Showing papers on "Debye published in 2005"

Diffuse mismatch model of thermal boundary conductance using exact phonon dispersion

Abstract: The acoustic mismatch model (AMM) and the diffuse mismatch model (DMM) have been traditionally used to calculate the thermal boundary conductance of interfaces. In these calculations, the phonon dispersion relationship is usually approximated by a linear relationship (Debye approximation). This is accurate for wave vectors close to the zone center, but deviates significantly for wave vectors near the zone edges. Here, we present DMM calculations of the thermal conductance of Al–Si, Al–Ge, Cu–Si, and Cu–Ge interfaces by taking into account the full phonon dispersion relationship over the entire Brillouin zone obtained using the Born-von Karman model (BKM). The thermal boundary conductance thus calculated deviates significantly from DMM predictions obtained using the Debye model in all cases.

Electric modulus approach to the analysis of electric relaxation in highly conducting (Na 0.75 Bi 0.25 )(Mn 0.25 Nb 0.75 )O 3 ceramics

Abstract: Broadband dielectric spectroscopy is applied to investigate the electrical properties of disordered perovskite-like ceramics in a wide temperature range. From the x-ray diffraction analysis it was found that the newly obtained (Na0.75Bi0.25) (Mn0.25Nb0.75)O3 ceramics consist of two chemically different phases. The major perovskite one has an orthorhombic structure described by the Pbcm space group (No 57, in yxz setting). The minor phase shows an orthorhombic symmetry, all-face-centred lattice F, with the lattice parameters a = 10.797(4) A, b = 7.601(3) A and c = 7.691(3) A. The electric modulus M* formalism used in the analysis enabled us to distinguish and separate the relaxation processes, dominated by marked conductivity in the e*(ω) representation. In the ceramics studied, the relaxation times are thermally activated and the dipole process has a clearly non-Debye behaviour. The relaxation process described with the use of the activation energy of approximately 0.4 eV and the characteristic relaxation time, τ0 = 1 × 10−11 s, was found to be related to oxygen vacancies. The low frequency relaxation shows Debye behaviour with a slightly lower activation energy and a longer characteristic time.

Diffusion-Controlled Electron Transfer Processes and Power-Law Statistics of Fluorescence Intermittency of Nanoparticles

Abstract: A mechanism involving diffusion-controlled electron transfer processes in Debye and non-Debye dielectric media is proposed to elucidate the power-law distribution for the lifetime of a blinking quantum dot. This model leads to two complementary regimes of power law with a sum of the exponents equal to 2, and to a specific value for the exponent in terms of a distribution of the diffusion correlation times. It also links the exponential bending tail with energetic and kinetic parameters.

Energy landscape and rigidity.

Abstract: The effects of floppy modes in the thermodynamical properties of a system are studied From thermodynamical arguments, we deduce that floppy modes are not at zero frequency and thus a modified Debye model is used to take into account this effect The model predicts a deviation from the Debye law at low temperatures Then, the connection between the topography of the energy landscape, the topology of the phase space, and the rigidity of a glass is explored As a result, we relate the number of constraints and floppy modes to the statistics of the landscape We apply these ideas to a simple model for which we provide an approximate expression for the number of energy basins as a function of the rigidity This helps to understand certain features of the glass transition, like the jump in the specific heat or the reversible window observed in chalcogenide glasses

Analytical theories of transport in concentrated electrolyte solutions from the MSA.

Abstract: Ion transport coefficients in electrolyte solutions (e.g., diffusion coefficients or electric conductivity) have been a subject of extensive studies for a long time. Whereas in the pioneering works of Debye, Huckel, and Onsager the ions were entirely characterized by their charge, recent theories allow specific effects of the ions (such as the ion size dependence or the pair association) to be obtained, both from simulation and from analytical theories. Such an approach, based on a combination of dynamic theories (Smoluchowski equation and mode-coupling theory) and of the mean spherical approximation (MSA) for the equilibrium pair correlation, is presented here. The various predicted equilibrium (osmotic pressure and activity coefficients) and transport coefficients (mutual diffusion, electric conductivity, self-diffusion, and transport numbers) are in good agreement with the experimental values up to high concentrations (1−2 mol L-1). Simple analytical expressions are obtained, and for practical use, the...

