Collective modes and ballistic expansion of a Fermi gas in the BCS-BEC crossover

TL;DR

This paper investigates the collective oscillations and expansion dynamics of a Fermi superfluid across the BCS-BEC crossover, providing theoretical predictions that align with experimental observations.

Contribution

It introduces a microscopic mean-field approach to compute collective mode frequencies and expansion rates across the crossover, confirming non-monotonic behavior.

Findings

Quantitative agreement with experimental measurements of breathing modes.

Confirmation of non-monotonic behavior in collective mode frequencies.

Validation of the mean-field scaling ansatz for superfluid dynamics.

Abstract

We evaluate the frequencies of collective modes and the anisotropic expansion rate of a harmonically trapped Fermi superfluid at varying coupling strengths across a Feshbach resonance driving a BCS-BEC crossover. The equations of motion for the superfluid are obtained from a microscopic mean-field expression for the compressibility and are solved within a scaling ansatz. Our results confirm non-monotonic behavior in the crossover region and are in quantitative agreement with current measurements of the transverse breathing mode by Kinast {\it et al.} [Phys. Rev. Lett. {\bf 92}, 150402 (2004)] and of the axial breathing mode by Bartenstein {\it et al.} [Phys. Rev. Lett. {\bf 92}, 203201 (2004)].