bluuebunny/crossref_metadata_2025 huggingface dataset:
Melting line of the Lennard-Jones system, infinite size, and full potential
Authors: Mastny Ethan A. , de Pablo Juan J.
Published: September 2007

Literature estimates of the melting curve of the Lennard-Jones system vary by as much as 10%. The origin of such discrepancies remains unclear. We present precise values for the Lennard-Jones melting temperature, and we examine possible sources of systematic errors in the prediction of melting points, including finite-size and interaction-cutoff effects. A hypothetical thermodynamic integration path is used to find the relative free energies of the solid and liquid phases, for various system sizes, at constant cutoff radius. The solid-liquid relative free energy and melting temperature scale linearly as the inverse of the number of particles, and it is shown that finite-size effects can account for deviations in the melting temperature (from the infinite-size limit) of up to 5%. An extended-ensemble density-of-states method is used to determine free energy changes for each phase as a continuous function of the cutoff radius. The resulting melting temperature predictions exhibit an oscillatory behavior as the cutoff radius is increased. Deviations in the melting temperature (from the full potential limit) arising from a finite cutoff radius are shown to be of comparable magnitude as those resulting from finite-size effects. This method is used to identify melting temperatures at five different pressures, for the infinite-size and full potential Lennard-Jones system. We use our simulation results as references to connect the Lennard-Jones solid equation of state of van der Hoef with the Lennard-Jones fluid equation of state of Johnson. Once the references are applied the two equations of state are used to identify a melting curve. An empirical equation that fits this melting curve is provided. We also report a reduced triple point temperatureTtr=0.694.
Langevin dynamics of molecules with internal rigid fragments in the harmonic regime
Authors: Essiz Sebnem G. , Coalson Rob D.
Published: September 2007

An approximation scheme is developed to compute Brownian motion according to the Langevin equation for a molecular system moving in a harmonic force field (corresponding to a quadratic potential energy surface) and characterized by one or more rigid internal fragments. This scheme, which relies on elements of the rotation translation block (RTB) method for computing vibrational normal modes of large molecules developed by Sanejouand and co-workers [Biopolymers 34, 759 (1994); Proteins: Struct., Funct., Genet. 41, 1 (2000)], provides a natural and efficient way to freeze out the small amplitude, high frequency motions within each rigid fragment. The number of dynamical degrees of freedom in the problem is thereby reduced, often dramatically. To illustrate the method, the relaxation kinetics of the small membrane-bound ion channel protein gramicidin-A, subjected to an externally imposed impulse, is computed. The results obtained from all-atom dynamics are compared to those obtained using the RTB-Langevin dynamics approximation (treating eight indole moieties as internal rigid fragments): good agreement between the two treatments is found.
Brillouin-scattering determination of the acoustic properties and their pressure dependence for three polymeric elastomers
Authors: Stevens Lewis L. , Orler E. Bruce , Dattelbaum Dana M. , Ahart Muhtar , Hemley Russell J.
Published: September 2007

The acoustic properties of three polymer elastomers, a cross-linked poly(dimethylsiloxane) (Sylgard® 184), a cross-linked terpolymer poly(ethylene-vinyl acetate-vinyl alcohol), and a segmented thermoplastic poly(ester urethane) copolymer (Estane® 5703), have been measured from ambient pressure to approximately 12GPa by using Brillouin scattering in high-pressure diamond anvil cells. The Brillouin-scattering technique is a powerful tool for aiding in the determination of equations of state for a variety of materials, but to date has not been applied to polymers at pressures exceeding a few kilobars. For the three elastomers, both transverse and longitudinal acoustic modes were observed, though the transverse modes were observed only at elevated pressures (>0.7GPa) in all cases. From the Brillouin frequency shifts, longitudinal and transverse sound speeds were calculated, as were the C11 and C12 elastic constants, bulk, shear, and Young’s moduli, and Poisson’s ratios, and their respective pressure dependencies. P-V isotherms were then constructed, and fit to several empirical/semiempirical equations of state to extract the isothermal bulk modulus and its pressure derivative for each material. Finally, the lack of shear waves observed for any polymer at ambient pressure, and the pressure dependency of their appearance is discussed with regard to instrumental and material considerations.
Ferroelectric BaTiO3 phase of orthorhombic crystal structure contained in nanoparticles
Authors: Ram S. , Jana A. , Kundu T. K.
Published: September 2007

Ferroelectric BaTiO3 phase of a Pnma orthorhombic crystal structure is synthesized from a chemical method using a polymer complex of Ba2+ and Ti4+ with polyvinyl alcohol (PVA). After burning out the polymer at temperature as high as 550 °C in air for 2 h results in an average 15 nm crystallite BaTiO3 size, with lattice parameters a=0.6435 nm, b=0.5306 nm, c=0.8854 nm, and density 5.124 g∕cm3. The relationship between dielectric constant εr and temperature showed a single Curie transition temperature TC=131 °C, with as large a εr value as 263 at TC. A low value of dissipation factor tan δ, as small as 0.033 at room temperature (frequency f=1 kHz), with good insulating properties made the sample promising for use in uncooled infrared detectors and thermal imaging applications. The εr value is nearly independent of f value up to 100 kHz. A spectrum of sharp EPR signals of g values 2.21 to 1.88 characterizes three major kinds of VBa−, VTi3−, and Ti3+−Vo2+ paramagnetic species present in the imperfections.
Energy transfer of highly vibrationally excited naphthalene. I. Translational collision energy dependence
Authors: Liu Chen-Lin , Hsu Hsu Chen , Hsu Yuan Chin , Ni Chi-Kung
Published: September 2007

