There are few inelastic neutron scattering (INS) reports on the superconducting single crystals of 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 the optimally electron-doped CaFe$_{ 0.88}$Co$_{ 0.12}$AsF single crystals with $T_\mathrm{ c}$ = 21 K by INS. The resonance energy of the superconducting spin resonant mode with $E_\mathrm{ r}$ = 12 meV amounts to 6.6 $k_\mathrm{ B}$$T_\mathrm{ c}$, which constitutes the largest $E_\mathrm{ r}$/$k_\mathrm{ B}$$T_\mathrm{ c}$ ratio among iron-based superconductors reported to date. The large ratio implies a strong coupling between conduction electrons and magnetic excitations in CaFe$_{ 0.88}$Co$_{ 0.12}$AsF. The resonance possesses a magnonlike upward dispersion along transverse direction due to the anisotropy of spin-spin correlation length within $ab$ plane in the normal-state, which points to a spin fluctuation mediated sign-reversed ${ s}\mathbf\pm$ wave pairing in CaFe$_{ 0.88}$Co$_{ 0.12}$AsF.
Categories: cond-mat.supr-con