Dc Field Strength Research Articles

Theoretical insights into laser-assisted field evaporation of ionic compounds

This study addresses the kinetic process of field evaporation of MgO assisted by ultrafast laser pulses combining density functional theory and molecular dynamics. A quantitative model is presented to describe the competitive evaporation of Mg and O ions under various conditions by comparing the activation barriers. The coordination number has a significant impact on the evaporation kinetics. The evaporation ratio of Mg to O rises with increasing DC field strength and laser intensity. Moreover, the energetics of evaporation is in correlation with photo-induced field ionization, revealing distinct mechanisms of evaporation for Mg and O. While Mg undergoes further ionization and field evaporation simultaneously, the evaporation of O is coupled with the relaxation of excited carriers. The final charge state of evaporated O is determined by the DC field strength rather than the laser intensity. Our findings provide insights into laser–matter interactions in ionic compounds and contribute to the development of atom probe techniques.

AC Loss Estimation Method for REBCO Superconducting Tapes in AC+DC Fields for Field Windings in Fully Superconducting Synchronous Machine for Electric Aircraft

Abstract The estimation of AC loss in REBa2Cu3O y (REBCO; RE = rare earth, such as Y, Eu, and Gd) superconducting tapes within superconducting synchronous motors and generators for electric aircraft is imperative during the design stage phase to optimize efficiency and cooling considerations. Our previous studies clarified the AC loss characteristics, focusing on estimating the AC loss in armature windings. While the magnetic field applied to the armature windings is exclusively as an AC field, the field windings are subject to a composite field comprising both AC and DC components. Notably, the amplitude of the AC component is markedly smaller than the magnitude of DC component. This study experimentally investigated to consider the AC loss properties in field windings. The experimental results were as follows: When subjected to operational conditions of field windings within synchronous motor and generator, the AC loss increased with the DC field strength. This property is attributed to a reduction in the critical current induced by the external field. Hence, we modified the method for estimating AC loss. Subsequently, we validated the effectiveness of the modified method.

Nonlinear response of magnetization obeying inertial dynamics to superimposed dc and ac fields and dynamic magnetic hysteresis

The nonlinear ac stationary responses of magnetization and the dynamic magnetic hysteresis of ferromagnetic particles with uniaxial anisotropy acted on by both external dc and ac magnetic fields are evaluated by employing the inertial Landau–Lifshitz–Gilbert equation. Analytical expressions are obtained for the components of the second- and third-order nonlinear susceptibility tensors using the perturbation method. The dynamic nonlinear susceptibility strongly depends on the dc field strength. The effect of inertia on the area and slope of the dynamic magnetic hysteresis loop is demonstrated. In addition to nutation resonance, the main nonlinear effects previously observed in the ferromagnetic resonance region (frequency doubling and appearance of weak subharmonic resonance peaks) are predicted in the nutation resonance frequency region.

Influence of a bias dc field and an ac field amplitude on the dynamic susceptibility of a moderately concentrated ferrofluid.

In this paper, we study the effect of a bias dc field on the dynamic response of a moderately concentrated ferrofluid to an ac magnetic field of arbitrary amplitude. The ferrofluid is modeled by an ensemble of interacting moving magnetic particles; the reaction of particle magnetic moments to ac and dc magnetic fields occurs according to the Brownian mechanism; and the ac and dc magnetic fields are parallel. Based on a numerical solution of the Fokker-Planck equationfor the probability density of the orientation of the magnetic moment of a random magnetic particle, dynamic magnetization and susceptibility are determined and analyzed for various values of the ac field amplitude, the dc field strength, and the intensity of dipole-dipole interactions. It is shown that the system's magnetic response is formed under the influence of competing interactions, such as dipole-dipole, dipole-ac field, and dipole-dc field interactions. When the energies of these interactions are comparable, unexpected effects are observed: the system's susceptibility can either increase or decrease with increasing ac field amplitude. This behavior is associated with the formation of nose-to-tail dipolar structures under the action of the dc field, which can hinder or promote the system's dynamic response to the ac field. The obtained results provide a theoretical basis for predicting the dynamic properties of ferrofluids to improve their use in biomedical applications, such as, in magnetic induction hyperthermia.

