Determination of the effective dielectric function of silver powders produced by inert gas evaporation

Silver powder samples were prepared by evaporation of silver in an inert gas atmosphere. They were investigated by dc-conductivity-, reflectance- and photoacoustic (PAS) measurements. Model spectra based on the Bergman ansatz for the effective dielectric function ε_eff, the effective thermal and electrical conductivities λ_eff, and σ_eff, were fitted to the experiments. Good agreement has been found assuming a spongily packed, only weakly percolating topology and an increased damping of the free carriers in the small particles (classical size effect).     

Effective biaxial modulus and strain energy density of ideally (h k l)-fiber-textured cubic polycrystalline films

The effective biaxial modulus (M_eff) and strain energy density (W) of cubic polycrystalline films with ideally (h k l) fiber textures are estimated using Vook-Witt (VW) grain interaction model and the data are compared with those derived from Voigt, Reuss and Voigt-Reuss-Hill (VRH) models. Numerical results show that the VW average of M_eff for ideally (1 0 0)- or (1 1 1)-fiber-textured films is identical to the VRH average of M_eff. For (1 1 0) and (1 1 2) planes, however, the VW average of M_eff for (1 1 0)-fiber-textured film is larger than that of (1 1 2)-fiber-textured film when the Zener anisotropic factor (A_R) is not equal to 1. Furthermore, M_eff and W exhibit incremental tendencies with the increase of the orientation factor (Γ_h k l) for the [h k l] axis when A_R > 1, implying that M_eff and W have the minimums on the (1 0 0) plane. Reversely, M_eff and W decrease with the increasing Γ_h k l when A_R < 1. This means that M_eff and W on (1 1 1) plane have the minimums.     

Conductivity of a periodic two-component system of rhombic type

The conductivity and the distribution of electric field, current, and charge density in a periodic two-component system composed of rhombs with an arbitrary vertex angle of 2α are investigated. The effective conductivity of such a medium is represented by a tensor with components σ _eff ¹¹ (α) and σ _eff ²² (α) in the principal axes that satisfy the Dykhne relation σ _eff ¹¹ (α) σ _eff ²² (α)=σ ₁σ₂, where σ₁, σ₂ are the isotropic conductivities of media 1 and 2. In addition, the relation σ _eff ²² (α)=σ _eff ¹¹ (π/2?α) is satisfied. The principal axes are directed along the diagonals of the rhombs. It is shown that there are three lines in the rectangle 0<α ≤π/2,?1<Z<1((Z=σ₁?σ₂)/(σ ₁+σ₂)) on which the charge density is expressed in terms elliptic functions. An explicit expression is obtained for all physical quantities on these lines.     

Frequency and amplitude alterations of sound modes in a space-dependent random mass density field

Abstract

We investigate the effect of a space-dependent random mass density field on small amplitude acoustic modes that are settled in a semi-infinite medium of a temperature growing linearly with depth. Using a perturbation method, the dispersion relation is derived in the form of Hill's determinant. Numerical solutions of this equation lead to the following conclusions: (a) a weak random field (with σ_eff?=?0.05) essentially affects long waves which experience attenuation and a frequency reduction; (b) for a stronger random field (with σ_eff?=?0.1), high-order sound modes behave as sound waves as they are attenuated and their frequencies are increased; (c) for a sufficiently strong random field (with σ_eff?=?0.2), mode coupling occurs, as a result of which the dispersive curves cross each other, the sound modes loose their identities, and some modes are amplified. Here σ_eff denotes the effective strength of a random field.     

Features of electrical conductivity in an incommensurate phase of graphite intercalation compound C10HNO3

Electrical conductivity and conduction-electron spin resonance (CESR) have been studied in stage-2 acceptor α-graphite-nitric acid intercalation compound C₁₀HNO₃. It is found that the electrical conductivity σ_c along the c axis in the structurally incommensurate phase of this compound is temperature independent, whereas the electrical conductivity σ_a along carbon layers exhibits “metallic” temperature behavior. Analysis of the temperature dependences of σ_c, σ_a, and the CESR linewidth demonstrates that, in the incommensurate phase of the graphite intercalation compound, the electrical conductivity along the c axis is realized through a nonband mechanism—the transfer of free charge carriers along thin high-conductivity channels shunting the carbon layers adjacent to the intercalate.     

