bluuebunny/crossref_metadata_2025 huggingface dataset:
The separation distance distribution in electron-donor–acceptor systems and the wavelength dependence of free ion yields
Authors: Zhou Jinwei , Findley Bret R. , Braun Charles L. , Sutin Norman
Published: July 2002

We recently reported that free radical ion quantum yields for electron-donor–acceptor (EDA) systems of alkylbenzenes-tetracyanoethylene (TCNE) exhibit a remarkable wavelength dependence in dichloromethane, a medium polarity solvent. We proposed that weak absorption by long-distance, unassociated or “random” D⋯A pairs is mainly responsible for the free radical ion yield. Here a model for the wavelength dependence of the free ion yield is developed for four systems in which differing degrees of EDA complex formation are present: 1,3,5-tri-tert-butylbenzene-TCNE in which only random pairs exist due to the bulky groups on the electron donor, and toluene—TCNE, 1,3,5-triethylbenzene-TCNE and 1,3,5-trimethylbenzene-TCNE. Mulliken–Hush theory is used to determine the excitation distance distribution of unassociated, random pairs at different wavelengths. For each absorption distribution, free radical ion yields at different wavelengths are then calculated using Onsager’s result for the ion separation probability. Encouraging agreement between the calculated yields and our experimental results is obtained. As far as we are aware, this is the first time that photoexcitation of unassociated donor/acceptor pairs has been invoked as the source of separated radical ion pairs.
Bonding structure in amorphous carbon nitride: A spectroscopic and nuclear magnetic resonance study
Authors: Sánchez-López J. C. , Donnet C. , Lefèbvre F. , Fernández-Ramos C. , Fernández A.
Published: July 2002

Since the prediction of Liu and Cohen [Science 245, 841 (1989)] of the potential extraordinary mechanical properties of crystalline β-C3N4, many authors have attempted its synthesis. However, in most cases, the obtained materials are amorphous phases with a complex bonding structure. Their characterization is complicated due to the absence of a reference compound, the lack of long-range order, and the poor knowledge about their bonding structure. In this article, we present H1, C13, and N15 solid-state nuclear magnetic resonance (NMR) measurements for the determination of the bonding types in amorphous CNx films. NMR measurements do not require long-range order and are able to clearly identify the signals from the sp2- and sp3-bonded phases. The analysis of the data obtained by other characterization techniques, such as infrared spectroscopy, x-ray photoelectron spectroscopy, electron energy-loss spectroscopy, and x-ray absorption near-edge spectroscopy on the same sample, based on the information acquired by NMR, enables the description of a structure model for the studied amorphous-CNx phase prepared by dc-magnetron sputtering and to revise the interpretation found in the literature.
Collisions of ions with surfaces at chemically relevant energies: Instrumentation and phenomena
Authors: Grill Verena , Shen Jianwei , Evans Chris , Cooks R. Graham
Published: July 2002

An overview of gaseous ion/surface collisions is presented, with special emphasis on the behavior of polyatomic projectile ions at hyperthermal collision energies (1–100 eV) and the instrumentation needed for such studies. The inelastic and reactive processes occurring during ion/surface collisions are described in terms of several archetypes, viz., elastic and quasielastic scattering, chemical sputtering leading to release of surface material, inelastic scattering leading to surface-induced dissociation (SID) of the projectile, ion/surface reactions, and soft landing. Parameters that are important in ion/surface interactions are discussed, including the interaction time, the conversion of translational to internal energy, the translational energies of the scattered ions, the effects of scattering angle, and the influence of the nature of the surface. Different types of tandem mass spectrometers, built specifically to study ion/surface collision phenomena, are discussed and the advantages and disadvantages of the individual designs are compared. The role of SID as a technique in bioanalytical mass spectrometry is illustrated and this inelastic collision experiment is compared and contrasted with gas-phase collision-induced dissociation, the standard method of tandem mass spectrometry. Special emphasis is placed on reactive scattering including the use of ion/surface reactions for surface chemical analysis and for surface chemical modification.
Implantation-produced structural damage in InxGa1−xN
Authors: Kucheyev S. O. , Williams J. S. , Zou J. , Pearton S. J. , Nakagawa Y.
Published: July 2002

