Properties of 2D Bose gases at non-zero temperatures

Abstract

In this work, we study the Berezinskii–Kosterlitz–Thouless (BKT) transition in two-dimensional ultracold Bose gases using the stochastic Gross–Pitaevskii equation at finite temperature. Through numerical simulations, we analyze several physical observables across the critical region, including the quasi-condensate density, vortex population, first-order correlation function g(1)(r), and superfluid density. Our results show clear signatures of the BKT transition: the onset of algebraic order in g(1)(r), the proliferation of free vortices above the critical temperature, and a universal jump in the superfluid density. We also examine the extent to which the energy distribution obeys the classical equipartition theorem. These findings demonstrate the effectiveness of stochastic Gross–Pitaevskii dynamics in capturing the essential features of 2D Bose gases across the BKT transition and provide insight into the interplay between coherence, topological defects, and superfluidity in low-dimensional systems.