bluuebunny/crossref_metadata_2025 huggingface dataset:
Interaction between charged anisotropic macromolecules: Application to rod-like polyelectrolytes
Authors: Chapot David , Bocquet Lydéric , Trizac Emmanuel
Published: February 2004

In this paper we propose a framework allowing one to compute the effective interactions between two anisotropic macromolecules, thereby generalizing the Derjaguin, Landau, Verwey, and Overbeek theory [E. J. W. Verwey and J. T. G. Overbeek, Theory of the Stability of Lyophobic Colloids (Elsevier, Amsterdam, 1948)] to nonspherical finite size colloids. We show in particular that the effective interaction potential remains anisotropic at all distances and provide an expression for the anisotropy factor. We then apply this framework to the case of finite rod-like polyelectrolytes. The calculation of the interaction energy requires the numerical computation of the surface charge profiles, which result here from a constant surface potential on the rod-like colloids. However, a simplified analytical description is proposed, leading to an excellent agreement with the full numerical solution. Conclusions on the phase properties of rod-like colloids are proposed in this context.
Properties of selected diatomics using variational Monte Carlo methods
Authors: Datta S. , Alexander S. A. , Coldwell R. L.
Published: February 2004

Using variational Monte Carlo and highly accurate trial wave functions optimized by Filippi and Umrigar, we calculate a number of molecular properties for the ground state of Li2, Be2, B2, C2, N2, O2, and F2. This is the first time that many of these properties have been computed.
Water formation on Pd(111) by reaction of oxygen with atomic and molecular hydrogen
Authors: Pauer G. , Winkler A.
Published: February 2004

In this work we have studied the steady-state reaction of molecular and atomic hydrogen with oxygen on a Pd(111) surface at a low total pressure (<10−7 mbar) and at sample temperatures ranging from 100 to 1100 K. Characteristic features of the water formation rate Φ(pH2; pO2; TPd) are presented and discussed, including effects that are due to the use of gas-phase atomic hydrogen for exposure. Optimum impingement ratios (OIR) for hydrogen and oxygen for water formation and their dependence on the sample temperature have been determined. The occurring shift in the OIR could be ascribed to the temperature dependence of the sticking coefficients for hydrogen (SH2) and oxygen (SO2) on Pd(111). Using gas-phase atomic hydrogen for water formation leads to an increase of the OIR, suggesting that hydrogen abstraction via hot-atom reactions competes with H2O formation. The velocity distributions of the desorbing water molecules formed on the Pd(111) surface have been measured by time-of-flight spectroscopy under various conditions, using either gas-phase H atoms or molecular H2 as reactants. In all cases, the desorbing water flux could be represented by a Maxwellian distribution corresponding to the surface temperature, thus giving direct evidence for a Langmuir–Hinshelwood mechanism for water formation on Pd(111).
Surface vibrations in alkanethiol self-assembled monolayers of varying chain length
Authors: Rosenbaum A. W. , Freedman M. A. , Darling S. B. , Popova I. , Sibener S. J.
Published: February 2004

The effect of chain length on the low-energy vibrations of alkanethiol striped phase self-assembled monolayers on Au(111) was studied. We have examined the low-energy vibrational structure of well-ordered, low-density 1-decanethiol (C10), 1-octanethiol (C8), and 1-hexanethiol (C6) to further understand the interaction between adsorbate and substrate. Dispersionless Einstein mode phonons, polarized perpendicularly to the surface, were observed for the striped phases of C10, C8, and C6 at 8.0, 7.3, and 7.3 meV, respectively. An overtone at 12.3 meV was also observed for C6/Au(111). These results, in concert with molecular dynamics simulations, indicate that the forces between the adsorbate and substrate can be described using simple van der Waals forces between the hydrocarbon chains and the Au substrate with the sulfur chemisorbed in the threefold hollow site.
Phase stability of nanocarbon in one dimension: Nanotubes versus diamond nanowires
Authors: Barnard A. S. , Snook I. K.
Published: February 2004

Since their discovery in 1990, the study of sp2 bonded carbon nanotubes has grown into a field of research in it’s own right; however the development of the sp3 analog, diamond nanowires, has been slow. A number of theoretical models have been proposed to compare the relative stability of diamond and graphite at the nanoscale; and more recently, to compare nanodiamonds and fullerenes. Presented here is a study of the phase stability of nanocarbon in one-dimension. The structural energies of carbon nanotubes and diamond nanowires have been calculated using density functional theory within the generalized gradient approximation, and used to determine the atomic heat of formation as a function of size.
Multicomponent nucleation: Thermodynamically consistent description of the nucleation work
Authors: Kashchiev Dimo
Published: February 2004

A thermodynamically consistent formula is derived for the nucleation work in multicomponent homogeneous nucleation. The derivation relies on the conservative dividing surface which defines the nucleus as having specific surface energy equal to the specific surface energy σ0 of the interface between the macroscopically large new and old phases at coexistence. Expressions are given for the radius of the nucleus defined by the conservative dividing surface and by the surface of tension. As a side result, the curvature dependence of the surface tension σT of the nucleus defined by the surface of tension is also determined. The analysis is valid for nuclei of any size, i.e., for nucleation in the whole range of conditions between the binodal and the spinodal of the metastable old phase provided the inequality σT⩽σ0 is satisfied. It is found that under the conditions of validity of the analysis the nucleation rate is higher than the nucleation rate given by the classical nucleation theory. The general results are applied to nucleation of unary liquids or solids in binary gaseous, liquid or solid mixtures.
Momentum representation of the solute/bath interaction in the dynamic theory of chemical processes in condensed phase
Authors: Basilevsky M. V. , Davidovich G. V. , Voronin A. I.
Published: February 2004

