Enhanced current transport at grain boundaries in high-Tc superconductors

Against the grain Soon after the discovery of high-temperature superconductors in 1986, it became clear that their polycrystalline nature has an unfortunate consequence: the superconducting properties are compromised by an orders of magnitude reduction in the critical current across these interfaces. Much work has gone into attempts to remove such boundaries. But a recent report suggested that doping can enhance the grain boundary critical current by an order of magnitude, providing a simple route towards practical use of YBa 2 Cu 3 O 7-δ as a high-temperature superconductor. Klie et al . present an atomic-scale description of the mechanism behind the dramatic improvement of grain boundary properties. This work should lead to rational guidelines for the choice of suitable dopants. Large-scale applications of high-transition-temperature (high- T c ) superconductors, such as their use in superconducting cables, are impeded by the fact that polycrystalline materials (the only practical option) support significantly lower current densities than single crystals 1 , 2 , 3 , 4 , 5 , 6 . The superconducting critical current density ( J c ) across a grain boundary drops exponentially if the misorientation angle exceeds 2°–7°. Grain texturing reduces the average misorientation angle, but problems persist 7 , 8 . Adding impurities (such as Ca in YBa 2 Cu 3 O 7- δ ; YBCO) leads to increased J c (refs 9 , 10 ), which is generally attributed to excess holes introduced by Ca 2+ substituting for Y 3+ (ref. 11 ). However, a comprehensive physical model for the role of grain boundaries and Ca doping has remained elusive. Here we report calculations, imaging and spectroscopy at the atomic scale that demonstrate that in poly-crystalline YBCO, highly strained grain-boundary regions contain excess O vacancies, which reduce the local hole concentration. The Ca impurities indeed substitute for Y, but in grain-boundary regions under compression and tension they also replace Ba and Cu, relieving strain and suppressing O-vacancy formation. Our results demonstrate that the ionic radii are more important than their electronic valences for enhancing J c .