bluuebunny/crossref_metadata_2025 huggingface dataset:
Local expansion of N-representable one-particle density matrices yielding a prescribed electron density
Authors: Pipek János , Nagy Szilvia
Published: October 2003

Multiresolution (or wavelet) analysis offers a strictly local basis set for a systematic introduction of new details into Hilbert space operators. Using this tool we have previously developed an expansion method for density matrices. The set of density operators providing a given electron density plays an essential role in density functional theory, in the minimization of energy expectation values with the constraint that the electron density is fixed. In this contribution, using multiresolution analysis, we present an excellent quality density matrix expansion yielding a prescribed electron density, and compare it to other known methods. Due to the strictly local nature of the applied basis functions, our construction has the specific advantage that the resulting density matrix is correlated and N-representable in the infinite resolution limit. As a further consequence of this scheme we can conclude that the deviation of the exact kinetic energy functional from the Weizsäcker term is not a necessary consequence of the particle statistics.
Interfacial instability in non-Newtonian fluid layers
Authors: Balmforth N. J. , Craster R. V. , Toniolo C.
Published: October 2003

Superposed layers of fluid flowing down an inclined plane are prone to interfacial instability even in the limit of zero Reynolds number. This situation can be explored by making use of a lubrication-style approximation of the governing fluid equations. Two versions of the lubrication theory are presented for superposed layers of non-Newtonian fluid with power-law rheology. First, the fluids are assumed to have comparable effective viscosities. The approximation then furnishes a simplified model for which the linear stability problem can be solved analytically and concisely. Weakly nonlinear analysis and numerical computations indicate that instabilities saturate at low amplitude beyond onset and form steady wavetrains. Further from onset, secondary instabilities arise that destroy trains of widely spaced wave trains. Patterns of closely spaced waves, on the other hand, coarsen due to wave merger events. The two mechanisms select steady wavetrains with a characteristic spatial scale. The second lubrication theory assumes that the upper layer is far more viscous than the lower layer. As a result, the upper fluid flows almost rigidly, and extensional stresses can become promoted into the leading-order balance of forces. Interfacial instability still arises in Newtonian fluid layers, and the nonlinear dynamics is qualitatively unchanged. Significant complications arise when the upper fluid is non-Newtonian due to the behavior of the viscosity at zero strain rate.
Theoretical predictions of the decomposition mechanism of 1,3,3-trinitroazetidine (TNAZ)
Authors: Alavi Saman , Reilly Lisa M. , Thompson Donald L.
Published: October 2003

A theoretical study of the decomposition pathways of 1,3,3-trinitroazetidine (TNAZ) is described. Possible decomposition transition-states, intermediates, and products are identified and structures, energies, and vibrational frequencies are determined at the B3LYP/6-31G(d,p) level for these species. Four major pathways are apparent. Two pathways are initiated by the fission of the N–NO2 and C–NO2 bonds to yield radical intermediates, while the other two pathways involve the molecular elimination of HONO. Energy profiles for the pathways and possible routes to some of the experimentally observed species of TNAZ decomposition are presented. The energy required to initiate the NO2 bond fission pathways are 4–8 kcal/mol lower than the HONO elimination pathways. In the gas phase, the NO2 elimination pathways will be the dominant routes for TNAZ decomposition. In the condensed phase, however, this trend may be reversed.
Density functional and Monte Carlo studies of sulfur. II. Equilibrium polymerization of the liquid phase
Authors: Ballone P. , Jones R. O.
Published: October 2003

The equilibrium polymerization of sulfur is investigated by Monte Carlo simulations. The potential energy model is based on density functional results for the cohesive energy, structural, and vibrational properties as well as reactivity of sulfur rings and chains [Part I, J. Chem. Phys. 118, 9257 (2003)]. Liquid samples of 2048 atoms are simulated at temperatures 450⩽T⩽850 K and P=0 starting from monodisperse S8 molecular compositions. Thermally activated bond breaking processes lead to an equilibrium population of unsaturated atoms that can change the local pattern of covalent bonds and allow the system to approach equilibrium. The concentration of unsaturated atoms and the kinetics of bond interchanges is determined by the energy ΔEb required to break a covalent bond. Equilibrium with respect to the bond distribution is achieved for 15⩽ΔEb⩽21 kcal/mol over a wide temperature range (T⩾450 K), within which polymerization occurs readily, with entropy from the bond distribution overcompensating the increase in enthalpy. There is a maximum in the polymerized fraction at temperature Tmax that depends on ΔEb. This fraction decreases at higher temperature because broken bonds and short chains proliferate and, for T⩽Tmax, because entropy is less important than enthalpy. The molecular size distribution is described well by a Zimm–Schulz function, plus an isolated peak for S8. Large molecules are almost exclusively open chains. Rings tend to have fewer than 24 atoms, and only S8 is present in significant concentrations at all T. The T dependence of the density and the dependence of polymerization fraction and degree on ΔEb give estimates of the polymerization temperature Tf=450±20 K.
Raman band shape analysis of a low temperature molten salt
Authors: Cavalcante Ary O. , Ribeiro Mauro C. C.
Published: October 2003

