Dc Field Strength Research Articles

A theoretical study of harmonic generation in a short period AlGaN/GaN superlattice induced by a terahertz field

Based on an improved energy dispersion relation, the terahertz field induced nonlinear transport of miniband electrons in a short period AlGaN/GaN superlattice is theoretically studied in this paper with a semiclassical theory. To a short period superlattice, it is not precise enough to calculate the energy dispersion relation by just using the nearest wells in tight binding method: the next to nearest wells should be considered. The results show that the electron drift velocity is 30% lower under a dc field but 10% higher under an ac field than the traditional simple cosine model obtained from the tight binding method. The influence of the terahertz field strength and frequency on the harmonic amplitude, phase and power efficiency is calculated. The relative power efficiency of the third harmonic reaches the peak value when the dc field strength equals about three times the critical field strength and the ac field strength equals about four times the critical field strength. These results show that the AlGaN/GaN superlattice is a promising candidate to convert radiation of frequency ω to radiation of frequency 3ω or even higher.

Analysis of the E-J Curve of HTS Tapes Under DC and AC Magnetic Fields at 77 K

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The evaluation of the electrical characteristics of technical HTS tapes are of the key importance in determining the design and operational features of superconducting power apparatuses as well as to understand the external factors which affect the superconducting performance. In this work we report the systematic measurements of the electric field versus current density, <formula formulatype="inline"><tex Notation="TeX">$E-J$</tex></formula> relation of short samples for three commercial HTS tapes (BSCCO-2223 tapes, with and without steel reinforcement, and YBCO-coated conductor) at 77 K. In order to get sensitive and noiseless voltage signals the measurements were carried out with DC transport current and subjecting the broad surface tape to DC (0–300 mT) and AC (0–62 mT, 60 Hz) magnetic fields. The voltage is measured by a sensitive nanovoltmeter and the applied magnetic field is monitored by a Hall sensor placed on the tape broad surface. The comparison between the <formula formulatype="inline"> <tex Notation="TeX">$E-J$</tex></formula> results obtained from the three tapes was done by fitting a power-law equation for currents in the vicinity of the critical current. For the current regime below the critical one a linear correlation of the electric field <formula formulatype="inline"><tex Notation="TeX">$E$</tex> </formula> against the current density is observed. The BSCCO samples presented the same behavior, i.e., a decreasing of <formula formulatype="inline"><tex Notation="TeX">$n$</tex></formula>-index with the increasing DC and AC magnetic field strength. Under AC field the decreasing slope of <formula formulatype="inline"> <tex Notation="TeX">$n$</tex></formula>-index is steeper as compared to DC field. The <formula formulatype="inline"><tex Notation="TeX">$n$</tex></formula>-index curve for the YBCO tape showed similar behavior for AC field, however under DC field in the 0–390 mT range exhibited a slight decreasing of the <formula formulatype="inline"><tex Notation="TeX">$n$</tex></formula>-index. </para>

High sensitivity sensor for continuous direct measurement of bipolar charged aerosols

The disruptive generation of aerosols is known to cause particles to carry electrostatic charges. Anthropogenic aerosol may have a standard deviation of charges when generated that is different to other sources or equilibrated aerosols. A simple, low cost ‘Bipolar Charged Aerosol Sensor’ (BCAS) has been developed to continuously measure charged aerosol in the ambient environment in real-time. Direct measurement of the current released from the charged aerosol particles when they deposit onto an electrode in a DC field is achieved using custom designed, sensitive electrometers. The mobility range of particles collected is defined by the DC field strength, air flow rate through the instrument and the electrode geometry. The mobility range of interest has been selected based on measurements made previously with a complex Laser Doppler Velocimetry (LDV) based instrument. The BCAS has been assessed in laboratory. The sensor design and initial measurement data will be discussed.

Suppression of Photoionization by a Static Field

The dc field Stark effect is studied theoretically for atoms in high intensity laser fields. We prove that the first-order perturbation corrections for the energy and photoionization rate vanish when the dc field strength serves as a perturbational strength parameter. Our calculations show that by applying a dc field in the same direction as the polarization direction of the ac field, the photoinduced ionization rate is almost entirely suppressed. This suppression is attributed to changes in the phase shift of the continuum atomic wave functions which can be controlled by the dc field.

Radioelectric effect in a superlattice under the action of an elliptically polarized electromagnetic wave

The density of the current associated with the drag of charge carriers in a superlattice by an elliptically polarized electromagnetic wave is calculated. Two cases of the mutual orientation of the Umov-Poynting vector and the superlattice axis, i.e., their parallel and perpendicular orientations, are analyzed. It is shown that, for the parallel orientation, the radioelectric effect can change sign. The influence of the longitudinal dc electric field on the radioelectric effect is investigated under the conditions where a circularly polarized electromagnetic wave propagates along the superlattice axis. The current density is studied as a function of the electric field strength and the electromagnetic wave intensity. It is demonstrated that the direction of the electric current is changed at specific values of the dc field strength and the wave intensity.

