Surface Impedance of YBa2Cu3O7-δ Films Grown on MgO Substrate as a Function of Film Thickness

The surface impedance characteristics of epitaxial YBa2Cu3O7-delta films of thickness d (f) = 75, 150, 300, 600 nm, produced by magnetron thermal co-evaporation onto single crystal MgO substrates was studied using measurement technique based on Ka-band whispering gallery mode (WGM) dielectric resonator (DR) fabricated from single crystal sapphire. Characterization of the unpatterned films was carried out in temperature interval from 20 K to 90 K. It was shown that the effective surface resistance approaches the minimum value for d (f) > 300 nm. At the same time, intrinsic impedance properties are practically independent on d (f) in the studied interval of d (f) values. The temperature dependence of London penetration depth was estimated experimentally and approximated with the model expressions. Effect of reducing the surface resistance of approximately two times at low temperatures one year later after their manufacture was registered for all films(except the film of 75 nm thickness). The effect may be explained by changes of the film parameters in time after the film light overdoping.