Characterizating the penetration depth and coherence length of superconductor-normal superlattices

Ferromagnetic Josephson junctions are a viable candidate for a low power cryogenic memory alternative compared to traditional CMOS technologies. There have been many reports concerning the supercurrent passing through the Josephson junction, including the discovery of spin triplet Cooper pairs in these systems [1]. Previously, we have found that by replacing the bottom Nb layer with a Nb/Al or Nb/Au superlattice, where the Al and Au layers are quite thin, the surface roughness is reduced compared to Nb. This decrease in surface roughness yields improved areal dependence of the critical current [2]. Previously, the effects on the penetration depth and coherence length from the insertion of a thin normal metal between Nb layers have not been fully explored, given that the use of such superlattices in Josephson junctions yielded behavior that was consistent with Nb. In this work, we report on our observation of an increased penetration depth and decreased coherence length in Nb/Al and Nb/Au superlattices utilizing polarized neutron reflectometry and electrical transport, respectively.

[1] Norman O. Birge, Phil. Trans. R. Soc. A 376, 20150150 (2018)
[2] Yixing Wang, W. P. Pratt, Jr. and Norman O. Birge, Phys. Rev. B 85 214522 (2012)