Spin filtering of free electrons by magnetic multilayers: towards an efficient self-calibrated spin polarimeter

An asymmetrical ferromagnetic cobalt bilayer (18 nm Au/0.8 nm Co/2.2 nm Au/1.3 nm Co/1.5 nm Au) operates as a self-calibrated spin polarimeter with a high spin selectivity for free electrons injected at a few eV above the Fermi level. We present the analysis of transmitted currents as a function of the incident energy, based on a model of spin polarization dilution into the first gold layer and ballistic transport close to the vacuum level throughout the sample.     

N23-core losses and autoionization emissions of clean and oxygen exposed zirconium

The electron energy loss spectra associated with N₂₃-excitation and the low energy N₂₃VV Auger emission have been studied for both the clean and oxygen exposed zirconium. In the high energy side of the N₂₃VV Auger spectrum, autoionization emission of electrons of the valence band due to the decay of 4p electrons excited to states ≈9eV above the Fermi level has been identified. The excitation process can be also observed in the electron energy loss spectra. This is the first time that an autoionization feature is observed in a electron excited Auger spectrum of a 4d transition metal.     

Fermi energy of a metal nanowire with elliptical cross section

The influence of the geometrical shape of the cross section on the energy characteristics of a metal nanowire has been investigated theoretically. The size oscillations of the Fermi energy have been calculated within the model of an infinite potential well in terms of the perturbation theory. The calculations have been carried out for Au and Al. It has been shown that the cross-sectional ellipticity with a low eccentricity can be taken into account in the first-order perturbation theory by modifying the boundary conditions for the radial wave function of electrons.     

Chemisorbed hydrogen: Electron scattering,resonance levels,and vibration frequency

Several physical properties of atomic hydrogen chemisorbed on tungsten are computed on the basis of our earlier density functional calculations. The differential scattering cross-section of 50–100 eV electrons by a chemisorbed hydrogen atom is computed and compared with that of an isolated hydrogen atom. The relevance of this cross-section to LEED experiments is discussed. An electronic resonance level associated with the hydrogen atom is found at 5.6 eV below the Fermi level, in very good agreement with recent photo-emission data. The vibrational excitation energy perpendicular to the metal plane is calculated to be 200 meV, compared to the observed 140 meV.     

Effective 4ƒ energy level in virtual excitation and in photoemission processes in Ce-pnictides

Effects of relaxation of occupied band electrons to the ?-hole state through the hybridization between ? and band states are studied based on a detailed model for Ce-monopnictides. The effective 4? level is shifted about 1–2 eV to shallow energy side from the unrenormalized bare level in processes in which the 4? electron is only virtually excited, such as in excitation to the vacant p band states through the p-? mixing. Photoemission spectra show two peaks, one near the Fermi energy and the other about 3 eV below it. The latter is shifted to deep energy side about 0.5–1 eV from the bare level when it lies near the bottom of the valence band. The discrepancy between the 4? level estimated from the low energy phenomena and that from photoemission is resolved.     

Doping graphene with metal contacts

Making devices with graphene necessarily involves making contacts with metals. We use density functional theory to study how graphene is doped by adsorption on metal substrates and find that weak bonding on Al, Ag, Cu, Au, and Pt, while preserving its unique electronic structure, can still shift the Fermi level with respect to the conical point by approximately 0.5 eV. At equilibrium separations, the crossover from p-type to n-type doping occurs for a metal work function of approximately 5.4 eV, a value much larger than the graphene work function of 4.5 eV. The numerical results for the Fermi level shift in graphene are described very well by a simple analytical model which characterizes the metal solely in terms of its work function, greatly extending their applicability.     

