Showing papers on "Mean free path published in 1981"

Band-structure-dependent transport and impact ionization in GaAs

Abstract: We have performed a Monte Carlo simulation of high-field transport in GaAs including a realistic band structure to study the band-structure dependence of electron transport and impact ionization. The band structure has been calculated using the empirical pseudopotential method. Unlike previous theories of impact ionization, our method is capable of calculating various parameters, such as mean free path, from first principles. The calculated electron mean free path, drift velocity, and impact ionization rate are in reasonable agreement with the experimental data in spite of several simplifications of the model. Within statistical uncertainty we do not observe any orientation dependence of the ionization rate in contradiction to the interpretation of recently reported experimental results. We also find that the contribution of ballistic electrons to impact ionization is negligibly small. Based on the results of the calculation, a general discussion of impact ionization is given.

Elecron Energy Transport in Steep Temperature Gradients in Laser-Produced Plasmas

Abstract: The Fokker-Planck equation has been solved numerically in one spatial and two velocity dimensions in order to study thermal conduction in large temperature gradients occurring in laser-produced plasmas. The heat flow is an order of magnitude smaller than that predicted by the classical theory when the temperature scale length is a few electron mean free paths.

Mean free path in a nucleus

Abstract: It is shown that proper treatment of the nonlocality of the nuclear optical potential resolves much of the apparent discrepancy between previous theoretical calculations and empirical values of the nucleon mean free path.

On Pinning of Superconducting Flux Lines by Grain Boundaries

Abstract: A simple model is developed which describes the reduction of the mean free path of conduction electrons in metals near a grain boundary. This leads to a decrease of the self-energy of flux lines in a layer which is considerably thicker than the perturbed zone of the boundary itself. The model yields pinning forces which agree, within an order of magnitude, with recent measurements on niobium bicrystals, and with observed values of grain boundary pinning in Nb/sub 3/Sn.

Effects of particle drift on the transport of cosmic rays. IV. more realistic diffusion coefficients

Abstract: Results from numerical simulations of cosmic-ray modulations by the solar wind are presented which show that the scattering mean free path should be larger than the particle gyroradius in the average magnetic field. It is found that the difference between drift and no-drift solutions is not as great as in previous simulations, which violated the mean free path constraint stated. Profound effects are still noted for the drifts, which determine the origin of the bulk of the cosmic rays seen at any given time in the inner solar system. Accordingly, during the 1975 solar minimum, the positively charged cosmic rays seen in the inner solar system came primarily from the outer boundary near the heliospheric poles while negative particles came from the equatorial regions of the boundary.

Effective collision frequency of electrons in noble gases

Abstract: The electron-neutral collision frequency in the noble gases has been calculated using recent numerical results for momentum transfer cross sections by assuming a Maxwellian distribution of electron velocities. In all these gases, except for argon, good agreement is obtained with most previously published experimental and theoretical data. Mean free path, mobilities and diffusion coefficients are also calculated from the resulting effective collision frequencies. The empirical formulae are presented for an electron temperature dependence of the electron-neutral collision frequency for all noble gases up to Te

On electrical transport in CoWO4 single crystals

Abstract: CoWO4 is a p-type semiconductor with conductivity in the range 10−8 to 10−3 ohm−1 cm−1. The solid exhibits extrinsic behaviour up to 750 K and intrinsic behaviour above 750 K. In this solid conduction by hopping of small polarons seems to be dominating up to 750 K and above 750 K conduction becomes band type. The energy band gap of the solid has been found to be 2.80 eV. The values of mobility and the mean free path are estimated. The variation of dielectric constant with temperature has been attributed to changes in atomic and ionic polarization and space charge polarization of thermally generated charge carriers.

New model of electron free path in multiple layers for Monte Carlo simulation

Abstract: A new model of an electron free path in multiple layers is proposed for Monte Carlo simulation of electron trajectories. In this model, the free path is calculated taking into account not only the scattering probability in the layer involving the initial scattering point but also that in the layers along the scattering direction. The result, simulated with the new model, agrees with the experimental result much better than results obtained with conventional models for backscattered electron intensity. It is also suggested that the simulation accuracy for the electron beam lithography is improved using the new model.

