Orbital degeneracy and the microscopic model of the FeAs plane in the iron-based superconductors

A microscopic model for the FeAs plane in the newly discovered iron-based superconductors is proposed with the emphasis on the role of the orbital degeneracy between the Fe 3d(xz) and 3d(yz) orbitals in the crystal field environment. The model predicts that the Fe 3d(xz) and 3d(yz) orbitals have the largest itinerancy among the five Fe 3d orbitals as a result of their hybridization with the As 4p orbital. The covalence with the As 4p orbital also explains naturally the large geometric frustration which is necessary for the formation of the magnetic order with a wavevector at (pi, 0) as found in experiments. An extended s-wave pairing is proposed for the superconducting state based on symmetry considerations and the local magnetic correlation pattern.