Granular Josephson and quantum interference effects in HTC ceramic superconductors

Josephson effects in high-T
c
superconductors are briefly reviewed, with specific reference to granular ceramic materials and SQUID device applications. It is suggested that the inductance associated with intergranular current loops may play an important role, even in determining the bulk superconductivity properties, as in weak-link superconducting rings. Evidence for quantum interference effects within intergranular current loops is presented. In ultra-low fields, the observed temperature dependence of thermally activated flux creep cannot be described by a simple granular superconductor model of equally spaced pinning centers, but would seem to imply a hierarchy of pinning sites of variable strength. The development of liquid-nitrogen-cooled rf and dc SQUIDs is described, and the noise levels currently achieved are presented.