title: 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 γ=1.08(9) and a bare transverse spin diffusivity D(0)(⊥)=2.3(4)ℏ/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 γ=0 at unitarity, where two-body scattering is purely dissipative. We observe γ→0 as temperature is increased towards the Fermi temperature, consistent with calculations that show the degenerate Fermi sea restores a nonzero γ. Tuning the scattering length a, we find that a sign change in γ occurs in the range 0<(k(F)a)(-1)≲1.3, where k(F) is the Fermi momentum. We discuss how γ reveals the effective interaction strength of the gas, such that the sign change in γ indicates a switching of branch between a repulsive and an attractive Fermi gas.
authors.affiliation: Department of Physics, University of Toronto, Ontario M5S 1A7, Canada.
authors.forename: S
authors.initials: S
authors.lastname: Trotzky
authors.affiliation: Department of Physics, University of Toronto, Ontario M5S 1A7, Canada.
authors.forename: S
authors.initials: S
authors.lastname: Beattie
authors.affiliation: Department of Physics, University of Toronto, Ontario M5S 1A7, Canada.
authors.forename: C
authors.initials: C
authors.lastname: Luciuk
authors.affiliation: Department of Physics, University of Toronto, Ontario M5S 1A7, Canada.
authors.forename: S
authors.initials: S
authors.lastname: Smale
authors.affiliation: Department of Physics, University of Toronto, Ontario M5S 1A7, Canada.
authors.forename: A B
authors.initials: AB
authors.lastname: Bardon
authors.affiliation: Institut für Theoretische Physik, Universität Heidelberg, 69120 Heidelberg, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Enss
authors.affiliation: Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada.
authors.forename: E
authors.initials: E
authors.lastname: Taylor
authors.affiliation: Department of Physics, Center of Theoretical and Computational Physics, University of Hong Kong, Hong Kong, China.
authors.forename: S
authors.initials: S
authors.lastname: Zhang
authors.affiliation: Department of Physics, University of Toronto, Ontario M5S 1A7, Canada and Canadian Institute for Advanced Research, Toronto, Ontario M5G 1Z8, Canada.
authors.forename: J H
authors.initials: JH
authors.lastname: Thywissen