Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z36: Focus Session: Non-equilibrium Dynamics in Quantum Systems |
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Sponsoring Units: DAMOP Chair: Stefan Natu, University of Maryland, College Park Room: 211 |
Friday, March 6, 2015 11:15AM - 11:51AM |
Z36.00001: Locality in quenched systems with long-range interactions Invited Speaker: Michael Foss-Feig For more than a decade, ultracold atomic and molecular systems have been exploited to simulate canonical models of strongly correlated materials. However, the extremely low (often sub nano-kelvin) temperatures required to realize the most interesting equilibrium behaviors of such models have proven extremely difficult to achieve. When these ultracold systems are driven far-from equilibrium, however, very small temperatures get traded in for very long time-scales, which enable the observation of dynamic phenomena that were never even envisioned in the context of real materials. In this talk, I will describe some recent experimental and theoretical explorations of non-equilibrium dynamics in quenched AMO systems, and will discuss some of the interesting questions that arise naturally from their remarkable tunability. In particular, I will describe recent efforts to understand the fate of locality --- i.e. constraints on the propagation of information/entanglement --- as interactions become increasingly long-ranged. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:03PM |
Z36.00002: Slowest local operators in quantum spin chains Hyungwon Kim, Mari Carmen Banuls, Ignacio Cirac, Matthew Hastings, David Huse We numerically construct slowly relaxing local operators in a nonintegrable spin-1/2 chain. Restricting the support of the operator to M consecutive spins along the chain, we exhaustively search for the operator that minimizes the Frobenius norm of the commutator with the Hamiltonian and show that the Frobenius norm bounds the time scale of relaxation of the operator. We find operators with significantly slower relaxation than the slowest simple ``hydrodynamic'' mode due to energy diffusion. Using both exhaustive search and tensor network techniques, we find similar slowly relaxing operators for a Floquet spin chain and for quantum circuits on spin chains; these systems are hydrodynamically ``trivial,'' with no conservation laws restricting their dynamics. We argue that such slow relaxation may be a generic feature following from locality and unitarity. [Preview Abstract] |
Friday, March 6, 2015 12:03PM - 12:15PM |
Z36.00003: Limit cycle phase in driven-dissipative spin systems Ching-Kit Chan, Tony Lee, Sarang Gopalakrishnan Quantum simulator experiments based on trapped ions and atomic ensembles offer an attractive platform to study nonequilibrium many-body phases and phase transitions. We theoretically explore the phase diagram of a driven and dissipative Heisenberg spin system featured by a time-dependent limit cycle phase in which the magnetization oscillates in time. We present a Gaussian-Floquet theory to study the fluctuation of this phase that spontaneously breaks time-translational symmetry. As a time-dependent generalization of the Mermin-Wagner theory, we show how spatial fluctuations destroy the limit cycle ordering for dimension $\leq 2$. We also demonstrate how the limit-cycle phase leads to new features in the power spectrum measurable in fluorescence experiments. [Preview Abstract] |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z36.00004: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z36.00005: Ramping through a topological critical point in two dimensions Marin Bukov, Phillip Weinberg, Michael Kolodrubetz The recent realisation of Floquet Chern insulators has resulted in a prolific study of periodically driven models. In order to probe equilibrium physics, the driving protocol is gently ramped up, in the process of which the system undergoes a dynamical phase transition to a topologically non-trivial state. Since such transitions are controlled by closing and re-opening a band gap, the notion of adiabaticity inevitably breaks down and the system gets excited. In this talk, I shall present recent results based on scaling arguments within Kibble-Zurek theory to study the excitations due to a ramp through a topological critical point in 2 dimensions. I shall show convincing evidence that the occupation of the chiral edge modes follows similar universal scaling as the bulk as a function of the ramp speed and the system size. Further, I shall apply these results to study the build-up of magnetisation due to the non-adiabatic population of the edge states in Haldane's model of graphene, which has recently been proposed to detect the topological character of the state of the system. Finally, I shall show that the quantisation of magnetisation is robust against non-adiabaticity due to crossing the critical point. [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z36.00006: The Floquet Adiabatic Theorem revisited Phillip Weinberg, Marin Bukov, Luca D'Alessio, Michael Kolodrubetz, Shainen Davidson, Anatoli Polkovnikov The existance of the adiabatic theorem for Floquet systems has been the subject of an active debate with different articles reaching opposite conclusions over the years. In this talk we clarify the situation by deriving a systematic expansion in the time-derivatives of a slow parameter for the occupation probabilities of the Floque states. Our analysis shows that the in a certain limit the transition between Floquet eigenstates are suppressed and it is possible to define an adiabatic theorem for Floquet systems. Crucially we observe however that the conditions for adiabaticity in ordinary and Floquet systems are different and that this difference can become important when the amplitude of the periodic driving is large. We illustrate our results with specific examples of a periodically driven harmonic oscillator and cold atoms in optical lattices which are relevant in current experiments. [Preview Abstract] |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z36.00007: Thermal Steady States in Fermionic Dissipative Floquet Systems Karthik Seetharam, Charles-Edouard Bardyn, Mark Rudner, Netanel Lindner, Gil Refael The possibility to drive quantum systems periodically in time offers unique ways to deeply modify their fundamental properties, as exemplified by Floquet topological insulators. It also opens the door to a variety of non-equilibrium