Ac Amplitudes Research Articles

Reversible to irreversible transitions for ac driven skyrmions on periodic substrates

Abstract Using atomistic simulations, we investigate the dynamical behavior of magnetic skyrmions in dimer and trimer molecular crystal arrangements, as well as bipartite lattices at 3/2 and 5/2 fillings, under ac driving over a square array of anisotropy defects. For low ac amplitudes, at all fillings reversible motion appears in which the skyrmions return to their original positions at the end of each ac drive cycle and the diffusion is zero. We also identify two distinct irreversible regimes. The first is a translating regime in which the skyrmions form channels of flow in opposing directions and translate by one substrate lattice constant per ac drive cycle. The translating state appears in the dimer and trimer arrangements, and produces pronounced peaks in the diffusivity in the direction perpendicular to the external drive. For larger ac amplitudes, we find chaotic irreversible motion in which the skyrmions can randomly exchange places with each other over time, producing long-time diffusive behavior both parallel and perpendicular to the ac driving direction.

An AC and DC Electric Field Effect Based Charged Particle Focusing Scheme for Particle Beam Mass Spectrometer

Particle Beam Mass Spectrometer (PBMS) is a mass analyzer based particle sizer, features a few to hundreds of nanometer (nm) sizing capabilities in environmental pressures above hundreds of mTorr. The above feature makes the PBMS be used as in-situ process monitoring schemes for several kinds of semiconductor process equipment (e.g. Chemical Vapor Deposition). However, it still cannot be used in other process equipment (e.g. Plasma etching, Ion implantation) which requires lower process pressure conditions than the PBMS operation limits. The bottleneck of pressure limits comes from an aerodynamic lens component, which focuses incoming particles in background gas using repetitive fluidic contraction and expansion effects. These effects hard to be happened when the target particles are surrounded in molecular or transition background gas regions. In order to lower the operating pressure limits, other focusing schemes are required.In this study, an alternative particle focusing scheme which uses AC and DC electric field effects is proposed. The proposed scheme uses a stacked-ring ion funnel structure with variable AC and DC electric sources, for driving AC and DC potential variations along the funnel’s axial direction. While radial electric potential wells produced by AC frequency components induce particles’ radial position concentration to the funnel axis center, potential well depth and axial well depth steepness is controlled by AC amplitudes and DC source values.Simulation results show that with the proposed scheme, more than 50% of particle transmission efficiencies are obtained for 5 nm – 100 nm sized charged particles moving with Maxwellian velocities in molecular background region. The results come from AC electric sources with 200 Hz - 20 kHz frequency ranges and within 50 V amplitude ranges, and DC electric sources within 50 V ranges which are feasible values for hardware implementation.

Pinning Energy and Evidence of Granularity in the AC Susceptibility of an YBa2Cu3O7-x Superconducting Film

The study of granularity in superconducting films by using AC susceptibility has a crucial role in the development of and improvement in the ReBCO-coated conductors, which are a constantly evolving reality in the modern power applications of superconductivity. Specifically, the study of the granularity is essential because the ReBCO superconducting wires and tapes are far from the regularity of a single crystal while they often present an inter- and intragranular contribution to the critical current density. On the other hand, the AC susceptibility is a key part of the characterization of a granular sample because this technique is very sensitive to the presence of granularity in the superconductors and, moreover, the study of its first harmonic allows for determining pivotal properties such as the pinning energy as well as the dissipation processes acting in the sample. The pinning energy values and the granularity of an YBCO thin film have been studied by means of AC susceptibility measurements as a function of the AC amplitude, temperature, and DC field. In particular, the first harmonic imaginary component of the AC susceptibility χ1″ related to the dissipation processes of the sample has been studied. First, starting from the Brandt approach, the critical current density Jc and the pinning energy U of the sample have been extracted at 77 K by using the χ1″ measurements as a function of the AC amplitude at different AC frequencies and DC fields. From these measurements, a first signal of granularity appears. In order to confirm it, the temperature dependence of the χ1″ at different DC fields has been studied and a contribution deriving from the inter- and intragranular part of the sample has emerged. By taking the temperature corresponding to the crossover between the two contributions at the different DC fields, the intergranular and intragranular response has been separated. Successively, the temperature has been fixed to 77 K, together with an AC frequency equal to 1597.9 Hz, and the χ1″ as a function of the DC field at different AC amplitudes has been analyzed showing a clear presence of granularity in all the curves. By drawing the contour plot of the χ1″ with the DC and AC values, it was possible to determine the best parameters to put at 77 K in order to exploit the material for applications.

