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 (3 x 3) phase formed at similar to 200 K, reverts to a new (root 3 x root 3)R30 degrees phase below 30 K. The vertical distortion characteristic of the (3 x 3) phase is lost across the phase transition, which is fully reversible. Angle-resolved photoemission experiments show that, concomitantly with the structural phase transition, a metal-insulator phase transition takes place. The (root 3 x root 3)R30 degrees ground state is interpreted as the formation of a Mott insulator for a narrow half-filled band in a two-dimensional triangular lattice.