Elucidating the nature of the magnetic ground state of iron-based superconductors is of paramount importance in unveiling the mechanism behind their high-temperature superconductivity. Until recently, it was thought that superconductivity emerges only from an orthorhombic antiferromagnetic stripe phase, which can in principle be described in terms of either localized or itinerant spins. However, we recently reported that tetragonal symmetry is restored inside the magnetically ordered state of certain hole-doped compounds, revealing the existence of a new magnetic phase at compositions close to the onset of superconductivity. Here, we present Mössbauer data that show that half of the iron sites in this tetragonal phase are non-magnetic, establishing conclusively the existence of a novel magnetic ground state with a non-uniform magnetization that is inconsistent with localized spins. Instead, this state is naturally explained as the interference between two commensurate spin-density waves, a rare example of collinear double-Q magnetic order. Our results demonstrate the itinerant character of the magnetism of the iron pnictides, and the primary role played by magnetic degrees of freedom in determining their phase diagram.
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Work at Argonne (J.M.A., K.M.T., D.E.Bugaris, M.J.K., D.Y.C., H.C., M.G.K., S.R., O.C. and R.O.) was supported by the US Department of Energy, Office of Science, Materials Science and Engineering Division. X-ray experiments were performed at the Advanced Photon Source, which is supported by the Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357. Neutron experiments were performed at the High Flux Isotope Reactor and Spallation Neutron Source. R.M.F. and J.K. were supported by the US Department of Energy, Office of Science, Basic Energy Sciences, under award number DE-SC0012336. The work of I.E. was supported by the Focus Program 1458 Eisen-Pniktide of the DFG, and by the German Academic Exchange Service (DAAD PPP USA no. 57051534). I.E. also acknowledges the financial support of the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST MISiS (N 22014015). The authors thank A. A. Aczel, A. Huq, M. J. Kirkham and P. S. Whitfield for experimental assistance, E. E. Alp for use of his Mössbauer spectrometer, and B. M. Andersen, A. V. Chubukov, M. N. Gastiasoro, A. Yaresko and Y. Zhao for fruitful discussions.
The authors declare no competing financial interests.
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Allred, J., Taddei, K., Bugaris, D. et al. Double-Q spin-density wave in iron arsenide superconductors. Nature Phys 12, 493–498 (2016). https://doi.org/10.1038/nphys3629
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