Single-electron charging effects and implications for tunneling measurements of the high-Tc superconductors

A theory is presented for the dynamics of two voltage-biased ultra-small capacitance tunnel junctions connected in series when one or more electrodes are superconducting, and experiments performed on parallel arrays of such junctions are reported. Using the semiclassical model, the authors find that the I-V characteristics display steps and therefore multiple peaks in dI/dV, corresponding to the time-average occupation of the interjunction region by integral numbers of electrons. The voltage at which the first step is located depends on the superconducting gap and the capacitances of the junctions. The spacing between subsequent steps depends solely on the capacitances. A discussion is presented of electron tunneling results performed on metal/Al₂O₃/2-10 nm-diameter metal particles/Al₂O₃/metal junctions where this multiple-peak structure is observed. The authors present preliminary tunneling results in junctions using Pb-particles. These results indicate that the multiple-peak structure commonly observed in tunneling data of high-T_c oxide superconductors can be explained in terms of charging effects in a material with a single superconducting gap