Showing papers on "Mean free path published in 2003"
TL;DR: In this article, a geometrical probability measure is proposed for calculating the effective conduction electron mean free path of an arbitrary shape convex particle, which is shown to be consistent with exact quantum mechanical widths for simple particle shapes.
Abstract: A geometrical probability measure is proposed for calculating the effective conduction electron mean free path of an arbitrary shape convex particle. It is shown that the plasmon widths determined from this mean free path are consistent with exact quantum mechanical widths for simple particle shapes. We use the mean free path formula to evaluate size and shape dependent dielectric functions and extinction spectra for silver spheroids, square prisms, truncated tetrahedrons, and cylinders.
410 citations
TL;DR: In this article, the behavior of spherical solid particles in a horizontal channel flow is analyzed using numerical calculations based on the Lagrangian approach and integral properties are presented in order to reveal the effects of wall roughness and inter-particle collisions.
219 citations
TL;DR: In this paper, the density and distribution function of trapped and untrapped ions, the potential profile, the ion and electron current to the collector, and the floating potential and charge of the collector were analyzed.
Abstract: The problem of electrostatic shielding around a small spherical collector immersed in nonflowing plasma, and the related problem of electron and ion flow to the collector, date to the origins of plasma physics. Calculations have typically neglected collisions, on the grounds that the mean free path is long compared to the Debye length. However, it has long been suspected that negative-energy trapped ions, created by occasional collisions, could be important. This paper presents self-consistent analytic calculations of the density and distribution function of trapped and untrapped ions, the potential profile, the ion and electron current to the collector, and the floating potential and charge of the collector. Under typical conditions for dust grains immersed in a discharge plasma, trapped ions are found to dominate the shielding near the grain, substantially increase the ion current to the grain, and suppress the floating potential and grain charge, even when the mean free path is much greater than the Debye length.
183 citations
01 Jul 2003-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this article, values of the gamma-ray mass-attenuation coefficient, the photon mean free path (MFP), the effective atomic number and the effective electron density have been determined experimentally for x ZnO·2 x PbO·(1−3 x )B 2 O 3 (x = 0.1−0.26) glasses at photon energies 511, 662, 1173 and 1332 keV and compared with theoretical data.
Abstract: Values of the gamma-ray mass-attenuation coefficient, the photon mean free path (MFP), the effective atomic number and the effective electron density have been determined experimentally for x ZnO · 2 x PbO · (1−3 x )B 2 O 3 ( x =0.1–0.26) glasses at photon energies 511, 662, 1173 and 1332 keV and compared with theoretical data. The specific volume of the glasses has been derived from density measurements and studied as a function of composition. It is pointed out that these glasses have potential applications in radiation shielding.
130 citations
TL;DR: In this paper, the relative role of the residual strain and dimensional scaling on heat transport in nanostructures is investigated by molecular dynamics simulations of a model Lennard-Jones solid, and it is observed that tensile (compressive) strains lead to a reduction of the lattice thermal conductivity.
Abstract: The relative role of the residual strain and dimensional scaling on heat transport in nanostructures is investigated by molecular dynamics simulations of a model Lennard-Jones solid. It is observed that tensile (compressive) strains lead to a reduction (enhancement) of the lattice thermal conductivity. A nonhydrostatic strain induces thermal conductivity anisotropy in the material. This effect is due to the variation with strain of the stiffness tensor and lattice anharmonicity, and therefore of the phonon group velocity and phonon mean free path. The effect due to the lattice anharmonicity variation appears to be dominant. The size effect was studied separately in unstrained thin films. Phonon scattering on surfaces leads to a drastic reduction of the thermal conductivity effect which is much more important than that due to strain in the bulk. It is suggested that strain may be used to tailor the phonon mean free path which offers an indirect method to control the size effect.
113 citations
TL;DR: In this article, the mean free path of cosmic ray particles in partially turbulent electromagnetic fields is calculated for two particular turbulence models: slab-like dynamical and random sweeping turbulence, and the rigidity dependence and absolute value of the mean-free path for these specific turbulence models are calculated for a turbulence power spectrum with finite wave amplitude at very small wavenumbers.
