Trends in the Theory of Quantum Materials (TTQM) 2026

19 Oct 2026, 08:00 → 21 Oct 2026, 18:00 Europe/Zurich

Auditorium (Paul Scherrer Institut)

Christopher Mudry (Paul Scherrer Institut)

TTQM2026 continues the long-running series of bilateral workshops between Japanese and Swiss physicists in the field of theoretical condensed matter physics and quantum information. It brings together the leading members of both communities, and also aims at nurturing scientific exchanges, collaborations, and friendships among young physicists working in the two countries.

Monday 19 October

1 Arrival and registration

2 Welcome talk and information

Speaker: Prof. Christopher Mudry (PSI)

3 Transport and optical responses emergent from spin chirality

Noncollinear spin textures, such as spin spirals and skyrmions, have attracted considerable attention in the past decade due to their rich functionalities. In particular, electrons coupled to these textures experience emergent electromagnetic fields associated with vector and scalar spin chirality, which break spatial-inversion and time-reversal symmetries, respectively. These emergent fields endow the electronic bands with nontrivial quantum geometry and give rise to characteristic transport phenomena, such as the topological Hall effect in skyrmion crystals.
In this talk, we present our recent findings on unconventional transport and optical responses in electron systems reflecting spin chirality. The electronic band structure in spin spirals exhibits odd-parity spin splitting with spin polarization. We show that spin spirals induce the unconventional anomalous Hall effect, which originates from the Berry curvature concentrated along the spin-nodal lines [1]. In a noncoplanar conical state, the band structure becomes asymmetric, leading to quadratic optical responses, such as nonrelativistic photocurrent and second harmonic generation [2]. Recently, based on a similar mechanism, chirality-dependent nonreciprocal transport has been reported theoretically and experimentally [3]. Furthermore, we explore the optical responses in skyrmion textures with finite scalar spin chirality. Under irradiation by linearly polarized light, a skyrmion insulator exhibits the topological magneto-optical Kerr effect and an injection photocurrent in the absence of spin-orbit coupling, originating purely from the Berry curvature and quantum metric, respectively [4]. These results open a new avenue for emergent transport and optical responses driven by quantum geometry in noncollinear spin textures.

[1] R. Yamada et al., Nature 646, 837 (2025); S. Okumura et al., arXiv:2512.14071 (2025).
[2] S. Okumura et al., Phys. Rev. B 104, L180407 (2021).
[3] D.-A. Deaconu et al., Phys. Rev. Lett. 135, 096701 (2025); A. H. Mayo et al., PNAS 122, e2405839122 (2025).
[4] R. Shimizu, S. Okumura et al., Commun. Phys. (2026).

Speaker: Shun Okumura (The University of Tokyo)

4 1/9 magnetization plateau of the spin-1/2 kagome Heisenberg antiferromagnet from large-unit-cell variational tensor networks

The $1/9$ magnetization plateau of the spin-$1/2$ kagome Heisenberg antiferromagnet has attracted considerable attention, yet its microscopic nature remains unresolved. Several competing states have been proposed, including quantum spin liquids and valence-bond crystals (VBCs) with 9- and 27-site unit cells. We investigate this plateau using infinite projected entangled-pair states (iPEPS). We find a VBC state with a 27-site unit cell and a spatial modulation of the local magnetization, distinct from previously proposed VBC states and exhibiting a lower variational energy. The 27-site structure possesses threefold rotational symmetry and consists of three 9-site regions related by $2\pi/3$ rotations. The plateau remains stable over the field range $0.25\lesssim h/J\lesssim0.46$, where $J$ is the Heisenberg exchange coupling.

Speaker: Pratyay Ghosh (Ecole Polytechnique Fédérale de Lausanne)

5 Competing symmetry breaking and topology in quantum spin chains

We investigate how spontaneous symmetry breaking and symmetry-protected topology compete and cooperate in one-dimensional quantum spin-1/2 systems. We introduce two families of local Hamiltonians with ℤ2×ℤ2×ℤT2 symmetry which host a zoo of broken-symmetry phases with and without gapless boundary spins, namely, six long-range-ordered gapped phases that differ by their ℤ2-symmetry-breaking patterns (FMx,y,z and Neelx,y,z), their partner symmetry-protected topological phases (FMSPTx,y and NeelSPTx,y), and two gapless phases (gFMz and gNeelz) with incommensurate ℤT2-breaking magnetic long-range order and central charge 𝖼=1. Direct transitions between competing gapped phases can be continuous or weakly first order. We demonstrate that continuous phase transition lines have exotic critical properties. For example, a line of deconfined quantum critical points (between the Neelx and Neely phases) described by Tomonaga-Luttinger liquid theory is terminated by the gFMz phase at a Lifshitz tricritical point which exhibits spontaneous breaking of the continuous U(1) symmetry despite the locality and one-dimensionality of the Hamiltonian, contrary to a naive expectation from the Hohenberg-Mermin-Wagner-Coleman theorem. Furthermore, critical lines between symmetry-protected topological phases with different ℤ2-symmetry breaking (e.g., NeelSPTx and NeelSPTy) additionally retain low-energy boundary degrees of freedom, realizing symmetry-enriched criticality (or a gapless SPT phase). Our results reveal unconventional mechanisms by which competing symmetry breaking and topology generate ordered critical phases and multicriticality.

Speaker: Akira Furusaki (Riken)

10:30 Coffee break

6 Tangent equations of motion for extracting nonlinear response functions from real-time dynamics

Nonlinear response functions are essential for describing nonlinear transport and spectroscopy, but extracting order-resolved responses from real-time dynamics is challenging because contributions from different perturbative orders are intrinsically mixed. We develop a real-time framework for evaluating nonlinear response functions as field-functional derivatives of dynamical observables. Building on an earlier finite-difference extraction protocol [1], we formulate tangent equations of motion (TEOM) by taking Gateaux derivatives of the equation of motion with respect to the external field [2]. The resulting closed hierarchy directly propagates field derivatives of the dynamical state at each desired perturbative order, without contamination from unwanted orders. This direct propagation enables the reconstruction of frequency-resolved nonlinear response functions up to fifth order for a solid-state electron model. The TEOM framework offers a general and practical route for extracting nonlinear response functions from modern real-time simulations of correlated quantum systems and other nonlinear dynamical systems.

[1] A. Ono, Phys. Rev. Lett. 135 026401 (2025).
[2] A. Ono, Phys. Rev. Research 8, 033131 (2026).

