NEUTRON SPECTROSCOPY IN 6-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-7) AND LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.2) - CHARGE-TRANSFER, CLUSTER FORMATION AND PERCOLATIVE SUPERCONDUCTIVITY

Neutron spectroscopic data obtained for ErBa2Cu3Ox(6 less than or equa l to x less than or equal to 7) and Pr2-xCexCuO4-delta(O less than or equal to x less than or equal to 0.2) are discussed. Since the rare-ea rth ions in these compounds are situated in close proximity to the sup erconducting CuO2-planes, crystal-held spectroscopy constitutes an ide al local probe of the charge distribution and therefore monitors direc tly changes of the carrier concentration induced by doping. Clear evid ence for cluster formation and percolative superconductivity is provid ed by the observed separation of crystal-field transitions - observed in hole- and electron-doped superconductors - into different local com ponents whose spectral weights depend on the doping level.