Quantum Matter Seminar
30 events

Seminar
Quantum skyrmion Hall effect
September 14 (Thu) at 17:00  18:15, 2023
Ashley Cook (Group Leader, Correlations and Topology, Max Planck Institute for the Physics of Complex Systems and Max Planck Institute for Chemical Physics of Solids, Germany)
Field: condensed matter physics Keywords: topology, electronbased quantum skyrmions, spin, Berry curvature Abstract: Topological skyrmion phases of matter are recentlyintroduced topological phases of electronic systems in equilibrium, in which a system with more than one degree of freedom (e.g. spin and orbital degrees of freedom) realizes a topological state for a subset of the degrees of freedom (e.g. only spin). For topological skyrmion phases of spin, this topology is relevant even if spin is not conserved due to nonnegligible atomic spinorbit coupling, and is distinguished by a skyrmion forming in the spin texture over the Brillouin zone, distinct from a skyrmion forming in the texture of the projector onto occupied states over the Brillouin zone. We present results on three band Bloch Hamiltonians realizing this nontrivial spin topology, and outline some bulkboundary correspondence features, such as gapless edge states corresponding to zero net charge—but finite spin angular momentum—pumped across the bulk gap. Tracing out the orbital degree of freedom, we can identify this spin pumping with pumping of spin point charges, and local curvature of the kspace spin skyrmion with a Berry curvature of these spin point charges. That is, the spin pumping is identified with pumping of spin magnetic skyrmions, which reduce to point magnetic charges after tracing out the orbital degree of freedom. We therefore identify topological skyrmion phases as lattice counterparts of quantized transport of quantum magnetic skyrmions, a quantum skyrmion Hall effect. This indicates that the theory of the quantum Hall effect must be generalized, by relaxing the assumption of point charges.
Venue: via Zoom
Event Official Language: English

Seminar
Electronic instabilities emerging from higherorder van Hove singularities
July 24 (Mon) at 17:00  18:15, 2023
Xinloong Han (Postdoctoral Fellow, Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences, China)
Time: 5pm ~ 6:15pm (JST); 10am ~ 11:15am (CET); 4pm ~ 5:15pm (Taiwan) Field: condensed matter physics Keywords: topological superconductor, Van Hove singularity, Hubbard model, Kagome lattices Abstract: Competing correlated electronic states are a central topic in condensed matter physics. A typical example is the close competition between spin density wave and dwave superconductivity in the Hubbard model on the square lattice near half filling where the band structures have saddle points at which the Fermi surface topology changes from hole type to electron type. The saddle points are called van Hove singularity (VHS) points, and host diverging density of states with powerlaw behavior in the two dimensions. Recently, another type of VHS, namely the higherorder VHS was investigated in ABCstacked trilayer graphene and twisted bilayer graphene. In this talk, I will first introduce the higherorder VHS, and make comparisons to the conventional VHS. Then I will discuss the enhanced nematicity driven by large flavor number with higherorder VHSs on the square and Kagome lattices. Finally, I will show that robust topological superconductivity can emerge on the square lattice due to interplay of spinorbital coupling and higherorder VHSs.
Venue: Hybrid Format (Common Room 246248 and Zoom)
Event Official Language: English

Seminar
Quantum skyrmion lattices in Heisenberg ferromagnets
June 8 (Thu) at 17:00  18:15, 2023
Andreas Haller (Postdoctoral Researcher, Department of Physics and Materials Science, University of Luxembourg, Luxembourg)
Skyrmions are topological magnetic textures that can arise in noncentrosymmetric ferromagnetic materials. In most systems experimentally investigated to date, skyrmions emerge as classical objects. However, the discovery of skyrmions with nanometer length scales has sparked interest in their quantum properties. In this talk, I present our (numeric) results on the ground states of unfrustrated twodimensional spin1/2 Heisenberg lattices with DzyaloshinskiiMoriya interactions, where we discovered a broad region in the zerotemperature phase diagram which hosts quantum skyrmion lattices. The simulations are based on an established variational optimization algorithm for matrix product states called density matrix renormalization group, which can faithfully approximate the ground states of small 2D clusters well beyond system sizes amenable for exact diagonalization. We argue that the quantum skyrmion lattice phase can be detected experimentally in the magnetization profile via local magnetic polarization measurements as well as in the spin structure factor via neutron scattering experiments. Deep in the skyrmion ordered phase, we find that the quantum skyrmion lattice state is only weakly entangled with ‘domain wall' entanglement between quasiparticles and environment localized near the boundary spins of the skyrmion. In this ordered regime of weakly entangled entities, large clusters of O(1000) sites can be simulated with great efficiency. Field: condensed matter physics Keywords: quantum spin systems, topology, density matrix renormalization group
Venue: via Zoom
Event Official Language: English