Diffusing wave spectroscopy of dense colloids: Liquid,crystal and glassy states

Using intensity autocorrelation of multiply scattered light, we show that the increase in interparticle interaction in dense, binary colloidal fluid mixtures of particle diameters 0.115μm and 0.089μm results in freezing into a crystalline phase at volume fractionφ of 0.1 and into a glassy state atφ=0.2. The functional form of the field autocorrelation functiong ⁽¹⁾(t) for the binary fluid phase is fitted to exp[−γ(6k ₀ ² D _eff t)^1/2] wherek ₀ is the magnitude of the incident light wavevector andγ is a parameter inversely proportional to the photon transport mean free pathl*. TheD _eff is thel* weighted average of the individual diffusion coefficients of the pure species. Thel* used in calculatingD _eff was computed using the Mie theory. In the solid (crystal or glass) phase, theg ⁽¹⁾(t) is fitted (only with a moderate success) to exp[−γ(6k ₀ ² W(t))^1/2] where the mean-squared displacementW(t) is evaluated for a harmonically bound overdamped Brownian oscillator. It is found that the fitted parameterγ for both the binary and monodisperse suspensions decreases significantly with the increase of interparticle interactions. This has been justified by showing that the calculated values ofl* in a monodisperse suspension using Mie theory increase very significantly with the interactions incorporated inl* via the static structure factor.     

Coupled matter-wave solitons in optical lattices

We make use of a potential model to study the dynamics of two coupled matter-wave or Bose-Einstein condensate (BEC) solitons loaded in optical lattices. With separate attention to linear and nonlinear lattices we find some remarkable differences for response of the system to effects of these lattices. As opposed to the case of linear optical lattice (LOL), the nonlinear lattice (NOL) can be used to control the mutual interaction between the two solitons. For a given lattice wave number k, the effective potentials in which the two solitons move are such that the well (V_eff(NOL)), resulting from the juxtaposition of soliton interaction and nonlinear lattice potential, is deeper than the corresponding well V_eff(LOL). But these effective potentials have opposite k dependence in the sense that the depth of V_eff(LOL) increases as k increases and that of V_eff(NOL) decreases for higher k values. We verify that the effectiveness of optical lattices to regulate the motion of the coupled solitons depends sensitively on the initial locations of the motionless solitons as well as values of the lattice wave number. For both LOL and NOL the two solitons meet each other due to mutual interaction if their initial locations are taken within the potential wells with the difference that the solitons in the NOL approach each other rather rapidly and take roughly half the time to meet as compared with the time needed for such coalescence in the LOL. In the NOL, the soliton profiles can move freely and respond to the lattice periodicity when the separation between their initial locations are as twice as that needed for a similar free movement in the LOL. We observe that, in both cases, slow tuning of the optical lattices by varying k with respect to a time parameter τ drags the oscillatory solitons apart to take them to different locations. In our potential model the oscillatory solitons appear to propagate undistorted. But a fully numerical calculation indicates that during evolution they exhibit decay and revival.     

Effective carrier lifetime and base-widening in shortp + nn +-diodes

The paper shows the charge storage behavior of short psn-silicon diodes under the condition of quasi-stationary equilibrium. Recombination in such diodes in the region of high current injection is characterized by an effective lifetime τ _F which may be determined simply by measuring the chargeQ _s stored in the forward biased diode. For short base widthsw _B the recombination in the weaklyn-doped middle region is negligibly small in relation to that in the high-doped contacts. The effective lifetime of the charge carriers injected into the middle region is, except for an additive quantity, identical with the mean transit timeT $$\tau _F = T + T_0 .$$ The injected charge carriers travel without recombination during a transit timeT through the base widthw, given by the doping profile, and then recombine in the high-doped contacts. A parameterw ₀ interpreted as ‘base-widening’ leads in a simplified concept to an effective storage regionw _eff =w+w ₀. The parameterw ₀ can be associated in the experiment with an additional storage effect in then ⁺-substrate. The storage effects in all three layers of the psn-structure are discussed theoretically.     

Carrier collection in a semiconductor quantum well

Data are presented showing that a GaAs quantum well, sandwiched between two epitaxial Al_xGa_1-xAs(x ～ 0.4) confining layers, loses its effectiveness as a collector of excess carriers and as a source of recombination radiation for well dimensions L_z < 100 Å. It is shown that this behavior is expected because of the difficulty in scattering carriers to the bottom of the quantum well as L_z → l_p, the path length for scattering (LO phonon).     

