CRITICAL-BEHAVIOR OF A 2-DIMENSIONAL XY MODEL - A STUDY BY THE MICROCANONICAL MONTE-CARLO TECHNIQUE

We investigate the critical behavior of the classical XY model on the square lattice. The system has a continuous U(1) symmetry corresponding to the global rotation of spins and undergoes the well-known Kosterlitz-Thouless transition associated with vortex-antivortex dissociation. We study several critical quantities and improve them via the microcanonical Monte Carlo technique. We show that the vortex density gives the effective size, as well as the chemical potential, of a vortex pair in the low-temperature region. We also find by extracting only the vortex part through a simple approximation, that the anomalous specific heat behavior is closely related to the energy-vorticity correlation.