effects. Resonant driving fields, in particular, lead to excitations which can expose the system to heating. Inspired by existing studies of photoexcited semiconductors, we demonstrate that the analog of thermal states can be achieved in a fermionic Floquet system including carrier-carrier interactions, phonon scattering, and spontaneous emission. We show that inelastic ``Floquet-Umklapp'' processes are responsible for non-thermal heating effects, and identify practical conditions under which they are suppressed. We propose to use suitably engineered external reservoirs of carriers to further stabilize thermal features and control the effective chemical potential of the resulting Floquet distributions. [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z36.00008: Dynamical preparation of Floquet Chern insulators: A no-go theorem and the experiments Luca D'Alessio, Marcos Rigol Recently, it has been proposed that time-periodic perturbations can induce topological properties in otherwise non-topological materials, opening the exciting possibility of studying non-equilibrium topological transitions. Here we address what should happen in an experiment when one turns on the periodic driving. On the one hand, for infinite (translationally invariant) systems we prove a no-go theorem. We show that the Chern number is conserved under unitary evolution, i.e., it is impossible to change the topological character of the initial wavefunction. On the other hand, for systems with boundaries, we show that the properly defined topological invariant, the Bott index, can change and it is possible to dynamically prepare a topological wavefunctions starting from a non-topological one. [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z36.00009: Universal post-quench prethermalization at a quantum critical point Peter P. Orth, Pia Gagel, Joerg Schmalian We consider an open system near a quantum critical point that is suddenly moved towards the critical point. The bath-dominated diffusive non-equilibrium dynamics after the quench is shown to follow scaling behavior, governed by a critical exponent that emerges in addition to the known equilibrium critical exponents. We determine this exponent and show that it describes universal prethermalized coarsening dynamics of the order parameter in an intermediate time regime. Implications of this quantum critical prethermalization are a powerlaw rise of order and correlations after an initial collapse of the equilibrium state and a crossover to thermalization that occurs arbitrarily late for sufficiently shallow quenches. [1] P. Gagel, P. P. Orth, J. Schmalian, Phys.Rev. Lett. (in press) arXiv:1406.6387 [Preview Abstract] |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z36.00010: Does the eigenstate thermalization hypothesis hold for non-local operators? James R. Garrison, Tarun Grover The eigenstate thermalization hypothesis (ETH) posits that given a single finite energy density eigenstate of a Hamiltonian, the expectation values of certain operators will match the values they would take in the canonical ensemble. Although ETH does not hold in all systems (notable exceptions include those that are integrable or exhibit many-body localization), even in systems where it does hold it is not obvious for which class of operators it is satisfied. Here, we study a non-integrable spin model via exact diagonalization and employ some general arguments to better understand which non-local operators satisfy or fail to satisfy ETH. [Preview Abstract] |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z36.00011: Absence of Quantum Time Crystals in Ground States Haruki Watanabe, Masaki Oshikawa In analogy with crystalline solids around us, Wilczek recently proposed the idea of ``time crystals'' as phases that spontaneously break the continuous time translation into a discrete subgroup. The proposal stimulated further studies and vigorous debates whether it can be realized in a physical system. However, a precise definition of the time crystal is needed to resolve the issue. Here we first present a definition of time crystals based on the time-dependent correlation functions of the order parameter. We then prove a no-go theorem that rules out the possibility of time crystals defined as such, in the ground state of a general Hamiltonian which consists of only short-range interactions. [Preview Abstract] |
Friday, March 6, 2015 1:51PM - 2:03PM |
Z36.00012: Quantum quench with hard wall boundary conditions Garry Goldstein, Natan Andrei In this work we present analysis of a quench for the Lieb Liniger gas contained in a large box with hard wall boundary conditions. We study the time average of local correlation functions. We show that both the quench action logic and the GGE are applicable. We show that the time average of the system corresponds to an eigenstate of the Lieb Liniger Hamiltonian. We show that this eigenstate is related to an eigenstate of a Lieb Liniger Hamiltonian with periodic boundary conditions on an interval of twice the length and with twice as many particles (a doubled system). We further show that local operators with support far away from the boundaries of the hard wall Lieb Liniger gas have the same expectation values as corresponding operators for the doubled system. We present an example of a quench where the Lieb Liniger gas is initially confined in several traps and then released into a bigger container, an approximate description of the Newton cradle experiment. [Preview Abstract] |
Friday, March 6, 2015 2:03PM - 2:15PM |
Z36.00013: Imaginary-time evolution following a quantum quench between distinct symmetric phases Keola Wierschem, Ying-Jer Kao Symmetry protected topological phases are a new class of distinct symmetric phases in the presence of a protecting symmetry. An early example of a nontrivial symmetry protected topological state is the ground state of the spin-1 Heisenberg antiferromagnet--the so-called Haldane phase. The Haldane phase is distinct from the symmetric product state of zero spin projection along the $z$ axis $|{\cal D}\rangle=\prod_i|0\rangle_i$ that is adiabatically connected to the so-called large-$D$ phase. In this work, we explore the imaginary-time evolution of the state $|{\cal D}\rangle$ after a quantum quench into the Haldane phase and present details of a quantum Monte Carlo method that can easily be extended to studies in higher dimensions. [Preview Abstract] |
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