Titre : Subgap leakage and bound states in normal metal-superconductor tunnel junctions
Institut Néel, Salle E424 (Salle Louis Weil)
Résumé : Aluminum oxide tunnel junctions are the fundamental building blocks for Josephson junctions, superconductingqubitsandmanyotherapplicationsincludingthermometryandon-chip refrigerators.Despitetheirwidespreaduseandeminentimportance,pervasiveimportant experimental signatures are not currently understood. In particular, the subgap leakage is much larger than theoretically estimated for opaque junctions, giving rise to dissipation, causing e.g. relaxation of superconducting qubits, losses in tunable resonators, and limiting the accuracy in current turnstiles. Despite decades of research and use in applications, its origin remains unknown and is posing a key challenge. We investigate normal metal-insulator-superconductor tunnel junctions (see Fig.), in a heavily shielded and filtered environment at temperatures down to a few mK. The devices exhibit a hard gap with very low leakage about 5 orders of magnitude below the normal state conductance – the Dynes leakage. On top of that, discrete finite bias current steps appear, symmetrically around zero-bias at random, cooldown dependent energies, ruling out Shapiro steps as the origin. These steps are splitting with a g-factor of 2 in an in-plane magnetic field (Fig. below), exhibit thermal broadening and cool down to temperatures as low as 4 mK. Further, a parabolic evolution in parallel field is seen (Fig. below), 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. We present numerical transport simulations finding that the steps result from geometry and disorder defined bound states with enhanced Andreev reflections. Finally, the simulations show leakage with slope proportional to the disorder strength, thus providing a possible microscopic origin of the Dynes leakage in the intrinsic regime where microwave absorption is negligible due to strong filtering. This mechanism thus opens potential avenues for a new generation of junctions with suppressed leakage and lower dissipation.
