Electron correlation effects in transport and tunneling spectroscopy of the Si(111)-7 x 7 surface

The electronic properties of the Si(111)-7 x 7 surface are studied using four-and two-probe conductivity measurements and tunneling spectroscopy. We demonstrate that the temperature dependence of the surface conductivity corresponds to the Efros-Shklovskii law at least in the 10-100 K temperature range. The energy gap at the Fermi level observed in tunneling spectroscopy measurements at T >= 5 K vanishes by thermal fluctuations at T approximate to 30 K, without any sign of a metal-insulator transition. We show that the low-temperature energy gap observed by the tunneling spectroscopy technique is actually a consequence of the Coulomb blockade effect.