Investigating the atomic scale structure and chemistry of grain boundaries in high-Tc superconductors

The short superconducting coherence length in high-T-c materials makes them extremely susceptible to the deleterious effect of atomic scale defects. Perhaps the most important of these defects for large-scale technological applications, are grain boundaries. Here we describe an atomic resolution investigation of structural and chemical changes that occur at grain boundaries in high-T-c materials using scanning transmission electron microscopy (STEM). STEM is ideally suited to this analysis, as atomic resolution Z-contrast images and electron energy loss spectra (EELS) can be acquired simultaneously, This permits a direct correlation between the structural images and the local electronic structure information in the spectrum. From this detailed experimental characterization of the grain boundaries, simple theoretical models can be derived that allow the structure-property relationships in high-T-c superconductors to be inferred. Results obtained from YBa2Cu3O7-delta and (Bi/Pb)(2)Sr2Ca2Cu3O10 show that there is a charge depletion zone formed at grain boundaries. This charge depletion zone can act as a tunnel barrier to the flow of superconducting charge carriers and appears to increase in width with increasing misorientation angle, The magnitude of the critical current across grain boundaries in high-T-c materials predicted from these models is in excellent agreement with the widely reported electrical transport results. (C) 1999 Elsevier Science Ltd. All rights reserved.