Investigating the Atomic Scale Superconducting Properties of Grain Boundaries in High-T(Sub c) Superconductors
Presented at Fourteenth International Congress on Electron Microscopy Cancun, Mexico, August 31-September 4, 1998, and published in Proceedings Over ten years after the discovery of high-TC superconductors, their widespread application into viable device structures is still limited by the deleterious effect of grain boundaries. One of the main difficulties associated with understanding this effect is that transport measurements are usually performed on the micron scale. However, the critical parameter for superconductivity, the coherence length, is only ~lnm. To understand grain boundaries on a fundamental level it is therefore necessary to investigate the properties on this atomic scale; a scale attainable only by electron microscopy [12]. As an example of the observed properties of grain boundaries in YB~C~07d (YBCO), the V(I) curves recorded across a 24o boundary for several magnetic fields are shown m figure 1, To explain these properties, a model where the grain boundary is composed of equally sized and spaced dislocation cores separated by a very small fraction of much stronger links has been developed (figure 1). These strong links may carry either the depairing current, the JC of the grains or another Josephson current (a depairing current seems unlikely in view of the field dependence of the experimental data). The simulated behavior obtained for this model, where the fraction of strong links is x=O.005 and JC is the observed J=(B) of the grains, exhibits qualitatively similar behavior to the experimental data (figure 1). However, the fit is not perfect, suggesting that the strong links are more likely to be regions of grain boundary with a higher Josephson current, rather than links with the JC(B) of the grains. Using electron microscopy we can look for the origin of these stronger coupled regions at the grain boundary. Figure 2 shows a Z-contrast image of a similar high-angle [001] tilt grain boundary in YBCO. The image shows that there are some regions where the boundary plane is symmetric, while other regions where it is asymmetric. EELS measurements [1] at such boundaries have shown that the symmetry of the boundary plane plays an important role in determining the properties. Asymmetric high-angle grain boundaries show significant hole depletion whereas symmetric high angle grain boundaries show very little (Figure 3). This effect can be understood using bond valence sum analysis [3]. Figure 4 shows the Cu valence plots across regions of both high angle symmetric and asymmetric boundaries. The asymmetric boundaries show a dramatic drop in the copper valence (charge carrying holes are formed by hybridization of the O 2p and Cu 3d bands), whereas the symmetric regions show areas of dramatic decrease in valence and areas where there is no valence change. The origin of this behavior is that the asymmetric boundaries always show a reconstruction on the Cu sub-lattice while symmetric boundaries show a reconstruction on both the Cu and Y/Ba sub-lattices (figure 2). Regions of the boundary plane where the reconstruction exists on the Y/Ba sub-lattice may be the strong links seen in the transport measurements. Work is continuing to investigate this supposition [4].
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Oak Ridge, TN
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- AC05-96OR22464
- OSTI ID:
- 1738
- Report Number(s):
- ORNL/CP-100165; KC 02 02 04 0; ON: DE00001738
- Resource Relation:
- Conference: 14th International Conference on Electron Microscopy, Cancun, Mexico, Aug. 31-September 4, 1998
- Country of Publication:
- United States
- Language:
- English
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