Surface-State Bipolaron Formation on a Triangular Lattice in the sp-Type Alkali-Metal/Si(111) Mott Insulator

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 root 3 X root 3 -> 2(root 3 X root 3) charge and lattice ordering below 270 K evidence a surface charge density wave in the strong electron-phonon coupling limit (g approximate to 8). Both the adiabatic ratio (h) over bar omega(0)/t approximate to 0.8 and the effective pairing energy V-eff = U - 2g (h) over bar omega(0) approximate to -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.