Vortex motion in two-dimensional arrays of small, underdamped Josephson junctions

We report measurements on vortex motion in two-dimensional arrays of small, underdamped, superconducting tunnel junctions, having charging energies on the order of the Josephson coupling energy. Our measurements are consistent with a simple model of the array as a periodic pinning potential, with measured values of the depinning current and the barrier energy in general agreement with the predicted values. Although the individual junctions are underdamped, the vortices move in an overdamped manner, with a critical velocity. We report preliminary evidence for macroscopic quantum tunneling of vortices.