title: Surface-state bipolaron formation on a triangular lattice in the sp-type alkali-metal/Si(111) Mott insulator.
abstract: We report on new low-energy electron diffraction, scanning tunneling microscopy, and angle-resolved photoemission spectroscopy studies of alkali-metal/Si(111) previously established as having a Mott-insulating ground state at surface. The observation of a strong temperature dependent Franck-Condon broadening of the surface band together with the novel sqrt[3] x sqrt[3] --> 2(sqrt[3] x sqrt[3]) charge and lattice ordering below 270 K evidence a surface charge density wave in the strong electron-phonon coupling limit (g approximately 8). Both the adiabatic ratio variant Planck's over 2piomega_{0}/t approximately 0.8 and the effective pairing energy V_{eff} = U - 2gvariant Planck's over 2piomega_{0} approximately -800 meV are consistent with the possible formation of a bipolaronic insulating phase consisting of alternating doubly occupied and unoccupied dangling bonds as expected in the Holstein-Hubbard model.
authors: Institut Jean Lamour, UMR 7198, Nancy Université/CNRS, B.P. 239 F-54506 Vandoeuvre-lès-Nancy, France.
authors: L A
authors: LA
authors: Cardenas
authors: Y
authors: Y
authors: Fagot-Revurat
authors: L
authors: L
authors: Moreau
authors: B
authors: B
authors: Kierren
authors: D
authors: D
authors: Malterre