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 $T_c$.
Highlights
We use polarized inelastic neutron scattering to study the spin-excitation anisotropy in the bilayer iron-based superconductor CaKFe4As4 (Tc = 35 K)
We find that the high-energy even mode (∼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
Considering the fact that the spin excitations in these materials have both local-moment and itinerant characteristics [1,2,3], either by approximating to the parent compound with strong single-ion anisotropy [8,9,10] or being affected by the orbital ordering via spin-orbit coupling (SOC) [11,12,13], a spin anisotropy can be present in the spin resonance mode
Summary
To further understand the spin anisotropy of the spin resonance and the role of the SOC in iron-based superconductivity, we have performed polarized neutron-scattering experiments on a stoichiometric iron-based superconductor CaKFe4As4 with Tc = 35 K [Fig. 1(a)] [27,28].
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