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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-Universität München (LMU)
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Dark Stars: Dark Matter Annihilation can power the first stars
Katherine Freese The University of Texas at Austin
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Moduli of Vacua and Categorical representations
David Ben-Zvi The University of Texas at Austin
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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-Universität 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. -
The new ultracold neutron facility at TRIUMF
Beatrice Franke TRIUMF
A permanent non-zero electric dipole moment of the free neutron (nEDM) violates CP-symmetry. The search for an nEDM contributes to understanding the Baryon asymmetry,
as well as it has a high discovery potential for Beyond Standard Model physics. The tool of choice to investigate the nEDM are ultracold neutrons (UCN), since they have such low energies that they can be stored in traps and allow observation times of hundreds of seconds.
The distinct feature of TRIUMFs UCN facility is the combination of a neutron spallation source with a superfluid helium UCN converter - unique among all existing and planned UCN sources worldwide. The goal of the UCN project at TRIUMF is to provide a density of several hundreds of UCN per cubic cm to experiments at up to two ports, whereas one will be dedicated to determine the nEDM to the 10-27 e·cm level of precision.
This presentation shall update the audience on the current status of the new UCN facility at TRIUMF. Additionally, a brief status update on the work of the CREMA collaboration (Charge Radius Experiments with Muonic Atoms) shall be given. Recent experiments performing laser spectroscopy on light muonic atoms shed new light on the Proton Radius Puzzle. -
Dark Stars: Dark Matter Annihilation can power the first stars
Katherine Freese The University of Texas at Austin
The first phase of stellar evolution in the history of the Universe may be Dark Stars (DS), powered by dark matter heating rather than by nuclear fusion. Weakly Interacting Massive Particles, which may be their own antipartners, collect inside the first stars and annihilate to produce a heat source that can power the stars. A new stellar phase results, a Dark Star, powered by dark matter annihilation as long as there is dark matter fuel, with lifetimes from millions to billions of years. Dark stars are very bright diffuse puffy objects during the DS phase, and grow to be very massive. In fact, we have found they can to grow to 10^5-10^7 solar masses with luminosities 10^9-10^11 solar luminosities. Such objects will be observable with James Webb Space Telescope (the sequel to HST). Once the dark matter fuel is exhausted, the DS becomes a heavy main sequence star; these stars eventually collapse to form massive black holes that may provide seeds for supermassive black holes observed at early times as well as in galaxies today.
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Moduli of Vacua and Categorical representations
David Ben-Zvi The University of Texas at Austin
I will present some results on three-dimensional gauge theory from the point of view of extended topological field theory. In this setting a theory is specified by describing its collection of boundary conditions - in our case, a collection of categories (standing in for 2d TFTs) with a prescribed symmetry group G. We will apply ideas from Seiberg-Witten geometry to construct a new commutative algebra of symmetries for categorical representations (or line operators in the gauge theory) - a categorification of Kostant's description of the center of the enveloping algebra. (Joint with Sam Gunningham and David Nadler)
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Gravity and non-locality
Pablo Diaz Benito University of Lethbridge
I will talk about the relation between non-local theories and gravity. The main thesis is that non-local field theories naturally induce gravity, even at the classical level. Supporting this idea, I will study bi-local scalar field theories, which involve minimal deviations from locality. We will treat them both, bi-local theories and gravity perturbatively. We will see that bi-local theories encode gravity together with higher spin fields.