The role of bond orientational order and the nature of the melting transition in an incommensurate physisorbed monolayer: argon on graphite at completion

• Published in: Surface science 1999-11, Vol.441 (2-3), p.270-282
• Main Authors: Roth, M.W., Salazar, M.
• Format: Article
• Language: English
• Subjects: Argon; Condensed matter: structure, mechanical and thermal properties; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Molecular dynamics; Physical adsorption; Physics; Rare gas solids; Solid surfaces and solid-solid interfaces; Solid-liquid transitions; Specific phase transitions; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Two-dimensional melting
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/S0039-6028(99)00738-4

A constant-temperature, constant density molecular-dynamics (MD) method is utilized to study the behavior of bond-orientational, translational and thermodynamic parameters in the solid, melting transition and fluid regimes of argon physisorbed onto graphite (Ar/gr) at complete coverage. For Ar/gr, completion occurs near ρ=1.26 in units of 0.0636atoms/Å2, where ρ=1 corresponds to the complete 3×3 R30° commensurate adlayer. Several bond-orientational and newly introduced pair-distribution order parameters and effective densities prove especially useful in helping evaluate the temperature behavior of the system's structural order. A broad melting transition accompanied by vacancy formation through large vertical fluctuations in the adsorbate atoms is observed at Tm=135±5K, spanning a temperature range of roughly 40K. The melting transition accompanies a rapid formation of a low-density second layer. Substantial bond-orientational order persists into the high-temperature fluid (T≈300K), which is not hexatic in nature and which exhibits infrequent desorption from the first layer. The behavior and interaction of the neighbor shells are compared and contrasted to those observed in Kr/gr.