Abstract: The parallel mean free path of cosmic ray particles in partially turbulent electromagnetic fields is calculated for two particular turbulence models: slab-like dynamical and random sweeping turbulence. Using the general results for the pitch-angle Fokker-Planck coefficient from Teufel & Schlickeiser (2002) the rigidity dependence and the absolute value of the mean free path for these specific turbulence models are calculated for a turbulence power spectrum with finite wave amplitude at very small wavenumbers. We demonstrate that this modification affects especially the mean free path at very large rigidities. We also derive approximations for the mean free path for realistic Kolmogorov-type turbulence power spectra which include the steepening at high wavenumbers due to turbulence dispersion and/or dissipation.
106 citations
TL;DR: It is shown that freeze- out occurs when the mean free path of pions lambda(f) reaches a value of about 1 fm, which is much smaller than the spatial extent of the system at freeze-out.
Abstract: Based on an evaluation of data on pion interferometry and on particle yields at midrapidity, we propose a universal condition for thermal freeze-out of pions in heavy-ion collisions We show that freeze-out occurs when the mean free path of pions lambda(f) reaches a value of about 1 fm, which is much smaller than the spatial extent of the system at freeze-out This critical mean free path is independent of the centrality of the collision and beam energy from the Alternating Gradient Synchrotron to the Relativistic Heavy Ion Collider
81 citations
TL;DR: Noncontacting (laser source and detector) measurements of ultrasonic wave propagation in randomly heterogeneous rock samples are described and the intensity data are well fit by a radiative transfer model, and the scattering mean free path is inferred.
Abstract: Lasers can be used to excite and detect ultrasonic waves in a wide variety of materials. This allows the measurement of absolute particle motion without the mechanical disturbances of contacting transducers. In an ultrasound transmission experiment, the wave field is usually accessible only on the boundaries of a sample. Using optical methods, one can measure the surface wave field, in effect, within the scattering region. Here, we describe noncontacting (laser source and detector) measurements of ultrasonic wave propagation in randomly heterogeneous rock samples. By scanning the surface of the sample, we can directly visualize the complex dynamics of diffraction, multiple scattering, mode conversion, and whispering gallery modes. We will show measurements on rock samples that have similar elastic moduli and intrinsic attenuation, but different grain sizes, and hence, different scattering strengths. The intensity data are well fit by a radiative transfer model, and we use this fact to infer the scattering mean free path.
66 citations
TL;DR: In this article, the authors construct two simple models with a test particle approximation, where the maximum energy of accelerated electrons is determined by the age of SN1006 (age-limited model) or the energy loss (energy loss-limited case), and constrain the magnetic field configuration and the diffusion coefficients of accelerated particles.
Abstract: Characteristic scale lengths of nonthermal X-rays from the SN1006 NE rim, which are observed by Chandra, are interpreted in the context of the diffusive shock acceleration on the assumption that the observed spatial profile of nonthermal X-rays corresponds to that of accelerated electrons with energies of a few tens of TeV. To explain the observed scale lengths, we construct two simple models with a test particle approximation, where the maximum energy of accelerated electrons is determined by the age of SN1006 (age-limited model) or the energy loss (energy loss-limited model), and constrain the magnetic field configuration and the diffusion coefficients of accelerated electrons. When the magnetic field is nearly parallel to the shock normal, the magnetic field should be in the range of 20-85 micro Gauss and highly turbulent both in upstream and downstream, which means that the mean free path of accelerated electrons is on the order of their gyro-radius (Bohm limit). This situation can be realized both in the age-limited and energy loss-limited model. On the other hand, when the magnetic field is nearly perpendicular to the shock normal, which can exist only in the age-limited case, the magnetic field is several micro Gauss in the upstream and 14-20 micro Gauss in the downstream, and the upstream magnetic field is less turbulent than the downstream.
61 citations
TL;DR: In this article, the transverse resistivity was measured in a reconnecting current sheet where the mean free path for the Coulomb collision is smaller than the thickness of the sheet.
Abstract: Measurement of the transverse resistivity was carried out in a reconnecting current sheet where the mean free path for the Coulomb collision is smaller than the thickness of the sheet. In a collisional neutral sheet without a guide field, the transverse resistivity is directly related to the reconnection rate. A remarkable agreement is found between the measured resistivity and the classical value derived by Spitzer. In his calculation the transverse resistivity for the electrons is higher than the parallel resistivity by a factor of 1.96. The measured values have verified this theory to within 30% errors.
