Charging effects in coupled superconducting tunnel junctions and their implications for tunneling measurements of high-Tcsuperconductors

We calculate the quasiparticle current for two mesoscopic superconducting tunnel junctions connected in series using the semiclassical model. The I-V characteristics exhibit a sequence of steps whose spacing is determined by the superconducting gap energy and the electrostatic charging energy of the junctions. These steps are reminiscent of those observed in point-contact tunneling experiments performed on granular high-$T_c$ superconductors. We propose that the ‘‘multiple-gap structure’’ observed in such experiments arises from charging effects in a material with a single gap, and suggest experiments to verify this.