bluuebunny/crossref_metadata_2025 huggingface dataset:
Critical exponents of a two-reaction model between monomers
Authors: da Costa E. C. , Figueiredo W.
Published: July 2002

We study in this work a competitive reaction model between monomers on a catalytic surface. The surface is represented by a square lattice and we consider the following reactions: A+A(B)→A2(AB), where A and B are two monomers that arrive at the surface with probabilities yA and yB, respectively. The model is studied in the adsorption controlled limit, and every time a monomer A or B lands on the surface it occupies a single empty site of the lattice. When a A monomer sits on the surface, it stays there unless it finds another A or B monomer. In this case the reaction occurs instantaneously leaving two new vacant sites on the lattice. The reactions between two A monomers and between A and B monomers are assumed to happen with the same probability. The model is studied in the site and pair mean-field approximations as well as through Monte Carlo simulations. We show that the model exhibits a continuous phase transition between an active and a B-absorbing state, when the parameter yA is varied through a critical value. Monte Carlo simulations and finite-size scaling analysis at the critical point are used to determine the critical exponents β, ν⊥, and ν∥. Our results seem to confirm that this reaction model is in the same universality class of the directed percolation in (2+1) dimensions.
Phase relaxation in a many-body system of diffusing spins: Slow motional limit
Authors: Nevzorov Alexander A. , Freed Jack H.
Published: July 2002

The echo amplitude decay of a diffusing electron-spin-bearing molecule interacting with a diffusing many-spin bath of proton-containing molecules has been studied theoretically in the slow motional limit. Closed-form asymptotic expressions for the short- and long-time behavior of the echo envelope have been obtained. In contrast to the well-studied fast-motional limit, the echo envelope cannot be described by a simple exponential decay, and its rate exhibits a DT1/3 dependence on the relative translational diffusion coefficient, DT. For low proton concentrations and long pulse delay times, τ, an exponential τ9/8-decay of the echo amplitude is found, whereas for high proton concentrations and short τ’s the decay exhibits an exponential τ3-behavior. These limiting analytical results are compared with the exact numerical solutions to establish their range of validity.
Fabrication and mechanical behavior of dye-doped polymer optical fiber
Authors: Jiang Changhong , Kuzyk Mark G. , Ding Jow-Lian , Johns William E. , Welker David J.
Published: July 2002

The purpose of this article is to study the materials physics behind dye-doped polymethyl metharcylate (PMMA) that is important for the optical fiber drawing process. We report effects of the fabrication process on the mechanical properties of the final fiber. The qualitative degree of polymer chain alignment is found to increase with the drawing force, which in turn decreases with the drawing temperature and increases with the drawing ratio. The chain alignment relaxes when the fibers are annealed at 95 °C with a commensurate decrease in fiber length and increase in diameter. The annealed fiber has higher ductility but lower strength than the unannealed fiber. Both the yield and tensile strengths are dependent on the strain rate. The relationship between tensile strength, σb, and fiber diameter, d, is found empirically to be σb∝d−0.5. The yield strength appears to be less sensitive to the fiber diameter than the tensile strength. For PMMA doped with disperse red 1 azo dye, the yield strength, tensile strength, and Young’s modulus peak at a dye concentration of 0.0094 wt %. These results are useful for designing polymer optical fibers with well-defined mechanical properties.
Stokes-flow computation of the diffusion coefficient and rotational diffusion tensor of lysozyme, a globular protein
Authors: Zhao Hong , Pearlstein Arne J.
Published: July 2002

Based on a closed surface of triangles fitted to atomic coordinates determined crystallographically, Brune and Kim [Proc. Natl. Acad. Sci. USA 90, 3835–3839 (1993)] proposed a boundary-element Stokes-flow technique for ab initio computation of a translational diffusion coefficient and the rotational diffusion tensor Dr of globular proteins. They applied their approach to atomic coordinates for a tetragonal structure of hen egg-white lysozyme, and reported that computed values of a translational diffusion coefficient and Dr=tr(Dr)/3 agreed well with experiment. After establishing the identity between the infinite-dilution tracer diffusion coefficient of the protein macroion (D+ for lysozyme cation) and the “translational diffusion coefficient” computed by Brune and Kim, we adopt a somewhat different computational approach and show how convergence of D+ and Dr for tetragonal lysozyme depends on two computational parameters characterizing the fidelity of the geometric approximation to the protein surface and two others characterizing the accuracy of the Stokes-flow computations. We then compute D+ and Dr for lysozyme using atomic coordinates for the triclinic crystal structure, three structures determined by nuclear magnetic resonance spectroscopy in the liquid phase (presumably corresponding more closely to in vivo structures), the solvated tetragonal structure (with 108 water molecules) considered by Brune and Kim, and a “dry” version of the same structure. These computations show that D+ and Dr computed for all of the dry crystal structures are in excellent agreement with those for the liquid-phase conformations. Values of D+ and Dr computed for the solvated structure are lower, consistent with the larger volume and area of the corresponding polyhedral surface. We also show that several choices of the origin of the force system [discussed by Brenner, J. Colloid Interface Sci. 23, 407–436 (1967)] give rise to nearly identical translational diffusion coefficients. Finally, we show how to estimate the thickness of the “solvation shell” contributing to the hydrodynamic resistance of the protein cation, and use the binary Nernst–Hartley equation to then estimate the effective cation charge at the two pH values at which the binary diffusion coefficient has been accurately measured in recent interferometric experiments.
Integrable variant of the one-dimensional Hubbard model
Authors: Guan X.-W. , Foerster A. , Links J. , Zhou H.-Q , Prestes Tonel A. , McKenzie R. H.
Published: July 2002