Electron affinity of the hydrogen atom and a resonance state of the hydrogen negative ion embedded in Debye plasmas

Abstract: The 2s 21 S e autoionization resonance state of the hydrogen negative ion embedded in Debye plasmas is determined by calculating the density of resonance states using the stabilization method. The electron affinity of the hydrogen atom is also estimated for various Debye lengths.A screened Coulomb potential obtained from the Debye model is used to represent the interaction between the charged particles. A correlated wave function consisting of a generalized exponential expansion has been used to represent the correlation effect between the three charged particles. The screening effect is taken care of forallpairsofthechargedparticles.Thecalculatedresonanceenergiesandwidths for various Debye parameters ranging from infinity to a small value along with the electron affinity are reported.

Debye type dielectric relaxation and the glass transition of alcohols.

Abstract: Many hydrogen-bonded liquids, especially glass-forming cases, display a dielectric relaxation behavior that differs qualitatively from that of other simple liquids. The majority of models aimed at explaining this unusual dielectric behavior associate the prominent Debye process with structural relaxation, viscous flow, and the glass transition. We perform dielectric and calorimetric studies of glass-forming mixtures of 2-ethylhexylamine and 2-ethyl-1-hexanol across the entire composition range. The kinetic glass transition temperature derived from the large dielectric Debye peak decreases, whereas that of the much smaller and asymmetrically broadened peak increases upon addition of amine. Only the latter feature coincides with the calorimetric glass transition results, implying that molecular structure and dielectric polarization fluctuate on time scales that can differ by orders of magitude in many hydrogen-bonding liquids.

Photons in gapless color-flavor-locked quark matter

Abstract: We calculate the Debye and Meissner masses of a gauge boson in a material consisting of two species of massless fermions that form a condensate of Cooper pairs. We perform the calculation as a function of temperature, for the cases of neutral Cooper pairs and charged Cooper pairs, and for a range of parameters including gapped quaisparticles, and ungapped quasiparticles with both quadratic and linear dispersion relations at low energy. Our results are relevant to the behavior of photons and gluons in the gapless color-flavor-locked phase of quark matter. We find that the photon's Meissner mass vanishes, and the Debye mass shows a non-monotonic temperature dependence, and at temperatures of order the pairing gap it drops to a minimum value of order sqrt(alpha) times the quark chemical potential. We confirm previous claims that at zero temperature an imaginary Meissner mass can arise from a charged gapless condensate, and we find that at finite temperature this can also occur for a gapped condensate.

Tokamak turbulence computations on closed and open magnetic flux surfaces

Abstract: We study turbulence on closed and open flux surfaces in a comparative manner using the three-dimensional electromagnetic gyrofluid turbulence code GEM. A magnetic field on a tokamak is doubly periodic and sheared. This leads to the so-called field line connection, which ensures a finite parallel response for every degree of freedom. In contrast, in the scrape-off layer (SOL), the field lines end on plates, breaking this constraint and allowing the existence of convective cell modes. Since the parallel electron response provides a path to dissipation, whether or not it is allowed to vanish is important. For the SOL case, a standard Debye sheath model is used to provide the parallel boundary conditions. A zero loss model (no fluxes into the plates) is also used to assess the importance of the Debye currents. Turbulence on closed and open flux surfaces at the same parameters is found to be very different, a property which basic transport models should take into account.

Photons in gapless colour-flavour-locked quark matter

Abstract: We calculate the Debye and Meissner masses of a gauge boson in a material consisting of two species of massless fermions that form a condensate of Cooper pairs. We perform the calculation as a function of temperature, for the cases of neutral Cooper pairs and charged Cooper pairs, and for a range of parameters including gapped quasiparticles, and ungapped quasiparticles with both quadratic and linear dispersion relations at low energy. Our results are relevant to the behaviour of photons and gluons in the gapless colour–flavour-locked phase of quark matter. We find that the photon's Meissner mass vanishes, and the Debye mass shows a non-monotonic temperature dependence, and at temperatures of order the pairing gap it drops to a minimum value of order times the quark chemical potential. We confirm previous claims that at zero temperature an imaginary Meissner mass can arise from a charged gapless condensate, and we find that at finite temperature this can also occur for a gapped condensate.

