NEUTRON SPECTROSCOPIC EVIDENCE FOR CLUSTER FORMATION AND PERCOLATIVE SUPERCONDUCTIVITY IN PR2-XCEXCUO4-DELTA LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.2)

We used neutron crystal field spectroscopy as a local probe to investi gate cluster formations in the electron-doped compound Pr2-xCexCuO4-de lta. The observed energy spectra were indeed found to result from a su perposition of three different components which we interpret in terms of local regions of undoped, intermediately doped and highly doped cha racter. The volume fraction of the doped clusters is shown to reach th e percolation limit for x approximate to 0.14 where the onset of super conductivity is observed. The highly doped clusters are presumably con trolled by a charge transfer mechanism.