THERMAL-ACTIVATION AND INJECTION OF CHARGE SOLITONS IN 2D-ARRAYS OF SMALL JOSEPHSON-JUNCTIONS

We have measured the zero bias resistance, R0, and the threshold volta ge, V(t), of 2D arrays of small Josephson junctions as functions of te mperature and magnetic field. At low temperature, the Coulomb blockade dominates due to the relatively large charging energy E(C)=e2/2C (C b eing the junction capacitance). We find that the zero bias resistance may be described by thermal activation of charge solitons in most case s, i.e., R0 almost-equal-to k exp(E(a)/k(B)T). In the normal state, th e activation energy E(a) is close to 0.25 E(C). The measured activatio n energy at low magnetic field is less than 0.25E(C) + DELTA (where DE LTA is the superconducting gap), but larger than E(C) for all arrays. In a few samples, where the Josephson coupling energy E(J) is relative ly large, E(a) oscillates with the magnetic field. The period of the o scillation corresponds to one flux quantum per unit cell and the ampli tude is roughly E(J). In these samples the threshold voltage also osci llates at low magnetic fields. Such behavior of both E(a) and V(t) is a clear indication that also Cooper pair solitons contribute to the ch arge transport.