Abstract
We study Josephson junctions between a multi-band iron-pnictide Ba1-xNaxFe2As2 and conventional s-wave superconductors Nb and Cu/Nb bilayer. We observe that junctions with a Cu interlayer exhibit much larger IcRn, despite a weaker proximity-induced superconductivity. This counterintuitive result is attributed to the difference in Fermi surface geometries of Nb and Cu, which leads to a selective one-band tunneling from Cu and a non-selective multi-band tunnelng from Nb. The latter leads to a mutual cancellation of supercurrents due to the sign-reversal s+- symmetry of the order parameter in the pnictide. Our results indicate that Fermi surface geometries play a crucial role for pnictide-based junctions. This provides a new tool for phase sensitive studies and paves a way to a conscious engineering of such junctions.
Highlights
The electronic structure of superconductors is usually quite complicated, even for low-Tc materials, such as the transition metal Nb
We observe that the insertion of a Cu interlayer in such junctions leads to a dramatic enhancement of the IcRn product, despite the weaker proximity-induced superconductivity of Cu
This counterintuitive phenomenon is attributed to the differences in Fermi surface geometries of Nb and Cu, which affect the selectivity of tunneling in sign-reversal s± bands of pnictide
Summary
The electronic structure of superconductors is usually quite complicated, even for low-Tc materials, such as the transition metal Nb. A simple description of the Josephson effect, which does not take into account complex Fermi surface (FS) geometry, works remarkably well for conventional superconductors [1,2] This happens because the probabilities of electron and Cooper pair tunneling are similar [3]. A drastic deviation should happen in the case of the sign-reversal order parameter [5,6,7,8] This occurs in cuprate- and iron-based superconductors, which are believed to have d-wave [9,10] and s± [11,12,13,14,15,16,17,18] symmetries, respectively. Ic depends on the gap values in each band, and the IcRn is band structure sensitive and not universal [5,6,7,8] (more discussion can be found in Appendix E)
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