Structure of grain boundaries: Correlation to supercurrent transport in textured Bi₂Sr₂Ca_n-1Cu_nO_x bulk material

Two kinds of characteristic grain boundaries were observed in textured Bi₂Sr₂Ca_n-1Cu_nO_x (n = 2 and 3) bulk material: one (P-type) is nearly parallel to the (001) plane, and the other (N-type) is approximately normal to the (001) plane. Low-angle till N-type boundaries are composed of arrays of dislocations. However, for a small c-axis misorientation, the regions between the dislocation cores are still well connected, providing "pathways" for supercurrents crossing the boundary plane. The P-type boundaries exhibit compositionally and structurally modulated faceting. Although we see local regions of the low T_c (2201) phase at low-angle tilt (<10°) P-type boundaries, there are also "pathways" crossing the boundary plane made up of the high T_c (2212) and (2223) phases. The characteristics of such low-angle tilt grain boundary structures can therefore be modeled to provide general insight inio the correlation between high critical current densities and low-texture breadths. On the other hand, a weak link could be formed at high-angle (> 10°) boundaries where there are the low T_c (2201) or insulating phases.