Doping-induced enhancement of the critical currents of grain boundaries in YBa ₂ Cu ₃ O 7 − δ

The critical current density of grain boundaries in YBa 2 Cu 3 O 7 − δ films is increased beyond the hitherto established limit by overdoping the superconductor. Using Ca-doping, values for the critical current density are obtained, that exceed the highest values reported ever for the undoped material by more than a factor of seven. Further, it is found that the overdoping strongly reduces the grain boundary normal-state resistivity. This systematic improvement of the grain boundary transport properties by doping is of great importance for applications of the high- T c cuprates and gives insight into the mechanisms controlling the grain boundary behavior.