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We present the multifrequency measurements of the surface resistance of spark-plasma-sintered MgB2 performed through a dielectric loaded resonator operating at 16.5 and 26.7 GHz. By normally applying magnetic fields ≤1.2 T to the sample surface, we drove it in the mixed state. By means of data-rooted analysis, we found that the sample vortex dynamics could be fully described within a single-component approach. Pinning phenomena were present and characterized by a depinning frequency smaller than the measurement ones. The multiband nature of the superconductor emerged in the flux-flow resistivity, whose field dependence could be interpreted well within theoretical models. By exploiting them, the upper critical field was extracted in the low-temperature range, which exhibited a consistent temperature trend with the values obtained at the onset of the resistive transition near Tc, and was well in line with literature data on other polycrystalline samples.

Particle physics and radio-frequency (RF) superconductivity have driven each other on since the 1970s. The unique properties of superconductors (SC) have been the enabling keys for the realization of accelerators with always increased performances thanks to the realization of all-superconducting cavities. The use of increasingly pure superconducting coatings for accelerating cavities, with lower and lower RF losses, determined such high-quality factors [1] that the need to operate at low temperatures (below the superconducting transition temperature $T_{c}$ ) was well paid for. Recently, with respect to the path followed by the high frequency superconductivity [2], a new field opened since SCs are being considered for GHz operation in high dc magnetic fields, and measurements (and optimization) of totally different quantities are needed. The possibility of successfully using SCs in high magnetic fields for these purposes is far from obvious and it depends on the outcome of accurate measurements of usually overlooked quantities.

We present the multifrequency measurements of the surface resistance of spark-plasma-sintered MgB2 performed through a dielectric loaded resonator operating at 16.5 and 26.7 GHz. By normally applying magnetic fields ≤1.2 T to the sample surface, we drove it in the mixed state. By means of data-rooted analysis, we found that the sample vortex dynamics could be fully described within a single-component approach. Pinning phenomena were present and characterized by a depinning frequency smaller than the measurement ones. The multiband nature of the superconductor emerged in the flux-flow resistivity, whose field dependence could be interpreted well within theoretical models. By exploiting them, the upper critical field was extracted in the low-temperature range, which exhibited a consistent temperature trend with the values obtained at the onset of the resistive transition near Tc, and was well in line with literature data on other polycrystalline samples.

The inkjet printing for flexible electronics is an emerging technology that is continuously expanding in different fields, such as healthcare, sports, space science, and, in general, where the traditional rigid electronics is not adequate. In this paper, the design and test of an inkjet-printed device on a flexible substrate are presented. The device is an interdigital capacitor (IDC) fabricated by deposition of conductive ink on a 127 mum-thick polyimide (Kapton) film. First, the electrical characterization of the substrate material is presented. The obtained results are then used for the IDC design. Finally, the prototype is fabricated by means of an inkjet printer and tested in a frequency range from 1 GHz to 5 GHz.

The surface impedance of superconductors in function of intense (~10 T) static magnetic field at varying orientations with respect to the crystallographic axes is of large interest for both characterization purposes, in view of the material engineering, and from the point of view of devices to be used in fundamental physics experiments. A varying field orientation is customarily obtained with mechanically rotating magnets, whose complexity limits the attainable maximum fields to a few T. Here we propose a special proof of concept design for a measuring cell in which the magnetic field orientation is fixed, while the sample orientation can be changed through its incorporation in a rotating portion of the cell. The main design choices are thus proposed for the measuring cell: a cylindrical dielectric loaded resonator, to be used within the surface perturbation approach for the measurement of the surface impedance of flat samples with typical size 10×10 mm2. The main focus in this preliminary work is put on the sensitivity attainable on the surface resistance, i.e. the real part of the surface impedance.

Nb 3 Sn is a superconductor of great relevance for perspective radiofrequency (RF) applications. We present for the first time surface impedance (Z9) measurements at 15 GHz and low-RF field amplitude on Nb 3 Sn in high magnetic fields up to 12 T, with the aim of increasing the knowledge of Nb 3 Sn behavior in such conditions. Z 9 is a fundamental material parameter that directly gives useful information about the dissipative and reactive phenomena when the superconductor is subjected to high-frequency excitations. Therefore, we present an analysis of the measured Z 9 with the aim of extracting interesting data about pinning in Nb 3 Sn at high frequencies. From Z 9 , we extract the vortex motion complex resistivity to obtain the r-parameter and the depinning frequency up in high magnetic fields. The comparison of the results with the literature shows that the measured up on bulk Nb 3 Sn is several times greater than that of pure Nb. This demonstrates how Nb 3 Sn can be a good candidate for RF technological applications, also in high magnetic fields.

