Phonon contribution to the shear viscosity of a superfluid Fermi gas in the unitarity limit

Understand

We present a detailed analysis of the contribution of small-angle Nambu-Goldstone boson (phonon) collisions to the shear viscosity, $\eta$, in a superfluid atomic Fermi gas close to the unitarity limit.

• We show that the experimental values of the shear viscosity coefficient to entropy ratio, $\eta/s$, obtained at the lowest reached temperature can be reproduced assuming that phonons give the leading contribution to $\eta$.
• The phonon contribution is evaluated considering $1 \leftrightarrow 2$ processes and taking into account the finite size of the experimental system.
• In particular, for very low temperatures, $T \lesssim 0.1 T_F$, we find that phonons are ballistic and the contribution of phonons to the shear viscosity is determined by the processes that take place at the interface between the superfluid and the normal phase.