LARGE ENHANCEMENT OF CRITICAL-CURRENT DENSITY DUE TO VORTEX MATCHING AT THE PERIODIC FACET STRUCTURE IN YBA2CU3O7-DELTA BICRYSTALS

We observed pronounced peaks in the critical-current density J(b)(B) v ersus applied magnetic field B in YBa2Cu3O7-delta bicrystals well abov e the lower critical field. These peaks correspond to the fields for w hich the spacing between intragrain vortices is commensurate with the wavelength of the periodic grain boundary facet structure observed in the same bicrystals. The peaks were observed on a group of grain bound aries that were isolated from four bulk [001]-tilt bicrystals with mis orientation angles around 15 degrees, which lie In the transition regi on from strong to weak coupling. The matching effect provides direct e vidence that faceting strongly modulates the coupling strength of grai n boundaries and can substantially increase J(b)(B) in high magnetic f ields. Periodic modulation of the intergrain coupling by the strain fi elds observed at the facet junctions by transmission electron microsco py is proposed as the origin of the J(c)(B) enhancement. [S0163-1829(9 8)07917-X].