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

TL;DR

This study measures the shear viscosity of a resonantly interacting Fermi gas across the superfluid transition, revealing preformed pairs influence and critical behavior near the transition temperature.

Contribution

It provides the first detailed local shear viscosity measurements across the superfluid transition, uncovering features like the rapid decrease of viscosity above T_c and the maximum of its derivative at T_c.

Findings

Shear viscosity decreases rapidly above T_c.

Derivative of shear viscosity peaks at T_c.

Data aligns with certain microscopic theories.

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.