How grain boundaries limit supercurrents in high-temperature superconductors

The interface properties of high-temperature (high-T-c) copper oxide superconductors have been of interest for many years, and play an essential role in Josephson junctions, superconducting cables and microwave electronics. In particular, the maximum critical current achievable in high-T-c wires and tapes is well known to be limited by the presence of grain boundaries, regions of mismatch between crystallites with misoriented crystalline axes. Studies of single artificially fabricated grain boundaries have revealed that the critical current J(c) of a grain boundary junction depends exponentially on the misorientation angle. Until now microscopic understanding of this apparently universal behaviour has been lacking. We present here the results of a microscopic evaluation based on a construction of fully three-dimensional YBa2Cu3O7-delta grain boundaries using molecular dynamics. With these structures, we calculate an effective tight-binding Hamiltonian for the d-wave superconductor with a grain boundary. The critical current is then shown to follow an exponential suppression with grain boundary angle alpha. We identify the build-up of charge inhomogeneities as the dominant mechanism for the suppression of the supercurrent.