Elliptic flow in a strongly interacting normal Bose gas

Abstract

We study the anisotropic, elliptic expansion of a thermal atomic Bose gas released from an anisotropic trapping potential, for a wide range of interaction strengths across a Feshbach resonance. We show that in our system this hydrodynamic phenomenon is for all interaction strengths fully described by a microscopic kinetic model with no free parameters. The success of this description crucially relies on taking into account the reduced thermalising power of elastic collisions in a strongly interacting gas, for which we derive an analytical theory. We also perform time-resolved measurements that directly reveal the dynamics of the energy transfer between the different expansion axes.

Universal prethermal dynamics of Bose gases quenched to unitarity

“…In particular, they give access to the unitary regime where the interactions are as strong as allowed by quantum mechanics. Following years of experiments on unitary Fermi gases [1,2], unitary Bose gases have recently emerged as a new experimental frontier [3][4][5][6][7][8][9][10][11]. They promise exciting new possibilities [12], including universal physics solely controlled by the gas density [13,14] and novel forms of superfluidity [15][16][17].…”

How many particles do make a fluid? Qualifying collective behavior in expanding ultracold gases

“…The disappearance of elliptic flow is observed for such a scenario, due to the fact that the critical velocity for the breakdown of superfluidity decreases with decreasing interaction strength, leading eventually to a ballistic expansion. Subsequent experiments with expanding ultracold gases have investigated, among others, elliptic flow for both repulsive and attractive interactions [35,36], in the expansion of Fermi-Fermi mixtures [37], as a probe of viscous corrections [38,39,40], in the expansion of normal Bose systems [41,42,43] and dipolar gases [44]. Of particular relevance for our analysis is the study by Fletcher et al [43].…”

“…Subsequent experiments with expanding ultracold gases have investigated, among others, elliptic flow for both repulsive and attractive interactions [35,36], in the expansion of Fermi-Fermi mixtures [37], as a probe of viscous corrections [38,39,40], in the expansion of normal Bose systems [41,42,43] and dipolar gases [44]. Of particular relevance for our analysis is the study by Fletcher et al [43]. This experiment analyzes elliptic flow in an expanding cloud of 39 K atoms for different choices of the interaction strength.…”

Observing the loss and revival of long-range phase coherence through disorder quenches

“…Coherent hydrodynamics originates from the existence of a macroscopic wave function, hence long-range coherence, and facilitates collective behavior such as quadrupole excitations. By contrast, the collisional hydrodynamic behavior in nondegenerate systems with strong interactions ( 35 , 36 ), such as unitary gases, is caused by frequent scattering events during expansion and is therefore not connected to a macroscopic wave function.…”