Spin-Excitations Anisotropy in the Bilayer Iron-Based Superconductor CaKFe${}_4$As${}_4$

Abstract

We use polarized inelastic neutron scattering to study the spin-excitations anisotropy in the bilayer iron-based superconductor CaKFe${}_4$As${}_4$ ($T_c$ = 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 (T. Xie {\it et al.} Phys. Rev. Lett. {\bf 120}, 267003 (2018)). Here we find that the high-energy even mode ($\sim 18$ meV) 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 mode of spin resonance below $T_c$.

6 pages, 4 figures. Accepted by Physical Review Research

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Cited by in corpus (2)

• Preferred Spin Excitations in the Bilayer Iron-Based Superconductor CaK(Fe${}_{0.96}$Ni${}_{0.04}$)${}_4$As${}_4$ with Spin-Vortex Crystal Order
• Nematic fluctuations in the non-superconducting iron pnictide BaFe${}_{1.9-x}$Ni${}_{0.1}$Cr${}_x$As${}_2$