristive switching are very small and occur primarily at the electrode interface or within nanoscale filaments. By employing different approaches of X-ray based spectromicroscopy, we could prove the formation of an oxygen vacancy enriched filament in epitaxial SrTiO3 thin film devices, which occurs preferentially at preformed positions such as extended defects. For high current operation, electric biasing goes along with the formation of SrO at the electrode interface which has a significant impact on the stabilization of the resistance states of the devices. In operando studies of SrTiO3 devices with photoelectron-transparent graphene electrodes enabled us to detect reversible changes of the O K-edge spectra within spatially confined regions of the devices. Based on these results, we obtain for the first time a quantitative estimate of the amount of oxygen vacancies shifted during the switching process.

Inelastic X-ray scattering at third generation synchrotron sources is a technique gaining an increasing interest from a growing community of users. Referring to the capabilities of beamline ID20 at the ESRF, I will recall the main fields of activity for IXS to probe electronic excitations in condensed matter, focusing in particular on resonant inelastic X-ray scattering experiments at the Ir L3 edge.

The magnetic properties of single atoms are significantly affected by details in the atomic- scale surrounding. One strategy to control the spin state and magnetic anisotropy is to embed the atoms into specifically designed organic ligands. Moreover, the surface plays a pivotal role in the magnetic properties when it interacts with the magnetic core. Here, we utilize scanning tunneling microscopy and spectroscopy to characterize the magnetic anisotropy and lifetimes of excited states of single paramagnetic molecules on a surface and point out possibilities to tune these.

After the discovery of superconductivity in a quasi-two-dimensional LaFeAs(OF) [1], the oxygen-free K- doped BaFe2As2 with the well-known quasi-two-dimensional ThCr2Si2-type (I4/mmm) structure becomes superconducting at 38 K [2]. The parent compound, AFe2As2 (A = Ca, Eu, Sr, and Ba), undergoes a commensurate spin density wave (SDW) antiferromagnetic transition. This SDW transition in AFe2As2 is suppressed by applied pressure, giving rise to superconductivity at low temperature. Since pressure tuning of a stoichiometric compound is a powerful technique for understanding phase transitions, AFe2As2 is promising candidate for studying the mechanism of superconductivity in these iron arsenide compounds. The 57Fe and 151Eu nuclear forward scattering (NFS) experiments under high pressure at low temperatures were carried out using the EuFe2As2 single-crystal sample on the NE1 beamline at the accumulation ring of the High Energy Accelerator Research Organization and on the BL09XU beamline at SPring-8 to investigate the correlation between magnetism and superconductivity in the pressure-induced EuFe2As2 superconductor. We find a microscopic coexistence of magnetism at the Fe and Eu sublattices and bulk superconductivity under pressure and at low temperature. The magnetic structure of the Fe sublattice changes to a modified antiferromagnetic structure in the superconducting state from the commensurate stripe-type antiferromagnetic one in the normal conducting state below 3 GPa. This modified antiferromagnetic structure in the superconducting state has never been observed in the magnetic ordered states in AFe2As2 under pressure and in substituted AFe2As2. The modified antiferromagnetic state with superconductivity in EuFe2As2 under pressure most likely is caused by the change of the electronic state of the Fe atom due to the effective As-As hybridization induced by pressure in the tetragonal symmetry of the crystal structure [3]. Furthermore, the order parameter of the modified antiferromagnetic state does not compete with that of superconductivity in the pressure-induced EuFe2As2 superconductor. References [1] Y. Kamihara, et al., J. Am. Chem. Soc. 130, 3296 (2008). [2] M. Rotter, et al., Phys. Rev. Lett. 101, 107006 (2008). [3] H. Kobayashi, et al., J. Phys.: Condens. Matter 25, 022201 (2013).

While epitaxial growth secures a source of high-quality graphene for large-scale applications, it also provides ample possibilities to study and tune the properties of graphene in terms of its structure, electronic bands or electron correlations. In particular, the choice of the substrate and growing conditions can be considered as a tool to form more complex structures based on epitaxial graphene. In this talk I will first present low energy electron microscopy characterization of graphene on Ir(111) and discuss the "intrinsic" stress-relaxation features, wrinkles, which are a result of graphene growth process on many substrates. It will be shown how the specific substrate interaction leads to a macroscopic quasi-hexagonal network of wrinkles. Also, the frustration within the network is identified as a governing force for the formation of complex wrinkle profiles with multiple lobes. Furthermore, the use of vicinal substrates for the growth of epitaxial graphene will be explored as a route for the uniaxial engineering of graphene. Based on scanning tunneling microscopy and angle-resolved photoemission spectroscopy experimental results as well as van der Waals-density functional theory calculations, we propose a general mechanism for the observed vicinal substrate restructuring induced by graphene. Moreover, by comparing graphene grown on flat and vicinal Ir substrates, we observe locally variable doping, the Dirac cone anisotropy, and a pronounced localization of states at step edges. In particular, the anisotropy is associated to a nanoscale potential modulation induced by the uniaxial pattern, and the pronounced states to an enhanced chemical binding and charge transfer. We will also show by atomic force microscopy and Raman spectroscopy that upon the transfer of such epitaxial graphene to a flat substrate, the uniaxial strain imposed by the growth process on a vicinal substrate is macroscopically preserved.

Currently 5d transition-metal oxides receive considerable attention due to their possibly significant spin-orbit coupling (SOC). In this talk, we will show, using ab initio calculations, that the SOC is indeed important in one-dimensional (1D) and 2D 5d transition-metal oxides. This is exemplified by addressing the varying magnetic moments in Fe or Co doped Sr2IrO4 [1] and the SOC tuning magnetism in Sr3NiIrO6 [2]. However, the SOC turns insignificant in 3D double perovskites Ca2FeOsO6 and Sr2NiIrO6 due to the band effect. In contrast to the high-temperature ferrimagnetism in Ca2FeOsO6 [3], an unusual magnetic frustration appears in Sr2NiIrO6 [4]. Finally, we will discuss how to ‘engineer’ a maximum magnetic anisotropy in Co, Ru, and Os adatoms on MgO (001) surface [5]. [1] X. D. Ou & H. Wu, PRB 89, 035138 (2014). [2] X. D. Ou & H. Wu, Sci. Rep. 4, 4609 (2014). [3] H. B. Wang et al., PRB 90, 054406 (2014). [4] X. D. Ou et al., Sci. Rep. 4, 7542 (2014). [5] X. D. Ou et al., submitted (2015).

