Analytical framework for non-equilibrium phase transition to Bose--Einstein condensate

TL;DR

This paper develops an analytical framework for describing non-equilibrium Bose--Einstein condensates, accounting for infinite states, and investigates how different conditions affect condensation and coherence in 2D and 3D systems.

Contribution

It introduces a novel analytical approach for non-equilibrium BEC that includes continuous states and covers both equilibrium and non-equilibrium scenarios.

Findings

Ground state occupation depends on temperature and pumping rate.

Coherence buildup can occur at different temperatures than ground state occupation.

Condensate linewidth behavior varies between 2D and 3D BECs.

Abstract

The theoretical description of non-equilibrium Bose--Einstein condensate (BEC) is one of the main challenges in modern statistical physics and kinetics. The non-equilibrium nature of BEC makes it impossible to employ the well-established formalism of statistical mechanics. We develop a framework for the analytical description of a non-equilibrium phase transition to BEC that, in contrast to previously developed approaches, takes into account the infinite number of continuously distributed states. We consider the limit of fast thermalization and obtain an analytical expression for the full density matrix of a non-equilibrium ideal BEC which also covers the equilibrium case. For the particular cases of 2D and 3D, we investigate the non-equilibrium formation of BEC by finding the temperature dependence of the ground state occupation and second-order coherence function. We show that for a given pumping rate, the macroscopic occupation of the ground state and buildup of coherence may occur at different temperatures. Moreover, the buildup of coherence strongly depends on the pumping scheme. We also investigate the condensate linewidth and show that the Schawlow--Townes law holds for BEC in 3D and does not hold for BEC in 2D.