PHASE-LOCKING OF JOSEPHSON-JUNCTIONS IN 2-DIMENSIONAL ARRAYS

We studied the phase-locking behavior of Josephson junctions in two-di mensional arrays under microwave irradiation or coupled to a resonant cavity. We are able to identify the phase-locked junctions by means of spatially resolving measurements. We focus on the degree of coherence in the arrays, changing from incoherent to coherent oscillations of t he junctions when the microwave power is increased. We interpret our r esults in terms of a second order phase transition to the phase-locked state.