The electron pairing of KxFe2-ySe2

We studied the pairing instabilities in K_xFe_2-ySe₂ using a two-stage functional renormalization group (FRG) method. Our results suggest the leading and subleading pairing symmetries are nodeless d_x²-y² and nodal extended s, respectively. In addition, despite having no Fermi surfaces we find the buried hole bands make important contributions to the final effective interaction. From the bandstructure, spin susceptibility and the FRG results we conclude that the low-energy effective interaction in K_xFe_2-ySe₂ is well described by a J₁-J₂ model with dominant nearest-neighbor antiferromagnetic interaction J₁ (at least as far as the superconducting pairing is concerned). In the end we briefly mention several obvious experiments to test whether the pairing symmetry is indeed d_x²-y².