Showing papers on "Debye published in 2006"

Debye temperature and stiffness of carbon and boron nitride polymorphs from first principles calculations

Abstract: A theoretical investigation has been made on the phonon spectrum and heat capacity of polymorphs of carbon and boron nitride with special interests on the variation of Debye temperature and stiffness with temperature. A part of optical phonon branches of graphite exhibits higher frequencies than those of diamond. As a consequence, graphite shows smaller heat capacity and higher Debye temperature than diamond above a crossover temperature of $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This supports experimental reports of heat capacity although available experimental data are widely scattered. The higher Debye stiffness of graphite at above $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is not contradictory to the fact that conventional stiffness of diamond is much larger than that of graphite, since the Debye stiffness is determined by both acoustic and optical phonons, whereas only acoustic phonons contribute to the conventional stiffness. The same trend was found between hexagonal and cubic boron nitrides with a crossover temperature of $600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Fast focus field calculations.

Abstract: We present a fast calculation of the electromagnetic field near the focus of an objective with a high numerical aperture (NA). Instead of direct integration, the vectorial Debye diffraction integral is evaluated with the fast Fourier transform for calculating the electromagnetic field in the entire focal region. We generalize this concept with the chirp z transform for obtaining a flexible sampling grid and an additional gain in computation speed. Under the conditions for the validity of the Debye integral representation, our method yields the amplitude, phase and polarization of the focus field for an arbitrary paraxial input field on the objective. We present two case studies by calculating the focus fields of a 40×1.20 NA water immersion objective for different amplitude distributions of the input field, and a 100×1.45 NA oil immersion objective containing evanescent field contributions for both linearly and radially polarized input fields.

High Thermal Conductivity Materials

Abstract: Every university student becomes familiar with the concept of thermal conductivity, a fundamental physical property of materials, through his or her textbooks. Initial work on high thermal conductivity was carried out in 1911 by Eucken, who discovered that diamond was a reasonably good conductor for heat at room temperature. Theoretical support for this discovery was established by Debye in 1914.

Fractional Stokes-Einstein and Debye-Stokes-Einstein relations in a network-forming liquid.

Abstract: We study the breakdown of the Stokes-Einstein (SE) and Debye-Stokes-Einstein (DSE) relations for translational and rotational motion in a prototypical model of a network-forming liquid, the ST2 model of water We find that the emergence of fractional SE and DSE relations at low temperature is ubiquitous in this system, with exponents that vary little over a range of distinct physical regimes We also show that the same fractional SE relation is obeyed by both mobile and immobile dynamical heterogeneities of the liquid

Photoionization of Li and Na in Debye plasma environments

Abstract: A calculation of the photoionization cross sections is presented for alkali-metal atoms such as Li and Na in plasma environments. The computational scheme is based on the complex coordinate rotation method. A model potential formalism has been used to simplify the computational complexity of the problems of making quantitative predictions of properties and interactions of many electron systems in Debye plasmas. The plasma environment is found to appreciably influence the photoionization cross sections. In this regard the photoionization cross sections of isolated atoms are also discussed that is found to be in good agreement with the previous theoretical results. It is observed that the strong plasma screening effect remarkably alters the photoionization cross sections near the ionization threshold. The Cooper minimum in the photoionization cross sections of Na shifts toward the higher energy as the plasma screening effect increases. For Li, the Cooper minimum is uncovered in strong plasma environments. This is the first time such structures have been determined.

Injection dependence of spontaneous radiative recombination in crystalline silicon : Experimental verification and theoretical analysis

Abstract: The radiative recombination coefficient B in crystalline bulk silicon is enhanced by the Coulomb attraction between electrons and holes. This effect is weakened at high carrier densities due to screening. We measure the resulting dependence of B on the free-carrier density (i) by reinterpreting published data and (ii) with photoluminescence and photovoltaic measurements. We calculate the Coulomb enhancement by determining the electron-hole pair correlation function at zero interparticle distance, assuming a Debye interaction potential. Both bound and scattering state contributions are fully taken into account. Due to screening, B decreases with increasing free-carrier density.

