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.

Optical absorption by small metallic particles

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.

The neutron scattering function for hard spheres

Abstract: Density, longitudinal, and transverse-current correlation functions for hard spheres at various densities and wavelengths have been generated by computer simulation and compared with both the generalized Enskog kinetic theory and wavelength-dependent hydrodynamics. It is shown that even for dense gases the generalized Enskog kinetic theory is quantitatively accurate in describing the thermal fluctuations at finite wavelengths and frequencies, as is wavelength-dependent hydrodynamics, as long as the wavelength is greater than the mean free path. At liquid densities neither theory can account for the viscoelastic relaxation effects, directly observed in the transverse-current correlation function by shear-wave propagation, at a wavelength somewhat above the first diffraction maximum. However, wavelength-dependent hydrodynamics quantitatively describes the neutron scattering function at wavelengths from this point (above the first diffraction maximum) through the diffraction maximum (where the de Gennes narrowing occurs) to the mean-free-path limit. Furthermore, viscoelastic effects in the long-wavelength regime can be accounted for by introducing into hydrodynamics time-dependent transport coefficients. At still longer wavelengths, viscoelastic relaxation times become short compared with hydrodynamic relaxation times and ordinary hydrodynamics with constant transport coefficients describes the neutron scattering function.

Prediction of Spectral Phonon Mean Free Path and Thermal Conductivity with Applications to Thermoelectrics and Thermal Management: A Review

Abstract: We give a review of the theoretical approaches for predicting spectral phonon mean free path and thermal conductivity of solids. The methods can be summarized into two categories: anharmonic lattice dynamics calculation and molecular dynamics simulation. In the anharmonic lattice dynamics calculation, the anharmonic force constants are used first to calculate the phonon scattering rates, and then the Boltzmann transport equations are solved using either standard single mode relaxation time approximation or the Iterative Scheme method for the thermal conductivity. The MD method involves the time domain or frequency domain normal mode analysis. We present the theoretical frameworks of the methods for the prediction of phonon dispersion, spectral phonon relaxation time, and thermal conductivity of pure bulk materials, layer and tube structures, nanowires, defective materials, and superlattices. Several examples of their applications in thermal management and thermoelectric materials are given. The strength and limitations of these methods are compared in several different aspects. For more efficient and accurate predictions, the improvements of those methods are still needed.

Measurement of ballistic phonon conduction near hotspots in silicon

Abstract: The Fourier law for lattice heat conduction fails when the source of heat is small compared to the phonon mean free path. We provide experimental evidence for this effect using heating and electrical-resistance thermometry along a doped region in a suspended silicon membrane. The data are consistent with a closed-form two-fluid phonon conduction model, which accounts for the severe departure from equilibrium at the hotspot. The temperature rise exceeds predictions based on the Fourier law by 60% when the phonon mean free path is a factor of 30 larger than the resistor thickness. This work is improving the constitutive modeling of heat flow in deep-submicron transistors.

Far-infrared response of free charge carriers localized in semiconductor nanoparticles

Abstract: A Monte Carlo method is employed to calculate the dynamical conductivity in the terahertz range of free charge carriers localized in semiconductor nanoparticles. The shape of the conductivity spectrum is essentially determined by the probability of carrier transition through interparticle boundaries and by the ratio of the nanoparticle size and carrier mean free path in the bulk. It is shown that the conductivity spectrum exhibits similar features as the classical extension of the Drude conductivity of electrons proposed by Smith [Phys. Rev. B 64, 155106 (2001)]. We find and discuss the link of this model to the results of our simulations which suggests an interpretation of the phenomenological parameters of the Drude-Smith model.