A TEM-EELS STUDY OF HOLE CONCENTRATIONS NEAR STRONGLY AND WEAKLY COUPLED GRAIN-BOUNDARIES IN ELECTROMAGNETICALLY CHARACTERIZED YBA2CU3O7-DELTA BICRYSTALS

High-spatial-resolution electron energy loss spectroscopy in a transmi ssion electron microscope (TEM-EELS) was used to obtain hole-concentra tion profiles near grain boundaries in flux-grown YBa2Cu3O7-delta bicr ystals with misorientation relationships of 7-degrees, 12-degrees, 28- degrees, and 31-degrees about their common [001 ] rotation axes. A key aspect of the work is that the electromagnetic properties and couplin g character of each of the bicrystals also were determined explicitly. The 28-degrees and 31-degrees bicrystals showed Josephson-junction-li ke weak coupling and evidence for severe hole depletion at the boundar y was present in the oxygen K near-edge fine structure (ELNES) of the EELS spectra obtained from them. The hole-deficient zone extended 20 t o 40 nm into each crystal. In striking contrast, the 7-degrees and 12- degrees bicrystals were strongly coupled and dramatically less hole de pleted at the grain boundary. These results show that the boundary reg ion within which the electronic structure differs from that of the bul k is substantially wider than both the YBa2Cu3O7-delta coherence lengt h and the grain-boundary structural and compositional width suggested by high-resolution TEM images and cation non-stoichiometry studies. Th ey also suggest a continuous transition from strongly to weakly couple d character.