Scanning-tunneling microscopy at small tip-to-surface distances

The scanning-tunneling microscopy (STM) of graphite at small tip-to-surface distances is investigated using the self-consistent-field pseudopotential method. We have calculated potential, charge density in the region between the tip and surface, and the force corrugation. Our results reveal that the tip at the close proximity to the surface disturbs the states near the Fermi level, and induces localized states. The STM images, which are usually related to the local density of states at the Fermi level of the clean surface, are affected by these localized states. The tunneling barrier is shown to collapse at small distances and a new mechanism for current is postulated. Some experimental evidence for this effect is presented. © 1987 The American Physical Society.