Correlated Electrons Virtual International Seminars (CEVIS)
June 18, 2020 - Dec 2, 2020
March 4, 2021 - May 27, 2021
Sep 27, 2021 - Dec 13, 2021
Past Seminars
Monday, Dec 13, 2021 - Cancelled.
10:00am (KST/JST)
Speaker: Hong Yao (Institute for Advanced Study, Tsinghua University, China)
Monday, Nov 29, 2021
10:00am (KST/JST)
Speaker: Bohm Jung Yang (Seoul National Univ, Korea)
Title: Wave function geometry and anomalous Landau levels of flat bands
Abstract: 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 flat-band 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 [1-5] 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.
[1] J. W. Rhim and B. -J. Yang, “Classification of flat bands according to the band-crossing singularity of Bloch wave functions”, PRB 99, 045107 (2019)
[2] J. W. Rhim, K. Kim, B. -J. Yang, “Quantum distance and anomalous Landau levels of flat bands”, Nature 584, 59-63 (2020)
[3] Y. Hwang, J. Jung, J. W. Rhim, B. -J. Yang, “Wave function geometry of band crossing points in two-dimensions”, PRB 103, L241102 (2021)
[4] Y. Hwang, J. W. Rhim, B. -J. Yang, “Flat bands with band crossing enforced by symmetry representation”, PRB 104, L081104 (2021); PRB 104, 085144 (2021)
[5] J. W. Rhim and B. -J. Yang, “Singular flat bands”, Advances in Physics X, 6:1, 1901606 (2021)
[6] Y. Hwang, J.-W. Rhim, B.-J. Yang, “Geometric characterization of anomalous Landau levels of isolated flat bands”, Nature Communications 12, 6433 (2021)
Chair: Gil Young Cho
Monday, Nov 15, 2021
10:00am (KST/JST)
Speaker: Kyoko Ishizaka (Univ of Tokyo, Japan)
Title: Detecting ultrafast dynamics in correlated materials by laser-based photoelectrons
Abstract: In strongly correlated systems, electron, spin, and phonon degrees of freedom are intricately intertwined, and various symmetry breaking phenomena occurs. These systems exhibit condensed states reflecting the interactions, thereby acquiring peculiar anisotropy and/or hierarchical structures which show unique dynamics in non-equilibrium states. Recently we have been working on observations of these dynamics by using new measurement techniques such as time- & angle-resolved photoelectron spectroscopy and ultrafast electron microscopy. In this talk, I will introduce our recent works on nematic iron-based superconductor FeSe and trimer-ordered “charge-density-wave” material VTe2. In FeSe, through the time-, energy-, momentum- and orbital-resolved photoelectron spectroscopy, we detected the ultrafast dynamics of the Fermi surface anisotropy, and found the short-lived nematic oscillation appearing right after the strong perturbation by light [1]. In VTe2, we first clarified the electronic structure and found that the trimer formation is intimately related with the band inversion and the Dirac surface state located at the Brillouin zone boundaries [2]. In addition, by using the ultrafast electron diffraction and imaging, we found the peculiar phononic responses related to the trimer dissolution by optical irradiation [3], suggesting the possibility of ultrafast control of band inversion.
[1] T. Shimojima, KI et al., Nat. Commun. 10, 1946/1-6 (2019).
[2] N. Mitsuishi, KI et al., Nat. Commun. 11, 2466/1-9 (2020).
[3] A. Nakamura, KI et al., Nano Lett. 20, 7, 4932–4938 (2020).
[4] T. Shimojima, KI et al, Sci. Adv. 7, eabg1322/1-8 (2021).
Chair: Masaki Oshikawa
Monday, Oct 18, 2021
10:00am (KST/JST)
Speaker: Hosho Katsura (Univ of Tokyo, Japan)
Title: Sine-square deformation of one-dimensional critical systems
Abstract: Sine-square deformation (SSD) is one example of smooth boundary conditions with significantly smaller finite-size effects than open boundary conditions. In one-dimensional chains with SSD, the interaction strength decreases from the center to the edges according to the sine-square function. Thus, the Hamiltonian describing such a system is inhomogeneous and lacks translational symmetry. Nevertheless, previous studies have revealed that the SSD leaves the ground state of a uniform chain with periodic boundary conditions (PBC) almost unchanged for critical systems. In particular, I showed in [1,2,3] that the correspondence is exact for critical XY and quantum Ising chains. The same correspondence between SSD and PBC holds for Dirac fermions in 1+1 dimension and more general conformal field theories. In this talk, I will review these results. If time permits, I will also talk about some recent developments.
