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(z2) orbitals are closer to the Fermi level epsilon(F) than the d(xy) 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 d(zx(y)) density of state at epsilon(F). Finally, in both pristine and doped compounds, the RKKY interaction is primarily mediated through bands due to Fe-d(z2) orbitals. Our calculations suggest the RKKY interaction mediated by d(z2) orbital is probably responsible for the magnetism in EuRbFe4As4 and doesn't change upon Ni-doping.