Doping-induced enhancement of grain boundary critical currents

The critical-current density of grain boundaries in high-T-c superconductor s was enhanced to values exceeding the previously known limits both at 4.2 K and at 77 K. Noting the importance of space-charge Layers and of the d(ch i)(2)-(2)(gamma)-wave pairing symmetry on grain-boundary transport, we have established a model that provides a comprehensive description of the grain boundaries and proposes ways for their improvement, such as overdoping of the grains and of their boundaries. Exploring as example the effects of ove rdoping of YBa2Cu3O7-delta with Ca, we enhanced significantly the critical current densities and decreased the normal-state resistivities of grain bou ndaries to unprecedented values. By introducing doping heterostructures to overdope grain boundaries selecti vely over a few nanometers by benefiting from grain boundary diffusion, the enhancement of the critical-current density is achieved at all temperature s up to T-c. At 77 K, critical current densities are obtained which before had been found only at 4.2 K. This concept is proposed as a practical and c ost-effective route to enhance the performance of high-T-c coated conductor s fabricated by ion beam assisted deposition (IBAD) [1]-[4] or by the rolli ng assisted biaxially aligned substrate process (RABITS) [5].