Showing papers on "Electron published in 2004"

A laser-plasma accelerator producing monoenergetic electron beams

Abstract: Particle accelerators are used in a wide variety of fields, ranging from medicine and biology to high-energy physics. The accelerating fields in conventional accelerators are limited to a few tens of MeV m(-1), owing to material breakdown at the walls of the structure. Thus, the production of energetic particle beams currently requires large-scale accelerators and expensive infrastructures. Laser-plasma accelerators have been proposed as a next generation of compact accelerators because of the huge electric fields they can sustain (>100 GeV m(-1)). However, it has been difficult to use them efficiently for applications because they have produced poor-quality particle beams with large energy spreads, owing to a randomization of electrons in phase space. Here we demonstrate that this randomization can be suppressed and that the quality of the electron beams can be dramatically enhanced. Within a length of 3 mm, the laser drives a plasma bubble that traps and accelerates plasma electrons. The resulting electron beam is extremely collimated and quasi-monoenergetic, with a high charge of 0.5 nC at 170 MeV.

High-quality electron beams from a laser wakefield accelerator using plasma-channel guiding

Abstract: Laser-driven accelerators, in which particles are accelerated by the electric field of a plasma wave (the wakefield) driven by an intense laser, have demonstrated accelerating electric fields of hundreds of GV m-1 (refs 1–3) These fields are thousands of times greater than those achievable in conventional radio-frequency accelerators, spurring interest in laser accelerators4,5 as compact next-generation sources of energetic electrons and radiation To date, however, acceleration distances have been severely limited by the lack of a controllable method for extending the propagation distance of the focused laser pulse The ensuing short acceleration distance results in low-energy beams with 100 per cent electron energy spread1,2,3, which limits potential applications Here we demonstrate a laser accelerator that produces electron beams with an energy spread of a few per cent, low emittance and increased energy (more than 109 electrons above 80 MeV) Our technique involves the use of a preformed plasma density channel to guide a relativistically intense laser, resulting in a longer propagation distance The results open the way for compact and tunable high-brightness sources of electrons and radiation

Monoenergetic beams of relativistic electrons from intense laser–plasma interactions

Abstract: High-power lasers that fit into a university-scale laboratory can now reach focused intensities of more than 10(19) W cm(-2) at high repetition rates. Such lasers are capable of producing beams of energetic electrons, protons and gamma-rays. Relativistic electrons are generated through the breaking of large-amplitude relativistic plasma waves created in the wake of the laser pulse as it propagates through a plasma, or through a direct interaction between the laser field and the electrons in the plasma. However, the electron beams produced from previous laser-plasma experiments have a large energy spread, limiting their use for potential applications. Here we report high-resolution energy measurements of the electron beams produced from intense laser-plasma interactions, showing that--under particular plasma conditions--it is possible to generate beams of relativistic electrons with low divergence and a small energy spread (less than three per cent). The monoenergetic features were observed in the electron energy spectrum for plasma densities just above a threshold required for breaking of the plasma wave. These features were observed consistently in the electron spectrum, although the energy of the beam was observed to vary from shot to shot. If the issue of energy reproducibility can be addressed, it should be possible to generate ultrashort monoenergetic electron bunches of tunable energy, holding great promise for the future development of 'table-top' particle accelerators.

Direct evidence for atomic defects in graphene layers

Abstract: Atomic-scale defects in graphene layers alter the physical and chemical properties of carbon nanostructures. Theoretical predictions have recently shown that energetic particles such as electrons and ions can induce polymorphic atomic defects in graphene layers as a result of knock-on atom displacements. However, the number of experimental reports on these defects is limited. The graphite network in single-walled carbon nanotubes has been visualized by transmission electron microscopy (TEM) and their chiral indices have been determined. But the methods used require a long image acquisition time and intensive numerical treatments after observations to find an 'average' image, which prevents the accurate detection and investigation of defect structures. Here we report observations in situ of defect formation in single graphene layers by high-resolution TEM. The observed structures are expected to be of use when engineering the properties of carbon nanostructures for specific device applications.

