The Sn/Si(111)-(√3×√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 LSDA+U calculations [ G. Profeta and E. Tosatti Phys. Rev. Lett. 98 086401 (2007).
