Scaling Flows and Dissipation in the Dilute Fermi Gas at Unitarity

TL;DR

This paper reviews experimental and theoretical efforts to measure the shear viscosity of the dilute Fermi gas at unitarity using scaling flow techniques, highlighting current bounds and future improvements.

Contribution

It analyzes various experimental approaches to determine shear viscosity and discusses necessary theoretical improvements for more definitive results.

Findings

Estimated eta/s ≤ 0.5 hbar/k_B from current data

Comparison of different flow experiments and their advantages

Identification of theoretical improvements needed for accuracy

Abstract

We describe recent attempts to extract the shear viscosity of the dilute Fermi gas at unitarity from experiments involving scaling flows. A scaling flow is a solution of the hydrodynamic equations that preserves the shape of the density distribution. The scaling flows that have been explored in the laboratory are the transverse expansion from a deformed trap ("elliptic flow"), the expansion from a rotating trap, and collective oscillations. We discuss advantages and disadvantages of the different experiments, and point to improvements of the theoretical analysis that are needed in order to achieve definitive results. A conservative bound based on the current data is that the minimum of the shear viscosity to entropy density ration is that eta/s is less or equal to 0.5 hbar/k_B.