The scaling behavior of the current-voltage (IV) characteristics of a two-dimensional proximity-coupled Josephson junction array (JJA) with quenched bond disorder was investigated for frustrations f = 1/5, 1/3, 2/5, and 1/2. For all these frustrations including 1/5 and 2/5 where a strongly first-order phase transition is expected in the absence of disorder, the IV characteristics exhibited a good scaling behavior. The critical exponent nu indicates that bond disorder may drive the phase transitions to be continuous but not into the Ising universality class, contrary to what was observed in Monte Carlo simulations. The dynamic critical exponent z for JJA's was found to be only 0.60-0.77.

We experimentally studied the effect of positional disorder on a Josephson junction array with f = n, 1 2 + n, or 2 5 + n flux quanta per unit cell for integral n. This system provides an experimental realization of a two-dimensional XY model with random phase shifts. Contrary to many earlier numerical and analytical investigations, our results suggest that low-temperature superconductivity is never destroyed by positional disorder. As the disorder strength increased, the Kosterltz-Thouless (KT) type order in the f = 0 and 1 2 systems changed to a non-KT type order with a long-range phase coherence, which persisted even in the maximal disorder limit. A possible finite-temperature glass transition is discussed.

To study the properties of frustrated two-dimensional Josephson junction arrays near incommensurability, we examine the current-voltage characteristics of a square proximity-coupled Josephson junction array at a sequence of frustrations f = 3/8, 8/21, 0.382 (≈ (3 − √ 5)/2), 2/5, and 5/12. Detailed scaling analyses of the current-voltage characteristics reveal approximately universal scaling behaviors for f = 3/8, 8/21, 0.382, and 2/5. The approximately universal scaling behaviors and high superconducting transition temperatures indicate that both the nature of the superconducting transition and the vortex configuration near the transition at the high-order rational frustrations f = 3/8, 8/21, and 0.382 are similar to those at the nearby simple frustration f = 2/5. This finding suggests that the behaviors of Josephson junction arrays in the wide range of frustrations might be understood from those of a few simple rational frustrations.

The superconducting transition of a two-dimensional (2D) Josephson junction array exposed to weak magnetic fields has been studied experimentally. Resistance measurements reveal a superconducting-resistive phase boundary in serious disagreement with the theoretical and numerical expectations. Critical scaling analyses of the IV characteristics indicate contrary to the expectations that the superconducting-to-resistive transition in weak magnetic fields is associated with a melting transition of magnetic-field-induced vortices directly from a pinned-solid phase to a liquid phase. The expected depinning transition of vortices from a pinned-solid phase to an intermediate floating-solid phase was not observed. We discuss effects of the disorder-induced random pinning potential on phase transitions of vortices in a 2D Josephson junction array.

Measurements of the IV characteristics of site-diluted Josephson-junction arrays have revealed intriguing effects of percolative disorder on the phase transition and the vortex dynamics in a twodimensional XY system. Different from other types of phase transitions, the Kosterlitz-Thouless (KT) transition was eliminated with the introduction of percolative disorder far below the percolation threshold. Even after the KT order had been removed, the system remained superconducting at low temperatures by establishing a different type of order. Near the percolation threshold, evidence was found that, as a consequence of the underlying fractal structure, the critical dynamics of the phase degrees of freedom persisted down to zero temperature.