Electric Field Configuration Research Articles

Characterization of magnetic states of graphene quantum dots of different shapes by application of electric field

In this work, we have demonstrated that the optical properties of magnetic states of graphene quantum dots (GQDs) is significantly influenced by their shape and application of external electric field. Our computations at the restricted Hartree-Fock and unrestricted Hartree-Fock level employing the Pariser-Parr-Pople (PPP) model based Hamiltonian have illustrated that the optical band-gap of the nonmagnetic (NM) state changes much more drastically than the antiferromagnetic (AFM) and ferromagnetic (FM) configurations as the shape of the GQD is altered. Further, the geometric configuration of GQDs and external electric field govern the energetic ordering of their magnetic states and consequently, nature of phase transition exhibited by these quantum dots. In addition, the optical spectra of AFM state of diamond shaped graphene quantum dot exhibits a spin-dependent splitting on application of external electric field while no such splitting is observed for the AFM phase of hexagonal shaped graphene quantum dot. This splitting was attributed to the type of spin-density distribution in GQDs. Further, the electro-absorption (EA) spectra exhibits distinct variations for the different magnetic phases of GQDs. Hence, our results have indicated that EA spectra is an efficient method to characterize the different magnetic states of GQDs of varied shapes.

Dielectric Anisotropy Sensor Using Coupled Resonators

In this article, a new approach is proposed for the measurement of the uniaxial anisotropic dielectric constant of different planar samples by means of a single sensor. The sensor is based on a couple of straight-line coupled resonators in microstrip technology that can be excited in odd and even propagation modes. This sensor is designed on an isotropic substrate at the design frequency. Due to the electric field configuration specific for each mode, it is possible to relate these modes to the dielectric constant in two different directions (parallel and perpendicular) of a dielectric material placed on top of the sensor. This technique is used for the successful characterization of the dielectric constant anisotropic of anisotropic dielectrics (FR4, Rogers 4350B, and Arlon Diclad 880), and the isotropic material PTFE.

The Effect of the Polarity and Design of the Discharge System on the X-ray Spectrum of the Micropinch Discharge Plasma

The spectral composition and X-ray radiation yield of a micropinch discharge plasma obtained at a facility of the low-inductance vacuum spark type are studied as a function of the polarity and design of the electrodes of the discharge system. It is shown that the X-ray radiation of the micropinch discharge plasma depends on the configuration of the electric field in the interval between the discharge electrodes, which determines the amount of erosion products involved in the micropinch process, resulting in the increase in the plasma density and its radiative characteristics.

Numerical simulation of a novel non-uniform electric field design to enhance the electrocoalescence of droplets

Abstract The behavior of binary droplet collision, exposed to an external electric field is studied numerically. Based on a fundamental understanding of the role of the electric field in the coalescence of aqueous drops in an immiscible oil phase, it is possible to optimize the design and operation of these external stimulators. In this research, numerical modeling of electrocoalescence is implemented, using Computational Fluid Dynamics (CFD) methods. Level set method is used to evaluate water droplet evolution in an oil phase under a novel concentric semi-elliptic non-uniform electric field configuration. The observations show that the shape of applied non-uniform fields can remarkably alter the electrocoalescence performance in a nontrivial way, which is sensitive to the drop-medium system. In addition to the shape of the medium, some other parameters such as droplets initial distance, initial skew angle of the droplets, applied voltage amplitude and oil viscosity are analyzed in the present paper. Our results can be useful in designing an optimum electrode configuration, achieving an efficient electrocoalescence.

