Incoming Electron Energy Research Articles

Ultrafast switching in spin field-effect transistors based on borophene nanoribbons.

Borophene, owing to the high mobility and long spin coherent length of its carriers, presents significant opportunities in ultrafast spintronics. In this research, we investigate the spin-dependent conductance of a Datta-Das field-effect transistor (FET) based on an armchair β12-borophene nanoribbon (BNR) using the tight-binding (TB) Hamiltonian in combination with the non-equilibrium Green's function (NEGF) method. The spin FET electrodes are magnetized by ferromagnetic (FM) insulators arranged in both parallel and anti-parallel configurations. This device acts as a controllable spin filter in the presence of Rashba spin-orbit coupling (SOC) for both configurations and its spin current is well modulated by a gate voltage and the strength of the Rashba SOC. For anti-parallel configurations, an energy gap emerges within a certain range of incoming electron energy which can disappear for electrons with flipped spin under the Rashba SOC. Furthermore, our findings indicate that the electron-electron (e-e) interaction helps the spin precession of electrons injected into the spin FET channel, thereby strengthening the Rashba SOC effect. Notably, a gate voltage can adjust the current-voltage (I-V) characteristics of this device. Finally, our calculations demonstrate that under the same conditions, the current magnitude and Ion/Ioff ratio of borophene spin FETs are several times higher than those of graphene and silicene spin FETs.

Effects of incoming particle energy and cluster size on the G-value of hydrated electrons

In hydrated electron (e−aq) dosimetry, absorbed radiation dose to water is measured by monitoring the concentration of radiation-induced e−aq. However, to obtain accurate dose, the radiation chemical yield of e−aq, G(e−aq), is needed for the radiation quality/setup under investigation. The aim of this study was to investigate the time-evolution of the G-values for the main generated reactive species during water radiolysis using GEANT4-DNA. The effects of cluster size and linear energy transfer (LET) on G(e−aq) were examined. Validity of GEANT4-DNA for calculation of G(e−aq) for clinically relevant energies was studied. Three scenarios were investigated with different phantom sizes and incoming electron energies (1 keV to 1 MeV). The time evolution of G(e−aq) was in good agreement with published data and did not change with decreasing phantom size. The time-evolution of the G-values increases with increasing LET for all radiolytic species. The particle tracks formed with high-energy electrons are separated and the resulting reactive species develop independently in time. With decreasing energy, the mean separation distance between reactive species decreases. The particle tracks might not initially overlap but will overlap shortly thereafter due to diffusion of reactive species, increasing the probability of e−aq recombination with other species. This also explains the decrease of G(e−aq) with cluster size and LET. Finally, if all factors are kept constant, as the incoming electron energy increases to clinically relevant energies, G(e−aq) remains similar to its value at 1 MeV, hence GEANT4-DNA can be used for clinically relevant energies.

Controllable spin filtering and half-metallicity in β12 -borophene nanoribbons

The experimental observation of the Dirac fermion states in $\beta_{12}$-borophene sheets and the discovery of their novel topological properties properties have made them a promising candidate for spintronic applications. Here, by combining non-equilibrium Green's function (NEGF) and tight-binding (TB) approximation, we study the charge and spin transport properties through a $\beta_{12}$-borophene nanoribbon (BNR) with the different edge shapes. We show when a BNR exposed to a nonlocal exchange magnetic field, the spin filtering occurs for both spin-up and spin-down so that the spin direction of transmitted electrons could be controlled by adjusting the energy of incoming electrons with the help of an external backgate voltage. It is found that an armchair BNR (ABNR) in the simultaneous presence of a transverse electric field and a nonlocal exchange field indicates a half-metallic nature which is electrically controllable. Moreover, the influence of local exchange field fields is evaluated by exposing the edges of ABNR to ferromagnetic strips with parallel and antiparallel configurations. Our findings show that the edge manipulations in ABNRs lead to the emergence of a giant magnetoresistance and a perfect spin filter. Finally, we studied the effects of edge vacancies and Anderson disorder on the spin-dependent conductance of an ABNR and find that the perfect spin polarization is not destroyed in the presence of Anderson disorder and various single vacancies. Our results reveal the outstanding spin transport properties of ABNRs for future spintronic devices.

