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

Abstract

We reconsider the problem of the measurements of the microwave complex
\nsurface impedance in thin superconducting films deposited on SrTiO3
\nsubstrates. We perform measurements of the complex surface impedance
\nZs = Rs + i X s of thin YBa2Cu3 O7−δ films deposited by laser ablation on
\nSrTiO3 substrates. The typical oscillations due to the strong temperature
\nvariation of the SrTiO 3 permittivity are confirmed in Rs and observed in
\nX s. The effects of the SrTiO3 substrate are evident even well below the
\nsuperconducting transition temperature of YBa2 Cu3O7−δ . Similarly to
\nprevious works, we describe the overall response in terms of impedance
\ntransformations. We extend the known results by (i) considering the
\nmeasurements of the imaginary part, (ii) comparing the measurements to the
\nabsolute dc resistivity measured on the same sample, and (iii) suggesting a
\nmethod for measuring the intrinsic thin film surface impedance by adjusting
\nthe substrate impedance. To demonstrate the feasibility of microwave
\nmeasurements of intrinsic properties of films grown onto SrTiO3 substrates,
\nwe check the proposed method by measuring the field dependent surface
\nimpedance before and after removal of the substrate resonance.