Predicting critical currents in grain-boundary limited superconductors

The critical current across grain boundaries is severely suppressed in high-temperature superconductors, such as the cuprate and iron-based compounds, if the grain boundary angle exceeds a few degrees. This is known from the low critical currents in untextured conductors and measurements on bicrystalline films. Textured conductors were developed to overcome this limitation; however, a quantitative understanding between the degree of texture and the macroscopic critical current is still missing. A model for the prediction of the self-field critical current as a function of grain alignment on the basis of experimental data obtained from bicrystals is presented. It is a mean-field approach based on percolation theory. Without any fit parameter, good agreement with recent studies on cuprates and iron-based superconductors is obtained, where the critical current and the texture were analyzed quantitatively. The simplified grain boundary physics hence describes the macroscopic properties of imperfectly textured materials.