Model dispersive media in finite-difference time-domain method with complex-conjugate pole-residue pairs

Abstract: In this letter, we show that both Debye poles and Lorentz pole pairs are special cases of complex-conjugate pole-residue pairs, and the general form of such pairs is in fact far more efficient than the commonly used Debye poles and Lorentz pole pairs for modeling real dispersive media with the finite-difference time-domain method. We first derive an alternative formulation of the auxiliary differential equation method for arbitrary dispersive media based on general complex-conjugate pole-residue pairs. We then numerically demonstrate the efficiency of using these pairs in modeling dispersive media

Effect of densification on the density of vibrational states of glasses

Abstract: We studied the effect of densification on the vibrational dynamics of a ${\mathrm{Na}}_{2}{\mathrm{FeSi}}_{3}{\mathrm{O}}_{8}$ glass. The density of vibrational states (DOS) has been measured using nuclear inelastic scattering. The corresponding changes in the microscopic, intermediate-range, and macroscopic properties have also been investigated. The results reveal that, in the absence of local structure transformations, the Debye level and the glass-specific excess of vibrational states above it have the same dependence on density, and the evolution of the DOS is fully described by the transformation of the elastic medium.

Debye length in non-Maxwellian plasmas

Abstract: In the present paper, comparison of characteristic shielding distance is determined by using a non-Maxwellian plasma. A modified version of the generalized Lorentzian distribution function, which is referred to as the (r, q) distribution, has been employed to derive the shielding distance with a modified power-law. The most surprising feature of a plasma containing superthermals is the strong dependence of plasma Debye length λD on spectral indices κ, r and q. It is observed that these spectral indices frustrate the Debye shielding distance. In the case of kappa, it is much smaller than that found for a Maxwellian plasma. We adopt the (r, q) distribution because it gives better data fit results, especially when there are shoulders in the profile of the distribution function along with a high-energy tail.

Bound states of helium atom in dense plasmas

Abstract: We have obtained the bound 1s21S, 1s2s1,3S, and 1s2p1,3P states energies of helium atom in dense plasma environments in accurate variation calculations. A screened Coulomb potential to represent the Debye model is used for the interaction between the charged particles. A correlated wave function consisting of a generalized exponential expansion has been used to take care of the correlation effect. The 1s21S, 1s2s1,3S, and 1s2p1,3P states energies along with the ionization potential, the energy splitting between the 1s2s3S, and 1s2s1S states, transition energies between the ground state and low-excited states of He estimated for various Debye lengths, are reported. The results show high degree of accuracy even under strong plasma conditions. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

High-frequency electrodynamics

Abstract: Preface.1 The Maxwell Equations.1.1 Complex amplitudes.1.2 The Maxwell equations.1.3 Idealized objects.1.4 Uniqueness and existence of solution.2 Plane Waves.2.1 Plane waves in an infinite homogeneous medium.2.2 Plane waves in a plano-layered medium.3 Closed Waveguides.3.1 Eigenmodes in nonfilled waveguides.3.2 Waves in waveguides with nonhomogeneous cross-section filling.3.3 Excitation of closed waveguides.3.4 Nonregular closed waveguides.4 Closed Resonators.4.1 Resonators with ideal-conducting walls..4.2 Resonators with impedance walls..5 Open Lines.5.1 Dielectric waveguides..5.2 The lines with surface wave.5.3 The wave beam.6 Backgrounds of Antenna Theory.6.1 Radiation of current set.6.2 Aperture antennas.6.3 Volume antennas.7 Diffraction on Metallic and Dielectric Objects.7.1 Diffraction of the plane wave on circular waveguide.7.2 Diffraction on metallic half-plane.7.3 The Debye potentials: diffraction on a metallic sphere.7.4 Small bodies large bodies.General References.Complementary References.Index.

Debye series for light scattering by a multilayered sphere

Abstract: We have derived the formula for the Debye-series decomposition for light scattering by a multilayered sphere. This formulism permits the mechanism of light scattering to be studied. An efficient algorithm is introduced that permits stable calculation for a large sphere with many layers. The formation of triple first-order rainbows by a three-layered sphere and single-order rainbows and the interference of different-order rainbows by a sphere with a gradient refractive index, are then studied by use of the Debye model and Mie calculation. The possibility of taking only one single mode or several modes for each layer is shown to be useful in the study of the scattering characteristics of a multilayered sphere and in the measurement of the sizes and refractive indices of particles.

Computational probes of molecular motion in the Lewis-Wahnstrom model for ortho-terphenyl.

