Surface Impedance Measurements Research Articles

Acoustical characterization of grass-covered ground

An investigation of acoustical properties of soil covered with live grass is presented. Measurements of normal surface impedance of such samples have been performed over a 200–2000 Hz frequency band using a vertical impedance tube. A phenomenological model is used to predict acoustic properties of the samples. In this work, the samples are considered as a three-component system: soil, grass roots, and foliage. Acoustic impedance of this composite material has components resulting from the different constituent elements. In order to differentiate the acoustic absorption contribution of each element, grass that was controlled for both water content and grass blade height was grown. The acoustic contribution of the soil was determined by performing measurements on unseeded ground with an equivalent watering protocol. Contributions of the roots and foliage were determined by making impedance measurements before and after shearing the mature foliage near the soil surface. The effect of water content in the soil was estimated by making measurements of the samples before and after oven desiccation. We show the effects of roots and foliage on acoustic absorption of grass-covered ground and the acoustic parameters of these complex media estimated using an equivalent fluid model.

Investigation of DC current injection effect on the microwave characteristics of HTS YBCO microstrip resonators

The DC current injection effect from a ferromagnetic (FM) La0.7Sr0.3MnO3 (LSMO) to a high temperature superconducting (HTS) Y1Ba2Cu3O7−x (YBCO) thin film was investigated by the microwave surface impedance measurements in a FM/HTS structure, formed as a microstrip resonator for improving the sensitivity of the experiments. The quality factor and the resonance frequency of this structure were found to strongly depend on the current strength, injected from the LSMO electrode into the HTS microstrip electrode. The magnetic penetration depth and the quasiparticle conductivity of the HTS component were determined to increase under DC current injection process, which in all probability stimulated breaking of Cooper pairs and led to a decrease of the superfluid concentration and an increase of the normal fluid concentration without significantly affecting the relaxation time of the quasiparticles.

Microwave conductivity and superfluid density in strongly overdoped Tl2Ba2CuO6+δ

We present measurements of the microwave surface impedance of the single-layer cuprate Tl${}_{2}$Ba${}_{2}$CuO${}_{6+\ensuremath{\delta}}$, deep in the overdoped regime, with ${T}_{c}\ensuremath{\approx}25$ K. Measurements have been made using cavity perturbation of a dielectric resonator at 17 discrete frequencies ranging from 2.50 to 19.16 GHz, and at temperatures from 0.12 to 27.5 K. From the surface impedance we obtain the microwave conductivity, penetration depth, and superfluid density. The superfluid density displays a strong linear temperature dependence from 2 to 14 K, indicative of line nodes in the energy gap. The microwave data are compared with theoretical predictions for a $d$-wave superconductor with pointlike impurities, with the conclusion that disorder in Tl${}_{2}$Ba${}_{2}$CuO${}_{6+\ensuremath{\delta}}$ acts predominantly in the weak-to-intermediate-strength scattering regime, and that small-angle scattering is important.

Characterizing a porous road pavement using surface impedance measurement: a guided numerical inversion procedure.

This paper deals with a numerical procedure to identify the acoustical parameters of road pavement from surface impedance measurements. This procedure comprises three steps. First, a suitable equivalent fluid model for the acoustical properties porous media is chosen, the variation ranges for the model parameters are set, and a sensitivity analysis for this model is performed. Second, this model is used in the parameter inversion process, which is performed with simulated annealing in a selected frequency range. Third, the sensitivity analysis and inversion process are repeated to estimate each parameter in turn. This approach is tested on data obtained for porous bituminous concrete and using the Zwikker and Kosten equivalent fluid model. This work provides a good foundation for the development of non-destructive in situ methods for the acoustical characterization of road pavements.

On the determination of the quasiparticle scattering rate in unconventional superconductors by microwave surface impedance

As found from numerous microwave experiments on the unconventional Fe-based superconductors, the temperature dependence of the quasiparticle scattering rate τ−1 cannot be accurately described within the framework of standard Drude module in the popular approximation of ωτ ≪ 1, where ω is the signal frequency. To account for the discrepancy, we have extended the classical Drude model for the case of arbitrary values of ωτ, and obtained the expression for τ−1 as a function of experimentally measurable quantities, namely the real and imaginary parts of the microwave surface impedance. We then show the temperature dependence of τ−1 in superconducting Ba(Fe1-xCox)2As2 single crystal pnictide derived from the Ka-band surface impedance measurements within the framework of the modified expression. The measurements indicate the extent to which assumption of ωτ ≪ 1 gives results different from those obtained without this restriction, i.e., incorrect results.

