Recent experiments on Zn-substituted 122-type iron-based superconductors (FeSCs) at electron- and hole-doped regions provide us with a testing ground for understanding the effect of Zn impurities in these systems. Our first-principles calculations of the electronic structure reveal that the Zn 3d orbitals are far below the Fermi level and are chemically inactive, while the Zn 4s orbital is partially occupied and its wave function overlaps with the 3d orbitals of neighboring Fe ions. This suggests that the impurity effect originates in the Zn 4s orbital, not its 3d orbitals. Employing a phenomenological two-orbital lattice model for 122-FeSCs and the self-consistent Bogoliubov-de Gennes equations, we study how the Zn impurities suppress the superconductivity in electron- and hole-doped compounds. Our obtained results qualitatively agree with the experimental measurements.
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