Identification of dielectric and structural relaxations in glass-forming secondary amides

Abstract: Dielectric relaxation dynamics of secondary amides is explored in their supercooled state near the glass transition temperature Tg by investigating N-ethylacetamide and its mixtures with N-methylformamide. All the samples are found to exhibit giant dielectric permittivities, reaching over 500 in N-methylformamide-rich mixtures around Tg. For both the neat and binary systems, the predominant relaxation peak is of the Debye-type throughout the viscous regime, which is an unexpected feature for a glass former with intermediate fragility. The present results combined with the earlier reported high-temperature data reveal that the dielectric strength delta epsilon(D) of the Debye relaxation extrapolates to zero at frequencies of 10(10)-10(11) Hz, which is about two orders of magnitude lower than the phonon frequency limit typical of the structural relaxation. This Debye process is remarkably similar to the dielectric behavior of many monohydroxy alcohols, which implies a common nature of purely exponential relaxation dynamics in these liquids. Based on the dielectric properties, we conclude that the Debye relaxation in the secondary amides is not a direct signature of the primary or alpha-relaxation, the latter being obscured at low temperatures due to the relatively low permittivity and close spectral proximity to the Debye peak. As in the case of monohydroxy alcohols, dielectric polarization and structure fluctuate on different time scales in secondary amides. The Kirkwood-Frohlich correlation factors for Debye-type liquids are also discussed.

Electronic structure of atoms in laser plasmas: a Debye shielding approach

Abstract: Spectral properties of the low-lying singlet states of a helium atom and those of the low-lying doublet states of a lithium atom in laser plasmas are calculated using a quantum chemical configuration interaction method with a Debye shielding model Hamiltonian. A large spherical Gaussian basis set is adopted to describe the wavefunctions of electrons bound in a non-Coulombic Yukawa-type potential with high accuracy. The relative energies and the first ionization potential are found to decrease as the shielding parameter ? increases and the number of bound states to decrease gradually as ? increases. The oscillator strengths for the lowest dipole-allowed (2p)1P?(1s)1S transition of He and the second lowest (3p)2P?(2s)2S transitions of Li are shown to decrease as ? increases and the lowest (2p)2P?(2s)2S transition of Li to increase.

Movement and Alignment of Microtubules in Electric Fields and Electric-Dipole-Moment Estimates

Abstract: The effects of both constant (up to 2 × 103 V/m) and high-frequency alternating fields (up to 2.1 × 105 V/m, 200 kHz to 2 MHz) on suspended microtubules are investigated. At pH 6.8 and 120 mM ionic strength, constant fields cause a motion of microtubules toward the anode. The electrophoretic mobility amounts to 2.6 × 10−4 cm2/Vs, reflecting a negative net charge of approximately 0.2 elementary charges per tubulin dimer. The moving microtubules are randomly space oriented. Alternating high-frequency fields induce electric dipoles and align the microtubules parallel to the field direction. By determining the angular velocity of the turning microtubules, we estimate a dipole moment of roughly 34,000 Debye at 2.1 × 105 V/m and 2 MHz. By comparing the potential energy of the dipole in the applied field with the thermal energy of microtubules, we obtain a minimum value of 6,000 Debye as necessary for an efficient alignment.

Diluent effects on the Debye-type dielectric relaxation in viscous monohydroxy alcohols.

Abstract: With the recognition that the Debye-type dielectric relaxation of liquid monohydroxy alcohols does not reflect the structural relaxation dynamics associated with the viscous flow and the glass transition, its behavior upon dilution is expected to differ from that of real alpha-processes. We have investigated the Debye-type dielectric relaxation of binary alcohol/alkane mixtures across the entire concentration range in the supercooled regimes. The focus is on 2-ethyl-1-hexanol in two nonpolar liquids, 3-methylpentane and squalane, which are more fluid and more viscous than the alcohol, respectively. The Debye relaxation is found to occur only for alcohol mole fractions x > 0.2 and is always accompanied by a non-Debye relaxation originating from the alcohol component. Prior to its complete disappearance, the Debye relaxation is subject to broadening. We observe that the Debye dynamics of 2-ethyl-1-hexanol is accelerated in the more fluid 3-methylpentane, while the more viscous squalane leads to longer Debye relaxation times. The present experiments also provide evidence that the breakdown of the Debye relaxation amplitude does not imply the absence of hydrogen-bonded structures.

