bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of driven three-dimensional thin films: From hydrophilic to superhydrophobic regimes
Authors: Ledesma-Aguilar R. , Hernández-Machado A. , Pagonabarraga I.
Published: July 2008

We study the forced displacement of a thin film of fluid in contact with vertical and inclined substrates of different wetting properties, that range from hydrophilic to hydrophobic, using the lattice-Boltzmann method. We study the stability and pattern formation of the contact line in the hydrophilic and superhydrophobic regimes, which correspond to wedge-shaped and nose-shaped fronts, respectively. We find that contact lines are considerably more stable for hydrophilic substrates and small inclination angles. The qualitative behavior of the front in the linear regime remains independent of the wetting properties of the substrate as a single dispersion relation describes the stability of both wedges and noses. Nonlinear patterns show a clear dependence on wetting properties and substrate inclination angle. The effect is quantified in terms of the pattern growth rate, which vanishes for the sawtooth pattern and is finite for the finger pattern. Sawtooth shaped patterns are observed for hydrophilic substrates and low inclination angles, while finger-shaped patterns arise for hydrophobic substrates and large inclination angles. Finger dynamics show a transient in which neighboring fingers interact, followed by a steady state where each finger grows independently.
Inclusion of explicit electron-proton correlation in the nuclear-electronic orbital approach using Gaussian-type geminal functions
Authors: Chakraborty Arindam , Pak Michael V. , Hammes-Schiffer Sharon
Published: July 2008

The nuclear-electronic orbital explicitly correlated Hartree-Fock (NEO-XCHF) approach for including electron-proton correlation in mixed nuclear-electronic wavefunctions is presented. A general ansatz for the nuclear-electronic wavefunction that includes explicit dependence on the nuclear-electronic distances with Gaussian-type geminal functions is proposed. Based on this ansatz, expressions are derived for the total energy and the electronic and nuclear Fock operators for multielectron systems. The explicit electron-proton correlation is incorporated directly into the self-consistent-field procedure for optimizing the nuclear-electronic wavefunction. This approach is computationally practical for many-electron systems because only a relatively small number of nuclei are treated quantum mechanically, and only electron-proton correlation is treated explicitly. Electron-electron correlation can be included by combining the NEO-XCHF approach with perturbation theory, density functional theory, and multiconfigurational methods. Previous nuclear-electronic orbital-based methods produce nuclear densities that are too localized, resulting in abnormally high stretching frequencies and inaccuracies in other properties relying on these densities. The application of the NEO-XCHF approach to the [He–H–He]+ model system illustrates that this approach includes the significant electron-proton correlation, thereby leading to an accurate description of the nuclear density. The agreement between the proton densities obtained with the NEO-XCHF and grid-based methods validates the underlying theory and the implementation of the NEO-XCHF method.
Mixed quantum/classical investigation of the photodissociation of NH3(Ã) and a practical method for maintaining zero-point energy in classical trajectories
Authors: Bonhommeau David , Truhlar Donald G.
Published: July 2008

The photodissociation dynamics of ammonia upon excitation of the out-of-plane bending mode (mode ν2 with n2=0,…,6 quanta of vibration) in the Ã electronic state is investigated by means of several mixed quantum/classical methods, and the calculated final-state properties are compared to experiments. Five mixed quantum/classical methods are tested: one mean-field approach (the coherent switching with decay of mixing method), two surface-hopping methods [the fewest switches with time uncertainty (FSTU) and FSTU with stochastic decay (FSTU/SD) methods], and two surface-hopping methods with zero-point energy (ZPE) maintenance [the FSTU∕SD+trajectory projection onto ZPE orbit (TRAPZ) and FSTU∕SD+minimal TRAPZ (mTRAPZ) methods]. We found a qualitative difference between final NH2 internal energy distributions obtained for n2=0 and n2>1, as observed in experiments. Distributions obtained for n2=1 present an intermediate behavior between distributions obtained for smaller and larger n2 values. The dynamics is found to be highly electronically nonadiabatic with all these methods. NH2 internal energy distributions may have a negative energy tail when the ZPE is not maintained throughout the dynamics. The original TRAPZ method was designed to maintain ZPE in classical trajectories, but we find that it leads to unphysically high internal vibrational energies. The mTRAPZ method, which is new in this work and provides a general method for maintaining ZPE in either single-surface or multisurface trajectories, does not lead to unphysical results and is much less time consuming. The effect of maintaining ZPE in mixed quantum/classical dynamics is discussed in terms of agreement with experimental findings. The dynamics for n2=0 and n2=6 are also analyzed to reveal details not available from experiment, in particular, the time required for quenching of electronic excitation and the adiabatic energy gap and geometry at the time of quenching.
The influence of thermostats and manostats on reverse nonequilibrium molecular dynamics calculations of fluid viscosities
Authors: Müller Thomas J. , Al-Samman Michael , Müller-Plathe Florian
Published: July 2008