A new integrable model which is a variant of the one-dimensional Hubbard model is proposed. The integrability of the model is verified by presenting the associated quantum R-matrix which satisfies the Yang–Baxter equation. We argue that the new model possesses the SO(4) algebra symmetry, which contains a representation of the η-pairing SU(2) algebra and a spin SU(2) algebra. Additionally, the algebraic Bethe ansatz is studied by means of the quantum inverse scattering method. The spectrum of the Hamiltonian, eigenvectors, as well as the Bethe ansatz equations, are discussed.
Surface effects on shape, self-organization and photoluminescence of InAs islands grown on InAlAs/InP(001)
Authors: Brault J. , Gendry M. , Grenet G. , Hollinger G. , Olivares J. , Salem B. , Benyattou T. , Bremond G.
Published: July 2002

InAs nanostructures were grown on In0.52Al0.48As alloy lattice matched on InP(001) substrates by molecular beam epitaxy using specific growth parameters in order to improve island self-organization. We show how the change in InAs surface reconstruction via growth temperature from (2×4) to (2×1) and/or the use of InAlAs initial buffer surface treatments improve the island shape homogeneity (either as quantum wires or as quantum dots). Differences in island shape and in carrier confinement are shown by atomic force microscopy and by photoluminescence measurements, respectively. We point out that such shape amendments induce drastic improvements to island size distribution and discernible changes in photoluminescence properties, in particular concerning polarization.
The application of the photopyroelectric method for measuring the thermal parameters of pyroelectric materials
Authors: Sahraoui A. Hadj , Longuemart S. , Dadarlat D. , Delenclos S. , Kolinsky C. , Buisine J. M.
Published: July 2002

The photopyroelectric calorimetry, in the standard (back) configuration, is applied in order to measure the thermal parameters of some pyroelectric materials. It is demonstrated that the method is able to simultaneously measure the thermal diffusivity and effusivity of a pyroelectric material. The information is obtained via a frequency scan of the amplitude or the phase of the pyroelectric signal; the measurements need no calibration. A combined amplitude-phase procedure, at a single frequency, leads to the same results. In the mean time, if the thermal parameters of the pyroelectric sensor are known, one can get the thermal effusivity of a sample acting in the experimental cell as a substrate. Investigations and theoretical simulations were performed on a well known pyroelectric material, LiTaO3, with various liquid substrates.
Studies of electron beams propagation in space-charge regime
Authors: Martina L. , Nassisi V. , Pedone A. , Pompa P. P. , Raganato G.
Published: July 2002

We report the experimental characteristics of electron beam propagation under space-charge regime. The electron beams were generated by a Nb polycrystalline photocathode illuminated by two different excimer lasers, a XeCl (308 nm) and a KrCl (222 nm). The laser photon energies were very close to the Nd work function. The cathode surface was mechanically worked in order to study the photoemission from a smooth and a rough surface. At low accelerating voltage the electron beam was dominated by the space-charge effect and its resulting pulse never clipped as predicted by the Child–Langmuir law. Instead, it presented as fast a rise time as the laser one, an intermediate zone, and a tail longer than the laser pulse one. On the other hand, under saturation regime the output current wave form was similar to the laser one. The quantum efficiency was higher for the rough cathode. It corresponded to 3.2×10−5 and 6.7×10−7 for the KrCl and XeCl irradiation, respectively. The maximum current value was an electron bunch containing 980 mA (8.9 nC), by means of 1.7 mJ KrCl laser energy and 10 kV of accelerating voltage.
Resonance and reversibility of vibrational relaxation of HF in high temperature Ar bath gas
Authors: Krems Roman V. , Buchachenko Alexei A. , Marković Nikola , Nordholm Sture
Published: July 2002

The Boltzmann averaged rate constants for total vibrational relaxation of HF(v=1) in collisions with Ar are computed in the range of temperatures between 100 and 1500 K. The computed rate constants overestimate the experimental measurements at high temperatures by a large factor. It is concluded that the deviation between theory and experiment cannot be explained by inaccuracy of the PES or dynamical approximations made. It is shown that increasing initial rotational energy enhances a resonant character of the vibrational energy transfer to a great extent. An assumption is made that total vibrational relaxation of HF(v=1) at high temperatures is determined by competition between vibrational relaxation to a resonant level (v=0,jres), vibrational excitation from the resonant level, and purely rotational relaxation of HF(v=0,jres). It is demonstrated that at high temperatures the latter process can be significantly slower than vibrationally inelastic transitions and rotational relaxation of HF(v=0,jres) may in fact be a rate-limiting stage of vibrational relaxation.
Photoexcitation of LiH2+ from selected initial states: A time-dependent model
Authors: Satta M. , Bodo E. , Martinazzo R. , Gianturco F. A.
Published: July 2002

