Showing papers on "Debye published in 1997"

Investigation of the temperature dependence of dielectric relaxation in liquid water by thz reflection spectroscopy and molecular dynamics simulation

Abstract: We report measurements of the real and imaginary part of the dielectric constant of liquid water in the far-infrared region from 0.1 to 2.0 THz in a temperature range from 271.1 to 366.7 K. The data have been obtained with the use of THz time domain reflection spectroscopy, utilizing ultrashort electromagnetic pulses generated from a photoconductive antenna driven by femtosecond laser pulses. A Debye model with an additional relaxation time is used to fit the frequency dependence of the complex dielectric constants. We obtain a fast (fs) and a Debye (ps) relaxation time for the macroscopic polarization. The corresponding time correlation functions have been calculated with molecular dynamics simulations and are compared with experimental relaxation times. The temperature dependence of the Debye relaxation time is analyzed using three models: Transition state theory, a Debye–Stoke–Einstein relation between the viscosity and the Debye time, and a model stating that its temperature dependence can be extrapolated from a singularity of liquid water at 228 K. We find an excellent agreement between experiment and the two latter models. The simulations, however, present results with too large statistical error for establishing a relation for the temperature dependence.

A quantum model for water: Equilibrium and dynamical properties

Abstract: Path integral molecular dynamics and centroid molecular dynamics have been applied to study and modify an empirical flexible model for water, the simple point charge/flexible (SPC/F) model. The quantum structural, thermodynamic and dynamical properties have been calculated and compared to their classical counterparts, as well as to experiment. The path integral molecular dynamics simulations demonstrate that the quantum liquid is less structured and exhibits less hydrogen bonding than its classical analog. Quantization also leads to a lower dielectric constant, relative to the corresponding classical value. Centroid molecular dynamics has been used to calculate single molecule time correlation functions, the Debye dielectric relaxation correlation function, and the power spectrum for the quantum model. These time correlation functions decay more rapidly than the classical ones, indicating that nuclear rotational tunneling occurs in the liquid. The power spectrum of the quantized liquid also exhibits red s...

Dielectric Dispersion Measurements of CdSe Nanocrystal Colloids: Observation of a Permanent Dipole Moment

Abstract: We measure the dielectric dispersion of CdSe nanocrystal colloids and show the existence of large dipole moments of 25 and 47 debye for 34 and 46 \AA{} diameter nanocrystals, respectively. The magnitude is consistent with the expected spontaneous polarization of the bulk wurtzite CdSe lattice and implies a potential drop of $\ensuremath{\approx}0.25\mathrm{V}$ across the nanocrystal. This effect, which is intrinsic to the wurtzite structure but has been largely overlooked, should be incorporated in the description of the quantum confined electronic states.

Wake potential of a dust grain in a plasma with ion flow

Abstract: Debye screening potential and wake potential for a moving dust grain in a collisionless plasma with ion flow is studied. When a relative velocity of the dust grain exceeds the ion acoustic velocity, the oscillatory wake potential is formed in a circular cone behind the particle and produces potential minima in a periodic manner. The ion acoustic collective effects on dust particles contribute to the formation of the periodic structure. The characteristic spacing between the potential minima are several times of Debye wavelength in height and in radius. Such a periodic structure may be relevant to the formation of Coulomb quasilattices (plasma crystals) observed in the dusty plasma laboratory experiments.

Qualitative properties of steady-state Poisson-Nernst-Planck systems: perturbation and simulation study

Abstract: Poisson--Nernst--Planck (PNP) systems are considered in the case of vanishing permanent charge. A detailed case study, based on natural categories described by system boundary conditions and flux, is carried out via simulation and singular perturbation analysis. Our results confirm the rich structure inherent in these systems. A natural quantity, the quotient of the Debye and characteristic length scales, serves as the singular perturbation parameter. The regions of validity are carefully analyzed by critical comparisons and contrasts between the simulation and the perturbation solution, which can be represented in closed form.

