Emergence and Dynamics in Quantum matter
February 2 - 5, 2021
Time table
Day 1: Physics of 2d Materials
Feb 2nd (KST) = Feb 1st (EST)
Talk 1
Title: 2D Electronic Systems in Bulk Materials
Abstract: Connecting theoretical models for exotic quantum states to real materials is a key goal in quantum materials synthesis. Two-dimensional model systems have been proposed to host a wide variety of exotic phases- historically a number of techniques have been used to realize these including thin film growth and mechanical exfoliation. We describe here our recent progress in experimentally realizing 2D model systems using bulk crystal synthesis including the kagome lattice model and 2D superconductivity. We discuss their structures and the new phenomena that they supported. We comment on the perspective for realizing further 2D model systems in complex material structures and their connections to other methods for realizing 2D systems.
Speaker: Joe Checkelsky (MIT)
Talk 2
Title: Discovery of van der Waals magnets and beyond
Abstract: The discovery of graphene in 2004 took the scientific community by surprise and virtually transformed the research landscape of materials science and engineering by creating a then-new field of 2d materials. However, despite the unique properties of many van der Waals materials since discovered, it has certain limitations in exploring novel and new physical properties. Magnetism is a case in point. Until I and a couple of other groups started to work on the much-neglected magnetic van der Waals materials, virtually nothing was known about it. However, with a series of publications, including those from my group [1-8], this field of magnetic van der Waals materials has become a fast emerging field in materials science. In this presentation, I would like to take you through the intellectual journey I made since 2010 and eventually discovering a novel quantum spin-entangled exciton NiPS 3 more recently [8]. I will end my talk by giving a personal view of the prospects for future research.
[1] Je-Geun Park, J. Phys. Condens. Matter 28, 301001 (2016)
[2] Cheng-Tai Kuo, et al., Scientific Reports 6, 20904 (2016)
[3] Jae-Ung Lee, et al., Nano Lett. 16, 7433 (2016)
[4] So Yeun Kim, et al., Phys. Rev. Lett. 120, 136402 (2018)
[5] K. S. Burch, D. Mandrus, and Je-Geun Park, Nature 563, 47 (2018)
[6] Kangwon Kim, et al., Nature Comm. 10, 345 (2019)
[7] H Chu, et al., Phys. Rev. Lett. 124, 027601 (2020)
[8] S. Kang, et al., Nature 583, 785 (2020)
Speaker: Je-Geun Park (SNU)
Talk 3
Title: Evidence of Higher Order Topology in Multilayer WTe2 from Josephson Coupling through Anisotropic Hinge States
Abstract: Non-centrosymmetric Td-WTe2, a type-II Weyl semimetal, is also expected to have higher order topological phases with topologically protected, helical one-dimensional (1D) hinge states when its Weyl points are annihilated. However, the detection of these hinge states is difficult due to the semimetallic behavior of its bulk. Here, we spatially resolve hinge states by analyzing the magnetic field interference of supercurrent in Nb-WTe2 -Nb proximity Josephson junctions. The Josephson current along the a-axis of the WTe2 crystal, but not along the b-axis, shows sharp enhancements at the edges of the junction; the amount of enhanced Josephson current was comparable to the upper limits of a single 1D helical channel. Our experimental observations suggest a higher order topological phase in WTe2 and its corresponding anisotropic topological hinge states, in agreement with theoretical calculations.
Speaker: Gil-Ho Lee (POSTECH)
Day 2: Quantum simulators & mACHINE lEARNING
Feb 3rd (KST) = Feb 2nd (EST)
Talk 1
Title: Inferring Quantum States from Projective Measurements
Abstract: Projective measurements from quantum systems such as available quantum computers and quantum simulators present new data-driven challenges. Projective measurements of many-body wave functions in their exponentially large Hilbert space present a difficult inverse problem when it comes to state reconstruction. While the full reconstruction of the density matrices may be restricted to small systems, expressive machine learning models may enable useful approximate characterization. Furthermore, quantum simulations driven by specific model Hamiltonian could be approached from a pattern recognition perspective. In this talk, I will present our recent efforts from the two angles of generative modeling and pattern recognition.