The salt tetra(n-butyl)ammonium croconate, [(n-C4H9)4N]2C5O5⋅4H2O, (TBCR), is a very viscous glassforming liquid which undergoes a glass transition at room-temperature. Raman band shape analysis of the totally symmetric ring breathing mode of the croconate dianion, C5O52−, was performed by Fourier analysis. The vibrational time correlation functions obtained from the isotropic Raman spectra were modelled with well-known models for vibrational dephasing. The time correlation functions of pure TBCR and of TBCR in acetonitrile solutions were compared with previous results for the simple salt Li2C5O5 in aqueous solution. It has been found remarkable changes of the dynamic parameters characterizing the vibrational dephasing of C5O52− in these different environments. Discontinuous temperature dependence of the dephasing parameters was observed at the glass transition temperature of pure TBCR. In glassy TBCR, however, common models for vibrational dephasing are not strictly valid because the Raman bands display clear asymmetric shapes. The experimental data in glassy TBCR were also reproduced with a model that considers the second and the third order terms in the cumulant expansion of the vibrational correlation function.
Photoluminescence and radiationless processes in Mn2+-doped Ca1−xSrxF2 fluorites as a function of pressure and temperature. A structural correlation study
Authors: Rodrı́guez F. , Hernández I. , Moreno M. , Alcalá R.
Published: October 2003

This work investigates the photoluminescence (PL) properties of Mn2+-doped fluorites in CaF2, BaF2, SrF2, and in solid solutions Ca1−xSrxF2. In particular, we focus on the radiationless processes leading to the surprising disappearance of the Mn2+ PL on passing from CaF2: Mn2+ to SrF2: Mn2+ or BaF2: Mn2+. For this purpose emission, excitation, lifetime, and time-resolved spectroscopy as a function of pressure and temperature are carried out in these compounds as well as in the Ca1−xSrxF2:Mn2+ (x=0–1) series using pressure spectroscopy. We show that the quenching of PL in these systems is associated with nonradiative thermal activated processes whose activation energy and pre-exponential rates strongly depend on the crystal volume irrespective of the chemical composition of the host crystal. A salient feature of this work deals with the increase of activation energy induced by pressure, whose variation with the lattice parameter is given by Ea(eV)=1.02–2.64 (a-5.46), with a(CaF2)=5.46 Å. It leads to a PL quantum yield enhancement, which favors appearance of Mn2+ PL even in the non-PL systems SrF2:Mn2+ and BaF2:Mn2+. Furthermore, the activation energy mainly depends on the crystal volume per molecule irrespective of the crystal structure or the local symmetry around the impurity. In this way, the relevance of the fluorite-to-cotunnite phase transition to enhance PL is analyzed. This enhancement is explained in terms of the large volume reduction at the phase transition, as well as by the presence of low-symmetry crystal fields attained at the cation site yielding an increase of the radiative transition rate by the electric-dipole mechanism.
Mode-correlated product pairs in the F+CHD3→DF+CHD2 reaction
Authors: Zhou Jingang , Lin Jim J. , Liu Kopin
Published: October 2003

The title reaction was investigated at three different collision energies in a pulsed, crossed-beam apparatus. The (2+1) REMPI spectra of the CHD2 products revealed, in addition to the anticipated 4mn vibronic bands, a hitherto unobserved feature. The new feature was shown and assigned to the 311 band. A time-sliced ion velocity imaging technique was applied to map out the coincident DF attributes of the two product states 42 and 31, whose energy levels lie nearly degenerate. Remarkably similar results were found for the two states in every aspect at all three collision energies. A simple model of Fermi-coupled states was proposed to rationalize this, at first sight, surprising finding. Implications to collisional processes which involve mixed molecular basis states in general are outlined. Possible quantum interference phenomenon is suggested.
Critical parameters of unrestricted primitive model electrolytes with charge asymmetries up to 10:1
Authors: Cheong Daniel W. , Panagiotopoulos Athanassios Z.
Published: October 2003

The phase behavior of charge- and size-asymmetric primitive model electrolytes has been investigated using reservoir grand canonical Monte Carlo simulations. The simulations rely on the insertion and removal of neutral ion clusters from a reservoir of possible configurations. We first validated our approach by investigating the effect of Rc, the maximum allowable distance between the central cation and its associated anions, on the critical parameters of 2:1 and 3:1 electrolytes. We have shown that the effect of Rc is weak and does not change the qualitative dependence of the critical parameters on size and charge asymmetry. The critical temperature for 2:1 and 3:1 electrolytes shows a maximum at Rc≈3, while the critical volume fraction decreases more or less monotonically, consistent with previous results for 1:1 electrolytes by Romero-Enrique et al. [Phys. Rev. E 66, 041204 (2002)]. We have used the reservoir method to obtain the critical parameters for 5:1 and 10:1 electrolytes. The critical temperature decreases with increasing charge asymmetry and shows a maximum as a function of δ, the size asymmetry parameter. The critical volume fraction however, defined as the volume occupied by ions divided by the total volume of the simulation box, increases with increasing charge asymmetry and exhibits a minimum as a function of δ. This trend is contrary to what is generally predicted by theories, although more recent approaches based on the Debye–Hückel theory reproduce this observed trend. Our results deviate somewhat from the predictions of Linse [Philos. Trans. R. Soc. London, Ser. A 359, 853 (2001)] for the scaling of the critical temperature for a system of macroions with point counterions.
Theoretical study of the static first hyperpolarizability of azo-enaminone compounds
Authors: de Oliveira H. C. B. , Fonseca T. L. , Castro M. A. , Amaral O. A. V. , Cunha S.
Published: October 2003

