Photoemission study of chemical bond nature of Pb-3212 epitaxial films

A change of electronic structure of Ca substituted (Pb₂Cu)Sr₂(Dy_1-xCa_x)Cu₂ O_8±δ (x=0.22-0.2 [Pb-3212] epitaxial films with oxidation-reduction treatment has been investigated by in-situ photoemission spectroscopy. The films with excess oxygen introduced by the cooling-procedure in post-deposition processing and annealing with an intensive active-oxygen beam were semiconducting. Coexistence of 4+ and 2+ states of Pb ions and a high binding energy component of Cu 2p _3/2 signals exceeding that of CuO were observed. Vacuum-annealing yielded a vanishing of the Pb⁴⁺ component and a decrease of the binding energy of Cu 2p_3/2 signals. The reduced films were converted to metallic and superconducting. It should be noted that, in ultraviolet photoemission spectra of the reduced films, the Fermi edge is observed for the first time for the Pb-system films. These results reveal that, in the excessively oxidized films, the major part of holes should be absorbed into (Pb₂Cu) ions in the charge-reservoir block. By contrast, in the reduced ones, electron-absorption into these cations should cause a rise of hole concentration of the CuO planes. This charge-redistribution should dominate the transport properties and be sensitive to post-deposition conditions