Spin resonance transport properties of a single Au atom in S–Au–S junction and Au–Au–Au junction

The spin transport properties of S–Au–S junction and Au–Au–Au junction between Au nanowires are investigated with density functional theory and the non-equilibrium Green's function. We mainly focus on the spin resonance transport properties of the center Au atom. The breaking of chemical bonds between anchor atoms and center Au atom significantly influences their spin transmission characteristics. We find the 0.8 eV orbital energy shift between anchor S atoms and the center Au atom can well protect the spin state stored in the S–Au–S junction and efficiently extract its spin state to the current by spin resonance mechanism, while the spin interaction of itinerant electrons and the valence electron of the center Au atom in the Au–Au–Au junction can extract the current spin information into the center Au atom. Fermi energy drift and bias-dependent spin filtering properties of the Au–Au–Au junction may transform information between distance, bias,and electron spin. Those unique properties make them potential candidates for a logical nanocircuit.     

Electron-stimulated desorption of lithium atoms from oxidized molybdenum surface

The yield and energy distributions of lithium atoms upon electron-stimulated desorption from lithium layers adsorbed on the molybdenum surface coated with an oxygen monolayer have been measured as functions of the impact electron energy and lithium coverage. The measurements are performed using the time-of-flight technique and a surface ionization detector. The threshold of the electron-stimulated desorption of lithium atoms is equal to 25 eV, which is close to the ionization energy of the O 2s level. Above a threshold of 25 eV, the yield of lithium atoms linearly increases with an increase in the lithium coverage. In the coverage range from 0 to 0.45, an additional threshold is observed at an energy of 55 eV. This threshold can be associated with the ionization energy of the Li 1s level. At the electron energies above a threshold of 55 eV, as the coverage increases, the yield of lithium atoms passes through a maximum at a coverage of about 0.1. Additional thresholds for the electron-stimulated desorption of the lithium atoms are observed at electron energies of 40 and 70 eV for the coverages larger than 0.6 and 0.75, respectively. These thresholds correlate with the ionization energies of the Mo 4s and Mo 4p levels. Relatively broad peaks in the range of these thresholds indicate the resonance excitation of the bond and can be explained by the excitation of electrons toward the band of free states above the Fermi level. The mean kinetic energy of the lithium atoms is equal to several tenths of an electronvolt. At electron energies less than 55 eV, the energy distributions of lithium atoms involve one peak with a maximum at about 0.18 eV. For the lithium coverages less than 0.45 and electron energies higher than 55 eV, the second peak with a maximum at 0.25 eV appears in the energy distributions of the lithium atoms. The results obtained can be interpreted in the framework of the Auger-stimulated desorption model, in which the adsorbed lithium ions are neutralized after filling holes inside inner shells of the substrate and lithium atoms.     

Hole dynamics in noble metals

Hole dynamics in noble metals (Cu and Au) is investigated by means of first-principles many-body calculations. While holes in a free-electron gas are known to live shorter than electrons with the same excitation energy, our results indicate that d holes in noble metals exhibit longer inelastic lifetimes than excited sp electrons, in agreement with experiment. The density of states available for d-hole decay is larger than that for the decay of excited electrons; however, the small overlap between d and sp states below the Fermi level increases the d-hole lifetime. The impact of d-hole dynamics on electron-hole correlation effects is also addressed.     

Auger-spectroscopic appearance of electron correlation at the Fermi surface of graphite

It is shown that, in Auger-electron spectra of three-dimensional semimetal graphite and two-dimensional graphite (a zero band-gap semiconductor), an energy gap should be observed between the thresholds (edges) of the forward and inverse processes (threshold gap). In the one-electron approximation, this gap is zero, since the threshold for the Auger spectrum of the forward process is the minimum hole energy in the valence band, while the threshold for the spectrum of the inverse process is the minimum energy of conduction electrons. Inclusion of the electron correlation at the Fermi surface within the quantum-chemical approximation of a single open electron shell for multiplet structures of the restricted Hartree-Fock method makes it possible to determine the threshold gap as 1.5 eV for a 48-atom cyclic model of three-dimensional graphite and as 2.0 eV for a 24-atom model of two-dimensional graphite. The threshold gap does not contain the Fermi energy, in contrast to the Auger spectrum thresholds, where \(\frac{1}{2}(4.0 eV - \varepsilon _F )\) for the forward Auger spectrum (holes) and \(\frac{1}{2}( - 1.1 eV + \varepsilon _F )\) for the inverse spectrum (conduction electrons), the sum of which gives this gap. The results of calculations for the forward Auger spectra of three-dimensional graphite (including the conclusion that electron correlation of holes in the top valence bands is weak in the Auger process) are shown to agree with the experimental data.     