The influence of In content on the accumulation of structural damage in InxGa1−xN films (with x=0.0–0.2) under heavy-ion bombardment is studied by a combination of Rutherford backscattering/channeling spectrometry and transmission electron microscopy. Results show that an increase in In concentration strongly suppresses dynamic annealing processes and, hence, enhances the buildup of stable lattice disorder in InGaN under ion bombardment, A comparison of the damage buildup behavior and defect microstructure in InGaN with those in GaN is presented. Results of this study may have significant technological implications for estimation and control of implantation-produced damage in InGaN/GaN heterostructures.
Detailed molecular dynamics simulation of the self-diffusion of n-alkane and cis-1,4 polyisoprene oligomer melts
Authors: Harmandaris V. A. , Doxastakis M. , Mavrantzas V. G. , Theodorou D. N.
Published: July 2002

Results are presented for the self-diffusion properties of monodisperse n-alkanes and cis-1,4 polyisoprene (PI) oligomer melts, as obtained through detailed atomistic molecular dynamics (MD) simulations. The simulations have been conducted in the NVT statistical ensemble on model systems thoroughly pre-equilibrated through an efficient Monte Carlo (MC) algorithm. Results for the self-diffusion coefficient D as a function of molecular weight M support a scaling law of the form D∼Mb, with b strongly depending on temperature T, for both the n-alkanes and the cis-1,4 PI melts. The simulation results have been fitted to an expression for D involving elements of Rouse dynamics and Cohen–Turnbull–Bueche chain-end (excess free volume) effects, proposed recently by von Meerwall et al. [J. Chem. Phys. 108, 4299 (1998)]. Using a geometric analysis involving tessellation of space in Delaunay tetrahedra developed by Greenfield and Theodorou [Macromolecules 26, 5461 (1993)], we have also calculated the excess chain-end free volume of the alkane and cis-1,4 PI melts. Calculated self-diffusivities and apparent activation energies for the two different polymers as a function of their molecular weight M are in excellent agreement with the experimental measurements of von Meerwall et al. (1998).
Determining the shear viscosity of model liquids from molecular dynamics simulations
Authors: Hess Berk
Published: July 2002

Several methods are available for calculating shear viscosities of liquids from molecular dynamics simulations. There are equilibrium methods based on pressure or momentum fluctuations and several nonequilibrium methods. For the nonequilibrium method using a periodic shear flow, all relevant quantities, including the accuracy, can be estimated before performing the simulation. We compared the applicability, accuracy and efficiency of this method with two equilibrium methods and another nonequilibrium method, using simulations of a Lennard-Jones fluid and the SPC and SPC/E [(extended) simple point charge] water models.
Photonic crystals of core-shell colloidal particles
Authors: Velikov Krassimir P. , Moroz Alexander , van Blaaderen Alfons
Published: July 2002

We report on the fabrication and optical transmission studies of thin three-dimensional photonic crystals of high-dielectric ZnS-core and low-dielectric SiO2-shell colloidal particles. These samples were fabricated using a vertical controlled drying method. The spectral position and width of a stopgap depend on the core-to-shell ratio, in a manner consistent with numerical calculations. Both experiments and calculations show that the relative L-stopgap width in the case of high-index core low-index shell particles can be larger in comparison to the case of homogeneous particles of either material. The core-shell morphology gives additional control over the photonic stopgap characteristics.
Kinetics of the late stage of first-order phase transitions under laser irradiation
Authors: Ambrok A. G. , Kalashnikov E. V. , Kukushkin S. A.
Published: July 2002

The effect of radiation on the kinetics of the late stage of first-order phase transitions (Ostwald’s ripening) was studied. In particular, features of the growth kinetics of nuclei were revealed in relation to their size and time of exposure to radiation. Possible approaches to controlling the nucleus growth kinetics were shown.
High-quality single-crystal Nb films and influences of substrates on the epitaxial growth
Authors: Oya Gin-ichiro , Koishi Masanori , Sawada Yasuji
Published: July 2002

Highly pure single-crystal Nb films (thickness, ∼2000 Å) with high superconducting transition temperatures Tc of ∼9.3 K and high resistance ratios R300/R10 up to ∼200 are successfully grown epitaxially on single-crystal sapphire (α-Al2O3) and MgO substrates at ∼500–∼700 °C by using an electron-beam evaporation technique. The most high-quality single-crystal Nb film (with the maximum Tc of 9.45 K and the maximum R300/R10 of 199) is obtained on a sapphire (11̄02) substrate, which has a thermal expansion coefficient very close to that of Nb as well as a small lattice misfit to Nb. The quality of obtained single-crystal Nb films is found to be lowered by the deposition of the films on substrates with thermal expansion coefficients different far from that of Nb because of the production of internal strains and lattice defects in the films cooled down.
Study of fluorine in silicate glass with 19F nuclear magnetic resonance spectroscopy
Authors: Duncan T. M. , Douglass D. C. , Csencsits R. , Walker K. L.
Published: July 2002

