A systematic investigation of the structure and properties of ͓001͔ twist boundaries was made in Bi 2 Sr 2 CaCu 2 O 8ϩ␦ bicrystals. Contrary to conventional wisdom, all these boundaries, regardless of their misorientation angle, carried almost the same critical current as their constituent single crystals at magnetic fields up to 9 t. The origin of this misorientation-independent superconducting behavior at twist boundaries was sought by detailed structural characterization using high-resolution and nanoprobe transmission-electron microscopy. The robust electromagnetic properties of these grain boundaries were mainly attributed to the high anisotropy of the crystals, and to the softness of the double BiO layers at the boundaries which allow the CuO 2 layers adjacent to the boundary plane to remain undisturbed. The structural characteristics of these boundaries are identical to those found in Bi 2 Sr 2 CaCu 2 O 8ϩ␦ and Bi 2 Sr 2 Ca 2 Cu 3 O 10ϩ␦ tapes, suggesting that the large-angle ͓001͔ twist boundaries are not a current-limiting obstacle in this important conductor.
We systematically investigated the structure and properties of [001] twist boundaries using Bi2Sr2CaCu2O8+δ (Bi/22112) bicrystals. Contrary to conventional wisdom, all these boundaries, regardless of their misorientation angle, carried the same critical current as their constituent single crystals in magnetic fields up to 9 tesla. Fig. 1 shows the ratio of the critical currents across a grain boundary to that within the grain interior as a function of misorientation of the boundaries. In striking contrast to the results of Dimos et al. with YBa2Cu3O7−δ, the twist boundaries in our bicrystals are not a limiting obstacle for supercurrent.The origin of the robust superconducting behavior at these twist boundaries was sought by detailed structural characterization using various TEM techniques. Several notable structural features were observed: 1) all the boundaries were clean, structurally intact without any visible amorphous materials; 2) nano-probe EDS and EELS measurements showed that there was no detectable off-stochiometric composition, including oxygen/hole concentration along and across the boundaries; 3) HREM image simulation revealed that the boundaries were located in the middle of the double BiO layers without exception (Fig.2); 4) there was no detectable boundary expansion, contrary to general expectation, and the inter-planar distance of the double BiO layer {dBio =0.309± 0.005nm, measured with line-scan (Fig.3)) at the boundary was the same as those far from the boundary within measurement error; and 5) very often, there was an intercalation of a Ca/CuO2 bi-layer near the boundary, either on one, or both sides, forming a local Bi/2223 structure (Fig.2).
Bulk BizSr2CaCuzOg+8 (Bi2212) bicrystals containing a single high quality [OOlJ twist grain boundary junction were prepared in order to investigate the orbital symmetry of the superconducting order parameter in highly anisotropic Bi-based high temperature superconductors. The misorientation angles of the bicrystals ranged from 0' to 180' . The microstructure in the vicinity of the junction was characterized using high-resolution, nano-probe analytical microscopy. We found that some high angle twist junctions were able to carry a critical w e n t density similar to their constituent single crystals. These results cannot be interpreted in terms of a pure d,z,z-wave order parameter for superconducting Bi2212.
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