Anharmonic correlated einstein-model debye-waller factors

Abstract: An anharmonic correlated Einstein model is derived for local vibrational amplitudes in x-ray-absorption fine structure ~XAFS! that takes into account all near neighbors of absorber and backscattering atoms. The model is based on quantum thermodynamic perturbation theory and includes anharmonic effects based on empirical potentials. Calculations are presented for the second and third cumulants in XAFS as well as the net thermal expansion and thermal expansion coefficient. This model avoids full lattice dynamical calculations yet provides reasonable agreement with experiment. The generalization to displacement-displacement correlation functions and multiple-scattering Debye-Waller factors is also discussed. @S0163-1829~97!03125-1#

Nonperturbative Debye Mass in Finite Temperature QCD

Abstract: QCD matter, a spatially and temporally extended sys- tem of matter described by the laws of Quantum Chro- modynamics, goes at high temperatures into a quark- gluon plasma phase, in which color is no more confined and chiral symmetry is restored An essential quantity, describing coherent static interactions in the plasma, is the inverse screening length of color electric fields, the Debye mass mD The Debye mass enters in many essen- tial characteristics of static properties of the plasma Its numerical value is important for phenomenological dis- cussions of formation of the quark-gluon plasma, for the analysis of J/� andsuppression in heavy ion colli- sions, for the computation of parton equilibration rates, etc (see, eg (1)) The definition and computation of the Debye mass for abelian QED plasma is well understood (2) The electro- magnetic current jµ is a gauge-invariant quantity, and the Debye mass can be extracted from the 2-point gauge invariant correlation function of j0 in the plasma There are no massless charged particles in QED, which allows an infrared-safe perturbative computation of the Debye mass in powers of the electromagnetic coupling e This has been done to order e 5 (3) The situation in QCD is much more complicated First, the corresponding cur- rent in QCD, j a , is not a gauge invariant quantity Sec- ond, there are massless charged gluons which give rise to infrared divergences and prevent the perturbative deter- mination of the Debye mass beyond leading order A non-perturbative gauge invariant definition of the Debye mass in vectorlike theories with zero chemical po- tential was suggested in (4) According to it, mD can be defined from the large distance exponential fall-off of correlators of gauge-invariant time-reflection odd opera- tors O, h O(τ, ~ x)O(τ,0)i ∼ C|~

Frequency Dependent Electrorheological Properties: Origin and Bounds

Abstract: We present a unified framework for the first-principles calculation of the frequency dependent shear modulus, static yield stress, and structures of dielectric electrorheological systems. It is shown that a strong (applied field) frequency dependence of the static yield stress, in good quantitative agreement with those measured experimentally, can arise from Debye relaxational effects that are typical of poor insulators. Physical upper bounds on the yield stress and the shear modulus, as well as frequencyinduced structural soft modes, are predicted. [S0031-9007(96)01165-9] PACS numbers: 61.90.+ d, 41.20.Cv, 62.20.‐x Electrorheological (ER) fluids are a class of materials whose rheological characteristics are controllable through the application of an electric field. In this work, we consider a particular type of ER fluids, the dielectric electrorheological (DER) systems, defined as colloidal dispersions of dielectric particles in which the electrical response of both the solid and the liquid components is governed by linear electrostatics. Besides being a topic of general theoretical interest in itself, the DER model has been widely invoked to explain the various aspects of the ER phenomenon, such as the mechanism of chain formation [1], the solid structure under an electric field [2], and the widely observed quadratic field dependence of the yield stress [3]. In spite of these successes, however, serious gaps still exist. Among them are the lack of quantitative understanding for the observed (applied electric field) frequency and conductivity dependencies of the yield stress, and the question concerning the upper bounds of ER shear modulus and yield stress. In the absence of a first-principles account for those issues, the gaps in our understanding are the source of much speculation about the basic mechanism of the ER effect and its potential limitation(s).

Effective Dielectric Properties of Solvent Mixtures at Microwave Frequencies

Abstract: We investigated the dielectric properties of various organic solvents and binary solvent mixtures at 21.4 °C over the frequency range of 200 MHz−13.5 GHz. These solvent mixturesnitrobenzene−N,N-dimethylformamide, 1-butanol−formamide, nitrobenzene−toluene, ethanol−1-butanol, and nitrobenzene−chlorobenzeneas well as the pure components display a Debye or near Debye dispersion. Their frequency-dependent dielectric properties can be summarized by three parameters in the Debye equation: a static dielectric constant, a high-frequency limiting dielectric constant, and a dielectric relaxation time constant. On the basis of electrostatics, rate theory, and solution thermodynamics, we have developed dielectric “mixing rules” that describe the frequency-dependent dielectric properties of the solvent mixtures based on solution composition and the dielectric parameters for the solution components. These “mixing rules” yield good agreement between the predicted and experimental dielectric properties for the binary sol...

