Spin-excitation anisotropy in the nematic state of detwinned FeSe

The mechanism that drives nematic behaviour in iron-based superconductors is still unclear. Now, nematicity and anisotropy in spin excitations are shown to disappear at the same temperature, indicating that the transition is primarily spin-driven. The origin of the electronic nematicity in FeSe is one of the most important unresolved puzzles in the study of iron-based superconductors. In both spin- and orbital-nematic models, the intrinsic magnetic excitations at Q(1) = (1, 0) and Q(2) = (0, 1) of twin-free FeSe are expected to provide decisive criteria for clarifying this issue. Although a spin-fluctuation anisotropy below 10 meV between Q(1) and Q(2) has been observed by inelastic neutron scattering at low temperature, it remains unclear whether such an anisotropy also persists at higher energies and associates with the nematic transition T-s. Here we use resonant inelastic X-ray scattering to probe the high-energy magnetic excitations of detwinned FeSe. A prominent anisotropy between the magnetic excitations along the H and K directions is found to persist to E approximate to 200 meV, which decreases gradually with increasing temperature and finally vanishes at a temperature around T-s. The measured high-energy spin excitations are dispersive and underdamped, which can be understood from a local-moment perspective.Taking together the large energy scale far beyond the d(xz)/d(yz) orbital splitting, we suggest that the nematicity in FeSe is probably spin-driven.