Quantum quenches & holography
Robert Myers Perimeter Institute for Theoretical Physics
PIRSA:13110074
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Dynamical Emergence of Universal Horizons during the formation of Black Holes
Mehdi Saravani Goldman Sachs (United States)
Entanglement at strongly-interacting quantum critical points
Roger Melko University of Waterloo
PIRSA:13110071Unparticles and Fermi Arcs in the Cuprates
Philip Phillips University of Illinois Urbana-Champaign
PIRSA:13100131Hofstadter’s Butterfly and interaction driven quantum Hall ferromagnetism in graphene
Philip Kim Columbia University
PIRSA:13110070Staring into the Abyss
Avery Broderick University of Waterloo
Fractional statistics in two-dimensions: Anyon there?
Smitha Vishveshwara University of Illinois Urbana-Champaign
Quantum quenches & holography
Robert Myers Perimeter Institute for Theoretical Physics
PIRSA:13110074We employ holographic techniques to study quantum quenches at finite temperature, where the quenches involve varying the coupling of the boundary theory to a relevant operator with an arbitrary conformal dimension. The evolution of the system is studied by evaluating the expectation value of the quenched operator and the stress tensor throughout the process. The time dependence of the new coupling is characterized by a fixed timescale and the response of the observables depends on the ratio of the this timescale to the initial temperature. The observables exhibit universal scaling behaviours when the transitions are either fast or slow, i.e., when this ratio is very small or very large. For fast quenches, we uncover a universal scaling behaviour in the response of the system, which depends only on the conformal dimension of the quenched operator in the vicinity of the ultraviolet fixed point of the theory.Matter matters in asymptotically safe quantum gravity
The Functional Renormalisation Group technique has received great attention in recent times proving itself as a powerful tool to describe the high energy behaviour of gravitational interactions.
Its key ingredient is a nontrivial fixed point of the theory renormalization group flow which controls the behavior of the coupling constants in the ultraviolet regime and ensures that physical quantities are safe from divergences. I will briefly review the main ingredients of the gravitational asymptotic safety framework before focusing on the effect of massless minimally coupled scalars, fermions and vector fields on the gravitational fixed point. I will then set bounds on the type and number of such fields requiring the existence of a UV attractive fixed point. I will also discuss the dynamically generated quantum-gravity scale in asymptotic safety, which is the transition scale to the fixed-point regime, and its variation as a function of matter degrees of freedom. To conclude I will also consider the case of higher dimensional models.Topological Phases in Transition Metal Oxides
PIRSA:13110072Certain varieties of transition metal oxides possess both significant interactions and strong spin-orbit coupling. In this talk I will describe materials-motivated models that predict topological phases in heterostructured and bulk transition metal oxides. We find Z2 topological insulators, Chern insulators, topological crystalline insulators, and interaction-driven topological phases not adiabatically connected to non-interacting topological phases.Dynamical Emergence of Universal Horizons during the formation of Black Holes
Mehdi Saravani Goldman Sachs (United States)
In many theories with fundamental preferred frame, such as Einstein-Aether or Gravitational Aether theories, K-essence, Cuscuton theory, Shape Dynamics, or (non-projectable) Horava-Lifshitz gravity, the low energy theory contains a fluid with superluminal or incompressible excitations. In this talk, I study the formation of black holes in the presence of such a fluid. In particular, I focus on the incompressible limit of the fluid (or Constant Mean Curvature foliation) in the space-time of a spherically collapsing shell within an asymptotically cosmological space-time. In this case, I show that an observer inside 3/4 of the Schwarzschild radius cannot send a signal outside, after a stage in collapse, even using signals that propagate infinitely fast in the preferred frame. This confirms the dynamical emergence of universal horizons that have been previously found in static solutions [arXiv:1110.2195, arXiv:1104.2889, arXiv:1212.1334].Entanglement at strongly-interacting quantum critical points
Roger Melko University of Waterloo
