Vortex pinning and flux flow microwave studies of coated conductors

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.Comment: 5 pages, IEEE Trans. Appl. Supercond., accepted for publication (2019

Frequency span optimization for asymmetric resonance curve fitting

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 conferenc

Surface impedance measurements on Nb3_{3}Sn at high magnetic fields

Nb$_{3}$Sn is a superconductor of great relevance for perspective RF applications. We present for the first time surface impedance $Z_s$ 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. $Z_s$ 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 $Z_s$ with the aim of extracting interesting data about pinning in Nb$_{3}$Sn at high frequencies. From $Z_s$ we extract the vortex motion complex resistivity to obtain the $r$-parameter and the depinning frequency $\nu_p$ in high magnetic fields. The comparison of the results with the literature shows that the measured $\nu_p$ 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.Comment: ASC 2018 conference, accepted in IEEE Trans Appl Supercon

Pinning, flux flow resistivity and anisotropy of Fe(Se,Te) thin films from microwave measurements through a bitonal dielectric resonator

We report on the anisotropy of the vortex motion surface impedance of a \fst thin film grown on a CaF$_2$ substrate. The dependence on the magnetic field intensity up to 1.2 T and direction, both parallel and perpendicular to the sample $c$-axis, was explored at fixed temperature at two distinct frequencies, $\sim16\;$GHz and $\sim27\;$GHz, by means of bitonal dielectric resonator. The free flux flow resistivity $\rho_{ff}$ was obtained by exploiting standard models for the high frequency dynamics, whereas the angle dependence was studied in the framework of the well known and widely used Blatter-Geshkenbein-Larkin (BGL) scaling theory for anistropic superconductors. Excellent agreement with the scaling law prescription by the fluxon flux flow resistivity was obtained. From the scaling analysis, a low-field mass anisotropy $\sim1.8$ was obtained, well within the value ranges reported in literature. The angular dependence of the pinning constant suggests that pinning is dominated by random, isotropic point pins, consistently with critical current density measurements.Comment: 5 pages, 5 figures, conference ASC202

Pinning properties of FeSeTe thin film through multifrequency measurements of the surface impedance

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 figure

Microwave measurements of the high magnetic field vortex motion pinning parameters in Nb3_3Sn

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 figure

Microwave investigation of pinning in Te-and cubic-BN-added MgB2

Abstract 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

Experimental microwave properties of innovative superconductors

This thesis summarizes a three-year experimental study. The work is focused on the investigation of the high frequency electrical transport properties of complex superconducting systems. The experimental part included the development, setup and testing of new experimental systems, the improvement of the performance of the existing setups. First, a new dielectric resonator (DR), which operates on the TE011 mode at 8.2 GHz was designed and put into operation. Second, the existing system, based on a sapphire DR excited on the TE011 at the frequency 48 Hz was upgraded for transmission measurements. Third, the design of a rectangular DR for the study of anisotropic properties was investigated and a prototype was tested at low temperature. Two classes of complex superconducting structures were then studied: ferromagnetic/superconductor/ferromagnetic (S/F/S) Nb/Pd0.81Ni0.19/Nb multilayers, driven by the interest in the phenomena occurring due to the interaction of ferromagnetism and superconductivity, and YBa2Cu3O7-x (YBCO) samples with nanosize BaZrO3 (BZO) columnar pinning centers because of the interest in the potential applications. The investigation of Nb/PdNi/Nb and bulk Nb film samples with different PdNi layer thickness in the temperature range T=2.4-15 K and magnetic field range H=0- 3 T was performed by combining wideband (2-20 GHz) and resonant (8 GHz) measurements. The temperature dependencies of the surface resistivity and of the London penetration depth were measured, and the role of structural disorder was assessed by EXAFS spectroscopy. The evolution of the field dependence of the finite resistivity due to the free flow of magnetic flux lines was studied and compared with present theories. A previously unreported field dependence was detected and characterized for various ferromagnetic layer thickness. It was, finally, determined that for the samples with larger ferromagnetic layer, a reduction of the superfluid density induces a reduction in magnetic fields lines pinning and points to a reduction of the superfluid. YBCO/BZO samples in the temperature range T=60-120 K in magnetic field up to 0.8T were studied using resonator technique at 48 GHz. Samples prepared using different growing techniques (Pulsed laser deposition method, PLD, and Metalorganic decomposition method, MOD, were studied). The effect of the BZO concentration on flux pinning in YBCO samples was studied. The study of the field orientation revealed the flux pinning, important for the reduction of the losses was due to a dynamic (“flux caging”) effect. In summary, it was shown that microwave technique is the effective tool for the study of the microscopic properties of the various superconducting systems, which are important as for the application as for understanding of the nature of the superconductivity

Surface impedance measurements in thin conducting films: Substrate and finite-thickness-induced uncertainties

Precise measurements of the surface impedance Zsof conducting, semiconducting and superconducting materials is a common requirement for research, metrology and industry. The interest is often devoted to thin films because of (i) the possibility to grow nearly-perfect (single-crystalline) materials, essential for research and selected applications, or (ii) ease of reproducibility for, e.g., metrological standard, or (iii) direct applications of thin films in the electronic industry. However, in finite-thickness films the probing electromagnetic field does not vanish in the volume, and a leakage arises. Thus, the substrate where the film is grown gives a substantial contribution to the measured surface impedance. While the electromagnetic problem is well known, analyses in terms of the evaluation of the uncertainty involved are scarce. It is then accepted that microwave measurements are affected 'necessarily' by some uncertainties that cannot be easily evaluated. In this paper we report an extensive numerical study of the commonly used approximations of the full electromagnetic expression for the surface impedance of finite-thickness (super)conducting films backed by insulating substrates. We take into account the most common simplified expressions, and we estimate the uncertainties involved as a function of the thickness and of the complex conductivity of the film. We find that ranges in frequency and film thickness exist, where the use of approximate expressions gives rise to a negligible error. However, the identification of such combined ranges is not trivial, and it is particularly critical for superconducting films