We report an application of nuclear magnetic resonance (NMR) spectroscopy to the study of fluorine-doped silicate glass prepared by the modified chemical vapor deposition process, prior to drawing the rod into fibers. The silica contains 1.03-wt. % fluorine, as determined by the calibrated intensity of the 19F NMR spectrum. The isotropic chemical shift of the 19F spectrum shows that fluorine bonds only to silicon; there is no evidence of oxyfluorides. Analysis of the distribution of nuclear dipolar couplings between fluorine nuclei reveals that the relative populations of silicon monofluoride sites [Si(O–)3F] and species having near-neighbor fluorines, such as silicon difluoride sites [Si(O–)2F2], are nearly statistically random. That is, to a good approximation, the fluorine substitutes randomly into the oxygen sites of the silica network. There is no evidence of local clusters of fluorine sites, silicon trifluoride sites [Si(O–)F3], or silicon tetrafluoride (SiF4).
Evidence for asymmetric well shapes in post-growth modified GaAs/AlGaAs quantum wells
Authors: Koteles Emil S. , Elman B. , Armiento C. A. , Melman P. , Chi J. Y. , Holmstrom R. J. , Powers J. , Owens D. , Charbonneau S. , Thewalt M. L. W.
Published: July 2002

Evidence has been obtained indicating asymmetry in the shapes of as-grown symmetric coupled double GaAs/AlGaAs quantum wells (CDQW) which were modified by capping with silicon dioxide followed by rapid thermal annealing. The sensitivity of the optical properties of CDQWs to perturbations, in contrast to single quantum wells, permitted this observation. The asymmetry may result from the thermally driven unidirectional diffusion of vacancies generated near the surface which enhances intermixing of barrier and well materials in the quantum wells.
Forces in magnetostatics and their computation
Authors: Bossavit A.
Published: July 2002

The use of unconventional (edge-based) finite elements is proposed for the computation of a static magnetic field in a system of deformable conductors and ferromagnetic bodies (with a linear b-h law). Thanks to a virtual-work approach, the associated forces can be computed at the element level.
Calculations of Mott scattering cross section
Authors: Czyżewski Zbigniew , MacCallum Danny O’Neill , Romig Alton , Joy David C.
Published: July 2002

Calculations of Mott elastic scattering cross section of electrons for most elements of the periodic table up to element number 94 in the energy range 20 eV–20 keV have been performed. The Dirac equation transformed to a first-order differential equation was solved numerically. The influence of the choice of atomic potential on the scattering factor was studied in comparison to a simple muffin-tin approximation of the atomic potential in solids. The application of calculated cross sections to a conventional Monte Carlo model for electron scattering using modified Bethe equation is described and results concerning the electron backscattering for different atomic potentials are compared.
Texture formation in polycrystalline diamond films
Authors: Wild Ch. , Herres N. , Koidl P.
Published: July 2002

Structure and morphology of polycrystalline diamond films prepared by chemical vapor deposition (CVD) have been studied using x-ray texture analysis, angle-resolved optical reflection, and scanning electron microscopy. The films under investigation exhibit a pronounced 110 fiber texture, i.e., a preferential alignment of {110} planes perpendicular to the growth direction. By thinning a 180-μm-thick CVD diamond film in an oxygen discharge the dependence of the degree of 110 texture on the film thickness has been investigated. It was found that the crystals formed at the beginning of the film growth are randomly oriented, and that a preferential orientation of {110} planes develops with increasing film thickness. Computer simulations show that this behavior can be explained by evolutionary selection, i.e., competing growth of differently oriented crystals, which implies that 〈110〉 is the direction of fastest growth. In addition, angle-resolved optical reflection and scanning electron micrographs show that the surface of the diamond films consists of {111} faces. Possible explanations for these findings are discussed.
An investigation of product distributions in microwave plasma for diamond growth
Authors: Mitomo T. , Ohta T. , Kondoh E. , Ohtsuka K.
Published: July 2002

