Microwave properties of in situ grown MgB2 superconducting thin films

The electrodynamic response at 20 GHz of c-axis oriented MgB2 superconducting thin films is reported. Mg-rich Mg-B precursor samples were grown on MgO and Al2O3 single crystal substrates by a d.c. planar magnetron sputtering technique, and subsequently annealed in situ at 800degrees C for 10 min in a In-sealed Nb box in the presence of saturated Mg vapor. The films were characterized by a variety of structural and electronic techniques including XRD, EDS, STM-AFM analyses, and transport measurements. The dependence of the surface impedance from temperature and radiofrequency (r.f.) field amplitude was measured via a dielectric resonator technique. Temperature data clearly confirm the s-wave nature of the newly discovered superconductor, even if the value of the energy gap is smaller than BCS prediction. An effective two-band model can be applied to quantitatively explain the experimental results. In spite of previous reports claiming the absence of weak link behavior in MgB2, the power dependence show that granularity governs the performance of these films in the microwave region.