Electrical controllability of gate-defined quantum dots (QDs) has brought significant developments in the coherent manipulation of electron spins and two-qubit gate operation toward scalable qubits for quantum computations. Moreover the suitability of gate-defined QDs to quantum information technologies would be considerably enhanced if spin states in the gate-defined QDs could couple to photon states coherently. Here we show that the photon polarization can couple to the spin degree of freedom in gate-defined GaAs QDs. Double QDs were fabricated in AlGaAs/GaAs quantum wells[1]. By synchronizing a pulse laser irradiation with a charge sensing measurement we performed the real-time single photoelectron spin detection in the double QD. First we show that the resonant inter-dot tunneling can offer a robust detection scheme of the single photoelectrons trapped in the double QDs [2.3]. In the two-electron regime, the inter-dot tunneling of the photoelectrons strongly depends on the relative spin orientation (parallel or anti-parallel) of the two QDs. Therefore by combining the resonant inter-dot tunneling scheme with the Pauli spin effect, we have realized the nondestructive detection of single photoelectron spins. Finally, we demonstrate the angular momentum conversion from single photons to single electron spins in the double QD from the dependence of the detected spins on the incident photon polarization. We also discuss the future prospects of the photon-spin quantum interface. This work was done in collaboration with T. Fujita, K. Morimoto, G. Allison, M. Larsson, H. Kiyama, S. Teraoka, S. Haffouz, D. G. Austing, A. Ludwig, A. D.Wieck and S. Tarucha. [1] G. Allison et al., Phys. Rev. B 90, 235310 (2014). [2] T. Fujita et al., Phys. Rev. Lett. 110, 266803 (2013). [3] K. Morimoto et al., Phys. Rev. B 90, 085306 (2014).

Topological insulators (TIs) realize a new quantum state of matter where insulating bulk states coexist with metallic surface states due to a nontrivial Z2 topology of the bulk valence band. I will discuss two prominent examples of novel physics found in TIs for which high-quality thin films grown by molecular beam epitaxy (MBE) are indispensable: I. A striking manifestation of the topological protection revealed in a systematic evolution of the surface conductance as a function of thickness in a series of high-quality Bi2Se3 films. II. The observation of unusual anisotropic magneto-resistance (AMR) and planar Hall effect (PHE) in bulk-insulating (Bi1-xSbx)2Te3 thin films, for which the concentration of Dirac-like surface carriers, and consequently the magnitude of AMR and PHE, can be controlled by electrostatic gating.

Neutron Spin Echo spectroscopy (NSE) is extensively used for the study of a large variety of slow motion phenomena (critical slowing down, relaxation effects, disordered dynamics, soft matter), which occur on a mesoscopic time scale between microscopic collision times and macroscopic dynamics. NSE applications in the field of magnetism benefit from the unique combination of high energy resolution with neutron polarization analysis, which allows a direct and unambiguous separation of the (often) weak magnetic scattering from all other structural contributions. The technique will be introduced and new results obtained in collaboration with the Cologne group on the multiferroics TbMnO3 and MnWO3 will be discussed.

The application of ideas developed in topology to the electronic band structure led to an intriguing discovery: materials can conduct electricity at the surface while remaining insulating at the bulk. These materials, called topological insulators, will have transformative impact on spintronics, low-power transport, and quantum computing. The search for a true topological insulator took years because even best candidates exhibited significant bulk conductivity. Only recently, literally in the past few months, several experimental groups established that samarium hexaboride, discovered in 1969 in Bell Labs, is a first topological insulator in its bulk form. In my talk, I review the theory which paved the way for this discovery. I will explain how crystalline symmetry, electron-electron interactions, and orbital degeneracy contribute to protecting the topological states in SmB6. In addition, I will discuss the experimental signatures of metallic surface states as well as the phenomena driven by interactions between the chiral electrons. Finally, I formulate the theoretical principles for discovering new topological insulators with designable properties

I’ll present recent exact results on the finite temperature and magnetic field transport in the one dimensional spin-1/2 Heisenberg model obtained by the Bethe ansatz (BA) method. Furthermore, I’ll discuss the thermodynamics, thermal transport and ESR of the spin S=1 easy-plane quasi-one dimensional quantum magnet NiCl2-4SC(NH2)2 (DTN) as well as induced magnetisation by light.

The quantum spin liquid is an exotic quantum state of matter in magnets, where conventional symmetry breaking is prevented down to the lowest temperature by strong quantum fluctuations. It has attracted interest from not only condensed matter physics but also quantum information, owing to the anticipated topological properties. In order to clarify the nature of the enigmatic state and explore its realization, we investigate thermodynamic properties of the Kitaev model and its variants, in which the exact solutions prove the quantum spin liquids in the ground state. We will discuss the surprisingly rich behaviors of quantum spin liquids revealed by our numerical simulations, including (i) thermal fractionalization of quantum spins into Majorana fermions, (ii) ``liquid-gas" transition by proliferation of loops of excited fluxes, and (iii) phase diagram of ``magnetic three states of matter".

In this presentation, I will introduce the field of time domain studies of correlated electron systems focusing on the interaction between phonons and high-energy excitation of electronic origin in cuprates. I will review our recent results in archetypal strongly correlated charge-transfer insulator (La2CuO4) revealing that photo-excitation in correlated materials pilots the formation of itinerant quasi-particles which are suddenly dressed (<100 fs) by an ultrafast reaction of the bosonic field. I will introduce our new approach to address correlated electron systems by a full quantum state reconstruction of ultrashort light pulses. I will explain Time Resolved Quantum Tomography and show how this provide a new perspective for addressing by time domain studies the quantum state of low energy boson in complex materials. References Nature Comm. 5, 5112 (2014) New J. Phys. 16 043004 (2014)

Functionality in materials bases not only on the possibility to affect their properties with external stimuli, but also on the time and length scales such switching occurs on. Time-resolved experimental techniques explore switching behavior directly and furthermore provide access to coupling mechanisms between different electronic and structural degrees of freedom. In the field of magnetic dynamics, the laser-induced suppression of helical spin order in heavy lanthanide metals reveals a new and very efficient angular-momentum transfer channel. For the antiferromagnetic semiconductor EuTe we studied the interplay between magnetic and structural dynamics. For the photon-pulse-induced Verwey transition in magnetite, we could further narrow down the time scale on which orbital order decays as well as gain further insight into the character of the low-temperature phase.

I will discuss several cases where far out of equilibrium dynamics of many-body systems gives rise to emergent quasistationary states, that can be loosely termed as prethermalized. Examples include dynamics of spin models with either short or long range interactions, quench dynamics of Fermi systems starting from a non-interacting state, systems with strong disorder, and systems close to integrability, such as low dimensional condensates. This suggests that emergent slow dynamics is a universal phenomenon of quantum systems. I will review possible realizations of these scenarios in experiments with ultracold atoms.

