Hole formation and charge transfer in Y_1-xCa_xSr₂Cu₂GaO₇, a new oxide superconductor

The technique of transmission electron energy loss spectrometry (EELS) in parallel detection has been utilized to analyze the fine structures associated with the O K and Cu L₃ absorption edges of recently discovered Y_1-xCa_xSr₂Cu₂GaO₇ (0.1 ≤ x ≤ 0.4) series of compounds, which exhibit superconductivity when annealed under high-pressure oxygen atmosphere. We find subtle but significant changes in EELS spectral features of nonsuperconducting and superconducting specimens of these compounds. A broad pre-edge feature is observed below the O K absorption edge, at about 528.2 eV which emerges with Ca-doping alone. Evidence is presented for another O K secondary pre-edge feature, at about 1.1 eV below the first O K pre-edge feature in only those specimens which have undergone high-pressure oxygen treatment (i.e. superconducting specimens). We interpret the O K pre-edge feature as due to formation of holes on oxygen sites and propose that the first broad pre-edge feature (at 528.2 eV) is associated with holes on oxygen sites other than the CuO₂ planes, which are responsible for normal conductivity. The second smaller pre-edge feature (at 527.1 eV) is most probably associated with holes in the CuO₂ planes, which are associated with superconductivity in this system. The presence of a secondary smaller O K pre-edge feature in the superconducting specimens appears to correlate well with a broad shoulder on the high energy side of the Cu L₃ edge. The results are interpreted in terms of oxidation of CuO₂ planes through charge transfer between copper and oxygen in the CuO₂ planes, i.e. covalent mixing of O and Cu orbitals at the Fermi level as a result of high oxygen pressure annealing.