Study On Electronic Stucture Of Bismuthate And Ferrum-Based Oxide Superconductor

Since the discovery of the La-Ba-Cu-0 high-T_c superconductor in 1986, extensive research activities have been devoted to this system due to the high T_c. However, the underlying physical mechanism for the high-T_c superconductivity (SC) of copper oxides is still under debate. It has been one of the most challenging problems in condensed material physics to clarify the mechanism, and many scientists hope to find new high-T_c superconductor other than the copper oxides to explain the high-T_c superconductivity. Although the T_c of the copper-free oxide superconductors is much lower than that of copper oxides, they provides a more clear system to uncover the mechanism of the high-T_c superconductivity and to explore other structural characteristics supporting the superconductivity. Therefore, it is very important to fully understand the structural characteristics of the copper-free oxide superconductors, which will broaden the research scope of superconductive materials.This dissertation mainly focuses on three parts. The first part is to investigate the effects of the doping behaviors of BaBi_1-xPb_xO₃ and Ba_1-xK_xBiO₃ superconductors on the electronic structures and superconductivity. The second part is to examine the effects of doping behaviors of NaCoO_x on the electro-magnetic properties. The last parts is to discuss the superconductivity and the electronic structures of the novel iron pnictides superconductor. The main contents are presented as follows:In chapter 1, a brief introduction was made to the fundamental properties of superconductors such as the superconductivity, crystal structure, mechanism models, significance of the current research and main contents of this dissertation.In chapter 2, the copper-free high-T_c superconductors were summarized. The structural characteristics, element substitute effect and preparation methods of the BaBiO₃ superconductors were introduced. The crystal structures, electronic structures, superconductivity phase maps and magnetism of the cobalt oxide superconductor system were briefly reviewed. Emphases were placed to introduce the research status, structural characteristics, element substitute effect, and mechanism of the novel iron based superconductive materials.In chapter 3, the electronic structures of BaPb_0.75-xHg_xBi_0.25O₃ (BPHBO) system were studied with x-ray photoelectron spectroscopy (XPS). It was indicated that the core levels of Pb4f and O1s states shifted towards lower region with the mercury doping, and that of Bi4f state shifted towards higher region. The results revealed that the mercury doping induced systematic changes of the electronic structures, consequently affecting the superconductivity. The suppression of superconductivity at low doping levels (x<0.2) was attributed to the suppressed charge-disproportionate state, whereas the recovery in the doping level of 0.31-xF_x were examined with the resistivity measurement and XPS. With the increase in the fluorine doing levels, the system changed from a metallic spin-density-wave state to a superconductive state. The small peak centered at 0.2 eV below the Fermi level in the valence band gradually disappeared and a broad plateau near the Fermi level formed. The shift of the Fermi level with a rate of dE_F/dx≌0.03 eV/at% after fluorine doping was obtained from the shift of the binding energy of O1s and Sm 3d core-levels. The results revealed that the spin-density-wave state originated from the low-spin state of the Fe3d electrons, which was gradually suppressed with the band width expansion after fluorine doping.In chapter 5, the electronic structures of Fe-based superconductor SmFe_1-xIr_xAsO were studied with the magnetism measurement and XPS. It indicated that element Ir was doped on the Fe site, which directly introduced electrons to the conductive Fe-As layer. Therefore, compare to F doping, Ir doping inicated more significant effects on the superconductivity. The magnetism order of SmFeAsO was closely related with the Fe3d low spin state, which was suppressed by Ir doping through the suppressing magnetism of the 3d itinerant electrons as mentioned previously. In superconductivity samples the 3d itinerant electrons were mainly in the high spin state. All the results suggested the strong correlation occurred between the electrons in this system.In chapter 6, the electronic structures of copper oxide superconductor were compared with those of copper-free oxide superconductor.In chapter 7, single phase polycrystalline samples Na_0.7Co_1-xAl_xO₂ (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) were prepared by solid state reaction. The magnetic properties were characterized by dc and ac magnetic susceptibility measurements from 5 to 300 K. Samples with lower doping amounts (x = 0, 0.05, and 0.10) showed paramagnetic behaviors, but those with higher doping amounts (x = 0.20, 0.25, and 0.30) showed spin-glass behaviors with a freezing temperature (T_f) of about 13 K.In chapter 8, the whole dissertation was sum-up.