# Anisotropic spin fluctuations in detwinned FeSe

## TL;DR

This study reveals that in FeSe, spin fluctuations are highly anisotropic in the nematic phase, with a strong directional dependence that influences the superconducting gap, as shown by inelastic neutron scattering on detwinned crystals.

## Contribution

It demonstrates the anisotropic nature of spin excitations in detwinned FeSe and links this to the nematic phase and superconducting properties, providing new insights into the pairing mechanism.

## Key findings

- Spin excitations are most intense at QAF = (1, 0) in the normal state.
- A gapped four-fold (C4) mode appears at lower energies.
- Strong nematic anisotropy persists in the superconducting state, affecting the spin resonance.

## Abstract

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.