[1] H. Katsura, J. Phys. A: Math. Theor. 44, 252001 (2011).
[2] H. Katsura, J. Phys. A: Math. Theor. 45, 115003 (2012).
[3] I. Maruyama, H. Katsura, T. Hikihara, Phys. Rev. B 84, 165132 (2011).
Chair: Eun Gook Moon
Monday, Nov 1, 2021
10:00am (KST/JST)
Speaker: Ehud Altman (UC Berkeley, USA)
Title: Phase transitions and critical states of monitored quantum systems
Abstract: In a closed system, thermalization occurs through unitary evolution, which encodes information in increasingly nonlocal degrees of freedom. Recent work on random quantum circuits has clarified how this non-local encoding is affected by an external observer: at a critical measurement rate the system undergoes a phase transition from an encoding state with volume law entanglement to an area law state. I will review the current understanding of the transition using a mapping to statistical mechanics models and use this description to predict two new phenomena. First, I will argue that large scale entanglement, with sub-volume critical power law scaling, is left in the monitored system when it is coupled to a decoherence channel at its edge. Second, I will show that the capacity of certain quantum circuits to facilitate quantum teleportation over infinite distances has a critical onset at a finite time.
Chair: Yuki Motome
Monday, Sep 27, 2021
10:00am (KST/JST)
Speaker: Takahiro Morimoto (Univ of Tokyo, Japan)
Title: Nonlinear optical effects in noncentrosymmetric magnets
Abstract: The responses of materials to high intensity light, i.e., nonlinear optical responses, constitute a vast field of physics and engineering. One of nonlinear optical responses that is attracting a recent attention is a bulk photovoltaic effect called shift current which arises from Berry phase of a Bloch wave function and has a close relationship to the modern theory of electric polarization [1]. In this talk, I will present a bulk photovoltaic effect supported by magnets nonlinear through the shift current mechanism [2]. In noncentrosymmetric magnets, magnons generally accompany electric polarization due to their multiferroic nature and couple to an external electric field. Constant excitation of such magnons causes an increase of electric polarization and results in dc photocurrent. I will demonstrate that a shift current appears in the cycloidal phase in J1-J2 spin chains with broken inversion symmetry.
[1] T. Morimoto, and N. Nagaosa, Sci. Adv. 2, e1501524 (2016).
[2] T. Morimoto, S. Kitamura, S. Okumura, Phys. Rev. B 104, 075139 (2021).
Chair: Yong Baek Kim
Thursday, May 27, 2021
10:30am (KST/JST)
Speaker: Junyeong Ahn (Harvard Univ)
Title: Quantum Geometry of Light-Matter Interactions
Abstract: The concept of the geometry of quantum states has been useful for characterizing responses of electronic systems to static electromagnetic fields. On the other hand, however, it has been challenging to relate quantum geometry with resonant optical responses. The main obstacle is that optical transitions are properties of a pair of states, while geometrical properties are usually defined for a single state. In this seminar, I will reveal the quantum geometric meaning of resonant optical transitions. This result suggests that light-matter interactions are in general manifestations of the geometry of quantum states. I will take linear, second-order, and third-order responses as examples to demonstrate our theory.
Speaker: Kazuaki Takasan (UC Berkeley)
Title: Current-induced second harmonic generation in inversion-symmetric topological semimetals
Abstract: The rapid developments of the experimental techniques enable us to drive quantum materials into the nonequilibrium states. This opens up a new route to control the states of matter. For example, controls of topological phases by strong laser light have been extensively studied [1]. In this talk, I will present our recent work about a nonlinear response in nonequilibrium states of matter [2]. We studied theoretically the second harmonic generation (SHG) in inversion-symmetric topological semimetals under a DC electric current. While inversion symmetry prohibits even-order responses including SHG, the finite current breaks the symmetry and the SHG is allowed in the nonequilibrium steady state. Based on analytic and numerical calculations, we find that Dirac and Weyl semimetals exhibit strong SHG upon application of finite current. Our experimental estimation for a Dirac semimetal Cd$_3$As$_2$ and a magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$ suggests that the induced susceptibility is 100-10000 times larger than those of typical nonlinear optical materials. We also discuss experimental approaches to observe the phenomena and comment on current-induced SHG in other topological semimetals in connection with recent experiments [3]. If time permits, I will briefly introduce our more recent paper about a nonlinear static response in an interacting many-body system [4].