Explaining the temperature effects on the magnetization dynamics of non-interacting antiferromagnetic nanoparticles across wide ranges of frequencies and damping

Owing to their ultrafast spin dynamics, antiferromagnetic nanoparticles based materials are very promising for electronic memory devices that require a specific control of interaction between the spin current and local magnetization of the sublattice. Hence, the elucidation of the dynamics of the magnetization of antiferromagnetic nanoparticles is an essential. In this work, we report an exploration of the dynamics of the magnetization of antiferromagnetic nanoparticles using Fokker–Planck equation susceptive to describe the behavior of the distribution function of the magnetization of non-interacting nanoparticles. The dynamic equation is treated within the framework of matrix continued fractions approach using numerical method for which the magnetization correlation function as well as the complex susceptibility are assessed for different values of temperature across wide ranges of frequencies and damping. The analysis performed within these circumstances reveal that the application of strong magnetic field at any arbitrary angle to the easy axis of the nanoparticle trains a substantial modification of the magnetization dynamics of the corresponding nanoparticle. Moreover, using the simple asymptotic formulae, the obtained results relative to the estimation of the greatest relaxation time reveal a decent agreement with the relaxation time calculated from the infinite hierarchy of linear differential-recurrence equations for the statistical moments. We show that the great sensitivity of the magnetization dynamics to the thermal agitation provides unique information regarding the coupling between the precession of the magnetization and its thermally activated reversal over the saddle point, resulting from influence of the dc field orientation and strength as well as the damping.

Theory of field emission from dielectric coated surfaces

This paper presents an exact analytical theory for field emission from dielectric coated cathode surfaces, by solving the one-dimensional (1D) Schrödinger equation with a double-triangular potential barrier introduced by the coating. The effects of the cathode material (work function and Fermi energy), dielectric properties (dielectric constant, electron affinity, and thickness), applied dc field strength, and cathode temperature are analyzed in detail. For 1D flat cathode surfaces with coating, it is found that the emission current density can be larger than the uncoated case when the dielectric constant is smaller than a certain value ɛdielth and the dielectric thickness is larger than the threshold value dth[nm]≈ɛdielW/eF with the dielectric constant ɛdiel<ɛdielth, where W is the work function of the cathode material, F the applied dc field, and e the elementary charge. Our quantum model is also compared with a modified Fowler-Nordheim equation for a double barrier, showing qualitatively good agreement. Our study provides insights for designing field emitters with higher efficiency and better stability.1 MoreReceived 19 October 2020Accepted 14 December 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.043439Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasField emissionStrong electromagnetic field effectsThermionic emissionPhysical SystemsElectron sourcesTechniquesField emission & field-ion microscopySchroedinger equationAccelerators & BeamsGeneral PhysicsCondensed Matter, Materials & Applied PhysicsInterdisciplinary Physics

Rotational dynamics of magnetic nanoparticles in different matrix systems

Abstract Dynamic magnetic measurements on magnetic nanoparticle (MNP) samples have been widely used for the determination of structural MNP parameters as well as for the realization of bioassays. On the other hand, proposed that the MNPs are thermally blocked, i.e., that the dynamics are dominated by the Brownian rotation, and knowing the distribution of their hydrodynamic size, information on the matrix properties can be obtained. In contrast to conventional rheology, the local environment of the MNPs is sensed on the nanoscale so that important information on the embedding of MNPs in the matrix and thus the particle-matrix interaction is obtained. Depending on the characteristic length scales of the matrix and the size of the MNPs, rheological parameters, such as viscosity and shear modulus, derived from nanorheological measurements can differ from the values obtained from conventional rheology. To measure the MNP dynamics, different experimental techniques can be applied. In this contribution, the focus lies on ac susceptometry and fluxgate magnetorelaxometry. The analysis of the complex ACS spectra is generally carried out within a modified Debye model. Different approaches for the estimation of rheological parameters from the complex ACS spectra will be presented. Two model systems will exemplarily be discussed in detail. As a Newtonian matrix system, water-glycerol mixtures were studied. It is demonstrated that the dynamic viscosity can accurately be estimated from ACS measurements on well thermally blocked single-core as well as on multicore MNP systems, which include Brownian and Néel dynamics. As a viscoelastic matrix system, aqueous gelatin solutions were studied. Gelatin is known to be a Voigt-Kelvin model system, in which elastic and viscous forces are parallel. In particular, we studied the gelation dynamics by repetitive measurements of the complex ACS spectrum. Different approaches to derive viscosity and shear modulus are applied and compared. In order to identify magnetoviscous effects in dynamic magnetic measurements, the magnetic field dependence of the Brownian relaxation time has to be eliminated. ACS measurements on various sufficiently strongly diluted aqueous MNP suspensions were performed in dependence of ac field amplitude and superimposed dc field strength and compared to theory. Excellent agreement was found.

