Searching for Perfect Fluids: Quantum Viscosity in a Universal Fermi Gas

TL;DR

This study measures the shear viscosity of a universal Fermi gas across different temperatures, providing insights into quantum fluid behavior and the viscosity-to-entropy ratio near the conjectured perfect fluid limit.

Contribution

It introduces a new temperature calibration method and provides comprehensive viscosity measurements across a broad temperature range in a strongly interacting Fermi gas.

Findings

Shear viscosity varies with temperature and is measured from near ground state to unitary regime.

High temperature data fits well with a model assuming zero bulk viscosity.

Estimated shear viscosity to entropy density ratio approaches the conjectured perfect fluid limit.

Abstract

We measure the shear viscosity in a two-component Fermi gas of atoms, tuned to a broad s-wave collisional (Feshbach) resonance. At resonance, the atoms strongly interact and exhibit universal behavior, where the equilibrium thermodynamic properties and the transport coefficients are universal functions of the density $n$ and temperature $T$. We present a new calibration of the temperature as a function of global energy, which is directly measured from the cloud profiles. Using the calibration, the trap-averaged shear viscosity in units of $\hbar\,n$ is determined as a function of the reduced temperature at the trap center, from nearly the ground state to the unitary two-body regime. Low temperature data is obtained from the damping rate of the radial breathing mode, while high temperature data is obtained from hydrodynamic expansion measurements. We also show that the best fit to the high temperature expansion data is obtained for a vanishing bulk viscosity. The measured trap-averaged entropy per particle and shear viscosity are used to estimate the ratio of the shear viscosity to the entropy density, which is compared that conjectured for a perfect fluid.