In this work the static electric properties of azo-enaminones, with special emphasis to the vector component of the first hyperpolarizability βvec, are determined at the Hartree–Fock (HF) level with the electron correlation (EC) effects included through the second-order Møller–Plesset perturbation theory (MP2). The ab initio results, in accordance with previous semiempirical calculations, show that appropriate choices of substituents to be incorporated to the molecular structure can have a marked influence on the first hyperpolarizability. An initial study about the changes on the βvec values of these compounds, as a result of the incorporation of different donor groups, indicates that this property increases as function of the donor group strength tending to a saturated value. A comparison between our HF and MP2 results, for all compounds studied here, show that the βvec values are strongly affected by the effects of the electron correlation correction.
Ab initio studies of He–HCCCN interaction
Authors: Akin-Ojo Omololu , Bukowski Robert , Szalewicz Krzysztof
Published: October 2003

Five two-dimensional potential energy surfaces for the interaction of He with cyanoacetylene (HCCCN) are presented, obtained from ab initio calculations using symmetry-adapted perturbation theory and the supermolecular method at different levels of electron correlation. HCCCN is taken to be a rigid linear molecule with the interatomic distances fixed at the experimental “r0” geometry extracted from ground-state rotational constants. The complex was found to have a global minimum at a T-shaped configuration and a secondary minimum at the linear configuration with the He atom facing the H atom. Two saddle points were also located. There is good agreement between the positions of the stationary points on each of the five surfaces though their energies differ by up to 19%. Rovibrational bound state calculations were performed for the He-HCCCN4 and He-HCCCN3 complexes. Spectra (including intensities) and wave functions of He-HCCCN4 obtained from these calculations are presented. The effective rotational constant of HCCCN solvated in a helium droplet was estimated by minimizing the energy of Hen–HCCCN for n=2–12, selecting the n=7 complex as giving the largest magnitude of interaction energy per He, and shifting the resulting ring of He atoms to the position corresponding to the average geometry of the ground state of the He–HCCCN dimer. This estimate is within 4.8% of the measured value.
Quantum and classical structures for He4 clusters with the H− impurity
Authors: Sebastianelli F. , Di Paola C. , Baccarelli I. , Gianturco F. A.
Published: October 2003

The structural properties and the energetics of some of the smaller HenH− clusters, with n varying from 2 to 14, are examined both with classical and quantum treatments, making also a comparison with the corresponding neutral systems Hen. The results of the calculations, the physical reliability of the employed interaction modeling, and the comparison with previous results are discussed. The emerging picture shows that for clusters of this size the dopant ion H− always locates itself outside the Hen moiety.
Resonant two-photon ionization spectroscopy of the van der Waals complex C6H5CH3⋯N2: Structure, binding energy, intermolecular vibrations, and internal rotation
Authors: Hu Yihua , Yang Shihe
Published: October 2003

Resonant two-photon ionization spectrum of C6H5CH3⋯N2 has been recorded, which exhibits rich information about the van der Waals vibrational modes and internal rotation of N2. An ab initio calculation shows a Cs point group for the complex, in which the mass center of N2 is ∼0.35 nm above the benzene ring and the molecular axis of N2 is in the symmetry plane but tilted slightly away from the CH3 group by ∼2°. With the help of isotopic substitution, quantum mechanics calculations, and systematic comparisons, we have tentatively assigned all the observed spectral features. The van der Waals bonding is shown to have an inhibitory effect on the internal rotation of the CH3 group. Bond dissociation energies for the complex C6H5CH3⋯N2 in the excited electronic state and in the ground electronic state are estimated to be 494 and 474 cm−1, respectively, in good agreement with the calculated value.
Tunneling dissociation from a double well via path integrals
Authors: Douvropoulos Theodosios G. , Nicolaides Cleanthes A.
Published: October 2003

It is shown how the semiclassical theory of path integrals can be implemented in a practical manner for the analysis of a potential that combines the two-state system of a double well potential (DWP) with decay into a continuous spectrum. This potential may correspond to a variety of physical situations in physics and chemistry. The structure of the formalism and of the results is such that it allows computation not only for analytic but also for numerically given potentials. The central theme is the determination of the energy-dependent Green’s function, which is shown to consist of a regular part and a part containing simple and double complex poles. These poles represent the position of the energy levels, as well as the energy widths and shifts due to the interaction with the continuous spectrum. When applied to the bound DWP without tunneling, the theory is shown to reduce in certain limits to known results from the Jeffreys–Wentzel–Kiamers–Bhrillouin approximation. If the system is taken to be prepared in the first well, the interactions with the remaining of the potential lead to two types of transition rates. One represents the transient motion toward a virtual equilibrium state of the DWP. It emerges as a positive imaginary part of the self-energy. The other represents the decay into the continuum and emerges as a negative imaginary part of the pole. Comparison of the two mechanisms of nonstationarity is made for different magnitudes of the second barrier relative to the first one. Since the system decays to the continuum while oscillating, the theory obtains a correction to the frequency of oscillation in the DWP due to the interaction with the continuum. This phenomenon is observable in real two-state systems, if an external perturbation which affects mainly one state converts it into a resonance state.
Indirect (J) coupling of inequivalent As75 nuclei in crystalline and glassy As2Se3 and As2S3
Authors: Whitaker J. , Ahn E. , Hari P. , Williams G. A. , Taylor P. C. , Facelli J. C.
Published: October 2003

