* Lunch box will be served to all participants during the workshop period.

 

 

Simon Trbest (University of Cologne)

Title: Nishimori physics — classical statistical mechanics, quantum measurements, and non-unitary dynamics

Abstract: The preparation of quantum states across many qubits is an essential building block to unlock the full potential of quantum computers. However, a key challenge is to realize efficient preparation protocols which are stable to noise and gate imperfections. In this talk, I will discuss how monitored quantum circuits allow to dynamically create, actively manipulate and decode entanglement structures. A principal example will be the preparation of macroscopic cat states, their stability and the unexpected connections to Nishimori physics that play out in multiple ways. 

 

 

Gil Young Cho (POSTECH)

Title: Pseudochaotic Many-Body Dynamics as a Pseudorandom State Generator

Abstract: We introduce a new class of quantum many-body dynamics in quantum simulations, namely 'pseudochaotic dynamics,' which generates computationally indistinguishable states from Haar-random states within the limited access to the measurement outcomes and runtime of quantum computations. While it is not chaotic, a defining characteristics of many-body quantum chaos, namely out-of-time-ordered correlators, fail to differentiate the pseudochaotic dynamics from chaotic one. We systematically construct such pseudochaotic unitary evolution and investigate their nature through extensive numerical and analytic calculations. Remarkably, we show that the pseudochaotic dynamics can generate a representative pseudo-quantum state, specifically a random subset-phase state, from initial computational states with a depth tightly bound by polylog(n) with the system size n, which opens up a practical route to realize pseudorandom states in near term quantum devices.

 

 

Luca Delacretaz (Chicago)

Title: Universal Thermalization Bound and Intermediate-Time Dynamics

Abstract: Interacting systems thermalize. They can do so arbitrarily slowly, but not arbitrarily fast: motivated by the strange metallic phase of high-Tc superconductors, the time scale necessary for a quantum many-body system to reach local thermal equilibrium has been conjectured to be bounded below by the Planckian time, hbar/T. I will show that consistency of fluctuating diffusion, which generically emerges at late times, implies that this local equilibration time indeed has a lower bound. The key tool is the derivation of universal intermediate time corrections to diffusion using EFT techniques: when these corrections are large the system cannot have thermalized. In quantum critical fans, combining this argument with causality constraints establishes the conjectured Planckian bound. I will also discuss spin chains and Floquet systems, where the knowledge of these universal corrections to diffusion can allow for precision tests of thermalization, and a more accurate identification of a thermalizing system's dissipative universality class with limited numerical resources.

 

 

Liujun Zou (NUS)

Title: Symmetry-enforced exotic quantum matter and beyond

Abstract : It is widely appreciated that symmetry is an important organizing principle of nature. In this talk, I will discuss situations where symmetry can lead to even more surprising consequences that have been largely overlooked before. In particular, taking Kitaev spin-S models as examples, I will argue that certain symmetry there can force the system to realize exotic quantum matter, and such exotic quantum matter is robust even if this symmetry is slightly broken. This idea of symmetry-enforced exotic quantum matter sheds novel insights on the understanding of strongly correlated systems and the realization of exotic quantum matter. If time allows, I will also briefly review some of my previous work that addresses multiple condensed matter problems using a systematic powerful framework based on quantum anomalies, which are ultimately of the same spirit as the analysis of the Kitaev spin-S models.

 

 

Takashi Oka (Tokyo/ISSP)

Title: Novel Floquet states in Dirac electrons induced by AC-magnetic fields and propagating waves

Abstract: Floquet engineering, which involves controlling systems through time-periodic driving, provides a powerful method for coherently manipulating quantum materials and realizing dynamical states with novel functionalities. This presentation reports our recent experimental and theoretical findings on the dynamical states realized in Dirac electrons under various fields. Circularly Polarized Laser [1]: Nonlinear optical response experiments suggest the generation of chiral gauge fields and the creation of emergent Weyl points. AC-Magnetic Fields [2]: We demonstrate the emergence of π-Landau levels and the chiral anomaly-induced homodyne effect when Dirac electrons are subjected to a time-oscillating magnetic field Bcos(Ωt). Propagating Fields [3]: Dirac electrons in a propagating wave Vcos(Qx−Ωt) exhibit sensitivity to the field speed v=Ω/Q. We classify these states and explore the associated topological phase transitions.