One of the highlights in the study of geometrically frustrated magnets is the emergence of nontrivial excitations in spin liquid states, in particular through the interplay between different degrees of freedom, e.g. between spins and mobile electrons. Such interesting phenomena have been found in Pr$_2$Ir$_2$O$_7$, a metallic spin ice with two electronic sectors. One is Pr based spin ice, where strong quantum effects are expected for ferromagnetic exchange coupling and many similarities are found to the quantum spin ice candidate Pr$_2$Zr$_2$O$_7$. Another sector is the 5$d$ conduction electrons for which novel topological phases have been predicted based on the semimetallic half-filled $J_{¥rm eff} = 1/2$ band due to strong spin-orbit coupling. We will discuss our observations of chiral spin liquid and quantum criticality in Pr$_2$Ir$_2$O$_7$ and topological effects due to semimetallic electronic structure.

On misoriented ("vicinal") surfaces, the terrace-width (spacing between adjacent steps) distribution can be related to the spacing distribution of repelling spinless fermions in one dimension, and thence to generalizations of the Wigner surmise (GWS) from random-matrix theory. The GWS expression also emerges as the steady-state solution of a Fokker-Planck description of step evolution. Subsequently we applied this approach to the areas of proximity cells (capture zones) of islands and quantum dots on surfaces; going beyond mean-field is necessary to get the correct relation between the characteristic GWS exponent and the critical nucleus size. A fragmentation model offers further insights. I compare analyses of island-size distributions and of scaling of island density vs. flux. I discuss several computer simulations and various experimental examples. In particular, for 6P on mica we need a novel quantitative rate equation treatment of "hot precursors". Applications to social phenomena include the distribution of Metro stations in Paris and the areal distribution of secondary administrative units (e.g., Landkreise and arrondissements).

The presentation concerns spatially dispersive optical metamaterials, for which the refractive index and impedance depend on the wave propagation direction. This dependence – often considered as unwanted – can as I will show bring new interesting applications to the field of metamaterials. In particular, it can be used to control the propagation characteristics of optical beams. I will also present a model that allows describing the interaction of optical beams with spatially dispersive metamaterials by making use of a vector angular-spectrum representation. Then, by applying the model, I will show that metamaterial slabs can be designed to have quite demanding functional abilities, for example, to act as an apertureless spatial filter or a diffraction-cancelling beam waveguide.

Following isolation of a single sheet of graphene, many other 2D systems were manufactured. Among them, single transition metal dichalcogenides (TMD) sheets have received a particular attention, as these materials exhibit intriguing electronic, optical and mechanical properties which can be controlled by varying material composition. Moreover, the properties can further be tuned by introduction of defects and impurities. In this lecture, after a brief introduction to the structure and properties of TMDs, I will present the results of our first-principles theoretical studies of the morphology, electronic and optical properties of 2D TMDs obtained in collaboration with several experimental groups. I will further discuss how defects and impurities can be used to tune the characteristics of 2D TMDs. I will also touch upon the stability and electronic characteristics of mixed TMDs, such as MoS2xSe2(1−x), which can be referred to as 2D random alloys. We demonstrated that 2D mixed ternary MoS2/MoSe2/MoTe2 compounds are thermodynamically stable at room temperature, and that the gap in such systems is direct and can continuously be tuned, as confirmed now by the experiments carried out by several groups.

In 2004 graphene ceased being a theoretical chimera to become the object of desire of the scientific community. In just few years, extraordinary properties have being demonstrated and many others are emerging as a result of the tremendous experimental and theoretical efforts devoted to this material. In this talk I will show how we use a scanning tunneling microscope to explore and manipulate graphene physics at an atomic level. I will mainly concentrate in two topics: -The investigation, at the atomic scale, of the impact that point defects like vacancies or atomic H have in the structural, electronic and magnetic properties of graphene layers grown on different substrates, where the pure bidimensionality of graphene gives to these atomic defects a critical role. -The study of the coupling of graphene with its local environment and how this affects to graphene’s quasiparticle dispersion, pseudospin, van Hove singularities..., which is absolutely critical to be able to integrate it in tomorrow's electronic devices.

Graphene as two-dimensional conductor and topologial insulators which possess two-dimensional metallic surface states around an insulating volume share also quasirelativistic properties of their charge carriers. Their characteristic linear E(k) band dispersions can be observed as so-called Dirac cones in angle-resolved photoelectron spectroscopy. In graphene, they are caused by pseudospin properties, in topological insulators such as Bi2Se3 by the electron spin. This opens the possibility to manipulate transport properties by opening a surface band gap at the Dirac crossing point through breaking of the sublattice symmetry of graphene or the time-reversal symmetry of topological insulators. In the talk, examples are shown where these attempts lead to surprising results. In a further step, graphene is made spin dependent by an extrinsic spin orbit interaction. The resulting spin polarization is in the surface plane - the same is the case with topological insulators. The opening of the Dirac point by symmetry breaking should in both cases turn the spin perpendicular to the surface. This is demonstrated for graphene. For topological insulators, we show that the spin can be turned out of the plane by polarized light in the photoemission process.

The large spectrum of perovskite oxides provides virtually unlimited possibilities to design and fabricate interfaces with functional properties. The ability to produce clean and sharp interfaces between oxide materials in a controlled manner provides appealing possibilities to generate novel electronic phases. Crucial steps in achieving high quality epitaxial oxide heterostructures have been the use of pulsed-laser deposition and molecular-beam epitaxy for the growth of multicomponent oxides, the ability to terminate oxide substrates at well-defined ionic planes, and the development of high pressure reflection high-energy electron diffraction to monitor the deposition of individual atomic layers. In consequence now epitaxial perovskite heterostructures can be fabricated with atomic-layer precision. In my presentation I will show examples of physical properties of epitaxial oxide heterostructures I have fabricated by pulsed-laser deposition. The emphasis will be on justifying the choice of the perovskite oxides that were interfaced (ferromagnetic manganites and ruthenates and ferroelectrics) and on the role of the interfaces, and their quality, in establishing the physical properties of these particular heterostructures.

