Ac Response Research Articles

Dynamic crossover across the peak effect in BaK x Fe2As2 superconductor for H-axis and H-plane

The vortex matter properties of a BaK x Fe2As2 single crystal ( K) were studied, by employing both isofield and isothermal ac-susceptibility measurements, , in a wide range of frequencies and amplitudes of the applied ac magnetic field. The irreversibility line (), formally defined by the onset of the third harmonic, is recorded for both -axis and -plane. It can be reproduced from the empirical equation, , with and Tesla, Tesla. The isofield measurements of the first harmonic revealed a narrow diamagnetic peak, related to a local peak of the critical current below the irreversibility line for both -axis and -plane. The local peak for Tesla is transformed to a sudden drop before it completely disappears. Detailed ac-susceptibility measurements were conducted for frequencies ranging within f = 0.1–10 kHz. From these data, the pinning potential, U, is deduced both as function of temperature and dc magnetic field. These results revealed that the ac response of vortex matter exhibits three distinct dynamic behaviors. By employing a model proposed by Mikitik and Brandt (2001 Phys. Rev. B 64 184514), that is based on a Lindemann type criterion and the collective pinning theory, we reproduced the experimentally recorded vortex matter phase diagram by taking into account both thermal fluctuations and random point disorder. To this effect, we adopted a pinning mechanism, , = 1.1–1.5 and a Ginzburg number .

Ac losses in field-cooled type I superconducting cavities

As superconductors are cooled below their critical temperature, stray magnetic flux can become trapped in regions that remain normal. The presence of trapped flux facilitates dissipation of ac current in a superconductor, leading to losses in superconducting elements of microwave devices. In type II superconductors, dissipation is well-understood in terms of the dynamics of vortices hosting a single flux quantum. In contrast, the ac response of type I superconductors with trapped flux has not received much attention. Building on Andreev’s early work (Andreev 1967 Sov. Phys. JETP 24 1019), here we show theoretically that the dominant dissipation mechanism is the absorption of the ac field at the exposed surfaces of the normal regions, while the deformation of the superconducting/normal interfaces is unimportant. We use the developed theory to estimate the degradation of the quality factors in field-cooled cavities, and we satisfactorily compare these theoretical estimates to the measured field dependence of the quality factors of two aluminum cavities. We also identify a regime in which the dissipated power depends weakly on the Ginzburg-Landau parameter; this makes it possible to apply our findings to cavities made of other materials, such as niobium.

A Personal, Portable Electric Circuit Lab

A portable setup consisting of a USB oscilloscope, an Arduino board and accessories is used for performing electricity experiments encountered in physics education: the AC response of circuits, charging/discharging of capacitors as well as inductive coupling between circuits. For documentation, analysis and plotting of the data, Jupyter notebooks are used as electronic lab-books. A range of typical experiments is presented in order to show the versatility of the setup and the lab-book. We strongly argue in favor of such portable measurement solutions in order to give students the possibility to do in-depth experiments outside the traditional laboratory classes.

Optical spin transport theory of spin- 12 topological Fermi superfluids

We theoretically investigate optical (frequency-dependent) bulk spin transport properties in a spin-1/2 topological Fermi superfluid. We specifically consider a one-dimensional system with an interspin {\it p}-wave interaction, which can be realized in ultracold atom experiments. Developing the BCS-Leggett theory to describe the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensate (BEC) evolution and the $\mathbb{Z}_2$ topological phase transition in this system, we show how the spin transport reflects these many-body aspects. We find that the optical spin conductivity, which is a small AC response of a spin current, shows the spin gapped spectrum in the wide parameter region and the gap closes at $\mathbb{Z}_2$ topological phase transition point. Moreover, the validity of the low-energy effective model of the Majorana zero mode is discussed along the BCS-BEC evolution in connection with the scale invariance at {\it p}-wave unitarity.

Quantitative Mapping of Unmodulated Temperature Fields with Nanometer Resolution.

