Progress in Josephson Junctions, Arrays, Chains, and Surface Interfaces
Subgap leakage and bound states in normal metal superconductor tunnel junctions
1:42 pm – 1:54 pmWe investigate normal metal-insulator-superconductor tunnel junctions in a heavily shielded and
filtered environment at temperatures down to a few mK. The subgap tunneling regime exhibits a
hard gap, with very low leakage down to about ∼ 1E-5 of the normal state conductance. This is
independent of the presence or absence of a ground plane below the junction shunting radiation, thus
indicating negligible microwave contribution. On top of the leakage, finite bias current steps appear
symmetrically around zero-bias at random, cooldown dependent energies, ruling out Shapiro steps
as the origin. The steps are splitting with a g-factor of 2 in an in-plane magnetic field and exhibit
thermal broadening and cool down to temperatures as low as 4 mK. Further, a parabolic evolution
in parallel field is seen, consistent with a diamagnetic shift due to confinement on a 10-50 nm scale,
thus ruling out atomic Yu-Shiba-Rusinov states. Further, Caroli-de Gennes-Matricon states are also
excluded due to the observed insensitivity to flux jumps.
The results are in qualitative agreement with numerical transport simulations where the finite
bias steps result from the sample specific geometry and disorder defined bound states arising from
enhanced Andreev reflection. Finally, the simulations show a linear subgap leakage with slope
proportional to the disorder strength, thus providing a microscopic origin of the Dynes leakage in
the intrinsic regime where microwave absorption is negligible due to sufficient filtering