Ac Response Research Articles

Nonlinear magnetization relaxation of superparamagnetic nanoparticles in superimposed ac and dc magnetic bias fields

The nonlinear ac response of the magnetization $\mathbf{M}(t)$ of a uniaxially anisotropic superparamagnetic nanoparticle subjected to both ac and dc bias magnetic fields of arbitrary strengths and orientations is determined by averaging Gilbert's equation augmented by a random field with Gaussian white-noise properties in order to calculate exactly the relevant statistical averages. It is shown that the magnetization dynamics of the uniaxial particle driven by a strong ac field applied at an angle to the easy axis of the particle (so that the axial symmetry is broken) alters drastically leading to different nonlinear effects due to coupling of the thermally activated magnetization reversal mode with the precessional modes of $\mathbf{M}(t)$ via the driving ac field.

Numerical Analysis of Alternating-Current Small-Signal Response in Graphene Nanoribbons

Alternating-current small-signal admittances of armchair graphene nanoribbons are investigated using the method of non-equilibrium Green's function. The calculated ac admittances show an oscillatory response between inductive and capacitive behaviors, which is a result of the finite length of the graphene nanoribbon. The effects of hydrogen-passivated edges on ac response are demonstrated. At low frequency, the edge effects transform the inductive behavior in a metallic graphene nanoribbon into a capacitive one. Finally, the effects of variations in the width and bandgap of a graphene nanoribbon on its dynamic response are investigated.

Microwave-assisted transport via localized states in degenerately doped Si single electron transistors

Resonant microwave-assisted and dc transport are investigated in degenerately doped silicon single electron transistors. A model based on hopping via localized impurity states is developed and first used to explain both the dc temperature dependence and the ac response. In particular, the non-monotonic power dependence of the resonant current under irradiation is proved to be consistent with spatial Rabi oscillations between these localized states.

Scaling and universality of ac conductivity and dielectric response in disordered materials under pressure

Scaling of physical quantities describing the ac response is a common feature of disordered matter, while in some cases, empirical functions have been proposed. The exact forms of analytic functions that scale complex conductivity and permittivity are derived in this work by exploring the effect of pressure at various temperatures on the ac conductivity of polypyrrole. The scaling behavior is visualized within the frame of percolation and self-similarity.

AC response of 2H–NbSe2 single crystals with electron-irradiation-induced defects

The generation of defects in NbSe2 single crystals by electron irradiation has been investigated by a combination of acsusceptibility and structural measurements. Remarkably, thanks to the layered structure ofNbSe2, we show that electronic irradiation cannot only create point defects butalso in-plane extended defects, which modify anisotropically the ac response.Indeed, the analysis of the onset of the nonlinear susceptibility response,Hacl(T), as a function of irradiation dose and field orientation shows a correlatedincrease in the density of anisotropic defects induced by electron irradiation.Also, we measured a decrease in the strength of the pinning (Labusch) constantαL accounting for elastic vortex oscillations within the linear Campbell regime forhigh-dose-irradiated samples in a transverse field, again compatible with the presenceof planar defects hindering vortex pinning. X-ray powder diffraction and TEMelectron diffraction measurements suggest these in-plane defects may result fromthe rupture of Se–Se bonds and the formation of nanorods and nanowires byNbSe2 sheet rolling.

Time‐Domain Response Simulation of ZnO Varistors by Voronoi Network with an Actual Grain Boundary Model

A special simulation model for the time‐domain responses of ZnO varistors is proposed in this paper. This model is based on the Voronoi network and contains the actual conduction mechanism of the grain boundaries, as well as the capacitances of the grain boundaries. The AC responses of the actual varistor samples were simulated and compared with the experimental results. The simulated waveforms agree with the measured ones very well, which indicates that the simulation model is quite reasonable.

