Detailed Schedule

Chiral antiferromagnets such as Mn₃Sn provides a unique platform for research and development of correlated topological physics and ultrafast spintronic functionalities as they exhibit the large transverse responses due to the combination of magnetic octupoles without magnetization and Weyl fermions. Here we present our recent work on three different ways of manipulation of correlated Weyl fermions in Mn₃Sn; 1) by using strain through piezomagnetic effect, 2) by using electrical current through spin-orbit torque, and 3) by using intense pulse laser through doping photocarrier. Our work clarifies the energy hierarchy of spin and charge degrees of freedom of correlated Weyl fermions and paves the way for developing the ultrafast nonvolatile memory.

I will discuss the recently constructed unified theory of order parameters in two dimensional Dirac systems such as graphene and its cousins. For N two-component Dirac fermions the maximal symmetry of the low-energy theory is SO(2N), and the order parameters that preserve Lorentz symmetry and provide mass-gaps for fermions belong either to the singlet or to the two-component irreducible symmetric tensor representation of this group. The remaining nematic states which break Lorentz symmetry fall into the antisymmetric tensor representation. The role of the chemical potential in the ground state selection, connections to basic fermionic models such as the Gross-Neveu, possible new universality class of Dirac semimetal-insulator transition, and features of the large-N approach will be addressed.

Pioneering heat transport measurements have established that the ν = 5/2 quantum Hall state in the half-filled first Landau level exhibits a thermal Hall conductance K quantized in half-integer multiples of K₀ = π² k_B²T) / 3h. A half-integer value is a signature of neutral Majorana edge modes, in turn linked to the presence of non-Abelian anyon excitations in the bulk, which numerical studies indicate is likely in the Moore-Read 'Pfafian' quantum Hall state, or its particle-hole conjugate 'AntiPfaffian'. However, the observed value of K/K0 = 5/2 corresponds to the particle-hole symmetric 'PH-Pfaffian' state. This is in tension with the expectation from the bulk-boundary correspondence, since particle-hole symmetry is not expected to be present in the bulk: the Pfaffian would yield K/K0 = 7/2, and the AntiPfaffian would yield K/K0 = 3/2. A variety of mechanisms have been invoked to explain this discrepancy, but have been either ruled out by further experiments or else involve fine-tuned scenarios. In this talk, I will propose an alternative resolution of this puzzle via a new type of ‘edge reconstruction’ involving only the neutral Majorana sector, and argue that it is both physically natural and consistent with various features of the experiments.

Moire superlattices in two-dimensional semiconductors have enabled the observation of a wealth of physical phenomena driven by strong electronic correlations, ranging from Mott-Wigner states to fractional Chern insulators. After reviewing electronic and optical properties in this new playground for condensed matter physics, I will describe recent experiments in a frustrated triangular lattice, where we measured magnetic correlations in the vicinity of a Mott-insulator state of electrons. By observing electronic magnetization through the strength of the polarization-selective attractive polaron resonances, we find that when the Mott state is doublon doped, the system exhibits ferromagnetic correlations in agreement with the Nagaoka model. Our observations, which are supported by DMRG calculations, provide a direct evidence for itinerant magnetism with a kinetic origin.

Magic-angle twisted graphene superlattices have shown robust superconducting states with orders of magnitude higher transition temperatures than those in other graphene-based superconductors, displayed ultrastrong coupling and large violations of the Pauli limit, and exhibited signatures of unconventional pairing in tunneling spectroscopy. However, the exact nature and origin of the superconductivity are yet to be known. Here, we create a novel structure that vertically couples two magic-angle graphene superlattices, where the carrier density in each magic-angle graphene layer, as well as the tunneling between the two layers, can be controlled independently. Interactions between the two layers in both the tunneling regime and the Josephson regime reveal the intricate nature of superconductivity and correlated states. Our results pave a new way to study 2D quantum materials and provide a potential platform for realizing a clean vertical tunnel junction.

Photonic systems form a special platform for topological physics, that is relatively easily designable and accessible. Photonic crystals can be fabricated with spatial symmetry designed at will. Eigenwavefunctions, band structures and correlations often can be measured directly through spectroscopic methods. Going beyond topological photonics in the non-interacting limit, dissipation and nonlinearity introduced through matter medium can lead to complex phases and rich topological phenomena. We will discuss a few examples of experimental, dissipative nonlinear topological photonic systems.

In our quest of finding new interacting topological phases, we are introducing a new platform of quantum spins with fractional topological numbers. This quantum platform can be realized in circuit quantum electrodynamics architecture. This allows us to measure the presence of a Einstein-Podolsky-Rosen pair through a pair of one-half topological numbers and also to realize topologically protected qubits for quantum information. For quantum spin arrays, this also builds an analogy with Kitaev p-wave superconducting wires and with other topological fractional numbers. This also allows us to propose a new characterization of interacting topological matter through light and transport. We show applications in condensed-matter systems through novel phases of matter e.g. a topological semimetal in two dimensions and Majorana liquids in one dimension.

Coaxial cable networks are an excellent experimental platform for the study of the topological properties of finite structures, because
their radio-frequency properties map directly onto a tight-binding model. The lattice sites correspond to the network junctions, where the variables are the voltages, and the hopping terms are determined by the impedances of the connecting cables. By swapping between cables with different impedances, it is straightforward to fabricate an ensemble of disordered structures, In this talk, I will give an overview of our recent experimental and theoretical work on cable networks. This includes demonstrations of topologically protected states in disordered SSH and graphene structures, the observation of a topological phase transition and Dyson singularity in random one-dimensional chains, and a full
topological classification of finite chiral networks based on concepts from graph theory.

