Negative differential resistance and quasiperiodic vortex-antivortex motion in a superconducting constriction

We report the observation of the negative differential resistance (NDR) in the current-driven regime of superconducting MoN/Cu constrictions. Our calculations in framework of time-dependent Ginzburg-Landau equation reveal that in the constriction at large current there is a channel with dynamically suppressed superconducting order parameter along which vortices and antivortices move quasiperiodically over time. As the current increases the constriction switches to the less resistive state with much faster vortices moving periodically over time (quasiphase slip-line state). Microwave radiation experiments demonstrate the presence of Shapiro steps at low voltage, where a slow, time-periodic vortex motion is expected. Conversely, Shapiro steps are absent at high voltage, where the NDR appears, indirectly confirming the quasiperiodic regime.