Abstract

In this talk I will give an overview of our research into the newly discovered iron-based superconductors. The work has two main threads. (1) Investigation of the normal state electronic structure using high field measurements of the de Haas-van Alphen effect, and (2) Investigation of the anisotropy of the superconducting energy gap using measurements of magnetic penetration depth. I will argue that the penetration depth data suggest that there are two competing pairing channels in these materials – one which produces a gap with nodes and another which produces a fully gapped (nodeless) pairing state. It seems that these different pairing channels can both produce high Tc superconductivity. The dHvA data will be compared to band-structure calculations to reveal the extent of the many body corrections. For the series BaFe_2(As_(1-x)P_x)_2 we are able to track the changes in the Fermi surface parameters as the composition is tuned towards that optimal for superconductivity. As the maximal Tc is approached we find both the electron and hole Fermi surfaces shrink and the effective mass increases strongly. This data suggests a direct quantitative link between spin-fluctuation induced changes to the normal state and the microscopic superconducting pairing mechanism.