Large scale simulations of melting in two dimensional Lennard-Jones systems: Evidence for a metastable hexatic phase

Large scale molecular dynamics simulations have been performed for two-dimensional Lennard-Jones systems on massively parallel computers. The calculations were done in the NPT ensemble as recently reformulated by Melchionna et al., avoiding problems associated with mixed phases and artificial treatment of vacancies and interstitials. In the largest systems studied (36864 and 102400 atoms), a metastable hexatic phase was found between solid and liquid, in agreement with the predictions of the theory of melting in two dimensions developed by Kosterlitz and Thouless, Halperin and Nelson, and Young. The hexatic phase was not seen in smaller samples, calling into question the conclusions of many previous simulations performed on smaller systems and shorter time scales.