We use inelastic neutron scattering to study the low-energy spin excitations of the 112-type iron pnictide ${\mathrm{Ca}}_{0.82}{\mathrm{La}}_{0.18}{\mathrm{Fe}}_{0.96}{\mathrm{Ni}}_{0.04}{\text{As}}_2$ with bulk superconductivity below $T_c=22\mathrm{K}$. A two-dimensional spin resonance mode is found around $E=11\mathrm{meV}$, where the resonance energy is almost temperature independent and linearly scales with $T_c$ along with other iron-based superconductors. Polarized neutron analysis reveals the resonance is nearly isotropic in spin space without any $L$ modulations. Because of the unique monoclinic structure with additional zigzag arsenic chains, the As $4p$ orbitals contribute to a three-dimensional hole pocket around the $\mathrm{\Gamma }$ point and an extra electron pocket at the $X$ point. Our results suggest that the energy and momentum distribution of the spin resonance does not directly respond to the $k_z$ dependence of the fermiology, and the spin resonance intrinsically is a spin-1 mode from singlet-triplet excitations of the Cooper pairs in the case of weak spin-orbital coupling.

• Received 8 December 2017

DOI:https://doi.org/10.1103/PhysRevLett.120.137001