Strong to weak coupling transition in low misorientation angle thin film YBa2Cu3O7-x bicrystals

Detailed transport measurements were made of YBa2Cu3O7-x thin-film [001] ti lt bicrystals with misorientation angles of 3 degrees, 5 degrees, 7 degrees , 10 degrees, 15 degrees, and 20 degrees, encompassing the angular regime w here the transition from strong to weak coupling occurs. The study includes measurements of intragrain and intergrain extended voltage-current charact eristics in applied magnetic fields that range from zero to well above the irreversibility field. The results show that the strong-to-weak coupling tr ansition is progressive at 77 K, occurring at misorientation angles between 7 degrees and 10 degrees in zero field, and between 10 degrees and 15 degr ees in higher magnetic fields. The shapes of the voltage-current characteri stics of the 7 degrees [:001] bicrystals and the ratio of the inter- and in tragranular critical current densities are particularly sensitive to indivi dual sample preparation conditions, suggesting that substrate-film interdif fusion along the grain-boundary dislocations is controlling the effective s ize of the superconducting channels between the dislocation cores. The line ar decline of the intergranular critical current density with misorientatio n angle predicted from present dislocation core overlap models is not found , showing that additional features of the grain-boundary nanostructure and the mechanism of intergranular current flow need to be invoked in order to explain transport across low-angle YBa2Cu3O7-x grain boundaries. [S0163-182 9(99)02626-0].