Low-Energy Spin Excitations in Detwinned FeSe

Abstract Antiferromagnetic spin fluctuation is regarded as the leading driving force for electron pairing in high-T_c superconductors. In iron-based superconductors, spin excitations at low energy range, especially the spin-resonance mode atE_R ∼ 5k_BT_c, are important for understanding the superconductivity. Here, we use inelastic neutron scattering (INS) to investigate the symmetry and in-plane wave-vector dependence of low-energy spin excitations in uniaxial-strain detwinned FeSe. The low-energy spin excitations (E< 10 meV) appear mainly atQ= (±1, 0) in the superconducting state (T≲ 9 K) and the nematic state (T≲ 90 K), confirming the constantC₂ rotational symmetry and ruling out theC₄ mode atE≈ 3 meV reported in a prior INS study. Moreover, our results reveal an isotropic spin resonance in the superconducting state, which is consistent with thes^± wave pairing symmetry. At slightly higher energy, low-energy spin excitations become highly anisotropic. The full width at half maximum of spin excitations is elongated along the transverse direction. TheQ-space isotropic spin resonance and highly anisotropic low-energy spin excitations could arise fromd_yzintra-orbital selective Fermi surface nesting between the hole pocket aroundΓpoint and the electron pockets centered atM_X point.