Abstract

The recently discovered iron arsenide superconductors present a very rich phase diagram, displaying superconducting, antiferromagnetic and structural order. In this seminar, I present a microscopic theory to study the interplay between electronic and magnetic degrees of freedom. In this model, electrons sharing the same bands are responsible for both superconductivity and itinerant magnetism, causing these two states to compete. Then, two distinct outcomes are possible: either these two states are mutually exclusive and phase separate or they can coexist microscopically. Using a weak coupling approach, I analyze the relationship between these two scenarios and the symmetry of the Cooper-pair wave function. I demonstrate that the static staggered magnetization plays the role of an intrinsic inter-band Josephson coupling in the free energy expansion, making the phase diagram sensitive to the symmetry of the gap function. In particular, I show that while the phonon-mediated s ⁺⁺ state is generally incompatible with the antiferromagnetic phase, the unconventional s ^+- state is able to coexist with magnetism. I also analyze the optical spectrum of the magnetically ordered superconducting phase, discussing its relation to the experimentally observed suppression of superfluid density in the coexistence state.