Grain formation, stringlike collective motion, and the onset of diffusion in disordered two-dimensional crystals

Two-dimensional crystals in random potentials are studied by molecular-dynamics methods. The ground state of the system is characterized by a transition of the random-potential-induced defect configurations, which evolve from a dilute gaslike state at weak random potentials to a grain-boundary state at strong random potentials. The dynamics of the system is strongly affected by the defect structure. Stringlike collective hopping of particles along the grain boundaries greatly assists the onset of diffusion in the system at low temperatures.