Electrorheology of a dilute emulsion of surfactant-covered drops

We investigate the effects of surfactant coating on a deformable viscous drop under the combined action of shear flow and a uniform electric field. Employing a comprehensive three-dimensional approach, we analyse the non-Newtonian shearing response of the bulk emulsion in the dilute suspension regime. Our results reveal that the location of the peak surfactant accumulation on the drop surface may get shifted from the plane of shear to a plane orthogonal to it, depending on the tilt angle of the applied electric field and strength of the electrical stresses relative to their hydrodynamic counterparts. The surfactant non-uniformity creates significant alterations in the flow perturbation around the drop, triggering modulations in the bulk shear viscosity. Overall, the shear-thinning or shear-thickening behaviour of the emulsion appears to be greatly influenced by the interplay of surface charge convection and Marangoni stresses. We show that the balance between electrical and hydrodynamic stresses renders a vanishing surface tension gradient on the drop surface for some specific shear rates, rendering negligible alterations in the bulk viscosity. This critical condition largely depends on the electrical permittivity and conductivity ratios of the two fluids and orientation of the applied electric field. Also, the physical mechanisms of charge convection and surface deformation play their roles in determining this critical shear rate. As a consequence, we obtain new discriminating factors, involving electrical property ratios and the electric field configuration, which govern the same. Consequently, the surfactant-induced enhancement or attenuation of the bulk emulsion viscosity depends on the electrical conductivity and permittivity ratios. The concerned description of the drop-level flow physics and its connection to the bulk rheology of a dilute emulsion may provide a fundamental understanding of a more complex emulsion system encountered in industrial practice.

Electric field assisted amplification of magnetic fields in tilted Dirac cone systems

We show that the continuum limit of the tilted Dirac cone in materials such as $8Pmmn$-borophene and layered organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ deformation of the Minkowski spacetime of Dirac materials. From its Killing vectors we construct an emergent tilted-Lorentz (t-Lorentz) symmetry group for such systems. With t-Lorentz transformations we are able to obtain the exact solution of the Landau bands for a crossed configuration of electric and magnetic fields. For any given tilt parameter $0\le\zeta<1$ if the ratio $\chi=v_FB_z/cE_y$ of the crossed magnetic and electric fields that satisfies $\chi\ge 1+\zeta$ one can always find appropriate t-boosts in both valleys labeled by $\pm$ in such a way the electric field can be t-boosted away, whereby the resulting pure effective magnetic field $B^\pm_z$ governs the Landau level spectrum around each valley. The effective magnetic field in one of the valleys is always larger than the applied perpendicular magnetic field. This amplification comes at the expense of of diminishing the effective field in the opposite valleyand can be detected in various quantum oscillation phenomena in tilted Dirac cone systems. Tuning the ratio of electric and magnetic fields to $\chi_{\rm min}=1+\zeta$ leads to valley selective collapse of Landau levels. Our geometric description of the tilt in Dirac systems reveals an important connection between the tilt and an incipient "rotating source" when the tilt parameter can be made to depend on spacetime in certain way.

Performance of optically readout GEM-based TPC with a 55Fe source

Optical readout of large Time Projection Chambers (TPCs) with multiple Gas Electron Multipliers (GEMs) amplification stages has shown to provide very interesting performances for high energy particle tracking. Proposed applications for low-energy and rare event studies, such as Dark Matter search, ask for demanding performance in the keV energy range. The performance of such a readout was studied in details as a function of the electric field configuration and GEM gain by using a 55Fe source within a 7 litre sensitive volume detector developed as a part of the R&D for the CYGNUS project. Results reported in this paper show that the low noise level of the sensor allows to operate with a 2 keV threshold while keeping a rate of fake-events lesser than 10 per year. In this configuration, a detection efficiency well above 95% along with an energy resolution (σ) of 18% is obtained for the 5.9 keV photons demonstrating the very promising capabilities of this technique.

Spin-torque resonance due to diffusive dynamics at the surface of a topological insulator

We investigate spin-orbit torques on magnetization in an insulating ferromagnetic (FM) layer that is brought into a close proximity to a topological insulator (TI). In addition to the well-known field-like spin-orbit torque, we identify an anisotropic anti-damping-like spin-orbit torque that originates in a diffusive motion of conduction electrons. This diffusive torque is vanishing in the limit of zero momentum (i. e. for spatially homogeneous electric field or current), but may, nevertheless, have a strong effect on spin-torque resonance at finite frequency provided external field is neither parallel nor perpendicular to the TI surface. The required electric field configuration can be created by a grated top gate.

