Physics – Condensed Matter – Other Condensed Matter
Scientific paper
2006-09-08
Phys. Rev. A 74, 063626 (2006)
Physics
Condensed Matter
Other Condensed Matter
30 pages
Scientific paper
10.1103/PhysRevA.74.063626
We derive an expression for the superfluid density of a uniform two-component Fermi gas through the BCS-BEC crossover in terms of the thermodynamic potential in the presence of an imposed superfluid flow. Treating the pairing fluctuations in a Gaussian approximation following the approach of Nozi\`eres and Schmitt-Rink, we use this definition of $\rho_s$ to obtain an explicit result which is valid at finite temperatures and over the full BCS-BEC crossover. It is crucial that the BCS gap $\Delta$, the chemical potential $\mu$, and $\rho_s$ all include the effect of fluctuations at the same level in a self-consistent manner. We show that the normal fluid density $\rho_n \equiv n - \rho_s$ naturally separates into a sum of contributions from Fermi BCS quasiparticles ($\rho^F_{n}$) and Bose collective modes ($\rho^B_{n}$). The expression for $\rho^F_{n}$ is just Landau's formula for a BCS Fermi superfluid but now calculated over the BCS-BEC crossover. The expression for the Bose contribution $\rho^B_{n}$ is more complicated and only reduces to Landau's formula for a Bose superfluid in the extreme BEC limit, where all the fermions have formed stable Bose pairs and the Bogoliubov excitations of the associated molecular Bose condensate are undamped. In a companion paper, we present numerical calculations of $\rho_s$ using an expression equivalent to the one derived in this paper, over the BCS-BEC crossover, including unitarity, and at finite temperatures.
Fukushima Noboru
Griffin Allan
Ohashi Yoji
Taylor Edward
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