For the system consisting of the chemically reactive solute immersed in the oscillator bath, we consider an approach based on the solute/medium interaction expressed in terms of momenta rather than coordinates. In the adiabatic representation the medium reorganization effects are suppressed, being hidden in the solute renormalized potential and new spectral density function. The advantage proposed by the bilinear interaction in momentum representation is its spatial uniformity important for approximate dynamical treatments. The procedure of explicit transforming a standard spectral density (coordinate representation of interaction) into the spectral density in adiabatic representation (momentum representation of interaction) is the main new result of the present study. Illustrative calculations for several types of spectral functions are performed. Special discussion is devoted to clarifying the nature of the slow diffusion coordinate, to which the present approach is mainly addressed.
Intermolecular complexes of HArF and HKrF with CO
Authors: McDowell Sean A. C.
Published: February 2004

Stable linear weakly bound hydrogen-bonded complexes of HArF and HKrF with the CO molecule have been predicted by ab initio computations at the MP2/6-311++G(2d,2p) level of theory. The complexes, having stabilities in the order, FArH⋯CO>FKrH⋯CO>FArH⋯OC>FKrH⋯OC are compared. They exhibit unusual blueshifts of the Ar–H (Kr–H) stretching frequency, as well as contraction of the Ar–H (Kr–H) bond, and these effects are rationalized mainly in terms of shifts in the electron density of HArF (HKrF) on complexation, caused mainly by a combination of the intermolecular electrostatic interaction, electron–electron (Pauli) and nuclear–nuclear repulsion and charge density transferred from the CO molecule to the rare-gas-containing molecule.
Low-temperature dynamics of weakly localized Frenkel excitons in disordered linear chains
Authors: Bednarz M. , Malyshev V. A. , Knoester J.
Published: February 2004

We calculate the temperature dependence of the fluorescence Stokes shift and the fluorescence decay time in linear Frenkel exciton systems resulting from the thermal redistribution of exciton population over the band states. The following factors, relevant to common experimental conditions, are accounted for in our kinetic model: (weak) localization of the exciton states by static disorder, coupling of the localized excitons to vibrations in the host medium, a possible nonequilibrium of the subsystem of localized Frenkel excitons on the time scale of the emission process, and different excitation conditions (resonant or nonresonant). A Pauli master equation, with microscopically calculated transition rates, is used to describe the redistribution of the exciton population over the manifold of localized exciton states. We find a counterintuitive nonmonotonic temperature dependence of the Stokes shift. In addition, we show that depending on experimental conditions, the observed fluorescence decay time may be determined by vibration-induced intraband relaxation, rather than radiative relaxation to the ground state. The model considered has relevance to a wide variety of materials, such as linear molecular aggregates, conjugated polymers, and polysilanes.
Picosecond acoustic transmission measurements. I. Transient grating generation and detection of acoustic responses in thin metal films
Authors: Slayton Rebecca M. , Nelson Keith A.
Published: February 2004

The technique of impulsive stimulated thermal scattering is extended to backside measurement of acoustic wave packets that have propagated through thin metal films following their generation by pulsed optical excitation, heating, and thermal expansion at the front side. The acoustic transmission measurement at the backside substantially isolates the acoustic responses from thermal and electronic responses of the metal film that often dominate acoustic reflection signals measured from the front side, and permits straightforward measurement of the acoustic response generated by optical excitation at a substrate-thin film interface. It can thus better distinguish among different factors that limit the bandwidth of the acoustic wave packet, an issue of concern in the measurement of high frequency responses. The paper that follows demonstrates the application of the backside measurement to a study of high frequency structural relaxation in the glass-forming liquid glycerol.
Picosecond acoustic transmission measurements. II. Probing high frequency structural relaxation in supercooled glycerol
Authors: Slayton Rebecca M. , Nelson Keith A.
Published: February 2004

The high frequency acoustic response of liquids is measured in a manner directly analogous to conventional ultrasonic measurements. Two thin metal films act as acoustic transducer and receiver for a liquid layer between them. Pulsed optical excitation generates high bandwidth wave packets in the transducer, and these are detected in the receiver after damping and dispersion by the liquid. This initial measurement probes structural relaxation dynamics of glycerol in the frequency range 2–20 GHz, for temperatures between 235 and 291 K. The analysis presented here demonstrates the presence of excess relaxation, not accounted for by either the alpha or beta relaxation of the mode-coupling theory, and suggests the presence of constant loss in the susceptibility spectrum of supercooled glycerol.
Oxide-free oxygen incorporation into Ru(0001)
Authors: Blume Raoul , Niehus Horst , Conrad Horst , Böttcher Artur
Published: February 2004

