  In iron-based superconductors, high critical temperature (Tc)
superconductivity over 50 K has only been accomplished in electron-doped
hREFeAsO (hRE = heavy rare earth (RE) element). While hREFeAsO has the highest
bulk Tc (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 (NPD) study of 154SmFeAsO1-xDx, and the
discovery of a new long-range antiferromagnetic ordering with x >= 0.56 (AFM2)
accompanying a structural transition from tetragonal to orthorhombic.
Surprisingly, the Fe magnetic moment in AFM2 reaches a magnitude of 2.73
muB/Fe, which is the largest in all non-doped iron pnictides and chalcogenides.
Theoretical calculations suggest that the AFM2 phase originates in kinetic
frustration of the Fe-3dxy orbital, in which the nearest neighbor hopping
parameter becomes zero. The unique phase diagram, i. e., highest-Tc
superconducting phase is adjacent to the strongly correlated phase in
electron-overdoped regime, yields important clues to the unconventional origins
of superconductivity.
