Zero-field singularities in Josephson tunnel junctions of intermediate length

Zero-field singularities (Z.F.S.) appear in the d.c. current-voltage characteristic of Josephson tunnel junction longer than the Josephson penetration depth λj. This modes are currently explained in terms of periodical oscillations of fluxons in the junction, and show potential applications in microwave devices. A set of one dimensional overlap junctions of length between 5.6λj and 1.6λj have been studied. Detailed experimental investigations of the current-voltage d.c. characteristics of Z.F.S. are reported. We find that the I-V shape becomes steeper very regularly either by increasing the Z.F.S. order number or by decreasing the junction length. A similar behavior is measured for 2π -kink oscillations on a mechanical analog. Moreover in real junctions anomalous structures appear in high order singularities.