Effects of Inorganic ZnO Particle Doping on Crystalline Polymer Morphology and Space Charge Behavior

This study further investigated the synergistic effect of micro- and nanofiller doping on matrix material space charges and breakdown characteristics. Accordingly, low-density polyethylene (LDPE) was used as the matrix material, and spherical ZnO particles with sizes of 30 nm and 1 µm were used as additives. Micro-ZnO/LDPE, nano-ZnO/LDPE, and micro-nano-ZnO/LDPE composites were prepared through melt blending. The crystalline morphologies of the composites were observed via polarized light microscopy. The composite crystallinity and melting peak temperature were measured via differential scanning calorimetry, and the micro- and nanoparticle dispersions in the matrix were observed via scanning electron microscopy. The test results showed that the particles were uniformly dispersed in the polyethylene matrix. The filler acted as a heterogeneous nucleation agent in the matrix. The crystal size decreased, thereby increasing the crystal quantity. The doping of inorganic ZnO particles improved the composite crystallinity. The ZnO/LDPE composites were subjected to DC breakdown, space charge, and dielectric spectrum tests. When the crystal arrangement of the sample was loose and its size was large, the breakdown process developed along a shorter path, and the field strength of the composite breakdown decreased. The order of AC and DC breakdown field strengths of the samples was as follows: micro-ZnO/LDPE &lt; pure LDPE &lt; micro-nano-ZnO/LDPE &lt; nano-ZnO/LDPE. The DC and AC breakdown field strengths of the micro- and nano-ZnO/LDPE were 4.7% and 3.2% higher than those of the pure LDPE, respectively. Moreover, the DC and AC breakdown field strengths of the nano-ZnO/LDPE were 11.02% and 15.8% higher than those of the pure LDPE, respectively. The doping of inorganic ZnO particles restrained the space charge accumulation, and the residual charges decreased after short-circuit treatment. The dielectric constant of all nanocomposites was lower than that of LDPE, and the dielectric loss of all composites was higher than that of LDPE.

Laser induced resistivity of chiral single wall carbon nanotubes

Using the Boltzmann transport equation within the semi-classical approximation with constant relaxation time, we theoretically studied the dynamics of electrons in chiral single wall nanotubes (SWNTs) subjected to a temperature gradient (∇T) in the presence of a combined direct current and high frequency alternating fields. We obtained an expression for the resistivity (ρc) of the SWNTs which varies with temperature and depends among others on material’s chiral angle (θh), dc field strength (Eo) and ac field amplitude (Es). Our results show that chiral SWNTs exhibit metallic behavior with resistivity increasing approximately linearly with temperature over a wide temperature range well above 100 K. Based on the low chiral resistivity obtained for the SWNTs at room temperatures, we propose these materials as good candidates for possible optoelectronic applications.

Anomalous transport in a topological Wannier-Stark ladder

A dc (e.g. electric) field with commensurate lattice direction turns a single particle band structure in $d=3$ dimensions into an infinite set of equally spaced irreducible $(d-1)=2$-dimensional Wannier-Stark (WS) band structures that are spatially localized along the field direction. Particle transport is expected to be suppressed once the WS bands are gapped in energy. The topological character of the irreducible band structure leads to one-dimensional sets of boundary states which fill the energy gaps. As a result, eigenmodes are smoothly connected in energy and space and yield anomalous particle transport throughout the ladder. The number of chiral boundary modes can be tuned by the dc field strength and manifests through the distribution of dissipated energy and spatial motion, and the temperature dependence of angular momentum carried by particles.

A Simple Printed Circuit Board-Based Ion Funnel for Focusing Low m/z Ratio Ions with High Kinetic Energies at Elevated Pressure.

Ion funnels are one of the key components for transferring ions from higher pressure into the vacuum. Typically, ion funnels are constructed of several different plate ring electrodes with a decreasing inner diameter where radio frequency (RF) voltages and electric DC fields are applied to the electrodes to focus and transport ion clouds. In this work, we developed and investigated a simple and low-cost ion funnel design that is based on standard printed circuit boards (PCB) with integrated planar electrodes including the signal distribution network. This ion funnel is capable of withstanding high electric fields with superimposed RF voltages due to its buried capacitors. To evaluate the ion focusing efficiency of the ion funnel, we simulated the movement of ions inside this funnel and experimentally evaluated the ion transfer. Our simulations show that a rectangular ion funnel like the PCB ion funnel has similar performance compared with conventional stacked ring funnels. Due to the hundredfold lower parasitic capacitance between the planar electrodes compared with conventional ion funnels, high RF voltage amplitudes up to 195V and reduced electric DC field strengths up to 100Td can be reached at a frequency of about 5MHz. Thus, the funnel is appropriate to focus light ions at elevated pressures up to 20mbar. Graphical Abstract .

