Showing papers on "Mean free path published in 2005"
TL;DR: In this article, the dependence of superlattice thermal conductivity on period length is investigated by molecular dynamics simulation, and the simulation results are consistent with phonon transmission coefficient calculations, which indicate increased stop bandwidth and thus strongly enhanced Bragg scattering.
Abstract: The dependence of superlattice thermal conductivity on period length is investigated by molecular dynamics simulation. For perfectly lattice matched superlattices, a minimum is observed when the period length is of the order of the effective phonon mean free path. As temperature decreases and interatomic potential strength increases, the position of the minimum shifts to larger period lengths. The depth of the minimum is strongly enhanced as mass and interatomic potential ratios of the constituent materials increase. The simulation results are consistent with phonon transmission coefficient calculations, which indicate increased stop bandwidth and thus strongly enhanced Bragg scattering for the same conditions under which strong reductions in thermal conductivity are found. When nonideal interfaces are created by introducing a 4% lattice mismatch, the minimum disappears and thermal conductivity increases monotonically with period length. This result may explain why minimum thermal conductivity has not been observed in a large number of experimental studies.
192 citations
TL;DR: In this paper, the authors used optical extinction spectra to determine the size of nearly spherical gold nanoparticles suspended in solution, produced by a reverse micelles process, and fitted Mie's theory if the optical constants from bulk material values are modified by introducing the limitation of the mean free path due to collisions of conduction electrons with the boundary of the nanoparticles.
Abstract: The measurement of optical extinction is used to determine the size of nearly spherical gold nanoparticles suspended in solution, produced by a 'reverse micelles' process. The contrast between the maximum and the minimum in the extinction spectra around 450 and 520 nm shows a linear dependence with the mean radius of the gold particles less than 3 nm; however, the method can be used to size particles up to 7 nm. Experimental results for extinction spectra can be fitted by Mie's theory if the optical constants from bulk material values are modified by introducing the limitation of the mean free path due to collisions of conduction electrons with the boundary of the nanoparticles.
146 citations
TL;DR: It is concluded that electron inelastic distributions for liquid water, although in some respects distinctively different from the vapor phase, have associated uncertainties that are comparable in magnitude to the phase differences.
Abstract: Emfietzoglou, D. and Nikjoo, H. The Effect of Model Approximations on Single-Collision Distributions of Low-Energy Electrons in Liquid Water. Radiat. Res. 163, 98–111 (2005). The development of cross sections for the inelastic interaction of low-energy electrons with condensed tissue-like media is best accomplished within the framework of the dielectric theory. In this work we investigate the degree to which various model approximations, used in the above methodology, influence electron single-collision distributions. These distributions are of major importance to Monte Carlo track structure codes, namely, the energy-loss spectrum, the inelastic inverse mean free path, and the ionization efficiency. In particular, we make quantitative assessment of the influence of (1) the optical data set, (2) the dispersion algorithm, and (3) the perturbation and exchange Born corrections. It is shown that, although the shape and position of the energy-loss spectrum remains almost fixed, its peak height may var...
144 citations
TL;DR: In this paper, the authors proposed to use l_0/(l_0+L) for the energy transmission covering both ballistic and diffusive regimes, where l is the mean free path and L is system length.
Abstract: We propose to use l_0/(l_0+L) for the energy transmission covering both ballistic and diffusive regimes, where l_0 is mean free path and L is system length. This formula is applied to heat conduction in carbon nanotubes (CNTs). Calculations of thermal conduction show: (1) Thermal conductance at room temperature is proportional to the diameter of CNTs for single-walled CNTs (SWCNTs) and to the square of diameter for multi-walled CNTs (MWCNTs). (2) Interfaces play an important role in thermal conduction in CNTs due to the symmetry of CNTs vibrational modes. (3) When the phonon mean free path is comparable with the length L of CNTs in ballistic-diffusive regime, thermal conductivity \kappa goes as L^{\alpha} . The effective exponent \alpha is numerically found to decrease with increasing temperature and is insensitive to the diameter of SWCNTs for Umklapp scattering process. For short SWCNTs (<0.1 \mu m) we find \alpha \approx 0.8 at room temperature. These results are consistent with recent experimental findings.
135 citations
TL;DR: In this paper, the second-order quasilinear theory is used to calculate corrections of the unperturbed orbit and these corrections can be resubstituted into transport theory to calculate a secondorder pitch-angle Fokker-Planck coefficient.
