ONE-DIMENSIONAL ORDERING OF IN ATOMS IN A CU(100) SURFACE

A Monte Carlo study of the ordering of In atoms embedded in the top layer of a Cu(100) surface is presented. The interaction energies between the In and Cu atoms were derived from atom-embedding calculations, with Finnis-Sinclair potentials. It was found that the interaction between In atoms in the close-packed [110] rows is repulsive, while it is attractive in the [100] direction. The strengths of the repulsive and attractive interactions were found to be such that it is energetically favorable for In atoms to form one-dimensional rows in the [100] direction instead of a two-dimensional c (2 X 2) structure. The ordering of In atoms was studied for In coverages between 5% and 25% of a monolayer and for temperatures between 50 and 2000 K. Below a critical temperature, one of the two [100] diagonal directions prevails. Above this critical temperature, the system consists of two types of domains in each of which one diagonal direction dominates. Close to the critical temperature these domains are large and become smaller with increasing coverage. From a statistical analysis of the Monte Carlo simulation results, the entropy, internal energy, and average In-row length were calculated as a function of temperature and In coverage. The results were compared with the results of perturbed-angular-correlation experiments. The main trends observed in the experiments were reproduced by our calculations.