 

References: [1] N. Yoshikawa et al., arXiv:2209.11932; Y. Hirai et al., arXiv:2301.06072

[2] S. Kitamura and T. Oka, arXiv:2407.08115

[3] T. Oka, arXiv:2407.21458

 

 

Zhong Wang (Tsinghua)

Title: Non-Hermitian energy spectrum: the stable and the fragile

Abstract: Non-Hermitian systems are not well described by the usual Bloch band theory, and a non-Bloch band theory is necessary to characterize many aspects of their properties. The recent “amoeba formulation” offers a non-Bloch band theory in arbitrary spatial dimensions, which generalizes the initial theory in one dimension. In this talk, I will explain the amoeba formulation, and show that the amoebic spectrum is the stable energy spectrum under local perturbations to the Hamiltonian. In the framework of amoeba formulation, I will also explain the mechanism why the energy spectra of certain non-Hermitain systems are fragile under local perturbations. Finally, I show that the fragile spectrum can be characterized by the Green's function exhibiting unconventional asymptotic behaviors.

 

References:

[1] H. Y. Wang, F. Song, Z. Wang, Physical Review X 14, 021011 (2024)

[2] F. Song, H. Y. Wang, Z. Wang, arXiv:2410.23175

 

 

Moonjip Park (HYU)

Title: Non-Hermitian topological phase and many-body physics

Abstract: Open quantum systems provide a plethora of exotic topological phases of matter that have no Hermitian counterpart. Non-Hermitian skin effect, macroscopic collapse of bulk states to the boundary, has been extensively studied in various experimental platforms. However, it remains an open question whether such topological phases persist in the presence of many-body interactions. Previous studies have shown that the Pauli exclusion principle suppresses the skin effect. In this study, we present a counterexample by demonstrating the presence of the skin effect in doublon-holon excitations. While the ground state of the spin-half Hatano-Nelson model shows no skin effect, the doublon-holon pairs, as its collective excitations, display the many-body skin effect even in strong coupling limit. We establish the robustness of this effect by revealing a bulk-boundary correspondence mediated by the point gap topology within the many-body energy spectrum. Our findings underscore the existence of non-Hermitian topological phases in collective excitations of many-body interacting systems.

 

 

Laura Classen (MPI Stuttgart and Tech Univ of Munich)

Title: Electronic mechanisms for charge density waves

Abstract: Interaction-induced charge orders with electronic origin occur as states of spontaneously broken symmetry in several materials platforms. We study  unconventional  charge density waves (CDWs), which  possess a non-zero angular momentum and correspond to bond or loop current orders on a lattice. We analyse electronic mechanisms for their formation and their interplay with other ordering tendencies  in square- and triangular-lattice model systems with SU(N) flavour symmetry, as well as in the magnetic kagome metal FeGe. For large flavour numbers, we work out an electronic Kohn-Luttinger-like mechanism that induces the required attraction in the effective CDW vertex.  We extend our analysis via an unbiased functional renormalisation group (FRG) calculation for the SU(4) triangular lattice Hubbard model and show that a novel loop current state with wave vector K/4 can win the competition against pairing. Furthermore, we analyse possible electronic CDWs in FeGe by combining ab-initio results with FRG calculations. We argue that the leading contribution to electronic correlations are approximately 2D and that intra-band processes are dominated by Van Hove points at the projected M-points. 

 

 

Kwang-Yong Choi (SKKU)

Title: Two case studies on s=1/2 and s=1 kagome antiferromagnets

Abstract: Kagome antiferromagnets have garnered significant interest in condensed matter physics for their potential to host a rich variety of many-body quantum phases. These phases include quantum spin liquids (QSLs), spin nematicity, supersolidity, and magnetization plateaus. The S=1/2 kagome antiferromagnet has emerged as a prominent platform for realizing QSLs, with candidates ranging from gapped Z2 to U(1) Dirac QSLs. In contrast, S=1 kagome antiferromagnets typically display characteristics of classical spin liquids, resulting in complex magnetic phase transitions under varying external conditions. Additionally, theoretical studies predict that kagome lattices exhibit fractional magnetization plateaus, which involve both entangled and unentangled states that vary with spin number. These intriguing phenomena highlight the need for experimental validations to confirm the theoretical predictions. In this presentation, I will share our group's recent work on both s=1/2 and s=1 kagome antiferromagnets. In the first part, I will present thermodynamic and spectroscopic signatures of Dirac spinons in YCu3(OH)6+xBr3-x (x~0.5), which forms a nearly perfect kagome lattice with an exchange interaction of J~60 K. In addition, I will dicuss the observation of the sought-after 1/9 and 1/3 plateaus under fields up to 60 T. Unlike the m=1/3 plateau, attributed to the crystallization of magnons, the nature of the m=1/9 plateau remains a topic of debate, with possibilities including a topological Z3 spin liquid or valence bond crystal. Remarkably, the angular dependence of our thermodynamic data reveals the emergence of threefold symmetry and gapless quantum states within the 1/9 plateau phase. In the second part, I will explore the ground-state properties and spin dynamics of the S=1 kagome antiferromagnet (CH3NH3)NaV3F12. μSR and specific heat measurements indicate weak spin freezing around 4 K, which can be quenched by applying an external magnetic field. The low-temperature specific heat follows a power-law behavior C~Tn (n~1.5), raising the possibility of a gapless spin liquid. Moreover, the pulsed-field magnetization exhibits the presence of a m=1/3 plateau, making this compound suitable for investigating the predicted spin nematicity this plateau.

