Gaussian fluctuations in the two-dimensional BCS-BEC crossover: finite temperature properties

TL;DR

This paper theoretically investigates Gaussian fluctuations in a two-dimensional ultracold Fermi gas across the BCS-BEC crossover, emphasizing their impact on thermodynamics, sound velocity, superfluid density, and the Berezinskii-Kosterlitz-Thouless transition.

Contribution

It provides a detailed theoretical analysis of Gaussian fluctuations in 2D Fermi gases, highlighting their importance in the strong-coupling limit and finite temperature superfluid properties.

Findings

Gaussian fluctuations are essential for accurate equation of state in strong coupling.

The theory aligns with experimental data on 2D Fermi gases.

Superfluid density and BKT transition temperature are characterized across regimes.

Abstract

The role of fluctuations is enhanced in lower dimensionality systems: in a two dimensions off-diagonal long-range order is destroyed by the fluctuations at any finite temperature, drastically modifying the critical properties with respect to the three-dimensional counterpart. Recently two-dimensional systems of interacting fermions have been the subject of Montecarlo studies and experimental investigations, in particular an ultracold gas of attractive fermions with a widely tunable interaction due to a Feshbach resonance has been realized and the Berezinskii-Kosterlitz-Thouless transition has been observed. The present work deals with the theoretical description of an ultracold Fermi gas: we discuss the role of Gaussian fluctuations of the order parameter in the equation of state, in particular we take into account the first sound velocity, showing that the inclusion of order parameter fluctuations is needed in order to get the correct composite-boson limit in the strong-coupling regime. The theory is also compared with experimental data. Finally we focus on the superfluid density in the weak-coupling, intermediate and strong-coupling regimes at finite temperature, through which the Berezinskii-Kosterlitz-Thouless critical temperature is obtained.