[1] For example, M. S. Rudner and N. H. Lindner, Nat. Rev. Phys. 2, 229–244 (2020). [2] KT, T. Morimoto, J. Orenstein and J. E. Moore, arXiv:2007.08887. [3] N. Sirica, et al., arXiv:2005.10308. [4] Y. Tanikawa, KT, and H. Katsura, arXiv:2103.05838.
Speaker: Jong Yeon Lee (KITP, UCSB)
Title: A fermionic Quantum Monte Carlo study of Twisted Bilayer Graphene
Abstract: We present a quantum Monte Carlo (QMC) study of twisted bilayer graphene (TBG) near the magic angle at charge neutrality. Due to its strongly interacting nature as well as topological obstructions for a simple lattice model, an unbiased numerical study of a continuum model of TBG has been a challenging problem. In the momentum space, we establish the absence of the sign problem for this model in the determinant QMC method and describe a computationally tractable formulation of this problem. We demonstrate the method by studying the TBG systems up to 9 x 9 Moire unit cells with varying physical parameters, illustrating the hierarchy of symmetry broken phases and the onset of spontaneous symmetry breaking instability as a function of temperature.
Chair: Gil Young Cho
Thursday, May 13, 2021
10:30am (KST/JST)
Speaker: Jun Sung Kim (POSTECH)
Title: Large magnetotransport responses of topological van der Waals magnets
Abstract: Topological semimetals, whose low energy electronic structure possesses several band contact points or lines, are generally expected to exhibit novel transport responses. Particularly, when combined with magnetism, topological band degeneracy can be readily tuned by spin configurations or orientations, offering an efficient magnetic control of electronic conduction. In this talk, I will introduce van der Waals (vdW) magnets, where combination of magnetism, spin-orbit interaction, and orbital-driven topological band degeneracy gives rise to large magnetotransport responses and magnetic tunability. These findings demonstrate that topological vdW magnets have great potential for various spin-dependent electronic functionalities.
Chair: Yong Baek Kim
*Tuesday*, April 27, 2021
10:30am (KST/JST)
Speaker: Youichi Yanase (Kyoto Univ)
Title: Parity transition, parity violation, and topological superconductivity in heavy fermion superconductors
Abstract: Symmetry and topology are fundamental properties of systems. In particular, the space inversion parity often plays an essential role for emergent phenomena in quantum phases. Historically, superconductors have been classified based on the space inversion parity, as spin-singlet (spin-triplet) Cooper pairs lead to the even-parity (odd-parity) superconductivity in the usual setup. The parity gives rise to a strong constraint on the topology, and odd-parity superconductors are promising candidates for the topological superconductivity. In the talk, I will present our studies on recently discovered superconductors UTe2 and CeRh2As2. Parity violation in UTe2 is shown by the analysis of an effective 24 band model, and parity transition in the superconducting phases of CeRh2As2 is discussed with focusing on the locally noncentrosymmetric crystal structure. Topological superconductivity is predicted in both UTe2 and CeRh2As2 based on first-principles band calculations.
Chair: Masaki Oshikawa
Thursday, April 8, 2021
10:30am (KST/JST)
Speaker: Mikito Koshino (Osaka Univ)
Title: Physics of moiré materials
Abstract: When 2D materials having different periodicities are overlaid with each other, the moiré interference pattern caused by the lattice mismatch always leads to unusual electronic properties. For instance, the twisted bilayer graphene exhibits dramatic phenomena such as the flat band formation and emergent superconductivity. In this talk, I present our recent theoretical works on moiré systems and their novel physical properties, together with recent experimental results. Using a general theoretical framework to treat incommensurate systems, I describe various exotic phenomena such as flat-band formation at the magic angle, graphene quasicrystal at 30 degree, and twist-dependent spintronics in graphene/TMDC system. I also discuss about the phonon properties of the moire materials, where I show that the original acoustic phonons of the single-layer material are completely mixed up and reconstructed into effective vibration modes of the moiré pattern, leading to a significant enhancement of the electron-phonon coupling.
