Critical Point of a Weakly Interacting Two-Dimensional Bose Gas

We study the Berezinskii-Kosterlitz-Thouless transition in a weakly interacting 2D quantum Bose gas using the concept of universality and numerical simulations of the classical $|\psi|^4$-model on a lattice. The critical density and chemical potential are given by relations $n_c=(mT/2\pi \hbar^2) \ln(\xi \hbar^2/ mU)$ and $\mu_c=(mTU/\pi \hbar^2) \ln(\xi_{\mu} \hbar^2/ mU)$, where $T$ is the temperature, $m$ is the mass, and $U$ is the effective interaction. The dimensionless constant $\xi= 380 \pm 3$ is very large and thus any quantitative analysis of the experimental data crucially depends on its value. For $\xi_{\mu}$ our result is $\xi_{\mu} = 13.2 \pm 0.4 $. We also report the study of the quasi-condensate correlations at the critical point.