Effects of Disorder on the Performance of Magnesium Diboride Microwave Resonators

A comprehensive understanding of the role of disorder in two-band superconducting magnesium diboride is an essential requirement for its use in potential applications. Within this framework, we investigated the role of disorder in modifying the performance of microwave coplanar resonators, characterized as a function of temperature and input power, before and after successive uniform irradiations with 250 MeV Au ions. Also sample aging has been considered as a further source of disorder. We show the main features of the peculiar phenomenology concerning the effects of disorder on the electromagnetic response of the resonator, due to surface resistance reduction at low-temperatures and to non-monotonic residual resistance as a function of disorder level. The analysis is mainly focused on the nonlinear response of the device. These results are discussed in the framework of the two-gap model with significant interband scattering, taking also into account the grain-boundary network underlying polycrystalline films.