Kostiantyn Torokhtii

A rectangular dielectric resonator for measurements of the anisotropic microwave properties in planar conductors

We designed, realized and tested a TE011-mode rectangular, high-permittivity, dielectric resonator, in order to obtain a measuring device capable of applying straight microwave currents to conductive samples. We characterized the prototype by means of extensive simulations that we compared to experimental results. Finally, we tested the device on a strongly anisotropic graphite sample. We found that the rotation of the sample produced significant changes in the response of the device. We conclude that the proposed rectangular dielectric resonator can be used as a device to detect the anisotropy of conductive samples.

Analysis of Transport Properties of MOD YBCO Films With BaZrO3 as Artificial Vortex Pinning Centers

The transport properties of nanocomposite YBa2Cu3O7-x (YBCO) films grown by metal-organic decomposition (MOD) and prepared with a low-fluorine-coating solution on SrTiO3 single-crystal substrates were studied with dc and microwave measurement techniques. As demonstrated in previous studies, the combination of these two techniques is a powerful tool for vortex pinning investigation. The BaZrO3 inclusions (BZO) as artificial pinning centers were introduced into the YBCO matrix by the excess of corresponding Ba and Zr precursor salts to the initial YBCO precursor solution up to 5 mol%. The effect of BZO was clearly observed in magnetic field dependence relations of critical current density (Jc) and the complex surface impedance, i.e., ΔZ, at fixed temperature and magnetic field parallel to the c-axis. The result shows that BZO particles mainly act as isotropic pinning centers. The short-range vortex dynamics was probed by the ~47.7-GHz microwave surface impedance technique. The derived vortex parameters r and kp were sensitive to the presence of nanoparticles, indicating the effect of BZO on pinning. The joint field dependence relations of Jc and kp help in elucidating the pinning regime in our nanocomposite MOD YBCO/BZO films.

Wideband Surface Impedance Measurements in Superconducting Films

We discuss calibration issues in wideband (2-20 GHz) surface impedance measurements in a cryogenic and high magnetic field environment. We show that, in the case of measurements taken on superconducting thin films, the substantial impossibility of a full, three-standard calibration can be partially overcome by exploiting appropriate approximations and the basic properties of superconductors. We discuss a modification of the so-called short-only calibration, in order to increase the accuracy of the measurements. We relate the material properties of a sample under study to the measured surface impedance, which is affected by the geometry of the measurement, and we discuss the effects on the proposed approximate calibration procedure. We experimentally check the applicability of the approximations at the ground of the method, and we thoroughly discuss the uncertainties related to the approximations employed. Finally, we report the sample measurements of the surface impedance of two MgB2 superconducting films.

Measurement of Vortex Pinning in YBCO and YBCO/BZO Coated Conductors Using a Microwave Technique

In order to study the effect of pinning in coated conductors, we have used pulsed laser ablation to grow YBa2Cu3O7-x (YBCO) and YBa2Cu3O7-x/BaZrO3 (YBCO/ BZO) films on metallic templates. Despite the complexity of the structure, after the optimization of the deposition procedure, the YBCO/BZO showed only a moderate reduction of Tc with respect to the pure YBCO sample. We use a very high frequency (~47.7 GHz) dielectric resonator technique in order to induce very short distance vortex shaking, thus probing the steepness of the potential well. We report on measurements of the microwave response of the superconducting structures in terms of the quality factor Q and the frequency resonance ν0 as a function of a moderate dc magnetic field H, at different temperatures T. To keep as much contact as possible to the raw data (Q and ν0), in agreement with a generalized model for vortex dynamics, we determine the pinning parameter r and the upper limit for the creep factor xM. Based on the field dependence of the pinning parameters, the vortex pinning regimes were discussed. We find that the improvement of pinning by BZO is much reduced with respect to the effect observed in films grown on single crystals. In particular, the reduction of the critical temperature must be taken into account in order to appreciate the results.

Analysis, design, realization and test of a sensor network for aerospace applications

A sensor network for aerospace applications is presented. The single node has been designed to follow important requirements in aerospace environments such as low power consumption, lightweight and reliability. Moreover, the network does not use radiofrequency transmitter/receiver to send the information, using infrared technology instead. The network has been designed, realized and tested. Some sample measurements are shown.

Cryogenic microwave wideband measurements of superconducting thin films

We discuss calibration issues in wideband (2-20 GHz) surface impedance measurements in a cryogenic and high-magnetic-field environment. We show that, in the case of measurements taken on superconducting thin films, the substantial impossibility of a full, three-standard calibration can be partially overcome by exploiting appropriate approximations and the basic properties of superconductors. We discuss a modification of the so-called “short-only” calibration, in order to increase the accuracy of the measurements. We check experimentally the applicability of the approximations at the ground of the method, and we report sample measurements of the surface impedance of a MgB2 superconducting film.

Superconductors in a DC magnetic field: Parameters derived from microwave measurements

The behavior of a superconductor in a DC magnetic field is dictated by the dissipative motion of flux quanta. Pinning of flux quanta improve the current carrying of superconductors, so that an essential part of the design of the devices and systems resides in proper modelling of such physical phenomenon. We present a method to determine the physical parameters affecting flux motion in superconducting materials in a DC magnetic field using nondestructive, high-frequency measurements of the field-induced surface impedance ΔZ(H). We discuss the reliability of the measurements of the flux motion parameters when thermal activation is present. Motivated by the extensive study of high-Tc superconductors with engineered, linearly extended pinning centers, we extend the analysis to include anisotropy and anisotropic properties. Finally, we give examples of the method using measurements of ΔZ(H) on YBa2Cu3O7-x-based materials.

Robustness of the 0-π transition against compositional and structural ageing in superconductor/ferromagnetic/superconductor heterostructures

We have studied the temperature induced

Measurement of Vortex Parameters and Pinning in YBa2Cu3O7−x with BaZrO3 Nanoinclusions.

0 -\pi

Critical Current and Vortex Pinning Properties in YBa2Cu3O7-x Films with Ba2YTaO6+Ba2YNbO6 and BaZrO3 Nano-inclusions by dc Transport and Microwave Measurements

thermodynamic transition in Nb/PdNi/Nb Superconductor/Ferromagnetic/Superconductor (SFS) heterostructures by microwave measurements of the superfluid density. We have observed a shift in the transition temperature with the ageing of the heterostructures, suggesting that structural and/or chemical changes took place. Motivated by the electrodynamics findings, we have extensively studied the local structural properties of the samples by means of X-ray Absorption Spectroscopy (XAS) technique, and the compositional profile by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). We found that the samples have indeed changed their properties, in particular for what concerns the interfaces and the composition of the ferromagnetic alloy layer. The structural and compositional data are consistent with the shift of the