This award is for the renewal of funding of the Materials Research Group at the University of Wisconsin at Madison on the topic of weak links, interfaces, and mechanisms of high temperature superconductivity. The original award (DMR-8911332) provided funding for a period of forty two months. The research to be performed under this award is a combination of continuation of research started under the original award plus new areas under the same general topic. Research to be continued includes coupled electromagnetic and microstructural characterization of flux-grown yttrium-barium-copper oxide bicrystals, cation-doped bicrystals, doped and undoped single crystals, melt-processed and solid state sintered polycrystals, as well as development of a high-speed position-sensitive atom probe. New programs starting under this award include coupled local electrical and microstructural characterization by patterning of thin-film and bulk-scale superconducting materials, coupled electromagnetic and microstructural characterization of step-edge superconducting junctions, coupled electromagnetic and microstructural characterization of grain boundaries in bismuth-strontium-calcium- copper oxide bicrystals, studies of local intergranular and intragranular compositional variations using the position- sensitive atom probe, characterization of the grain boundaries using high-spatial-resolution electron energy loss spectroscopy, and investigation of the superconducting gap as a function of temperature and location in the Brillouin zone. %%% The research is aimed at understanding why and how grain boundaries limit the transport critical current density of high temperature superconductivity. This is one of the most important problems in high temperature superconductivity. The challenging experimental procedures required for this research have been demonstrated in the first three years of the Materials Research Group. The group is the only one to have performed coupled electromagnetic and microstructural characterization on the same specimen of superconducting material. The issue of critical current density must be solved to achieve the full potential of high temperature superconductivity for national technological competitiveness.