Chair: Eun-Gook Moon
Thursday, March 25, 2021
10:30am (KST/JST)
Speaker: Haruki Watanabe (Univ of Tokyo)
Title: Multipole moments and fractional corner charges of insulating materials
abstract: We ingest tiny crystals of table salt every day. How well do we understand such commonplace substances? More broadly, how do we characterize and classify the insulating states of matter? Recent advances in the topological approach in condensed-matter physics offer a classification based on the winding and the quantum entanglement in the ground-state wavefunction. The nontrivial bulk topology is often manifested as anomalous surface states, but only corners and hinges exhibit gapless modes in the case of “higher-order” topology.
In this talk, we discuss that even absolutely topologically trivial materials may exhibit fractional charges on their corners and hinges. To predict these boundary signatures from the bulk, we develop a general formulation of bulk multipole moments, directly generalizing the “modern theory” formulation of the bulk polarization. As an example, we discuss e/8 fractional corner charges of grains of table salt and discuss their direct measurement using atomic force microscopy.
Chair: Gil Young Cho
Thursday, March 4, 2021
10:30am (KST/JST)
Speaker: Zi-Yang Meng (IOP, CAS)
Title: Model design and Computational solution for quantum many-body systems
Abstract: Correlated quantum lattice models and their computational solutions constitute an important research direction in contemporary condensed matter physics. In this talk, I will present few examples of such models motivated from field theoretical analysis and their solutions by employing state-of-the-art numerical tools, ranging from quantum Monte Carlo, tensor renormalization group, stochastic analytical continuation, etc. Such joint efforts have proved to greatly help us to understand few fundamental questions in non-Fermi-liquid, frustrated magnets, twisted bilayer graphene and topological state of matter. I will try to convey the message that through the fruitful dialogue between theoretical and numerical communities, a bridge connecting together theory, numerical and experimental studies of correlated electron systems can be built.
References:
[1] Phys. Rev. X 11, 011014 (2021)
[2] npj Quantum Materials 5, 65 (2020)
[3] arXiv:2007.05543
[4] arXiv:2005.07337
Chair: Yuki Motome
Wednesday, December 2, 2020
10:30am (KST/JST)
Speaker 1: Byungmin Kang (KIAS)
Title: Superuniversality from disorder at two-dimensional topological phase transitions
Abstract: In this talk, I will investigate the effects of quenched randomness on topological quantum phase transitions in strongly-interacting two-dimensional systems. Using a combination of microscopically exact duality transformations and asymptotically exact real-space renormalization group techniques applied to these two-dimensional disordered gauge theories, I will argue that the resulting critical scaling behavior is `superuniversal' across a wide range of such condensation transitions, and is controlled by the same infinite-randomness fixed point as that of the 2D random transverse-field Ising model. I will then validate this claim using large-scale quantum Monte Carlo simulations that allow us to extract zero-temperature critical exponents and correlation functions in (2+1)D disordered interacting systems. If time permits, I will discuss generalizations of these results to a large class of ground-state and excited-state topological transitions in systems with intrinsic topological order as well as those where topological order is either protected or enriched by global symmetries. When the underlying topological order and the symmetry group are Abelian, the results provide prototypes for topological phase transitions between distinct many-body localized phases.
Reference: https://arxiv.org/abs/2008.09617
Speaker 2: Kohei Kawabata (University of Tokyo)
Title: Non-Hermitian topology
Abstract: While topological phases were mainly investigated for Hermitian systems at equilibrium, non-equilibrium topological phases of non-Hermitian systems are attracting growing interest. One of the remarkable consequences of non-Hermiticity is new types of topological phases without Hermitian analogs. Here, we show that such intrinsic non-Hermitian topology leads to the non-Hermitian skin effect [1], which is anomalous localization due to non-Hermiticity. Furthermore, we develop a field-theoretic description of intrinsic non-Hermitian topological phases [2]. From the field-theoretic perspective, the non-Hermitian skin effect is shown to be a signature of an anomaly.
[1] N. Okuma, K. Kawabata, K. Shiozaki, and M. Sato, Phys. Rev. Lett. 124, 086801 (2020).
[2] K. Kawabata, K. Shiozaki, and S. Ryu, arXiv:2011.11449.