Abstract: The problem of pitch-angle diffusion close to 90° is well known in cosmic ray astrophysics. If the pitch-angle Fokker–Planck coefficient for pure slab geometry is calculated, the quasilinear approximation results in vanishing pitch-angle scattering. For a realistic wave spectrum with a steep dissipation range this vanishing coefficient generates an infinitely large parallel mean free path. It is well known from numerical simulations that the 90° problem is a problem of quasilinear theory and not a problem of reality. In the current paper quasilinear theory is used to calculate corrections of the unperturbed orbit. These corrections can be resubstituted into transport theory to calculate a second-order pitch-angle Fokker–Planck coefficient. The second-order quasilinear theory is an applicable theory which agrees with simulations for pitch-angle diffusion.
104 citations
TL;DR: In this article, the plasmon polariton resonances in the nanoparticulate regime are effectively modelled by a Lorentz oscillator, and the resonance energy of the oscillator is observed to reduce to zero shortly after the percolation threshold, whereby the oscillation is described by Drude free electron theory.
Abstract: In situ real-time spectroscopic ellipsometry is used to monitor the growth of magnetron sputtered silver nanoparticles on SiO2 substrates, through the percolation threshold and into the bulk film regime. The plasmon polariton resonances in the nanoparticulate regime are effectively modelled by a Lorentz oscillator. The resonance energy of the oscillator is observed to reduce to zero shortly after the percolation threshold, whereby the oscillation is described by Drude free electron theory. From the Drude theory, the electronic mean free path is observed to increase dramatically at the percolation threshold, to a value of 16 nm in the bulk regime, in good agreement with x-ray diffraction and transmission electron microscope measurements of the crystallite size in the films. Shortly before the percolation threshold the data is better modelled by two Lorentz oscillators, attributed to coupling between the plasmon polaritons. The onset of the coupling is determined to occur at a surface area coverage of 52%.
102 citations
TL;DR: With the wall equilibrium boundary condition and the appropriate relation defined for the Knudsen number and the relaxation time, the computed slip velocity and nonlinear pressure distribution along the microchannel are in excellent agreement with analytical solutions.
Abstract: A wall equilibrium boundary condition for an implicit lattice-Boltzmann-equation method is proposed to simulate gas flows in a microchannel with rough surface on the characteristic length of gas molecules. The boundary condition is based on the assumption that impinging molecules reach equilibrium with the surface. The molecular mean free path used to define the Knudsen number is determined by the lattice speed and the relaxation time of the lattice-Boltzmann equation. With the wall equilibrium boundary condition and the appropriate relation defined for the Knudsen number and the relaxation time, the computed slip velocity and nonlinear pressure distribution along the microchannel are in excellent agreement with analytical solutions.
98 citations
TL;DR: Both propagation and polarization directions of light are found to isotropize, following a power law of the number of scattering events, and the characteristic length scale governing light isotropization and linear depolarization is derived.
Abstract: We study the propagation of polarized light in turbid media as a random walk of vector photons. Both propagation and polarization directions of light are found to isotropize, following a power law of the number of scattering events. The characteristic length scale governing light isotropization and linear depolarization, the isotropization length , is derived using the exact Mie scattering for spherical particles. A simple relation is obtained for Rayleigh-Gans scatterers where is the transport mean free path and is the mean cosine of scattering angles.
75 citations
TL;DR: In this paper, the production/absorption rate of particles in compressed and heated asymmetric matter is studied using a Relativistic Mean Field (RMF) transport model with an isospin-dependent collision term.
68 citations
TL;DR: In this article, the mean free path and shear viscosity in the color-flavor locked (CFL) phase of dense quark matter at low temperature T, when the contributions of mesons, quarks and gluons to the transport coefficients are Boltzmann suppressed.
Abstract: We compute the mean free path and shear viscosity in the color-flavor locked (CFL) phase of dense quark matter at low temperature T, when the contributions of mesons, quarks and gluons to the transport coefficients are Boltzmann suppressed. CFL quark matter displays superfluid properties, and transport phenomena in such cold regime are dominated by phonon-phonon scattering. We study superfluid phonons within thermal field theory and compute the mean free path associated to their most relevant collision processes. Small-angle processes turn out to be more efficient in slowing transport phenomena in the CFL matter, while the mean free path relevant for the shear viscosity is less sensitive to collinear scattering due to the presence of zero modes in the Boltzmann equation. In analogy with superfluid He4, we find the same T power law for the superfluid phonon damping rate and mean free path. Our results are relevant for the study of rotational properties of compact stars, and correct wrong estimates existing in the literature.
