Collisions and expansion of an ultracold dilute Fermi gas

TL;DR

This paper investigates how collisions influence the expansion dynamics of an ultracold dilute Fermi gas, revealing that even at zero temperature, collisions can significantly affect the density distribution and entropy during expansion.

Contribution

It introduces a Boltzmann equation approach to quantify collisional effects on the expansion and entropy of a degenerate Fermi gas, highlighting conditions where collisions are impactful.

Findings

Aspect ratio remains close to ballistic predictions in dilute gases.

Thermal broadening due to entropy increase can be significant in elongated traps.

Collisions are effective even at zero temperature in a Fermi gas.

Abstract

We discuss the effects of collisions on the expansion of a degenerate normal Fermi gas, following the sudden removal of the confining trap. Using a Boltzmann equation approach, we calculate the time dependence of the aspect ratio and the entropy increase of the expanding atomic cloud taking into account the collisional effects due to the deformation of the distribution function in momentum space. We find that in dilute gases the aspect ratio does not deviate significantly from the predictions of ballistic expansion. Conversely, if the trap is sufficiently elongated the thermal broadening of the density distribution due to the entropy increase can be sizeable, revealing that even at zero temperature collisions are effective in a Fermi gas.