The tight-binding Su-Schrieffer-Heeger (SSH) model describes 1D periodic chains of resonators with alternating coupling strengths. For certain configurations, topologically protected collective states occur at the edges. Here, we propose a plasmonic chain SSH-system with alternating few-nanometer gaps, fabricated from mono-crystalline Au-microplatelets by means of He-ion beam milling. Full FDTD simulations show the occurrence of edge modes for such geometries in real space. The frequency at which edge states occur can be fully controlled via particle size and the next-neighbour coupling via the gaps. Scattering scanning near-field optical microscopy (sSNOM) experiments provide first evidence for the presence of topologically protected edge states.

I will give a broad overview of our recent progress on Landau-level spectroscopy of Dirac and Weyl semimetals. With infrared light, one can excite carriers from one Landau level into another, causing inter-Landau-level transitions. This technicque, known as Landau-level spectroscopy, has been widely employed since the early 1950s as an extremely sensitive probe of semimetal and semiconductor band structures.

Through recent advances, one can resolve intricate complexities of topological materials’ bands, all while discovering new physics. I will present highly detailed inter-Landau-level transition maps in extreme magnetic fields, focusing on select topological materials: Dirac semimetals, a weak topological insulator ZrTe₅, and a Weyl semimetal TaAs. I will discuss how we can apply magneto-optical techniques to confirm or eliminate possible magnetic topological semiconductors.

Two-dimensional coherent nonlinear optical spectroscopy (2DCS) is of great interest in order to deconvolute excitation continua in correlated magnets, potentially allowing to analyze individual quasiparticles, including those of fractionalized magnets. We study 2DCS susceptibilities for two central models of frustrated quantum magnetism, i.e., the Kitaev magnet and the J₁-J₂ chain. The Kitaev magnet hosts a quantum spin-liquid, featuring fractionalization in terms of mobile Majorana fermions and static flux-visons. We show that 2DCS does not only probe characteristic features of both fractional excitations, but also allows to extract single quasiparticle lifetimes from the multi-particle continua of the 2DCS response functions. These properties will be discussed over a wide range of temperatures. For the J₁-J₂ chain, we show that in a parameter range of spiral correlations, 2DCS displays anti-diagonal spectral features which are very similar to those of the Kitaev magnet.

Conventionally, we think of Fermi surfaces as being the hallmark of normal metals, whereas superconductors have a gap towards quasiparticle excitations, which may have point or line nodes in unconventional superconductors. However, not only can superconductors have Fermi surfaces but these appear generically for inversion-symmetric multiband superconductors that break time-reversal symmetry. I will give an introduction to the physics of this phenomenon, including its physical explanation in terms of a pseudomagnetic field, and discuss potential instabilities towards more conventional superconducting states and against lattice deformations.

Beyond the spin, electronic states in crystalline solids can exhibit finite expectation values of orbital angular momentum (OAM). This phenomenon has attracted considerable attention in recent years and can particularly be traced back to the following key applications: (i) It appears as an interesting quantum degree of freedom raising the potential of orbitronics, i.e., orbital analogs to spintronic phenomena. (ii) OAM has been proposed to be a useful observable to assess nontrivial topology in the band structure of topological quantum matter. In this talk, I will shed light on the OAM from the perspective of angle-resolved photoemission spectroscopy (ARPES). Combining ARPES with light-polarization-dependent measurements allows us to address the momentum-dependent orbital textures in quantum materials. Based on the experimental results presented, I will discuss the potential of utilizing OAM (i) towards orbitronic transport and (ii) to detect unexpected topological features.

The THz frequency domain offers unique capabilities for applications in wireless communication, spectroscopy, and imaging. Communication technologies rely on frequency mixing, but traditional methods using nonlinear components such as diodes and transistors are limited to sub-THz bands.

This presentation will discuss our demonstration of THz upconversion using a high-mobility HgTe-based topological insulator, characterized by its exceptional third-order nonlinearity. We employ intense sub-THz radiation, facilitating efficient nonlinear mixing with signals from a photoconductive antenna. This method results in a strong THz response at the sum and difference frequencies, achieving a field conversion efficiency of approximately 2% with a tensile-strained HgTe nanolayer at room temperature.

When fermions are in equilibrium with a heat bath they occupy states according to the well-known Fermi-Dirac distribution. When the bath is very cold, this distribution displays a discontinuous jump defining the location of the Fermi surface. But how should fermions occupy states when they are driven away from equilibrium by a time dependent periodic force? would their occupation still have sharp jumps? or would the periodic drive simply heat them up and smear their fermi surfaces?

We have found that the non-equilibrium steady states of periodically driven fermions can retain sharp fermi surfaces and remain much more quantum than previously anticipated. Interestingly, the non-equilibrium steady states of fermions can be very different in a grand-canonical setting where the system exchanges particles and energy with the bath (i.e. fermions coupled to a fermionic bath) and a canonical setting where the system only exchanges energy with the bath (i.e. fermions coupled to a bosonic bath). In the grand-canonical setting there is a non-equilibrium fermi-liquid-like steady state with an occupation that displays multiple jumps resembling a staircase shape, and therefore features multiple non-equilibrium fermi surfaces. In contrast, in the canonical setting there is a non-equilibrium non-fermi-liquid steady state where the occupation does not have jumps but rather multiple sharp kinks, which, remarkably, remain sharp even when the bath is at finite temperature. Some of the platforms and regimes to realize experimentally these states are readily accessible, and include ultra-clean and cold two-dimensional metallic systems such as Gallium arsenide hetero-structures or graphene irradiated with microwaves.