Doubly excited 2 s 2 p P o 1 , 3 resonance states of helium in dense plasmas

Abstract: We have made an investigation on the 2s2p {sup 1,3}P{sup o} resonance states of helium embedded in dense plasma environments. A screened Coulomb potential obtained from the Debye model is used to represent the interaction between the charge particles. A correlated wave function consisting of a generalized exponential expansion has been used to represent the correlation effect. Resonance energies and widths for the doubly excited He embedded in plasmas with various Debye lengths are determined using the stabilization method by calculating the density of resonance states. The resonance energies and widths for various Debye parameters ranging from infinity to a small value for the lowest {sup 1,3}P{sup o} resonance states are reported.

Stark shifts and widths of a hydrogen atom in Debye plasmas

Abstract: A computational scheme has been developed and used to investigate the influence of the plasma environments on modified atomic autoionization for isolated atoms/ions by using the complex coordinate rotation method which is proved to be a very simple and powerful tool to analyze the position and the width of a resonance. The Debye screening potential is employed to describe the effects of the plasma environments. Stark shifts and widths on the ground state of hydrogen are reported for field strength up to F=0.12a.u. Slater-type basis wave functions are used to describe the system and angular-momentum states up to L=11 are included when the external electric field is turned on. Converged results are obtained by using different maximum angular-momentum states. The modified autoionization for various Debye lengths ranging from infinite to a small value of 0.86 are reported. It has been observed that for a given temperature and under the influence of a given external electric field, the resonance energy and the...

Ideal mixing behavior of the debye process in supercooled monohydroxy alcohols.

Abstract: Glass-forming monohydroxy alcohols exhibit two dielectric relaxation signals with super-Arrhenius temperature dependence: a Debye peak and an asymmetrically broadened α-process. We explore the beh...

S-wave resonances in the positron–hydrogen system with screened Coulomb potentials

Abstract: We have made a first calculation for S-wave resonances in positron–hydrogen scattering under the influence of Debye plasma environments. A screened Coulomb potential obtained from the Debye model is used to represent the interaction between the nucleus, the electron and the positron. Hylleraas-type wavefunctions are used to represent the correlation effect between the charge particles. The stabilization method is used to calculate the density of resonance states, from which resonance energies and widths are deduced. We have found two resonances of which one is associated with the hydrogen n = 2 threshold, and the other one is found lying below the Ps(2S) threshold. The resonance parameters (energies and widths) calculated for various Debye lengths are reported.

The efficiency of electrokinetic pumping at a condition of maximum work.

Abstract: Numerical methods are employed to examine the work, electric power input, and efficiency of electrokinetic pumps at a condition corresponding to maximum pump work. These analyses employ the full Poisson-Boltzmann equations and account for both convective and conductive electric currents, including surface conductance. We find that efficiencies at this condition of maximum work depend on three dimensionless parameters, the normalized zeta potential, normalized Debye layer thickness, and a fluid property termed the Levine number indicating the nominal ratio of convective to conductive electric currents. Efficiencies at maximum work exhibit a maximum for an optimum Debye layer thickness when the zeta potential and Levine number are fixed. This maximum efficiency increases with the square of the zeta potential when the zeta potential is small, but reaches a plateau as the zeta potential becomes large. The maximum efficiency in this latter regime is thus independent of the zeta potential and depends only on the Levine number. Simple analytical expressions describing this maximum efficiency in terms of the Levine number are provided. Geometries of a circular tube and planar channel are examined.

Electrohydrodynamics of binary electrolytes driven by modulated surface potentials.

