Phase transitions of soft disks in external periodic potentials: A Monte Carlo study

The nature of freezing and melting transitions for a system of model colloids interacting via the Derjaguin, Landau, Verwey, and Overbeek potential in a spatially periodic external potential is studied using extensive Monte Carlo simulations. Detailed finite size scaling analyses of various thermodynamic quantities, such as the order parameter, its cumulants, etc., are used to map the phase diagram of the system for various values of the reduced screening length kappaa(s) and the amplitude of the external potential. We find clear indication of a reentrant liquid phase over a significant region of the parameter space. Our simulations therefore show that the system of soft disks behaves in a fashion similar to charge stabilized colloids, which are known to undergo an initial freezing, followed by a remelting transition as the amplitude of the imposed, modulating field produced by crossed laser beams is steadily increased. The detailed analysis of our data shows several features consistent with a recent dislocation unbinding theory of laser induced melting.