Chair: Gil Young Cho
Wednesday, November 25, 2020
10:30am (KST/JST)
Speaker: Xiao-Liang Qi (Stanford University)
Title: Emergent classicality in general multipartite states and channels
Abstract: In a quantum measurement process, classical information about the measured system spreads throughout the environment. Meanwhile, quantum information about the system becomes inaccessible to local observers. Here we prove a result about quantum channels indicating that an aspect of this phenomenon is completely general. We show that for any evolution of the system and environment, for everywhere in the environment excluding an O(1)-sized region we call the “quantum Markov blanket,” any locally accessible information about the system must be approximately classical, i.e. obtainable from some fixed measurement. The result strengthens the earlier result of Brandao et al. (Nat. comm. 6:7908 ) in which the excluded region was allowed to grow with total environment size. It may also be seen as a new consequence of the principles of nocloning or monogamy of entanglement. Our proof offers a constructive optimization procedure for determining the “quantum Markov blanket” region, as well as the effective measurement induced by the evolution. Alternatively, under channel-state duality, our result characterizes the marginals of multipartite states.
Reference: https://arxiv.org/abs/2001.01507
Chair: Gil Young Cho
Wednesday, November 11, 2020
10:30am (KST/JST)
Speaker: Hiroaki Ishizuka (Tokyo Institute of Technology)
Title: Asymmetric magnetic scattering and transport phenomena in chiral magnets
Abstract: Magnetism has a significant effect on electron transport phenomena. A classic example is the anomalous Hall effect, where the magnetism produces a Hall current proportional to the ferromagnetic moment. The studies on magnetism-related transport phenomena have mostly focused on two cases. The first is the low-temperature magnetically-ordered phase, in which the deformation of the electronic band and electron-magnon scattering are dominant. Another is the high-temperature paramagnetic phase, where the effect of magnetic moments is well described as the scattering by a single magnetic impurity. In contrast, recent experiments find unique transport phenomena at an intermediate temperature comparable to the magnetic transition temperature. In this talk, I discuss the magnetic scattering by two- and three-spin clusters, which gives the leading order correction to the magnetic scattering theory in the high-temperature limit. These corrections contribute to the electron transport in the intermediate temperature where the short-range magnetic correlation develops. We find that the spin correlation in the spin cluster brings about various asymmetric scattering. The asymmetric scatterings produce various transport phenomena such as extrinsic-type anomalous Hall effect and nonreciprocal current. These results are consistent with recent experiments and suggest a new direction in realizing a large anomalous Hall effect and nonreciprocal response.
Chair: Gil Young Cho
Wednesday, October 21, 2020
10:30am (KST/JST)
Speaker: Tarun Grover (UC San Diego)
Title: Simulatable models of nodal superconductivity and Kondo breakdown
Abstract: The fermion sign problem often stymies exploration of strongly correlated quantum systems. Recently, new ideas have emerged on how to construct "designer Hamiltonians" which do not suffer from the sign problem, while still hosting highly entangled phases of matter. In this talk, I will present results on two new directions: (a) A model of competing antiferromagnetism and nodal d-wave superconductivity. (b) Models of non-Fermi liquids in Kondo lattice systems. In both cases, the phase diagram hosts interesting critical phenomena, and I will also discuss field-theory approaches to understand the associated universal properties.
Chair: Masaki Oshikawa
Wednesday, October 7, 2020
10:30am (KST/JST)
Speaker: Ryusuke Matsunaga (ISSP, University of Tokyo)
Title: Terahertz nonlinear response and efficient frequency conversion in a superconductor and a Dirac semimetal
Abstract: Nonlinear response in THz frequency has attracted considerable attention since highly-intriguing phenomena can emerge in many-body systems far out of equilibrium. We have studied the Higgs amplitude mode in superconductors and found that THz third harmonic generation appears very strongly due to the resonance with the Higgs mode assisted with impurity scattering. Recently we also found the efficient THz third harmonic generation in a Dirac semimetal at room temperature originating from coherent intraband acceleration of massless electrons. The result would open a new route for efficient frequency conversion and mixing in high-speed electronics.
Chair: Eun-Gook Moon
Thursday, September 17, 2020
10:30am (KST/JST)
Speaker: Masaki Oshikawa (ISSP, University of Tokyo, Japan)
Title: Adiabatic vs Sudden Flux Insertion and Nonlinear Electric Conductivity
Abstract: The adiabatic insertion of the magnetic flux through a ring can be regarded as an application of an infinitesimal uniform electric field on the ring-shaped system over a long period of time.
This can induce a “persistent current” which does not decay in the ideal limit. The coefficient of proportionality corresponds to Drude weight. On the other hand, we can consider the limit of instantaneous flux insertion, which produces a delta-function pulse of the electric field. This may be also regarded as a particular type of a quantum quench process. By considering these two limits of the flux insertion, we can understand the Kohn formula for the Drude weight and the f-sum rule of the AC conductivity in a unified manner. In this picture, both the Kohn formula and the f-sum rule can be naturally extended to nonlinear electric conductivity at all orders.