Quantum devices characterized by non-Hermitian topology are predicted to show very robust and potentially useful properties, but realizing them has remained a daunting experimental task. This is because non-Hermiticity is often associated with gain and loss, which would require precise tailoring to produce the signatures of nontrivial topology.

We use the nonreciprocity of quantum Hall edge states to directly observe the non-Hermitian topology of a multi-terminal ring. Our transport measurements evidence a robust, non-Hermitian skin effect: currents and voltages show an exponential profile, which persists also across Hall plateau transitions. Our observation of non-Hermitian topology in a quantum device introduces a scalable experimental approach to construct and investigate generic non-Hermitian systems.

We transpose the concepts introduced for the QHE device to classical electronic systems where the non-Hermitian topology allow us to build a non-Hermitian ohmmeter.

As light can mediate interactions between atoms in a photonic environment, engineering it for endowing the photon-mediated Hamiltonian with desired features, like robustness against disorder, is crucial in quantum research. We provide general theorems on the topology of photon-mediated interactions in terms of both Hermitian and non-Hermitian topological invariants, unveiling the phenomena of topological preservation and reversal, and revealing a system-bath topological correspondence. Depending on the Hermiticity of the environment and the parity of the spatial dimension, the atomic and photonic topological invariants turn out to be equal or opposite. Consequently, the emergence of atomic and photonic topological boundary modes with opposite group velocities in two-dimensional Hermitian topological systems is established. Owing to its general applicability, our results can guide the design of topological systems.

Topological Photonics is an emerging and novel field of research, adapting concepts from condensed matter physics to photonic systems adding new degrees of freedom. After the first demonstrations of topological photonic insulators, the field has moved on to study and exploit the inherent non-hermiticity of photonic systems and the interplay with their topological nature. By choosing precise lattice geometries we can tailor optical band structures realizing novel photonic lattices. The specific geometry as well as the hybrid light-matter nature allow for ways to break time-reversal symmetry and implement topologically non-trivial systems. Using a novel design technique for polaritonic optical lattices, so-called corner modes, fully localized higher-order topological defects in a two-dimensional lattice in breathing Kagome and 2D-SSH lattices are realized and discussed, with a particular focus on the robustness against (deterministic) fabrication imperfections. Finally, recent advances in using polarization degrees of freedom in the context of artificial gauge fields and the realization of the (pseudo-)spin quantum Hall effect of light are briefly discussed.

Controlled spatial variations within crystals of topological and/or correlated materials are expected to induce novel, gradient driven physics. I will review our departments current observations and efforts on micromechanical structures carved from single crystals. Emphasis will be on resonant detection of elastic moduli and nematic states in pnictide high-Tc superconductors as well as correlated nickelates. First steps towards artificial gauge fields in 3D topological semimetals prove as a promising path to translate mechanical stress gradients into non-standard Landau-level physics.

A quantum anomalous Hall (QAH) insulator is characterized by quantized Hall resistance and zero longitudinal resistance at zero magnetic field, unaffected by local perturbations and sample specifics. This insensitivity renders global transport measurements blind to the local current distribution, which remains unknown. I will discuss how we use magnetic imaging to visualize the transport current in the QAH regime. By tuning through the QAH plateau with electrostatic gating, we identify a regime where current flows mainly through the bulk rather than the edges. At high currents within and outside the QAH regime, we detect a weak magnetization response to applied current, explained by current-induced electron heating. This allows imaging of local dissipation in the QAH regime. As an example, I will show images of hotspots localized in the corners of the electrical contacts through which the transport current enters our devices.

The discovery of the quantum Hall effect founded the field of topological condensed matter physics. Its amazingly accurate quantisation of the Hall conductance is stable against any reasonable perturbation. Conversely, any local information is hidden from the observer. The spatial distribution of the current in the sample is such a piece of information, which however has now become accessible thanks to spectacular experimental advances. We demonstrate the possibility of a broad `edge state' meandering away from the sample boundary deep into the sample bulk. Further, we show that varying experimental parameters permits continuously tuning between narrow edge states and meandering channels all the way to incompressible bulk transport.

We consider magnetic impurities modeled as classical unit-length spins that are exchange coupled to the spinful Haldane model and study the spectral flow of bound states with the coupling strength J. In addition to conventional k-space topology, an additional, spatially local topological feature is available, based on the space of impurity-spin configurations. Global k-space and local S-space topology are represented by different Chern invariants. We demonstrate that there is a local S-space topological transition as a function of J associated with a change in the spin-Chern number and work out the implications for the J-dependent local electronic structure close to the impurities and, in particular, for in-gap bound states. The critical exchange couplings’ dependence on the parameters of the Haldane model, and thus on the k-space topological state, is obtained numerically to construct local topological phase diagrams for systems with R = 1 and 2 impurity spins.

I discuss the interplay between non-Fermi liquid behaviour and pairing near a quantum-critical point (QCP) in a metal. These tendencies are intertwined in the sense that both originate from the same interaction mediated by gapless fluctuations of a critical order parameter. The two tendencies compete because fermionic incoherence destroys the Cooper logarithm, while the pairing eliminates scattering at low energies and restores fermionic coherence. I show that the pairing wins, but the pairing mechanism is fundamentally different from the BCS one and is associated with the emergence of complex exponents for the pairing susceptibility rather than with Cooper logarithm. I discuss topological aspects of such non-BCS pairing.

