Showing papers on "Free electron model published in 1998"

Fully self-consistent GW self-energy of the electron gas

Abstract: We present fully self-consistent results for the self-energy of the electron gas within the GW approximation. This means that the self-consistent Green’s function G, as obtained from Dyson’s equation, is used not only for obtaining the self-energy but also for constructing the screened interaction W within the random-phase approximation. Such a theory is particle and energy conserving in the sense of Kadanoff and Baym. We find an increase in the weight of the quasiparticle as compared to ordinary non-self-consistent calculations but also to calculations with partial self-consistency using a fixed W. The quasiparticle bandwidth is larger than that of free electrons and the satellite structure is broad and featureless; both results clearly contradict the experimental evidence. The total energy, though, is as accurate as that from quantum Monte Carlo calculations, and its derivative with respect to particle number agrees with the Fermi energy as obtained directly from the pole of the Green’s function at the Fermi level. Our results indicate that, unless vertex corrections are included, non-self-consistent results are to be preferred for most properties except for the total energy. (Less)

Atomic and Free Electrons in a Strong Light Field

Abstract: Introduction to the theory of field-induced atomic transitions multiphoton stimulated bremsstrahlung multiphoton Compton scattering and ponderomotive forces in an inhomogeneous light field free-electron lasers laser acceleration of electrons wave packets above-threshold ionization stabilization of atoms in a strong ionizing field.

Work functions of elements expressed in terms of the Fermi energy and the density of free electrons

Abstract: On the basis of Brodie's definition of the work function and the length of spontaneous polarization of plasma, the following new formula for calculation of the work functions of elements has been derived: , where is the electron density parameter expressed in units of the Bohr radius, is the Fermi energy and is an empirical constant ( for the alkali metals, Ca, Sr, Ba, Ra and Tl, whereas for the remaining elements). The density parameter was calculated from the atomic mass, the bulk density of the element and the assessed number of free electrons per atom which is equal to the nominal valence of the element or, in the case of transition metals, close to this number (within ). The values obtained by using the above formula are in excellent agreement with experimental data for pure-metal polycrystalline surfaces, within 5% in most cases. A table with the work functions and with complete input data for most of the elements is presented.

Nonequilibrium Electron Interactions in Metal Films

Abstract: Ultrafast relaxation dynamics of an athermal electron distribution is investigated in silver films using a femtosecond pump-probe technique with 18 fs pulses in off-resonant conditions. The results yield evidence for an increase with time of the electron-gas energy loss rate to the lattice and of the free electron damping during the early stages of the electron-gas thermalization. These effects are attributed to transient alterations of the electron average scattering processes due to the athermal nature of the electron gas, in agreement with numerical simulations.

Nonlinear propagation of dust-acoustic waves in a magnetized dusty plasma with vortex-like ion distribution

Abstract: A theoretical investigation has been made of the nonlinear propagation of dust-acoustic waves in a magnetized three-component dusty plasma consisting of a negatively charged dust fluid, free electrons and vortex-like distributed ions. It is found that, owing to the departure from the Boltzmann ion distribution to a vortex-like one, the dynamics of small- but finite-amplitude dust-acoustic waves in a magnetized dusty plasma is governed by the modified Korteweg–de Vries equation. The latter admits a stationary dust-acoustic solitary-wave solution that has larger amplitude, smaller width and higher propagation velocity than that involving adiabatic ions. The effects of external magnetic field, trapped ions and free electrons on the properties of these dust-acoustic solitary waves are briefly discussed.

Electrical properties of iron aluminides

Abstract: Electrical properties of iron aluminides covering a wide range of Al contents have been investigated using a four-probe electrical resistivity method at room temperature. Room temperature electrical resistivities of iron aluminides indicate a monotonic increase with Al content up to ∼ 33 at.% Al, followed by a steep decrease with further increase of Al content. Heat treatments in a vacuum at 1100°C for 1 h, and at 700°C for 2 h followed by cooling to room temperature in helium and argon exhibited an identical trend, with resistivity values comparable to the values obtained after a prolonged annealing. A similar trend was observed with the fine grained sheets of FeAl obtained by roll compaction and sintering. Interpretation of literature band calculations with the help of the Mott and Jones model suggests that the electrical resistivity should increase with an increase in Al content until ∼ 33 at.% Al is reached, due to the filling up of holes by the electrons from Al in the narrow d-bands. Further addition of Al above 33 at.% value essentially adds more electrons to a nearly free electron conduction band and the resistivity falls sharply thereafter approaching the resistivity of Al.

