Pair correlations of an expanding superfluid Fermi gas

TL;DR

This paper studies the pair correlation function of an expanding superfluid Fermi gas across the BEC-BCS crossover, revealing interaction-induced bunching effects and proposing a local equilibrium model predicting isotropic correlations during expansion.

Contribution

It introduces a local equilibrium ansatz for the pair correlation function that remains isotropic during expansion, highlighting the impact of interactions on scaling behavior.

Findings

Interaction causes a sizable bunching effect at unitarity.

The pair correlation function remains isotropic during expansion despite anisotropic trapping.

Interactions violate the scaling of the pair correlation function.

Abstract

The pair correlation function of an expanding gas is investigated with an emphasis on the BEC-BCS crossover of a superfluid Fermi gas at zero temperature. At unitarity quantum Monte Carlo simulations reveal the occurrence of a sizable bunching effect due to interactions in the spin up-down channel which, at short distances, is larger than that exhibited by thermal bosons in the Hanbury-Brown and Twiss effect. We propose a local equilibrium ansatz for the pair correlation function which we predict will remain isotropic during the expansion even if the trapping potential is anisotropic, contrary to what happens for non-interacting gases. This behavior is understood to be a consequence of the violation of scaling of the pair correlation function due to interactions.