Energy transfer between highly vibrationally excited naphthalene and Kr atom in a series of translational collision energies (108–847cm−1) was studied separately using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. Highly vibrationally excited naphthalene in the triplet state (vibrational energy: 16194cm−1; electronic energy: 21400cm−1) was formed via the rapid intersystem crossing of naphthalene initially excited to the S2 state by 266nm photons. The collisional energy transfer probability density functions were measured directly from the scattering results of highly vibrationally excited naphthalene. At low collision energies a short-lived naphthalene-Kr complex was observed, resulting in small amounts of translational to vibrational-rotational (T→VR) energy transfer. The complex formation probability decreases as the collision energy increases. T→VR energy transfer was found to be quite efficient at all collision energies. In some instances, nearly all of the translational energy is transferred to vibrational-rotational energy. On the other hand, only a small fraction of vibrational energy is converted to translational energy. The translational energy gained from vibrational energy extend to large energy transfer (up to 3000cm−1) as the collision energy increases to 847cm−1. Substantial amounts of large V→T energy transfer were observed in the forward and backward directions at large collision energies.
Atomiclike ionization and fragmentation of a series of CH3–X (X: H, F, Cl, Br, I, and CN) by an intense femtosecond laser
Authors: Tanaka Michinori , Murakami Masanao , Yatsuhashi Tomoyuki , Nakashima Nobuaki
Published: September 2007

Methane derivatives of CH3–X (X: H, F, Cl, Br, I, and CN) were ionized and fragmented by an intense femtosecond laser with a 40fs pulse at 0.8μm in intensities of 1013–1015Wcm−2. The curves of the ionization yields of CH3–X versus laser intensities have been found to be fitted with an atomic ionization theory (the theory of Perelomov, Popov, and Terent’ev) that has been established to reproduce experimental results well for rare gas atoms. The saturation intensities have been reproduced within a factor of 1.6 of the calculated ones. For molecules with low ionization potentials such as amines, another atomic ionization theory (the theory of Ammosov, Delone, and Krainov) reproduced the saturation intensities. The atomiclike ionization behavior of molecules indicates that the fragmentation occurs after the ionization. The fragmentation mechanisms after the ionization of some molecular ions are discussed.
Statistical approaches to the transient populations and to the macroscopic energy transfer rate for the upconversion phenomenon in monodoped amorphous solid
Authors: Hadad C. Z.
Published: September 2007

In this article, statistical approaches to the first and the second excited state transient populations and to the temporal macroscopic energy transfer rate for the upconversion process in amorphous solid generic systems monodoped with trivalent lanthanide ions are reached. The plots of the expressions show general tendencies reported in the literature. The derivation and the analysis of the formalism allowed us to fulfill our main objective, that is, to make a theoretical study about the microscopic and statistical mechanisms present in the phenomenon and their relation with the classic kinetic analysis. The study shows that the inclusion of the minimum possible radius between two optical centers in a solid affects the initial slopes of the decay curves of the luminescence from the intermediate state. We also corroborate that the usual treatment of experimental data using direct equations for the dynamics of the populations in laser pulsed excitation experiments falls in the mistake of not considering the temporality of the macroscopic energy transfer rate. Finally, physical explanations are formulated about this temporal behavior and about the main factors that generate the characteristic simple exponential decay loss of the luminescence from the intermediate state.
Electronic spectroscopy of jet-cooled HCP+: Molecular structure, phosphorus hyperfine structure, and Renner-Teller analysis
Authors: Sunahori Fumie X. , Zhang Xiaopeng , Clouthier Dennis J.
Published: September 2007

Laser-induced fluorescence spectra of jet-cooled HCP+ and DCP+ have been obtained with the pulsed discharge technique using HCP∕DCP and argon precursor mixtures. Transitions involving all of the excited state vibrations have been observed and a set of vibrational constants has been obtained. High-resolution spectra of the Π3∕22 components of the 000 bands of both isotopomers have been recorded, and these spectra show resolved phosphorus hyperfine structure which allowed the determination of the excited state Fermi contact parameter. The B values were used to obtain the ground and excited state effective geometric parameters as r0″(CH)=1.077(2)Å, r0″(CP)=1.6013(3)Å, r0′(CH)=1.082(2)Å, and r0′(CP)=1.5331(3)Å. A Renner-Teller analysis of the ground state vibrational energy levels obtained from the literature was attempted. All of the observed levels of DCP+ and the majority of those of HCP+ were satisfactorily fitted with a standard Renner-Teller model, but three HCP+ levels showed large systematic deviations which could not be accommodated by reassignments or improvements in the Fermi resonance Hamiltonian. Further improvements in the theory or in the experimental data will be needed to resolve this discrepancy.
Raman spectroscopy of intermolecular charge transfer complex between a conjugated polymer and an organic acceptor molecule
Authors: Bruevich V. V. , Makhmutov T. Sh. , Elizarov S. G. , Nechvolodova E. M. , Paraschuk D. Yu.
Published: September 2007

