RESONANT PHOTOEMISSION FROM COMPLEX CUPRATES AND NICKELATES

Synchrotron-excited resonant-photoemission measurements at rare-earth 4d --> 4f and transition-metal 3p --> 3d thresholds have been carried out using a variety of complex cuprates and nickelates on stations 6.1 (grazing-incidence monochromator) and 6.2 (toroidal-grating monochrom ator) at the SRS CLRC Daresbury Laboratory. The systems studied are Nd 2Ni1-xCuxO4, La2-xSrxNi1-yFeyO4+delta and Bi2Sr2Ca1-xYxCu2O8+delta. A combination of EDC and constant-initial-state data is used to examine the 4f and 3d contributions to the valence-band density of states and their binding-energy positions relative to the Fermi energy. This allo ws the study of the valence states of the transition-metal ions and th eir modulation on doping. For La2-xSrxNi1-yFeyO4+delta?, this approach is used to infer a valence state of greater than or equal to 3.0 for Fe. In the case of Bi2Sr2Ca1-xYxCu2O8+delta, the effect of Cu valence modulation on the 3p resonance is observed as x is varied. This is dis cussed in the light of controversy surrounding shifts in core-level ph otoemission with doping for this system.