Unifying treatment of nonequilibrium and unstable dynamics of cold bosonic atom system with time-dependent order parameter in Thermo Filed Dynamics

TL;DR

This paper develops a self-consistent theoretical framework using Thermo Field Dynamics to describe the nonequilibrium and unstable dynamics of Bose-Einstein condensates with a time-dependent order parameter, including decay processes.

Contribution

It introduces a novel, self-consistent set of coupled equations for Bose-Einstein condensates that handle both nonequilibrium and unstable dynamics within Thermo Field Dynamics.

Findings

Derived self-consistent evolution equations for condensates.

Constructed a time-dependent quasiparticle picture.

Described decay mechanisms due to Landau and dynamical instabilities.

Abstract

The coupled equations which describe the temporal evolution of the Bose-Einstein condensed system are derived in the framework of nonequilibrium Thermo Field Dynamics. The key element is that they are not the naive assemblages of presumable equations, but are the self-consistent ones derived by appropriate renormalization conditions. While the order parameter is time-dependent, an explicit quasiparticle picture is constructed by a time-dependent expansion. Our formulation is valid even for the system with a unstable condensate, and describes the condensate decay caused by the Landau instability as well as by the dynamical one.