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

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 ${\mathrm{CaFe}}_{0.88}{\mathrm{Co}}_{0.12}\mathrm{AsF}$.