Magnetic resonance from the interplay of frustration and superconductivity

Motivated by 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 wave vector Q1=(π,0) and s± superconductivity. The onset of superconductivity leads to the appearance of a magnetic resonance near the wave vector Q2=(0,π), where magnetic order is absent. This resonance is isotropic in spin space, unlike the excitations near Q1, where the magnetic Goldstone mode resides. We discuss several features which can be observed experimentally. In particular, we find that the additional subtle effects of microscopic coexistence of AF and s± SC order, such as damping of the spin waves at Q1 and the modified dispersing behavior of the spin resonance around Q2, can be used by INS to distinguish the coexistence phenomenon from phase separation effects.