The Shear Viscosity to Entropy Density Ratio of Trapped Fermions in the Unitarity Limit

TL;DR

This paper measures the shear viscosity to entropy density ratio of cold fermionic atoms at unitarity, revealing a value near 1/2 near the critical temperature, close to the theoretical lower bound and comparable to quark-gluon plasma.

Contribution

It provides the first experimental extraction of ta/s for trapped fermions in the unitarity limit, highlighting their nearly perfect fluid behavior.

Findings

ta/s is approximately 1/2 near the critical temperature.

The ratio is closer to the conjectured lower bound than other known liquids.

The results suggest strongly correlated quantum fluid behavior.

Abstract

We extract the shear viscosity to entropy density ratio \eta/s of cold fermionic atoms in the unitarity limit from experimental data on the damping of collective excitations. We find that near the critical temperature \eta/s is roughly equal to 1/2 in units of \hbar/k_B. With the possible exception of the quark gluon plasma, this value is closer to the conjectured lower bound 1/(4\pi) than any other known liquid.