Speaker: Dr Atsushi Ono (Tohoku University)

7 A Quantum Algorithm for Real-Time Evolution in the Single-Impurity Anderson Model

Simulating the real-time dynamics of strongly correlated quantum systems is a major challenge for near-term quantum computers. In conventional digital simulation, the depth of Trotter circuits grows with evolution time, causing errors to accumulate and limiting the timescales accessible on noisy hardware.

We address this challenge for the single-impurity Anderson model (SIAM), a paradigmatic model of a magnetic impurity coupled to a fermionic bath. In the Kondo regime, the SIAM captures the many-body screening of a local moment and provides a basic setting for understanding correlated impurity physics in quantum dots, heavy-fermion materials, and other strongly correlated systems. More importantly for our purposes, the SIAM is also the canonical quantum impurity problem underlying dynamical mean-field theory (DMFT), where solving the impurity dynamics is a central computational step in describing strongly correlated quantum matter out of equilibrium.

In this talk, we introduce an approach to SIAM dynamics that reorganizes the time-evolution operator into a sum of exponentially many operator strings. Rather than implementing these strings sequentially, we exploit their algebraic structure to compress each string into a quantum circuit whose depth depends on system size but not on the simulated time. We then reduce the sampling complexity of the exponentially large operator space using a stochastic sector-based scheme that approximates the full sum with polynomial cost and controllable statistical error. Because physical observables involve interference between different operator strings, we additionally introduce an ancilla-free interferometric protocol for measuring the required off-diagonal contributions without costly controlled operations. We demonstrate the approach using classical simulations, noisy-device simulations, and IBM quantum hardware. The resulting dynamics show substantially improved error control compared with standard Trotter evolution and remain in good agreement with exact results in the regimes studied.

Speaker: Utso Bhattacharya (ETH/IBM Postdoctoral Research Fellow, Institute of Theoretical Physics, ETH Zürich)

8 Generalized Keldysh formalism for nonequilibrium correlation functions and its application to fluctuation dynamics

Recent progress in time-resolved Raman scattering and resonant inelastic X-ray scattering has made it possible to follow how low-energy charge, spin, and phonon excitations evolve in time. These measurements have also been shown to provide higher-order quantities beyond the spectra, like the quantum Fisher information, which is drawing attention as a new way to characterize nonequilibrium states [1].

From a theoretical point of view, both the spectra and such quantities are described by nonequilibrium two-particle correlation functions. Although their dynamics could in principle be computed from the nonequilibrium Bethe–Salpeter equation, the required computational time and memory are prohibitive. In practice, conventional approaches therefore neglect vertex corrections, thereby omitting collective effects.

To address this problem, we present a framework in which vertex corrections are included naturally. It builds on the generalized Keldysh formalism [2], applied to a Hamiltonian supplemented by a virtual external probe field [3]. We derive a linear integral equation for the correlation functions and develop an efficient solver for it based on quantics tensor trains [4], a tensor network technique [5].

In this presentation, we introduce the formalism and demonstrate it by studying the fluctuation dynamics of nonequilibrium antiferromagnetic states in the Hubbard model. We find that including vertex corrections alters the dynamics qualitatively, and that fluctuations grow in the vicinity of a nonthermal critical point [6].

[1] D. R. Baykusheva et al., Phys. Rev. Lett. 130, 106902 (2023).
[2] E. Canovi, P. Werner, and M. Eckstein, Phys. Rev. Lett. 113, 265702 (2014).
[3] O. P. Matveev, A. M. Shvaika, and J. K. Freericks, Phys. Rev. B 113, 195148 (2026).
[4] H. Shinaoka et al., Phys. Rev. X 13, 021015 (2023).
[5] K. Inayoshi, H. Shinaoka, and Y. Murakami, arXiv:2607.11055.
[6] N. Tsuji, M. Eckstein, and P. Werner. Phys. Rev. Lett. 110, 136404 (2013).

Speaker: Ken Inayoshi (Department of Physics, Saitama University)

12:30 Lunch

9 Two-dimensional coherent spectroscopy of superconductors

Two-dimensional coherent spectroscopy (2DCS) has recently emerged as a powerful probe of nonlinear optical responses in condensed matter systems, providing access to dynamical information beyond conventional linear and nonlinear spectroscopies. The 2DCS is also useful in detecting collective modes in an ordered phase of materials including quantum magnets and superconductors. In superconductors, there exists a Higgs mode, i.e., an amplitude mode of the superconducting order parameter, which does not linearly couple to electromagnetic fields in ordinary situations. Motivated by recent experiments of 2DCS for superconductors, we study 2DCS signals of superconductors [1]. We examine the behavior of 2DCS signals for narrow-band and broad-band pulse waveforms, and discuss the physical origin of the resonance and threshold structures observed in the 2D frequency map. We also mention our recent studies of 2DCS for multiband superconductors [2].
[1] N. Tsuji, Phys. Rev. B 113, 214502 (2026).
[2] S. Neri, R. Shimano, D. Manske, N. Tsuji, in preparation.

Speaker: Naoto Tsuji (The University of Tokyo)

10 Entropy transport through a superfluid quantum point contact: A Keldysh field-theory approach

We study the matter and entropy transport between two ultra-cold neutral Fermi-gas reservoirs linked by a quantum point contact under a chemical-potential gradient. We describe the two leads with a BCS mean-field model and derive the current-bias characteristics for both particle and entropy transport. We compute the out of equilibrium steady-state currents by using the Keldysh formalism. In accordance with previous works in the literature, we confirm the well-known behavior for the particle current and extend the computation to the entropy current in the BCS regime. The entropy current shows an oscillatory behavior at low voltage in the ballistic junction limit. We analyze the results for a wide range of values of the junction's transparency. We also compare our findings with experimental results in cold atomic gases in the unitary regime.

Speaker: Davide Bertolusso (University of Geneva)

11 Anomalous charge fluctuations in low dimensional quantum systems

The advent of quantum simulators and processors has enabled the measurement of not only the expectation values of local observables, but also of their full quantum statistics, including fluctuations and space-time correlations. In this talk, I will present a theoretical framework for the dynamics of charge fluctuations in generic many-body quantum systems, and show how non-equilibrium fluctuations can be used experimentally to extract transport coefficients such as conductivities and diffusion constants. I will then discuss notable deviations from this conventional behavior in a broad class of systems, including Dirac fluids describing the hydrodynamic regime of graphene and Dirac semimetals at charge neutrality. In particular, I will argue that charge noise in Dirac fluids, as well as in certain two-component classical gases, is parametrically enhanced compared to that in conventional diffusive metals.

