一种用来精确测定材料价电子分布和界面点阵位移的电子衍射技术

A novel electron diffraction technique to accurately measure valence electron distribution and interfacial lattice displacement

作者开发了一种新的电子衍射技术来测量大单胞复杂晶体的电荷密度。该技术是在透射电子显微镜中把电子束聚焦在样品的上方从而在很多Bragg衍射盘中得到阴影像(简称为PARODI)。对于楔型晶体,该技术提供了可同时记录许多衍射的厚度条纹的方法,它确保了所有衍射盘中的厚度变化和入射方向等实验条件是相同的。PARODI技术还被进一步扩展到使用相干光源这一新领域并用来精确测定晶体中缺陷的位移矢量。本文用该技术研究了Bi-2212高温超导体中的层错和扭转晶界并观察到了由面缺陷引起的相干条纹。通过使用相干PARODI技术,测量的Bi-2212中面缺陷的位移矢量的精度达到了1皮米(10-12m),这是目前在测量缺陷位移方面达到的最高精度,比以前的技术提高了将近一个数量级。

We developed a novel electron diffraction technique by focusing a small probe above （or below） the sample in a transmission electron microscope to measure charge density in complex compound with large unit cell, e.g. high-dielectric-constant perovskite CaCu3 Ti4O12 , The method features the simultaneous acquisition of shadow images within many Bragg reflections, resulting in Parallel Recording of Dark-field Images （denoted as PARODI）. For wedge-shaped crystals, this procedure provides a means of recording thickness or Pendellosung fringes of many reflections simultaneously, which ensures that the experimental conditions with respect to thickness variations and crystallographic direction of the incident beam are the same for all reflections. The PARODI technique has been further extended to study defects with coherent electron source （denoted as coherent PARODI）, e.g. in transmission electron microscope equipped with a field emission gun. We studied Bibased high-temperature superconductors containing stacking faults and twist grain boundaries with accompanying displacements perpendicular to the boundary plane. We observed strong interference fringes from the planar faults even when they were viewed edge-on, in agreement with model calculations. With coherent PARODI, we determined displacement vectors of the planar faults in Bi-2212 with an accuracy down to 1 picometer （ 10^-12 m）, which is the highest accuracy that has been achieved in measuring displacement vectors.