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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.
The deformation of a Fermi surface is a fundamental phenomenon leading to a plethora of exotic quantum phases. Understanding these phases, which play crucial roles in a wealth of systems, is a major challenge in atomic and condensed-matter physics. H
We present measurements of the local (homogeneous) density-density response function of a Fermi gas at unitarity using spatially resolved Bragg spectroscopy. By analyzing the Bragg response across one axis of the cloud we extract the response functio
We study the stability of a thermal $^{39}$K Bose gas across a broad Feshbach resonance, focusing on the unitary regime, where the scattering length $a$ exceeds the thermal wavelength $lambda$. We measure the general scaling laws relating the particl
The low temperature unitary Bose gas is a fundamental paradigm in few-body and many-body physics, attracting wide theoretical and experimental interest. Here we first present a theoretical model that describes the dynamic competition between two-body
The contact ${cal I}$, introduced by Tan, has emerged as a key parameter characterizing universal properties of strongly interacting Fermi gases. For ultracold Fermi gases near a Feshbach resonance, the contact depends upon two quantities: the intera