Dichotomy of metallic electron density and charge density wave in 1T-TiSe2

1T-TiSe2 has been intensely investigated for its intriguing charge density wave (CDW) phase, which competes with emerging superconductivity. However, the mechanism of the CDW transition has been elusive with the possibility of strong excitonic interaction. Here, we investigate, using angle-resolved photoelectron spectroscopy and density functional theory calculations, the evolution of the CDW band structure upon electron doping into the surface layer by alkali metal adsorbates. Alkali metal adsorption induces substantial electron donation into the Ti 3d conduction band with strong band renormalization while the Se 4p valance band is contrastingly intact. The density functional theory calculations reveal that only the spectator-type Ti 3d band is selectively doped with the CDW band gap largely intact. This result indicates that the CDW formation is not critically related to the metallic electron density and, in turn, to the excitonic coupling due to its unique multiband configuration.