Interpretation of high-order commensurate phases for an argon monolayer adsorbed on Pt(111)

Publisher Website
Google Scholar
Abstract
Recent helium atom scattering experiments have shown that Ar monolayers form high-order commensurate (HOC) phases on Pt(111) substrate at low temperature and that HOC-HOC transitions proceed as functions of coverage and temperature. This leads to the realization of a two-dimensional devil’s staircase. Potential calculations using effective pairwise Pt-Ar and Ar-Ar interactions and substrate-mediated Ar-Ar/Pt contributions are performed to determine the equilibrium geometries of the monolayer vs Ar coverage. The free energy calculated at finite temperature (T≤50 K) shows that five metastable hexagonal HOC structures occur within the range 31 ≤T≤46 K. When T increases in this range, the nonrotated structures appear with increasing lattice parameters 3.71, 3.78, 3.82, 3.84, and 3.89 Å. The most stable geometry is a (4×4) unit cell with 9 Ar atoms. Rotated phases exhibit the same behavior with, however, a slightly shifted temperature range and shifted values for the lattice parameter. These results are in close agreement with the experimental findings and they corroborate the devil’s staircase event for such a system.