Showing papers on "Mean free path published in 1997"
TL;DR: In this paper, a microscopic definition of the exciton mean free path (Lf) and localization size (Lρ) is given in terms of the off-diagonal elements of the green function and density matrix, respectively.
Abstract: The pump−probe signal from the light-harvesting antenna LH2 of purple bacteria is analyzed using a Green function expression derived by solving the nonlinear exciton-oscillator equations of motion (NEE). A microscopic definition of the exciton mean free path (Lf) and localization size (Lρ) is given in terms of the off-diagonal elements of the exciton Green function and density matrix, respectively. Using phonon-induced (homogeneous) and disorder-induced (inhomogeneous) line widths compatible with superradiane measurements, we find that at 4.2 K the localization size is Lρ = 15 and that the shift ΔΩ between the positive and negative peaks in the differential absorption is determined by a different effective size Lf/2 = 5.6 associated with the exciton mean free path. Our model further predicts the recently observed superradiance coherence size determined by Lρ.
176 citations
TL;DR: In this article, the dependence of absorption by small metallic particles on particle shape and wave polarization in the IR frequency range was studied theoretically, and it was shown that for asymmetric particles smaller than the mean free path, the light-induced conductivity is a tensor.
Abstract: We study theoretically the dependence of absorption by small metallic particles on particle shape and wave polarization in the IR frequency range. We examine the electric and magnetic absorption by small particles. The particles may be either larger or smaller than the electron mean free path. We show that for asymmetric particles smaller than the mean free path the light-induced conductivity is a tensor. We also show that the total absorption and the electric-to-magnetic absorption ratio are strongly dependent on particle shape and wave polarization. Finally, we construct curves representing the dependence of the ratio of the electric and magnetic contributions to absorption on the degree of particle asymmetry for different wave polarizations. Similar curves are constructed for the ratio of the components of the light-induced conductivity tensor.
109 citations
TL;DR: In this paper, the authors developed one spatial dimension and two velocity variables, fully kinetic, collisional, and time-dependent particle-in-cell code for scrape-off layer plasmas.
Abstract: The short mean-free path expansion used in fluid modeling of scrape-off layer plasmas is often violated for typical discharge parameters, especially by the superthermal particles, which carry most of the heat flux. Thus, the tail of the distribution function can strongly depart from Maxwellian due to nonlocal mean-free path effects, which can modify plasma transport, impurity radiation, and plasma–neutral gas interactions. These nonlocal effects become particularly pronounced for detached plasma conditions that are characterized by sharp gradients in the plasma parameters along the magnetic field. These problems are being addressed by developing one spatial dimension and two velocity variables, fully kinetic, collisional, and time-dependent particle-in-cell code, W1 [Contrib. Plasma Phys. 34, 436 (1994)], and its parallel-computer version, PW1 [Contrib. Plasma Phys. 34, 424 (1996)]. Comparisons are made with the Fokker–Planck code ALLA [Phys. Plasmas 3, 1634 (1996)] and with experimental results. Kinetic ...
88 citations
TL;DR: In this paper, the thermal variations of ESR, dc, and microwave resistivity of unoriented bulk single wall carbon nanotube samples were compared, and it was shown that the system remains metallic even at low temperature where dr/dT, 0. 0! is an intrinsic property of the bulk material.
