Collective modes and emergent symmetry of superconductivity and magnetism in the iron pnictides

We consider the two-pocket model of the iron pnictides with generic interactions, and show that the low-energy effective theory of this model belongs to a wide class of Hamiltonians with enhanced symmetry. We consider the effect of symmetry breaking perturbations around such systems and explore the physical consequences. Our analysis reveals that the presence of the spin density wave with ordering wave vector (pi, 0) necessarily implies the existence of odd-sign s-wave superconductivity. We predict a spin-triplet resonance at the wave vector (pi, 0) whose frequency depends linearly on applied field and is independent of temperature. We also discuss the possible presence of a novel orbital density wave state as a close competing order encompassed by the emergent SO(6) symmetry. Copyright (C) EPLA, 2009