Quantitative mapping of temperature fields with nanometric resolution is critical in various areas of scientific research and emerging technology, such as nanoelectronics, surface chemistry, plasmonic devices, and quantum systems. A key challenge in achieving quantitative thermal imaging with scanning thermal microscopy (SThM) is the lack of knowledge of the tip-sample thermal resistance (RTS), which varies with local topography and is critical for quantifying the sample temperature. Recent advances in SThM have enabled simultaneous quantification of RTS and topography in situations where the temperature field is modulated enabling quantitative thermometry even when topographical features cause significant variations in RTS. However, such an approach is not applicable to situations where the temperature modulation of the device is not readily possible. Here we show, using custom-fabricated scanning thermal probes (STPs) with a sharp tip (radius ∼25 nm) and an integrated heater/thermometer, that one can quantitatively map unmodulated temperature fields, in a single scan, with ∼7 nm spatial resolution and ∼50 mK temperature resolution in a bandwidth of 1 Hz. This is accomplished by introducing a modulated heat input to the STP and measuring the AC and DC responses of the probe's temperature which allow for simultaneous mapping of the tip-sample thermal resistance and sample surface temperature. The approach presented here─contact resistance resolved scanning thermal microscopy (CR-SThM)─can greatly facilitate temperature mapping of a variety of microdevices under practical operating conditions.

Synthesizing Electrically Equivalent Circuits for Use in Electrochemical Impedance Spectroscopy through Grammatical Evolution

Electrochemical impedance spectroscopy (EIS) is an important electrochemical technique that is used to detect changes and ongoing processes in a given material. The main challenge of EIS is interpreting the collected measurements, which can be performed in several ways. This article focuses on the electrical equivalent circuit (EEC) approach and uses grammatical evolution to automatically construct an EEC that produces an AC response that corresponds to one obtained by the measured electrochemical process(es). For fitting purposes, synthetic measurements and data from measurements in a realistic environment were used. In order to be able to faithfully fit realistic data from measurements, a new circuit element (ZARC) had to be implemented and integrated into the SPICE simulator, which was used for evaluating EECs. Not only is the presented approach able to automatically (i.e., with almost no user input) produce a more than satisfactory EEC for each of the datasets, but it also can also generate completely new EEC configurations. These new configurations may help researchers to find some new, previously overlooked ongoing electrochemical processes.

Anomalous out-of-phase magnetic ac response in superconducting wires

We describe and explain two competing regimes of ac magnetic response in current-carrying type-II superconducting wires which were observed experimentally. In the usual regime, voltage $V(t)$, induced by vortex motion across the wire, is ``in phase'' with the external magnetic field $H(t)\ensuremath{\propto}sin(\ensuremath{\omega}t)$. However, as frequency $\ensuremath{\omega}$ grows up or transport current $I$ decreases, an anomalous, ``out-of-phase'' peak in $V(t)$ appears. If these two regimes coexist, then two peaks in voltage are observed per half period of $H(t)$ in both experiment and numerical simulations. At certain combinations of $\ensuremath{\omega}, I$ and the amplitude of the external field, the out-of-phase mechanism even overwhelms the usual, in-phase one. It is shown that the out-of-phase maximum in $V(t)$ is due to the inhibition effect of zero-field (annihilation) lines on flux motion. Such lines, if present in the sample, significantly decelerate magnetic relaxation and dramatically affect the induced voltage. A phase diagram enabling one to distinguish between the in-phase and out-of-phase regimes is constructed.

Conceptual framework on teaching capacitors and inductors

The objective of this work is to suggest a conceptual framework on teaching capacitors and inductors in order to improve teaching abilities and to eliminate some fundamental misconceptions. The effort initially deals with resolution of physical structure and structural concepts/parameters of the devices and then resolves DC and AC responses of the inductors and capacitors. The resolutions are achieved by considering basic serious RL and RC circuits and resolving the circuits both theoretically and experimentally. As a result, an alternative teaching approach is offered and also a novel instructional material is developed and suggested. Some teaching clues are briefly inserted to guide teachers/instructors to reach a higher comprehension level. It is believed that the implications of the developed teaching material would be quite high and beneficial.

Analysis and experimental verification of improved two-level converter behaviors under unbalanced AC conditions

An improved two-level converter (I2LC) represents a practical design trade-off between the conventional two-level converter (C2LC) and modular multilevel converter (MMC), in which the MMC like AC and DC fault responses and relative simplicity of C2LC power circuit and control are prioritized. Therefore, this paper presents theoretical analysis, which helps to develop broader understandings of the I2LC, and identification of the needed countermeasures to ensure correct operation during balanced and unbalanced grid conditions. The correctness of the findings and observations drawn from the presented analysis has been validated with the aid of scaled-down simulations and experimentations of the I2LC. Unlike the C2LC, it has been revealed that the I2LC can offer two concurrent operational objectives at system level during unbalanced AC grid conditions, such as balanced output currents and ripple-free DC link current. Further investigation conducted on the performance of the I2LC based two-terminal DC link during asymmetrical AC faults corroborates the theoretical analysis and scaled-down simulation and experimental studies.

