Grain boundary weak links in high Tc oxide superconductors

It is now well established that superconducting grains in polycrystalline samples of high Tc materials are weakly coupled at the grain boundaries and the flow of current between the superconducting grains is through Josephson tunneling. Josephson weak links may be caused by small coherence length, by the misalignment of neighboring grains, by a disturbance of the structure at the grain surfaces or by the presence of non-superconducting phases at the grain boundaries. This weak link problem is the main obstacle in achieving high critical transport currents in high Tc superconductors. On the other hand these weak links have been utilized to fabricate SQUIDs and other devices. During the last three years a large number of papers have been published on the study of grain boundary weaklinks in high Tc superconductors. This article briefly reviews the work done in this area