Coupled Hartree-Fock-Bogoliubov kinetic equations for a trapped Bose gas

TL;DR

This paper derives coupled Hartree-Fock-Bogoliubov kinetic equations for a trapped Bose gas using non-equilibrium Green's functions, including anomalous correlations, and discusses their equilibrium solutions and oscillation behavior.

Contribution

It generalizes previous uniform Bose gas models to trapped gases by including off-diagonal correlations and deriving self-consistent kinetic equations.

Findings

Derived coupled HFB kinetic equations for trapped Bose gases.

Identified conditions under which the equations reduce to semi-classical form.

Confirmed the equilibrium solution corresponds to in-phase oscillations at trap frequency.

Abstract

Using the Kadanoff-Baym non-equilibrium Green's function formalism, we derive the self-consistent Hartree-Fock-Bogoliubov (HFB) collisionless kinetic equations and the associated equation of motion for the condensate wavefunction for a trapped Bose-condensed gas. Our work generalizes earlier work by Kane and Kadanoff (KK) for a uniform Bose gas. We include the off-diagonal (anomalous) pair correlations, and thus we have to introduce an off-diagonal distribution function in addition to the normal (diagonal) distribution function. This results in two coupled kinetic equations. If the off-diagonal distribution function can be neglected as a higher-order contribution, we obtain the semi-classical kinetic equation recently used by Zaremba, Griffin and Nikuni (based on the simpler Popov approximation). We discuss the static local equilibrium solution of our coupled HFB kinetic equations within the semi-classical approximation. We also verify that a solution is the rigid in-phase oscillation of the equilibrium condensate and non-condensate density profiles, oscillating with the trap frequency.