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.