In copper oxides (cuprates) with single CuO \(_2\) layer such as La \(_{2-x}\) Ba(Sr) \(_x\) CuO \(_4\) , antiferromagnetism coexists with superconductivity at small doping levels x, where chemical disorders are significant. Here, we report that superconductivity occurs in a uniform and fully ordered Néel state in a single-layer cuprate La \(_2\) CuO \(_{4+\delta }\) with a small amount of excess oxygen \((\delta = 0.015)\) as demonstrated by the 139La nuclear quadrupole resonance measurement. A uniform oxygen distribution in the crystal is crucial for achieving microscopic phase coexistence and overcoming the miscibility gap associated with the staging instability; self-organized periodic oxygen arrangement driven by mobile oxygen atoms. This finding prompts the reconsideration of superconductivity in cuprates, highlighting that it can emerge in a robust Néel state that retains sizable magnetic moments and hosts only a small carrier density.
