CHARGE SOLITONS AND QUANTUM FLUCTUATIONS IN 2-DIMENSIONAL ARRAYS OF SMALL JOSEPHSON-JUNCTIONS

We have measured the current-voltage (IV) characteristics of several t wo-dimensional arrays of small Josephson junctions as a function of te mperature, T and magnetic field B. The junctions have relatively large charging energies E(C) almost-equal-to 1 K, and normal-state resistan ces R(N) in the range of 4-150 kOMEGA. From the IV characteristics we can deduce the zero-bias resistance R0 and the threshold voltage V(t) which reveal important information about the dynamics and statics of c harge solitons in the array. R0(T) increases with decreasing temperatu re and may be described by thermal activation of charge solitons, char acterized by an activation energy E(a). When the electrodes are in the normal state, E(a) is close to 1/4 Ec. At low T, the thermal activati on behavior breaks down, and R0(T) levels off to a value that can be a ttributed to the quantum fluctuations in the array. This interpretatio n places limitations on the observability of the charge unbinding, Kos terlitz-Thouless-Berezinskii transition for single electrons. When the electrodes are superconducting, E(a) is much larger and dependent on B. In several samples, both E(a) and V(t) oscillate with B, having a p eriod corresponding to one flux quantum per unit cell. For increasing magnetic fields, V(t) increases until B almost-equal-to 250-450 G wher e it starts to decrease rapidly. We interpret the B dependence of E(a) and V(t) as a result of competition between Cooper-pair solitons and single-electron solitons.