Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe4As4

The dynamical spin susceptibility in the new-structure-type iron-based superconductor CaKFe$_4$As$_4$ was investigated by using a combination of inelastic neutron scattering (INS) measurements and random phase approximation (RPA) calculations. Powder INS measurements show that the spin resonance at $Q_\text{res}=1.17(1)$~$\text{\AA}^{-1}$, corresponding to the $(\pi,\pi)$ nesting wave vector in tetragonal notation, evolves below $T_\text{c}$. The characteristic energy of the spin resonance $E_\text{res}=12.5$~meV is smaller than twice the size of the superconducting gap ($2\Delta$). The broad energy feature of the dynamical susceptibility of the spin resonance can be explained by the RPA calculations, in which the different superconducting gaps on different Fermi surfaces are taken into account. Our INS and PRA studies demonstrate that the superconducting pairing nature in CaKFe$_4$As$_4$ is the $s_\pm$ symmetry.