Shear viscosity and strong-coupling corrections in the BCS–BEC crossover regime of an ultracold Fermi gas

We theoretically investigate the shear viscosity η 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_c. We show that, while η diverges in both the weak-coupling BCS and strong-coupling BEC limits, it becomes small in the unitary regime. The minimum of η is obtained, not at the unitarity, but slightly in the strong-coupling BEC side. This deviation is consistent with the recent experiment on a ⁶Li Fermi gas. In the weak-coupling BCS regime, we also find that η exhibits anomalous temperature dependence near T_c, which is deeply related to the pseudogap phenomenon originating from strong pairing fluctuations.