Collective excitations of a charged Fermi superfluid in the BCS-BEC crossover

TL;DR

This paper investigates the collective excitations in a charged Fermi superfluid across the BCS-BEC crossover, revealing interactions between plasma oscillations, Higgs modes, and phonon-like excitations, with implications for experimental detection.

Contribution

It develops a unified Gaussian fluctuation formalism combining path integral methods for superfluid Fermi gases and Coulomb gases, describing all collective excitation branches in charged superfluids.

Findings

Resonant avoided crossing of modes observed

Enhanced response due to pair-breaking mode interaction with plasma oscillations

Spectral functions and propagator poles used to determine excitation spectra

Abstract

We consider collective excitations in the superfluid state of Fermi condensed charged gases. The dispersion and damping of collective excitations at nonzero temperatures are examined, and the coexistence and interaction of different branches of collective excitations: plasma oscillations, pair-breaking Higgs modes, and Carlson-Goldman phonon-like excitations are taken into account. The path integral methods for superfluid Fermi gases and for Coulomb gas are combined into a unified formalism that extends the Gaussian fluctuation approximation to account for plasmonic modes. This approximation of Gaussian pair and density fluctuations is able to describe all branches of collective excitations existing in a charged superfluid. The spectra of collective excitations are determined in two ways: from the spectral functions and from the complex poles of the fluctuation propagator. A resonant avoided crossing of different modes is shown. It is accompanied by resonant enhancement of the response provided by the pair-breaking modes due to their interaction with plasma oscillations. This may facilitate the experimental observation of the pair-breaking modes.