Modification of transport properties in low-angle grain boundaries via calcium doping of YBa2Cu3O delta thin films

The limiting factor for current transport in high-T-c superconductors including grain boundaries is the intergranular critical current density. There have been various attempts to enhance the transport properties of multicrystalline high-T-c superconductors and to understand the mechanisms leading to a reduction of the critical currents over grain boundaries (GB's). We have extensively studied the effects of Ca doping on textured thin films, establishing only a rather small overdoping of the bulk samples. Nevertheless, doping the YBa2Cu3Odelta films with 20% Ca, symmetrical [001] tilt GB's with misorientation angle theta =4 degrees and 8 degrees showed strong benefits to Ca additions. We found an increase in the critical current density [J(gb)(theta)] up to 40% and 100% for the 4 degrees and 8 degrees GB's, respectively. Considering a model of Gurevich and Pashitskii for current transport in low-angle GB's, the improvements can be attached to a reduction of the strain fields epsilon and the localized charges Q, leading to a reduced built-in potential \V-bi\ in the dislocation cores.