ELECTRONIC-STRUCTURE STUDIES ON THE N-TYPE DOPED SUPERCONDUCTORS R2-XMXCUO4-DELTA (R=PR,ND,SM-M=CE,TH) AND ND2CUO4-XFX BY ELECTRON-ENERGY-LOSS SPECTROSCOPY

The electronic structure of the high-T(c) superconductors R2-xM(x)CuO4-delta (R = Pr, Nd, Sm; M = Ce, Th) and Nd2CuO3.8F0.2 has been studied by high-energy electron-energy-loss spectroscopy in transmission. Core-level spectroscopy, particularly on O 1s and Cu 2p levels yields information on the partial unoccupied density of states near the Fermi level, while that on R and Ce 3d and 4d levels may serve as a valence monitor for these constituents. A pre-edge structure in the O 1s spectrum is polarized in the CuO2 plane and assigned to transition into a two-dimensional conduction band consisting of Cu 3d hybridized to O 2p states. In the Cu 3d9 configuration we found a slight admixture (approximately 10%) of Cu3d3z2-r2 to the Cu 3d(x2-y2) hole states similar to the p-type doped superconductors. Upon doping we recognized a reduction (approximately 14%) of the excitonic Cu 2p03/2 line, indicating that the introduced electrons go onto the Cu sites, thus producing charge carriers having predominantly Cu 3d character.