Anomalous distance dependence in scanning tunneling microscopy

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Abstract
In this work it is found experimentally that the appearance of surfaces in scanning tunneling microscope (STM) images can change drastically as the distance between the STM tip and sample is varied. Defects are found on gold-sputtered graphite samples which appear as protrusions in charge density when the spacing exceeds a critical value. At smaller distances the protrusions are not evident in the images. It is possible to model these defects as gold atoms which lie just below the surface layer. We discuss possible mechanisms that give rise to the distance dependence.