Electronic structure of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ studied by soft-x-ray-fluorescence spectroscopy with tunable excitation

The O Kα x-ray fluorescence of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ (x=0.02, 0.04, 0.07, 0.1, 0.15) has been recorded using tunable and high-resolution monochromatized synchrotron-radiation excitation. A strong variation in the shape of the O Kα emission spectra depending on incidence photon energy is observed. This variation results from different in-plane and apical oxygen contributions to the emission line. The x-ray emission bands which reflect the partial density of 2p states pertaining to the two inequivalent oxygen sites are distinguished. The experimental results show that holes induced by the Sr doping are mainly located on in-plane oxygen atoms and suggest the existence, at least at x≥0.07, of doped holes which have Cu 3${\mathit{d}}_{\mathit{x}\mathit{z},}$ ${\mathit{yz}}_{\mathrm{-}}$O 2${\mathit{p}}_{\mathit{z}}$π character.