Grain boundaries in the cuprate superconductors: tapes and tunneling spectroscopy

Grain boundaries in the high temperature superconducting cuprates have played a central role in their development for practical applications and in the fundamental understanding of the nature of superconductivity in these materials. Tapes for energy use, SQUIDS, symmetry of the wavefunction, Qbits, applications related to the AC Josephson effect, and tunneling spectroscopy are some areas of current research. In this brief paper, the authors first summarize what we know about what limits the critical current densities of tapes and suggest a few experiments to further understand these limits to critical current densities and, secondly, discuss the use of grain boundary for carrying out tunneling spectroscopy in optimally doped La1.84Sr0.16CuO4 (LSCO). This includes new data and comparisons with theory and experiments. The background material and review were presented at the EUCAS 09 conference in Dresden as one of the plenary talks and are available from the authors.