THE MICROWAVE SURFACE IMPEDANCE OF DYBA2CU3O7-X VERY THIN-FILMS

Measurements of the surface impedance of DyBa2Cu3O7-delta (DyBCO) thin films have been made around 6 GHz by using the parallel plate resonat or method with sapphire dielectrics. The films were deposited by RF ma gnetron sputtering on LaAlO3 substrates to thicknesses between 20 and 320 nm. The change of the London penetration depth as a function of te mperature is deduced from resonance frequency shifts. A generalized tw o fluid model is used to fit the data in terms of an effective penetra tion depth lambda(eff)(0) at 0 K, taking into account the effect of gr ain boundary weak links. This analysis give an interpretation of the c hanges in lambda(eff)(0) as a function of the film thickness and yield s values for lambda(eff)(0) around 160 nm for films thinner than 120 n m.