*The above dates and times are based on Korea Standard Time (KST) Time Zone (UTC/GMT + 9 hours)

 

• December 18 (Mon)

14:00-14:40 Sungbin Lee (KAIST)

Title: Exotic magnetism in quasicrystals

Abstract: In quasicrystals, the non-crystallographic symmetries lead to multipolar degrees of freedom, only allowed in quasicrystals but forbidden in conventional crystals. For both Kramers and non-Kramers doublets, the characteristics of multipoles are classified and the effective spin Hamiltonian on symmetry grounds are derived. Based on the self-similar triangular structure of the icosahedron, we argue the long-range frustration in terms of the Ising model and discuss quantum fluctuation effects. Next, we point out the anomalous RKKY interaction in quasicrystals. In quasicrystals, rather than the uniform decaying, the RKKY interaction becomes significant between the moments in non-local region. It realizes a strong coupling between the moments which are far separated, where their distance can be controlled via quasi-periodicity of the systems. It turns out that such anomalous RKKY interaction is originated from the critical states of itinerant electrons, which are neither localized nor extended wave functions and mediate the RKKY interactions on behalf of the extended itinerant electrons. Finally, we also discuss a possible non-local manipulation of the magnetic moments due to such anomalous RKKY coupling in quasicrystals.

 

14:45-15:30 Yuan-Ming Lu (Ohio State University)

Title: New ways to detect the pairing symmetry in superconductors

Abstract: The pairing symmetry, mathematically defined as the representation of the unbroken symmetry group carried by the pairing order parameter, is considered as a classification and characterization of the broken symmetries in a superconductor. In this talk, we will introduce another concept, known as the projective symmetry group (PSG) of a superconductor. We will show that the PSG is closely related to the pairing symmetry, and in many cases imposes a strong constraint on the pairing symmetry. This constraint on pairing symmetry from PSG allows us to identify two new ways to detect pairing symmetry in a superconductor: (1) through new selection rules in optical/THz and Raman spectroscopy, (2) through the presence/absence of Majorana zero modes in vortex cores centered at high-symmetry locations of the crystal. I will illustrate the results using examples of superconductors w/ or w/o spin-orbit interactions.

 

16:00-16:45 Suk Bum Chung (University of Seoul)

Title: Interaction physics in doped quantum paraelectric

Abstract: Recent experiments on Nb-doped SrTiO3 have shown that the superconducting energy gap to the transition temperature ratio maintains the Bardeen–Cooper–Schrieffer (BCS) value throughout its superconducting dome. Motivated by these and related studies, I will show that the Cooper pairing mediated by a single soft transverse-optical phonon is the most natural mechanism for such a superconducting dome given experimental constraints, and present the microscopic theory for this pairing mechanism. I will then discuss how these phonons can also mediate spin current response to inhomogeneous electric field.

 

16:45-17:30 Jieun Lee(Seoul National University)

Title: Two-dimensional quantum materials with atomic defects for quantum information science

Abstract: Atomic defects in solid-state materials provide an interesting platform to study quantum information science with applications in quantum sensing, computing and communication. Two-dimensional materials with atomic defects have recently been introduced as emerging systems that can host single-photon emitters which are particularly promising for quantum communication. Interplay between charge, optical, and spin states of defects allows various experimental observations that are critical to understand and manipulate the properties of single-photon emitters. In this talk, we introduce the electrical engineering and charge state control of single-photon emitters in two-dimensional host materials embedded in van der Waals heterostructures. Also, we will discuss the possibility to observe single-photon emission in a wider range of van der Waals crystals through tailoring their material properties.