53 citations
TL;DR: In this article, the effect of diamagnetic drifts and long electron mean free path on the shielding of resonant magnetic perturbations by plasma rotation is investigated, and it is shown that the force exerted on a moving plasma by a resonant perturbation is qualitatively altered by both drift and long mean free-path effects.
Abstract: The effect of diamagnetic drifts and of long electron mean free path on the shielding of resonant magnetic perturbations by plasma rotation is investigated. The nature of the force exerted on a moving plasma by a resonant perturbation is qualitatively altered by both drift and long mean-free-path effects. The force is found to have three minima, each of which is a possible locus for discontinuous transitions in plasma velocity. Between these minima are two points where the force exerted by the perturbation is resonant. These points describe locked states where shielding is ineffective and a magnetic island will grow. They correspond to rotation velocities such that either the electrons or the ions are at rest in the frame of the perturbation. The ion root, however, is unstable.
TL;DR: In this paper, the electrical resistivity and piezoresistance of some metal films, such as the Pd, Au, and Cu films, were measured by using an atomic force microscope.
TL;DR: It is found that the energy inelasticity can take on values greater than one, which implies that the rotational degrees of freedom play an important role in energy transfer.
Abstract: We present an experimental investigation of the statistical properties of spherical granular particles on an inclined plane that are excited by an oscillating side wall. The data is obtained by high-speed imaging and particle tracking techniques. We identify all particles in the system and link their positions to form trajectories over long times. Thus, we identify particle collisions to measure the effective coefficient of restitution and find a broad distribution of values for the same impact angles. We find that the energy inelasticity can take on values greater than one, which implies that the rotational degrees of freedom play an important role in energy transfer. We also measure the distance and the time between collision events in order to directly determine the distribution of path lengths and the free times. These distributions are shown to deviate from expected theoretical forms for elastic spheres, demonstrating the inherent clustering in this system. We describe the data with a two-parameter fitting function and use it to calculate the mean free path and collision time. We find that the ratio of these values is consistent with the average velocity. The velocity distributions are observed to be strongly non-Gaussian and do not demonstrate any apparent universal behavior. We report the scaling of the second moment, which corresponds to the granular temperature, and higher order moments as a function of distance from the driving wall. Additionally, we measure long-time correlation functions in both space and in the velocities to probe diffusion in a dissipative gas.
TL;DR: In this paper, the authors applied the inhomogeneous nonequilibrium molecular dynamics (NEMD) scheme to model heat conduction in thin silicon films and found that in a film thickness range of about 2-32 nm, the calculated thermal conductivity decreases almost linearly as the film thickness is reduced, exhibiting a remarkable reduction compared with the bulk experimental data.
Abstract: Molecular dynamics simulations are performed to explore the thermal conductivity in the cross-plane direction of single-crystal thin silicon films. The silicon crystal has diamond structure, and the Stillinger-Weber potential is adopted. The inhomogeneous nonequilibrium molecular dynamics (NEMD) scheme is applied to model heat conduction in thin films. At average temperature T = 500 K, which is lower than the Debye temperature ΘD = 645 K, the results show that in a film thickness range of about 2–32 nm, the calculated thermal conductivity decreases almost linearly as the film thickness is reduced, exhibiting a remarkable reduction as compared with the bulk experimental data. The phonon mean free path is estimated and the size effect on thermal conductivity is attributed to the reduction of phonon mean free path according to the kinetic theory.
TL;DR: In this article, the electrical transport properties of bismuth nanowire arrays embedded in an alumina matrix which are capped with layers of bulk Bi to produce a very low contact resistance were studied.
