Electronic correlation effects and Coulomb gap in the tunneling experiments on low-conductive Si(111)-(√3x√3)-Sn and Si(111)-7x7 surfaces

The electronic transport properties of the 2D surfaces Si(111)-(√3x√3)-Sn and Si(111)-7x7 formed on low-doped Si substrates are studied via measurements of the temperature dependence of the conductivity and scanning tunneling spectroscopy (STS). We show that the real electronic band structure not always corresponds to the measured tunneling dI/dV spectra which should provide the local density of states. This happens on low-conductive surfaces with sheet conductance 1/Rh << e2/h. This effect modifies the tunneling dI/dV spectra at low voltages and suppresses the differential tunnel conductance dI/dV due to Coulomb  blockade in a single-junction system. We demonstrate that this effect dominates in STS study of the Si(111)-(√3x√3)-Sn and Si(111)-7x7 surfaces at low temperatures and converts the soft Coulomb gap in surface band structure at the Fermi level into a hard gap, observed in dI/dV spectra. We demonstrate that the value of the Coulomb gap depends on the localization length of the charge carrier ξ, which we obtained from transport measurements.