Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Y50: Anomalous Transport of Low-Dimensional SystemsRecordings Available
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Sponsoring Units: DCMP Chair: Pieter Claeys, University of Cambridge Room: McCormick Place W-474A |
Friday, March 18, 2022 8:00AM - 8:12AM |
Y50.00001: Anomalous diffusion in nonlinear sigma models Javier Lopez-Piqueres, Romain Vasseur Recent studies of integrable systems that are invariant under a continuous nonabelian global symmetry have shown anomalous finite-temperature transport with dynamical exponent z=3/2. The origin of this superdiffusive behavior is still unclear, despite the fact that a self-consistent framework based on the spreading of long-lived giant quasiparticles dressed by thermal fluctuations has been put forward. In this work, we study the fate of superdiffusive transport in the paradigmatic O(N) nonlinear sigma model using a combination of field theoretic methods and the toolbox of integrability. |
Friday, March 18, 2022 8:12AM - 8:24AM |
Y50.00002: Subdiffusive hydrodynamics of nearly-integrable anisotropic spin chains Brayden A Ware, Jacopo De Nardis, Sarang Gopalakrishnan, Romain Vasseur We address spin transport in the easy-axis Heisenberg spin chain subject to integrability-breaking perturbations. We find that spin transport is subdiffusive with dynamical exponent z=4 up to a timescale that is parametrically long in the anisotropy. In the limit of infinite anisotropy, transport is subdiffusive at all times; for large finite anisotropy, one eventually recovers diffusion at late times, but with a diffusion constant independent of the strength of the integrability breaking perturbation. We provide numerical evidence for these findings, and explain them by adapting the generalized hydrodynamics framework to nearly integrable dynamics. Our results show that the diffusion constant of near-integrable interacting spin chains is generically not perturbative in the integrability breaking strength. |
Friday, March 18, 2022 8:24AM - 8:36AM |
Y50.00003: Universal KPZ dynamics in integrable quantum systems Bingtian Ye, Francisco Machado, Jack Kemp, Norman Y Yao A broad class of integrable spin chains with a non-Abelian symmetry have recently been proven to exhibit anomalous, superdiffusive transport with a dynamical exponent of 3/2. In the specific context of the integrable, SU(2)-symmetric Heisenberg spin-half chain, both numerics and experiments, have recently shown that these superdiffusive dynamics fall into the Kardar–Parisi–Zhang (KPZ) universality class. Leveraging a novel numerical technique, termed density matrix truncation, we combine and generalize these previous results to show that KPZ transport occurs not only in non-Abelian symmetric integrable models, but also in their periodically-driven and supersymmetric counterparts. Moreover, by analyzing symmetry-breaking perturbations, we observe direct evidence for the purported microscopic mechanism underlying this anomalous transport behavior. |
Friday, March 18, 2022 8:36AM - 8:48AM |
Y50.00004: Spin and energy transport in a solid-state quantum simulator Pai Peng, Bingtian Ye, Norman Y Yao, Paola Cappellaro Characterizing the emergence of classical hydrodynamics from strongly correlated quantum systems remains an important experimental challenge. Nuclear spins in solids provide an intriguing platform to study such emergent hydrodynamics; in particular, the ability to use Floquet engineering to tune the effective Hamiltonian can allow for the exploration of different hydrodynamical behaviors using nuclear magnetic resonance techniques. However, a remaining challenge in such systems is to achieve local measurement with collective RF controls, as needed to characterize hydrodynamics. Here, we introduce a novel technique that exploits intrinsic disorder in such systems to enable the measurement of local autocorrelation functions with only global control. With this toolset, we measure both the magnetization and energy transport of nuclear spins in fluorapatite. By tuning the effective Hamiltonian, we demonstrate that the spin transport can be tuned between ballistic and diffusive behavior, while the energy transport remains ballistic throughout the time-scales measured in our experiment. Our work opens the door to studying interacting integrable systems using nuclear magnetic resonance. |
Friday, March 18, 2022 8:48AM - 9:00AM |
Y50.00005: Equilibration time in many-body quantum systems Talía Lezama Mergold Love, E. Jonathan Torres-Herrera, Francisco Pérez-Bernal, Yevgeny Bar Lev, Lea F Santos Isolated many-body quantum systems quenched far from equilibrium can eventually equilibrate, but it is not yet clear how long they take to do so. To answer this question, we use exact numerical methods and analyze the entire evolution, from perturbation to equilibration, of a paradigmatic disordered many-body quantum system in the chaotic regime and as we approach the many-body localization transition. We investigate how the equilibration time depends on the system size and observables. We show that if dynamical manifestations of spectral correlations in the form of the correlation hole ("ramp") are taken into account, the time for equilibration scales exponentially with system size, while if they are neglected, the scaling is better described by a power law with system size, although with an exponent larger than what is expected for diffusive transport. |
Friday, March 18, 2022 9:00AM - 9:12AM |
Y50.00006: Searching for anomalous spin transport in the disordered XXZ Heisenberg chain Izabella Lovas, Tibor Rakovszky, Curt von Keyserlingk, Frank Pollmann We study the spin transport in the ergodic phase of the disordered XXZ Heisenberg chain, an archetypal model for many-body localization proposed to show subdiffusive transport. We analyze the statistics of the spin autocorrelation function by applying the dissipation-assisted operator evolution method, granting access to large system sizes. We find that our results lie on the boundary between diffusion and weak subdiffusion. We gain more insight into the quantum data by considering a classical single particle random walk, displaying a tunable subdiffusive behavior. Our analytical and numerical results for this classical model reveal three distinct transport regimes, characterized by subiffusive behavior, by diffusion with anomalous power law fluctuations in the statistics of the return probability, and by standard diffusion, respectively. By comparing to the behavior of the disordered Heisenberg chain, we find that the distribution of the spin autocorrelation function shows a power law tail, consistent with weak subdiffusion or diffusion with anomalous fluctuations. Our results highlight the challenges in detecting anomalous transport from finite size and finite time quantum simulations. |
