We have performed Monte Carlo (MC) simulations on systems of L\ifmmode\times\else\texttimes\fi{}L classical planar unit spins on square lattices, for L=6, 15, 30, 60, 90, and 200. The interaction between any two given spins S${\ensuremath{\rightarrow}}_{1}$ and S${\ensuremath{\rightarrow}}_{2}$ is given by -JS${\ensuremath{\rightarrow}}_{1}$\ensuremath{\cdot}S${\ensuremath{\rightarrow}}_{2}$ if
S1 and
S2 are nearest neighbors and vanishes otherwise. In order to make sure that our results correspond to equilibrium values, we have looked into the time-dependent properties of this model in the vicinity of critical temperature (
Tc). We have found that the diffusion constant for vortex motion is given at
Tc by D\ensuremath{\simeq}0.2 (in units of nearest-neighbor distance squared per MC step per spin). The values of the relaxation times follow from the value of D. Our computer running times were typically
105 MC steps per spin, larger than any relaxation time for the system sizes we deal with. We use a procedure based on finite-size scaling to establish the value of
Tc=0.89J/
kB, the value of \ensuremath{\nu}=0.5\ifmmode\pm\else\textpm\fi{}0.1, and the value of ${\ensuremath{\eta}}_{c}$=0.24\ifmmode\pm\else\textpm\fi{}0.03, in agreement with the values predicted by the Kosterlitz-Thouless theory.
The temperature scale of relevance for the vortex excitations in the two-dimensional XY model is identified. The size dependence of the helicity modulus at constant vortex temperature is then obtained by Monte Carlo simulations. The Monte Carlo data are then compared with the results for the length-dependent dielectric function from the Kosterlitz renormalization-group equations. The agreement is excellent for temperatures up to and slightly above
Tc. The critical temperature is determined to be
Tc=0.892 13(10). The temperature dependence of the characteristic length right above
Tc is also obtained and found to be in agreement with recent results from high-temperature series expansions.
A method of critical index calculation is proposed. The method is based on matching the temperature values of two different size lattices by finding the optimized solution of the equation t'=
bTYt, where t and t' are the reduced temperatures of the lattices, b is the ratio of lattice sizes, and
YT is the thermal critical index. The values of t and t' are obtained from the fourth cumulant of the order parameter, which satisfies the relation
Qr(t,L)=
Qr(t'L') around the critical point. By using this method, the critical indices of the two-dimensional Ising model and the (2+1)-dimensional lattice gauge theory are calculated. The advantage of the method is that one can obtain the critical point and the critical indices with high accuracy by performing a relatively small number of Monte Carlo iterations.
The properties of phase transition and the lower-temperature phase of the plane rotator model are studied in the square lattice with Monte Carlo method. Physical results thus obtained for this model are quite similar to those of the two-dimensional quantum XY-model,1) that is, the susceptibility diverges and the specific heat does not diverge at the critical point. Furthermore the size-dependence of physical quantities and scaling relations among some temperature-dependent indices are also investigated.