Generation of oblique electromagnetic wave by hot injection electron beam with parallel AC electric field in the magnetosphere of Saturn

The Cassini radio and plasma wave investigation is designed to study radio emissions, plasma waves, thermal plasma, and dust in the vicinity of Saturn. Plasma particles, waves and fields are the major factors on which present knowledge about Saturn’s magnetosphere is based from the three flyby missions. For the present work, our examination is focused to study those waves who have frequency lower than electron cyclotron frequency in Saturnian magnetosphere. In this paper, we investigate the growth rate of whistler mode waves in magneto-plasma of Saturn. A dispersion relation for parallel propagating whistler mode waves has been applied to the magnetosphere of Saturn. The effect of electron density, temperature anisotropy, number density and A.C frequency on the temporal growth rate of the whistler mode emission is studied. For the given magnetic field and external electric field configuration, the general dispersion relation has been derived.

Characterisation of a small electrode HPGe detector

© 2019 Elsevier B.V. Small electrode HPGe detectors in an inverted coaxial geometry are increasingly in use in applications where both high efficiency and excellent energy resolution are required. The unusual electric field configuration of these detectors results in extremely long charge collection times compared to planar and coaxial devices. In this work we have characterised such a detector using gamma-ray coincidence measurements and optimised an electric field simulation to reproduce the positional variation of detector response. We show that, alongside accurate crystal geometry and applied electric potential, a temperature correction is crucial to correctly determining appropriate charge carrier mobility parameters. This work will help to guide the future development of HPGE detectors for applications including radioactive waste assay, radio-isotope dating, and fundamental nuclear physics.

Chapter 37 - Transcranial magnetic stimulation

This review presents the neurophysiologic principles and clinical applications of transcranial magnetic stimulation (TMS) and other related techniques of noninvasive cortical stimulation. TMS can serve various purposes for diagnosis or treatment. Regarding diagnosis, TMS is mainly dedicated to the recording of motor evoked potentials (MEPs). MEP recording allows investigation of corticospinal conduction time and cortical motor control in clinical practice. Especially when using image-guided neuronavigation methods, MEP recording is a reliable method to perform functional mapping of muscle representation within the motor cortex. Using various types of paired-pulse paradigms, TMS allows the assessment of brain circuit excitability or plastic changes affecting these circuits. In particular, paired-pulse TMS paradigms are able to appraise the intracortical balance between inhibitory controls mediated by GABAergic neurotransmission and excitatory controls mediated by glutamatergic neurotransmission. Finally, TMS delivered as repetitive trains of stimulation (rTMS) may activate, inhibit, or otherwise interfere with the activity of neuronal cortical networks, depending on stimulus frequency and intensity, and brain-induced electric field configuration. Therefore by modifying brain functions, with after-effects lasting beyond the time of stimulation, rTMS opens exciting perspectives for therapeutic applications, especially in the domain of depression and chronic pain syndromes.

Tunable edge bands and optical properties in black phosphorus nanoribbons under electric field**Project supported by the National Natural Science Foundation of China (Grant No. 10947004) and the Jiangsu Government Scholarship for Overseas Studies.

For several types of the applied electric field configuration on the normal-zigzag black phosphorus nanoribbon (nZZ-BPNR) we investigate the band structure and the linear optical absorption spectrum, especially for the edge states and the corresponding low-energy absorption peaks. The obtained results show that the applied electric field can not only open another band gap at k = 0.5 point, but also can change completely the spacial probabilities of edge states in the two edge bands. The strength of electric field can tune the two band gaps at the Γ point and 0.5 point. Further, one remarkable feature is that the forbidden transition E12 and E21 are allowed. The lowest-excited-energy linear absorption peak E11 originates from the transition between two edge bands at the Γ point. Finally, in comparison with the lowest-excited-energy peaks among various configurations, the second type of electric field configuration can move this peak blue-shift larger than otherconfigurations.