A smooth Ru(0001) surface prepared under ultra-high vacuum conditions has been loaded with oxygen under high-pressure (p∼1 bar) and low-temperature (T<600 K) conditions. Oxygen phases created in this way have been investigated by means of thermal desorption spectroscopy, low-energy electron diffraction, and ultraviolet photoelectron spectroscopy. The exposure procedures applied lead to oxygen incorporation into the subsurface region without creation of RuO2 domains. For oxygen exposures ranging from 1011 to 1014 L oxygen contents up to about 4 monolayer equivalent could be achieved. The oxygen incorporation is thermally activated. The CO oxidation reaction conducted at mild temperatures (T<500 K) at a sample loaded with subsurface oxygen reaches CO→CO2 conversion probabilities of 10−3.
Thermal expansion and impurity effects on lattice thermal conductivity of solid argon
Authors: Chen Yunfei , Lukes Jennifer R. , Li Deyu , Yang Juekuan , Wu Yonghua
Published: February 2004

Thermal expansion and impurity effects on the lattice thermal conductivity of solid argon have been investigated with equilibrium molecular dynamics simulation. Thermal conductivity is simulated over the temperature range of 20–80 K. Thermal expansion effects, which strongly reduce thermal conductivity, are incorporated into the simulations using experimentally measured lattice constants of solid argon at different temperatures. It is found that the experimentally measured deviations from a T−1 high-temperature dependence in thermal conductivity can be quantitatively attributed to thermal expansion effects. Phonon scattering on defects also contributes to the deviations. Comparison of simulation results on argon lattices with vacancy and impurity defects to those predicted from the theoretical models of Klemens and Ashegi et al. demonstrates that phonon scattering on impurities due to lattice strain is stronger than that due to differences in mass between the defect and the surrounding matrix. In addition, the results indicate the utility of molecular dynamics simulation for determining parameters in theoretical impurity scattering models under a wide range of conditions. It is also confirmed from the simulation results that thermal conductivity is not sensitive to the impurity concentration at high temperatures.
Transport properties of nitrogen in single walled carbon nanotubes
Authors: Sokhan V. P. , Nicholson D. , Quirke N.
Published: February 2004

Transport properties including collective and tracer diffusivities of nitrogen, modeled as a diatomic molecule, in single walled carbon nanotubes have been studied by equilibrium molecular dynamics at different temperatures and as a function of pressure. It is shown that while the asymptotic decay of the translational and rotational velocity autocorrelation function is algebraic, the collective velocity decays exponentially with the relaxation time related to the interfacial friction. The tracer diffusivity in the nanochannel, which is comparable in magnitude with diffusivity in the equilibrium bulk phase, depends only weakly on the conditions at the fluid–solid interface, whereas the collective diffusivity is a strong function of the hydrodynamic boundary conditions and is found to be three orders of magnitude higher than self-diffusivity in carbon nanotubes and for the comparatively rough surface of the rare-gas tube it is one order of magnitude greater. A relationship between the collective diffusivity and the Maxwell coefficient describing wall collisions is obtained. The transport coefficients appear to be insensitive to the long-range details of the potential function.
A polarizable continuum model for molecules at diffuse interfaces
Authors: Frediani Luca , Cammi Roberto , Corni Stefano , Tomasi Jacopo
Published: February 2004

In this work we illustrate an extension of the polarizable continuum model to describe solvation effects on molecules at the interface between two fluid phases (liquid/liquid, liquid/vapor). This extension goes beyond the naive picture of the interface as a plane dividing two distinct dielectrics, commonly employed in continuum models. The main feature of the model is the use of a diffuse interface with an electric permittivity depending on the position. This characteristic clearly allows the study of simple interfaces as well as more complex membrane or multilayer structures. Moreover the smooth variation of the permittivity in the diffuse interface, in contrast to the sharp boundary between two regions, overcomes the numerical divergences due to charges placed at the boundary. The implementation of the model relies on the integral equation formalism, which allows one to calculate the reaction field acting on a molecule immersed in a dielectric environment once the proper Green’s function is known. In the present case, such a Green’s function is obtained numerically, allowing a large flexibility in the choice of the dielectric permittivity profile. The applications have been selected with the aim of illustrating the capabilities of the model; its present limitations are also discussed.
Elastic moduli of multiblock copolymers in the lamellar phase
Authors: Thompson R. B. , Rasmussen K. O/. , Lookman T.
Published: February 2004

We study the linear elastic response of multiblock copolymer melts in the lamellar phase, where the molecules are composed of tethered symmetric AB diblock copolymers. We use a self-consistent field theory method, and introduce a real space approach to calculate the tensile and shear moduli as a function of block number. The former is found to be in qualitative agreement with experiment. We find that the increase in bridging fraction with block number, that follows the increase in modulus, is not responsible for the increase in modulus. It is demonstrated that the change in modulus is due to an increase in mixing of repulsive A and B monomers. Under extension, this increase originates from a widening of the interface, and more molecules pulled free of the interface. Under compression, only the second of these two processes acts to increase the modulus.
Coupled folding–binding versus docking: A lattice model study
Authors: Gupta Nitin , Irbäck Anders
Published: February 2004