 

 

Yuan Wan (IOP)

Title: Time-domain interferometry of electron weak localization through two-dimensional coherent spectroscopy

Abstract : Weak localization is the quintessential quantum interference phenomenon that features prominently in disordered conductors. Since its discovery, the canonical diagnostic for electron weak localization has been the magnetoresistance, which works as a space-domain interferometry. In this work, we propose an ultrafast diagnostic for electron weak localization based on the nonlinear optical response in the terahertz regime. Our analytical and numerical calculations reveal that, in orthogonal/symplectic class systems, two consecutive, phase coherent optical pulses generate an electric current echo that appears after the second pulse, and at a time equal to the pulse delay time. The current echo reflects the quantum interference between a self-intersecting electron path and its time reversal partner, and, therefore, provide a time-domain interferometry of weak localization. Our results can be potentially tested on disordered metal films by using terahertz two-dimensional coherent spectroscopy.

 

 

So Yeun Kim (DGIST)

Title: Ultrafast investigation on emergent charge-density-wave collective excitations in (TaSe4)2I

Abstract: One of the major goals of condensed matter physics is to understand the novel phase transition mechanism where various degrees of freedom are strongly coupled. Measuring quasiparticles intrinsic to the phase is therefore a good way to monitor and diagnose the phase of solids. Some of these collective modes (quasiparticles) found in solid systems are analogs of elementary particles predicted in high-energy physics, such as Dirac fermion, Weyl fermion, and axion, in the sense that they follow the same equation of motion. In these aspects, investigating quasiparticles in solids provides an exciting opportunity to explore and simulate novel physical phenomena. In this talk, we discuss the collective modes that arise in quantum materials hosting charge-density-wave (CDW) phases originating from strong charge-lattice coupling. We investigate a non-magnetic chiral chain lattice system in (TaSe4)2I, which exhibits incommensurate CDW phase transition near 260 K. As highlighted a few years ago [1], the coexistence of CDW wavevector and Weyl fermions suggested that (TaSe4)2I may host quasiparticle that follows axion-like electrodynamics that allows E·B in analog with axion particles in high-energy physics, which is yet controversial [2]. An important thing to point out is that much basic information about CDW modes in (TaSe4)2I has been unclear despite the intense amount of research over the past few decades. Here we discuss some advancements of different types of collective modes explored via ultrafast spectroscopy, including massive phason [3] and composite amplitude modes [4]. The detailed experiment and data and future search plan for axion-like quasiparticles in the material will be discussed. Our approaches using femtosecond light pulses shed light on elusive collective mode detection that was inaccessible before, thereby demonstrating the essential role of ultrafast spectroscopy in resolving complex electronic and lattice structures in quantum materials.

 

[1] J. Gooth et al., Nature 575, 315 (2019).

[2] A. A. Sinchenko et al., Appl. Phys. Lett. 120, 063102 (2022).

[2] S. Kim et al., Nat. Mater. 22, 429 (2023).

[3] Q.L. Nguyen, R. Duncan et al., Phys. Rev. Lett. 131, 076901 (2023).

 

 

Nic Shannon (OIST)

Title: Using ideas from quantum information to learn about quantum magnets

Abstract: The very things which make exotic phases in quantum magnets interesting: the absence of conventional magnetic order parameters; the emergence of rational excitations; their topological and entanglement properties, also make them difficult to distinguish in experiment. This is particularly true of quantum spin liquids, which are notoriously difficult to diagnose, especially in the presence of structural or chemical disorder. In this talk we explore how ideas from quantum information could be used to distinguish quantum spin liquids from other competing phases of matter. We find that, used carefully, experiments based on entanglement witnesses can be used identify quantum spin liquids in disordered samples.