[1] https://arxiv.org/abs/2006.12469
[2] https://arxiv.org/abs/2011.03474
Speaker: Eun-Ah Kim (Cornell)
Talk 2
Title: Reconstructing quantum states with generative models
Abstract: Generative models are a powerful tool in unsupervised machine learning, where the goal is to learn the unknown probability distribution that underlies a data set. Recently, it has been demonstrated that modern generative models adopted from industry are capable of reconstructing quantum states, given projective measurement data on individual qubits. These virtual reconstructions can then be studied with probes unavailable to the original experiment. In this talk I will outline the strategy for quantum state reconstruction using generative models, and show examples on real experimental data from a Rydberg atom quantum simulator. I will discuss the continuing theoretical development of the field, including the exploration of powerful autoregressive models for the reconstruction of sign-problematic and mixed quantum states.
Speaker: Roger Melko (Perimeter)
Talk 3
Title: Dynamics of Quantum Entanglement and Information
Abstract: We will discuss novel quantum phases and phase transitions that emerge in the dynamics of quantum circuits and exhibit distinct modes of information flow. We consider a quantum system undergoing time evolution generated by random unitary gates which are occasionally interrupted by local measurements. It was previously found that such evolution leads to a phase transition in the entanglement of the wavefunction: at steady state it is characterized by volume law entanglement for small measurement rate, while it is an area law above a threshold rate. We investigate this phase transition by mapping the quantum dynamics into classical statistical mechanics models in equilibrium. Furthermore, we show that more phases are possible if we impose symmetries on the dynamics. To identify the phases, we develop a mapping between the dynamics in a broad class of quantum circuits to the ground states of effective spin Hamiltonians, which possess a higher symmetry than the original circuit. The different ground state phases admitted by the higher symmetry correspond to distinct patterns of information flow and scrambling in the circuit dynamics. We illustrate these ideas with multiple examples. Our work is a first step toward the classification of information flow in generic quantum circuits with symmetries.
Speaker: Soonwon Choi (MIT)
Day 3: Magnetism & correlated Electrons
Feb 4th (KST) = Feb 3rd (EST)
Talk 1
Title: Hybridization and interaction between quantum multipolar fluctuations in a conventional magnet
Abstract: One of the scientific frontiers in quantum magnetism is the discovery and understanding of quantum entangled and topologically ordered states in real bulk materials. At the focal point of the experimental investigation of these quantum spin networks is the identification of fractionalized excitations in transport and spectroscopic measurements. Inelastic neutron scattering has proved a powerful technique to reveal such signatures in a variety of systems ranging from quasi-1D magnets to kagome compounds and more. Recent and on-going developments with neutron scattering instrumentation have allowed the characterization of magnetic excitations in entire volumes of momentum-energy space with high resolution. This has prompted revisiting long overlooked materials in search for exotic spin dynamics despite seemingly classical magnetically ordered ground-states. In this talk, I will discuss such experiments on a long-known material, FeI2, and show how high-fidelity modeling brings new insights on its spin dynamics [1]. I will describe the mechanism that endows low energy quadrupolar fluctuations in FeI2 with large spectral weight and how these can be completely understood using a SU(3) representation of spin degrees of freedom. I will discuss the consequence of the having several quasiparticles as the low-energy degrees of freedom in this system [2]. This work was supported by DOE/BES under award DE-SC-0018660.
[1] X. Bai, S.-S. Zhang, Z. L. Dun, H. Zhang, Q. Huang, H. D. Zhou, M. B. Stone, A. I. Kolesnikov, F. Ye, C. D. Batista, M. Mourigal, “Hybridized quadrupolar excitations in the frustrated and spin-anisotropic magnet FeI2 ”, Nature Physics (January 2021), https://doi.org/10.1038/s41567-020-01110-1, https://arxiv.org/abs/2004.05623.