Using a simple hydrophobic/polar protein model, we perform a Monte Carlo study of the thermodynamics and kinetics of binding to a target structure for two closely related sequences, one of which has a unique folded state while the other is unstructured. We obtain significant differences in their binding behavior. The stable sequence has rigid docking as its preferred binding mode, while the unstructured chain tends to first attach to the target and then fold. The free-energy profiles associated with these two binding modes are compared.
Nonequilibrium thermodynamic description of the three-phase contact line
Authors: Bedeaux D.
Published: February 2004

In order to give a solid foundation of the description of the motion of the three-phase contact line, we develop the nonequilibrium thermodynamic description of the contact line. It is postulated that during its motion the contact line is featured as a separate thermodynamic system. Conservation laws are given for the excess densities of the components, the momentum and the energy along the line. The Gibbs law is formulated for the contact line and using this law the excess entropy production density along the line is constructed. This identifies the conjugate thermodynamic forces and fluxes for the contact line. Linear laws relating these quantities can then be given. The special case considered by Shikmurzaev, who gave the first satisfactory description of the motion of the contact line, is considered in more detail. A new boundary condition is found, which was not used by Shikhmurzaev. The need for such an additional boundary condition for this case was discussed in recent work by Billingham.
Three-body problem in quantum mechanics: Hyperspherical elliptic coordinates and harmonic basis sets
Authors: Aquilanti Vincenzo , Tonzani Stefano
Published: February 2004

Elliptic coordinates within the hyperspherical formalism for three-body problems were proposed some time ago [V. Aquilanti, S. Cavalli, and G. Grossi, J. Chem. Phys. 85, 1362 (1986)] and recently have also found application, for example, in chemical reaction theory [see O. I. Tolstikhin and H. Nakamura, J. Chem. Phys. 108, 8899 (1998)]. Here we consider their role in providing a smooth transition between the known “symmetric” and “asymmetric” parametrizations, and focus on the corresponding hyperspherical harmonics. These harmonics, which will be called hyperspherical elliptic, involve products of two associated Lamé polynomials. We will provide an expansion of these new sets in a finite series of standard hyperspherical harmonics, producing a powerful tool for future applications in the field of scattering and bound-state quantum-mechanical three-body problems.
Experimental and theoretical study of proton spin-lattice relaxation in H2–Ar gas mixtures: Critical examination of the XC(fit) potential energy surface
Authors: Sabzyan Hassan , McCourt Frederick R. W. , Power William P.
Published: February 2004

Proton nuclear magnetic resonance spin–lattice relaxation time measurements have been carried out at 500 MHz proton Larmor frequency on two hydrogen–argon gas mixtures with 1.90% and 3.93% hydrogen at four different temperatures in the range 225 K<T<337 K and at two different number densities. The results for different hydrogen mole percentages have been extrapolated to infinite dilution to obtain the contributions to the overall relaxation times arising from the hydrogen–argon interaction. The extrapolated relaxation times fall in the reciprocal regime in which relaxation times are inversely proportional to the density. Relaxation times have also been calculated using quantum mechanical close-coupled computations based on the H2–Ar XC(fit) potential energy surface obtained by Bissonnette et al. [J. Chem. Phys. 105, 2639 (1996)]. Significant differences found between the experimental and theoretical results indicate that the short-range anisotropy of the XC(fit) potential surface is too weak. The reciprocal regime is shown to have a much higher sensitivity to changes in the anisotropic component of the intermolecular potential energy surface.
Elasticity of polyelectrolyte multilayer microcapsules
Authors: Lulevich Valentin V. , Andrienko Denis , Vinogradova Olga I.
Published: February 2004

We present a novel approach to probe elastic properties of polyelectrolyte multilayer microcapsules. The method is based on measurements of the capsule load-deformation curves with the atomic force microscope. The experiment suggests that at low applied load deformations of the capsule shell are elastic. Using elastic theory of membranes we relate force, deformation, elastic moduli, and characteristic sizes of the capsule. Fitting to the prediction of the model yields the lower limit for Young’s modulus of the polyelectrolyte multilayers of the order of 1–100 MPa, depending on the template and solvent used for its dissolution. These values correspond to Young’s modulus of an elastomer.
Electrorheology in nanopores via lattice Boltzmann simulation
Authors: Melchionna Simone , Succi Sauro
Published: February 2004

The conductance of ionic species through a cylindrical nanochannel is explored by means of a mesoscopic lattice Boltzmann numerical method. It is shown that in the absence of any external voltage, the ionic profiles develop a considerable amount of structure within the channel. The details of this structure depend on the ionic density, channel length, and Debye length. In the presence of a trans-membrane electrostatic potential, less structure is observed and Ohm’s law is found to hold.
Microsolvation of N2H+: The nature of interactions in N2H+–(H2)n (n=1–14) complexes
Authors: Sheng Yinghong , Roszak Szczepan , Leszczynski Jerzy
Published: February 2004

Experimental studies of the consecutive growth of N2H+(H2)n clusters led to the discovery of an unusual bonding pattern for species with n=2–4. Theoretical studies revealed that the ligands are located within five well-separated solvation shells that are visible in structures, values of successive enthalpies and entropies of clustering reactions, vibrational motions, the distribution of atomic charges, and interaction energy decomposition components. The pattern of consecutive enthalpy changes for the second shell (n=2–5) is complicated. This pattern shows anomalous behavior, although its interpretation is not univocal. A large part of consecutive enthalpies for the clustering reactions is a contribution due to the rotational and vibrational properties of clusters which are difficult for adequate modeling in large systems. The structures of clusters are rationalized based on interaction energy contributions of a different nature. Geometries of complexes are determined by prevailing covalent forces.
Inertial solvent dynamics and the analysis of spectral line shapes: Temperature-dependent absorption spectrum of β-carotene in nonpolar solvent
Authors: Burt Jim A. , Zhao Xihua , McHale Jeanne L.
Published: February 2004