Abstract: We compare the thermal variations of ESR, dc, and microwave resistivity of unoriented bulk single wall carbon nanotube samples. We conclude that the ‘‘metallic’’ high- T behavior (dr/dT. 0! is an intrinsic property of the bulk material, and that the system remains metallic even at low temperature where dr/dT, 0. The spin susceptibility is also independent of T, and a long mean free path implies transport predominantly along the tube axes in bulk material. @S0163-1829~97!08440-3# Nonchiral single-wall carbon nanotubes ~SWNT! with an armchair wrapping 1 @N,N# are attracting great interest, as they are predicted by band theory to be intrinsically metallic. 2,3 Laser ablation of graphite targets doped with Co and Ni produces samples in which @10,10# SWNT are predominant. 4 In this process, the SWNT self-organize into crystalline ropes consisting of tubes close-packed on a twodimensional ~2D! triangular lattice. 1 The conducting behavior of both unoriented bulk material and oriented single ropes has been experimentally confirmed. 1,5,6 An explanation for the linear resistivity r versus T behavior observed at high temperature has been proposed, in which the dominant process is electron backscattering by low-energy twists of the tube. 6 Measurements of the temperature dependence of r agree qualitatively with this prediction down to a sampledependent crossover temperature T* below which dr/dT becomes negative. Here we report experiments performed on unoriented bulk material using electron spin resonance, microwave and dc resistivity measurements. In the high-temperature regime, all our results are consistent with the reported metallic behavior, and establish that the observed resistivity is intrinsic to the bulk material. By estimating transport parameters from the ESR data and comparing with r(T), we also show that the system remains metallic below T* despite the sign change in dr/dT, ruling out a metal-insulator transition. 7 ESR spectra
85 citations
TL;DR: The distribution functions of interstellar pick-up hydrogen and helium exhibit fluctuations which are correlated as mentioned in this paper, and these fluctuations reveal that they may be the result of variations in, and provide additional evidence for, large mean free paths for these low rigidity particles.
Abstract: The distribution functions of interstellar pick-up hydrogen and helium exhibit fluctuations which are correlated An analysis of these fluctuations reveals that they may be the result of variations in, and provide additional evidence for, large mean free paths for these low rigidity particles An analysis of the properties of the distribution function of pick-up hydrogen reveals that these particles do suffer considerable adiabatic cooling, and that the likely cause for the long mean free path is the inability of the particles to scatter through 90° pitch angle
61 citations
TL;DR: In this article, a nanoscale-tip field emission system was proposed with multiple emitter tips of radii about 1 nm within an extractor aperture of diameter 50 nm.
Abstract: Novel field emission devices with electron path lengths an order of magnitude less than the elastic mean free path of electrons in air have been fabricated and tested at atmospheric pressure. The nanoscale-tip field emission system consisted of multiple emitter tips of radii about 1 nm within an extractor aperture of diameter 50 nm. The extractor turn-on voltage was approximately 7.5 V; field-emitted currents of up to 10 nA were collected at extractor voltages of less than 10 V. Maximum current densities of over 1011 A m−2 have been observed, and the emission stability in air at atmospheric pressure is better than 3%.
54 citations
TL;DR: In this article, the authors developed new methods to quantify the data acquired by electron spectroscopic imaging (ESI) which are based on recording series of energy filtered images across inner shell loss edges or in the low loss region.
53 citations
TL;DR: It is newly found that, for all solid loads, depolarization sets in for path lengths longer than approximately five transport mean free paths.
Abstract: We report on depolarization measurements of femtosecond pulses retroreflected from dense suspensions of silica microspheres with solid loads increasing from 5% to 54%. Backscattered pulse shapes compare well with predictions of the diffusion theory for all volume fractions, and the inferred values of the transport mean free path agree with independent measurements of enhanced backscattering. The measured degree of polarization decays exponentially with temporal rates that scale with the solid load. It is newly found that, for all solid loads, depolarization sets in for path lengths longer than approximately five transport mean free paths.
51 citations
TL;DR: In this paper, the authors investigated multiple scattering of polarized electromagnetic waves in diffusive media by means of radiative transfer theory, which is restricted to Rayleigh scattering, and obtained the polarization-dependent, angle-resolved mean diffuse intensity in reflection and in transmission, and the shape of the cone of the enhanced backscattering, in terms of solutions to Schwarzschild-Milne equations.
