bluuebunny/crossref_metadata_2025 huggingface dataset:
Interactions between brush-coated clay sheets in a polymer matrix
Authors: Wang Rong , Qiu Feng , Zhang Hongdong , Yang Yuliang
Published: May 2003

The interactions between clay sheets with lateral length L that are grafted by polymer chains of N monomers immersed in a chemically identical polymer melt of polymerization index P are calculated by using Edwards’ self-consistent field theory. The calculation is carried out in two dimensions and shows that, as expected, for short grafted chains (aN1/2≪L, with a the segment size), the interactions and concentration profiles of the grafted layers are that of stretched brushes at flat interface; while for long grafted chains (aN1/2≫L), the interactions and concentration profiles are characteristic of star polymers. In the practically useful (but analytically untractable) case of intermediate grafted chain lengths, where aN1/2∼L, we have found that the lateral length of the clay sheets is a new relevant length scale in determining the structure and interactions of the grafted layers. These results indicate that the structure and interactions of the brush-coated clay sheets can be tailored by varying the grafted chain length and/or the lateral length of the clay sheets to benefit the fabrication of polymer/clay nanocomposites.
Vacancy hardening in single-crystal TiNx(001) layers
Authors: Shin C.-S. , Gall D. , Hellgren N. , Patscheider J. , Petrov I. , Greene J. E.
Published: May 2003

We investigate the effect of N vacancies on the mechanical properties of epitaxial δ-TiNx(001) layers with x=0.67–1.0. The relaxed lattice parameter increases linearly with x in good agreement with ab initio density functional calculations, indicating that deviations from stoichiometry are entirely due to anion vacancies. Hardness values increase continuously, while the elastic modulus decreases with increasing N-vacancy concentration. We attribute the observed vacancy hardening to a reduced dislocation mobility arising from an increase in the rate-limiting activation energy for cation migration.
Circular susceptibilities of CoFeSiB amorphous wires determined by inductance and second harmonic longitudinal magnetization
Authors: Chen D.-X. , Pascual L.
Published: May 2003

After a positive high field premagnetizing, the circular susceptibility χφ of annealed nearly nonmagnetostrictive amorphous wires determined by measuring ac inductance is anomalously large and asymmetric with respect to the sign of the dc bias field Hz. This phenomenon is explained by comparing with another circular susceptibility χφ,V2 obtained by measuring second harmonic longitudinal magnetization in terms of a model of connected (for low Hz) or coupled (for elevated Hz) donut structures induced by alinged magnetic inclusions.
Light-induced defects in KTaO3
Authors: Laguta V. V. , Glinchuk M. D. , Bykov I. P. , Cremona A. , Galinetto P. , Giulotto E. , Jastrabik L. , Rosa J.
Published: May 2003

Photoconductivity (PC), thermally stimulated conductivity (TSC), photoluminescence (PL), thermoluminescence (TL), and electron spin resonance (ESR) measurements have been made on single crystals of potassium tantalate over the temperature range 4.2–290 K. We revealed two sorts of O− shallow hole centers which are responsible for the two temperature regions of PL and PC enhancement: T<70 K and 100–150 K. Both O− centers were identified by their ESR spectra. While at low temperatures PL and PC have a rather intrinsic origin, i.e., they do not depend essentially on the sort or quality of crystals, at 100–150 K both quantities strongly depend on the defect content and vanish in well-oxidized crystals. We show that O− centers serve as radiative electron–hole recombination centers. Their energy levels are situated at 0.08 and 0.16 eV above the top of the valence band. Measurements of TSC and TL after UV irradiation revealed several glow peaks at temperatures 18–30 K and 65–70 K. There is a good correlation between TSC and TL intensity in different samples as well as after annealing in O2 and H2 atmospheres. Because electrons are mobile species in KTaO3, we attribute both TSC and TL to the thermal ionization of the same shallow donor centers related with isolated oxygen vacancies. The experimental data were treated in a one-trap/one-recombination center model, which takes into account the presence of “thermally disconnected” deep electron traps.
Lateral current density fronts in asymmetric double-barrier resonant-tunneling structures
Authors: Rodin Pavel , Schöll Eckehard
Published: May 2003

We present a theoretical analysis and numerical simulations of lateral current density fronts in bistable resonant-tunneling diodes with Z-shaped current–voltage characteristics. The bistability is due to the charge accumulation in the quantum well of the double-barrier structure. We focus on asymmetric structures in the regime of sequential incoherent tunneling and study the dependence of the bistability range, the front velocity, and the front width on the structure parameters. We propose a sectional design of a structure that is suitable for experimental observation of front propagation and discuss potential problems of such measurements in view of our theoretical findings. We point out the possibility to use sectional resonant-tunneling structures as controllable three-terminal switches.
Electrical and optical properties of the CdS quantum wells of CdS/ZnSe heterostructures
Authors: Dremel M. , Priller H. , Grün M. , Klingshirn C. , Kažukauskas V.
Published: May 2003