Stable compounds in microwave plasmas of various source gases for vapor-phase diamond growth have been measured by using in situ Fourier transform infrared spectroscopy. Each gas mixture of CH4+H2, C2H2+H2, C2H4+H2, CH3OH+H2, C2H5OH+H2, CO+H2, CCl4+H2, and CH4+O2+H2 was introduced into the plasma discharge region as a source gas. The detected carbonaceous compounds were CH4, C2H2, C2H4, and CO, and no other carbonaceous compounds were observed. CH4 and C2H2 were observed in plasmas of all source gases, whereas C2H4 was detected only at higher concentrations of carbonaceous compounds in source gases. CO was produced from source molecules containing oxygen atoms. The source molecules of CH3OH, C2H5OH, and CCl4 disappeared in the microwave plasma. Concentrations of products in the plasmas of CH4+H2, C2H2+H2, and C2H4+H2 were very similar. This result suggests that an equilibrium among CH4, C2H2, C2H4, H2, H, electrons, and various radicals is kept in the hydrogen excess plasma. Moreover, it was found that the addition of O2 to the CH4+H2 plasma reduced the concentrations of CH4 and C2H2 in the plasma.
Texture formation in titanium nitride films prepared by chemical vapor deposition
Authors: Cheng Hsyi-En , Hon Min-Hsiung
Published: July 2002

The 〈100〉, 〈110〉, and 〈211〉 textured TiN films were obtained in a chemical vapor deposition system using TiCl4, N2, and H2 as reaction gases. The microstructure of these textured films was investigated by scanning electron microscopy and transmission electron microscopy. The results show that the 〈211〉, 〈110〉, and 〈100〉 textured films consist of laminated twinned grains, multiply twinned grains, and nearly thermodynamic equilibrium grains respectively at the growing stage. The formation of these textures is discussed based on nucleation and growth mechanisms. At a lower deposition temperature with higher nitrogen concentration, the decahedral multiply twinned and laminated twinned crystals dominate over crystals with single crystalline structure. After competitive growth of these crystals, the multiply twinned crystals with 〈110〉 twin axis perpendicular to substrate surface survive. At an intermediate deposition temperature with lower nitrogen concentration, the laminated twinned crystals dominate the growth and the 〈211〉 orientation along the (111) twin planes possesses the fastest growth rate and form 〈211〉 textured films. At a higher deposition temperature above 1200 °C, the single crystalline structure dominates and forms 〈100〉 textured films. The effect of surface adsorption on texture formation is discussed.
Structure and magnetic properties of Sm2(Fe,Mn)17Nx coarse powders with high coercivity
Authors: Majima K. , Ito M. , Katsuyama S. , Nagai H.
Published: July 2002

The influence of a substitution of Mn for Fe in the Sm2(Fe,Mn)17Nx alloys on the structure and the magnetic properties was studied. Sm2(Fe,Mn)17Nx alloy powders of 32–74 μm in particle size with several values of x were successively synthesized using NH3+H2 gas. It was found that the primary phase of the powders maintained the Th2Zn17-type structure in the wide range for 0<x<6. On the other hand, the coercivity of the powders remarkably increased for x>3. The Sm2(Fe0.95Mn0.05)17N5 powder proved to have a coercivity of 0.62 MA/m (7.7 kOe), without a fine grinding. Microstructural analysis by transmission electron microscopy and energy-dispersive x-ray analysis revealed the powder consisted of Sm2(Fe,Mn)17N3 nanocrystalline grains of 10–30 nm in diameter surrounded by Mn-enriched amorphous phase.
Dielectric properties of (PMN)(1−x)(PT)x single crystals for various electrical and thermal histories
Authors: Colla Eugene V. , Yushin Nikolai K. , Viehland Dwight
Published: July 2002

Dielectric investigations of the (PMN)(1−x)(PT)x crystalline solution have been performed for various electrical histories. For x<0.3, relaxor behavior was observed, however the degree of diffuseness in the dielectric maximum and degree of relaxor characteristics were decreased with increasing x. Application of dc bias for x<0.33 (xc) resulted in an induced transformation to a long-range ferroelectric state. This transition had a critical threshold field (Eth). The value of Eth depended on x. For all x<xc, glasslike behavior was observed in the nonergodic temperature range for E<Eth.
A law of mixtures for transport properties in binary particulate composites
Authors: Duncan K. L. , Lodenquai J. F. , Wagh A. S. , Goretta K. C.
Published: July 2002

