We have measured the zero bias resistance, R0, and the threshold voltage, Vt, 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 EC= e2 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., R0≈k exp( Ea kBT). In the normal state, the activation energy Ea is close to 0.25 EC. The measured activation energy at low magnetic field is less than 0.25EC+Δ (where Δ is the superconducting gap), but larger than EC for all arrays. In a few samples, where the Josephson coupling energy EJ is relatively large, Ea oscillates with the magnetic field. The period of the oscillation corresponds to one flux quantum per unit cell and the amplitude is roughly EJ. In these samples the threshold voltage also oscillates at low magnetic fields. Such behavior of both Ea and Vt is a clear indication that also Cooper pair solitons contribute to the charge transport.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Electrical and Electronic Engineering