Microwave spectroscopy of vortex dynamics in ortho-II YBa2Cu3O6.52

We present measurements of the vortex-state surface impedance, Z_s = R_s + i X_s, of a high quality, ortho-II-ordered single crystal of the cuprate high temperature superconductor YBa2Cu3O6.52 (T_c = 59K). Measurements have been made at four microwave frequencies (\omega/2\pi = 2.64, 4.51, 9.12 and 13.97 GHz), for magnetic fields ranging from 0 to 7 T. From these data we obtain the field, frequency and temperature dependence of the vortex viscosity, pinning constant, depinning frequency and flux-flow resistivity. The vortex viscosity, \eta(\omega,T), has a surprisingly strong frequency dependence and bears a striking resemblance to the zero-field quasiparticle conductivity, \sigma_qp(\omega,T), suggesting that the dominant dissipative mechanism for the flux lines is induced electric fields coupling to bulk, long-lived d-wave quasiparticles outside the vortex cores. This is in sharp contrast to the conventional Bardeen-Stephen picture, in which dissipation takes place inside quasi-normal vortex cores. The strong frequency dependence of the vortex viscosity in the microwave range requires us to treat it as a complex response function, with an imaginary part that is predicted to contribute to the apparent pinning force on the vortices. Measurements of the frequency dependence of the pinning force confirm that this term is present, and in a form consistent with the requirements of causality. At low temperatures the flux-flow resistivity, \rho_ff \propto 1/\eta, has the form \rho_ff(T) = \rho_0 + \rho_1 \ln(1/T), reminiscent of the DC resistivity of cuprates in the pseudogap regime.

Development of an Anatomically Realistic Forward Solver for Thoracic Electrical Impedance Tomography.

Electrical impedance tomography (EIT) has the potential to provide a low cost and safe imaging modality for clinically monitoring patients being treated with mechanical ventilation. Variations in reconstruction algorithms at different clinical settings, however, make interpretation of regional ventilation across institutions difficult, presenting the need for a unified algorithm for thoracic EIT reconstruction. Development of such a consensual reconstruction algorithm necessitates a forward model capable of predicting surface impedance measurements as well as electric fields in the interior of the modeled thoracic volume. In this paper, we present an anatomically realistic forward solver for thoracic EIT that was built based on high resolution MR image data of a representative adult. Accuracy assessment of the developed forward solver in predicting surface impedance measurements by comparing the predicted and observed impedance measurements shows that the relative error is within the order of 5%, demonstrating the ability of the presented forward solver in generating high-fidelity surface thoracic impedance data for thoracic EIT algorithm development and evaluation.

Surface Impedance Measurement of Resistive Coatings at Microwave Frequencies

A simple approach to estimate the surface impedance of a thin ink deposition on dielectric substrates is described. The measured sample is transversally accommodated in a waveguide, and the scattering parameters are measured. Then, the surface impedance value is recovered by recurring to an efficient transmission-line approach. Closed-form relations are proposed, which allow removing the dispersive effect of the substrate on the estimated surface impedance. The robustness of the retrieving procedure is analyzed, showing that the one based on the transmission coefficient provides more accurate results than the reflection-based one. The positioning of the sample is also investigated, highlighting that the placement inside the waveguide leads to incorrect estimations of the surface impedance with fictitious high imaginary parts because of the presence of air gaps between the sample and the waveguide walls, which transform the uniform layer to a capacitive resistive frequency selective surface. A modified setup with a uniform resistive ink deposition pressed between the waveguide flanges is therefore adopted. Additional experimental results on the effect of the dielectric substrate and on the homogeneity of the ink deposition are reported.

Microwave surface-impedance measurements of the electronic state and dissipation of magnetic vortices in superconducting LiFeAs single crystals

LiFeAs is one of the iron-based superconductors having multiple gaps with the possible sign reversal. To clarify how those novel natures affect the energy dissipation of magnetic vortices, we investigated the microwave surface impedance of LiFeAs single crystals under finite magnetic fields. The flux-flow resistivity enhanced rapidly at low magnetic fields, which is similar to the case of MgB$_{2}$. This is probably the consequence of the multiple-gap nature and the gap anisotropy. This suggest that the sign-reversal is not important for the flux-flow even for multiple-gap superconductors. As for the electronic state, the vortex core of LiFeAs turned out to be "moderately clean". Furthermore, the mean free path inside the vortex core was much shorter than that outside, and was close to the core radius. These results strongly suggest a process specific to the core boundary is important for a scattering mechanism inside the vortex core.

