High-Temperature Expansion of the Free Energy in the Two-Dimensional XY Model

Abstract

We extend the high-temperature series of the free energy for the XY model in two dimensions to order $\beta^{48}$ from the previous order of $\beta^{22}$ by applying the improved algorithm of the finite lattice method. The long series obtained enables us to conclude that the behavior of the free energy is consistent in high accuracy with what is expected when the phase transition of the model is of the Kosterlitz-Thouless type .