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

Superconductivity allows one to build unique devices, which can be used for a variety of purposes: transporting electrical power, medical examination, scientific research, electronics, etc. Since 1986, superconductivity at unexpectedly high temperature has been observed in two broad classes of materials. However, their critical temperatures are still considerably lower than room temperature, and these materials require cooling with cryogenic liquids to be superconducting. This restricts superconductivity from being widely used. Knowledge of a mechanism of superconductivity is essential for designing new materials, which superconduct at a higher temperature.However, the mechanism of high-temperature superconductivity remains unknown. Knowledge of the details of the electronic structure of existing superconducting materials can help to shed light on it. In this thesis, we analyze the electronic structure of two materials: FeSe and LiFeAs, which belong to iron-based superconductors, a relativly new family of superconductors. To access the electronic structure, we use angle-resolved photoemission spectroscopy. In our analysis, we focus on the following aspects of the electronic structure: structure of the superconducting gap and 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 smallerthan it was believed earlier. Also, we have observed an anomalous shift ofthe 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.