Master's Defence: Maria Chatzieleftheriou

Iron-based superconductors: electronic correlations and orbital selectivity.

A common belief is that the multi-orbital character of the high-Tc iron-based superconductors plays a major role in the physical processes. In this thesis we apply two methods to assess the strength of electronic correlations in these materials. We use a local slave spin method to show the presence of a strong mass renormalization, as well as the emergence of an orbital selective phase, with co-existence of weakly and strongly correlated electrons. In order to include the momentum dependence of the self-energy and estimate its impact on the correlations, we consider the effect of spin fluctuations. Furthermore, motivated by a recent experiment carried out on iron selenide, we investigate whether a small energy difference between the xz and yz iron d-orbitals in the nematic phase of the system, can be accompanied by a large discrepancy in their quasiparticle weights, thus in their degree of correlation. We show qualitatively that indeed, if the system in the nematic state is close to a magnetic instability, such an effect can be observed.