A connected-grain model was developed earlier to explain mechanical and thermal properties of porous ceramics and sedimentary rocks. We have now generalized this model for binary particulate composites, based on simulation of a connected-grain structure of individual components of the composites by randomly selecting individual grains and shrinking them. Repetition of this procedure results in a structure of a binary particulate composite that contains channels of individual components, through which transport occurs. We developed a generalized law of mixtures in which transport properties are expressed as scaling relationships that depend on the shrinking parameter expressed as an exponent. This parameter provides the skewness of the distribution of the grains. The model is compared with various transport properties of binary composites reported in the literature. In addition, the model is tested on YBa2Cu3Ox superconductors and Ag composites that were fabricated in our laboratory and tested for electrical conductivity and elastic modulus. This test demonstrates how the model predicts two entirely different transport properties through their common microstructure and grain-size distribution.
The effect of pressure on the thermal hysteresis of the first-order spin transition in bis(1,10-phenanthroline-2-carbaldehyde phenylhydrazone) iron (II) complexes
Authors: König E. , Ritter G. , Waigel J. , Goodwin H. A.
Published: July 2002

The effect of pressure on the thermal hysteresis of the high-spin (5T2) ∏ low-spin (1A1) transition in [Fe(phy)2](BF4)2 and [Fe(phy)2](ClO4)2 has been studied (phy=1, 10-phenanthroline-2-carbaldehyde phenylhydrazone). An increase of the hysteresis width ΔTc as well as of the residual fractions nres5T2 and (1−n5T2)res with pressure is observed. The quantity (ΔTc)1/2 shows a linear dependence on pressure p as expected from the Landau theory. The increase of the residual fractions is explained by the assumption of decreasing size of the cooperative region with pressure.
Theory of thermal unimolecular reactions at high pressures. II. Analysis of experimental results
Authors: Cobos C. J. , Troe J.
Published: July 2002

The simplified version, proposed in Part I, of the statistical adiabatic channel model for thermal unimolecular bond fission reactions and the reverse radical association reactions is applied to 26 experimental systems in which limiting high pressure rate coefficients have been determined. From the analysis, information on the global character of the potential energy surfaces involved is obtained and expressed by the characteristic looseness parameter α of the angular part of the potential and the Morse parameter β of the radial part. It is investigated to what extent empirical ‘‘standard values’’ of α, or of α/β, can be defined, by which limiting high pressure rate coefficients become predictable without knowing further details of the potential energy surfaces.
Quantum simulation study of the hydrated electron
Authors: Schnitker Jürgen , Rossky Peter J.
Published: July 2002

An excess electron in a sample of classical water molecules at room temperature has been simulated using path integral techniques. The electron–water interaction is modeled by a pseudopotential with effective core repulsion and further terms for the Coulomb interaction and polarization effects. Various discretizations of the electron path, up to 1000 points, are examined. The charge distribution of the electron is found to be compact and to occupy a cavity in the water, in agreement with the conventional picture. The solvation shell structure is similar to that of relatively large solvated atomic anions, but the radial electron-solvent correlations are largely smeared out due to fluctuations of the electronic density distribution. In parts of the simulation the structure of the first solvation shell corresponds on the average to the structure proposed for hydrated electrons by Kevan. The computed solvation energy and the estimated energy of the first optical excitation agree reasonably well with experimental data.
Electron thermalization distances and free ion yields in liquid nitrogen from 77 K to near T
c
Authors: Ramanan G. , Freeman G. R.
Published: July 2002

The free ion yield in x-irradiated liquid nitrogen at zero applied field strength increases from G0fl=0.0034 ion pairs/100 eV at 77.1 K to 0.043 at 123.7 K, while the density-normalized thermalization distance of the secondary electrons increases from bEPd=4.4×10−6 to 5.6×10−6 kg/m2. The ratio of the GEfi values at 123.7 and 77.1 K decreases from 13 at E=0 to 3.1 at E=5.2 MV/m. Yields calculated from the extended Onsager model agree with the measured values only when the electron thermalization distance distribution is assumed to be exponential with an added power tail (YEP): F(y)=YE, 0<(y−y0)<3.5bEP; F(y)=YE+0.74b2EP/(y−y0)3, (y−y0)>3.5bEP; where YE=b−1EP exp[(y0−y)/bEP], y>y0. The electron thermalization process in liquid nitrogen appears to be truncated by electron capture to form an anion.
Dynamic Monte Carlo with a proper energy barrier: Surface diffusion and two-dimensional domain ordering
Authors: Kang H. C. , Weinberg W. H.
Published: July 2002

