Structural origin of misorientation-independent superconducting behavior at [001] twist boundaries in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\mathrm{\delta }}$

A systematic investigation of the structure and properties of [001] twist boundaries was made in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\mathrm{\delta }}$ 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 ${\mathrm{CuO}}_2$ layers adjacent to the boundary plane to remain undisturbed. The structural characteristics of these boundaries are identical to those found in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\mathrm{\delta }}$ and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{10+\mathrm{\delta }}$ tapes, suggesting that the large-angle [001] twist boundaries are not a current-limiting obstacle in this important conductor.