 

17:30-18:15 Kyusung Hwang(KIAS)

Title: Topological Quantum Dimers Emerging from Kitaev Spin Liquid Bilayer: Anyon Condensation Transition

Abstract: Quantum Spin Liquids (QSLs) are many-body quantum entangled states supporting anyon quasiparticles, proposed as one of the best platforms for quantum science and technology. In particular, Kitaev Spin Liquid (KSL) and Resonating Valence Bond (RVB) states are active subjects of research, stimulated by recent advances in experimental platforms including the quantum magnet RuCl3, quantum processors, and Rydberg atom arrays. Transition mechanism between distinct QSLs is one of the outstanding problems in the field of topological quantum matter. Anyon condensation was theoretically proposed as a mechanism for such transitions, providing global insights on how a variety of topological phases can be connected via anyon condensation transitions. However, it has been elusive to confirm the mechanism in quantum spin systems because of the scarcity of appropriate microscopic models and the difficulty with defining an order parameter for anyon condensation in terms of local spin operators. In this work, I introduce a bilayer spin model that illuminates the mechanism of anyon condensation transition. KSL bilayer state and RVB state are stabilized in different limits of the model, connected by an anyon condensation transition. By performing explicit calculations of the order parameter, this work provides numerical evidence for the anyon condensation and uncovers an intimate connection between the KSL bilayer and RVB states. [1] K. Hwang, arXiv preprint arXiv:2301.05721.

 

• December 19 (Tue)

09:00-09:45 Cenke Xu (UC Santa Barbara)

Title: Title: A peep into Quantum Information at the Temporal Boundary

Abstract: The bridge between the quantum nature of the microscopic world and our classical daily life is a process called quantum decoherence, during which a quantum system gradually loses quantum information to the environment and becomes classical. The decoherence can also be viewed as the quantum system being weakly-measured by the environment. We apply quantum field theory techniques to understand quantum decoherence and weak measurement. This question can be mapped to a problem on the temporal boundary (or defect) of a quantum field theory, and many universal results can be obtained through standard field theory techniques such as renormalization group. As examples, we investigate quantum critical points and topological states of matter, and their behaviors when they lose certain quantum features through decoherence.

 

10:15-11:00 Jae Hoon Kim (Yonsei University)

Title: Terahertz Dynamics of the Quantum Spin Liquid Candidate TbInO3

Abstract: Terahertz absorption spectra of the quantum spin liquid candidate TbInO3 have been experimentally measured and analyzed on the basis of U(1) Dirac spin liquid theory. The in-plane optical conductivity exhibits a quadratic frequency dependence in the low frequency regime, and this behavior is maintained at temperatures as high as 300 K and at magnetic fields as high as 7 T. The continuum excitation interacts strongly with an optical phonon mode, leading to a Fano resonance. All these features strongly point to the quantum spin liquid nature of TbInO3.

 

11:00-11:45 Dung Nguyen Xuan (IBS-PCS)

Title: Tkachenko Wave in superfluid vortex crystal

Abstract:I will take you on a journey through the history of the Tkachenko wave, the vibration of the vortex lattice in a rotating superfluid. It is a special Goldtone boson shared by several spontaneous broken symmetries in the vortex lattice phase. This special Goldstone boson excitation has many unique properties that emerge at low energy. I will provide insight into its unusual characteristics by examining this wave through the lens of modern non-commutative quantum field theory.

 

14:00-14:45 Jun Sung Kim (POSTECH)

Title: Novel magnetotransport properties of isolated nodal line fermions

Abstract: Topology and electron correlation are two key ingredients in quantum materials producing novel electronic properties and phases yet-to-be-discovered. Of particular interest are topological nodal-line materials that host unique quasiparticles near the one-dimensional band-degeneracy in the momentum space and exhibit unusual transport properties highly sensitive to electromagnetic perturbations. In this talk, I will present unusual magnetotransport properties of nodal-line fermions when they are isolated from topologically trivial electronic states and strongly correlated. Various magnetotransport phenomena will be discussed, including quantum oscillations with toroidal pseudospin texture, significantly enhanced two-dimensional weak antilocalization, and an ultralow electric-field-driven metal-insulator transition. These findings highlight that topological nodal-line materials represent a promising material platform for developing unprecedented functionalities with potential applications in next-generation electronic devices.

