Superconducting state in iron-based materials and spin-fluctuation pairing theory

The multiorbital spin-fluctuation theory of pairing can explain many observed features of iron-based superconductors, in particular, the different variants of the experimentally examined behaviors of the superconducting gap. Soon after the discovery of superconductivity in pnictides, estimates were made of the possibility of pairing due to the electron-phonon interaction. The coupling constant appears to be even smaller than that for aluminum. This led to the conclusion that it is unlikely that the pairing caused by electron-phonon interaction could play a leading role in the emergence of superconductivity. The anisotropic s∓state and its nodal structure on Fermi surfaces are quite sensitive to some details of the electronic structure, such as the orbital character of the bands, spin-orbit interaction, and changes in the band structure due to the doping.