Unique crystal field splitting and multiband RKKY interactions in Ni-doped EuRbFe4As4

The relationship between magnetism and superconductivity has been one of the most discussed topics in iron-based superconductors. Using first-principles calculations, we have studied the electronic structure of 1144-type iron-based superconductor EuRbFe4As4. We find the crystal field splitting of EuRbFe4As4 is unique, such that the \(d_{z^2}\) orbitals are closer to the Fermi level \(\epsilon _{\mathrm F}\) than the dxy orbitals. The Ruderman？Kittel？Kasuya？Yosida (RKKY) interaction strength is approximately 0.12？meV in pristine EuRbFe4As4. Upon Ni-doping on the Fe site, the RKKY interaction strength is barely changed upon Ni-doping due to the highly anisotropic Fermi surfaces and multiband effect, despite the drastically reduced dzx(y) density of state at \(\epsilon _{\mathrm F}\). Finally, in both pristine and doped compounds, the RKKY interaction is primarily mediated through bands due to Fe-\(d_{z^2}\) orbitals. Our calculations suggest the RKKY interaction mediated by \(d_{z^2}\) orbital is probably responsible for the magnetism in EuRbFe4As4 and doesn’t change upon Ni-doping