High-transition-temperature superconductivity arises in copper oxides whenholes or electrons are doped into the CuO2 planes of their insulating parentcompounds. While hole-doping quickly induces metallic behavior andsuperconductivity in many cuprates, electron-doping alone is insufficient inmaterials such as R2CuO4 (R is Nd, Pr, La, Ce, etc.), where it is necessary toanneal an as-grown sample in a low-oxygen environment to remove a tiny amountof oxygen in order to induce superconductivity. Here we show that themicroscopic process of oxygen reduction repairs Cu deficiencies in the as-grownmaterials and creates oxygen vacancies in the stoichiometric CuO2 planes,effectively reducing disorder and providing itinerant carriers forsuperconductivity. The resolution of this long-standing materials issuesuggests that the fundamental mechanism for superconductivity is the same forelectron- and hole-doped copper oxides.
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