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 $\sqrt{ 3}\times\sqrt{ 3}\to2(\sqrt{ 3}\times\sqrt{ 3})$ charge and lattice ordering below 270 K evidence a surface charge density wave (SCDW) in the strong e-ph coupling limit ($g\approx8$). Both the adiabatic ratio $\hbar\omega_0/t\approx0.8$ and the effective pairing energy $V_{ eff}=U-2g\hbar\omega_0\approx-800$ $meV$ 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.
Categories: cond-mat.str-el cond-mat.supr-con