### Abstract

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 Nozières and Schmitt-Rink, we use this definition of ρs 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 Δ, the chemical potential μ, and ρs all include the effect of fluctuations at the same level in a self-consistent manner. We show that the normal fluid density ρn n- ρs naturally separates into a sum of contributions from Fermi BCS quasiparticles (ρnF) and Bose collective modes (ρnB). The expression for ρnF is just Landau's formula for a BCS Fermi superfluid but now calculated over the BCS-BEC crossover. The expression for the Bose contribution ρnB 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 ρs using an expression equivalent to the one derived in this paper, over the BCS-BEC crossover, including unitarity, and at finite temperatures.

Original language | English |
---|---|

Article number | 063626 |

Journal | Physical Review A - Atomic, Molecular, and Optical Physics |

Volume | 74 |

Issue number | 6 |

DOIs | |

Publication status | Published - 2006 |

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### ASJC Scopus subject areas

- Atomic and Molecular Physics, and Optics
- Physics and Astronomy(all)

### Cite this

*Physical Review A - Atomic, Molecular, and Optical Physics*,

*74*(6), [063626]. https://doi.org/10.1103/PhysRevA.74.063626

**Pairing fluctuations and the superfluid density through the BCS-BEC crossover.** / Taylor, E.; Griffin, A.; Fukushima, N.; Ohashi, Yoji.

Research output: Contribution to journal › Article

*Physical Review A - Atomic, Molecular, and Optical Physics*, vol. 74, no. 6, 063626. https://doi.org/10.1103/PhysRevA.74.063626

}

TY - JOUR

T1 - Pairing fluctuations and the superfluid density through the BCS-BEC crossover

AU - Taylor, E.

AU - Griffin, A.

AU - Fukushima, N.

AU - Ohashi, Yoji

PY - 2006

Y1 - 2006

N2 - 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 Nozières and Schmitt-Rink, we use this definition of ρs 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 Δ, the chemical potential μ, and ρs all include the effect of fluctuations at the same level in a self-consistent manner. We show that the normal fluid density ρn n- ρs naturally separates into a sum of contributions from Fermi BCS quasiparticles (ρnF) and Bose collective modes (ρnB). The expression for ρnF is just Landau's formula for a BCS Fermi superfluid but now calculated over the BCS-BEC crossover. The expression for the Bose contribution ρnB 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 ρs using an expression equivalent to the one derived in this paper, over the BCS-BEC crossover, including unitarity, and at finite temperatures.

AB - 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 Nozières and Schmitt-Rink, we use this definition of ρs 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 Δ, the chemical potential μ, and ρs all include the effect of fluctuations at the same level in a self-consistent manner. We show that the normal fluid density ρn n- ρs naturally separates into a sum of contributions from Fermi BCS quasiparticles (ρnF) and Bose collective modes (ρnB). The expression for ρnF is just Landau's formula for a BCS Fermi superfluid but now calculated over the BCS-BEC crossover. The expression for the Bose contribution ρnB 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 ρs using an expression equivalent to the one derived in this paper, over the BCS-BEC crossover, including unitarity, and at finite temperatures.

UR - http://www.scopus.com/inward/record.url?scp=33846384237&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=33846384237&partnerID=8YFLogxK

U2 - 10.1103/PhysRevA.74.063626

DO - 10.1103/PhysRevA.74.063626

M3 - Article

AN - SCOPUS:33846384237

VL - 74

JO - Physical Review A

JF - Physical Review A

SN - 2469-9926

IS - 6

M1 - 063626

ER -