Effective AC response of a nonlinear spherical coated composite

An effective nonlinear response of a nonlinear composite with spherical coated inclusions randomly embedded in a host medium under the action of an external AC electric field, Ea = E1 sin(ωt) + E3 sin(3ωt), is investigated using a perturbation method. The local potentials of the composite at higher harmonics are given both in the region of local inclusion particles and in the local host region under the external AC electric field. All effective nonlinear responses of the composite and the relationship between the effective nonlinear responses at the fundamental frequency and third harmonics are also studied for spherical coated inclusion in a dilute limit.

Study of the electrical properties of bi-layer ion-exchange membrane systems

The electrical properties of ion-exchange membrane systems with a bi-layer distribution of the fixed-charge inside the membrane, have been analyzed by using the network simulation method. The system under study is constituted by an inhomogeneous membrane with a bi-layer distribution of the fixed-charge and two diffusion boundary layers on both sides of the membrane, the ionic transport of a symmetric 1:1 electrolyte being described by the Nernst-Planck and Poisson equations. The steady-state, transient and small-amplitude ac responses of the membrane system, have been obtained. The influence of the degree of inhomogeneity of the membranes on the steady-state current–voltage characteristic, the electrochemical impedance and the chronoamperometric response of the system, has been analyzed and discussed.

Charge and spin dynamics in interacting quantum dots

The transient response of a quantum dot with strong Coulomb interaction to a fast change in the gate potential, as well as the stationary ac response to a slow harmonic variation in the gate potential are computed by means of a real-time diagrammatic expansion in the tunnel-coupling strength. We find that after a fast switching, the exponential relaxation behavior of charge and spin are governed by a single time constant each, which differ from each other due to Coulomb repulsion. We compare the response to a step potential with the $RC$ time extracted from the ac response.

Micromagnetic simulations of spinel ferrite particles

This paper presents the results of simulations of the magnetization field ac response (at 2–12 GHz) of various submicron ferrite particles (cylindrical dots). The ferrites in the present simulations have the spinel structure, expressed here by M 1− n Zn n Fe 2O 4 (where M stands for a divalent metal), and the parameters chosen were the following: (a) for n=0: M={Fe, Mn, Co, Ni, Mg, Cu }; (b) for n=0.1: M = {Fe, Mg} (mixed ferrites). These runs represent full 3D micromagnetic (one-particle) ferrite simulations. We find evidences of confined spin waves in all simulations, as well as a complex behavior nearby the main resonance peak in the case of the M = {Mg, Cu} ferrites. A comparison of the n=0 and n=0.1 cases for fixed M reveals a significant change in the spectra in M = Mg ferrites, but only a minor change in the M=Fe case. An additional larger scale simulation of a 3 by 3 particle array was performed using similar conditions of the Fe 3O 4 (magnetite; n=0, M = Fe) one-particle simulation. We find that the main resonance peak of the Fe 3O 4 one-particle simulation is disfigured in the corresponding 3 by 3 particle simulation, indicating the extent to which dipolar interactions are able to affect the main resonance peak in that magnetic compound.

Magnetic study of low-temperature phase of

Abstract Magnetization and phase sensitive AC susceptibility was measured for the annealed and as-cast samples of ferromagnetic UCu2Ge2, T C = 107 K . Special attention was paid to the low temperature range, T 50 K , which is interesting due to the magnetization decrease and frozen AC response. The hysteresis loops showed a remanence of ≈87% and a fast, thermally activated decrease in the coercivity field. Thermoremanent magnetization at 4.3 K decayed with the relaxation time ≈1∗103 s. The strong increase of the χ A C with the amplitude of the driving field was observed. All these results point to a very large anisotropy of the compound at the lowest temperatures. Phase transition to the randomly canted ferromagnetic phase at T ≈ 55 K was detected in the measurement of the third order non-linear susceptibility. It is concluded that the origin of the transition is the strong temperature dependence of magnetic anisotropy.

