bismuth and iron-based electronic structure of oxide superconductors

Title: bismuth and iron-based electronic structure of oxide superconductorsAuthor: Zhang YongDegree granted by: Southwest Jiaotong UniversityKeywords: non-copper-oxide superconductors;; electronic structure;; BaBiO_3 system;; iron-based superconducting materials;; NaCoO_x systemSummary:Since 1986, discovered the first high-temperature superconducting materials - lanthanum barium copper oxide, copper-based superconducting materials physicists around the world to become a research hotspot. However, until now, superconducting materials for high temperature superconducting copper-based mechanism, the unanimous view of the physics Neodymium Magnets community has not yet formed, which also makes high-temperature superconductivity in condensed matter physics become one of the greatest mysteries, many scientists hope to copper based superconducting materials other than to find new high-temperature superconducting materials, high-temperature superconductivity mechanism in order to make more clear. Although the discovery of non-copper-oxide superconductors T_c much lower than the copper-oxygen compounds, but the study helps to understand the system of high-temperature superconductivity mechanism for high-temperature superconducting mechanism to explore a clearer system, look for other support superconductivity The structure of the law, for the final reveal the underlying mechanism of superconductivity provides new conditions and opportunities. Therefore, to fully understand some of the typical non-copper oxide superconductor structure the law, such materials will greatly broaden the scope of exploration and research, which for the discovery of new superconductors is important.This paper mainly divided into three parts, the first part of BaBi_ (1-x) Pb_xO_3 doping behavior of superconductivity and electronic structures; second part of the new iron-based superconducting materials and superconducting properties of electronic structure; The third part NaCoO_x doping behavior of the electromagnetic properties. Main contents are as follows:第一章概述 the basic physical properties of superconductors; then describes the structural features of copper oxides, the superconducting phase diagram and superconductivity mechanism; the last topics of this paper is the significance and thesis research.Chapter II of the non-copper-oxide superconductors are reviewed, describes the high-temperature superconductors BaBiO_3 system structure, element substitution effect and preparation methods; describes the layered cobalt oxide superconductors, the http://www.everbeenmagnet.com/en/products/110-sintered-neodymium-magnets crystal structure, electronic structure, the superconducting phase diagrams and magnetic properties; highlights the new iron-based superconducting materials research status, structure, element substitution effect and the superconducting mechanism.Chapter X-ray photoelectron spectroscopy (XPS) studies BaPb_ (0.75-x) Hg_xBi_ (0.25) O_3 (BPHBO) series of electronic structure of superconductors, discovered Pb4f and O1s core level to the low-level mixed with Hg direction, while Bi4f Hg core level with the incorporation of moving to the high energy level, results and analysis showed that the electronic structure of Hg-doped system caused by changes in the system ultimately affect the superconductivity. In the low doping range (x <0.2) superconductivity is suppressed due to reduced carrier concentration due to the doping range 0.3 <x ≤ 0.4 the recovery of superconductivity is due to the new metal - insulator transition to system from an insulator into a type of carrier-based metallic state with hole.Chapter resistance measurement and XPS methods used on the new iron-based superconducting materials SmFeAsO_ (1-X) F_X of superconductivity and electronic structure of the study, results show that with increasing F doping, the spin system from the metallic density wave state into a superconducting state, while valence band under the Fermi level is located at the 0.2eV small peak gradually disappeared near the Fermi level in the formation of a wider platform. Resulting from the F-doped and Sm 3d core level O1s peak displacement values ​​derived from the Fermi level displacement speed dE_F / dx ≌ 0.03 eV / at% F atoms. The results showed that the spin-density-wave state from the low-spin state Fe3d electronics, with F doping increases, the spin density wave state is suppressed and the level gradually broadening.Chapter by magnetic measurements and XPS study of Ir doping on SmFeAsO superconductivity and band structure of the experimental results show that the Ir-doped Fe-bit, directly to the Fe-As conductive layer into the electron, and fluorine-doped compared to show its effect on superconductivity is more direct, a small amount of doping that have a strong influence on the superconducting properties, it produces superconductivity doping range adjustable small. Reveals SmFeAsO magnetic ordering and Fe3d low spin state of electrons associated with, and Ir-doped low-spin state of its suppression. When the Ir mixed system, Fe3d low-spin state of electrons is suppressed, the formation of the superconducting sample SmFe_ (1-x) Ir_xAsO in Fe3d electron spin state from low into high-spin state. All results show that the system has a strong electron correlation effects.Chapter VI of copper and copper oxide superconducting materials, non-electronic structure for comparison.Research NaCoO_x Chapter VII with the doping of magnetic variation of element types. Successfully synthesized by rapid heating of polycrystalline Na_ (0.7) Co_ (1-x) Al_xO_2, nominal doping up to 0.3. Measured in 5-300K doped polycrystalline samples of different dc magnetic susceptibility, dc magnetic susceptibility of more doping effect relationship. Found that low doped paramagnetic state for the overall sample, the amount of high-doped samples there is about 13K of the spin-freezing temperature glass transition.Chapter VIII of the full text of the work are summarized.Degree Year: 2009