Low Temerature Electron Microscopy of High T_c Superconductors

这项工作最初于五年前在日美合作组织的支持下开始。JSPS和NSF科学合作项目“相变和固相反应的低温高分辨率电子显微镜”。在该项目下，探索了低温HRTEM研究超导金属间化合物的技术。由于双方的合作卓有成效，本研究项目启动，目的是开发高Tc氧化物超导体，不仅使用低温技术，而且还使用分析电子显微镜和环境温度下的原子尺度电子显微镜。现将本工作取得的主要成果总结如下：用高分辨电子显微镜研究了Bi_2Sr_2CaCu_2Ox的晶体结构，特别注意了它的不适应调制。通过大范围的详细考察，可以很好地解释正交Bi_4Ti_3O_<12>型晶格沿b轴的不均匀周期性，其不均匀周期性是正交Bi_4Ti_3O_<12>型晶格基本单位b_o的4.8倍，该结构模型由周期为4.5 b_o和5.0 b_o的两种域组成。通过高分辨率观测，直接提出了不适应调制结构中的原子排列。利用电子能量损失谱（EELS）平行检测系统，测量了超导Y-、Bi-和tl -氧化物中氧K边附近的精细光谱。我们发现在530 eV处有一个峰表示空穴态的存在，峰的强度与晶体取向和氧浓度有关。结果表明，空穴浓度随Tc的增加而减小，且空穴与Cu-O平面上的2p_<xy>轨道有关。以熔融淬火非晶Bi-Sr-Ca-Cu-O体系为前驱体制备了高密度、高Tc超导陶瓷膜。用高分辨电镜分析了其微观结构及其与高温超导性的关系。结合EDX和EELS分析电子显微镜的详细研究正在进行中。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