Chair: Yong-Baek Kim
Thursday, September 3, 2020
10:30am (KST/JST)
Speaker: Mike Zaletel (UC Berkeley, USA)
Title: Skyrmion Superconductivity: DMRG evidence for a topological route to superconductivity
Abstract: It was recently suggested that the topology of magic-angle twisted bilayer graphene's (MATBG) flat bands could provide a novel mechanism for superconductivity distinct from both weakly-coupled BCS theory and the d-wave phenomenology of the high-T_c cuprates. In this talk I will examine this possibility using a DMRG study of a phenomenological model which captures the essential features of MATBG's symmetry and topology. We find evidence for all-electronic, skyrmion-mediated superconductivity, even in the regime where the unscreened Coulomb repulsion is by-far the largest energy scale.
Chair: Masaki Oshikawa
Thursday, August 27, 2020
10:30am (KST/JST)
Speaker: Gang Chen (University of Hong Kong and Fudan Univeristy, China)
Title: Thermal Hall effects in Mott insulators and a non-Fermi liquid
Abstract: Strongly correlated Mott insulators could harbor various magnetic excitations that are charge neutral and hidden from usual electrical transport measurement, while thermal transport is powerful to detect the elementary excitations therein. Besides the more conventional and well-understood magnon excitations in the ordered state, here we discuss thermal Hall effects of the appealing fractionalized excitations in the quantum disordered spin liquid phase, which reveal the intriguing couplings between emergent gauge fields and fractionalized spinon or “magnetic monopole”, and provide strong evidence to confirm spin liquids.
In the second part of the talk, we discuss a novel type of non-Fermi liquid (NFL) in itinerant magnets in 3D. Unlike the conventional hotspot and patch theory, the gapless fermions from the conduction electrons are coupled with infinitely many critical boson modes. We show that the magnetic moments fluctuate near a 2D boson surface in the momentum space at low energies when the system approaches an ordering transition. The infinite number of critical modes on the boson surface strongly scatter the conduction electrons on the Fermi surface and convert the metallic sector into a NFL. Various physical consequences of in this NFL are explored, including the thermodynamic, spectroscopic, transport properties.
Chair: Gil Young Cho
Thursday, August 20, 2020
10:30am (KST/JST)
Speaker: Sungbin Lee (KAIST, Korea)
Title: Fractionalization and Hidden phases in Quantum Spin Liquids
Abstract: Quantum spin liquids are one of the most intriguing phenomena with emergent gauge structures and fractionalized excitations. Here, we discuss how such emergent gauge fields and fractionalized particles can be controlled in the presence of a lattice strain effect and an external magnetic field. In particular focusing on a U(1) quantum spin liquid, we argue possible frustration in gauge fields and large thermal Hall effect as a signal of such frustration. In addition, considering the proper physical Hilbert space, we derive unique coupling between fractionalized particles in a U(1) spin liquid and discuss discovery of the `hidden phases' such as a nematic Z2 spin liquid and a supersolid.
Chair: Masaki Oshikawa
Thursday, August 13, 2020
10:30am (KST/JST)
Speaker: Takashi Oka (ISSP, University of Tokyo, Japan)
Title: Twisted Schwinger Effect
Abstract: We study the nonperturbative pair production of particles induced by strong rotating electric fields [1]. The excitations by tunneling become strongly chirality dependent due to nonadiabatic geometric effects. The threshold, i.e., Schwinger limit, even vanishes for particles with an optically allowed chirality. We explain these phenomena through the twisted Landau-Zener model proposed by M. V. Berry and provide a quantitative understanding in terms of the geometric amplitude factor. As a condensed matter application, we make a nonperturbative analysis on the optically induced valley polarization in 2D Dirac materials. Furthermore, in 3D Dirac and Weyl materials with spin-orbit coupling, we predict the generation of a nonlinear spin or charge current in the direction of the laser propagation.