Ferroelectrics exhibit a spontaneous dipole moment whose orientation can be determined by the application of electric fields. This allows for the controlled fabrication of ferroelectric domain patterns which are, for instance, in use for efficient frequency doubling using quasi phase matching. Although known since more than fifty years, and intensively investigated for applications, lots of basic knowledge and understanding of ferroelectrics is still missing. To start with, I will briefly introduce ferroelectrics as a material itself, and in particular show the potential of domain-structured ferroelectrics. In the main part of my talk I will discuss the possibilities of scanning probe microscopy (SFM) for a better understanding of ferroelectrics and also ferroelectric domains. Finally, I will present our results on structuring lithium niobate surfaces.

Starting from the archetypal FERMI externally seeded FEL, recent theoretical and experimental progress has shown the possibility of producing fully coherent, variable polarization and tunable, soft-X-ray, ultra-short pulses at high repetition rate. This ultimate achievement will unlock the gate for performing X-ray-based experiments that are qualitatively different from those available at any current or planned X-ray source. Here we will review the experiments and the ideas that represent the science frontier in soft X-ray, time-resolved spectroscopy, coherent imaging and scattering experiments. These studies will lead to an understanding of fundamental dynamics, occurring on the ultrafast time and nanometer spatial scales, needed for addressing a broad range of science essential for resolving our complex and long-term energy challenges, environmentally urgent questions and demanding problems in bioscience and novel materials.

For a long time, rare-earth (RE) intermetallic materials have attracted considerable interest be-cause of their exotic properties at low temperatures which include complex magnetic phases, valence fluctuations, heavy-fermion states, Kondo behavior and many others. It is widely believed that all of these properties stem from the delicate interplay between almost localized 4f electrons and itinerant valence band states. Our experiments aim to disclose details of this interaction, and reveal the fine electronic struc-ture of such materials near the Fermi level. To this end, we apply Angle-Resolved Photoemission Spectroscopy (ARPES) technique, which in spite of its surface sensitivity when working in UV mode, indeed, allows to gain deep insight not only into surface electron structure, but to probe the bulk de-rived states, too. In that regard, the particular point of our experiments is the proper discrimination of (sub-) surface and bulk related phenomena. In the past years we performed extensive ARPES studies on the RERh2Si2 (RE = Ce, Eu and Yb) series of compound as well as on YbIr2Si2 With our approach we were able to address some of the topics at the core of strongly correlated 4f electron systems: (i) observation of crystal-electric field (CEF) splitting of the 4f states and their fine dispersion induced by f-d hybridization; (ii) insight into the Fermi surface and manifestation of its strong 4f character. Disclosing its topology and features reflecting f-d coupling at the surface and bulk of the material. Studying its temperature dependence for a Kondo lattice (YbRh2Si2); (iii) clear evidence of the interplay of Dirac fermions and heavy quasi-particles; (iv) manifestation of unusual and strong ferromagnetism at the Si-terminated surface of an antiferromagnet (EuRh2Si2) caused by buried 4f magnetic moments of Eu lying 4-layers below the surface. [1] A. Chikina et al., Strong ferromagnetism at the surface of an antiferromagnet caused by buried magnetic moments, Nature Communications 5, 3171 (2014). [2] M. Höppner et al., Interplay of Dirac fermions and heavy quasiparticles in solids, Nature Commu-nications 4, 1646 (2013). [3] S. Danzenbächer et al., Insight into the f-derived Fermi surface of the heavy-fermion compound YbRh2Si2, Phys. Rev. Lett. 107, 267601 (2011). [4] D.V. Vyalikh et al., k-dependence of the crystal-field splittings of 4f states in rare-earth systems, Phys. Rev. Lett. 105, 237601 (2010). [5] D.V. Vyalikh et al., Tuning the hybridization at the surface of a heavy-fermion system, Phys. Rev. Lett., 103, 137601 (2009).

The interaction between light and the topologically protected spin polarized Dirac particles at the surface of topological insulators (TIs) constitutes the key issue for the development of opto-spintronics devices. Laser-based time and angle resolved photoelectron spectroscopy (tr-ARPES) in the sub-ps time domain represents an important experimental tool to investigate the ultrafast light-induced modification of the electronic properties in TIs. Several studies recently disclosed a very rich scenario, characterized by long-lived electronic population [ref. Sobota], Floquet-Bloch states [ref. Gedik science], reduction of the electron-phonon coupling [ref Crepaldi2] and the possibility to tune the Schottky barrier [ref. Marsi]. Despite the large number of experimental investigations of the out-of-equilibrium electronic properties [ref. Sobota JP, Chulkov, Perfetti, Crepaldi1], a detailed description of the microscopic scattering mechanisms within the surface state and between the bulk conduction band and the surface state is lacking, so far. Here an overview of the most recent discoveries relevant to the evolution of the electron population optically-injected in the topologically protected state at the surface is reported. In particular, we prove that only by measuring the spin-intensity signal over several orders of magnitude in spin- time- and angle resolved photoemission spectroscopy (s-tr-ARPES) experiments it is possible to comprehensively describe the optically excited electronic states in TIs materials. Remarkably, the experimental data, interpreted on the base of ab-initio fully relativistic spin resolved photoemission calculations and spin dynamics modeling, unambiguously show that two non-interacting electronic systems, derived from the excited surface and bulk states, are characterizing the non-equilibrium electronic properties of the archetypal TI, Bi2Se3.

In this talk I will present my research in supercontinuum light generation in photonic crystal fibers (PCF) and describe two applications of these fibers in high resolution interferometry. In one application PCF is utilized in a light source for high resolution stroboscopic white-light interferometry, the technique used for characterization of surface acoustic vibrations. Another application uses hollow-core PCF as a waveguide and gas cell for simultaneous measurement of the complex refractive index in gaseous analytes.

Since the initial discovery of skyrmion lattices in chiral magnets [1], there has been a tremendous growth in this field as an increasing number of compounds are found to have extended regions of stable skyrmion lattices [2] even close to room temperature [3]. These systems have significant promise for applications due to their size scale and the low currents or drives needed to move the skyrmions [4]. Another interesting aspect of skyrmions is that the equations of motion have significant non-dissipative terms or a Magnus effect which makes them unique in terms of collective driven dynamics as compared to other systems such as vortex lattices in type-II superconductors, sliding charge density waves, and frictional systems. We examine the driven dynamics of skyrmions interacting with random and periodic substrate potentials using both continuum based modelling and particle based simulations. In clean systems we examine the range in which skyrmion motion can be explored as a function of the magnetic field and current and show that there can be a current-induced creation or destruction of skyrmions. In systems with random pinning we find that there is a finite depinning threshold and that the Hall angle shows a strong dependence on the disorder strength. We also show that features in the transport curves correlate with different types of skyrmion flow regimes including a skyrmion glass depining/skyrmion plastic flow region as well as a transition to a dynamically reordered skyrmion crystal at higher drives. We find that increasing the Magnus term produces a low depinning threshold which is due to a combination of skyrmions forming complex orbits within the pinning sites and skyrmion-skyrmion scattering effects. If the skyrmions are moving over a periodic substrate, with increasing drive the Hall angle changes in quantized steps which correspond to periodic trajectories of the skyrmion that lock to symmetry directions of the substrate potential. [1] S. Muhlbauer et al Science 323 915 (2009). [2] X. Z. Yu et al. Nature 465, 901–904 (2010). [3] X.Z. Yu et al Nature Materials, 10, 106 (2011). [4] A. Fert, V. Cros, and J. Sampaio Nature Nanotechnology 8, 152 (2013).

