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

Abstract

With the discovery of high-temperature superconductivity, 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 Jc of the supercurrents that can pass through these polycrystalline materials without destroying superconductivity is remarkably small. In many materials, Jc is suppressed at grain boundaries, by phenomena such as interface charging and bending of the electronic band structure. Partial replacement ('doping') of the yttrium in YBa2Cu3O(7-delta) with calcium has been used to increase grain-boundary Jc 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 Jc 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.

Drastic improvement of surface structure and current-carrying ability in YBa2Cu3O7 films by introducing multilayered structure

“…It is also well established that the surface roughness and single-crystalline structure of the films degrade as the thickness of the films increases. 5 Gain in smoothening of the surface roughness is minimal for films deposited at slower rates. 4,5 Larger surface inhomogeneity reduces total supercurrent and obstructs fabrication of various types of JJs.…”

Bulk nanocrystalline superconducting YBa2Cu3O7-x

“…This is attributed to weak transmission across d-wave grain boundaries at large misorientation angles [1][2][3][4][5]. Several techniques have been investigated to increase J C , including increasing the flux pinning through nano-particle additions [6] and improving the superconducting properties of the grain boundary [7][8][9]. Unfortunately pinning effects that come from the nanoscale disorder in the materials are drastically weakened by a large anisotropy in YBCO [10], and it is not clear that pinning within the grains will increase the macroscopic J C .…”

Degenerate Ground State in a MesoscopicYBa2Cu3O7−x

“…An interesting possibility seems to be to increase the barrier transparency by an appropriate doping of the grain boundary. 22 Partial support by the DFG and INTAS is gratefully acknowledged. MG and RH were supported in part by the Slovak Grant Agency VEGA under Grant No.…”