Abstract
We discovered iron-based layered superconductors: F-doped LaFePnO (Pn=P, As) in 2006. Several superconductors containing square iron lattice have been reported in 2008. Superconducting transition temperature (Tc) of F-doped LnFeAsO (Ln=Nd, Sm, ...) reaches ~55 K. LaFePnO is a 3d transition-metal (TM) based oxypnictides: LnTMPnO. The crystal structure is composed of carrier conducting TM-Pn layer and insulating Ln-O layer. Undoped LnFePO (Ln=La, Sm) shows Tc ~3-5 K, while undoped LnFeAsO shows no Tc. The undoped LnFeAsO takes a orthorhombic crystallographic phase at temperatures below ~160 K, F-doping suppresses the structural transition from tetragonal to orthorhombic phase, stabilizing the tetragonal lattice of LnFeAsO. F-doped LnFeAsO with tetragonal symmetry exhibits Tc.
