Phase diagram and critical behavior of the adsorption system O/Ru(001): Comparison with lattice-gas models

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Abstract
The chemisorption of oxygen on the Ru(001) surface shows ordered p(2×2) and p(1×2) phases at coverages of 1/4 and 1/2 monolayer, respectively, that are observed to undergo apparently continuous transitions to a disordered state upon heating. We present here details of the experimental determination of the O/Ru phase diagram, lattice-gas models that describe it, and some (but not all) of the observed critical behavior. Monte Carlo simulations based on a phenomenological Hamiltonian describing pairwise interactions between adatoms are used to calculate the phase boundaries and (effective) critical exponents. Restricting adsorption of oxygen atoms to the triangular lattice of the hcp-type hollow sites as seen experimentally, repulsive first- and second-nearest-neighbor interactions are sufficient to describe the observed phase diagram at coverages Θp(2×1) ground states.