Evolution of pairing interactions in Fe-based superconductors*

The superconducting properties of the ferro-pnictides and -chalcogenides vary substantially across the families. In particular one finds almost clean as well as strongly anisotropic energy gaps suggesting that small changes in the electronic structure entail strong variations in the pairing. Raman spectroscopy offers a window into the structure of the energy gaps and, in addition, unveils fingerprints of sub-leading pairing interactions. We studied the energy gaps in various families including Ba(Fe_1-xCo_x)₂As₂ and Ba_1-xK_xFe₂As₂ and found values similar to those obtained by other methods. In materials with clean gaps such as Ba_1-xK_xFe₂As₂ solely light scattering allows the observation of collective in-gap modes which, for their line shapes, temperature and doping dependences, and evolution of spectral weights, can be identified as Bardasis-Schrieffer excitons. The binding energies of these modes encode the relative coupling strengths in the sub-leading pairing channels and show along with theoretical RPA and fRG calculations that spin fluctuations contribute substantially to Cooper pairing. So far it is open whether this conclusion can be generalized to all Fe-based systems.