Quasicondensation and coherence in the quasi-two-dimensional trapped Bose gas

TL;DR

This paper uses simulations to explore the phase transition and coherence properties of a trapped quasi-two-dimensional Bose gas, identifying the BKT transition and coherence development above it.

Contribution

It provides a detailed characterization of the phase diagram and coherence phenomena in a trapped quasi-2D Bose gas using classical field simulations.

Findings

Density fluctuations are suppressed above $T_{BKT}$ when a quasi-condensate forms.

Coherence develops as a finite-size effect above $T_{BKT}$, seen as bimodality in momentum distribution.

At $T_{BKT}$, algebraic decay of correlations occurs with an exponent of 0.25.

Abstract

We simulate a trapped quasi-two-dimensional Bose gas using a classical field method. To interpret our results we identify the uniform Berezinskii-Kosterlitz-Thouless (BKT) temperature $T_{BKT}$ as where the system phase space density satisfies a critical value. We observe that density fluctuations are suppressed in the system well above $T_{BKT}$ when a quasi-condensate forms as the first occurrence of degeneracy. At lower temperatures, but still above $T_{BKT}$, we observe the development of appreciable coherence as a prominent finite-size effect, which manifests as bimodality in the momentum distribution of the system. At $T_{BKT}$ algebraic decay of off-diagonal correlations occurs near the trap center with an exponent of 0.25, as expected for the uniform system. Our results characterize the low temperature phase diagram for a trapped quasi-2D Bose gas and are consistent with observations made in recent experiments.