Forced response and dynamic hysteresis of magnetic nanoparticles with mixed uniaxial and cubic anisotropy in superimposed strong ac and dc bias fields

The stationary response of ferromagnetic nanoparticles with mixed uniaxial and cubic anisotropy to superimposed external ac and dc bias fields of arbitrary strength is investigated using Brown's continuous diffusion of orientations model [W. F. Brown Jr., Phys. Rev. 130, 1677 (1963)]. The susceptibility and dynamic magnetic hysteresis (DMH) loops of an individual nanoparticle are calculated in extensive ranges of the superimposed fields, their orientation, damping, temperature, and specific mixed anisotropy parameter. The DMH loops and nonlinear dynamic susceptibility strongly depend on the dc and ac field strengths, the polar angle between the easy axis of the particle and the external field vectors, as well as the temperature and damping, simultaneously displaying a marked dependence on the mixed anisotropy parameter.

Transient Dynamics of Super Bloch Oscillations of a 1D Holstein Polaron under the Influence of an External AC Electric Field

AbstractTheoretical formalism for DC‐field polaron dynamics is extended to the dynamics of a 1D Holstein polaron in an external AC electric field using multiple Davydov trial states. Effects of carrier–phonon coupling on detuned and resonant scenarios are investigated for both phase and nonzero phase. For slightly off‐resonant or detuned cases, a beat between the usual Bloch oscillations and an AC driving force results in super Bloch oscillations, that is, rescaled Bloch oscillations in both the spatial and the temporal dimension. Super Bloch oscillations are damped by carrier–phonon coupling. For resonant cases, if the carrier is created on two nearest‐neighboring sites, the carrier wave packet spreads with small‐amplitude oscillations. Adding carrier–phonon coupling localizes the carrier wave packet. If an initial broad Gaussian wave packet is adopted, the centroid of the carrier wave packet moves with a certain velocity and with its shape unchanged. Adding carrier–phonon coupling broadens the carrier wave packet and slows down the carrier movement. Our findings may help provide guiding principles on how to manipulate the dynamics of the super Bloch oscillations of carriers in semiconductor superlattice and optical lattices by modifying DC and AC field strengths, AC phases, and detuning parameters.

Cadmium zinc telluride as a mid-infrared variable retarder

The electro-optic behavior of cadmium zinc telluride is examined in the mid-infrared region between 3 and 11 μm, for applied DC field strengths of up to 106 V/m. The measurements performed here include full characterization of the polarization state of the transmitted light by means of the Stokes vector. We demonstrate the suitability of this material for DC variable retarder applications such as those achieved by quarter- or half-wave retardation. A comparison of two different metallic coatings for electrodes, gold and indium, reveals important differences in performance that are attributed to the homogeneity of the field through the bulk of the crystal. We illustrate that, in the case of both metals, the same electro-optic coefficients are measured, but regions of higher and lower retardation result in significant depolarization in the case of gold. Such depolarization may adversely affect the contrast ratio in a light valve, or increase the voltage necessary for the operation of an arbitrary polarization state generator.

Topological flat Wannier-Stark bands

We analyze the spectrum and eigenstates of a quantum particle in a bipartite two-dimensional tight-binding dice network with short range hopping under the action of a dc bias. We find that the energy spectrum consists of a periodic repetition of one-dimensional energy band multiplets, with one member in the multiplet being strictly flat. The corresponding macroscopic degeneracy invokes eigenstates localized exponentially perpendicular to the dc field direction, and super-exponentially along the dc field direction. We also show that the band multiplet is characterized by a topological winding number (Zak phase), which changes abruptly if we vary the dc field strength. These changes are induced by gap closings between the flat and dispersive bands, and reflect the number of these closings.