Earlier we reported the investigation of the electrical properties of selectively doped and degenerate CdS/ZnSe quantum heterostructures grown by molecular beam epitaxy [V. Kažukauskas, M. Grün, St. Petillon, A. Storzum, and C. Klingshirn, Appl. Phys. Lett. 74, 395 (1999)]. The maximum Hall mobilities in these heterostructures were found to be less than 400 cm2/Vs. In the present work we analyze in detail the scattering mechanisms in order to increase the carrier mobility and to optimize these quantum structures. We demonstrate that the Hall mobility can reach in the CdS quantum wells at low temperatures 2800 cm2/V s for slightly doped structures, having an effective sheet carrier density 2.6×1011 cm−2. In these structures the mobility is mostly limited by interface alloying scattering. At high doping levels carriers become redistributed between the quantum well and the ZnSe doped layer. This causes the parallel conductivity phenomena, which diminishes the effective mobility. Near room temperature the scattering by optical phonons prevails which is superimposed by dislocation scattering.
Influence of the temperature on the carrier capture into self-assembled InAs/GaAs quantum dots
Authors: Duarte C. A. , da Silva E. C. F. , Quivy A. A. , da Silva M. J. , Martini S. , Leite J. R. , Meneses E. A. , Lauretto E.
Published: May 2003

Photoluminescence (PL) spectroscopy and atomic-force microscopy (AFM) were used to investigate the size evolution of InAs quantum dots on GaAs(001) as a function of the amount of InAs material. Different families of islands were observed in the AFM images and unambiguously identified in the PL spectra, together with the signal of the wetting layer. PL measurements carried out at low and intermediate temperatures showed a thermal carrier redistribution among dots belonging to different families. The physical origin of this behavior is explained in terms of the different temperature dependence of the carrier-capture rate into the quantum dots. At high temperatures, an enhancement of the total PL-integrated intensity of the largest-sized quantum dots was attributed to the increase of diffusivity of the photogenerated carriers inside the wetting layer.
The ultrafast dynamics of HBr–water clusters: Influences on ion-pair formation
Authors: Hurley S. M. , Dermota T. E. , Hydutsky D. P. , Castleman A. W.
Published: May 2003

The ultrafast dynamics of HBr–water clusters have been investigated using pump–probe spectroscopy coupled with reflectron time-of-flight mass spectrometry. HBr clusters, mixed HBr–water clusters, and protonated water clusters are observed in the mass spectra. Dynamic studies reveal that when an HBr chromophore of a cluster with less than five solvent molecules is excited electronically, solvent reorganization occurs to form the solvent separated ion-pair [S. M. Hurley et al., Science 298, 202 (2002)]. The present paper focuses on the influence of clustering on the dynamics of the C and D states of HBr. In addition, further evidence is presented which confirms that complete dissolution of HBr requires five solvent molecules in the isolated species found in complexes comprised of pure water or HBr/H2O mixtures.
Two-vibron bound states in CO/Ru(001): An effective Hamiltonian approach
Authors: Pouthier V.
Published: May 2003

Two-vibron bound states in the CO monolayer adsorbed on Ru(001) are investigated by using an attractive Hubbard Hamiltonian for boson with long-range lateral interactions. The parameters which characterize this Hamiltonian are determined through an accurate comparison between the present theory and previous experimental results. In a perfect agreement with experimental data, an anharmonic shift equal to 31.12 cm−1 is obtained. It is pointed out that the anharmonicity depends on the CO coverage which originates in a chemical modification of the intramolecular CO potential due to surrounding molecules. The anharmonicity is responsible for the occurrence of a two-vibron bound states band located at approximately 20 cm−1 below the free states continuum. This band exhibits a rather weak dispersion, equal to 3.28 cm−1, almost 30 times smaller than the two-vibron free states continuum bandwidth equal to 100 cm−1. Although long-range lateral interactions favor a large free states continuum, it is shown that the range of the coupling does not modify significantly the bound states band.
Exciton–bath coupling and inhomogeneous broadening in the optical spectroscopy of semiconductor quantum dots
Authors: Salvador Mayrose R. , Hines Margaret A. , Scholes Gregory D.
Published: May 2003

The homogeneous line shape and inhomogeneous broadening of colloidal quantum dots are measured at ambient temperatures and in solution. Three-pulse echo peak shift (3PEPS) measurements of nanocrystalline CdSe semiconductor quantum dots in toluene solvent at 298 K are reported. Modeling of the 3PEPS data using a detailed model designed to account for the optical properties of semiconductor quantum dots, suggests that Debye-type relaxations attributed to reorientation of the trioctylphosphine and trioctyl phosphine oxide capping ligands are significant. It is shown that 3PEPS spectroscopy can quantify precisely the inhomogeneous broadening of the optical spectrum, brought about mainly by the distribution of nanocrystal sizes.
A spin-complete version of the spin-flip approach to bond breaking: What is the impact of obtaining spin eigenfunctions?
Authors: Sears John S. , Sherrill C. David , Krylov Anna I.
Published: May 2003

Spin-complete versions of the spin-flip configuration-interaction-singles (SF-CIS) approach have been investigated to determine the impact of making the wave function an eigenfunction of Ŝ2. The method has been implemented within an extended restricted active space configuration interaction formalism. Spin-complete results are presented for excitation energies, equilibrium geometries, and potential energy curves for dissociation of a single bond in several small molecules. The effect of different orbital choices has also been investigated. The spin-complete results are compared both to results using the original spin-flip method and to more computationally expensive benchmarks. Using spin eigenfunctions dramatically improves upon the accuracy of the SF-CIS approach.
The two-dimensional character of spin-up in a rectangular container
Authors: Zavala Sansón L.
Published: May 2003

