Competitions of magnetism and superconductivity in FeAs-based materials

Using the numerical unrestricted Hartree--Fock approach, we studythe ground state of a two-orbital model describing newly discoveredFeAs-based superconductors. We observe the competition of a $(0,pi)$ mode spin-density wave and the superconductivity as the dopingconcentration changes. There might be a small region in theelectron-doping side where the magnetism and superconductivitycoexist. The superconducting pairing is found to be spin singlet,orbital even, and coexisting $s_{xy} + d_{x^{2}-y^{2}}$ wave (evenparity).