Dynamical processes of interstitial diffusion in a two-dimensional colloidal crystal

1) Point defects, such as vacancies and interstitials, play an important role in the thermodynamics of 2D solids.
2) Interstitial diffusion constants are larger than those of vacancies, with diinterstitials being slower than monointerstitials.
3) Equilibrium behavior is observed for mono-interstitials, while local melting is suggested for di-interstitials, opening up opportunities for microscopic studies on the dynamics of melting in colloidal model systems.

Abstract

Defects in crystalline materials are of broad and fundamental interest. In condensed-matter physics, defect dynamics contain essential information about the microscopic processes of crystal formation and melting. Two-dimensional melting (2D) is widely accepted to be mediated by the proliferation of edge dislocations. In 2D crystals, vacancies and interstitials are, in fact, bound pairs of edge dislocations and disclinations. They are expected to play critical roles in 2D melting. We demonstrated significant progress in quantifying the dynamical processes of interstitials. We also propose a simple yet powerful method in visualizing the time-averaged configurations of the defects, providing a direct tool to assess whether the detailed balance is obeyed or violated in the fluctuating processes in a lattice. In two-dimensional (2D) solids, point defects, i.e., vacancies and interstitials, are bound states of topological defects of edge dislocations and disclinations. They are expected to play an important role in the thermodynamics of the system. Yet very little is known about the detailed dynamical processes of these defects. Two-dimensional colloidal crystals of submicrometer microspheres provide a convenient model solid system in which the microscopic dynamics of these defects can be studied in real time using video microscopy. Here we report a study of the dynamical processes of interstitials in a 2D colloidal crystal. The diffusion constants of both mono- and diinterstitials are measured and found to be significantly larger than those of vacancies. Diinterstitials are clearly slower than monointerstitials. We found that, by plotting the accumulative positions of five- and sevenfold disclinations relative to the center-of-mass position of the defect, a sixfold symmetric pattern emerges for monointerstitials. This is indicative of an equilibrium behavior that satisfies local detailed balance that the lattice remains elastic and can be thermally excited between lattice configurations reversibly. However, for diinterstitials the sixfold symmetry is not observed in the same time window, and the local lattice distortions are too severe to recover quickly. This observation suggests a possible route to creating local melting of a lattice (similarly one can create local melting by creating divacancies). This work opens up an avenue for microscopic studies of the dynamics of melting in colloidal model systems.

Access paper here