Seminar
Groundstate phases of the onedimensional SU(N)symmetric Kondo lattice model
May 11 (Thu) at 17:00  18:15, 2023
Keisuke Totsuka (Associate Professor, Yukawa Institute for Theoretical Physics, Kyoto University)
The Kondolattice model and its variants (e.g., the KondoHeisenberg model), in which itinerant fermions interact with immobile magnetic moments via spinexchange coupling (Kondo coupling), have been playing an important role in understanding the physics of heavyfermion systems. In this talk, I begin by quickly explaining how the SU(N) Kondolattice model, in which the spin SU(2) symmetry is generalized to SU(N), is realized in actual physical systems (e.g., cold fermions and twisted bilayer graphene), and then I focus on the groundstate properties of its onedimensional version. Specifically, when the Kondo coupling is sufficiently large, we find ferromagnetic metallic phases that can be established rigorously as well as several insulating ones. I also show that the SU(N) Kondolattice model provides a natural condensedmatter realization of supersymmetric [i.e., SU(N1)] models. Various (insulating) phases at small Kondo coupling are then explored using the machinery of bosonization and various conformal field theory (CFT) techniques, and the results are compared with the predictions of the LiebSchultzMattistype (or anomalymatching) argument. Field: condensed matter physics Keywords: Kondo lattice model, SU(N) symmetry, supersymmetry, heavyfermion systems, bosonization, conformal field theory
Venue: via Webex
Event Official Language: English

Seminar
Topological Kondo superconductors
March 2 (Thu) at 17:00  18:15, 2023
YungYeh Chang (Postdoctoral Researcher, National Center for Theoretical Sciences & National Chiao Tung University, Taiwan)
Spintriplet pwave superconductors are promising candidates for topological superconductors. They have been proposed in various heterostructures where a material with strong spinorbit interaction is coupled to a conventional swave superconductor by proximity effect. However, topological superconductors existing in nature and driven purely by strong electron correlations are yet to be studied. Here we propose a realization of such a system in a class of Kondo lattice materials in the absence of proximity effect. Therein, the oddparity Kondo hybridization mediates ferromagnetic spinspin coupling and leads to spintriplet resonantvalencebond (tRVB) pairing between local moments. Spintriplet p±p’ wave topological superconductivity is reached when Kondo effect coexists with tRVB [1]. We identify the topological nature by the nontrivial topological invariant and the Majorana zero modes at edges. Our results on the superconducting transition temperature, Kondo coherent scale, and onset temperature of Kondo hybridization not only qualitatively but also quantitatively agree with the observations for UTe2, a Ubased ferromagnetic heavyelectron superconductor. *This work is supported by the National Science and Technology Council, Taiwan. Field: condensed matter physics Keywords: strongly correlated systems, topological superconductor, Kondo effect, resonant valence bond, heavyfermion compounds
Venue: via Webex
Event Official Language: English

Seminar
Entanglement in nonHermitian quantum systems and nonunitary conformal field theories
February 9 (Thu) at 17:00  18:15, 2023
Chang PoYao (Assistant Professor, Department of Physics, National Tsing Hua University, Taiwan)
Time: 5pm ~ 6:15pm (JST); 9am ~ 10:15am (CET); 4pm ~ 5:15pm (Taiwan) Entanglement is a powerful tool to diagnose manybody quantum systems. One example is the critical system where the low energy property can be described by conformal field theories (CFTs), and the central charge which uniquely characterizes the CFT can be perfectly extracted from the entanglement entropy. However, the entanglement properties for nonunitary CFTs are not well understood. Moreover, the entanglement properties in manybody microscopic models which can be described by nonunitary CFTs have not been explored. In this talk, I would like to demonstrate several nonHermitian systems which can be described by nonunitary CFTs, and show their entanglement properties can be correctly obtained by the proposed generic entanglement entropy. Field: Condensed Matter Physics Keywords: nonHermitian systems, conformal field theory, manybody systems, entanglement entropy
Venue: via Webex
Event Official Language: English

