High-Frequency Sound in a Unitary Fermi Gas

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 Tc. Below Tc, 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 Tc, 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.

Strumming a Strongly Interacting Fermi Gas

“…They found that the damping rate drops suddenly when the gas transitions from the normal state to a superfluid. This feature agrees with the team's theory calculations [1], and its size is strikingly similar to the decrease observed in bosonic helium-4.…”

“…In addition, a material undergoing a phase transition may attenuate sound more strongly, providing a signature of the change. Taking advantage of sound's qualities as a material probe, two independent teams have used it to understand the properties of the so-called unitary Fermi gas [1,2]. This form of quantum matter contains very strongly interacting fermions, and it is a model system for describing some superconductors and neutron stars.…”

“…One group, led by Chris Vale of Swinburne University in Australia, studied sound waves with a relatively short wavelength. At these wavelengths, the superfluid phase is in the so-called collisionless regime, where sound waves are weakly damped by kicks from thermally excited particles [1]. The team focused two lasers with slightly different frequencies on a small region of a trapped atom cloud.…”

Thermal conductivity of an ultracold Fermi gas in the BCS-BEC crossover

“…Therefore, the system is in a weak dissipation region in the superfluid phase, which is also consistent with the studies on shear viscosity 11,21,22 . A very recent experiment on sound waves studies a similar circumstance 16 . It observes excitation modes at frequency ω 0 ∼ (0.35 − 0.5)E F , which lie in the collisionless regime in superfluid phase and the crossover between the hydrodynamic and collisionless regime above T c .…”

“…Black solid line is our calculations. Blue circles and the cyan dashed line are experimental data and quasiparticle random phase approximation (QRPA) theory, respectively, from 16 . Inset: the thermal relaxation rate τ −1 versus T/T c .…”