The wide application of the modern resonant measurement techniques makes all the steps of the measuring process, including data acquisition more efficient and reliable. Here we investigate the multidimensional space of the parameters to determine the optimum span for resonant measurements. The study concentrated on experimental systems with standard performance and capabilities. We determine the range of the optimum span for the resonant frequency and quality factor by simulating and fitting resonant curves with different levels of asymmetry., Comment: 7 pages, 5 figures, Accepted to IEEE I2MTC 2021 conference

We present numerical and experimental results aimed at reducing the uncertainties in measurements of the quality factor and resonance frequency of microwave resonators. We focus on the optimization of the frequency span and on the role of uncalibrated transmission lines. We find that optimal frequency spans are different for measurements of the quality factor and of the resonance frequency. The uncertainty introduced by uncalibrated lines, unavoidable in particular kinds of measurements, is experimentally determined. We give practical indications on best compromises for a wide range of quality factors.

High frequency applications of superconductors in a dc magnetic field rely on the estimate of the characteristic crossover frequency ν c between low and high losses. Customarily, high sensitivity resonant techniques, intrinsically operating at discrete frequencies, are used to estimate ν c . We exploit here a method based on a dual frequency resonator. We show that single-frequency evaluations of ν c lead to heavy underestimations of the superconductor surface resistance. We describe a combined analytical and experimental approach that gives more accurate estimates for ν c , and additionally allows to test the underlying physical model.

We present high frequency measurements of the vortex dynamics of a FeSe$_x$Te$_{1-x}$ ($x=0.5$) thin film grown on a CaF$_2$ substrate and with a critical temperature $T_c\simeq18\;$K, performed by means of a dual frequency dielectric resonator at 16.4 GHz and 26.6 GHz. We extract and discuss various important vortex parameters related to the pinning properties of the sample, such as the characteristic frequency $\nu_c$, the pinning constant $k_p$ and the pinning barrier height $U$ relevant for creep phenomena. We find that the vortex system is in the single-vortex regime, and that pinning attains relatively high values in terms of $k_p$, indicating significant pinning at the high frequencies here studied. The pinning barrier energy $U$ is quite small and exhibits a non-monotonous temperature dependence with a maximum near 12 K. This result is discussed in terms of core pinning of small portion of vortices of size $\propto\xi^3$ jumping out of the pinning wells over very small distances, a process which is favoured in the high frequency, short ranged vortex oscillations here explored., Comment: 18 pages, 7 figures

Anisotropy is an intrinsic factor that dictates the magnetic properties of YBa2Cu3O7−δ and therefore has a great impact on many applications. Artificial pinning centres are often introduced in an attempt to mitigate its effect, resulting in less anisotropic electrical and magnetic properties. However, the nanoengineering of superconductors makes the quantification of the anisotropy itself uncertain: the intrinsic anisotropy due to the layered structure, quantified by the anisotropy factor γ, mixes with the additional anisotropy due to pinning. As a consequence, there is no consensus on the experimental anisotropy factor γ that can result in YBa2Cu3O7−δ when directional (twin planes, nanorods) or isotropic defects are present. We present measurements of the magnetic field and angular dependent surface impedance in very different nanostructured YBa2Cu3O7−δ films, grown by a chemical route and by pulsed laser deposition with different kind of defects (nanorods, twin planes, nanoparticles). We show that the surface impedance measurements are able to disentangle the intrinsic anisotropy from the directional pinning anisotropy, thanks to the possibility of extracting the true anisotropic flux–flow resistivity and by correctly exploiting the angular scaling. We find in all films that the intrinsic anisotropy γ = 5.3 ± 0.7. By contrast, the pinning anisotropy determines a much more complex, feature– rich and nonuniversal, sample–dependent angular landscape., This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom Research and Training Programme 2014-2018 and 2019-2020 under grant agreement No 633 053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. We thank Benedetta Belli and Raffaella Rogai for help in data acquisition. We acknowledge financial support from Spanish Ministry of Economy and Competitiveness through the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496), SUMATE project RTI2018-095853-B-C21, co-financed by the European Regional Development Fund, COST-Nanohybri CA16218, DWARFS project (MAT2017-83468-R) and Catalan Government with 2017-SGR-1519 and XRE4S.This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom Research and Training Programme 2014-2018 and 2019-2020 under grant agreement No 633 053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. We thank Benedetta Belli and Raffaella Rogai for help in data acquisition. We acknowledge financial support from Spanish Ministry of Economy and Competitiveness through the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496), SUMATE project RTI2018-095853-B-C21, co-financed by the European Regional Development Fund, COST-Nanohybri CA16218, DWARFS project (MAT2017-83468-R) and Catalan Government with 2017-SGR-1519 and XRE4S.

