Observation of the Leggett-Rice effect in a unitary Fermi gas

Currents can reveal essential qualities of a system that are not evident from equilibrium measurements. In a trapped cloud, spin currents are natural to study because they can exist without net mass transport. Spin diffusivity, like conductivity, is a measure of the scattering rate. Precession of spin current, also called the Leggett-Rice effect [1], is a measure of the coherent interactions between excitations. In a degenerate Fermi gas of potassium tuned to a Feshbach resonance, we measure the dynamics of a superposition of two hyperfine states. Using a spin-echo sequence, we probe both the phase and amplitude of magnetization dynamics due to transverse spin currents. Transport coefficients are measured as a function of temperature and of scattering length, at and near unitarity.\\[4pt] [1] A.\ J.\ Leggett and M.\ Rice, {\it PRL} {\bf 20}, 586 (1968); A.\ J.\ Leggett, {\em J.\ Phys.\ C} {\bf 3}, 448 (1970).