Photoemission Studies of High Temperature Superconductors and Related Materials.

The discovery of the high temperature superconductors has challenged our previous understanding of mechanisms of the superconductivity. A key obstacle that prevents us from understanding the origin of the superconductivity in the high-temperature superconductors is the lack of understanding of their electronic structures. This theses presents the results of a photoemission study of the electronic structures of the high-temperature superconductors and their related materials. The experimental techniques used in this study include ultraviolet photoemission, x-ray photoemission, resonance photoemission and angle-resolved photoemission. For the cuprate superconductors, an important result is the observation of Cu satellites in both the valence band the Cu atomic core states for all the compounds studied. We found that one-electron pictures cannot adequately explain the experimental data which were then fitted by a cluster version of the Anderson Hamiltonian. The results indicate that the Coulomb repulsion between the two electrons on the same Cu site is very strong which signals the importance of the correlation effects, and the charge transfer energy between the Cu 3d states and the O 2p states is much smaller than the Cu on-site Coulomb interaction energy so that the Cu rightarrow O charge transfer process dominates the low energy excitations. We also investigated the electronic structure changes of the cuprate superconductors upon various doping. Examples include the Y_{1-x}Pr _{x}Ba _2Cu_3O_ {7-delta} system with various x and the Bi-Sr-Ca-Cu-O system with different number of CuO _2 planes in the unit cell. Furthermore, we have performed angle-resolved photoemission studies on simpler, but related materials of NiO and CoO. The results showed that the lattice effects are important for the photoemission data from these highly correlated materials. Thus, we suggested that only a lattice Hamiltonian which takes the correlation effects into account explicitly can fully explain our photoemission data from the cuprate superconductors. For BaBiO_3, which is a parent compound of superconductors without Cu, our data were consistent with the predictions of band calculations.