The Shear Viscosity in an Anisotropic Unitary Fermi Gas

TL;DR

This paper proposes that an anisotropic shear viscosity tensor can be studied in ultracold unitary Fermi gases under harmonic confinement, potentially violating the KSS bound and revealing new quantum fluid behaviors.

Contribution

It introduces a novel experimental setup to explore anisotropic shear viscosity and suggests possible violations of the KSS bound in ultracold Fermi gases.

Findings

Anisotropic shear viscosity can be engineered in ultracold gases.

Potential violation of the KSS bound in anisotropic conditions.

Reduction of shear viscosity to entropy ratio below the bound is possible.

Abstract

We consider a system consisting of a strongly interacting, ultracold unitary Fermi gas under harmonic confinement. Our analysis suggests the possibility of experimentally studying, in this system, an anisotropic shear viscosity tensor driven by the anisotropy in the trapping potential. In particular, we suggest that this experimental setup could mimic some features of anisotropic geometries that have recently been studied for strongly coupled field theories which have a gravitational dual. Results using the AdS/CFT correspondence in these theories show that in systems with a background linear potential, certain viscosity components can be made much smaller than the entropy density, parametrically violating the bound proposed by Kovtun, Son and Starinets (KSS). This intuition, along with results from a Boltzmann analysis that we perform, suggests that a violation of the KSS bound can perhaps occur in the unitary Fermi gas system when it is subjected to a suitable anisotropic trapping potential which may be approximated to be linear in a suitable range of parameters. We give a concrete proposal for an experimental setup where an anisotropic shear viscosity tensor may arise. In such situations, it may also be possible to observe a reduction in the spin one component of the shear viscosity from its lowest value observed so far in ultracold Fermi gases. In extreme anisotropic situations, the reduction may be enough to reduce the shear viscosity to entropy ratio below the proposed KSS bound, although this regime is difficult to analyze in a theoretically controlled manner.