Studies of static magnetic order in electron-superconductors and their parent compounds

UNTIL recently, all known high- Tc superconductors involved conduction by holes; the doping of holes into the parent compound, for example by the substitution of Sr2+ or Ba2+ for La3+ in La2CuO4–y, results in a metallic state whose electronic ground state is superconducting. Recently, Tokura et al1 have discovered a series of superconducting copper oxides of general formula Ln2–xCexCuO4–y (where Ln represents Pr, Nd, Sm), with transition temperatures as high as Tc ≈ 24 K, in which the charge carriers are electrons. These compounds have the Nd2CuO4–y structure, which is similar to that of La2CuO4–y, except for a different arrangement of the oxygen atoms that results in the absence of Cu–O octahedra. Superconductivity is achieved by doping Ce4+ for Ln3+ and making the samples oxygen-deficient. Here we show, using muon spin rotation, that the parent compounds Ln2CuO4–y of these electron-superconductors exhibit static magnetic order below 300 K, which is similar to that observed in the parent compounds of hole-superconductors. In addition, fairly random static magnetic order has been observed in the non-superconducting material Nd1.90Ceo0.10CuO4–y below about 240 K, but no detectable static magnetic order was observed above 4 K in a superconducting sample Nd1.8Ce0.16CuO4–y (with a Tc of 24 K). Hence, electron doping seems not to fully destroy the magnetic order until the material becomes superconducting. The observation of static magnetic order in these new systems demonstrates that the electronic properties of these copper oxide planar systems are at least partially symmetric with respect to electron and hole doping.