Strong-Coupling Effects and Shear Viscosity in an Ultracold Fermi Gas

We theoretically investigate the shear viscosity eta, as well as the entropy density s, in the normal state of an ultracold Fermi gas. Including pairing fluctuations within the framework of a T -matrix approximation, we calculate these quantities in the Bardeen-Cooper-Schrieffer (BCS)-Bose-Einstein condensation (BEC) crossover region. We also evaluate eta/s, to compare it with the lower bound of this ratio, conjectured by Kovtun, Son, and Starinets (KSS bound). In the weak-coupling BCS side, we show that the shear viscosity eta is remarkably suppressed near the superfluid phase transition temperature T-c, due to the so-called pseudogap phenomenon. In the strong-coupling BEC side, we find that, within the neglect of the vertex corrections, one cannot correctly describe eta. We also show that eta/s decreases with increasing the interaction strength, to become very close to the KSS bound, n/4 pi k(B), on the BEC side.