Microscopic Approach to Shear Viscosities in Superfluid Gases: From BCS to BEC

TL;DR

This paper calculates shear viscosity across the BCS-BEC crossover in superfluid gases, revealing low-temperature behavior and aligning well with experimental data, thus constraining microscopic theories.

Contribution

It introduces a conservation-law consistent microscopic calculation of shear viscosity across the BCS-BEC crossover, emphasizing the role of pairing gaps and pair degrees of freedom.

Findings

Shear viscosity decreases at low temperatures due to pairing effects.

Theoretical results agree with recent experimental measurements.

Normal state pairing gap influences viscosity behavior.

Abstract

We compute the shear viscosity, $\eta$, at general temperatures $T$, in a BCS-BEC crossover scheme which is demonstrably consistent with conservation laws. The study of $\eta$ is important because it constrains microscopic theories by revealing the excitation spectra. The onset of a normal state pairing gap and the contribution from pair degrees of freedom imply that $\eta$ at low $T$ becomes small, rather than exhibiting the upturn predicted by most others. Using the local density approximation, we find quite reasonable agreement with just-published experiments.