The $\mathrm{Sn}/\mathrm{Si}(111)\mathrm{\text{−}}(\sqrt{3}\times \sqrt{3})R30°$ surface was so far believed to be metallic according to the electron counting argument. We show, by using tunneling spectroscopy, scanning tunneling microscopy, photoemission, and photoelectron diffraction, that below 70 K this surface has a very low density of states at the Fermi level and is not appreciably distorted. The experimental results are compatible with the insulating Mott-Hubbard ground state predicted by $\mathrm{LSDA}+U$ calculations [G. Profeta and E. Tosatti, Phys. Rev. Lett. 98, 086401 (2007)].

• Received 31 August 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.126401