Surface state bi-polarons formation on a triangular lattice in the sp-type alkali/Si(111) Mott insulator

Abstract

We report on new LEED, STM and ARPES studies of alkali/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 $\sqrt{3}\times \sqrt{3}\to 2(\sqrt{3}\times \sqrt{3})$ charge and lattice ordering below 270 K evidence a surface charge density wave (SCDW) in the strong e-ph coupling limit ($g\approx 8$). Both the adiabatic ratio $\hslash {\omega }_0/t\approx 0.8$ and the effective pairing energy $V_{eff}=U-2g\hslash {\omega }_0\approx -800$ $meV$ are consistent with the possible formation of a bi-polaronic insulating phase consisting of alternating doubly-occupied/unoccupied dangling bonds as expected in the Holstein-Hubbard model.

4 pages, 3 figures. accepted in Physical Review Letters (2009)