 

[1] S. Sabharwal, T. Shimokawa and N. Shannon, arXiv:2407.20797

 

 

Yasir Iqbal (IIT)

Title: U(1) and Z2 Dirac Spin Liquids

Abstract: I will present the construction of monopole excitations in U(1) Dirac spin liquids. Evidence of their stability for Heisenberg antiferromagnets on the kagome and triangular lattices will be discussed. Some recent results on Dirac spin liquids on the Shastry-Sutherland lattice will be presented in the context of recent material realizations.

 

 

Eun-Gook Moon (KAIST)

Title: Manipulation of topological phase transitions in quantum spin liquids

Abstract: A quantum spin liquid is one of the most entangled states of many-body systems whose identification is known to be notoriously difficult in experiments. Any symmetry order parameters are absent, and experimental probes with external symmetry operations are commonly believed to be inapplicable. Here, we demonstrate that a class of quantum spin liquids may be identified by using external symmetry operations associated topological phase transitions. Specifically, we show that quantum spin liquids with quantum criticality, including Kitaev quantum spin liquids and Dirac quantum spin liquids, may be controlled by external symmetry operations such as electromagnetic fields and strains.

 

 

Gil-Ho Lee (POSTECH)

Title: Steady Floquet-Andreev state and multi-dimensional Andreev band

Abstract: Andreev bound states (ABS), which form in proximity Josephson junctions, govern the physics of the Josephson devices. Consequently, understanding and manipulating the ABS is essential for optimizing device functionality. In this regard, two topics will be discussed; one is Floquet physics of ABS, and another is ABS of multi-terminal Josephson junction. First, we will introduce the generation of steady Floquet Andreev (F-A) states in graphene Josephson junctions by continuous microwave application and direct measurement of their spectra by superconducting tunnelling spectroscopy. This can be a unique platform to study non-equilibrium quantum physics at a device level. Second, we will discuss the artificial topological band structure of three-terminal graphene Josephson junctions by using superconducting tunneling spectroscopy. We controlled the superconducting phase configurations by applying the flux gates and obtained the ABS energy spectrum as a function of two independent phase differences and energy. Such quasi-momentum v.s. energy map of ABS unveils the transition between gapped and gapless states, corresponding to the topological band structure of 2D-Dirac semimetals.

 

 

Sid Parameswaran (Oxford)

Title: Majorana edge reconstruction and  the ν=5/2 non-Abelian thermal Hall puzzle

Abstract: Pioneering  thermal transport measurements on two-dimensional electron gases in high magnetic fields have demonstrated that the quantized Hall state  at filling factor ν=5/2  has a thermal Hall conductance $K$ quantized in half-integer multiples of K_0 = π^2k_B^2T/3h. Half-integer K/K_0 is a signature of neutral Majorana edge modes, in turn linked to the presence of non-Abelian anyon excitations in the bulk.  However, the  observed value  of K corresponds to the  `PH-Pfaffian' state, in tension with numerical studies which instead favor either the Pfaffian or the AntiPfaffian.  A variety of mechanisms have been invoked to explain this discrepancy, but have been either ruled out by further experiments or else involve fine-tuning. Building on density-matrix-renormalization group studies of physically realistic edges and analytic calculations of edge structure, I will discuss an alternative resolution of this puzzle involving an `edge reconstruction' solely involving the neutral Majorana sector of the theory. Such a Majorana edge reconstruction can ``screen'' a Pfaffian or AntiPfaffian bulk, so that transport signatures at the {it physical} edge  are indistinguishable from those of the PH-Pfaffian. I will argue that this physically natural scenario is  consistent with experiment. 

 

 

Yeongkwan Kim (KAIST)

Title: Charge density waves in quantum materials

Abstract: The charge density wave (CDW), the ununiform distribution of itinerant charge density, has been found and studied in various low-dimensional systems for decades. Despite extensive investigations, the many characteristics of CDW remain unknown; a microscopic mechanism other than the Pierls instability is required, and how CDW intertwines with other competing phases, particularly with the superconductivity, should be investigated. Furthermore, the recent compelling evidence has cast light on the unexpected aspect of CDW, additional symmetry breakings that accompany or occur concurrently with the CDW transition. In this talk, I will discuss various aspects of CDWs in different quantum materials, i) the additional symmetry breaking in CDW phase of 1T-TiSe2 and CsV3Sb5 systems, which are footprinted in the intensity of angle-resolved photoemission spectroscopy, ii) the possible role of CDW fluctuation on Cooper pairing, which is accomplished by analyzing the low-energy electronic structure of 2H-TaSe2.