Inductive detection of magnetostrictive resonance

We report an inductive method detecting a magnetostrictive resonance signal, which is applied to an ultrasonic magnetostrictive transducer sample. Slab shaped ferrite samples are mounted in a rf coil and actuated by a pulsed rf magnetic field. A dc magnetic field is also applied and the resonance signal from the sample is detected by the same coil after the rf field is turned off. The detector system is identical to a conventional pulse NMR system with quadrature detection. The detected signal is Fourier transformed and wide band spectrum data are obtained. The resonance spectrum data show strong dependencies on the bias dc field strength and direction as well as the dimension of the sample.

Helium atom in presence of DC and AC electric fields

We investigate the effect of a static electric field on photoionization of the He atom in the ground 1S and low-lying 2S and 2P excited states. The field-affected ionization potential and photoionization cross-section are determined from the complex eigenvalues of the time-dependent Schrodinger equation solved by the complex rotation method in the Floquet ansatz. Accuracy of the method is enhanced by the use of the Hylleraas basis set. For the ground state of helium, we find that the total photoionization cross-section remains constant or decreases as a function of the DC field strength until this field reaches a certain critical value. For the low-lying excited states, effect of the static field is similar to the ordinary DC Stark effect.

Dielectric nonlinearity of the glassy material Bi1.8Pb0.3Sr2Ca2Cu2.7K0.3Oz

AbstractThe dependence of the dielectric permittivity on the strength of a dc electric field in the glassy dielectric material Bi1.8Pb0.3Sr2Ca2Cu2.7K0.3Oz was studied. It was revealed that the value of the dielectric constant is decreased by a quadratic law as the dc field strength is increased. Such behavior of the dielectric permittivity under external bias electric field speaks in favor of a dielectric nonlinearity, which can be described in frames of a thermodynamic approach. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Spectrum of electromagnetic excitations in a dc-biased semiconductor superlattice

We analyze the spectrum and stability of transverse electromagnetic excitations in a dc-biased semiconductor superlattice in a microstrip line geometry. We show the existence of a critical dc field strength that corresponds to equality of the Bloch frequency and the electron plasma frequency at the bottom of the miniband. At this field strength the frequency–wave-number dispersion curves change their character. For a subcritical dc field, the spectrum of electromagnetic excitations consists of propagating transverse electromagnetic wave and quasistatic Bloch oscillations. In the case of overcritical dc field, transverse electromagnetic wave and Bloch oscillations are hybridized. There is a low-frequency branch that is unstable, whereas the high-frequency one is always stable. Instability of the low-frequency mode makes possible the creation of a tunable Bloch-oscillator-based tetrahertz (THz) amplifier and generator using microstrip line and microcavity devices. We also find that the low-frequency branch displays a negative refraction region which is associated with the backward wave excitation in a superlattice; this gives the potential possibility of the subwavelength focusing of electromagnetic radiation in THz frequency domain by means of a superlattice flat slab.

Fingerprint Pattern and Dielectric Property in Pb(Zn1/3Nb2/3)O3

The effects of an electric field on the fingerprint pattern of the relaxor Pb(Zn1/3Nb2/3)O3 (PZN) were investigated using atomic force microscopy (AFM). It was found that the pattern of the domain wall structure in the relaxor, called the fingerprint pattern, strongly depends on the dc field strength, and disappears for a weak field of 0.2 kV/cm, which is about 1/10 of the coercive field in the D–E hysteresis loop. The dc field dependence of the dielectric constant in PZN was also investigated. It was found that the dielectric constant under the dc field, being of the same order as the coercive field, gradually decreases to 84% with time, and the characteristic dispersion of the relaxor becomes remarkably small with the appearance of the piezoelectric response. We discussed the contribution from the small domain wall structure to the dielectric property of the relaxor.

A predictive model for the permeability tensor of magnetized heterogeneous materials

This paper describes a mathematical model enabling the calculation of the effective permeability tensor of heterogeneous magnetic materials set in various magnetization states. The theoretical approach we propose permits to predict the microwave behavior of unsaturated magnetic loaded composites as well as those of unsaturated polycrystalline ferrites. The model gives all the complex components of the permeability tensor in a single calculation phase whatever the magnetization state of the material is. This is useful for the computer-aided design of microwave magnetic devices. The calculation of the effective permeability tensor is carried out from a limited number of parameters: saturation magnetization, anisotropy field, damping factor, external dc field strength, concentration, and shape of the magnetic particles or domains. The model is predictive in the way that the magnetization state of the material is determined from the strength of the external dc magnetic field applied on the material.