Surface Plasmon Enhanced Second Harmonic Response from Gold Clusters Embedded in an Alumina Matrix

Abstract: The second order nonlinear response from 4 nm diam gold clusters embedded in an alumina matrix deposited on a pure silica substrate has been recorded as a function of the wavelength of the fundamental incident beam. The spectrum exhibits a narrow resonance band peaked at 520 nm as a result of the coupling of the second harmonic field with the surface plasmon of the particles. The nonlinear second harmonic response of gold clusters is found to be blueshifted compared to the bulk, due to the finite size effects on the cluster optical properties, as already observed with the linear response. Furthermore, this nonlinear response appears to be well described with a simple free electron model where the valence electrons only participate through the screening of the ionic cores, owing to the weakness of the interband transition contribution.

Nonlinear propagation of ion-acoustic waves in a hot magnetized plasma with vortexlike electron distribution

Abstract: Effects of external magnetic field, ion temperature, and vortexlike electron distribution are incorporated in the study of nonlinear ion-acoustic waves in a hot magnetized plasma that consists of a positively charged, hot ion fluid and trapped, as well as, free electrons. It is found that, owing to the departure from the Boltzmann electron distribution to a vortexlike one, the dynamics of small but finite amplitude ion-acoustic waves is governed by a nonlinear equation of K-dV (Korteweg-de Vries) type. The latter admits a stationary ion-acoustic solitary wave solution, which has larger amplitude, smaller width, and higher propagation velocity, than that involving isothermal electrons. The effects of external magnetic field and ion temperature on the properties of these ion-acoustic solitary structures are also discussed.

Solitary Waves in a Three-component Dusty Plasma with Trapped Ions

Abstract: A theoretical investigation has been made of dust-acoustic solitary waves in an unmagnetized three-component dusty plasma which consists of a negatively charged dust fluid, free electrons and trapped as well as free ions. It is found that owing to the departure from the Boltzmann ion distribution to a trapped ion one, the dynamics of small but finite amplitude dust-acoustic waves is governed by a modified Korteweg-de Vries equation. The latter admits a stationary dust-acoustic solitary wave solution which has larger amplitude, smaller width, and higher propagation velocity, than that involving adiabatic ions. The effects of trapped ions and free electrons on these dust-acoustic solitary waves are discussed. The present investigation should be useful in understanding some nonlinear features of the dust-acoustic waves which have been observed in a recent numerical simulation study.

Recombination of with free electrons at very low energies

Abstract: Recombination of ions with free electrons has been investigated in a merged-beams experiment at the UNILAC of GSI in Darmstadt. Special emphasis was put on the recombination at very low energies in the electron-ion centre-of-mass frame. At we found an enormous recombination rate exceeding the theoretical expectations for radiative recombination by a factor of 365. For further investigation of this enhancement, the electron density and the magnetic field guiding the electron beam were varied. While an increase of the electron density by a factor of 10 had little influence, the measured rate coefficient increased significantly with the magnetic field strength.

Characterization of the Fermi surface of the organic superconductor \(\)-\(\) by measurements of Shubnikov-de Haas and angle-dependent magnetoresistance oscillations and by electronic band-structure calculations

Abstract: The electronic structure of the quasi two-dimensional (2D) organic superconductor \(\)-(ET)2SF5CH2CF2SO3 was examined by measuring Shubnikov-de Haas (SdH) and angle-dependent magnetoresistance (AMRO) oscillations and by comparing with electronic band-structure calculations. The SdH oscillation frequencies follow the \(\) angular dependence expected for a 2D Fermi surface (FS), and the observed fundamental frequency shows that the 2D FS is 5% of the first Brillouin zone in size. The AMRO data indicate that the shape of the 2D FS is significantly non-circular. The calculated electronic structure has a 2D FS in general agreement with experiment. From the temperature and angular dependence of the SdH amplitude, the cyclotron and band effective masses were estimated to be \(\) and \(\),where g is the conduction electron g factor and \(\) the free electron mass. The band effective mass is estimated to be \(\) from the calculated electronic band structure.

An explicit-solvent dynamic-dielectric screening model of electron-hole interactions in conjugated polymers

Abstract: The effects of interchain interactions on the exciton-binding energy of conjugated polymers are explored theoretically, using rigid polyacetylene chains as a model system. An explicit quantum chemical description is used to describe the polarization that an electron and hole induce in the surrounding polymer chains. The motivation for explicitly including interchain interactions is to allow the standard parameters of semiempirical quantum chemistry to be used to make predictions for solid-state polymers. The model includes the time scales of both the electron-hole motion and the dielectric polarization. A free electron or hole forms an electronic polaron, in which the bare electron or hole delocalizes over about four unit cells before developing a polarization cloud. In the 1 1Bu exciton state, the time scale for electron-hole motion is comparable to that of the polarization. (If a fast dielectric response is assumed, the polarization energy is overestimated by about 60%.) For the Pariser-Parr-Pople Hamil...