Speaker: Romain Vasseur (University of Geneva)

15:30 Coffee break

12 Nonequilibrium Green's functions meet quantics tensor trains: Momentum-resolved electron-phonon dynamics

Time-resolved photoemission shows that relaxing photoexcited electrons experience bottlenecks when transferring energy to the lattice. The widely used two-temperature model misses these bottlenecks. A microscopic description requires nonequilibrium Green's functions (NEGFs), which depend on two times and must be stored for every momentum. Previous NEGF studies were therefore restricted to local phonons and local self-energy approximations. This memory bottleneck can be overcome by compressing the two-time functions as quantics tensor trains (QTTs) [1,2] and propagating them using block time-stepping [3,4]. Here, we use QTT-NEGF simulations with full electron-phonon feedback on lattices of up to 256×256 sites to revisit electron-phonon relaxation with high momentum resolution [5]. We find that the well-known phonon-window bottleneck turns into a hierarchy of bottlenecks. For optical phonons, electron-hole recombination opens a reduced window at half the phonon energy, which separates momentum-space regions with an excess and a deficit of electrons, and we find that phonon modes which cannot decay into particle-hole pairs remain overpopulated. For acoustic phonons, simultaneous energy and momentum conservation make the window momentum dependent and the reduced window asymmetric, and low-momentum phonons relax most slowly. Our results provide momentum-resolved signatures of these bottlenecks that time-resolved spectroscopies can test.

[1] H. Shinaoka, M. Wallerberger, Y. Murakami, K. Nogaki, R. Sakurai, P. Werner, and A. Kauch, Phys. Rev. X 13, 021015 (2023).
[2] M. Środa, K. Inayoshi, H. Shinaoka, and P. Werner, Phys. Rev. Lett. 135, 226501 (2025).
[3] K. Inayoshi, M. Środa, A. Kauch, P. Werner, and H. Shinaoka, arXiv:2509.15028.
[4] M. Środa, K. Inayoshi, M. Schüler, H. Shinaoka, and P. Werner, arXiv:2509.22177.
[5] M. Środa and P. Werner, in review.

Speaker: Maksymilian Sroda (University of Fribourg)

13 Critical aspects of the Kasteleyn transition on the anisotropic honeycomb-lattice monomer-dimer problem

The Kasteleyn transition is a strongly asymmetric phase transition, characterized by a fluctuationless low-temperature phase and the proliferation of string excitations above the transition point ($T_K$). The critical behavior above $T_K$ is characterized by inverse square-root singularities in both the specific heat and the correlation length.

These anomalous features can be closely approached in the low-energy sector of a weakly anisotropic spin-$1/2$ Heisenberg antiferromagnet on a diamond-decorated honeycomb lattice. For a certain parameter range, this system exhibits a highly degenerate manifold of low-lying dimer-tetramer liquid states. Its low-energy behavior is accurately described by an effective monomer-dimer model with anisotropic activities. In a particular limit of the spin-$1/2$ model, the monomer activity can be made small and the system approaches the Kasteleyn transition.

Making use of the Corner Transfer Matrix Renormalization Group (CTMRG), a Tensor Network method working directly in the thermodynamic limit, we were able to accurately describe the gradual smoothing of the Kasteleyn transition in the effective monomer-dimer model. Surprisingly, our results also reveal that, when the monomer activity is small, the monomer density rises singularly below $T_K$. This constitutes a novel critical aspect of the Kasteleyn transition, arising from the unbinding of monomer pairs connected by strings of dimers. The corresponding critical exponent can be found by describing the system as a gas of bound monomer pairs.

Speaker: Afonso Rufino (Ecole Polytechnique Fédérale de Lausanne)

14 Quantum Criticality on the Fuzzy Sphere: XY and Heisenberg Universality

Continuous phase transitions exhibit universal behavior described by conformal field theories. Characterizing these theories beyond the leading critical exponents remains a central challenge in the study of quantum criticality.

I will present fuzzy-sphere realizations of the XY and Heisenberg conformal field theories in $(2+1)$ dimensions. Preserving spatial rotational symmetry enables direct access to conformal data through the state-operator correspondence. Combining exact diagonalization and DMRG with conformal perturbation theory, we extract scaling dimensions and selected OPE coefficients, finding agreement with conformal bootstrap results. I will highlight connections to Goldstone excitations in the XY theory and corrections to scaling in dimerized Heisenberg antiferromagnets.

Speaker: Arjun Dey (PSI - Paul Scherrer Institut)

15 Closing the gap between simulations of paradigmatic and realistic models in tensor networks

A key driver in the development of numerical approaches to correlated electrons is to move beyond paradigmatic lattice models toward realistic Hamiltonians, typically effective multi-orbital Hubbard-like models derived from ab initio calculations. Unlike the celebrated single-band Hubbard model, these Hamiltonians contain hopping and interaction terms that couple several orbitals and extend well beyond nearest neighbors. This poses a major challenge for tensor network (TN) methods, whose computational cost depends on how local the interactions are.

In this talk, I will present recent advances in TN methodology that together push the frontier of classically simulable systems, and I will illustrate them with application to unconventional phases in two-dimensional material. I will conclude by identifying the bottlenecks that remain. These define the current limits of state-of-the-art classical simulation, provide natural benchmarks for emerging hybrid quantum-classical methods, and point to where new approaches will be needed to reach larger and more complex systems.

Speaker: Juraj Hasik (University of Zurich)

16 Spectral flow and the Josephson effect in clean SNS junctions

In long, ballistic superconductor–normal metal–superconductor (SNS) junctions at zero temperature, the Josephson current-phase relation takes the form of a sawtooth. The standard derivation relies on Green's function methods, leading to a sum over Matsubara frequencies, which makes it difficult to see how individual energy states contribute to the current. In this talk, we present an alternative picture based on spectral flow and show how both the linear slope and the jump of the sawtooth emerge from the motion of states in the spectrum.

Speaker: Silvan Aepli (ETH Zurich)

Tuesday 20 October

17 Non-Hermitian Topology in Hermitian Topological Matter

Non-Hermiticity gives rise to distinctive topological phenomena absent in Hermitian systems. However, connection between such intrinsic non-Hermitian topology and Hermitian topology has remained largely elusive. Here, considering the bulk and boundary as an environment and system, we demonstrate that anomalous boundary states in Hermitian topological insulators exhibit non-Hermitian topology [1, 2]. We study the self-energy capturing the particle exchange between the bulk and boundary, and show that it detects Hermitian topology in the bulk and induces non-Hermitian topology at the boundary. As an illustrative example, we reveal the non-Hermitian topology and concomitant skin effect inherently embedded within chiral edge states of Chern insulators. We also identify the emergence of hinge states within effective non-Hermitian Hamiltonians at surfaces of three-dimensional topological insulators. Furthermore, we comprehensively classify our correspondence across all the tenfold symmetry classes of topological insulators and superconductors.

