QUANTUM PHASE-TRANSITIONS IN 2 DIMENSIONS - EXPERIMENTS IN JOSEPHSON-JUNCTION ARRAYS

We have studied two-dimensional superconducting networks coupled by Jo sephson junctions in the regime where the Josephson coupling energy is comparable in magnitude to the capacitive energy of charging an islan d with a Cooper pair. We have mapped out the dependence of quantum pha se transitions on the ratio of these two energies (for different value s of the applied magnetic field) and on the applied field for arrays b oth with square and triangular cells. Our experimental results are com pared with existing theoretical predictions.