Observation of Bifurcations and Hysteresis in Nonlinear NbN   Superconducting Microwave Resonators
  In this paper we report some extraordinary nonlinear dynamics measured in the resonance curve of NbN superconducting stripline microwave resonators. Among the nonlinearities observed: aburpt bifurcations in the resonance response at relatively low input powers, asymmetric resonances, multiple jumps within the resonance band, resonance frequency drift, frequency hysteresis, hysteresis loops changing direction and critical coupling phenomenon. Weak links in the NbN grain structure are hypothesized as the source of the nonlinearities. 
Baleegh Abdo, Eran Segev, Oleg Shtempluck, and Eyal Buks