The influence of solvent dynamics on optical spectra is often described by a stochastic model which assumes exponential relaxation of the time-correlation function for solvent-induced frequency fluctuations. In contrast, theory and experiment suggest that the initial (subpicosecond) phase of solvent relaxation, resulting from inertial motion of the solvent, is a Gaussian function of time. In this work, we employ numerical and analytical calculations to compare the predicted absorption line shapes and the derived solvent reorganization energies obtained from exponential (Brownian oscillator) versus Gaussian (inertial) solvent dynamics. Both models predict motional narrowing as the ratio κ=Λ/Δ is increased, where Λ and Δ are the frequency and variance, respectively, of the solvent-induced frequency fluctuations. However, the motional narrowing limit is achieved at lower values of κ for the Brownian oscillator model compared to the inertial model. For a given line shape, the derived value of the solvent reorganization energy λsolv is only weakly dependent on the solvent relaxation model employed, though different solvent parameters Λ and Δ are obtained. The two models are applied to the analysis of the temperature-dependent absorption spectrum of β-carotene in isopentane and CS2. The derived values of λsolv using the Gaussian model are found to be in better agreement with the high temperature limit of Δ2/2kBT than are the values obtained using the Brownian oscillator model. In either approach, the solvent reorganization energy is found to increase slightly with temperature as a result of an increase in the variance Δ of the solvent-induced frequency fluctuations.
Rate constants for diffusive processes by partial path sampling
Authors: Moroni Daniele , Bolhuis Peter G. , van Erp Titus S.
Published: February 2004

We introduce a path sampling method for the computation of rate constants for complex systems with a highly diffusive character. Based on the recently developed transition interface sampling (TIS) algorithm this procedure increases the efficiency by sampling only parts of complete transition trajectories. The algorithm assumes the loss of memory for diffusive progression along the reaction coordinate. We compare the new partial path technique to the TIS method for a simple diatomic system and show that the computational effort of the new method scales linearly, instead of quadratically, with the width of the diffusive barrier. The validity of the memory loss assumption is also discussed.
Particle rearrangements during transitions between local minima of the potential energy landscape of a binary Lennard-Jones liquid
Authors: Vogel Michael , Doliwa Burkhard , Heuer Andreas , Glotzer Sharon C.
Published: February 2004

The potential energy landscape (PEL) of binary Lennard-Jones (BLJ) mixtures exhibits local minima, or inherent structures (IS), which are organized into metabasins (MBs). We study the particle rearrangements related to transitions between both successive IS and successive MB for a small 80:20 BLJ system near the mode-coupling temperature TMCT. The analysis includes the displacements of individual particles, the localization of the rearrangements, and the relevance of string-like motion. We find that the particle rearrangements during IS and MB transitions do not change significantly at TMCT. In particular, an onset of single particle hopping on the length scale of the interparticle distance is not observed. Further, it is demonstrated that IS and MB dynamics are spatially heterogeneous and facilitated by string-like motion. To investigate the mechanism of string-like motion, we follow the particle rearrangements during suitable sequences of IS transitions. We find that most strings observed after a series of transitions do not move coherently during a single transition, but subunits of different sizes are active at different times. Several findings suggest that, though string-like motion is of comparable relevance when the system explores a MB and when it moves from one MB to another, the occurrence of a successful string enables the system to exit a MB. Moreover, we show that the particle rearrangements during two consecutive MB transitions are basically uncorrelated. In particular, different groups of particles are highly mobile. We further find the positions of strings during successive MB transitions weakly but positively correlated, supporting the idea of dynamic facilitation. Finally, the relation between the features of the potential energy landscape and the relaxation processes in supercooled liquids is discussed.
Particle dynamics and the development of string-like motion in a simulated monoatomic supercooled liquid
Authors: Gebremichael Y. , Vogel M. , Glotzer S. C.
Published: February 2004

The microscopic details of local particle dynamics is studied in a glass-forming one component supercooled liquid modeled by a Dzugutov potential developed for simple metallic glass formers. Our main goal is to investigate particle motion in the supercooled liquid state, and to ascertain the extent to which this motion is cooperative and occurring in quasi-one-dimesional, string-like paths. To this end we investigate in detail the mechanism by which particles move along these paths. In particular, we show that the degree of coherence—that is, simultaneous motion by consecutive particles along a string—depends on the length of the string. For short strings, the motion is highly coherent. For longer strings, the motion is highly coherent only within shorter segments of the string, which we call “microstrings.” Very large strings may contain several microstrings within which particles move simultaneously, but individual microstrings within a given string are temporally uncorrelated with each other. We discuss possible underlying mechanism for this complex dynamical behavior, and examine our results in the context of recent work by Garrahan and Chandler [Phys. Rev. Lett. 89, 035704 (2002)] in which dynamic facilitation plays a central role in the glass transition.
Gas-phase reactions of organic radicals and diradicals with ions
Authors: Zhang Xu , Bierbaum Veronica M. , Ellison G. Barney , Kato Shuji
Published: February 2004