We present the results of nuclear quadrupole resonance (NQR) measurements in crystalline and glassy As2Se3 and As2S3. As a function of pulse separation, the decays of the NQR Hahn echoes following a 90°–180° pulse sequence exhibit damped oscillations superimposed on an exponential decay. These damped oscillations can be explained by an indirect coupling (J coupling) between two As nuclei through the polarized electrons on the chalcogen atom. Experimental values of the J couplings were obtained from the periods of the oscillations and calculations of the most probable transitions using second-order perturbation theory. The value estimated by this method for the J2(75As–S–75As) in crystalline As2S3 compares well with empirical estimates, which are obtained using an existing value of J2(31P–S–31P) and known scalings with atomic number from the literature. Unfortunately, there is insufficient experimental data on J2(X–Se–X) couplings to make any empirical estimate of J2(75As–Se–75As).
Microwave spectrum, structure, and internal motion of the open-shell van der Waals complex Ar–ClO2
Authors: Schäfer Martin , Ha Tae-Kyu , Bauder Alfred
Published: October 2003

The rotational spectrum of the open-shell complex argon–chlorine dioxide has been measured between 5–24 GHz with a pulsed nozzle Fourier transform microwave spectrometer. This complex displays a- and c-type spectra, the latter of which is shifted by internal motion of the ClO2 unit. Structure information has been gained from the analysis of the rotational constants and of the fine and hyperfine structure parameters of the Ar–35ClO2 and Ar–37ClO2 complexes. Changes of the electronic structure of ClO2 upon complexation are shown to be very small. The observed internal motion effect was analyzed with a model of an internal rotation of the ClO2 subunit around its a axis. In addition, the structure of the complex and the barrier hindering internal motion has also been explored by ab initio calculations. The results of the analysis of the structure and internal motion of Ar–ClO2 were compared with those of the similar argon complexes of SO2, O3, and NO2.
Analyzing the biopolymer folding rates and pathways using kinetic cluster method
Authors: Zhang Wenbing , Chen Shi-Jie
Published: October 2003

A kinetic cluster method enables us to analyze biopolymer folding kinetics with discrete rate-limiting steps by classifying biopolymer conformations into pre-equilibrated clusters. The overall folding kinetics is determined by the intercluster transitions. Due to the complex energy landscapes of biopolymers, the intercluster transitions have multiple pathways and can have kinetic intermediates (local free-energy minima) distributed on the intercluster pathways. We focus on the RNA secondary structure folding kinetics. The dominant folding pathways and the kinetic partitioning mechanism can be identified and quantified from the rate constants for different intercluster pathways. Moreover, the temperature dependence of the folding rate can be analyzed from the interplay between the stabilities of the on-pathway (nativelike) and off-pathway (misfolded) conformations and from the kinetic partitioning between different intercluster pathways. The predicted folding kinetics can be directly tested against experiments.
Glass transition of small polystyrene spheres in aqueous suspensions
Authors: Sasaki Takashi , Shimizu Atsushi , Mourey Thomas H. , Thurau Courtney T. , Ediger M. D.
Published: October 2003

The glass transition temperature Tg of polystyrene spheres in aqueous suspension was investigated by differential scanning calorimetry. Spheres with diameters of 42–548 nm show an unambiguous glass transition very near the Tg of bulk polystyrene. The magnitude of the observed heat capacity jump ΔCp at the transition decreases as the sphere size decreases. These results are interpreted as indicating that the center portion of a sphere has bulklike dynamics while an outer shell has substantially faster dynamics than the bulk. The ΔCp values are consistent with a mobile layer approximately 4 nm thick. In contrast, free-standing polystyrene films with thicknesses similar to these sphere diameters have been reported to show a single glass transition substantially below the bulk Tg value.
Intermolecular exchange of vibrational population in a binary liquid mixture
Authors: Graener H. , Patzlaff T. , Paradowska-Moszkowska K. , Seifert G.
Published: October 2003

Using a picosecond infrared pump–probe technique, we have studied the decay of transient excess population of the CH stretch vibrational modes in binary liquid mixtures of CHBr3 and CHCl3. Analyzing the concentration dependence of the observed decay curves on the basis of a simple rate equation model gives strong evidence for an effective resonant transfer of vibrational energy between the two different types of molecules. A value of κ=[(22±5) ps]−1 is determined for the rate constant κ describing this process. This value is larger than the total relaxation rates of both neat bromoform and chloroform indicating that even there CH stretching quanta can be transferred from molecule to molecule several times before they are relaxing.
Resonance gas oscillations in closed tubes: Numerical study and experiments
Authors: Alexeev A. , Gutfinger C.
Published: October 2003