This study presents two-dimensional (2D) numerical simulations of the spin-up of a homogeneous fluid in a rectangular container, compared with the corresponding three-dimensional (3D) simulation presented by other authors. A 2D simulation is based on a recently derived model [Zavala Sansón and van Heijst, J. Fluid Mech. 412, 75 (2000)] including nonlinear Ekman friction effects, in addition to the well-known linear damping term. It is found that the inclusion of nonlinear Ekman terms provides a remarkable agreement between 2D and 3D simulations for times comparable with the Ekman timescale, which implies that this formulation is able to capture the essential viscous features of the 3D situation. In contrast, a simulation based on a 2D model with conventional linear friction is not able to represent the 3D case correctly.
Numerical calculation of the melting phase diagram of low molecular-weight polyethylene
Authors: Das Chinmay , Frenkel Daan
Published: May 2003

Using thermodynamic integration, we calculate free energies of the melt and the crystalline phases of a model system of C198H398 with a realistic all-atom potential. We use the Gibbs–Duhem integration scheme to calculate the melting curve over the experimentally relevant pressure range. The crystal structure and the melting curve obtained from our simulation are in good quantitative agreement with the available experimental results.
Labyrinthine instability of miscible magnetic fluids
Authors: Igonin M. , Cebers A.
Published: May 2003

The paper treats theoretically an inhomogeneous magnetic fluid (MF), modeling a miscible MF pair, in a Hele-Shaw cell subjected to a perpendicular magnetic field. As the existing experimental evidence indicates, a miscible form of the labyrinthine instability may occur in this system, with diffusion of magnetic particles playing the key role. Linear stability analysis is performed in the present paper: Analytically for a sharp interface and numerically for a diffused concentration distribution. For the sharp interface, assuming the Darcy law governs the flow, the neutral curves and the stability diagram are found along with the critical wavelength and the critical field intensity. Oscillatory and stationary instabilities are shown to substitute each other under certain conditions. For the diffused interface the viscous effects due to the flow nonuniformity in the plane of the cell are allowed for and found significant. Therefore, the conventional Darcy law that takes into account only the near-wall friction must be replaced by the Brinkman (Darcy–Stokes) equation. With the latter, the most unstable wavelength in strong fields tends to the limit of a few gap widths that quite weakly depends on the basic concentration gradient. A mechanism of the oscillatory instability is explained physically. Self-oscillations occur through the interplay between diffusion and advection driven via a magnetic body force by concentration inhomogeneity.
Crater formation in gold nanoislands due to MeV self-ion irradiation
Authors: Satyam P. V. , Kamila J. , Mohapatra S. , Satpati B. , Goswami D. K. , Dev B. N. , Cook R. E. , Assoufid Lahsen , Wang J. , Mishra N. C.
Published: May 2003

The modification of gold nanoislands, grown on silicon substrates under high-vacuum conditions, by MeV self-ion irradiation has been studied by using scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and x-ray reflectivity. Upon irradiation with 1.5 MeV Au2+, two types of craters are observed on the Au islands: Empty craters and craters with a central hillock. The contribution of plastic flow, pressure spike, and sputtering to the crater formation during the ion impacts on the gold islands is analyzed. Thermal spike confinement within the gold islands is also proposed to be one of the possible reasons for crater formation in nanoislands.
Theory of single molecule line shapes of multichromophoric macromolecules
Authors: Jang Seogjoo , Silbey Robert J.
Published: May 2003

A theoretical framework is developed for the line shapes of the single molecule spectroscopy (SMS) of multichromophoric macromolecules by combining the Hamiltonian-based line shape theory with a kinetic equation formulation of single molecule photon counting processes. The theory allows a consistent treatment of two different line broadening mechanisms, one originating from the fast quantum dynamical degrees of freedom and the other originating from slow stochastic fluctuations that can be substantial within the SMS measurement time scale. A general expression is derived for the photon counting probability, which enables simulation of SMS line shape as an average over a renewal process and makes it possible to study the effects of radiation induced fluctuations. Then conditions are clarified under which the line shape reduces to a simple average over the distribution function of slow fluctuations. Model calculations for a bichromophoric system illustrate the subtle interplay between the fast and slow line broadening mechanisms.
Critical behavior of dielectric permittivity and electric conductivity in temperature and pressure studies above and below the critical consolute point
Authors: Malik P. , Rzoska S. J. , Drozd-Rzoska A. , Jadżyn J.
Published: May 2003

