Electronic Structure of Ce-Doped and -Undoped Nd2CuO4 Superconducting Thin Films Studied by Hard X-Ray Photoemission and Soft X-Ray Absorption Spectroscopy.

In order to realize superconductivity in cuprates with the T'-type structure, not only chemical substitution (Ce doping) but also postgrowth reduction annealing is necessary. In the case of thin films, however, well-designed reduction annealing alone without Ce doping can induce superconductivity in the T'-type cuprates. In order to unveil the origin of superconductivity in the Ce-undoped T'-type cuprates, we have performed bulk-sensitive hard x-ray photoemission and soft x-ray absorption spectroscopy on superconducting and nonsuperconducting Nd2-xCexCuO4 (x=0, 0.15, and 0.19) thin films. By postgrowth annealing, core-level spectra exhibited dramatic changes, which we attributed to the enhancement of core-hole screening in the CuO2 plane and the shift of chemical potential along with changes in the band filling. The result suggests that the superconducting Nd2CuO4 film is doped with electrons despite the absence of the Ce substitution. [ABSTRACT FROM AUTHOR]