Value Of Prefactor Research Articles

Temperature activated conductivity of Ge2Sb2Te5: connection to the variation of Fermi level and implications on resistance drift

The non-Arrhenius behaviour of Ge2Sb2Te5 conductivity is attributed to the non-linear temperature dependence of the Fermi level, assuming extended state conduction by free holes. The temperature-dependent Fermi level is shown to alter values of activation energy and prefactor for conductivity so that their exact determination becomes impossible using the conductivity data alone. However, if one assumes the temperature dependence of the Fermi level to be a parabolic function. Then, two of three model parameters can be retrieved; to obtain the third parameter one has to make an assumption about the value of prefactor for conductivity. Applying the proposed analysis scheme to Ge2Sb2Te5, reasonable values of Fermi level position are obtained and the change of Fermi level as a result of resistance drift phenomenon is demonstrated.

Morphology of soot sampled from N2-diluted methane/air counterflow flames at elevated pressures via TEM imaging

An experimental work is carried out to investigate the influence of pressure on morphological parameters of soot in counterflow flames of N2-diluted methane and air. Flames are stabilized at 3, 5, 7 and 10 atm in a pressure vessel and a global strain rate of 30 s−1 is maintained at all pressures by adjusting the inlet mass flux. The mole fraction of methane is maintained at 0.7. The entire soot zone of the counterflow flames are sampled using a thermophoretic sampling device attached to the pressure vessel. Our sampling method minimizes the flow disturbances to a level that they are visually negligible during the sampling process. Collected samples are analyzed under a transmission electron microscope and information about mean primary particle diameter, fractal dimension (Df), fractal prefactor (kf) and aggregate size distribution is inferred at different pressures. To investigate the effects of carbon flux on primary particle size, fuel mole fraction is decreased to 0.5 and primary particle size is investigated at 5, 7 and 10 atm. Mean primary particle size increases by 70% when pressure is changed from 5 to 10 atm and remains independent of fuel mole fractions. Geometric mean and geometric width of aggregate size distributions also increase by increasing the pressure. Fractal properties of soot aggregates are found to be insensitive to the pressure. Fractal dimension varies between 1.56 and 1.65 while fractal prefactor values range between 1.96 and 2.1.

Understanding the Binding of Aromatic Hydrocarbons on Rutile TiO2(110)

The adsorption of cyclohexane, benzene, and alkyl-substituted benzene derivatives is studied on rutile TiO2(110) by a combination of molecular beam dosing, temperature-programmed desorption, and density functional theory (DFT). An inversion analysis is used to extract the coverage-dependent desorption energies from TiO2(110). The values of the suitable prefactors are derived from simple statistical mechanical models assuming different limits in the adsorbate mobility on the surface. The prefactor values determined using the vibrational frequencies from DFT calculations corroborate this analysis and show that the adsorbates are mobile in one or two dimensions on a corrugated TiO2(110) surface. The adsorption of benzene derivatives is found to be dominated by the dative Lewis acid–base interactions of the π system with the surface Ti ions. While the desorption energy generally increases with increasing the length and the number of substituents, the difference between the desorption energies decreases as the...

A charge-transfer resistance model and Arrhenius activation analysis for hydrogen ion transmission across single-layer graphene

Transmission rates for protons and deuterons across single-layer graphene embedded in Nafion | graphene | Nafion sandwich structures are measured as a function of temperature in electrochemical hydrogen pump cells. Rates of ion transmission through graphene are obtained in the form of area-normalized ion-transfer resistances, and are interpreted in terms of ion-exchange current densities and standard heterogeneous ion-transfer rate constants. An encounter pre-equilibrium model for the ion-transfer step is then used to provide rate constants for the fundamental microscopic step of ion (proton or deuteron) transmission across graphene. Application of this rate model to interpret variable-temperature data on proton and deuteron transmission rates provides values for the activation energy and pre-exponential factor for the fundamental ion transmission step across graphene. Activation energies obtained from the Arrhenius plots for proton and deuteron transmission are as follows; for proton, Eact = 48 ± 2 kJ/mole (0.50 ± 0.02 eV) and for deuteron, Eact = 53 ± 5 kJ/mole (0.55 ± 0.05 eV). The difference between these two values of approximately 5 kJ/mole is in good agreement with the expected difference in vibrational zero-point energies for OH and O-D bonds, albeit with some uncertainty given the uncertainties in the activation energy values. Pre-exponential frequency factor values of 8.3 ± 0.4 × 1013 s−1 and is 4.7 ± 0.5 × 1013 s−1 were obtained for proton and deuteron transmission respectively across graphene. These pre-factor values are both quite large, on the order of the values predicted from the Eyring – Polanyi equation with a transmission coefficient near one. The ratio of 1.8 for the rate pre-factors (H/D) is in reasonable agreement with the value of 1.3 for the ratio of bond vibrational frequencies for OH and O-D stretching, respectively. Taken together, these data support a model in which proton and deuteron transmission across graphene are largely adiabatic processes for which the differences in transmission rate at room temperature are due largely to differences in activation energies.