Results of comprehensive temperature (T) and pressure (P) studies of static dielectric permittivity (ε′) and electric conductivity (σ) in the one- and two-phase regions of critical nitrobenzene–dodecane mixture are presented. A strong asymmetry of determined coexistence curves causes a strong violation of the law of rectilinear diameter. The obtained critical anomalies in the homogeneous phase [εhomo′(T),εhomo′(P),σhomo(P)] or the diameter of the binodal [εmean′(T),εmean′(P),σmean(T),σmean(P)] are associated with the same critical exponent φ=1−α ≈0.88, where α is the specific heat critical exponent. Critical anomalies for the isothermal, pressure path exhibit a set of favorite in comparison with results obtained in σ(T) and ε(T) tests. They are: the negligible influence of the critical Maxwell–Wagner effect, the hardly visible appearance of the correction-to-scaling term, a more pronounced manifestation of critical anomalies, and a reduced number of fitted parameters. Particularly noteworthy is the evidence for the σhomo(P) anomaly in the homogeneous phase, hardly obtained up to now in a mixture of a low electric conductivity. Results presented suggest the isomorphic behavior of ε′(T), σ(T) and ε′(P), σ(P) critical anomalies for the homogeneous phase and for the diameter of the binodal.
Efficient calculation of electron paramagnetic resonance g-tensors by multireference configuration interaction sum-over-state expansions, using the atomic mean-field spin–orbit method
Authors: Brownridge Scott , Grein Friedrich , Tatchen Jörg , Kleinschmidt Martin , Marian Christel M.
Published: May 2003

Using the multireference configuration interaction method due to Grimme and Waletzke, combined with the atomic mean-field approximations for the efficient calculation of spin–orbit matrix elements, the g-tensors in second-order perturbation theory have been calculated for the main group radicals CO+, CN, BO, BS, MgF, AlO, O2, HCO, H2O+, NO2, CO2−, NF2, NO22−, O3−, ClO2, and H2CO+, and for the transition metal compounds ZnH, ZnF, and TiF3, using explicit sum-over-state expansions for up to 20 excited states. In most cases, a valence triple-zeta basis set with polarization functions has been employed. It is shown that the addition of diffuse functions to this basis set does not improve the g-tensor results, and in several instances leads to slower convergence of the sum-over-state expansion. The calculated g-tensors are in good agreement with experimental values, and with our previous multireference configuration interaction results available for 9 of the 19 radicals. Our results are shown to be equivalent to, or better than, values obtained by other theoretical methods. Examples of radicals for which g-tensor calculations presented problems in the past are AlO and TiF3. For AlO, we obtain Δg⊥=−1530 ppm (parts per million), compared with an experimental value of −1900 ppm in Ne matrix. Using the SVP (valence double-zeta plus polarization) basis set, Δg⊥ of TiF3 is calculated to be −115.3 ppt (parts per thousand), compared with experimental values of −111.9 and −123.7 ppt.
Ga vacancies as dominant intrinsic acceptors in GaN grown by hydride vapor phase epitaxy
Authors: Oila J. , Kivioja J. , Ranki V. , Saarinen K. , Look D. C. , Molnar R. J. , Park S. S. , Lee S. K. , Han J. Y.
Published: May 2003

Positron annihilation measurements show that negative Ga vacancies are the dominant acceptors in n-type gallium nitride grown by hydride vapor phase epitaxy. The concentration of Ga vacancies decreases, from more than 1019 to below 1016 cm−3, as the distance from the interface region increases from 1 to 300 μm. These concentrations are the same as the total acceptor densities determined in Hall experiments. The depth profile of O is similar to that of VGa, suggesting that the Ga vacancies are complexed with the oxygen impurities.
Short-time kinetics of an irreversible bimolecular solution reaction: Asymptotic prediction by a non-Markovian Smoluchowski approach
Authors: Naumann Wolfgang
Published: May 2003

The effect of the nondiffusional short-time liquid dynamics on the kinetics of an irreversible bimolecular solution reaction (A+B→products) is investigated using a non-Markovian Smoluchowski approach. It is shown that due to the convolution-type basic equation in this concept, asymptotic rate coefficient predictions can be deduced from exact results of the Smoluchowski–Collins–Kimball theory. For the case that only the inertial effect plays a significant part in the nondiffusional reactant pair dynamics, an asymptotic short-time expansion of the rate coefficient was derived in this way. It explicitly indicates as a new reaction-kinetic result how the rate coefficient in the short-time region depends on the AB potential of mean force. Specified for the conditions of contact reactions in a simple hard-sphere fluid, the kinetic predictions of the new concept are compared with the results of other approaches.
Vibrational spectroscopy of HOD in liquid D2O. V. Infrared three-pulse photon echoes
Authors: Piryatinski A. , Lawrence C. P. , Skinner J. L.
Published: May 2003

Using molecular dynamics simulation and the self-consistent renormalization scheme for liquid-state vibrational spectroscopy developed earlier by us, we have calculated the infrared integrated three-pulse photon echo signal (and in particular, the peak shift) for the OH stretch of dilute HOD in D2O at 300 K. Our results are in reasonable agreement with preliminary experimental results of Tokmakoff and co-workers. We show that the second-cumulant approximation to the integrated three-pulse echo intensity works well when t1 (the delay time between the first two pulses) is less than 100 fs, even though the frequency fluctuations are not Gaussian. We discuss the reason for, and generality of, this result, and the physical origin of the non-Gaussian fluctuations. Our results indicate that the standard approaches (peak shift, initial slope, and direct determination by nonlinear fitting) for determining the frequency time-correlation function from integrated three-pulse echo data, all of which implicitly assume the validity of this approximation, are useful and adequate.
An extension of generalized Taylor dispersion in unbounded homogeneous shear flows to run-and-tumble chemotactic bacteria
Authors: Bearon R. N.
Published: May 2003

