APPARENT BARRIER HEIGHT IN SCANNING-TUNNELING-MICROSCOPY REVISITED

The apparent barrier height phi(ap), that is, the rate of change of th e logarithm of the conductance with tip-sample separation in a scannin g tunneling microscope (STM), has been measured for Ni, Pt, and Au sin gle crystal surfaces. The results show that phi(ap) is constant until point contact is reached rather than decreasing at small tunneling gap distances, as previously reported. The findings for phi(ap) can be ac counted for theoretically by including the relaxations of the tip-surf ace junction in an STM due to the strong adhesive forces at close prox imity. These relaxation effects are shown also to be generally relevan t under imaging conditions at metal surfaces.