Neutron spin resonance in the 112-type iron-based superconductor

TL;DR

This study uses inelastic neutron scattering to investigate the spin excitations in a 112-type iron-based superconductor, revealing a nearly isotropic spin resonance mode that scales with the critical temperature and is unaffected by the material's unique monoclinic structure.

Contribution

It provides new insights into the nature of the spin resonance in 112-type iron pnictides, highlighting its isotropic character and independence from the Fermi surface's three-dimensional features.

Findings

Discovery of a 11 meV spin resonance mode in the superconductor

Resonance energy is temperature independent and scales with T_c

Resonance is nearly isotropic in spin space

Abstract

We use inelastic neutron scattering to study the low-energy spin excitations of 112-type iron pnictide Ca$_{0.82}$La$_{0.18}$Fe$_{0.96}$Ni$_{0.04}$As$_{2}$ with bulk superconductivity below $T_c=22$ K. A two-dimensional spin resonance mode is found around $E=$ 11 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. Due to the unique monoclinic structure with additional zigzag arsenic chains, the As $4p$ orbitals contribute to a three-dimensional hole pocket around $\Gamma$ point and an extra electron pocket at $X$ point. Our results suggest that the energy and momentum distribution of spin resonance does not directly response to the $k_z$ dependence of 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.