Excess Dissipation in a Single-Electron Box: The Sisyphus Resistance

We present measurements of the ac response of a single-electron box (SEB). We apply a radio frequency signal with a frequency larger than the tunneling rate and drive the system out of equilibrium. We observe much more dissipation in the SEB then one would expect from a simple circuit model. We can explain this in terms of a mechanism that we call the Sisyphus resistance. The Sisyphus resistance has a strong gate dependence which can be used for electrometery applications.

AC Response to Gas Exposure in Vertically Aligned Multiwalled Carbon Nanotube Electrode

Vertically aligned multiwalled carbon nanotubes (MWCNTs) as an interdigital electrode were fabricated to investigate AC response to NO2 gas. The response mechanism was explained in terms of the pol...

Electrical Recording of Otoacoustic Emission

Conclusion: Otoacoustic emissions (OAEs) could be detectable as cochlear AC potentials. Spontaneous otoacoustic emissions (SOAEs) were detected either electrically or acoustically, while evoked otoacoustic emissions (EOAEs) could be detected electrically but not acoustically. Objective: Many lines of evidence support the hypothesis that SOAEs are produced by spontaneous mechanical oscillation within the cochlea, and perhaps motile properties of the outer hair cells. If this is the case, SOAEs, emitted acoustically in the external auditory meatus, could also be recorded electrically as cochlear AC potential. EOAE is also thought to be produced by vibration of the basilar membrane, generated by the backward-traveling waves. EOAE thus seems to be detectable electrically as cochlear AC potential. In the present study, SOAE and EOAE were recorded both acoustically and electrically in the guinea pigs to examine the correlation between electrically recorded SOAE (ER-SOAE) and acoustically recorded SOAE (AR-SOAE). In addition, a microphonics response (MPR) to tone pip was recorded to analyze the characteristics of the non-linear component and linear component of the AC responses. Results: (1) In 4 out of 20 guinea pigs (20%), SOAE could be detected both acoustically and electrically. (2) Electrical signals of SOAE had a better S/N ratio than acoustical signals. Generally, only some ER-SOAE could be detected acoustically. (3) Almost without exception, the prominent frequencies of multiple ER-SOAEs corresponded to the intermodulation distortion product, or harmonics. (4) ER-SOAEs were suppressed by hypoxia or intense sound exposure and reappeared upon rebreathing or discontinuation of the external tone. During recovery, prominent frequencies showed a transient downward shift in frequency. (5) SOAEs were synchronized in phase with an external tone in the spectral neighborhood of SOAE. The averaged waveform of SOAE synchronized with the external tone was the same with either acoustic or electrical signals. (6) The MPR to tone pip is composed of two components with different frequency characteristics and input/output functions. (7) The non-linear component delayed to cochlear microphonics was markedly saturated at the intensity level of 40 dB peak equivalent SPL. This component was a phase-lock response, not a frequency-locked one. (8) The non-linear component could be separated with Probst’s non-linear differential extraction technique. In the MPR to a 4-kHz tone pip, high-cut filtration at 3.5 kHz produced a waveform similar to the non-linear component separated by Probst’s method.

Modelling And Simulation of Direct Torque Control For Multi-Phase Switched Reluctance Motor Drive

Switched reluctance motors (SRMs) are rugged, simple in construction and economical compared with synchronous motor and induction motor. They are known to have high-peak torque-to-inertia ratio and the rotor mechanical structure is well suited for high-speed applications. Direct torque control (DTC) is known to produce quick and robust response in AC drives. However, during steady state notable torque, flux and current pulsations occur. They are reflected in speed estimation, speed response and also in increased acoustic noise. This paper presents a novel DTC scheme where fast torque response with low ripples in the stator flux and torque of five-phase SRM drive can be achieved. In comparison with DTC of three-phase, the five-phase DT C system has more space vectors providing greater flexibility in selecting the inverter switching states, thus accomplishing a more precise control of the stator flux and torque. Simulation results clearly demonstrate that the combination of DT C with multi-phase SRM realizes higher performance with greater prospects in industries.