A DFT study the solvent effects of H2 adsorption on Cu(h k l) surface

Density functional theory (DFT) combined with conductor-like solvent model (COSMO) have been performed to study the solvent effects of H₂ adsorption on Cu(h k l) surface. The result shows H₂ can not be parallel adsorbed on Cu(h k l) surface in gas phase and only vertical adsorbed. At this moment, the binding energies are small and H₂ orientation with respect to Cu(h k l) surfaces is not a determining parameter. In liquid paraffin, when H₂ adsorbs vertically on Cu(h k l) surface, solvent effects not only influences the adsorptive stability, but also improves the ability of H₂ activation; When H₂ vertical adsorption on Cu(h k l) surface at 1/4 and 1/2 coverage, H-H bond is broken by solvent effects. However, no stable structures at 3/4 and 1 ML coverage are found, indicating that it is impossible to get H₂ parallel adsorption on Cu(h k l) surfaces at 3/4 and 1 ML coverages due to the repulsion between adsorbed H₂ molecules.     

Measurements of effective delayed neutron fraction in a fast neutron reactor using the perturbation method

A perturbation method is proposed to obtain the effective delayed neutron fraction β_(eff) of a cylindrical highly enriched uranium reactor.Based on reactivity measurements with and without a sample at a specified position using the positive period technique,the reactor reactivity perturbation △ρ of the sample in β_(eff) units is measured.Simulations of the perturbation experiments are performed using the MCNP program.The PERT card is used to provide the difference dκ of effective neutron multiplication factors with and without the sample inside the reactor.Based on the relationship between the effective multiplication factor and the reactivity,the equation β~(eff)=dκ/△ρ is derived.In this paper,the reactivity perturbations of 13 metal samples at the designable position of the reactor are measured and calculated.The average β_(eff) value of the reactor is given as 0.00645,and the standard uncertainty is 3.0%.Additionally,the perturbation experiments for β_(eff) can be used to evaluate the reliabilities of the delayed neutron parameters.This work shows that the delayed neutron data of ~(235)U and ~(238)U from G.R.Keepin's publication are more reliable than those from ENDF-B6.0.ENDF-B7.0,JENDL3.3 and CENDL2.2.     

Remarks on compact spacelike hypersurfaces in de Sitter space with constant higher order mean curvature

It is shown that a compact spacelike hypersurface which is contained in the chronological future (or past) of an equator of de Sitter space is a totally umbilical round sphere if one of the mean curvatures H_l does not vanish and the ratio H_k/H_l is constant for some k, l, 1≤l<k≤n. This extends the previous result in [J. Geom. Phys. 31 (1999) 195, Theorem 7].     

Temperature dependence of the radiation-stimulated conductivity of CsI crystals upon picosecond excitation by electron beams

The temperature dependence of the pulse conductivity for CsI crystals upon excitation with an electron beam (0.2 MeV, 50 ps, 400 A/cm²) at a time resolution of 150 ps is investigated. Under experimental conditions, the time of bimolecular recombination of electrons and holes (V _k centers) is directly measured in the temperature range 100–300 K. This made it possible to calculate the temperature dependence of the effective recombination cross section S(T)=7.9×10^?8 T² cm². The temperature dependence of the conductivity σ(T) is interpreted within the model of the separation of genetically bound electron-hole pairs. The activation energy of this process is found to be E _G =0.07 eV.     

Quasiparticle recombination and 2Δ-phonon-trapping in superconducting tunneling junctions

The experimental recombination lifetime τ_eff of quasiparticles in superconducting films in general exceeds the intrinsic recombination lifetime τ _R by phonon trapping. On the basis of geometric acoustic propagation and reabsorption of phonons emitted in quasiparticle recombination, τ_eff is calculated as a function of film thicknessd taking into account longitudinal and transverse phonon reabsorption, bulk loss processes and acoustical phonon transmission into the substrate. With increasing thicknessd three characteristic ranges are found: range 1 with film thicknessd small compared to the phonon reabsorption mean free path Λ _w range 2 withd larger than Λ _w and dominating boundary losses, and range 3, also withd larger than Λ _w but with dominating bulk losses. For very smalld the relation between τ_eff and τ _R , the intrinsic recombination lifetime, contains only the limiting angle of total reflection of phonons within the superconducting film. Therefore, τ _R can be directly obtained by τ _eff measurements and from the sound velocities of the film-substrate system. Range 2 is characterized by a linear dependence of τ _eff ond. In this range it is not possible to obtain τ _R from τ _eff measurements, however, τ _eff allows a determination of the phonon boundary transmission. Range 3 shows no thickness dependence of τ _eff ond in the limit of larged values. In this range a further method for obtaining τ _R from τ _eff values is suggested.     

