Phase transitions of model colloids in external fields

The nature of the melting transition for a system of hard disks with translational degrees of freedom in two spatial dimensions has been analyzed by a combination of computer simulation methods and a finite size scaling technique. The behavior of the system is consistent with the predictions of the Kosterlitz-Thouless-Halperi-Nelson-Young (KTHNY) theory. Hard and soft disks in external periodic (light-) fields show rich phase diagrains including freezing and melting transitions when the density of the system is varied. Monte Carlo simulations for detailed finite size scaling analysis of various thermodynamic quantities like the order parameter, its cumulants etc., have been used in order to map the phase diagram of the system for various values of the density and the amplitude of the external potential. For hard disks we find clear indication of a, reentrant liquid phase over a, significant region of the parameter space. The simulations therefore show that the system of hard 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. Detailed analysis of the simulation data shows several features consistent with a recent dislocation unbinding theory of laser induced melting. The differences and similarities of systems with soft potentials (DLVO, 1/r(12), 1/r(6)) and the relation to experimental data, is analyzed. Interpreting hard disks as the simplest model of an atomic fluid, quantum effects on the phase diagram are investigated by path integral Monte Carlo simulations.