Investigating the atomic scale structure and chemistry of grain boundaries in high-T c superconductors N.D. Browning a, * , J.P. Buban a , C. Prouteau a,b,1 , G. Duscher a,b , S.J. Pennycook b a Department of Physics (M/C 273), University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607-7059, USA b Solid State Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6030, USA Received 20 October 1998; accepted 13 January 1999 Abstract 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 YBa 2 Cu 3 O 72d and (Bi/Pb) 2 Sr 2 Ca 2 Cu 3 O 10 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. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: Grain boundaries; Atomic structure; Charge carrier depletion; Scanning transmission electron microscopy (STEM); Electron energy loss spectra (EELS) 1. Introduction Grain boundaries have long been known to have a deleterious effect on the transport properties of high-T c superconductors. For example, in systematic studies of high-angle [001] tilt grain boundaries in YBa 2 Cu 3 O 72d (YBCO) thin-film bicrystals, it has been found that there is an exponential decrease in the critical current as the misorientation angle of the boundary increases (Dimos et al., 1990; Gross and Mayer, 1991; Ivanov et al., 1991). While this overall trend in the data is clear, attempts to ascertain the fundamental origin of the transport properties have been confused by the considerable scatter in the experimental measurements (results can vary by several orders of magnitude for the same misorientation angle). Many explanations for the transport properties have been linked to the chemistry of these ceramic oxide materials. It may be that as YBCO is highly susceptible to the formation of oxygen vacancies, an oxygen deficient boundary layer is formed, thereby reducing the charge carrier concentration (Browning et al., 1992; Zhu et al., 1993; Babcock et al., 1994). Alternatively, there may be second phases, precipi- tates or the preferential segregation of impurities to the boundary that cause a barrier to transport. However, as all of these possibilities are a function of the processing condi- tions in ceramic oxides, it is not clear why they should cause the overall trend of an exponential decrease in critical current with misorientation angle. The large scatter in the experimental transport measurements is an obvious conse- quence of variable processing conditions and the chemistry of the oxide systems, but if it is the dominant effect, high conductivity should be readily achieved by preparing clean, oxygen-rich grain boundaries. This has not been reported in the literature, despite many attempts to oxidize boundaries with excellent cation stoichiometry. The widely observed transport behavior of grain bound- aries in high-T c superconductors therefore suggests that there is an underlying mechanism responsible for the exponential decrease in critical current with increasing Micron 30 (1999) 425–436 PERGAMON 0968-4328/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0968-4328(99)00044-X www.elsevier.com/locate/micron * Corresponding author. Tel.: 1 1-312-413-8164; fax: 1 1-312-996- 9016. E-mail address: browning@uic.edu (N.D. Browning) 1 Current address: University of Birmingham, Birmingham, UK.