A new quantum time-dependent treatment has been employed to model the photoexcitation of LiH2+ by looking at the process both in a constrained configuration and in the full three-dimensional space. The study has been carried out by monitoring at the end of the process the fragmentation probabilities, the final vibrational distributions of the molecular fragments, and the angular evolution of the wave functions of the complex on the excited electronic surface. The comparison between different initial conditions is able to shed light on the microscopic mechanism of the energy redistribution, with particular reference to the role of the angular coordinate that turns out to provide efficient energy channeling during the evolution. The possibility of extending the method to larger systems is briefly discussed.
The N2–N2 system: An experimental potential energy surface and calculated rotovibrational levels of the molecular nitrogen dimer
Authors: Aquilanti V. , Bartolomei M. , Cappelletti D. , Carmona-Novillo E. , Pirani F.
Published: July 2002

An accurate new representation for the potential energy surface for the N2–N2 dimer has been obtained from the analysis of scattering experiments from our laboratory, and of available second virial coefficient data. A harmonic expansion functional form describes the salient geometries of the dimer and accounts for the relative contributions to the intermolecular interaction from components of different nature. The equilibrium geometry is a T conformation with well depth 13.3 meV (107.14 cm−1) and at a distance of 4.03 Å. In order to assist in the analysis of spectra, we calculated the bound rotovibrational states for the (N2)2 system for J⩽6 by solving a secular problem over the exact Hamiltonian, considering the N2 monomers as rigid rotors, and where the Coriolis coupling is included.
Tetrahedrally invariant discrete variable representation basis on the sphere
Authors: Cargo Matthew , Littlejohn Robert G.
Published: July 2002

This paper explores the difficulties of constructing multidimensional discrete variable representation (DVR) basis sets and the strategies that can be used to overcome them. A parameter count shows that the conditions on a DVR basis set cannot be satisfied on most spaces of wave functions. One-dimensional, orthogonal polynomials are an exception, but the Ylm’s on the sphere only go 3/4 of the way, in a certain sense, toward supplying enough parameters to satisfy the DVR conditions. It is shown that DVR sets involving rotationally invariant subspaces of wave functions on the sphere (consisting of complete subshells only) exist only for small values of the angular momentum cutoff. However, an exploration of DVR sets invariant under subgroups of the full rotation group leads to the discovery of a 12-point DVR set that is invariant under the tetrahedral group, whose grid points are the vertices of an icosahedron.
Electronic excited states of conjugated cyclic ketones and thioketones: A theoretical study
Authors: Serrano-Andrés Luis , Pou-Amérigo Rosendo , Fülscher Markus P. , Borin Antonio Carlos
Published: July 2002

Absorption spectra of a series of cyclic conjugated ketones and thioketones have been computed at the multiconfigurational second-order multistate perturbation level of theory, the CASSCF/MS-CASPT2 method. Excitation energies, transition dipole moments, oscillator strengths, and static dipole moments are reported and discussed for excited states with energies lower than ≈7–8 eV. The main bands of the spectra have been assigned and characterized in most cases for the first time. The spectroscopy of the different systems is compared in detail. Thioketones in particular have low-energy and intense ππ* transitions which suggest corresponding enhanced nonlinear molecular optical properties. Additionally, some of the methods used to estimate these properties from spectroscopic data have been considered in order to analyze the main contributions to the nonlinear optical properties.
n-type doping of oxides by hydrogen
Authors: Kılıç Çetin , Zunger Alex
Published: July 2002

First-principles total-energy calculations suggest that interstitial hydrogen impurity forms a shallow donor in SnO2, CdO, and ZnO, but a deep donor in MgO. We generalize this result to other oxides by recognizing that there exist a “hydrogen pinning level” at about 3.0±0.4 eV below vacuum. Materials such as Ag2O, HgO, CuO, PbO, PtO, IrO2, RuO2, PbO2, TiO2, WO3, Bi2O3, Cr2O3, Fe2O3, Sb2O3, Nb2O5, Ta2O5, FeTiO3, and PbTiO3, whose conduction band minimum (CBM) lie below this level (i.e., electron affinity>3.0±0.4 eV) will become conductive once hydrogen is incorporated into the lattice, without reducing the host. Conversely, materials such as BaO, NiO, SrO, HfO2, and Al2O3, whose CBM lie above this level (i.e., electron affinity<3.0±0.4 eV) will remain nonconductive since hydrogen forms a deep impurity.
Origin and consequences of aromatic back-bonding at a transition metal surface: Benzyne on Ir{100}
Authors: Yamagishi S. , Jenkins S. J. , King D. A.
Published: July 2002

Recent low energy electron diffraction experiments have shown that partial dissociation of benzene at the Ir{100} surface yields an ordered overlayer of ortho-benzyne radicals (C6H4) with ring-planes inclined at 47.2° to the normal. The primary molecule-surface interaction may be attributed to σ bonds between radical C and surface Ir atoms, but this characteristic alone does not uniquely constrain the adsorbate to the observed orientation. Through first-principles density functional theory which gives good agreement with the experimental structure, we now demonstrate that the molecular tilt arises due to interaction of the aromatic π orbitals with the surface d orbitals, and not because of any dispersive intermolecular forces.
Influence of protective layers on the blinking of fluorescent single molecules observed by confocal microscopy and scanning near field optical microscopy
Authors: Vargas F. , Hollricher O. , Marti O. , de Schaetzen G. , Tarrach G.
Published: July 2002