Despite phase diagrams being the central objects of study in condensed matter and statistical physics, the rules by which phases and critical surfaces are organized are unclear and expectations are largely guided by lore. One of these is the belief that a stable second order critical surface that can be reached by tuning a single parameter represents a genuine transition between distinct phases of matter, in contrast to first-order surfaces which can abruptly terminate. However, this is not true and many counter-examples have recently been discovered. When present, these 'unnecessary' critical surfaces are surprisingly stable and are accompanied by robust boundary phenomena in its vicinity. In this poster, based on upcoming work with S.A. Parameswaran, I demonstrate that unnecessary criticality is protected by the encircling family of gapped Hamiltonians forming a non-trivial charge pump and the accompanying boundary modes the consequence of a generalized bulk boundary correspondance.

In twisted van der Waals heterostructures, atomically thin crystals are assembled together twisted relative to each other developing striking material properties. From the perspective or opto-electronics, their spectral responses can be tuned over an ultra-broad wavelength range bridging visible and terahertz technologies, while their giant superlattice cells amplify quantum effects that can be leveraged for a new generation of quantum photovoltaic devices– rectification driven by the wavefunction properties of electrons. In this talk, I will present emerging types of quantum photovoltaic responses in twisted van der Waals heterostructures. I will discuss THz photocurrent experiments performed in Moiré superlattices where we probe the interplay of quantum geometry with interaction phenomena intrinsic to flat-band systems. Following, we’ll present high-field experiments in Moiré superlattices introducing a new platform for single-photon detection with long-wavelength capabilities.

Characterizing the topology of bound electron-hole states is needed to address the global features of the charge distribution of the excitonic state. Such information is essential to understand observable exciton properties and to supplement the numerical investigation of various properties of excitons in ab initio approaches. I will discuss a framework that accounts for the quantum geometry and topology of two-dimensional exciton states in a way that pinpoints how interactions can introduce nontrivial topology to the exciton bands beyond the topology in the single-particle bands. In addition, I will discuss the effect of the single-particle band topology on excitons bound to the defects. I will describe a numerical study showing that topological electronic defect states' properties are translated into modified excitonic states, a difference that is reflected in the excitons' binding energy and their spatial shape.

Time-reversal symmetry is central to the theory of superconductivity as it is the reason for why the Fermi surface is unstable towards the formation of Cooper pairs already at weak coupling. Consequently, breaking it in the normal state leads to non-trivial consequences for superconductivity. In this talk, we will discuss some of our recent theoretical works on superconductivity in the absence of time-reversal symmetry. This will include the superconducting diode effect as a result of spontaneous ferromagnetism as well as induced by altermagnetic order, and the interplay of strong on-site repulsion and the topology of Chern bands for the formation of a superconducting ground state.

Floquet theory allows the design of quantum phases with exotic properties through time-periodic driving, enabling control of band topology to realize topological phases like Chern insulators dynamically. Extending this to topological superconductivity is challenging because typical superconductors' gap functions do not couple directly to electromagnetic fields, making it hard to induce topological phase transitions. This study overcomes the challenge by considering correlation effects, showing that d-wave superconductivity in the doped Hubbard model can transform into topological d+id one under circularly-polarized light. By deriving the Floquet t–J model using Floquet theory and Schrieffer-Wolff transformation, the Hamiltonian exhibits chiral many-body interactions with broken time-reversal symmetry, which modulate the pairing symmetry.

The interplay between superconductivity and topological surface states in t-PtBi₂ makes this compound a fascinating representative of quantum materials. Intriguingly, ARPES shows that in t-PtBi₂ superconductivity is intrinsically intertwined with, as well as confined to, the Weyl
Fermi arcs, and it exhibits  > 5 K - more than one order of magnitude higher than measured by transport. Via low-temperature scanning tunneling spectroscopy and quasi-particle interference investigations, we experimentally address the topological and superconducting properties of the surface of t-PtBi₂. The revealed scattering channels can be attributed to the existence of Fermi arcs. Furthermore, spectroscopic signatures provide clear evidence of the superconductivity’s surface nature. Our findings thus experimentally corroborate the non-trivial topology and variable surface superconductivity in t-PtBi₂.

On the example of graphene, we will discuss various contributions to circular dichroism in angle-resolved photoemission (CD-ARPES) which include phase shifts of the participating partial waves, the interatomic phase shifts, and the CD due to elastic scattering of an excited electron. Multiple scattering calculations are performed using the EDAC cluster code. Subsequently, we perform similar analysis for WSe₂, a material where orbital characters are relatively well-defined. Finally, a simple interatomic interference model that qualitatively explains asymmetric spin-polarized ARPES (spin-ARPES) spin texture from WTe₂ single crystal surface is presented. This study aims to investigate how CD-ARPES and spin-ARPES techniques can contribute to a better understanding of topological materials.

Neutron spectroscopy offer a unique insight into the emergent quantum phases and entangled dynamics in quantum materials. A textbook example is offered by the compound SrCu₂(BO₃)₂ realizing the theoretical Shastry-Sutherland model, which reveal a plethora of intriguing phenomena including: bosonic flat bands; a zoo of entangled bound states; correlated decay of magnons; valence bond solid of plaquette singlets; a quantum equivalent to the critical point of water; a putative deconfined quantum critical point; fractional magnetization plateaus and bosonic BEC of triplet bound states. Exploring this rich physics in parallel illustrates the challenges and rewards of technological advancements in neutron instrumentation and pushing the capabilities of extreme condition sample environments.

