Phase transitions in a quasi-two-dimensional system

We report the results of molecular dynamics simulations of a quasi-two-dimensional system designed to mimic the quasi-two-dimensional colloid suspensions studied by Marcus and Rice Phys. Rev. E 55, 637 (1997). The simulations duplicate all of the important qualitative findings of Marcus and Rice, in particular the occurrence of first order liquid-to-hexatic and hexatic-to-solid transitions. At higher densities this system also exhibits an isostructural solid-to-solid transition and a buckling transition, both of which are continuous. We find that the dislocation pair, free dislocation, and free disclination concentrations do not satisfy the predictions of the Kosterlitz-Thouless-Halperin-Nelson-Young theory. Our results cast light on the role of the out of plane motion in determining the global character of the phase diagram of a quasi-two-dimensional system, and they require reconsideration of the suggestion by Bladon and Frenkel Phys. Rev. Lett. 74, 2519 (1995) of the character of the driving force for the first order liquid-to-hexatic and hexatic-to-solid transitions.