Reactions of polyatomic organic radicals with gas phase ions have been studied at thermal energy using a flowing afterglow-selected ion flow tube (FA-SIFT) instrument. A supersonic pyrolysis nozzle produces allyl radical (CH2CHCH2) and ortho-benzyne diradical (o-C6H4) for reaction with ions. We have observed: [CH2CHCH2+H3O+→C3H6++H2O], [CH2CHCH2+HO−→no ion products], [o-C6H4+H3O+→C6H5++H2O], and [o-C6H4+HO−→C6H3−+H2O]. The proton transfer reactions with H3O+ occur at nearly every collision (kII≅10−9 cm3 s−1). The exothermic proton abstraction for o-C6H4+HO− is unexpectedly slow (kII≅10−10 cm3 s−1). This has been rationalized by competing associative detachment: o-C6H4+HO−→C6H5O+e−. The allyl+HO− reaction proceeds presumably via similar detachment pathways.
Molecular dynamics of glucose in solution: A quasielastic neutron scattering study
Authors: Smith Luis J. , Price David L. , Chowdhuri Zema , Brady John W. , Saboungi Marie-Louise
Published: February 2004

The molecular dynamics of glucose dissolved in heavy water have been investigated at 280 K by the technique of quasielastic neutron scattering. The scattering was described by a dynamic structure factor that accounts for decoupled diffusive jumps and free rotational motions of the glucose molecules. With increasing glucose concentration, the diffusion constant decreases by a factor five and the time between jumps increases considerably. Our observations validate theoretical predictions concerning the impact of concentration on the environment of a glucose molecule and the formation of cages made by neighboring glucose molecules at higher concentrations.
Tunneling-induced spin alignment at low and zero field
Authors: Tomaselli M. , Meier U. , Meier B. H.
Published: February 2004

The transfer of rotational to spin angular momentum of CH3 groups according to the Haupt effect is shown to be independent of magnetic field strength, including zero field. Haupt enhanced pulsed nuclear resonance signals of γ-picoline have been observed at fields below 50 mT with a sensitivity enhancement of more than 3 orders of magnitude over thermally polarized experiments.
Morphology of femtosecond laser-induced structural changes in bulk transparent materials
Authors: Schaffer Chris B. , Jamison Alan O. , Mazur Eric
Published: February 2004

Using optical and electron microscopy, we analyze the energy and focusing angle dependence of structural changes induced in bulk glass by tightly focused femtosecond laser pulses. We observe a transition from small density variations in the material to void formation with increasing laser energy. At energies close to the threshold for producing a structural change, the shape of the structurally changed region is determined by the focal volume of the objective used to focus the femtosecond pulse, while at higher energies, the structural change takes on a conical shape. From these morphological observations, we infer the role of various mechanisms for structural change.
Measurement of the effective electron mass in GaInNAs by energy-loss spectroscopy
Authors: Gass M. H. , Papworth A. J. , Joyce T. B. , Bullough T. J. , Chalker P. R.
Published: February 2004

Imaging of the electronic structure of a GaInNAs/GaAs quantum well has been investigated by mapping the variation in the plasmon frequency using an electron energy-loss spectrometer on a dedicated field emission gun scanning transmission electron microscope. Kramers–Kronig analysis of the single scattered low-loss region yields a measure of the joint effective valence electron density. The average electron density has been used to provide a direct measurement of the electron effective mass of GaInNAs. The reduced mass was found to be 0.0874m0 for a 7 nm thick Ga0.9In0.1N0.04As0.96 quantum well.
Synthesis of InN/InP core/sheath nanowires
Authors: Yin Long-Wei , Bando Yoshio , Zhu Ying-Chun , Golberg Dmitri , Li Mu-Sen
Published: February 2004

Bulk quantities of InN/InP core/sheath nanowires with a diameter of 60–90 nm and a length of several micrometers were synthesized through a vapor reaction route. The nanowires consisted of single-crystalline InN core with 30–40 nm in diameter and amorphous InP sheath with a thickness of 20–25 nm. High-resolution transmission electron microscopy images indicated that most of the InN core is perfect crystalline; there also exists a high density of stacking faults and twins in the crystalline InN core in some of the synthesized nanowires. Indium nanoparticles found at the tip of the nanowires indicated a vapor-liquid-solid growth mechanism for formation of the InN/InP nanostructures. This type of InN/InP nanowire is important and may be further developed to be utilized as nanoscale field effect transistor and light-emitting diode.
Nonlinear emission in II–VI pillar microcavities: Strong versus weak coupling
Authors: Obert M. , Renner J. , Forchel A. , Bacher G. , André R. , Le Si Dang D.
Published: February 2004

Pillar microcavities based on CdMnTe/CdMgTe heterostructures have been investigated by means of spatially resolved optical spectroscopy. Strong coupling with a Rabi splitting of about 15.8 meV is demonstrated by performing microreflectivity measurements on a single pillar with three-dimensional optical confinement. Analyzing the temperature and the power dependent photoluminescence signal after nonresonant excitation, clear evidence of strong coupling in the nonlinear emission regime is obtained even at elevated temperatures.
Phonon confinement in oxide-coated silicon nanowires
Authors: Bhattacharyya Somnath , Samui Saumyadip
Published: February 2004

