Measurements and removal of substrate effects on the microwave surface impedance of YBCO films on SrTiO3

We reconsider the problem of the measurements of the microwave complex surface impedance in thin superconducting films deposited on SrTiO3 substrates. We perform measurements of the complex surface impedance Z(s)(') = R-s(') + i Delta X-s(') of thin YBa2Cu3O7-delta films deposited by laser ablation on SrTiO3 substrates. The typical oscillations due to the strong temperature variation of the SrTiO3 permittivity are confirmed in R-s(') and observed in Delta X-s('). The effects of the SrTiO3 substrate are evident even well below the superconducting transition temperature of YBa2Cu3O7-delta. Similarly to previous works, we describe the overall response in terms of impedance transformations. We extend the known results by (i) considering the measurements of the imaginary part, (ii) comparing the measurements to the absolute dc resistivity measured on the same sample, and (iii) suggesting a method for measuring the intrinsic thin film surface impedance by adjusting the substrate impedance. To demonstrate the feasibility of microwave measurements of intrinsic properties of films grown onto SrTiO3 substrates, we check the proposed method by measuring the field dependent surface impedance before and after removal of the substrate resonance.