Electronic structure of Ce-doped and undoped Nd$_2$CuO$_4$ superconducting thin films studied by hard x-ray photoemission and soft x-ray absorption spectroscopies

摘要

In order to realize superconductivity in cuprates with the T'-type structure,not only chemical substitution (Ce doping) but also post-growth reductionannealing is necessary. In the case of thin films, however, well-designedreduction annealing alone without Ce doping can induce superconductivity in theT'-type cuprates. In order to unveil the origin of superconductivity in theCe-undoped T'-type cuprates, we have performed bulk-sensitive hard x-rayphotoemission and soft x-ray absorption spectroscopies on superconducting andnon-superconducting Nd$_{2-x}$Ce$_x$CuO$_4$ ($x = 0, 0.15$, and 0.19) thinfilms. By post-growth annealing, core-level spectra exhibited dramatic changes,which we attributed to the enhancement of core-hole screening in the CuO$_2$plane and the shift of chemical potential along with changes in the bandfilling. The result suggests that the superconducting Nd$_2$CuO$_4$ film isdoped with electrons and that the electronic states are similar to those ofCe-doped superconductors.