Raman spectroscopy of micron-long crystalline Si nanowires covered with a thick SiO2 layer showed a downshift and asymmetric broadening of the Raman first order TO phonon peak when compared with the bulk (q=0) mode. The Raman shift and broadening were attributed to phonon confinement in the nanowires. A phenomenological phonon confinement model, incorporating the effects of nanowire diameter distribution, is presented. This model is shown to accurately describe the broadening of the Raman peak and is consistent with the microstructure of Si nanowires. In addition to the work a distribution of the phonon wave vector was directly taken into consideration replacing the diameter distribution to fit the Raman TO peak. The effects of the nano-Si:SiO2 boundary on the Raman spectra are discussed in terms of softening of the phonon confinement.
Highly-efficient blue electroluminescence based on two emitter isomers
Authors: Kan Ying , Wang Liduo , Duan Lian , Hu Yuanchuan , Wu Guoshi , Qiu Yong
Published: February 2004

Highly-efficient blue organic light-emitting devices with a nondoped device structure have been fabricated with two anthracene derivatives, 2,3,6,7-tetramethyl-9,10-(1-dinaphthyl)-anthracene (α-TMADN) and 2,3,6,7-tetramethyl-9,10-(2-dinaphthyl)-anthracene (β-TMADN). The homemade α-TMADN, β-TMADN, or their blend were used as the light-emitting materials, and 4,7-diphenyl-1,10-phenanthroline was used as the hole blocking and electron transporting material, N,N′-biphenyl-N,N′-bis-(1-naphenyl)-[1,1′-biphenyl]-4,4′-diamine was used as the hole transporting material. The brightness of the device with β-TMADN as the light-emitting material is up to 10 000 cd/m2 at 12 V with the maximum efficiency of 4.5 cd/A, which is better than that of the device with α-TMADN as the light-emitting material. The brightness of the same structure device with the blend of α-TMADN and β-TMADN as the light-emitting material, in which the ratio of α-TMADN to β-TMADN is 9:1, is more than 12 000 cd/m2 at 15 V with the maximum efficiency of 5.2 cd/A.
Investigation of azobenzene side group orientation in polymer surface relief gratings by means of photoelectron spectroscopy
Authors: Henneberg O. , Geue Th. , Pietsch U. , Saphiannikova M. , Winter B.
Published: February 2004

The molecular orientation of azobenzene side groups in polymer films before (nonpatterned) and after (patterned) development of a surface relief grating has been investigated by photoelectron spectroscopy using synchrotron radiation. The photoemission spectra obtained for 60–100 eV photons of a patterned and a nonpatterned surface are similar when the polarization vector of the synchrotron light is parallel to the grating vector. However, for perpendicular excitation, considerable spectral intensity differences can be observed for 9–14 eV electron binding energy. The observed changes are attributed to the formation of well-oriented azobenzenes at the surface.
Epitaxial GaN1−yAsy layers with high As content grown by metalorganic vapor phase epitaxy and their band gap energy
Authors: Kimura Akitaka , Paulson C. A. , Tang H. F. , Kuech T. F.
Published: February 2004

GaN 1−y As y epitaxial alloy samples with [N]≫[As] were grown by metalorganic vapor phase epitaxy. The range of As content achieved, up to y=0.067, greatly extends the range of achievable As levels to values that are well within the miscibility gap of the GaN–GaAs system. The single-phase epitaxial nature of the alloy samples was confirmed by x-ray diffraction. The As-content dependence of the band gap was determined by optical absorption measurements. A highly-bowed bandgap was observed as a function of the As content, and a refined value of the bowing parameter of 16.9±1.1 eV was determined.
Nonlinear optical properties of an organic film formed of dye aggregates with absorption in the telecommunication wavelength range
Authors: Tatsuura Satoshi , Tian Minquan , Furuki Makoto , Sato Yasuhiro , Iwasa Izumi , Mitsu Hiroyuki
Published: February 2004

An organic film formed of newly synthesized dye aggregates with an absorption peak in telecommunication wavelength range exhibited large third-order nonlinear optical susceptibility χ(3) and fast recovery of bleached absorption. A derivative with a di(benzofuranonyl)methanolate structure forms stable aggregates in a spin-coated film and shows an absorption maximum at 1.24 μm. An imaginary part χI(3) of the film assumes a maximum at 1.30 μm, which is −0.83×10−7 esu. The time evolution of differential transmission spectra indicates that the time constant of the recovery of bleached absorption is 80–240 fs for a fast component and 2.2–6.5 ps for a slow one. This organic film has considerable advantages for future planar optical devices.
Low resistance and transparent Ni–La solid solution/Au ohmic contacts to p-type GaN
Authors: Song June-O , Leem Dong-Seok , Kwak J. S. , Lee S. N. , Nam O. H. , Park Y. , Seong Tae-Yeon
Published: February 2004

