Role of Lanthanide in the Electronic Properties of RbLn₂Fe₄As₄O₂ (Ln = Sm and Ho) Superconductors

We focus on the effect of ionic radius of lanthanides and the number of electrons in 4f orbitals on the superconducting temperature in 12442-type iron-based superconductors RbLn₂Fe₄As₄O₂ (Ln = Sm and Ho). Electronic properties of RbSm₂Fe₄As₄O₂ and RbHo₂Fe₄As₄O₂ with the largest differences of ionic radii and numbers of electrons in 4f orbital, and the largest difference of superconducting temperatures by using first-principles calculations. We predict that the ground state of RbLn₂Fe₄As₄O₂ is spin-density-wave-type in-plane striped antiferromagnet, and the magnetic moment around each Fe atom is about 2μ_B. RbSm₂Fe₄As₄O₂ has a great influence on the energy band near the Γ point, and a Dirac-like dispersion energy band appears. This band is mainly contributed by the $d_{z^2}$ orbital of Fe, which proves that RbSm₂Fe₄As₄O₂ has a stronger three-dimensionality. At the same time, this extra Fermi surface appears at the Γ point, which also shows that Sm can effectively enhance the coupling strength within Fe₂As₂ bilayers. This is also confirmed by the charge density difference ρ(RbHo₂Fe₄As₄O₂) −ρ(RbSm₂Fe₄As₄O₂). It increases the internal coupling strength of the bilayer Fe₂As₂ layers, which in turn leads to a higher T_c of RbSm₂Fe₄As₄O₂ than RbHo₂Fe₄As₄O₂. Determining the details of their electronic structure, which may be closely related to superconductivity, is crucial to understanding the underlying mechanism. Such microscopic studies provide useful clues for our further research of other high-temperature superconductors.