Transitions of fluorescent Rhodamine 6G dye molecules into metastable dark states with lifetimes of several seconds were observed by single-molecule detection (SMD) using far-field confocal microscopy. The samples were protected with different organic thin films and were characterized using atomic force microscopy. The data are compared to published models: The best fit is found with a model suggesting that oxygen migration or polarity changes are responsible for the transitions. For further studies with molecules close to each other, we studied the capability of cantilever-SNOM sensors for SMD. Using an α-SNOM we were able to demonstrate 40 nm optical resolution of these new near-field tips for single-molecule fluorescence imaging in direct comparison with confocal microscopy on samples of Rhodamine 6G molecules protected by PMMA.
A vibrational spectroscopic investigation of the CO+O2 reaction on Pt{110}
Authors: Miners J. H. , Cerasari S. , Efstathiou V. , Kim M. , Woodruff D. P.
Published: July 2002

The CO coverage of a Pt{110} surface in both the high and low reaction rate branches of the bistable CO oxidation reaction has been determined by Infrared Reflection-Absorption Spectroscopy (IRAS), first performing extensive calibration experiments on the various factors determining the absorbance and frequency associated with the C–O vibrational stretching mode. The same two states of the surface are shown to be present under steady-state low and high reaction rates and when the surface is undergoing pattern formation and homogeneous reaction rate oscillations. Using the CO coverages determined by IRAS, the intensities observed in a series of photoelectron emission microscopy images have been used to elucidate the oxygen coverage in both coadsorption states. The low reaction rate branch is found to be associated with a high CO coverage (0.5±0.1 ML) and very low O coverage (0.03±0.01 ML) consistent with the (1×1) unreconstructed phase. In the high rate branch the surface has a low CO coverage (0.05±0.03 ML) and O coverages in the range 0.3–0.7 ML [(1×2) reconstructed phase]. No evidence for bridged CO, oxide, or subsurface oxygen, variously proposed to play a role in the reaction rate bistability, was found under the conditions measured. These findings are consistent with the site blocking and reconstruction model. Coadsorption experiments of CO and oxygen under nonreactive conditions, performed as part of the IRAS calibration process, demonstrate that CO and O can occupy a mixed adlayer and identify two different chemical environments for CO adsorption.
Magnetic rope structures in the electromagnetic interchange mode
Authors: Vranješ J. , Tanaka M. Y. , Kono M.
Published: July 2002

Nonlinear electromagnetic interchange mode in a magnetized plasma, with the spatially dependent intensity of the magnetic field, is studied. The equilibrium conditions in such a system require the existence of a spatially nonuniform ion drift, and the effects of such a drift on the nonlinear evolution of unstable modes are investigated. The existence of spatially localized structures is demonstrated for the density, electric, and magnetic field. In the case of the magnetic field these structures have a twisted, or rope form.
Highly sensitive strain sensors based on magnetic tunneling junctions
Authors: Löhndorf M. , Duenas T. , Tewes M. , Quandt E. , Rührig M. , Wecker J.
Published: July 2002

Micrometer-sized highly sensitive strain sensors are presented. The sensors are based on magnetic tunneling junctions (MTJs) incorporating magnetostrictive free layers. The influence of mechanical strain upon the free layer is explained by a model taking into account the total free energy of the sensing layer. Those MTJ devices prepared in situ with magnetostrictive Fe50Co50 layers exhibit a tunneling magnetoresistance (TMR) ratio of 48%. The changes in strain Δε on the order of 0.4 parts per thousand (‰) result in resistance changes of 24%, which in turn leads to gauge factors [(ΔR/R)/Δε] on the order of 600, whereas gauge factors of 2–4 are typical for metal based, and 40–180 for piezoresistive semiconductor strain gauges.
Analysis of the molecular density: STO densities
Authors: Rico J. Fernández , López R. , Ema I. , Ramı́rez G.
Published: July 2002

A partition of the molecular density for Slater basis sets (STO), which parallels one previously developed for Gaussian basis sets (GTO), is reported. The atomic fragments are expanded in spherical harmonics times radial factors. Each fragment contains all the one-center charge distributions centered in the atom plus the part of every two-center distribution assigned to the atom by the partition criterion. The performance of the procedure is analyzed, concluding that the analysis gives highly accurate representations of the molecular density at a very low cost. Moreover, the results of the analysis are illustrated with the study of the densities in CO and H2O and the comparison of the atomic densities obtained from STO and GTO molecular calculations.
Theoretical investigations of I=5/2 quadrupolar spin dynamics in the sudden-passage regime
Authors: Walls Jamie D. , Lim Kwang Hun , Logan John W. , Urban Jeffry T. , Jerschow Alexej , Pines Alexander
Published: July 2002

