Subgap leakage and bound states in normal metal superconductor tunnel junctions

We 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