Measurement of stress-induced sympathetic nervous activity using multi-wavelength PPG

The onset of stress triggers sympathetic arousal (SA), which causes detectable changes to physiological parameters such as heart rate, blood pressure, dilation of the pupils and sweat release. The objective quantification of SA has tremendous potential to prevent and manage psychological disorders. Photoplethysmography (PPG), a non-invasive method to measure skin blood flow changes, has been used to estimate SA indirectly. However, the impact of various wavelengths of the PPG signal has not been investigated for estimating SA. In this study, we explore the feasibility of using various statistical and nonlinear features derived from peak-to-peak (AC) values of PPG signals of different wavelengths (green, blue, infrared and red) to estimate stress-induced changes in SA and compare their performances. The impact of two physical stressors: and Hand Grip are studied on 32 healthy individuals. Linear (Mean, s.d.) and nonlinear (Katz, Petrosian, Higuchi, SampEn, TotalSampEn) features are extracted from the PPG signal’s AC amplitudes to identify the onset, continuation and recovery phases of those stressors. The results show that the nonlinear features are the most promising in detecting stress-induced sympathetic activity. TotalSampEn feature was capable of detecting stress-induced changes in SA for all wavelengths, whereas other features (Petrosian, AvgSampEn) are significant (AUC ≥ 0.8) only for IR and Red wavelengths. The outcomes of this study can be used to make device design decisions as well as develop stress detection algorithms.

Vortex creep activation energies and depinning currents in CaKFe4As4 and Ba0.6K0.4Fe2As2 revealed by AC susceptibility measurements

Systematic ac susceptibility measurements have been carried out to study the vortex dynamics in CaKFe4As4 and Ba0.6K0.4Fe2As2 single crystals under various temperatures, dc magnetic fields, ac field frequencies and amplitudes. The field-temperature phase diagrams were shown, and the characteristics of irreversibility line were also derived. The specific expressions of activation energy on the parameters of temperature (T), current density (J), and dc magnetic field (H) are obtained according to these data. The results indicate that both superconductors have similar functional expressions of activation energy and flux pinning behaviors. Though both CaKFe4As4 and Ba0.6K0.4Fe2As2 superconductors exhibit very strong flux pinning ability, the vortex pinning potential in CaKFe4As4 is slightly smaller than that in Ba0.6K0.4Fe2As2, which may result from its distorted FeAs4-tetrahedron in CaKFe4As4. The depinning critical current densities at the limit of low temperature and low field were also extrapolated, yielding the corresponding values of Jc0(0) ∼ 1.0 × 108 and 2.2 × 108 A cm−2 for CaKFe4As4 and Ba0.6K0.4Fe2As2 superconductors, respectively, which suggest potential applications.

Time evolution of the vortex configuration associated with dynamic ordering detected by dc drive

When a periodic shear force with a small amplitude dinp is applied to vortex assemblies having a random distribution, the vortices gradually self-organize to avoid future collisions and transform into an organized configuration. We showed recently that this random-organization or dynamic-ordering process can be detected from the time-evolution of voltage V(t) that increases to a steady-state voltage. We also showed from the subsequent readout experiment of V(t) using various ac amplitudes d that the transient vortex configuration during random organization is not microscopically homogeneous but consists of the disordered and organized regions. In this work, we develop an alternative readout method using a dc drive. It is found that the dc method gives the same results as obtained from the ac one, which further supports our view of the coexistence regions. It is expected that both methods will be applied complementarily to detect the vortex configuration over a wide range of disorder.

On the nonlinear dynamics of a circular dielectric elastomer oscillator

Dielectric elastomer actuators (DEAs) are an emerging soft actuation technology that have advantages in large actuation strain, inherent compliance and low cost. Resonant actuation of DEAs has been shown to increase the actuation stroke and power output and has been utilized in applications such as robotic locomotion and loudspeakers. In this work, we present a planar circular dielectric elastomer oscillator (DEO) that can exert out-of-plane deformation as large as 80% of its own diameter at resonance. By experiments and model simulation we show that this simple DEO design exhibits complex nonlinear responses, including multiple solutions, sub- and super-harmonics, and exhibits multiple resonant peaks. The DEO performance is characterized against membrane pre-stretch and the weight of the attached mass and its actuation signals (DC and AC amplitudes). The resonant frequency and the amplitude are found to be easily tuned by varying these parameters. Such large stroke output and highly tunable resonance make this DEO a promising candidate for applications such as active vibration absorption, embedded loudspeakers, soft pumps and robotic locomotion.

