Medium effects and the shear viscosity of the dilute Fermi gas away from the conformal limit

TL;DR

This paper investigates how medium effects influence the shear viscosity of a dilute Fermi gas near unitarity, revealing a shift of the viscosity minimum towards the BEC side, consistent with recent experimental findings.

Contribution

It introduces a kinetic theory-based calculation of shear viscosity incorporating medium effects, showing a shift of the viscosity minimum away from unitarity.

Findings

Shear viscosity minimum occurs at unitarity in leading order.

Medium effects shift the viscosity minimum to the BEC side.

Results align with recent experimental observations.

Abstract

We study the shear viscosity of a dilute Fermi gas as a function of the scattering length in the vicinity of the unitarity limit. The calculation is based on kinetic theory, which provides a systematic approach to transport properties in the limit in which the fugacity $z=n\lambda^3/2$ is small. Here, $n$ is the density of the gas and $\lambda$ is the thermal wave length of the fermions. At leading order in the fugacity expansion the shear viscosity is independent of density, and the minimum shear viscosity is achieved at unitarity. At the next order medium effects modify the scattering amplitude as well as the quasi-particle energy and velocity. We show that these effects shift the minimum of the shear viscosity to the Bose-Einstein condensation (BEC) side of the resonance, in agreement with the result of recent experiments.