Elastic scattering of slow electrons by carbon nanotubes

In this paper we calculate the cross sections for elastic scattering of slow electrons by carbon nanotubes. The corresponding electron–nanotube interaction is substituted by a zero-thickness cylindrical potential that neglects the atomic structure of real nanotubes, thus limiting the range of applicability of our approach to sufficiently low incoming electron energies. The strength of the potential is chosen to be the same as was used in describing the scattering of electrons by fullerene C60. We present results for total and partial electron scattering cross sections as well as their respective angular distributions, all with account of the five lowest angular momenta contributions. In the calculations we assumed that the incoming electron moves perpendicular to the nanotube axis, since an incoming electron along the axis moves freely.

Controlled electron transmission by lead chalcogenide barrier potential

Transmission of electrons across a rectangular barrier of IV–VI semiconductor compounds is considered. Conduction electrons arrive at the barrier and are reflected or transmitted through it depending on the relative values of the barrier potential V b and the electron energy E. The theory, in close analogy to the Dirac four component spinors, accounts for the boundary conditions on both sides of the barrier. The calculated transmission coefficient TC is an oscillatory function of the barrier voltage varying between zero (for full electron reflection) and unity (for full electron transmission). Character of electron wave functions outside and inside the barrier is studied. There exists a total current conservation, i.e. the sum of transmitted and reflected currents is equal to the incoming current. The transmission TC is studied for various barrier widths and incoming electron energies. Finally, the transmission coefficient TC is studied as a function of V b for decreasing energy gaps E g of different Pb1 − x Sn x Se compounds in the range of 150 meV 2 meV. It is indicated that for very small gap values the behavior of TC closely resembles that of the chiral electron tunneling by a barrier in monolayer graphene. For E g = 0 (Pb0.81Sn0.19Se) the coefficient TC reaches the value of 1 independently of V b .

Elucidating the capability of electron backscattering for 3D nano-structure determination

Reflection electron energy loss spectroscopy (REELS) is well established for the study of homogeneous materials with flat surfaces. Here we extend the use of this technique to nano-structures consisting of silicon and silica and show that the experimentally-observed peculiar dependence of the REELS spectra on the sample orientation can be reproduced by Monte Carlo simulations using the known sample morphology. A sample with a 3D structure, resembling those found in FinFET transistors, was analyzed through electron Rutherford backscattering (ERBS, revealing the mass of the atoms near the surface) and REELS (revealing the electronic structure). ERBS/REELS spectra were taken at two incoming electron energies (5 and 40 keV) and in two experimental geometries with the component of the outgoing propagation direction along the surface being either parallel or perpendicular to the fins. The measured spectra were different for the two geometries due to attenuation effects within the fins, especially at 5 keV where the inelastic mean free path is of the order of the fin dimensions. This means that the 3D structure induces shadowing effects which suppress the elastic peaks and enhanced the inelastic signal. A Monte Carlo code was used to simulate multiple elastic and inelastic interactions of the electrons with these 3D structures and was indeed able to reproduce these experimental results, including the shadowing effects. A sub-angstrom layer of Au was evaporated on the sample and the changes induced by the Au layer were dependent on the orientation of the fins and were again reproduced by the simulation.

Secondary electron emission of tin and tin-lithium under low energy helium plasma exposure

Secondary electron emission (SEE) yields of tin (Sn) and tin-lithium (SnLi) eutectic (20 at.% Li) samples were measured in He-plasma at a mean incoming electron energy up to 120 eV. SnLi shows a maximum yield of about 1.45 at 110 eV electron energy while the yield of the Sn surface was measured to be maximally 1.05 at 120 eV. X-ray photoelectron spectroscopy (XPS) analysis demonstrated the segregation effect of Li to the surface of the eutectic, both after melting in the argon atmosphere and in molten state with simultaneous He-plasma exposure. At least the top 10 nm of the SnLi samples were heavily enriched with Li, and Sn/Li ratios varied in the range 0.8–5% depending on eutectic treatment conditions. After the plasma exposure Sn3d is detected predominantly in the oxidized state while after extended atmospheric oxidation there was still a significant amount of Sn3d detected in the metallic state. The liquid surface of SnLi indicated a possible decrease of SEE yield. All measurements gave values of SEE yield close to or above unity. Such values can lead to significant plasma sheath disturbances and subsequent additional heat flux from electrons on such a plasma-facing material, thus, should be accounted for in designing fusion reactors using these components.

