Essential finite-size effect in the 2D XY model

The thermodynamics of the 2D XY model is formulated by a transfer matrix method and analyzed by a density matrix renormalization group. The finite-size scaling and the beta function of the model are studied by the Roomany-Wyld renormalization group theory. It is found that the 2D XY model has an essential finite-size effect and the Berezinskii-Kosterlitz-Thouless transition with the critical temperature TBKT = 0.892 appears in a finite system of 2000 - 3000 spins as a massless to massive transition with the effective critical temperature Tc = 1.07 " 0.01.