New scaling behavior has been both predicted and observed in the spontaneous production of fluxons in quenched $\mathrm{Nb}\mathrm{\text{−}}\mathrm{Al}/{\mathrm{Al}}_{\mathrm{ox}}/\mathrm{Nb}$ annular Josephson tunnel junctions (JTJs) as a function of the quench time, ${\tau }_Q$. The probability $f_1$ to trap a single defect during the normal-metal–superconductor phase transition clearly follows an allometric dependence on ${\tau }_Q$ with a scaling exponent $\sigma =0.5$, 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 $\sigma =0.25$, 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.

• Received 22 February 2006

DOI:https://doi.org/10.1103/PhysRevLett.96.180604