Frequency Of Ac Field Research Articles

Modeling Tumor Treating Fields (TTFields) application in single cells during metaphase and telophase.

Effects of electric fields on biological cells have been extensively studied but primarily in the low and high frequency regimes. Low frequency AC fields have been investigated for applications to nerve and muscle stimulation or to examine possible environmental effects of 60 Hz excitation. High frequency fields have been studied to understand tissue heating and tumor ablation. Biological effects at intermediate frequencies (in the 100-500 kHz regime) have only recently been discovered and are now being used clinically to disrupt cell division, primarily for the treatment of recurrent glioblastoma multiforme. In this study, we develop a computational framework to investigate the mechanisms of action of these Tumor Treating Fields (TTFields) and to understand in vitro findings observed in cell culture. Using Finite Element Method models of isolated cells we show that the intermediate frequency range is unique because it constitutes a transition region in which the intracellular electric field, shielded at low frequencies, increases significantly. We also show that the threshold at which this increase occurs depends on the dielectric properties of the cell membrane. Furthermore, our models of different stages of the cell cycle and of the morphological changes associated with cytokinesis show that peak dielectrophoretic forces develop within dividing cells exposed to TTFields. These findings are in agreement with in vitro observations, and enhance our understanding of how TTFields disrupt cellular function.

AC Electric Field-Induced Trapping of Microparticles in Pinched Microconfinements.

The trapping of charged microparticles under confinement in a converging-diverging microchannel, under a symmetric AC field of tunable frequency, is studied. We show that at low frequencies, the trapping characteristics stem from the competing effects of positive dielectrophoresis and the linear electrokinetic phenomena of electroosmosis and electrophoresis. It is found, somewhat unexpectedly, that electroosmosis and electrophoresis significantly affect the concentration profile of the trapped analyte, even for a symmetric AC field. However, at intermediate frequencies, the microparticle trapping mechanism is predominantly a consequence of positive dielectrophoresis. We substantiate our experimental results for the microparticle concentration distribution, along the converging-diverging microchannel, with a detailed theoretical analysis that takes into account all of the relevant frequency-dependent electrokinetic phenomena. This study should be useful in understanding the response of biological components such as cells to applied AC fields. Moreover, it will have potential applications in the design of efficient point-of-care diagnostic devices for detecting biomarkers and also possibly in some recent strategies in cancer therapy using AC fields.

Photon assisted long-range tunneling

We analyze long-range transport through an ac driven triple quantum dot with a single electron. Resonant transitions between separated and detuned dots are mediated by the exchange of n photons with the time-dependent field. An effective model is proposed in terms of second order (cotunneling) processes which dominate the long-range transport between the edge quantum dots. The ac field renormalizes the inter dot hopping, modifying the level hybridization. It results in a non-trivial behavior of the current with the frequency and amplitude of the external ac field.

Low temperature study of mechanically alloyed Fe67.5Ni32.5 Invar sample

The study at low temperatures of powder of the Invar alloy, Fe67.5Ni32.5, produced by mechanical alloying, shows that the sample presents two structural phases, the Fe–Ni BCC and the Fe–Ni FCC. The 57Fe Mössbauer spectra obtained in this sample at different temperatures were fitted considering two hyperfine magnetic field distributions. The first one having the larger mean field and only one peak (at ca. 35T, varying with T), is associated with the BCC phase, and the second one, presenting several broad peaks (distributed between 10 and 35T), is associated to the FCC phase. A singlet, which is associated to low spin Fe sites of the FCC phase, was also considered. The mean hyperfine magnetic field of the BCC phase increases monotonically as temperature decreases, while that of the FCC phase presents an anomaly near 75K. The real part of the ac magnetic susceptibility temperature scans presents a peak whose position increases from 31 to 39K, when the ac field frequency increases from 100 to 5000Hz. These results permit to associate the detected anomaly to the occurrence of a reentrant spin glass transition.

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.