Friday, March 18, 2022 9:12AM - 9:24AM Withdrawn |
Y50.00007: Confinement induced impurity states in spin chains Joseph W Vovrosh, Johannes Knolle, Alvise Bastianello, hongzheng zhao Quantum simulators hold the promise of probing central questions of high-energy physics in tunable condensed matter platforms, for instance the physics of confinement. Local defects can be an obstacle in these setups harming their simulation capabilities. However, defects in the form of impurities can also be useful as probes of many-body correlations and may lead to fascinating new phenomena themselves. Here, we investigate the interplay between impurity and confinement physics in a basic spin chain setup, showing the emergence of new exotic excitations as impurity-meson bound states with a long lifetime. For weak confinement, semiclassical approximations can describe the capture process in a meson-impurity scattering event. In the strong-confining regime, intrinsic quantum effects are visible through the quantization of the emergent bound state energies which can be readily probed in quantum simulators. |
Friday, March 18, 2022 9:24AM - 9:36AM |
Y50.00008: Dephasing induced steady state refrigeration in the Fibonacci chain Cecilia Chiaracane, Archak Purkayastha, John Goold Understanding quantum transport in low dimensional systems has become of obvious relevance for the design of mesoscopic thermoelectrics. Quasiperiodic disorder, incommensurate with the underlying periodicity of the lattice, induces fractality in the energy spectra and non-trivial transport properties even in one dimension, which can be exploited in the context of quantum heat engines. As an example, the Fibonacci model exhibits critical states and anomalous diffusion, with a continuously varying dynamical exponent. When dephasing is introduced, it always renders transport standard diffusive, however, the competition with quasiperiodicity gives rise to a maximum in the diffusion constant at finite dephasing. We study electric and heat currents in the Fibonacci model at finite temperature in the Landauer-Büttiker framework, with dephasing incorporated through the action of Büttiker probes. We observe a remarkably different sensitivity of the position of the maxima of thermal and electric conductivities to the thermodynamic parameters of the set-up. We show that by tuning the strength of the dephasing we can enhance the performance of the device in regimes where it acts as refrigerator. |
Friday, March 18, 2022 9:36AM - 9:48AM |
Y50.00009: Absence of superdiffusion in certain random spin models Pieter W Claeys, Austen Lamacraft, Jonah Herzog-Arbeitman The dynamics of spin at finite temperature in the spin-1/2 Heisenberg chain was found to be superdiffusive in numerous recent numerical and experimental studies. Theoretical approaches to this problem have emphasized the role of nonabelian SU(2) symmetry as well as integrability, but the associated methods cannot be readily applied when integrability is broken. Here I will examine spin transport in such a spin-1/2 chain in which the exchange couplings fluctuate in space and time, breaking integrability but not spin symmetry, showing that operator dynamics in the strong noise limit can be analyzed using conventional perturbation theory. I will argue that the spin dynamics undergo enhanced diffusion with some interesting transient behavior rather than superdiffusion, comparing the dynamics with both a hydrodynamic approach and tensor network simulations. |
Friday, March 18, 2022 9:48AM - 10:00AM |
Y50.00010: Dephasing enhanced transport in boundary-driven quasiperiodic chains Artur Machado Lacerda, Gabriel Landi, John Goold We study dephasing-enhanced transport in boundary-driven quasi-periodic systems. Specifically we consider dephasing modelled by current preserving Lindblad dissipators acting on the non-interacting Aubry-André-Harper (AAH) and Fibonacci bulk systems. The former is known to undergo a critical localization transition with a suppression of ballistic transport above a critical value of the potential. At the critical point, the presence of non-ergodic extended states yields anomalous sub-diffusion. The Fibonacci model, on the other hand, yields anomalous transport with a continuously varying exponent depending on the potential strength. By computing the covariance matrix in the non-equilibrium steady-state, we show that sufficiently strong dephasing always renders the transport diffusive. The interplay between dephasing and quasi-periodicity gives rise to a maximum of the diffusion coefficient for finite dephasing, which suggests the combination of quasi-periodic geometries and dephasing can be used to control noise-enhanced transport. |
Friday, March 18, 2022 10:00AM - 10:12AM |
Y50.00011: Breaking of Huygens-Fresnel principle in inhomogeneous Tomonaga-Luttinger liquids Marek Gluza, Per Moosavi, Spyros Sotiriadis Tomonaga-Luttinger liquids (TLLs) can be used to effectively describe one-dimensional quantum many-body systems such as ultracold atoms, charges in nanowires, superconducting circuits, and gapless spin chains. Their properties are given by two parameters, the propagation velocity and the Luttinger parameter. Here we study inhomogeneous TLLs where these are promoted to functions of position and demonstrate that they profoundly affect the dynamics: In general, besides curving the light cone, we show that propagation is no longer ballistically localized to the light-cone trajectories, different from standard homogeneous TLLs. Specifically, if the Luttinger parameter depends on position, the dynamics features pronounced spreading into the light cone, which cannot be understood via a simple superposition of waves as in the Huygens-Fresnel principle. This is the case for ultracold atoms in a parabolic trap, which serves as our main motivation, and we discuss possible experimental observations in such systems. |
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