Intermolecular donor-acceptor charge transfer complex (CTC) formed in the electronic groundstate between poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and 2,4,7-trinitrofluorenone (TNF) has been investigated by Raman and optical absorption spectroscopies. Blending of MEH-PPV and TNF results in appearance of the CTC absorption band in the optical gap of the both components and in changes in the characteristic MEH-PPV Raman bands including shifts, change in bandwidth, and intensity. The experimental data are similar in films and solutions indicating the CTC formation in both. We associate the low-frequency shift of the strongest MEH-PPV Raman band at ∼1580cm−1 reaching 5cm−1 with partial electron transfer from MEH-PPV to TNF amounting ∼0.2e−. We suggest that polymer conjugated segments can form the CTC of variable composition MEH-PPV:TNF=1:X, where X⩽0.5 is per MEH-PPV monomer unit. Our Raman data indicate that MEH-PPV conjugated segments involved in the CTC become more planar; however, their conjugation length seemingly does not change.
Rotational dynamics of water and benzene controlled by anion field in ionic liquids: 1-butyl-3-methylimidazolium chloride and hexafluorophosphate
Authors: Yasaka Yoshiro , Wakai Chihiro , Matubayasi Nobuyuki , Nakahara Masaru
Published: September 2007

The rotational correlation time (τ2R) is determined for D2O (polar) and C6D6 (apolar) in 1-butyl-3-methylimidazolium chloride ([bmim][Cl]) and hexafluorophosphate ([bmim][PF6]) by measuring H2 (D) nuclear magnetic resonance spin-lattice relaxation time (T1) in the temperature range from −20to110°C. The τ2R ratio of water to benzene (τW∕B) was used as a measure of solute-solvent attraction. τW∕B is 0.73 and 0.52 in [bmim][Cl] and [bmim][PF6], respectively, whereas the molecular volume ratio is as small as 0.11. The slowdown of the water dynamics compared to the benzene dynamics in ionic liquids is interpreted by the Coulombic attractive interaction between the polar water molecule and the anion. As for the anion effect, the rotational dynamics of water solvated by Cl− is slower than that solvated by PF6−, whereas the rotational dynamics of benzene is similar in the two ionic liquids. This is interpreted as an indication of the stronger solvation by the anion with a larger surface charge density. The slowdown of the water dynamics via Coulombic solvation is actually significant only at water concentrations lower than ∼9moldm−3 at room temperature, and it is indistinguishable at temperatures above ∼100°C. The quadrupolar coupling constants determined for D2O and C6D6 in the ionic liquids were smaller by a factor of 2–3 than those in the pure liquid state.
Effects of the DNA state fluctuation on single-cell dynamics of self-regulating gene
Authors: Okabe Yurie , Yagi Yuu , Sasai Masaki
Published: September 2007

A dynamical mean-field theory is developed to analyze stochastic single-cell dynamics of gene expression. By explicitly taking account of nonequilibrium and nonadiabatic features of the DNA state fluctuation, two-time correlation functions and response functions of single-cell dynamics are derived. The method is applied to a self-regulating gene to predict a rich variety of dynamical phenomena such as an anomalous increase of relaxation time and oscillatory decay of correlations. The effective “temperature” defined as the ratio of the correlation to the response in the protein number is small when the DNA state change is frequent, while it grows large when the DNA state change is infrequent, indicating the strong enhancement of noise in the latter case.
Asymmetric internal solitary waves with a trapped core in deep fluids
Authors: Derzho Oleg G. , Grimshaw Roger
Published: September 2007

We describe an asymptotic model for long large-amplitude internal solitary waves with a trapped core, propagating in a narrow layer of nearly uniformly stratified fluid embedded in an infinitely deep homogeneous fluid. We consider the case of a mode one asymmetric wave with an amplitude slightly greater than the critical amplitude, for which there is incipient overturning, that is wave-breaking. We then incorporate a vortex core located near the point at which this incipient breaking occurs. The effect of the vortex core is to introduce into the governing equation for the wave amplitude an extra nonlinear term proportional to the 3∕2 power of the difference between the wave amplitude and the critical amplitude. Thus the derived new equation for the wave amplitude incorporates both the nonlinearity arising due to the flow over the recirculation core, and the nonlinearity associated with the ambient stratification; the dispersion term however remains of the Benjamin-Ono-type. We find that as the wave amplitude increases above the critical amplitude, the wave broadens, which is in marked contrast to the case of small amplitude waves where a sharpening of the wave crest normally occurs. The limiting form of the broadening wave is “a deep fluid bore.” The wave speed is found to depend nonlinearly on the wave amplitude and the traditional linear dependence underestimates this speed.
Finite Larmor radius effects on the coupled trapped electron and ion temperature gradient modes
Authors: Sandberg I. , Isliker H. , Pavlenko V. P.
Published: September 2007