 

14:45-15:30 Bohm Jung Yang(Seoul National University)

Title: Odd-parity topological superconductivity in magnetic metals

Abstract: I am going to talk about the topological aspects of the superconductivity that coexists with stable magnetism. In the first part of this talk, we propose a route to achieve odd-parity spin-triplet superconductivity in metallic collinear antiferromagnets with inversion symmetry[1]. Owing to the existence of hidden antiunitary symmetry, which we call the effective time-reversal symmetry (eTRS), the Fermi surfaces of ordinary antiferromagnetic metals are generally spin-degenerate, and spin-singlet pairing is favored. However, by introducing a local inversion symmetry breaking perturbation that also breaks the eTRS, we can lift the degeneracy to obtain spin-polarized Fermi surfaces. In the weak-coupling limit, the spin-polarized Fermi surfaces constrain the electrons to form spin-triplet Cooper pairs with odd-parity. Furthermore, we find that the odd-parity superconducting states host nontrivial band topologies manifested as chiral topological superconductors, second-order topological superconductors, and nodal superconductors. In the second part, I am going to extend the related idea to the case of non-collinear antiferromagnets[2]. In particular, I will discuss how the relative direction between d-vector and Fermi surface spin texture leads to nodal superconductivity.

References [1] "Odd-Parity Spin-Triplet Superconductivity in Centrosymmetric Antiferromagnetic Metals", Seung Hun Lee, Hong Chul Choi, and Bohm-Jung Yang, Physical Review Letters 126, 067001 (2021) [2] "Fermi surface spin texture and topological superconductivity in spin-orbit free non-collinear antiferromagnets", Seung Hun Lee, Bohm-Jung Yang, arXiv:2308.09925.

 

16:00-16:45 Debanjan Chowdhury (Cornell University)

Title: On the origin of superconductivity in the non-perturbative flat-band regime

Abstract: Superconductivity in the limit of a vanishing bandwidth in isolated bands is a classic example of a non-perturbative problem, where BCS theory does not apply. This question has become especially relevant with the discovery of superconductivity in moiré materials. What sets the superconducting phase stiffness, and relatedly the transition temperature, in this limit is of both fundamental and practical interest. This talk will focus on two complementary approaches to address this problem. I will begin by proving a low-energy formulation of a “projected” f-sum rule that relates the integrated optical spectral weight for the active electronic degrees of freedom to their diamagnetic susceptibility. In the strong-coupling chiral flat-band limit of twisted bilayer graphene, I will use the sum-rule to derive the Coulomb interaction-induced low-energy optical spectral weight at all integer fillings, which provides an estimate of the maximum superconducting transition temperature. I will also present numerically exact results for the interplay between superconductivity and various competing orders in models of interacting flat-bands, focusing specifically on the effects of band topology and geometry. 

 

16:45-17:30 Vic Kam Teun Law (Hong Kong University of Science and Technology)

Title: Josephson diode effect and flat band superconductivity with quantum metric in moiré materials

Absrtact: Recently, it has been observed that superconducting and interaction-driven quantum anomalous Hall states can appear simultaneously in gate-defined Josephson junctions in twisted bilayer graphene [1]. The interaction-driven state serves as the weak link in the superconductor/correlated state/superconductor Josephson junction. In this talk, we will discuss how the interaction-driven valley polarization is essential for the Josephson diode effect observed in experiments [2]. Moreover, many of the superconducting properties of moiré superconductors with ultra-flat bands deviate greatly from conventional BCS theory predictions [3]. In the second half of the talk, I would like to present a Ginzburg-Landau theory derived from a microscopic flat band Hamiltonian, which incorporates the quantum metric effects of moiré flat band superconductors [4,5]. The theory explains how the quantum metric length is critically important in determining the properties of moiré flat band superconductors.

 

• December 20 (Wed)

09:00-09:45 Yuto Ashida (University of Tokyo)

Title: Universality in system-environment entanglement

Absrtact: I will talk about universality and phase transitions of the entanglement inherent to open many-body systems, namely, the entanglement between a system of interest and its environment. We consider the Tomonaga-Luttinger liquid under a local measurement and analyze its unconditioned nonunitary evolution, where the measurement outcomes are averaged over. Time permitting, I will also talk about its relation to recent discussions about the fate of the dissipative quantum phase transition in the resistively shunted Josephson junction.