The Kitaev spin liquid realizes an emergent static Z₂ gauge field with vison excitations coupled to Majorana fermions. While static in the idealized Kitaev model, single visons and vison pairs become dynamical degrees of freedom in the presence of perturbations. We develop a concise theory of the universal properties of single visons [1] and vison pairs [2] in Kitaev spin liquids. We focus both on single layer models and on multilayer systems. When Kitaev models are stacked on top of each other, weakly coupled by Heisenberg interaction ∝J⊥, a rich zoo of dynamical gauge excitations emerge whose dynamics is strongly constraint by topology and residual conservation laws.

In magic angle twisted bilayer graphene, transport, thermodynamic and spectroscopic experiments pinpoint to a competition between distinct low-energy states with and without electronic order, as well as a competition between localized and delocalized charge carriers. In this talk I will discuss these observation in the context of a heavy-fermion-like description. We will show through a combination of Hartree-Fock (HF) and Dynamical Mean Field Theory (DMFT) that such a heavy-fermion picture is able to describe in a unified way many of the experimentally observed features in these materials.

Diode behavior in superconducting junctions describes the phenomenon of dissipationless current flow in one direction, while the current in the other direction underlies dissipation. Such non-reciprocal behavior has been found in Josephson junctions where inversion and time-reversal symmetry are broken. Here, we create atomic-scale Josephson junctions in a scanning tunneling microscope and investigate their transport properties in current-biased mode. This allows characterization of the switching and retrapping currents, which separate the dissipationless from the dissipative branch. Plain Pb-Pb junctions show hysteretic and reciprocal behavior. By insertion of single magnetic adatoms the retrapping current adopts nonreciprocity. We show that the nonreciprocity of the retrapping current depends on the particle-hole asymmetry of the Yu-Shiba-Rusinov (YSR) states inside the superconducting energy gap.

Using a highly sensitive, giant magnetoresistance sensor, we have measured the gate voltage–dependent magnetization of a single graphene layer. The signal exhibits a sharp diamagnetic peak at the Dirac point and is a fundamental signature of the characteristic Berry phase of graphene’s electronic wave functions. This experiment paved the way for the investigation of orbital currents in 2D materials and inspired experiments on graphene with a Moiré potential. Beside the sharp diamagnetic peak at the Dirac point, we find sharp paramagnetic peaks revealing the existence of paramagnetic current loops in 2D crystals when the Fermi energy is tuned at the saddle points of the Moiré band structure. These results motivate the investigation of twisted graphene bilayers in which an intriguing topological orbital ferromagnetism is expected. Experiments in 2D topological insulators are also in progress for the detection of phase coherent current loops along protected helical edges.

We present a multi-probe transport analysis for effectively separating bulk and edge currents in large Hall-bar devices with carefully designed contacts. Being applied to the transport measurements involving all four-probe measurements of six-probe Hall-bar devices made from inverted three-layer InAs/GaInSb quantum wells (QWs), our analysis not only reveals the presence of dissipative edge currents in the topological gap but also allows evaluating separately the temperature dependencies of the bulk and edge conductivity.

The temperature dependence of the edge conductivity for Hall bar channels of 10 µm and 70 µm length in the range from 1.5 K to 45 K is consistent with the theoretical expectation of weakly interacting helical edge electrons with the backscattering due to localized magnetic moments of charge impurities. We argue that these charge impurities are naturally associated with intrinsic Ga-antisite defects, acting as double acceptors in InAs/Ga(In)Sb-based QWs.

In recent years, experiments have reported superconductivity in various Weyl semimetals. The low-energy electromagnetic response of Weyl semimetals is governed by the axion term due to the chiral anomaly. Recently it has been demonstrated that time-reversal-invariant Weyl superconductors (SCs) exhibit a chiral Meissner state. We explore the influence of this state on tunnel junctions and SQUIDs made of time-reversal invariant Weyl SCs. We derive a modified Fraunhofer interference pattern in such a junction and show how the axion term affects Josephson energy in asymmetric SQUIDs and Berry phase in charge qubits. The effect of the chiral Meissner state manifests as a temperature-dependent deficit flux, providing a new tuning parameter compared to ordinary SQUIDs.

Quantum materials are hoped to change technology in various aspects. However, most of the desired applications are hindered by the lack of suitable materials. In my group we are using concepts from chemistry to understand, predict and synthesize new quantum materials. In this talk, I will show how simple concepts, such as measuring bond distances, allow us to make predictions about electronic structures of materials, which we can then use to find new topological materials. We then can combine this with structural building blocks containing magnetic elements to design materials with non-colinear or even non-coplanar magnetism. Thinking about the degree of delocalization in a chemical bond can be helpful to find kagome or linear-chain materials with band structures that better resemble simple tight binding models. I will give a general overview how powerful chemical concepts are in materials discovery and highlight a flute of materials that were discovered in this light.

Naturally occurring materials exhibiting complex quantum phases often have intricate atomic structures, defects, impurities, and dopants, hindering the systematic manipulation of their electronic properties. This can be overcome through van der Waals (vdW) heterostructures, where the layers interact through vdW forces, allowing them to maintain their inherent properties. Nevertheless, proximity effects may lead to the "leakage" of properties between adjacent layers, giving rise to unique quantum-mechanical phases.
I will discuss our recent work on artificial heavy-fermion systems in vdW heterostructures and visualizing emergent excitations in multiferroic vdW monolayers. These examples underscore the adaptability of vdW heterostructures in realizing elusive quantum states that cannot be readily found in naturally occurring materials.