The properties of the coupled trapped electron and toroidal ion temperature gradient modes are investigated using the standard reactive fluid model and taking rigorously into account the effects attributed to the ion polarization drift and to the drifts associated with the lowest-order finite ion Larmor radius effects. In the flat density regime, where the coupling between the modes is relatively weak, the properties of the unstable modes are slightly modified through these effects. For the peak density regions, where the coupling of the modes is rather strong, these second-order drifts determine the spectra of the unstable modes near the marginal conditions.
Transition on the entropic elasticity of DNA induced by intercalating molecules
Authors: Rocha M. S. , Ferreira M. C. , Mesquita O. N.
Published: September 2007

We use optical tweezers to perform stretching experiments on DNA molecules when interacting with the drugs daunomycin and ethidium bromide, which intercalate the DNA molecule. Our results show that the persistence length of the DNA-drug complexes increases strongly as the drug concentration increases up to some critical value. Above this critical value, the persistence length decreases abruptly and remains approximately constant for larger drug concentrations, at least in the concentration range used in our experiments. Measured intercalators critical concentrations for the persistence length transition coincide with the reported values for the helix-coil transition of DNA-drug complexes obtained from sedimentation experiments. The contour length of the molecules increases monotonically and saturates as the drug concentration increases. The neighbor exclusion model fits to our results for the total drug concentration as a function of the relative increase of the contour length.
Counterion-counterion correlation in the double layer around cylindrical polyions: Counterion size and valency effects
Authors: Piñero J. , Bhuiyan L. B. , Reščič J. , Vlachy V.
Published: September 2007

Monte Carlo simulation and Poisson-Boltzmann results on some aspects of structure and thermodynamics of aqueous polyelectrolyte solutions are presented. The polyelectrolyte solution is described by an infinitely long cylindrical polyion surrounded by counterions modeled as rigid ions moving in a continuum dielectric. Ion-ion correlations in the form of volume average of the counterion-counterion distribution function in the double layer surrounding the polyion are reported for mono- and divalent counterions and for a range of polyion concentrations and charge density parameters in each case. These results confirm again strong influence of the charge density parameter of polyions on properties of polyelectrolyte solutions. The structural information is supplemented by the calculated thermodynamic properties such as osmotic coefficients and heats of dilutions; the latter quantity has not been examined yet in detail by computer simulations. The results are discussed in view of the existing experimental data from the literature for these properties.
High lattice temperature effects on the ultrafast electron transport in 4H-SiC
Authors: Maia F. F. , Caetano E. W. S. , da Costa J. A. P. , Freire V. N.
Published: September 2007

High lattice temperature effects on the electron transport transient in the 4H-SiC c-parallel direction are studied within a single equivalent isotropic valley picture in the momentum and energy relaxation time approximation. The ultrafast transport regime occurs in a subpicosecond scale (<0.2ps), during which an overshoot in the electron drift velocity starts to be evident for high electric fields (≳60kV∕cm), depending on the lattice temperature. An increase of the electric field strength shifts the overshoot peak of the electron drift velocity to an earlier time. For a strong enough electric field, a higher lattice temperature cannot eliminate the electron drift overshoot effect, but can reduce it considerably due to a stronger electron-phonon scattering.
Flow-induced forces on two nearby spheres
Authors: Yoon Dong-Hyeog , Yang Kyung-Soo
Published: September 2007

Flow-induced forces on two identical nearby spheres at Re=300 were numerically studied. We consider all possible arrangements of the two spheres in terms of the distance between the spheres and the angle inclined with respect to the main flow direction. It turns out that significant changes in the characteristics of vortex shedding are noticed depending on how the two spheres are positioned, resulting in quantitative changes of force coefficients on both spheres. Collecting all the numerical results obtained, we present the diagrams for the force coefficients on the distance versus angle plane for each of the two spheres. The perfect geometrical symmetry implied in the flow configuration allows one to use those diagrams to estimate flow-induced forces on two identical spheres arbitrarily positioned in physical space with respect to the main flow direction.
Influence of the miniband on emission mechanism in Zn1−xCdxSe/ZnSe quantum wells
Authors: Jen J. Y. , Anderson J. R. , Gorska M.
Published: September 2007

The stimulated emission has been investigated in Zn1−xCdxSe/ZnSe quantum wells with a constant sample width of 60 Å and barrier widths from 47 Å to 500 Å. A redshift of the stimulated emission peak with increasing excitation intensity was observed in samples with barrier widths narrower than 350 Å, and the stimulated emission mechanism in these samples is found to be dominated by exciton-exciton scattering. In contrast, no redshifts were observed for the sample with a barrier width of 500 Å, and the stimulated emission in this sample appears to be dominated by biexcitons. Time-resolved spectroscopy measurements were also performed on samples with barrier width of 47 and 500 Å below the threshold of stimulated emission, and it was found that the exciton lifetime decreases with increasing exciton energy for both of the samples. With a ten times smaller barrier width in the 47 Å sample, the lifetime of high energy exciton is significantly shorter than that of the other sample, while the low energy lifetime becomes longer. This implies that with a narrower barrier width of the quantum wells the excitons move faster to lower energy regions. The narrower the barrier, the easier it is for excitons to move through the miniband. This freedom of movement increases the exciton concentration at low energies in localized areas in the wells. High exciton concentration made exciton-exciton scattering more probable and reduce the stimulated emission threshold value.
A new model for Overhauser enhanced nuclear magnetic resonance using nitroxide radicals
Authors: Armstrong Brandon D. , Han Songi
Published: September 2007

