TWO TECHNIQUES FOR BROADBAND MEASUREMENT OF THE SURFACE IMPEDANCE OF HIGH CRITICAL TEMPERATURE SUPERCONDUCTING THIN FILMS

Abstract

Measurements of the surface impedance of high critical temperature superconductors (HTS) over a wide frequency range provide important information on the electrodynamic properties of these materials and a severe test for any related theoretical model. We present two experimental techniques for the measurement of the surface impedance of HTS thin films in the microwave range, based on the detection of the reflection coefficient of the sample, connected to a vector network analyzer through a coaxial line. In one case the film forms an electrical short across the terminal section of the coaxial line, according to the so called Corbino geometry. In the other, a small circular gap separates the film from the inner conductor of the cable. In the latter case, the absence of direct electrical contact between the sample and the coaxial core simplifies realization and avoids contact instability always present with large temperature variations. We describe the two necessary steps to extract the film impedance from measured data, namely the study of electromagnetic field propagation in the two structures and the calibration of the coaxial measurement line at cyogenic temperatures. Finally, we present measurements performed on YBa 2 Cu 3 O 7-δ thin films with the two techniques.