Phase transitions in two planar lattice models and topological defects: a Monte Carlo study

Monte Carlo simulation has been performed in the planar P2 and P4 models to investigate the effects of the suppression of topological defects on the phase transition exhibited by these models. Suppression of the 1/2 defects on the square plaquettes in the P2 model leads to complete elimination of the phase transition observed in this model. However, in the P4 model, on suppressing the single 1/2 defects on square plaquettes, the otherwise first order phase transition changes to a second order one which occurs at a higher temperature, and this is due to the presence of a large number of 1/2 pair defects which are left within the square plaquettes. When we suppressed these charges too, complete elimination of the phase transition was observed.