Spatial Distribution Of Electrons Research Articles

Environmental benefits of recycling waste electrical and electronic equipment and spatial distribution

Environmental benefits of recycling waste electrical and electronic equipment and spatial distribution

Visualization of spontaneous electronic polarization in Pb ion of ferroelectric PbTiO3 by synchrotron-radiation x-ray diffraction

In this study, we accurately visualized the valence electron density distributions of ferroelectrics PbTiO3 and BaTiO3 by analyzing synchrotron-radiation powder x-ray diffraction (SXRD) data using the Rietveld method and the maximum entropy method. Clear SXRD evidence indicates that the Pb ion in PbTiO3 is polarized in the ferroelectric phase, whereas the Ba ion in BaTiO3 is not polarized and fully ionized as a Ba2+ ion. The large electronic polarization of the Pb ion is attributed to the anisotropic spatial distribution of the lone-pair electrons of the Pb2+ ion, particularly the anisotropic p-like orbital of the lone-pair electrons caused by the formation of Pb–O covalent bonds. In PbTiO3, the contribution of the electronic polarization of the Pb ion to the spontaneous polarization is significant. We experimentally evaluate the contributions of the ionic polarization and the electronic polarization, respectively, from the valence electron density distribution map and demonstrate that the spontaneous polarization can be calculated from the SXRD data as the sum of them. One of the Ti–O covalent bonds is broken at the phase transition in PbTiO3, whereas no change in the atomic coordination is observed in BaTiO3. When focusing on the covalent bonding network, we propose using a layered material for PbTiO3 in the ferroelectric phase. These results agree well with those of first-principles calculations. We expect further comprehensive valence electron density studies can be performed by combining SXRD experiments and first-principles calculations to better understand the emergence of ferroelectricity.

Breathing mode of relativistic twisted electron beams under periodic magnetic field

Transmission electron microscopy typically works with relativistic electron beams of energy 200–300 keV whose wavelength is on the picometer scale. Here, we study theoretically the relativistic dynamics of twisted Dirac-electron beams driven by a time-oscillating magnetic field. The spatial density distribution and the spinor wave-functions of twisted electron beams at each instant time t are obtained. It is demonstrated that the spatial density distribution of electrons can be changed periodically by the external field, whose change is very similar to the “Breathing”. Correspondingly, the azimuthal component of the current changes periodically with the magnetic field. The central “spot size” of spatial distribution of is directly related to the magnitude of the magnetic field. Our results suggest that twisted electron beams are promising for testing the magnitude and frequency of ac magnetic fields created by microsources.

Electron-beam broadening in electron microscopy by solving the electron transport equation

Scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM) are prominent techniques for the structural characterization of materials. STEM in particular provides high spatial resolution down to the sub-angstrom range. The spatial resolution in STEM and SEM is ultimately limited by the electron-beam diameter provided by the microscope's electron optical system. However, the resolution is frequently degraded by the interaction between electron and matter leading to beam broadening, which depends on the thickness of the analyzed sample. Numerous models are available to calculate beam broadening. However, most of them neglect the energy loss of the electrons and large-angle scattering. These restrictions severely limit the applicability of the approaches for large sample thicknesses in STEM and SEM. In this work, we address beam broadening in a more general way. We numerically solve the electron transport equation without any simplifications, and take into account energy loss along the electron path. For this purpose, we developed the software package CeTE (Computation of electron Transport Equation). We determine beam broadening, energy deposition, and the interaction volume of the scattered electrons in homogeneous matter. The calculated spatial and angular distributions of electrons are not limited to forward scattering and small sample thicknesses. We focus on low electron energies of 30 keV and below, where beam broadening is particularly pronounced. These electron energies are typical for SEM and STEM in scanning electron microscopes.

Application of the Green Function Method for Calculating the Spatial Distribution of Electrons in the Finite System

The paper presents a numerical implementation of the Green’s function method for calculating the spatial distribution of the macroscopic number of free electrons in a finite system. The obtained results are compared with the results obtained by direct summation of the wave functions. The quantitative coincidence is demonstrated.