59 citations
TL;DR: Theoretical derivations of the inelastic differential inverse mean free path (DIMFP) for electrons crossing solid surfaces were made for different crossing angles and electron distances relative to the crossing point at the surface.
TL;DR: In this paper, the resistivity of tungsten interconnects manufactured by a damascene CVD process was studied for a wide range of line widths (40-1000nm).
TL;DR: It is shown that above 100 K the mean free path of the phonons is considerably smaller than the lattice parameter and is no longer dependent on temperature.
Abstract: The thermal conductivity of tetrahydrofuran hydrate has been measured in the temperature region 2–220 K by the steady-state potentiometric method. The temperature dependence of the thermal conductivity exhibits behavior typical of amorphous substances. It is shown that above 100 K the mean free path of the phonons is considerably smaller than the lattice parameter and is no longer dependent on temperature.
TL;DR: In this article, the energy dependence of the mean free path in graphite at low kinetic energies s below,50 eVd is studied using the synchrotron radiation excited Si 2p core level photoemission signal from a SiC substrate attenuated by an epitaxial graphite overlayer.
Abstract: The energy dependence of the mean free path lsEd in graphite at low kinetic energies sbelow ,50 eVd is studied using the synchrotron radiation excited Si 2p core level photoemission signal from a SiC substrate attenuated by an epitaxial graphite overlayer. Diffraction structure in lsEd, appearing as strong intensity minima in the Si 2p signal, is found to reflect band gaps in the unoccupied states of graphite. Furthermore, lsEd is derived based on analysis of very-low-energy electron diffraction data supported by calculations of the complex band structure of unoccupied states, where lsEd appears from the Bloch wave damping factor. Conceptually different, the two methods yield equivalent lsEd. The strength of the diffraction structure in lsEd manifests a significant elastic contribution to electron scattering at low energies, sharply increasing in the band gaps of the unoccupied states.
TL;DR: In this paper, the normalized differential mean free path for volume scattering and the differential surface excitation probability for medium energy electrons travelling in Fe, Pd and Pt are extracted from reflection electron energy loss spectra (REELS).
TL;DR: In this article, a detailed microscopic transport theory for light in strongly scattering disordered systems whose constituent materials exhibit linear absorption or gain is presented, which is based on a fully vectorial treatment of the generalized kinetic equation and utilizes an exact Ward identity.
Abstract: We present a detailed, microscopic transport theory for light in strongly scattering disordered systems whose constituent materials exhibit linear absorption or gain. Starting from Maxwell's equations, we derive general expressions for transport quantities such as energy transport velocity, transport mean free path, diffusion coefficient, and absorption/gain length. The approach is based on a fully vectorial treatment of the generalized kinetic equation and utilizes an exact Ward identity (WI). While for loss- and gainless media the WI reflects local energy conservation, the effects of absorption or coherent gain are implemented exactly by additional terms in the WI. As a result of resonant (Mie) scattering from the individual scatterers, all transport quantities acquire strong, frequency-dependent renormalizations, which are, in addition, characteristically modified by absorption or gain. We illustrate the influence of various experimentally accessible parameters on these quanitities for dilute systems. The transport theory presented here may set the stage for a theory of random lasing in three-dimensional disordered media.
TL;DR: The results show that systematic lidar probings with the proposed multiple-scattering technique can provide valuable physical information on cloud formation and evolution.
Abstract: Recent developments in the search for a practical method of exploiting the multiple-scattering contributions to lidar returns are consolidated in a robust retrieval algorithm. The theoretical basis is the small-angle diffusion approximation. This implies that the algorithm is limited to media of sufficient optical thickness to generate measurable multiple scattering and to geometries for which the receiver’s footprint diameter is less than the scattering mean free path. The primary retrieval products are the range-resolved extinction coefficient and the effective particle diameter from which secondary products such as the particle volume mixing ratio and the extinction at other wavelengths can be calculated. We recall briefly earlier validation tests and present new data and analysis that demonstrate and quantify the solutions’ accuracy. The results show that systematic lidar probings with the proposed multiple-scattering technique can provide valuable physical information on cloud formation and evolution.
