Understanding the Emergence of Chiral Spin Liquids in Mott Insulators
Ciaran Hickey University of Toronto
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Many-body physics in a trapped ion quantum simulator
Kazi-Rajibul Islam Institute for Quantum Computing (IQC)
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The i epsilon prescription in the SYK model
Razvan Gurau Universität Heidelberg
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Topological states in honeycomb materials
Hae-Young Kee University of Toronto
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Entanglement area law in superfluid 4He
Chris Herdman Institute for Quantum Computing (IQC)
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NMR signature of charge order in high Tc cuprates revisited
Takashi Imai McMaster University
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Delafossite layered metals: intriguing physics in the high purity limit
Andrew Mackenzie Max Planck Institute
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Synchronous correlations, Bell inequalities, and categories of nonlocal games
Brad Lackey University of Maryland, College Park
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Many-Body Localization Through the Lens of Ultracold Quantum Gases
Pranjal Bordia Ludwig-Maximilians-Universitiät München (LMU)
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Understanding the Emergence of Chiral Spin Liquids in Mott Insulators
Ciaran Hickey University of Toronto
In recent years, there has been a resurgence of interest in the study of chiral spin liquids (CSLs), topologically ordered states of matter that are closely related to the celebrated fractional quantum Hall states. This resurgence has been driven by the introduction of exact parent Hamiltonians and a number of numerical studies that have identified CSLs in local spin models. However, our understanding of how and why these states emerge is still lacking. I will discuss evidence supporting one particularly intuitive mechanism in which they arise as "quantum-disordered" descendants of certain non-coplanar magnetic parent states, uniting many of the CSLs found so far under a common framework. -
Many-body physics in a trapped ion quantum simulator
Kazi-Rajibul Islam Institute for Quantum Computing (IQC)
Many-body quantum systems are often hard to simulate on a computer, due to the computational complexity generated by non-classical correlations or entanglement between parts of these systems. An alternate platform is to experimentally simulate non-trivial quantum models on synthetic quantum matter composed of cold atomic systems. These systems exhibit excellent quantum coherence properties due to their isolation from environment, and hence faithfully evolve in time under the prescribed quantum Hamiltonian. In this talk, I will focus on simulating models of quantum magnetism with laser-cooled trapped atomic ions. Two internal states of each ion represent a spin-1/2 system that can be initialized, manipulated, and detected with near perfection by laser beams. Precisely tuned optical dipole forces couple individual spins to the collective vibrational (phonon) states of the ions, which mediate effective spin-spin interactions. By suitably tailoring these couplings, the interactions can be tuned dynamically in magnitude, sign, and range, allowing for the investigation of a large class of problems in quantum many-body physics. -
The i epsilon prescription in the SYK model
Razvan Gurau Universität Heidelberg
In this talk I will discuss the SYK model from a quantum field theoretical perspective and present an i epsilon prescription which regularizes the divergences of the model.
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Topological states in honeycomb materials
Hae-Young Kee University of Toronto
The topological Haldane model (THM) on a honeycomb lattice is a prototype of systems hosting topological phases of matter without external fields. It is the simplest model exhibiting the quantum Hall effect without Landau levels, which motivated theoretical and experimental explorations of topological insulators and superconductors. Despite its simplicity, its realization in condensed matter systems has been elusive due to a seemingly difficult condition of spinless fermions with sublattice-dependent magnetic flux terms. While there have been theoretical proposals including elaborate atomic-scale engineering, identifying candidate THM materials has not been successful, and the first experimental realization was recently made in ultracold atoms. Here we suggest that a series of Fe-based honeycomb ferromagnetic insulators, AFe2(PO4)2 (A=Ba,Cs,K,La) possess Chern bands described by the THM. -
Nearly fractionalized excitations in 2D quantum antiferromagnets
Anders Sandvik Boston University
The 2D S = 1/2 square-lattice Heisenberg model is a keystone of theoretical studies of quantum magnetism. It also has very good realizations in several classes of layered insulators with localized electronic spins. While spin-wave theory provides a good understanding of the antiferromagnetic ground state and low-lying excitations of the Heisenberg model, an anomaly in the excitations at higher energy around wave-number q = (\pi, 0) has been diffi_cult to explain. At first sight, the anomaly is just a suppression of the excitation energy by a few percent, but it also represents a more dramatic shift of spectral weight in the dynamic spin structure factor from the single- magnon (spin wave) pole to a continuum. Recent neutron scattering experiments on the quasi-2D material Cu(DCOO)2_.4D2O (the best realization so far of the 2D Heisenberg model) were even interpreted as a complete lack of magnon pole at the anomaly; instead it was suggested that the excitations there are fractional (spinons) [1]. I will discuss recent quantum Monte Carlo and stochastic analytic continuation results pointing to the existence of fragile q~(\pi,0) magnon excitations in the Heisenberg model [2], which can be fractionalized by interactions competing with the nearest-neighbor exchange coupling. This phenomenon can be understood phenomenologically within a simple theory of magnon-spinon mixing. -
Entanglement area law in superfluid 4He
Chris Herdman Institute for Quantum Computing (IQC)
