Perfect-fluid behavior of a dilute Fermi gas near unitary

TL;DR

This paper uses quantum Monte Carlo calculations to show that a dilute Fermi gas near unitarity behaves as a nearly perfect fluid with minimal shear viscosity, outperforming other known fluids like quark-gluon plasma.

Contribution

It provides the first ab initio calculation of shear viscosity in a Fermi gas near unitarity, revealing its near-perfect fluid behavior and minimal viscosity.

Findings

Shear viscosity decreases approaching the Bose-Einstein condensation limit.

Minimum eta/s ratio is approximately 0.2 hbar/k_B, smaller than quark-gluon plasma.

Fermi gas near unitarity can sustain quantum turbulence at low temperatures.

Abstract

We present an ab initio calculation of the shear viscosity as a function of interaction strength in a two-component unpolarized Fermi gas near the unitary limit, within a finite temperature quantum Monte Carlo (QMC) framework and using the Kubo linear-response formalism. The shear viscosity decreases as we tune the interaction strength 1/ak_F from the Bardeen-Cooper-Schrieffer side of the Feshbach resonance towards Bose-Einstein condensation limit and it acquires the smallest value for 1/ak_F approx 0.4, with a minimum value of (eta/s)_min approx 0.2 hbar/k_B, which is about twice as small as the value reported for experiments in quark-gluon plasma (eta/s)_QGP lesssim 0.4 hbar/k_B. The Fermi gas near unitarity thus emerges as the most "perfect fluid" observed so far in nature. The clouds of dilute Fermi gas near unitarity exhibit the unusual attribute that, for the sizes realized so far in the laboratory or larger (less than 10^9 atoms), can sustain quantum turbulence below the critical temperature, but at the same time the classical turbulence is suppressed in the normal phase.