Low Temerature Electron Microscopy of High T_c Superconductors

这项工作最初是在五年前由JSPS和美国国家科学基金会赞助的日美科学合作计划的支持下开始的，该计划的主题是“相变和固态反应的低温高分辨率电子显微镜”。在此基础上，探索了用于超导金属间化合物研究的低温高分辨电子显微镜技术。由于合作工作已经如此有效地完成，这项研究计划不仅利用低温技术，而且还利用常温下的分析电子显微镜和原子尺度电子显微镜，开始了开发高T_c氧化物超导体的研究计划。1.用高分辨电子显微镜研究了Bi2Sr2CaCu2Ox的晶体结构，特别是它的无公度调制。基于对大范围的详细检查，沿b轴w…的无公度周期性用周期为4.5b_o和5.0b_o的两种磁区组成的结构模型可以很好地解释正交型Bi4Ti3O12&gt；12&gt；型晶格的基本单元bo的4.8倍以上的原子排列。2.利用电子能量损失谱(EELS)的平行探测系统，测量了Y-、Bi-和Tl-氧化物的氧K边附近的精细光谱。我们发现，在约530 eV处的峰表明存在空穴状态，并且峰的强度取决于晶体取向和氧浓度。结果表明，空穴浓度随Tc值的增加而减小，空穴与CuO平面上的2p&lt；xy&gt；轨道有关。3.以熔体急冷非晶态BiSrCaCuO系为前驱体制备了高密度、高Tc值的超导陶瓷薄膜。用高分辨电子显微镜研究了样品的微观结构及其与高T_c超导电性的关系。结合使用EDX和EELS的分析电子显微镜的详细调查目前正在进行中。较少

This work has originally started five years ago by the support of Japan-U.S. Science Cooperation Program under the auspices of JSPS and NSF on "Low temperature high-resolution electron microscopy of phase transition and solid state reactions". Under this program, the techniques of HRTEM at low temperature were explored for studying superconducing intermetallic compounds. Because the cooperative efforts have been done so effectively, this research program was initiated for development of high Tc oxide superconducters with the use of not only the low temperature technique but also analytical electron microscopy as well as atomic-scale electron microscopy at ambient temperature. Major results obtained in this work are summarized as follows.1: Crystal structure of Bi_2Sr_2CaCu_2Ox has been studied by high resolution electron microscopy with particular attention to its incommensurate modulation. Based on detailed examination over wide areas, the incommensurate periodicity along the b-axis w … More hich is 4.8 times of the basic unit b_o of the orthorhombic Bi_4Ti_3O_<12> type lattice can be interpreted well with a structure model consisting of two kinds of domain with the periods of 4.5 b_o and 5.0 b_o. The atomic arrangement in the incommensurate modulated structure is directly proposed from the high resolution observation.2: Using a parallel detection system of electron energy loss spectroscopy (EELS), fine spectra near oxygen K- edge in superconducting Y-, Bi- and Tl-oxides have been measured. We found that a peak at about 530 eV indicates the presence of a hole state, and the peak intensity depends on the crystallographic orientation and the oxygen concentration. It is concluded that the hole concentration decreases with the increase of Tc, and the hole is associated with the 2p_<xy> orbit in the Cu-O plane.3: High density, high Tc superconducting ceramic films have been prepared from precursor of melt-quench amorphous Bi-Sr-Ca-Cu-O system. Microstructure and its correlation with the high Tc superconductivity are clarified by high resolution electron microscopy. Detailed investigations combining with analytical electron microscopy using EDX and EELS are now in progress. Less