Determination of fractal dimension and prefactor of agglomerates with irregular structure

Agglomerates are often composed of amorphous and irregular primary particles, especially in the food industry. The spatial morphology of this kind of soft agglomerate, here maltodextrin, can be quantified by fractal dimension. Previous research in this regard was focused on simulated agglomerates or studied 2D projected images of real agglomerates. In this work 3D volume images of agglomerates are generated with the help of X-ray computed tomography. Primary particles are distinguished and separated by means of a sequence of image processing steps. Thus center coordinates and volume of each particle are extracted. Based on this information, the radius of gyration is calculated and compared for either monodisperse or polydisperse primary particles. The primary particles comprising the maltodextrin agglomerates follow a broad size distribution, hence considering the polydispersity is highly recommended. Next, radii of primary particles are determined in order to calculate 3D fractal dimension and prefactor from power law equation. Due to the irregular shape of primary particles, two different ways of calculating primary particle radius are investigated. It is observed that differences in primary particle radius affect the partial overlapping of particles which mostly influences the prefactor value, while only slight changes are noticed in the fractal dimension. Further, the gyration radius and fractal dimension are obtained directly from voxel data. Though voxel based method is more accurate, it requires more effort and time. Therefore, by considering some error in the values of fractal dimension and gyration radius, the separated polydisperse primary particle model is suggested as a proper option. Finally, fractal dimension is also calculated by the box counting method. The proper implementation of this method for 3D structures is discussed and the results are compared with the classical power law function.

Weak Antilocalization in Granular Sb2Te3 Thin Films Deposited by MOCVD

Topological insulator (TI) properties of granular Sb2Te3 thin films, grown by MOCVD at room temperature on 4″ SiO2/Si substrates, are investigated. The negative magneto‐conductance (MC), analyzed in terms of the Hikami–Larkin–Nagaoka theory, reveals the presence of a clear weak antilocalization (WAL) effect, thus demonstrating the TI character of our system. The extracted WAL prefactor (α) values are explained in terms of the film conductance, in agreement with the existing models for WAL in transport disordered systems. The dephasing length () dependence on temperature is affected by the grain size of the film and might in turn explain the observed temperature dependence of α and of the conductance. These results demonstrate the feasibility of technologically‐relevant processes to synthesize TIs films, at the same time providing a tool to investigate the effects of the film microstructure on the topological character of their transport properties.

Bouncing universe of entropy-corrected Friedmann equations

In this paper, we investigate the possibility of obtaining bouncing-oscillating solution in modified Friedmann equations with logarithmic entropy corrected, S=frac{A}{4G}+alpha ln frac{A}{4G}+beta frac{4G}{A}, for positive, negative and zero values of (alpha ,beta ) pre-factors and all kinds of curved universes. The results are argued using the dynamical system techniques and by employing the phase plane analysis for full classification of the nonsingular evolutions. Our analysis indicates that it is possible to have an oscillating universe as well as a bounce universe for k=1 and k=- 1 curvatures. In k=1 case, both positive and negative values of alpha and beta can make bouncing-oscillating solution, while in k=- 1 case, only the positive value of alpha with negative value of beta can make a bounce. Also the flat universe have no bounce solution.

Universality of finite-size corrections to geometrical entanglement in one-dimensional quantum critical systems

Recently, the finite-size corrections to the geometrical entanglement per lattice site in the spin-1/2 chain have been numerically shown to scale inversely with system size, and its prefactor b has been suggested to be possibly universal [Q-Q. Shi et al., New J. Phys. 12, 025008 (2010)]. As possible evidence of its universality, the numerical values of the prefactors have been confirmed analytically by using the Affleck-Ludwig boundary entropy with a Neumann boundary condition for a free compactified field [J-M. Stephan et al., Phys. Rev. B 82, 180406(R) (2010)]. However, the Affleck-Ludwig boundary entropy is not unique and does depend on conformally invariant boundary conditions. Here, we show that a unique Affleck-Ludwig boundary entropy corresponding to a finitesize correction to the geometrical entanglement per lattice site exists and show that the ratio of the prefactor b to the corresponding minimum groundstate degeneracy gmin for the Affleck- Ludwig boundary entropy is a constant for any critical region of the spin-1 XXZ system with the single-ion anisotropy, i.e., b/(2 log2 g min ) = −1. Previously studied spin-1/2 systems, including the quantum three-state Potts model, have verified the universal ratio. Hence, the inverse finite-size correction to the geometrical entanglement per lattice site and its prefactor b are universal for one-dimensional critical systems.

