Sound modes at the BCS-BEC crossover

TL;DR

This paper calculates first and second sound speeds in a superfluid Fermi gas across the BCS-BEC crossover, revealing how these modes depend on temperature, density, and interaction strength, and highlighting their experimental detectability.

Contribution

It provides a comprehensive calculation of sound modes throughout the BCS-BEC crossover, including the effects of mode mixing and avoided crossing, which was not thoroughly explored before.

Findings

Second sound speed signals superfluidity and determines critical temperature.

First and second sound undergo avoided crossing on the BEC side.

Sound modes are detectable as density and thermal waves in traps.

Abstract

First and second sound speeds are calculated for a uniform superfluid gas of fermi atoms as a function of temperature, density and interaction strength. The second sound speed is of particular interest as it is a clear signal of a superfluid component and it determines the critical temperature. The sound modes and their dependence on density, scattering length and temperature are calculated in the BCS, molecular BEC and unitarity limits and a smooth crossover is extrapolated. It is found that first and second sound undergo avoided crossing on the BEC side due to mixing. Consequently, they are detectable at crossover both as density and thermal waves in traps.