In the absence of flow, the biased random walk of bacteria such as Escherichia coli is modeled by straight runs punctuated by random changes in direction, tumbles. We model chemotaxis by allowing the tumble rate of a run to depend on the component of swimming velocity in the direction of the chemoattractant gradient. In the well-studied situation of weak bias in tumble rate, bacteria disperse over a diffusive time scale, and the evolution of density satisfies the classic Keller–Segel advection-diffusion equation. In this paper, we model swimming bacteria being advected and rotated by an unbounded homogeneous shear flow. The flow field alters the trajectories of individuals and thus affects the macroscopic dispersion of a population. We adapt the formal framework of generalized Taylor dispersion theory to make it applicable for run-and-tumble bacteria with an arbitrary bias in tumble rate. This enables us to obtain a macroscopic description of the dispersion of bacteria. For the particular case of simple shear flow, we calculate explicitly the effect of flow on the diffusion tensor and mean swimming velocity.
Laminar flamelet decomposition for conditional source-term estimation
Authors: Bushe W. Kendal , Steiner Helfried
Published: May 2003

A new decomposition approach to conditional source-term estimation (CSE) is proposed and discussed. The new approach is tested in the a priori sense using direct numerical simulations (DNS). It is found that—where CSE had previously been found to provide closure for chemical source-terms with arbitrary chemistry in the large eddy simulation paradigm—it can provide this closure in the Reynolds averaged Navier–Stokes paradigm as well. Using the proposed decomposition improves the predictions of CSE considerably. Only the assumptions that gradients in conditional averages are small and that the probability density function of mixture fraction can be adequately approximated using a presumed functional form are needed. The computational cost of the new laminar flamelet decomposition approach to CSE is also substantially lower than that of the original approach.
On conditional velocity increment statistics
Authors: Zhang Hengbin , Tong Chenning
Published: May 2003

Previous studies have shown that conditional velocity increment statistics for inertial-range separations conditional on the locally averaged energy dissipation rate εr are generally quasi-Gaussian. Kolmogorov’s refined similarity hypotheses further predict that they are independent of εr. We study conditional statistics of velocity increments with the local turbulent kinetic energy kr and εr as the conditioning variables. Experimental data obtained in the fully developed region of an axisymmetric turbulent jet are used to compute the statistics. The conditional probability density functions (PDFs) conditional on the kr are found to be sub-Gaussian and bimodal for kr values large compared to its mean, indicating existence of local velocity fields that are less intermittent than a Gaussian field. Furthermore, the bimodality becomes stronger as kr increases. For small kr the conditional PDFs are close to Gaussian. Thus the conditional statistics are not universal. When εr is included as a second conditioning variable, the conditional PDFs change toward bimodal shapes as εr decreases. However, we found that the bimodality can largely be characterized by a single parameter, the ratio of εr to its conditional mean 〈εr|kr〉. This dependence on kr and εr differs from predictions using Kolmogorov’s refined hypotheses, and is argued to be related to the nonequilibrium of the local velocity fields.
Electronic structure of the 1,3,5-tridehydrobenzene triradical in its ground and excited states
Authors: Slipchenko Lyudmila V. , Krylov Anna I.
Published: May 2003

The ground and low-lying electronic states of the 1,3,5-tridehydrobenzene triradical are characterized by electronic structure calculations. It is found that the ground state is the A12 doublet of C2v symmetry. Another doublet state lies 0.1–0.2 eV higher in energy, and the lowest quartet state of D3h symmetry is 1.2–1.4 eV higher in energy. Both doublets are degenerate at D3h geometries and undergo different Jahn–Teller distortions. Structurally, the triradical is tighter than the parent neutral molecule (benzene), because the interaction among the unpaired electrons results in additional bonding even in the high-spin state (quartet). The adiabatic doublet–quartet energy gap and the excitation energies calculated at the equilibrium geometries of the 3,5-dehydrophenyl anion are provided to aid in the design and interpretation of photoelectron experiments.
Density distribution for a self-interacting polymer adsorbed at an oil–water interface
Authors: Cai Jun , Prausnitz John M.
Published: May 2003

To obtain the density distribution of a polymer at an oil–water interface, the interaction between a polymer segment and the interface is represented by a symmetric square-well potential or a symmetric δ-well potential. The polymer is modeled as a Gaussian chain with an effective segment–segment interaction that depends on the liquid phase (oil or water). Self-consistent field theory is used to calculate the density distribution of the polymer along a coordinate perpendicular to the interface. If the polymer segment–segment repulsion is strong in one phase and weak in the other, the polymer density distribution is relatively sharp in the first phase and relatively flat in the other. Contrary to what we expect for a polymer in the bulk, a polymer near the interface prefers the phase where the polymer–polymer repulsive interactions are relatively weak.
Isomers of Ge2N2: Production and infrared absorption of GeNNGe in solid N2
Authors: Bahou Mohammed , Sankaran K. , Wu Yu-Jong , Lee Yuan-Pern , Rayner David , Simard Benoit
Published: May 2003

