  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.