Crystalline films of small organic semiconductors offer attractive potential for electronic and optoelectronic applications. To realize such applications on flexible substrates, graphene as a transparent electrode material comes into play. We studied the growth of the rod-like oligophenylene molecule para-hexaphenyl (6P) on metal supported as well as on exfoliated graphene. On clean graphene, the molecules grow in a lying fashion on the substrate whereas contaminations cause the molecules to stand up. Islands of upright 6P molecules are also observed on ion-bombarded surfaces. We found recently that on ion bombarded rutile(110) under oblique incidence the resulting nanometer surface ripples significantly change the shape of the 6P islands.

In this talk I will argue that the physics of the fractional quantum Hall effect my arise, under certain conditions and limitations, directly in one dimensional systems. The primary example that will be discussed is that of one dimensional wires with spin-orbit coupling. We will show that in the presence of Zeeman field and strong electron-electron interaction, a clean wire may form fractional helical liquid states with phenomenology similar to fractional quantum Hall liquids. Most notably, the wire's two terminal conductance is predicted to show fractional quantized conductance plateaus at low electron density. The elementary charge excitation of the wire is fractional and affects the shot noise in a nontrivial way. When the system is proximity-coupled to a superconductor, fractional Majorana bound states may be stabilized, giving rise to 1D analog of non-abelian anyons. I will discuss how disorder destabilizes these fractional phases. Possible numerical checks of the predicted effects will be discussed.

Fascinating phenomena such as superconductivity, charge density waves, Luttinger liquids or topologically protected edge states, can be realized on surfaces with atomic precision. Strongest correlation effects are expected in 1D structures and I will present in this talk two recent examples of geometrically and electronically confined systems fabricated by lithography and self-assembly, respectively, where the spectroscopic signatures and transport properties are determined by their edges and step structures: Pb adsorbed on Si(557) leads to formation of long range ordered quantum wire ensembles. At temperatures below 78K the system undergoes a refacetting transition accompanied by a metal-insulator transition. The insulating behavior found in the direction across the wires can be explained in terms of Fermi nesting [1]. Recently, the spin-orbit coupling in this system and its impact to magneto-transport measurements have been analyzed in detail. The spin polarization of the Fermi surface, manifested already in the suppression of spin-orbit coupling along the wires within the 1D transport regime, has been confirmed by spin-resolved ARPES measurements. The magnitude of the Rashba-splitting in this strongly anisotropic system evidences the formation of a spin density wave, which is the ground state of a CDW system electron-electron interaction [2]. The regime of mesoscopic transport physics has been entered with graphene nanoribbon structures (GNR) grown on SiC-MESAs. By means of a 4-tip STM/SEM system ballistic transport channels with mean free path lengths up to 10 µm at room temperature have been identified [3]. The existence of edge-localized states near the zig-zag edges have been confirmed by first STS spectra taken across the ribbon [4]. [1] C. Tegenkamp, et.al. PRL 95, 176804 (2005) [2] C. Tegenkamp, et.al. PRL 109, 266401 (2012) [3] J. Baringhaus ,et.al., Nature 506, 349 (2014) [4] J. Baringhaus et.al. JPCM, 25, 392001 (2013).

After nearly three decades of research on high temperature superconductivity, the transition temperature remains stagnant and our understanding incomplete. In this talk, I will argue that finding a novel material system that mimics the cuprate physics can provide a breakthrough. Ruddelsden-popper series iridates display remarkable similarity to high-Tc cuprates in terms of crystal, lattice, and magnetic structures, which lead to similarity in the effective low-energy physics as manifested by the unique cuprate fermiology reproduced in Sr2IrO4. How superconductivity emerges or fails in this new class of materials will reveal essential features of high temperature superconductivity; in particular the long-sought connection between the pseudogap and superconductivity.

The recent discovery of superconductivity at high temperature in iron-based materials have highly renewed the interest in the scientific community for the comprehension of the physical properties of superconductors and their applications. In this talks, we will give an overview of some new experimental results in this field and their theoretical interpretations by means of density functional theory calculations. In particular, we will present results on the effects of charge doping, pressure and reduced dimensionality on the electronic, Fermi surface topology and magnetic properties in conventional and non-conventional superconductors, highlighting their debated role in the appearance of the superconducting phase.

In this talk, I will present recent results on the unconventional superconductor LiFeAs obtained by scanning tunneling micrsopcopy/spectroscopy (STM/STS). STM. STM/STS is an important probe for studying such superconductors. On the one hand it provides the possibility to directly measure the spatial variation of the order parameter in the presence of a magnetic vortex lattice, and thus the coherence length. On the other hand, the quasiparticle interference (QPI) which arises from quasiparticle scattering off impurities, and which sensitively depends on the superconducting wave function can be probed directly. LiFeAs belongs to the class of iron arsenide superconductors which have been discovered in 2008. This material possesses very clean and charge neutral cleaved surfaces without a surface state and is thus perfectly suited for STM/STS. Our QPI data are perfectly consistent with band structure data derived from angular reolved photoemission spectroscopy [1]. However, the further analysis yields incompatibility with elementary s- or d-wave order parameters, suggestive of a more complex scenario [2]. I will discuss these findings in the context of further experimental data and theoretical results. [1] C. Hess, S. Sykora, T. Hänke, R. Schlegel, D. Baumann, V. B. Zabolotnyy, L. Harnagea, S. Wur­mehl, J. van den Brink, B. Büchner, Phys. Rev. Lett. 110, 017006 (2013) [2] T. Hänke, S. Sykora, R. Schlegel, D. Baumann, L. Harnagea, S. Wurmehl, M. Daghofer, B. Büch­ner, J. van den Brink, C. Hess, Phys. Rev. Lett. 108, 127001 (2012)

