Spin-excitation anisotropy in the nematic state of detwinned FeSe

The origin of the electronic nematicity in FeSe is one of the most important unresolved puzzles in the study of iron-based superconductors (FeSC). In either spin- or orbital-nematic models, the intrinsic symmetry of the magnetic excitations at (1, 0) and (0, 1) of twin-free FeSe are believed to be integral for unveiling the origin of the nematic state and superconductivity. Although anisotropic spin fluctuations below 10 meV between (1, 0) and (0, 1) have been observed by inelastic neutron scattering around T_c. Here we use resonant inelastic x-ray scattering to probe the high-energy magnetic excitations of detwinned FeSe. Strong anisotropy between the magnetic excitations along the H and K directions is found to persist to ∼200 meV, which is even more pronounced than the anisotropy of the spin waves for BaFe₂As₂ with collinear spin ordering. This anisotropy decreases gradually with increasing temperature and finally vanishes at a temperature around the nematic transition temperature T_s. Our results reveal an unprecedentedly strong spin-excitation anisotropy with a large energy scale well above the d_xz/d_yz orbital splitting, suggesting that the nematic phase transition is primarily spin-driven. Moreover, the measured high-energy spin excitations are dispersive and underdamped, which can be understood from a local moment perspective. Our findings provide the much-needed understanding of the mechanism for the nematicity of FeSe and point to a unified description of the correlation physics across seemingly distinct classes of Fe-based superconductors.