The electronic structure of ultrathin Ag films with 1/3 monolayer ͑ML͒ of Pb alloyed at the surface was investigated by angle-resolved photoemission spectroscopy. Compared to clean ultrathin Ag films, the energy positions of the quantum-well states ͑QWSs͒ moved closer to the Fermi level due to the change of the potential barrier at the film/vacuum interface. We found that the parabolic band dispersion of the QWSs become disturbed where they cross the surface-state bands, and furthermore, they were shown to follow the periodicity that is only present at the surface. Our results suggest that it is possible to tune the properties of the QWSs whose thickness is more than 10 ML just by submonolayer deposition of heteroelements at the surface.

The transport properties of the Si͑111͒ ͱ 3 ϫ ͱ 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 ͓G. Profeta and E. Tosatti, Phys. Rev. Lett. 98, 086401 ͑2007͔͒ predicting this surface to be a Mott insulator.