TRANSPORT PROCESSES AND NOISE IN YBA2CU3O7-DELTA GRAIN-BOUNDARY JUNCTIONS

In the high temperature superconductors the weak link nature of extended defects as grain boundaries is responsible for the deterioration of the superconducting transport properties. From the study of individual grain boundaries in thin-film bicrystals of YBa2Cu3O7-delta general relations describing the superconducting transport and noise characteristics of grain boundaries in the high temperature superconductors were found. Firstly, the critical current density J(c)gb decreases as the misorientation angle between the adjoining grains increases. Secondly, the J(c)gb rho-N products, where rho-N is the normal resistance times unit area of the grain boundaries, scale proportional to about (1/rho-N)q with q ranging between 1 and 1.5. Thirdly, all grain boundaries show large amounts of low frequency 1/f noise. The transport and noise characteristics can be explained by a junction model which is based on an insulating layer at the grain boundary interface containing a large number of localized defect states.