[2] A. Legros, S.-S. Zhang, X. Bai, H. Zhang, Z. L. Dun, W. A. Phelan, C. D. Batista, M. Mourigal, and N. P. Armitage, “Observation of 4- and 6-magnon bound-states in the spin-anisotropic frustrated antiferromagnet FeI2”, Submitted (November 2020), https://arxiv.org/abs/2012.04205.
Speaker: Martin Mourigal (Georgia Tech)
Talk 2
Title: Chiral magnetism: a geometric perspective
Abstract: Chiral ferromagnets have spatially modulated magnetic order exemplified by helices, spirals, and more complex patterns such as skyrmion crystals. The theoretical understanding of these states is based on a competition of a strong Heisenberg exchange interaction favoring uniform magnetization and a weaker Dzyaloshinskii-Moriya interaction promoting twists in magnetization. We offer a geometric approach, in which chiral forces are a manifestation of curvature in spin parallel transport [1]. The resulting theory is a gauged version of the Heisenberg model, with the Dzyaloshinskii-Moriya vectors serving as background SO(3) gauge fields. This geometrization of chiral magnetism is akin to the treatment of gravity in general theory of relativity, where gravitational interactions are reduced to a curvature of spacetime. An immediate benefit of this geometrical perspective is a simple way to define a conserved spin current in the presence of spin-orbit interaction. We show that the ground state of the gauged Heisenberg model in 2 spatial dimensions is a hexagonal skyrmion crystal in a wide range of applied magnetic fields. The simplicity of the model allows for an efficient analytical treatment of this problem using standard field-theoretic methods. Monte Carlo simulations confirm our analytical arguments.
[1] D. Hill, V. Slastikov, and O. Tchernyshyov, arXiv:2008.08681.
Speaker: Oleg Tchernyshyov (Johns Hopkins)
Talk 3
Title: Two-Channel Kondo Problem in Non-Kramers Doublet of Praseodymium-Based Compounds with 4f^2 Configuration
Abstract: There has been considerable interest in two-channel Kondo problem which manifests itself as quadrupole Kondo effect in the non-Kramers 3 doublet of cubic praseodymium-based compounds with 4f^2 configuration [1]. The 3 doublet for the total angular momentum of J = 4 has no magnetic dipole but electric quadrupoles and magnetic octupole. Thereby, the local quadrupole of the 4f^2 electrons could interact with two equivalent conduction bands to give rise to anomalous metallic state involving a so-called non-Fermi liquid (NFL) behavior. Among the Pr-based compounds, PrT2Zn20 (T = Rh, Ir) and PrT2Al20 (T =Ti, V) crystallizing in the cubic CeCr2Al20 -type structure exhibit coexistence of quadrupole order and superconducting state, suggesting that the quadrupole fluctuations in the 3 doublet play a role in formation of the superconducting pair [2]. Moreover, NFL behaviors of the specific heat C and electrical resistivity were observed above the quadrupole ordering temperature, which indicates a possible formation of the quadrupole Kondo lattice [3,4]. On the other hand, for the Pr diluted system Y(Pr)Ir2Zn20 , NFL behaviors of and C/T –log T variations were observed for 0.08 <T<0.3 K [5]. These temperature variations are almost the same as those predicted by the single-site quadrupole Kondo effect [1]. The common characteristics of the NFL behaviors were also verified in the isostructural Pr diluted systems of Y(Pr)Co2Zn20 [6] and La(Pr)Ti 2 Al 20 [7]. Moreover, in Y(Pr)Ir2Zn20 , the elastic constant of the (C_{11} -C_{12})/2 mode, which corresponds to the 3 -symmetry quadrupole susceptibility , was measured, and the logT variation of (C_{11}- C_{12})/2 was observed, confirming the manifestation of the single-site NFL behavior due to the quadrupole Kondo effect [8].
