We discuss and propose a complete data treatment, in close contact to typical microwave experimental data, in order to derive vortex parameters, such as pinning constant and viscous drag coefficient (also referred to as “vortex viscosity”), in a way as model independent as possible. We show that many of the accepted models for the complex resistivity can be described by a single, very general analytical expression. Using typical measurements of real and imaginary resistivity as a function of the applied field, we show that, even for single-frequency measurements, it is always possible to obtain (a) estimates of viscous drag coefficient and pinning constant with well-defined upper and lower bounds and (b) quantitative information about thermal creep. It turns out that neglecting thermal creep, in particular and counterintuitively at low temperatures, might result in a severe overestimation of the viscous drag coefficient. We also discuss the impact of thermal creep on the determination of the pinning constant. The present results might lead to a reconsideration of several estimates of the vortex parameters.

Dielectric loaded resonators, by virtue of their high sensitivity, are a de facto standard for the measurement of the microwave surface impedance Z of good conductors, including superconductors. However, new perspective applications of superconductors request measurements in increasingly extreme conditions: cryogenic and magnetic environments, often combined together. Careful evaluations of the dielectric resonator (DR) design are necessary to operate in these conditions. Moreover, the evaluation of the uncertainties and the issues related to an appropriate calibration become problematic. We present here a performance analysis of DRs designed for cryogenic temperature and high magnetic field operation and some sample experimental results on Z measurements on the most technologically interesting superconductors.

We observe a strong reduction of the field induced thin film surface resistance measured at high microwave frequency $(\nu =47.7\mathrm{GHz})$ in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ thin films grown on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates, as a consequence of the introduction of submicrometric $\mathrm{Ba}\mathrm{Zr}{\mathrm{O}}_3$ particles. The field increase of the surface resistance is smaller by a factor of $\sim 3$ in the film with $\mathrm{Ba}\mathrm{Zr}{\mathrm{O}}_3$ inclusions, while the zero-field properties are not much affected. Combining surface resistance and surface reactance data, we conclude (a) that $\mathrm{Ba}\mathrm{Zr}{\mathrm{O}}_3$ inclusions determine very deep and steep pinning wells and (b) that the pinning changes nature with respect to the pure film.

The hypothetical axion particle is an excellent cold dark matter candidate. Models predict an axion mass of tens of μeV (GHz frequency range). It is in principle possible to convert galactic axions into a monochromatic microwave signal or to detect them with an electron-spin flip. In both cases, to measure the resulting vanishing signal ( $P\approx10^{-23} W$ ) a high-Q cavity is needed, with $Q\sim10^6$ . Superconducting RF technology can satisfy the requirement of operation in moderate-to-high DC magnetic fields. In this contribution we present an experimental study of the behavior of superconducting cavities in the vortex state. This study has been realized using 14 GHz Cu-NbTi and $Cu-NbTiN_x$ film cavities. A new cavity geometry is presented. We measured the magnetic field dependence of the Q-factor and $\Delta f_{res}/f_{res}$ where $f_{res}$ is resonant frequency, in the range B=0-6 T at 4.2 K. Also Q versus temperature at fixed B in ZFC and FC setups has been considered. A comparison was made with Cu and Nb cylindrical bulk cavities. The data for Q(B) were analyzed in terms of the elastic and dissipative motion of flux lines, thus including flux flow and flux pinning. We discuss whether the field-induced quasiparticle contribution is relevant in our field and temperature range.

The high frequency vortex motion in Nb ₃ Sn was analyzed in this work up to 12 T. We used a dielectric loaded resonator tuned at 15 GHz to evaluate the surface impedance Z of a Nb ₃ Sn bulk sample (24.8 at.%Sn). From the field induced variation of Z , the high frequency vortex parameters (the pinning constant k _p , the depinning frequency ν _p and the flux flow resistivity ρ _ff ) were obtained over a large temperature and field range; their field and temperature dependencies were analyzed. Comparison with other superconducting materials shows that high frequency applications in strong magnetic fields are also feasible with Nb ₃ Sn. In the present work, we report the first measurements about the microwave response in Nb ₃ Sn in strong magnetic fields.