Observation of a Mott insulating ground state for Sn/Ge(111) at low   temperature
  We report an investigation on the properties of 0.33 ML of Sn on Ge(111) at temperatures down to 5 K. Low-energy electron diffraction and scanning tunneling microscopy show that the (3x3) phase formed at 200 K, reverts to a new (root-3xroot-3)R30 phase below 30 K. The vertical distortion characteristic of the (3x3) phase is lost across the phase transition. Angle-resolved photoemission experiments show that concomitantly with the structural phase transition, a metal-insulator phase transition takes place. In agreement with theoretical predictions, the (root-3xroot-3)R30 ground state is interpreted as the experimental realization of a Mott insulator for a narrow half-filled band in a two-dimensional triangular lattice. 
R. Cortes, A. Tejeda, J. Lobo, C. Didiot, B. Kierren, D. Malterre,   E.G. Michel, and A. Mascaraque