Superconductivity in Iron Oxypnictide Induced by F-Doping

In the history of superconductors, Fe-based superconductor LnFePnO_1-xF_x (1111) has the highest T_c at ~58K except Cu-based oxide superconductors. Electron doping by substitution of O^2- with F^- was essential to induce superconductivity. Local structures around iron are found to have a good empirical relation to T_c. Maximum T_c is observed for 1111 materials having nearly regular tetrahedral FeAs₄ local structures. The T_c and upper critical field of 1111 compounds are very attractive for power cable under high magnetic fields, although fabrication of practical superconducting wires is a tough challenge due to the difficulty in controlling the F contents during a processing for ex-situ powder-in-tube method.