Evolution of superconductivity in Fe-based systems with doping

We study the symmetry and the structure of the gap in Fe-based superconductors by decomposing the pairing interaction obtained in the RPA into s- and d-wave components and into contributions from scattering between different Fermi surfaces. We show that each interaction is well approximated by the lowest angular harmonics and use this simplification to analyze the origin of the attraction in the two channels, the competition between s- and d-wave solutions, and the origin of superconductivity in heavily doped systems, when only electron or only hole pockets are present.