Reference:
[1] Takayoshi, Wu, Oka, arXiv:2005.01755
Chair: Eun-Gook Moon
Thursday, August 6, 2020
10:30am (KST/JST)
Speaker: BJ Kim (Postech, IBS, Korea)
Title: Observation of excitonic instability in Ta2NiSe5
Abstract: Excitonic insulator is an elusive phase of matter predicted many decades ago to occur in a narrow gap semiconductor or a semi-metal. Analogous to Cooper pairs in superconductors, Coulomb attractions bind electrons and holes in pairs to form charge-neutral excitons, which undergo a Bose-Einstein condensation at a sufficiently low temperature. However, unambiguous identification of an excitonic insulator remains challenging because candidate materials invariably display simultaneous structural phase transitions. In this talk, I will discuss the case of Ta2NiSe5, for which a fierce debate continues for more than a decade on the physical origin of its semimetal-to-insulator transition. Using Raman scattering, we have observed an incipient divergence in the uniform static electronic susceptibility. Critical fluctuations of the excitonic order give rise to quasi-elastic scattering of B2g symmetry, whose intensity grows inversely with temperature toward the Weiss temperature of Tw~237 K, which is arrested by a structural phase transition driven by an acoustic phonon of the same symmetry at Tc=325 K. Concurrently, a B2g optical phonon becomes heavily overdamped to the extent that its trace is almost invisible around TC, which manifests a strong electron- phonon coupling that has obscured the identification of the low-temperature phase as an excitonic insulator for more than a decade. Our result unambiguously reveals the electronic origin of the phase transition.
Chair: Yuki Motome
Thursday, July 30, 2020
10:30am (KST/JST)
Speaker: Jason Alicea (Caltech, USA)
Title: Towards topological quantum computing with Kitaev materials
Abstract: Recent experiments on the Kitaev material RuCl_3 support the emergence of a magnetic-field-induced spin liquid phase featuring non-Abelian anyons, which underlie intrinsically fault-tolerant topological quantum computation schemes. This talk will explore two classes of measurements that probe the hallmark chiral Majorana edge state and bulk anyons born in such a spin liquid. The first class utilizes low-voltage electrical probes – despite the fact that RuCl_3 realizes a good Mott insulator! To this end I will introduce circuits that interface electrically active systems with RuCl_3 to perfectly convert physical fermions in the former into emergent fermions in the latter, enabling analogues of transport probes for topological superconductors. The second class of measurements utilizes ancilla spins to implement a time-domain analog of anyon interferometry developed for quantum Hall systems. Together, these results illuminate a partial pathway towards exploiting Kitaev materials for topological quantum computation.
Chair: Yong-Baek Kim
Monday, July 20, 2020 (Note special day)
10:30am (KST/JST) - Session for "Young" physicists
Speaker 1: Yuto Ashida (University of Tokyo, Japan)
Title: Quantum Rydberg central spin problem: remnant of integrability
Abstract: I will talk about a recently proposed variational approach to quantum impurity problems and its application to analyze dynamics of a Rydberg impurity undergoing spin-exchanging collisions in ultracold atoms [1,2]. This system realizes a new type of many-body problems combining both features of the central spin model and the Kondo problem. In particular, we predict long-lasting spin precession of the Rydberg electron spin, which is reminiscent of the (integrable) central spin problem.
[1] PRL 121, 026805; PRB 98, 024103 (2018).
[2] PRL 123, 183001; PRA 100, 043618 (2019).
Speaker 2: Moonjip Park (KAIST, Korea)
Title: Higher-Order topology in Twisted Bilayer Graphene
Abstract: There have been numerous works attempting to develop a general theory of the twisted bilayer graphene. So far, most of the previous theoretical works rely on the small-angle limit, where a simple Dirac model is approximable. However, there exist, in principle, infinitely many numbers of the large commensurate angles that do not fit in this small angle limit. In this talk, we tackle the problem of the twisted bilayer graphene at arbitrary angles. We show that all moiré patterns of twisted bilayer graphene share the same non-trivial higher-order band topology, which is irrespective of the specific angles as long as the underlying symmetries are intact. Besides, we propose a novel transport effect, which we refer to as higher-order topological instanton tunneling. We show that this tunneling effect can distinguishes the topological corner states from trivial impurity bound states.
Chair: Gil Young Cho
Thursday, July 16, 2020
10:30am (KST/JST)
Speaker: Takasada Shibauchi (University of Tokyo, Japan)
Title: Experimental signatures of a Kitaev quantum spin liquid in α-RuCl3
Abstract: In this presentation I will show our recent results of thermal conductivity and specific heat measurements in the high-field paramagnetic state of Kitaev material α-RuCl3. The field dependence of thermal Hall conductivity shows a plateau behavior with the value close to one half of quantized thermal Hall effect of electron systems [1], whose sign changes with field angle as expected from the Chern number in the Kitaev model [2]. The most recent specific heat measurements provide thermodynamic evidence for field-angle dependent Majorana gap, which demonstrates the bulk-edge correspondence in a Kitaev quantum spin liquid.
