| In this dissertation, measurements on two different niobium (Nb) based systems, namely s&barbelow;uperconductor-n&barbelow;ormal-metal-s&barbelow;uperconductor (SNS) Josephson junction arrays (JJA), and thin Nb films with nanoscale size magnetic dot arrays, are presented.; Without high frequency (rf) signals at nearly zero magnetic field, these measurements show similarities for the two systems. These similarities are explained by proposing that very close to the critical temperature (Tc) of the Nb film sample, the stray magnetic field of the dots reduces the superconductivity in the film to make it a s&barbelow;uperconductor-weaker-s&barbelow;uperconductor-s&barbelow;uperconductor (SS'S) JJA.; With rf signals, their dc-voltage-current characteristics (VI's) show the appearance of constant voltage steps known as Shapiro steps. For SNS-JJA, the dependence of the width of the steps (DeltaIn) on the amplitude of the rf current (Irf) is a Bessel-function-like. The maximum width of the first step ([DeltaI1]MAX) was found to increase as a function of rf frequency (nurf) reaching a maximum for nurf∼1.8 times the array's Josephson frequency (nuc). This maximum was maintained for nurf ∼ 4.9v,. Also, in JJA with defects (missing junctions), [DeltaI1]MAX was smaller, disappearing for nu rf ∼ nuc.; For Nb films near Tc, no Bessel-function-like oscillations were observed for DeltaIn. Also, DeltaI0 (or critical current, Ic) showed a maximum for Irf ≠ 0, contrary to what was found in SNS JJA, falling to zero for higher Irf. This enhancement of Ic with Irf was attributed to non-equilibrium superconductivity mechanisms that are found near Tc in superconducting structures. Observation of higher order steps (n = 1, 2, 3, etc.) became evident only from the dynamic resistance (dV/dI) vs. V plots (where a step corresponds to a minima for this curve). The first step was most evident for Irf values that made DeltaI0 = 0. At these values, V1 (the voltage that corresponded to the first minimum in the dV/dIvs.V curve) satisfied the Josephson relation V 1 = N(h/2e)nurf, where N is the number of "junctions" in the direction of the current. At fixed nurf, V1 was found to increase with Irf. This result was interpreted as having more "junctions" participating in the step's formation, which was attributed to magnetic interactions among the dot's magnetic moments and with the Irf to minimize the total energy of the system. |