Logical spin-filtering in a triangular network of quantum nanorings with a Rashba spin-orbit interaction

The spin-resolved electron transport through a triangular network of quantum nanorings is studied in the presence of Rashba spin-orbit interaction (RSOI) and a magnetic flux using quantum waveguide theory. This study illustrates that, by tuning Rashba constant, magnetic flux and incoming electron energy, the triangular network of quantum rings can act as a perfect logical spin-filtering with high efficiency. By changing in the energy of incoming electron, at a proper value of the Rashba constant and magnetic flux, a reverse in the direction of spin can take place in the triangular network of quantum nanorings. Furthermore, the triangular network of quantum nanorings can be designed as a device and shows several simultaneous spintronic properties such as spin-splitter and spin-inverter. This spin-splitting is dependent on the energy of the incoming electron. Additionally, different polarizations can be achieved in the two outgoing leads from an originally incoming spin state that simulates a Stern-Gerlach apparatus.

Transport Properties of a Nonequilibrium Quantum Dot Connected to Ferromagnetic Leads

In this paper, transmission resonance and conductance properties of nonequilibrium quantum dot connected by ferromagnetic leads are investigated. Thermoelectric properties are also studied. Using the tight-binding formalism and numerically solving the Schrodinger equation, spin-dependent transmissions are obtained and plotted as a function of incoming electron energy. Transmissions of spin up and spin down electrons change in different ways as voltage bias and tilt angle change. Current spin polarization can be sensitively tuned by adjusting voltage bias. These research indicates the possible methods to modulate tilt angle or the bias voltage to obtain spin-dependent transmission, spin polarized current and effective Seebeck coefficients.

Aharonov-Bohm effect in a helical ring with long-range hopping: Effects of Rashba spin-orbit interaction and disorder

We study Aharonov-Bohm effect in a two-terminal helical ring with long-range hopping in presence of Rashba spin-orbit interaction. We explore how the spin polarization behavior changes depending on the applied magnetic flux and the incoming electron energy. The most interesting feature that we articulate in this system is that zero-energy crossings appear in the energy spectra at $\Phi=0$ and also at integer multiples of half-flux quantum values ($n \Phi_0/2$, $n$ being an integer) of the applied magnetic flux. We investigate the transport properties of the ring using Green's function formalism and find that the zero energy transmission peaks corresponding to those zero energy crossings vanish in presence of Rashba spin-orbit interaction. We also incorporate static random disorder in our system and show that the zero energy crossings and transmission peaks are not immune to disorder even in absence of Rashba spin-orbit interaction. The latter prevents the possibility of behaving these helical states in the ring like topological insulator edge states.

Measurement of the band gap by reflection electron energy loss spectroscopy

We investigate the possibilities of measuring the band gap of a variety of semiconductors and insulators by reflection electron energy loss spectroscopy without additional surface preparation. The band gap is a bulk property, whereas reflection energy loss spectroscopy is generally considered a surface sensitive technique. By changing the energy of the incoming electrons, the degree of surface sensitivity can be varied. Here, we present case studies to determine the optimum condition for the determination of the band gap. At very large incoming electron energies recoil effects interfere with the band gap determination, whereas at very low energies surface effects are obscuring the band gap without surface preparation. Using an incoming energy of 5keV a reasonable estimate of the band gap is obtained in most cases.

Determining the band gap and mean kinetic energy of atoms from reflection electron energy loss spectra.

Reflection electron energy loss spectra from some insulating materials (CaCO3, Li2CO3, and SiO2) taken at relatively high incoming electron energies (5-40 keV) are analyzed. Here, one is bulk sensitive and a well-defined onset of inelastic excitations is observed from which one can infer the value of the band gap. An estimate of the band gap was obtained by fitting the spectra with a procedure that includes the recoil shift and recoil broadening affecting these measurements. The width of the elastic peak is directly connected to the mean kinetic energy of the atom in the material (Doppler broadening). The experimentally obtained mean kinetic energies of the O, C, Li, Ca, and Si atoms are compared with the calculated ones, and good agreement is found, especially if the effect of multiple scattering is taken into account. It is demonstrated experimentally that the onset of the inelastic excitation is also affected by Doppler broadening. Aided by this understanding, we can obtain a good fit of the elastic peak and the onset of inelastic excitations. For SiO2, good agreement is obtained with the well-established value of the band gap (8.9 eV) only if it is assumed that the intensity near the edge scales as (E - Egap)(1.5). For CaCO3, the band gap obtained here (7 eV) is about 1 eV larger than the previous experimental value, whereas the value for Li2CO3 (7.5 eV) is the first experimental estimate.

