van der Waals atomic trap in a scanning-tunneling-microscope junction: Tip shape, dynamical effects, and tunnel current signatures

The growing interest in the study of artificial nanoscale structures stabilized by a corrugated surface calls for specific models adapted to the low symmetry of such systems. In the case of physisorbed species, such atomic patterns can be realized by controlling the magnitude of the van der Waals trap generated by the apex of a thin metal tip. In this work, the van der Waals interaction between a Cu(110) surface, a xenon atom, and the copper probe tip of a scanning tunneling microscope (STM) is investigated. The dispersion energy contribution between the xenon atom and the discrete tip apex is determined at the N-body order by solving Dyson's equation. From this procedure, we investigate the stability of the adsorbate for different shapes and sizes of the probe. When we consider the entire STM junction, a van der Waals trap occurs within a small tip-surface spacing. The magnitude of this trap can reach about 30 meV, which has to be compared with the physisorption energy of similar to 180 meV of a single xenon atom on the Cu(110) surface. From this model system the important question of the atomic displacement with a STM tip, as realized experimentally by Eigler and Schweizer [Nature 344, 524 (1990)], is revisited. To achieve this purpose, we have studied the dynamical atomic dragging with the [100], [110], and [111] oriented tips: We have found that the adsorbate is pulled by the [110] tip and is displaced in front of the two other types of tip. Finally, by calculating the tunnel current during the motion of the adsorbate in the junction, we were able to extract a current signature directly related to the nature of the moving process.