The design of Maxwellian absorbers for numerical boundary conditions and for practical applications using engineered artificial materials

Abstract: A Maxwellian material interpretation of the Berenger (see J. Computat. Phys., vol.114, p.185-200, 1994) perfectly matched layer (PML) is developed using polarization and magnetization fields. The PML material is found to be a passive lossy electric and magnetic medium with particular conductivity and Debye dispersion characteristics. Although it is recognized that the PML medium is physically unrealizable, this polarization and magnetization field interpretation reveals the necessary characteristics of a perfect electromagnetic absorber. A Maxwellian material that has perfect absorption properties and may be physically realizable is derived with these concepts. This Maxwellian absorber is based upon a time-derivative Lorentz material (TD-LM) model for the dispersive and absorptive electric and magnetic properties of a material. This TD-LM model represents a straightforward generalization of the standard Lorentz material model to include the time derivatives of the fields as driving mechanisms for the polarization and magnetization fields. The numerical implementation of the perfect absorber is given and the resulting reflection coefficients from a perfect electric conductor-backed slab of this material are characterized. It is shown for broad bandwidth pulsed fields that this Maxwellian TD-LM slab, like the non-Maxwellian PML, has absorption characteristics in the 70-110-dB range for large angles of incidence. Strategies are discussed for engineering this dispersive electric and magnetic TD-LM absorber artificially with a substrate that has an array of pairs of appropriately designed small coil-loaded dipole radiating elements embedded in it.

Nonequilibrium phase transition in the kinetic Ising model: Critical slowing down and the specific-heat singularity

Abstract: The nonequilibrium dynamic phase transition, in the kinetic Ising model in the presence of an oscillating magnetic field has been studied both by Monte Carlo simulation and by solving numerically the mean-field dynamic equation of motion for the average magnetization. In both cases, the Debye ''relaxation'' behavior of the dynamic order parameter has been observed and the ''relaxation time'' is found to diverge near the dynamic transition point. The Debye relaxation of the dynamic order parameter and the power law divergence of the relaxation time have been obtained from a very approximate solution of the mean-field dynamic equation. The temperature variation of appropriately defined ''specific heat'' is studied by the Monte Carlo simulation near the transition point. The specific heat has been observed to diverge near the dynamic transition point.

Debye-series analysis of the first-order rainbow produced in scattering of a diagonally incident plane wave by a circular cylinder

Abstract: We derive the Debye-series expansion of the partial-wave scattering and interior amplitudes for the interaction of a diagonally incident beam of arbitrary profile with an infinitely long homogeneous dielectric circular cylinder. We then discuss the physical interpretation of the various terms of the series. We also consider the one-internal-reflection Debye-series terms for diagonal plane-wave incidence and examine the first-order rainbow extinction transition as a function of the tilt angle of the incident plane wave. We experimentally observe the first-order rainbow extinction transition and compare our observations with the Debye-series predictions.

Variations on a theme by Debye and Waller: from simple crystals to proteins.

Abstract: Debye and Waller showed how to adjust scattering intensities in diffraction experiments for harmonic motions of atoms about an average, static reference configuration. However, many motions, particularly in biological molecules as compared to simple crystals, are far from harmonic. We show how, using a variety of simple anharmonic, multiconformational models, it is possible to construct a variety of Generalized Debye-Waller Factors, and understand their meaning. A central result for these cases is that, in principle, intensity factors cannot be obtained from true total mean square displacements of the atoms. We make the distinction between the intensity factors for unimodal quasiharmonic motions and those for the anharmonic, multimodal (valley hopping) motions. Only the former affect the conventional B factors.

Accurate Analytic Formulae for the Far Field Effective Potential and Surface Charge Density of a Uniformly Charged Sphere

Abstract: Simple yet accurate formulae are presented for the far field effective potential and surface charge density of a uniformly charged spherical colloidal particle immersed in a symmetric z : z electrolyte. Unlike previous formulations, these results are valid for all values of κ a (where κ is the Debye screening parameter and a is the radius of the particle) and maintain good accuracy for all surface potentials of practical interest (≤200 mV).

Counterion condensation on ionic oligomers

Abstract: The Ramanathan-Woodbury formulas representing the charge density critical for the onset of counterion condensation on finite-length polymers are derived by three alternate methods, an extension of Debye-Huckel theory, a theory of end effects, and by density functional theory. For charged oligomers with length of the same order as the Debye length, the threshold for condensation is the same as for polymers of length much greater than the Debye lenght. However, the threshold depends both on length and salt concentration if the oligomer is shorter than the Debye length, in such a way as to recede to infinity as the ratio of oligomer length to Debye length tends to zero (i.e., condensation vanishes in this limit). The extended Debye-Huckel theory additionally provides a new result for the partition function of the condensed layer, showing that the free energy of the condensed counterions is different on an oligomer and a polymer, even when the fractional extent of condensation is the same. The end effect theory discloses a hitherto unnoticed connection between the number of counterions condensed at the ends of a long polymer and the number condensed on a short oligomer.

