Microstructure and critical current of superconducting YBa₂Cu₃O_7−x

Abstract

In the few months that have passed since the discovery of high-transition-temperature superconducting oxides^1–3, remarkable progress has been made in identifying the structure of superconducting phases, improving our understanding of factors that control their synthesis, and determining their basic properties. In one area, however, progress has been disappointingly slow; that is, in the control and optimization of the superconducting critical current density, a property vital for effective technological application of these materials. Bulk samples display a critical current that is generally very low and, furthermore, drops steeply in an applied magnetic field. There is persuasive evidence, however, that low critical currents are not an inherent characteristic of the material^4,5, and recent work on thin epitaxial films⁶ has indicated that values as high as 10⁶ A cm⁻² can be achieved. It is also generally established that the critical current that can be carried by any superconductor depends predominantly on the local micro-structure, its degree of uniformity and the nature of its characteristic defect structures⁷. We present here the main results of a detailed electron microscopical study of sintered YBa₂Cu₃O_7−x⁸, and discuss their relevance to observed critical current and its modification by increased silicon impurity levels⁹. In the light of these results we assess the prospects for controlling microstructure development so as to optimize the critical current for technological application.