Abstract: Multiple scattering of polarized electromagnetic waves in diffusive media is investigated by means of radiative transfer theory. This approach amounts to summing the ladder diagrams for the diffuse reflected or transmitted intensity, or the cyclical ones for the cone of enhanced backscattering. The method becomes exact in several situations of interest, such as a thick-slab experiment (slab thickness L » mean free path l >> wavelength λ). The present study is restricted to Rayleigh scattering. It incorporates in a natural way the dependence on the incident and detected polarizations, and takes full account of the internal reflections at the boundaries of the sample, due to the possible mismatch between the mean optical index n of the medium and that n1 of the surroundings. This work does not rely on the diffusion approximation. It therefore correctly describes radiation in the skin layers, where a crossover takes place between free and diffusive propagation, and vice-versa. Quantities of interest, such as the polarization-dependent, angle-resolved mean diffuse intensity in reflection and in transmission, and the shape of the cone of enhanced backscattering, are predicted in terms of solutions to Schwarzschild-Milne equations. The latter are obtained analytically, both in the absence of internal reflections (n = n1), and in the regime of a large index mismatch (n/n1 > 1).
50 citations
TL;DR: In this article, a tube-like structure with a cross section of 25 nm × 25 nm has been created in a semiconductor and the mean free path of the electrons exceeds 10 μm -more than 400 times the confinement dimension.
49 citations
TL;DR: In this article, it was shown that a photoelectron in n-BSO constitutes the clearest case of a strongly coupled "large" polaron, i.e., a polaron whose motion can be explained by a "polaron" theory.
Abstract: collision model were used to describe this drift, the thermal mean free path (,0.2 nm) would be less than the de Broglie wavelength of the electron, and the collision rate times the Planck constant (the uncertainty in the electron energy) would be an order of magnitude greater than the thermal energy. In this “strong coupling” case the Boltzmann equation cannot be expected to apply. We report measurements of mobility vs temperature in n-BSO which can be explained well by a “polaron” theory that avoids the limitations of the Boltzmann equation [4‐7]. An electron in a polar crystal polarizes the lattice in its neighborhood. The electron moving with its accompanying lattice distortion is called a polaron. We argue that a photoelectron in n-BSO constitutes the clearest case of a strongly coupled “large” polaron, i.e., a polaron whose
TL;DR: In this article, the authors formulated the multiple scattering process in a scattering medium for an impulsive radiation from a point-shear dislocation source on the basis of the radiative transfer theory.
TL;DR: A new probabilistic approach is proposed for the present case of dense, random, weak scatterers involving the otherwise rare and ignorable sub-mean-free-path scatterings, now made effective by the high gain in the medium, which is consistent with experimentally observed features.
Abstract: We report enhanced emission and gain narrowing in Rhodamine 590 perchlorate dye in an aqueous suspension of polystyrene microspheres. A systematic experimental study of the threshold condition for and the gain narrowing of the stimulated emission over a wide range of dye concentrations and scatterer number densities showed several interesting features, even though the transport mean free path far exceeded the system size. The conventional diffusive-reactive approximation to radiative transfer in an inhomogeneously illuminated random amplifying medium, which is valid for a transport mean-free path much smaller than the system size, is clearly inapplicable here. We propose a new probabilistic approach for the present case of dense, random, weak scatterers involving the otherwise rare and ignorable sub-mean-free-path scatterings, now made effective by the high gain in the medium, which is consistent with experimentally observed features.
TL;DR: In this article, the frequency and temperature dependences of acoustic-phonon damping in vitreous silica is reconsidered and the essential role played by Rayleigh scattering of the phonons, in quantitative agreement with thermal properties at low temperatures.
Abstract: Direct spectroscopic information on the frequency and temperature dependences of acoustic-phonon damping in vitreous silica is reconsidered. It points to the essential role played by Rayleigh scattering of the phonons, in quantitative agreement with thermal properties at low temperatures. A picture is proposed that consistently unifies thermal properties with all the available spectroscopic results.
