Energy gap in tunneling spectroscopy: Effect of the chemical potential shift

We study the effect of a shift of the chemical potential level on the tunneling conductance spectra. In the systems with gapped energy spectra, significant chemical-potential dependent distortions of the differential tunneling conductance curves, $dI/dV$, arise in the gap region. An expression is derived for the correction of $dI/dV$, which in a number of cases is found to be large. The sign of the correction depends on the chemical potential level position with respect to the gap. The correction of associated with the $dI/dV$ chemical potential shift has a nearly linear dependence on the tip-sample separation $z$ and vanishes at $z\to0$.