We have simulated the two-dimensional classical planar spin model using the Metropolis Monte Carlo technique. The loss of long-range order as a function of the size of the lattice was confirmed. The energy and specific heat were calculated for a square lattice of 900, 3 600, and 10 000 spins. A sharp specific-heat peak was found at $\frac{k_{B}T}{J}=1.02\mathrm{}$ $J$ is the nearest-neighbor coupling), 15% above the transition temperature $\frac{k_B{T}_c}{J}=0.89\mathrm{}$. $T_c$ was determined by fitting the spin-spin correlation function and the susceptibility to the forms of the Kosterlitz-Thouless theory. The density of vortex pairs was computed and found to increase exponentially with inverse temperature. At $T_c$ vortex pairs begin to unbind and also larger clusters of vortices appear and unbind as the temperature is increased. These larger clusters may be responsible for the specific-heat peak being sharper and closer to $T_c$ than simple theories predict.

• Received 10 May 1979

DOI:https://doi.org/10.1103/PhysRevB.20.3761