First Sound Damping in the Imbalanced Fermi Spin Mixture

Strongly interacting spin-imbalanced Fermi gases are ubiquitious in nature - for example magnetized electrons, nuclear matter with unequal proton-neutron components, and quark mixtures - but are difficult to understand due to the notorious sign problem. Ultracold atomic gases near a Feshbach resonance, in addition to providing a highly clean system with well-understood microscopic interactions and accessible probes, also allow the spin imbalance to be freely tuned; they are thus an optimal tool for studying these physics. In this work, we prepare homogeneous, highly degenerate two-species mixtures of 6Li near unitarity with controllable levels of spin imbalance. Using our box trap, we excite and probe collective modes of density oscillations - first sound - in order to understand damping properties in the hydrodynamic regime. We observe a dramatic increase in the first sound damping with increasing imbalance, consistent with an increase in the mean free path.