Large-moment antiferromagnetic order in overdoped high- superconductor SmFeAsO D .

In iron-based superconductors, high critical temperature () superconductivity over 50 K has only been accomplished in electron-doped FeAsO ( is heavy rare earth () element). Although FeAsO has the highest bulk (58 K), progress in understanding its physical properties has been relatively slow due to difficulties in achieving high-concentration electron doping and carrying out neutron experiments. Here, we present a systematic neutron powder diffraction study of SmFeAsO D , and the discovery of a long-range antiferromagnetic ordering with ≥ 0.56 (AFM2) accompanying a structural transition from tetragonal to orthorhombic. Surprisingly, the Fe magnetic moment in AFM2 reaches a magnitude of 2.73 μ/Fe, which is the largest in all nondoped iron pnictides and chalcogenides. Theoretical calculations suggest that the AFM2 phase originates in kinetic frustration of the Fe-3 orbital, in which the nearest-neighbor hopping parameter becomes zero. The unique phase diagram, i.e., highest- superconducting phase adjacent to the strongly correlated phase in electron-overdoped regime, yields important clues to the unconventional origins of superconductivity.