 

 

Xueyang Song (HKUST)

Title: Transition out of quantum Hall and exotic superconductivity upon doping critical points

Abstract:  I will talk about two models that realized bosonic Laughlin state-superfluid and fermionic quantum Hall - chiral spin liquid transitions, respectively. The critical theories are closely related, and given by quantum electrodynamics in 2+1D with Chern-Simons coupling. The numerical calculation from density matrix renormalization group and signature of emergent SO(3) symmetry further verify the critical theory. In the fermionic case with SU(2) spin, upon doping the system, topological superconductivity is shown to emerge. This is demonstrated by 2e bound states in zero doping case. The relation to celebrated anyon superconductivity will be discussed. The generalization to SU(N) spin system holds similar phase diagrams and a mean-field calculation will be presented.

 

 

Trithep Devakul (Stanford)

Title: Emergent phases of electrons in unbounded Chern bands

Abstract: Strongly interacting electrons in topological bands can organize into correlated states with remarkable emergent properties, as exemplified by the fractional quantum Hall effect. Recent developments in rhombohedral graphene have introduced a new paradigm for correlated topological physics: unbounded Chern bands. Unlike conventional Chern bands with discrete translation symmetry, these unbounded bands possess high Berry curvature but have continuous translation symmetry (and hence unbounded momentum). In this talk, I will explore the exotic topological states that can arise in this setting.

 

 

Young-Woo Son (KIAS)

Title: Tunable incommensurability, complete structural phase diagrams and peculiar electronic properties in trilayer moiré-of-moiré systems

Abstract: In this talk, we present a comprehensive structural analysis and the unique electronic properties of twisted trilayer graphene (TTG), a minimal multilayer configuration beyond twisted bilayer graphene. We demonstrate a complete catalog of reconstructed moiré-of-moiré structural phases by manipulating combinations of two twist angles. Our findings reveal cascades of spontaneous symmetry breaking as a function of the twist angles, resulting in a diverse array of large-scale moiré lattices, including triangular, kagome, and corner-shared hexagram-shaped domain patterns. Furthermore, our analysis emphasizes the crucial role of long-range interactions across entire layers, alongside the well-known contributions of twist angles and strain between adjacent layers, in realizing various domain lattices. Building on our results for TTG, we apply our methods to other twisted trilayer (TTL) systems made from two-dimensional semiconductors, demonstrating how external fields can control domain lattices. The diverse tessellation of distinct domains, whose topological network can be adjusted by modifying the twist angles, positions TTL systems as a platform for exploring the interplay between emerging quantum properties and controllable nontrivial lattices.

 

 

Yves Kwan (Princeton)

Title: When could fractional topological insulators appear in twisted MoTe$_2$ and other systems?

Abstract: The experimental observations of fractional Chern insulators in moir’e heterostructures motivate the investigation of their time-reversal invariant generalizations, fractional topological insulators (FTIs), which have been significantly less explored. I will present recent work where we perform exact diagonalization studies of twisted bilayer MoTe$_2$, as well as idealized Landau level models, with the goal of extracting general principles for engineering such exotic phases. Our results reveal that a large suppression of the short-range part of the Coulomb repulsion is crucial for realizing FTIs in realistic systems. I will examine how aspects such as the screening environment, band-mixing, Landau level character and finite-size effects affect the FTI phases, and also discuss potential sample-engineering routes to improve their stability.

 

 

Shuichi Murakami (Tokyo Inst Tech)

Title: Theory of quantized corner charges of three-dimensional crystals

Abstract: Crystals of obstructed atomic insulators can have fractional corner charges, depending on the crystal shapes, and in two dimensions, it is known to be described in terms of the topological invariant called filling anomaly. Here we construct a complete theory of the quantized corner charges in three-dimensional crystals. We exhaust all the crystal shapes and space groups having quantized corner charges, and construct their formulae in real space and in momentum space for all the cases. In some cases, a new Z2 topological invariant is needed to characterize nontrivial corner charges. We also discuss candidate materials with nontrivial corner charges.

 

 

Adrian Po (HKUST)

Title: Cascade of strongly correlated quantum states in a partially filled kagome flat band

Abstract: Coulomb interactions among charge carriers occupying an electronic flat band significantly influence the macroscopic properties of materials. The nontrivial wave functions associated with the partially filled band can further enrich the range of possible phases that emerge. In this talk, we first present experimental results obtained from scanning tunneling microscopy measurements on Fe-doped CoSn. When the filling of the flat band is tuned through chemical doping, a cascade of strongly correlated states, including nematic order, an orbital-selective Mott phase, and a pseudogap phase, are discovered. We also discuss the theoretical implications of the characteristic kagome flat-band wave functions on the experimental observations.