Iron-based Superconductor Electronic Structure Of The High Resolution Angular Resolution Photoelectron Spectroscopy Study

The newly discovered iron-based superconductors, which are the second high-temperature superconductor besides the cuprates, could be divided into iron-pnictide and iron-selenide superconductors with many series, thus exhibiting rich phase diagrams. Meanwhile, the superconducting mechanism of the cuprates is still unclear, which requires us to find another way to search for the superconducting mechanism of the high-temperature superconductors. Therefore, the iron-based superconductors have received continuous and considerable interest around the world. The valence electrons near the Fermi energy not only dominate the electrical, magnetic, optical, and thermal properties, but also constitute the cooper pairs when entering superconducting state, thus their study would enhance the understanding of the superconducting mechanism. Up to now, angle-resolved photoemission spectroscopy (ARPES) has been turn out to be the sole tool to simultaneously detect the electron’s energy, moving direction, and scattering property near Fermi energy in solids, and it would be more efficient for investigating their novel electronic structures, phase transitions, and various orderings if combing with the synchrotron radiation technique. In this thesis, we mainly focus on the electronic structure of iron-based superconductors, the mechanisms of superconductivity, and spin-density-wave (SDW) ordering, etc., and utilize the high-resolution ARPES to study these properties. The corresponding results are listed as follows.1. We study the electronic structure of CaFe2As2in the normal state and spin-density-wave state by ARPES. The temperature-dependence of band structure in CaFe2As2indicates the band shift and splitting occurs under the SDW transition. Since the band shift and splitting even exists at high binding energy well below the Fermi energy, and there is no gap opening near the Fermi surface, the band folding and Fermi surface nesting scenario couldn’t explain such significant reconstruction of band structure below SDW transition. Furthermore, there exists a pair of electron-hole pockets around the M point in the SDW state, which could be explained by the kz-dependence of the small Fermi pocket. 2. We firstly study the electronic structure of a slightly overdoped BaFe2-xCoxAs2and its superconducting gap distribution through the entire Fermi surface. The superconductivity in this iron-based superconductor is very robust against the variation of Fermi surface topology with the disappearance of two hole pockets around the zone center, and the superconducting gaps of all the bands are nearly isotropic in the plane, which naturally excludes the possibility of pairing symmetry variation proposed by spin fluctuation theory. On the other hand, only the superconducting gap of certain band exhibits certain k-dependence out of the plane.3. We firstly study the detailed electronic structure of11series with boths ARPES and band calculation. The superconductor of11series has the simplest layer structure among all the iron-based superconductors, however, its parent compound exhibits distinct antiferromagnetic ordering. Thus, it’s significant to study its electronic structures, and then compare it with other iron-based superconductor. Our ARPES experiment and density-functional theory calculation reveals the low-lying electronic structure, and its orbital characters are illustrated by the polarization-dependent ARPES. Compared with iron-pnictide compounds, most of their electronic structures are similar, except the electronic structure around the Γ point in the normal state. Our results provide comprehensive electronic structure of Fe1.04Te0.66Se0.34, and illustrate how the anion affects the electronic structure.4. Our study firstly resolves these four phases for AxFe2-ySe2with electronic identification, including two insulating parental phases, one semiconducting parental phase, and one superconducting phase by ARPES. These two insulating phases exhibit Mott-insulator-like signatures, and one of the insulating phases is even present in the superconducting and semiconducting KxFe2.ySe2compounds. However, it is mesoscopically phase separated from the superconducting or semiconducting phase, and the sizes of these phases are on the order of nm according to our transmission electron microscopy (TEM) study. Thus these studies also directly prove the existence of phase separation in KxFe2-ySe2superconductor and semiconductor for the first time, and we could study the intrinsic electronic structures of superconducting and semiconducting phases by the charging effect. Moreover, we find that both the superconducting and semiconducting phases are free of the magnetic and vacancy orders present in the insulating phases, and the electronic structure of the superconducting phase could be developed by doping the semiconducting phase with electrons. These results not only give a comprehensive understanding of various anomalous properties of this material, but also provide the foundation for a microscopic understanding of this new class of iron-based high-temperature superconductors.5. Exploiting polarization-dependent ARPES, we have firstly determined the detailed orbital characters of band structure in a KxFe2-ySe2superconductor. To a large extent, we find that KxFe2-ySe2superconductor shares similar orbital characters with other iron-based superconductors, but with its own characteristics. Moreover, we resolve two highly degenerate electron pockets around the zone corner in the s and p geometries, respectively, indicating negligible interactions between them. The determined low-energy electron structure and its orbital characters would help to construct a realistic model for KxFe2-ySe2.