Electronic flat-band systems have gained considerable attention as a platform to realize quantum matter phenomena. Electrons in a flat band have reduced kinetic energy, a diverging effective mass, and localized wave functions, leading to strong correlation effects. Significant efforts have been directed at realizing electronic flat bands in special ‘line graph’ lattices, which possess compact localized states that are spatially ‘trapped’ by the destructive interference of hopping pathways.
In this talk, I will discuss how flat bands can support strong correlation physics in two paradigmatic lattices – the kagome and pyrochlore lattice. Using ARPES, we probe the electronic band structure in crystalline materials hosting these lattices. I will first present the general phenomenology of flat bands in kagome and pyrochlore intermetallics. I will then discuss the role of flat bands for the heavy fermion-like physics in two ‘f electron-free’ compounds, kagome metal Ni3In and spinel LiV₂O₄.

The kagome lattice has been enigmatic to various quantum many-body ground states ranging from quantum spin liquids and fractional Chern insulators to, most recently, exotic charge order and nematic electronic order descending from a metallic parent state on a kagome lattice. From a synopsis of progress that has been reached thus far, we formulate an agenda for expanding the kagome paradigm. We do this by further broadening the landscape of correlated electronic states of matter on the kagome lattice through intertwining topology, magnetism, and quantum liquid electronic phases.

Insulators have been discovered that exhibit thermal properties of metals as if they hosted quasiparticles carrying entropy but not charge. There are even insulators that further display quantum oscillations which allow mapping fully-fledged Fermi surfaces. This phenomenon has been notably observed in topological Kondo insulators SmB₆ and YbB₁₂. Prompted by their mixed-valence nature, involving f and d shells of the lanthanide, we advance a novel explanation inspired by the pseudo gapped underdoped cuprates. We argue that f and d subsystems, taken separately, act, respectively, as electron- and hole-doped Mott insulators, featuring Fermi pockets and Luttinger surfaces responsible for the pseudogap. When coupled to each other, they turn into a topological insulator with chiral edge modes, though Luttinger surfaces persist and may support neutral quasi-particles. This scenario, aligned with DCA simulations, effectively accounts for the challenging phenomenology of SmB₆ and YbB₁₂.

The n-body reduced density matrix (n-RDM) characterizes higher-order correlations in many-body systems. This quantity can be used to compute any n-body observable and is experimentally measurable. Analytically, the problem of computing higher-order density matrices becomes increasingly challenging as the number of coordinates grows. However, within the Luttinger-liquid regime, correlation functions can be computed from bosonization. We outline the derivation of the exact 2-RDM for fermions. We then present an analytic result for the density-density correlation function, investigating finite size lattice effects. We further demonstrate the applicability of our expression to the J-V model of interacting, spinless fermions in one dimension. Our results agree with those obtained from density matrix renormalization group calculations. Finally, we discuss the computation of two-body observables such as lattice energies, particle entanglement, and structure factors.

I will present universal bounds on the energy gap and structure factor of topological states of matter, such as integer/fractional Chern insulators, quantum spin liquids and topological superconductors. These bounds are shown to be fairly tight for Chern insulator states that were predicted and observed in twisted bilayer transition metal dichalcogenides. Next, I will show a universal inequality relation between the energy gap and dielectric constant of all solids. Our results are derived from fundamental principles of physics and apply to all electronic materials. I will end by outlining new directions involving topology, geometry and energy in quantum materials research.

A band with a nonzero Chern number cannot be fully localized by weak disorder. There must remain at least one extended state, which “carries the Chern number.” Here we show that a trivial band can behave in a similar way. Instead of fully localizing, arbitrarily weak disorder leads to the emergence of two sets of extended states, positioned at two different energy intervals, which carry opposite Chern numbers. Thus, a single trivial band can show the same behavior as two separate Chern bands. We show that this property is predicted by a topological invariant called a “localizer index.” Our work points to a previously overlooked manifestation of topology, which impacts the response of systems to impurities beyond the information included in conventional topological invariants.

We present generalizations of the AdS/CFT correspondence to discrete systems involving hyperbolic lattices. Insights from AdS/CFT are used to determine the stability of fluctuation modes. Also, the bulk theory leads to predictions for aperiodic spin chain theories defined at the boundary. These display phase transitions that reflect changes in the entanglement structure. We also discuss how to obtain discretized gravity in terms of spin models on hyperbolic lattices. Furthermore, we propose realizations of these results in hyperbolic metamaterials involving optical waveguides or electric circuits.

The Landau-Ginzburg-Wilson paradigm has led to a deep understanding of a great number of phase transitions over the last 50 years. During the last three decades, however, the paradigm has been challenged various times. One such instance was the discovery of deconfined quantum phase transitions, i.e. continuous order-to-order quantum phase transitions enabled by a fractionalization mechanism. Here, we propose a different mechanism generating unambiguously continuous order-to-order quantum phase transitions. The proposed mechanism is based solely on a fixed point annihilation and therefore does not rely on fractionalization. We exemplify the new mechanism in Luttinger semimetals, i.e. three-dimensional strongly-spin-orbit-coupled systems, in which valence and conduction bands touch quadratically at the Fermi level. Our results are relevant for the low-temperature behavior of rare-earth pyrochlore iridates, such as Pr₂Ir₂O₇ or Nd₂Ir₂O₇.

