X-ray spectroscopy study on the electronic structure of hole-doped edge-shared chains in Ca2+xY2−xCu5O10

X--ray absorption (XAS) and emission (XES) spectroscopy near O K edge has been performed on edge--shared CuO chain compounds Ca2+xY2--xCu5O10 (0*0*?x*?2*0) using poly-- and single--crystalline samples. Two large peaks are observed at 528*?eV (PH) and 530*?eV (PU) in the XAS spectra. The intensity of the PH peak monotonically increases and that of the PU monotonically decreases with hole doping. The two characteristic peaks in the XAS spectra at PH and PU, 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 PU peak decreases by hole doping. The spectral weight transfer from the PU peak to the PH peak of the edge--shared chain by hole doping is strongly suppressed in comparison with that of the CuO2 plane. From the single crystal measurements of undoped Ca2Y2Cu5O10, the spectra of XAS and XES are interpreted in terms of a square--like CuO4 plaquette for the compound. As for the hole--doped Ca3YCu5O10 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.