Search

Axions, hypothetical particles theorized to solve the strong CP-problem, are presently being considered as strong candidates as cold dark matter constituents. The signal power of resonant-based axion detectors, known as haloscopes, is directly proportional to their quality factor $Q$. In this paper, the impact of the use of superconductors in the performances of the haloscopes is studied by evaluating the obtainable $Q$. In particular, the surface resistance $R_s$ of NbTi, Nb$_3$Sn, YBa$_2$Cu$_3$O$_{7-\delta}$ and FeSe$_{0.5}$Te$_{0.5}$ is computed in the frequency, magnetic field and temperature ranges of interest, starting from the measured vortex motion complex resistivity and screening lengths of these materials. From $R_s$ the quality factor $Q$ of a cylindrical haloscope with copper conical bases and superconductive lateral wall, operating with the TM$_{010}$ mode, is evaluated and used to perform a comparison of the performances of the different materials. Both YBa$_2$Cu$_3$O$_{7-\delta}$ and FeSe$_{0.5}$Te$_{0.5}$ are shown to improve the measurement sensitivity by almost an order of magnitude with respect to a whole Cu cavity, while NbTi is shown to be suitable only at lower frequencies (<10 GHz). Nb$_3$Sn can give an intermediate improvement in the whole spectrum of interest., Comment: 15 pages, 5 figures, submitted to Instruments journal (published by MDPI)

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 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 ASC2020

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 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

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)

There is a renewed interest in superconductors for high-frequency applications, leading to a reconsideration of already known low-$T_c$ and high-$T_c$ materials. In this view, we present an experimental investigation of the millimeter-wave response in moderate magnetic fields of Tl$_2$Ba$_2$CaCu$_2$O$_{8+x}$ superconducting films with the aim of identifying the mechanisms of the vortex-motion-induced response. We measure the dc magnetic-field-dependent change of the surface impedance, $\Delta Z_s(H) = \Delta R_s(H) + i\Delta X_s(H)$ at 48 GHz by means of the dielectric resonator method. We find that the overall response is made up of several contributions, with different weights depending on the temperature and field: a possible contribution from Josephson or Abrikosov-Josephson fluxons at low fields; a seemingly conventional vortex dynamics at higher fields; a significant pair breaking in the temperature region close to $T_c$. We extract the vortex motion depinning frequency $f_p$, which attains surprisingly high values. However, by exploiting the generalized model for relaxational dynamics we show that this result come from a combination of a pinning constant $k_p$ arising from moderate pinning, and a vortex viscosity $\eta$ with anomalously small values. This latter fact, implying large dissipation, is likely a result from a peculiar microscopic structure and thus poses severe limits to the application of Tl$_2$Ba$_2$CaCu$_2$O$_{8+x}$ in a magnetic field., Comment: Presented at Applied Superconductivity Conference, Seattle (US) 2018. Accepted for publication on IEEE Trans. Appl. Supercond

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 Supercond

Measurements of the high-frequency complex resistivity in superconductors are a tool often used to obtain the vortex parameters, such as the vortex viscosity, the pinning constant and the depinning frequency. In anisotropic superconductors, the extraction of these quantities from the measurements faces new difficulties due to the tensor nature of the electromagnetic problem. The problem is specifically intricate when the magnetic field is tilted with respect to the crystallographic axes. Partial solutions exist in the free-flux-flow (no pinning) and Campbell (pinning dominated) regimes. In this paper we develop a full tensor model for the vortex motion complex resistivity, including flux-flow, pinning, and creep. We give explicit expressions for the tensors involved. We obtain that, despite the complexity of the physics, some parameters remain scalar in nature. We show that under specific circumstances the directly measured quantities do not reflect the true vortex parameters, and we give procedures to derive the true vortex parameters from measurements taken with arbitrary field orientations. Finally, we discuss the applicability of the angular scaling properties to the measured and transformed vortex parameters and we exploit these properties as a tool to unveil the existence of directional pinning., Comment: 21 pages, 3 figures. arXiv admin note: text overlap with arXiv:1402.3164

We report a detailed study performed on La2Zr2O7 (LZO) pyrochlore material grown by Metal-Organic Decomposition (MOD) method as buffer layers for YBa2Cu3O7-x (YBCO) coated conductors. High quality epitaxial LZO thin films have been obtained on single crystal (SC) and Ni-5%at.W substrates. In order to evaluate structural and morphological properties, films have been characterized by means of X-ray diffraction analyses (XRD), atomic force microscope (AFM) and scanning electron microscope (SEM). Precursors solutions and heat treatments have been studied by thermogravimetric analyses (TG-DTA-DTG) and infrared spectra (FT-IR) with the aim of optimizing the annealing process. Thin films of YBCO have been deposited by pulsed laser ablation (PLD) on this buffer layers. The best results obtained on SC showed YBCO films with critical temperature values above 90 K, high self field critical current density values (Jc > 1 MA/cm2) and high irreversibility field values (8.3 T) at 77 K together with a rather high depinning frequency vp (0.5 T, 77 K)>44 GHz as determined at microwaves. The best results on Ni-5%at.W has been obtained introducing in the heat treatment a pyrolysis process at low temperature in air in order to remove the residual organic part of the precursor solution.

We discuss the excess conductivity at nonzero frequencies in a superconductor above T_c within the gaussian approximation. We focus the attention on the temperature range not too close to T_c: within a time-dependent Ginzburg-Landau formulation, we phenomenologically introduce a short wavelength cutoff (of the order of the inverse coherence length) in the fluctuational spectrum to suppress high momentum modes. We treat the general cases of thin wires, anisotropic thin films and anisotropic bulk samples. We obtain in all cases explicit expressions for the finite frequency fluctuational conductivity. The dc case directly follows. Close to T_c the cutoff has no effect, and the known results for Gaussian fluctuations are recovered. Above T_c, and already for epsilon = ln(T/T_c) > 10^{-2}, we find strong suppression of the paraconductivity as compared to the gaussian prediction, in particular in the real part of the paraconductivity. At high epsilon the cutoff effects are dominant. We discuss our results in comparison with data on high-T_c superconductors., Comment: Changed RevTex