Why some Iron-based superconductors are nodal while others are nodeless
The symmetry of the order parameter in iron-based superconductors, especially the presence or absence of nodes, is still a question of debate. While contradictory experiments can be explained by appropriately tuned theories of nodeless superconductivity in the iron-arsenide compounds, for 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 two sets of compounds, and use functional renormalization group (fRG) techniques to analyze it in detail. Our results show that, due to the orbital content of the electron and hole bands, nodal superconductivity on the electron pockets (hole pocket gaps are always nodeless) can naturally appear when the third hole pocket which lies at wavevector (pi,pi) in the unfolded Brillouin zone is absent, as is the case in LaOFeP. When present, the third hole pocket has overwhelming d_{xy} orbital character, and the intra-orbital interaction with the d_{xy} dominated part of the electron Fermi surface is enough to drive the superconductivity nodeless (of s^+- form). However, in its absence, pair hopping, inter-orbital, and electron-electron intra-orbital interactions render the gap on the electron pockets softly nodal.