Non-invasive magnetic detection of human injury currents

Injury currents are a hallmark of acute lesions in polarized cells. Our objective was to develop a non-invasive technique for monitoring human near-DC injury currents in vivo. Using diagnostic muscle biopsy as controlled paradigm, injury-related magnetic DC-fields were mapped for 60 min postsurgery over leg muscle lesions of 9 subjects. A 49-channel magnetometer was used in combination with a mechanical horizontal modulation of the subject beneath the sensor array. Magnetic DC-field maps showed salient differences between biopsy and contralateral legs in 8/9 patients with a characteristic slowly decaying field in all biopsy legs. A variety of anomalous DC-field patterns was recorded over the biopsy sites, corresponding to theoretically predicted geometric variations of equivalent DC-current dipoles, i.e. wound surfaces, pointing into opposing muscle fibre ends. By contrast, all control measurements showed an elongated dipolar DC-field pattern. Additionally, mean global DC-field strengths were significantly higher over biopsy legs compared to the contralateral site. Our pilot data illustrate that human injury currents can be detected using non-invasive magnetometry. Thus, DC-magnetometry may provide an essential new tool for clinical monitoring of injury currents, possibly also in brain tissue, e.g. in case of anoxic or peri-infarct depolarizations.

Electric field regulating behaviour of microwavepropagation in ER fluids

The model of microwave propagation through electrorheological (ER) fluids has been established according to the experimental facts that the structure and the dielectric properties of ER fluids change with the external electric field. The theoretical simulation shows that the microwave transmittance increases with the dielectric constant of ER fluids if the dielectric constant of the ER fluids is larger than that of the container. Otherwise it decreases with the dielectric constant. The experimental results revealed that the microwave transmittance in the ER fluids could be adjusted by changing the DC field strength. The microwave transmittance increases with the field strength when the concentration of the fluid is low, but it decreases when it is high. The phenomenon that the microwave transmission could be adjusted in the ER fluids can be attributed to the changes of the structure and the dielectric properties of the ER fluids under the external electric field.

Mathematical properties of the rotational diffusion equation

An iterative scheme for the approximative solution of the Galerkin system of the linear rotational diffusion equation for the chromophore distribution in nonlinear optical polymers is formulated. Uniformly valid asymptotic steady-state solutions are obtained in terms of power series expansions in the normalized dc field and the convergence properties of the scheme are discussed. Moreover, by neglecting the effect of the ac field, it is proved by means of the Galerkin-system approach that the equilibrium solution of the rotational diffusion equation acts as a global attractor for any initial distributions. In addition, the decay towards this distribution is a purely exponential decay for small and moderate values of the dc-field strength. When the dc-field strength exceeds a certain threshold, this relaxation process is characterized as a damped oscillation.

Breakup ofH2+by one photon in the presence of a strong dc field

We calculate the frequency profile of the rate for breakup of ${\mathrm{H}}_{2}^{+}$ by one photon in the presence of a strong dc field. After photoexcitation of the molecular ion from its ground $1s\ensuremath{\sigma}$ electronic state to the $2p\ensuremath{\sigma}$ electronic state, the nuclei are temporarily trapped by a dc field-induced barrier with turning point ${R}_{\mathrm{max}}.$ However, the nuclei are released through either rotation of the internuclear axis (which, for the sake of tractability, we ignore) or dc-field ionization involving resonant transfer from the $2p\ensuremath{\sigma}$ state to a more highly excited state. The most prominent resonance occurs at an internuclear separation that is less than ${R}_{\mathrm{max}}$ and, in a.u., is approximately ${3/(8F}_{\mathrm{dc}})+3,$ where ${F}_{\mathrm{dc}}$ is the dc field strength. We interpret the change in the frequency profile, as ${F}_{\mathrm{dc}}$ varies, in terms of different aspects of the dynamics of the breakup process, in particular the dispersion of the probability distribution of the nuclei due to over-the-barrier dissociation from the $1s\ensuremath{\sigma}$ state.

DC Field Dependence of the Dielectric Dispersion in the 2/7 Phase of Betaine Calcium Chloride Dihydrate

DC field dependence of the dielectric dispersion in the 2/7 phase of betaine calcium chloride dihydrate was measured in the frequency region between 100 Hz and 10 MHz. It was found that there exists a dielectric dispersion, and both the relaxation frequency and the reciprocal strength are proportional to the DC field strength, therefore their ratio is independent of the DC field strength. We discussed these results on the basis of the phenomenological consideration taking the interaction between domain walls into account.