Crystalline germanium was ablated with light at 532 nm from a frequency-doubled Nd:YAG laser, and the resultant plume reacted with N2 before deposition onto a substrate at 20 K. Multiple absorption lines at 891.8, 890.8, 890.4, 889.9, 889.4, 888.9, 888.4, 887.9, 887.5, and 887.0 cm−1 that become enhanced after annealing of the matrix at 31 K are attributed to a species containing two Ge atoms in their isotopic variants: Ge70, Ge72, Ge73, Ge74, and Ge76. Replacing N214 with a mixture of N214 and N215 yields an additional multiplet in the region 867.1–862.1 cm−1, whereas reaction of the same mixture subjected to microwave discharge yields a further multiplet in the region 878.9–874.0 cm−1. The isotopic pattern indicates that the vibrational mode is associated with a Ge–N stretching motion that involves two equivalent Ge atoms and two equivalent N atoms. Theoretical calculations with density-functional theories (B3LYP/6-311+G* and B3LYP/aug-cc-pVTZ) predict nine stable isomers of Ge2N2, with linear GeNNGe and asymmetric angular Ge(GeNN) having the least energies. Among calculated vibrational wave numbers, IR intensities, and isotopic shifts for all isomers of Ge2N2, only those predicted for the asymmetric Ge–N stretching mode of linear GeNNGe fit satisfactorily with experimental results. GeNNGe is likely formed from direct reaction of Ge2 and N2 rather than from two-step reactions involving GeNN or GeN.
Dynamics of hydrogen–HF complexes in helium nanodroplets
Authors: Moore David T. , Miller Roger E.
Published: May 2003

High-resolution infrared spectra are reported for ortho- and para-H2, ortho- and para-D2, and HD binary complexes with HF, solvated in helium nanodroplets. These rotationally resolved spectra provide detailed information on the rotational and vibrational dynamics of these systems. The para-H2–HF complex is observed, indicative of the unusual growth conditions present in the droplets. In the case of the HD–HF complex, extensive line broadening is observed, which we tentatively explain in terms of the near resonance between the HF and HD vibrational frequencies, which presumably facilitates vibrational relaxation.
Vibrational analysis for the nuclear–electronic orbital method
Authors: Iordanov Tzvetelin , Hammes-Schiffer Sharon
Published: May 2003

The methodology for a vibrational analysis within the nuclear–electronic orbital (NEO) framework is presented. In the NEO approach, specified nuclei are treated quantum mechanically on the same level as the electrons, and mixed nuclear–electronic wave functions are calculated variationally with molecular orbital methods. Both electronic and nuclear molecular orbitals are expressed as linear combinations of Gaussian basis functions. The NEO potential energy surface depends on only the classical nuclei, and each point on this surface is optimized variationally with respect to all molecular orbitals as well as the centers of the nuclear basis functions. The NEO vibrational analysis involves the calculation, projection, and diagonalization of a numerical Hessian to obtain the harmonic vibrational frequencies corresponding to the classical nuclei. This analysis allows the characterization of stationary points on the NEO potential energy surface. It also enables the calculation of zero point energy corrections and thermodynamic properties such as enthalpy, entropy, and free energy for chemical reactions on the NEO potential energy surface. Illustrative applications of this vibrational analysis to a series of molecules and to a nucleophilic substitution reaction are presented.
The bound state spectrum of HOBr up to the dissociation limit: Evolution of saddle-node bifurcations
Authors: Azzam Tarek , Schinke Reinhard , Farantos Stavros C. , Joyeux Marc , Peterson Kirk A.
Published: May 2003

Based on an accurate potential energy surface [J. Chem. Phys. 113, 4598 (2000)] we calculated ca. 700 bound state energies and wave functions of nonrotating HOBr using the filter-diagonalization method. Similar to HOCl, a 1:2 anharmonic resonance between the HOBr bending and the OBr stretching mode determines the general structure of the level spectrum. One of the results of this resonance is a saddle-node bifurcation at which a new class of states (“dissociation states”) comes into existence, which advance along the HO–Br dissociation path. Because the resonance condition at low energies is better fulfilled for HOBr, the bifurcation occurs at considerably lower energies than for HOCl. The results of the quantum mechanical calculations are interpreted in terms of classical periodic orbits (continuation/bifurcation diagram) and a semiclassical analysis based on a spectroscopic Hamiltonian, which is fitted to the exact energy levels and also taking into account the wave functions.
Interference between stationary and vibrating cylinder wakes
Authors: Lai W. C. , Zhou Y. , So R. M. C. , Wang T.
Published: May 2003

The flow behind two side-by-side tubes of identical diameter, d, one stationary and the other forced to oscillate in the lateral direction at an amplitude of A=0.1∼0.5d was examined. Two values of T/d, i.e., 2.2 and 3.5 were investigated, where T is the cylinder center-to-center spacing. The Reynolds number Re ranges from 150 to about 1000. The effect of A/d, T/d, and fe/fs, where fe is the cylinder oscillating frequency and fs is the vortex shedding frequency of an isolated stationary cylinder, on the vortex shedding and the wake structure was examined. Specific attention was given to the occurrence of lock-in, where vortex shedding from the oscillating cylinder synchronizes with fe. Significant influence of these parameters has been observed on the flow behind the cylinders in terms of the predominant vortex patterns and interactions between vortices. It has been found that the shedding frequency associated with the oscillating and the stationary cylinder can be modified as fe/fs approaches unity. Furthermore, the flow regime may change under the conditions of T/d=2.2 and A/d=0.5 from two distinct coupled streets to the combination of one narrow and one wide street. Subsequently, the lock-in state is considerably extended probably because multiple dominant frequencies in the asymmetrical flow regime can all be locked in with the structural oscillation.
Positron lifetime spectroscopic studies of nanocrystalline ZnFe2O4
Authors: Nambissan P. M. G. , Upadhyay C. , Verma H. C.
Published: May 2003

