First Sound Damping in the Imbalanced Unitary Fermi Gas*

The behavior of strongly-interacting spin-imbalanced Fermi systems is essential to an understanding of magnetized electrons or isospin-imbalanced nuclear matter, and these systems are also expected to exhibit exotic phases, such as Fulde-Farrell-Larkin-Ovechnikov superfluidity. Ultracold neutral atomic gas platforms allow a clean, highly tunable implementation of such systems, with the added advantage that the microscopic interactions are well understood. 

We prepare a degenerate spin mixture of fermionic 6Li in a homogeneous blue-detuned box potential, with s-wave interactions between spin components tuned to unitarity via a Feshbach resonance. We measure resonances of density waves - first sound - in this box potential, extracting the first sound diffusivity as a proxy for momentum and heat transfer in the liquid. We observe a dramatic increase in diffusivity with increasing imbalance, as well as signatures of non-monotonicity with temperature at high imbalance, in qualitative agreement with Fermi liquid theory.