Low-energy spin excitations in detwinned FeSe

Abstract

Abstract Antiferromagnetic spin fluctuation is regarded as the leading driving force for electron pairing in high-Tc superconductors. In iron-based superconductors, spin excitations at low energy range, especially the spinresonance mode at ER ~5kBTc , 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 of uniaxial-strain detwinned FeSe. The low-energy spin excitations (E ＜ 10 meV) appear mainly at Q = (±1; 0) in the superconducting state (T ≲ 9 K) and the nematic state (T ≲ 90 K), confirming the constant C 2 rotational symmetry and ruling out the C 4 mode at E ≈ 3 meV reported in a prior INS study. Moreover, our results reveal an isotropic spin resonance in the superconducting state, which is consistent with the s ± wave pairing symmetry. At slightly higher energy, low-energy spin excitations become highly anisotropic. The full width at half maximum (FWHM) of spin excitations is elongated along the transverse (TR) direction. The Q-space isotropic spin resonance and highly anisotropic low-energy spin excitations could arise from dyz intraorbital selective Fermi surface nesting between the hole pocket around Γ point and the electron pockets centered at M X point.