Seminar
Phantom Bethe excitations and spin helices in integrable spin chains
September 15 (Thu) at 17:00  18:15, 2022
Vladislav Popkov (University Wuppertal, Germany)
We demonstrate the existence of a special chiral “phantom” mode with some analogy to a Goldstone mode in the anisotropic quantum XXZ Heisenberg spin chain. The phantom excitations contribute zero energy to the eigenstate, but a finite fixed quantum of momentum. The mode exists not due to symmetry principles, but results from nontrivial scattering properties of magnons with momentum k given by the anisotropy via cos (k) = Jz/Jx. The mode originates from special stringtype solutions of the Bethe ansatz equations with unbounded rapidities, the phantom Bethe roots. All such Bethe states are chiral (the simplest representative being factorized state with helicoidal magnetization profile) and exist in both periodic and open XXZ spin chain under finetuning. I show how phantom Bethe states can be generated dissipatively, by setting a polarization gradient via coupling the ends of the open spin chain to suitable dissipative baths. Spin helix eigenstates were observed and used in recent cold atom experiments, and led to further surprising findings.
Venue: via Zoom
Event Official Language: English

Seminar
Adiabatic pumps in quantum spin systems
July 12 (Tue) at 16:00  17:15, 2022
Ken Shiozaki (Assistant Professor, Yukawa Institute for Theoretical Physics, Kyoto University)
The Thouless pump is a oneparameter cycle of 1dimensional gapped quantum systems with U(1) symmetry, which is classified by integers. In this talk, I introduce a generalization of the Thouless pump to quantum spin systems in any dimension with any finite group onsite symmetry. I show a simple model with Z_2 onsite symmetry, and how it is nontrivial via boundary degrees of freedom. Using the framework of the injective matrix product state, one can construct the topological invariant in a way similar to the Berry phase. If time allows, I will briefly introduce a group cohomology model by Roy and Harper for generic space dimensions and discuss its properties.
Venue: via Zoom
Event Official Language: English

Seminar
Topological quantum effects in lowdimensional spin systems  The power of the boundary
June 30 (Thu) at 17:00  18:15, 2022
Thore Posske (Group Leader, I. Institute for Theoretical Physics, University of Hamburg, Germany)
Manipulating the boundary of lowdimensional magnetic structures could grant control about topological magnetic quantum sates. I will discuss the creation of one and twodimensional topological quantum magnets by manipulating the boundary magnetization, address their stability against external perturbations, and discuss their possible application to quantum information processing.
Venue: via Zoom
Event Official Language: English

Seminar
Topological aspects of nonHermitian physics
June 21 (Tue) at 16:00  17:15, 2022
Nobuyuki Okuma (Assistant Professor, Yukawa Institute for Theoretical Physics, Kyoto University)
The past decades have witnessed an explosion of interest in topological materials, and a lot of mathematical concepts have been introduced in condensed matter physics. Among them, the bulkboundary correspondence is the central topic in topological physics, which has inspired researchers to focus on boundary physics. Recently, the concepts of topological phases have been extended to nonHermitian Hamiltonians, whose eigenvalues can be complex. Besides the topology, nonHermiticity can also cause a boundary phenomenon called the nonHermitian skin effect, which is an extreme sensitivity of the spectrum to the boundary condition. In this talk, I will explain recent developments in nonHermitian topological physics by focusing mainly on the boundary problem. As well as the competition between nonHermitian and topological boundary phenomena, I will discuss the topological nature inherent in nonHermiticity itself. Field: condensed matter physics Keywords: topological materials, nonHermitian systems, skin effect, bulkboundary correspondence
Venue: via Zoom
Event Official Language: English

Introduction to Topological Insulators: The Tenfold Classification of Topological Insulators and Superconductors Part.2
June 13 (Mon) at 14:00  15:30, 2022
ChingKai Chiu (Senior Research Scientist, iTHEMS)
Venue: via Zoom
Event Official Language: English

