We have found the 1/8 anomaly, namely, the anomalous suppression of superconductivity at p (the hole concentration per Cu) $=1/8\mathrm{}$ in the excess-oxygen-doped ${\mathrm{La}}_{1.8}{\mathrm{Nd}}_{0.2}{\mathrm{Cu}}_{1-y}{\mathrm{Zn}}_y{\mathrm{O}}_{4+\delta },$ 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 $p=1/8.\mathrm{}$ 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 $p=1/8.\mathrm{}$ These results are consistent with the stripe-pinning model.

• Received 10 March 2003

DOI:https://doi.org/10.1103/PhysRevB.68.024524