2021 journal article
Dissipative superfluid hydrodynamics for the unitary Fermi gas
PHYSICAL REVIEW A, 104(2).
In this work we establish constraints on the temperature dependence of the shear viscosity $\ensuremath{\eta}$ in the superfluid phase of a dilute Fermi gas in the unitary limit. Our results are based on analyzing experiments that measure the aspect ratio of a deformed cloud after release from an optical trap. We discuss how to apply the two-fluid formalism to the unitary gas and provide a suitable parametrization of the equation of state. We show that in expansion experiments the difference between the normal and superfluid velocities remains small and can be treated as a perturbation. We find that expansion experiments favor a shear viscosity that decreases significantly in the superfluid regime. Using an exponential parametrization, we find $\ensuremath{\eta}({T}_{c}/2{T}_{F})\ensuremath{\lesssim}0.37[\ensuremath{\eta}({T}_{c}/{T}_{F})]$, where ${T}_{c}$ is the critical temperature and ${T}_{F}$ is the local Fermi temperature of the gas.