Modification of transport properties in low-angle grain boundaries via calcium doping ofYBa2Cu3Oδthin films

Abstract

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 ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{\ensuremath{\delta}}$ films with 20% Ca, symmetrical [001] tilt GB's with misorientation angle $\ensuremath{\theta}=4\ifmmode^\circ\else\textdegree\fi{}$ and $8\ifmmode^\circ\else\textdegree\fi{}$ showed strong benefits to Ca additions. We found an increase in the critical current density $[{J}_{\mathrm{gb}}(\ensuremath{\theta})]$ up to 40% and 100% for the $4\ifmmode^\circ\else\textdegree\fi{}$ and $8\ifmmode^\circ\else\textdegree\fi{}$ 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 $\ensuremath{\varepsilon}$ and the localized charges Q, leading to a reduced built-in potential $|{V}_{\mathrm{bi}}|$ in the dislocation cores.