Characteristic study of a quadruple GEM detector in different electric field configurations

A quadruple GEM detector has been assembled and operated using Ar/CO2 90:10 and 70:30 gas mixtures. Detailed study of the effective gain and energy resolution of the detector are performed using the X-ray spectrum of 55Fe source. The gain and energy resolution of the detector are measured and compared for two different electric field configurations, the standard field setting and the ion backflow (IBF) field setting. It is observed that for the standard field configuration the quadruple GEM detector can operate at lower operating voltages and has better energy resolution compared to IBF field setting.

Simulation and experiment of asymmetric electrode placement for electrophoretic exclusion in a microdevice.

Electrophoretic exclusion (EE) is a counterflow gradient technique that exploits hydrodynamic flow and electrophoretic forces to exclude, enrich, and separate analytes. Resolution for this technique has been theoretically examined and the smallest difference in electrophoretic mobilities that can be completely separated is estimated to be 10-13 cm2 /Vs. Traditional and mesoscale systems have been used, whereas microfluidics offers a greater range of geometries and configurations towards approaching this theoretical limit. To begin to understand the impact of seemingly subtle changes to the entrance flow and the electric field configurations, three closely related microfluidic interfaces were modeled, fabricated, and tested. These interfaces consisted of systematically varying placement of an asymmetric electrode relative to a channel entrance: leading electrode placed outside the channel entrance, leading electrode aligned with the channel, and leading electrode placed within the channel. A charged fluorescent dye is used as a sensitive and accurate probe for the model and to test the concentration variation at these interfaces. Models and experiments focused on visualizing the concentration profile in areas of high temporal dynamics, thus providing a severe test of the models. Experimental data and simulation results showed strong qualitative agreement. The complexity of the electric and flow fields about this interface and the agreement between models and testing suggests the theoretical assessment capabilities can be used to faithfully design novel and more efficient interfaces.

Silicon PIN Diodes as Neganov–Trofimov–Luke Cryogenic Light Detectors

Cryogenic rare event searches based on heat and light composite calorimeters have a common need for large area photon detectors with high quantum efficiency, good radiopurity and high sensitivity. By employing the Neganov–Trofimov–Luke effect, the phonon signal of particle interactions in a semiconductor absorber operated at cryogenic temperatures can be amplified by drifting the photogenerated electrons and holes in an electric field. We present here the most recent results of a Neganov–Trofimov–Luke effect light detector with an electric field configuration optimized to improve the charge collection within the absorber.

Effect of electric fields on the ion current signals in a constant volume combustion chamber

Ion current sensing has the potential to become a promising combustion diagnostic technique for mass productive engines. In this paper, the effect of electric fields on ion current signals measured from a series of methane/air flames in a constant volume combustion chamber (CVCC) is investigated both experimentally and numerically. Based on simultaneous flame Schlieren imaging and ion current measurement, the relation between the flame/electrodes contact area and the ion current signal waveform is explored under different electric field configurations. A CFD model, which incorporates flame plasma hydrodynamics, neutral/charged species reaction kinetics and ion-electric field interactions, is constructed. The effect of the electric field on the ion distribution and the charged species flux are analyzed, and the signal amplitude and timing are well predicted under the equivalence ratio range of Ф = 0.7–1.1. Besides, the behavior of electrons, which is normally neglected in previous studies, is also analyzed in this work. The results show that it will affect the signal as well. The electron produced in the flame front zone can hardly diffuse into the pre-flame zone (< 350 K) even its mobility is 3-4 order higher than those of the positive ions. Therefore, the anode of the ion probe, which placed in the pre-flame zone, cannot detect the ion current signal until it contacts the flame front.

