Multiparticle tunneling observed in superconducting junctions

Abstract

Asymmetric (Δ1 ≠ Δ2) Nb/oxide/PbBi and symmetric (Δ1 = Δ2 ≡ Δ) Sn/oxide/Sn superconducting tunnel junctions show distinct excess current steps in their current-voltage characteristics. The step-like shape and the relative amplitudes of the observed subharmonic gap structure, in particular, that the current step at the larger of the two gap voltages Δ2/e and Δ1/e was found to be the higher one, are in excellent agreement with the multiparticle tunneling model. An increase in step height at ΔSn/e with decreasing temperature can be accounted for with an enhanced, nonequilibrium quasiparticle density generated by the two-particle current. © 1981.