Normal and superfluid fractions of inhomogeneous nonequilibrium quantum fluids

TL;DR

This paper investigates the behavior of normal and superfluid fractions in nonequilibrium quantum fluids under various potentials, revealing that traditional definitions break down and can lead to unexpected results such as negative fractions.

Contribution

It introduces a theoretical framework for analyzing superfluidity in nonequilibrium quantum fluids and shows that standard equilibrium concepts do not directly apply.

Findings

Normal and superfluid fractions can become negative in nonequilibrium conditions.

The sum of normal and superfluid fractions does not necessarily equal one.

Definitions based on response to vector potential lose their physical meaning out of equilibrium.

Abstract

We present a theoretical analysis of the normal and superfluid fractions of quantum fluids described by a nonequilibrium extension of the Gross-Pitaevskii equation in the presence of an external potential. Both disordered and regular potentials are considered. The normal and superfluid fractions are defined by the response of the nonequilibrium quantum fluid to a vector potential, in analogy with the equilibrium case. We find that the physical meaning of these definitions breaks down out of equilibrium. The normal and superfluid fractions no longer add up to one and for some types of external potentials, they can even become negative.