Model-independent determination of the shear viscosity of a trapped unitary Fermi gas: Application to high temperature data

TL;DR

This paper introduces a new anisotropic fluid dynamics method to accurately determine the shear viscosity of a trapped unitary Fermi gas from expansion experiments, reducing systematic uncertainties and validating results against theoretical predictions.

Contribution

The paper presents a novel anisotropic fluid dynamics approach that bridges Navier-Stokes and ballistic regimes, improving shear viscosity measurements in ultracold gases.

Findings

Shear viscosity at high temperature: η=0.282(mT)^{3/2}

Method validation against Boltzmann equation solutions

Agreement within 5% with theoretical predictions

Abstract

Determinations of the shear viscosity of trapped ultracold gases suffer from systematic, uncontrolled uncertainties related to the treatment of the dilute part of the gas cloud. In this work we present an analysis of expansion experiments based on a new method, anisotropic fluid dynamics, that interpolates between Navier-Stokes fluid dynamics at the center of the cloud and ballistic behavior in the dilute corona. We validate the method using a comparison between anisotropic fluid dynamics and numerical solutions of the Boltzmann equation. We then apply anisotropic fluid dynamics to the expansion data reported by Cao et al. In the high temperature limit we find $\eta=0.282(mT)^{3/2}$, which agrees within about 5\% with the theoretical prediction $\eta=0.269(mT)^{3/2}$.