Based on this correspondence and K-theory, we complete the tenfold classification of Wannier localizability and detachable topological boundary states [3, 4]. While topology can impose obstructions to exponentially localized Wannier functions, certain topological insulators are exempt from such Wannier obstructions. The absence of the Wannier obstructions can further accompany topological boundary states that are detachable from the bulk bands. Here, we elucidate a close connection between these detachable topological boundary states and non-Hermitian topology. Identifying topological boundary states as non-Hermitian topology, we demonstrate that intrinsic non-Hermitian topology leads to the inevitable spectral flow. By contrast, we show that extrinsic non-Hermitian topology underlies the detachment of topological boundary states and clarify anti-Hermitian topology of the detached boundary states.

References
[1] F. Schindler, K. Gu, B. Lian, and K. Kawabata, PRX Quantum 4, 030315 (2023).
[2] S. Hamanaka, T. Yoshida, and K. Kawabata, Phys. Rev. Lett. 133, 266604 (2024).
[3] D. Nakamura, K. Shiozaki, K. Shimomura, M. Sato, and K. Kawabata, Phys. Rev. Lett. 135, 096601 (2025).
[4] K. Shiozaki, D. Nakamura, K. Shimomura, M. Sato, and K. Kawabata, Phys. Rev. B 112, 075152 (2025).

Speaker: Kohei Kawabata (Institute for Solid State Physics, University of Tokyo)

18 Charge ices as model systems for exploring correlated disorder

A pyrochlore charge-ice consisting of two magnetic cation types gives rise to a Heisenberg magnet with an interesting type of long range correlated disorder. If the antiferromagnetic exchange among like ions dominates over the inter-species exchange, closed anti-ferromagnetic chains following the fully packed loops of like cations emerge as non-local effective low temperature degrees of freedom. Monte Carlo simulations reveal a fluctuation driven first order phase transition from an algebraically correlated liquid to a long range ordered spin-nematic, which mean field theory captures with very high accuracy. In this talk I will discuss this simplified system and how it might relate to the properties of experimental realizations (such as CsNiCrF6) which show a hierarchical freezing rather than the predicted nematic phase transition.

Speaker: Peter Derlet (Paul Scherrer Institut)

19 Dynamics and Non-Hermitian Skin Effects in Two-Dimensional Electron Systems

Because of non-Hermiticity, open quantum systems exhibit unique topological phenomena. A representative example is the non-Hermitian skin effect, a localization phenomenon induced by point-gap topology[1]. This type of non-Hermitian localization phenomena has been extended to the Liouvillian skin effects[2] which result in dynamical particle accumulation around one of the edges in one dimension. These remarkable non-Hermitian phenomena have been reported for a wide range of platforms from photonic metamaterials[3] to cold atoms with dissipation[4].

In spite of this significant progress, Liouvillian skin effects, which affect dynamics, are limited to synthetic systems and thus are largely unexplored in electron systems in solids. In addition, effects of finite temperatures and Fermi surfaces which are ubiquitous in solids remain unclear.

In the presentation, we discuss Liouvillian skin effects in two-dimensional electron systems with Rashba spin-orbit coupling on a substrate[5] subject to a transverse magnetic field. In this system, breaking mirror and time-reversal symmetries induces a Liouvillian skin effect characterized by a winding number. This Liouvillian skin effect becomes pronounced at temperatures lower than the energy scale of the spin-orbit coupling and the magnetic field. We also demonstrate that the Liouvillian skin effect results in dynamical electron accumulation around the edge under the quench dynamics of the magnetic field.
If time allows, we also address topology of multifold exceptional points[6,7]. In particular, we discuss interaction-enabled multifold exceptional points (or higher-order exceptional points)[6] which are allowed only at the interacting level.

References
[1] N. Okuma, K. Kawabata, K. Shiozaki, and M. Sato, Phys. Rev. Lett. 124, 086801 (2020).
[2] T. Haga, M. Nakagawa, R. Hamazaki, and M. Ueda, Phys. Rev. Lett. 127, 070402 (2021).
[3] S. Weidemann et al., Science 368, 6488 (2020).
[4] Q. Liang et al., Phys. Rev. Lett. 129, 070401 (2022).
[5] Y. Shigedomi and T. Yoshida Phys. Rev. B 113, 035121 (2026).
[6] M. Kato and T. Yoshida arXiv: 2602.14056.
[7] T. Yoshida arXiv: 2604.00366.

Speaker: Tsuneya Yoshida (Kyoto University)

10:30 Coffee break

20 Thermal transport in the 1/3 plateau phase of a kagome antiferromagnet

Geometric frustration in magnets can produce quantum states whose elementary excitations differ qualitatively from conventional spin flips. Although even the ground state remains elusive in many frustrated magnets, the 1/3 plateau phase of kagome antiferromagnets offers a concrete starting point since its ground state can be described by a product state of singlet loops on hexagons and surrounding polarized spins. There, excitations on each hexagon are localized due to the destructive interference between different hopping paths, suggesting the strong suppression of thermal transport. Recent measurements on the kagome antiferromagnet In-Kapellasite, however, reveal both a nearly field-independent longitudinal thermal conductivity with a power-law temperature dependence and a sizable thermal Hall conductivity within the 1/3 plateau phase. These observations raise the question of what excitations carry heat in this seemingly localized state.

In this talk, we address this question by formulating a bond-operator theory around a 1/3 plateau state. We identify two distinct types of quasiparticles with contrasting roles in heat transport. The first involves rearrangements of the quantum superposition within each hexagon without changing its total magnetization. This magnetically neutral excitation forms a weakly dispersive band and contributes to the field-insensitive longitudinal thermal conductivity. The second originates from spin flips on the polarized sites. Their propagation via chiral hexagons produces dispersive bands with nonzero Berry curvature, giving rise to the thermal Hall effect. We also compare the calculated thermal conductivities with experiment and discuss how further-neighbor exchange interactions, relevant to Kapellasite-type materials, affect these transport properties.