Abstract: We use molecular dynamics simulations to investigate translational and rotational diffusion in a rigid three-site model of the fragile glass former ortho-terphenyl, at 260K⩽T⩽346K and ambient pressure. An Einstein formulation of rotational motion is presented, which supplements the commonly used Debye model. The latter is shown to break down at supercooled temperatures as the mechanism of molecular reorientation changes from small random steps to large infrequent orientational jumps. We find that the model system exhibits non-Gaussian behavior in translational and rotational motion, which strengthens upon supercooling. Examination of particle mobility reveals spatially heterogeneous dynamics in translation and rotation, with a strong spatial correlation between translationally and rotationally mobile particles. Application of the Einstein formalism to the analysis of translation-rotation decoupling results in a trend opposite to that seen in conventional approaches based on the Debye formalism, namely, an...

Analysis of stability and dispersion in a finite element method for Debye and Lorentz dispersive media

Abstract: This is the pre-peer reviewed version of the following article: H T Banks, V A Bokil and N L Gibson, Analysis of Stability and Dispersion in a Finite Element Method for Debye and Lorentz Media, Numerical Methods for Partial Differential Equations, 25(4), pp 885-917, July 2009, which has been published in final form at http://www3intersciencewileycom/journal/122341241/issue

Einstein and Debye models for EXAFS parallel and perpendicular mean-square relative displacements.

Abstract: The correlated Einstein and Debye models for EXAFS parallel mean-square relative displacement (MSRD) are derived from the general expression in terms of eigenfrequencies and eigenvectors of the dynamical matrix, without ad hoc assumptions. The two models are generalized to parameterize also the EXAFS perpendicular MSRD. The physical meaning of Einstein frequencies, as well as the application of the Debye model to crystals with more than one atom per cell, are critically discussed.

Computational probes of molecular motion in the Lewis and Whanstrom model for ortho-terphenyl

Abstract: We use molecular dynamics simulations to investigate translational and rotational diffusion in a rigid three-site model of the fragile glass former ortho-terphenyl, at 260 K < T < 346 K and ambient pressure. An Einstein formulation of rotational motion is presented, which supplements the commonly-used Debye model. The latter is shown to break down at supercooled temperatures as the mechanism of molecular reorientation changes from small random steps to large infrequent orientational jumps. We find that the model system exhibits non-Gaussian behavior in translational and rotational motion, which strengthens upon supercooling. Examination of particle mobility reveals spatially heterogeneous dynamics in translation and rotation, with a strong spatial correlation between translationally and rotationally mobile particles. Application of the Einstein formalism to the analysis of translation-rotation decoupling results in a trend opposite to that seen in conventional approaches based on the Debye formalism, namely an enhancement in the effective rate of rotational motion relative to translation upon supercooling.

Eddies in a bottleneck: An arbitrary Debye length theory for capillary electroosmosis

Abstract: Using an applied electrical field to drive fluid flows becomes desirable as channels become smaller Although most discussions of electroosmosis treat the case of thin Debye layers, here electroosmotic flow (EOF) through a constricted cylinder is presented for arbitrary Debye lengths (κ−1) using a long wavelength perturbation of the cylinder radius The analysis uses the approximation of small potentials The varying diameter of the cylinder produces radially and axially varying effective electric fields, as well as an induced pressure gradient We predict the existence of eddies for certain constricted geometries and propose the possibility of electrokinetic trapping in these regions We also present a leading-order criterion which predicts central eddies in very narrow constrictions at the scale of the Debye length Eddies can be found both in the center of the channel and along the perimeter, and the presence of the eddies is a consequence of the induced pressure gradient that accompanies electrically driven flow into a narrow constriction

Dynamics of glass-forming liquids. XI. Fluctuating environments by dielectric spectroscopy.

Abstract: The dielectric relaxation of a 1wt% mixture of di-n-butylether in 3-methylpentane has been measured across a range of eight decades, in which the characteristic relaxation time varies from 5sto50ns. Each loss spectrum is a superposition of the dispersive solvent peak and a Debye peak which is one decade slower and readily assigned to the larger and more dipolar solute molecules. Fluctuating environments or rate exchange is made responsible for the Debye nature of probe rotation, implying that the environmental relaxation times fluctuate on time scales which are faster than the rotational correlation decay of the probe molecule. Within the experimental range from 2.2sto42ns regarding the mean α-relaxation time, the results are consistent with the exchange time matching the upper limit of structural relaxation times or two to three times their average value. As Tg is approached, no indication for a variation in exchange behavior or for slower environmental fluctuations is found.

Density of vibrational states of a hyperquenched glass.

