Observation Of The Leggett-Rice Effect In A Unitary Fermi Gas

We observe that the diffusive spin current in a strongly interacting degenerate Fermi gas of K-40 precesses about the local magnetization. As predicted by Leggett and Rice, precession is observed both in the Ramsey phase of a spin-echo sequence, and in the nonlinearity of the magnetization decay. At unitarity, we measure a Leggett-Rice parameter gamma = 1.08(9) and a bare transverse spin diffusivity D-0(perpendicular to) = 2.3(4)h/m for a normal-state gas initialized with full polarization and at one-fifth of the Fermi temperature, where m is the atomic mass. One might expect gamma = 0 at unitarity, where two-body scattering is purely dissipative. We observe gamma -> 0 as temperature is increased towards the Fermi temperature, consistent with calculations that show the degenerate Fermi sea restores a nonzero gamma. Tuning the scattering length a, we find that a sign change in. occurs in the range 0 < (k(F)a)(-1) less than or similar to 1.3, where k(F) is the Fermi momentum. We discuss how gamma reveals the effective interaction strength of the gas, such that the sign change in. indicates a switching of branch between a repulsive and an attractive Fermi gas.