Transport Properties of Liquids I. Self-Diffusion, Viscosity and Density of Nearly Spherical and Disk Like Molecules in the Pure Liquid Phase

Abstract: Self-diffusion coefficient D, viscosity n and density p of nearly spherical molecules (Tetraalkyl-, Tetraalkoxy compounds) and of nearly disk like molecules (crown ethers) were measured (250 ≦ T/K < 370). - 1H-NMR spin-echo spectroscopy was applied for the measurements of D. n was determined via a Ubbelohde micro-viscosimeter and p via a vibrating densimeter. - D and n follow the Stokes-Einstein-Gierer-Wirtz relation. The free-volume theory of Cohen and Turnbull (CT) was applied to describe the experimental data quantitatively. By introducing geometrical relations for the mean free path λ and the critical free volume v*f the CT-model was extended and applied to the experimental results. Thereby a very satisfactory description of the measurements is attained. D of the substances investigated can be calculated as function of the melting point and p (T). Empirically the temperature dependence of D (T) follow an Arrhenius law. Application of the hybrid equation of Macedo and Litovitz to the experimental data shows that the temperature dependence of the transport properties in van-der-Waals liquids composed of spherical or disk like molecules is mainly determined by the creation of the free volume with increasing temperature.

The effect of sample thickness on the resistivity of aluminium

Abstract: The authors report measurements of the electrical resistivity of many high-purity aluminium samples in the thickness range 0.1 mu m to 7 mm over the temperature interval 2 to 300K. It is shown that surface scattering of the conduction electrons affects the temperature dependence of the resistivity for samples less than 0.5 mm in thickness up to 70K. Above about 30K the simple Bloch-Gruneisen theory agrees well with the data obtained using the thickest samples, while, for thinner samples, application of a surface scattering theory due to Soffer (1967) leads to satisfactory agreement between theory and experiment. Fitting of this theory allows the determination of the surface roughness parameter defined as the ratio of the root mean height of surface asperities, a, divided by the electron wavelength at the Fermi surface, lambda e. The authors determine 1

Theory of lattice thermal conductivity: Role of low-frequency phonons

Abstract: The lattice thermal conductivity arises from contributions by phonons of all frequencies. The mean free path l(ω) is limited mainly by three-phonon interactions, and l i(ω)∝ω−2 T −1 where ω is the phonon frequency, and T is the absolute temperature. Since the spectral specific heat varies as ω 2, the spectral thermal conductivity is independent of frequency, and low frequencies play a larger role than they do in the heat content. The effect of additional scattering processes due to defects must be compared to intrinsic scattering, not just at the highest frequency, but over the full spectral range. This enhances the resistance due to grain boundaries and large obstacles, and reduces the effect of point defects. Some typical examples are discussed. The role of low-frequency phonons may be even further enhanced if longitudinal low-frequency phonons have their interaction with other phonons reduced by wave vector conservation. Such modes would then contribute substantially to the overall thermal conductivity, and this contribution would be sensitive to grain size and to large-scale defects. However, the mean free path must be consistent with ultrasonic attenuation data. This enhanced sensitivity may be observable.

Temperature Dependence of Ionization Coefficients for InP and 1.3 µm InGaAsP Avalanche Photodiodes

Abstract: Investigations were made on the temperature dependence of excess noise characteristics for InP and 1.3 µm InGaAsP APDs for the case where electrons are injected into the high field region. The effective ratio of the ionization coefficients Keff(β/α) was found to decrease with decreasing temperature for both APDs. Theoretical investigations on hot carriers are combined with McIntyre's theory for the excess noise characteristics to explain the experimental results. The dominant scattering process for hot electrons is the intervalley scattering; this leads to the fact that the temperature dependence of the mean free path is greater in the case of electrons than holes. The theoretical analysis provides the best fit to the experimental results. On the basis of the theoretical results, the compositonal dependence of the mean free path for hot electrons is discussed.