First principles study of the electronic structures of erbium silicides with non-frozen 4f treatment

The electronic structures (especially 4f states) of hexagonal and tetragonal erbium silicides are investigated within density functional theory. Contrary to previous theoretical studies on these compounds, Er 4f electrons are treated as valence state electrons, explicitly taking into account the on-site Coulomb interactions. Total energy calculations show that the relaxed hexagonal ErSi_1.7 is more stable than the tetragonal structure, consistently with related experimental observations. The calculated total density of states of the hexagonal ErSi_1.7 agrees well with the experimental valence-band spectrum in a wide energy range from 0 to 12 eV below the Fermi level. In addition, our study indicates that the occupied 4f states in erbium silicides can also locate in the energy range of 0–4.0 eV below the Fermi energy, much different from the prediction of the previously adopted Er ion model.     

Fermi energy and optical conductivity of metal quantum wires

The size dependence of the Fermi energy of conduction electrons has been analytically evaluated and its oscillations have been calculated in the framework of the cylindrical well model. The conductivity tensor components for a metal wire with the radius ρ₀ have been calculated with the use of the expansion in powers of ρ₀/λ, where λ is the wavelength of incident light. The influence of the dimension of the systems has been established by comparing the results of the calculations of the optical conductivity of thin wires and films for Au, Al, and Pb metals. According to the results of the calculations, the real and imaginary parts of the conductivity depend substantially on the size and frequency. The difference between the results obtained for Pb and Au metals has been explained by the different relaxation times of electrons in nanowires that are homogeneous in the thickness.     

Al掺杂对Mg_2Si介电性质影响的第一性原理研究

采用基于密度泛函理论的第一性原理平面波超软赝势方法,计算了本征Mg_2Si和Al掺杂Mg_2Si的形成能、电子结构和介电函数.结果表明,Al掺杂Mg_2Si后,Al以替位杂质(Al替Mg位,即AlMg)或填隙杂质(Al位于晶胞间隙位,即Ali)的形式进入Mg_2Si晶格.费米能级进入导带,体系呈n型导电.掺杂后体系的介电函数的实部和虚部在低频时比未掺杂时均增大,主要是由于晶格中的施主杂质Al离子束缚着附近的过剩电子,在外加交变电场的作用下,束缚在Al离子周围的电子要克服一定的势垒不断地往复运动造成松弛极化和损耗.另外,掺杂体系相对于未掺杂体系,介电函数的虚部在0.5 eV附近出现了一个额外的介电峰,该峰主要是由电子从价带跃迁到Al杂质能级引起的.计算结果为Mg_2Si基光电子器件的设计和应用提供了理论依据.     

Resonant valence band satellite in palladium

Normal incidence photoemission has been used to investigate the existence of a valence band satellite as in the case of Ni. It is observed at 8.5 eV below the Fermi level and has an intensity which is only measurable when a photon energy is reached which corresponds to the threshold for excitation of an electron from the 4p levels to the Fermi level.     

Addition of monochromated UV discharge lamp to X-ray photoemission spectrometer

To combine in a single system the benefits of XPS and UPS, a noble gas cold cathode discharge lamp and a grazing incidence grating monochromator have been attached to a Hewlett-Packard 5950A ESCA spectrometer. Since the HP multichannel detection system was designed for the relatively low count rates associated with a monochromator dispersed X-ray source some customarily unavailable low intensity UV lines can be used. Photoemission spectra for 9 excitation lines of He and Ne spaced approximately every 4 eV between 16.7 and 48.4 eV are presented for a polycrystalline Au test sample. Operation of the ESCA spectrometer's electron optics and hemispherical analyzer for low kinetic energy (< 50 eV) electrons is described.     