Reverse nonequilibrium molecular dynamics to calculate the shear viscosity of Lennard-Jones liquids was extended to simulations at constant number of particles, constant volume, and constant pressure using a Berendsen thermostat and a Berendsen manostat. Using additional systems such as water and hexane, we also report on the performance of shear viscosity calculations of systems with electrostatic and nontrivial intramolecular interactions when a manostat is applied. We compare the shear viscosities of simulations using no coupling, only temperature coupling, and temperature and pressure coupling and characterize discrepancies, where observed. From this, we deduce guidelines for when and how manostats can be usefully applied in reverse nonequilibrium simulations.
Shear thinning and shear dilatancy of liquid n-hexadecane via equilibrium and nonequilibrium molecular dynamics simulations: Temperature, pressure, and density effects
Authors: Tseng Huan-Chang , Wu Jiann-Shing , Chang Rong-Yeu
Published: July 2008

Equilibrium and nonequilibrium molecular dynamics (MD) simulations have been performed in both isochoric-isothermal (NVT) and isobaric-isothermal (NPT) ensemble systems. Under steady state shearing conditions, thermodynamic states and rheological properties of liquid n-hexadecane molecules have been studied. Between equilibrium and nonequilibrium states, it is important to understand how shear rates (γ̇) affect the thermodynamic state variables of temperature, pressure, and density. At lower shear rates of γ̇<1×1011s−1, the relationships between the thermodynamic variables at nonequilibrium states closely approximate those at equilibrium states, namely, the liquid is very near its Newtonian fluid regime. Conversely, at extreme shear rates of γ̇>1×1011s−1, specific behavior of shear dilatancy is observed in the variations of nonequilibrium thermodynamic states. Significantly, by analyzing the effects of changes in temperature, pressure, and density on shear flow system, we report a variety of rheological properties including the shear thinning relationship between viscosity and shear rate, zero-shear-rate viscosity, rotational relaxation time, and critical shear rate. In addition, the flow activation energy and the pressure-viscosity coefficient determined through Arrhenius and Barus equations acceptably agree with the related experimental and MD simulation results.
The H2O2+OH→HO2+H2O reaction in aqueous solution from a charge-dependent continuum model of solvation
Authors: Ginovska Bojana , Camaioni Donald M. , Dupuis Michel
Published: July 2008

We applied our recently developed protocol of the conductorlike continuum model of solvation to describe the title reaction in aqueous solution. The model has the unique feature of the molecular cavity being dependent on the atomic charges in the solute and can be extended naturally to transition states and reaction pathways. It was used to calculate the reaction energetics and reaction rate in solution for the title reaction. The rate of reaction calculated using canonical variational transition state theory in the context of the equilibrium solvation path approximation, and including correction for tunneling through the small curvature approximation, was found to be 3.6×106 M−1 s−1, significantly slower than in the gas phase in accord with experiment. These results suggest that the present protocol of the conductorlike continuum model of solvation with the charge-dependent cavity definition captures qualitatively and quantitatively the solvation effects at transition states and allows for quantitative estimates of reaction rates in solutions.
Self-diffusion in molecular liquids: Medium-chain n-alkanes and coenzyme Q10 studied by quasielastic neutron scattering
Authors: Smuda Christoph , Busch Sebastian , Gemmecker Gerd , Unruh Tobias
Published: July 2008

A systematic time-of-flight quasielastic neutron scattering (TOF-QENS) study on diffusion of n-alkanes in a melt is presented for the first time. As another example of a medium-chain molecule, coenzyme Q10 is investigated in the same way. The data were evaluated both in the frequency and in the time domain. TOF-QENS data can be satisfactorily described by different models, and it turned out that the determined diffusion coefficients are largely independent of the applied model. The derived diffusion coefficients are compared with values measured by pulsed-field gradient nuclear magnetic resonance (PFG-NMR). With increasing chain length, an increasing difference between the TOF-QENS diffusion coefficient and the PFG-NMR diffusion coefficient is observed. This discrepancy in the diffusion coefficients is most likely due to a change of the diffusion mechanism on a nanometer length scale for molecules of medium-chain length.
Single-layer metallodielectric nanostructures as dual-band midinfrared filters
Authors: Tang Yan , Bossard Jeremy A. , Werner Douglas H. , Mayer Theresa S.
Published: July 2008

We report a design and fabrication strategy for creating single-layer metallodielectric nanostructures with dual-band filtering properties at midinfrared wavelengths. Genetic algorithm optimization was used to determine an arrangement of nanometer-scale metal pixels within one unit cell of a two-dimensional periodic array that best satisfied the user-specified filter response and nanofabrication design rule constraints. Infrared transmission and reflection spectra measured on an optimized nanostructure array had two narrow stop bands blueshifted by 0.2μm from the designed center wavelengths of 3.3 and 4.1μm, with transmission attenuation greater than −20dB and reflection attenuation less than −1.5dB in each band. This strategy provides a practical and efficient approach to design metallodielectric nanostructures needed for photonic device applications as well as for future low-loss refractive index engineered metamaterials.
Using free energy perturbation to predict effects of changing force field parameters on computed conformational equilibriums of peptides
Authors: Cao Zanxia , Liu Haiyan
Published: July 2008

