ELECTRON-ENERGY LEVELS IN ND2-XCEXCUO4 - A STUDY BY VALENCE-LEVEL AND CORE-LEVEL PHOTOEMISSION SPECTROSCOPY

Valence- and core-level photoemission spectra of vacuum-annealed polycrystalline Nd2-xCexCuO4 ceramics have been studied over the composition range 0 < x < 0.20. The cleaned surfaces give simple O1s core line shapes in x-ray photoemission spectroscopy (XPS) and metallic samples with x values in excess of 0.1 display x-ray and Ne(I) excited valence-region spectra showing a distinct Fermi-edge cutoff. Valence-band and core-level binding energies are inconsistent with filling of a rigid conduction band. Moreover, there is little change in the Cu 2p XPS line shape across the series. This observation is discussed in relation to the Larsson-Sawatzky model. The Cu 2p core-level data for Nd2-xCexCuO4 are compared with those for La2-xSrxCuO4. The experimental work reveals a stabilization of Cu 3d levels in Nd2CuO4 as compared with La2CuO4 and suggests a negative charge-transfer parameter DELTA in the former compound.