Atomic transient recorder

Abstract: In Bohr's model of the hydrogen atom, the electron takes about 150 attoseconds (1 as = 10(-18) s) to orbit around the proton, defining the characteristic timescale for dynamics in the electronic shell of atoms. Recording atomic transients in real time requires excitation and probing on this scale. The recent observation of single sub-femtosecond (1 fs = 10(-15) s) extreme ultraviolet (XUV) light pulses has stimulated the extension of techniques of femtochemistry into the attosecond regime. Here we demonstrate the generation and measurement of single 250-attosecond XUV pulses. We use these pulses to excite atoms, which in turn emit electrons. An intense, waveform-controlled, few cycle laser pulse obtains 'tomographic images' of the time-momentum distribution of the ejected electrons. Tomographic images of primary (photo)electrons yield accurate information of the duration and frequency sweep of the excitation pulse, whereas the same measurements on secondary (Auger) electrons will provide insight into the relaxation dynamics of the electronic shell following excitation. With the current approximately 750-nm laser probe and approximately 100-eV excitation, our transient recorder is capable of resolving atomic electron dynamics within the Bohr orbit time.

Quantum Transport in Semiconductor Nanostructures

Abstract: I. Introduction (Preface, Nanostructures in Si Inversion Layers, Nanostructures in GaAs-AlGaAs Heterostructures, Basic Properties). II. Diffusive and Quasi-Ballistic Transport (Classical Size Effects, Weak Localization, Conductance Fluctuations, Aharonov-Bohm Effect, Electron-Electron Interactions, Quantum Size Effects, Periodic Potential). III. Ballistic Transport (Conduction as a Transmission Problem, Quantum Point Contacts, Coherent Electron Focusing, Collimation, Junction Scattering, Tunneling). IV. Adiabatic Transport (Edge Channels and the Quantum Hall Effect, Selective Population and Detection of Edge Channels, Fractional Quantum Hall Effect, Aharonov-Bohm Effect in Strong Magnetic Fields, Magnetically Induced Band Structure).

CdSe/CdS/ZnS and CdSe/ZnSe/ZnS Core−Shell−Shell Nanocrystals

Abstract: We report the synthesis and characterization of highly luminescent colloidal nanocrystals consisting of CdSe cores protected with double inorganic shells (core−shell−shell nanocrystals). The outer ZnS shell provides efficient confinement of electron and hole wave functions inside the nanocrystal as well as high photochemical stability. Introducing the middle shell (CdS or ZnSe) sandwiched between CdSe core and ZnS outer shell allows considerable reducing strain inside nanocrystals because CdS and ZnSe have the lattice parameter intermediate to those of CdSe and ZnS. In contrast to CdSe/ZnS core−shells, in the core−shell−shell nanocrystals ZnS shell grows nearly defect free. Due to high quality of the ZnS shell, the core−shell−shell nanocrystals exhibit PL efficiency and photostability exceeding those of CdSe/ZnS nanocrystals. Preferential growth of the middle CdS shell in one crystallographic direction allows engineering the shape and luminescence polarization of the core−shell−shell nanocrystals.

Towards more accurate First Principles prediction of redox potentials in transition-metal compounds with LDA+U

Abstract: First-principles calculations within the Local Density Approximation (LDA) or Generalized Gradient Approximation (GGA), though very successful, are known to underestimate redox potentials, such as those at which lithium intercalates in transition metal compounds. We argue that this inaccuracy is related to the lack of cancellation of electron self-interaction errors in LDA/GGA and can be improved by using the DFT+$U$ method with a self-consistent evaluation of the $U$ parameter. We show that, using this approach, the experimental lithium intercalation voltages of a number of transition metal compounds, including the olivine Li$_{x}$MPO$_{4}$ (M=Mn, Fe Co, Ni), layered Li$_{x}$MO$_{2}$ ($x=$Co, Ni) and spinel-like Li$_{x}$M$_{2}$O$_{4}$ (M=Mn, Co), can be reproduced accurately.

Production of a keV x-ray beam from synchrotron radiation in relativistic laser-plasma interaction.

