A study was made of the temperature and field dependences of the microwave surface impedance Zs in thin films. Samples were prepared by the e-beam evaporation of B onto r-plane sapphire, followed by ex situ annealing in Mg vapour. The critical temperature values ranged from 26 to 38K. Surface impedance measurements (Zs = Rs + iXs) were performed from 2K close to TC in the microwave region up to 20GHz via parallel plate or dielectrically loaded resonators in symmetrical (two MgB2 films) and asymmetrical (MgB2 film and commercial YBCO control film) configurations. At high microwave power, frequency domain measurements exhibited a characteristic signature associated with weak links. This appeared to be the limiting factor governing the performance of these films.

A Study on the Nonlinear Microwave Electrodynamic Response of e-Beam Evaporated MgB2 Superconducting Thin Films. A.Andreone, E.Di Gennaro, G.Lamura, M.Salluzzo, A.Purnell, L.F.Cohen, L.Hao, J.Gallop, C.Cantoni, M.Paranthaman: Superconductor Science and Technology, 2003, 16, 260-3