Simple Model for Striped-incommensurate Phase Formed by Pb Adsorption on Si(111)

Striped-incommensurate (SIC) phase is formed on Si(111) around lead (Pb) coverage of 1.30. The structure of the SIC phase is difficult to analyze, since it is incommensurate. Even an approximate model requires a large unit cell. In this paper, we propose a simple model based on a ball model, and show that the driving force which leads to SIC phase is compressive stress inherent to a hypothetical perfect (√3×√3)R30° Pb arrangement on Si(111). The stress is relieved by uniaxial [-110] expansion of the Pb layer and subsequent slight twin-like deformation. The modification enables Pb atoms to occupy stable H3 sites or T4 sites as many as possible, and makes stable the resultant arrangement. We show that these models well account for various features observed with scanning tunnelling microscopy. We also apply our model to devil’s staircase reported around similar Pb coverages, and show that higher-coverage phases in devil’s staircase family should be understood with our model.