Dielectric Resonators for the Measurements of the Surface Impedance of Superconducting Films

Abstract

We present the development, realization and setup of dielectric resonators, for the purpose of measuring the surface impedance at microwave frequencies of superconducting thin films. We focus on resonators designed to operate in dc magnetic fields, optimized for the measurements of the variation of the surface impedance with the applied field. Two resonators, operating at 8 and 48 GHz, are presented. We discuss different approaches to the measurement of the resonator parameters, with particular attention to the nonidealities of real setups in a cryogenic environment. Finally, we present some sample measurement of high-T c and low-T c superconducting films.

Vortex Pinning and Flux Flow Microwave Studies of Coated Conductors

“…2 The surface impedance Z measurements have been performed by means of a dielectric resonator used in the endwall replacement configuration. By exciting the TE 011 mode (resonant frequency f res ∼ 47.9 GHz) through a two-port connection (transmission operation), the frequency dependent resonant curve is measured and fitted, yielding the unloaded quality factor Q and f res [12] from which, through standard e.m. theory [13], Z is computed as:…”

Design and test of a microwave resonator for the measurement of resistivity anisotropy

“…Within a perturbation approach, the sample to be measured is placed either inside the volume of the resonator or as part of its conducting enclosure [4]. For each electromagnetic mode, by measuring the variations of the resonator quality factor Q and resonant frequency f res due to the insertion of the sample, one extracts the sample properties with a sensitivity which is essentially dictated by Q (the higher, the better) [2,5]. Typical measurement applications are the contactless determination of the conductivity of semiconductors [1], including resistance mapping, in the integrated electronics industry; the characterization of dielectric materials for electronics and telecommunications [6,7,8], including wood [9]; the investigation of dielectric properties of nanoparticles [10]; the study of the photoconductivity in insulating materials to be used in scintillators, phosfors for fluorescent tubes and plasma panels [11]; the study of the complex impedance of superconductors [12]; the study of penetration of the microwaves in several materials [13].…”

“…For the special case of the study of anisotropic materials, the rectangular resonators have the correct symmetry to induce straight currents on the sample [17] (other however powerful configurations [18] may require patterning or shap- 5 ing of the material under study). In a previous work [19] we have presented the design (including a discussion of the straightness of the currents), realization and characterization of a prototype rectangular rutile (TiO 2 ) resonator, which gave promising results.…”

“…The minimum measurable R a is determined by the system sensitivity S = ∂Q/∂R s = Q 2 /G s[5], R a,min ∼ S −1 δQ. The relatively low values of Q, typical of the rectangular metallized structures, allow to neglect the dielectric with respect to ohmic losses; 170 moreover, the main source of uncertainty is the Q itself, whereas the uncertainties on the various geometrical factor can be safely ignored (neglecting, for now, centering and positioning mechanical issues).…”

Robustness of the0−πtransition against compositional and structural ageing in superconductor/ferromagnetic/superconductor heterostructures

“…The details of the setup and of the procedure have been given elsewhere. 11,12 In short, the sample was placed at the bottom of the resonator. Measurements of the quality factor Q(T ) and of the resonant frequency ν 0 (T ) yielded the variation of the effective surface resistance ∆R s and of the effective penetration depth ∆λ ef f with T , according to: and…”