Abstract: We present a study of the electrical transport properties of 270-nm-diameter bismuth nanowire arrays embedded in an alumina matrix which are capped with layers of bulk Bi to produce a very low contact resistance. The resistance of the Bi nanowires has been measured over a wide range of temperatures (1.8--300 K) and magnetic fields (-8--8 T) for the longitudinal and transverse orientation. At low magnetic fields, the longitudinal magnetoresistance exhibits field-periodic modulations whose periods are consistent with theoretical predictions for the Aharonov-Bohm ``whispering gallery'' modes of electrons with long mean free path. At high magnetic fields, as the carrier cyclotron radius becomes smaller than the wire diameter, we observe Shubnikov--de Haas oscillations associated with both holes and electrons. These represent a detailed study of magnetoquantum oscillations in high-density nanowire arrays. Overall, the hole periods are increased by 5% and the carrier density is decreased by 13% from the values for bulk Bi, which is consistent with recent theoretical estimates of the effect of confinement on the carrier's Fermi surface.
TL;DR: In this article, the second order slip model was derived using a more physical approach, in which the requirement that the expansion length scale be the mean free path of the gas molecules was relaxed and the coefficients of the correction terms in the derived lubrication equation depend on that length scale.
Abstract: In the original derivations of the first order and the second order slip models of the generalised Reynolds equation in the literature [3,4], a length scale equal to the mean free path of the gas molecules was used in a Taylor series expansion of the mean velocity field. The coefficients of the correction terms in the derived lubrication equation depend on that length scale. This choice of the length scale is arbitrary to some extent. In this paper, new first order and the second order slip models are derived using a somewhat more physical approach, in which the requirement that the expansion length scale be the mean free path is relaxed. In this approach the momentum transfer rate across each surface element is obtained by summing up the contributions from each group of molecules impinging on the surface at an angle θ to the surface normal within a solid angle dω. The new second order slip lubrication equation appears to be preferable to the original one when the inverse Knudsen number is small, and it is free of any contact pressure singularity, whereas the new first order slip model continues to contain the unacceptable pressure singularity in the limit as the spacing approaches zero, as does the original first order model.
TL;DR: In this paper, the authors used analytical and numerical techniques to find the rate of the two-dimensional electron-hole capture process in bilayer organic light-emitting diodes.
Abstract: We model the two-dimensional recombination of electrons and holes in a system where the mean free path is short compared with the thermal capture radius. This recombination mechanism is relevant to the operation of bilayer organic light-emitting diodes (LED's), where electrons and holes accumulate on either side of the internal heterojunction. The electron-hole recombination rate can be limited by the time taken for these charge carriers to drift and diffuse to positions where electrons and holes are directly opposite to each other on either side of the interface, at which point rapid formation of an emissive neutral state can occur. In this paper, we use analytical and numerical techniques to find the rate of this two-dimensional electron-hole capture process. Where one species of carrier is significantly less mobile than the other, we find that the recombination rate depends superlinearly on the density of the less mobile carrier. Numerical simulations allow the effects of disorder to be taken into account in a microscopic hopping model. Direct solution of the master equation for hopping provides more efficient solutions than Monte Carlo simulations. The rate constants extracted from our model are consistent with efficient emission from bilayer LED's without requiring independent hopping of electrons and holes over the internal barrier at the heterojunction.
TL;DR: In this paper, a new analysis of reflection electron energy-loss spectroscopy (REELS) spectra is presented, which provides the distribution of energy losses in a single surface excitation in absolute units without the use of any fitting parameters.
Abstract: A new analysis of reflection electron energy-loss spectroscopy (REELS) spectra is presented. Assuming inelastic scattering in the bulk to be quantitatively understood, this method provides the distribution of energy losses in a single surface excitation in absolute units without the use of any fitting parameters. For this purpose, REELS spectra are decomposed into contributions corresponding to surface and volume excitations in two steps: first the contribution of multiple volume excitations is eliminated from the spectra and subsequently the distribution of energy losses in a single surface scattering event is retrieved. This decomposition is possible if surface and bulk excitations are uncorrelated, a condition that is fulfilled for medium-energy electrons because the thickness of the surface scattering layer is small compared with the electron elastic mean free path. The developed method is successfully applied to REELS spectra of several materials. The resulting distributions of energy losses in an individual surface excitation are in good agreement with theory. In particular, the so-called begrenzungs effect, i.e. the reduction of the intensity of bulk losses due to coupling with surface excitations near the boundary of a solid-state plasma, becomes clearly observable in this way. Copyright © 2003 John Wiley & Sons, Ltd.
