Theory of adsorbate-induced surface reconstruction on W(100)

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Abstract
We report results of a theoretical study on an adsorbate-induced surface reconstruction. Hydrogen adsorption on a W(100) surface causes a switching transition in the symmetry of the displacements of the W atoms within the ordered c(2×2) phase. This transition is modeled by an effective Hamiltonian, where the hydrogen degrees of freedom are integrated out. Based on extensive Monte Carlo renormalization-group calculations we show that the switching transition is of second order at high temperatures and of first order at low temperatures. This behavior is qualitatively explained in terms of an XY model where there is an interplay between fourfold and eightfold anisotropy fields. We also compare the calculated phase diagrams with a simple mean-field theory.
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