In a triclinic iron pnictide superconductor, a spin resonance precursor, Nernst signal, and NMR density-of-states reduction all point to preformed Cooper pairs persisting to T* = 45 K, above Tc = 30 K.
Low-energy spin excitations in optimally doped CaFe$_{0.88}$Co$_{0.12}$AsF superconductor studied with inelastic neutron scattering
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abstract
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.
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2025 1verdicts
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Preformed Cooper Pairs in a Triclinic Iron Pnictide Superconductor
In a triclinic iron pnictide superconductor, a spin resonance precursor, Nernst signal, and NMR density-of-states reduction all point to preformed Cooper pairs persisting to T* = 45 K, above Tc = 30 K.