Superfluid Density in the BCS-BEC Crossover Regime of a Two-Component Fermi Gas at Finite Temperatures

We investigate the superfluid carrier density ρs in the BCS-BEC crossover regime of a superfluid Fermi gas at finite temperatures. Including pairing fluctuations within the gaussian fluctuation approximation, we determine the superfluid order parameter Δ, Fermi chemical potential μ, as well as ρs, in a consistent manner. In the weak-coupling regime, the normal fluid density ρn≡ρ−ρs is dominated by quasi-particle excitations associated with the dissociation of Cooper pairs. As one approaches the BCS-BEC crossover regime, effects of pairing fluctuations on ρn become strong. In the strong-coupling BEC regime, ρn is shown to be dominated by Bogoliubov phonon in a molecular Bose superfluid.