We report on the formation of reliable Ni–La solid solution (8 nm)/Au (8 nm) ohmic contacts to p-type GaN (Na=5×1017 cm−3). The as-deposited contact reveals nonlinear current–voltage (I–V) characteristics. However, the contacts show considerably improved I–V behavior when annealed at temperatures of 350–550 °C for 1 min in air ambient. For example, the specific contact resistance as low as 7.2×10−5 Ω cm2 is obtained from the samples annealed at 550 °C, whose transmittance is measured to be 82.5% at a wavelength of 470 nm. Based on Auger electron spectroscopy and x-ray photoemission spectroscopy results, possible ohmic formation mechanisms for the annealed contacts are described and discussed.
AlGaN/GaN polarization-doped field-effect transistor for microwave power applications
Authors: Rajan Siddharth , Xing Huili , DenBaars Steve , Mishra Umesh K. , Jena Debdeep
Published: February 2004

We discuss an AlGaN/GaN metal–semiconductor field-effect transistor (MESFET) structure grown without any impurity doping in the channel. A high-mobility polarization-induced bulk channel charge was created by grading the channel region linearly from GaN to Al0.3Ga0.7N over 1000 Å. This polarization-doped FET (PolFET) was fabricated and tested under dc and rf conditions. Current density of 850 mA/mm and transconductance of 93 mS/mm was observed under dc conditions. The 0.7 μm gate length devices had a cutoff frequency, fτ=19 GHz, and maximum oscillation frequency, fmax=46 GHz. We demonstrate that the PolFETs perform better than comparable MESFETs with impurity-doped channels, and are suitable for high power microwave applications. An important advantage of these devices over AlGaN/GaN high electron mobility transistors is that the transconductance versus gate voltage profile can be tailored by compositional grading for better large-signal linearity.
Radio frequency potential of inductive plasma immersed in a weak magnetic field
Authors: Godyak V. A. , Alexandrovich B. M.
Published: February 2004

Radio frequency potential has been revealed in inductively coupled plasma (ICP) immersed in a weak magnetic field. The rf plasma potential measured in a wide range of driving frequency and argon pressure reached tens of volts at a relatively weak magnetic field and was found falling with an increase in the driving frequency and gas pressure. The appearance of rf plasma potential in magnetized ICP is interpreted as a result of rf Hall effect caused by the electron rf drift in a crossed magnetic field.
A 7-nm nanocolumn structure fabricated by using a ferritin iron-core mask and low-energy Cl neutral beams
Authors: Kubota Tomohiro , Baba Tomohiro , Samukawa Seiji , Kawashima Hiroyuki , Uraoka Yukiharu , Fuyuki Takashi , Yamashita Ichiro
Published: February 2004

A 7-nm silicon column structure was fabricated by using a Cl neutral beam we developed. The neutral beam achieved a high etching selectivity to a ferritin iron-core mask by using charge-free and damage-free etching processes. The silicon etching selectivity ratio to the iron core was measured to be about 59. The iron core in the ferritin was 7 nm in diameter, which was identical to that of the etched nanocolumn. This indicates that neutral-beam etching transferred the structure and size of the iron core to the silicon substrate.
Inhomogeneous magnetization of a superconducting film measured with a gradiometer
Authors: Stalzer H. , Cosceev A. , Sürgers C. , Löhneysen H. v. , Brosi J.-M. , Chakam G.-A. , Freude W.
Published: February 2004

The magnetic flux generated by the screening currents of a superconducting Nb film in the Meissner state is measured with a coaxial gradiometer in a weak magnetic field B. A peculiar position dependence is found when the sample surface is only slightly tilted away from the parallel field orientation and shifted with respect to the center axis of the gradiometer. This is due to the inhomogeneous magnetization with a strong out-of-plane contribution measured by the two pick-up coils as confirmed by numerical finite-element calculations. The unusual position dependence has important consequences for interpreting the diamagnetic screening of superconducting films measured by a coaxial gradiometer near the parallel-field orientation.
Anthropogenic impacts on the ecosystems of coastal lagoons: modelling fundamental biogeochemical processes and management implications
Authors: Webster Ian T. , Harris Graham P.
Published: February 2004

This paper presents a biogeochemical model of a coastal lagoon intended to be representative of lagoons occurring along the south-east and south-west coasts of Australia. Many of these lagoons are threatened by increased nutrient loads because of land use change, by alterations to their freshwater inflows and by modification to their tidal flushing regimens. The model simulates the biogeochemical response of the lagoon to nutrient (nitrogen) loading and includes nutrient transformation processes in the sediments, as well as in the water column. The paper focuses on the response of primary producers to increasing and decreasing nutrient loads and how the response is altered by changes in the flushing rate of the lagoon with the sea. In common with lakes, the modelled lagoon exhibits alternative stable states representing macrophyte or phytoplankton dominance depending on nutrient loading and history. A third state representing severe degradation occurs when denitrification shuts down. A characteristic of Australian coastal lagoon systems is that, due to highly sporadic rainfall patterns, nutrient inflows are dominated by intermittent extreme events. The modelling demonstrates that such a loading regimen may be expected to generally increase the vulnerability of the lagoon to increasing nutrient loads. The results of the analysis presented are pertinent to several questions raised by coastal managers, such as what are the expected benefits of improving flushing by dredging and what are the consequences of altering the timing and magnitudes of the loads reaching the lagoons?