Abstract: We demonstrate that a beam of x-ray radiation can be generated by simply focusing a single high-intensity laser pulse into a gas jet. A millimeter-scale laser-produced plasma creates, accelerates, and wiggles an ultrashort and relativistic electron bunch. As they propagate in the ion channel produced in the wake of the laser pulse, the accelerated electrons undergo betatron oscillations, generating a femtosecond pulse of synchrotron radiation, which has keV energy and lies within a narrow (50 mrad) cone angle.

A Measure of Electron Localizability

Abstract: A functional of the same-spin electron pair density is proposed as a measure of electron localizability. This functional yields the average number of same-spin electron pairs in a region Ω enclosing a fixed charge. The functional equals zero if the fixed charge in Ω originates from one electron only, with all other same-spin electrons outside the region Ω. Then, the correlation of the electronic motion in Ω and thus the localizability of an electron is high. If the motion of the same-spin electrons becomes less correlated, more electrons participate in the fixed charge contained in Ω, the average number of same-spin electron pairs (the functional) increases. In the Hartree–Fock approximation the Taylor expansion of the proposed localizability functional can be related to the electron localization function of Becke and Edgecombe without using an arbitrary reference to the uniform electron gas. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Direct Sub-Angstrom Imaging of a Crystal Lattice

Abstract: Despite the use of electrons with wavelengths of just a few picometers, spatial resolution in a transmission electron microscope (TEM) has been limited by spherical aberration to typically around 0.15 nanometer. Individual atomic columns in a crystalline lattice can therefore only be imaged for a few low-order orientations, limiting the range of defects that can be imaged at atomic resolution. The recent development of spherical aberration correctors for transmission electron microscopy allows this limit to be overcome. We present direct images from an aberration-corrected scanning TEM that resolve a lattice in which the atomic columns are separated by less than 0.1 nanometer.

Interpretation of the Time Constants Measured by Kinetic Techniques in Nanostructured Semiconductor Electrodes and Dye-Sensitized Solar Cells

Abstract: The processes of charge separation, transport, and recombination in dye-sensitized nanocrystalline TiO2 solar cells are characterized by certain time constants. These are measured by small perturbation kinetic techniques, such as intensity modulated photocurrent spectroscopy (IMPS), intensity modulated photovoltage spectroscopy (IMVS), and electrochemical impedance spectroscopy (EIS). The electron diffusion coefficient, Dn, and electron lifetime, τn, obtained by these techniques are usually found to depend on steady-state Fermi level or, alternatively, on the carrier concentration. We investigate the physical origin of such dependence, using a general approach that consists on reducing the general multiple trapping kinetic-transport formalism, to a simpler diffusion formalism, which is valid in quasi-static conditions. We describe in detail a simple kinetic model for diffusion, trapping, and interfacial charge transfer of electrons, and we demonstrate the compensation of trap-dependent factors when formin...

Superconductivity Modulated by Quantum Size Effects

Abstract: We have fabricated ultrathin lead films on silicon substrates with atomic-scale control of the thickness over a macroscopic area. We observed oscillatory behavior of the superconducting transition temperature when the film thickness was increased by one atomic layer at a time. This oscillating behavior was shown to be a manifestation of the Fabry-Perot interference modes of electron de Broglie waves (quantum well states) in the films, which modulate the electron density of states near the Fermi level and the electron-phonon coupling, which are the two factors that control superconductivity transitions. This result suggests the possibility of modifying superconductivity and other physical properties of a thin film by exploiting well-controlled and thickness-dependent quantum size effects.

Identification of Reactive Species in Photoexcited Nanocrystalline TiO2 Films by Wide-Wavelength-Range (400−2500 nm) Transient Absorption Spectroscopy

Abstract: Reactive species, holes, and electrons in photoexcited nanocrystalline TiO2 films were studied by transient absorption spectroscopy in the wavelength range from 400 to 2500 nm. The electron spectrum was obtained through a hole-scavenging reaction under steady-state light irradiation. The spectrum can be analyzed by a superposition of the free-electron and trapped-electron spectra. By subtracting the electron spectrum from the transient absorption spectrum, the spectrum of trapped holes was obtained. As a result, three reactive speciestrapped holes and free and trapped electronswere identified in the transient absorption spectrum. The reactivity of these species was evaluated through transient absorption spectroscopy in the presence of hole- and electron-scavenger molecules. The spectra indicate that trapped holes and electrons are localized at the surface of the particles and free electrons are distributed in the bulk.