Nanoscale materials science has been dominated by graphene for the past ten years. The presentation will discuss the way forward by looking at a number of emerging novel materials that are expected to define the next decade of research in two-dimensional materials. The presentation will focus on two classes of semiconducting nanomaterials: hexagonal chalcogenides and silicene/germanene derivatives. Hexagonal gallium chalcogenides and indium chalcogenides are structurally quite similar to transition metal dichalcogenides while exhibiting very unique electronic properties. Their valence band will be discussed with emphasis on their use to engineer a very robust Lifshitz transition, and their optical properties will be examined to project their future use as UV photodetectors [1,2,3]. Silicane and germanane are fully hydrogenated covalent derivatives of silicene and germanene, respectively. Their stability will be discussed and their electronic structure presented. The talk will show that a tight-binding description of their valence and conduction bands requires the inclusion of second-nearest neighbor interaction and present a model with parameters obtained from first principles [4,5]. [1] Zólyomi, V.; Drummond, N. D.; Fal'ko, V. I.; Phys. Rev. B 87, 195403 (2013) [2] Liu, F. C.; Shimotani, H.; Shang, H.; Kanagasekaran, T.; Zólyomi, V.; Drummond, N. D.; Fal'ko, V. I.; Tanigaki, K.; ACS Nano 8, 752-760 (2014) [3] Zólyomi, V.; Drummond, N. D.; Fal'ko, V. I.; arXiv:1403.4389 [4] Drummond, N. D.; Zólyomi, V.; Fal'ko, V. I.; Phys. Rev. B 85, 075423 (2012) [5] Zólyomi, V.; Wallbank, J. R.; Fal'ko, V. I.; 2D Mater. 1, 011005 (2014)

Spin ice materials such as Dy2Ti2O7 and Ho2Ti2O7 provide a rare instance of emergent gauge symmetry and fractionalisation in three dimensions. Their elementary excitations carry a fraction of the magnetic moment of the microscopic spin degrees of freedom and they can be thought of as magnetic monopoles. Spin ice ground states are highly degenerate yet locally constrained and in the absence of monopole excitations, effectively no dynamics is possible. At low temperatures, the monopoles are sparse and dynamics becomes very slow. These systems are therefore prone to falling out of equilibrium at low temperatures, for instance following comparatively rapid changes in temperature or applied magnetic field. In this regime, a wealth of dynamical phenomena occur, including reaction diffusion behaviour, slow dynamics due to kinematic constraints, as well as behaviour that mimic the deposition of interacting dimers on a lattice. The situation is further complicated by the presence of disorder that, even at small densities, appears to have a sizeable effect on the low-temperature dynamics of these systems. Here we investigate some of these phenomena and we propose how to effectively extend existing theories to describe spin ice out of equilibrium. Nonequilibrium physics in spin ice is a novel setting which combines kinematic constraints, emergent topological defects, and magnetic long range Coulomb interactions.

In the on-going process of electronic device miniaturization new concepts are developed by utilizing hybrid systems. For this purpose the tailoring of the magnetic properties in these nanoscale systems is essential. We make use of hybrid systems that consist of a combination of magnetic molecules, graphene, thin films as well as magnetic nanopillars. Surface and interface effects are crucial to achieve new functionalities in these structures. This shall be demonstrated by means of two examples: 1) The magnetic coupling of paramagnetic molecules to ferromagnetic surfaces can be tailored by the help of an intermediate layer of atomic oxygen or graphene [1-4]. The fundamental understanding of the relevant interactions for the coupling is possible by combination of experimental and theoretical results utilizing ab initio calculations. 2) Ferrimagnetic CoFe2O4 (CFO) nanopillars embedded in a ferroelectric BaTiO3 (BTO) matrix are an example for a two-phase magnetoelectrically coupled system [5]. In particular, the [1] H. Wende et al., Substrate-induced magnetic ordering and switching of iron porphyrin molecules, Nature Materials 6, 516 (2007). [2] H. Wende, Molecular magnets: How a nightmare turns into a vision, Nature Materials 8, 165 (2009). [3] M. Bernien et al., Tailoring the Nature of Magnetic Coupling of Fe-Porphyrin Molecules to Ferromagnetic Substrates, Phys. Rev. Lett. 102, 047202 (2009). [4] S. Bhandary et al., Graphene as a Reversible Spin Manipulator of Molecular Magnets, Phys. Rev. Lett. 107, 257202 (2011). [5] C. Schmitz-Antoniak et al., Electric in-plane polarization in multiferroic CoFe2O4/BaTiO3 nanocomposite tuned by magnetic fields, Nature Communications 4:2051 doi: 10.1038/ncomms3051 (2013).

The description of strongly coupled systems presents a challenge for theoretical physics, since usual weak-coupling methods based upon perturbation theory typically fail. Therefore, new tools are required to describe strongly interacting quantum matter, like the quark gluon plasma created in heavy ion collisions or cold fermionic atoms at unitarity. Holographic correspondences between gravity theories and quantum field theories can provide such tools. I review basic aspects of holography and summarize our recent results in flat space holography

Some magnets are exceptional solid-state model systems for high-precision studies of fundamental quantum phenomena in physics like Bose-Einstein condensates and Tomonaga-Luttinger liquids [1,2]. Further examples include complex ground states and correlations realized in low-dimensional and frustrated systems [3,4], exciting physics of impurities and quenched disorder [5], and dramatic events near quantum phase transitions [6]. In this colloquium studies of model oxides and halides by neutron scattering and complementary experimental techniques will be presented. Classical phase transitions in such compounds can be controlled easily by temperature, interactions and quantum fluctuations by chemistry and pressure application, and correlations and particle numbers by magnetic field. The experimental results will be discussed in the context of recent developments of powerful computational methods enabling fully quantitative analysis and of related work on other model systems like gases of ultracold atoms. [1] T. Giamarchi et al., Nature Physics 4, 198 (2008). [2] B. Thielemann et al., Phys. Rev. Lett. 102, 107204 (2009). [3] Y. Kohama et al., Phys. Rev. Lett. 109, 167204 (2012). [4] F. Casola et al., Phys. Rev. Lett. 110, 187201 (2013). [5] S. Ward et al., J. Phys.: Condens. Matter 25, 014004 (2013). [6] P. Merchant et al., Nature Physics, in press (2014).