LQR Control of Single-Phase Grid-Tied PUC5 Inverter With LCL Filter

This paper presents the current control design procedure of a single-phase grid-tied five-level packed U-cell inverter (PUC5) with an LCL output filter. The PUC5 inverter is used as an interface of renewable energy sources, such as solar applications. The LCL filter is calculated according to the grid-tied operation and converter ratings. An optimal controller, based on a linear quadratic regulator with integral action, is designed to inject a sinusoidal current with low harmonic distortion at unity power factor. For that design, the PUC5 inverter is modelled in the D – Q frame. The sensorless voltage control is incorporated into the switching technique to balance the PUC5 capacitor voltage and generate a five-level waveform at the output. Experimental tests are performed on a laboratory benchmark to confirm the theoretical design. The results prove the efficiency and accuracy of the adopted control strategy in a steady state and under transients of grid current, grid inductance, ac and dc voltage amplitudes.

Nonlinear dynamics in a terahertz-driven double-layer graphene diode * * Project supported by the National Natural Science Foundation of China (No. 11604126), the National Natural Science Foundation of China (No. 61601205), the Basic Research Program of Jiangsu Province (Natural Science Foundation for Young Scholars) (No. BK20160541), and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No.

By using the time-dependent hydrodynamic equations, we carry out a theoretical study of nonlinear dynamics in an n+nn+ double-layer graphene diode driven by terahertz radia-tion. A cooperative nonlinear oscillatory mode shows up due to the negative differential conductance effect. We use different chaos-detecting methods, such as the Poincaré bifurcation diagram and the first return map, to examine the transitions between the periodic and chaotic states. The double-layer graphene diode shows typical nonlinear dynamical behavior with the DC bias, AC amplitudes and the AC frequency as the control parameters.

Monel Contribution to AC Losses in MgB2 Wires in Frequencies Up To 18 kHz

AC losses for a wide range of ac amplitudes and frequencies have been studied in magnesium diboride (MgB2 ) superconducting wire with 36 filaments and Monel sheath at different temperatures and dc current levels. The results show a strong nonlinear frequency dependence below 1 kHz, which crosses over to a more moderate linear behavior at frequencies up to 18 kHz. Surprisingly, the introduction of dc current causes a significant reduction in the ac losses. Finite element simulations yield ac losses consistent with that observed experimentally. The simulations show that the magnetic Monel sheath is a dominant source for ac losses in zero dc current and that nonzero dc current saturates the magnetization, thus reducing the ac losses.

Precision PGC demodulation for homodyne interferometer modulated with a combined sinusoidal and triangular signal.

A precision PGC demodulation for homodyne interferometer modulated with a combined sinusoidal and triangular signal is proposed. Using a triangular signal as additional modulation, a continuous phase-shifted interference signal for ellipse fitting is generated whether the measured object is in static or moving state. The real-time ellipse fitting and correction of the AC amplitudes and DC offsets of the quadrature components in PGC demodulation can be realized. The merit of this modulation is that it can eliminate thoroughly the periodic nonlinearity resulting from the influences of light intensity disturbance, the drift of modulation depth, the carrier phase delay, and non-ideal performance of the low pass filters in the conversional PGC demodulation. The principle and realization of the signal processing with the combined modulation signal are described in detail. The experiments of accuracy and rate evaluations of ellipse fitting, nanometer, and millimeter displacement measurements were performed to verify the feasibility of the proposed demodulation. The experimental results show that the elliptical parameters of the quadrature components can be achieved precisely in real time and nanometer accuracy was realized in displacement measurements.

Normal form of nonlinear oscillator model relevant to spin-torque nano-oscillator theory

We introduce a normal form for the equation which describes nonlinear oscillations of spin-torque nano-oscillators (STNO). The general form of the equation might be reduced to the equation for STNO with uniformly magnetized free layer or to the equation for STNO with free layer in a magnetic vortex state. The equation is used to study the injection-locking problem. The bifurcation diagram for this problem is derived by using as bifurcation parameters the frequency and the amplitude of the ac excitation. Bifurcations lines corresponding to local bifurcations, i.e. the Saddle-node and the Hopf bifurcations, are derived by using linearization, while the lines corresponding to the Homoclinic bifurcation, which is global in nature, are derived numerically. The diagram exhibits universal features in terms of the shape and position of bifurcation lines. When the frequency of the ac excitation is relatively close to the free-running frequency of the oscillator the system is in synchronized state. For small ac amplitudes, the synchronization is lost for values of the ac frequency which imply the crossing of saddle-node bifurcations. For sufficiently larger values of the ac amplitude and for ac frequency smaller that the free-running frequency, the loss of the synchronized state occurs through a more complex bifurcation pattern which involves a sequence of two homoclinic bifurcations and an Hopf bifurcation. The value of frequency corresponding to loss of stability of the synchronized state can be determined analytically through linearization. The above sequence of bifurcations is connected to the presence of a special point in the bifurcation diagram, referred to as Takens-Bogdanov bifurcation.

