Structure, superconductivity and XPS studies of the Bi_2.1Sr_1.93Ca_0.97-xY_xCu₂O_8+y system

Abstract

A systematic study of the effect of Y doping at the Ca site in the high-T_c superconductor Bi2212 has been made. Starting with an off-stoichiometric composition Bi_2.1Sr_1.93(Ca_0.97-xY_x)Cu₂ O_8+y (BiY2212) by power X-ray diffraction, resisti vity, AC and DC susceptibility and XPS measurements have shown the single-phase nature for all values of x. The decrease in T_c observed systematically by both resistivity and AC susceptibility measurements with increasing Y concentration, and the subsequent metal-semiconductor cross-over, is explained as due to the decrease in the average valence of the copper brought about by the excess charge introduced by Y. The Cu 2p core level shows that the relative intensity of the satellite, the energy separation between main line and satellite, and the binding energy of the main line vary in a systematic way. A decrease in charge-transfer energy and an increase in the hybridisation strength is predicted for higher x with the configuration interaction model. The presence of internal strain in this superconductor makes the BVS calculation not applicable.