Subgap Structure as Function of the Barrier in Atom-Size Superconducting Tunnel Junctions

Abstract

Using a mechanically controllable breakjunction we study superconducting tunnel junctions of atom size. The vacuum barrier is adjusted by controlling the distance between the electrodes to give barrier transparencies ranging over 2 orders of magnitude. We observe a systematic variation of the subgap current steps at $eV=\frac{2\ensuremath{\Delta}}{n}$, for $n=2$ and $n=3$ in excellent agreement with theories of multiparticle tunneling and multiple Andreev reflection.