Reversible vector ratchets for skyrmion systems

We show that ac driven skyrmions interacting with an asymmetric substrate provide a realization of a class of ratchet system which we call a vector ratchet that arises due to the effect of the Magnus term on the skyrmion dynamics. In a vector ratchet, the dc motion induced by the ac drive can be described as a vector that can be rotated clockwise or counterclockwise relative to the substrate asymmetry direction. Up to a full ${360}^{\ensuremath{\circ}}$ rotation is possible for varied ac amplitudes or skyrmion densities. In contrast to overdamped systems, in which ratchet motion is always parallel to the substrate asymmetry direction, vector ratchets allow the ratchet motion to be in any direction relative to the substrate asymmetry. It is also possible to obtain a reversal in the direction of rotation of the vector ratchet, permitting the creation of a reversible vector ratchet. We examine vector ratchets for ac drives applied parallel or perpendicular to the substrate asymmetry direction, and show that reverse ratchet motion can be produced by collective effects. No reversals occur for an isolated skyrmion on an asymmetric substrate. Since a vector ratchet can produce motion in any direction, it could represent a method for controlling skyrmion motion for spintronic applications.

AC Induced Corrosion of Underground Steel Pipelines under Cathodic Protection: III. Theoretical Approach with Electrolyte Resistance and Double Layer Capacitance for Mixed Corrosion Kinetics

In this series of papers, AC induced corrosion of steel pipe under cathodic protection was investigated. In this Part III, the most realistic model was devised, i.e., the cathodic current was composed of the reductions of dissolved oxygen partly controlled by diffusion and of the water following the Tafel law. The cathodic protection potential necessary to mitigate the corrosion for various AC amplitudes were evaluated. In this modeling approach, no change of soil properties due to AC corrosion is considered. Mean DC values of corrosion potential shift and the corrosion current density in presence of AC signal were calculated in reduced scale making possible field applications by introducing an appropriate set of corrosion kinetics parameter collected without AC signal. In a highly corrosive soil, the reduction of dissolved oxygen is slow compared with that of the water reduction, and then the corrosion behavior becomes similar to the case examined in Part II.

Experimental Characterization of Secular Frequency Scanning in Ion Trap Mass Spectrometers.

Secular frequency scanning is implemented and characterized using both a benchtop linear ion trap and a miniature rectilinear ion trap mass spectrometer. Separation of tetraalkylammonium ions and those from a mass calibration mixture and from a pesticide mixture is demonstrated with peak widths approaching unit resolution for optimized conditions using the benchtop ion trap. The effects on the spectra of ion trap operating parameters, including waveform amplitude, scan direction, scan rate, and pressure are explored, and peaks at black holes corresponding to nonlinear (higher-order field) resonance points are investigated. Reverse frequency sweeps (increasing mass) on the Mini 12 are shown to result in significantly higher ion ejection efficiency and superior resolution than forward frequency sweeps that decrement mass. This result is accounted for by the asymmetry in ion energy absorption profiles as a function of AC frequency and the shift in ion secular frequency at higher amplitudes in the trap due to higher order fields. We also found that use of higher AC amplitudes in forward frequency sweeps biases ions toward ejection at points of higher order parametric resonance, despite using only dipolar excitation. Higher AC amplitudes also increase peak width and decrease sensitivity in both forward and reverse frequency sweeps. Higher sensitivity and resolution were obtained at higher trap pressures in the secular frequency scan, in contrast to conventional resonance ejection scans, which showed the opposite trend in resolution on the Mini 12. Mass range is shown to be naturally extended in secular frequency scanning when ejecting ions by sweeping the AC waveform through low frequencies, a method which is similar, but arguably superior, to the more usual method of mass range extension using low q resonance ejection. Graphical Abstract ᅟ.

Homodyne laser vibrometer capable of detecting nanometer displacements accurately by using optical shutters.

This paper describes a homodyne laser vibrometer with optical shutters. The parameters that define the nonlinearity of the quadrature signals in a vibrometer can be pre-extracted before the measurement, and can then be used to compensate for nonlinear errors, such as unequal AC amplitudes and DC offsets. The experimental results indicated that the homodyne laser vibrometer developed has the ability to accurately detect the vibration state of the object to be measured, even when the amplitude is ≤λ/4. The displacement residual error can be reduced to a value under 0.9 nm.

