Two-dimensional roughening of adsorbate islands in thermodynamic equilibrium

Equilibrium fluctuations of islands of adsorbed O atoms on Ru(0001) were investigated by scanning tunneling microscopy (STM), density functional theory calculations (DFT) and Monte Carlo (MC) simulations. Very ramified (2 x 2)-O islands were observed by high-speed STM that point to complex interactions between the O atoms. The DFT calculations show that, in addition to pairwise attractive interactions between third-nearest neighbors, a repulsive three-body interaction exists between these. MC simulations that include three-body interactions reproduce the observed ordering behavior.