Enhanced supercurrent density in polycrystalline YBa2Cu3O7-delta at 77 K from calcium doping of grain boundaries

With the discovery of high-temperature superconductivity(1), it seemed that the vision of superconducting power cables operating at the boiling point of liquid nitrogen (77 K) was close to realization. But it was soon found t hat the critical current density J(c) of the supercurrents that can pass th rough these polycrystalline materials without destroying superconductivity is remarkably small(1,2). In many materials, J(c) is suppressed at grain bo undaries(2-4), by phenomena such as interface charging and bending of the e lectronic band structure(5-9). Partial replacement (`doping') of the yttriu m in YBa2Cu3O7-delta with calcium has been used to increase grain-boundary J(c) values substantially, but only at temperatures much lower than 77 K (r ef. 9). Here we show that preferentially overdoping the grain boundaries, r elative to the grains themselves, yields values of J(c) at 77 K that far ex ceed previously published values. Our results indicate that grain-boundary doping is a viable approach for producing a practical, cost-effective super conducting power cable operating at liquid-nitrogen temperatures.