Magnetic resonance from the interplay of frustration and superconductivity

Motivated by the iron-based superconductors, we develop a self-consistent electronic theory for the itinerant spin excitations in the regime of coexistence of the antiferromagnetic stripe order with wavevector ${\bf Q}{1} = (\pi,0)$and$s^{±}$ superconductivity. The onset of superconductivity leads to the appearance of a {\em magnetic} resonance near the wavevector ${\bf Q}{2} = (0,\pi)$ where magnetic order is absent. This resonance is isotropic in spin space, unlike the excitations near ${\mathbf{Q}}_1$ where the magnetic Goldstone mode resides. We discuss several features which can be observed experimentally.