[1] D. L. Cox and A. Zawadowski, Adv. Phys. 47, 599 (1998).
[2] T. Onimaru and H. Kusunose, J. Phys. Soc. Jpn. 85, 082002 (2016).
[3] A. Tsuruta and K. Miyake, J. Phys. Soc. Jpn. 84, 114714 (2015).
[4] T. Onimaru et al., Phys. Rev. B 94, 075134 (2016).
[5] Y. Yamane, T. Onimaru et al., Phys. Rev. Lett. 121, 077206 (2018).
[6] Y. Yamane, T. Onimaru et al., JPS Conf. Ser. 29, 015001 (2020).
[7] S. Asatani, T. Onimaru et al., JPS Conf. Proc. 30, 011159 (2020).
[8] T. Yanagisawa, T. Onimaru et al., Phys. Rev. Lett. 123, 067201 (2019).
Speaker: Takahiro Onimaru (Hiroshima)
Day 4: Quantum Dynamics
Feb 5th (KST) = Feb 4th (EST)
Talk 1
Title: Thwarting long-lived dark states to speed up dynamic polarization
Abstract: Long-lived dark states, in which an experimentally accessible qubit is not in thermal equilibrium with a surrounding spin bath, are pervasive in solid-state systems. In the first half of the talk, I will explain the ubiquity of dark states in a large class of inhomogenous central spin models using the proximity to integrable lines with exact dark eigenstates. I will develop a picture of the eigenstates away from the integrable lines in terms of many-body resonances, and argue that relaxation times are exponentially large in system size at accessible sizes.
Long-lived dark states stymie hyperpolarization protocols that aim to transfer spin polarization from the central qubit to the surrounding spin bath. In the second half of the talk, I will describe fast and efficient hyperpolarization protocols that exploit certain integrability-breaking terms to reduce the statistical weight on dark states. These protocols use approximate counter-diabatic driving and are experimentally accessible through Floquet engineering.
Speaker: Anushya Chandran (BU)
Talk 2
Title: Superdiffusion and KPZ hydrodynamics in isotropic spin chains
Abstract: Finite-temperature spin transport in the quantum Heisenberg spin chain is known to be superdiffusive, and has been conjectured to lie in the Kardar-Parisi-Zhang (KPZ) universality class. In this talk, I will review the numerical and experimental evidence for this surprising anomalous transport property, and propose a theory in terms of “giant”, soft quasiparticles stabilized by integrability. I will argue that anomalous transport is `superuniversal’ in integrable spin chains with continuous non-abelian symmetry. Finally, I will discuss the stability of this phenomenon against integrability-breaking perturbations, and argue that it is surprisingly robust to perturbations preserving non-abelian symmetries.
Speaker: Romain Vasseur (UMass)
Talk 3
Title: Topological edge modes without symmetry in quasiperiodically driven spin chains
Abstract: Time-dependent driving enables new types of intrinsically dynamical topological phases with no equilibrium counterparts. A striking example are Floquet symmetry protected topological (FSPT) phases of periodically driven 1d spin-chains, which host edge states that can coherently store quantum information protected against stray-fields, and uncontrolled cross-talk by a combination of dynamical decoupling and symmetry. Unfortunately, the required symmetries are often unnatural for many physical realizations, and greatly restrict the class of perturbations against which 1d FSPT edge states are protected. In this talk, I will introduce a model of a quasiperiodically driven spin-chain that enjoys dynamical and topologically protected edge states without the need for any symmetry protection. Instead, the edge dynamics are protected by purely emergent symmetries, that are robust against generic perturbations. I will also present numerical evidence for a new type of dynamical boundary-criticality in which the 0d edge-mode dynamics can transition between quasiperiodic to chaotic, causing the spin-chain to heat from the edge in. This behavior is special to quasiperiodic drives, has no analog in Floquet systems, and introduces important modifications to the notion of phases and stability of quasiperiodically driven systems.
Speaker: Andrew Potter (UT Austin)