Reduced Critical Currents and Anomalous Zero-Bias Finite Resistance in SIS Junctions

Al/AlOx/Al tunnel junctions play a critical role in superconducting qubit devices. The inherent nonlinear inductance and tunability of the Josephson energy allows for control over qubit transition states. Consequently, it is important to characterize the properties of the junction as they impact the coherence time of the qubit. Here we report on the results of dc transport measurements on Al/AlOx/Al junctions, both in stand-alone structures and transmon configurations. The critical currents of these junctions are almost an order of magnitude smaller than estimates based on measurements of the superconducting gap and normal state resistance. These devices also show a finite differential resistance at zero current bias even at temperatures as low as 25 mK, similar to what has been observed earlier by Iansiti et al. [1] and Kautz and Martinis [2]. We will discuss possible explanations for the reduced critical currents and the finite zero-bias differential resistance.