Origin of Robust Superconductivity at Twist Boundary in Bi/2212 Bicrystals

Extract

We systematically investigated the structure and properties of [001] twist boundaries using Bi₂Sr₂CaCu₂O_8+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 YBa₂Cu₃O_7−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 {d_Bio =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).