TL;DR: In this article, the conductance of single-shell carbon nanotubes is numerically calculated in the effective mass approximation by treating scattering from impurities exactly, and it is shown that the magnetic field dependence of conductance depends sensitively on the mean free path.
Abstract: The conductance of single-shell carbon nanotubes is numerically calculated in the effective-mass approximation by treating scattering from impurities exactly. The magnetic-field dependence of the conductance depends sensitively on the mean free path. When the mean free path is sufficiently large, undoped metallic nanotubes exhibit a large positive magnetoresistance in agreement with the prediction of the Boltzmann transport equation. This positive magnetoresistance disappears when the mean free path becomes comparable to the circumference length.
TL;DR: In this article, the electron inelastic mean free path (IMFP) was estimated to be 1.00nm, which was one half of the usually accepted length, for 1630eV kinetic energy electrons in Ag from the reduction factor of the backgrounds of the total reflection X-ray photoelectron spectra.
Abstract: X-Ray photoelectron spectra were measured when the excited X-rays were impinged on a Ag surface with a grazing angle. The spectral backgrounds owing to the electron inelastic scattering in solids were lower when the incident X-rays were totally reflected than when the X-rays were not totally reflected. We developed a new method to estimate the electron inelastic mean free path (IMFP). The IMFP was estimated to be 1.00nm, which was one half of the usually accepted length, for 1630eV kinetic energy electrons in Ag from the reduction factor of the backgrounds of the total reflection X- ray photoelectron spectra.
TL;DR: In this article, the authors measured the probability distributions of a microwave field transmitted through a random medium in the crossover from ballistic to diffusive propagation and found that the joint distribution of in-phase and out-of-phase components of the field is nearly Gaussian, even in samples with thickness less than the scattering mean free path.
Abstract: We have measured the probability distributions of a microwave field transmitted through a random medium in the crossover from ballistic to diffusive propagation. The joint distribution of in-phase and out-of-phase components of the field is nearly Gaussian, even in samples with thickness less than the scattering mean free path. In these samples, the phase distribution of the residual field, which is the difference between the field and its ensemble average, is nearly flat. The extent of the deviation of the residual field from complete randomization depends upon the frequency, sample length, and modes of excitation.
TL;DR: In this paper, the activation energy for conduction (in annealed films) is found to be thickness dependent and this can be explained with the help of the grain-boundary trapping model.
Abstract: thin films of different thicknesses were deposited by the flash evaporation method onto cleaned glass plates held at room temperature. Structural characterization was carried out using x-ray diffraction and transmission electron microscopy which revealed that the films are polycrystalline and the grain size increases with increasing thickness. Electrical resistivity was measured in the temperature range 300 - 450 K during two cycles of heating and cooling. During the first heating, irreversible behaviour of conductivity has been observed. Semiconductor-like behaviour has been observed in the annealed films and also during the first cooling and subsequent heating - cooling cycles. The activation energy for conduction (in annealed films) is found to be thickness dependent and this can be explained with the help of the grain-boundary trapping model. The thickness dependence of electrical resistivity (in annealed films) has been analysed using the effective mean free path model. From the analysis, important material constants like the mean free path and the electron concentration have been evaluated.
TL;DR: In this article, a review of the development of the anomalous skin effect in normal metals is discussed, where the dependence of the impedance on the mean free path of conduction electrons becomes anomalous for certain directions of electromagnetic wave propagation if there are singular points (points of parabolic type, points with zero curvature) on the Fermi surface.
TL;DR: In this paper, the theory of transport properties and vibration modes of the two-dimensional Wigner solid coupled to surface dimples on normal and superfluid 3 He is presented.