By carrying out positron lifetime measurements in zinc ferrite (ZnFe2O4) samples of various grain sizes down to 5 nm, the defect microstructures have been identified. In the bulk samples composed of grains of large sizes, positrons were trapped by monovacancies in the crystalline structure. Upon reduction of the grain sizes to nanometer dimensions, positrons get trapped selectively at either the diffused vacancies on the grain surfaces and the intergranular regions. Below about 9 nm, the grains undergo the transformation from the normal spinel structure to the inverse phase. A concomitant lattice contraction results in substantial reduction of the octahedral site volume, and hence, a fraction of the Zn2+ ions with larger ionic radius fails to occupy these sites. This leaves vacancies at the octahedral sites which then turn out to be the major trapping sites for positrons. ZnFe2O4 samples prepared through different routes were investigated, which showed similar qualitative features, although those synthesized through the hydrothermal precipitation method showed remarkably larger lifetimes for trapped positrons upon nanocrystallization in comparison to the samples prepared through the citrate route.
Mechanical analysis of interconnected structures using process simulation
Authors: Senez Vincent , Hoffmann Thomas , Le Duc Philippe , Murray Frank
Published: May 2003

An elastoplastic model has been implemented in a technology computer-aided design (TCAD) program with the aim of predicting the mechanical behavior of polycrystalline materials used in silicon-based technology in microelectronics. In order to analyze microstructures combining both nonlinear viscoelastic and elastoplastic materials, we propose a computational process with quick convergence. This model is now included in a stress simulation system, thus allowing the prediction of variations in stress according to the stage of the process. The modeling takes into account not only thermal stress but also intrinsic/extrinsic stresses and etching-related stress. The stress evolution of aluminum copper (Al–Cu) periodic lines embedded within silicon dioxide has been examined. The tensile and compressive yield strengths of thin-film Al–Cu have been characterized using the von Mises criterion for various film thicknesses. Comparisons with experimental results, based on passivated and unpassivated line structures, show that von Mises yield stress is independent of linewidth. It was also found that the correct prediction of the principal stresses strongly depends on the accurate characterization of thin-film yield strength. The use of incorrect values can lead to large errors in the determination of the line aspect ratio giving the maximum principal stresses. Finally, the analysis of an industrial back end of line process is performed to demonstrate what we can now carry out with TCAD to solve stress-related reliability problems in interconnects.
Dynamic failure of steel under hypervelocity impact of polycarbonate up to 9 km/s
Authors: Moritoh Tatsumi , Matsuoka Shohei , Ogura Toshiyuki , Nakamura Kazutaka G. , Kondo Ken-ichi , Katayama Masahide , Yoshida Masatake
Published: May 2003

We performed hypervelocity impact experiments on SS400 steel with a polycarbonate projectile at velocities up to 9 km/s. Spall fracture damages were observed near a rear surface of the impacted target. The microstructure and microdamages were examined using optical microscopy and scanning electron microscopy. The α−ε phase transition region was observed near the crater. Cracks parallel to the impact direction were observed below the crater, and radial cracks grew from the α−ε phase interface at high velocity impact tests, especially above 6 km/s. Cleavage was the dominant mechanism for a spall fracture surface, and ductile fracture structures were also observed at the edge of spall plane. Geometric spall behaviors were well reproduced by numerical simulations using a hydrocode. These simulation results also showed that the cracks below the crater would be due to dynamic tensile stresses. The calculated results using the value of 13 GPa as the transition pressure showed that the duration necessary for the phase transition is about 150 ns for impact velocity of 8.8 km/s.
Nucleation and growth of polytypic-layered crystals from the network liquid zinc chloride
Authors: Wilson Mark
Published: May 2003

The liquid to solid crystallization for zinc (II) chloride is studied by molecular dynamics computer simulation. The transition is unusual in that it involves a change from a three-dimensional network liquid structure to a pseudo-two-dimensional layered crystal. The crystallization events are observed from four distinct liquid starting configurations and are identified by reference to the time evolution of the system energetics and Bragg peaks associated with the cation layering. Order parameters and molecular graphics are applied to understand the transitions at an atomistic length scale. Mechanisms are presented for the initial layer growth, the coherent joining of the layered crystallites, and the destruction of high-energy grain boundaries. The growth kinetics are analyzed by defining times for catastrophic and critical nucleation. The final crystal structures are shown to have essentially random anion close-packed stacking sequences consistent with the large number of experimentally observed polytypic structures. The formation of grain boundary stacking faults is also observed.
A simple and realistic model system for studying hydrogen bonds in β-sheets
Authors: Rossmeisl Jan , Hinnemann Berit , Jacobsen Karsten W. , Nørskov Jens K. , Olsen Ole H. , Pedersen Jan T.
Published: May 2003

