Observation of Anomalous Spin Segregation in a Trapped Fermi Gas

We report the observation of spin segregation, i.e., time-dependent separation of the spin density profiles of two spin states, in a trapped, coherently prepared Fermi gas of Li-6 with a magnetically tunable scattering length a(12) close to zero. For vertical bar a(12)vertical bar similar or equal to 5 bohr, as the cloud profiles evolve, the measured difference in the densities at the cloud center increases in 200 ms from 0 to similar or equal to 60% of the initial mean density and changes sign with a(12). The data are in disagreement in both amplitude and temporal evolution with a spin-wave theory for a Fermi gas. In contrast, for a Bose gas, an analogous theory has successfully described previous observations of spin segregation. The observed segregated atomic density profiles are far from equilibrium, yet they persist for similar or equal to 5 s, long compared to the axial trapping period of 6.9 ms. We find the zero crossing in a(12)=0, where spin segregation ceases, at 527.5 +/- 0.2 G.