Details of 3D electronic structure of some Fe-based superconductors and their superconducting order parameters

In this thesis, the results of analyzing the electronic structure of two iron-based superconductors: FeSe and LiFeAs are presented. To access the electronic structure, angle-resolved photoemission spectroscopy was used. In our analysis, we focus on the structure of the superconducting gap and the influence of nematicity on the electronic structure. We have revealed changes in the electronic structure of FeSe caused by nematicity in all parts of the Brillouin zone. A scale of these changes is smaller than it was believed earlier. Also, we have observed an anomalous shift of the dispersions in opposite directions with temperature in this material. We have observed anisotropic superconducting gap on all sheets of the Fermi surfaces of both: FeSe and LiFeAs. We have shown that in LiFeAs, rotational symmetry is broken in the superconducting state, which manifests not only in the gap symmetry but also in the shapes of the Fermi surfaces sheets. This result indicates a realization of a novel phenomenon of superconductivity-induced nematicity:1 Iron-based superconductors 1.1 Introduction to iron-based superconductors 1.2 LiFeAs - special iron-based superconductor 1.3 FeSe - structurally simplest iron-based superconductor 2 Angle-Resolved Photoemission 3 Temperature evolution of the electronic structure of FeSe 3.1 Effects of nematicity from low-temperature measurements 3.2 Temperature dependent shift of the dispersions 3.3 Discussion and conclusions 4 Three-dimensional superconducting gap in FeSe 4.1 Superconducting gap on the electron-like pockets 4.2 Superconducting gap on the hole-like pocket 4.3 Discussion and conclusions 5 Superconductivity-induced nematicity in LiFeAs 5.1 Superconducting gap 5.2 Nematicity 5.3 Discussion and conclusions Summar