Surface Charge Ordering Transition: α Phase of Sn/Ge(111)

We have identified and characterized a charge-density wave transition (Tc~-60^∘C) in the low-coverage {α} phase of the Sn/Ge(111) interface both experimentally and theoretically. Charge ordering is accompanied by a structural distortion from (√3×√3)R30^∘ to (3×3) 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.