Characterization of anomalous pair currents in Josephson junction networks

Abstract

Measurements performed on superconductive networks shaped in the form of planar graphs display anomalously large currents when specific branches are biased. The temperature dependences of these currents evidence that their origin is due to Cooper pair hopping through the Josephson junctions connecting the superconductive islands of the array. The experimental data are discussed in terms of theoretical models which predict, for the system under consideration, an inhomogeneous Cooper pair distribution on the superconductive islands of the network as a consequence of a Bose-Einstein condensation phenomenon.

Conditioning of Superconductive Properties in Graph-Shaped Reticles

“…Phenomenona similar to those visible in Fig. 2a have been reported previously [8][9][10] , but the fact that we now note is that even the gap voltages of the junctions of the array embedded in the graph structure result higher than those of the reference array. In Fig.…”

“…The value of 6.57 K is not far from the "singular" temperature that could be extracted in ref. 10 (see Fig. 3 of that paper), for samples with higher current densities, which was slightly above 7 K. As we have just seen, for the temperature dependence the "singular" increase of the backbone embedded currents does occur when the thermal energy equals, within 1%, the Josephson energy.…”

“…3 of ref. 10 in Fig. 3a of that paper, one can see that excess currents gets as high as allowed by the superconducting state and this phenomenon still deserves more careful analyses and measurements.…”

“…A peculiar "hybrid" fabrication technique was used for the samples of ref. 10 . We judged that for the present experiments a very "standard" Nb trilayer technology would be advisable.…”

Josephson Currents and Gap Enhancement in Graph Arrays of Superconductive Islands

“…Several experiments have been performed in which evidence of boson distribution gradients in planar arrays, with specific topological structures, has been provided [ 18 , 19 , 20 , 21 ]. In these experiments the distribution of bosons on the islands was probed measuring current-voltage characteristics of series arrays of the Josephson junctions originated by oxide barriers between the superconductive islands; the junctions were also providing the coupling potential.…”