Mean free path

About: Mean free path is a research topic. Over the lifetime, 4412 publications have been published within this topic receiving 114418 citations.

Shear viscosity of a hot pion gas

Abstract: The shear viscosity of an interacting pion gas is studied using the Kubo formalism as a microscopic description of thermal systems close to global equilibrium. We implement the skeleton expansion in order to approximate the retarded correlator of the viscous part of the energy-momentum tensor. After exploring this in gφ 4 theory we show how the skeleton expansion can be consistently applied to pions in chiral perturbation theory. The shear viscosity η is determined by the spectral width, or equivalently, the mean free path of pions in the heat bath. We derive a new analytical result for the mean free path which is well conditioned for numerical evaluation and discuss the temperature and pion-mass dependence of the mean free path and the shear viscosity. The ratio η/s of the interacting pion gas exceeds the lower bound 1/4π from AdS/CFT correspondence.

The ion energy distribution in front of a negative wall

Abstract: The ion energy distribution of ions bombarding a negative wall depends on the sheath potential and on the ion kinetics in the boundary layer. In most discharges the electron Debye length lambda D is small compared to the ion mean free path lambda C. For weakly negative walls the sheath is, therefore, nearly collision free. In the case of highly negative walls, however, the sheath thickness increases and collisions in the sheath may become important. The effect of collisions on the ion distribution function and sheath potential variation is investigated theoretically and experimentally. The theory starts from the charge exchange model of ion kinetics, presents a convenient self-consistent description of the unipolar ion sheath and extends a former asymptotic analysis lambda D/ lambda C to 0 to the Debye sheath. In the experimental part a retarding field analyser is used to determine the ion energy distribution. Measurements were made in the negative glow of a DC glow discharge at pressures of several Pa in argon, nitrogen and oxygen.

Measurement of mean free paths for inelastic electron scattering of Si and SiO2

Abstract: The effects of accelerating voltage and collection angle on the mean free path for all inelastic electron scattering (lambdap), which is an important parameter for determining specimen thickness by using electron energy-loss spectroscopy, were investigated with crystalline Si and amorphous SiO2. First, thickness of Si film was measured with the convergent-beam electron diffraction method, while thickness of SiO2 particles was estimated from their spherical shape. Then from electron energy-loss spectra, lambdap was evaluated for Si film and SiO2 particles by changing the accelerating voltage (100 to approximately 300 kV) and the collection angle for the scattered electrons. Under the condition of no objective aperture, lambdap for Si film and SiO2 particles was found to increase with the increase of accelerating voltage and to take values of 180+/-6 nm (Si) and 247+/-8 nm (SiO2) at 300 kV. Also, it was found that lambdap in both cases decreases drastically with the increase of collection angle in the range smaller than 25 mrad, while it tends to take a constant value at the collection angle larger than 25 mrad at 200 kV.

Anderson localization in two-dimensional graphene with short-range disorder: One-parameter scaling and finite-size effects

Abstract: We study Anderson localization in graphene with short-range disorder using the real-space Kubo-Greenwood method implemented on graphics processing units. Two models of short-range disorder, namely, the Anderson on-site disorder model and the vacancy defect model, are considered. For graphene with Anderson disorder, localization lengths of quasi-one-dimensional systems with various disorder strengths, edge symmetries, and boundary conditions are calculated using the real-space Kubo-Greenwood formalism, showing excellent agreement with independent transfer matrix calculations and superior computational efficiency. Using these data, we demonstrate the applicability of the one-parameter scaling theory of localization length and propose an analytical expression for the scaling function, which provides a reliable method of computing the two-dimensional localization length. This method is found to be consistent with another widely used method which relates the two-dimensional localization length to the elastic mean free path and the semiclassical conductivity. Abnormal behavior at the charge neutrality point is identified and interpreted to be caused by finite-size effects when the system width is comparable to or smaller than the elastic mean free path. We also demonstrate the finite-size effect when calculating the two-dimensional conductivity in the localized regime and show that a renormalization group $\ensuremath{\beta}$ function consistent with the one-parameter scaling theory can be extracted numerically. For graphene with vacancy disorder, we show that the proposed scaling function of localization length also applies. Last, we discuss some ambiguities in calculating the semiclassical conductivity around the charge neutrality point due to the presence of resonant states.

Propagation conditions of relativistic electrons in the inner heliosphere

Abstract: The intensity and anisotropy time profiles of six relatively small relativistic electron events observed within 0.65 AU by one of the Helios spacecraft were fitted with the model of focused transport under the assumption of a particle mean free path λ independent of the radial distance. For ∼0.5 MeV electrons it is found that the amount of interplanetary scattering varies from one solar event to the other, with local mean free paths λ between 0.02 and 0.15 AU. Comparison with previous results obtained close to 1 AU shows that there is no marked variation in the average scattering conditions with radial distance