Suppression and emergence of charge-density waves at the surfaces of layered 1T-TiSe2 and 1T-TaS2 by in situ Rb deposition

In situ real-time angle-resolved photoelectron spectroscopy is used to measure the electronic structure changes at the surfaces of the layered charge-density-wave materials 1T-TiSe2 and 1T -TaS2 during Rb deposition. For 1T-TiSe2, Rb adsorption at a sample temperature of 80K causes a transition from a dirty semiconductor to a metal, in the course of which the p(2 x 2 x 2) charge-density wave is completely suppressed. For 1T-TaS2, Rb adsorption at room temperature is rapidly followed by intercalation, leading to a pronounced metal-to-insulator transition that is correlated with a structural change from the pristine nearly commensurate p(root 13 x root 13)R13.9 degrees superlattice to a commensurate c(2 root 3 x 4)rect. charge-density-wave phase. The implications of the results on the charge-density-wave mechanisms in both compounds are discussed with particular reference to the possible interplay of electron-phonon and electron-electron interactions. It is suggested that strong electron-phonon coupling drives the charge-density wave (CDW) phase transitions in pristine 1T-TiSe2 as well as in Rb intercalated 1T-TaS2, i.e. the transitions are interpreted as primarily Peierls-like instabilities or, in other words, Jahn-Teller band instabilities.