Disorder and critical current variability in Josephson junctions

Abstract

We investigate theoretically the origins of observed variations in the critical currents of Nb/Al-AlOx/Nb Josephson junctions in terms of various types of disorder. We consider the following disorder sources: vacancies within the Al layer; thickness variations in the AlOx layer; and "pinholes" (i.e., point contacts) within the AlOx layer. The calculations are all performed by solving the microscopic Bogoliubov-de Gennes Hamiltonian self-consistently. It is found that a small concentration of vacancies within the Al layer is sufficient to suppress the critical current, while the presence of a small number of thick regions of the oxide layer induces a similar effect as well. The pinhole scenario is found to result in anomalous behavior that resembles neither that of a pure tunnel junction nor that of an SNS junction, but a regime that interpolates between these two limits. We comment on the degree to which each of the three scenarios describes the actual situation present in these junctions. arXiv:1908.08360v1 [cond-mat.supr-con]

High-Quality Ferromagnetic Josephson Junctions Based on Aluminum Electrodes

“…The characteristic voltage V c = I c R N is about 25% of the expected Ambegaokar-Baratoff value [ 35 ]. The suppression of I c could be related to the presence of paramagnetic impurities [ 36 ] or oxygen vacancies in the insulating barrier [ 37 ]. The suppression of I c was observed in JJs employed in quantum circuits and does not seem to affect their coherence times [ 38 ].…”

Critical Currents in Conventional Josephson Junctions With Grain Boundaries

“…Finally, we set up a Hamiltonian in real space using this information and we then calculate the critical current after obtaining self-consistent mean field solutions using the Bogoliubov-de Gennes approach. 6,[15][16][17][18][19][20][21][22][23][24][25][26][27] We outline these steps in detail in the following subsections.…”

“…as t(|r ij |). To connect to earlier investigations 6 based on a single band model, we obtain the function t(|r ij |) by computing the expectation value of the kinetic energy −∇ 2 /(2m * ) from overlapping atomic s-orbitals (see Fig. 2).…”

“…4 These scenarios were explored in detail in our previous work, which demonstrated how realistic amounts of device-to-device variability can naturally result from the presence of these forms of disorder. 6 However, one explanation that has not been examined in similar detail is the presence of grain boundaries within the superconducting niobium leads. This has been floated as a possible explanation for the aforementioned critical current variation.…”