A new model is presented and discussed that allows Monte Carlo simulations to be carried out with a proper energy barrier crossing. Results are presented for the surface diffusion coefficient and the growth exponent of domain ordering of a half-monolayer of adatoms experiencing nearest- and next-nearest-neighbor repulsive lateral interactions (equal in magnitude), both on a square lattice. The results are compared with those derived using both Kawasaki dynamics and a Metropolis walk. The reasons why neither of the latter methods can be expected, in general, to describe thermally excited, time-dependent phenomena are explained and discussed.
Pulsed neutron diffraction study on the structures of glassy 7LiX–KX–CsX–BaX2 (X=Cl, Br, and I)
Authors: Kinugawa Kenichi , Ohtori Norikazu , Kadono Kohei , Tanaka Hiroshi , Okazaki Susumu , Misawa Masakatsu , Fukunaga Toshiharu
Published: July 2002

The structures of glassy 7LiX–KX–CsX–BaX2 (X=Cl, Br, and I) have been investigated by means of pulsed neutron diffraction experiment. The obtained total radial distribution functions were discussed with the help of the molecular dynamics simulation based on the Tosi–Fumi potential. The Li–X (X=Cl, Br, and I) distances in the three glasses are 2.37, 2.49, and 2.70 Å, respectively, which are 0.2–0.3 Å shorter than those in the rocksalt-type LiX crystals. For each system, it is found that the coordination number of the X− ions around the Li+ ions is about four when only the closely contacting X− ions are counted. Because of the 1:1 monovalent character of the present systems, the first coordination shell around the Li+ ions has the ill-defined outer extent, compared with the 4:2 and 2:1 charged systems such as SiO2 and ZnCl2. This causes the increase of the coordination number, from four to six, when all the near-neighbor X− ions are counted as the surrounding ions.
Selective detection of O(1S) following electron impact dissociation of O2 and N2O using a XeO* conversion technique
Authors: LeClair Lance R. , McConkey J. W.
Published: July 2002

We have observed the metastable species O(1S0) following the dissociation of molecules by pulsed electron impact. A crossed beam apparatus was used to obtain time-of-flight spectra of the fragments. We have employed a novel detector which consists of a layer of freshly deposited Xe. O(1S0) atoms impinging on the surface quickly form XeO* excimers which rapidly decay (<1 μs) producing easily detected photons. Our present results indicate that this method is sensitive (with high quantum efficiency) solely to the 1S state of oxygen. Other metastables (such as 5S and 1D) and ground state oxygen atoms have not been detected. Low resolution optical spectra of the XeO emission reveal bands at 375, 550, and 725 nm. Time-of-flight and fragment kinetic energy spectra for both target gases at various electron impact energies are presented together with excitation functions from threshold to 1000 eV. These have been made absolute using a Bethe–Born calibration technique for N2O. O2 data are calibrated relative to N2O. Maximum cross sections for O(1S) production are 2.25×10−17 cm2 at 45 eV and 2.1×10−18 cm2 at 80 eV for N2O and O2 targets, respectively.
Structure and spectroscopy of phosphorus cluster anions: Theory (simulated annealing) and experiment (photoelectron detachment)
Authors: Jones R. O. , Ganteför G. , Hunsicker S. , Pieperhoff P.
Published: July 2002

Photoelectron detachment measurements have been performed on singly charged phosphorus cluster anions with up to nine atoms, generated by a pulsed arc cluster ion source (PACIS). Transitions between the anion ground states and states of the neutral clusters are observed for all clusters, and vibrational fine structure in both dimer and trimer. A comparison with the results of density functional calculations with simulated annealing—an extension to negative ions of earlier work on neutral and positively charged clusters—provides a consistent overall picture for all cluster sizes and the first experimental structural information on several.
Tricritical points in bimodal polymer solutions
Authors: Szleifer I. , ten Brinke G.
Published: July 2002

