Quasi-free Electrons Research Articles

Synthesis and dielectric properties of nano Si/C/N powders

Nano Si/C/N powders with different configurations were prepared by Chemical Vapor Deposition (CVD). The crystallinity of the products increased with increasing reaction temperature. The powder prepared at 1200 °C consisted of amorphous spherical particles with a particle size of 20 to 50 nm. The nanocrystalline particles produced at 1400 °C, with a mean size from 50 to 100 nm, were also spherical. The powder prepared at 1600 °C consisted of whiskers with a diameter from 20 to 50 nm, and the length ranged from 100 to 200 µm. XRD results indicated the whisker sample was mainly β-SiC, which was in good agreement with the SAED pattern. The permittivity of the powders was measured in the frequency range from 8.2 GHz to 12.4 GHz and loss tangents were calculated also. The permittivity and loss tangent of the amorphous Si/C/N nano powder were the lowest of the three samples studied. The ε′, ε″ and tanδ of the Si/C/N powders all decreased with increasing frequency. Impurity conduction, charged defects, quasi-free electrons, and π-bonds caused by nitrogen dissolved in the SiC lattice were the reasons for the dielectric loss of the Si/C/N powders.

Free-Ion Yield and Electron Mobility in Liquid Hydrocarbons: A Consistent Correlation

A correlation is sought between the free-ion yield and electron mobility in liquid hydrocarbons in terms of the elastic and the inelastic scattering mean free paths of epithermal (≤0.2 eV) electrons. These determine the thermalization distance distribution and consequently the free-ion yield. The thermal quasi-free electron mobility, μqf, can also be obtained from the same cross sections. Finally, the effective mobility is derived from μqf using the electron trap concentration and binding energy, thereby establishing a relationship between free-ion yield and mobility. Thus, given the input data for trapping and the elastic and inelastic cross sections, both the free-ion yield and the effective mobility may be obtained from the interactions of epithermal electrons. In very low mobility liquids (μ < 0.1 cm2 v-1 s-1), transport is governed by trapping and detrapping rates, being relatively independent of μqf. As found by Jay-Gerin et al. (Can. J. Chem. 1993 , 71, 287) the free-ion yield in such cases is virtually independent of mobility, a phenomenon which is naturally explained in the quasiballistic model of the author (Chem. Phys. Lett. 1993, 207, 245 and 1995, 233, 167) but not in the usual trapping model. For low mobility liquids, the elastic mean free path, L, is found to be ca. 1−5 A and there is a probability of ∼0.1−0.3 inelastic scatterings per elastic mean free path. These values increase progressively with mobility, where, in the high mobility cases, L is on the order of a few tens of angstroms and inelastic collisions outnumber elastic ones by a factor of ∼2−4. The situation is reminiscent of liquefied rare gases.

Collective excitations in a linear periodic array of cylindrical nanotubes

A theory of collective plasma excitations in a linear periodic array of multishell nanotubes is presented. The electron system in the nanotubes is modeled by a quasifree electron gas confined to the surface of an infinitely long cylinder. The plasmon dispersion equation is derived in the random-phase approximation neglecting the electron tunneling between the individual cylindrical tubules. The dispersion equation is solved numerically for a single-wall nanotube array, and the plasmon excitation energies are obtained as a function of the wave vector in the direction of the nanotube axes and in the transverse direction. It is found that the intertube Coulomb interaction couples the modes with different angular momenta m in individual nanotubes and, in particular, lifts the $\ifmmode\pm\else\textpm\fi{}m$ degeneracy of the single-nanotube modes. This effect is analyzed numerically as a function of the separation between the tubules. We show that the translational symmetry of the lattice is maintained in the plasmon spectrum, and the plasmon energies have a periodic dependence on the transverse wave vector.

Valence excitations in individual single-wall carbon nanotubes

We report on measurements of the plasmon losses of individual single-wall carbon nanotubes by electron energy-loss spectroscopy in a high-resolution transmission electron microscope. The experimental data are compared to simulated excitation probabilities calculated using the hydrodynamic theory of the interaction between a probe electron and a two-dimensional quasifree electron gas confined on a cylindrical shell. Depending on the nanotube geometry, the first- or the second-order oscillation mode dominates the loss spectrum. The resonance energy of the dominant resonance mode is found to depend on the radius of the nanotube.

