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 (pi, 0)-(0, pi) 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.