We propose a method that may allow data about the conformational equilibriums of peptides to enter the parameter calibration phase in force field developments. The method combines free energy perturbation with techniques for extensive sampling in the conformational space. It predicts shifts in computed conformational equilibriums in response to separate or combined perturbations of force field parameters. As an example we considered a force field associated with an implicit solvent model. We considered two different approaches to define conformational states of four peptides. One is based on reaction coordinates and two-dimensional free energy surfaces. The other is based on the clustering analysis of sampled conformations. Effects of perturbing various model parameters on the equilibriums between nativelike states with other conformational states were considered. For one type of perturbation predicted to have consistent effects on different peptides, the predictions have been verified by actual simulations using a perturbed model.
Very strong hydrogen bonds in a bent chain structure of fluorohydrogenate anions in liquid Cs(FH)2.3F
Authors: Kohara Shinji , Takata Masaki , Matsumoto Kazuhiko , Hagiwara Rika , Suzuya Kentaro , Morita Hidetoshi , Siewenie Joan E. , Benmore Chris J.
Published: July 2008

The structure of liquid Cs(FH)2.3F was revealed using a combination of high-energy x-ray and neutron diffraction measurements. We found that the strongest intermolecular H–F hydrogen bonds at an average distance of 1.36Å are accompanied by the formation of a high degree of bending of the oligomer chain in the melt, with ∠FHF=150°. A reverse Monte Carlo simulation showed that the average number of atoms per chain is 4.4. A detailed chain analysis of the atomic configuration revealed that (FH)2F− oligomer chains are the major entities in the liquid, and asymmetrical FHF− are formed owing to the strong H–F hydrogen bonds. The results suggest that an average of one or two HF molecules bond to each of the 11 fluorine atoms surrounding a cesium ion.
Coverage-dependent sticking probability and desorption kinetics of water molecules on Rh(111)
Authors: Beniya Atsushi , Mukai Kozo , Yamashita Yoshiyuki , Yoshinobu Jun
Published: July 2008

The adsorption and desorption kinetics of water molecules on Rh(111) were investigated using temperature programed desorption (TPD). Water molecules on Rh(111) show coverage-dependent sticking probability; the initial sticking probability is estimated to be 0.46. In the desorption process, a dilute gaslike phase and two-dimensional islands of water coexist on the surface. Based on the model proposed by Kreuzer and Payne [Surf. Sci.200, L433 (1988)], the apparent fractional-order TPD spectra can be interpreted as first-order desorption from the coexistence of two phases on which the sticking probabilities are different. Based on this, the previous estimation of pre-exponential factors assuming half-order desorption [A. Beniya et al., J. Chem. Phys.125, 054717 (2006)] should be revised.
Spherical tensor gradient operator method for integral rotation: A simple, efficient, and extendable alternative to Slater–Koster tables
Authors: Giese Timothy J. , York Darrin M.
Published: July 2008

We present a novel alternative to the use of Slater–Koster tables for the efficient rotation and gradient evaluation of two-center integrals used in tight-binding Hamiltonian models. The method recasts the problem into an exact, yet implicit, basis representation through which the properties of the spherical tensor gradient operator are exploited. These properties provide a factor of 3 to 4 speedup in the evaluation of the integral gradients and afford a compact code structure that easily extends to high angular momentum without loss in efficiency. Thus, the present work is important in improving the performance of tight-binding models in molecular dynamics simulations and has particular use for methods that require the evaluation of two-center integrals that involve high angular momentum basis functions. These advances have a potential impact for the design of new tight-binding models that incorporate polarization or transition metal basis functions and methods based on electron density fitting of molecular fragments.
Linear stability of a compressible coaxial jet with continuous velocity and temperature profiles
Authors: Perrault-Joncas Dominique , Maslowe Sherwin A.
Published: July 2008

This paper describes an investigation of the stability of a jet with velocity and temperature profiles characteristic of the exit region for a turbofan engine. Because the bypass stream mixes with both the exhaust and the ambient air, these profiles contain thin layers in which the velocity and temperature may vary rapidly. As a consequence, multiple instability modes are possible. In accordance with Rayleigh’s theorem for axisymmetric incompressible shear flows, it turns out that there are three possible modes, only two of which are unstable. We consider the effect on spatial growth rates of varying the diameter and velocity ratios, compressibility, and azimuthal wavenumber. Radiating modes, that are possible when the primary jet is heated, are also studied.
Correlation between hardness and elastic moduli of the ultraincompressible transition metal diborides RuB2, OsB2, and ReB2
Authors: Chung Hsiu-Ying , Weinberger Michelle B. , Yang Jenn-Ming , Tolbert Sarah H. , Kaner Richard B.
Published: July 2008

The ultraincompressible transition metal diborides RuB2, OsB2, and ReB2 were synthesized by arc melting the elemental metals and boron under an argon atmosphere at ambient pressure. The hardness and Young’s modulus were measured using nanoindentation with a Berkovich diamond indenter. The bulk modulus and shear modulus were derived based on an isotropic model and then plotted as a function of hardness. A strong correlation is observed between the hardness and shear modulus for these transition metal diborides.
Do ionic and hydrophobic probes sense similar microenvironment in Triton X-100 nonionic reverse micelles?
Authors: Dutt G. B.
Published: July 2008

