PHASE-DIAGRAM AND PHASE-TRANSITIONS IN THE SYSTEM H/RU(0001) - A MONTE-CARLO STUDY

We study a lattice gas model with pair and three-body interactions up to the third neighbour shell on a triangular lattice by Monte Carlo methods to simulate the adsorbate system hydrogen/ruthenium(0001). This system has an asymmetric theta-T phase diagram with respect to coverage theta = 1/2. To get qualitative agreement we need only first and second neighbour pair interactions and one three-body term, while the quantitative agreement requires an additional three-body term. We investigate the nature of the order-disorder phase transitions by calculating critical exponents to compare our model with the experiment. Two continuous phase transitions and one weak first-order transition were found. The numerical results are accurate within 10% to the experimental data.