Strong coupling effects in (Nb-Al-AlOx)(2)-Nb stacked Josephson junctions

Stacked (Nb-Al-AlOx)(2)-Nb long Josephson junctions with very thin intermed iate Nb-Al superconducting layer are investigated experimentally and theore tically. Stable coherent in-phase zero-field steps (ZFS's) are observed; bo th the critical current and the maximum ZFS current dependence on magnetic field are measured to prove the in-phase nature of this mode. The dependenc es are in good agreement with the inductive coupling model. The Swihart vel ocities for in-phase (<(c)over bar (+)>) and antiphase (<(c)over bar (-)>) modes are measured for different lengths of the stacks. In order to make a proper interpretation of the experimental results, an extension of the exis ting model is developed, taking into account the fact that the middle elect rode consists of two different superconductors (Nb and proximized Al). A co mparison of a new model with the conventional model and experimental data i s made. The extended model gives the <(c)over bar (+)>/<(c)over bar (-)> ra tio as 30% different from what the conventional model predicts. For thin Nb and Al layers the correction factor depends only on the ratio of the magne tic field penetration depths in Nb and proximized Al. [S0163-1829(98)04746- 8].