Inter- and Intra-Granular Flux Pinning Properties in Ba(Fe $$_{0.91}$$ 0.91 Co $$_{0.09})_{2}$$ 0.09 ) 2 As $$_{2}$$ 2 Superconductor in AC and DC Magnetic Fields

Flux pinning dynamics are studied in a Ba(Fe\(_{0.91}\)Co\(_{0.09})_{2}\)As\(_{2}\) (\(T_\mathrm{{c}}=25.3\) K) bulk samples via ac susceptibility measurements. Ac susceptibility curves shift to higher temperatures as the frequency of small ac fields is increased from 75 to 1997 Hz in all magnetic fields ranging from 0 to 18 T. The temperature profile of the ac susceptibility curves shows narrower ac loss distribution in temperature for higher frequencies and gradually narrowing frequency shift as the temperature sweeps the full range from 2 K to the upper critical field temperature. The frequency (\(f)\) shift of the susceptibility curves is modeled by the Anderson–Kim Arrhenius law \(f = f_{0} \mathrm {exp}(- {E}_\mathrm{{a}} /kT)\) to determine flux activation energy \(E_\mathrm{{a}}/k\) as a function of magnetic field. Extensive mapping of the irreversibility lines shows broad dependence on the magnitude and the frequency of the ac field, in addition to the dc magnetic field. The irreversibility lines were just below the upper critical field \(H_\mathrm{{c2}}\) lines at 0 T in the \(H-T\) plane, but they moved significantly below the \(H_\mathrm{{c2}}\) line at higher magnetic fields, placing constraints on the use of these materials at higher magnetic fields such as 10 T and above.

Magnetization losses in superconducting YBCO conductor-on-round-core (CORC) cables

Described are the results of magnetization loss measurements made at 77 K on several YBCO conductor-on-round-core (CORC) cables in ac magnetic fields of up to 80 mT in amplitude and frequencies of 50 to 200 Hz, applied perpendicular to the cable axis. The cables contained up to 40 tapes that were wound in as many as 13 layers. Measurements on the cables with different configurations were made as functions of applied ac field amplitude and frequency to determine the effects of their layout on ac loss. In large scale devices such as e.g. Superconducting Magnetic Energy Storage (SMES) magnets, the observed ac losses represent less than 0.1% of their stored energy.

Detection of the flux dynamical regimes in Bi4O4S3 by multiharmonic AC susceptibility

The temperature dependent first and third harmonics of the AC magnetic susceptibility χ1,3(T) have been measured on Bi4O4S3. The χ1(T) magnetic response is due to the intergranular contribution with a transition temperature Tc≈4.4K. The dependencies of the χ3(T) on the AC field amplitude and frequency reveal a critical state with the presence of the flux creep and the flux flow regimes. In fact, the field dependencies of the effective activation energy and of the frequency dependent critical current density have been found to be described with power laws characteristic of the collective creep regime involving large flux bundles.

Nonstationary holographic currents in neutron-irradiated SiC crystal

We report the excitation of nonstationary holographic currents in semi-insulating 6H-SiC crystal preliminarily irradiated by reactor neutrons. The currents are studied for the cases of zero and non-zero external electric fields. The dependences of the signal amplitude versus the frequency of phase modulation, light intensity, spatial frequency and amplitude of the external ac field are measured. The frequency transfer functions of the effect demonstrate unusual behaviour, namely, there are two frequency-independent regions in the diffusion regime of signal excitation and low-frequency maximum in the experiments with the applied ac field. These peculiarities are explained in the frames of a two-level model of semiconductor. Photoconductivity, diffusion length of electrons, thermal excitation rate and ionization cross section for shallow traps are estimated for the wavelength λ = 532 nm.

Nonlinear dynamics of Josephson vortices in a film screen under dc and ac magnetic fields

We present detailed numerical simulations of Josephson vortices in a long Josephson junction perpendicular to a thin film screen under strong dc and ac magnetic fields. By solving the sine-Gordon equation, we calculated the threshold magnetic field for penetration of fluxons as a function of frequency, and the power dissipated by oscillating fluxons as functions of the ac field amplitude and frequency. We considered the effects of superimposed ac and dc fields, and a bi-harmonic magnetic field resulting in a vortex ratchet dynamics. The results were used to evaluate the contribution of weak-linked grain boundaries to the nonlinear surface resistance of polycrystalline superconductors under strong electromagnetic fields, particularly thin film screens and resonator cavities.

