Topology-induced critical current enhancement in Josephson networks

Abstract

We investigate the properties of Josephson junction networks with inhomogeneous architecture. The networks are shaped as "square comb" planar lattices on which Josephson junctions link superconducting islands arranged in the plane to generate the pertinent topology. Compared to the behavior of reference linear arrays, the temperature dependencies of the Josephson currents of the branches of the network exhibit relevant differences. The observed phenomena evidence new and surprising behavior of superconducting Josephson arrays as well as remarkable similarities with bosonic junction arrays.

Characterization of anomalous pair currents in Josephson junction networks

“…6 and were well characterized in ref. 7, however now, due to fact that in our new design the BA is "topologically" consistent with the theoretical model we record very uniform currents for all the backbone junctions at all temperatures, which was not the case in ref. 7 (see Fig.…”

“…In conclusion, we have reported evidence that topology-induced effects in arrays of Josephson junctions can generate relevant gradients in the distribution of Cooper pairs on the superconductive islands of the arrays. Our results do not contradict previous experimental observations [6,7] and provide further quantitative input toward a full characterization of this intriguing phenomenon.…”

“…For a maximum pair current of 1 μA, the Josephson energy would be 3x10 -22 J and comparable with the thermal energy which, in the (1-9)K temperature interval ranges in the (10 -22 -10 -23 )J interval. We conclude that a critical current of the order of few microamperes could be appropriate for the experimental observations [10]; for this reason indeed in previous investigations [6,7] the interval (1-10)μA for the maximum critical currents of the junctions was targeted.…”

“…Their theoretical analysis showed that in Josephson networks shaped in the form of a symmetric comb, or other graph-like structures, the hopping between neighbour superconducting islands generates a nonuniform distribution of bosons, physically represented by Cooper pairs, along the branches. A thorough mathematical formulation of this problem has been recently published [5] and measurements have pointed toward a remarkable qualitative agreement of this theoretical model with experimental reality [6,7]. We have undertaken a systematic analysis of inhomogeneous distribution of Cooper pairs on a comb graph by measuring the current-voltage characteristics (I-V curves) of specific arrays and determining temperature and magnetic field-induced dependencies of the curves.…”

“…The samples that we fabricated contain 400 backbone superconducting islands and 400 finger arrays; the high number of fingers was designed in the attempt to approach the thermodynamic limit of the number of bosons considered by the theory [3,4]. It is worth noting that the present design contains a relevant correction with respect to previously fabricated samples [6,7]: the BA was now designed in a way to meet more closely the requirements of a symmetric comb topology. In fact we see in Fig.…”