Periodic gas oscillations in closed tubes are investigated experimentally and numerically. At resonance, these oscillations are accompanied by shock waves traveling back and forth along the tube. Results of gas temperature and pressure measurements are reported. It is found that the gas temperature changes substantially along the tube. A two-dimensional numerical model of turbulent gas oscillations is developed and verified by a comparison with experiments. It is found that the experimental data of temperature and pressure inside the resonance tube are well correlated by the numerical model. Using the numerical model, turbulence and acoustic streaming at resonance are investigated. It is shown that the direction of gas streaming at resonance is opposite to that in nonresonant oscillations. A parametric investigation of resonant flow is performed in terms of the tube parameter ε, acoustic Reynolds number Rea and dimensionless tube length Λ. In particular, it is found that the normalized pressure amplitude, as well as other flow characteristics, are functions of a single parameter E=Rea1/3/Λ. Useful correlations for flow characteristics are proposed in terms of tube dimensionless parameters.
Control of vortex breakdown by temperature gradients
Authors: Herrada Miguel Angel , Shtern Vladimir
Published: October 2003

An axial gradient of temperature can either suppress or enhance vortex breakdown (VB). The underlying mechanism of such VB control is centrifugal or/and gravitational convection. An additional thermal-convection flow directed oppositely to the base flow suppresses VB while a co-flow enhances VB. Our numerical simulations of a compressible flow in a sealed cylinder induced by a rotating bottom disk clearly reveal these effects. We vary the temperature gradient (ε), Mach (Ma), Froude (Fr), and Reynolds (Re) numbers, and the aspect ratio (h). As ε increases (ε>0 corresponding to a temperature gradient parallel to the downward near-axis flow), the VB “bubble,” which occurs at ε=0, diminishes and then totally disappears. The opposite temperature gradient (ε<0) enlarges the VB bubble and makes the flow unsteady. These effects of centrifugal convection become more prominent with increasing Ma and Re. Density variations induced by the temperature gradients are more important for VB control than those induced by the increase in Ma. A new efficient time-evolution code for axisymmetric flows of an ideal gas has facilitated these simulations.
Conditional and unconditional acceleration statistics in turbulence
Authors: Sawford B. L. , Yeung P. K. , Borgas M. S. , Vedula P. , La Porta A. , Crawford A. M. , Bodenschatz E.
Published: October 2003

In this paper we study acceleration statistics from laboratory measurements and direct numerical simulations in three-dimensional turbulence at Taylor-scale Reynolds numbers ranging from 38 to 1000. Using existing data, we show that at present it is not possible to infer the precise behavior of the unconditional acceleration variance in the large Reynolds number limit, since empirical functions satisfying both the Kolmogorov and refined Kolmogorov theories appear to fit the data equally well. We also present entirely new data for the acceleration covariance conditioned on the velocity, showing that these conditional statistics are strong functions of velocity, but that when scaled by the unconditional variance they are only weakly dependent on Reynolds number. For large values of the magnitude u of the conditioning velocity we speculate that the conditional covariance behaves like u6 and show that this is qualitatively consistent with the stretched exponential tails of the unconditional acceleration probability density function (pdf). The conditional pdf is almost identical in shape to the unconditional pdf. From these conditional covariance data, we are able to calculate the conditional mean rate of change of the acceleration, and show that it is consistent with the drift term in second-order Lagrangian stochastic models of turbulent transport. We also calculate the correlation between the square of the acceleration and the square of the velocity, showing that it is small but not negligible.
A model for the reduction of the turbulent energy redistribution by compressibility
Authors: Heinz Stefan
Published: October 2003

Compressibility may strongly reduce the redistribution of turbulent kinetic energy. The question of how such effects can be taken into account in turbulence models is addressed here. First, a corresponding stochastic turbulence model is developed on the basis of a simplification of the generalized Langevin model for turbulent velocities. This model is then reduced to a deterministic model that extends existing methods by the consideration of structural compressibility effects. Combined with stochastic models for scalars, these velocity models may be used for compressible turbulent reacting flow simulations where the consideration of chemical reactions does not require approximations.
Intermittent dry granular flow in a vertical pipe
Authors: Bertho Yann , Giorgiutti-Dauphiné Frédérique , Hulin Jean-Pierre
Published: October 2003

The intermittent compact flow of glass beads in a vertical glass pipe of small diameter is studied experimentally by combining particle fraction, pressure, and air, and grain flow rates measurements with a spatiotemporal analysis of the flow. At the onset of the flow, a decompaction front is observed to propagate from the bottom to the top of the tube at a velocity much larger than that of the grains. The blockage front also propagates upward and at a still higher velocity. The decompaction induces a decreasing pressure wave strongly amplified as it propagates upward toward the top of the tube. Pressure variations of 3000 Pa or more are detected in this region while particle fraction variations are of the order of 0.02. Grain velocities during the flow period also increase strongly at the top of the tube while the corresponding fraction of total time decreases. A one-dimensional numerical model based on a simple relation between the effective acceleration of the grains and the particle fraction variations reproduces the amplification effect and provides predictions for its dependence on the permeability of the packing.
Formation and separation of merged liquid sheets developed from the mixing of coaxial swirling liquid sheets
Authors: Sivakumar D. , Raghunandan B. N.
Published: October 2003

