Collisionless sound in a uniform two-dimensional Bose gas

TL;DR

This paper demonstrates that sound can propagate in a collisionless 2D Bose gas due to mean-field interactions, supported by theoretical analysis and numerical simulations, aligning with recent experimental findings.

Contribution

It reveals the existence of collisionless sound in a 2D Bose gas and highlights the role of Landau damping, supported by both theoretical and numerical approaches.

Findings

Collisionless sound propagates in 2D Bose gases due to mean-field effects.

Landau damping significantly influences sound propagation.

Numerical simulations agree with experimental observations.

Abstract

Using linear response theory within the Random Phase Approximation, we investigate the propagation of sound in a uniform two dimensional (2D) Bose gas in the collisionless regime. We show that the sudden removal of a static density perturbation produces a damped oscillatory behavior revealing that sound can propagate also in the absence of collisions, due to mean-field interaction effects. Our analysis points out the crucial role played by Landau damping. We support our predictions by performing numerical simulations with the stochastic (projected) Gross-Pitaevskii equation. The results are consistent with the recent experimental observation of sound in a weakly interacting 2D Bose gas both below and above the superfluid Berezinskii-Kosterlitz-Thouless transition.