Modeling dark photon oscillations in our inhomogeneous Universe

A dark photon may kinetically mix with the Standard Model photon, leading to observable cosmological signatures. The mixing is resonantly enhanced when the dark photon mass matches the primordial plasma frequency, which depends sensitively on the underlying spatial distribution of electrons. Crucially, inhomogeneities in this distribution can have a significant impact on the nature of resonant conversions. We develop and describe, for the first time, a general analytic formalism to treat resonant oscillations in the presence of inhomogeneities. Our formalism follows from the theory of level crossings of random fields and only requires knowledge of the one-point probability distribution function (PDF) of the underlying electron number density fluctuations. We validate our formalism using simulations and illustrate the photon-to-dark photon conversion probability for several different choices of PDFs that are used to characterize the low-redshift Universe.

Inevitable imprints of patchy reionization on the cosmic microwave background anisotropy

ABSTRACT Reionization of the cosmic neutral hydrogen by the first stars in the Universe is an inhomogeneous process, which produces spatial fluctuations in free electron density. These fluctuations lead to observable signatures in cosmological probes like the cosmic microwave background (CMB). We explore the effect of the electron density fluctuations on CMB using photon-conserving seminumerical simulations of reionization named SCRIPT. We show that the amplitude of the kinematic Sunyaev–Zeldovich (kSZ) and the B-mode polarization signal depends on the patchiness in the spatial distribution of electrons along with the dependence on mid-point and extent of the reionization history. Motivated by this finding, we provide new scaling relations for the amplitude of kSZ and the B-mode polarization signal which can capture the effects arising from the mean optical depth, width of reionization, and spatial fluctuations in the electron density arising from patchy reionization. We show that the amplitude of the kSZ and the B-mode polarization signal exhibits different dependency on the width of reionization and the patchiness of reionization, and hence a joint study of these CMB probes will be able to break the degeneracy. By combining external data sets from 21-cm measurements, the degeneracy can be further lifted by directly exploring the sizes of the ionized regions.

Forward Modeling of Particle Acceleration and Transport in an Individual Solar Flare

Abstract The aim of this study is to generate maps of the hard X-ray emission produced by energetic electrons in a solar flare and compare them with observations. The ultimate goal is to test the viability of the combined MHD/test-particle approach for data-driven modeling of active events in the solar corona and their impact on the heliosphere. Based on an MHD model of X-class solar flare observed on 2017 September 8, we calculate trajectories of a large number of electrons and protons using the relativistic guiding-center approach. Using the obtained particle trajectories, we deduce the spatial and energy distributions of energetic electrons and protons, and calculate bremsstrahlung hard X-ray emission using the “thin-target” approximation. Our approach predicts some key characteristics of energetic particles in the considered flare, including the size and location of the acceleration region, energetic particle trajectories and energy spectra. Most importantly, the hard X-ray bremsstrahlung intensity maps predicted by the model are in good agreement with those observed by RHESSI. Furthermore, the locations of proton and electron precipitation appear to be close to the sources of helioseismic response detected in this flare. Therefore, the adopted approach can be used for observationally driven modeling of individual solar flares, including manifestations of energetic particles in the corona, as well as the inner heliosphere.

Effect of an electric field on liquid helium scintillation produced by fast electrons

The dependence on applied electric field ($0 - 40$ kV/cm) of the scintillation light produced by fast electrons and $\alpha$ particles stopped in liquid helium in the temperature range of 0.44 K to 3.12 K is reported. For both types of particles, the reduction in the intensity of the scintillation signal due to the applied field exhibits an apparent temperature dependence. Using an approximate solution of the Debye-Smoluchowski equation, we show that the apparent temperature dependence for electrons can be explained by the time required for geminate pairs to recombine relative to the detector signal integration time. This finding indicates that the spatial distribution of secondary electrons with respect to their geminate partners possesses a heavy, non-Gaussian tail at larger separations, and has a dependence on the energy of the primary ionization electron. We discuss the potential application of this result to pulse shape analysis for particle detection and discrimination.