A detailed electrochemical analysis of sensitized AISI 410 martensitic stainless steel

In the present work, the effect of sensitization heat treatment on the intergranular corrosion (IGC) behavior of AISI 410 martensitic stainless steel was investigated using potentiodynamic electrochemical impedance spectroscopy (PDEIS) in conjunction with conventional methods including acid etching test, anodic potentiodynamic polarization, double-loop electrochemical potentiodynamic reactivation (DLEPR) and electrochemical noise (EN). Microstructural studies by scanning electron microscopy revealed that increasing tempering time up to 2h would be followed by more severe localized attack, but desensitization state attributed to specimens undergone prolonged tempering cannot be interpreted based on the standard test method. Susceptibility to IGC via DLEPR experiments indicates differences in the degree of sensitization (DOS) of specimens with varying tempering time, revealing a healing effect arising from prolonged tempering. The EN results elucidated the alloy tendency for active dissolution, but no obvious correlation can be established between the EN results and IGC behavior. The PDEIS data clarified that the charge transfer resistance values related to the AC responses in the middle of transpassive potential region can be utilized to investigate and compare the IGC susceptibility of the alloy. Moreover, the surface morphology of corroded samples under such experimental conditions can be used to qualitatively compare the severity of intergranular attack.

Genetic Factors Associated With Pain Severity, Daily Opioid Dose Requirement, and Pain Response Among Advanced Cancer Patients Receiving Supportive Care

BackgroundCurrent understanding of genetic factors associated with pain severity, and improvement of pain with opioids in advanced cancer patients (AC) is inadequate for delivery of personalized pain therapy (PPT). Therefore, the aim of this study was to determine the genetic factors associated with pain severity, daily opioid dose, and pain response in AC patients receiving supportive care. MethodsIn this prospective study, AC patients were eligible if they had cancer pain ≥4/10 on Edmonton Symptom Assessment Scale (ESAS) - Pain Item and needed opioid rotation for pain control by specialist at the outpatient supportive care center. Association of genetic factors with pain phenotype was assessed using logistic regression models and SKATO (Gene-block) analysis. ResultsAbout 174/178 (98%) patient samples were analyzed. After adjustment for demographic and clinical variables, pain severity was negatively associated with intron variant alleles in OPRM1 rs9322446, P = 0.02; rs2270459, P = 0.038; rs62052210, P = 0.038. Opioid daily dose was positively associated NFKBIA rs2233419, P = 0.008; rs2233417, P = 0.007; rs3138054, P = 0.008; rs1050851, P = 0.015; ORPM1 rs9479759, P = 0.046; rs2003185, P = 0.047; rs636433, P = 0.044; COMT (rs9306234, P = 0.014; rs165728, P = 0.014; rs2020917, P = 0.036; rs165728, P = 0.034); ARRB2 (rs1045280, P = 0.045); and pain response to opioids was negatively associated OPRM1 rs1319339, P = 0.024; rs34427887, P = 0.048; and COMT rs4646316, P = 0.03; rs35478083, P = 0.028, respectively. SKATO analysis showed association between pain severity and CXCL8 (P = 0.0056), and STAT6 (P = 0.0297) genes respectively, and pain response with IL-6 (P = 0.00499). ConclusionsThis study identified that SNPs of OPRM1, COMT, NFKBIA, CXCL8, IL-6, STAT6, and ARRB2 genes were associated with pain severity, opioid daily dose, and pain response in AC receiving supportive care. Additional studies are needed to validate our findings for PPT.