The theoretical approach utilizing bimodal Floquet theory in the quadrupolar/central-transition interaction frame, presented in an earlier article [J. D. Walls, K. H. Lim, and A. Pines, J. Chem. Phys. 116, 79 (2002)], is extended to describe the more complicated spin dynamics of I=5/2 spin systems. Rotary resonance effects occur when the strength of the radio-frequency irradiation, ω1, matches the sample spinning speed, ωr, at the conditions ω1=23nωr (n integral). At these conditions, conversions of both triple-quantum and five-quantum coherences to central-quantum coherence are observed. Between rotary resonance conditions [2n3ωr<ω1<[2(n+1)]/3ωr], five-quantum as well as triple-quantum coherences can be created from equilibrium z-magnetization via a nutation mechanism. In addition, effective transfer between five-quantum and triple-quantum coherences also is observed in between rotary resonance conditions. These effects have been investigated theoretically and verified by both numerical calculations and experimental results.
Time-resolved scavenging and recombination dynamics from I:e− caged pairs
Authors: Kloepfer Jeremiah A. , Vilchiz Victor H. , Lenchenkov Victor A. , Chen Xiyi , Bradforth Stephen E.
Published: July 2002

The competition between geminate recombination of electrons with their parent radicals and electron scavenging with H+ is directly time resolved with ∼100 fs resolution at several acid concentrations. Electrons were produced from iodide photodetachment or two-photon ionization of H2O. With regards to those produced from iodide photodetachment, the separation between primary and secondary I:e− recombination is established using a full numerical solution to the diffusion equation. Electron ejection is found to be short range and a potential well of ∼3kbT depth stabilizing the solvent caged pair is required to yield a satisfactory fit to experiment. From time-resolved scavenging data up to 5 M HCl, it is shown that the electron can be scavenged both inside and outside the caged pair by H+ with nearly equal efficiency. The steady-state scavenging yield as a function of scavenger concentration is then predicted based on the determined time-dependent recombination function. Reassessment of several benchmark scavenging experiments from the 1960’s leads to the conclusion that the primary yield of electrons after excitation of iodide is near unity.
The effect of position along the chain on the dynamic properties of hard chain segments
Authors: McCormick Julie A. , Hall Carol K. , Khan Saad A.
Published: July 2002

Discontinuous molecular dynamics simulations are performed on systems containing 32 hard chains of length 192 at three volume fractions, φ=0.40, 0.45, and 0.50, to investigate the effect of position on the segmental mean squared displacement. The mean squared displacements of various sized blocks of segments at different positions along the chain are calculated. First, the effect of block size on the dynamics of end and middle blocks is considered. It is found that small blocks provide a greater difference between the mean squared displacements of middle blocks, end blocks, and the whole chain than larger equal-sized blocks. Next, the portions of the chain exhibiting end and middle behavior are determined. It is found that a large portion of the chain displays middle behavior, while a small portion displays end behavior. Finally, the dynamics of segment relaxation along the chain are studied. The relaxation of small blocks of segments at different positions along the chain starts at the chain ends and progresses toward the chain middle with time, as the tube model predicts. The minor chain length, the portion of the chain that has relaxed, follows a power law with time, but the power is somewhat less than predicted.
Polymer induced depletion potentials in polymer-colloid mixtures
Authors: Louis A. A. , Bolhuis P. G. , Meijer E. J. , Hansen J. P.
Published: July 2002

The depletion interactions between two colloidal plates or between two colloidal spheres, induced by interacting polymers in a good solvent, are calculated theoretically and by computer simulations. A simple analytical theory is shown to be quantitatively accurate for the case of two plates. A related depletion potential is derived for two spheres; it also agrees very well with direct computer simulations. Theories based on ideal polymers show important deviations with increasing polymer concentration: They overestimate the range of the depletion potential between two plates or two spheres at all densities, with the largest relative change occurring in the dilute regime. They underestimate the well depth at contact for the case of two plates, but overestimate it for two spheres. Depletion potentials are also calculated using a coarse graining approach which represents the polymers as “soft colloids;” good agreement is found in the dilute regime. Finally, the effect of the polymers on colloid–colloid osmotic virial coefficients is related to phase behavior of polymer–colloid mixtures.
Models for turbulent plane Couette flow using the proper orthogonal decomposition
Authors: Moehlis J. , Smith T. R. , Holmes P. , Faisst H.
Published: July 2002

We model turbulent plane Couette flow (PCF) by expanding the velocity field as a sum of optimal modes calculated via the proper orthogonal decomposition from numerical data. Ordinary differential equations are obtained by Galerkin projection of the Navier–Stokes equations onto these modes. For a minimal truncation including only the most energetic modes having no streamwise variation, we show under quite general conditions the existence of linearly stable nontrivial fixed points, corresponding to a state in which the mean flow is coupled to streamwise vortices and their associated streaks. When the two next most energetic modes, still lacking streamwise variations, are included, chaos and heteroclinic cycles associated with the fixed points are found. The attractors involve repeated visits near unstable fixed points and periodic orbits corresponding to steady and periodically varying vortices, and account for a self-sustaining process in which vortices interact with the mean flow. The models considered in this paper can also serve as a foundation for more sophisticated ordinary differential equation models for turbulent PCF, including those which include modes with streamwise variations.
Intermixing-promoted scaling of Ge/Si(100) island sizes
Authors: De Seta M. , Capellini G. , Evangelisti F. , Spinella C.
Published: July 2002