Liquid–liquid coaxial swirl atomizers are used in liquid rocket engines to achieve an efficient mixing between the fuel and oxidizer sprays. The characteristics of the mixed spray are mainly controlled by the flow behavior of merged liquid sheet originating at the contact point of inner and outer swirling liquid sheets. With an intention of identifying various flow regimes of merged liquid sheet at different conditions of inner and outer liquid sheets, we report here a fundamental experimental investigation on the characteristics of merged liquid sheets using water as the experimental liquid. The physical processes involved in the formation and separation of a merged liquid sheet are described from the experimental measurements. For a given outer liquid sheet condition, the merged liquid sheet forms and separates at specific inner liquid sheet flow conditions. At low outer liquid sheet flow conditions with Weber number less than 50, the merged liquid sheet exhibits a self-sustaining periodic separation process, whose frequency increases with increasing inner liquid sheet Weber number for a given outer liquid sheet Weber number. Experimental measurements are presented to show that the dynamics of the contact point plays a major role in governing the characteristics of merged liquid sheets.
Accurate determination of SiO2 film thickness by x-ray photoelectron spectroscopy
Authors: Takahashi K. , Nohira H. , Hirose K. , Hattori T.
Published: October 2003

We established the number of Si 2p photoelectrons emanating from a Si(100) substrate covered with a silicon–oxide film as a function of azimuthal and polar angles using the oxide film thickness as a parameter. The elastic and inelastic scattering cross sections of Si 2p photoelectrons in silicon oxide were deduced by reproducing the experimental results with Monte Carlo simulation for the path of Si 2p photoelectrons in silicon oxide. Based on the simulation, we found that the elastic scattering of Si 2p photoelectrons in silicon oxide could effectively be neglected in several specific directions. We also found that an emitting direction different to these specific directions is indispensable when precisely determining the thickness using XPS with a large receiving angle.
Stability diagrams for disconnected capillary surfaces
Authors: Slobozhanin Lev A. , Alexander J. Iwan D.
Published: October 2003

Disconnected free surfaces (or interfaces) of a connected liquid volume (or liquid volumes) occur when the boundary of the liquid volume consists of two or more separate surface components Γi (i=1,…,m) that correspond to liquid–gas (or liquid–liquid) interfaces. We consider disconnected surfaces for which each component Γi is axisymmetric and crosses its own symmetry axis. In most cases, the stability problem for an entire disconnected equilibrium capillary surface subject to perturbations that conserve the total liquid volume reduces to the same set of problems obtained when separately considering the stability of each Γi to perturbations that satisfy a fixed pressure constraint. For fixed pressure perturbations, the stability of a given axisymmetric Γi can be found through comparison of actual and critical values of a particular boundary parameter. For zero gravity, these critical values are found analytically. For non-zero gravity, an analytical representation of the critical values is not generally possible. In such cases, a determination of stability can be accomplished by representing all possible equilibrium surface profiles on a dimensionless “height–radius” diagram. This diagram is contoured with critical values of the boundary parameter. The stability diagram can, in most cases, be used to determine the stability of a disconnected surface (subject to perturbations that conserve the total volume) that is composed of components that are represented by given equilibrium profiles on the diagram. To illustrate this approach, solutions of stability problems for systems consisting of a set of sessile or pendant drops in contact with smooth planar walls or with the edges of equidimensional perforated holes in a horizontal plate are presented.
Flatness of tracer density profile produced by a point source in turbulence
Authors: Gleeson James P. , Pullin D. I.
Published: October 2003

The average concentration of tracers advected from a point source by a multivariate normal velocity field is shown to deviate from a Gaussian profile. The flatness (kurtosis) is calculated using an asymptotic series expansion valid for velocity fields with short correlation times or weak space dependence. An explicit formula for the excess flatness at first order demonstrates maximum deviation from a Gaussian profile at time t of the order of five times the velocity correlation time, with a t−1 decay to the Gaussian value at large times. Monotonically decaying forms of the velocity time correlation function are shown to yield negative values for the first order excess flatness, but positive values can result when the correlation function has an oscillatory tail.
Crystallographic influence on nanomechanics of (100)-oriented silicon resonators
Authors: Wang Dong F. , Ono Takahito , Esashi Masayoshi
Published: October 2003

The influence of crystallographic orientation on the nanomechanical properties of 50 nm thick (100)-oriented single-crystal silicon resonators was investigated by examining the effects of surface treatments, such as flash-heating, on the mechanical quality factors (Q-factors) and resonant frequencies. The measured Q-factors were found to vary periodically with crystallographic orientation and were shown to have higher values in the 〈110〉 direction. A 1500 nm thick (100)-oriented cantilever array was also studied for comparison, for which no obvious periodic change was observed. Since the energy dissipation in vibrating resonators cannot be explained by the support loss or thermoelastic loss, a surface-related mechanism should be considered.
Growth and characteristics of TaN/TiN superlattice structures
Authors: Wang H. , Zhang X. , Gupta A. , Tiwari Ashutosh , Narayan J.
Published: October 2003

Epitaxial B1 NaCl-structured TaN(3 nm)/TiN(2 nm) superlattice structures were grown on Si(100) substrates with a TiN buffer layer, using pulsed-laser deposition. A special target assembly was used to manipulate the thickness of each layer. X-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy (Z contrast) studies confirmed the single-crystalline nature of the superlattice with a uniform layer structure. Nanoindentation results suggest the high hardness of these superlattice structures. Four-point-probe resistivity measurements show low resistivity of the heterostructures and a Cu diffusion characteristic study proved this superlattice system can be a promising diffusion barrier and can withstand 700 °C annealing for 30 min.
Susceptibility of atomic force microscope cantilevers to lateral forces: Experimental verification
Authors: Sader John E. , Sader Raymond C.
Published: October 2003