Relativistic electron transport through an oscillating barrier: Wave-packet generation and Fano-type resonances

Transport properties of massive Dirac particles are investigated through an oscillating barrier. The Floquet quasienergies related to the time-dependent potential appear both in transmission and reflection as sidebands around the incoming electron's energy. We take all relevant sidebands into account and present time averaged transmission and reflection probabilities in a wide energy range. Most qualitative features of scattering on a static barrier---like Klein paradox---are still visible, but the transmission probability in the evanescent regime observably increases due to the oscillation of the potential. The strongly inelastic scattering process is shown to lead to multiple Fano-type resonances and temporal trapping of the particles inside the oscillating potential. We also present a detailed study of the time evolution of the wave packets generated in the scattering process. Our results can be relevant for graphene with an induced energy gap.

A high-energy electron scattering study of the electronic structure and elemental composition of O-implanted Ta films used for the fabrication of memristor devices

High-energy electron scattering is used to investigate Ta films implanted with 10 keV O ions. These films are of interest as they have been used for the fabrication of memristors. High-energy electron scattering is used with incoming electron energies ranging from 5 to 40 keV. The inelastic mean free path, and hence the probing depth, is at these energies of the same order as the range of the implanted ions. At the same time, we can distinguish the mass of the atom that scattered the electron elastically, due to the dependence of the recoil energy on the mass of the scatterer. This allows us to determine quantitatively the atomic composition near the surface from the signal of electrons that have scattered elastically but not inelastically. Electrons that have scattered inelastically as well as elastically provide us with information on the possible electronic excitations. Their signal is used to monitor the presence of the Ta2O5 phase near the surface (characterised by a significant band gap of ≃4.5 eV), and estimate at what depth below the surface pure Ta metal is present. In this way, we obtain a fairly detailed picture of the elemental composition and electronic properties of these films.

A critical overview of measurement techniques of spacecraft charging in space plasma

Under certain conditions in space there are physical processes which may cause misinterpretation of spacecraft potential measurements using some very common techniques. The use of long booms to measure the potential difference between a spacecraft and the tip of a boom has at least two deficiencies. First, when the spacecraft potential is high, the tip of the boom may be engulfed by the sheath of the spacecraft. Second, the tip of the boom itself may charge. Even if materials of high secondary emission coefficients are used for the boom tips, charging can still occur when the space plasma is energetic. Another common technique for measuring the energy shift of the incoming electron energy distribution function also has deficiencies. First, if ionization is abundant in the vicinity of the spacecraft (for example, during electron beam emissions), the energy gap resulting from the shift becomes blurred. Second, if the angular momentum of the incoming electrons is significant, the shift should not be attributed to potential energy only. The technique of using Langmuir probes, which are located at short distances from the spacecraft surface, is often deficient in measuring spacecraft potential. It may be too naive to apply either the attractive regime or the repulsive regime of Langmuir. For example, while the probe potential may be negative relative to that of the space plasma, it may be positive relative to that of the spacecraft. In that case, the probe is repelling electrons from the space plasma but attracting secondary electrons from the spacecraft.

Electron transport through an extended nano-ring in presence of magnetic field: An analytical approach

We analytically describe the influence of magnetic field on the electronic transport properties of an extended nano-ring by using Green's function technique in the nearest neighbor tight-binding approximation. We obtain exact analytic formulas for the electronic transmission coefficient, the total and local contact density of states as functions of incident electron energy, magnetic flux crossing over the ring and all the system parameters. Our formalism gives the ability to study the extended nano-rings of any size under certain conditions which provides useful tools to analyze electronic transport of these systems, much faster and more easily. The results show that the electronic transport quantities of a system consisting of a nano-ring are strongly sensitive to incoming electron energy, magnetic flux and contact hopping energies. The present approach may be useful to design nano-devices measuring magnetic field and magnetic based nano-switches.

Resonant phenomena in laser-assisted radiative attachment or recombination

Resonant enhancements are predicted in cross sections σn for laser-assisted radiative attachment or electron–ion recombination accompanied by absorption of n laser photons. These enhancements occur for incoming electron energies at which the electron can be attached or recombined by emitting μ laser photons followed by emission of a spontaneous photon upon absorbing n + μ laser photons. The close similarity between rescattering plateaus in spectra of resonant attachment/recombination and of high-order harmonic generation is shown based on a general parametrization for σn and on numerical results for e − H attachment.

