Collective modes of an imbalanced unitary Fermi gas

TL;DR

This paper theoretically investigates the collective breathing modes of an imbalanced unitary Fermi gas in a trap, showing good agreement with experiments and exploring the transition between hydrodynamic and collisionless regimes.

Contribution

It introduces a kinetic equation approach to describe collective modes in an imbalanced Fermi gas, accounting for collisions and polaron effects, bridging hydrodynamic and collisionless behaviors.

Findings

Good quantitative agreement with experimental measurements of mode frequencies and damping.

Identification of a strongly damped out-of-phase mode at finite temperature.

Interpolation between hydrodynamic and collisionless regimes as polarization varies.

Abstract

We study theoretically the collective mode spectrum of a strongly imbalanced two-component unitary Fermi gas in a cigar-shaped trap, where the minority species forms a gas of polarons. We describe the collective breathing mode of the gas in terms of the Fermi liquid kinetic equation taking collisions into account using the method of moments. Our results for the frequency and damping of the longitudinal in-phase breathing mode are in good quantitative agreement with an experiment by Nascimb\`ene et al. [Phys. Rev. Lett. 103, 170402 (2009)] and interpolate between a hydrodynamic and a collisionless regime as the polarization is increased. A separate out-of phase breathing mode, which for a collisionless gas is sensitive to the effective mass of the polaron, however, is strongly damped at finite temperature, whereas the experiment observes a well-defined oscillation.