Electron-Microscopy Investigation of the Structure of Defects

Abstract

Since that moment when the superconducting compounds of Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O, Tl-Ba-Ca-Cu-O type were discovered, the quasi-two-dimensional structure of the superconducting phases has attracted much interest. In Y-Ba-Cu-0 compounds there are twin boundaries of high bulk density, in Bi-Sr-Ca-Cu-O compounds there are structural modulations, and in the Tl-Ba-Ca-Cu-O compounds they exhibit polytypism. Up to the present time, a number of works have been devoted to the influence of these two-dimensional defects, as supposed pinning centers, on the superconducting properties and as special crystal planes where 2D and ID models of superconductivity may be realized. Since the role of quasi-2D defects on the physical properties of superconducting compounds is not yet quite clear, the study of defect structures in these materials is of significant value. In particular, the study of oxygen atoms and oxygen vacancies inside and in the vicinity of planar defects presents an especially interesting problem, since, in the majority of existing models of high-temperature superconductivity, the localization of oxygen ions is one of the basic factors affecting superconductivity parameters.