Effects of Ce substitution and reduction on conduction in Nd_2-xCe_xCuO₄ single crystals

Electrical resistivity and magnetoresistivity of as-grown and reduced single crystals of Nd_2-xCe_xCuO₄ have been investigated to elucidate the effects of the Ce substitution and the reduction on the electrical conduction. The resistivity of the as-grown Ce-substituted samples shows a minimum as a function of temperature, indicating electron localization in the low-temperature range. The Ce concentration dependence of the localization behavior demonstrates that the Ce ions increase the electron carriers and cause the random potential on the conduction plane as well. The reduced superconducting sample has no localization behavior and a larger temperature coefficient of resistivity than those of the reduced samples. The results strongly suggest that excess oxygen, which creates random potential as in the case of the Ce ions, exists in the as-grown samples and that the superconductivity is induced by the carrier increase and the removal of the random potential due to the reduction.