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 th
e novel $sqrt{3}timessqrt{3}to2(sqrt{3}timessqrt{3})$ charge and lattice ordering below 270 K evidence a surface charge density wave (SCDW) in the strong e-ph coupling limit ($gapprox8$). Both the adiabatic ratio $hbaromega_0/tapprox0.8$ and the effective pairing energy $V_{eff}=U-2ghbaromega_0approx-800$ $meV$ 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.
