Spectral function of the K/Si(1Â 1Â 1):B surface state: the bipolaronic CDW scenario

Very recently, new LEED, STM and ARPES measurements on the (1/3)ML K/Si(1 1 1):B interface have shed the light on the role of phonons and force to reconsider the nature of the ground state, formerly proposed to be a Mott insulator. In this paper, we present simulations of the one-electron spectral function A(E). These are based on the atomic Holstein-Hubbard model, which is relevant in the strong electron-phonon coupling limit. The exact spectral function has been analysed carefully with special attention on the gap behaviour as a function of the on-site repulsion. We show that the spectral funcion is the same for opposite values of the effective on-site repulsion. The observed reconstruction favours the negative-Ueff case, which corresponds to a bipolaronic charge-ordered ground state. Finally, the ARPES lineshape is reproduced correctly with fitting parameters in agreement with the literature.