Speaker: Masataka Kawano (Department of Basic Science, University of Tokyo)

21 Lighting up superconductivity

Photo-induced, superconducting-like behaviors have been reported in a number of materials driven by intense pump fields. Recent experiments in YBCO observed a transient magnetic field outside the sample, which is proportional to and in the same direction as an applied field, suggestive of flux exclusion due to the Meissner effect. We explore theoretical models in which a strong terahertz pump pulse applied to systems with short-ranged superconducting correlations can enhance, rather than destroy, coherence. We also consider an alternative interpretation of experiments in YBCO based on a novel Floquet instability of the sine-Gordon model.

Speaker: Eugene Demler (ETH Zurich)

22 Exceptional Majorana bosons

For many years, Majorana fermions have attracted considerable attention in the condensed matter physics community. Majorana fermions can emerge in topological superconductors, and their non-Abelian statistics provide a promising route toward topological quantum computation. In contrast, their bosonic counterparts, Majorana bosons, remain largely unexplored.

In this talk, based on our recent papers, we propose the concept of Majorana bosons characterized by exceptional points. We show that Majorana bosons can emerge at the critical points of a certain class of phase transitions in Bose-Einstein condensates (BECs). Specifically, we demonstrate their emergence in two distinct settings: a ferroelectric transition in a magnon BEC and a nonequilibrium BEC transition in a bosonic Kitaev chain. These results establish Majorana bosons as a concept extending beyond a specific physical platform.

Speaker: Kazuki Yamamoto (Kyoto University)

12:30 Lunch

23 Semiclassical approach to multiorbital Hubbard model

To quantitatively describe both equilibrium and nonequilibrium properties of strongly correlated electron systems, it is important to incorporate detailed electronic-structure information obtained from first-principles calculations. By combining SU(N) coherent-state formulations with field-theoretical techniques in statistical mechanics, we aim to develop a quantitative framework for strongly correlated electron systems, including nonequilibrium dynamics, while retaining material-specific information.

We first consider correlated (Mott) insulating states described by multi-orbital Hubbard models in the strong-coupling limit, for which effective Hamiltonians can be constructed from realistic tight-binding models [1,2]. We further demonstrate that the SU(N) coherent-state formalism can be consistently extended to fermionic systems, providing a route toward a broader semiclassical description of interacting electron systems [3]. This formalism can capture both itinerant and localized electron behavior at a reasonable computational cost owing to the semiclassical approximation.

[1] R. Iwazaki, H. Shinaoka, and S. Hoshino, Phys. Rev. B 108, L241108 (2023).
[2] R. Pohle, Y. Motome, T. Tadano, and S. Hoshino, arXiv:2507.19764 (2025).
[3] Y. Yamasaki, H. Suwa, C. D. Batista, and S. Hoshino, arXiv:2604.02769 (2026).

Speaker: Shintaro Hoshino (Chiba University)

24 Nonperturbative Nonlinear Hall Effect in Nonequilibrium Steady States

The nonlinear Hall effect in quantum materials has attracted broad interest, yet most existing studies focus on the weak-field, perturbative regime. Here we develop a nonperturbative approach based on nonequilibrium steady-state Green's functions for dc-field-driven lattice systems, with dissipation and interactions incorporated through self-energies beyond the constant relaxation-time approximation and interband transitions treated alongside their intraband counterparts. Applied to a two-band semimetal model, our approach provides direct access to the strong-field Hall response beyond the nonperturbative crossover where the edge of the nonequilibrium distribution reaches Berry-curvature hot spots, a regime in which constant relaxation-time estimates and Berry curvature dipole calculations become unreliable. We further demonstrate that interaction and electron-phonon self-energies within dynamical mean-field theory can substantially change the Hall signal. Our framework enables quantitative simulations of nonequilibrium nonlinear Hall phenomena and provides guidance for strong-field transport experiments.

Speaker: Lei Geng (University of Fribourg)

25 Dimensional effects of high-harmonic generation in strongly correlated electron systems

Dimensionality strongly affects elementary excitations in correlated quantum materials, yet its impact on extreme nonlinear optical responses remains largely unexplored. Here, we combine high-harmonic generation (HHG) experiments on the quasi-one-dimensional (1D) cuprate SrCuO$_2$ and the quasi-two-dimensional (2D) cuprate Pr$_2$CuO$_4$ with nonequilibrium simulations of the single-band Hubbard model [1]. We have found a pronounced dimensional contrast. SrCuO$_2$ exhibits a robust plateau-like structure in high-harmonic spectrum with weak temperature dependence. In contrast, Pr$_2$CuO$_4$ shows a monotonic decrease in harmonic yield with increasing order and a strong thermal suppression that becomes more intense at higher harmonics. Simulations using finite-temperature infinite time-evolving block decimation for the 1D chain and time-dependent dynamical mean-field theory for the 2D square lattice qualitatively reproduce these trends. Furthermore, subcycle analysis demonstrates that the doublon-holon (D-H) pairs remain coherent over long times in 1D owing to spin-charge separation, whereas in 2D their motion disturbs the surrounding spin background and causes rapid D-H dephasing. These results establish dimensionality and spin-charge coupling as key control parameters for extreme nonlinear optical responses in correlated insulators and suggest that HHG can serve as a time-domain probe of many-body charge dynamics.
[1] K. Uchida, S. Takayoshi, Y. Murakami, D. Song, A. M. Hallas, M. Watanabe, T. Konishi, A. Nakano, and K. Tanaka, arXiv:2609.02466

Speaker: Shintaro Takayoshi (Konan University)

26 Koopman Nonlinear Non-Hermitian Skin Effect

Non-Hermitian skin effects are conventionally understood as boundary localization of eigenstates in linear systems [1]. In realistic experimental platforms, however, nonlinearity is ubiquitous [2], and eigenstates are no longer well defined, raising a fundamental question of how skin effects should be characterized beyond the linear regime. Previous approaches have mainly focused on stationary solutions, reducing the problem to a spatial dynamical system at a fixed frequency [3, 4]. While successful in certain contexts, such approaches may probe a restricted subset of the full nonlinear dynamics, leaving the underlying nature of nonlinear skin effects obscured.
In this talk, I propose a Koopman-based characterization of nonlinear skin effects, in which localization is defined in terms of Koopman eigenfunctions in a lifted observable space, rather than physical states [5]. Using a minimal nonlinear extension of the Hatano-Nelson model, I show that dominant Koopman eigenfunctions localize sharply on higher-order observables, in stark contrast to linear skin effects confined to linear observables. This lifted-space localization governs the boundary selective nonlinear response providing a distinct dynamical signature of the nonlinear skin effect. These results establish the Koopman framework as a natural setting in which skin effects unique to nonlinear non-Hermitian systems can be identified.
References:
[1] S. Yao and Z. Wang, Phys. Rev. Lett. 121, 086803 (2018).
[2] S. Wang et al., Phys. Rev. Lett. 134, 243805 (2025).
[3] C. Yuce, Physics Letters A 408, 127484 (2021); Phys. Rev. B 111, 054201 (2025).
[4] K. Kawabata and D. Nakamura, Phys. Rev. Lett. 135, 126610 (2025).
[5] S. Hamanaka, arXiv: 2601.03636.