TL;DR: Using coherent backscattering technique, this paper measured the transport mean-free-path length for photons in NdAl3(BO3)4 powder, a promising solid-state random laser material.
Abstract: Using the coherent backscattering technique, we have measured the transport mean-free-path lengths lt for photons in NdAl3(BO3)4 powder, a promising solid-state random laser material The developed model allows one to calculate lt as a function of the mean particle size s in tightly packed powder or sintered ceramic The experimentally determined value of lt is in good agreement with the model prediction
TL;DR: The measured mean free path of clean, undamaged arc-produced multiwalled carbon nanotubes is several orders of magnitude greater than that for metals, making this perhaps the most significant property of carbon Nanotubes.
Abstract: The electrical transport in multiwalled carbon nanotubes is shown to be ballistic at room temperature with mean free paths on the order of tens of microns. The measurements are performed both in air and in the transmission electron microscope by contacting the free end of a nanotube pointing out of a fiber to a liquid metal and measuring the dependence of the nanotube resistance between the contacts. For a specific representative nanotube the resistance per unit length is found to be Rt = 31 +/- 61 omega/micron and the contact resistance with the liquid metal, Rc = 165 +/- 55 omega microns, corresponding to a mean free path l = 200 microns. Current-to-voltage characteristics are in accord with the electronic structure. The nanotubes survive high currents (up to 1 mA, i.e., current density on the order of 10(9) A/cm2). In situ electron microscopy shows that a relatively large fraction of the nanotubes do not conduct (even at high bias), consistent with the existence of semiconducting nanotubes. Discrepancies with other measurements are most likely due to damage caused to the outer layer(s) of the nanotubes during processing. The measured mean free path of clean, undamaged arc-produced multiwalled carbon nanotubes is several orders of magnitude greater than that for metals, making this perhaps the most significant property of carbon nanotubes.
TL;DR: In this paper, the authors calculated the thermal conductivity of nano-electro-mechanical (NEMS) systems using molecular dynamics (MD) simulations compared with the available bulk experimental data when possible.
Abstract: lm plays an important role in the design of nano-electro-mechanical systems (NEMS) or micro-electro-mechanical systems (MEMS) since heat removal from these devices is a crucial factor for their intended proper operations. The values calculated by using molecular dynamics (MD) simulations are compared with the available bulk experimental data when possible. It is conrmed that there is apparently a size eect on the thermal conductivity, which indicates that the microscale system has a lower thermal conductivity than that of the bulk material in the heat transfer direction. The dependence of the thermal conductivity on the system size is the result of a reduction in the phonon mean free path (MFP) as the system size becomes microscaled, and the MD simulations can be used to predict the phonon MFP of such a system.
TL;DR: In this paper, the Landau parameters of nuclear matter and neutron matter are extracted from the Brueckner theory including three-body forces, and the dynamical response function to weak neutrino current is calculated in terms of the landau parameters in the RPA limit.
Abstract: The Landau parameters of nuclear matter and neutron matter are extracted from the Brueckner theory including three-body forces. The dynamical response function to weak neutrino current is calculated in terms of the Landau Parameters in the RPA limit. Then, the neutrino mean free path in neutron stars is calculated for different conditions of density and temperature.
TL;DR: The anomalous thermal conductivity in spin chains observed in experiments is studied for the low-temperature regime in this article, and the Drude weight which is the prefactor in the divergent conductivity is calculated, and the temperature dependence is discussed.
Abstract: The anomalous thermal conductivity in spin chains observed in experiments is studied for the low-temperature regime. In the effective dynamics with most realistic perturbations, the so-called umklapp terms are irrelevant to reduce the mean free path in the energy transport at even finite temperatures. This is consistent with the large conductivities found in recent experiments. The Drude weight which is the prefactor in the divergent conductivity is calculated, and the temperature dependence is discussed.
TL;DR: In this article, the neutrino mean free path and the Equation of State of pure neutron matter at finite temperature within a selfconsistent scheme based on the Brueckner-Hartree--Fock approximation were calculated.
