Universal Quantum Viscosity in a Unitary Fermi Gas

TL;DR

This paper investigates the universal behavior of shear viscosity in a unitary Fermi gas across different temperature regimes, demonstrating quantum scale dependence and precise temperature scaling through experimental measurements.

Contribution

It provides the first experimental measurements of shear viscosity in a unitary Fermi gas at both low and high temperatures, confirming universal hydrodynamic predictions.

Findings

Viscosity scales with $ abla$ at low T and with $T^{3/2}$ at high T.

Experimental data matches universal hydrodynamic models.

Ratio of shear viscosity to entropy density approaches that of a perfect fluid.

Abstract

A Fermi gas of atoms with resonant interactions is predicted to obey universal hydrodynamics, where the shear viscosity and other transport coefficients are universal functions of the density and temperature. At low temperatures, the viscosity has a universal quantum scale $\hbar n$ where $n$ is the density, while at high temperatures the natural scale is $p_T^3/\hbar^2$ where $p_T$ is the thermal momentum. We employ breathing mode damping to measure the shear viscosity at low temperature. At high temperature $T$, we employ anisotropic expansion of the cloud to find the viscosity, which exhibits precise $T^{3/2}$ scaling. In both experiments, universal hydrodynamic equations including friction and heating are used to extract the viscosity. We estimate the ratio of the shear viscosity to the entropy density and compare to that of a perfect fluid.