Nanofabricated SNS junction series arrays in superconductor-normalmetal bilayers

Abstract

We have applied our existing focused ion beam based nanoscale planarSNS bridge junction technology to the fabrication of series arrays, with aview to lumped array applications. Single junctions are created in amicrometre-width bilayer track (125 nm Nb on 75 nm Cu) by milling a trench50 nm wide in the upper superconducting layer. Individual junctions withnon-hysteretic, resistively-shunted current-voltage characteristics, criticalcurrent IC~1 mA and characteristic voltage ICRN~50 µV at 4.2 K can be fabricated routinely. The characteristics of 10-junctionseries arrays at junction spacings of 0.2 µm to 1.6 µmhave been studied at 4.2 K and above. Locking of all the junctions in thearray under an applied microwave field is observed as the transitiontemperature is approached. This effect is achieved at a lower temperature forshorter junction spacings, suggesting a penetration depth-dependentelectromagnetic coupling mechanism. Measurements of differential resistanceversus current reveal the IC distribution of individual junctions withinthe array. Spreads of critical current δIC = ICmax/ICmin~1.5at 4.2 K are typically observed. The spread in normal state resistance, RNbetween junctions is negligible in comparison, as the unbroken normal metallayer shunts all of the junctions in the array. This allows locking to beachieved in spite of the appreciable spread in IC.