The performance of the atomic force microscope (AFM) is underpinned by the properties of its force-sensing microcantilever. Due to the universal belief that V-shaped cantilevers are more resistant to lateral forces than rectangular cantilevers, V-shaped cantilevers are used widely in the AFM. A recent theoretical study [J. E. Sader, Rev. Sci. Instrum. 74, 2438 (2003)], however, discounts this entrenched operating principle by theoretically establishing that V-shaped AFM cantilevers offer less resistance to lateral forces than rectangular cantilevers. Since this finding is difficult to test on (microscopic) AFM cantilevers, we conduct detailed measurements on model macroscopic cantilevers, and thus verify this theoretical prediction.
Proton beams generated with high-intensity lasers: Applications to medical isotope production
Authors: Fritzler S. , Malka V. , Grillon G. , Rousseau J. P. , Burgy F. , Lefebvre E. , d’Humières E. , McKenna P. , Ledingham K. W. D.
Published: October 2003

Proton beams of up to 10 MeV have been obtained by the interaction of a 10 Hz “table-top” laser, focused to intensities of 6×1019 W/cm2, with 6-μm-thin foil targets. Such proton beams can be used to induce B4(p,n)C11 reactions, which could yield an integrated activity of 13.4 MBq (0.36 mCi) after 30 min laser irradiation. This can be extended to GBq levels using similar lasers with kilohertz repetition rates, making this positron-emission tomography isotope production scheme comparable to the one using conventional accelerators.
Terahertz imaging with nanometer resolution
Authors: Chen Hou-Tong , Kersting Roland , Cho Gyu Cheon
Published: October 2003

We report on the application of scanning near-field optical microscopy for terahertz imaging. We demonstrate a spatial resolution of 150 nm for 2.0 THz pulses. Our experiments show the feasibility of submicron THz microscopy for imaging of biologic tissues on the cell level or for the investigation of individual submicron semiconductor devices.
Role of boron for defect evolution in hydrogen-implanted silicon
Authors: Lee J. K. , Höchbauer T. , Averitt R. D. , Nastasi M.
Published: October 2003

The mechanism underlying the exfoliation phenomenon in B+H coimplanted Si is presented. Compared with only H implantation, H-implanted Si samples that received a B preimplant were observed to have a decrease in implantation-induced lattice damage, in spite of enhanced blistering behavior, which was more pronounced for nonactivated B. Infrared spectroscopy showed that the concentration of Si–H multivacancy defects was also decreased in the B+H coimplanted Si relative to H-only-implanted Si. These decreases in the damage and H-defect density suggest that the lower damage in the B+H coimplanted Si results from a reduction in H-trap density, which in turn enhances H diffusion and the kinetics of blistering. This reduction in lattice damage and H–Si-defect complexes is attributed to the formation of a kinetically active B–Si interstitial complex that is prevalent in nonactivated B-implanted Si.
Efficient generation of subpicosecond terahertz radiation by phase-matched optical rectification using ultrashort laser pulses with tilted pulse fronts
Authors: Stepanov Andrei G. , Hebling János , Kuhl Jürgen
Published: October 2003

We report on the generation of subpicosecond terahertz (THz) electromagnetic radiation in a nonlinear crystal by ultrashort laser pulses with a tilted pulse front. Free-space THz pulses with an energy of 98 pJ and a repetition rate of 200 kHz have been obtained by optical rectification using 2.3 μJ near-infrared laser pulses in a LiNbO3 crystal cooled down to 77 K. The emitted THz beam has a divergence smaller than 40 mrad, which is close to the diffraction limit. Spectral analysis of the generated THz beam with a Michelson interferometer reveals an approximately 1.5 THz broad asymmetric peak centered at 2 THz and a pulse duration of less than 500 fs.
High mobility organic transistors fabricated from single pentacene microcrystals grown on a polymer film
Authors: Wang Guanzhong , Luo Yi , Beton Peter H.
Published: October 2003

Isolated multilayer, bilayer, and monolayer crystals of pentacene are grown on polymer thin films. Source and drain contacts are formed to individual crystallites and transistor operation, based on the formation of an inversion layer at the pentacene/polymer interface is demonstrated for multilayers, trilayers, and bilayers. Hole inversion layers are formed with mobilities up to 1.2 cm2/V s for multilayer crystals, while a reduced mobility is observed for trilayers and bilayers.
ac conductivity and conduction mechanism of NaNbO3 semiconductor antiferroelectric ceramic: A relaxational approach at high temperature
Authors: Nobre M. A. L. , Lanfredi S.
Published: October 2003

The electric properties of the sodium niobate perovskite ceramic were investigated by impedance spectroscopy in the frequency range from 5 Hz to 13 MHz and from room temperature up to 1073 K, in a thermal cycle. Both capacitance and conductivity exhibit an anomaly at around 600 K as a function of the temperature and frequency. The electric conductivity as a function of angular frequency σ(ω) follows the relation σ(ω)=Aωs. The values of the exponent s lie in the range 0.15⩽s⩽0.44. These results were discussed considering the conduction mechanism as being a type of polaron hopping.
Growth, branching, and kinking of molecular-beam epitaxial 〈110〉 GaAs nanowires
Authors: Wu Z. H. , Mei X. , Kim D. , Blumin M. , Ruda H. E. , Liu J. Q. , Kavanagh K. L.
Published: October 2003

