High Frequency Sound in a Unitary Fermi Gas

TL;DR

This study combines experimental measurements and theoretical modeling to analyze the phonon mode in a unitary Fermi gas, revealing its temperature-dependent behavior and similarities to liquid helium.

Contribution

It provides the first detailed experimental and theoretical characterization of the phonon mode across the superfluid transition in a unitary Fermi gas.

Findings

Below T_c, the dominant excitation is the Bogoliubov-Anderson phonon.

The phonon damping is consistent with quasiparticle collision models.

Above T_c, the phonon becomes a strongly damped collisional mode.

Abstract

We present an experimental and theoretical study of the phonon mode in a unitary Fermi gas. Using two-photon Bragg spectroscopy, we measure excitation spectra at a momentum of approximately half the Fermi momentum, both above and below the superfluid critical temperature $T_\mathrm{c}$. Below $T_\mathrm{c}$, the dominant excitation is the Bogoliubov-Anderson (BA) phonon mode, driven by gradients in the phase of the superfluid order parameter. The temperature dependence of the BA phonon is consistent with a theoretical model based on the quasiparticle random phase approximation in which the dominant damping mechanism is via collisions with thermally excited quasiparticles. As the temperature is increased above $T_\mathrm{c}$, the phonon evolves into a strongly damped collisional mode, accompanied by an abrupt increase in spectral width. Our study reveals strong similarities between sound propagation in the unitary Fermi gas and liquid helium.