Competing pairing symmetries in a generalized two-orbital model for the pnictide superconductors

We introduce and study an extended "t-U-J" two-orbital model for the pnictides that includes Heisenberg terms deduced from the strong coupling expansion. Including these J terms explicitly allows us to enhance the strength of the (π,0)-(0,π) spin order which favors the presence of tightly bound pairing states even in the small clusters that are here exactly diagonalized. The A_1g and B_2g pairing symmetries are found to compete in the realistic spin-ordered and metallic regime. The dynamical pairing susceptibility additionally unveils low-lying B_1g states, suggesting that small changes in parameters may render any of the three channels stable.