This talk discusses two examples of how a combination of analytical and computational methods can serve to connect basic theoretical ideas about correlated states to quantum information quantities and neutron scattering experiments. We introduce the Heisenberg model’s novel fluid-like dynamical regime at high temperatures and describe its realization in a variety of recent experiments ranging from neutron scattering on crystals to optical lattice emulation with atoms. It turns out that the dynamics of spins in this canonical model are described by the Kardar-Parisi-Zhang dynamical universality class, which is well known from classical problems such as driven interfaces. We present theoretical arguments first, that a chiral spin liquid is likely to appear near the Mott transition in some triangular lattice materials, and second, that other kinds of spin liquids and quantum critical points are suggested in recent experiments on Yb-based triangular-lattice compounds.

The interplay of quantum fluctuations and interactions can yield to novel quantum phases of matter with fascinating properties. Understanding the physics of such system is a very challenging problem as it requires to solve quantum many body problems—which is generically exponentially hard on classical computers. In this context, universal quantum computers are potentially an ideal setting for simulating the emergent quantum many-body physics. Here we discuss applications to the study the dynamics of topologically ordered systems: We prepare the ground state of the toric code Hamiltonian using an efficient quantum circuit on a superconducting quantum processor. We measure a topological entanglement entropy near the expected value of ln(2), and simulate anyon interferometry to extract the braiding statistics of the emergent excitations.

The standard phenomenology for fermions to acquire a mass requires a spontaneous symmetry breaking, and it is understood in terms of the Anderson-Higgs mechanism. Typically, topological phase transitions are linked to the formation of gapless symmetry-protected states. However, electronic correlations can alter this scenario. Here we solve a Bernevig-Hughes-Zhang model with local interactions, demonstrating that dynamical quantum fluctuations can open a gap without any symmetry breaking. Specifically, we observe that beyond Mean-Field approximation the continuous gapless topological transition becomes first-order. This change is associated with the emergence of a massive Dirac fermion at the transition point, showing a Gross-Neveu critical behavior near the quantum critical endpoint. We identify this gap opening as a condensed matter analog of the Coleman-Weinberg mechanism of mass generation.

The eigenstate thermalization hypothesis (ETH) connects the field of statistical physics with quantum mechanics. It suggests that an eigenstate of a quantum many-body system acts as a (micro)canonical ensemble for small local subsystems. While the ETH was applied to and tested in a variety of physical systems, its microscopic origin is still not completely known. In this presentation, we discuss how it can be understood more fundamentally by employing concepts from random matrix theory. In particular, we report on random matrix computations for a spin system with random couplings and compare the results to our numerical findings.

Conventional BCS theory of superconductivity provides important insights, yet fails to reproduce many key signatures of high-temperature superconductors (HTSCs): dome-shape of the critical temperature (T_c) vs doping, very weak isotope effect, nearly universal value of the optimal doping, the Uemura plot to name a few. We consider layered HTSCs such as cuprate ceramics, where the electrons are localized within 2D CuO planes that are only weakly coupled. As such electrons can be effectively considered two-dimensional, we model them by a 2DEG with a circular Fermi surface with the Fermi momentum k_F. Attractive interaction is mainly mediated by optical phonons polarized across the CuO planes. We apply our theory to the electron-phonon interaction in HTSCs and find the dome-shape structure of T_c vs doping, strongly suppressed isotope effect, stable value of the optimal doping within a wide range of parameters, and qualitative agreement with the Uemura plot.

Controlling and understanding electron correlations in quantum matter is one of the most challenging tasks in materials engineering. In the past years a plethora of new puzzling correlated states have been found by carefully stacking and twisting two-dimensional van der Waals materials of different kind. Unique to these stacked structures is the emergence of correlated phases not foreseeable from the single layers alone. In Ta-dichalcogenide heterostructures, recent reports have evidenced a cross-breed itinerant and localized nature of the electronic excitations, similar to what is typically found in heavy fermion systems.
We put forward a new interpretation based on first-principles calculations which indicates a sizeable charge transfer of electrons. We accurately quantify the strength of the interlayer hybridization which allows us to unambiguously determine that the system is much closer to a doped Mott insulator than to a heavy fermion scenario.

In this theory talk, I will present our results on the quantum phase diagram of twisted bilayer graphene at charge neutrality as function of twist angle, employing a microscopic tight-binding model with screened Coulomb interactions. At large twist angles, the electron and hole bands near the Fermi level are dispersive and touch only at isolated points in the moiré Brillouin zone, leading to a stable Dirac semimetal state at low temperatures. Upon reducing the twist angle, the band width of the low-energy bands decreases, culminating in an interaction-induced instability towards a symmetry-broken state. We find a direct transition between the Dirac semimetal and a Kramers intervalley-coherent insulator as function of twist angle for fixed interaction parameters. The nature of this twist-tuned quantum phase transition will be discussed as well. Our results may in principle be experimentally tested using setups based on the quantum twisting microscope.

I will present the recent progresses on novel quantum phases and phase transitions in both fermionic and bosonic fractional Chern insulator (FCI) models. The intertwinement of topological order and Landau order and the discovery of the emergent forms of FCI states, and the collective excitations in FCIs and their role in tuning these transitions, will be discussed.