Thermal Treatment and Dielectric Properties of Na0.5Bi0.5TiO3 Single Crystal

Dielectric properties of heat treated in air and in vacuum Na0.5Bi0.5TiO3 single crystals are studied. It is shown, that above 350°C ɛ and tanδ exhibit sufficient temperature hysteresis and strongly depend on AC field frequency and heat treatment conditions. Low frequency dielectric dispersion around 400°C and temperature growth of dielectric loss above 500°C are observed in as grown and heat treated in vacuum samples. Frequency dispersion of ɛ and temperature growth of tanδ strongly diminish after annealing in air. It is supposed that slow relaxing dipolar complexes and mobile structural defects are created by oxygen vacancies.

Dielectrophoresis Response and Manipulation of Silica and its Application to Micro/Nanosensors

In this paper, silica microspheres and several novel μm scale DEP (dielectrophoresis) micro electrode array chips were prepared. A DEP experimental system was assembled to investigate the the DEP response of silica microspheres and micrometer particles. The effects of pattern of electrodes and frequency of AC field on the manipulation of silica microspheres were analyzed. The results showed that silica microspheres were manipulated by negative DEP (nDEP) in all types of electrodes, the position of silica microspheres in the electrodes changed when the frequency was changed and the phenomenon of nDEP was more obvious at higher frequency. A gas sensor was fabricated by dielectrophoretically assembling SiO2 micrometer particles. The sensor showed good response to SO2 and NH3 gas. The measurement results confirmed DEP could be a quicker method for constructing gas sensor.

Photon-Assisted Heat Generation in a Quantum Dot Device with the Charging Effect

Electrical current induced heat generation through an ultrasmall quantum dot–leads coupled system under AC fields is investigated by employing the nonequilibrium Green’s function technique. The external AC fields, charging energy and junction capacitances influence the heat generation sensitively. We find negative differential heat generation in our system. The reason for this is the Coulomb interaction, and the negative differential heat generation becomes larger as the frequency of AC fields \(\omega \) increases. In addition, the peak caused by charging energy in \(Q{/}\omega _0\) curve increases with \(\omega \) rapidly. A region in which the value \(Q{/}I\) is low emerges due to the charging effect. It is an ideal region for device operation. This region will enlarge as the parameter \(R\) and the charging energy \(E_c\) increase.

Dynamic characteristics of double-barrier nanostructures with asymmetric barriers of finite height and widths in a strong ac electric field

The theory of the interaction of a monoenergetic flow of injected electrons with a strong high-frequency ac electric field in resonant-tunneling diode (RTD) structures with asymmetric barriers of finite height and width is generalized. In the quasi-classical approximation, electron wavefunctions and tunneling functions in the quantum well and barriers are found. Analytical expressions for polarization currents in RTDs are derived in both the general case and in a number of limiting cases. It is shown that the polarization currents and radiation power in RTDs with asymmetric barriers strongly depend on the ratio of the probabilities of electron tunneling through the emitter and collector barriers. In the quantum mode, when δ = ɛ − ɛ r = ħω ≪ Γ (ɛ is the energy of electrons injected in the RTD, ħ is Planck’s constant, ω is the ac field frequency, ɛ r and Γ are the energy and width of the resonance level, respectively), the active polarization current in a field of E ≈ 2.8ħω/ea (e is the electron charge and a is the quantum-well width) reaches a maximum equal in magnitude to 84% of the direct resonant current, if the probability of electron tunneling through the emitter barrier is much higher than that through the collector barrier. The radiation-generation power at frequencies of ω = 1012–1013 s−1 can reach 105–106 W/cm2 in this case.

