Thin Film Theory of Inert Gases and Isotopic Series of Molecular Hydrogen.
Theoretical questions arise concerning two dimensional physics which may be approached in physisorption systems. These include the possibility and nature of long range order in two dimensions, the question of the thin film growth modes, and the nature of 2D melting. Thin physisorbed layer of xenon on graphite and silver (111) surfaces, and neon and molecular deuterium on graphite have been studied with harmonic lattice dynamics. Realistic pair potentials and Lennard Jones LJ(12,6) potential models are used as the interaction between adatoms. The substrate is treated as an inert system providing a holding potential and substrate-mediated interaction between adatoms. The calculations are based on the quasiharmonic theory and special points method of evaluating Brillouin Zone sums. For Xe/Ag(111), Xe/Gr and Ne/Gr, the conditions for the coexistence of monolayer and bilayer, and bilayer and trilayer are obtained. For molecular deuterium the coexistence of monolayer and bilayer on graphite is studied with different lattice structures. The xenon trilayers are found to be stable with respect to formation of the bulk solid of the adsorbate. The Ne/Gr system shows an overcompression at the trilayer condensation. The D(,2)/Gr system shows a severe overcompression at the bilayer condensation. The quasiharmonic method failed to treat the trilayer of D(,2). Crystals of Ne, He and molecular isotopes of hydrogen have large zero point motion. The 2D Hartree aprroximation is used to calculate the lowest energy of the monolayer using a variational treatment. In order to include both the effects of correlation and symmetry, a Jastrow-type wave function is used, and a Monte Carlo method is used to simulate the particle system. The variational energies of both liquid and solid phase are obtained. It is shown that the stable phase of 2D Helium system is liquid, and the 2D molecular isotopes of hydrogen are solid at zero temperature. The melting and solidification of the 2D Helium system at zero temperature are calculated from the double tangent construction. A critical deBoer parameter is derived for the 2D boson system where the liquid and solid phase coexist at zero temperature and pressure.
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- Physics: Condensed Matter