Speaker: Shu Hamanaka (Kyoto University)

15:30 Coffee break

27 Unconventional Excitons from Topological Bands

Collective excitations in quantum magnets can inherit key features of their underlying electron or quasiparticle bands, including their dispersion, energetics, and quantum geometry, while being strongly reshaped by interactions. In this talk, I will discuss examples ranging from topological excitons, their condensation, and the resulting unconventional magnons in the interacting Haldane model [1], to thermal effects on topological Dirac magnons in van der Waals ferromagnets [2]. These examples illustrate how band structure, topology, and interactions conspire to produce unconventional collective excitations in strongly correlated systems.
[1] Eto & Knolle, arXiv:2606.05837
[2] Eto, Salgado-Linares, Mochizuki, Knolle, & Mook, Phys. Rev. X 16, 021053 (2026).

Speaker: Rintaro Eto (Technical University of Munich)

28 Three-path interferometry of quantum Hall edge chirality and anyonic statistics

Fractional quantum Hall edges carry electric charge downstream, while neutral excitations may propagate in the opposite direction. How can one probe this directional structure and learn about the statistics of tunneling quasiparticles?

I will discuss a three-path interferometer in which a direct tunneling process interferes with a coherent two-step alternative. The resulting Aharonov–Bohm oscillations appear in the average electrical currents. In the local weak-tunneling regime, equal drain voltages make one output dark for a purely downstream tunneling excitation, while the other remains bright. This provides a simple probe of the directional correlations of quantum Hall quasiparticles.

By combining the bias dependence and relative amplitudes of the interference signals, one can extract the quasiparticle scaling dimension and, under controlled conditions, its anyonic exchange angle. Finally, I will show how spatially nonlocal interactions can activate the otherwise dark signal even in the absence of upstream modes.

Speaker: Eugene Sukhorukov (University of Geneva)

29 Spin currents in crystals with spin-orbit coupling: Multiband effects in an effective Hamiltonian formalism

When focusing on a few essential bands in an effective description of a material to calculate observable quantities, the respective operators have to be adjusted accordingly. Ignoring contributions arising from integrating out remote bands can lead to qualitatively wrong results. We present a detailed analysis of the interband mixing effects on spin currents. Specifically, we calculate the intrinsic spin current in a time-reversal invariant noncentrosymmetric crystal in the presence of electron-lattice spin-orbit coupling. Starting from formally exact microscopic expressions, we derive the spin current operator restricted to one or more essential bands by iterative elimination of the contributions from distant bands. We show that the standard definition of the spin current operator in terms of the group velocity obtained from an effective band Hamiltonian cannot be justified using a microscopic theory. The modified expression for the spin current operator contains additional terms, which dominate the equilibrium spin current in a uniform crystal. We show that the magnitude of these additional terms can considerably exceed the spin current obtained using the standard
definition.

Speaker: Mark Fischer (University of Zurich)

30 Flavour selection in the trigonal warped electron gas

Strongly interacting electrons in 2D materials can polarise into discrete flavours, spin or valley, before crystallisation. Experiments show that different materials give opposite answers for which flavour is chosen: AlAs chooses valley polarisation first, while graphene multilayers choose spin first in a cascade of polarised metals and lifshitz transitions tied to trigonal warping. Hartree–Fock leaves the candidate states exactly degenerate and so correlations decide, with perturbation theory suggesting the flavour choice depends on the symmetry relating the valleys. I study a minimal two-valley electron gas with a tuneable warped dispersion, the valleys being time-reversed partners, using variational Monte Carlo to treat the metal and Wigner crystal on equal footing. I will show how the warping affects the flavour choice, whether the crystal remembers it, and ask what this might mean for the experiments.

Speaker: Samuel Bird (ETH Zurich)

31 Pauli Crystal Superradiance

Pauli crystals are geometric many-body correlations that emerge in noninteracting fermions solely from Fermi statistics and confinement. In this talk, I will show how coupling such a system to an optical cavity can make these hidden correlations observable. Confinement-induced degeneracies allow cavity-mediated interactions to select symmetry-breaking states, leading to zero-threshold superradiance and a quantum crystalline state with modulated density. Analytical results and many-body simulations reveal how statistics, geometry, and light-mediated interactions combine to turn Pauli correlations into quantum crystallization.

Speaker: Daniel Ortuño-Gonzalez (ETH Zurich)

32 Three-dimensional Foliated Fractional Quantum Hall Phases

Foliated topological orders in three dimensions are layered systems in which anyons are free to move within a layer but cannot hop between them. A simple model with such a phase is a stack of decoupled two-dimensional electron gases in a strong magnetic field, each in the same fractional quantum Hall state. By focusing on the case of filling ν=1/3 of the lowest Landau level in each layer, we show that (i) the limit of decoupled Laughlin states is stable upon introducing interlayer interactions and (ii) the system can enter a spontaneously layer-trimerized foliated non-Abelian Fibonacci phase. We support our claims by numerical exact diagonalization of up to 10 layers as well as perturbative analytical calculations. Specifically, we show that the foliated Fibonacci phase exists in the 9-layer system with pseudopotential interactions within and between neighboring layers. We identify the phase via quasihole counting and by calculating the overlap with a model wave function which we derive from the associated conformal field theory. Our numerical results suggest the possibility of realizing these phases in layered van der Waals crystals in strong magnetic fields, as well as in multilayer heterostructures.

Speaker: Andrea Kouta Dagnino (University of Zurich)

33 Poster session

20:00 Social Dinner

Wednesday 21 October

34 Quantum geometry in correlated electron phases: from flat band to dispersive band

Bloch electrons are characterized by two ingredients, the energy dispersion and the Bloch wave function. In the conventional single-band picture, correlated electron phases such as superconductivity and itinerant magnetism are governed by the energy dispersion. By contrast, in flat-band systems, where the energy dispersion is quenched, geometric properties of the Bloch wave function in momentum space, namely quantum geometry, play an essential role, as exemplified by flat-band superconductivity and flat-band ferromagnetism. In this talk, we first give a brief introduction to quantum geometry in correlated electron phases, focusing on the connection between flat-band and dispersive-band systems [1]. We then present our recent studies on superconductivity and magnetism in dispersive-band systems [2–4], showing that the quantum metric stabilizes ferromagnetism and induces ferromagnetic fluctuations that mediate spin-triplet superconductivity.