Nitroxide free radicals are the most commonly used source for dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) experiments and are also exclusively employed as spin labels for electron spin resonance (ESR) spectroscopy of diamagnetic molecules and materials. Nitroxide free radicals have been shown to have strong dipolar coupling to H1 in water, and thus result in large DNP enhancement of H1 NMR signal via the well known Overhauser effect. The fundamental parameter in a DNP experiment is the coupling factor, since it ultimately determines the maximum NMR signal enhancements which can be achieved. Despite their widespread use, measurements of the coupling factor of nitroxide free radicals have been inconsistent, and current models have failed to successfully explain our experimental data. We found that the inconsistency in determining the coupling factor arises from not taking into account the characteristics of the ESR transitions, which are split into three (or two) lines due to the hyperfine coupling of the electron to the N14 nuclei (or N15) of the nitric oxide radical. Both intermolecular Heisenberg spin exchange interactions as well as intramolecular nitrogen nuclear spin relaxation mix the three (or two) ESR transitions. However, neither effect has been taken into account in any experimental studies on utilizing or quantifying the Overhauser driven DNP effects. The expected effect of Heisenberg spin exchange on Overhauser enhancements has already been theoretically predicted and observed by Bates and Drozdoski [J. Chem. Phys. 67, 4038 (1977)]. Here, we present a new model for quantifying Overhauser enhancements through nitroxide free radicals that includes both effects on mixing the ESR hyperfine states. This model predicts the maximum saturation factor to be considerably higher by the effect of nitrogen nuclear spin relaxation. Because intramolecular nitrogen spin relaxation is independent of the nitroxide concentration, this effect is still significant at low radical concentrations where electron spin exchange is negligible. This implies that the only correct way to determine the coupling factor of nitroxide free radicals is to measure the maximum enhancement at different concentrations and extrapolate the results to infinite concentration. We verify our model with a series of DNP experimental studies on H1 NMR signal enhancement of water by means of N14 as well as N15 isotope enriched nitroxide radicals.
Implicit subgrid-scale modeling for large-eddy simulation of passive-scalar mixing
Authors: Hickel S. , Adams N. A. , Mansour N. N.
Published: September 2007

Further development of large-eddy simulation (LES) faces as major obstacles the strong coupling between subgrid-scale (SGS) modeling and the truncation error of the numerical discretization. One can exploit this link by developing discretization methods where the truncation error itself functions as an implicit SGS model. The name “implicit LES” is used for approaches that merge the SGS model and numerical discretization. In this paper, the implicit SGS modeling environment provided by the adaptive local deconvolution method is extended to LES of passive-scalar mixing. The resulting adaptive advection algorithm is discussed with respect to its numerical and turbulence-theoretical background. We demonstrate that the new method allows for reliable predictions of the turbulent transport of passive scalars in isotropic turbulence and in turbulent channel flow for a wide range of Schmidt numbers.
Self-written gradient double claddlike optical guiding channels of high stability
Authors: Zohrabyan Armen , Tork Amir , Birabassov Rouslan , Galstian Tigran
Published: September 2007

Composite photopolymerizable material and self-writing method are developed, allowing the creation of very stable (in time) gradient double-clad optical waveguides with W form of refractive index profile. The guide is formed by the mutual counter diffusion of reactive and nonreactive molecules followed by photopolymerization. After the guide formation by a Gaussian intensity shaped beam, the mixture is simply fixed by uniform irradiation without additional processing. More than 2cm length permanent self-guiding channels are created. The obtained results could be useful for “optical bonding” of dissimilar optical devices.
Phenomenological model for the confined dynamics in semicrystalline polymers: the multiple α relaxation in cold-crystallized poly(ethylene terephthalate)
Authors: Pieruccini M. , Ezquerra T. A. , Lanza M.
Published: September 2007

The segmental relaxation in poly(ethylene terephthalate), crystallized from either an isotropic or a cold-drawn glass, is investigated by means of dielectric spectroscopy. It is shown that there exist two distinct α relaxation modes: a slow one, characterized by a rather wide frequency interval, and a faster, much narrower one. A simple phenomenological model is developed in order to analyze the polarization autocorrelation functions ϕ(t)’s associated with these relaxation modes. The model is based on the idea that the growth of crystalline domains causes a progressive confinement of the amorphous regions where, eventually, the observed α processes take place. The mechanism of confinement is accounted for by applying to the case of constrained density fluctuations, well known concepts introduced by Adam and Gibbs [J. Chem. Phys. 43, 139 (1965)] concerning the relaxation dynamics in liquids close to the glass transition. Randomness on confining conditions is then introduced, leading to the derivation of analytical expressions which are used afterwards to fit the asymptotic behavior of the ϕ’s for long-time tails. It is found that the slow, broad α process takes place in regions where the confining effect of crystals is strong, whereas the amorphous domains relaxing via the fast mode are those where the confinement effect of crystals is weak. The analysis of the ϕ’s by means of this model allows us to relate the fitting exponents to the dispersion in the free energy associated with structural rearrangement.
Lagrangian avenues of transport in the Earth’s mantle
Authors: Schneider Judit , Schmalzl Joerg , Tél Tamás
Published: September 2007