Rotational diffusion of two structurally similar ionic probes, rhodamine 110 and fluorescein, has been examined in nonionic reverse micellar system of Triton X-100/benzene–n-hexane/water as a function of mole ratio of the water to surfactant, W. This study has been undertaken to find out whether ionic and hydrophobic probes experience similar microenvironment in these reverse micelles. Experimental results indicate that, from W=0 to 3, the average reorientation time, which is a measure of the microviscosity experienced by the probe molecule, increases by 90% and 40% for rhodamine 110 and fluorescein, respectively, and from W=3 to 8, it decreases by 20% for both the probes. The increase in the average reorientation time with W has been rationalized on the basis of the flexible oxyethylene chains of the TX-100 surfactant being hydrogen bonded by the water molecules, which makes the core region less fluid. However, once the hydration of the oxyethylene chains is complete, further addition of water results in formation of water droplet; which renders the micelle-water interface in the core region less compact leading to a marginal decrease in the average reorientation time of the probe molecules. These explanations are consistent with the location of the probes and the structure of the Triton X-100/benzene–hexane/water reverse micelles. To compare how the microenvironment experienced by these ionic probes is different from the hydrophobic ones, results from our earlier work [J. Phys. Chem. B 108, 7944 (2004)] have been considered. Such a comparison revealed that both ionic and hydrophobic probes experience similar microenvironment in these reverse micelles until the hydration of the oxyethylene chains is complete. In case of hydrophobic probes, however, the onset of water droplet formation does not alter their microenvironment, which is due to their location in the reverse micellar cores.
Electric-field gradient sorts molecules according to their shape
Authors: Miller Johanna
Published: July 2008

Physically distinct but chemically identical molecules can have different electric dipole moments and therefore can respond differently to an inhomogeneous electric field.
Scattered countries revive hope for european spallation source
Authors: Feder Toni
Published: July 2008

In realizing the European Spallation Source—which even in its scaled-down form would be the world’s strongest neutron source—the partner nations hope to forge mechanisms for multinational decision making.
Lawmakers fret over health, safety impacts of nanotechnology
Authors: Kramer David
Published: July 2008

As Congress reauthorizes the National Nanotechnology Initiative, groups petition the Environmental Protection Agency to step up its regulation of nanomaterials.
The strategic offense initiative? the soviets and star wars
Authors: Westwick Peter J.
Published: July 2008

Historians of the Cold War have paid too little attention to Soviet fears of “space-strike weapons”—that is, possible offensive uses of President Ronald Reagan’s Strategic Defense Initiative.
Brown dwarfs: Failed stars, super jupiters
Authors: Burgasser Adam J.
Published: July 2008

The recently discovered brown dwarfs that bridge the gap between giant planets and hydrogen-fusing stars are enabling unique insights into low-temperature atmospheres, star and planet formation, and the properties of our galaxy.
Self-consistent implementation of a nonlocal van der Waals density functional with a Gaussian basis set
Authors: Vydrov Oleg A. , Wu Qin , Van Voorhis Troy
Published: July 2008

Nearly all common density functional approximations fail to properly describe dispersion interactions responsible for binding in van der Waals complexes. Empirical corrections can fix some of the failures but cannot fully grasp the complex physics and may not be reliable for systems dissimilar to the fitting set. In contrast, the recently proposed nonlocal van der Waals density functional (vdW-DF) was derived from first principles, describes dispersion interactions in a seamless fashion, and yields the correct asymptotics. Implementation of this functional is somewhat cumbersome: Nonlocal dependence on the electron density requires numerical double integration over the space variables and functional derivatives are nontrivial. This paper shows how vdW-DF can be implemented self-consistently with Gaussian basis functions. The gradients of the energy with respect to nuclear displacements have also been derived and coded, enabling efficient geometry optimizations. We test the vdW-DF correlation functional in combination with several exchange approximations. We also study the sensitivity of the method to the basis set size and to the quality of the numerical quadrature grid. For weakly interacting systems, acceptable accuracy in semilocal exchange is achieved only with fine grids, whereas for nonlocal vdW-DF correlation even rather coarse grids are sufficient. The current version of vdW-DF is not well suited for pairing with Hartree–Fock exchange, leading to considerable overbinding.
Electrostatic similarity of proteins: Application of three dimensional spherical harmonic decomposition
Authors: Długosz Maciej , Trylska Joanna
Published: July 2008

We present a method for describing and comparing global electrostatic properties of biomolecules based on the spherical harmonic decomposition of electrostatic potential data. Unlike other approaches our method does not require any prior three dimensional structural alignment. The electrostatic potential, given as a volumetric data set from a numerical solution of the Poisson or Poisson–Boltzmann equation, is represented with descriptors that are rotation invariant. The method can be applied to large and structurally diverse sets of biomolecules enabling to cluster them according to their electrostatic features.
Single-photon Mach–Zehnder interferometer for quantum networks based on the single-photon Faraday effect
Authors: Seigneur H. P. , Leuenberger Michael N. , Schoenfeld W. V.
Published: July 2008

