Superconductivity emerging from the Néel state in infinite-stage single-layer cuprate La[Formula: see...

In copper oxides (cuprates) with single CuO[Formula: see text] layer such as La[Formula: see text]Ba(Sr)[Formula: see text]CuO[Formula: see text], 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[Formula: see text]CuO[Formula: see text] with a small amount of excess oxygen [Formula: see text] 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.