Local thermal bistability in MgB2 microwave coplanar resonators: Opposite jumpwise response to weak-link switching and to vortex avalanches

We report on specific nonlinear features in the response of microwave coplanar resonators obtained from magnesium diboride thin films. Different jumpwise behaviors observed in the resonance curves are found to be generated by either weak-link switching or vortex avalanches. Both mechanisms can be explained in terms of local-heating-induced thermal bistability, but they result in opposite effects, i.e., opposite signs in the abrupt variations of the quality factor and of the resonance frequency, due to the rf-current distribution. The weak-link-switching response shows promising characteristics (jumpwise signal, bistability, and hysteresis) for applications of MgB2 films in nonlinear devices.