Impact on the Conductance Method of the Asymmetry in the AC Response Induced by Interface Trap Levels

The conductance method has been extensively used to analyze the dielectric-semiconductor interfaces by assuming a continuous density of the interface traps. If dangling bonds are dense at the interface, the wavefunction tails of electrons in the dangling bonds overlap each other to form a continuous density of interface traps. However, as the advanced fabrication process improves the interface quality, the interface traps become dilute. Hence, the density of interface traps may be interpreted as a composition of distinct peaks with less overlapping of trapped electrons, i.e., the discrete localized interface traps. In this regard, we assume that peaks apart from the Fermi level more than the thermal energy affect the conductance method. This is a novel approach to the conventional conductance method. Present model derived in this study is applied on the zirconium oxide—silicon interface fabricated using the atomic layer deposition. As result, we successfully reproduced the asymmetrical conductance in frequency spectrum and obtained the distribution of interface trap density with time constants correspondent to the extracted energy levels. Inclusion of the peak levels shows larger fluctuation in time constants, and the proposed method compensates underestimation in the conventional conductance method.

AC conductivity and dielectric studies in polypyrrole wrapped tungsten disulphide composites

Using in situ chemical polymerization method, polypyrrole/tungsten disulphide (PPy/WS2) composites were synthesized in various compositions viz. 10, 20, 30, and 40 wt% of WS2 in polypyrrole. Composites were structurally and morphologically characterized by X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectra, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy techniques. AC response studies of the composites at room temperature were carried out in the frequency range 50 Hz–1 MHz. When compared with pristine PPy, the composites have exhibited enhanced conductivity and the same has been reflected in the complex plane impedance plots. This indicates that the conduction mechanism is due to the hopping of charge carriers. The loss tangent curves of the composites have been explained using Rezlescu model.

DTC based on SVM for induction motor sensorless drive with fuzzy sliding mode speed controller

By using the direct torque control (DTC), robust response in ac drives can be produced. Ripples of currents, torque and flux are oberved in steady state. space vector modulation (SVM) applied in DTC and used for a sensorless induction motor (IM) with fuzzy sliding mode speed controller (FSMSC) is studied in this paper. This control can minimize the torque, flux, current and speed pulsations in steady state. To estimate the rotor speed and stator flux the model reference adaptive system (MRAS) is used that is designed from identified voltages and currents. The FSMSC is used to enhance the efficiency and the robustness of the presented system. The DTC transient advantage are maintained, while better quality steady-state performance is produced in sensorless implementation for a wide speed range. The drive system performances have been checked by using Matlab Simultaion, and successful results have been obtained. It is deduced that the proposed control system produces better results than the classical DTC.

History Erase Effect of Real Memristors

Different from the static (power-off) nonvolatile property of a memristor, the history erase effect of a memristor is a dynamic characteristic, which means that under the excitation of switching or different signals, the memristor can forget its initial value and reach a unique stable state. The stable state is determined only by the excitation signal and has nothing to do with its initial state. The history erase effect is a desired effect in memristor applications such as memory. It can simplify the complexity of the writing circuit and improve the storage speed. If the memristor’s response depends on the initial state, a state reset operation is required before each writing operation. Therefore, it is of great theoretical and practical significance to judge whether the memristor has a history erase effect. Based on the study of the history erase effect of real memristors, this paper focuses on the history erase effect of a Hewlett-Packard (HP) TiO2 memristor and the Self-Directed Channel (SDC) memristor of Knowm Company. The DC and AC responses of the HP TiO2 memristor are given, and it is pointed out that there is no AC history erase effect. However, considering the parasitic memcapacitance effect, it is found that it has the effect. Based on the theoretical model of the SDC memristor, its history erase properties with and without considering parasitic effects are studied. It should be noted that this study method can be useful for other materials such as Al2O3 and MoS2.

The effect of temperature on the structural, dielectric and magnetic properties of cobalt ferrites synthesized via hydrothermal method

Abstract A series of cobalt ferrite nanoparticles were prepared using hydrothermal process by varying the reaction temperature. The structural, magnetic and dielectric properties have been studied with the help of X-ray diffractometer (XRD), vibrating sample magnetometer (VSM) and impedance analyzer respectively. XRD spectra of all samples confirmed the formation of cobalt ferrite (CoFe2O4) nanoparticles (NPs). The lattice constant ‘a’ for temperature series samples is averaged around 8.4023 Å. Crystallite size of temperature series is calculated by Debye–Scherer formula that lies in the range of 15.04–20.49 nm. Its values increase because the chance of coalescence increases by increasing temperature. The maximum packing factor is obtained for the sample with highest reaction temperature. From VSM data, we get the M–H hysteresis curves for complete temperature series which confirms the magnetic nature. The maximum saturation magnetization 150.67 emu/g is obtained for the sample prepared at highest temperature. Different magnetic parameter such as saturation magnetization, coercivity, retentivity, squareness ratio, anisotropy constant and magneton number has been calculated from VSM data. AC response of all prepared ferrites was studied with impedance analyzer of frequency range 20 Hz to 20 MHz. Ferrites are the insulating materials, so, eddy current does not induce in transformer cores made of ferrite materials. In the medical field cobalt ferrite is used for drug delivery, as a biosensor and in MRI.

