Abstract
Large-scale applications of high-transition-temperature (high-Tc) superconductors, such as their use in superconducting cables, are impeded by the fact that polycrystalline materials (the only practical option) support significantly lower current densities than single crystals1,2,3,4,5,6. The superconducting critical current density (Jc) across a grain boundary drops exponentially if the misorientation angle exceeds 2°–7°. Grain texturing reduces the average misorientation angle, but problems persist7,8. Adding impurities (such as Ca in YBa2Cu3O7-δ; YBCO) leads to increased Jc (refs 9, 10), which is generally attributed to excess holes introduced by Ca2+ substituting for Y3+ (ref. 11). However, a comprehensive physical model for the role of grain boundaries and Ca doping has remained elusive. Here we report calculations, imaging and spectroscopy at the atomic scale that demonstrate that in poly-crystalline YBCO, highly strained grain-boundary regions contain excess O vacancies, which reduce the local hole concentration. The Ca impurities indeed substitute for Y, but in grain-boundary regions under compression and tension they also replace Ba and Cu, relieving strain and suppressing O-vacancy formation. Our results demonstrate that the ionic radii are more important than their electronic valences for enhancing Jc.
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Acknowledgements
We thank D.W. Gillette for the sample preparation and A.R. Lupini for his help in acquiring the images of Ca-doped YBCO. This work is supported by the US Department of Energy, Division of Materials Sciences, Office of Basic Energy Science and the National Energy Research Scientific Computing Center, supported by the Office of Science of the US Department of Energy.
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Supplementary information
Supplementary Figure S1
The relative pre-peak intensity of the oxygen K-edge as a function of position within the dislocation core for pristine and Ca-doped YBa2Cu3O7-δ. (DOC 97 kb)
Supplementary Figure S2
Two EELS spectra containing the Ca L-edge and the O K-edge for Ca-doped YBa2Cu3O7-δ taken from the centre of the grain boundary dislocation core and the bulk of the sample. (DOC 187 kb)
Supplementary Figure S3
Two EELS spectra containing the O K-edge and the Ba M-edge for pristine YBa2Cu3O7-δ taken from the centre of the grain boundary dislocation core and the bulk of the sample. (DOC 157 kb)
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Klie, R., Buban, J., Varela, M. et al. Enhanced current transport at grain boundaries in high-Tc superconductors. Nature 435, 475–478 (2005). https://doi.org/10.1038/nature03644
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DOI: https://doi.org/10.1038/nature03644
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