| The 2008 discovery of the iron pnictide superconductors triggered enormous theoretical and experimental effort in the condensed matter physics community, as they represent the second class of high temperature superconductors with Tc up to 55 K. The phase diagrams of these materials contain a number of intriguing features that hint at a close link between high temperature superconducting behavior and their electronic structure. The present thesis presents experimental studies on the electronic structures of the pnictides, using the technique of angle resolved photoemission spectroscopy (ARPES). These studies reveal a comprehensive picture of their band structures. We performed some of the first ARPES measurements on the pnictides, followed by detailed ARPES studies on a wide range of members, including the carrier-doped AEFe2As2 (122) and RFeAs(O,F) (1111) systems. Our experimental results show the quasi two dimensionality of electronic structure in the 1111 systems, versus complete three dimensional Fermi surfaces in the 122 systems; a surface-driven electronic structure with the presence of a large s-wave-like superconducting gap in the 1111 systems; a magnetically reconstructed electronic structure revealing an unexpected nesting condition in undoped as well as underdoped 122 systems. Most importantly, a comprehensive survey on cobalt-doped BaFe2As2 led to the discovery of several Lifshitz transitions (topological changes in the Fermi surface due to doping-driven rigid band shifting) that have direct impact on their superconducting properties. Both the low doping emergence and the high doping disappearance of superconductivity are linked with such Lifshitz transitions. For the first time, we found that superconductivity favors a specific Fermiology in high-Tc superconductors. |