Low-frequency dispersion of the effective transverse conductivity in inhomogeneous media in strong magnetic field

On crystalline silicon specimens with a nonuniform carrier concentration distribution produced by an optical method, a dispersion of the effective transverse conductivity σ _⊥ ^eff (ω) is observed near the frequency ω≈ω_c=τ _⊥ ^?1 ≡ε/4πσ _⊥ ^eff . At ω<ω_c, an anomalous transverse effective conductivity is observed: σ _⊥ ^eff (ω) is greater than the transverse conductivity of a homogeneous specimen σ _⊥ ^h (ω) (in the frequency range studied in the experiment σ _⊥ ^h (ω) = const). Near ω≈ω_c, the conductivity σ _⊥ ^eff decreases, and, at ω>ω_c it coincides with σ _⊥ ^h .     

Capture cross-section and density of deep gap states in a−SiHx schottky barrier structures

Recombination of charge carriers in a?SiH_x Schottky barriers with density of states near mid-gap ranging from 2.8×10¹⁵?7×10¹⁶cm^-1eV^-1 is attributed to recombination centers with hole capture cross-section of 1.3×10^-15cm².     

The equivalent vector boson approximation for intermediate mass Higgs boson production

We consider the equivalent vector boson approximation (EVBA) for the processqq→qqWW→qqH andqq→qqZZ→qqH. It is shown that the contributions σT and σL, of the transversely and longitudinally polarized vector bosonsV respectively, are comparable with each other for intermediate values ofm _H viz. 0.3 TeV to 0.6 TeV. σT can be as large as 1.5–2 times σL in this mass range. As a result the leading EVBA for σ=σL+σT overestimates the exact total cross-section by a factor, upto 2–3 even, at high energies. σT is negligible with respect to the leading contribution σL only form _H≧0.6 TeV, where EVBA is correct within 15–20%. Further the effect of the corrections to EVBA, which are naively of nonleading order, on these conclusions in the framework of the EVBA is discussed.     

Stimulated Raman scattering of an extraordinary mode in a solid state plasma

This paper presents an investigation of stimulated Raman scattering of an extraordinary mode in a solid state plasma, n In Sb. As the pump wave (w₀, k₀) propagates in the semiconductor the electrons acquire an oscillatory drift velocity and the magnetic field of the pump interacts with a low frequency perturbation (w_l, k_l) to give rise to high frequency side bands (w_l ± w₀, k_l ± k₀). The side band (w_l − w₀, k_l − k₀) interacts with the pump to produce a low frequency ponderomotive force responsible for driving the original density perturbation. The expressions for the growth rate and threshold for the instability have been obtained. For typical plasma parameters of n In Sb and laser radiation of frequency 1.778 × 10¹⁴s⁻¹, the growth rate turns out to be ~ 10¹¹s⁻¹ for the scattering angle θ = 0°. The growth rate is found to reduce with increasing values of scattering angle. A magnetic field enhances the growth rate and tends to reduce the threshold for the instability. The present investigation may be used to obtain useful information about the nature of elementary excitations in solid state plasmas, and the estimate of the growth rate may help in diagnostics and in the characterization of semiconductors.     

Fe deep level optical spectroscopy in InP

The optical cross-section σ_n⁰(hv) and σ_p⁰(hv) associated with the (Fe³⁺ ? Fe²⁺) deep level have been measured by Deep Level Optical Spectroscopy in n-type Fe doped samples of InP. Optical transitions are interpreted as transitions from the Fe²⁺ ground state to the Γ and L point minima of the conduction band for σ_n⁰(hv) and from the valence band to the ground and excited state for Fe²⁺ for σ_p(hv). A theoretical model which accounts for the main features of the experimental data is proposed.