S-wave superconductivity with orbital dependent sign change in the checkerboard models of iron-based superconductors

We study three different multi-orbital models for iron-based superconductors (iron-SCs) in the solvable limit of weakly coupled square plaquettes. The strongest superconducting (SC) pairing is in the $A_{1g}$ $s$-wave channel and its development is correlated with the emergence of the next-nearest-neighbour antiferromagnetism (NNN-AFM). For the models with more than three orbitals, this study suggests that the signs of the intra-orbital pairing order parameters of the $d_{xy}$ and the $d_{xz}$ (or $d_{yz}$) orbitals must be {\it opposite}. Such sign difference stems from the intrinsic symmetry properties of inter-orbital hoppings and might, ultimately, lead to the sign-change of the SC orders between the hole Fermi pockets at the $Γ$ point and produce anisotropic or even gapless SC gaps in the electron Fermi pockets around the $M$ point in reciprocal space, as restoring back to the homogeneous limit.

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