1/8 anomaly in the excess-oxygen-doped La_1.8Nd_0.2Cu_1−yZnyO_4+δ

We have found the 1/8 anomaly, namely, the anomalous suppression of superconductivity at p (the hole concentration per Cu) =1/8 in the excess-oxygen-doped La_1.8Nd_0.2Cu_1−yZnyO₄+δ, where the excess oxygen is doped by the electrochemical technique and the phase separation of the excess oxygen is suppressed. The 1/8 anomaly has become marked by the 1% substitution of Zn for Cu. The muon-spin-relaxation measurements have revealed that the magnetic correlation develops at low temperatures below about 50 K in both Zn-free and 1% Zn-substituted samples with p=1/8. Clear precession of muon spins suggesting the formation of a long-range magnetic order has been observed below 1 K in the 1% Zn-substituted sample with p=1/8. These results are consistent with the stripe-pinning model.