Comment on "Collective excitations of a degenerate gas at the BEC-BCS crossover"

TL;DR

This paper discusses recent experimental observations of collective excitations in ultracold lithium gases across the BEC-BCS crossover, highlighting how axial modes reveal the equation of state and addressing unexpected radial mode results.

Contribution

It clarifies the interpretation of collective mode measurements in strongly interacting Fermi gases and emphasizes the relevance of hydrodynamics near unitarity.

Findings

Axial mode measurements provide direct access to the equation of state.

Radial mode results are consistent with superfluid hydrodynamics.

Experimental data near unitarity offers insights into strongly interacting quantum gases.

Abstract

Very recent experiments have studied for the first time collective excitations of an ultracold $^6$Li gas covering in particular the BEC-BCS crossover domain. We point out that the results for the axial mode, through hydrodynamics, give direct access to the (3D) equation of state of the strongly interacting gas, mostly near the unitarity limit. On the other hand the surprising results found for the radial mode are actually not necessarily in contradiction with the expectations from superfluid hydrodynamics.