Field-induced superconductor-to-insulator transitions in Josephson-junction arrays

We have studied phase transitions in fabricated 2D arrays of Josephson junctions where the charging and Josephson coupling energies are about equal. We find a crossover from superconducting behavior in low fields to insulating behavior for fields above a critical value of 0.1--0.2 flux quanta per unit cell. The quantitative aspects agree well with predictions from a recent theory for quantum vortex motion by M. P. A. Fisher and with measurements on films. Similar transitions are found around values f=n/m, where f is the flux per cell and n,m=1,2,3. The field dependence is periodic in f with period 1.