Studies of 2D cryocrystals by STM techniques

Scanning tunneling microscopy (STM) studies of two-dimensional (2D) cryocrystals (He-4, Kr, Xe) physisorbed on graphite surfaces are presented. Individual helium atoms, usually thought to be invisible with STM, were recently observed on graphite surfaces at a density corresponding to the root 3 x root 3 commensurate solid. Here we show that a local elastic deformation seems to be the principal mechanism responsible to render the atoms visible. Recent tight-binding calculations of the local density of states (LDOS) of graphite which predict the appearance of an energy gap support this picture. I-z curve measurements for the case of He-4 show a sharp drop (increase) of the tunneling current I at a certain tip-surface distance z during retraction (approach) of the tip. This drop (increase) may be associated with the tunneling of a single Lie adatom, opening new possibilities to study the quantum tunneling of atoms via STM.