Spin-excitation anisotropy in the bilayer iron-based superconductor CaKFe 4 As 4

The author investigates the spin-excitations anisotropy for both odd and even resonance modes in a bilayer iron-based superconductor. The spin anisotropy only emerges in the low-energy odd mode for the spin-orbit coupling in the spin-vortex-type fluctuations of this unique compound. Such anisotropy is weak in the normal state at low energy, but it is significantly enhanced by the superconductivity and pushed into the spin resonance mode.