Cumulative Identical Spin Rotation Effects in Collisionless Trapped Atomic Gases

We discuss the strong spin segregation in a dilute trapped Fermi gas recently observed by Du et al. with an "anomalous" large time scale and amplitude. In a collisionless regime, the atoms oscillate rapidly in the trap and average the inhomogeneous external field in an energy dependent way, which controls their transverse spin precession frequency. During interactions between atoms with different spin directions, the identical spin rotation effect transfers atoms to the up or down spin state, depending on their motional energy. Since low energy atoms are closer to the center of the trap than high energy atoms, the final outcome is a strong correlation between spins and positions.