PIRSA:13110071At a quantum critical point (QCP) in two or more spatial dimensions, leading-order contributions to the scaling of entanglement entropy typically follow the "area" law, while sub-leading behavior contains universal physics. Different universal functions can be access through entangling subregions of different geometries. For example, for polygonal shaped subregions, quantum field theories have demonstrated that the sub-leading scaling is logarithmic, with a universal coefficient dependent on the number of vertices in the polygon. Although such universal quantities are routinely studied in non-interacting field theories, it requires numerical simulation to access them in interacting theories. In this talk, we discuss numerical calculations of the Renyi entropies at QCPs in 2D quantum lattice models. We calculate the universal coefficient of the vertex-induced logarithmic scaling term, and compare to non-interacting field theory calculations. Also, we examine the shape dependence of the Renyi entropy for finite-size lattices with smooth subregion boundaries. Such geometries provide a sensitive probe of the gapless wavefunction in the thermodynamic limit, and give new universal quantities that could be examined by field-theoretical studies in 2+1D.Unparticles and Fermi Arcs in the Cuprates
Philip Phillips University of Illinois Urbana-Champaign
PIRSA:13100131One of the open problems in strong correlation physics is whether or not Luttinger's theorem works for doped Mott insulators, particularly in the pseudo gap regime where the pole-like excitations form only a Fermi arc. I will begin this talk by using this theorem to count particles and show that it fails in general for the Mott state. The failure stems from the divergent self energy that underlies Mottness. When such a divergence is present, charged degrees of freedom are present that have no particle interpretation. I will argue that such excitations are governed by a non-trivial IR fixed point and the propagator of which is of the unparticle form proposed by Georgi. I will show how a gravity dual can be used to determine the scaling dimension of the unparticle propagator. I will close by elucidating a possible superconducting instability of unparticles and demonstrate that unparticle stuff is likely to display fractional statistics in the dimensionalities of interest for strongly correlated electron matter. Time permitting, an underlying theory of the strongly coupled fixed point will be outlined.Hofstadter’s Butterfly and interaction driven quantum Hall ferromagnetism in graphene
Philip Kim Columbia University
PIRSA:13110070Electrons moving in a periodic electric potential form Bloch energy bands where the mass of electrons are effectively changed. In a strong magnetic field, the cyclotron orbits of free electrons are quantized and Landau levels forms with a massive degeneracy within. In 1976, Hofstadter showed that for 2-dimensional electronic system, the intriguing interplay between these two quantization effects can lead into a self-similar fractal set of energy spectrum known as “Hofstadter’s Butterfly.” Experimental efforts to demonstrate this fascinating electron energy spectrum have continued ever since. Recent advent of graphene, where its Bloch electrons can be described by Dirac feremions, provides a new opportunity to investigate this half century old problem experimentally. In this presentation, I will discuss the experimental realization Hofstadter’s Butterfly via substrate engineered graphene under extremely high magnetic fields controlling two competing length scales governing Dirac-Bloch states and Landau orbits, respectively. In addition, the strong Coulomb interactions and approximate spin-pseudo spin symmetry are predicted to lead to a variety of integer quantum Hall ferromagnetic and fractional quantum Hall states and the quantum phase transition between them in graphene. I will discuss several recent experimental evidences to demonstrate the role of the electron interaction in single and bilayer graphene.Staring into the Abyss
Avery Broderick University of Waterloo
Nearly a century after their discovery, black holes remain one of the most striking, and problematic predictions of general relativity. Even more unsettling is the fact that they actually appear to exist! With only a handful of exceptions, every galaxy contains a supermassive behemoth, millions to billions as massive as the sun, at their center. These supermassive black holes are hardly incidental, they gravitationally power enormous outflows that rule the fates of their hosts. Despite the critical role they play in our understanding of gravity and impact upon the visible universe, actually testing their nature has remained beyond our reach - until now. Dr. Broderick will describe how astronomers are currently constructing (and operating) facilities that will image the horizons of black holes, and what we can already say about these enigmatic monsters in the dark.Fractional statistics in two-dimensions: Anyon there?
Smitha Vishveshwara University of Illinois Urbana-Champaign
A fascinating aspect of the two dimensional world is the possible existence of anyons, particles which obey 'fractional' statistics different from fermionic and bosonic statistics. In this colloquium, following an introduction to fractional particles in the context of quantum Hall systems, some of the tantalizing experiments for detecting the fractional charge of these particles will be described. Probes of fractional statistics in these systems will be discussed, drawing from analogies with the bosonic behavior of light studied by Hanbury Brown and Twiss in the 1950's as well as in the more recent beam-splitter settings in quantum optics. Finally, the exciting prospect of detecting fractional statistics in the context of superconductors will be explored.