Hydrodynamics and universality in cold atomic gases

S. A. Simmons

Manas Kulkarni, Alexander G. Abanov

Dynamics of interacting cold atomic gases have recently become a focus of both experimental and theoretical studies. Often cold-atom systems show hydrodynamic behavior and support the propagation of nonlinear dispersive waves. Although this propagation depends on many details of the system, great insight can be obtained in the rather universal limit of weak nonlinearity, dispersion, and dissipation. In this limit, using a reductive perturbation method we map some of the hydrodynamic models relevant to cold atoms to well-known chiral one-dimensional equations such as the Korteweg--de Vries (KdV), Burgers, KdV-Burgers, and Benjamin-Ono equations. These equations have been thoroughly studied in the literature. The mapping gives us a simple way to make estimates for the original hydrodynamic equations and to study the interplay between nonlinearity, dissipation, and dispersion which are the hallmarks of nonlinear hydrodynamics.

K. Li, Hong-Fang Song, H. Xu +2 more

Azimuthal anisotropy has been ubiquitously observed in high-energy nuclear (heavy-ion) collisions, perceived to require hydrodynamic interactions. This work reports a study of anisotropic expansion of cold 6 Li Fermi gases, released from anisotropic potential traps, under tunable interaction strength by a magnetic field. A universal scaling of the momentum anisotropy response to geometry is observed for the first time between cold-atom and heavy-ion systems as a function of opacity—the average number of collisions per particle ( n coll ), despite their vast differences in scale and physics. The anisotropy response increases quickly at small opacity, without the need of a large amount of interactions, and shows no sign of saturation in the observed range, with an approximate power-law dependence of n coll , characteristic of random walks. This universality potentially unifies a variety of vastly different physical systems, from dilute atomic gases to the hot and dense quark-gluon plasma of the early universe.

Jeff Leiberton, Marcos Rigol

Hydrodynamization refers to the remarkably rapid process in relativistic heavy-ion collisions by which hydrodynamic descriptions become applicable. Following the observation of analogous behavior in ultracold one-dimensional (1D) Bose gases, hydrodynamization has been conjectured to be a universal dynamical phenomenon in quantum systems following high-energy quenches. Theoretical studies in this cold-atom setting have so far been restricted to quenches from ground states. Here we study how nonzero temperatures affect hydrodynamization. Specifically, using a homogeneous 1D gas of hard-core bosons, we explore how the initial temperature affects the timescales associated with hydrodynamization and prethermalization following a Bragg-pulse quench. We find that while the hydrodynamization coherence time remains unchanged, increasing temperature shortens both the damping time of the hydrodynamization oscillations and the prethermalization time. We argue that this is mainly the result of the broadening of the initial rapidity distribution, and introduce a nonzero-temperature dephasing time defined in terms of the extent of the rapidity distribution.

J. Petricka

V. V. Flambaum, Evgeny A. Kuznetsov