Letters to Nature

Nature 407, 162-164 (14 September 2000) | doi:10.1038/35025014; Received 21 March 2000; Accepted 16 August 2000

Enhanced supercurrent density in polycrystalline YBa2Cu 3O7-delta at 77 K from calcium doping of grain boundaries

G. Hammerl1, A. Schmehl1, R. R. Schulz1, B. Goetz1, H. Bielefeldt1, C. W. Schneider1, H. Hilgenkamp1,2 & J. Mannhart1

  1. Experimental Physics VI, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany
  2. Low Temperature Division and MESA + Institute, University of Twente, Box 217, 7500 AE Enschede, The Netherlands

Correspondence to: J. Mannhart1 Correspondence and requests for materials should be addressed to J.M. (e-mail: Email: jochen.mannhart@physik.uni-augsburg.de).

With the discovery of high-temperature superconductivity1, 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 that the critical current density J c of the supercurrents that can pass through these polycrystalline materials without destroying superconductivity is remarkably small1, 2. In many materials, J c is suppressed at grain boundaries2, 3, 4, by phenomena such as interface charging and bending of the electronic band structure5, 6, 7, 8, 9. Partial replacement ('doping') of the yttrium 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 (ref. 9). Here we show that preferentially overdoping the grain boundaries, relative to the grains themselves, yields values of J c at 77 K that far exceed previously published values. Our results indicate that grain-boundary doping is a viable approach for producing a practical, cost-effective superconducting power cable operating at liquid-nitrogen temperatures.