The shape evolution and the effect of deposition temperature on size and composition of chemical vapor deposition grown Ge/Si(100) islands have been investigated in the deposition temperature range 450–850 °C. It is found that the increase of the growth temperature above 600 °C entails a strong island enlargement due to an increased Si/Ge intermixing. The crystallographic structure of the islands was investigated by transmission electron microscopy. The analysis of the resulting Moiré pattern reveals that the island lattice deformation decreases with increasing island size and that the effective mismatch ε between the silicon substrate and the epilayer decreases with increasing deposition temperature. The island nucleation size, the mean size of coherent islands and the critical size for the insertion of misfit dislocations have been found to scale as ε−2, ε−2, and ε−1, respectively. The agreement of our experimental scaling results with the predictions of theoretical calculation performed for homogeneous heterostructures suggests that, although the Si distribution inside the islands is not homogeneous, the island growth is driven by the mean effective strain.
Hirshfeld surfaces as approximations to interatomic surfaces
Authors: Pendás A. Martı́n , Luaña V. , Pueyo L. , Francisco E. , Mori-Sánchez P.
Published: July 2002

A simple algebraic model is used to show that Hirshfeld surfaces in condensed phases may be understood as approximations to the interatomic surfaces of the theory of atoms in molecules. The conditions under which this similarity is valid are explored, and both kinds of surfaces are calculated in the LiF and CS2 crystals to illustrate the main results. The link between Hirshfeld and interatomic surfaces provides a physical ground to understand the usage of the former to visualize intermolecular interactions.
The resonance spectrum of the cusp map in the space of analytic functions
Authors: Antoniou I. , Shkarin S. A. , Yarevsky E.
Published: July 2002

We prove that the Frobenius–Perron operator U of the cusp map F:[−1,1]→[−1,1], F(x)=1−2|x| (which is an approximation of the Poincaré section of the Lorenz attractor) has no analytic eigenfunctions corresponding to eigenvalues different from 0 and 1. We also prove that for any q∈(0,1) the spectrum of U in the Hardy space in the disk {z∈C:|z−q|<1+q} is the union of the segment [0,1] and some finite or countably infinite set of isolated eigenvalues of finite multiplicity.
Radial macroscopic model of a plasma flowing along annular dielectric walls
Authors: Ahedo E.
Published: July 2002

The classical model of the transverse interaction of a collisionless, partially ionized plasma with two insulator walls confining it, is here generalized to plasmas flowing parallel to the walls. A macroscopic formulation is proposed, where the contributions of the axial flow are included into the radial equations through source terms for particle production, ion friction, and ion heating. These contributions compete with those due to ionization, and add three new parameters to the classical model. Zero-ionization cases are presented to show that the net production (and loss) of plasma can be independent of ionization. Friction effects on the radial ion acceleration is found to reduce plasma fluxes into the walls. The ion temperature is determined from the competition between heating due to ionization and cooling due to radial rarefaction; isothermal and isentropic behaviors are recovered only for particular situations. Planar and annular geometries are studied. For the latter one, the differences in the inner and outer plasma fluxes are evaluated. The application of the model to the plasma discharge in an annular Hall thruster is commented.
A method to simultaneously determine sorption isotherms and sorption enthalpies with a double twin microcalorimeter
Authors: Wadsö Lars , Markova Natalia
Published: July 2002

Sorption of vapors of water, ethanol, and other liquids on solids like pharmaceuticals, textiles and food stuffs are of both practical and theoretical importance. In this article we present a technique to simultaneously measure sorption isotherms and sorption enthalpies. The sample is contained in one end of a sorption vessel. In the other end a vaporizable liquid is introduced to start the measurement. Mass transfer from the liquid to the sample is by vapor diffusion and the rate of mass transfer is calculated from the measured thermal power of vaporization. Simultaneously, the thermal power of sorption is measured and from this one may calculate the differential enthalpy of sorption. The thermal power measurements are made by inserting the sorption vessel in an isothermal double twin microcalorimeter.
Ultrasonic device for measuring periodontal attachment levels
Authors: Lynch J. E. , Hinders M. K.
Published: July 2002

Periodontal disease is manifested clinically by a degradation of the ligament that attaches the tooth to the bone. The most widely used diagnostic tool for assessment of periodontal diseases, measurement of periodontal attachment loss with a manual probe, may overestimate attachment loss by as much as 2 mm in untreated sites, while underestimating attachment loss by an even greater margin following treatment. Manual probing is also invasive, which causes patient discomfort. This work describes the development and testing of an ultrasonographic periodontal probe designed to replace manual probing. It uses a thin stream of water to project an ultrasonic beam into the periodontal pocket, and then measures echoes off features within the pocket. To do so, the ultrasonic beam must be narrowed from 2 (the diameter of the transducer) to 0.5 mm (the approximate width of the periodontal pocket at the gingival margin). The proper choice of transducer frequency, the proper method for controlling water flow from the probe, and a model for interpreting these echoes are also addressed. Initial results indicate that the device measures echoes from the hard tissue of the tooth surface, and that the periodontal attachment level can be inferred from these echoes.
Novel diffractometer optimized for the study of weak superlattice reflections using crossed parabolic mirrors
Authors: Wilkins S. B. , Spencer P. D. , Hatton P. D. , Tanner B. K. , Lafford T. A. , Spence J. , Loxley N.
Published: July 2002

