The study of electronic structure in new Hg-based Sr-containing 1212-typed (Hg_0.5Pb_0.5)Sr₂(Ca_1-xY _x)Cu₂O₇ superconductors

High resolution O K-edge X-ray-absorption near-edge-structure (XANES) spectra for the series of new Hg-based Sr-containing 1212-typed (Hg_0.5Pb_0.5)Sr₂(Ca_1-xY _x)Cu₂O₇ superconductors (x = 0.3 ∼ 0.7) were measured using a bulk-sensitive total-X-ray-fluorescence-yield technique. Near the O 1s edge, the pre-edge peak with maxima at ∼ 528.3 eV is ascribed to transitions into O 2p holes located in the CuO₂ planes. The intensity of this pre-edge peak monotonically increases with increasing doping level of Ca²⁺ into the Y³⁺ sites. The effect of chemical substitution of Ca²⁺ for Y³⁺ in (Hg_0.5Pb_0.5)Sr₂(Ca_1-xY _x)Cu₂O₇ is to induce O 2p hole states in the CuO₂ planes near the Fermi level which cause an increase of T_c from 15 K for x = 0.6 to 90 K for x = 0.3. Moreover, the generation of O 2p holes within the CuO₂ planes is probably responsible for inducing a transition from a semiconductor to a superconductor.