Dielectric resonators for the measurement of superconductor thin films surface impedance in magnetic fields at high microwave frequencies

We present the design and realization of cylindrical dielectric resonators operating in the 40– 60 GHz frequency range, designed for the measurement of the surface resistance and of the surface reactance shift in High-T c Superconductors (HTS) thin films in a dc magnetic field. The resonators are single tone, based on the TE011 mode, and multiple tone, the latter al- lowing in principle to exploit the simultaneous determination of the surface impedance at different frequencies. As an application example, we report the temperature and field depen- dencies of the effective surface impedance of some cuprate superconductors thin films. The results are compared with those obtained through the use of a standard metal cavity with a similar Q-factor and operating in the same frequency range. The comparison highlights a su- perior stability and a higher sensitivity, resulting in an increase of about two orders of magni- tude in the resolution of the surface impedance measurement. By contrast, the dissipative part of the superconducting transition (above T/T c = 0.97) is better studied with the metal cavity. We also present measurements in the vortex state in YBa2Cu3O7−δ and Tl2Ba2 CaCu2O8+x that show significantly different physics.