Propagation of Second sound in a superfluid Fermi gas in the unitary limit

TL;DR

This paper investigates the propagation of second sound in a superfluid Fermi gas at unitarity, demonstrating that density perturbations can effectively excite second sound, which signals superfluidity.

Contribution

It applies Landau's two-fluid hydrodynamics and NSR theory to calculate second sound velocities and response amplitudes in a unitary Fermi gas, highlighting the excitation of second sound by density pulses.

Findings

Second sound exists in the unitary Fermi gas.

Density perturbations can excite second sound with appreciable amplitude.

The response function analysis confirms the detectability of second sound signals.

Abstract

We study sound propagation in a uniform superfluid gas of Fermi atoms in the unitary limit. The existence of normal and superfluid components leads to appearance of two sound modes in the collisional regime, referred to as first and second sound. The second sound is of particular interest as it is a clear signal of a superfluid component. Using Landau's two-fluid hydrodynamic theory, we calculate hydrodynamic sound velocities and these weights in the density response function. The latter is used to calculate the response to a sudden modification of the external potential generating pulse propagation. The amplitude of a pulse which is proportional to the weight in the response function, is calculated the basis of the approach of Nozieres and Schmitt-Rink (NSR) for the BCS-BEC crossover. We show that, in a superfluid Fermi gas at unitarity, the second sound pulse is excited with an appreciate amplitude by density perturbations.