References:

[1]. From Fractional Quantum Anomalous Hall Smectics to Polar Smectic Metals: Nontrivial Interplay Between Electronic Liquid Crystal Order and Topological Order in Correlated Topological Flat Bands, Hongyu Lu, Han-Qing Wu, Bin-Bin Chen, Kai Sun, Zi Yang Meng, Rep. Prog. Phys. 87 108003 (2024)

[2]. Vestigial Gapless Boson Density Wave Emerging between nu=1/2 Fractional Chern Insulator and Finite-Momentum Supersolid,
Hongyu Lu, Han-Qing Wu, Bin-Bin Chen, Zi Yang Meng, arXiv:2408.07111

[3]. Thermodynamic Response and Neutral Excitations in Integer and Fractional Quantum Anomalous Hall States Emerging from Correlated Flat Bands, Hongyu Lu, Bin-Bin Chen, Han-Qing Wu, Kai Sun, Zi Yang Meng, Phys. Rev. Lett. 132, 236502 (2024)

Chiral superconductors represent an exotic state of matter where the angular-momentum state of the Cooper pairs is ‘unconventional’ and time-reversal symmetry is broken. While there are several candidate materials, conclusive evidence for the existence of chiral superconductivity has yet to be established. Here, I present evidence suggesting the presence of a chiral d-wave superconducting state in a dilute monatomic Sn layer on the Si(111) surface. This triangular adatom lattice becomes superconducting upon hole doping, reaching a maximum Tc of about 9 K. Quasi-particle interference spectra below Tc indicate that time-reversal symmetry is broken, while edge-state spectra are consistent with the presence of topological order. The simplicity and experimental control of these (and related) surface-science platforms provides a powerful testbed for theoretical models and discovery of elusive phases of quantum matter. Interesting analogies with cuprate physics will be discussed.

I will overview efforts at the Quantum Science Center to understand honeycomb-based frustrated magnets and their potential for Kitaev physics. These compounds order at zero magnetic field and low temperatures, however exhibit complex phase diagrams. A prediction is that in the presence of non-Kitaev terms the Kitaev phase could still appear where the ordering is suppressed at high magnetic fields. I will discuss our obtained phase diagrams and physical properties in high magnetic fields and the role of electric polarization in probing symmetry-breaking of magnetic order, disordered phases and phase transitions. I will also talk about measurements and quantum simulations on different Ising Co materials, that lend themselves to e.g. D-wave and QuERA quantum simulators. These materials with slow frustrated dynamics can be simulated out of equilibrium.

A magnetic hedgehog lattice (HL) features three-dimensional hedgehog and antihedgehog spin textures, acting as emergent magnetic monopoles and antimonopoles. Recently, HLs were experimentally discovered in both noncentrosymmetric and centrosymmetric metals, which exhibit unconventional transport such as a topological Hall effect (THE). While the antisymmetric spin interactions play crucial roles in the noncentrosymmetric case, the stabilization mechanism and topological properties including the emergent magnetic field remain elusive in the centrosymmetric case. In this work, we numerically study the ground state of an effective spin model with long-range interactions mediated by itinerant electrons. We find that a HL is stabilized by the synergy of the bilinear and biquadratic interactions in a centrosymmetric and isotropic case. Incorporating the anisotropic interactions from spin-orbit coupling, we reveal modified HLs hosting more monopoles and antimonopoles, contributing to the THE.

Magnetic skyrmion crystals (SkX) hold promise for novel electromagnetic phenomena. Although SkX has been observed in many noncentrosymmetric materials such as MnSi, SkX has also been observed in materials with spatial inversion symmetry, such as Gd2PdSi3 and GdRu₂Si₂. In particular, in GdRu₂Si₂ a square-lattice SkX appears, and it has been suggested that bond-dependent anisotropy is required in addition to the frustration of the exchange interaction and higher-order exchange interactions in itinerant electron systems. In this study, we constructed an effective real-space spin model from a wave-space spin model based on machine learning to search for a new stabilization mechanism of the square-lattice SkX. By performing Monte-Carlo simulations for the obtained spin model, we find that the square-lattice SkX is stabilized even without the bond-dependent anisotropy. These results indicate a new stabilization mechanism for the square-lattice SkX, through machine learning-assisted analysis.

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We perform a projective-symmetry-group classification of fermionic mean-field Ansätze of U(1) and Z₂ quantum spin liquids on the Shastry-Sutherland lattice, and a map a subset of these states to the symmetric spin liquids on the square lattice. Employing DMRG calculations we provide evidence that the S=1/2 Shastry-Sutherland spin liquid is adiabatically connected to the spin-liquid phase of the J₁-J₂ Heisenberg antiferromagnet on the square lattice. Within a variational Monte-Carlo approach we show that the putative spin liquid reported in the Shastry-Sutherland lattice is indeed a gapless Z₂ Dirac spin liquid which interpolates between both models.

Classical spin liquids are paramagnetic phases that feature nontrivial patterns of spin correlations within their ground-state manifold whose degeneracy scales with system size. Often they harbor fractionalized excitations, and their low-energy fluctuations are described by emergent gauge theories. In this work, we discuss a model composed of chiral three-body spin interactions on the pyrochlore lattice that realizes a novel classical chiral spin liquid whose excitations are fractonalized while also displaying a fracton-like behavior. We demonstrate that the ground-state manifold of this spin liquid is given by a subset of the so-called color-ice states. We demonstrate that the low-energy states are captured by an effective gauge theory which possesses a divergence-free condition and an additional chiral term that constrains the total flux of the fields through a single tetrahedron.