Magnon-mediated pairing and isotope effect in iron-based superconductors

Within a minimal model for the iron-based superconductors in which itinerant electrons interact with a band of local moments, we derive a general conclusion for multiband superconductivity. In a multiband superconductor, due to the Adler theorem, the interband scattering dominates the intraband scattering at the long wavelength limit as long as both interactions are induced by the Goldstone boson (which is the magnon in our case) and the transferred momentum is non-zero. Such an interaction leads to a well known sign-reversing superconductivity even if the interband and intraband interaction are repulsive. This effect can be modeled as arising from an internal Josephson link between the Fermi surface sheets. Our model is also consistent with the recently discovered coexistence of superconductivity and magnetic order in iron-pnictides. Although the experimentally observed isotope effect is large, α = 0.4, we show that it is consistent with a non-phononic mechanism in which it is the isotope effects which result in a change in the lattice constant and, as a consequence, the zero-point motion of the Fe atoms.