Overdamped excitations of the free electron gas in GaN layers studied by Raman spectroscopy

Abstract: Raman spectra of $n$-type GaN on GaAs are compared with line-shape calculations for excitations of the free electron gas. The spectral features in the frequency range of the optical phonons are well explained by plasmon-phonon scattering from an overdamped plasma system taking into account wave-vector nonconservation. The charge-density-fluctuation mechanism is found to be important for off-resonant excitation. For excitation closer to the fundamental band gap of GaN, the impurity-induced Fr\"ohlich mechanism becomes dominant. In the latter case, the observation of a relatively narrow line at the longitudinal-optical phonon frequency is consistent with the presence of a high-density electron gas.

Photoluminescence and free carrier interactions in erbium-doped GaAs

Abstract: The photoluminescence properties of GaAs:Er doped with a new pyrazole and pyridine-based Er source, tris(3,5-di-tert-butylpyrazolato)bis(4-tert-butylpyridine)erbium(III), were investigated. These samples showed significantly stronger and sharper 1.54 μm Er3+ luminescence than in GaAs:Er samples doped with cyclopentadienyl-based Er sources. The efficient luminescence was associated with the Er–2O center, formed with unintentional oxygen impurities. The Er3+ emission was greatly reduced in n-type samples, whereas the emission remained strong in p-type samples. This trend suggests that either the free hole concentration is very important to the Er3+ excitation efficiency, and/or there is a strong Auger quenching mechanism which involves free electrons. A model based on the results of a two-beam experiment indicates the presence of strong Auger energy transfer from the Er-bound exciton to a free electron. Auger energy transfer from the excited Er3+ ion to a free electron was found to be much less important. T...

Free-electron model for mesoscopic force fluctuations in nanowires

Abstract: When two metal electrodes are separated, a nanometer sized wire (nanowire) is formed just before the contact breaks. The electrical conduction measured during this retraction process shows signs of quantized conductance in units of G_0=2e^2/h. Recent experiments show that the force acting on the wire during separation fluctuates, which has been interpreted as being due to atomic rearrangements. In this report we use a simple free electron model, for two simple geometries, and show that the electronic contribution to the force fluctuations is comparable to the experimentally found values, about 2 nN.

Half-Cycle Pulse Assisted Electron-Ion Recombination

Abstract: Unipolar ``half-cycle'' electric field pulses (HCPs) have been used to recombine free electrons and calcium ions. The field assisted process is very similar to controlled three-body recombination in plasmas. We report on experiments that utilize HCP assisted recombination to probe the probability distribution of continuum electron wave packets and produce bound wave packets that are highly localized in three spatial dimensions.

Electrical and optical properties of cdgeas2 single crystals irradiated with fast electrons

Abstract: The effects of 10 MeV fast electron irradiation on conductivity type, resistivity, and optical absorption of CdGeAs2 single crystals were studied Irradiation of as-grown p-type crystals with a fluence of 1×1017 cm−2 caused compensation of the semiconductor and a marked improvement in the optical transmission A level of absorption below 01 cm−1 at 5⩽λ⩽11 μm was registered Increase in the fluence to 2×1017 cm−2 led to conversion to n-type, a drop in the resistivity and deterioration of the transmission Analysis of the absorption in p-type crystals showed that transitions from the V2 and V3 split-off bands to the V1 valence band represent the major absorption mechanism Splitting energies ΔE2−1=015 eV and ΔE3−1=035 eV have been calculated Absorption in n-type crystals increased as λ24 indicating absorption by free electrons and their scattering by optical phonons The effect of irradiation on the optical absorption of CdGeAs2 is assigned to the radiation-induced changes in the compensation level

A strongly correlated electron model for the layered organic superconductors kappa-(BEDT-TTF)2X

Abstract: The fascinating electronic properties of the family of layered organic molecular crystals kappa-(BEDT-TTF)2X where X is an anion (e.g., X=I3, Cu[N(CN)2]Br, Cu(SCN)2) are reviewed. These materials are particularly interesting because of similarities to the high-$T_c$ cuprate superconductors including unconventional metallic properties and competition between antiferromagnetism and superconductivity. The temperature dependence of electrical transport, optical, and nuclear magnetic resonance properties deviate significantly from those of a conventional metal. In particular, there appears to be an effective Fermi energy of the order of 100 K which is an order of magnitude smaller than predicted by band structure calculations. The results of quantum chemistry calculations suggest that a minimal theoretical model that can describe these materials is a Hubbard model on an anisotropic triangular lattice with one electron per site. Appropriate parameter values for the model imply that the electronic correlations are strong, significant magnetic frustration is present, and the system is close to a metal-insulator transition. Insight into the physics of this model can be obtained from recent studies of the Hubbard model using a dynamical mean-field approximation. They are consistent with a low effective Fermi energy and the unconventional temperature dependence of many of the properties of the metallic phase. Future directions are suggested for both theoretical and experimental studies.

