Large steps in long Josephson junctions

Large constant voltage-current steps were observed in the I-V 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 corresponding roughly to three to five times the voltage spacing expected between adjacent Fiske steps. Models addressing 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, leading to large steps. A model which involves pinning of fluxons in the junction is given. Due to the granularity of NbN and its very small coherence length, there will be pinning sites in the junction preventing single fluxons from moving at low bias currents. At particular junction parameter values the steps become very regular and sharp. The sharp steps can be used as a voltage standard without external RF bias or as a high-power oscillator when the device is suitably biased.<>