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] tilt bicrystals with misorientation angles of 3 degrees, 5 degrees, 7 degrees, 10 degrees, 15 degrees, and 20 degrees, encompassing the angular regime where the transition from strong to weak coupling occurs. The study includes measurements of intragrain and intergrain extended voltage-current characteristics in applied magnetic fields that range from zero to well above the irreversibility field. The results show that the strong-to-weak coupling transition is progressive at 77 K, occurring at misorientation angles between 7 degrees and 10 degrees in zero field, and between 10 degrees and 15 degrees in higher magnetic fields. The shapes of the voltage-current characteristics of the 7 degrees [:001] bicrystals and the ratio of the inter- and intragranular critical current densities are particularly sensitive to individual sample preparation conditions, suggesting that substrate-film interdiffusion along the grain-boundary dislocations is controlling the effective size of the superconducting channels between the dislocation cores. The linear decline of the intergranular critical current density with misorientation 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. [S0163-1829(99)02626-0].