Electronic structure in (Hg0.5Pb0.5)Sr-2(Ca1-xYx)Cu2O7 compounds studied by soft X-ray absorption spectroscopy

We report O K-edge and Cu L(23)-edge X-ray-absorption near-edge-structure (XANES) spectra for the series of (Hg0.5Pb0.5)Sr-2(Ca1-xYx)Cu2O7 compounds (x = 0.3-0.7) obtained using a bulk-sensitive total-X-ray-fluorescence-yield technique. Near the O Is edge, the pre-edge peak with maxima at similar to 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 (Hg0.5Pb0.5)Sr-2(Ca1-xYx)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))(-1)3d(10)L 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 similar to 528.3 eV in the O K-edge absorption spectra.