Utilizing a handheld electrode array for localized muscle impedance measurements

Electrical impedance myography (EIM) is a noninvasive technique used for assessment of muscle health in which a high-frequency, low-amplitude electric current is applied to the skin overlying a muscle, and the resulting surface voltage is measured. We have previously used adhesive electrodes, application of which is inconvenient. We present data using a handheld electrode array (HEA) that we devised to expedite the EIM procedure in a clinical setting. Thirty-four healthy volunteers and 24 radiculopathy subjects underwent EIM testing using the HEA and adhesive electrodes. The HEA was shown to have good test-retest reproducibility, with intraclass correlation coefficients as high as 0.99. HEA data correlated strongly with data from adhesive electrodes, ρ = 0.85 in healthy volunteers (P < 0.001) and ρ = 0.75 in radiculopathy subjects (P < 0.001). These data support the potential use of a handheld array for performing rapid localized surface impedance measurements.

Surface impedance measurements of single crystal MgB2 films for radiofrequency superconductivity applications

We report microstructure analyses and superconducting radiofrequency (SRF) measurements of macroscopic scale epitaxial MgB2 films. MgB2 films on 5 cm diameter sapphire disks were fabricated by a hybrid physical chemical vapor deposition (HPCVD) technique. The electron-beam backscattering diffraction (EBSD) results suggest that the film is a single crystal complying with a MgB2(0001)∥ Al2O3(0001) epitaxial relationship. The SRF properties of different film thicknesses (200 and 350 nm) were evaluated under different temperatures and applied fields at 7.4 GHz. A surface resistance of 9 ± 2 μΩ has been observed at 2.2 K.

Microwave surface impedance measurements on reduced graphene oxide

Here we report a non-contact method for microwave surface impedance measurements of reduced graphene oxide samples using a high Q dielectric resonator perturbation technique, with the aim of studying the water content of graphene oxide flakes. Measurements are made before, during and after heating and cooling cycles. We have modelled plane wave propagation of microwaves perpendicular to the surface of graphene on quartz substrates, capacitively coupled to a dielectric resonator. Analytical solutions are derived for both changes in resonant frequency and microwave loss for a range of water layer thicknesses. In this way we have measured the presence of adsorbed water layers in reduced graphene oxide films. The water can be removed by low temperature annealing on both single and multilayer samples. The results indicate that water is intercalated between the layers in a multilayer sample, rather than only being adsorbed on the outer surfaces, and it can be released by applying a mild heating.

IMPEDANCE PREDICTION FOR SEVERAL POROUS LAYERS ON A MOVING PLATE: APPLICATION TO A PLATE COUPLED TO AN AIR CAVITY

This paper presents the problem of modeling poroelastic material on a moving structure. It presents the ambiguity of using estimated or measured surface impedance on a rigid structure as opposed to transfer impedance, which takes into account the movement of the structure to which a porous material is fixed. Biot's model for prediction of surface impedance is presented, and the transfer impedance is defined on the basis of this model. A case where several layers of porous material are fixed to a moving wall is also studied. Finally, it presents the level of agreement with experimental measurements in a coupled fluid–structure problem when an impedance considering a fixed structure and an impedance considering a moving structure are used.

Field-angle dependence of the quasiparticle scattering inside a vortex core in unconventional superconductors

We theoretically investigate the quasiparticle scattering rate Γ inside a vortex core in the existence of non-magnetic impurities distributed randomly in a superconductor. We show that the dependence of Γ on the magnetic field direction is sensitive to the sign of the pair potential. The behavior of Γ is quite different between an s-wave and a d-wave pair potential, where these are assumed to have the same amplitude anisotropy, but a sign change only for the d-wave one. It is suggested that measurements of the microwave surface impedance with changing applied-field directions would be used for the phase-sensitive identification of pairing symmetry.

Microwave surface impedance measurements of LiFeAs, LiFe(As,P) and FeSe1−xTex single crystals

We study microwave surface impedance measurements of LiFeAs, LiFe(As,P) and FeSe1−xTex single crystals. The in-plane penetration depths of LiFeAs and LiFe(As,P) depend on temperature exponentially at low temperatures, which is a strong indication that these two materials have the nodeless superconducting gap. The temperature dependence of the superfluid density indicates that LiFeAs and LiFe(As,P) are multi-gap superconductors with at least two isotropic gaps. In FeSe1−xTex, on the other hand, the quadratic temperature dependence of the in-plane penetration depth appears, which indicates the existence of some sort of quasiparticle excitations. In addtion, the real part of the microwave conductivity exhibits a large enhancement below Tc in all three materials, which results from the increase of the quasiparticle relaxation time, τ, below Tc.

