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Higher current superconductors: why Ca-doped grain boundaries improve critical current

R. F. Klie1, J. P. Buban2, M. Varela3, A. Franceschetti3,4, C. Jooss5, Y. Zhu1
S. T. Pantelides4,3, S. J. Pennycook3,4

Nature, 435, 475 (2005)

Aberration-corrected scanning transmission electron microscopy in conjunction with first-principles theoretical calculations has revealed the atomistic mechanism whereby calcium enhances current transport across grain boundaries in YBa2Cu3O7-x (YBCO). It was previously assumed that the calcium would take the same atomic sites as in bulk YBCO, namely replacing yttrium, where it would be expected to increase the local hole concentration and improve the current-carrying capacity. However, Z-contrast images of grain boundaries showed significant changes in grain boundary structure that were inconsistent with this simple picture. Theoretical calculations were then carried out and predicted an entirely different scenario, which was subsequently confirmed by atomic-resolution electron energy loss spectroscopy: calcium only takes the place of yttrium in the perfect material. In the pure boundary, which is highly strained, oxygen is expelled and the boundary is not superconducting. When calcium is added, it replaces copper, a smaller atom, in regions that are expanded, and replaces barium, a larger atom, in compressed regions. The local stresses are reduced, the oxygen returns, and the grain boundary critical current is raised.

Large-scale applications of high-transition-temperature superconductors, such as their use in superconducting cables, is impeded by the unavoidable presence of grain boundaries. Grain boundaries reduce the current that can cross by a factor of 2-1000, depending on misorientation. Ca was known empirically to improve grain boundary critical current but reduced the transition temperature in the grain itself, which is obviously a problem. This work has revealed the key requirements for successful doping to improved grain boundary currents. A suitable dopant should have comparable size to Ca, but be insoluble in the grain. A possible candidate would be silver. Furthermore, a range of elements with different sizes could be even more effective for relieving strains at the three or four different sites that are important. This work opens a new path toward higher current superconductors with the potential to improve wire current carrying capacity by a factor of about two, a factor sufficient to make large-scale applications a reality.

Energy required to substitute Ca at different sites in YBCO as a function of strain. Ca chooses the site that reduces the strain, allowing O to return to the grain boundary and improve the critical current, a cooperative doping effect.

1Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973
2Institute of Engineering Innovation, The University of Tokyo, Tokyo, 113-8656, Japan
3Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
4Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
5University of Göttingen, 37073 Göttingen, Germany


 Oak Ridge National Laboratory