[1] Y. Kasahara et al., Nature 559, 227-231 (2018).
[2] T. Yokoi et al., arXiv:2001.01899.
Chair: Yuki Motome
Thursday, July 9, 2020
10:30am (KST/JST)
Speaker: Yuan Wan (Institute of Physics, Chinese Academy of Sciences, China)
Title: Two-dimensional coherent spectroscopy: concepts and opportunities for strongly-correlated electronic systems
Abstract: Two-dimensional coherent spectroscopy (2DCS) is a powerful experimental technique that probes the nonlinear optical response of materials. In essence, the traditional “1D” spectroscopy, which measures the linear response, reveals the excitations in a system, whereas the 2DCS unveils the interplay between these excitations. The 2DCS in the infrared frequency range has been widely used in chemistry and biology to unravel the complex structure of molecules. The advent of tetrahertz 2DCS makes it now possible to apply this technique to solid state systems and, in particular, strongly correlated electronic systems. In this talk, I will give an exposition to the basic concepts of the 2DCS and demonstrate theoretically its potential utilities in quantum spin systems. Specifically, I will show that the 2DCS can resolve the “spinon continuum” from the gapped fractional excitations. For the Luttinger spin liquid, the 2DCS can directly reveal the coherent propagation of the gapless phonon modes. Finally, I will briefly discuss some of the challenges in the development of 2DCS theory.
Chair: Eun-Gook Moon
Thursday, July 2, 2020
10:30am (KST/JST)
Speaker: Nic Shannon (OIST, Japan)
Title: A route to finding fractons ? Rank-2 U(1) spin liquid on the breathing pyrochlore lattice
Abstract: Fractons, dimensionally-challenged excitations with roots in theories of quantum glasses and quantum information, offer an intriguing bridge between gravity, liquid crystals, elasticity, and higher-rank generalisations of electromagnetism. However, despite rapid progress in the understanding the phenomenology of these novel excitations, concrete proposals as to where fractons might be found in experiment remain scarce.
In this talk we show how a spin liquid, described by a rank-2 U(1) gauge theory known to support fractons, arises in a simple and realistic model of a magnet on the breathing pyrochlore lattice. Candidate materials, and the experimental signatures of this spin liquid, are also discussed.
[1] Han Yan, Owen Benton, L. D. C. Jaubert and Nic Shannon, Phys. Rev. Lett. 124, 127203 (2020)
Chair: Masaki Oshikawa
Thursday, June 25, 2020
10:30am (KST/JST)
Speaker: Gil Young Cho (Postech, Korea)
Title: Non-Fermi Liquids and Dimensional Crossover in Conducting Network Systems
Abstract: We will consider novel electronic systems consisting of conducting one-dimensional segments, which form a regular two-dimensional network. Such network is naturally realized in nearly commensurate charge-density wave states of 1T-TaS2 and 1T-TiSe2, Helium atoms adsorbed on graphite, and twisted bilayer graphene at small twisting angles. In this talk, we will show that the network systems support intriguing non-Fermi liquid phases, which crossover to the truly two-dimensional states at low temperatures.
Chair: Yuki Motome
Thursday, June 18, 2020
10:30am (KST/JST)
Speaker: Shuichi Murakami (Tokyo Institute of Technology, Japan)
Title: Axion insulators and topological chiral hinge states protected by inversion symmetry
Abstract: In this talk I theoretically study axion insulators, which are also known as higher-order topological insulators. This phase is a topological phase protected by inversion symmetry but without time-reversal symmetry. In such systems, hinge states are known to appear along hinges of the crystals. Here we discuss how the appearance of hinge states results from the bulk Z4 topological number. Furthermore, we show how the positions of the hinge states are determined in systems under various symmetries. In particular we apply our theory to a layered antiferromagnet such as EuIn2As2 to show that the positions of hinge states show novel dependence on the parity of the number of layers. If the time allows, I also introduce our recent research on nonhermitian systems. In nonhermitian systems, we recently establish Bloch band theory in crystals, and show that the Brillouin zone acquires various new properties such as presence of cusps, as compared with Hermitian cases.
Chair: Yong-Baek Kim