The structure and melting transition of two-dimensional colloidal alloys

We study theoretically the structure and melting transition of two-dimensional (2D) binary mixtures of colloidal particles interacting via a dipole-dipole potential. Using a lattice sum method, we find that at zero temperature (T = 0) the system forms a rich variety of stable crystalline phases whose structure depends on the composition and dipole moment ratio. Using Monte Carlo (MC) simulations, we also find that the melting temperature of the different T = 0 structures is a very strong and non-monotonic function of composition. For example, from a direct analysis of the radial distribution function vs.temperature, we find that the melting temperature of hexagonal AB₂ and AB₆ phases is three orders of magnitude higher than that of hexagonal AB₅. Finally the melting transition for our binary colloidal system is found to proceed via at least two stages for hexagonal AB₂ and AB₆ and at least three stages for hexagonal AB₅ and is thus much richer compared to the melting transition of 2D one component colloidal systems.