Leggett collective excitations in a two-band Fermi superfluid at finite temperatures

TL;DR

This paper investigates the properties of Leggett collective excitations in a two-band Fermi superfluid at finite temperatures, providing a nonperturbative analysis relevant for cold atomic gases and future experiments.

Contribution

It introduces a nonperturbative method to determine eigenfrequencies and damping of Leggett modes beyond low-energy approximations in two-band Fermi superfluids.

Findings

Eigenfrequencies and damping factors calculated for Leggett modes.

Analysis applicable to cold atomic gases at Feshbach resonances.

Method extends to energies near the pair-breaking continuum.

Abstract

The Leggett collective excitations for a two-band Fermi gas with s-wave pairing and Josephson interband coupling in the BCS-BEC crossover at finite temperatures are investigated within the Gaussian pair fluctuation approach. Eigenfrequencies and damping factors for Leggett modes are determined in a nonperturbative way, using the analytic continuation of the fluctuation propagator through a branch cut in the complex frequency plane, as in Phys. Rev. Lett. 122, 093403 (2019). The treatment is performed beyond the low-energy expansion, which is necessary when the collective excitation energy reaches the pair-breaking continuum edge. The results are applied in particular to cold atomic gases at the orbital Feshbach resonance and in a regime far from BEC, which can be relevant for future experiments.