Bifurcation magnetic resonance in films with differently oriented crystallographic axes

The magnetization dynamics in single-crystal films with cubic anisotropy has been investigated based on numerical simulation. It is shown that the application of an in-plane bias magnetic field along hard magnetization axes gives rise to an additional (bifurcation) resonance, which is due to the existence of bistability, i.e., two closely located equilibrium magnetization states. The specific features of bifurcation resonance are revealed in films with the three main crystallographic orientations: (100), (110), and (111). Changes in the resonance range and corresponding dynamic modes with a change in the ac field frequency are investigated. States of dynamic bistability and multistability are obtained in the bifurcation resonance range.

Double Landau-Zener interferometry in a dc-ac–driven Bloch-Zener oscillation

The effect of an additional high frequency alternating field on the Bloch-Zener oscillation of a quantum particle is investigated theoretically. The effective magnitude of the band gap is reduced by the ac field, and even collapsed to zero by appropriately tuning the amplitude and the frequency of the oscillation. This results in the enhancement of the Zener tunneling with dramatic changes of the Bloch-Zener oscillation. The temporal behavior of the wave packet is clarified as due to the interplay of the Landau-Zener interference in the repeated level crossings with two different time scales, namely, the period of the Bloch oscillation, and the period of the high frequency ac field.

Frequency dependence of magnetization reversal in thin Fe(100) films

We have studied the magnetization reversal of epitaxial Fe(100) films in an ac magnetic field as a function of frequency between 0.1 and 1.2 MHz by polarized neutron reflectivity. Different modes of magnetization reversal are excited depending on the ac field frequency, those with domain nucleation and propagation, and those with a coherent magnetization rotation. At low frequencies the magnetization follows the external field adiabatically via nucleation and propagation of antiparallel 180${}^{\ensuremath{\circ}}$ domains. At higher frequencies additional 90${}^{\ensuremath{\circ}}$ domain walls are being nucleated and propagate. Beyond 0.5 MHz, the magnetization reversal becomes progressively suppressed and finally comes to a stop at about 1 MHz.

Bistability suppression and precession mode switching under condition of bifurcation resonance

We describe the magnetization dynamics in a single-crystal film in the area of bifurcation resonance caused by being positioned between the two equilibrium magnetization states that are in close proximity to one another. It is shown that the dynamic bistability can be suppressed by an additional AC field and the precession mode can be selected by an adjustment of the AC field frequency. Multiple direct switching from one mode to another is obtained by varying the frequency of the additional field. Also, we report the state when the system becomes noise-sensitive, i.e., it performs regular switching between precession modes that are different in amplitude because of the presence of the additional noise signal.

AC-field-induced polarization for uncharged colloids in salt solution: A dissipative particle dynamics simulation

We study the response of a spherical colloid under alternating electric fields (AC fields) by mesoscopic simulation method, accounting in full for hydrodynamic and electrostatic interactions. We focus on a special case of uncharged colloids. The main polarization mechanism is the "volume polarization", where ionic currents are deflected by the core of the uncharged colloid. Specifically, we compute the polarizability of a single colloid and systematically investigate the effect of AC field frequency and salt concentration. The simulation results are compared with predictions from classical Maxwell-Wagner theory and electrokinetic theory.

Reservoir‐based dielectrophoresis for microfluidic particle separation by charge

The separation of particles from a complex mixture is important to a wide range of applications in industry, biology, medicine etc. This work demonstrates a microfluidic approach to separate similar-sized fluorescent and nonfluorescent particles based upon the difference in their surface charges inside a reservoir. Such a separation exploits the reservoir-based dielectrophoresis, which is induced by the inherent electric field gradient formed at the reservoir-microchannel junction, to isolate the trapped fluorescent particles within the reservoir from the streaming nonfluorescent particles. The effects of the DC field magnitude (or equivalently the electrokinetic flow magnitude) and the AC field frequency of DC-biased AC electric fields are investigated on particle separation. A numerical model is also developed to simulate the electrokinetic transport behaviors of the two types of particles. This demonstrated reservoir-based dielectrophoresis particle sorter can operate in parallel to increase the flow throughput. It is suitable for integration with other functional parts into lab-on-a-chip devices for diverse particle handling.