The Study Of Microstructure In Sr₂Cuo_3+δ High Temperature Superconducting Material By Synchrotron Radiation

The energy shortage today is found to be a great challege in the development of modern society. The energy loss in transportation, which make people puzzle for a long time, is one of the most important energy problems. Developing superconducting material, who has a lot of excellent characteristics, i. e. zero resistance property, Meissner effect, Josephson effect et al, provide an important solution for resolving this problem. In recent years, the superconducting material, especially high temperature superconducting material, is widely applied in almost all fields, i. e. military, medicine, industry, electricity et al. However, corresponding cooling method is still expensive for using the high temperature superconducting material. The most highest value of superconducting critical temperature (Tc) for the available superconducting material is just about160K. In order to further cut the operating cost, the study of new superconducting material with higher superconductive critical temperatureor even room-temperature, is necessary. However, still now, there is no a general theory governing the high temperature superconductivity. The main difficulty for interpreting high temperature superconductivity is the strongly interaction between charge, orbit, spin and lattice order parameters, which causes many kinds of microstructures show up. Lots of studies show that the evolution of microstructure is highly correlated with high temperature superconductivity. Therefore, synchrotron radiation studies are performed on typical high temperature superconducting material with sing CuO2layer for further understanding the relationship between microstructure and high temperature superconductivity. Firstly, the evolution of superstructure is studied by temperature-dependent resonant x-ray diffraction around Cu K-edge in Sr2CuO3+s powder sample. At the same time, temperature-dependent Cu K-edge x-ray absorption fine structure spectroscopy is performed on Sr2CuO3+δ powder sample for determining the local lattice distortion. Based on these results, we discuss the origin of superstructure and the relationship with superconductivity. Then, we perform synchrotron radiation polarized Cu K-edge x-ray absorption fine structure spectroscopy on Sr2CuO3+δ superconducting single crystal specimen to determine the location of oxygen vacancies. Finally, the anisotropic electronic structure at CuO2plane of Sr2CuO3+δ single crystal is studied by polarized soft x-ray absorption spectroscopy at Cu L23-edge.In chapter one, we introduce the development of superconducting material and the remaining challenges. Firstly, we review the research development of superconducting material and current state of high temperature cuprate superconducting material. Then, the research progress of Sr2CuO3+δ high temperature superconducting material with single CuO2layer is introduced and the necessity of the research is pointed out. Finally, we illustrate the research object, research contents and the scientific problems we will deal with.In chapter two, we mainly introduce synchrotron radiation characterizing methods used in this paper, i.e., polarized x-ray absorption fine structure spectroscopy, polarized soft x-ray absorption spectroscopy, and resonant x-ray diffraction.In chapter three, we performed x-ray absorption fine structure spectroscopy and resonant x-ray diffraction on Sr2CuO3+δ powder sample for studying the evolution of microstructure around Tc. Temperature-dependent resonant x-ray diffraction shows that a new modulated structure show up just above Tc. Temperature-dependent x-ray absorption fine structure also shows local lattice distortion and weak ordering signal at the temperature where the new modulated structure appears. These indicate that the new modulated structure is correlated with local lattice distortion. At the same time, we found that the new modulated structure always exist below Tc, suggesting that the new modulated struture is also superconducitvity-related.In chapter four, we choose Sr2CuO3+δ single crystal sample for the study. Polarized Cu K-edge x-ray absorption fine structure spectroscopy and Cu L23-edge x-ray absorption spectroscopy are performed for studying anisotropic local crystal structure and electronic structure around Cu site of CuO2plane, respectively. Polarized x-ray absorption fine structure suggests that oxygen vancancies in Sr2CuO3+δ locates both in the CuO2plane and at the apical site. Among the total17.5%oxygen vacancies, about10%locates in the CuO2plane and7.5%at the apical site. Polarized Cu L23-edge x-ray absorption spectra show that the unoccupied states is dominated by the in-plane component of Cu3dx2-y2. The signal from the unoccupied out-of-plane Cu3dz2states also appear. It indicates that the unoccupied Cu3dz2states also have an effect on superconductivity of Sr2CuO3+δ.