Crossover of positive and negative magnetoconductance in composites of nanosilica glass containing dual transition metal oxides.

A facile sol–gel approach to prepare composites of nanosilica glass containing dual transition metal oxides with compositions xCoO·(20 − x)NiO·80SiO2 comprising x values 5 (NC-1), 10 (NC-2) and 15 (NC-3) within hexagonal pores of SBA-15 template has been demonstrated. The synergistic effect of dual transition metal oxide ions on MD properties and crossover of positive and negative magnetoconductance phenomena were observed in these nanocomposite systems. The physical origin of magnetoconductance switching is explained based on the factors: nanoconfinement effect, wave-function shrinkage and spin polarized electron hopping. DFT calculations were performed to understand the structural correlation of the nanoconfined system. The static (dc) and dynamic (ac) responses of magnetization revealed the spin-glass behaviour of the investigated samples. Both scaling law and Vogel–Fulcher law provide a satisfactory fit to our experimental results which are considered as a salient feature of the spin-glass system. Our studies indicate the possibilities of fabricating magnetically controlled multifunctional devices.

Structural and magnetic studies of mononuclear lanthanide complexes derived from N-rich chiral Schiff bases

A new family of mononuclear lanthanide complexes with the formula [CeIII(L)(NO3)3(MeOH)] (1) and [LnIII(L)(NO3)3]·MeOH where Ln = Gd (2) or Dy (3) and L = N,N'-bis(pyridin-2-ylmethylene)cyclohexane-1,2-diamine has been obtained with the use of enantiomerically pure Schiff bases. Dynamic magnetic studies indicate that 1-3 present field-induced slow relaxation of the magnetization and their response has been compared with the magnetically diluted complexes 2d and 3d. Structural studies have been carried out by single crystal X-ray and powder diffraction.

Nonlinear electrical transport in Fe3O4-decorated graphene nanoplatelets

This paper investigates the nonlinear properties of graphene nanoplatelets (GNPs), decorated with Fe3O4 magnetic nanoparticles (MaNPs). Nanocomposite MaNP/GNP samples were prepared by a solvothermal method with three different MaNP loading concentrations of 17 wt%, 28 wt% and 40 wt%, and deposited on a metallic interdigitated electrode (IDE). Three different models are proposed to assess measurements, with the objective to explain the electronic transport in the nanocomposites. At first, a thermionic transport model is proposed to fit the DC nonlinear current–voltage characteristics for the three concentrations. It is observed that the barrier height decreases to 0.312, 0.310 and 0.281 eV, following a decrease in the MaNP loading. A second model, dynamic random resistor network (DRRN) further shows that the impedance of IDE increases following the decreasing MaNP loading rate, 40 wt% > 28 wt% > 17 wt%, and that charge transport takes place through a resistor–capacitor (RC) rectifying percolating network. Finally, impedance spectroscopy performed at different applied DC biases shows that a constant phase element (CPE) is necessary in the equivalent circuit in order to fit the Cole–Cole plot AC response of the IDE, instead of the classical parallel RC circuit. The presence of the CPE confirms the hypothesis of random phenomena occurring in the transport according to the DRRN model. CPE is associated with a spatial distribution of different RC circuits, due to disorderness that arises from inhomogeneities in the Fe3O4–GNP samples.

Anisotropic time-dependent London approach: Application to the ac response in the Meissner state

The anisotropic London equations taking into account the normal currents are derived and applied to the problem of the surface impedance in the Meisner state of anisotropic materials. It is shown that the complex susceptibility of anisotropic slab depends on the orientation of the applied microwave field relative to the crystal axes. In particular, the anisotropic sample in the microwave field is subject to a torque, unless the field is directed along with one of the crystal principle axes.