 

10:15-11:00 Kohei Kawabata (Institute for Solid State Physics, University of Tokyo)

Title: Lieb-Schultz-Mattis Theorem in Open Quantum Systems

Abstract: The Lieb-Schultz-Mattis (LSM) theorem provides a general constraint on quantum many-body systems and plays a significant role in the Haldane gap phenomena and topological phases of matter. Here, we extend the LSM theorem to open quantum systems and establish a general theorem that restricts the steady state and spectral gap of Liouvillians based solely on symmetry. Specifically, we demonstrate that the unique gapped steady state is prohibited when translation invariance and U(1) symmetry are simultaneously present for noninteger filling numbers. As an illustrative example, we find that no dissipative gap is open in the spin-1/2 dissipative Heisenberg model while a dissipative gap can be open in the spin-1 counterpart---an analog of the Haldane gap phenomena in open quantum systems. Furthermore, we show that the LSM constraint manifests itself in a quantum anomaly of the dissipative form factor of Liouvillians. We also find the LSM constraints due to symmetry intrinsic to open quantum systems, such as Kubo-Martin-Schwinger symmetry. Reference: K. Kawabata, R. Sohal, and S. Ryu, arXiv:2305.16496.

 

11:00-11:45 Seung Sup Lee (Seoul National University)

Title: Two-stage screening in Hund metals and heavy fermions

Abstract: The dynamical mean-field theory (DMFT) framework has been successful in understanding the physics of strongly correlated metals in terms of the Kondo screening, in that local fluctuations of different degrees of freedom (spin, orbital, etc.) are screened by surrounding electrons. In this presentation, I will show that Hund metals and heavy-fermion systems, two seemingly distinct classes of materials, exhibit a common phenomenon of the two-stage Kondo screening, yet different degrees of freedom are involved. The screening picture explains the smallness of the Fermi-liquid coherence scales of these systems as well as the non-Fermi liquid regime appearing at intermediate energies between the two screening energy scales. Moreover, I will show that the two-stage screening picture resolves open questions in heavy-fermion systems, including Fermi surface reconstruction, non-Fermi liquid, and quantum critical scaling, to name a few.

 

14:00-14:45 Hyun Yong Lee(Korea University Sejong)

Title: Quantum phases and quench dynamics of constrained dipolar bosons

Abstract: In this presentation, we discuss the quantum phase diagram and dynamic properties of a one-dimensional Dipolar Bose-Hubbard Model (DBHM), a system characterized by the conservation of both boson number and dipole moment. Our first focus is on the quantum phase diagram, particularly in strongly tilted optical lattices where boson dipole moment conservation dramatically alters the landscape [1]. Through the Density Matrix Renormalization Group (DMRG) simulations, we found incompressible dipole-condensed phases, fractured boson droplet, and featureless Mott phases. Our second focal point is the quench dynamics within DBHM under dipole conservation constraints [2]. Here, we uncover intriguing fractonic dynamics, especially notable when transitioning from deep Mott phases to weaker ones. This transition is characterized by a light-cone-like spread in the dipole correlation function, contrasting with the suppressed single-boson correlator due to dipole conservation. Our analysis extends to the examination of phase and group velocities, finding remarkable alignment with the Time-Dependent Variational Principle (TDVP) outcomes. Further, we investigate quenches between dipole-condensed and Mott phases, highlighting peculiarities in Loschmidt echo and Bose-condensed peaks in dipole momentum distribution functions, indicative of dynamical quantum phase transitions. Contrarily, quenches from Mott to dipoled-condensed phases exhibit none of these features, despite crossing the equilibrium quantum critical point.

[1] E. Lake, Hyun-Yong Lee, Jung Hoon Han, and T. Senthil, Phys. Rev. B 107, 195132 (2023)

[2] Yun-Yak Oh, Jung Hoon Han, and HYun-Yong Lee, arXiv:2311.13156

 

14:45-15:30 Jung Hoon Han(Sungkyunkwan University)

Title: Dipolar SPT and dipolar quantum Hall systems

Absrtact: One-dimensional SPT model protected with Z_n x Z_n global symmetries are generalized to dipolar SPT model of the same symmetry which are however modulated in space. An exactly solvable model embodying the dipolar SPT is constructed and its ground state wave function is derived. MPS analysis gives rise to new conditions on the tensors used to construct the dipolar SPT state. In the second part of the talk, we use the wire construction to construct dipolar analogue of the integer quantum Hall effect. The resulting edge states are chiral quadrupole channels. The accompanying effective response theory is constructed.