The Kondo lattice model, if properly generalized, provides microscopic understanding of rich varieties of ordering phenomena in strongly correlated electrons. Moreover, the model can predict new orders that can be tested experimentally. We demonstrate novel electronic orders in the Kondo lattice by including multi-bands of conduction electrons. The resultant two-channel Kondo lattice at zero temperature cannot remain disordered because of the remaining fractional entropy at each site, and unconventional orders are realized. The hidden symmetry SO(5) of the model at half-filling of conduction bands causes degeneracy between multipole composite orders with three components, and an odd-frequency superconducting order with a complex order parameter. Detailed numerical calculation shows that deviation from the half-filling makes the superconductivity most stable for a certain range of the density. We discuss peculiar features of the order parameters.

This talk focuses on aspects of Bose-Einstein condensation in a trapped, dilute atomic gas beyond the Gross-Pitaevskii mean field regime. By use of the formalism of pair excitation introduced by Wu, based on earlier works by Lee, Huang and Yang, the description of many-body bound states is addressed via formal asymptotics. There are two related questions to be discussed. (i) What is a plausible modeling of pair correlations by use of Partial Differential Equations, beyond the Gross-Pitaevskii limit? (ii) What is the effective depletion of the macroscopic quantum state (condensate) for a spatially varying scattering length having a periodic microstructure?

Resonant plasmonic metal nanoparticles allow manipulating optical fields on the nanoscale. In analogy to the radio frequency domain such particles are called optical antennas. They have led to a multitude of breakthroughs, e.g., in enhancement of light emission and radiation engineering of single photons, as well as ultrasensitive sensing. I will present the results of our work on enhancing the optical properties of single solid-state quantum emitters using optical antennas and on applying plasmonic nanoantennas in point-to-point links. Coupling a quantum emitter to the antenna allows enhancing both its absorption efficiency as well as emission rate. Meanwhile, point-to-point links based on optical antennas are a promising concept to transmit optical signals between nano-objects. I will show the first realization of such a link.

The present focus on quantum critical phenomena in correlated matter is driven by the puzzling enigma concerning the origin of unconventional superconductivity (SC). In a weak coupling picture, the magnetic ordering is a spin density wave (SDW) induced by Fermi surface (FS) nesting of itinerant electrons. Consequently, superconductivity results from pair scattering between the electron - and holelike FS pockets. A drastically different viewpoint proposes that magnetism and superconductivity develop out of localized moments which are generated by strong electron - electron correlations (strong coupling limit). The similarities in the superconducting phase diagrams of correlated materials tempt us to believe that there may be a certain fundamental aspects shared between the iron pnictides, chalcogenides, cuprate superconductors and the HF materials. Against this backdrop, heavy Fermion (HF) materials acquire a new significance. This class of materials is inherent close to a magnetic quantum phase transition frequently in conjunction with emergence of unconventional superconductivity [1]. Recently it was shown that strong and weak coupling scenarios are complementary if taking into account the "dimensionality" of the critical fluctuations which in most cases mirrors the structural dimensionality of the compound [2-4]. In this presentation I will discuss the global phase diagram of heavy fermions with an experimental focus on the CenTmIn3n+2m (T = transition metal, n = 1, 2,..; m = 0, 1,..) family. This family is predestinated in order to understand how magnetism and superconductivity is influenced when going from 3D to 2D HF structures. CenTmIn3n+2m compounds display both attributes with cubic CeIn3 at the 3D and CeCoIn5 as an example of the opposite (2D) end of the series. I will present first results on single crystals of two new members Ce3PdIn11 and Ce3PtIn11 located in between [5]. At ambient pressure superconductivity emerges in the complex antiferromagnetic state below Tc = 0.42 K (Tc = 0.32 K) in Ce3PdIn11 (Ce3PtIn11). Both phenomena coexist in a wide range of the pressure - temperature phase diagrams. The critical pressure where TN -> 0 and Tc is maximum equals pc ~= 1.6 GPa. [1] H. von Löhneysen et al., Rev. Mod. Phys. 79, 1015 - 1075 (2007). [2] Q. Si, Physica B 378-380, 23 (2008) [3] J. Custers et al. Phys. Rev. Lett. 104, 186402 (2010) [4] J. Custers et al. Nature Mat. 11, 189 (2012) [5] J. Prokleska et al. submitted to Science (2013)

Atoms with bosonic `electrons' have always been more of a theoretical fancy than a model of real physical matter, at best suited to illustrate how important the Pauli principle is in our world, but mainly of only academic interest to mathematical physicists. All this is about to change in the not-too-distant future if the recently discovered anti-alpha particles (anti-He nuclei which are twice positively charged bosons in their nuclear ground state) can be produced in sufficiently large numbers, captured, and substituted for the conventional electrons in the atomic hull around conventional nuclei (a similar feat has already been accomplished with anti-protons, but these are fermions, not bosons). In my talk I will try to survey what is mathematically rigorously known about atoms with bosonic `electrons' and, more importantly, present a list of interesting open problems which still await their solution.

Eigenfunctions of the Schrodinger operator (or a Laplacian on a domain or manifold) represent modes of vibrations of drums and membranes. In quantum mechanics they represent stationary states of atoms. Understanding shapes and sizes of eigenfunctions allows one to visualize these objects. An intriguing problem is to relate the shapes and sizes of eigenfunctions to the underlying classical mechanics, such as the geodesic flow of (M, g) or the dynamics of billiard trajectories on a billiard table. In this talk we will explain the role of eigenfunctions in quantum mechanics and discuss both classic and new results describing nodal (zero) sets of eigenfunctions. The new results relate nodal sets to classical dynamics. No prior knowledge of quantum mechanics is assumed.

Due to the locking of the electron spin to the momentum at the conducting surface of a 3D topological insulator, new physical phenomena arise in transport and superconductivity. The topological nature of surface states can be evidenced by measuring the Berry phase of quantum oscillations in magnetotransport. I will quickly review the current status of the Bi-based class of 3D topological insulators in terms of how good a topological insulator one can make nowadays. Also the conducting interface between insulating LaAlO3 and SrTiO3 bears great resemblance to these topological surfaces. When superconductivity is induced into a topological state by using the proximity effect or a Josephson junction configuration, the superconducting order parameter can have a large p-wave component, leading to Andreev bound states at zero energy, the so-called Majorana states. I will review the status of our experiments on topological Josephson junctions based on the Bi-based 3D topological insulators, as well as conducting interfaces between LaAlO3 and SrTiO3.

