Surface charge ordering transition: alpha phase of Sn/Ge(111)

We have identified and characterized a charge-density wave transition (T-c similar to -60 degrees C) in the low-coverage ct phase of the Sn/Ge(111) interface both experimentally and theoretically. Charge ordering is accompanied by a structural distortion from (root 3 x root 3)R30 degrees to (3 x 3) symmetry. Density-functional theory calculations are unable to correctly reproduce the observed ground state and, more importantly, indicate that Fermi surface nesting does not play a role in this transition. Both signal the importance of many-body effects in this system. Experiment and theory indicate that the Sn/Ge(111) overlayer is fundamentally different from the Pb/Ge(111) overlayer previously reported.