Nonlinear dynamics in the resonance line shape of NbN superconducting resonators

We report on unusual nonlinear dynamics observed in the resonance response of NbN superconducting microwave resonators. The nonlinear dynamics which occurs at relatively low input powers (2-4 orders of magnitude lower than Nb) includes among others, jumps in the resonance line shape, hysteresis loops changing direction, and resonance frequency shift. These effects are measured herein using varying input power, applied magnetic field, white noise, and rapid frequency sweeps. Based on these measurement results, we consider a hypothesis according to which local heating of weak links forming at the boundaries of the NbN grains is responsible for the observed behavior, and we show that most of the experimental results are qualitatively consistent with such a hypothesis.