Abstract: We study the electrohydrodynamics of the Debye screening layer that arises in an aqueous binary solution near a planar insulating wall when applying a spatially modulated ac voltage. Combining this with first order perturbation theory we establish the governing equations for the full nonequilibrium problem and obtain analytic solutions in the bulk for the pressure and velocity fields of the electrolyte and for the electric potential. We find good agreement between the numerics of the full problem and the analytics of the linear theory. Our work provides the theoretical foundations of circuit models discussed in the literature. The nonequilibrium approach also reveals unexpected high-frequency dynamics not predicted by circuit models.

Localized parallel electric fields associated with inertial Alfvén waves

Abstract: Measurements of intense, localized parallel electric fields are reported in association with inertial Alfven waves and accelerated electron fluxes in a space plasma. The parallel electric fields are localized to several hundreds of Debye lengths and carry potentials on the order of the energy of the accelerated electron fluxes. The structures are observed at sharp maxima or minima of the perturbation magnetic field of the Alfven wave, indicating high shear or rapid changes in the parallel current but near-zero, large-scale parallel currents. The localized electric-field structures are not entirely consistent with published double-layer models or observations in the aurora and may represent a new class of parallel electric-field structures or double layer, which are an important feature of the nonlinear evolution of Alfven waves and of the electron acceleration processes. These data also represent a possible example of cross-scale coupling from Alfven wavelengths to Debye scales.

On electro-kinetic fluids: one dimensional configurations

Abstract: Electro-kinetic fluids can be modeled by hydrodynamic systems describing the coupling between fluids and electric charges. The system consists of a momentum equation together with transport equations of charges. In the dynamics, the special coupling between the Lorentz force in the velocity equation and the material transport in the charge equation gives an energy dissipation law. In stationary situations, the system reduces to a Poisson-Boltzmann type of equation. In particular, under the no flux boundary conditions, the conservation of the total charge densities gives nonlocal integral terms in the equation. In this paper, we analyze the qualitative properties of solutions to such an equation, especially when the Debye constant $\epsilon$ approaches zero. Explicit properties can be derived for the one dimensional case while some may be generalized to higher dimensions. We also present some numerical simulation results of the system.

On the difference in scattering behavior of cyclic and linear polymers in bulk.

Abstract: It has been suggested that, due to topological constraints, rings in the melt may assume a more compact shape than Gaussian chains. In this paper, we exploit the availability of narrow fractions of perdeuterated linear and cyclic polydimethylsiloxane sPDMSd and, through the analysis of the small angle neutron scattering sSANSd profiles, demonstrate the difference in scattering properties of linear and cyclic PDMS molecules. As expected for Gaussian chains, for the H/D linear PDMS samples, log-log plots of the scattered intensity versus scattering vector Q display a Q s˛2d dependence. However, for H/D cyclic blends, the scaling exponent is higher than 2, as predicted by computer simulations reported in the literature. We show that cyclic molecules in bulk display the characteristic maximum in plots of scattered intensity versus Q s˛2d that is expected on the basis of Monte Carlo calculations and from the Casassa equation fE. F. Casassa, J. Polym. Sci. A 3, 605 s1965dg. It is also shown that, for rings, the Debye equation fP. Debye, J. Appl. Phys. 15, 338 s1944dg is no longer appropriate to describe the SANS profiles of H/D cyclic blends, at least up to Mw< 10 000. For these samples, the Casassa form factor gives a better representation of the SANS data and we show that this function which was developed for monodisperse cyclics is still adequate to describe our slightly polydisperse samples. Deviations from all above observations are noted for Mw. 11 000 and are attributed to partial contamination of cyclic samples with linear chains. The failure of both the Debye and the Casassa form factors could be due to contamination of the cyclic fractions by linear polymers or to a real conformational change. © 2005 American Institute of Physics. fDOI: 10.1063/1.1849162g

Method and apparatus for ion mobility spectrometry with alignment of dipole direction (IMS-ADD)

Abstract: Techniques and instrumentation are described for analyses of substances, including complex samples/mixtures that require separation prior to characterization of individual components. A method is disclosed for separation of ion mixtures and identification of ions, including protein and other macromolecular ions and their different structural isomers. Analyte ions are not free to rotate during the separation, but are substantially oriented with respect to the drift direction. Alignment is achieved by applying, at a particular angle to the drift field, a much stronger alternating electric field that 'locks' the ion dipoles with moments exceeding a certain value^ That value depends on the buffer gas composition, pressure, and temperature, but may be as low as ~ 3 Debye under certain conditions. The presently disclosed method measures the direction- specific cross-sections that provide the structural information complementing that obtained from known methods, and, when coupled to those methods, increases the total peak capacity and specificity of gas-phase separations. Simultaneous 2-D separations by direction-specific cross sections along and orthogonally to the ion dipole direction are also possible.