The tricritical point of polymer solutions composed by two polymer homologs of different molecular weight in a solvent is studied using the single-chain mean-field theory. The tricritical point is found for a ratio of molecular weights of the two polymers r=N1/N2, which decreases as a function of the short chain length N2 from 51 to approximately 40 for N2 increasing from 1 to 8. This last value is still much larger than the experimentally measured value for mixtures of polystyrene in cyclohexane, where it was found that r≂25. Although a further reduction of r as a function of N2 seems likely, the chain lengths of the corresponding long chains are beyond the reliability limit of the SAW’s simulation involved. The addition of a small effective repulsion between the polymer homologs, is found to bring down the predicted value to 25.
Molecular adsorption on unrelaxed and relaxed ionic steps. Application to Ar, CO, CO2, and NH3 adsorbed on MgO(001)
Authors: Briquez S. , Girardet C. , Goniakowski J. , Noguera C.
Published: July 2002

The adsorption energy and the equilibrium valley of various molecules adsorbed on a step of MgO(001) substrate are determined on the basis of interatomic potentials. The step acts as a trapping site for all molecules adsorbed on the lower terrace and as a barrier for molecules on the upper terrace. The height of the potential barrier increases or decreases when compared to the diffusion barrier on a perfect surface, depending on the electric nature of the molecule. While nonpolar admolecules experience a potential barrier at the upper step edge, this is a characteristic of the strong ionic substrate surface field to drastically reduce the potential barrier for polar molecules. The relaxation and the rumpling of the step ions change the adsorption energy by about 10%, resulting in an increase for a rare-gas adatom and in a decrease for all polar species, but they have little influence on the molecular orientation.
How do the properties of a glass depend on the cooling rate? A computer simulation study of a Lennard-Jones system
Authors: Vollmayr Katharina , Kob Walter , Binder Kurt
Published: July 2002

Using molecular dynamics computer simulations we investigate how the glass transition and the properties of the resulting glass depend on the cooling rate with which the sample has been quenched. The system we study is a two component Lennard-Jones model which is coupled to a heat bath whose temperature is decreased from a high temperature, where the system is a liquid, to zero temperature, where the system is a glass. The temperature Tb of this heat bath is decreased linearly in time, i.e. Tb=Ti−γt, where γ is the cooling rate, and we study the cooling rate dependence by varying γ over several orders of magnitude. In accordance with simple theoretical arguments and with experimental observations we find that the glass transition, as observed in the specific heat and the thermal expansion coefficient, becomes sharper when γ is decreased. A decrease of the cooling rate also leads to a decrease of the glass transition temperature Tg and we show that the dependence of Tg on γ can be rationalized by assuming that the temperature dependence of the relaxation times of the system is given by either a Vogel–Fulcher law or a power law. By investigating the structural properties of the glass, such as the radial distribution functions, the coordination numbers and the angles between three neighbor-sharing particles, we show how the local order of the glass increases with decreasing cooling rate. The enthalpy H and the density ρ of the glass decrease and increase, respectively, with decreasing γ. By investigating the γ dependence of clusters of nearest neighbors, we show how the cooling rate dependence of H and ρ can be understood from a microscopic point of view. Furthermore we demonstrate that the frequency of icosahedral-like structures is decreasing with decreasing cooling rate. We also show that the spectrum of the glass, as computed from the dynamical matrix, shows a shift towards higher frequencies when γ is decreased. All these effects show that there is a significant dependence of the properties of glasses on the cooling rate with which the glass is produced.
Influence of long-range interactions in the diffusion of multiparticle systems
Authors: Lepore J. R. , Albano E. V.
Published: July 2002

The diffusion of multiparticle systems with long-range dipolar repulsion and long-range dipolar repulsion perturbed by randomly distributed dipolar impurities is studied by means of computer simulations. Our investigation is motivated by experimental studies of the diffusion of alkali atoms on clean and contaminated (e.g. by oxygen atoms) single crystal metal and semiconductor surfaces. Concentration profiles of the diffusion fronts are in qualitative agreement with the experimental findings. Comparing to the behavior of non-interacting particles, it is found that dipolar repulsion considerably enhance the chemical diffusion coefficient, particularly at lower coverages where a sharp peak is observed close to θ≈0.09. In contrast, the chemical diffusion coefficient of non-interacting particles exhibits a smooth maximum close to θ≃0.5. The presence of random dipolar impurities causes a delay of the diffusion process and the low coverage peak of the diffusion coefficient becomes shifted to θ≈0.16 . The number of distinct sites visited by the diffusing particles, which is relevant for the evaluation of the rate constant for diffusion-limited reactions, is also studied and the results are compared with those of non-interacting particles.
From molecular clusters to bulk matter. I. Structure and thermodynamics of small CO2, N2, and SF6 clusters
Authors: Maillet Jean-Bernard , Boutin Anne , Buttefey Séverine , Calvo Florent , Fuchs Alain H.
Published: July 2002