We present the development of a novel laboratory-based diffractometer optimized for high intensity and resolution matched to that of flux grown single crystal transition metal–oxide samples studied in solid state physics. This has been implemented using crossed graded d-spacing parabolic multilayer mirrors, a severely off-cut asymmetric analyzer crystal, and a microminiature cryostat. We demonstrate that the wider bandpass of the multilayer mirrors provides a significant increase in intensity compared to the use of silicon and germanium optics, but still provides the necessary resolution to obtain accurate measurements for inverse correlation lengths. The increase in flux allows the observation of features that were previously only visible with third generation synchrotron x-ray sources, shown by the observation of the very weak charge order peak without the use of a synchrotron source. Results on samples previously studied show an increase of a factor of 10 in intensity, coupled together with a factor of 7.5 increase in resolution over the previous system employed using a rotating anode source and flat pyrolytic graphite (0001) crystals. With such a diffractometer it is now possible to carry out detailed studies of charge ordering in transition metal oxides in the laboratory and the benefits of this are threefold. First, this will improve the quality of any preparation work, for subsequent experiments at synchrotron sources. This has the advantage of maximizing the efficiency of synchrotron measurements. Second, the enhanced intensity and resolution will allow experiments to be conducted in the laboratory which previously required access to synchrotron sources. Third, it will allow for synchrotron time to be used for its unique properties, i.e., wavelength tunability and polarization, which are necessary for the observation of magnetic reflections and orbital ordering by means of resonant enhancements and polarization analysis. This means that laboratory based measurements can be easily incorporated with synchrotron data, maximizing the potential of both techniques and receiving a net gain in data output.
The lowest doublet and quartet potential energy surfaces involved in the N(4S)+O2 reaction. II. Ab initio study of the C2v-symmetry insertion mechanism
Authors: González Miguel , Oliva Carolina , Sayós R.
Published: July 2002

In the present work we have carried out ab initio complete active space self-consistent field (CASSCF) and second-order perturbation theory on CASSCF wave function (CASPT2) calculations and also some density functional theory calculations with the aug-cc-pVTZ Dunning’s basis set on the lowest A1, B1, A2, and B2 doublet and quartet potential energy surfaces (PES) that could be involved in the title reaction. Thus, several minima, transition states, and surface crossings have been found for the C2v-insertion reaction mechanism. The results agree very well with available experimental data [i.e., for NO2 (2A1), MIN2 (2B2), NO2 (2Πu)] and with other previous ab initio calculations. Six A′/A′- and four A′/A″-type surface crossings were located and classified for these PES’, whose only one (i.e., B22/2A1) has been previously reported in theoretical and experimental studies. High-energy barriers were found for the direct C2v-insertion mechanism (3.11 and 2.54 eV for the lowest doublet and quartet PES’ at the CASPT2/aug-cc-pVTZ level, respectively), clearly showing that this competitive mechanism is much less favorable than the direct Cs-abstraction or the indirect Cs-insertion reaction mechanisms reported in Paper I.
Energy landscapes of model polyalanines
Authors: Mortenson Paul N. , Evans David A. , Wales David J.
Published: July 2002

A parallel searching algorithm based on eigenvector-following is used to generate databases of minima and transition states for all-atom models of Ac(ala)12NHMe and Ac(ala)16NHMe. The AMBER95 force field of Cornell et al. [J. Am. Chem. Soc. 117, 5179 (1995)] is employed both with and without a simple implicit solvent. We use a master equation approach to analyze the dynamics of both systems, and relate the results to the potential energy landscapes using disconnectivity graphs. The low-lying regions of both energy landscapes are compared and found to be remarkably similar. α-helix formation occurs via an ensemble of pathways involving both the N- and C-termini. The global minima of the two systems are also located using the CHARMM22 force field of Mackerell et al. [J. Phys. Chem. B 102, 3586 (1998)], for comparison with AMBER95.
Dust-lower-hybrid instability in the presence of dust charge fluctuation in a streaming dusty plasma
Authors: Islam M. K. , Salahuddin M. , Banerjee A. K. , Salimullah M. , Nambu M.
Published: July 2002

A detailed theoretical investigation has been made on the properties and excitation of low-frequency and low-phase-velocity electrostatic dust-modes in a magnetized dusty plasma including the streaming of ions, dust charge fluctuation, and the kinetic effects using the Vlasov–Poisson analysis. In the case of high-density dusty plasma, the dust-lower-hybrid (dlh) mode propagating nearly transverse to the external static magnetic field is obtained. On the other hand, for the low-density dusty plasma, the dust-acoustic (DA) wave propagating nearly perpendicular to the external magnetic field has been obtained. It is found that in any case of the above dust-modes, the Landau damping linearly adds to the damping due to dust charge fluctuation. These low-frequency modes can be excited when the ion streaming velocity exceeds the parallel phase velocity of the mode provided the damping due to the dust charge fluctuation is smaller than the Landau damping term of the respective mode. Moreover, it is found that the DA mode is damped rapidly compared to the dlh-mode through charge fluctuation and Landau damping processes.
C
13
nuclear Overhauser polarization nuclear magnetic resonance in rotating solids: Replacement of cross polarization in uniformly C13 labeled molecules with methyl groups
Authors: Takegoshi K. , Terao Takehiko
Published: July 2002

