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

Abstract Changes in the electronic structure for the (Pb 0.5 Cd 0.5 )(Sr 2− x Ba x )(Ca 0.5 Y 0.5 )Cu 2 O 7 (0 ≤ x ≤ 0.5) system associated with an semiconductor-superconductor transition have been investigated by means of X-ray photoelectron spectroscopy. The main line of Cu 2 p 3 2 core level and the valence-band peak in the range of 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 → Cu 3d x 2 − y 2 and apical O 2p z → Cu 3d 3 z 2 − r 2 , 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 ls 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 → Cu 3d 3 z 2 − r 2 hybridization mechanism.