LAUSR

Jiaxun Hou and Thomas Schäfer Department of Physics, North Carolina State University, Raleigh, NC 27695 Abstract In this work we establish constraints on the temperature dependence of the shear viscosity η 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 η(Tc/(2TF )) ∼< η(Tc/TF )/e, where Tc is the critical temperature, TF is the local Fermi temperature of the gas, and e is Euler’s number.