Stabilizing a two-dimensional colloidal solid by light-induced substrates: pinning, depinning and the effect of disorder

We present Monte Carlo simulations of two-dimensional colloidal solids interacting with disordered and ordered substrate potentials which in practice are created by interfering laser beams. The filling factor η, the number of colloids per potential minimum of the substrate, is taken to be either 1 or 1/9. For an ordered and commensurate two-dimensional substrate with η = 1, the solid, being pinned to the periodicity of the substrate, always adopts the perfect order of the substrate, irrespective of the strength of the pinning potential. For η = 1/9, a solid phase ('floating-solid') with the same translational order decay characteristic as the free solid can form. We explore the nature of this phase and show phase-diagrams containing all three transitions: liquid to pinned-solid, liquid to floating-solid and floating-solid to pinned-solid. We also consider the case of a disordered substrate with a filling factor η = 1/9 and show that a floating-solid phase can also exist above such a glassy substrate.