Abstract
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 ${\mathrm{CaFe}}_{0.88}{\mathrm{Co}}_{0.12}\mathrm{AsF}$ 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 a large ${E}_{\mathrm{r}}/{k}_{\mathrm{B}}{T}_{\mathrm{c}}$ ratio among iron-based superconductors. The large ratio implies a strong coupling between conduction electrons and magnetic excitations in ${\mathrm{CaFe}}_{0.88}{\mathrm{Co}}_{0.12}\mathrm{AsF}$. The resonance possesses a magnonlike upward dispersion along the transverse direction due to the anisotropy of the spin-spin correlation length within the $ab$ plane in the normal state, which points to a spin fluctuation mediated sign-reversed $s\ifmmode\pm\else\textpm\fi{}$ wave pairing in ${\mathrm{CaFe}}_{0.88}{\mathrm{Co}}_{0.12}\mathrm{AsF}$.
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