Combining the recent progress in semiconductor nanostructures along with the versatility of photonic crystals in confining and manipulating light, quantum networks allow for the prospect of an integrated and low power quantum technology. Within quantum networks, which consist of a system of waveguides and nanocavities with embedded quantum dots, it has been demonstrated in theory that many-qubit states stored in electron spins could be teleported from one quantum dot to another via a single photon using the single-photon Faraday effect. However, in addition to being able to transfer quantum information from one location to another, quantum networks need added functionality such as (1) controlling the flow of the quantum information and (2) performing specific operations on qubits that can be easily integrated. In this paper, we show how a single-photon Mach–Zehnder interferometer (SMZI), that uses the concept of the single-photon Faraday effect to manipulate the polarization of a single photon, can be operated both as a switch to control the flow of quantum information inside the quantum network and as various single-qubit quantum gates to perform operations on a single photon. Given that the X gate, the Z gate, and the XZ gate are essential for the implementation of quantum teleportation, we show explicitly their implementation by means of our proposed SMZI. We also present the implementation of the Hadamard gate and the single-qubit phase gate, which are needed to complete the universal set of quantum gates for integrated quantum computing in a quantum network. Finally, the expected fidelity and robustness of the proposed SMZI are quantitatively explored by considering the phase errors within the SMZI.
Study on the boson peak in bulk metallic glasses
Authors: Li Yong , Yu Peng , Bai H. Y.
Published: July 2008

The low-temperature specific heat (low-T Cp) and other properties of a series of representative bulk metallic glasses (BMGs) were studied. The pronounced low-T Cp anomalies associated with the boson peak (BP) in the representative BMGs were observed. The BMG samples were annealed and quenched near the glass transition temperatures, and the effect of isothermal annealing for different times and quenching at different cooling rates on the low-T Cp anomalies were studied. We observed that the BP is affected by annealing and quenching processes. It is a possible explanation that the BP mainly originated from the random dense cluster-packing structure. The origin of the Cp anomalies is interpreted with the harmonic localized modes based on the random dense-packed atomic cluster structure of BMGs. The clear correlation between the maximum value of BP [(Cp−γT)/T3]max and its position Tmax further supports the model.
Phase-field modeling of solute precipitation and dissolution
Authors: Xu Zhijie , Meakin Paul
Published: July 2008

A phase-field approach to the dynamics of liquid-solid interfaces that evolve due to precipitation and/or dissolution is presented. For the purpose of illustration and comparison with other methods, phase-field simulations were carried out assuming first order reaction (dissolution/precipitation) kinetics. In contrast to solidification processes controlled by a temperature field that is continuous across the solid/liquid interface (with a discontinuous temperature gradient), precipitation/dissolution is controlled by a solute concentration field that is discontinuous at the solid/liquid interface. The sharp-interface asymptotic analysis of the phase-field equations for solidification [A. Karma and W.-J. Rappel, Phys. Rev. E 57, 4323 (1998)] has been modified for precipitation/dissolution processes to demonstrate that the phase-field equations converge to the proper sharp-interface limit. The mathematical model has been validated for a one-dimensional precipitation/dissolution problem by comparison with the analytical solution.
Quantum dot nanocolumn photodetectors for light detection in the infrared
Authors: Böberl M. , Kovalenko M. V. , Pillwein G. , Brunthaler G. , Heiss W.
Published: July 2008

Colloidal quantum dots absorbing in the infrared are filled into nanoporous alumina membranes to form narrow columns with aspect ratios of 300:1. The columns define the charge carrier path vertical through the ordered pore structure of the membrane. Electrical transport and photocurrent of various quantum dot column photodetectors based on two different nanomaterials, namely, HgTe and PbS quantum dots, with different quantum dot sizes are investigated. Photocurrents up to long wavelengths of 3μm are demonstrated.
Frequency autocorrelation function of stochastically fluctuating fields caused by specific magnetic field inhomogeneities
Authors: Ziener C. H. , Kampf T. , Herold V. , Jakob P. M. , Bauer W. R. , Nadler W.
Published: July 2008

Signal formation in NMR is due to incoherent dephasing of nuclear spins. Of particular practical importance is the situation of nuclear spins undergoing independent stochastic motion in inhomogeneous local magnetic fields, e.g., created by magnetized objects. Since it was demonstrated recently that the frequency correlation function of nuclear spins can be measured directly, a theoretical analysis of such functions is of interest. Here, we provide a numerically exact analysis of that correlation function for the inhomogeneous fields around two particular geometries: cylinders and spheres. The functional form exhibits three regimes: after an initial transient, there is an algebraic regime with a t−d/2 time dependence (d being the space dimension), followed by an exponential cutoff due to microscopic system size effects. The main parameter controlling the range of the individual regimes is the volume fraction of the magnetized objects. In addition to our numerical analysis, which is based on eigenfunction expansions, we provide analytical results and approximations based on the generalized moment expansion.
Monte Carlo simulations of microchannel plate detectors. I. Steady-state voltage bias results
Authors: Wu Ming , Kruschwitz Craig A. , Morgan Dane V. , Morgan Jiaming
Published: July 2008

X-ray detectors based on straight-channel microchannel plates (MCPs) are a powerful diagnostic tool for two-dimensional, time-resolved imaging and time-resolved x-ray spectroscopy in the fields of laser-driven inertial confinement fusion and fast Z-pinch experiments. Understanding the behavior of microchannel plates as used in such detectors is critical to understanding the data obtained. The subject of this paper is a Monte Carlo computer code we have developed to simulate the electron cascade in a MCP under a static applied voltage. Also included in the simulation is elastic reflection of low-energy electrons from the channel wall, which is important at lower voltages. When model results were compared to measured MCP sensitivities, good agreement was found. Spatial resolution simulations of MCP-based detectors were also presented and found to agree with experimental measurements.
Luminescence from 3,4,9,10-perylenetetracarboxylic dianhydride on Ag(111) surface excited by tunneling electrons in scanning tunneling microscopy
Authors: Ino Daisuke , Yamada Taro , Kawai Maki
Published: July 2008

