Subgap leakage and interface states in superconductor–insulator–superconductor tunnel junctions

Abstract

We use tunneling spectroscopy in Al and Nb based superconductor–insulator–superconductor (SIS) junctions to demonstrate the existence of metallic quasiparticle states at the interface of a superconducting electrode and an oxide layer, introducing a new method to directly evaluate the interface quasiparticle areal density of states. Current–voltage (I–V) characteristics typically observed in metal–insulator–superconductor (MIS) junctions are observed in the subgap voltage region. The turn-on voltage of the MIS-type leakage is found to be determined by the superconducting electrode with a smaller gap energy (Δ), and its magnitude depends on the nature of the interface. Our experiment suggests that the interface plays a key role in contributing to the subgap leakage influencing the performance of superconducting circuits.