Electron-impact excitation of the(2p2)1Dand(2s2p)1Poautoionizing states of helium

An experimental study of the excitation of the ($2{p}^{2}$) ${}^{1}D$ and ($2s2p$) ${}^{1}{P}^{o}$ autoionizing states of helium by 250-eV electron impact is presented. The ejected-electron angular distributions and energy spectra are measured in coincidence with the corresponding scattered electrons for a scattering angle of $\ensuremath{-}{13}^{\ensuremath{\circ}}$ and for a range of ejected-electron angles in both the forward and backward directions. Resonance profiles are analyzed in terms of the Shore-Balashov parametrization to obtain the resonance asymmetry ${A}_{\ensuremath{\mu}}$ and yield ${B}_{\ensuremath{\mu}}$ parameters and the direct ionization cross section $f$. The spectra and their parameters are compared to the previous measurements of Lower and Weigold [J. Phys. B. 23, 2819 (1990)] and McDonald and Crowe [J. Phys. B 26, 2887 (1993)]. Comparison is also made with the recent theoretical triply differential cross-section calculations based on the first and second Born approximations. In general, good qualitative agreement is found between the experimental results. Some differences are found at the forward and backward directions. These differences in the shape and magnitude of the cross sections are attributed to the different incoming electron energies used in the experiments. The second Born approximation with inclusion of the three-body Coulomb interaction in the final state agrees reasonably well with experiments in the binary region. However, the ${}^{1}{P}^{o}$ resonance yield parameter ${B}_{\ensuremath{\mu}}$ is significantly overestimated at the recoil region, giving a relatively large recoil peak, in contradiction to the experiment. There is also a discrepancy between the two theories available for the ${}^{1}D$ resonance yield parameter ${B}_{\ensuremath{\mu}}$ in this region. Remaining discrepancies between theories and experiments are also discussed.

DoubleL3Mionization of Pd induced by impact with medium-energy electrons

The electron-induced ${L}_{3}M$ two-step double ionization cross sections of metallic Pd were determined experimentally for incident electron beam energies ranging from the double ionization threshold up to 18 keV. The double ${L}_{3}M$ ionization cross sections were derived from the intensity ratios (${I}_{L\ensuremath{\alpha}M}$:${I}_{L\ensuremath{\alpha}}$) of the resolved $M$ satellites to the parent diagram lines. The sample was bombarded with monoenergetic electrons from an energy-tunable 20-kV electron gun. The diagram and $M$-satellite x-ray lines were measured by means of high-resolution x-ray spectroscopy, using a reflection-type von Hamos bent crystal spectrometer. The two-step partial cross sections were determined by subtracting from the measured total double ionization cross sections the contributions due to the shake process and ${L}_{1}$-${L}_{3}{M}_{4,5}$ Coster-Kronig transitions. Despite the thick target employed in the present study, the dependence of the two-step cross sections on the incoming electron energy could be derived using a target slice decomposition method. It is shown that the obtained energy dependence can be well reproduced by the semiempirical parametrization model of Pattard and Rost.

Ballistic spin-dependent transport of Rashba rings with multi-leads

Using the Landauer–Büttiker formula with the transfer matrix technique, we develop a formalism of the ballistic spin-dependent electron transport in the multi-lead Rashba rings. We give analytic formulas of the total conductance G j , spin- σ conductance g j σ and spin polarization P j of each outgoing lead j, and their resonant and antiresonant conditions. Analytic studying with numerical investigating Rashba rings with several symmetric and asymmetric leads, we find that G j , g j σ and P j oscillate with the incoming electron energy and the spin–orbit interaction (SOI) strength, and their antiresonances depend on the incoming electron energy, the SOI strength and the outgoing-lead angle with the incoming lead. For the symmetric-lead rings, G j , g j σ and P j have some symmetries, G j = G N - j , g j σ = g N - j - σ , and P j = − P N− j for symmetric leads, j and N − j, where the angles between the symmetric outgoing leads j and N − j and the incoming lead are γ N− j = 2 π − γ j . The spin polarization of the outgoing lead with γ j = π is exactly zero for even- N-symmetric-lead rings. These symmetries originate from the lead symmetry and time reversal invariance. For asymmetry-lead rings these symmetries vanish.