Large steps in long Josephson junctions

Large constant voltage current-steps were observed in the IV-characteristics of long NbN-MgO-NbN Josephson junctions in the presence of external magnetic fields of a few Gauss. The steps are separated by a voltage AV corresponding roughly to 3 to 5 times the voltage spacing expected between adjacent Fiske-steps. Models discussing the origin of such steps are presented. Simultaneous nucleation of fluxons at several sites along the junction with thin electrodes, favored by the large London penetration depth of NbN, would introduce a large amount of magnetic flux into the junction at once which would lead to large steps. A new model is presented here; involves pinning of fluxons in the junction. Due to the granularity of NbN and its very small coherence length there will be pinning sites in the junction preventing single fluxons to move at low bias currents. At particular junction parameter values the steps become very regular and sharp. Such characteristics are well suited for a voltage standard and oscillators.