Component Modeling of 2DOF Comb Transducer for Equivalent Circuit Using Built-in Displacement Detection

In this paper, we report on a component modeling of in-plane 2DOF comb transducer for equivalent circuit analysis of electrostatic MEMS. The proposed model does not depend on connected components such as voltage sources, external forces, and springs since the model contained a dummy spring that can detect the comb displacement in itself. Using the model, we can analyze not only small-signal ac response but also dc operating points which means that once the parameters are calculated for a comb transducer, we need not to change the model parameters by changing any connected components. This model was adapted to a 2DOF SOI resonator and the measured frequency responses agreed well with the simulated responses.

Low Frequency AC Conduction in Polyaniline/Zinc Tungstate (PANI/ZnWO4) Composites

In the present paper, we report low frequency ac conductivity and dielectric studies on the composites of conducting polyaniline (PANI) with crystalline zinc tungstate (ZnWO4) powder. These composites have been synthesized by single step in situ polymerization technique by placing fine grade powder of ZnWO4 during the polymerization of aniline. The composites thus formed were characterized by FTIR spectra and scanning electron microscopy (SEM), which conforms the presence of ZnWO4 in polyaniline matrix and the formation of the composite. AC conductivity and dielectric response of these composites were investigated in the frequency ranging from 50 Hz to 5 MHz. it was found that the ac conductivity obeyed the power law index, where the ac conductivity increases with frequency. PANI/ZnWO4 composites have shown high dielectric constant, which could be related to conductivity relaxation phenomenon. It is observed that both dielectric constant and dielectric loss have decreased with increase in frequency. Variations in measured parameters of AC response with increase frequency of the composites follow systematic trends that are similar to those observed with decreasing temperature and level of doping.

Field-Effect and Frequency Dependent Transport in Semiconductor-Enriched Single-Wall Carbon Nanotube Network Device

The electrical and optical response of a field-effect device comprising a network of semiconductor-enriched single-wall carbon nanotubes, gated with sodium chloride solution is investigated. Field-effect is demonstrated in a device that uses facile fabrication techniques along with a small-ion as the gate electrolyte-and this is accomplished as a result of the semiconductor enhancement of the tubes. The optical transparency and electrical resistance of the device are modulated with gate voltage. A time-response study of the modulation of optical transparency and electrical resistance upon application of gate voltage suggests the percolative charge transport in the network. Also the ac response in the network is investigated as a function of frequency and temperature down to 5 K. An empirical relation between onset frequency and temperature is determined.

DYNAMIC DEFORMATION OF VORTEX LATTICE IN THE HOLLOW SUPERCONDUCTING YBaCuO CYLINDER

The elastic and viscous properties of vortex lattice in ceramic YBaCuO are studied by the measurements of ac response U in the cavity of the hollow cylinder placed in the magnetic field H aligned along the cylinder's axis. It is observed that the U(H) dependence is reaching saturation with increase of magnetic field. We interpret this effect by nonlocality of the vortex lattice elastic constants. Based on the analysis of the response dependence on excitation frequency, we conclude that vortex lattice deformation vector decreases at higher frequencies. The amplitude–frequency characteristics of the response indicate that vortices perform overdamped oscillations. The estimated damping coefficient value exceeds the evaluation by Bardeen–Stephen theory.

AC Response of Cu0.5Tl0.5Ba2Ca2Cu3−x Sn x O10−δ Superconductor

We have studied the ac response of Sn doped Cu0.5Tl0.5Ba2Ca2Cu3−x Sn x O10−δ superconductor samples from their ac-susceptibility measurements under different magnitudes of ac magnetic fields; H ac=0.4, 4, 16 A/m. The samples with x=0.5 and 1.0 have shown strong flux pinning and intergrain coupling. However, the sample with Sn doping of x=1.5 has shown very poor flux pinning characteristics.