Abstract: We present the theory of transport properties and vibration modes of the two-dimensional Wigner solid (WS) coupled to surface dimples on normal and superfluid 3 He. Compared to the case of liquid 4 He, Fermi liquid properties of the substrate crucially affect the low-frequency dynamics of the electron solid. At ultralow temperatures, WS transport is shown to be determined mostly by the reflection of ballistic quasiparticles of the bulk liquid at the dimple sublattice. The long mean free path regime of normal 3 He is characterized by the temperature-independent conductivity. Below the superfluid transition, the appearance of the quasiparticle energy gap sharply reduces the momentum adsorbed by moving dimples, which leads to the rapid increase of the WS conductivity. The analysis presented shows that the WS can serve as a new powerful experimental probe for superfluid phases of 3 He.
TL;DR: In this article, the authors examined the features of electron-atom elastic scattering over the range of energy and atomic number important in XPS and AES analysis, and showed that the third and fourth partial waves, together with Levinson's theorem, give rise to the structure in the transport mean free path.
Abstract: Quantitative Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) depend on an accurate knowledge of the correct depth scale of emission of the signal electrons. This depends on both inelastic and elastic scattering processes occurring in the specimen under analysis. A previous paper showed that the depth scale in AES and XPS is significantly influenced by unexpected structure in the transport mean free path for electrons in high-atomic-number elements over the approximate energy range 100-400 eV. This behaviour is implicit in Mott cross-sections, which are known to be reliable and have been used in low-energy electron diffraction (for example) for decades. This paper examines the features of electron-atom elastic scattering over the range of energy and atomic number important in XPS and AES analysis. In particular, the third and fourth partial waves, together with Levinson's theorem are shown to give rise to the structure in the transport mean free path. This is a transport cross-section analogue of the Ramsauer-Townsend effect exhibited at low energies in total elastic scattering cross-sections.
TL;DR: In this article, the authors explain in detail how the notion of mean free path is related to the mathematical results obtained in their previous paper. But they do not explain how the results of the present paper are related to those obtained in this paper.
Abstract: In this note, we explain in detail how the notion of mean free path is related to the mathematical results obtained in our previous paper.
TL;DR: In this paper, the convective loss is shown to be independent of particle size, dependent on the ambient pressure, and independent of the gas phase thermal conductivity, which is a consequence of three results from continuum theory.
Abstract: The standard theories of particle combustion rely upon continuum gasdynamic relationships. These theories predict that small reacting particles are always essentially in thermal equilibrium with the ambient gas phase. This is a consequence of three results from continuum theory: 1) the convective loss is inversely proportional to particle size, 2) the convective loss is independent of pressure, and 3) the convective loss depends upon the thermal conductivity of the ambient gas. Typical soot particles are smaller than the mean free path of the ambient gas. Energy and mass conservation equations are derived assuming free molecular conditions. Under these assumptions, the convective loss is shown to be independent of particle size, dependent on the ambient pressure, and independent of the gas phase thermal conductivity. These equations are solved for a variety of O 2 /O/N 2 gas mixtures, pressures, and temperatures. High O 2 mole fractions at low pressure result in significant gas/particle thermal nonequilibrium. The particle surface temperature is shown to elevate significantly above the gas temperature. This result contradicts the behavior of small reacting particles predicted by continuum theory.
TL;DR: In this article, the effects of a short mean free path on the properties of a well defined superfluid Fermi liquid with anisotropic pairing were studied. But the authors focused on aerogel.
Abstract: Superfluid $^3$He confined in aerogel offers a unique chance to study the effects of a short mean free path on the properties of a well defined superfluid Fermi liquid with anisotropic pairing. Transport coefficients and collective excitations, e.g. longitudinal sound, are expected to react sensitively to a short mean free path and to offer the possibility for testing recently developed models for quasiparticle scattering at aerogel strands. Sound experiments, together with a theoretical analysis based on Fermi liquid theory for systems with short mean free paths, should give valuable insights into the interaction between superfluid $^3$He and aerogel.
TL;DR: In this article, a Monte Carlo simulation code has been developed to account for the mass transfer in the sputtering deposition, where the velocity distribution of gas atoms at ambient temperature is taken into consideration, while the gas atoms have been regarded cold enough compared to the sputtered particle.