The electronic excitations induced with tunneling electrons into adlayers of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on Ag(111) have been investigated by in situ fluorescence spectroscopy in scanning tunneling microscopy (STM). A minute area of the surface is excited by an electron tunneling process in STM. Fluorescence spectra strongly depend on the coverage of PTCDA on Ag(111). The adsorption of the first PTCDA layer quenches the intrinsic surface plasmon originated from the clean Ag(111). When the second layer is formed, fluorescence spectra are dominated by the signals from PTCDA, which are interpreted as the radiative decay from the manifold of first singlet excited state (S1) of adsorbed PTCDA. The fluorescence of PTCDA is independent of the bias polarity. In addition, the fluorescence excitation spectrum agrees with that by optical excitation. Both results indicate that S1 is directly excited by the inelastic impact scattering of electrons tunneling within the PTCDA adlayer.
Molecular dynamics in the isothermal-isobaric ensemble: The requirement of a “shell” molecule. III. Discontinuous potentials
Authors: Uline Mark J. , Corti David S.
Published: July 2008

Based on the approach of Gruhn and Monson [Phys. Rev. E 63, 061106 (2001)], we present a new method for deriving the collisions dynamics for particles that interact via discontinuous potentials. By invoking the conservation of the extended Hamiltonian, we generate molecular dynamics (MD) algorithms for simulating the hard-sphere and square-well fluids within the isothermal-isobaric (NpT) ensemble. Consistent with the recent rigorous reformulation of the NpT ensemble partition function, the equations of motion impose a constant external pressure via the introduction of a shell particle of known mass [M. J. Uline and D. S. Corti, J. Chem. Phys. 123, 164101 (2005); 123, 164102 (2005)], which serves to define uniquely the volume of the system. The particles are also connected to a temperature reservoir through the use of a chain of Nosé-Hoover thermostats, the properties of which are not affected by a hard-sphere or square-well collision. By using the Liouville operator formalism and the Trotter expansion theorem to integrate the equations of motion, the update of the thermostat variables can be decoupled from the update of the positions of the particles and the momentum changes upon a collision. Hence, once the appropriate collision dynamics for the isobaric-isenthalpic (NpH) equations of motion is known, the adaptation of the algorithm to the NpT ensemble is straightforward. Results of MD simulations for the pure component square-well fluid are presented and serve to validate our algorithm. Finally, since the mass of the shell particle is known, the system itself, and not a piston of arbitrary mass, controls the time scales for internal pressure and volume fluctuations. We therefore consider the influence of the shell particle algorithm on the dynamics of the square-well fluid.
W
-band TE01 gyrotron backward-wave oscillator with distributed loss
Authors: Chang T. H. , Yu C. F. , Hung C. L. , Yeh Y. S. , Hsiao M. C. , Shin Y. Y.
Published: July 2008

Distributed wall loss is proposed to enhance the stability and tunability of a W-band TE01 gyrotron backward-wave oscillator (gyro-BWO). Simulation results reveal that loss effectively suppresses the unwanted transverse modes as well as the high-order axial modes (HOAMs) without degrading the performance of a gyro-BWO that operates at the fundamental axial mode. Linear and nonlinear codes are used to calculate the interaction properties. The effects of the distributed loss on the starting currents of all of the modes of interest are discussed in depth. The interacting structure is optimized for stability. The calculated peak output power is 102kW, corresponding to an efficiency of 20%. The 3dB tuning bandwidth is 1.8GHz, centered at 94.0GHz when using 5A and 100kV electron beam.
Artifact-free dynamic atomic force microscopy reveals monotonic dissipation for a simple confined liquid
Authors: Kaggwa G. B. , Kilpatrick J. I. , Sader J. E. , Jarvis S. P.
Published: July 2008

We present definitive interaction measurements of a simple confined liquid (octamethylcyclotetrasiloxane) using artifact-free frequency modulation atomic force microscopy. We use existing theory to decouple the conservative and dissipative components of the interaction, for a known phase offset from resonance (90° phase shift), that has been deliberately introduced into the experiment. Further we show the qualitative influence on the conservative and dissipative components of the interaction of a phase error deliberately introduced into the measurement, highlighting that artifacts, such as oscillatory dissipation, can be readily observed when the phase error is not compensated for in the force analysis.
Nanodrop on a nanorough solid surface: Density functional theory considerations
Authors: Berim Gersh O. , Ruckenstein Eli
Published: July 2008

