Probing two-fluid hydrodynamics in a trapped Fermi superfluid at unitarity

TL;DR

This paper develops a variational method to calculate the density response of a trapped Fermi superfluid near unitarity, proposing a way to observe second sound-like modes via Bragg scattering to confirm superfluidity.

Contribution

It introduces a variational approach to analyze two-fluid modes in trapped Fermi superfluids and suggests experimental detection of second sound modes at unitarity.

Findings

Out-of-phase modes are observable at finite temperatures.

Two-photon Bragg scattering spectrum relates to the density response.

Provides a method to test superfluid thermodynamics at unitarity.

Abstract

We develop a variational approach to calculate the density response function at finite temperatures of the lowest-lying two-fluid modes in a trapped two-component Fermi superfluid close to a Feshbach resonance. The out-of-phase oscillations, which are the analogue in trapped gases of second sound in uniform superfluids, have so far not been observed in cold-atom experiments. At unitarity, we show that these modes are observable at finite temperatures via two-photon Bragg scattering, whose spectrum is related to the imaginary part of density response function. This provides direct evidence for superfluidity and a promising way to test microscopic results for thermodynamics at unitarity.