# Shear viscosity and Strong-Coupling Corrections in the BCS-BEC crossover Regime of an Ultracold Fermi Gas

## TL;DR

This paper theoretically studies shear viscosity in an ultracold Fermi gas across the BCS-BEC crossover, revealing how strong interactions influence transport properties and aligning with recent experimental findings.

## Contribution

It provides a detailed analysis of shear viscosity behavior in the crossover regime using a strong-coupling self-consistent T-matrix approach, highlighting the minimum near the BEC side.

## Key findings

- Shear viscosity diverges in weak and strong coupling limits.
- Minimum shear viscosity occurs slightly on the BEC side, not at unitarity.
- Anomalous temperature dependence of shear viscosity near T_c in the BCS regime.

## Abstract

We theoretically investigate the shear viscosity $\eta$ in the BCS-BEC crossover regime of an ultracold Fermi gas with a Feshbach resonance. Within the framework of the strong-coupling self-consistent $T$-matrix approximation, we examine how a strong pairing interaction associated with a Feshbach resonance affects this transport coefficient, in the normal state above the superfluid phase transition temperature $T_{\rm c}$. We show that, while $\eta$ diverges in both the weak-coupling BCS and strong-coupling BEC limits, it becomes small in the unitary regime. The minimum of $\eta$ is obtained, not at the unitarity, but slightly in the strong-coupling BEC side. This deviation is consistent with the recent experiment on a $^6$Li Fermi gas. In the weak-coupling BCS regime, we also find that $\eta$ exhibits anomalous temperature dependence near $T_{\rm c}$, which is deeply related to the pseudogap phenomenon originating form strong pairing fluctuations.