6533b81ffe1ef96bd1278f2b

RESEARCH PRODUCT

Phase transitions and quantum effects in adsorbed monolayers

subject

description

Phase transitions in absorbed (two-dimensional) fluids and in absorbed layers of linear molecules are studied with a combination of path integral Monte Carlo (PIMC), Gibbs ensemble Monte Carlo (GEMC), and finite size scaling techniques. For a classical (nonadditive) hard-disk fluid the “critical” nonadditivities, where the entropy-driven phase separations set in, are presented. For a fluid with internal quantum states the gas-liquid coexistence region, tricritical, and triple points can be determined, and a comparison with density functional (DFT) results shows good agreement for the freezing densities. LinearN 2 molecules adsorbed on graphite (in the √3 × √3 structure) show a transition from a high-temperature phase to a low-temperature phase withherringbone ordering of the orientational degrees of freedom. The order of the transition is determined in the anisotropic planar rotor model as a weak first-order transition. The effect of quantum fluctuations on the herringbone transition is quantified by PIMC and classical simulational methods. The values of the order parameter at low temperatures and the transition temperature are both lowered by roughly 10% due to quantum effects. Rounding effects of the phase transition in adsorbed layers of (N2) x (CO)1, for× < 7% are analyzed by Monte Carlo ( MC) methods, and the ground state ordering for the transition in the adsorbed pure CO system is discussed, from ab initio potentials.