Thermal activation and injection of charge solitons in 2D-arrays of small Josephson junctions.

Abstract

We have measured the zero bias resistance, R 0, and the threshold voltage, V t, of 2D arrays of small Josephson junctions as functions of temperature and magnetic field. At low temperature, the Coulomb blockade dominates due to the relatively large charging energy E C=e 2/2C (C being the junction capacitance). We find that the zero bias resistance may be described by thermal activation of charge solitons in most cases, i.e., R 0≈k exp(E a/k BT). In the normal state, the activation energy E a is close to 0.25 E C. The measured activation energy at low magnetic field is less than 0.25E C+Δ (where Δ is the superconducting gap), but larger than E C for all arrays. In a few samples, where the Josephson coupling energy E J is relatively large, E a oscillates with the magnetic field. The period of the oscillation corresponds to one flux quantum per unit cell and the amplitude is roughly E J. In these samples the threshold voltage also oscillates 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 charge transport.