PERTURBATIVE DENSITY-FUNCTIONAL THEORY FOR PHASE-TRANSITIONS IN A 2-DIMENSIONAL ANTIFERROMAGNETIC FLUID

We study the temperature-density phase diagram of a fluid in two dimen sions consisting of hard disks which, in addition, possess an internal (Ising) spin degree of freedom. The Ising spin of each disk couples w ith those of its neighbors via a short-ranged antiferromagnetic (AF) i nteraction. Recent Monte Carlo simulations have shown that this system undergoes a gas-liquid transition followed by a gas-AF-ordered-square -solid sublimation transition at low temperatures. Using a perturbativ e density functional approach we obtain, in addition to the observed t ransitions, a freezing transition at high density to a frustrated tria ngular solid phase. Interestingly, the calculated phase diagram sugges ts that at low temperatures, this transition is suppressed so that ove r a range of parameters, the system refuses to crystallize.