Structure of monolayer Ar on Pt(111): Possible realization of a devil’s staircase in two dimensions

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Abstract
The structure of Ar physisorbed on Pt(111) in the submonolayer-coverage regime has been explored by thermal He-atom scattering. On the clean Pt(111) surface Ar condenses in a hexagonal close-packed phase aligned with the substrate. At coverages Θ≲0.7 ML the Ar adlayer lattice parameter is about 3.81 Å. When the Ar coverage increases above 0.75 ML a first-order phase transition into a compressed hexagonal Ar phase with a lattice constant of 3.70 Å is observed. From thermal-expansion measurements and the appearance of superlattice diffraction peaks, the compressed phase is demonstrated to be a high-order commensurate phase, characterized by a (4×4)R0° commensurate unit cell containing nine argon atoms. We have found experimental evidence for further high-order commensurate phases also in the low-Ar-coverage regime. Therefore the phase diagram of the Ar/Pt(111) physisorption system is likely to be a realization of a devil’s staircase in two dimensions.