Zero Dimensionality and Josephson Coupling in Granular Niobium Nitride

Ultrathin granular niobium nitride films have been found to behave like an array of zerodimensional superconducting grains coupled through grain boundaries by a Josephsontype interaction. The temperature dependence of the current at which a voltage first appears fits the Josephson-junction-like critical-current predictions remarkably well. The films exhibit a paraconductivity which extends to 24 K, more than twice the critical temperature, TcG, of the grains and varying as (T-TcG)^-2.