NEUTRON SPECTROSCOPY IN RBA(2)CU(3)O(X) (R=RARE EARTH, 6-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-7) AND R(2-X)CE(X)CUO(4-DELTA) LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.2) COMPOUNDS - CHARGE-TRANSFER, CLUSTER FORMATION, PERCOLATIVE SUPERCONDUCTIVITY, CHARGE FLUCTUATIONS

Neutron spectroscopic experiments performed for the high-T-c supercond ucting hole-doped RBa(2)Cu(3)Ox (R=rare earth; 6 less than or equal to x less than or equal to 7) and electron-doped R(2-x)Ce(x)CuO(4-delta) (0 less than or equal to x less than or equal to 0.2) compounds are d iscussed. In these systems the R ions are situated close to the superc onducting CuO2 planes, thus the crystal-field interaction at the R sit e constitutes an ideal local probe of the charge distribution and ther eby monitors directly changes of the carrier concentration induced by doping. For several compounds the observed crystal-field spectra separ ate into different local components whose spectral weights distinctly depend on the doping level, i.e., there is clear experimental evidence for cluster formation. The onset of superconductivity can be shown to result from percolation, i.e., the superconductivity is an inhomogene ous property in the persovskite-type compounds. From a line-width anal ysis of the observed crystal-field transitions we derive the evolution of the fractal sizes of the clusters versus doping. At high doping th e neutron spectroscopic data reveal anomalies which are interpreted in terms of copper-oxyde charge fluctuations.