Automated determination of size and morphology information from soot transmission electron microscope (TEM)-generated images

The thermophoretic sampling of particulates from hot media, coupled with transmission electron microscope (TEM) imaging, is a combined approach that is widely used to derive morphological information. The identification and the measurement of the particulates, however, can be complex when the TEM images are of low contrast, noisy, and have non-uniform background signal level. The image processing method can also be challenging and time consuming, when the samples collected have large variability in shape and size, or have some degree of overlapping. In this work, a three-stage image processing sequence is presented to facilitate time-efficient automated identification and measurement of particulates from the TEM grids. The proposed processing sequence is first applied to soot samples that were thermophoretically sampled from a laminar non-premixed ethylene-air flame. The parameter values that are required to be set to facilitate the automated process are identified, and sensitivity of the results to these parameters is assessed. The same analysis process is also applied to soot samples that were acquired from an externally irradiated laminar non-premixed ethylene-air flame, which have different geometrical characteristics, to assess the morphological dependence of the proposed image processing sequence. Using the optimized parameter values, statistical assessments of the automated results reveal that the largest discrepancies that are associated with the estimated values of primary particle diameter, fractal dimension, and prefactor values of the aggregates for the tested cases, are approximately 3, 1, and 10 %, respectively, when compared with the manual measurements.

Adsorption energies and prefactor determination for CH3OH adsorption on graphite

In this paper, we have studied adsorption and thermal desorption of methanol CH3OH on graphite surface, with the specific aim to derive from experimental data quantitative parameters that govern the desorption, namely, adsorption energy Eads and prefactor ν of the Polanyi-Wigner law. In low coverage regime, these two values are interconnected and usually the experiments can be reproduced with any couple (Eads, ν), which makes intercomparison between studies difficult since the results depend on the extraction method. Here, we use a method for determining independently the average adsorption energy and a prefactor value that works over a large range of incident methanol coverage, from a limited set of desorption curves performed at different heating rates. In the low coverage regime the procedure is based on a first order kinetic law, and considers an adsorption energy distribution which is not expected to vary with the applied heating rate. In the case of CH3OH multilayers, Eads is determined as 430 meV with a prefactor of 5 × 10(14) s(-1). For CH3OH submonolayers on graphite, adsorption energy of 470 ± 30 meV and a prefactor of (8 ± 3) × 10(16) s(-1) have been found. These last values, which do not change between 0.09 ML and 1 ML initial coverage, suggest that the methanol molecules form island-like structure on the graphite even at low coverage.

Electromagnetic coupling of spins and pseudospins in bilayer graphene

We present a detailed theoretical study of bilayer-graphene's electronic properties in the presence of electric and magnetic fields. Using group-theoretical methods, we derive an invariant expansion of the Hamiltonian for electron states near the K point of the Brillouin zone. In contrast to known materials, including single-layer graphene, any possible coupling of physical quantities to components of the external electric (magnetic) field has a counterpart where the analogous component of the magnetic (electric) field couples to exactly the same combination of quantities. For example, a purely electric spin splitting appears as the magneto-electric analogue of the familiar magnetic Zeeman spin splitting. The measurable thermodynamic response induced by magnetic and electric fields is thus completely symmetric. The Pauli magnetization induced by a magnetic field takes exactly the same functional form as the polarization induced by an electric field. Our findings thus reveal unconventional behavior of spin and pseudospin degrees of freedom in their coupling to external fields. We explain how these counterintuitive couplings are consistent with fundamental principles such as time reversal symmetry. For example, only a magnetic field can give rise to a macroscopic spin polarization, whereas only a perpendicular electric field can induce a macroscopic polarization of the sublattice-related pseudospin degree of freedom characterizing the intravalley orbital motion in bilayer graphene. These rules enforced by symmetry for the matter-field interactions clarify the nature of spins versus pseudospins. We also provide numerical values of prefactors for relevant coupling terms. While our theoretical arguments use bilayer graphene as an example, they are generally valid for any material with similar symmetries.