In this paper we present a short overview on the results that can be obtained through the study of vortex motion at high frequencies. The phenomenological force balance for isolated-like vortices shaken by microwave currents and subjected to viscous drag, pinning forces and thermal creep is recalled and physically presented. The derived vortex motion resistivity, together with the main vortex parameters (viscosity η and flux flow resistivity p ff, pinning constant kp, creep factor χ), is then commented. Sample measurements are reported to illustrate the main aspects of the involved physical models.

In high frequency applications of superconductors, a relevant parameter used to synthetically describe their performances in terms of surface resistance (proportional to power losses) dependence on the operating frequency is the so-called pinning frequency νp. Customarily, high sensitivity resonant techniques, intrinsically operating at discrete frequencies, are used to estimate νp. By exploiting a dual frequency resonator, we show here that the inaccurate evaluation of νp in the common approach based on single-frequency measurements leads to heavy underestimations of the superconductor surface resistance.

We present here a microwave (16 GHz) investigation of the complex surface impedance in a dc magnetic field (up to 1 T) of Fe(Se,Te) thin ( 300 nm) films. We derive the vortex parameters: the flux-flow resistivity yields information on the dynamics of the quasiparticles, the Labusch parameter yields a measure of the steepness of the pinning potential wells, and the depinning frequency assesses the frequency range where the material is suitable for high-frequency applications in a dc magnetic field. We compare the results to the data obtained in YBCO, Nb and Nb3Sn.

3D-printer materials are becoming increasingly appealing, especially for high frequency applications. As such, the electromagnetic characterisation of these materials is an important step in evaluating their applicability for new technological devices. We present a measurement method for complex permittivity evaluation based on a dielectric loaded resonator (DR). Comparing the quality factor Q of the DR with a disk-shaped sample placed on a DR base, with Q obtained when the sample is substituted with an air gap, allows a reliable determination of the loss tangent.

Demanding microwave applications in a magnetic field require the material optimization not only in zero-field but, more important, in the in-field flux motion dominated regime. However, the effect of artificial pinning centers (APC) remains unclear at high frequency. Moreover, in coated conductors the evaluation of the high frequency material properties is difficult due to the complicated electromagnetic problem of a thin superconducting film on a buffered metal substrate. In this paper we present an experimental study at 48 GHz of 150-200 nm YBa$_2$Cu$_3$O$_{7-x}$ coated conductors, with and without APCs, on buffered Ni-5at%W tapes. By properly addressing the electromagnetic problem of the extraction of the superconductor parameters from the measured overall surface impedance $Z$, we are able to extract and to comment on the London penetration depth, the flux flow resistivity and the pinning constant, highlighting the effect of artificial pinning centers in these samples., 5 pages, IEEE Trans. Appl. Supercond., accepted for publication (2019)

We present an extended study on the uncertainty in resonant measurements. The uncertainty of the resonant frequency and quality factor was estimated. The effect of the use of uncalibrated resonant curve on uncertainty was extensively studied. For the uncalibrated data the systematic contribution to uncertainty was determined.

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.

3D-printer materials are becoming increasingly more appealing also for high frequency applications. Thus, the electromagnetic characterization of these materials is an important step in order to evaluate their applicability in new technological devices. We present a measurement method for the loss tangent evaluation based on a dielectric loaded resonator (DR). Comparing the quality factor Q of the DR with a disk-shaped sample placed on a DR base with Q obtained when the sample is substituted with an air gap, allows a reliable loss tangent determination.

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 $\nu_p$ and the flux flow resistivity $\rho_{ff}$) were obtained over a large temperature and field range; their field and temperature dependence 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., Comment: 24 pages, 12 figures

We present an extended study on the uncertainty in resonant measurements. The uncertainty of the resonant frequency and quality factor was estimated. Two different measurement systems and different fitting approaches were used. The effect of the use of uncalibrated resonant curves on uncertainty was extensively studied. For the uncalibrated data the systematic contribution to the uncertainty was determined.

