Structural origin of misorientation-independent superconducting behavior at [001] twist boundaries in Bi2Sr2CaCu2O8+δ

A systematic investigation of the structure and properties of [001] twist boundaries was made in Bi2Sr2CaCu2O8+delta 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 CuO2 layers adjacent to the boundary plane to remain undisturbed. The structural characteristics of these boundaries are identical to those found in Bi2Sr2CaCu2O8+delta and Bi2Sr2Ca2Cu3O10+delta tapes, suggesting that the large-angle [001] twist boundaries are not a current-limiting obstacle in this important conductor.