Viscosity-Entropy Ratio of the Unitary Fermi Gas from Zero-Temperature Elementary Excitations

We investigate the low-temperature behavior of the ratio between the shear viscosity eta and the entropy density s in the unitary Fermi gas by using a model based on the zero-temperature spectra of both bosonic collective modes and fermonic single-particle excitations. Our theoretical curve of eta/s as a function of the temperature T is in qualitative agreement with the experimental data of trapped ultracold Li-6 atomic gases. We find the minimum value eta/s similar or equal to 0.44 (in units of (h) over bar /k(B)) at the temperature T/T-F similar or equal to 0.27, with T-F the Fermi temperature.