Shear viscosity in a superfluid cold Fermi gas at unitarity

Details Shear viscosity in a superfluid cold Fermi gas at unitarity Shear viscosity in a superfluid cold Fermi gas at unitarity

2012-01-19

arxiv.org/abs/1201.4006v1

Physics

Condensed Matter

Quantum Gases

4 pages, 1 figure

Scientific paper

We compute the contribution of small-angle collision processes of Nambu-Goldstone modes to the shear viscosity, $\eta$, of a superfluid atomic Fermi gas close to the unitarity limit. Performing a calculation analogous to the one done for superfluid $^4$He, we show that the low temperature experimental values of the shear viscosity coefficient to entropy ratio, $\eta/s$, can be reproduced assuming that $\eta$ is dominated by small-angle splitting and joining processes, and considering an effective shear viscosity due to the finite size of the experimental system. Our results allow us to constrain the form of the dispersion law of the Nambu-Goldstone bosons and to predict that at very low temperatures $\eta/s$ should correlate with the size of the optical trap and decrease with decreasing temperature. At extremely low temperatures the predicted value of $\eta/s$ becomes smaller than the universal bound $1/4 \pi$ derived by string theory methods. Whether the effective description of the viscosity remains valid at very low temperatures deserves further investigation.

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