Diffusion of point defects in two-dimensional colloidal crystals

Abstract

Uniform colloidal microspheres dispersed in a solvent will, under appropriate conditions, self-assemble into ordered crystalline structures¹. Using these colloidal crystals as a model system, a great variety of problems of interest to materials science, physical chemistry, and condensed-matter physics have been investigated during the past two decades. Recently, it has been demonstrated² that point defects can be created in two-dimensional colloidal crystals³ by manipulating individual particles with optical tweezers. Direct imaging of these defects verified that their stable configurations have lower symmetry than the underlying triangular lattice, as predicted by numerical simulations for a number of two-dimensional systems^4,5,6,7. It was also observed that point defects can dissociate into pairs of well-separated dislocations, a topological excitation especially important in two dimensions. Here we use a similar experimental system to study the dynamics of mono- and di-vacancies in two-dimensional colloidal crystals. We see evidence that the excitation of point defects into dislocation pairs enhances the diffusion of di-vacancies. Moreover, the hopping of the defects does not follow a pure random walk, but exhibits surprising memory effects. We expect the results presented in this work to be relevant for explaining the dynamics of other two-dimensional systems.