Area laws were first discovered by Bekenstein and Hawking, who found that the entropy of a black hole grows proportional to its surface area, and not its volume. Entropy area laws have since become a fundamental part of modern physics, from the holographic principle in quantum gravity to ground state wavefunctions of quantum matter, where entanglement entropy is generically found to obey area law scaling. As no experiments are currently capable of directly probing the entanglement area law in naturally occurring many-body systems, evidence of its existence is based on studies of simplified theories. Using new exact microscopic numerical simulations of superfluid 4He, we demonstrate for the first time an area law scaling of entanglement entropy in a real quantum liquid in three dimensions. We validate the fundamental principles underlying its physical origin, and present an "entanglement equation of state" showing how it depends on the density of the superfluid. -
NMR signature of charge order in high Tc cuprates revisited
Takashi Imai McMaster University
In 1999, A. W. Hunt et al. discovered that all the NMR anomalies detected at the charge density wave (CDW) order transition Tcharge ~ 60 K of nearly non-superconducting La1.48Nd0.4Sr0.12CuO4 are shared by superconducting La1.88Sr0.12CuO4 (Tc ~ 30K) [1]. The unexpected finding inevitably led us to conclude that charge order must exist even in the superconducting La2-xSrxCuO4, sending a shockwave in the high-Tc community [2]. Subsequent search of charge order peaks based on scattering techniques, however, failed to detect additional evidence for charge order until very recently. In view of the recent confirmation of charge order in many superconducting cuprates by X-ray diffraction techniques, we revisit the old problem of charge order using newer NMR techniques that have become available in recent years. -
Delafossite layered metals: intriguing physics in the high purity limit
Andrew Mackenzie Max Planck Institute
In this talk I will introduce a relatively little-studied but intriguing family of metals, the delafossite series of layered oxides ABO2 in which the A site is occupied by Pd or Pt, and the B site by a transition metal. For reasons that are not perfectly understood, these materials have amazingly high electrical conductivity, with mean free paths of hundreds of angstroms (longer than even elemental copper or silver) at room temperature, growing to tens of microns at low temperatures. The electronic structure that yields these properties is in one way very simple, with a single half filled conduction band, but in another sense very rich, because the nearly free electrons originate mainly from the (Pt,Pd) layers in the crystal structure, while the adjacent transition metal oxide layers host Mott insulating states to which the conduction electrons also have some coupling. My group is interested in the delafossites for a number of reasons. Firstly, they are possible hosts for electronic transport at the crossover between ballistic and hydrodynamic regimes, which we investigate by fabricating size-restricted microstructures using focused ion beam techniques. As layered materials that can be cleaved at low temperatures, they are also well suited to study by angle resolved photoemission spectroscopy, and host a variety of interesting surface states in addition to a simple single-band bulk electronic structure. I will discuss our findings on non-magnetic PdCoO2, PtCoO2 and PdRhO2 and magnetic PdCrO2. -
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Synchronous correlations, Bell inequalities, and categories of nonlocal games
Brad Lackey University of Maryland, College Park
Given two sets X and Y, we consider synchronous correlations in a two-party nonlocal game with inputs X and outputs Y as a notion of generalized function between these sets (akin to a quantum graph homomorphism). We examine some structures in categories of synchronous classical, quantum, and nonsignalling strategies. We also provide analogues of Bell’s inequalities for such games when Y = {0,1}.
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Scaling with two divergent lengths in deconfined quantum criticality
Anders Sandvik Boston University
The existence of a deconfined quantum-critical point [1] between the standard antiferromagnet
and a valence-bond solid in 2D S=1/2 quantum magnets has been controversial, in part due to
anomalous finite-size scaling behaviors observed in quantum Monte Carlo simulations interpreted by some as signs of a first-order transition. I will discuss a new finite-size scaling hypothesis in which a scaling function of two divergent length scales [the standard correlation length and a length-scale related to an emergent U(1) symmetry of the valence-bond solid] has a limiting form implying unconventional finite-size scaling behaviors, while maintaining conventional scaling in the thermodynamic limit [2]. This proposal goes beyond the standard scenario of a dangerously irrelevant perturbation as a source of the second length scale in, e.g., classical 3D clock models. Quantum Monte Carlo simulations of the J-Q model (a spin-1=2 Heisenberg model extended with certain multi-spin interactions) are in full agreement with the proposed scaling form, suggesting that deconfined quantum-criticality is an even richer phenomenon than initially imagined. Since finite temperature T plays the role of a finite imaginary-time dimension in quantum systems, the anomalous scaling behavior impacts also the scaling in the quantum-critical \fan" at T > 0. This is also observed in the J-Q model.
[1] H. Shao, W. Guo, and A. W. Sandvik, Science 352, 213 (2016).
[2] T. Senthil, A. Vishwanath, L. Balents, S. Sachdev, M. P. A. Fisher, Science 303, 1490 (2004). -
Many-Body Localization Through the Lens of Ultracold Quantum Gases
Pranjal Bordia Ludwig-Maximilians-Universitiät München (LMU)
A fundamental assumption of quantum statistical mechanics is that closed isolated systems always thermalize under their own dynamics. Progress on the topic of many-body localization has challenged this vital assumption, describing a phase where thermalization, and with it, equilibrium thermodynamics, breaks down.
In this talk, I will describe how we can realize such a phase of matter with ultracold fermions in both driven and undriven optical lattices, with a focus on the relevance of realistic experimental platforms. Furthermore, I will describe new results on the observation of a regime exhibiting extremely slow scrambling, even for "infinite-temperature states" in one and two dimensions. Our results demonstrate how controlled quantum simulators can explore fundamental questions about quantum statistical mechanics in genuinely novel regimes, often not accessible to state-of-the-art classical computations.