Hidden SDW order and effective low-energy theory for FeAs superconductors

We propose a simple effective model to describe FeAs superconductors. This model is based on the assumption of a local spin-density-wave (SDW) order, with its magnetization direction allowed to fluctuate. It is shown that the long-range order with momentum Q=(\pi ,\pi) is generally unstable in competing with the kinetic energy of the charge carriers. A true weak SDW order is formed in the undoped case with an additional momentum shift Q_s=(\pi, 0) due to the peculiar Fermi surface nesting. In the doped case, the unstable long-range order driven by kinetic energy can naturally result in a d-wave superconducting condensation. Such low-energy physics is protected by the presence of the local SDW which sustains some kind of "Mott gags" for the multiband d-electrons near the Fermi energy.