The interplay between defects and collective motion in a 2D Yukawa crystal

2D materials offer an ideal platform to quantify collective motion. While defects are important for rearrangements in crystals, the role of collective motion is often overlooked. To understand the interplay between defects and collective motion, we perform molecular dynamics simulations of a charged colloidal crystal—a model extendable to 2D dusty plasma crystals. To unambiguously distinguish thermal vibrations from rearrangements, we map configurations from the real trajectories to the nearest energy-minimized configuration, or inherent structure (IS). In the resulting IS trajectory, the crystal has extended periods in the defect-free ground state interrupted by brief transitions to excited energy states where defects spawn replacing particle clusters with string-like geometries; small collective ring exchanges can also occur in the ground state without any defects. Defects, identified through the Voronoi tessellation as particles with 5 or 7 neighbors, must occur in pairs, and usually form clusters of 4 or 6 neighboring defects; these defect clusters also come in pairs and are randomly distributed in space. Collective particle exchanges link the defect clusters, ultimately healing them. Thus, a picture emerges where diffusion is primarily a result of collective motion, facilitated by defect clusters.