Switching response of MgB2 thin-film microwave resonators due to local nonlinear Joule heating

We investigated the switching response of MgB2 coplanar waveguide resonators to increase of rf currents. A model based on the heat balance equation is able to account for the observed behaviors, including the presence of a thermal bistability regime. The model also reproduces the temperature dependence of the limiting currents. Results are consistent with the hypothesis that a hot spot is formed due to local Joule heating in correspondence with a weak link. Weak links are modeled as long S-N-S junctions, originating from the transition to the normal state of rows of MgB2 grains. This thermal process is proposed as the main mechanism setting the performance limit of polycrystalline MgB2 films in microwave applications.