Extended voltage-current (V -I) characteristics of ͓001͔ tilt YBa 2 Cu 3 O 7Ϫx bicrystal films having misorientation angles, , of 3°to 20°showed a substantial and progressive change with , when measured in large fields. Transmission electron microscopy of the 10°bicrystal showed the grain boundary to contain edge dislocations separated by channels of relatively undisturbed lattice. A large drop in the intergrain irreversibility field, H*, occurred between 10°and 15°, and the characteristics became qualitatively different by 20°. Both the microscopy and the electromagnetic data support a heterogeneous description of the grain boundary, consisting of strongly coupled channels that close at a misorientation angle of around 15°. © 1996 American Institute of Physics.
͓S0003-6951͑96͒03630-3͔It is well established that the critical current density, J c , of ͓001͔ tilt thin film bicrystals of high temperature superconductors is strongly dependent on the misorientation angle, , of the grain boundary ͑GB͒. 1-4 Dimos et al. 1 proposed that the cores of primary grain boundary dislocations ͑PGBD͒ subdivide low angle grain boundaries. Chisholm and Pennycook 5 additionally proposed that the strain fields of the PGBDs extend the effective size of the dislocation cores. Both models postulate that the supercurrent is restricted to channels of good lattice between the PGBDs. The effective size of the PGBD cores should then determine the magnitude of J b /J c ͑the ratio of the inter-to intragranular critical current density͒, and the critical misorientation angle at which strong coupling properties disappear. Measurements on different kinds of samples suggest that this critical angle can vary over a wide angular range of 5°-20°. 1,4,6,7 Since this range covers a PGBD separation of 4.5-0.6 nm, a more explicit test of the above models is highly desirable. 8 This issue has regained importance because of renewed activity on making conductors with a strong biaxial texture from YBa 2 Cu 3 O 7Ϫx ͑YBCO͒, using an ion beam assisted deposition process. [9][10][11] Since such conductors would be useful for the production of strong fields, provided that the spread in the a/b grain axis misalignment is controlled, knowledge of the extended high field voltage current (V -I) characteristics of YBCO bicrystals of varying misorientations is of both fundamental and applied interest.This letter describes the first systematic measurements of the V -I curves of ͓001͔ tilt YBCO bicrystal thin films with misorientation angles in the low-to-high angle crossover regime of 3°-20°in large magnetic fields. Prior measurements of single crystal thin film 12,13 and bulk 14 high temperature superconductors have established that the irreversibility field, H*, above which J c tends to zero, can be experimentally defined by the change in sign of the curvature of the electric field-current density, E -J, characteristic plotted on a double logarithmic scale. In single crystal YBCO films, this transition occurs near 4 T at 77 K. 12 To test the channel models described...