Superelastic scattering of electrons from metastable He-likeC4+andO6+ions

Superelastic scattering, a collision process in which the scattered electron gains energy, has been investigated for metastable C 4 + (1 s2s 3 S) and O 6 + (1s2s 3 S) He-like ions colliding with quasifree H 2 -target electrons. The measured superelastic-scattering cross sections for 1s2s 3 S→1s 2 1 S deexcitation show a strong dependence on the collision energy and the atomic number of the He-like ion. R-matrix calculations for the time-reversed equivalent process of electron-impact excitation are compared with the measured cross sections through the principle of detailed balance. In these calculations, the impulse approximation is utilized in conjunction with the Compton profile of the target electrons for the collisions of ions with quasifree electrons studied here. Calculations were performed taking into account the anisotropy of the cross sections as a function of the scattering angle, which equals 180° in the projectile rest frame for the reported measurements, Comparison between theory and experiment was used to infer the metastable 1s2s s 3 S fraction in the incident He-like beam.

Charge states distribution of 3350 keV He ions channeled in silicon

When an ion beam is aligned along a major crystalline axis the dominant interaction is with valence electrons. In this condition the charge exchange processes mostly concern the interaction between the incident ion and a quasi-free electron gas and a strong reduction of the charge-changing probabilities is expected. In this work, 3350 keV He + and He 2+ ions were aligned at small tilt angles about the 〈1 1 0〉 axis of a 4650 Å silicon crystalline membrane. The charge state distribution (CSD) of the transmitted ions was detected by an electro-magnetic analyzer having a very small acceptance angle. In these conditions the equilibration of the CSD was not yet reached and this allowed, making use of simple approximations, for the measurement of the valence electron loss cross-section.

Simultaneous projectile-target ionization: a novel approach to (e, 2e) experiments on ions.

A kinematically complete experiment for simultaneous ionization of a projectile and target has been performed for 3.6 MeV/u C2+ on He collisions measuring the final vector momenta of the He1+ recoil ion and of two electrons (projectile, target) in coincidence with the emerging C3+ projectile. The feasibility of an event-by-event separation of the various reaction channels, among them the ionization of C2+ by the interaction with a quasifree target electron, is demonstrated in agreement with six-body classical trajectory Monte Carlo calculations, paving the way to kinematically complete electron-ion scattering experiments.

Influence ofs−dinterfacial scattering on the magnetoresistance of magnetic tunnel junctions

We propose the two-band s-d model to describe theoretically a diffuse regime of the spin-dependent electron transport in magnetic tunnel junctions (MTJ's) of the form F/O/F where F's are 3d transition metal ferromagnetic layers and O is the insulating spacer. We aim to explain the strong interface sensitivity of the tunneling properties of MTJ's and investigate the influence of electron scattering at the nonideal interfaces on the degradation of the TMR magnitude. The generalized Kubo formalism and the Green's functions method were used to calculate the conductance of the system. The vertex corrections to the conductivity were found with the use of "ladder" approximation combined with the coherent-potential approximation (CPA) that allowed to consider the case of strong electron scattering. It is shown that the Ward identity is satisfied in the framework of this approximation that provides the necessary condition for a conservation of a tunneling current. Based on the known results of ab-initio calculations of the TMR for ballistic junctions, we assume that exchange split quasi-free s-like electrons with the density of states being greater for the majority spin sub-band give the main contribution to the TMR effect. We show that, due to interfacial inter-band scattering, the TMR can be substantially reduced even down to zero value. This is related to the fact that delocalized quasi-free electrons can scatter into the strongly localized d sub-band with the density of states at the Fermi energy being larger for minority spins compared to majority spins. It is also shown that spin-flip electron scattering on the surface magnons within the interface leads to a further decrease of the TMR at finite temperature.

On the relationship between the ionization potential and the work function: Metals

A relationship is analyzed between the ionization energy of atoms I and the work function φ of a metal composed of these atoms. The transition energy ∑=I−φ is assumed to be the sum of kinetic K and coulombic C contributions. Contribution K is calculated in the framework of a model of uniform gas of quasi-free electrons, and C is determined from experimental values of Σ. Calculations performed for a wide range of metals have shown that the dimensionless coefficients determining the Coulombic contribution C differ insignificantly between various groups of metals. The relationships obtained have been used for determining the work function of binary alloys.

How many-particle interactions develop after ultrafast excitation of an electron-hole plasma.

