Superconductivity Induced By Co-ooping In Quaternary Fluoroarseniderncafeasf

摘要

Since the discovery of superconductivity in F-doped LaFeAsO with T_c = 26 K, FeAs-based layered compounds including RFeAsO (R = rare-earth) and AFe_2As_2 (A = alkali-earth) have been intensively studied as candidates for high-T_c superconductors. With these efforts, T_c is raised to 56 K in Th-doped GdFeAsO. The parent compounds for these superconductors consist of an (FeAs)~(σ+) layer forming a square iron lattice sandwiched by (RO)~(σ+) or A~(σ+) layers and suffer the crystallographic transition from the tetragonal to orthorhombic, stabilizing the antiferromagnetic spin order at 140-200 K. High concentration doping of the electron or hole to the (FeAs)~(σ-) layer through the doping of fluorine to (RO)~(σ+) or alkali-metal to A~(σ+) layers suppresses the crystallographic and magnetic phase transitions and causes the superconductivity. Therefore, the relation among the structural and magnetic instabilities and superconductivity has been studied, and it was recently found that the superconductivity coexists with the orthorhombic and/or magnetic phases in SmFeAsO for low concentration doping. Further, much effort has been made for the synthesis of related compounds composed of the similar iron lattice with a hope to raise the T_c.