Obliquely Propagating Electrostatic Solitary Structures in a Hot Magnetized Dusty Plasma

Abstract: A rigorous theoretical investigation has been made of obliquely propagating electrostatic solitary structures in a hot magnetized dusty plasma which consists of a negatively charged, extremely massive, hot, dust fluid and Maxwellian distributed ions and electrons. The reductive perturbation method has been employed to derive the Korteweg–deVries (K–dV) equation which admits a solitary wave solution for small but finite amplitude limit. It has been shown that the effects of dust-temperature, obliqueness, magnetic field and free electrons have changed the nature of the electro-static solitary structures. The result of the present investigation for unmagnetized case, parallel propagation, no free electron and cold dust fluid completely agrees with the existing published results (Mamun et al. [4]).

Generation of Compton harmonics by scattering linearly polarized light of arbitrary intensity from free electrons of arbitrary initial velocity

Abstract: We present an analytic classical relativistic derivation for a general expression of the harmonic power generated per unit laboratory solid angle due to Compton scattering of plane wave, linearly polarized light of arbitrary intensity from free electrons moving initially with arbitrary velocity. We show graphically the generated frequency as a function of the coordinates of the observation point for several initial electron kinetic energies and light field intensities.

Single and double ionization of helium by highly charged ions in the independent electron model

Abstract: The continuum-distorted-wave - eikonal-initial-state and classical trajectory Monte Carlo models are used to study single and double ionization of helium by highly charged ions at impact energy. The independent electron model, applied in order to calculate transition probabilities as a function of impact parameter, is found to give a correct description of the processes within these two approaches.

Electromagnetic-wave propagation and amplification in overdense plasmas: Application to free electron lasers

Abstract: Considering a propagation of electromagnetic (EM) waves through overdense homogeneous plasmas, a new regime of amplification of EM waves with frequencies below the plasma frequency has been found. An application to free electron lasers is suggested.

Self-interaction-corrected band structure calculations for intracavity electrons in electro-sodalite

Abstract: The zeolite sodalite is a crystalline compound consisting of cages. The windows connecting the cages are large enough to allow small molecules to be absorbed. The material studied here, called sodium electro-sodalite (SES), is prepared by absorbing one Na atom in each cage. Because of the large electric fields inside the cages, each alkali atom is ionized and the donated electron is shared among several ions. A noninteracting electron model, used in earlier work, calculated the absorption spectrum and the temperature dependence of the Al and Si NMR shifts and found good agreement with experiment. The model predicted that the material is a metal. However, recently published low-temperature magnetic susceptibility measurements show that the ground electronic state is antiferromagnetic. This is incompatible with a noninteracting electron model. In this paper we study the electronic properties of this material by using various levels of spin–density functional (local density approximation, local spin–density ...

Impact of the free electron distribution on the random telegraph signal capture kinetics in submicron n-metal–oxide–semiconductor field-effect transistors

Abstract: In this letter, the role of the free electron distribution on the capture kinetics of repulsive random telegraph signals in deep submicron n-channel metal–oxide–semiconductor field-effect transistors is studied. The inversion layer density and profile is varied by changing simultaneously the substrate and the gate bias of the transistor which is in linear operation at a constant drain current I. Detailed results are obtained for a class of repulsive trap centers when charged by an electron, which show a I−m variation of the capture time constant, with m>1. Such a nonstandard behavior can be understood in the framework of the Coulomb blockade model, whereby the image charge of the trapped carrier is stored on the gate electrode and in the inversion and depletion layer in the silicon substrate. As is shown here the capture time constant is a unique function of the ratio of the inversion layer surface density and the squared thickness of the inversion layer.

Ground-state characteristics of an acceptor center in wide-gap semiconductors with a weak spin-orbit coupling

Abstract: Spin-orbit splitting, strain-potential constants, and the g factor of the acceptor-center ground state described by a superposition of a Coulomb and a central-cell potential have been calculated for wide-band-gap semiconductors, such as GaN. Analytical expressions for these parameters, which depend only on the light-to heavy-hole mass ratio, have been obtained within the zero-range potential model. It is shown that the differences between these parameters for the limiting cases of purely Coulomb and zero-range potentials do not exceed 7%, thus permitting one to use for estimates simple analytical expressions. Calculation of the acceptor-center ground state made for the hexagonal modification of GaN suggests a strong anisotropy of the g factor, whereas measurements yield a practically isotropic value of g close to that of a free electron. This contradiction is removed if a spontaneous strain due to the Jahn-Teller effect appears perpendicular to the C6 axis of the crystal near the acceptor center.