Spin-excitation anisotropy in the bilayer iron-based superconductor CaKFe₄As₄

We use polarized inelastic neutron scattering to study the spin-excitation anisotropy in the bilayer iron-based superconductor CaKFe4As4 (Tc=35 K). In the superconducting state, both odd and even L modulations of spin resonance have been observed in our previous unpolarized neutron-scattering experiments [Xie et al., Phys. Rev. Lett. 120, 267003 (2018)]. Here we find that the high-energy even mode (∼18meV) is isotropic in spin space, but the low-energy odd modes consist of a c-axis polarized mode around 9 meV along with another partially overlapped in-plane mode around 12 meV. We argue that such spin anisotropy is induced by the spin-orbit coupling in the spin-vortex-type fluctuations of this unique compound. The spin anisotropy is strongly affected by the superconductivity, where it is weak below 6 meV in the normal state and then transferred to higher energy and further enhanced in the odd modes of spin resonance below Tc