The characteristic electron-phonon coupling time of unconventional superconductors and implications for optical detectors

Abstract: Superconductors are highly suitable materials for radiation detection. Several detector types have been proposed, with properties of fast detection or high wavelength resolution over a wide range of optical frequencies. Their performances depend on the relaxation processes involving phonons, quasiparticles and Cooper pairs occurring during the energy cascade following the absorption of the radiation in the superconductor. The energy down-conversion processes are related to the electron–phonon scattering strength λe−ph which is usually expressed in terms of the electron–phonon coupling time, τ0, which is characteristic for each material. In this paper we estimate the value of τ0 for several classes of superconducting materials not yet investigated. It is calculated in the framework of the McMillan model for the superconducting critical temperature Tc within the Debye approximation. The values obtained for τ0 are discussed as regards new possibilities for unexplored materials in the field of superconducting detectors. In particular, we focus our attention on materials with τ0 values that could play a significant role in both hot electron photodetectors and superconducting tunnel junction devices.

From quantum chemistry and the classical theory of polar liquids to continuum approximations in molecular mechanics calculations

Abstract: Biological macromolecules and other polymers belong to the class of mesoscopic systems, with characteristic length scale of the order of a nanometer. Although microscopic models would be the preferred choice in theoretical calculations, their use in computer simulations becomes prohibitive for large systems or long simulation times. On the other hand, the use of purely macroscopic models in the mesoscopic domain may introduce artifacts, with effects that are difficult to assess and that may compromise the reliability of the calculations. Here is proposed an approach with the aim of minimizing the empirical nature of continuum approximations of solvent effects within the scope of molecular mechanics (MM) approximations in mesoscopic systems. Using quantum chemical methods, the potential generated by the molecular electron density is first decomposed in a multicenter-multipole expansion around predetermined centers. The monopole and dipole terms of the expansion at each site create electric fields that polarize the surrounding aqueous medium whose dielectric properties can be described by the classical theory of polar liquids. Debye's theory allows a derivation of the dielectric profiles created around isolated point charges and dipoles that can incorporate Onsager reaction field corrections. A superposition of screened Coulomb potentials obtained from this theory makes possible a simple derivation of a formal expression for the total electrostatic energy and the polar component of the solvation energy of the system. A discussion is presented on the physical meaning of the model parameters, their transferability, and their convergence to calculable quantities in the limit of simple systems. The performance of this continuum approximation in computer calculations of amino acids in the context of an atomistic force field is discussed. Applications of a continuum model based on screened Coulomb potentials in multinanosecond simulations of peptides and proteins are briefly reviewed. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Thermally Developing Electro-osmotic Convection in Microchannels with Finite Debye-Layer Thickness

Abstract: Thermally developing electro-osmotically generated flow with in circular microtubes with finite Debye-layer thickness has been analyzed. This study focuses on finite Debye-layer effects, a scenario for which the velocity distribution across the tube cross section varies with the ratio of tube radius and Debye length (termed here the relative microtube radius). Numerical solution of the hydrodynamically developed, thermally developing transport for such a flow is presented in this article. The effect of variations in the relative microtube radius and strength of the Joule and viscous heating on the thermal transport are explored over the possible ranges of the governing parameters.

Reorientation of the H2O cage studied by terahertz time-domain spectroscopy

Abstract: The terahertz absorption spectrum of a thin film of liquid H2O of ∼14μm thickness showed a clear absorption mode at 1.56THz(∼53cm−1). This mode is attributed to the bending motion of the intermolecular hydrogen-bond coordinate in a water cage. The dielectric relaxation times of a slow τ1∼9ps and a fast τ2∼0.2ps component (0.4–2.0THz) were determined using a double Debye dielectric model.