Electrostatic coupling between particles is important in many microscopic phenomena found in nature. The interaction between two isolated point charges is described by the bare Coulomb potential, but in many-body systems this interaction is modified as a result of the collective response of the screening cloud surrounding each charge carrier. One such system involves ultrafast interactions between quasi-free electrons in semiconductors-which are central to high-speed and future quantum electronic devices. The femtosecond kinetics of nonequilibrium Coulomb systems has been calculated using static and dynamical screening models that assume the instantaneous formation of interparticle correlations. However, some quantum kinetic theories suggest that a regime of unscreened bare Coulomb collisions might exist on ultrashort timescales. Here we monitor directly the temporal evolution of the charge-charge interactions after ultrafast excitation of an electron-hole plasma in GaAs. We show that the onset of collective behaviour such as Coulomb screening and plasmon scattering exhibits a distinct time delay of the order of the inverse plasma frequency, that is, several 10(-14) seconds.

Slow-ion induced electron emission from clean metal surfaces: “Subthreshold kinetic emission” and “potential excitation of plasmons”

We discuss some recent advances in the understanding of potential (PE) and kinetic electron emission (KE) for impact of slow ions on clean metal surfaces. Below the “classical threshold” for KE by direct momentum transfer to quasi-free metal electrons, electron emission can already occur by three other mechanisms, i.e., electron promotion into the continuum in binary ion-target collisions, quasi-localization of electrons at the surface and, being only relevant at very low impact velocity, a partially localized electronic excitation in the impact zone. Furthermore, we regard potential emission of electrons (PE) from the one-electron decay of plasmons which are excited by impact of slow ions on quasi-free electron metal surfaces (e.g., poly- and mono-crystalline aluminum). Electron spectra for impact of 3–10 keV singly and doubly charged ions show that such plasmons result either from the ion potential energy (“potential excitation of plasmons – PEP”) or, in a secondary process, from sufficiently fast electrons from KE. Contributions from these processes to the respective total electron yield are typically in the percent range. A considerably more pronounced structure in the electron spectra resulting from impact of H + on Al(1 1 1), which has formerly been ascribed also to plasmon decay, is probably caused by diffraction of slow KE electrons undergoing multiple scattering in the uppermost target layers.

Long electron spin memory times in gallium arsenide

Extremely long electron spin memory times in GaAs are reported. It was established by the optical orientation method that the spin relaxation time of electrons localized at shallow donors in n-type gallium arsenide (N d −N A ≈1014 cm−3) is 290±30 ns at a temperature of 4.2 K. The exchange interaction of quasi-free electrons and electrons at donors suppresses the main spin-loss channel for electrons localized at donors—spin relaxation due to the hyperfine interaction with lattice nuclei.

On the nonequilibrium light-induced diamagnetism

A new interpretation of light-induced magnetization changes of a magnetic semiconductor, manganese arsenide (MnAs), observed by the authors of references [1,2], is proposed in this paper. Contrary to references [1,2], where the results of experiments were interpreted as the observation of light-induced phase transition, here we propose a completely different approach. It suggests that at least far from the vicinity of Tc, there are no real magnetization changes as in case of phase transition, but there are changes of the magnetic flux threading the MnAs-sample. These changes are due to non-equilibrium light-induced diamagnetic moments of quasi-free electrons of narrow d-subbands of the MnAs-conduction band. The other aspects of the experiments of [1,2] are also discussed and some similarity between this effect and the orbital diamgnetism due to persistent currents in mesoscopic structures is emphasised.

Metastable Electrons, High-Mobility Solvent Anions, and Charge Transfer Reactions in Supercritical Carbon Dioxide

Time-resolved dc photoconductivity has been used to observe quasifree electrons and solvent radical anions in supercritical CO2 (ρc ≈ 0.468 g/cm3; Tc ≈ 31 °C). The electrons have lifetime τe < 200 ps and mobility μe > 10 cm2/(V s). For ρ/ρc > 0.64, the product μeτe rapidly increases with the solvent density ρ, reaching 2.5 × 10-9 cm2/V at ρ/ρc ≈ 1.82. The mobility of the solvent anions exponentially increases with ρ, being 2−10 times higher than the mobilities of other ions. The activation energy of the solvent anion migration is 0.46 eV, whereas for solute ions, this energy is less than 20 meV. Electron detachment upon 1.0−3.5 eV photoexcitation of the solvent anion has been observed. The cross section of photodetachment linearly increases with photon energy above 1.76 eV end exhibits a shoulder at 2.82 eV; the spectrum is similar to photoelectron spectra of (CO2)n- clusters (n = 6−9) in the gas phase. Both the electrons and solvent anions react with nonpolar electron acceptor solutes whose gas-phase electron affinity (EAg) is greater than 0.4 eV. At T = 41 °C and ρ/ρc ≈ 1.77, the rates of the electron attachment and solvent anion scavenging correlate with each other and the solute EAg. Only solutes with EAg > 2 eV exhibit diffusion-controlled kinetics. For ρ/ρc > 0.85, the scavenging radii of these diffusion-controlled reactions decrease with ρ. For O2, the scavenging of the solvent anion is reversible (ΔH° ≈ −0.44 eV).

