Intraband light absorption in quasi-two-dimensional systems in external electric and magnetic fields

Intraband light absorption in parabolic quantum wells is studied with an electric field directed along the spatial quantization axis and a magnetic field parallel to the plane of the size-confined system. In such a geometry direct optical transitions between the quantum-well levels are possible, the peak light absorption coefficient reaches large values (∼3×10² cm⁻¹), and the frequency of the absorption maximum depends on magnetic field strength. It is shown for the normal incidence of electromagnetic waves that the level of absorption decreases with increasing electric field strength and that it is incorrect to confine the calculations to the Born approximation in strong magnetic fields. Fiz. Tekh. Poluprovodn. 33, 828–831 (July 1999)     

Second-order model of membrane electric field induced by alternating external electric fields

With biological cells exposed to ac electric fields below 100 kHz, external field is amplified in the cell membrane by a factor of several thousands (low-frequency plateau), while above 100 kHz, this amplification gradually decreases with frequency. Below 10 MHz, this situation is well described by the established first-order theory which treats the cytoplasm and the external medium as pure conductors. At higher frequencies, capacitive properties of the cytoplasm and the external medium become increasingly important and thus must be accounted for. This leads to a broader, second-order model, which is treated in detail in this paper. Unlike the first-order model, this model shows that above 10 MHz, the membrane field amplification stops decreasing and levels off again in the range of tens (high-frequency plateau). Existence of the high-frequency plateau could have an important impact on present theories of high-frequency electric fields effects on cells and their membranes.     

Pacemaker interference and low-frequency electric induction in humans by external fields and electrodes

The possibility of interference by low-frequency external electric fields with cardiac pacemakers is a matter of practical concern. For pragmatic reasons, experimental investigations into such interference have used contact electrode current sources. However, the applicability to the external electric field problem remains unclear. The recent development of anatomically based electromagnetic models of the human body, together with progress in computational electromagnetics, enable the use of numerical modeling to quantify the relationship between external field and contact electrode excitation. This paper presents a comparison between the computed fields induced in a 3.6-mm-resolution conductivity model of the human body by an external electric field and by several electrode source configurations involving the feet and either the head or shoulders. The application to cardiac pacemaker interference is also indicated.     

Response of a heavy electronic component to harmonic excitations applied to its external electric leads

Summary External electric leads, soldered into plated-through-holes (PTHs) of a printed circuit board (PCB), provide, in addition to electrical interconnection, also mechanical support for heavy (high mass) electronic components mounted on the board. The leads can possibly break, if the PCB support contour is subjected to extensive vibrations that are being transmitted to the heavy component. In this analysis we examine the dynamic response (forced vibrations) of a heavy component to harmonic (sine) excitations applied to its external electric leads. The vibrations are due to the angular oscillations of the clamped ends of the leads. These oscillations, in their turn, are due to the vibrations of the PCB support contour in the PCB through-thickness direction. The obtained results can be used to evaluate the stresses occurring in the electric leads because of the harmonic oscillations of the PCB support contour. These results can be used also for the assessment of the "output" spectrum of the dynamic stresses, when the oscillations of the PCB support contour are random and are described by an appropriate "input" power spectrum.      

Controlling a unified chaotic system to hyperchaotic

This brief presents a simple technique using a sinusoidal parameter perturbation control input to drive a unified chaotic system to hyperchaotic. The original chaotic system is a three-dimensional autonomous system that has a broad spectrum of chaotic behaviors with the Lorenz and the Chen systems as two extremes of the spectrum. The control input is a simple sinusoidal function cos(/spl omega/t) with a constant parameter /spl omega/. The hyperchaotic system is not only demonstrated by computer simulations but also verified with bifurcation analysis and implemented via an electronic circuit.     

Electron and hole ionization rates in epitaxial silicon at high electric fields

Ionization rates for electrons and holes are extracted from photomultiplication measurements on silicon p⁺n mesa diodes for electric fields of 2·0 × 10⁵?7·7 × 10⁵ V/cm at temperatures of 22, 50, 100 and 150°C. These results are particularly pertinent to the analysis of high-frequency (～ 100 GHz) silicon IMPATT diodes.The rates obtained here are in reasonable agreement with previously published data of van Overstraeten and DeMan, although slightly larger in magnitude. Calculated curves of breakdown voltage vs background doping level are presented using the room temperature ionization rates. Also a comparison is made to previously reported rates. The new rates provide a closer agreement between predicted and measured breakdown voltages for breakdown voltages less than 70 V.     

