We have identified and characterized a charge-density wave transition ͑T c ϳ 260 ± C͒ in the lowcoverage a phase of the Sn͞Ge(111) interface both experimentally and theoretically. Charge ordering is accompanied by a structural distortion from ͑ p 3 3p 3 ͒R30 ± to ͑3 3 3͒ 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 Sn͞Ge(111) overlayer is fundamentally different from the Pb͞Ge (111) overlayer previously reported. [S0031-9007(97)04249-X] PACS numbers: 73.20.At, 61.16.Ch, 68.35.Md, 71.45.LrA charge-ordered state, or charge-density wave (CDW), incorporates a symmetry-lowering periodic redistribution of valence charge driven by a reduction in the system's total electronic energy, resulting in a small periodic lattice distortion [1]. This phenomenon is most likely to occur in reduced dimensions [2,3], for example, in the layered perovskites [4]. The loss of coordination at a crystal surface might also be expected to invite CDW formation, but few genuine instances of surface charge-density waves have been reported to date. One clear example was recently discovered at the Pb͞Ge(111)-a vacuum interface [5]. This low-density overlayer transforms from the room-temperature (RT) ͑ p 3 3p 3 ͒R30 ± structure to the low-temperature (LT) ͑3 3 3͒ ground state. Charge ordering (ϳ0.5 Å corrugation) and accompanying lattice distortion occur gradually and reversibly with T c ϳ 220 ± C. Density function theory calculations indicate the chargeordered ͑3 3 3͒ structure is the ground state of this system [5]. The opening of a E g ϳ 65 meV band gap below T c (later confirmed by photoemission measurements [6]) must be the consequence of correlation effects [5]. We therefore conjectured that although enhanced electron-phonon coupling (enabled by Fermi surface nesting) drives this transition, many-body interactions stabilize the ground state.Our motivation to study the a phase of Sn͞Ge(111) was threefold. First, seeking verification that Pb͞Ge(111) charge ordering is not an isolated quirk of nature resulting from Pb's exotic properties, we considered the other isostructural adsorbate/substrate combinations that exist. Al, Ga, In, Sn, and Pb all form the same low-density overlayer atop both Si(111) and Ge(111) [7,8]. However, overlayers composed of the trivalent species (Al, Ga, In) will probably not undergo a CDW distortion, as they are semiconducting [7] with an even number of electrons per unit cell at RT.Second, it is imperative to test several theoretically derived concepts, including Fermi surface nesting, and the idea that the CDW ground state is a Mott-Hubbard insulator [5]. Intuitively, the Sn overlayer bandwidth w should be less than that of the Pb overlayer, because the metallic r Sn is ϳ10% smaller than r Pb . For the same reason one expects the on-site Coulomb repuls...
