Disorder-induced enhancement of local hexatic correlations in two-dimensional fluids

Quenched disorder affects translational and orientational correlations in two-dimensional interacting particle systems. Such disorder always suppresses orientational order in crystalline states. Surprisingly, in fluid phases of particles interacting with a power-law repulsive interaction, increasing the strength of a quenched Gaussian random pinning potential appears to enhance hexatic order locally. We propose that nearby pairs of pinned particles lock in the relative orientation of neighbours around them, propagating hexatic orientational order across larger distances than in the unpinned fluid. We test this idea using Monte Carlo simulations of interacting particles in their fluid phase in two dimensions, where two of the particles are constrained to be permanently pinned at a fixed distance from each other. We use Voronoi tesselations of instantaneous particle configurations to demonstrate that these pinned particles create hexatic neighbourhoods that can extend well beyond the range of their separation. This structuring is enhanced when the particle density is increased and is most prominent in the vicinity of the liquid-solid transition. Testing these ideas experimentally using experiments on 2D colloidal systems should be feasible. (paper)