A method of visualizing and characterizing stirring structures of high Rayleigh number geophysical flows whose time dependence is strongly aperiodic is presented. To this end, the system is leaked by defining a smaller region of the flow, so that a particle is considered to be escaped if it enters this region. By means of an ensemble of nonescaped tracers, we are able to characterize stirring and transport processes by visualizing the converging and stretching filamentations (stable and unstable manifolds) in the flow. The method indicates that the present-day Earth’s mantle is not well stirred because the time that has passed since the formation of the Earth has not been long enough for the flow of the mantle to generate efficient stirring, and observations reveal indeed the existence of reservoirs of different materials.
Effective interaction of nonuniformly charged colloid spheres in a bulk electrolyte
Authors: Lian Z. J. , Ma H. R.
Published: September 2007

The interaction energy between two like-charged colloid spheres with nonuniformly distributed surface charges immersed in a bulk electrolyte is calculated under the linearized Poisson-Boltzmann equation. The calculated results are sensitive to the relative orientations of the spheres due to the anisotropy of the distribution of charges on the sphere surface. The effective repulsive interaction after thermal average is weaker than the case when charges are uniformly distributed on the spheres, and with the increase of the nonuniformity, an attractive interaction between the two spheres emerges.
Low thermal budgetSiO2∕Si3N4∕SiO2 stacks for advanced SONOS memories
Authors: Saraf Meirav , Akhvlediani Roza , Edrei Rachel , Edelstein Ruth Shima , Roizin Yakov , Hoffman Alon
Published: September 2007

SiO 2 ∕ Si 3 N 4 ∕ SiO 2 (ONO) stacks are used as trapping media in industrial semiconductor memories. This memory device uses deep traps in the silicon oxynitride interlayer between the silicon nitride layer and the top oxide. In this research, low-thermal budget (low temperature) ONO layers were designed suitable for integration into advanced scaled down semiconductor technologies. The alternative growing procedure included oxidation of silicon nitride surface in oxygen plasma. Low temperature oxidation of the silicon nitride layer in oxygen plasma allows incorporation of oxygen into the silicon nitride layer, thus forming a silicon oxynitride layer. The chemical composition of the silicon oxynitride interlayer was similar to the oxynitride of thermally grown ONO. The plasma procedure did not cause any surface roughening. The memory devices with the low-temperature-oxygen-plasma ONO stack showed similar performance to memories with standard high temperature ONO. ONO structures were characterized chemically and structurally by the time of flight secondary ion mass spectrometry (TOF-SIMS), x-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM).
Time-dependent density functional theory based upon the fragment molecular orbital method
Authors: Chiba Mahito , Fedorov Dmitri G. , Kitaura Kazuo
Published: September 2007

Time-dependent density functional theory (TDDFT) was combined with the two-body fragment molecular orbital method (FMO2). In this FMO2-TDDFT scheme, the system is divided into fragments, and the electron density for fragments is determined self-consistently. Consequently, only one main fragment of interest and several fragment pairs including it are calculated by TDDFT. To demonstrate the accuracy of FMO2-TDDFT, we computed several low-lying singlet and triplet excited states of solvated phenol and polyalanine using our method and the standard TDDFT for the full system. The BLYP functional with the long-range correction (LC-BLYP) was employed with the 6-31G* basis set (some tests were also performed with 6-311G*, as well as with B3LYP and time-dependent Hartree-Fock). Typically, FMO2-TDDFT reproduced the full TDDFT excitation energies within 0.1eV, and for one excited state the error was about 0.2eV. Beside the accurate reproduction of the TDDFT excitation energies, we also automatically get an excitation energy decomposition analysis, which provides the contributions of individual fragments. Finally, the efficiency of our approach was exemplified on the LC-BLYP∕6-31G* calculation of the lowest singlet excitation of the photoactive yellow protein which consists of 1931 atoms, and the obtained value of 3.1eV is in agreement with the experimental value of 2.8eV.
Microstructure and defect investigations of the as-grown and annealed ZnO/Si thin films
Authors: Huang Jingyun , Lu Huanming , Ye Zhizhen , Wang Lei , Zhao Binghui , He Haiping
Published: September 2007

