Abstract
A lattice-gas model for the phase transitions of O monolayers on Mo(110) with interactions ranging up to fifth neighbors, and including a three-particle term, is studied by Bragg–Williams mean-field theory and finite-size scaling of data from Monte Carlo simulations and numerical transfer-matrix calculations. Attention is focused on the p(2×2) phase whose symmetry is that of an XY model with cubic anisotropy. The order–disorder transition lines are shown to exhibit the predicted nonuniversal behavior, and possess tricritical points. A phenomenological finite-size scaling theory for the Monte Carlo order-parameter data in the pertinent case of a third-order coupling in the Landau free energy is established. The order–order transition line between the p(2×2) and (2×1) phases is found to consist of two parts, a first-order line at low temperature, and a line at higher temperatures, along which the model has a pure Kosterlitz–Thouless phase. Good agreement is established with an experimental phase diagram based on low-energy electron diffraction (LEED) data.
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