ATOMIC-SCALE TIP SAMPLE INTERACTIONS AND CONTACT PHENOMENA

Tip-sample interactions become crucial owing to increased overlap at small tip-sample separation. The potential barrier collapses before the point of maximum attraction on the apex of the tip, but the effective barrier may remain significant owing to the strong confinement of current-carrying states to the constriction between tip and sample. At such separations the perpendicular tip force is still attractive and determined by ion-ion repulsion and redistribution of electronic charge. Electronic states are modified by the tip-induced perturbation of the potential in the vicinity of the tip. Self-consistent calculations reveal that local properties, such as elastic deformation, effective height and width of the tunneling barrier, electronic states and attractive tip force are site-dependent and reversible on the atomic scale. Numerical results suggest a relation between the perpendicular tip force and barrier height as a function of separation. A mechanical contact is formed with relatively strong bonds at separation near the point of zero force gradient. Whether the effective potential can collapse and hence the first channel can open to allow a transition from tunneling to ballistic conduction, and whether the conductance can show quantized steplike changes with increasing plastic deformation depends on material properties.