Structural characterizations of Bi₂Sr₂CaCu₂O_8+δ twist boundaries using advanced transmission electron microscopy

We systematically investigated the structure and properties of [001] twist boundaries in Bi₂Sr₂CaCu₂O_8+δ superconducting bicrystals. Contrary to conventional wisdom, all these boundaries, regardless of their misorientation angle, carried the same critical current as their constituent single crystals at magnetic fields up to 9 tesla. The origin of the robust superconducting behavior at the twist boundaries was sought by detailed structural characterization using high-resolution, nano-probe analytical microscopy, and strain analysis using anisotropic elasticity theory. The robust electromagnetic properties of these grain boundaries were mainly attributed to the high anisotropy of the crystals and the softness of the double BiO layers at the boundaries which allow the CuO₂ layers adjacent to the boundary plane to remain undisturbed. The structural characteristics of these boundaries are identical to those in large-scale superconducting tapes, suggesting that the large-angle twist boundaries are not a current-limiting obstacle in these important high-temperature superconductors.