Vortex Dynamics and Phase Transitions in a Two-Dimensional Array of Josephson Junctions

The ac response of large two-dimensional arrays of proximity-effect Josephson junctions to an oscillating driving field has been studied as a function of temperature, applied transverse magnetic field, and frequency. For an integer number of flux quanta per unit cell, a peak in dissipation and a drop in superfluid density are observed near the superconducting transition of the array. These features as well as their frequency dependence provide clear evidence for the vortex-unbinding transition predicted by the Kosterlitz-Thouless theory.