Topological Hall effect and Berry phase in magnetic nanostructures.

Abstract: We discuss the anomalous Hall effect in a two-dimensional electron gas subject to a spatially varying magnetization. This topological Hall effect does not require any spin-orbit coupling and arises solely from Berry phase acquired by an electron moving in a smoothly varying magnetization. We propose an experiment with a structure containing 2D electrons or holes of diluted magnetic semiconductor subject to the stray field of a lattice of magnetic nanocylinders. The striking behavior predicted for such a system (of which all relevant parameters are well known) allows one to observe unambiguously the topological Hall effect and to distinguish it from other mechanisms.

Role of structural defects on the half-metallic character of Co 2 MnGe and Co 2 MnSi Heusler alloys

Abstract: Heusler alloys, such as ${\mathrm{Co}}_{2}\mathrm{MnSi}$ and ${\mathrm{Co}}_{2}\mathrm{MnGe},$ have been predicted from first--principles to be half metallic and potential candidates for spintronic applications. However, spin polarizations of only 50--60 % were experimentally obtained for these compounds---a decrease attributed to defects in the Mn and Co sublattices. Accurate ab initio full-potential linearized augmented plane wave calculations are performed in order to determine the effects of several types of defects (such as antisites and atomic swaps) on the electronic and magnetic properties of the bulk Heusler compounds. Our findings, in general agreement with experiments, show that Mn antisites have the lowest formation energy and retain the half-metallic character. On the other hand, Co antisites have a slightly higher formation energy and a dramatic effect on the electronic properties: the defect states that locally destroy half metallicity are energetically localized and are screened out in a couple of atomic shells. In this case, the spin polarization at the Fermi level is strongly reduced, and the spin polarization due to the s electrons, responsible for the tunneling current, is in excellent agreement with experiment. Finally, both Mn-Si and Mn-Co atomic swaps have very high formation energies, keeping however the half-metallic character.

Dynamics of charged particles and their radiation field

Abstract: This book provides a self-contained and systematic introduction to classical electron theory and its quantization, non-relativistic quantum electrodynamics. The first half of the book covers the classical theory. It discusses the well-defined Abraham model of extended charges in interaction with the electromagnetic field, and gives a study of the effective dynamics of charges under the condition that, on the scale given by the size of the charge distribution, they are far apart and the applied potentials vary slowly. The second half covers the quantum theory, leading to a coherent presentation of non-relativistic quantum electrodynamics. Topics discussed include non-perturbative properties of the basic Hamiltonian, the structure of resonances, the relaxation to the ground state through emission of photons, the non-perturbative derivation of the g-factor of the electron and the stability of matter.

Spin Current and Polarization in Impure Two-Dimensional Electron Systems with Spin-Orbit Coupling

Abstract: We derive the transport equations for two-dimensional electron systems with Rashba spin-orbit interaction and short-range spin-independent disorder. In the limit of slow spatial variations, we obtain coupled diffusion equations for the electron density and spin. Using these equations we calculate electric-field induced spin accumulation and spin current in a finite-size sample for an arbitrary ratio between spin-orbit energy splitting $\ensuremath{\Delta}$ and elastic scattering rate ${\ensuremath{\tau}}^{\ensuremath{-}1}$. We demonstrate that the spin-Hall conductivity vanishes in an infinite system independent of this ratio.

Few-Electron Quantum Dots in Nanowires

Abstract: We demonstrate transport spectroscopy on bottom-up grown few-electron quantum dots in semiconductor nanowires The dots are defined by InP double barrier heterostructures in InAs nanowires catalytically grown from nanoparticles By changing the dot size, we can design devices ranging from single-electron transistors to few-electron quantum dots In the latter case, electrons can be added one by one to the dots from 0 to ∼50 electrons while maintaining an almost constant charging energy, with addition spectra of the devices displaying shell structures as a result of spin and orbital degeneracies The reduced dimensionality of the nanowire emitter gives rise to pronounced resonant tunneling peaks, where a gate can be used to control the peak positions

Tunable nonlocal spin control in a coupled-quantum dot system.

