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.