Development of high critical current density in multifilamentary round-wire Bi₂Sr₂CaCu₂O_8+δ by strong overdoping

Bi₂Sr₂CaCu₂O_8+δ is the only cuprate superconductor that can be made into a round-wire conductor form with a high enough critical current density J_c for applications. Here we show that the J_c(5 T,4.2 K) of such Ag-sheathed filamentary wires can be doubled to more than 1.4×10⁵ A/cm² by low temperature oxygenation. Careful analysis shows that the improved performance is associated with a 12 K reduction in transition temperature T_c to 80 K, an increase in flux pinning, and particularly a significant enhancement in intergranular connectivity. In spite of the macroscopically untextured nature of the wire, overdoping is highly effective in producing high J_c values.