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In the stoichiometric iron pnictide superconductor CaKFe₄As₄, we have discovered triple spin resonance modes with strong L-modulation by unpolarized neutron scattering experiments. The two odd modes appear around 9.5 meV and 13 meV, and the even mode locates around 18.3 meV. [1] To study the spin anisotropy of the three modes, we performed polarized inelastic neutron scattering experiments on the same CaKFe₄As₄ sample. Here, I will present our recent results. These results will give better understanding on the spin-orbit coupling in iron-based superconductors, which can also enlighten us about the nature of magnetism and its relation with unconventional superconductivity.

[1] Tao Xie et al., Phys. Rev. Lett. 120, 267003 (2018)