IR–microwave double resonance studies of dipole moments in the ν1 and ν3 states of ammonia

Abstract: Infrared laser–microwave double resonance spectroscopy is used to observe the Stark effect of tunneling transitions within the ν1 and ν3 hydrogen stretching states of NH3 Dipole moments for 11 J,K states of ν1 and 3 J,K states of ν3 are measured to high accuracy These data, combined with previous measurements in the ground and excited bending states, give a dipole moment function (in Debye) of: μv1v2v3v4=15610+72×10−3(v1+1/2)−2271×10−1(v2+1/2)+375×10−2(v3+1)−165×10−2(v4+1) μe=1561 D is the first experimental measurement of the NH3 equilibrium moment These results are also used to analyze and recalculate the dipole moment measured by Shimizu and co-workers in a vNH=5 excited state of NH3

Debye Shielding and Particle Correlations in Strongly Coupled Dusty Plasmas

Abstract: A particle-in-cell simulation method is shown effective in modeling strongly coupled plasmas, exhibiting good energy conservation properties and good resolution of the dust-particle interaction. For coupling parameters of order unity, the simulation dust particles exhibit Debye shielding on the spatial scale of the dust Debye length. When initialized with a large coupling parameter, the dust particles do not organize themselves into a crystalline structure as expected, but instead are scattered by the presence of substantial electrostatic wave activity. Liquid-like or crystal-like correlations among the dust particles occur only when annealing is imposed. {copyright} {ital 1997} {ital The American Physical Society}

Frequency and water content dependencies of electrorheological properties

Abstract: The static yield stress and complex dielectric constant of glass spheres dispersed in vacuum oil are measured as a function of (applied electric field) frequency and water content. Based on a model in which the dielectric constants of the solid and the liquid constituents are given by the Debye form with a log-normal distribution of relaxation times, it is shown that the electrical and the mechanical electrorheological responses can be consistently explained through first-principles calculations.

Frequency and Water Content Dependencies of Electrorheological Properties

Abstract: The static yield stress and complex dielectric constant of glass spheres dispersed in vacuum oil are measured as a function of (applied electric field) frequency and water content. Based on a model in which the dielectric constants of the solid and the liquid constituents are given by the Debye form with a log-normal distribution of relaxation times, it is shown that the electrical and the mechanical electrorheological responses can be consistently explained through first-principles calculations.

Mixing formulas in time domain

Abstract: This paper discusses the dispersive properties of dielectric materials both in the time and the frequency domains. Special emphasis is on the treatment of heterogeneous materials, in particular two-phase mixtures. A time domain Maxwell Garnett rule is derived which differs from the corresponding frequency domain formula in the respect that it is expressed in terms of convolutions and inverse operators of the susceptibility kernels of the materials. Much of the analysis deals with the question how the temporal dispersion of the dielectric response of various physical materials is affected by the mixing process. Debye, Lorentz, Drude, and modified Debye susceptibility models are treated in detail.

Correlation lengths for scattering potentials in two-dimensional electron gases

Abstract: Shubnikov - de Haas measurements have been made in a `bias-cooled' GaAs/GaAlAs heterojunction, i.e. a gated sample cooled with a negative bias voltage to suppress the formation of DX centres. It was found that the quantum lifetime was significantly smaller in the bias-cooled sample than for normal cooling and also that the field dependence of the amplitude was functionally different. This is attributed to a roughening of the scattering potential which results when correlations between the DX centres are suppressed. To explain the functional change in the field dependence it is necessary to consider the confinement effects associated with scattering potentials that have correlation lengths smaller than the Debye wavelength of the electrons. The bias cooling technique enables the characteristic length scale of the scattering potential to be changed, in the same sample, and at the same carrier density.

A new method for calculating kinetic constants within the Rayleigh - Gans - Debye approximation from turbidity measurements

Abstract: Using the Rayleigh - Gans - Debye (RGD) approximation and a von Smoluchowski kinetics, a simple theory has been developed which provides a complete fitting of the absorbance versus time curve in a homocoagulation process. Contributions from pairs of particles with zero, one, and two particles between them have been taken into account for aggregates of any size. A description of the curve in the first 20 seconds is obtained for all cases, and in general it is possible to reproduce the curve correctly for the first minute. Finally, we present a technique based on this theory for adjusting the kinetic constant in a homocoagulation process.