Controlling the dynamic behavior of light emitting electrochemical cells

Light emitting electrochemical cells (LECs) present an attractive route towards cost efficient lighting applications. By utilizing ionic phosphorescent transition metal complexes, efficient electroluminescence can be realized from a single layer device using air stable electrodes. These devices achieve efficient charge carrier injection due to ion accumulation at the interface upon driving, resulting in a dynamic response upon device operation. Here we investigate the device operation by using fast current and luminance versus voltage sweeps during normal fixed bias operating. A universal set of JL–V curves can be identified in which different regimes are observable. The speed and extent in which a LEC evolves through this set of curves can be controlled by varying the driving voltage, enabling the device to operate in it maximum efficacy state.     

Interband light absorption in size-confined systems in uniform electric fields

A simple approach to calculation of the interband absorption coefficient in a uniform electric field is developed. This approach provides a means for studying the special features of electroabsorption in a wide class of semiconductor systems on the basis of the most general relationships. The approach is used to study the electroabsorption in two-dimensional systems with different profiles of their one-dimensional potential, quantum wells, and superlattices in magnetic fields.     

Luminescence in quantum-confined cadmium selenide nanocrystals and nanorods in external electric fields

It is found that the absorption and luminescence spectra of CdSe nanocrystals and nanorods depend on the external electric field. It is shown that the external electric field quenches the P-polarized photoluminescence of CdSe nanorods to a degree higher than the degree of field-induced quenching of the S-polarized photoluminescence. It is established that the nanocrystals are more sensitive to the external electric field than the nanorods. The effect of the external electric field on the luminescence properties of the semiconductor nanorods is discussed.     

Experimental observation of splitting of the light and heavy hole bands in elastically strained GaAsN

Twin peak photoluminescence of a GaAsN solid solution grown on GaAs substrate has been observed at room temperature. The peak splitting increases with an increase in the nitrogen content of the ternary compound. The observed form of the spectra is attributed to the presence of two transitions involving light and heavy holes. The splitting of light-and heavy-hole levels is due to elastic strain in GaAsN layers grown on the GaAs surface.     

Prediction of chaotic behavior

This paper considers the prediction of chaotic behavior using a master-slave synchronization scheme. Based on the stability theory for retarded systems using a Lyapunov-Krasovskii functional, we derive a sufficient condition for perfect state prediction of the master system via a time-delayed output signal of the slave system. The obtained result is based on the delay-dependent stability of time-delay systems. In addition, we derive an upper bound of the admissible time delay by using linear matrix inequality techniques. Finally, we show the effectiveness of the proposed predictor by two numerical examples.     

Experimental corrections for the hot hole distribution function in germanium in crossed electric and magnetic fields

A procedure for finding corrections for the hot hole distribution functions obtained from absorption measurements on intersubband transitions of hot heavy and light holes in germanium in crossed electric and magnetic fields is proposed. This procedure is based on the multiple-valued dependence of the absorption on the photon energies of transitions from light holes to a subband split off as a result of the spin-orbit interaction. Taking these corrections into account improves the agreement between the gain for direct optical transitions between the light and heavy hole subbands calculated from measurements of the absorption in the near-infrared and direct measurements in the far-infrared. Fiz. Tekh. Poluprovodn. 32, 1197–1199 (October 1998)     

Calculation of electric fields due to lines of charge

The amplitude of the electric field due to a straight line segment of uniform electric charges is shown to be given by E=2q (4πDε₀)^-1 sen (α/2) where q is the linear charge density, α is the viewing angle (from the observation point to the filament extremities), and D is the distance to the filament; furthermore, the direction of the vector E lies on the bisector of the viewing angle. This result can greatly reduce the computation time in the analysis of EMC (electromagnetic compatibility) problems involving static or quasi-static charge distributions     

