Superfluidity and collective modes in a uniform gas of Fermi atoms with a Feshbach resonance

Abstract

We investigate strong-coupling superfluidity in a uniform gas of Fermi atoms attractively interacting via quasi-molecular bosons associated with a Feshbach resonance. This interaction is tunable by the threshold energy $2ν$ of the Feshbach resonance, becoming large as $2ν$ is decreased. In recent work, we showed that the enhancement of this tunable pairing interaction naturally leads to the BCS-BEC crossover, where the character of the superfluid phase transition changes from the BCS-type to a BEC of composite bosons consisting of preformed Cooper-pairs and Feshbach-induced molecules. In this paper, we extend our previous work and study both single quasi-particles and the collective dynamics of the superfluid phase below Tc. We show how the superfluid order parameter changes from the Cooper-pair amplitude to the square root of the number of condensed molecules associated with the Feshbach resonance, as the threshold energy $2ν$ is lowered. We also discuss the Goldstone mode associated with superfluidity, and show how its character smoothly changes from the Anderson-Bogoliubov phonon in the BCS regime to the Bogoliubov phonon in the BEC regime in the BCS-BEC crossover. This Goldstone mode is shown to appear as a resonance in the spectrum of the density-density correlation function, which is experimentally accessible.