Damping of Hydrodynamic Modes in a Trapped Bose Gas above the Bose-Einstein Transition Temperature

TL;DR

This paper calculates the damping of collective modes in a trapped Bose gas above the BEC transition, showing shear viscosity as the sole damping mechanism and highlighting the need for higher densities to observe hydrodynamic behavior.

Contribution

It provides a hydrodynamic analysis of damping mechanisms in a Bose gas above the transition temperature, accounting for the limitations of hydrodynamics in the outer regions.

Findings

Damping is solely due to shear viscosity.

Hydrodynamic expression diverges without cutoff.

Higher densities are needed for experimental observation.

Abstract

We calculate the damping of low-lying collective modes of a trapped Bose gas in the hydrodynamic regime, and show that this comes solely from the shear viscosity, since the contributions from bulk viscosity and thermal conduction vanish. The hydrodynamic expression for the damping diverges due to the failure of hydrodynamics in the outer parts of the cloud, and we take this into account by a physically motivated cutoff procedure. Our analysis of available experimental data indicates that higher densities than have yet been achieved are necessary for investigating hydrodynamic modes above the Bose-Einstein transition temperature.