[1] T. Kitamura, A. Daido, and Y. Yanase, Appl. Phys. Lett. 128, 130501 (2026).
[2] T. Kitamura, A. Daido, and Y. Yanase, Phys. Rev. Lett. 132, 036001 (2024).
[3] T. Kitamura, H. Nakai, A. Daido, and Y. Yanase, arXiv:2505.01089.
[4] T. Kitamura, H. Nakai, H. Katsura, and R. Arita, Phys. Rev. Lett. 137, 126502 (2026).

Speaker: 北村 泰晟 (東京大学)

35 Fractal Confinement and Magnetic Self-sabotage of Current Flow: Electrons near the Metal-insulator Crossover in a 2D δ-layer

The interplay of disorder, interactions, and magnetism in two-dimensional electron systems remains a long-standing problem in condensed matter physics. Positionally disordered $\delta$-doped semiconductors have recently reached a new level of experimental control, promising new insight into this old question.

We investigate theoretically the origin of anomalous low-temperature transport near the critical density in disordered two-dimensional semiconductors. Large-scale Hartree-Fock calculations are performed for a model with random hopping and on-site repulsion $U_0$, tracking transport and magnetic properties and their mutual correlation as the temperature is lowered.

Near the critical density, two transport regimes emerge. Conductivity initially increases below $T\sim U_0/10$, consistent with renormalization-group predictions. At lower temperatures ($T\sim U_0/100$), however, the increasingly fractal wavefunctions enhance interactions and promote the formation of local magnetic moments, that ultimately suppress conduction through dynamical $SU(2)$ symmetry breaking.
These results provide a microscopic explanation for a decades-old transport anomaly, stimulating further work to unveil the interesting aspects of magneto-transport seen at low temperatures.

Speaker: Markus Müller (PSI - Paul Scherrer Institut)

36 Anomalous crystal growth of topological materials

Understanding crystal growth and morphology is a fundamental challenge in condensed matter physics. While crystal morphology governed by the distribution and dynamics of the diffusion field has been extensively studied, the influence of intrinsic material properties on crystal morphology remains unclear. In this talk, we present two theoretical studies connecting electronic topology to nonequilibrium crystal morphology and interfacial dynamics.

First, we show that topological insulators can exhibit an intrinsic interfacial instability that amplifies small interface fluctuations, resulting in chaotic behavior during interface growth [1]. We find that the boundary states of topological insulators can induce negative surface stiffness. Surface stiffness quantifies how strongly a surface resists changes in its shape or orientation. We derive an effective equation governing interface growth in the presence of this negative stiffness and demonstrate that the interface dynamics is governed by the Kuramoto–Sivashinsky equation, a prototypical nonlinear equation exhibiting spatiotemporal chaos.

Second, we demonstrate that higher-order topological phases give rise to characteristic crystal morphologies through a previously unexplored mechanism that originates from the topological electronic properties of the material [2]. The resulting morphologies feature preferential corner growth and relatively smooth interfaces. We quantitatively characterize these crystal shapes by analyzing the fractal dimensions of both the crystal and its boundary.

Together, these studies demonstrate how topological boundary states can influence both the shape of a growing crystal and the dynamics of its interface.

Reference
[1] Y. Tanaka and A. Furusaki, Phys. Rev. B 114, 045404 (2026).
[2] Y. Tanaka, S. Zhang, T. Zhang, and S. Murakami, arXiv:2509.09875 (2025).

Speaker: Yutaro Tanaka (RIKEN)

10:30 Coffee break

37 Multifold band degeneracies and linking of nodal manifolds

Topological degeneracies of energy bands in crystalline matter are conventionally characterized by invariants computed on an enclosing sphere over which the spectrum remains gapped. Crucially, for higher-order degeneracies, the enclosing-sphere paradigm faces a fundamental obstruction: two $(n\,{-}\,1)$-fold degeneracy loci emanating from the $n$-fold node necessarily pierce every choice of enclosing sphere, leaving no uniform spectral gap and thus no standard homotopy classification. However, we can elevate this obstruction into the diagnostic itself. Namely, on the two nodal manifolds where the loci cross the sphere, complementary spectral gaps are restored, admitting conventional band invariants (Chern numbers, Stiefel-Whitney classes, and winding numbers). This observation establishes a general two-way correspondence: (1) the multifold node is topologically protected whenever the associated nodal manifolds are robustly linked, and (2) invariants on cycles of one manifold encode their linking numbers with cycles of the other. Carrying out this program, we can recast multifold band topology as the topology of linked nodal manifolds.

Speaker: Askar Iliasov (University of Zurich)

38 Quantum geometry, monopole, and strongly-correlated superconductivity in CeRh2As2

Strongly correlated superconductors provide a fertile platform for unconventional pairing mediated by spin and multipole fluctuations. Recently, the quantum geometry of Bloch wave functions has emerged as another key ingredient controlling electronic instabilities [1]. However, how strong correlations, multipole physics, superconductivity, and quantum geometry cooperate in real materials remains largely unexplored.
CeRh$_2$As$_2$ is an ideal system for addressing this issue. This locally noncentrosymmetric heavy-fermion superconductor exhibits two superconducting phases, suggesting competition between even- and odd-parity superconducting states [2]. NMR/NQR measurements have reported signatures of magnetic monopole order inside the superconducting phase, indicating the importance of odd-parity multipole degrees of freedom [3,4]. In addition, applied pressure significantly modifies the superconducting phase diagram and appears to bring the even- and odd-parity superconducting states close in stability [5,6], a behavior that cannot be captured by a simplified bilayer model [7,8]. These facts call for a realistic microscopic theory incorporating both strong correlations and material-specific electronic structure.
We construct a realistic Dirac-Anderson model for CeRh$_2$As$_2$ based on the low-energy electronic structure revealed by ARPES [9-11] and DFT+$U$ calculations [12]. We show that the quantum geometry of the $f$-electron Dirac band promotes ferroic magnetic monopole fluctuations through a mechanism distinct from the conventional density-of-states-driven Stoner scenario. By solving the linearized Eliashberg equation, we find parity-competitive superconducting states with dominant spin-triplet character associated with the Dirac band.
We further find that electron correlation effects renormalize this quantum-geometric monopole instability and modify the balance between even- and odd-parity superconductivity. This provides a natural interpretation of the pressure evolution of the superconducting phase diagram: pressure suppresses correlation-induced renormalization and brings the two superconducting states closer to degeneracy. Our results identify CeRh$_2$As$_2$ as a material platform where strong correlations, multipole physics, and quantum geometry cooperate to realize parity-competitive unconventional superconductivity [13].

