Two techniques for broadband measurement of the surface impedance of high critical temperature superconducting thin films

Measurements of the surface impedance of high critical temperature supercon ductors (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 m easurement of the surface impedance of HTS thin films in the microwave rang e, based on the detection of the reflection coefficient of the sample, conn ected 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 l ine, according to the so called Corbino geometry In the other, a small circ ular gap separates the film from the inner conductor of the cable. In the l atter case, the absence of direct electrical contact between the sample and the coaxial core simplifies realization and avoids contact instability alw ays present with large temperature variations. We describe the two necessar y steps to extract the film impedance from measured data, namely the study of electromagnetic field propagation in the two structures and the calibrat ion of the coaxial measurement line at cyogenic temperatures. Finally, we p resent measurements performed on YBa2Cu3O7-delta thin films with the two te chniques.