Direct measurements of electrostatic potentials at grain boundaries: mechanism for current improvement in high-T-c superconductors

Grain boundaries (GBs) in high-temperature superconductors represent the major factor limiting high current applications. Via magneto-optical imaging and direct transport measurements, we observed an increase of the intergranular J(c) in 4degrees and 8degrees [001] low-angle grain boundaries via substitution of Y by Ca in YBa2Cu3O7-x by up to 100%. Since Ca substitution leads to hole overdoping, these results suggest that charge depletion and band bending may be responsible for the suppression of superconductivity in GBs. By using electron holography, we directly observe ail electrostatic potential at the dislocation cores. The radius as well as the value of the potential are reduced in Ca-doped grain boundaries, directly reflecting the improved transport properties. (C) 2004 Elsevier B.V. All rights reserved.