Anisotropic spin fluctuations in detwinned FeSe

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 BaFe 2 As 2 substrate to demonstrate that spin excitations are most intense at the antiferromagnetic wave vectors Q AF = (±1, 0) at low energies E = 6–11 meV in the normal state. This two-fold ( C 2 ) anisotropy is reduced at lower energies, 3–5 meV, indicating a gapped four-fold ( C 4 ) mode. In the superconducting state, however, the strong nematic anisotropy is again reflected in the spin resonance ( E = 3.6 meV) at Q AF 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. Extreme electronic anisotropy is revealed in the high-temperature superconductor FeSe through tour de force experiments on detwinned crystals.