Focusing of electric-dipole waves in the Debye and Kirchhoff approximations

Abstract: We compare the Debye and Kirchhoff approximations for focusing of electromagnetic waves, with special emphasis on the focusing of converging electric-dipole waves. Numerical solutions for the spectral amplitude of the incident converging wave and for the focused field are provided in both approximations for a variety of Fresnel numbers and f-numbers. Comparisons of numerical solutions in Kirchhoff and Debye approximations show that for systems of Fresnel numbers larger than 20 and f-numbers less than 0.5, both approximations give indistinguishable results for focused electric-dipole waves. At low Fresnel numbers, the Kirchhoff approximation gives the best results irrespective of the f-number, as in the scalar case. The focal shift depends mainly on the Fresnel number, but at low Fresnel numbers, a reduction of the f-number gives an increase of the relative focal shift.

Dielectric Low-Frequency Dispersion and Crossover Phenomena of HCl-Doped Ice

Abstract: Experimental results for the dielectric measurement of HCl-doped ice are presented. A dielectric low-frequency dispersion was observed at a lower frequency side than the intrinsic Debye dispersion and is discussed on the basis of the temperature dependence of the relaxation time and the dielectric strength. In HCl-doped ice showing crossover phenomena between ac and dc conductivities on temperature dependence, the relaxation time of the low-frequency dispersion is constant above the crossover temperature. It increases with decreasing temperature below the crossover temperature, but HCl-doped ice does not clearly show crossover between relaxation times of the Debye and the low-frequency dispersions on temperature dependence.

Nuclear electron capture in a plasma

Abstract: We consider the electron density at the position of an ion of charge Ze in a plasma under conditions approximating those in the core of the Sun. Numerical calculations have shown that the plasma effects on the density, over and above the ordinary Coulomb factor for a single electron in the vicinity of the ion, are well represented by a reduction factor, exp (-Ze2βκD), where β is the inverse temperature and κD is the Debye wavelength. Although this factor is the direct analog of the Salpeter enhancement factor for the fusion rates in stars, the elementary considerations that establish it in the fusion case are not applicable to the determination of the electron density and the resulting electron-capture rates. We show analytically, through a sum rule that leads to a well-defined perturbative approach, that in the limit of Boltzmann statistics, the Salpeter factor indeed provides the leading correction. We estimate residual effects both from Fermi statistics and from short-range terms.

Temperature dependence of reflection high-energy electron diffraction intensity from Si(111)-7×7 superlattice

Abstract: Rocking curves of reflection high-energy electron diffraction (RHEED) were observed from Si(111)-(7×7) surface at temperature of 293, 523, and 653 K by a RHEED apparatus with a magnetic deflector to change the glancing angle. The temperature dependence of RHEED intensity was calculated with change of the real and imaginary part of the potential corresponding to the thermal diffuse scattering arising from a thermal vibration of atoms, which was obtained by the way proposed by Dudarev et al. [Surf. Sci. 330, 86 (1995)]. We assumed that the amplitude of the thermal vibration (Debye parameter) increases with temperature according to the Einstein model. By comparing with the experimental results, the atomic positions of the (7×7) structure satisfying the observed RHEED rocking curves in this temperature range is determined. The positions of the surface atoms are almost coincident with the results obtained by using ab initio molecular-dynamics scheme (Brommer et al. [Phys. Rev. Lett. 68, 1355 (1992)]) within the difference of 0.01 nm.

Vibrational Dynamics and Heat Capacity of Poly( L -lysine)

Abstract: Poly(L-lysine) is a polypeptide having a bulky hydrophobic side chain. A systematic study of the full vibrational dynamics, including phonon dispersion, for the α-helical form of this biopolymer has been reported using Wilson’s GF matrix method as modified by Higgs for an infinite polymeric chain. The calculated frequencies explain well the IR and Raman spectra. A comparison of the amide modes with other α-helical polypeptides is reported here. Spectral frequencies have been obtained for the N-deuterated species to check the validity of force field and correctness of assignments. Heat capacity has also been calculated from dispersion curves via density-of-states using Debye relation in the temperature range 50–500 K. The calculated values are found to be in close agreement with the recent experimental data of Roles et al. [Biopolymers, 33, 753 (1993)].