Phase coherence in a weakly coupled array of 20 000 Nb Josephson junctions

The properties of a very weakly coupled two-dimensional array of 20 000 1-μ${\mathrm{m}}^2$Nb Josephson junctions have been studied to 3 mK. Not only are many of the characteristics similar to those of granular films, but new effects, particularly resistance periodicities with magnetic field, provide direct evidence for the increase of two-dimensional phase coherence as the temperature is lowered through the resistive transition.