A study on the nonlinear microwave electrodynamic response of e-beam evaporated MgB2 superconducting thin films

We present a study on the temperature and field dependence of the microwave surface impedance Z(s) in thin films of the superconducting MgB2 compound. Samples were prepared by e-beam evaporation of boron on r-plane sapphire followed by an ex situ annealing in Mg vapour. Critical temperature values range between 26 and 38 K. Surface impedance measurements (Z(s) = R-s + iX(s)) were performed from 2 K close to T-c in the microwave region up to 20 GHz via parallel plate or dielectrically loaded resonators in 'symmetric' (two MgB2 films) and asymmetric (an MgB2 film and a commercial YBCO control film) configurations. At high microwave power, frequency domain measurements show a characteristic signature associated with weak links and this appears to be the limiting factor governing the performance of these films.