Effect of Au contact layer on the microwave performance of YBa2Cu3O7-x thin films

The microwave surface resistance (R_s) of YBCO films sputtered simultaneously on both sides of CeO₂ coated sapphire wafers of 3 inch diameter was measured using the disk resonator technique at a frequency of 1.92 GHz. Gold layers were deposited on the unpatterned side of the resonator, and the microwave losses were measured in the Au-contacted YBCO films. Although the Au layer was not directly exposed to the microwave power, it dramatically increased the effective R_s of the YBCO film, while leaving the effective surface reactance (X_s) almost unchanged. The R_s-enhancement was examined experimentally and theoretically in terms of the impedance transformation approach as a function of the Au and YBCO film properties. Furthermore, the technique appeared suitable for the determination of the absolute value of the London penetration depths (λ_L) in the double-sided YBCO films. The resulting temperature dependencies of λ_L were in a good agreement with those estimated by the conventional frequency-shift technique