Design and optimization of nanowire betavoltaic generators

A model used to simulate and optimize the performance of nanowire-based betavoltaic generators is developed. The optimum nanowire array geometry is established for devices made of silicon, gallium arsenide, and gallium phosphide for both nickel-63 and tritium sources by computing the energy capture efficiency for each case. The captured power for nanowire devices is shown to be drastically greater compared to planar betavoltaic generators with maximum improvement factors of approximately 7, 3, 5, and 9 for devices utilizing radioisotope sources of nickel-63, nickel citrate, tritium, and tritiated butyl, respectively. In addition, nanowire devices do not suffer from self-shielding effects, a large limitation in conventional, planar betavoltaics. By coupling the spatial distribution of electron–hole pair generation rate calculated from Monte Carlo simulations and a semiconductor charge-transport model, the diode design is optimized for the maximum power output. The top performing devices, utilizing a tritium source, exhibited an output power of approximately 4, 6, and 2 μW cm−2 for silicon, gallium arsenide, and gallium phosphide, respectively. Overall device efficiencies were found to range from 4% to 10%, surpassing several betavoltaic devices reported in the literature. It was also found that, contrary to the traditional betavoltaic design, semiconductors with higher bandgaps do not necessarily result in the best device performance due to additional material parameters, such as surface recombination velocity. Potential improvements for nanowire-based betavoltaic generators are suggested for additional investigation.

Electron bunching induced by betatron resonance in a picosecond laser driven plasma channel

In direct laser acceleration, the spatial distribution of electrons will become bunched periodically. This bunching has been regarded to be caused by forced oscillations driven by the laser field. In this paper we report on the generation of electron bunching in a picosecond laser driven plasma channel. In our case, the bunched electrons are caused by betatron resonance, not forced oscillations. Due to the resonance, electrons are kept in phase with the laser field, which leads to similar trajectories in the laser field for the most energetic electrons. Furthermore, these electrons will cross the laser axis with an equal slope induced by the equal electron transverse velocity in the direct laser acceleration regime, which also aids the formation of the bunched electrons. We find that a quasi-static longitudinal electric field exists in the second half of the plasma channel, which provides a pre-acceleration process for the electron before the resonance condition is matched. The electron transverse velocity decreases dramatically during the pre-acceleration process and the decrease of electron transverse velocity is the key point to match the resonance condition.

Effect of low-energy electron irradiation on voltage-capacity curves of Al/SiO2/Si structure

Charging of dielectric targets by electron irradiation is a well-known phenomenon which should be taken into account in characterization of dielectric materials and coatings with electron microscopy, in electron beam lithography, in development of dielectric coatings for spacecrafts and other fields of science and engineering. Charging kinetics is strongly affected by spatial distribution of electrons and holes formed by irradiation. At the experimental electron beam energy electron penetration depth is smaller than dielectric thickness and this allows identifying the contribution of excess carrier transport to trap formation at the SiO2/Si interface. Low-energy electron beams have been shown to substantially affect C–V curve slope, i.e., to form traps at the interface. We have studied the effect of bias applied to the test structure before and after electron beam irradiation. The experiment has shown that bias of either polarity applied to the test MOS structure before low-energy electron beam irradiation practically does not affect the C–V curves of the test structure. Positive bias applied to the metallization layer during low-energy electron beam irradiation has a strong effect on the C–V curve pattern while negative bias affects the C–V curves but slightly. Study of the stability of the changes caused by electron beam irradiation has shown that the C–V curves of the test structure restore slowly even at room temperature. Application of negative bias decelerated charge relaxation.

