Vanishing bulk viscosities and conformal invariance of unitary Fermi gas

TL;DR

This paper demonstrates that the unitary Fermi gas exhibits zero bulk viscosity in the normal phase due to conformal invariance, with specific viscosities vanishing in the superfluid phase, highlighting its unique hydrodynamic properties.

Contribution

It establishes the vanishing of bulk viscosities in the unitary Fermi gas based on symmetry principles, clarifying its hydrodynamic behavior across phases.

Findings

Bulk viscosity zeta=0 in the normal phase.

Two bulk viscosities zeta_1=zeta_2=0 in the superfluid phase.

The third bulk viscosity zeta_3 can be nonzero in the superfluid phase.

Abstract

By requiring general-coordinate and conformal invariance of the hydrodynamic equations, we show that the unitary Fermi gas has zero bulk viscosity, zeta=0, in the normal phase. In the superfluid phase, two of the bulks viscosities have to vanish, zeta_1=zeta_2=0, while the third one zeta_3 is allowed to be nonzero.