YBa2Cu3O7−x (YBCO) nanocomposites for wire applications need to operate in a broad range of frequencies, ranging from dc in magnets to GHz in cavities and screenings of future particle accelerators. We have investigated the in-field and angular vortex pinning performance in dc and at 50 GHz of two types of nanocomposites, pulsed laser deposition (PLD) YBCO with mixed Ba2YNbO6 + Ba2YTaO6 (BYNTO) nanorods and chemical solution deposited (CSD) YBCO with BaHfO3 (BHO) nanoparticles (NPs), and the pristine counterpart films, grown on top of single-crystalline substrates. Transport measurements performed up to 9 T between 5 and 77 K show that CSD nanocomposites exhibit a smooth field decay and increased single-to-collective crossover field H* compared to pristine samples, associated to the enhanced isotropic pinning contribution induced by the NPs, while PLD films exhibit unchanged H* and superior critical current densities up to higher irreversibility fields, associated to the anisotropic contribution introduced by the rods. Microwave in-field measurements of the pinning constant kp revealed CSD NCs exhibit a qualitatively similar, but smoother kp(H) than pristine samples, whereas for PLD samples, a growing kp(H) dependence is observed as a result of the increased relevance of the stiffness of the fluxons pinned by nanorods., This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom Research and Training Programme 2014–2018 and 2019–2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. We acknowledge financial support from Spanish Ministry of Economy and Competitiveness through the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496), SUMATE project RTI2018-095853-B-C21, cofinanced by the European Regional Development Fund, COST-Nanocohybri CA16218, DWARFS project (MAT2017-83468-R) and Catalan Government with 2017-SGR-1519 and XRE4S.

MgB2 has great potential for many applications, thanks to its relatively high critical temperature and low fabrication cost. Large efforts are done to improve the current carrying capabilities of bulks and tapes in view of different application fields, e.g. with the addition of Te and cubic-BN to MgB2. To elucidate the vortex pinning physics exploiting a different dynamic regime, we present here a microwave study of the pinning properties of spark plasma sintered bulk MgB2 with and without the addition of 0.01 % at. Te or cubic-BN. We show the surface resistance Rs of the MgB2 samples measured with a dielectric-loaded resonator at ~ 16.5 GHz and ~ 26.7 GHz in the 10 K-Tc temperature range at fields up to 1.0 T. Then, the MgB2 Rs is studied with high frequency vortex motion models in order to obtain the pinning constant (Labusch parameter) and the depinning frequency. Finally, the microwave behavior of MgB2 in the mixed state is compared with the recent results obtained on Nb3Sn.

We present a study of the uncertainties on the measures of the surface resistance, obtainable with different measurement methods based on the use of dielectric-loaded resonators. We consider the so-called round robin rotation method versus the single measurement technique and we evaluate the respective uncertainties. This comparison is done in the case of samples with surface resistance near that of the resonant cavity walls. We establish the limits of application of the round robin technique versus those of the single measurement.

In this article we present a study about using VNA calibrated or uncalibrated measurements for the purpose of measuring the Q–factor of a dielectric loaded microwave resonator. We evaluate the uncertainties in Q involved in both methods. We use a complete and robust transmission resonant model for the fit and parameter extraction. The Q values obtained from uncalibrated and calibrated data were compared by taking into account the S-parameter measurement uncertainty.

We studied pinning characteristics of YBa2Cu3O7−x (YBCO) with 5 mol.% Ba2YTaO6/Ba2YNbO6 (BYNTO). In contrast to widely used d.c methods, at microwaves very short oscillations of the fluxons are involved giving the information on both the pinning strength, through the pinning constant and on dissipation, related to the vortex core physics, through the viscosity η. We estimate the pinning constant kp , the vortex viscosity η and also the maximum creep factor χmax. The results were compared not only with the pristine YBCO sample but also with the effect of another type of columnar defects (BZO) in YBCO. All film samples were grown by Pulsed Laser Deposition (PLD) technique at nearly the same conditions to ensure the uniform. We detect an improvement of all pinning characteristics of YBCO-BYNTO sample in the temperature range 62 - 90 K and for field up to 0.75 T. While the average diameter of the defects is similar for YBCO-BYNTO and YBCO-BZO the key of the superior characteristics in YBCO-BYNTO could be both the concentration of columns and an addition of another phase of point-like defects.