Two-fluid dynamics for a Bose-Einstein condensate out of local equilibrium with the non-condensate

TL;DR

This paper develops a generalized two-fluid hydrodynamic theory for Bose-Einstein condensates that includes collisions between condensate and non-condensate atoms, capturing non-equilibrium dynamics and new relaxational modes.

Contribution

It extends existing two-fluid models by incorporating atom exchange collisions, allowing description of non-equilibrium states in Bose-Einstein condensates.

Findings

Introduces a more general two-fluid theory including condensate-non-condensate collisions.

Identifies a new relaxational mode associated with atom exchange.

Reduces to Landau two-fluid theory under equilibrium conditions.

Abstract

We extend our recent work on the two-fluid hydrodynamics of a Bose-condensed gas by including collisions involving both condensate and non-condensate atoms. These collisions are essential for establishing a state of local thermodynamic equilibrium between the condensate and non-condensate. Our theory is more general than the usual Landau two-fluid theory, to which it reduces in the appropriate limit, in that it allows one to describe situations in which a state of complete local equilibrium between the two components has not been reached. The exchange of atoms between the condensate and non-condensate is associated with a new relaxational mode of the gas.