GaAs nanowires were grown on GaAs (100) substrates by vapor–liquid–solid growth. About 8% of these nanowires grew in 〈110〉 directions with straight, Y-branched or L-shaped morphologies. The role of strain-induced reduction in surface free energy is discussed as a possible factor contributing to the evolution of 〈110〉 nanowires. Kinking and branching is attributed to growth instabilities resulting from equivalent surface free energies for 〈110〉 growth directions. Transmission electron microscopy verified that 〈110〉 nanowires are defect free.
Room-temperature preparation of β-FeSi2 microprecipitates by the KrF excimer laser ablation of an iron disilicide alloy target
Authors: Narazaki Aiko , Sato Tadatake , Kawaguchi Yoshizo , Niino Hiroyuki
Published: October 2003

We have fabricated β-FeSi2 microprecipitates at room temperature using KrF excimer laser ablation of an α-FeSi2 alloy target. Raman spectra with a spatial resolution of 1 μm confirmed that the micron-sized droplets generated by laser ablation precipitate as the β-FeSi2 crystalline phase on a silicon substrate maintained at RT, whereas the rest of the deposited film is amorphous. It was also found that films containing a high density of β-FeSi2 precipitates exhibited 1.55 μm photoluminescence at low temperature (up to 200 K) after annealing at 800 °C for 6 h in an argon atmosphere.
Determination of the electric field in 4H/3C/4H-SiC quantum wells due to spontaneous polarization in the 4H SiC matrix
Authors: Bai S. , Devaty R. P. , Choyke W. J. , Kaiser U. , Wagner G. , MacMillan M. F.
Published: October 2003

We report a low-temperature photoluminescence study of 4H/3C/4H-SiC single quantum wells. A quantum well consists of thirteen 3C-SiC bilayers as displayed in a high-resolution transmission electron microscope image. The optical emission energy of the quantum well is more than 200 meV below the exciton band gap of bulk 3C-SiC. A strong internal electric field on the order of 1 MV/cm leads to the large redshift of the emission energy due to the quantum-confined Stark effect. The origin of this field is discussed in terms of the spontaneous polarization difference between 3C- and 4H-SiC.
Role of surface plasmons in the optical interaction in metallic gratings with narrow slits
Authors: Sun Zhijun , Jung Yun Suk , Kim Hong Koo
Published: October 2003

We report an experimental study of the transmission of light through narrow slits in metallic gratings (Ag layer thickness of 100–400 nm, grating period of 370 or 780 nm, and slit width of 30–100 nm). Peak transmission of ∼60% is observed for TM polarization at a wavelength redshifted from the point of surface plasmon (SP) resonance at the metal/substrate interface. At the transmission minima, the angular dependence of reflection shows a sharp peak with minimum loss of optical power. Two types of surface plasmon excitation are found responsible for the observed transmission dips: (1) the SP resonance along the planes that comprise either the metal/air or metal/substrate interfaces and (2) the SP resonance localized along the surface that encloses each metal island separated by slits.
Critical pressure for weakening of size-induced stiffness in spinel-structure Si3N4 nanocrystals
Authors: Wang Zhongwu , Zhao Yusheng , Schiferl David , Zha C. S. , Downs Robert T. , Sekine T.
Published: October 2003

We demonstrate that the compressibility of spinel Si3N4 nanocrystals increases when the pressure is raised above ∼40 GPa. Si3N4 nanocrystals initially exhibit an extremely high bulk modulus of 685(45) GPa. But, above 40 GPa, the bulk modulus is reduced to 415(10) GPa. Thus, a critical pressure of ∼40 GPa was determined that signifies the onset of size-induced weakening of elastic stiffness in nanocrystalline Si3N4. Enhanced surface energy contributions to the shell layers of nanoparticles and the resulting effect on the corresponding large d-spacing planes are used to explain the observed phenomenon. Upon decompression, the bulk modulus of the Si3N4 spinel remained at its lower value of 415 GPa. This study provides a reasonable explanation for the different compressibility properties of numerous nanocrystals.
Ultrathin single-crystalline-silicon cantilever resonators: Fabrication technology and significant specimen size effect on Young’s modulus
Authors: Li Xinxin , Ono Takahito , Wang Yuelin , Esashi Masayoshi
Published: October 2003

Ultrathin resonant cantilevers are promising for ultrasensitive detection. A technique is developed for high-yield fabrication of single-crystalline-silicon cantilevers as thin as 12 nm. The formed cantilever resonators are characterized by resonance testing in high vacuum. Significant specimen size effect on Young’s modulus of ultrathin (12–170 nm) silicon is detected. The Young’s modulus decreases monotonously as the cantilevers become thinner. The size effect is consistent with the published simulation results of direct-atomistic model, in which surface effects are taken into consideration.
Temperature and strain rate effects on the strength and ductility of nanostructured copper
Authors: Wang Y. M. , Ma E.
Published: October 2003

We show that the yield strength for Cu with ultrafine grain sizes becomes obviously temperature and strain-rate dependent, in contrast to the temperature/rate insensitive behavior of conventional face-centered-cubic metals. A thermally activated deformation mechanism is operative at room temperature and especially at slow strain rates, but not at 77 K. In addition to the gain in strength, the tensile ductility and particularly uniform strains also increase at cryogenic temperatures and with increasing strain rate, as a result of improved strain hardening due to suppressed dynamic recovery.