Neutron scattering

Physics

Inelastic neutron scattering

Optics

Liquid crystal

Antiferromagnetism

Scattering

Materials science

Condensed matter physics

Spin (aerodynamics)

Phase (matter)

Superconductivity

Quantum mechanics

Anisotropy

Thermodynamics

Nature Materials

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. Extreme electronic anisotropy is revealed in the high-temperature superconductor FeSe through tour de force experiments on detwinned crystals.

Anisotropic spin fluctuations in detwinned FeSe