TL;DR: In this paper, the authors proposed a method to obtain the percentage of electrons elastically reflected by a substrate in elastic peak electron spectroscopy (EPES) using a reference curve.
Dissertation•
17 Oct 1997
TL;DR: In this article, the authors measured at low temperature the energy distribution function in several diffusive mesoscopic wires (a few micrometers long), placed out-of-equilibrium.
Abstract: The first part of the thesis is concerned with quasiparticle-quasiparticle interaction in diffusive films, that is films with an elastic mean free path much smaller than the film dimensions. In such films, the interaction is expected to be greater than in perfect metals, which are described by the Landau theory of Fermi liquids. We have measured at low temperature the energy distribution function in several diffusive mesoscopic wires (a few micrometers long), placed out-of-equilibrium. Depending on the ratio between the diffusion time across the wire and the interaction time, the distribution function is typical of a non-interacting regime, a strongly thermalized regime, or an intermediate regime. From the intermediate regime one can infer the energy dependence of the modulus of the interaction. The second part of the thesis is concerned with the proximity effect, that is the penetration in a non-superconducting (normal N) metal of superconducting (S) order, characterized by pair correlations between quasiparticles. The penetration of pair correlations in the normal metal modifies its density of states (DOS). We have measured this DOS at several distances from the interface with a superconductor: 100 nm away, the DOS at the Fermi energy is reduced by a factor two. The modification of the DOS persists as far as one micrometer into the normal metal. The subgap or Andreev current through a NS interface is another property which depends on the pair correlations induced in the normal metal. This current, which is due to the coherent passage of two electrons from the N metal into the S, is enhanced by confinement of the quasiparticle trajectories near the NS interface. In order to demonstrate the interferential origin of this enhancement, we have implemented an « NS-QUID », made of a superconducting fork, closed by a normal mesoscopic wire. The interference pattern corresponds to the current through the device, which is modulated by a perpendicular magnetic field.
TL;DR: In this paper, the inelastic mean free path and the transport mean free paths of the electron were investigated using the differential elastic scattering cross-section data of Czyzewski et al. The results identify regions of the periodic table where elastic scattering effects are particularly pronounced.
TL;DR: A soft X-ray emission spectroscopy (SXES) study under an energetic electron irradiation has been applied to a nondestructive buried interface analysis of a thin-film/Si (substrate) contact system, where the energy of primary electrons, E p, is less than 20 keV.
TL;DR: In this paper, an extended Drude dielectric function, which allows the characteristic oscillator strength, damping constant, and critical point energy for each subband of valence electrons, is employed to evaluate the position-dependent inelastic mean free paths of photoelectrons emitted from a solid surface.
Abstract: Monte Carlo simulations have been performed for studies of the influence of surface excitations on the angular distributions of photoelectron peak intensities. An extended Drude dielectric function, which allows the characteristic oscillator strength, damping constant, and critical-point energy for each subband of valence electrons, is employed to evaluate the position-dependent inelastic mean free paths of photoelectrons emitted from a solid surface. Elastic-scattering cross sections are calculated using phase-shift analysis and the finite difference method for a solid atom with the Hartree-Fock-Wigner-Seitz potential in the Dirac equation. Results of Monte Carlo simulations on the angular distribution of the four electron lines from gold, 4f{sub 7/2}, 4d{sub 5/2}, 4p{sub 3/2}, and 4s are presented. Our calculations reveal that surface effects leads to a reduction of the intensities at small detection angles and a sharp decrease at large angles since the surface excitation is most probable for glancing electrons. However, the difference between the results obtained with and without surface effects may nearly disappear for the ratio of two different photoelectron intensities at small escape angles. Nevertheless, the calculated results taking into account surface effects are in better agreement with the experimental data. {copyright} {ital 1997} {ital The American Physical Society}