The microstructure and defects of as-grown and annealed ZnO/Si thin films were investigated by high-resolution transmission electron microscopy (HRTEM). The cross-sectional bright-field TEM images showed that the ZnO thin films consisted of columnar grains. The selected-area electron diffraction pattern showed that the ZnO/Si thin film were c-axis oriented and the deviation angle along the ZnO (0001) direction with respect to the growth direction was no more than 5°. The grain boundaries can be classified into three types: low-angle boundaries, boundaries near a 30 deg angle with (101¯0) facet structures and large-angle boundaries. In addition to grain boundary, the as-grown and annealing processing defects were also investigated by HRTEM. The crystal quality of the ZnO/Si thin films can be improved by annealing at 600 °C. However, at too high an annealing temperature of 800 °C, the newly produced processing defects such as three layer stacking fault and double stacking fault were formed. The research was useful for us to find optimal growth conditions to improve ZnO/Si thin film quality.
Analytical model for subthreshold conduction and threshold switching in chalcogenide-based memory devices
Authors: Ielmini Daniele , Zhang Yuegang
Published: September 2007

Chalcogenide materials are receiving increasing interest for their many applications as active materials in emerging memories, such as phase-change memories, programmable metallization cells, and cross-point devices. The great advantage of these materials is the capability to appear in two different phases, the amorphous and the crystalline phases, with rather different electrical properties. The aim of this work is to provide a physically based model for conduction in the amorphous chalcogenide material, able to predict the current-voltage (I−V) characteristics as a function of phase state, temperature, and cell geometry. First, the trap-limited transport at relatively low currents (subthreshold regime) is studied, leading to a comprehensive model for subthreshold conduction accounting for (a) the shape of the I−V characteristics, (b) the measured temperature dependence, (c) the dependence of subthreshold slope on the thickness of the amorphous phase, and (d) the voltage dependence of the activation energy. The threshold switching mechanism is then explained by the nonequilibrium population in high-mobility shallow traps at high electric field and by the nonuniform field distribution along the amorphous layer thickness. A single analytical model is then shown which is able to account for subthreshold conduction, threshold switching, negative differential resistance region, and ON regime. The model can be applied for fast yet physically based computation of the current in chalcogenide-based devices (e.g., phase change memory cells and arrays) as a function of applied voltage, temperature, and programmed state.
Onset of oscillatory convection in a porous cylinder saturated with a viscoelastic fluid
Authors: Zhang Zhiyong , Fu Ceji , Tan Wenchang , Wang C. Y.
Published: September 2007

A stability analysis determining the onset of oscillatory convection in a bounded cylindrical cavity containing a viscoelastic fluid saturated porous medium is performed for insulated sidewalls, isothermal top wall, and bottom heated by constant heat flux or constant temperature. The dependence of the critical Darcy-Rayleigh number and the preferred convective motion on both the aspect ratio of the cylinder and the viscoelastic parameters is established. The differences between bottom constant heat flux heating and bottom constant temperature heating are compared.
Cluster scale composition determination in a boron-rich compound
Authors: Jiang Jun , Yuan Jun , Bleloch Andrew
Published: September 2007

Composition metrology is an important issue for compound nanostructure and devices. By imaging the internal structure of boron clusters in a boron-rich material using the high angle annular dark field technique in an aberration-corrected scanning transmission electron microscope, the authors have been able to determine quantitatively the local composition to be B6O1−x (x=0.30±0.04) through a relative image contrast analysis with the aid of a dynamical electron scattering simulation. Fluctuation of the oxygen occupancy is spatially resolved. This high precision and efficient measurement allows the study of the nonstoichiometry effect on an atomic scale for boron-rich materials.
On wavelength selection by stratification in the instability of settling fibers
Authors: Metzger Bloen , Butler Jason E. , Guazzelli Élisabeth
Published: September 2007

We have tested whether stratification of the suspension can explain the wavelength selection observed in the instability of settling fibers. Reexamination of the data of Metzger et al. [J. Fluid Mech. 575, 307 (2007)] to assess the scaling determined by the large-scale nonlinear simulations of Saintillan et al. [Phys. Fluids 18, 121503 (2006)] demonstrates that this mechanism does not account for the length scale reduction observed experimentally.
Point and ring defects in nematics under capillary confinement
Authors: De Luca Gino , Rey Alejandro D.
Published: September 2007

The textures exhibited by nematic liquid crystals confined to cylindrical capillaries under homeotropic anchoring have been studied for nearly thirty years. One of the reasons behind this maintained interest is that the processing of many high-performance fibers including carbon fibers and spider silks involves these textures. Three of these textures, the planar radial with line defect, the planar polar with two line defects (PPLD), and the escape radial (ER), are relatively well understood. A third one, the escape radial with point defects presents, however, some unresolved issues and recent studies have questioned the real nature and dimensionality of the defects involved in this texture. It seems that the defects are not in the form of points but rather in the form of closed lines or rings. This paper presents a detailed study on the connection between point and ring defects in a cylindrical cavity using three-dimensional simulations based on the continuum Landau–de Gennes theory. The results show that true point defects cannot exist in cylindrical cavities and that the merging of two ringlike defects may lead to two qualitatively different stable textures, namely, the ER and PPLD textures. The various results are in qualitative agreement with recent molecular dynamic studies and with theoretical predictions based on experimental observations. The predictions provide new insights on the structural connections between synthetic and biological superfibers.
Effect of microstructure on the dielectric properties of compositionally graded (Ba,Sr)TiO3 films
Authors: Sigman J. , Clem P. G. , Nordquist C. D. , Richardson J. J. , Dawley J. T.
Published: September 2007