Anisotropic Diffusion of Charges on Au Nanoclusters Embedded in Al2O3 Dielectrics

Recent transistors are one of state‐of‐the‐art products of semiconductor industry. As it has been required to be minimized to the nanoscale regime, the classical model from the microelectronics is often challenged by its discrepancy. Therefore, understanding the behavior of electrons in such regime is essential for transistor applications in nanoscale. Herein, the charge trapping and anisotropic diffusion behaviors of electrons in multistacked nanostructures composed of a blocking oxide, a charge storage layer of Au nanoclusters (NCs), a tunneling oxide, and silicon substrate are reported. Electrons are injected from the substrate to Au NCs via quantum tunneling effect. The charge retention and diffusion characteristics of the NCs are investigated by measuring surface potential. To explore the spatial dependence, a line‐ and dot‐shaped injection of charges is examined. Both of the accumulated electrons decay following an exponential function of time. However, the injected electrons with a line‐shape in Au NCs are found to have a larger time constant than those with a dot‐shape. This result implies that the initial spatial distribution of electrons plays a role in determining the equilibrium distribution of the charges in the nanostructure.

Genesis of non-uniformity of plasma fluxes over emissive wall in low-temperature plasmas

Origins of spatial modulation of stationary electron and ion currents to the wall in discharge plasma with/without magnetic field at low gas pressure are studied in the experiments and 2D PIC MCC simulations. It is shown that a non-uniformity of ion and electron fluxes to the wall is induced by (a) a non-planar topology on the emissive wall, (b) a difference in the secondary electron emission yields of materials in segmented wall or (c) an inclination of the external magnetic field. The transition in the sheath structure over the grooved emissive surface from a developed sheath to a collapsed one caused by the increase of electron energy enlarges the alteration of the ion and electron currents over the grooved or segmented surfaces. The experimental study of the plasma-emissive wall sheath transition was carried out with hexagonal boron nitride wall samples grooved with the characteristic size of 1 mm and 5 mm, which is about of the Debye length. In kinetic simulations, this phenomenon is analyzed in terms of the electron and ion energy distribution functions. An external oblique magnetic field applyed to the dc discharge is found to redistribute the plasma and the periodical structure with the spikes of electron and ion densities forms This phenomena causes the non-uniform spatial distribution of electron and ion currents to the wall. The spikes in electron and ion densities became more pronounced with an increase of magnetic field incidence angle.

Effects of Solar Wind Plasma Flow and Interplanetary Magnetic Field on the Spatial Structure of Earth's Radiation Belts

AbstractBased on the statistical data measured by Van Allen Probes from 2012 to 2016, we analyzed the effects of solar wind plasma flow and interplanetary magnetic field (IMF) on the spatial distribution of Earth's radiation belt electrons (>100 keV). The statistical results indicate that the increases in solar wind plasma density and flow speed can exert different effects on the spatial structure of the radiation belts. The high solar wind plasma density (≥6 cm−3)/flow pressure (≥2.5 nPa) and a large southward IMF (Bz < −6 nT) usually appear in the front of high‐speed solar wind streams (≥450 km/s), and they tend to narrow the outer radiation belt but broaden the slot region. In contrast, the increase in solar wind flow speed can broaden the outer radiation belt but narrows the slot region. When the solar wind speed exceeds 500 km/s, the outer radiation belt electrons can penetrate into the slot region (L < 3) and even enter the inner radiation belt (L < 2). The lower‐energy electrons penetrate into the deeper (smaller‐L) region than the higher‐energy electrons.

Monte-Carlo simulation of the ionization processes for discharges in the left branch of the Paschen curve

The paper deals with the development of the simulation methods applied to the low-pressure discharges in which the reduced electric field E/p is extremely high and there exists a problem for the correct description of the ionization processes in the classical electron avalanches. Here we use the PIC/MC code xoopic for calculation of the impact ionization coefficient α, the electron drift velocity and the electron diffusion coefficient D in the electron avalanches in nitrogen. The range of calculations covers the ratio E/p ≤ 1000 V·(cm·Torr)−1. The simulation method was tested on a simple analytical model of an electron avalanche and showed good agreement in low and moderate E/p. It is shown that in conditions of high E/p the spatial distribution of electrons in the avalanche head based on the concepts of electron mobility during their drift motion and diffusion coefficient is not quite correct. The work confirmed that at the very low pressures pre-breakdown phenomena and the transition to the next stage of high-current breakdown cannot be based on ideas about classical electron avalanches.

