Electronic structure of iron-based superconductors: BKFA and FeSe

The nature of high-temperature superconductivity (HTSC) remains unclear. There are many different theoretical models, which describe some of HTSC aspects and peculiarities, but none of them is neither fully predictive nor coherent enough. Because of the extremely complicated electronic structure, the iron-based superconductors (IBS) are one of the most interesting classes of HTSC to study. In particular, the effect of three-dimensionality of electronic structure on electronic properties of IBS is not clear at all. Using angle-resolved photoemission spectroscopy (ARPES) we have measured and studied the kz-dependence of electronic structure in BKFA. We focus on peak positions of unoccupied bands in different points of Brillouin zone, Fermi surface and renormalization dependencies. A number of unexpected effects have been observed such as the swap of known excitation energy values for G and Z points and renormalization dependence from kz in BKFA. In addition, temperature dependence of FeSe was carefully studied and leads to new explanation of dxy-band shift in the center of Brillouin's zone.