Quantitative Analysis of Local Structure in High Tc Superconductors by Energy Filtering Convergent-Beam Electron Diffraction

提高YBa_2Cu_3O_（YBCO）高温超导体的临界电流密度Jc是其实用化的重要课题之一<7-x>。晶体结构和超导性能敏感地取决于氧浓度，而氧浓度很容易根据大气而变化。会聚束电子衍射已经用于提取关于氧浓度和/或电荷分布和/或氧的信息。用电子衍射动力学理论分析了所观测到的衍射强度。用配有Ω型能量滤波器的FE-TEM：JEM-2010 FEF观察了大角度CBED谱图，测量的氧浓度的准确性取决于分析中使用的温度因子（B因子）的可靠性。用各向异性温度因子估计了电子束辐照下试样的实际温度。所测定的局部氧浓度因点而异。这一结果表明，氧浓度是不均匀的YBCO和缺氧波动的局部。在[001]衍射图中没有观察到超晶格点，表明氧缺陷随着氧沿着Cu-O链的随机减少而发生。氧缺位的波动是YBCO中峰效应产生的最可能原因。通过对CBED图的精细分析，提取了CuO_2和CuO面上的电荷分布信息。

Improvement of the critical current density, Jc is one of the important subjects for practical application of high Tc superconductor YBa_2Cu_3O_<7-x> (YBCO). The crystal structure and superconducting properties sensitively depend on oxygen concentration that easily changes depending on atmosphere. Convergent beam electron diffraction has been used to extract information on oxygen concentration and/or charge distribution and/or oxygen. The observed diffraction intensities are analyzed with the dynamical theory of electron diffraction. Large angle CBED patterns were observed with a FE-TEM : JEM-2010FEF equipped with an energy filter of omega-type.The accuracy of the measured oxygen concentration depends on the reliability of temperature factors (B-factors) used in the analysis. An actual temperature of a specimen under electron beam irradiation is estimated with anisotropic temperature factors. The local oxygen concentration determined vary from point to point. This result shows that oxygen concentration is not uniform in YBCO and oxygen deficiency fluctuates locally. No super lattice spots were observed in [001] diffraction patterns indicating that oxygen deficiency occurs with random depopulation of oxygen along a Cu-O chain. Fluctuation of oxygen deficiency is the most probable reason for the peak-effect in YBCO. Information on charge distribution on CuO_2 and CuO planes are extracted by more precise analysis of CBED patterns.