Mean-field description of pairing effects, BKT physics, and superfluidity in 2D Bose gases

TL;DR

This paper develops a mean-field theory for 2D Bose gases that incorporates pairing effects and BKT physics, revealing the conditions for superfluidity and phase coherence at finite temperatures.

Contribution

It introduces a mean-field framework that includes pairing and phase fluctuations, providing a comprehensive description of superfluidity and BKT physics in 2D Bose gases.

Findings

Superfluidity only persists at zero temperature without phase fluctuations.

Finite pairing amplitude leads to a superfluid density at finite temperatures.

The phase diagram shows superfluid and normal phases separated by the BKT transition.

Abstract

We derive a mean-field description for two-dimensional (2D) interacting Bose gases at arbitrary temperatures. We find that genuine Bose-Einstein condensation with long-range coherence only survives at zero temperature. At finite temperatures, many-body pairing effects included in our mean-field theory introduce a finite amplitude for the pairing density, which results in a finite superfluid density. We incorporate Berenzinskii-Kosterlitz-Thouless (BKT) physics into our model by considering the phase fluctuations of our pairing field. This then leads to the result that the superfluid phase is only stable below the BKT temperature due to these phase fluctuations. In the weakly interacting regime at low temperature we compare our theory to previous results from perturbative calculations, renormalization group calculations as well as Monte Carlo simulations. We present a finite-temperature phase diagram of 2D Bose gases. One signature of the finite amplitude of the pairing density field is a two-peak structure in the single-particle spectral function, resembling that of the pseudogap phase in 2D attractive Fermi gases.