Electronic structure in (Hg_0.5Pb_0.5) Sr₂(Ca_{1 - X}Y_x) Cu₂O₇ compounds studied by soft X-ray absorption spectroscopy

We report O K-edge and Cu L₂₃-edge X-ray-absorption near-edge-structure (XANES) spectra for the series of (Hg_0.5Pb_0.5)Sr₂(Ca_{1 - x}Y_x)Cu₂O₇ compounds (x = 0.3-0.7) obtained 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 the transitions to 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 - x}Y_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. In the Cu L-edge absorption spectra, high-energy shoulders at 933.2 and 953.0 eV are attributed to the transitions to the Cu(2p_3/2,1/2)^-13d¹⁰L states in the CuO₂ layers, where L denotes the O 2p ligand hole. The behavior of these shoulders correlates with that of the pre-edge peak at ∼ 528.3 eV in the O K-edge absorption spectra.