Resolving the coverage puzzle of the Pb/Si(111)-√7x√3 phase

The dense Pb/Si(111)-alpha-root 3 x root 3 has been extensively studied over the last 20 years with scanning tunneling microscopy (STM) and surface diffraction to determine its structure and its phase transformations with temperature T and coverage theta. Two apparently incompatible models have been proposed for the structure and have been debated in the literature. One model was based on a coverage assignment theta=1 ML. The other model was based on assigning coverage in the range 1.2 ML <theta < 4/3 ML. The current experiments show that there are two different root 7 x root 3 phases with the same unit cell symmetry but different stoichiometry. This hopefully answers the debated question.