The density distributions and contact angles of liquid nanodrops on nanorough solid surfaces are determined on the basis of a nonlocal density functional theory. Two kinds of roughness, chemical and physical, are examined. The former considers the substrate as a sequence of two kinds of semi-infinite vertical plates of equal thicknesses but of different natures with different strengths for the liquid-solid interactions. The physical roughness involves an ordered set of pillars on a flat homogeneous surface. Both hydrophobic and hydrophilic surfaces were considered. For the chemical roughness, the contact angle which the drop makes with the flat surface increases when the strength of the liquid-solid interaction for one kind of plates decreases with respect to the fixed value of the other kind of plates. Such a behavior is in agreement with the Cassie–Baxter expression derived from macroscopic considerations. For the physical roughness on a hydrophobic surface, the contact angle which a drop makes with the plane containing the tops of the pillars increases with increasing roughness. Such a behavior is consistent with the Wenzel formula developed for macroscopic drops. For hydrophilic surfaces, as the roughness increases the contact angle first increases, in contradiction with the Wenzel formula, which predicts for hydrophilic surfaces a decrease of the contact angle with increasing roughness. However, a further increase in roughness changes nonmonotonously the contact angle, and at some roughness, the drop disappears and only a liquid film is present on the surface. It was also found that the contact angle has a periodic dependence on the volume of the drop.
Virial series for fluids of hard hyperspheres in odd dimensions
Authors: Rohrmann René D. , Robles Miguel , López de Haro Mariano , Santos Andrés
Published: July 2008

A recently derived method [R. D. Rohrmann and A. Santos, Phys. Rev. E 76, 051202 (2007)] to obtain the exact solution of the Percus–Yevick equation for a fluid of hard spheres in (odd) d dimensions is used to investigate the convergence properties of the resulting virial series. This is done both for the virial and compressibility routes, in which the virial coefficients Bj are expressed in terms of the solution of a set of (d−1)/2 coupled algebraic equations which become nonlinear for d≥5. Results have been derived up to d=13. A confirmation of the alternating character of the series for d≥5, due to the existence of a branch point on the negative real axis, is found and the radius of convergence is explicitly determined for each dimension. The resulting scaled density per dimension 2η1/d, where η is the packing fraction, is wholly consistent with the limiting value of 1 for d→∞. Finally, the values for Bj predicted by the virial and compressibility routes in the Percus–Yevick approximation are compared with the known exact values [N. Clisby and B. M. McCoy, J. Stat. Phys. 122, 15 (2006)].
Enhanced bending stability of carbon-nanotube-reinforced indium tin oxide films on flexible plastic substrates
Authors: Chiba Kiyoshi , Futagami Atsushi
Published: July 2008

Carbon-nanotube (CNT)-reinforced indium tin oxide (ITO) films have been fabricated on polyethylene terephthalate substrates by sputter deposition of ITO onto spin-coated multiwalled CNTs. The bending electrical stability of the films was investigated. When bent to a curvature radius of 1.5mm, unreinforced film surface resistance increased by more than 100 times, while the resistance of CNT-reinforced films remained constant. Scanning electron microscopy of the films after bending revealed significant cracking and delamination in the unreinforced ITO films, while the CNT-reinforced films remained substantially intact. These results suggest that CNTs may prevent percolating clusters of defects in films under stress.
Anisotropic crystallographic properties, strain, and their effects on band structure of m-plane GaN on LiAlO2(100)
Authors: Liu B. , Zhang R. , Xie Z. L. , Kong J. Y. , Yao J. , Liu Q. J. , Zhang Z. , Fu D. Y. , Xiu X. Q. , Chen P. , Han P. , Shi Y. , Zheng Y. D. , Zhou S. M. , Edwards G.
Published: July 2008

The m-plane GaN films grown on LiAlO2(100) by metal-organic chemical vapor deposition exhibit anisotropic crystallographic properties. The Williamson–Hall plots point out they are due to the different tilts and lateral correlation lengths of mosaic blocks parallel and perpendicular to GaN[0001] in the growth plane. The symmetric and asymmetric reciprocal space maps reveal the strain of m-plane GaN to be biaxial in-plane compress εxx=−0.79% and εzz=−0.14% with an out-of-plane dilatation εyy=0.38%. This anisotropic strain further separates the energy levels of top valence band at Γ point. The energy splitting as 37meV as well as in-plane polarization anisotropy for transitions are found by the polarized photoluminescence spectra at room temperature.
Periodic solutions of the short pulse model equation
Authors: Matsuno Yoshimasa
Published: July 2008

We develop a systematic procedure for constructing periodic solutions of the short pulse (SP) model equation that describes the propagation of ultrashort pulses in nonlinear media. We first summarize a novel exact method of solution that consists of a hodograph transformation to convert the SP equation into the sine-Gordon (sG) equation. We then exemplify some one- and two-phase periodic solutions of the sG equation for which the system of linear partial differential equations governing the inverse mapping can be integrated analytically to obtain periodic solutions of the SP equation in the form of the parametric representation. We obtain a new class of two-phase periodic solutions and investigate their properties in detail. Of particular interest is a nonsingular periodic solution that reduces to the breather solution in the long-wave limit. It may play an important role in studying the propagation of ultrashort pulses in a finite-length optical system.
Quantitative determination of the dopant distribution in Si ultrashallow junctions by tilted sample annular dark field scanning transmission electron microscopy
Authors: Parisini A. , Morandi V. , Solmi S. , Merli P. G. , Giubertoni D. , Bersani M. , van den Berg J. A.
Published: July 2008