We investigate the interaction between peptide chains at the level of state-of-the-art ab initio density functional theory. We propose an interacting periodic polypeptide model for studying the interactions in β-sheets and apply this to glycine and alanine peptide chains in both parallel and antiparallel structures. The calculated structures of alanine are compared to x-ray structures of β-sheets and the model is found to reproduce the geometry of the hydrogen bonds very well both concerning parallel and antiparallel β-sheets. We investigate the structures of both the N–H⋯O=C and the Cα–H⋯O=C hydrogen bonds. The former is thoroughly investigated, whereas the structure of the latter still is the subject of much discussion. We show that the hydrogen bonds between peptide chains are considerably weaker than what is found in studies of smaller models, e.g., the N-methylacetamide molecule. By molecular mechanics calculations we study the effect of twisting, which is not included in our model. We estimate its contribution to the interaction energy to be small.
Vibrational dynamics of deuterium chloride in solid nitrogen probed by linear and nonlinear spectroscopy
Authors: Broquier M. , Crépin C. , Cuisset A. , Dubost H. , Galaup J. P. , Roubin P.
Published: May 2003

The vibrational dynamics of DCl isolated in nitrogen matrices is investigated by means of time-resolved one-color degenerate four-wave mixing experiments with the help of infrared absorption spectroscopy. Coherence times are found in the tens to hundreds of picoseconds, showing a strong coupling between the DCl stretch and the low-energy local phonons of the lattice. Spectral diffusion due to vibrational energy transfers between the guest molecules is also examined. Fast energy transfers between isotopic species explained the specific shape of the time-resolved signals. The efficiency of such processes is discussed, suggesting an enhancement of the DCl vibrational transition dipole moment induced by the nitrogen environment.
Hydration structure and diffusion of ions in supercooled water: Ion size effects
Authors: Chowdhuri Snehasis , Chandra Amalendu
Published: May 2003

We have performed a series of molecular dynamics simulations of alkali metal (Li+, Na+, K+, Rb+, and Cs+) and halide (F−, Cl−, Br−, and I−) ions in water at infinite dilution at T=258 K to investigate the effects of ion size on the hydration structure and diffusion of ions in supercooled water. Simulations are also performed at T=298 K in order to compare the results of the hydration structure and diffusion in supercooled water with those in ambient water. With increase of ion size, like in ambient water, in supercooled water also the diffusion coefficients of alkali metal and halide ions are found to fall in different curves with distinct maxima. However, the relative increases of the diffusion coefficients of larger ions compared to those of Li+ and F− are found to be significantly higher in the supercooled water.
Solvent-mediated interactions and solvation close to fluid–fluid phase separation: A density functional treatment
Authors: Archer A. J. , Evans R.
Published: May 2003

We apply a general density functional approach for calculating the force between two big particles immersed in a solvent of smaller ones to calculate the solvent-mediated (SM) potential between two big Gaussian core particles in a binary mixture of smaller Gaussian particles, a simple model of polymers in solution. Within a mean-field free-energy functional, which generates the random phase approximation (RPA) for the bulk pair direct correlation functions, the binary solvent exhibits fluid–fluid phase separation and we show that the theory for calculating the SM potential captures effects of thick adsorbed films surrounding the big solute particles. For a single big particle there is a first-order thin–thick adsorbed film transition and in the thick-film regime—i.e., for solvent-state points lying close to the binodal, on the side where the solvent is poor in the species which is favored by the big particles—we find extremely attractive, long-ranged SM potentials between the big particles whose range is determined by the film thickness. For state points away from the binodal in the thin film regime, or above the “wetting point”, the SM potentials are short ranged and less attractive. We show that the effects of the thick adsorbed films around the big particles are not included when the SM potential is obtained from the big–big radial distribution function gbb(r), calculated using the RPA closure to the Ornstein–Zernike equations. In the region of the solvent critical point we also find extremely attractive SM potentials whose range is now set by the bulk correlation length in the binary solvent and which increases and eventually diverges for state points approaching the critical point. We calculate the excess chemical potential of the big solute particle in the binary solvent as a function of the concentration of one of the smaller species and show that this quantity also reflects the formation of thick adsorbed films. The form of the excess chemical potential and, hence, the solvation for the soft Gaussian core fluid is contrasted with that expected for a hard-core solute.
Excluded volume effect on diffusion-influenced reactions in one dimension
Authors: Park Joonho , Kim Hyojoon , Shin Kook Joe
Published: May 2003

The excluded volume (EV) effect between nonreactive like-particles of diffusion-influenced pseudo-first-order reaction A+B→C is investigated by the hierarchical Smoluchowski approach of Kuzovkov and Kotomin [Rep. Prog. Phys. 51, 1479 (1988)] and the many-particle kernel formalism of Lee et al. [J. Chem. Phys. 113, 8686 (2000)] in one dimension. Contrary to the three-dimensional analysis, the latter theory can be formulated without additional approximations in one dimension so that more accurate results are obtained. Although formulations and resulting expressions are different, these two theories show almost identical results numerically. The EV effect becomes significant at higher concentrations of B molecules as in three dimensions. However, we found that the EV effect in one dimension is more pronounced than in three dimensions. A similar trend appears as the size of the B molecule increases. Theoretical results are compared with Monte Carlo simulations. The simulation results reveal much larger EV effect than that predicted by both theories. This behavior may be attributed to the “cage” effect which is not considered in both theories.