Collective modes and the speed of sound in the Fulde-Ferrell-Larkin-Ovchinnikov state

TL;DR

This paper investigates the collective excitations and anisotropic sound propagation in a spin-imbalanced ultracold Fermi gas within the FFLO state, providing potential experimental signatures of this exotic pairing phase.

Contribution

It presents the calculation of collective mode spectrum and sound speed in the FFLO state, highlighting anisotropy and damping effects in the density response.

Findings

Collective modes are damped but well-defined at zero temperature.

Sound speed is anisotropic due to FFLO pairing.

The anisotropic sound propagation can serve as an experimental signature.

Abstract

We consider the density response of a spin-imbalanced ultracold Fermi gas in an optical lattice in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. We calculate the collective mode spectrum of the system in the generalised random phase approximation and find that though the collective modes are damped even at zero tempererature, the damping is weak enough to have well-defined collective modes. We calculate the speed of sound in the gas and show that it is anisotropic due to the anisotropy of the FFLO pairing, which implies an experimental signature for the FFLO state.