IR Spectroscopic studies of charged–discharged crystalline WO 3 films

The structural changes of charged–discharged crystalline WO 3 films have been studied using ex-situ IR spectro-electrochemical measurements. The films were deposited on transparent conducting glass (F:SnO 2, Flabeg) and Si wafers by dip coating from a peroxopolytungstic acid solution. Annealing was then performed at 150 and 300°C (60 min) and at 500°C (15 min). At 150°C, an amorphous WO 3· nH 2O phase forms while annealing at 500°C produces a monoclinic WO 3 (m-WO 3) modification. XRD and IR spectroscopies were used to assess the film structure and crystallinity. Crystalline films were galvanostatically charged–discharged to a different level of insertion coefficient ( x) in 0.1 M HClO 4(aq), 1 M LiClO 4/propylene carbonate (PC) and 1 M NaClO 4/PC electrolytes. Ex-situ IR spectra of all intercalated films exhibit a pronounced increase in absorption between 8000–1000 cm −1 due to the quasi-free electrons. We observed differences between the IR spectra of WO 3 when charged to varying levels of x with H +, Li + and Na +. The intercalation of m-WO 3 independently of the inserted ion leads to the transformation from monoclinic to tetragonal and further to cubic phase. By using site symmetry group analysis we predicted the number of IR and Raman active WO stretching vibrations for H x WO 3, Li x WO 3 and Na x WO 3 in cubic, tetragonal and monoclinic phases. We observed a single WO stretching for the c-Li x WO 3 ( Pm3 m) and two for the c-H x WO 3 ( Im3) phase, which agrees with the lowering of the site symmetry of WO 6 groups in the latter phase. For the t-H x WO 3 and t-Li x WO 3 films, a similarity between the corresponding IR spectra indicates that the structure of the tetragonal phases does not differ significantly.

Thermalization of low energy electrons in liquid methylcyclohexane studied by the photoassisted ion pair separation technique

A spectral dependence of the charge carrier quantum yield in liquid methylcyclohexane was investigated on photoexcitation of geminate electrons at wavelengths between 425 and 867 nm. The electrons were produced by two photon ionization of the anthracene admixture with the UV laser pulse (wavelength 308 nm, duration about 20 ns) and, then, were excited by the dye laser pulse delayed for a time of 25 ns with respect to the UV one. The increase of photocurrent caused by an additional dye laser pulse action was observed as temperature decreased below 230 K. It has been found that the spectrum of the photocurrent enhancement in the range of dye laser radiation wavelengths from 425 to 867 nm is close to that of the trapped electron optical absorption. The increase of photocurrent has been attributed to photogeneration of “hot” quasi-free electrons arising from a photon absorption by geminate trapped electrons. Basing on the diffusion model of photostimulated ion pair dissociation, the thermalization length of 4.2–4.7 nm was determined for quasi-free electrons with an initial kinetic energy within the range from about 0.9 to 2.4 eV. The energy dissipation rate and effective mean free path were evaluated for the quasi-free electrons with the energy of about 1 eV in hydrocarbon matrix.

Dielectric properties of nano Si/C/N composite powder and nano SiC powder at high frequencies

The dielectric properties of nano Si/C/N composite powder and nano SiC powder at high frequencies have been studied. The nano Si/C/N composite powder and nano SiC powder were synthesized from hexamethyldisilazane ((Me 3Si) 2NH) (Me:CH 3) and SiH 4–C 2H 2, respectively, by a laser-induced gas-phase reaction. The complex permittivities of the nano Si/C/N composite powder and nano SiC powder were measured at a frequency range of 8.2–12.4 GHz. The real part ( ε′) and imaginary part ( ε″) of the complex permittivity, and dissipation factor ( tg δ=ε″/ε′) of nano Si/C/N composite powder are much higher than those of nano SiC powder and bulk SiC, Si 3N 4, SiO 2, and Si, especially the tg δ . The promising features of nano Si/C/N composite powder would be due to more complicated Si, C, and N atomic chemical environment than in a mixture of pure SiC and Si 3N 4 phase. The charged defects and quasi-free electrons moved in response to the electric field, diffusion or polarization current resulted from the field propagation. Because there exists graphite in the nano Si/C/N composite powder, some charge carries are related to the sp 3 dangling bonds (of silicon and carbon) and unsaturated sp 2 carbons. The high ε″ and tg δ of nano Si/C/N composite powder were due to the dielectric relaxation. The nano Si/C/N composite powder would be a good candidate for electromagnetic interface shielding material.

