J. Mannhart, P. Chaudhari, D. Dimos, C.C. Tsuei, T.R. McGuire, "Critical currents in  grains and across their tilt boundaries in YBa2Cu3O7 films," Physical Review Letters, 61, 2476-79, 21 November 1988.
J. Mannhart (IBM Research Division, Zurich Research Laboratory, Rüschlikon, Switzerland): "A detailed understanding of the electronic properties of the grain boundaries in YBaCuO is important both for fundamental reasons and for possible applications of this superconductor. The fact that the critical current density of polycrystalline YBaCuO is limited by the grain boundaries is a particularly crucial point.
"In this paper, the superconducting transport properties of individual grain boundaries in YBaCuO films are compared with the transport properties of the adjacent grains. These data show that the grain boundaries behave like standard superconductor-normal conductor-superconductor type weak links, whereas the behavior of the grains is in agreement with simple flux creep models."