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 transmission electron microscope (TEM-EELS) was used to obtain hole-concentration profiles near grain boundaries in flux-grown YBa2Cu3O7-delta bicrystals 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 coupling character of each of the bicrystals also were determined explicitly. The 28-degrees and 31-degrees bicrystals showed Josephson-junction-like weak coupling and evidence for severe hole depletion at the boundary was present in the oxygen K near-edge fine structure (ELNES) of the EELS spectra obtained from them. The hole-deficient zone extended 20 to 40 nm into each crystal. In striking contrast, the 7-degrees and 12-degrees bicrystals were strongly coupled and dramatically less hole depleted at the grain boundary. These results show that the boundary region within which the electronic structure differs from that of the bulk is substantially wider than both the YBa2Cu3O7-delta coherence length and the grain-boundary structural and compositional width suggested by high-resolution TEM images and cation non-stoichiometry studies. They also suggest a continuous transition from strongly to weakly coupled character.