MICROWAVE PERMITTIVITY OF NANO Si/C/N OMPOSITE POWDERS

The microwave permittivity of nano Si/C/N composite powders suspended in paraffin wax has been studied at the frequency range of 8.2—18GHz. The nano Si/C/N composite powders were synthesized from hexamethyldisilazane ((Me3Si)2NH) (Me:CH3) by a laser induced gas phase reaction. The dissipation factors of the nano Si/C/N composite powders are high at the microwave frequencies. The microwave permittivity of the mixture of nano Si/C/N composite powders and paraffin wax (or other dielectric materials) can be tailored by the content of the composite powders. And ε′, ε″ and tan δ increase with the volume filling factor ( v ) of nano Si/C/N composite powders. The ε′ and ε″ can be effectively modeled using second order polynomials ( ε′, ε″=Av2+Bv+C ). The ε′ and ε″ of the nano Si/C/N composite powders decrease with frequency at the frequency range of 8.2—18GHz. The difference being the microwave resonance is not sharply peaked but rather smeared out over a large frequency range. The promising features of nano Si/C/N composite powders would be due to more complicated Si, C, and N atomic chemical environment than in a mixture of pure SiC and Si3N4 phase. The SiC microcrystallines in the nano Si/C/N composite powders dissolve a great deal of nitrogen. The local structure around Si atoms changes by introducing N into SiC. Carbon atoms around Si are substituted by N atoms. So there exist a large number of charged defects and dangling bonds in the nano Si/C/N composite powders. Thus charged defects and quasi free electrons move in response to the electric field, diffusion or polarization current resulting from the field propagation. The high ε″ and dissipation factor tan δ(ε″/ε′) of Si/C/N composite powders are due to the dielectric relaxation.

Kinetic electron emission from clean polycrystalline gold induced by impact of slowC+,N+,O+,Ne+,Xe+,andAu+ions

Ion-induced kinetic electron emission is commonly attributed to collisions of an energetic projectile with quasifree electrons, and to the promotion of atomic levels in binary collisions of the projectile with atomic particles in the solid. The contribution of the promotion processes to the electron emission has been estimated theoretically for all studied systems from molecular-orbital correlation diagrams. As quasifree electron collisional excitations have a sharp threshold at relatively high velocities of the projectiles, their contribution to the electron emission at lower impact velocities should be negligible. We will show, however, that the partial localization of the quasifree electrons due to the presence of the solid surface ``washes out'' this sharp threshold. This can lead to one-electron excitations at low impact velocities that may be more significant than excitations due to promotion. At the lowest impact velocities the electron emission yields conspicuously level off in some studied cases. Such behavior cannot be reconciled with any existing one-electron model (including the one proposed here), as they all predict a rapid decrease of the electron emission with decreasing impact velocity. In this paper we interpret the leveling-off of the yield in terms of a many-electron excitation mechanism, based on the assumption of spatial and temporal localizations of electronic excitation in the impact zone. The models discussed in this paper will be compared with experimental data on kinetic electron emission from polycrystalline gold bombarded by ${\mathrm{C}}^{+},$ ${\mathrm{N}}^{+},$ ${\mathrm{O}}^{+},$ ${\mathrm{Ne}}^{+},$ ${\mathrm{Ne}}^{0},$ ${\mathrm{Xe}}^{+},$ and ${\mathrm{Au}}^{+},$ with kinetic energies below \ensuremath{\sim}15 keV, and perpendicular incidence on the surface.

The Perturbative Floquet Solution for Quasi-free Electrons

In this paper the quasi-energies and the reduced Floquet vectors corresponding to a particle in 2-dimensions interacting with a time-dependent, monochromatic, linearly polarized electro-magnetic field, and in the presence of a weak, static, spatial-periodic field, are calculated. The later is treated using the stationary perturbation theory in the extended Hilbert space adapted for the eigenvalue equation of the Floquet Hamiltonian. The results have similarities to those of the quasi-free electron model.