[1] T. Kitamura, A. Daido, and Y. Yanase, Appl. Phys. Lett. 128, 130501 (2026).
[2] S. Khim et al., Science 373, 1012 (2021).
[3] M. Kibune et al., Phys. Rev. Lett. 128, 057002 (2022).
[4] S. Ogata et al., Phys. Rev. B 110, 214509 (2024).
[5] H. Siddiquee et al., Phys. Rev. B 108, L020504 (2023).
[6] K. Semeniuk et al., Phys. Rev. B 110, L100504 (2024).
[7] T. Yoshida, M. Sigrist, and Y. Yanase, Phys. Rev. B 86, 134514 (2012).
[8] K. Nogaki and Y. Yanase, Phys. Rev. B 106, L100504 (2022).
[9] X. Chen et al., Phys. Rev. X 14, 021048 (2024).
[10] B. Chen et al., Phys. Rev. B 110, L041120 (2024).
[11] Y. Wu et al., Chin. Phys. Lett. 41 097403 (2024).
[12] J. Ishizuka, K. Nogaki, M. Sigrist, and Y. Yanase, Phys. Rev. B 110, L140505 (2024).
[13] K. Nogaki and Y. Yanase, Phys. Rev. B 113, 214506 (2026).

Speaker: Kosuke Nogaki (Niigata University)

39 Superconducting acoustogalvanic effect in moire superconductors

Since the discovery of magic-angle twisted bilayer graphene [1], research on moiré materials has rapidly expanded. Superconductivity in twisted bilayer transition metal dichalcogenides such as twisted WSe2 has also been realized [2,3]. While extensive studies have been conducted on the mechanism of superconductivity in these systems, the determination of pairing symmetry remains an open question.
To address this issue, we propose to take advantage of surface acoustic waves (SAWs) [4]. SAWs are elastic waves that propagate along the surface of a material, inducing spatio-temporally periodic strain in the crystal lattice. The strain effect induced by SAWs is known to be incorporated into the Hamiltonian as a pseudo gauge field, giving rise to a response analogous to nonlinear optical response, namely the acoustogalvanic effect (AGE) [5]. While AGE has been studied in the normal state, in this study, we extend AGE to superconductors. Unlike optical responses, superconducting acoustogalvanic effect (SAGE) allows us to probe the quantum geometry of the Bogoliubov quasiparticles. Moreover, since the frequency range of SAW lies in the GHz regime, which is much lower than that of optical excitations, it enables the investigation of low-energy physics. Since this energy scale is comparable to the superconducting gap of low-transition-temperature superconductors, SAGE is expected to serve as a probe for observing the superconducting gap structures.
In this presentation, we demonstrate that SAGE can directly observe the quantum geometry of the Bogoliubov quasiparticles and access pairing symmetry and apply this approach to twisted WSe₂ [6].

[1] Y. Cao, et al., Nature 556, 43 (2018).
[2] Y. Xia, et al., Nature 637, 833 (2025).
[3] Y. Guo, et al., Nature 637, 839 (2025).
[4] X. Nie, et al., Nanoscale Horizons 8(2), 158 (2023).
[5] P. O. Shkhachov and H. Rostami, Phys. Rev. Lett. 124, 126602 (2020).
[6] T. Matsumoto, et al., arXiv: 2505.21436.

Speaker: Tsugumi Matsumoto (Kyoto University)

40 Partial symmetry breaking and exotic multicriticality in a frustrated clock model

Clock and Potts models are paradigmatic generalisations of the Ising model to systems with higher-dimensional degrees of freedom, with qualitatively distinct phase diagrams: In two dimensions, the Potts model undergoes a first-order ordering transition, while the clock model exhibits an intermediate quasi-long-range-ordered liquid phase, bounded by BKT transitions.

In this talk [1], I will explore a family of models that interpolate between the well-studied six-state clock and Potts models via a multicritical point described by decoupled Ising and three-state Potts models. We find that this decoupling extends into stable phases that break only $\mathbb{Z}_2$ or $\mathbb{Z}_3$ symmetry. We also find that the liquid phase of the pure clock model extends into a finite region of the phase diagram, terminated by a $\mathbb{Z}_6$ parafermionic multicritical point. In order to conclusively identify this intricate conformal field theory, we develop correlation and spectral analysis tools highly adapted to DMRG simulations with both open and periodic boundaries.

Our work shows that parafermionic criticality readily emerges far from special integrable points. In particular, I will discuss an observation of parafermions in a frustrated magnetic model with intertwined Ising and three-sublattice ordering (i.e., no native six-state degrees of freedom), as well as potential applications for artificial spin ice.

[1] A. K. Dagnino, A. Szabó. Parafermionic and decoupled multicritical points in a frustrated $\mathbb{Z}_6$ clock chain. arXiv:2605.03001
[2] G. Rakala et al. Melting of three-sublattice order in triangular lattice Ising antiferromagnets: Power-law order, $Z_6$ parafermionic multicriticality, and weakly first order transitions. arXiv:2109.03178

Speaker: Attila Szabó (University of Zurich)

41 Fractional Chern insulators vs. charge order in twisted MoTe$_2$ around 3.7$^\circ$

The bandstructure of twisted MoTe$_2$ around 3.7$^\circ$ consists of spin-valley locked Chern bands with a bandwidth that is smaller than the interaction scale. As such, the strongly correlated phases familiar from Landau levels can be realized: Quantum Hall states, composite Fermi liquids and charge order. These states have in common that they are all favoured by strong interactions, however changing the quantum geometry and topology of the bands via the displacement field and twist angle tilts the balance between them. We provide a comprehensive exact diagonalization analysis of the competition between these states and in particular elucidate the nature of the charge order at fillings $\nu=-\frac14,-\frac13,-\frac12,-\frac23$. Furthermore, we analyze the effect of band mixing by going beyond a single band per valley. This effect of band mixing is crucial for reconciling the numerical phase diagrams with experimental observations.

Speaker: Glenn Wagner (ETH Zurich)

42 Closing

12:45 Lunch to go

13:00 Social activity