TL;DR: In this paper, the steady-state and ballistic transport properties of semiconducting zig-zag carbon nanotubes (CNTs) using semiclassical Monte Carlo simulation were investigated.
Abstract: We study the steady-state and ballistic transport properties of semiconducting zig-zag carbon nanotubes (CNTs) using semiclassical Monte Carlo simulation. Electron-phonon scattering is the only type of interaction included in the model. The band structure and phonon dispersion are derived from that of graphene by the zone folding method. Steady-state drift velocity and low-field mobility are calculated for CNTs with wrapping index ranging from n=10 to n=59, i.e., for a diameter range of 0.78−4.62nm. Principally, a transient analysis of transport under uniform driving field is realized and gives the fraction of ballistic electrons as a function of CNT length and the mean free path (MFP) for acoustic and optical phonons scattering. The probability to have ballistic electrons on a given distance appears to be higher for nanotubes of large diameter and depends on the field applied.
TL;DR: In this article, a brief description is given of three databases issued by the US National Institute of Standards and Technology (NIST), designed principally for applications in Auger-electron spectroscopy (AES) and X-ray photoelectron (XPS), providing electron elastic-scattering cross sections, electron inelastic mean free paths, and electron effective attenuation lengths.
Abstract: A brief description is given of three databases issued by the US National Institute of Standards and Technology (NIST). These databases, designed principally for applications in Auger-electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), provide electron elastic-scattering cross sections, electron inelastic mean free paths, and electron effective attenuation lengths. Examples are given of their use.
TL;DR: In this paper, the dependences of magnetic field penetration depth at zero temperature λ(0), microwave surface resistance Rs and π-band energy gap on the normal-state resistivity right above the critical temperature, ρ0, were studied for MgB2 thin films prepared by different techniques by employing a sapphire resonator technique.
Abstract: The dependences of magnetic field penetration depth at zero temperature λ(0), microwave surface resistance Rs and π-band energy gap at zero temperature Δπ(0) on the normal-state resistivity right above the critical temperature, ρ0, were studied for MgB2 thin films prepared by different techniques by employing a sapphire resonator technique. We found that the zero-temperature penetration depth λ(0) data could be well fitted by yielding a London penetration depth λL of 34.5 nm, where ξ0 is the coherence length, and is the mean free path determined from ρ0. The surface resistance Rs at 15 and 20 K increases roughly linearly with ρ0. The observed increase of Δπ(0) with ρ0 and the decrease of Tc indicate the expected effects of interband impurity scattering within an extended BCS approach. The low values of Rs and λ(0) in conjunction with the large coherence length for epitaxial films are potentially attractive for applications in electronics and microwave technology.
TL;DR: In this article, the authors model a solar energetic particle event observed by Wind STEP at 0.31-0.62 MeV nucleon-1, by solving the focused transport equation using the Markov stochastic process theory.
Abstract: The mean free path is widely used to measure the level of solar energetic particles' diffusive transport. We model a solar energetic particle event observed by Wind STEP at 0.31-0.62 MeV nucleon-1, by solving the focused transport equation using the Markov stochastic process theory. With different functions of the pitch angle diffusion coefficient Dμμ, we obtain different parallel mean free paths for the same event. We show that the different values of the mean free path are due to the high anisotropy of the solar energetic particles. This makes it problematic to use just the mean free path to describe the strength of the solar energetic particle scattering, because the mean free path is only defined for a nearly isotropic distribution. Instead, a more complete function of pitch angle diffusion coefficient is needed.
TL;DR: In this paper, the authors derived the temperature and doping dependence of the intrinsic phonon escape length l(e), which can be a direct measure of charge inhomogeneity, from data on the effective quasiparticle relaxation time tau(r) in La(2-x)SrxCuO4 and Nd(2x)Ce(x)CuO 4.
Abstract: Time-resolved optical experiments can give unique information on the characteristic length scales of dynamic charge inhomogeneity on femtosecond time scales. From data on the effective quasiparticle relaxation time tau(r) in La(2-x)SrxCuO4 and Nd(2-x)Ce(x)CuO4, we derive the temperature and doping dependence of the intrinsic phonon escape length l(e), which can be a direct measure of charge inhomogeneity. Remarkably, a common feature of both p- and n-type cuprates is that, as T --> Tc, l(e) approaches the superconducting coherence length l(e) --> xi(s)0. In the normal state l(e) is found to be in excellent agreement with the mean free path l(m) obtained from the resistivity data and structural coherence lengths l(s) from neutron scattering experiments, implying the existence of complex intrinsic textures on different length scales which may have a profound effect on the functional properties of these materials.
