Dislocation dipoles in rare-gas monolayers

Atomistic computer-modeling methods with realistic potentials have been used to calculate the energies associated with dislocation dipoles in krypton and xenon monolayers. The results are not as predicted by elasticity theory. In the ideal two-dimensional monolayers without substrate modulation short-vacancy dipoles are greatly favored and are tightly bound, behaving as single, immobile defects. Their core energy is very small and separations of the order of 30 lattice spacings are required before elastic behavior is realized. Even at these large separations the Peierls stresses are not negligible ({~}50 K). Significant Peierls stresses are also observed for interstitial dipoles although the energies are as predicted by continuum elasticity theory. When physisorbed on graphite the Xe monolayer remains similar to its floating counterpart but the Kr monolayer does not. The commensurate phase is dominated by the substrate and admits only small defects. The incommensurate Kr phase exhibits a striking coupling between the vacancy dipoles and domain walls, both of which move freely. In this case a continuous melting mechanism involving the progressive removal of the hexagonal domain-wall pattern is proposed.