Viewpoint Not all iron superconductors are the same

We are facing a new paradigm in superconductivity research with the discovery of superconductivity in iron pnictides (LaFeAsO, SrFe2As2, BaFe2As2, etc.) and the iron chalcogenides (FeTe and FeSe). Both classes of ironbased materials, for some as yet unknown reason, have proven to be fertile ground for novel superconductivity, with the transition temperature of the pnictides racing above 50 K. A paper published in Physical Review Letters[1] from Yu Qi Xia and colleagues at Princeton University and the Lawrence Berkeley Laboratory, both in the US, in collaboration with Jiao Tong University in Shanghai, and the Institute of Physics of the Chinese Academy of Sciences in Beijing, leads, in concert with other recent experimental work [2, 3], to a remarkable conclusion: iron chalcogenides, in seeming contrast to the iron pnictides, do not exhibit the characteristic Fermisurface-induced magnetism, a spin-density-wave ordering, that the parent compounds of nearly all other ironbased superconductors share (see Fig. 1). The implication for the iron chalcogenides is that the magnetism arises from a different interaction than in the iron pnictides. This also means that in the chalcogenides either the electron pairing that leads to superconductivity and the magnetism arise from different interactions, or that the pairing arises from an interaction distinct from the Fermi-surface-driven itinerant spin fluctuations that are believed by many to create pairing in the iron pnictides.