Superconductivity in FeSe emerges from a nematic phase that breaks four-fold rotational symmetry in the iron plane. This phase may arise from orbital ordering, spin fluctuations or hidden magnetic quadrupolar order. Here we use inelastic neutron scattering on a mosaic of single crystals of FeSe, detwinned by mounting on a BaFe2As2 substrate to demonstrate that spin excitations are most intense at the antiferromagnetic wave vectors QAF = (±1, 0) at low energies E = 6–11 meV in the normal state. This two-fold (C2) anisotropy is reduced at lower energies, 3–5 meV, indicating a gapped four-fold (C4) mode. In the superconducting state, however, the strong nematic anisotropy is again reflected in the spin resonance (E = 3.6 meV) at QAF with incommensurate scattering around 5–6 meV. Our results highlight the extreme electronic anisotropy of the nematic phase of FeSe and are consistent with a highly anisotropic superconducting gap driven by spin fluctuations.
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We thank D. Abernathy, Q. Wang, Y. Hao and H. Hu for useful discussions. The neutron-scattering work at Rice University was supported by the US Department of Energy, BES DE-SC0012311 (P.D.). The single-crystal synthesis work was supported by Robert A. Welch Foundation grant no. C-1839 (P.D.). X.L. is supported by the National Natural Science Foundation of China under Grant No. 11734002. C.B. and Y.C. are supported by the US Department of Energy grant no. DE-FG02-08ER46544. B.M.A. acknowledges financial support from the Carlsberg Foundation. P.J.H. was supported by the Department of Energy under grant no. DE-FG02-05ER46236. This research used resources at the High Flux Isotope Reactor and Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. Access to MACS was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under agreement No. DMR-1508249.
Supplementary Notes 1 and 2, Supplementary Figs. 1–10 and Supplementary References 1–12.