Note: Radio frequency surface impedance characterization system for superconducting samples at 7.5 GHz

A radio frequency (RF) surface impedance characterization (SIC) system that uses a novel sapphire-loaded niobium cavity operating at 7.5 GHz has been developed as a tool to measure the RF surface impedance of flat superconducting material samples. The SIC system can presently make direct calorimetric RF surface impedance measurements on the central 0.8 cm(2) area of 5 cm diameter disk samples from 2 to 20 K exposed to RF magnetic fields up to 14 mT. To illustrate system utility, we present first measurement results for a bulk niobium sample.

Acoustical characterization of perforated facings

An experimental method to estimate the acoustical parameters of perforated facings used for noise control applications is proposed. These perforating facings (also called screens) can be woven or non-woven fabrics or even micro-perforated plates (MPP). Following the work by Atalla and Sgard [J. Sound Vib. 303, 195-208 (2007)], the perforated facings are modeled as porous media composed with identical cylindrical perforations of circular cross-section. The acoustical parameters characterized with the proposed method are the radius of the perforations and the perforation rate (also named the open-porosity). These parameters are obtained from analytical expressions and a single measurement of the normal acoustic surface impedance of the perforated facing backed by an air cavity in a standing wave tube. The value of the static air flow resistivity can also be recovered with no additional assumption or measurement. In the case of a facing that contains perforations of an arbitrary shape, the radius parameter should be understood as a characteristic length of the visco-inertial dissipative effects. Results for two characterization examples (a low porosity screen and a high porosity one) are presented and discussed. Values of the estimated static air flow resistivity are compared with the results from direct measurements. Values of the predicted sound absorption coefficients are compared to the measured ones.

Microwave Surface Impedance Measurements of LiFeAs Single Crystals

We report results of microwave surface impedance measurements of LiFeAs single crystals. The in-plane penetration depth depends on temperature exponentially at low temperatures, which strongly suggests that this material has the nodeless superconducting gap. The temperature dependence of the superfluid density indicates that LiFeAs is a multi-gap superconductor with at least two isotropic gaps. In addtion, the real part of the microwave conductivity exhibits a large enhancement below $T_\mathrm{c}$, indicating that the quasi-particle relaxation time, $\tau$, increases rapidly below $T_\mathrm{c}$. We believe that this enhancement is rather common to all superconductors where an inelastic scattering is dominant above $T_\mathrm{c}$, irrespective of the strength of the electron correlation.

Assessment of Alterations in the Electrical Impedance of Muscle After Experimental Nerve Injury via Finite-Element Analysis

The surface measurement of electrical impedance of muscle, incorporated as the technique of electrical impedance myography (EIM), provides a noninvasive approach for evaluating neuromuscular diseases, including amyotrophic lateral sclerosis. However, the relationship between alterations in surface impedance and the electrical properties of muscle remains uncertain. In order to investigate this further, a group of healthy adult rats, a group of rats two weeks postsciatic crush, and a group of animals six months postcrush underwent EIM of the gastrocnemius-soleus complex. The animals were then killed and the conductivity and permittivity of the extracted muscle measured. Finite-element models based on MRI data were then constructed for each group. The characteristic EIM parameter, 50 kHz phase (±standard error), obtained with surface impedance measurements was 17.3° ± 0.3° for normal animals, 13.8° ± 0.7° for acutely injured animals, and 16.1° ± 0.5° for chronically injured animals. The models predicted parallel changes with phase values of 24.3°, 18.8°, and 21.2° for the normal, acute, and chronic groups, respectively. Other multifrequency impedance parameters showed similar alterations. These results confirm that surface impedance measurements taken in conjunction with anatomical data and finite-element models may offer a noninvasive approach for assessing biophysical alterations in muscle in neuromuscular disease states.

RF Surface Impedance and77Se-NMR Studies of FeSe: Effect of the Sample Quality

We report the results of RF surface impedance measurements and 77 Se-NMR measurements on improved-quality FeSe. The linewidth of 77 Se-NMR spectrum for the improved FeSe is one third of that for FeSe 0.92 , evidencing the homogeneity of the sample. From the field dependence of the R F surface impedance measurements, the upper critical field H c2 (0) is estimated to be ≈26 T. We also measured nuclear spin–lattice relaxation rate 1/ T 1 in the temperature range of 190–2 K under external magnetic fields of 2, 7, and 16 T. In the normal state below 100 K, 1/ T 1 T shows a Curie–Weiss like temperature dependence, suggesting antiferromagnetic spin fluctuations develop below the structural transition. In the superconducting state, 1/ T 1 S measured at H =2, 7, and 16 T exhibit marked decrease with decreasing temperature due to an opening of the superconducting gap.