Characteristics of oxygen over-reduced Nd_1.85Ce_0.15CuO_4-y films

We have studied the oxygen doping effect in thin Nd_1.85Ce_0.15CuO_4-y (NCCO) films. Contrary to the common believe that the disappearance of superconductivity in the over-reduced NCCO samples is a consequence of phase decomposition, we found that there exists an over-reduced region in which the crystallographic structure of the film remains essentially the same as in the optimum-oxygen doped region, but the film becomes insulating. Rutherford backscattering spectroscopy channeling analysis reveals that the oxygen over-reduction increases atomic distortion in the lattice. This work provides strong support to previous transport studies by Jiang et al. [Phys. Rev. Lett. 73, 1291 (1994)], which suggested the existence of both electron and hole carriers in superconducting NCCO.