The Feenstra parameter

One of the major problems in doing tunneling spectroscopy with STM is that the tunneling current exhibits a dramatic dependence on tip-sample separation 

Another possible definition of the relative dynamic conductance is 

For semiconductors, in the vicinity of V = 0, the conductance is very small, and the relative accuracy is low. A common practice to deal with this situation is to replace the inaccurate section by a small constant using interpolation. 

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