Superconductivity above 50K in LnFeAsO_<1-y> (Ln=Nd, Sm, Gd, Tb, and Dy) Synthesized by High-Pressure Technique(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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We have succeeded in synthesizing single-phase polycrystalline samples of oxygen-deficient oxypnictide superconductors, LnFeAsO_<1-y> (Ln: lanthanide elements) with Ln=La, Ce, Pr, Nd, Sm, Gd, Tb, and Dy using high-pressure technique. It is found out that the synthesis pressure is a key parameter for synthesizing samples, in particular for the heavier Ln's, such as Tb and Dy. The lattice parameters systematically decrease with the atomic number of Ln, reflecting the shrinkage of Ln ionic radius. For the lighter Ln's (La, Ce, Pr, Nd), T_c increases monotonously with decreasing the lattice parameters from 26K for Ln=La to 54K for Ln=Nd, then stays at the constant value around 53K for the heavier counterpart (Nd, Sm, Gd, Tb, and Dy). The results suggest the intimate relationship between the crystal structural parameters and the superconductivity, as well as the possible existence of the inherent maximum T_c, which is located around 50K in the LnFeAsO based materials.
社団法人日本物理学会の論文
2009-03-15