Tunable-excitation soft X-ray fluorescence spectroscopy of high-T-c superconductors: an inequivalent-site seeing story

The study of high-T-c superconductors and related compounds by means of sof t X-ray fluorescence spectroscopy with monochromatic photon excitation is r eviewed. It is shown that this spectroscopy provides a powerful tool for pr obing the local electronic structure at inequivalent O sites. The states in the valence band belonging to different sites are selectively studied by v irtue of large variations in the O Is X-ray absorption cross-section for th ese sites at certain excitation energies as well as by taking advantage of Is chemical shifts between the sites. The character of unoccupied states is analyzed by monitoring the changes in the shape of X-ray fluorescence spec tra with varying energies of incident photons. Issues are addressed, such a s symmetry of doping-induced states and the origin of charge-carriers with respect to inequivalent sites, as well as the influence of electron-electro n interaction. (C) 2000 Elsevier Science BN. All rights reserved.