Novel Self-Sustained Modulation in Superconducting Stripline Resonators

We study thermal instability in a driven superconducting NbN resonator integrated with a microbridge. A monochromatic input drive is injected into the resonator and the response is measured as a function of the frequency and amplitude of the drive. Inside a certain zone in the frequency-amplitude plane the system has no steady state, and consequently self-sustained modulation of the reflected power off the resonator is generated. A theoretical model, according to which the instability originates by a hot spot forming in the microbridge, exhibits a good quantitative agreement with the experimental results.