Shape oscillations in non-degenerate Bose gases - transition from the collisionless to the hydrodynamic regime

TL;DR

This study explores how collective oscillations in non-degenerate Bose gases transition from collisionless to hydrodynamic behavior as density increases, providing experimental data that aligns well with theoretical predictions.

Contribution

It experimentally investigates the crossover from collisionless to hydrodynamic regimes in Bose gases and analyzes the effects of trap anharmonicities on oscillation frequencies.

Findings

Good agreement between experimental results and theory.

Identification of the density at which the transition occurs.

Quantitative expressions for trap anharmonicity effects.

Abstract

We investigate collective oscillations of non-degenerate clouds of Rb-87 atoms as a function of density in an elongated magnetic trap. For the low-lying M=0 monopole-quadrupole shape oscillation we measure the oscillation frequencies and damping rates. At the highest densities the mean-free-path is smaller than the axial dimension of the sample, which corresponds to collisionally hydrodynamic conditions. This allows us to cover the cross-over from the collisionless to the hydrodynamic regime. The experimental results show good agreement with theory. We also analyze the influence of trap anharmonicities on the oscillations in relation to observed temperature dependencies of the dipole and quadrupole oscillation frequencies. We present convenient expressions to quantify these effects.