X-ray spectroscopy study on the electronic structure of hole-doped edge-shared chains in Ca_2+xY2-xCu_5O10

X-ray absorption (XAS) and emission (XES) spectroscopy near O K edge has been performed on edge-shared CuO chain compounds Ca_2+xY2-xCu _5O10 (0.0≤x≤2.0) using poly- and single-crystalline samples. Two large peaks are observed at 528 eV (P_H) and 530 eV (P _U) in the XAS spectra. The intensity of the P_H peak monotonically increases and that of the P_U monotonically decreases with hole doping. The two characteristic peaks in the XAS spectra at P _H and P_U, which are assigned for the polycrystalline samples to the hole and upper Hubbard band (UHB) states, respectively. The theoretical calculations reproduce the experimental XAS results, especially that the P_U peak decreases by hole doping. The spectral weight transfer from the P_U peak to the P_H peak of the edge-shared chain by hole doping is strongly suppressed in comparison with that of the CuO ₂ plane. From the single crystal measurements of undoped Ca _2Y2Cu_5O10, the spectra of XAS and XES are interpreted in terms of a square-like CuO₄ plaquette for the compound. As for the hole-doped Ca₃YCu_5O10 sample, the plaquette containing a doped hole is found to be square-like, and the plaquettes with no doped holes are found to be rectangle-like. We concluded that the doped hole is localized associated with the lattice deformation. The electric properties are discussed based on our result.