Abstract: We calculate the neutrino mean free path and the Equation of State of pure neutron matter at finite temperature within a selfconsistent scheme based on the Brueckner--Hartree--Fock approximation. We employ the nucleon-nucleon part of the recent realistic baryon-baryon interaction (model NSC97e) constructed by the Nijmegen group. The temperatures considered range from 10 to 80 MeV. We report on the calculation of the mean field, the residual interaction and the neutrino mean free path including short and long range correlations given by the Brueckner--Hartree--Fock plus Random Phase Approximation (BHF+RPA) framework. This is the first fully consistent calculation in hot neutron matter dedicated to neutrino mean free path. We compare systematically our results to those obtain with the D1P Gogny effective interaction, which is independent of the temperature. The main differences between the present calculation and those with nuclear effective interactions come from the RPA corrections to BHF (a factor of about 8) while the temperature lack of consistency accounts for a factor of about 2.
TL;DR: In this paper, the rate of relaxation of zigzag single-walled carbon nanotubes is calculated by consideration of three-phonon Umklapp process and the results show that the relaxation rate increases exponentially with phonon frequency at low frequency.
Abstract: The rate of relaxation of zigzag single-walled carbon nanotubes is calculated by consideration of three-phonon Umklapp process. The results show that the relaxation rate increases exponentially with phonon frequency at low frequency. The linear dependence of the relaxation rate on temperature is obtained. It is shown that the value of the phonon mean free path reaches a few micrometres, which is consistent with the estimated experimental result.
Abstract: The mobility of self-interstitials in α-zirconium (α-Zr) is studied with molecular dynamic (MD) and molecular static (MS) simulations, using Ackland’s many-body inter-atomic potential. The basal crowdion configuration is found to be the ground state. Four types of diffusion jumps can be identified via MS, in-plane in-line, in-plane off-line, out-of-plane in-line and out-of-plane off-line. The in-plane migration is dominated by one-dimensional crowdion motion along the [11\(\bar\Box2\Box\)0] directions, interrupted from time to time by off-line or out-of-plane jumps. Based on the MS results, the activation energies and pre-exponentials for the diffusion processes are determined by fits to the Arrhenius plots of Dc and Da. The diffusional anisotropy factor Dc/Da is also obtained, and compares well with experimental results. The mean frequency of each type of jumps is then found using Monte Carlo simulation, and is reported as a function of temperature. The mean lifetime and mean free path of the one-dimensional mobility are then obtained. The 1-D mean free path is found to be unimportant for sink separations involved under the usual irradiation damage conditions.
TL;DR: In this paper, the authors studied hydrodynamic behavior in expanding thermal clouds of {sup 87}Rb released from an elongated trap and presented an analytical model to describe the expansion and to estimate the cooling.
Abstract: We study hydrodynamic behavior in expanding thermal clouds of {sup 87}Rb released from an elongated trap. At our highest densities the mean free path is smaller than the radial size of the cloud. After release the clouds expand anisotropically. The cloud temperature drops by as much as 30%. This is attributed to isentropic cooling during the early stages of the expansion. We present an analytical model to describe the expansion and to estimate the cooling. Important consequences for time-of-flight thermometry are discussed.
29 Sep 2003
TL;DR: In this article, the microscopic details of Joule heating in bulk silicon with Monte Carlo simulations including acoustic and optical phonon dispersion are examined, providing an engineering tool for electro-thermal analysis of future nano-devices.
Abstract: As current device technologies advance into the sub-continuum regime, they operate at length scales on the order of the electron and phonon mean free path. The ballistic conditions lead to strong non-equilibrium at nanometer length scales. The electron-phonon interaction is not energetically or spatially uniform and the generated phonons have widely varying contributions to heat transport. This work examines the microscopic details of Joule heating in bulk silicon with Monte Carlo simulations including acoustic and optical phonon dispersion. The approach provides an engineering tool for electro-thermal analysis of future nano-devices.
TL;DR: In this paper, the influence of surface excitation effects on the measured electron elastic backscattering probability was investigated quantitatively, and it was shown that properly corrected measured elastic electron backscatter probabilities lead to inelastic mean free path values which compare well with the theory.
TL;DR: In this article, Monte Carlo computations for highly relativistic parallel shock particle acceleration are presented for upstream flow gamma factors, Γ =(1− V 1 2 / c 2 ) −0.5 with values between 5 and 10 3.