Monte Carlo Study of the Critical Behaviour of a System with Coupled Phase Transitions

Abstract

Using a 2-D lattice gas model of orientable molecules that exhibits two phase transitions, we have performed Monte Carlo simulations in order to study its critical behaviour. One of the phase transitions, associated with positional degrees of freedom, has been found to be second order, while the other, related to orientational degrees of freedom, is first order. Changing the parameters of the hamiltonian we can vary the separation between the two phase transition temperatures. When they are very close to each other the β critical exponent (or at least the effective β critical exponent) of the positional transition varies continuously from the 2-D Ising value to zero when the two transitions overlap. The model is also suitable for qualitatively explaining some experimental results found in Liquid Crystals and other systems showing coupling effects between orientational and positional degrees of freedom.