First and second sound in a strongly interacting Fermi gas

Understand

Using a variational approach, we solve the equations of two-fluid hydrodynamics for a uniform and trapped Fermi gas at unitarity.

• In the uniform case, we find that the first and second sound modes are remarkably similar to those in superfluid Helium, a consequence of strong interactions.
• In the presence of harmonic trapping, first and second sound become degenerate at certain temperatures.
• At these points, second sound hybridizes with first sound and is strongly coupled with density fluctuations, giving a promising way of observing second sound.