Mechanisms Limiting the Performance of ${\rm MgB}_{2}$ Polycrystalline Thin Film Microwave Resonators

We investigate the nonlinear behavior of magnesium diboride (MgB₂) coplanar waveguide resonators in order to identify the basic mechanisms limiting the performance of polycrystalline MgB₂ films in microwave applications. In particular, we focus on several kinds of switching response. Switching is observed due to increase of rf currents above a given threshold or to application of dc fields. In the former case a thermic model is able to account for the observed behaviors, including the presence of a thermal bistability regime. In the latter case, penetration of vortices at low fields is demonstrated and the effects of thermo-magnetic instabilities at higher fields are described.