Spin–orbit interaction in bent carbon nanotubes: resonant spin transitions

We develop an effective tight-binding Hamiltonian for spin–orbit (SO) interaction in bent carbon nanotubes (CNT) for the electrons forming the π bonds between the nearest neighbor atoms. We account for the bend of the CNT and the intrinsic spin–orbit interaction which introduce mixing of π and σ bonds between the pz orbitals along the CNT. The effect contributes to the main origin of the SO coupling—the folding of the graphene plane into the nanotube. We discuss the bend-related contribution of the SO coupling for resonant single-electron spin and charge transitions in a double quantum dot. We report that although the effect of the bend-related SO coupling is weak for the energy spectra, it produces a pronounced increase of the spin transition rates driven by an external electric field. We find that spin-flipping transitions driven by alternate electric fields have usually larger rates when accompanied by charge shift from one dot to the other. Spin-flipping transition rates are non-monotonic functions of the driving amplitude since they are masked by stronger spin-conserving charge transitions. We demonstrate that the fractional resonances—counterparts of multiphoton transitions for atoms in strong laser fields—occurring in electrically controlled nanodevices already at moderate ac amplitudes—can be used to maintain the spin-flip transitions.

The effect of vascular changes on the photoplethysmographic signal at different hand elevations

In order to further understand the contribution of venous and arterial effects to the photoplethysmographic (PPG) signal, recordings were made from 20 healthy volunteer subjects during an exercise in which the right hand was raised and lowered with reference to heart level. Red (R) and infrared (IR) PPG signals were obtained from the right index finger using a custom-made PPG processing system. Laser Doppler flowmetry (LDF) signals were also recorded from an adjacent fingertip. The signals were compared with simultaneous PPG signals obtained from the left index finger. On lowering the hand to 50 cm below heart level, both ac and dc PPG amplitudes from the finger decreased (e.g. 18.70 and 63.15% decrease in infrared dc and ac signals respectively). The decrease in dc amplitude most likely corresponded to increased venous volume, while the decrease in ac PPG amplitude was due to regulatory adjustments on the arterial side in response to venous distension. Conversely, ac and dc PPG amplitudes increased on raising the arm above heart level. Morphological changes in the ac PPG signal are thought to be due to vascular resistance changes, predominately venous, as the hand position is changed.

Investigation of the vortex dynamics of Fe1.02Se crystals by fundamental and 3rd harmonic ac magnetic susceptibility analysis

The superconducting properties of mm-sized Fe1.02Se crystals grown by a flux method are investigated. The structural and morphological features are studied by x-ray diffraction (XRD) and by scanning electron microscopy-energy dispersive x-ray spectroscopy SEM-EDX analysis, which identified a co-growth of a dominant superconducting tetragonal phase, with the minority of a non-superconducting hexagonal phase. The ac magnetic response is analyzed using a combined method of the fundamental and the 3rd harmonic ac magnetic susceptibility as a function of the temperature at different ac magnetic field amplitudes and frequencies and with various superimposed dc fields. The variation of the ac magnetic field and frequency in different ranges especially affects the 3rd harmonic components, which are more sensitive to the changes in the flux dynamic regimes. This allows a fine observation of the evolution of the different linear and non-linear processes responsible for the ac magnetic response of the Fe1.02Se crystals. At low enough ac amplitudes and frequencies, and even in high imposed dc magnetic fields, the Fe1.02Se crystals show a typical critical state behavior, marking a high stability of the pinning, with very small influence of the vortex dynamical processes. With the change of ac field amplitude and frequency a gradual crossover is observed from the initial stable pinning state through the domination of the intermediate regimes as flux creep and finally to the complete dominance of flux flow. The ac magnetic response is also influenced by geometric edge barrier effects arising from the plate-like geometry of the Fe1.02Se crystals. The changes of the dominant irreversible (non-linear) mechanism from surface pinning to bulk pinning or to prevailing dynamical regimes is also identified by analyzing the behavior of the 3rd harmonic components.

Effect of alternating voltage on corrosion of X80 and X100 steels in a chloride containing solution – Investigated by AC voltammetry technique

AC corrosion of the X80 and X100 steels in 0.1M NaCl solution were studied by the AC voltammetry technique. Corrosion electrochemical kinetics and solid/solution interface structure changes under the influence of AC voltage were characterized. Results illustrate that corrosion potential of the two steels shift negatively with the increase of AC amplitude and decrease of AC frequency. The anodic processes are under charge-transfer control and the anodic Tafel slopes increase with the increase of AC magnitude. The cathodic processes are under diffusion control at low AC amplitudes, while they become increasingly under charge-transfer control with higher AC amplitudes.