Monte Carlo Studies of Adsorption Phenomena
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MONTE CARLO STUDIES OF ADSORPTION PHENOMENA
W. Selke Institut fOr Festk~rperforschung der KFA, 5170 J~lich, Federal Republic of Germany
ABSTRACT Two different adsorption phenomena are studied using, in particular, Monte Carlo techniques: (i) Lattice gas models with competing interactions of ANNNI-type are shown to describe various uniaxial commensurate and incommensurate structures as observed in chemisorbed overlayers such as H on Fe(110) and 02 on Pd(110). (ii) At interfaces of multi-state models (Potts, spin-i Ising models) nonboundary states are adsorbed preferably; critical properties of this "wetting" phenomenon are investigated.
INTRODUCTION The Monte Carlo (MC) method is a widely used technique to study model Hamiltonians in statistical mechanics, in particular in connection with phase transitions and critical phenomena [i]. In this article we shall discuss recent applications which demonstrate one of the main advantages of this method. One may readily identify complex structures and new phenomena by just examining typical equilibrium MC configurations. To get a quantitative description averages have to be taken over these configurations for suitably defined quantities. Quite often such configurations may also provide a clue to a physically sound analytic approximate treatment of the problem. In the next section two lattice gas models with competing interactions of ANNNI-type (ANNNI stands for axial next-nearest neighbor Ising) [2] for adsorption on a centered rectangular lattice (H/Fe(110) [3,4]) and on a rectangular lattice (0 2 /Pd(110) [5,6]) are analysed. The role of topological defects, like walls and dislocations, in the melting of the commensurate and incommensurate phases is emphasized. (The corresponding simplified analytic description is the free fermion approximation [7].) Phase diagrams in the coverage-temperature plane are determined, in particular, from finite-size analysis of the structure factor. In the second part, a new phenomenon is introduced, the adsorption of nonboundary states at interfaces of multi-state models. The phenomenon has been found, indeed, by just looking at typical equilibrium MC configurations [8,9,10]. It motivates an approximate analytic calculation, a modification of the method of Mller-Hartmann and Zittartz, of phase diagrams for such models. The phenomenon resembles the appearance of a wetting layer in a two-fluid system in equilibrium with its vapor phase. Other realizations might be expected at interfaces of adsorption monolayers described by Potts models, like He on graphite.
LATTICE GAS MODELS WITH COMPETING INTERACTIONS Experiments [3,5] on the chemisorbed overlayers [ii],H on Fe(110) and 02 on Pd(110) show that both adsorbates display, as a function of the coverage of the gases, at sufficiently low temperatures uniaxially modulated commensurate (C) phases ((2x1) and (3xl) in the usual notation) as well as incommensurate (IC) structures in between the C phases. Mat. Res.
soc. Symp. Proc. Vol. 21 (1984) @Elsevier Science Publishing Co.,
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