Aspherical and covalent bonding character of d electrons of molybdenum from synchrotron x-ray diffraction

The occupancies and spatial distribution of electrons for 4d-orbitals in pure molybdenum have been experimentally determined by a charge density study from synchrotron radiation x-ray powder diffraction. There are valence charge density maxima in interatomic positions indicating bond formation. The electron deficiencies of Γ12 orbitals were visualized in the observed static deformation density. An electron deficiency of ∼0.5 was observed from the orbital population analysis through multipole refinement. The occupancies and spatial distribution have also been calculated by a density functional theoretical calculation using WIEN2k packages for comparison. The observed features agree well with the theoretical study. In addition, the observed charge density has more covalent bonding character than the theoretical one. The present study confirms that a state-of-the-art x-ray charge density study can reveal the spatial structure of d-electrons in 4d-system.

Measurements of the fast electron bremsstrahlung during lower hybrid current drive in the HL-2A tokamak

Physics related to fast electrons in lower hybrid (LH) current drive (LHCD) plasma is a very important issue, since these particles will play an important role in runaway electron (RE) generation and lower hybrid wave (LHW)-related physics. Utilizing a new hard X-ray (HXR) pinhole camera, recent HL-2A tokamak experiments have devoted to enhancing the understanding of the physics on fast electrons and LHW. The fast electron bremsstrahlung (FEB) emission in the HXR energy range between 20 and 200 keV was measured by the HXR camera. To study the conversion of LHW-produced fast electrons into REs, a very short pulse of LHW, so-called “blip”, with duration of 5 ms was injected into the plasma during the current flattop phase. A strong enhancement of REs was induced by the blip injection. Measurements from the HXR camera show that the fast electrons generated by LHWs is mainly concentrated in 40-60 keV, which is well consistent with the calculated value based on Landau damping theory. The energy of these seed electrons is higher than the critical runaway energy. This phenomenon may be come from the synergetic effects of Dreicer and avalanche RE generation. Moreover, the measurements indicate that the spatial distribution of the fast electrons during LHCD has a peaked profile, implying that the fast electrons are mainly produced in the plasma core. It also suggests that the energy of the LHW mainly deposited in the plasma core region.

Effects of reconnection downstream conditions on electron parallel acceleration during the merging start-up of a spherical tokamak

The axial merging method is one of the candidates to provide a center-solenoid-free start-up of high-beta spherical tokamak (ST) plasma. Two initially formed STs merge through magnetic reconnection in the presence of the guide (toroidal) magnetic field, which is perpendicular to the reconnection (poloidal) magnetic field. During ST merging start-up, electrons are effectively accelerated near the reconnection point where the reconnection electric field is almost parallel to the magnetic field. In order to evaluate the effectivity of this acceleration process on electron heating, the temporal and spatial distributions of generated energetic electrons are observed by a soft x-ray fast imaging system equipped on the UTST device. The energetic electrons were generated not only in the vicinity of the reconnection point but transiently in the inboard-side downstream region until the static electric field by charge separation grew to cancel the reconnection electric field component parallel to the magnetic field line. Adequate control of the downstream condition could enhance the generation of energetic electrons and provide a more effective conversion from the released magnetic energy to electron energy.

Magnetic excitations in molecular magnets with complex bridges: the tetrahedral molecule Ni4Mo12

We investigate the spectroscopic magnetic excitations in molecular magnets with complex intermediate structure among the magnetic ions. Our approach consists in introducing a modified spin Hamiltonian that allows for discrete coupling parameters accounting for all energetically favorable spatial distributions of the valence electrons along the exchange bridges connecting the constituent magnetic ions. We discuss the physical relevance of the constructed Hamiltonian and derive its eigenvalues. The model is applied to explore the magnetic excitations of the tetrameric molecular magnet Ni$_4$Mo$_{12}$. Our results are in a very good agreement with the available experimental data. We show that the experimental magnetic excitations in the named tetramer can be traced back to the specific geometry and complex chemical structure of the exchange bridges leading to the splitting and broadness of the peaks centered about 0.5 meV and 1.7 meV.