Electronic structure of the Ba-doped (Pb,Cd)-1212 cuprates

Changes in the electronic structure for the (Pb_0.5Cd_0.5)(Sr_2-xBa_x)(Ca _0.5Y_0.5)Cu₂O₇ (O ≤ x ≤ 0.5) system associated with an semiconductor-superconductor transition have been investigated by means of X-ray photoelectron spectroscopy. The main line of Cu2p_3/2 core level and the valence-band peak in the range 0 ∼ 8 eV for the samples are broadened with Ba doping, which suggest that there are two charge-transfer mechanisms; in-plane O 2p_x,y → Cu3d_x2-y2 and apical O 2p_z → Cu3d_3z2-r2, in the system. For the higher doped (x = 0.4 and 0.5) superconductive samples, the latter charge transfer seems to be more pronounced than that for the semiconductor samples (x ≤ 0.3). Pb4f and O 1s core-level spectra show that as the Ba doping content increases the oxidation state of Pb increases. The state of density in the Fermi level increases slightly, and a new state appears more clearly just below the Fermi level with the partial substitution of Ba for Sr, which can be attributed to the presence of apical O 2p_z derived holes in the (Sr,Ba)-O layers. The evidence suggests that the appearance of superconductivity in the (Pb,Cd)-1212 system has a relation with the changes in electronic structure, especially the apical O 2p_z → Cu3d_3z2-r2 hybridization mechanism.