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

TL;DR

This study experimentally observes the Leggett-Rice effect in a unitary Fermi gas, measuring spin precession and diffusivity, and explores how the interaction parameter gamma varies with temperature and scattering length, revealing interaction nature.

Contribution

First experimental observation of the Leggett-Rice effect in a strongly interacting Fermi gas, including measurements of gamma and spin diffusivity at unitarity and their dependence on temperature and scattering length.

Findings

Measured gamma = 1.08(9) at unitarity.

Observed gamma approaches zero as temperature increases.

Detected sign change in gamma with varying scattering length.

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 $D_0^\perp = 2.3(4)\,\hbar/m$ 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 $\gamma \rightarrow 0$ 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.