In many of the advanced solids exhibiting macroscopic quantum phenomena (e.g. superconductivity) their functional properties arise from a delicate balance between interactions among different degrees of freedom (electrons, phonons, spins). An example of such is the appearance of Charge Density Wave (CDW) ground state in several quasi one- or two-dimensional metals. According to the standard Peierls’ scenario the appearance of the CDW phase is driven by the divergent static electronic susceptibility at a wave vector q=2k­F. This classical description has been challenged [1], arguing that the appearance of CDWs, in particular in the transition-metal dichalcogenides, is due to strong (q-dependent) electron-phonon coupling [1]. I will review some recent studies where some of the open questions were addressed utilizing femtosecond time-resolved optical [2] and structural probes [3-5]. Using femtosecond electron diffraction we were able to directly follow the photo-triggered dynamics of the periodic lattice distortion (order parameter) in dichalcogenides [3,4], point out the importance of three-dimensionality for the existence of CDWs in these quasi-2D systems [4], and to track the different stages of the photo-induced phase transition between different CDW phases [5]. [1] e.g. M. Johannes and I. Mazin, Phys. Rev. B 77 (2008); L. Gor'kov, Phys. Rev. B 85, 165142 (2012); H-M. Eiter, PNAS 110, 64 (2013). [2] H. Schäfer, et al., Phys. Rev. Lett. 105, 066402 (2010); H. Schäfer, et al., EPJ-ST 222, 1005 (2013). [3] M. Eichberger, et al., Nature 468, 799 (2010). [4] N. Erasmus, et al., Phys. Rev. Lett. 109, 167402 (2012); E. Eichberger, et al., APL 102, 121106 (2013). [5] K. Haupt, et al., in preparation.

This talk will focus on experimental search and discovery of novel forms of quantum order in metallic and insulating magnets, intercalated compounds, ferroelectric systems and multi-ferroic materials. Particularly discussed will be the pressure-induced superconductivity and critical phenomena in the vicinity of quantum phase transitions. Materials tuned to the neighbourhood of a zero temperature phase transition often show the emergence of novel quantum phenomena. Much of the effort to study these new emergent effects, like the breakdown of the conventional Fermi-liquid theory in metals has been focused in narrow band electronic systems. But Spin or Charge ordered phases in insulating systems can also be tuned to absolute zero using hydrostatic pressure. Close to such a zero temperature phase transition, physical quantities like resistivity, magnetisation and dielectrics constant change into radically unconventional forms due to the fluctuations experienced in this region giving rise to new kinds ordered states including superconductivity in the metallic systems

Creation of isolated Majorana fermions, the simplest type of non-Abelian anyons, in one out of many superconductor-semiconductor hybrid systems seems reachable within a few years time. I will introduce a scheme for manipulation of Majorana fermions using existent superconducting circuits, namely transmon qubits. Within this framework I will consider in particular the minimal setup required to measure the unusual properties of Majorana fermions, and a possible implementation of a scalable quantum computer that relies on the topological properties of Majorana fermions.

Traditionally, condensed matter physicists have classified phases of matter according to their symmetries. Over the last few decades, it became clear that near zero temperature, there are plenty of phases which lie beyond this classification scheme. These intrinsically quantum mechanical states of matter lack any ordinary order parameter; they can be thought of as a strongly fluctuating quantum liquids. Nevertheless, they posses a hidden underlying order, known as "topological order". The quantum Hall effect is a celebrated example of such a phase; several others have been discovered recently, and many more have been predicted theoretically. The elementary excitations of topologically ordered states can be thought of as emergent particles; intriguingly, these particles can obey unusual exchange statistics rules which resemble neither those of bosons nor of fermions. This property makes topological phases potentially useful as building blocks for future decoherence-free quantum processing devices. In this talk, I will describe some modern insights into the nature of these phases, and their characterization in term of their quantum entanglement. I will also discuss a new route to realize novel phases that arise on the boundaries of other, previously known topologically ordered states.

Heavy fermion (HF) systems are metals where the weak hybridisation between nearly localized f-electrons and the mobile conduction electrons, i.e. the Kondo eect, leads to a Fermi liquid (FL) ground state with narrow bands and quasiparticles with strongly enhanced eective electronic masses. When the magnetic RKKY interaction becomes comparable to the Kondo interaction, magnetic order can appear, mostly at very low T. The magnetic order can be suppressed by an external parameter, e.g. pressure or magnetic eld, inducing a quantum phase transition (QPT) at T = 0. If this QPT is continuous, the associated quantum critical point (QCP) is surrounded by a non- FL regime of quantum critical fuctuations where unconventional superconductivity or novel phases of matter may arise [1]. The unambiguous observation of antiferromagnetic (AFM) QCPs in HF systems [2] has led to an increasing number of theoretical and experimental works in order to understand QPTs as deeply as their classical counterpart. Although it has been demonstrated that in antiferromagnets QCPs exist, in ferromagnets there is still no clear evidence. Intensive investigations have shown that metallic ferromagnets are inherently unstable [3, 4] and do not exhibit a FM QCP. However, in the recently discovered HF system YbNi4P2, a quasi-1D ferromagnet with a remarkably-low TC = 0:15K [5, 6], the T-divecgence in the Grüneisen ratio [7] points to the presence of a FM QCP. I will present a general overview of the state of the art of FM quantum criticality in HF systems, discussing in particular the cases of YbNi4P2 [5, 6], CeFePO [8], CePd1-xRhx [9] as well as the AFM system YbRh2Si2 [10] where FM order is induced by chemical pressure [11].

Nanoparticle superlattices can be considered as novel type of materials with controllable electronic, optical and magnetic properties. Their building blocks are nanoparticles (or 'nanocrystals') from a metallic, metal-oxide, or semiconducting material or hybrid between different materials. I will report about the structural and magnetic properties of 2- or 3-dimensional 'superlattices' of magnetic nanoparticles. Depending on the substrate and the self-assembly technique we can fabricate either 'superlattice films' showing various growth modes as known from classical thin films or 3d mesocrystals. The lateral ordering is quantified using electron microscopy and grazing incidence small angle X-ray scattering (GISAXS). The magnetic behavior and correlations are investigated by magnetometry, polarized neutron reflectometry (PNR) and grazing incidence small-angle neutron scattering (GISANS).

Rather commonly, Hamiltonian systems at nonzero temperature in one spatial dimension have short range static correlations, but long-lived excitations. The slowest modes are the locally conserved fields. To capture their universal behavior we develop a nonlinear version of fluctuating hydrodynamics. The best studied systems are classical anharmonic chains. We compare with MD simulations.

It is well known how to construct a formal semiclassical (or perturbative) quantization of a classical field theory on $\R^n$. The answer is given by formal power series in Feynman diagrams. In this talk we will focus on how to do it for space time manifolds with boundary. The main example is Chern-Simons classical field theory. The talk is based on joint work with A. Cattaneo and P. Mnev.