The demand for better performing radio frequency (RF) and microwave devices has led to research on lower dielectric loss and more temperature-stable dielectric materials. In this work, we report on an approach to decrease the temperature coefficient of capacitance (TCC) of (Ba,Sr)TiO3, which is desirable for improved stability of phase shifter elements, voltage-controlled oscillators, and other tunable devices. We have grown and characterized a series of single-composition and compositionally-graded films with both parallel plate and interdigitated (IDC) electrodes. Graded films with larger Ba2+ concentrations in the initially-grown layers, as well those films crystallized at lower temperatures, displayed the lowest TCC. An asymmetrically graded BaTiO3∕Ba0.50Sr0.50TiO3∕SrTiO3 film on polycrystalline alumina crystallized at 750 °C yielded the lowest observed TCC of −150 ppm∕°C, a dielectric constant of 240, 20% tuning of permittivity (0−40 VDC, 80 kV/cm), and losses below 1%, measured at 100 kHz. The microstructures of graded films were examined and found to play an important role in determining not only the TCC of graded films but also film dielectric constants. In particular, it was found that films with BaTiO3-rich compositions displayed overall finer grain structures that appear to suppress the ferroelectric behavior. The effect of electrode configuration on the TCC, dielectric constant, and dielectric losses in compositionally-graded films is also discussed.
Numerical study of ultrafast dynamics of femtosecond laser-induced periodic surface structure formation on noble metals
Authors: Wang Jincheng , Guo Chunlei
Published: September 2007

In this article, we perform a theoretical study to understand morphological differences in femtosecond laser induced periodic surface structures on noble metals. Our numerical study thoroughly examines the role of two competing ultrafast processes, electron-phonon energy coupling and hot electron diffusion, following femtosecond laser heating of metals. The calculation results confirm our previous experimental work on femtosecond laser induced periodic surface structures on noble metals, where electron-phonon coupling strength is believed to play the dominant role.
Exchange coupling between an amorphous ferromagnet and a crystalline antiferromagnet
Authors: Fecioru-Morariu Marian , Güntherodt Gernot , Rührig Manfred , Lamperti Alessio , Tanner Brian
Published: September 2007

We have investigated the exchange bias effect in bilayers of an amorphous ferromagnet (CoFeB) and a crystalline antiferromagnet (IrMn) in a top-pinned configuration. When the crystalline IrMn layer was deposited on top of the amorphous CoFeB layer, no exchange bias was observed. On insertion of a thin crystalline ferromagnetic layer of NiFe between the amorphous CoFeB and the crystalline IrMn, exchange coupling appeared and it was dependent on the thickness of the NiFe layer. An enhancement in the blocking temperature of the CoFeB/NiFe/IrMn layers was observed on increasing the thickness of the NiFe layer. These effects were directly correlated with the (111) texture in the antiferromagnetic phase of the IrMn layer, which developed progressively with increasing thickness of the NiFe layer. The blocking temperature was found to vary linearly with the intensity under the (111) IrMn X-ray diffraction peak. A NiFe interlayer can be used to introduce an additional source of anisotropy in a giant magnetoresistance sensor, by exchange coupling the free ferromagnetic (FM) layer of CoFeB in an orthogonal direction to the anisotropy direction of the pinned FM layer.
Effects of composition and phonon scattering mechanisms on thermal transport in MFI zeolite films
Authors: Hudiono Yeny , Greenstein Abraham , Saha-Kuete Carine , Olson Brandon , Graham Samuel , Nair Sankar
Published: September 2007

We report a systematic study that reveals the effect of composition (silicon-to-aluminum ratio) and the role of different phonon scattering processes on thermal transport in the nanoporous zeolite MFI. This is accomplished via synthesis of a series of films with graded compositions, thermal property measurements, and lattice dynamical modeling in the framework of the Boltzmann equation. MFI films with different Si/Al ratios (from infinity to 26) and constant (h0l) out-of-plane orientation were successfully synthesized by a seeded hydrothermal process. Three-omega measurements on these films allowed us to obtain comprehensive information on the thermal conductivity of MFI as a function of temperature (150–450 K) and Si/Al ratio. Detailed atomistic simulations (energy minimization and phonon dispersion calculations) were carried out for the MFI crystal structure with different Si/Al ratios and incorporated into a Boltzmann transport model along with approximate theoretical expressions for describing the rate of phonon scattering through umklapp, defect, and boundary scattering processes. The model predicts the observed thermal conductivity behavior very well across the entire range of temperature and composition investigated, with only a small number of fitting parameters of physical significance which allow us to distinguish the contributions of the different phonon scattering mechanisms. In particular, our results strongly suggest that the upper limit of thermal conductivity is defined by boundary-like scattering associated with the pore structure of the material. Below this limit, silicon substitution with aluminum allows considerable suppression of thermal conductivity by point defect scattering and a decrease in phonon velocity. These findings are important from the point of view of developing a robust platform for understanding thermal transport in complex crystalline materials with nanostructural features (such as an ordered nanopore network), which in turn serve as model systems for tuning of phonon transport properties in complex materials.