Microwave synthesis of carbon onions in fractal aggregates using heavy oil as a precursor

In this work we report a method for preparing carbon onions through microwave heating of heavy oil. It was shown that microwave heating of heavy oil mixed with a carbon catalyst at 300 W leads to the growth of a several-centimetre long fractal carbon structure in just 60 s. Scanning electron microscopy (SEM) images showed that the structure is predominantly made of networks of small particles with diameters ranging from tens to hundreds of nanometres. High-resolution transmission electron microscopy (HRTEM) images revealed the presence of disordered graphitic structures, including carbon onions with diameters as small as 30 nm. This was supported by the Raman spectroscopy which showed typical spectra for carbon onions with disordered graphitic structure. CO2 gas sorption results revealed a specific surface area of up to 164 m2/g. The carbon onions are believed to form through pyrolysis of the oil into light hydrocarbons followed by nucleation and growth of concentric graphitic layers. The carbon tree, subsequently, grew through aggregation of the carbon onions and further deposition of pyrolytic carbon, with the electric field configuration favoured the longitudinal growth of thin branches. This study demonstrates that carbon onions can be prepared from cheap unrefined liquid precursors.

Hyperactivation of left inferior parietal lobule and left temporal gyri shortens resting EEG microstate in schizophrenia

ObjectiveThe momentary spatial configuration of the brain electric field at the scalp reflects quasi-stable “functional microstates” caused by activity of different intracranial generators. There is paucity in literature on the intracranial generators of resting state EEG microstate alterations in stable patients with schizophrenia. The present study aimed to investigate resting state microstate alterations and their neural generators in patients with schizophrenia and their first-degree relatives as compared to healthy controls in an attempt to establish state and trait marker. MethodThirty-four patients with schizophrenia (DSM-5 criteria), 29 first-degree relatives and 25 matched healthy controls participated in the study. Brain activity during eyes closed condition was recorded using 128 channel electroencephalography. Microstates were clustered into 5 maps across groups according to their topography. Microstate map parameters and their cortical sources were compared among groups. ResultsMap 5 mean duration (χ2(2) = 7.617, p = 0.022) was significantly lower in patients compared to controls (U = 256, p = 0.010). Maximum activation was seen in left inferior parietal lobule (MNI coordinates: −65, −35, 25, Log-Fmax = 0.748). Suprathreshold cortical voxels with increased activations were found localized at left temporal gyri. ConclusionHyperactivation in left inferior parietal lobule and temporal gyri might have shortened Map 5 duration at rest in patients with schizophrenia. This could imply microstate alterations as the potential state marker of schizophrenia.

Analysis of triple-band binary metamaterial absorber based on low-permittivity all-dielectric resonance surface

Different from the conventional metamaterial absorbers (MAs), which used metal resonance surface and ternary structure (metal–dielectric-backplane), as an alternative route, the all-dielectric resonance surface (ADRS) made of single low-permittivity dielectric is proposed to design binary (dielectric backplane) metamaterial absorber (BMA). As an illustration, a triple-band BMA composed of ADRS with a metallic backplane is designed and fabricated, where the ADRS incorporates two dielectric layers with different hole-array structures. The absorbing mechanisms of this kind of absorber are analyzed via analyzing configurations of power loss density, electric field and magnetic field, as well as investigating dependences of absorbing properties on structure dimensions. The study indicates that the structural design of ADRS leads to reverse magnetic field rings distributed inside the dielectric, forming the strong couplings at the resonance frequencies. The proposed BMA relies on low-permittivity ADRS, with the rapid preparation and low cost greatly simplifying the design of MAs. The current concept is also suitable to design multi-band and broadband MAs worked at other bands, by changing the structural design of ADRS.

Pulse generation scheme for flying electromagnetic doughnuts

Transverse electromagnetic plane waves are fundamental solutions of Maxwell's equations. It is less known that a radically different type of solutions has been described theoretically, but has never been realized experimentally, that exist only in the form of short burst of electromagnetic energy propagating in free-space at the speed of light. They are distinguished from transverse waves by a donut-like configuration of electric and magnetic fields with a strong field component along the propagation direction. Here, we report that such 'Flying Donuts' can be generated from conventional pulses using a singular metamaterial converter designed to manipulate both the spatial and spectral structure of the input pulse. The ability to generate Flying Donuts is of fundamental interest, as they shall interact with matter in unique ways, including non-trivial field transformations upon reflection from interfaces and the excitation of toroidal response and anapole modes in matter, thus offering new opportunities for telecommunications, sensing, and spectroscopy.