Bond-orientational structure and melting signature in krypton physisorbed onto graphite at complete coverage

Abstract

A constant-temperature, constant-density molecular-dynamics method is utilized to study the bondorientational structure and behavior in Kr-gr for the solid, fluid, and melting transition regimes at monolayer completion (r51). Several bond-orientational order parameters are introduced to help monitor the system’s symmetry behavior, both with respect to the substrate as well as intrinsic. The firstand fourth-neighbor shells exhibit random thermal fluctuations in the absence of significant interaction with other neighbor shells in the solid. The fluctuations magnify as temperature increases, promoting the decay of structural order through melting and into the fluid. The second and third shells exhibit not only thermal fluctuations but also interaction via local shared-lattice defects ~vacancy/interstitial pairs! that begin in the low-temperature solid. Multipleconsecutive vacancies or interstitials are found to be far rarer in occurrence than single ones. The monolayer loses all bond-orientational order with respect to the graphite substrate upon melting, while floating ~intrinsic! bond-orientational order in the lattice itself is maintained through the high-temperature fluid (T'250 K). The intrinsic bond-orientational order of the fluid after melting is insensitive to change in temperature, and therefore local vacancy production does not increase with increasing temperature until the generation of vacancies created by the onset of desorption at T'200 K. Finite-size scaling results show that the algebraic exponent for the order parameter OB6 reaches the critical value of h65 1 4 simultaneously with the onset of melting around T5100 K illustrating that neither a hexatic phase nor an intermediate region is supported in the system in this study. @S0163-1829~98!07319-6#