Evidence of Long-Range Coherence in Superconducting Networks

Systematic experimental investigations are reported of charge transport in double-comb and star-shaped planar arrays obtained by coupling superconducting islands via Josephson tunnel junctions. The fabrication of the structures is based on a standard niobium trilayer technology with superconducting transition temperature close to 9 K. Noticeable enhancements of Josephson supercurrents and energy gap are recorded. Complementarity between experimental data and theoretical predictions is employed as a tool to clarify the role of different graph topologies in conditioning the singular behavior of synthetic graph-shaped networks. The predictions of the theoretical models are based on the Bose-Einstein topological condensation and a de Gennes-Alexander approach for granular superconductors. Evidence is shown of the dependence, for both gap and Josephson critical currents, on node-to-node connections in the networks, namely on array topology.