Apparent barrier height in scanning tunneling microscopy

The apparent barrier height in the scanning tunneling microscope, defined in terms of the rate of change (at constant bias) of the logarithm of current with tip-sample separation, is calculated as a function of separation. It is found to be substantially lower than the sample work function in the range of tip-sample separations commonly used experimentally, and becomes very small in the region just before contact between tip and sample. © 1988 The American Physical Society.