Introduction to Topological Insulators: The Tenfold Classification of Topological Insulators and Superconductors Part.1
May 24 (Tue) at 14:00  15:30, 2022
ChingKai Chiu (Senior Research Scientist, iTHEMS)
Venue: via Zoom
Event Official Language: English

Introduction to Topological Insulators: Topological Superconductors and Quantum Computing
May 9 (Mon) at 14:00  15:30, 2022
ChingKai Chiu (Senior Research Scientist, iTHEMS)
Venue: via Zoom
Event Official Language: English

Seminar
Local and global topology for Dirac points with multihelicoid surface states
March 24 (Thu) at 17:00  18:15, 2022
Tiantian Zhang (Specially Appointed Assistant Professor, School of Science, Tokyo Institute of Technology)
Though topological invariants defined for topological semimetals are usually local ones, they also have a global nature. For example, the Z type local monopole charge C for Weyl points, has a global nature, telling us its influence to the rest of the Brillouin zone, giving rise to bulksurface correspondence associated with helical surface states. In Dirac systems, helical surface states are not guaranteed due to C=0. However, a new bulksurface correspondence associated with double/quadhelicoid surface states (DHSSs/QHSSs) can be obtained for Dirac points with the protection of a Z2 type monopole charge Q, which is defined in terms of the timereversal (T)glide (G) symmetry (TG)2= 1. Here we study the topology of Q for Z2 Dirac points and establish its bulksurface correspondence with strict proofs. We find that Q is equivalent to the Gprotected Z2 invariant v mathematically and physically in Z2 Dirac systems. This result is counterintuitive, since v is always trivial in Tpreserving gapped systems, and was thought to be illdefined in gapless systems. We offer a gaugeinvariant formula for Q, which is associated with DHSSs in both the spinless and spinful systems with single G. Q is formulated in a simpler form in spinless systems with two vertical G, associated with QHSSs, which is also entangled with fillingenforced topological band insulators in three space groups when a Tbreaking perturbation is introduced. Since Q is illdefined in spinful systems with two vertical G, QHSSs will not be held. Material candidate Li2B4O7 together with a list of possible space groups preserving QHSSs are also proposed for demonstration on our theory and further studies. *Detailed information about the seminar refer to the email.
Venue: via Zoom
Event Official Language: English

Seminar
How is turbulence born: Spatiotemporal complexity and phase transition of transitional fluids
February 24 (Thu) at 17:00  18:15, 2022
HongYan Shih (Assistant Research Fellow, Institute of Physics, Academia Sinica, Taiwan)
How a laminar flow becomes turbulence has been an unsolved problem for more than a century and is important in various industrial applications. Recently precise measurements in pipe flow experiments showed nontrivial spatiotemporal complexity at the onset of turbulence. Based on numerical evidence from the hydrodynamics equations, we discovered the surprising fact that the fluid behavior at the transition is governed by the emergent predatorprey dynamics of the important longwavelength mode, leading to the mathematical prediction that the laminarturbulent transition is analogous to an ecosystem on the edge of extinction. This prediction demonstrates that the laminarturbulent transition is a nonequilibrium phase transition in the directed percolation universality class, and provides a unified picture of transition to turbulence emerging in systems ranging from turbulent convection to magnetohydrodynamics. *Detailed information about the seminar refer to the email.
Venue: via Zoom
Event Official Language: English

Seminar
Bethe ansatz and quantum computing
January 26 (Wed) at 22:00  23:15, 2022
Rafael I. Nepomechie (Professor, Physics Department, University of Miami, Florida, USA)
We begin with a brief review of the Heisenberg quantum spin chain and its remarkable solution found by Bethe. We then review a probabilistic algorithm for preparing exact eigenstates of this model on a quantum computer. An exact formula for the success probability is presented, and the computation of correlation functions is discussed. A generalization of the algorithm to open chains with boundaries is also noted.
Venue: via Zoom
Event Official Language: English

