title: Are "pinholes" the cause of excess current in superconducting tunnel junctions? A study of Andreev current in highly resistive junctions.
abstract: In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic pinholes in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlO(x)/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlO(x)/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement with experiment over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.
authors.affiliation: Department of Microelectronics and Nanoscience, Chalmers University of Technology, Göteborg, Sweden.
authors.forename: Tine
authors.initials: T
authors.lastname: Greibe
authors.forename: Markku P V
authors.initials: MP
authors.lastname: Stenberg
authors.forename: C M
authors.initials: CM
authors.lastname: Wilson
authors.forename: Thilo
authors.initials: T
authors.lastname: Bauch
authors.forename: Vitaly S
authors.initials: VS
authors.lastname: Shumeiko
authors.forename: Per
authors.initials: P
authors.lastname: Delsing