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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 $T_c$.
We report an inelastic neutron scattering study on the spin resonance in the bilayer iron-based superconductor CaKFe$_4$As$_4$. In contrast to its quasi-two-dimensional electron structure, three strongly $L$-dependent modes of spin resonance are foun
The upper critical field of multiband superconductors is an important quantity that can reveal the details about the nature of the superconducting pairing. Here we experimentally map out the complete upper critical field phase diagram of a stoichiome
The magnetic penetration depth anisotropy $gamma_lambda=lambda_{c}/lambda_{ab}$ ($lambda_{ab}$ and $lambda_{c}$ are the in-plane and the out-of-plane components of the magnetic penetration depth) in a CaKFe$_4$As$_4$ single crystal sample (the critic
Employing a combination of symmetry analysis, low-energy modeling, and ab initio simulations, we predict the presence of magnetic-field-induced Weyl points close to the Fermi level in CaKFe$_4$As$_4$. Depending on the relative strengths of the magnet
The optical properties of the new iron-based superconductor CsCa$_2$Fe$_4$As$_4$F$_2$ with $T_c sim 29$~K have been determined. In the normal state a good description of the low-frequency response is obtained with a superposition of two Drude compone