Modeling assemblies of biological cells exposed to electric fields

Gap junctions are channels through the cell membrane that electrically connect the interiors of neighboring cells. Most cells are connected by gap junctions, and gaps play an important role in local intercellular communication by allowing for the exchange of certain substances between cells. Gap communication has been observed to change when cells are exposed to electromagnetic (EM) fields. In this work, the authors examine the behavior of cells connected by gap junctions when exposed to electric fields, in order to better understand the influence of the presence of gap junctions on cell behavior. This may provide insights into the interactions between biological cells and weak, low-frequency EM fields. Specifically, the authors model gaps in greater detail than is usually the case, and use the finite element method (FEM) to solve the resulting geometrically complex cell models. The responses of gap-connected cell configurations to both dc and time harmonic fields are investigated and compared with those of similarly shaped (equivalent) cells. To further assess the influence of the gap junctions, properties such as gap size, shape, and conductivity are varied. The authors' findings indicate that simple models, such as equivalent cells, are sufficient for describing the behavior of small gap connected cell configurations exposed to dc electric fields. With larger configurations, some adjustments to the simple models are necessary to account for the presence of the gaps. The gap junctions complicate the frequency behavior of gap-connected cell assemblies. An equivalent cell exhibits lowpass behavior. Gaps effectively add a bandstop filter in series with the low-pass behavior, thus lowering the relaxation frequency. The characteristics of this bandstop filter change with changes to gap properties. Comparison of the FEM results to those obtained with simple models indicates that more complex models are required to represent gap-connected cells     

在外电场作用下BaTiO3单晶中铁电畴极化反转的透射电镜原位研究

通过在样品表面施加电场,利用透射电子显微镜原位观察和研究了BaTiO3单晶中铁电畴的极化反转过程。结果表明,在外加电场作用下,铁电畴发生重新取向,其极化方向逐渐向着与电场方向平行的方向转变．当撤去电场后,又趋向于恢复到初始状态。     

Intersubband far-infrared emission and magnetotransport of 2D hole gas in a strong in-plane electric field and transverse magnetic fields

We observe FIR emission and magnetotransport phenomena of modulation doped p-GaAs/AlGaAs multi-quantum wells structures of different well width in a streaming regime. This FIR emission originates from radiative transitions between the upper quantum subbands, where hot holes are trapped by the emission of LO-phonons and the ground quantum state. The importance of the light-heavy states mixing with an increase of confinement effect is shown experimentally. The magnetoresistance in a strong in-plane electric field applied along the [110] crystallographic direction is found to be negative, which is explainedDby strong warping of the constant hole energy surfaces in two-dimensional quantum wells.     

Resonances related to an array of InAs quantum dots and controlled by an external electric field

Photoluminescence of multilayer structures with InAs quantum dots grown in the p-n junction in GaAs by molecular-beam epitaxy is studied. Formation of vertical columns of quantum dots is verified by the data of transmission electron microscopy. It is shown that a natural increase in the size of quantum dots from layer to layer brings about their vertical coalescence at the upper part of a column. An unbalance of electronic levels caused by the enlargement of quantum dots was compensated by an external electric field, so that the resonance of ground electronic states in the column was attained. The onset of resonances was checked by the methods of steady-state and time-resolved photoluminescence. It is shown that, in the case of a resonance, the photoluminescence intensity and the radiative lifetime of excitons increase (up to 0.6–2 ns), while the time of tunneling of charge carriers becomes shorter (shorter than 150 ps). Outside the resonances, tunneling of electrons is appreciably enhanced owing to the involvement of longitudinal optical phonons. If only these phonons are involved, the time of nonresonance tunneling between quantum dots becomes shorter than the time of relaxation of charge carriers from the barrier (100 and 140 ps, respectively).     

Investigating membrane breakdown of neuronal cells exposed to nonuniform electric fields by finite-element modeling and experiments

High electric field strengths may induce high cell membrane potentials. At a certain breakdown level the membrane potential becomes constant due to the transition from an insulating state into a high conductivity and high permeability state. Pores are thought to be created through which molecules may be transported into and out of the cell interior. Membrane rupture may follow due to the expansion of pores or the creation of many small pores across a certain part of the membrane surface. In nonuniform electric fields, it is difficult to predict the electroporated membrane area. Therefore, in this study the induced membrane potential and the membrane area where this potential exceeds the breakdown level is investigated by finite-element modeling. Results from experiments in which the collapse of neuronal cells was detected were combined with the computed field strengths in order to investigate membrane breakdown and membrane rupture. It was found that in nonuniform fields membrane rupture is position dependent, especially at higher breakdown levels. This indicates that the size of the membrane site that is affected by electroporation determines rupture.     

Electron dynamics in superlattices subject to crossed magnetic and electric fields

The motion of charge particles in a superlattice structure subject to the crossed magnetic and electric fields, both applied in the in-plane direction, is discussed with respect to the novel type of terahertz oscillations suggested recently, [M. Orlita, R. Grill, L. Smr?ka, M. Zvára, Phys. Rev. B 74 (2006) 125312]. The effects of the finite size of the superlattice is considered in comparison with the standard Bloch oscillations.