Doping of a one-dimensional Mott insulator: Photoemission and optical studies of Sr₂CuO_3+δ

Angle-resolved photoemission and optical spectroscopy are used to probe the electronic structure of the one-dimensional Mott insulator Sr₂CuO_3+δ, both at half-filling and with small concentrations of excess oxygen doping. Spin-charge separation can be seen as evidenced by the existence of spinon and holon branches in the photoemission spectra. Optical studies reveal no significant doping dependence, while photoemission studies show a large energy shift in the spinon and holon states with respect to the main valence band states which remain nearly unaffected. The results suggest the excess dopant carriers are incorporated solely within the one-dimensional Hubbard band which is electronically isolated from the rest of the states in the system.