In this letter, it is shown how the application of a Z-contrast scanning transmission electron microscopy technique to the study of the dopant signal in ultrashallow junctions in Si can lead, in the case of As in Si, to a quantitative determination of the dopant depth profile at subnanometer resolution. The quantitative results thus obtained demonstrate that As accumulates on the Si side of the SiO2∕Si interface with a negligible loss of dopant into the oxide. Modeling of the effect indicates that segregation occurring during the recrystallization of the implanted layer is the dominant cause of this dopant pileup.
Mg-induced increase of band gap in Zn1−xMgxO nanorods revealed by x-ray absorption and emission spectroscopy
Authors: Chiou J. W. , Tsai H. M. , Pao C. W. , Chien F. Z. , Pong W. F. , Chen C. W. , Tsai M.-H. , Wu J. J. , Ko C. H. , Chiang H. H. , Lin H.-J. , Lee J. F. , Guo J.-H.
Published: July 2008

X-ray absorption near-edge structure (XANES) and x-ray emission spectroscopy (XES) measurements were used to investigate the effect of Mg doping in ZnO nanorods. The intensities of the features in the O K-edge XANES spectra of Zn1−xMgxO nanorods are lower than those of pure ZnO nanorods, suggesting that Mg doping increases the negative effective charge of O ions. XES and XANES spectra of O 2p states indicate that Mg doping raises (lowers) the conduction-band-minimum (valence-band-maximum) and increases the band gap. The band gap is found to increase linearly with the Mg content, as revealed by photoluminescence and combined XANES and XES measurements.
Impedance of curved rectangular spiral coils around a conductive cylinder
Authors: Burke S. K. , Ditchburn R. J. , Theodoulidis T. P.
Published: July 2008

Eddy-current induction due to a thin conformable coil wrapped around a long conductive cylinder is examined using a second-order vector potential formalism. Compact closed-form expressions are derived for the self- and mutual impedances of curved rectangular spiral coils (i) in free space and (ii) when wrapped around the surface of the cylindrical rod. The validity of these expressions was tested against the results of a systematic series of experiments using a cylindrical Al-alloy rod and conformable coils manufactured using flexible printed-circuit-board technology. The theoretical expressions were in very good agreement with the experimental measurements. The significance of the results for eddy-current nondestructive inspection using flexible coils and flexible coil arrays is discussed.
Theoretical study of terahertz current oscillation in GaAs1−xNx
Authors: Feng W. , Cao J. C.
Published: July 2008

We have theoretically investigated current self-oscillation in doped n+nn+ GaAs1−xNx diodes driven by direct current (dc) electric field. The current self-oscillation is associated with negative differential velocity effect in the highly nonparabolic conduction band of this unique material system. By solving a time-dependent drift-diffusion model that takes into account the negative differential velocity effect, we provide a detailed analysis of the current oscillations. The frequencies of current oscillations are in the gigahertz to terahertz region, depending on the doping concentration and the applied dc electric field. The calculated average current density is in qualitative agreement with the measured result.
Capillary filling of sub-10nm nanochannels
Authors: Haneveld Jeroen , Tas Niels R. , Brunets Nataliya , Jansen Henri V. , Elwenspoek Miko
Published: July 2008

We have developed a procedure for accurate fabrication of silicon-based nanochannels down to a few nanometer channel height, based on the use of a thin thermal silicon oxide spacer layer. Nanochannels with a predictable and carefully measured height between 5 and 50nm were successfully fabricated and filled with de-ionized water. For all channel heights the filling kinetics behaves according to the classical Washburn law for capillary filling, with a small correction for a loss of liquid at the moving front at a constant rate and a smaller than expected Washburn coefficient (up to a factor of 1.6 smaller for water in 5nm channels).
Possible electronic decay channels in the ionization spectra of small clusters composed of Ar and Kr: A four-component relativistic treatment
Authors: Pernpointner Markus , Kryzhevoi Nikolai V. , Urbaczek Sascha
Published: July 2008

In this work single and double ionization spectra of the homo- and heteronuclear argon/krypton dimers and trimers are calculated by means of propagator methods where a four-component implementation was employed for the single ionizations. Scalar relativistic effects play only a minor role for the outer valence spectral structure, whereas spin-orbit coupling and electron correlation have to be treated adequately in order to reproduce the features correctly. Nonradiative decay mechanisms of subvalence vacancies in the argon and krypton dimers and trimers are discussed both for the interatomic Coulombic decay and the electron transfer mediated decay (ETMD). In the heteronuclear triatomic system which serves as a model for larger clusters, a possible ETMD process of the Ar 3s vacancy is found for the linear arrangement of the atoms. In the bent configuration the ETMD channel is closed.
Superior nanoscale passive oxide layers synthesized under photon irradiation for environmental protection
Authors: Chang Chia-Lin , Engelhard Mark H. , Ramanathan Shriram
Published: July 2008

We report on synthesis and functional properties of ultrathin oxide layers synthesized on metal surfaces by room temperature photon irradiation. We show that the impedance of a passive aluminum oxide film synthesized under ultraviolet photon irradiation is an order of magnitude larger than that of native oxide in a 0.5M NaCl solution. Further, the structure and impedance of existing native oxide layers can be dramatically improved by minutes-long exposure to photon irradiation. Depth profiling studies with x-ray photoelectron spectroscopy shows that chlorine uptake in UV-synthesized oxides, compared to that of native oxides, is reduced which can contribute to the improvement in corrosion resistance. The results are of significance to synthesis of ultrathin passive layers on metal and alloy structures for environmental protection.