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 absolute value psi(4) model on a lattice. The critical density and chemical potential are given by relations n(c) = (mT/2piPlanck's(2))ln(xiPlanck's(2)/mU) and mu(c) = (mTU/piPlanck's(2))ln(xi(mu)Planck's(2)/mU), where T is the temperature, m is the mass, and U is the effective interaction. The dimensionless constant xi = 380+/-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+/-0.4. We also report the study of the quasicondensate correlations at the critical point.