Flux flow and vortex tunneling in two-dimensional arrays of small Josephson junctions

We have measured the temperature dependence and magnetic field dependence of the zero-bias resistance (R₀) as well as the current-voltage (I-V) characteristics for several two-dimensional arrays of small aluminum Josephson junctions. R₀(T) decreases with decreasing temperature, which can be described in terms of two types of vortex motion: flux, flow, and vortex tunneling. At temperatures higher than the Kosterlitz-Thouless transition temperature (T>Tc) or at a bias current greater than the current corresponding to the onset of the nonlinear I-V characteristics (I>Id), the effective damping resistance which characterizes flux-flow motion is found to be approximately equal to the junction normal-state resistance RN. At low temperatures and at small bias current, R₀ is temperature independent and remains finite down to our minimum attainable temperature. This finite resistance is found to be dependent on the array size as well as the junction parameters. {} 1996 The American Physical Society.