TL;DR: In this article, the electronic transport properties of the flash-evaporated n-type Bi 2 Te 2.4 Se 0.6 thin films have been investigated by the effective mean free path model.
TL;DR: In this article, the contribution of electron-phonon scattering and grain boundary scattering to the mid-IR properties of Au has been assessed by examining both bulk, single crystal samples and thin film, polycrystalline Au samples at 300 and 100 K by means of surface plasmon polariton excitation.
TL;DR: In this paper, single crystals of two-dimensional 2H-NbSe2, which undergoes a superconducting transition at Tc = 7.2 K and a charge-density wave (CDW) transition at tc = 30 K, were synthesized.
Abstract: Single crystals of two-dimensional 2H-NbSe2, which undergoes a superconducting transition at Tc = 7.2 K and a charge-density wave (CDW) transition at TCDW = 30 K, were synthesized. Measurements of the resistivity, Hall coefficient and magnetoresistance (MR) versus temperature were performed on NbSe2 crystals with different resistance residual ratios (RRR = R(300 K)/R(8 K)), chosen from different batches. The superconducting transition temperature hardly changes with the RRR, while the MR and Hall coefficient are strongly dependent on the RRR values of the samples. Moreover, the temperature and field dependence of the MR violate Kohler's rule, indicating that the scattering times of the charge carriers are no longer isotropic. A steep decrease of the Hall coefficient RH below 50 K is only observed for the high quality sample; this can be interpreted in terms of a drastic increase of the mean free path for the electron-type charge carriers. The effect of the CDW transition on the Hall coefficient is discussed using a two-band model and a sharp change in the scattering rate on an electron-like orbit below TCDW is suggested.
TL;DR: In this article, a numerical solution for weakly collisional cylindrical plasmas is presented, where the ion current density is reduced by approximately a factor of 0.45 when the mean free path is 0.1 of the plasma radius.
Abstract: Numerical solutions to the plasma-sheath problem are presented for weakly collisional cylindrical plasmas. For a constant mean free path model, the reduction of the ion flux to the wall due to charge-exchange collisions is calculated. The ion current density is reduced by approximately a factor of 0.45 when the mean free path is 0.1 of the plasma radius.
TL;DR: In this paper, the charging of small neutral and charged particles suspended in a weakly ionized plasma is investigated under the assumption that the Coulomb + image forces affect the ion transport in the carrier plasma and define the rate of charging processes.
TL;DR: In this article, the Goudsmit-Saunderson solution is applied to electron microscopy of samples composed of one or more materials, which yields reasonable parameter-free agreement with experimental data taken from the literature for the multiple scattering of 300-keV electrons through aluminum foils up to 25μm thick.
Abstract: Radiation transport theory is applied to electron microscopy of samples composed of one or more materials. The theory, originally due to Goudsmit and Saunderson, assumes only elastic scattering and an amorphous medium dominated by atomic interactions. For samples composed of a single material, the theory yields reasonable parameter-free agreement with experimental data taken from the literature for the multiple scattering of 300-keV electrons through aluminum foils up to 25μm thick. For thin films, the theory gives a validity condition for Beer’s law. For thick films, a variant of Moliere’s theory [V. G. Moliere, Z. Naturforschg. 3a, 78 (1948)] of multiple scattering leads to a form for the bright-field signal for foils in the multiple-scattering regime. The signal varies as [tln(e1−2γt∕τ)]−1 where t is the path length of the beam, τ is the mean free path for elastic scattering, and γ is Euler’s constant. The Goudsmit–Saunderson solution interpolates numerically between these two limits. For samples with ...
TL;DR: In this article, an experimental determination of the conduction channel distribution in lead nanoscale contacts with total conductances ranging from 1 to 15 G0, where G0 = 2e2/h.
Abstract: We present an experimental determination of the conduction channel distribution in lead nanoscale contacts with total conductances ranging from 1 to 15 G0, where G0 = 2e2/h. It is found that even for contacts having a cross section much smaller than the mean free path the distribution tends to be remarkably close to the universal diffusive limit. With the help of theoretical calculations we show that this behavior can be associated with the specific band structure of lead which produces a significant contribution of partially open channels even in the absence of atomic disorder.