Theory of scanning tunneling spectroscopy

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Abstract
Recent developments of the theory of Scanning Tunneling Spectroscopy (STS) are reviewed. The basis for the use of the Scanning Tunneling Microscope (STM) as a spectroscopic tool is first discussed. Then, we review the studies of the interface potential and its change as the gap width is varied; in this context, the calculations of the contact resistance are also considered. The close relationship between surface spectroscopy and topography, as viewed by means of the STM, is remarked upon. As a dramatic example we consider the unusual corrugations observed in systems such as Si(111)2 × 1 or graphite, apparently not related to their surface geometry. Of special interest is the spectroscopy of localized states (image or surface states). Relevant questions arise such as the way in which a tunneling current through states located at the surface can be built up; implications on I-V of the mechanisms which sustain the current are highlighted. The use of STS in the study of adsorbed species are also reviewed. Finally, recent theoretical efforts carried out to interpret STS data for superconductor materials are discussed.