Ultrahigh-density magneto-optical recording based on the effect of plasmon excitations

The analysis of ultrahigh-density recording in magneto-optical media formed with the use of granular CoAg(Au) films is performed. Due to excitation of plasma oscillations of electrons in granular CoAg(Au) films, the latter make it possible to perform additional focusing of recording radiation and to significantly decrease the size of memory cells. It is suggested that two-layer film CoAg(Au)/TbFeCo/substrate systems be used as recording media for magneto-optical recording. It is shown theoretically that the angle of the Kerr rotation of the plane of polarization θ_K upon magnetization of such a two-layer structure increases significantly in the range of incident-photon energy from 2 to 4 eV in comparison with the case of TbFeCo magnetic film. Some specific features were detected experimentally in the spectra of θ_K in the region of excitation of plasma oscillations in nanogranular CoAu and CoAg films at ? ω _P ≈ 2.5 and 3.5 eV, respectively, as well as in two-layer CoAg(Au)/TbFeCo film systems.     

Photoemission with polarized light from free-electron and transition metals

Photoemission from free electron like (Al, Mg) and transition (Mo, Au) metals has been studied using polarized light at different angles of incidence in a broad photon energy range (< 11.8, 16.8 and 21.2 eV). The results are interpreted in terms of both surface and volume photoexcitation mechanisms. Both the optical and the surface plasmon electromagnetic fields are considered as excitation sources for photoelectrons. Anisotropies due both to the surface and the volume excitation have been detected.     

Inelastic photoemission due to scattering by surface adsorbate vibrations

By measuring the very low energy photoemission spectra of the CO/Cu(001) surface with a high resolution, we have found the energy loss components due to inelastic scattering of electrons near the Fermi level by the CO vibrational modes. The main energy loss structure appears as a step at 254 meV below the Fermi edge for 12C16O. An isotope shift of the step to 240 meV was observed when 13C18O was adsorbed. This observation confirms that this step arises from the energy loss of photoelectrons near the Fermi level through the excitation of the C-O stretching mode.     

Inelastic mean free path, surface excitation parameter, and differential surface excitation parameter in Au for 300-3000 eV electrons

An analytical approach for simultaneously determining an inelastic mean free path (IMFP), a surface excitation parameter (SEP) and a differential SEP (DSEP) with absolute units was applied for the analysis of absolutely measured reflection electron energy loss spectra for Au. The IMFP, SEP and DSEP in Au for 300-3000 eV electrons are successfully obtained. The obtained DSEPs show a reasonable agreement with those theoretically calculated. The present SEPs were compared with those calculated by several empirical equations, revealing that the present SEPs are close to those calculated using the Oswald's equation. The IMFPs for Au determined by the present analysis were compared with those calculated by the TPP-2M predictive equation, revealing that the present IMFPs are in fairly good agreement with those calculated by the TPP-2M equation. The results confirmed that the present approach is effective for experimentally determining the SEP, DSEP, and IMFP for electrons in solids.     

Electron energy loss spectra and X-ray photoelectron spectra of Ca2V2O7

We have measured the electron energy loss spectra of Ca₂V₂O₇ in the reflexion mode, at incident energies between 200 and 2400 eV, and the X-ray photoelectron spectra excited by Al K α radiation. The abundant loss structures observed can be correlated with the possible interband transitions, collective oscillations, and excitation of O2s and V3p electrons within the V₂O₇^4- ion. The gap width and molecular orbital (MO) spread (or splitting) is about l eV larger in the V₂O₇^4- ion than in its component VO₄^3- ion. Excitation of O2s states, which may occur together with some MO over-gap transitions, displaces the collective oscillations about 7 eV towards lower energies. Deeper V3p electrons are excited with a maximum energy loss some 7 eV above their binding energy. Cross transitions from Ca3p levels into some empty states of the V₂O₇^4- ion, or direct transitions to available states of the Ca²⁺ ion could not be unambiguously identified. The energy dependence of the excitation cross section and of the electron penetration depth results in a significant variation of the relative intensity of various losses over the investigated energy range.