Low Temerature Electron Microscopy of High T_c Superconductors

这项工作最初于五年前开始，由JSPS和NSF主持的日美科学合作计划支持，题为“相变和固态反应的低温高分辨率电子显微镜”。在此基础上，对低温高分辨透射电镜技术在超导金属间化合物研究中的应用进行了探索。由于合作的努力已经做得如此有效，这项研究计划开始开发的高Tc氧化物超导体，不仅使用低温技术，但也分析电子显微镜，以及在环境温度下的原子尺度电子显微镜。主要结果如下：1.用高分辨电子显微术研究了Bi_2Sr_2CaCu_2Ox的晶体结构，特别是它的无公度调制。基于对大面积的详细检查，沿b轴的不相称周期性沿着 ...更多信息 这是正交Bi_4Ti_3O_型晶格基本单元B_o的4.8倍<12>，可以用周期为4.5 B_o和5.0 B_o的两种畴结构模型来解释。利用电子能量损失谱（EELS）并行探测系统，测量了Y-、Bi-和Tl-氧化物超导体中氧K边附近的精细谱。我们发现，在约530 eV的峰表明空穴状态的存在下，和峰强度取决于晶体取向和氧浓度。结果表明，空穴浓度随Tc的增加而降低，空穴与Cu-O面的2 p轨道有关<xy>。3.采用熔体急冷非晶Bi-Sr-Ca-Cu-O系前驱体制备了高密度、高Tc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温超导性的关系。详细的调查结合分析电子显微镜使用EDX和EELS现在正在进行中。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