Seminar
Topological exchange statistics in one dimension
November 17 (Wed) at 17:00  18:15, 2021
Harshman Nathan (Department of Physics, American University, USA)
In two dimensions, the topological approach to exchange statistics predicts the existence of anyons obeying statistics given by the braid group. However, in one dimension the topological approach is ambiguous because particles cannot exchange without coincidence and scattering. I will review the topological approach and show how old controversies can be resolved using orbifolds (roughly, manifolds with symmetry) to describe configuration space for onedimensional systems. Using orbifolds also predicts new topological physics, including possibilities for “traid group” statistics when there are threebody interactions in one dimension and nonabelian statistics for indistinguishable particles on a ring. *Detailed information about the seminar refer to the email.
Venue: via Zoom
Event Official Language: English

Seminar
Nonlinear response in strongly correlated systems
October 20 (Wed) at 17:00  18:15, 2021
Robert Peters (Lecturer, Department of Physics, Graduate School of Science, Kyoto University)
Nonlinear responses in condensed matter are intensively studied because they provide rich information about materials and hold the possibility of being applied in diodes or highfrequency optical devices [14]. While nonlinear responses in noninteracting models have been explored widely, the effect of strong correlations on the nonlinear response is still poorly understood. This talk will introduce a Green's function method to calculate nonlinear conductivities in strongly correlated materials [56]. Correlation effects are thereby included by the selfenergy of the material. I will then use this method to study the nonlinear conductivities in noncentrosymmetric felectron systems. The first system is a heavy Fermion system, where a nonreciprocal conductivity appears in the ferromagnetic phase. The nonreciprocal conductivity thereby always occurs perpendicular to the magnetization of the system and has a strong spin dependence, which might be advantageous for spintronic applications. The second system is a model corresponding to the WeylKondo semimetal Ce3Bi4Pd3, in which a giant spontaneous Hall effect without timereversal symmetry breaking has been observed [7]. This Hall effect can be explained as a nonlinear Hall effect in an inversionsymmetry broken Weylsemimetal. It has been shown that the nonlinear Hall effect is related to the Berry curvature dipole [4]. Our study shows that the magnitude of the experimentally observed nonlinear Hall effect can be explained by the strong correlations inherent in this felectron material [8]. *Detailed information about the seminar refer to the email.
Venue: via Zoom
Event Official Language: English

Seminar
Symmetrybased analysis for unconventional superconductors: Diagnosis of topological and nodal superconductivity
October 12 (Tue) at 16:00  17:15, 2021
Seishiro Ono (Department of Applied Physics, School of Engineering, The University of Tokyo)
The physics of unconventional superconductors has gained a new dimension in the past decade, thanks to the bloom in the understanding of topological quantum materials. Keeping in mind the success of the symmetrybased diagnosis in the largescale discovery of topological insulator and semimetal candidates [1], it is natural to ask whether the approach can be generalized to superconducting systems. In this talk, I provide a unified way to diagnose topology and superconducting nodes in unconventional superconductors. First, I review symmetryindicator theory for the topological insulators [2]. Also, I also discuss how to generalize the theory to superconductors [3,4,5]. Next, I show that the symmetrybased approach can extensively classify superconducting nodes pinned to highsymmetric momenta [6]. Finally, I show that these results enable us to derive the comprehensive correspondences between pairing symmetries and topological/nodal superconducting nature for each material [7]. *Detailed information about the seminar refer to the email.
Venue: via Zoom
Event Official Language: English

Seminar
Wave function geometry and anomalous Landau levels of flat bands
October 7 (Thu) at 16:00  17:15, 2021
BohmJung Yang (Associate Professor, Department of Physics and Astronomy, Seoul National University, Republic of Korea)
Semiclassical quantization of electronic states under magnetic field describes not only the Landau level spectrum but also the geometric responses of metals under a magnetic field. However, it is unclear whether this semiclassical idea is valid in dispersionless flatband systems, in which an infinite number of degenerate semiclassical orbits are allowed. In this talk, I am going to show that the semiclassical quantization rule breaks down for a class of flat bands including singular flat bands [15] and isolated flat bands [6]. The Landau levels of such a flat band develop in the empty region in which no electronic states exist in the absence of a magnetic field. The total energy spread of the Landau levels of flat bands is determined by the quantum geometry of the relevant Bloch states, which is characterized by their Hilbert–Schmidt quantum distance and fidelity tensors. The results indicate that flat band systems are promising platforms for the direct measurement of the quantum geometry of wavefunctions in condensed matter. *Detailed information about the seminar refer to the email.
Venue: via Zoom
Event Official Language: English
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