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

Abstract

We observe that the diffusive spin current in a strongly interacting degenerate Fermi gas of ${}^{40}$K 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 $D_0^{\perp }=2.3(4)\hslash /m$ 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 $\gamma \to 0$ as temperature is increased towards the Fermi temperature, consistent with calculations that show the degenerate Fermi sea restores a non-zero $\gamma$. Tuning the scattering length $a$, we find that a sign change in $\gamma$ occurs in the range $0<(k_{F}a{)}^{-1}\lesssim 1.3$, 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 $\gamma$ indicates a switching of branch, between a repulsive and an attractive Fermi gas.

9 pages, 5 figures; Changed to the more conventional kF=(3 pi^2 n)^1/3, instead of the polarized definition we used in v1

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• Nonlinear spin diffusion and spin rotation in a trapped Fermi gas
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