A new C13 polarization technique in solids is presented on the basis of a recently proposed C13–C13 recoupling sequence [13C–1H dipolar-assisted rotational resonance (DARR), K. Takegoshi, S. Nakamura, and T. Terao, Chem. Phys. Lett. 344, 631 (2001)] operative under fast magic angle spinning (MAS), in which a rf field is applied to H1H with a rotary resonance condition but none to C13. The H1 irradiation in DARR saturates H1 signals, leading to the C13 signal enhancement due to the nuclear Overhauser effect for fast rotating methyl groups, if any. If we use a uniformly C13 labeled sample, C13–C13 polarization transfer enhanced by DARR successively distributes the enhanced methyl carbon polarization to the other C13 spins, leading to uniform enhancement for all C13 spins even under very fast MAS. In uniformly C13 labeled rotating samples, the enhancement factor in cross polarization (CP) is about 2.4, while in the present nuclear Overhauser polarization (NOP), it is 3.0 in the fast rotation limit of the methyl groups. While the CP enhancement becomes smaller for molecules with short T1ρ of H1 or C13, NOP would work well for such mobile molecules, and also NOP enables us to acquire a signal with a short repetition time even if H1 T1 is long. Further, NOP has the advantage of quantitativeness, and is very easy to carry out, being insensitive to the adjustment of rf field intensity and requiring only very low rf power. These features are demonstrated for uniformly C13, N15-labeled L-threonine and uniformly C13, N15-labeled glycylisoleucine. NOP-MAS is also applied for a naturally abundant C13 sample.
Constant-temperature molecular-dynamics algorithms for mixed hard-core/continuous potentials
Authors: Houndonougbo Yao A. , Laird Brian B.
Published: July 2002

We present a set of second-order, time-reversible algorithms for the isothermal (NVT) molecular-dynamics (MD) simulation of systems with mixed hard-core/continuous potentials. The methods are generated by combining real-time Nosé thermostats with our previously developed Collision Verlet algorithm [Mol. Phys. 98, 309 (1999)] for constant energy MD simulation. In all we present five methods, one based on the Nosé–Hoover [Phys. Rev. A 31, 1695 (1985)] equations of motion and four based on the Nosé–Poincaré [J. Comput. Phys. 151, 114 (1999)] real-time formulation of Nosé dynamics. The methods are tested using a system of hard spheres with attractive tails and all correctly reproduce a canonical distribution of instantaneous temperature. The Nosé–Hoover based method and two of the Nosé–Poincaré methods are shown to have good energy conservation in long simulations.
Pinning of magnetic vortices in microfabricated permalloy dot arrays
Authors: Shima H. , Novosad V. , Otani Y. , Fukamichi K. , Kikuchi N. , Kitakamai O. , Shimada Y.
Published: July 2002

Temperature dependent magnetic properties of vortices trapped in a lithographically patterned Permalloy disk were examined. A large residual magnetization at 5 K was observed in hysteresis curves unlike theoretical prediction. The residual magnetization, coercive field, and initial susceptibility were found to be dependent on temperature. Escaping from the pinning potential was facilitated by the increase of temperature, and the pinning temperature Tpin was 9.6 K. The vortex is effectively pinned at the pinning potential when T<Tpin. This physical picture is well supported by the temperature variations of ac susceptibility for the biased dc field. The energy barrier is probably originated from defects such as the edge and surface roughnesses, and irregular grain boundaries in the disk.
Neutron diffraction studies of the magnetic phase transitions in Ce2Fe17 compound under pressure
Authors: Prokhnenko O. , Ritter C. , Arnold Z. , Isnard O. , Kamarád J. , Pirogov A. , Teplykh A. , Kuchin A.
Published: July 2002

The influence of hydrostatic pressure (up to 5 kbar) on the magnetic structure of Ce2Fe17 was investigated using neutron diffraction in the temperature range from 2 to 300 K. The existence of a collinear ferromagnetic phase below 95 K with a magnetic moment of Fe, mFe=2.0 μB, was confirmed at ambient pressure. Magnetic peaks present between 95 and 205 K correspond to an incommensurate antiferromagnetic structure with a wave vector changing its value from τ1=0.026 Å−1 at 100 K to τ1=0.034 Å−1 at 205 K. A helical model is used to describe the magnetic structure. Application of high pressures leads to significant changes of the magnetic structure. The ferromagnetic phase, suppressed in the studied temperature range by pressures higher than 3 kbar, gets substituted by a new incommensurate antiferromagnetic phase. This phase can be described as a superposition of the helical structure with a second antiferromagnetic coupling with propagation vector τ2≈0.078 Å−1 at 40 K under pressures above 3 kbar. The correlation with recent results of magnetization studies under pressure is analyzed and discussed.
𝒩-fold supersymmetry in quantum mechanics—analyses of particular models
Authors: Sato Masatoshi , Tanaka Toshiaki
Published: July 2002

We investigate particular models which can be 𝒩-fold supersymmetric at specific values of a parameter in the Hamiltonians. The models to be investigated are a periodic potential and a parity-symmetric sextic triple-well potential. Through the quantitative analyses on the nonperturbative contributions to the spectra by the use of the valley method, we show how the characteristic features of 𝒩-fold supersymmetry which have been previously reported by the authors can be observed. We also clarify the difference between quasi-exactly solvable and quasi-perturbatively solvable cases in view of the dynamical property, that is, dynamical 𝒩-fold supersymmetry breaking.