Abstract: The vibrational density of states of a hyperquenched and an annealed glass has been measured using nuclear inelastic scattering. The hyperquenched sample shows a higher number of vibrational states in the low-energy region with respect to the annealed glass. It reveals, however, lower density and sound velocity and, therefore, smaller Debye energy. After rescaling the energy axes in Debye energy units and area renormalization, the density of states of both samples becomes identical. Thus, the effect of quenching is described by the transformation of the continuous medium.

Electromagnetic inverse problems involving distributions of dielectric mechanisms and parameters

Abstract: : We consider electromagnetic interrogation problems for complex materials involving distributions of polarization mechanisms and also distributions for the parameters in these mechanisms. a theoretical and computational framework for such problems is given. Computational results for specific problems with multiple Debye mechanisms are given in the case of discrete, uniform, log-normal, and log-Bi-Gaussian distributions.

Doubly excited 1,3 P o resonance states of Ps - in weakly coupled plasmas

Abstract: The P resonance states of positronium negative ions embedded in dense plasma environments are determined by calculating the density of resonance states using the stabilization method. A screened Coulomb potential obtained from the Debye model is used to represent the interaction between the charge particles, together with employing highly correlated wave functions to represent the correlation effect between them. We have calculated one P and two P resonances associated with and below the n=2 threshold of the positronium atom. The resonance energies and widths for various Debye lengths ranging from infinity to a small value are reported.

Homogenization of Periodically Varying Coefficients in Electromagnetic Materials

Abstract: In this paper, we employ the periodic unfolding method for simulating the electromagnetic field in a composite material exhibiting heterogeneous microstructures which are described by spatially periodic parameters. We consider cell problems to calculate the effective parameters for a Debye dielectric medium in the case of a circular microstructure in two dimensions. We assume that the composite materials are quasi-static in nature, i.e., the wavelength of the electromagnetic field is much larger than the relevant dimensions of the microstructure.

Influence of atomistic physics on electro-osmotic flow: an analysis based on density functional theory.

Abstract: Molecular density profiles and charge distributions determined by density functional theory (DFT) are used in conjunction with the continuum Navier-Stokes equations to compute electro-osmotic flows in nanoscale channels. The ion species of the electrolyte are represented as centrally charged hard spheres, and the solvent is treated as a dense fluid of neutral hard spheres having a uniform dielectric constant. The model explicitly accounts for Lennard-Jones interactions among fluid and wall molecules, hard sphere repulsions, and short range electrical interactions, as well as long range Coulombic interactions. Only the last of these interactions is included in classical Poisson-Boltzmann (PB) modeling of the electric field. Although the proposed DFT approach is quite general, the sample calculations presented here are limited to symmetric monovalent electrolytes. For a prescribed surface charge, this DFT model predicts larger counterion concentrations near charged channel walls, relative to classical PB modeling, and hence smaller concentrations in the channel center. This shifting of counterions toward the walls reduces the effective thickness of the Debye layer and reduces electro-osmotic velocities as compared to classical PB modeling. Zeta potentials and fluid speeds computed by the DFT model are as much as two or three times smaller than corresponding PB results. This disparity generally increases with increasing electrolyte concentration, increasing surface charge density and decreasing channel width. The DFT results are found to be comparable to those obtained by molecular dynamics simulation, but require considerably less computing time.

First hints on a change of the 22Na βdecay half-life in the metal Pd

Abstract: For the β+-decay of 22Na in the metallic environment Pd cooled to T = 12 K the 22Na half-life was observed to be shorter by 1.2±0.2%. The result is consistent with --but lower than-- the prediction of the Debye plasma model.

On the heat capacities of M2AlC (M=Ti,V,Cr) ternary carbides

Abstract: In this paper, we report on the heat capacities cp of bulk polycrystalline samples of Ti2AlC, V2AlC, and Cr2AlC in the 3–260K temperature range. Given the structural and chemical similarities of these compounds it is not surprising that the cp’s and their temperature dependencies were quite similar. Nevertheless, at all temperatures the heat capacity of Cr2AlC was higher than the other two. The density of states at the Fermi level were 3.9, 7.5, and 14.6(eVunitcell)−1 for Ti2AlC, V2AlC, and Cr2AlC, respectively. The results obtained are analyzed using the Debye and Einstein model approximations for cp. Good description of cp is obtained if one assumes that nine phonon modes vibrate according to the Debye model approximation whereas the remaining 3 of 12 modes expected for M2AlC formula unit fulfill an Einstein-like phonon vibration pattern. Debye temperatures θD describing acoustic phonon and Einstein temperature θE describing optical phonon contributions have been estimated for the studied compounds. The...