Zurek-Kibble Mechanism for the Spontaneous Vortex Formation in Nb-Al/Al_ox/Nb Josephson Tunnel Junctions: New Theory and Experiment

New scaling behavior has been both predicted and observed in the spontaneous production of fluxons in quenched Nb-Al/Al_ox/Nb annular Josephson tunnel junctions (JTJs) as a function of the quench time, τ_Q. The probability f₁ to trap a single defect during the normal-metal–superconductor phase transition clearly follows an allometric dependence on τ_Q with a scaling exponent σ=0.5, as predicted from the Zurek-Kibble mechanism for realistic JTJs formed by strongly coupled superconductors. This definitive experiment replaces one reported by us earlier, in which an idealized model was used that predicted σ=0.25, commensurate with the then much poorer data. Our experiment remains the only condensed matter experiment to date to have measured a scaling exponent with any reliability.