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

Abstract

UNTIL recently, all known high- T_c 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 La₂CuO_4–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 Ln_2–xCe_xCuO_4–y (where Ln represents Pr, Nd, Sm), with transition temperatures as high as T_c ≈ 24 K, in which the charge carriers are electrons. These compounds have the Nd₂CuO_4–y structure, which is similar to that of La₂CuO_4–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 Ln₂CuO_4–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 Nd_1.90Ceo_0.10CuO_4–y below about 240 K, but no detectable static magnetic order was observed above 4 K in a superconducting sample Nd_1.8Ce_0.16CuO_4–y (with a T_c 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.