Viscous heating of expanding fireballs

Abstract: The effect of a finite mean free path on the expansion stage of central collisions between heavy nuclei is studied in a nonrelativistic fluid dynamical approach. A gas of point particles with localized interactions is used. Kinetic theory then provides the thermal conductivity and viscosity coefficients in terms of the nucleon-nucleon cross section. About 10% additional entropy is generated during the expansion stage of central collisions at a beam energy of 800 MeV/A due to viscous heating of the system. Surprisingly there is only a weak dependence on the mass of the colliding nuclei. These results are in substantial agreement with recent intranuclear cascade calculations, although at present the fluid dynamical results are slightly ambiguous due to an uncertainty in the breakup criterion.

The electrical properties of Fe2WO6

Abstract: Fe2WO6 was found to be a p-type semiconductor with an energy gap of 1.68 eV. The drift mobility (μd) and mean free path (L) of the charge carriers have been calculated by combining the electrical conductivity and thermoelectric power data. It is proposed, from the magnitude ofL andμd and the temperature dependence ofμd, that the charge carriers in Fe2WO6 are small polarons and that the charge transport process occurs by a hopping mechanism below 800 K. Above 800 K the results are explained using a narrow d-band model. The dielectric constant as a function of temperature exhibits two stages of increase: (i) a slow rise at lower temperature which is attributed mainly to an increase in ionic polarization and (ii) a fast rise at higher temperature which is ascribed to space charge polarization.

Energy Losses and Mean Free Paths of Electrons in Silicon Dioxide

Abstract: Theoretical models and calculations are combined with experimental optical data to determine a model energy-loss function for SiO2. Sum-rule checks and comparisons with experimental information are made to insure overall consistency of the model. The model energy-loss function is employed to calculate electron inelastic mean free paths and stopping powers for electrons with energies ? 10 keV in SiO2.

A direct indication for specular reflection of conduction electrons at the vacuum boundary of metal films

Abstract: Highly disordered films of Al, Cu, Ag and Au are deposited at low temperatures on top of an annealed layer of the same metal. Although two electric conductors are connected in parallel, the total conductance of the sandwich first decreases, then passes through a minimum, and finally increases with increasing thickness of the disordered overlayer. Such curves of the total conductance versus thickness of the second film are interpreted within the Fuchs-Sondheimer model of the geometrical size effect. This model is used to calculate the fraction of specularly scattered conduction electrons at the uncovered surface, and the mean free path of the conduction electrons in the quench condensed overlayer.

X‐ray photoelectron analysis of surface layers with composition gradients

Abstract: The problems of quantitative surface analysis using X-ray photoelectron spectroscopy (ESCA) are considered with respect to surface layers having composition gradients. Various cases of given elemental distributions are discussed. The ESCA spectra are shown to allow the determination of an average element concentration in a layer of thickness Cλ sin α, λ and α being the photoelectron mean free path and escape angle respectively, and C being equal to 1.2, 1.8 or 2.0 depending on the change in the intensity with varying escape angle. The method based on the presence of composition gradient is suggested as a way of evaluating the photoelectron mean free path and the rate of ion sputtering.

Boundary Effects in Solution of Boltzmann's Equation for Electron Swarms

Abstract: The combined effect of interaction of electrons with walls, neutral molecules and an electrostatic field is considered through analytic solution of Boltzmann's equation. In the first instance, we discuss a half-range decomposition in velocity space, corresponding to electrons moving to and from the walls, which is valid for all types of electron-molecule interactions. The half-range equations are solved in the steady state for zero field and the constant mean free path model, and it is shown that the familiar full-range 'two-term' approximation equations are adequate in this case, as far as estimating bulk properties of the electrons is concerned. For the nonzero field, again in the steady state, the full-range equations are solved for the constant collision frequency model.