High Rydberg states of argon: Stark effect and field-ionization properties

The Rydberg states with principal quantum number located below the ground state of the ion have been studied by pulsed field ionization following single-photon excitation out of the ground state of Ar. The linewidth of the tunable extreme ultraviolet (XUV) laser source used enabled high-resolution measurement of the Stark effect over a wide range of principal quantum numbers and electric field strengths. Particular attention was given to the ionization of high Rydberg states induced by DC and pulsed electric fields. The lowering (expressed in ) of the ionization threshold by DC electric fields is accurately described by when the electric field strength F is expressed in , a result that is in good agreement with predictions of the classical saddle-point model for field ionization. The field-ionization threshold is very sharp: its width decreases from 0.7 to when the DC field strength is reduced from 580 to . Apart from the Stark states located in a very narrow energy range around the saddle-point energy in the potential which are found to ionize very slowly, all Stark states located below the saddle-point energy have lifetimes exceeding several microseconds, whereas those located beyond the saddle-point energy ionize within less than 20 ns. The very slow field ionization that is observed in a narrow range of energies around the classical saddle point can be used to obtain high state selectivity in the pulsed field ionization. The pulsed field-ionization behaviour observed in argon suggests that the rule that is now commonly assumed in the analysis of pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) spectra to describe the low-wavenumber onset of a line relative to the position of the corresponding field-free ionization threshold must be used with caution.

Control of high-order harmonic generation in strong laser fields.

Using solutions of the time-dependent Schr\"odinger equation for the ${\mathrm{H}}_{2}^{+}$ molecular ion, we examine various scenarios for controlling high-order harmonic generation with combinations of laser fields. It is found that (i) the harmonic generation ``plateau'' can be modified by combining a laser field with its third harmonic field, (ii) even harmonics are controllable using the combination of a laser field and a weaker second harmonic field, and (iii) the phases of high-order harmonics in a strong driving field can also be changed by additional fields. In particular, when a weak dc electric field is applied, we prove that the phase shifts of all harmonics linearly depend on the dc field strength. In addition, when an ultrastrong static magnetic field is applied collinearly with the driving laser polarization, harmonic generation efficiency is enhanced and higher harmonics beyond the usual ``cutoff'' are generated as a result of the squeezing of the transversely spreading electron wave packets. The enhancement is a confirmation of the electron-ion recollision mechanism for high-order harmonic generation.

Nuclear transfer and electrofusion in bovine in vitro-matured/in vitro-fertilized embryos: effect of media and electrical fusion parameters.

In this study, micromanipulation and electrofusion conditions for the cloning of in vitro-produced bovine embryos (here after termed IVM/IVF embryos) derived from in vitro-matured (IVM) and in vitro-fertilized (IVF) oocytes were established. The effect of DC field strength on the fusion rate was tested in a model system using pronuclear stage embryos in which a cytoplasmic vesicle was removed and reinserted. Efficient fusion (80%) was obtained by applying a pulse of 1.75 kV/cm for 40 microseconds. In vitro development of manipulated pronuclear stage embryos was as efficient as that of unmanipulated control embryos. Different fusion media were compared in the cloning procedure, using IVM oocytes as recipients and blastomeres from day 6 IVM/IVF donor embryos. Zimmermann cell fusion medium reduced the lysis of nuclear transfer embryos compared to F300 (5% vs. 25%). The effects of drugs disrupting the microfilaments and microtubuli were determined. Neither the addition of cytochalasin B (CCB) for 1 hr in the postfusion medium nor incubation of donor blastomeres with nocodazole had a significant effect on the fusion or cleavage rate of the nuclear transfer embryos. Additional experiments demonstrated that there was no difference in developmental potential between nuclear transfer embryos allowed to develop in vitro or in vivo and that the embryos gave a 15% pregnancy rate in recipient cattle.

Ultrafast optical generation of carriers in a dc electric field: Transient localization and photocurrent.

Recent experimental observations of terahertz radiation produced by the ultrafast optical excitation of surface depletion layers of GaAs and InP have generated a lot of interest but its physical origins are still not fully understood. The source of the radiation is believed to be either the time-dependent transport current produced by the optically generated carriers in the depletion field, or the displacement current due to creation of polarized electron-hole pairs. We show that, in general, both mechanisms must be included. The ultrafast optical generation in a dc field is shown to result in the creation of carriers in transiently localized states that evolve into delocalized states, causing transport current in the process. By calculating the dipole moments of these localized states, we are able to determine the time-dependent polarization and photocurrent including both transport and displacement contributions. We find that the displacement contribution comes mainly from the virtual carriers while the real carriers are responsible for the transport current. The competition between transport and displacement current leads to a nontrivial dependence of the overall signal on the dc field strength, the excitation duration, and the detuning. In particular, we predict a sign reversal of the signal at sufficiently high detunings in agreement with recent experimental findings.