Ronny Thomale, Princeton
The symmetry of the order parameter in iron-based superconductors, especially the presence or absence of nodes, is still unknown. Seemlingly contradictory experiments for a large class of As-based compounds can be explained by highly fine-tuned theories of nodeless s-wave type superconductivity. However, for P-based compounds such as LaOFeP, all experiments clearly point to a nodal order parameter. We put forward a scenario that naturally explains the difference between the order-parameter character in these compounds, and use functional renormalization group techniques to analyze it in detail. Our results show that, due to the orbital content of the electron and hole bands, nodal superconductivity can naturally appear as a function of varying pnictogen height, i.e. chemical pressure in the compound. In addition, we describe further classes of iron-based superconductors where we predict d-wave order such as in hole doped KFe2As2, and furthermore analyze LiFeAs showing superconductivity in the presence of strong ferromagnetic fluctuations. In view of all the different fascinating phenomena appearing in these compounds, we give an outlook of certain promising future directions of the field in theory and experiment.