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

We have identified and characterized a charge-density wave transition $(T_c\sim -60\mathrm{}°\mathrm{C})$ 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 $(\sqrt{3}\times \sqrt{3})R30\mathrm{}°$ to $(3\times 3)$ 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.