 

16:00-16:45 Bum Jun Kim (POSTECH)

Title: Origin of chirality in the triple-q charge density wave semimetal 1T-TiSe2

Abstract: Chiral electronic orders provide a route to unconventional physical phenomena, but their realizations in achiral lattices pose a fundamental challenge to our current understanding of structure-property relationship in quantum matters. We investigate the case of the archetypal charge density wave (CDW) system 1T-TiSe2, in which charge density modulations and atomic displacements of the same wave-vectors lead to different space group symmetries owing to the different symmetry transformation properties of the scalar (charge) and the vector (displacement) orders. Specifically, these atomic displacements alone are incapable of breaking inversion symmetry when the CDW acquires chirality. We unravel the mechanism whereby this symmetry inconsistency is resolved through an induced lattice distortion that transforms as the combined symmetry of the two orders. Using Raman spectroscopy and inelastic x-ray scattering, we show that all but translation symmetries are broken at the onset of its triple-q CDW order, at a level unresolved by state-of-the-art diffraction techniques.

 

• December 21 (Thu)

09:00-09:45 Jaewook Ahn (KAIST)

Title: Rydberg-atom quantum simulation of magnon bound states

Absrtact: Recent progress in systems of programmable atom arrays makes Rydberg atoms a viable physical platform for quantum simulation, whereby intrinsic van der Waals interactions and dipole-dipole interactions are employed to simulate, e.g., the quantum Ising and XY models of magnetism [1]. Here we demonstrate engineered interaction to construct a widely tunable Heisenberg magnet, where the magnon density interaction can be made two orders of magnitude larger than the magnon hopping strength [2]. Such an extremely anisotropic regime gives rise to fundamentally new phenomena that have never been observed in the literature. As one of the most prominent examples, we experimentally demonstrate the formation of multiple magnon bound states with qualitatively distinct transport properties. [1] M. Kim et al., "Quantum computing with Rydberg atom graphs," JKPS 82, 827 (2023). [2] K. Kim et al., "Realization of an extremely anisotropic Heisenberg magnet in Rydberg atom arrays," arXiv: 2307.04342 (2023).

 

09:45-10:30 Masaki Oshikawa(ISSP, University of Tokyo)

Title: Construction of Symmetry-Protected Topological Phases with Duality Transformations

Abstract: In 1992, Kennedy and Tasaki constructed a non-local unitary transformation that maps between a ℤ2×ℤ2 spontaneously symmetry breaking phase and the Haldane gap phase, which is a prototypical Symmetry-Protected Topological phase in modern framework, on an open spin chain. In this talk, I will discuss how to define it on a closed chain, by sacrificing unitarity. The operator realizing such a non-unitary transformation satisfies non-invertible fusion rule, and implements a generalized gauging of the ℤ2×ℤ2 global symmetry. These findings connect the Kennedy-Tasaki transformation to numerous other concepts developed for SPT phases. Furthermore, it can be used to systematically construct various SPT phases, including gapless SPT phases.

 

10:30-11:15 Jong Yeon Lee (University of Illinois, Urbana-Champaign)

Title: Many Body Physics of Information

Abstract: Information can be viewed as a fundamental element that constitutes the essence of physical systems. From this perspective, phases can be characterized or defined based on the inherent information content and its extraction. Expanding on this idea, we demonstrate that, under decoherence, various quantum many-body systems exhibit a phase transition behavior in terms of information-theoretic metrics. These information-theoretic phases starkly contrast conventional phase descriptions relying on order parameters, which often fail to capture these transitions. Our work offers three primary contributions: (i) information theoretic phase diagrams for various symmetry-protected and intrinsic topological orders under decoherence and faulty measurements. (ii) a comprehensive framework for understanding decoding transitions in the presence of measurement errors, and (iii) elucidating the mechanism behind the decodability in various topological codes. Therefore, our work introduces a novel framework to understand various quantum many-body systems beyond those inspired by quantum codes through the lens of information theory.