Insulating conduction in Sn/Si(111): Possibility of a Mott insulating ground state and metallization/localization induced by carrier doping

The transport properties of the Si (111) √3×√3-Sn surface are investigated by micro-four-point-probe conductivity measurements. The temperature dependence of the surface-state conductivity showed an insulating behavior from room temperature down to 15 K although the surface was believed to be metallic. Furthermore, with changing the band filling by partially replacing Sn atoms with In or Na deposition, we found that the conductivity showed a metallic behavior down to 260 K and upon further cooling, the carriers became strongly localized possibly due to the dopants themselves. Our result suggests that the ground state of this surface is insulating with a very small energy gap, which is consistent with a recent theoretical study predicting this surface to be a Mott insulator.