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 = 3.7 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.
@article{arxiv.1905.08399,
title = {Anisotropic spin fluctuations in detwinned FeSe},
author = {Tong Chen and Youzhe Chen and Andreas Kreisel and Xingye Lu and Astrid Schneidewind and Yiming Qiu and J. T. Park and Toby G. Perring and J Ross Stewart and Huibo Cao and Rui Zhang and Yu Li and Yan Rong and Yuan Wei and Brian M. Andersen and P. J. Hirschfeld and Collin Broholm and Pengcheng Dai},
journal= {arXiv preprint arXiv:1905.08399},
year = {2019}
}