Shear viscosity of a universal Fermi gas near the superfluid phase transition

Abstract

We precisely measure the shear viscosity for a resonantly interacting Fermi gas as a function of temperature, from nearly the ground state through the superfluid phase transition at a critical temperature $T_c$. Using an iterative method to invert the data, we extract the {\it local} shear viscosity coefficient $\alpha_S(\theta)$ versus reduced temperature $\theta$, revealing previously hidden features. We find that $\alpha_S$ begins to decrease rapidly with decreasing $\theta$ well above $T_c$, suggesting that preformed pairs play an important role. Further, we observe that the derivative $\alpha_S'(\theta)$ has a maximum at $T_c$. We compare the local data to several microscopic theories. Finally, we determine the local ratio of the shear viscosity to the entropy density.