Shear viscosity in a superfluid cold Fermi gas at unitarity
Abstract
We analyze the contributions of Nambu-Goldstone modes to the shear viscosity of a superfluid atomic Fermi gas close to unitarity. We show that the low temperature experimental values of the shear viscosity to entropy ratio,
η/s
, are reproduced considering an effective shear viscosity which takes into account the collisions among Nambu-Goldstone bosons and the finite size of the optical trap. We predict that, for
T≲0.1
T
F
,
η/s
should correlate with the size of the trap and linearly decrease with decreasing temperature. For
T≳0.1
T
F
, we find agreement with the experimental data assuming that the Nambu-Goldstone modes have an anomalous dispersion law.