Abstract: The effective interaction between magnetic impurities in metals that can lead to various magnetic ground states often competes with a tendency for electrons near impurities to screen the local moment (known as the Kondo effect). The simplest system exhibiting the richness of this competition, the two-impurity Kondo system, was realized experimentally in the form of two quantum dots coupled through an open conducting region. We demonstrate nonlocal spin control by suppressing and splitting Kondo resonances in one quantum dot by changing the electron number and coupling of the other dot. The results suggest an approach to nonlocal spin control that may be relevant to quantum information processing.

Three‐dimensional collisionless magnetic reconnection in the presence of a guide field

Abstract: [1] Previous investigations of collisionless magnetic reconnection in a standard Harris neutral sheet configuration have demonstrated the importance of the Hall term for producing near-Alfvenic rates of reconnection and the existence of a very thin (∼c/ωpe) electron current layer and sharp density/pressure gradients on c/ωpi scales. The present work uses three-dimensional (3-D) particle-in-cell simulations with an open geometry to investigate the changes in the reconnection physics produced by a “guide field” component B0y of the magnetic field. With B0y ≲ B0, the nonlinear reconnection rate is not substantially modified from that for the Harris case. The properties of the reconnection fields and particle dynamics, however, are strongly altered. The familiar quadrupole By pattern is replaced by an enhancement of ∣By∣ between the separatrices. The enhanced parallel electric field and parallel electron velocity are confined to one pair of separatrix arms (which are positively charged), while the electron current peaks on the other pair (which are negatively charged). The ion outflow along the current sheet polarizes the separatrices, thereby creating large components of the in-plane electric field. The electrons are accelerated to form a beam structure with parallel speed limited by the electron Alfven speed. The beam-dominated electron distribution produces some y-dependent structures in E∥. For B0y ≫ B0, the reconnection rate is reduced by a factor of 2–3, and the parallel fields and velocities are somewhat smaller; the Hall current produced perturbations in By are considerably reduced.

How do small water clusters bind an excess electron

Abstract: The arrangement of water molecules around a hydrated electron has eluded explanation for more than 40 years. Here we report sharp vibrational bands for small gas-phase water cluster anions, (H 2 O) 4-6 – and (D 2 O) 4-6 – . Analysis of these bands reveals a detailed picture of the diffuse electron-binding site. The electron is closely associated with a single water molecule attached to the supporting network through a double H-bond acceptor motif. The local OH stretching bands of this molecule are dramatically distorted in the pentamer and smaller clusters because the excited vibrational levels are strongly coupled to the electron continuum. The vibration–to–electronic energy transfer rates, as revealed by line shape analysis, are mode-specific and remarkably fast, with the symmetric stretching mode surviving for less than 10 vibrational periods [50 fs in (H 2 O) 4 – ].

Adsorption of O2 and oxidation of CO at Au nanoparticles supported by TiO2(110)

Abstract: Density functional theory calculations are performed for the adsorption of O2, coadsorption of CO, and the CO+O2 reaction at the interfacial perimeter of nanoparticles supported by rutile TiO2(110). Both stoichiometric and reduced TiO2 surfaces are considered, with various relative arrangements of the supported Au particles with respect to the substrate vacancies. Rather stable binding configurations are found for the O2 adsorbed either at the trough Ti atoms or leaning against the Au particles. The presence of a supported Au particle strongly stabilizes the adsorption of O2. A sizable electronic charge transfer from the Au to the O2 is found together with a concomitant electronic polarization of the support meaning that the substrate is mediating the charge transfer. The O2 attains two different charge states, with either one or two surplus electrons depending on the precise O2 adsorption site at or in front of the Au particle. From the least charged state, the O2 can react with CO adsorbed at the edge sites of the Au particles leading to the formation of CO2 with very low (≈0.15 eV) energy barriers.

Inelastic electron tunneling spectroscopy in molecular junctions: peaks and dips.

