Low-energy spin excitations in the optimally doped CaFe0.88Co0.12AsF superconductor studied with inelastic neutron scattering

There are few inelastic neutron scattering (INS) reports on the superconducting single crystals of the FeAs-1111 system, even though it was first discovered in 2008, due to the extreme difficulty in large single-crystal growth. In this paper, we have studied the low-energy spin excitations in optimally electron-doped CaFe_0.88Co_0.12AsF single crystals with T_c=21 K by INS. The resonance energy of the superconducting spin resonant mode with E_r=12 meV amounts to 6.6 k_BT_c , which constitutes a large E_r/k_BT_c ratio among iron-based superconductors. The large ratio implies a strong coupling between conduction electrons and magnetic excitations in CaFe_0.88Co_0.12AsF . The resonance possesses a magnonlike upward dispersion along the transverse direction due to the anisotropy of the spin-spin correlation length within the a b plane in the normal state, which points to a spin fluctuation mediated sign-reversed s ± wave pairing in CaFe_0.88Co_0.12AsF .