Stimulation of oil palm (Elaeis guineensis) seed germination by exposure to electromagnetic fields

Difficult-to-germinate seeds are a common plant conservation, plantation and biotechnological problem. Oil palm (Elaeis guineensis) seed germination takes 6 months to 1year with ≈40% failure rate. We investigated the affect of various electromagnetic DC field strengths on oil palm germination. Kernels imbibed water more effectively when exposed to a magnetic field. The kernels were divided into three groups. The first group (50) was soaked (imbibed) in distilled water (2h) with no exposure to applied magnetic fields (blank control). The second group was exposed to magnetic fields with intensities of 2.5mT, 5.0mT, 7.0mT, 9.0mT and 11.0mT for 1, 2, 3, 4 and 5h (Dry Treated Kernels, DTK). The same electromagnetic protocol was conducted on a third group but with kernels which were immersed in water (Magnetically-Treated-Water-Kernels, MTWK). 96% germination was achieved by day-30 for the MTWK treatment using 9.0mT for 4h: slightly lower results were found for DTK kernels. None of the controls germinated within 30days. Young oil palm seedlings kept in a shade house (≈110μmol quanta m−2s−1 PPFD) watered every day with MTW grew 3 times faster (3.1mmday−1) than controls watered using unmagnetised water.

Effect of thermal ageing on space charge characteristics in double-layered polyester film

To understand the effect of thermal ageing mechanism on space charge formation and dissipation in double-layered polyester films (PET), the time dependence of space charge characteristics at physical interface and in the bulk of PET films have been investigated. The measurements of space charge characteristics in specimens of different thermal ageing conditions (80 and 130 °C) were conducted under different DC field strengths (polarization) and short circuit conditions (depolarization) using the Pulsed Electro Acoustic (PEA) measurement system. The influence of electric field distortion caused by the space charges on average AC breakdown strength of PET specimens was also investigated. Results indicate that the thermal ageing leads to polymer interface acts as positive charge traps at ageing temperature of 80 °C, but acts as electron traps at ageing temperature of 130 °C and provides deeper trapping level for positive charge carriers. Besides these, average breakdown strength of the composite decreases due to the electric field distortion caused by the space charges. A preliminary study of this phenomenon is given and further investigation is needed.

External dc bias-field effects in the nonlinear ac stationary response of dipolar particles in a mean-field potential.

External dc bias-field effects on the nonlinear dielectric relaxation and dynamic Kerr effect of a system of permanent dipoles in a uniaxial mean-field potential are studied via the rotational Brownian motion model postulated in terms of the infinite hierarchy of differential-recurrence equations for the statistical moments f_{n}(t)=〈P_{n}〉(t) (the expectation value of the Legendre polynomials P_{n}). By solving these equations, the nonlinear dielectric and Kerr-effect ac stationary responses are evaluated for arbitrary dc field strength via perturbation theory in the ac field. Simple analytic equations based on the large separation of the time scales of the fast intrawell and slow overbarrier (interwell) relaxation processes are also derived.

Torque density measurements on vortex fluids produced by symmetry-breaking rational magnetic fields.

We have recently reported on the discovery that an infinite class of triaxial magnetic fields is capable of producing rotational flows in magnetic particle suspensions. These triaxial fields are created by applying a dc field orthogonally to a rational biaxial field, comprised of orthogonal components whose frequencies form a rational ratio. The vorticity axis can be parallel to any of the three field components and can be predicted by a careful consideration of the symmetry of the dynamic field. In this paper we not only test the field-symmetry predictions, but also quantify fluid vorticity as a function of the field parameters (strength, frequency ratio, phase angle and relative dc field strength) and particle shape. These measurements validate the symmetry predictions and demonstrate that rational fields are as effective as vortex fields for producing strong fluid mixing, yet have the advantage that small changes in the frequency of one of the field components can change the vorticity axis. This approach extends the possibilities for noncontact control of fluid flows and should be useful in areas such as microfluidics, and the manipulation and mixing of microdroplets.

Nonlinear susceptibility and dynamic hysteresis loops of magnetic nanoparticles with biaxial anisotropy

The nonlinear ac susceptibility and dynamic magnetic hysteresis (DMH) of a single domain ferromagnetic particle with biaxial anisotropy subjected to both external ac and dc fields of arbitrary strength and orientation are treated via Brown's continuous diffusions model [W. F. Brown, Jr., Phys. Rev. 130, 1677 (1963)] of magnetization orientations. The DMH loops and nonlinear ac susceptibility strongly depend on the dc and ac field strengths, the polar angle between the easy axis of the particle, the external field vectors, temperature, and damping. In contrast to uniaxial particles, the nonlinear ac stationary response and DMH strongly depend on the azimuthal direction of the ac field and the biaxiality parameter Δ.