Influence of the prefactor to defect motion in α-Iron during long time scale simulations

We present a study of the influence of the prefactor in the Arrhenius equation for the long time scale motion of defects in α-Fe. It is shown that calculated prefactors vary widely between different defect types and it is thus important to determine these accurately when implementing on-the-fly kinetic Monte Carlo (otf-KMC) simulations. The results were verified by reproducing many events using Molecular Dynamics (MD) and Temperature-Accelerated Dynamics (TAD). The calculated prefactor was shown to increase the relative interstitial-vacancy diffusion rates by an order of magnitude compared to the assumption of a constant prefactor value and the ordering of the rate table for the interstitial defect migration mechanisms was also changed. In addition, low prefactor values were observed for the 4 interstitial dumbbells configuration with low barrier transitions.

Universal scaling of the formation factor in porous media derived by combining percolation and effective medium theories

The porosity dependence of the formation factor for geologic media is examined from the perspective of universal scaling laws from percolation and effective medium theories. Over much of the range of observed porosity, the expected percolation scaling is observed, but the values of the numerical prefactor do not conform to the simple predictions from percolation theory. Combining effective medium and percolation theories produces a numerical prefactor whose value depends on both the threshold porosity and the porosity above which the formation factor crosses from percolation to effective medium scaling. This change allows extraction of a numerical value of the prefactor, which is reasonably close to experimental values. Subsequent evaluation of the porosity dependence of the formation factor shows that difficulties in prior comparisons of theory and experiment are largely removed when percolation scaling is allowed to transition to effective medium scaling far above the percolation threshold.

Temperature dependence of current density and admittance in metal-insulator-semiconductor junctions with molecular insulator

Electrical transport in ultrathin Metal-insulator-semiconductor (MIS) tunnel junctions is analyzed using the temperature dependence of current density and admittance characteristics, as illustrated by Hg//C12H25 – n Si junctions incorporating n-alkyl molecular layers (1.45 nm thick) covalently bonded to Si(111). The voltage partition is obtained from J(V, T) characteristics, over eight decades in current. In the low forward bias regime (0.2–0.4 V) governed by thermionic emission, the observed linear T-dependence of the effective barrier height, qΦEFF(T)=qΦB+(kT)β0dT, provides the tunnel barrier attenuation, exp(-β0dT), with β0= 0.93 Å−1 and the thermionic emission barrier height, ΦB=0.53 eV. In the high-forward-bias regime (0.5–2.0 V), the bias dependence of the tunnel barrier transparency, approximated by a modified Simmons model for a rectangular tunnel barrier, provides the tunnel barrier height, ΦT=0.5 eV; the fitted prefactor value, G0 = 10−10 Ω−1, is four decades smaller than the theoretical Simmons prefactor for MIM structures. The density distribution of defects localized at the C12H25 – n Si interface is deduced from admittance data (low-high frequency method) and from a simulation of the response time τR(V) using Gomila’s model for a non equilibrium tunnel junction. The low density of electrically active defects near mid-gap (DS < 2 × 1011 eV−1.cm−2) indicates a good passivation of dangling bonds at the dodecyl – n Si (111) interface.

Direct Quantification of the Attempt Frequency Determining the Mechanical Unfolding of Ubiquitin Protein

Understanding protein dynamics requires a comprehensive knowledge of the underlying potential energy surface that governs the motion of each individual protein molecule. Single molecule mechanical studies have provided the unprecedented opportunity to study the individual unfolding pathways along a well defined coordinate, the end-to-end length of the protein. In these experiments, unfolding requires surmounting an energy barrier that separates the native from the extended state. The calculation of the absolute value of the barrier height has traditionally relied on the assumption of an attempt frequency, υ(‡). Here we used single molecule force-clamp spectroscopy to directly determine the value of υ(‡) for mechanical unfolding by measuring the unfolding rate of the small protein ubiquitin at varying temperatures. Our experiments demonstrate a significant effect of the temperature on the mechanical rate of unfolding. By extrapolating the unfolding rate in the absence of force for different temperatures, varying within the range spanning from 5 to 45 °C, we measured a value for the activation barrier of ΔG(‡) = 71 ± 5 kJ/mol and an exponential prefactor υ(‡) ∼4 × 10(9) s(-1). Although the measured prefactor value is 3 orders of magnitude smaller than the value predicted by the transition state theory (∼6 × 10(12) s(-1)), it is 400-fold higher than that encountered in analogous experiments studying the effect of temperature on the reactivity of a protein-embedded disulfide bond (∼10(7) M(-1) s(-1)). This approach will allow quantitative characterization of the complete energy landscape of a folding polypeptide from highly extended states, of capital importance for proteins with elastic function.

