In iron-based superconductors the interactions driving the nematic order (that breaks four-fold rotational symmetry in the iron plane) may also mediate the Cooper pairing1. The experimental determination of these interactions, which are believed to depend on the orbital or the spin degrees of freedom1,2,3,4, is challenging because nematic order occurs at, or slightly above, the ordering temperature of a stripe magnetic phase1,5. Here, we study FeSe (ref. 6)—which exhibits a nematic (orthorhombic) phase transition at Ts = 90 K without antiferromagnetic ordering—by neutron scattering, finding substantial stripe spin fluctuations coupled with the nematicity that are enhanced abruptly on cooling through Ts. A sharp spin resonance develops in the superconducting state, whose energy (∼4 meV) is consistent with an electron–boson coupling mode revealed by scanning tunnelling spectroscopy7. The magnetic spectral weight in FeSe is found to be comparable to that of the iron arsenides8,9. Our results support recent theoretical proposals that both nematicity and superconductivity are driven by spin fluctuations1,10,11,12,13.
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We thank D. H. Lee, Q. Si, F. Wang and H. Yao for useful discussions. This work is supported by the National Natural Science Foundation of China (Grant No. 11374059), the Ministry of Science and Technology of China (973 project: 2015CB921302) and the Shanghai Pujiang Scholar Program (Grant No. 13PJ1401100). M.M. and F.Z. acknowledge support from the National Natural Science Foundation of China (Grant No. 11190020). H.C. received support from the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. A.N.V. was supported in part by the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST 〈MISiS〉 (No. 2-2014-036). D.A.C. and A.N.V. also acknowledge the support of the Russian Foundation for Basic Research through Grants 13-02-00174, 14-02-92002, 14-02-92693.
The authors declare no competing financial interests.
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Wang, Q., Shen, Y., Pan, B. et al. Strong interplay between stripe spin fluctuations, nematicity and superconductivity in FeSe. Nature Mater 15, 159–163 (2016). https://doi.org/10.1038/nmat4492
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