Evidence for an orientationally ordered two dimensional fluid phase from molecular dynamics calculations

Results of molecular-dynamics calculations on melting in a two-dimensional (2D) Lennard-Jones system are presented. We find that this system loses its resistance to shear at a temperature T1 (≈0.36). However, long-range "orientational" order persists up to a higher temperature T2 (≈0.57). These observations are compatible with the existence of a liquid-crystal-like phase with sixfold anisotropy separated from both the solid and the isotropic-fluid phase by second-order phase transitions. Such two-stage melting behavior in 2D has been predicted by Halperin and Nelson.