Applied Physics Seminar

Systems of superconducting islands on normal metal films provide a tunable medium with which to study the superconducting proximity effect, phase transitions, and vortex dynamics. Such systems are predicted to exhibit 2D zero-temperature metallic states. Although there has been experimental evidence of such states, they cannot be explained by conventional transport theory. We report transport measurements on triangular arrays of mesoscopic, proximity-coupled Nb islands on normal metal Au films. The arrays undergo a two-step transition to a superconducting state; we characterize the superconducting transitions in these systems as a function of island thickness and spacing. The temperature of the first step of the transition linearly decreases with increasing island spacing, and the spacing-dependence of the second step deviates from a conventional theory for SNS arrays. Through a phenomenological model, we resolve these two transitions as a consequence of intra- and inter-island coupling between superconducting phases of individual Nb grains. Moreover, the trends of both steps suggest that the system is approaching zero-temperature metallic states, and the sparsest arrays studied show evidence of such states. To further understand these systems, we characterize the vortex dynamics intrinsic to the 2D superconducting ground state, as well as that in response to an externally applied current and magnetic field. We provide evidence that the superconducting state is characterized by bound vortex-antivortex pairs. Additionally, we study the current-voltage characteristics; applying a current induces a Lorentz force on vortices that competes with pinning in the arrays. Lastly, in response to sweeping the field, we observe resistance oscillations, manifestations of competing magnetic ground states and correlated vortex motion.