Impaired protein conformational landscapes as revealed in anomalous Arrhenius prefactors

A growing body of data supports a role for protein motion in enzyme catalysis. In particular, the ability of enzymes to sample catalytically relevant conformational substates has been invoked to model kinetic and spectroscopic data. However, direct experimental links between rapidly interconverting conformations and the chemical steps of catalysis remain rare. We report here on the kinetic analysis and characterization of the hydride transfer step catalyzed by a series of mutant thermophilic alcohol dehydrogenases (ht-ADH), presenting evidence for Arrhenius prefactor values that become enormously elevated above an expected value of approximately 10(13)s(-1) when the enzyme operates below its optimal temperature range. Restoration of normal Arrhenius behavior in the ht-ADH reaction occurs at elevated temperatures. A simple model, in which reduced temperature alters the ability of the ht-ADH variants to sample the catalytically relevant region of conformational space, can reproduce the available data. These findings indicate an impaired landscape that has been generated by the combined condition of reduced temperature and mutation at a single, active-site hydrophobic side chain. The broader implication is that optimal enzyme function requires the maintenance of a relatively smooth landscape that minimizes low energy traps.

Charge transport studies in thermally evaporated 2,2′,7,7′-tetrakis-( N, N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-MeOTAD) thin film

Charge transport mechanism in 2,2′,7,7′-tetrakis-( N, N-di-4-methoxyphenyl amino)-9,9′-spirobifluorene (spiro-MeOTAD) has been investigated as a function of temperature and organic layer thicknesses. Hole only devices of different thicknesses were fabricated in configuration ITO/spiro-MeOTAD/Au by vacuum evaporation technique. The hole current is space charge limited which provides a direct measurement of the hole mobility μ as a function of electric field and temperature. Gaussian disorder model has been used to explain the temperature and field dependent behavior of mobility. The values of energetic disorder ( σ = 0.088 eV), positional disorder ( Σ = 3.35) and mobility prefactor ( μ 0 = 0.0147 cm 2/V s) have been evaluated using this model.

Radar/rain‐gauge comparisons on squall lines in Niamey, Niger for the AMMA

AbstractMassachusetts Institute of Technology C‐band radar observations are integrated with rainfall measurements from an extensive network of gauges in Niamey, Niger, West Africa, for the African Monsoon and Multidisciplinary Analysis (AMMA). The large number of gauges available enabled Ze–R power‐law relationships for the convective and stratiform regions of individual squall lines. The Ze–R relationships based solely on radar measurements directly over the gauges were developed for the estimate of rainfall and attendant latent heat release (by other AMMA investigators) where gauges were unavailable. The low prefactor values of the Ze–R power laws relative to like values for Z–R disdrometer power laws have contributions of order 1–2 dB from the use of the lowest beam tilt (0.57° ) and ∼1–2 dB by the radar reading low. (The sphere calibration and the Tropical Rainfall Measuring Mission TRMM—radar calibration are inconsistent at the 1–2 dB level for unknown reasons.) Radar/gauge comparisons are also shown for individual storms. Accurate, unbiased results for the convective regime require adjustment of the radar‐to‐gauge radials for attenuation. Beam filling problems and aliasing issues can often be identified in the case of outlier points. Copyright © 2010 Royal Meteorological Society

Rheological investigation of soy protein hydrogels induced by Maillard-type reaction

Rheological measurements were used to determine the power law relationship between gel elastic modulus and protein concentration (solid volume fraction) for soy protein hydrogels. The slope calculated from the log–log plot was used to determine gel fractal dimension. The validity of the prefactor in the power law relationship was also studied. Soy protein at 6.5–9.5% was gelled using monofunctional (glyceraldehyde) or bifunctional (glutaraldehyde) Maillard cross-linkers with or without salt. Gel fractal dimension did not vary with cross-linker type or concentration, while prefactor values increased with cross-linker concentration and when salt was used. The invariability of the fractal dimension is believed related to the absence of effect of the cross-linking agents on protein ionic repulsions, while prefactor value is believed inversely proportional to the number of inter-floc particle–particle interactions in the three-dimensional network and to floc size. The prefactor value was thus shown to increase as cross-linking degree increased and gel lacunarity decreased.

ESR spectra modeling for ordered and fractal aggregates of metal nanoparticles

ESR spectra are calculated for aggregates of mono-and polydisperse particles designed by special algorithm and characterized by preset values of fractal dimensionality and prefactor. The dependence of ESR spectrum on the aggregate size, fractal dimensionality, external field (for anisotropic aggregates), and character of large and small particles packing in the aggregate in the case of polydisperse system is demonstrated.