Abstract: We study inelastic electron tunneling through a molecular junction using the nonequilibrium Green’s function formalism. The effect of the mutual influence between the phonon and the electron subsystems on the electron tunneling process is considered within a general self-consistent scheme. Results of this calculation are compared to those obtained from the simpler Born approximation and the simplest perturbation theory approaches, and some shortcomings of the latter are pointed out. The self-consistent calculation allows also for evaluating other related quantities such as the power loss during electron conduction. Regarding the inelastic spectrum, two types of inelastic contributions are discussed. Features associated with real and virtual energy transfer to phonons are usually observed in the second derivative of the current I with respect to the voltage Φ when plotted against Φ. Signatures of resonant tunneling driven by an intermediate molecular ion appear as peaks in the first derivative dI/dΦ and may show phonon sidebands. The dependence of the observed vibrationally induced lineshapes on the junction characteristics, and the linewidth associated with these features are also discussed.

Phenomenological theory of laser-plasma interaction in ``bubble'' regime

Abstract: The electron trapping in the “bubble” regime of laser-plasma interaction as proposed by Pukhov and Meyer-ter-Vehn [A. Pukhov and J. Meyer-ter-Vehn, Appl. Phys. B 74, 355 (2002)] is studied. In this regime the laser pulse generates a solitary plasma electron cavity: the bubble. It is free from the cold plasma electrons and runs with nearly light velocity. The present work discusses the form of the bubble and the spatial distribution of electromagnetic fields within the cavity. We extend the one-dimensional electron capture theory to the three-dimensional case. It is shown that the bubble can trap plasma electrons. The trapping condition is derived and the trapping cross section is estimated. Electron motion in the self-generated electron bunch is investigated. Estimates for the maximum of electron bunch energy and the bunch density are provided.

Carbon nanotube electron sources and applications

Abstract: In this review we give an overview of the present status of research on carbon nanotube (CNT) field emitters and their applications. Several different construction principles of field-emission devices with CNTs are summarized. The emission mechanism is introduced and a detailed overview is given of the measured emission properties and related topics of CNT electron sources. We give also several examples of field-emission devices with CNT electron emitters that are presently being investigated in the academic world as well as in industry. Carbon nanotube electron sources clearly have interesting properties, such as low voltage operation, good stability, long lifetime and high brightness. The most promising applications are the field-emission display and high-resolution electron-beam instruments. But several hurdles remain, such as the manufacture of an electron source or an array of electron sources with exactly the desired properties in a reproducible manner.

Heating and current drive by electron cyclotron waves

Abstract: The physics model of electron cyclotron heating (ECH) and current drive (ECCD) is becoming well validated through systematic comparisons of theory and experiment. This work has shown that ECH and ECCD can be highly localized and robustly controlled in toroidal plasma confinement systems, leading to applications including stabilization of magnetohydrodynamic instabilities like neoclassical tearing modes, control and sustainment of desired profiles of current density and plasma pressure, and studies of localized transport in laboratory plasmas. The experimental work was supported by a broad base of theory based on first principles which is now well encapsulated in linear ray tracing codes describing wave propagation, absorption, and current drive and in fully relativistic quasilinear Fokker–Planck codes describing in detail the response of the electrons to the energy transferred from the wave. The subtle balance between wave-induced diffusion and Coulomb relaxation in velocity space provides an understandin...

Hydrated Electron Dynamics: From Clusters to Bulk

Abstract: The electronic relaxation dynamics of size-selected (H2O)n–/(D2O)n[25 ≤ n ≤ 50] clusters have been studied with time-resolved photoelectron imaging. The excess electron ( \(e_{c}^{-}\) ) was excited through the \(e_{c}^{-}(p){\leftarrow}e_{c}^{-}(s)\) transition with an ultrafast laser pulse, with subsequent evolution of the excited state monitored with photodetachment and photoelectron imaging. All clusters exhibited p-state population decay with concomitant s-state repopulation (internal conversion) on time scales ranging from 180 to 130 femtoseconds for (H2O)n– and 400 to 225 femtoseconds for (D2O)n–; the lifetimes decrease with increasing cluster sizes. Our results support the “nonadiabatic relaxation” mechanism for the bulk hydrated electron ( \(e_{aq}^{-}\) ), which invokes a 50-femtosecond \(e_{aq}^{-}(p){\rightarrow}e_{aq}^{-}(s^{{\dagger}})\) internal conversion lifetime.