The thermodynamics and structural properties of small molecular XN clusters (X=N2, CO2, and SF6) are investigated using molecular dynamics simulations. In this paper we compare the behavior of carbon dioxide, nitrogen, and sulfur hexafluoride for a given cluster size of N=13. Evidence is provided for “dynamical coexistence” between solidlike and liquidlike forms of the cluster, in a finite energy range, in the case of (CO2)13 and (N2)13 but not (SF6)13. In addition (N2)13 exibits a solid–solid phase transition characterized by the release of the molecular orientational degree of freedom. A systematic use of the dynamic quenching method enables us to interpret these different behaviors in terms of the energy distribution of minima in the potential energy surface of the systems. A comparison of the strain energies of these clusters, using a model recently proposed by Wales and co-workers, enables us to understand why different molecular clusters exhibit different crossover points from icosahedral to bulk properties.
A self-guided algorithm for learning control of quantum-mechanical systems
Authors: Phan Minh Q. , Rabitz Herschel
Published: July 2002

This paper presents a general self-guided algorithm for direct laboratory learning of controls to manipulate quantum-mechanical systems. The primary focus is on an algorithm based on the learning of a linear laboratory input–output map from a sequence of controls, and their observed impact on the quantum-mechanical system. This map is then employed in an iterative fashion, to sequentially home in on the desired objective. The objective may be a target state at a final time, or a continuously weighted observational trajectory. The self-guided aspects of the algorithm are based on implementing a cost functional that only contains laboratory-accessible information. Through choice of the weights in this functional, the algorithm can automatically stay within the bounds of each local linear map and indicate when a new map is necessary for additional iterative improvement. Finally, these concepts can be generalized to include the possibility of employing nonlinear maps, as well as just the laboratory control instrument settings, rather than observation of the control itself. An illustrative simulation of the concepts is presented for the control of a four-level quantum system.
Energy transfer with migration. Generalization of the Yokota–Tanimoto model for any kind of multipole interaction
Authors: Martı́n I. R. , Rodrı́guez V. D. , Rodrı́guez-Mendoza U. R. , Lavı́n V. , Montoya E. , Jaque D.
Published: July 2002

The dynamics of luminescence decay is analyzed within the general scenario of randomly distributed ions with whatever multipole interaction, taking into account energy transfer and migration processes among them. By applying Padé approximants to the Allinger and Blumen expression for the excited state population, a simple expression has been obtained which can be easily fitted to experimental data. This expression can be considered a generalization of the dipole–dipole Yokota–Tanimoto equation. On the other hand, the decay curves from the D35 level of the Tb3+ ions in fluorozincate glasses have been measured for different Tb3+ concentrations. The results are analyzed in the framework of the proposed expression.
Alternative definition of exchange-correlation charge in density functional theory
Authors: Liu Shubin , Ayers Paul W. , Parr Robert G.
Published: July 2002

A new definition of the exchange-correlation charge is presented. This charge is related to the exchange-correlation potential of density functional theory by the Poisson equation. It is illustrated how, using the Zhao–Morrison–Parr method, this new exchange-correlation charge can be evaluated numerically, thus providing direct input for the ongoing process of finding improved approximate density functionals. Several properties of this new charge, including the sum rule and Coulombic-like behavior, are derived. Both atomic shell and subshell structures are observed. Exchange-correlation charges generated from various approximate functionals are calculated and compared with numerically accurate data for a few atoms.
Transition matrix of point interactions as the scaling limit of integrable potentials on the real line
Authors: Pittner L. , Valjavec M.
Published: July 2002

On the real line, the transition matrix corresponding to the nonrelativistic one-particle Hamilton operator for a finite number of zero-range interaction points is the scaling limit of the transition matrix for corresponding integrable potentials.
Tadpole graph in covariant closed string field theory
Authors: Zemba Guillermo , Zwiebach Barton
Published: July 2002

A one-loop tadpole graph in covariant closed string field theory must generate all inequivalent tori with one puncture. In order to determine the region of integration in moduli space we map the tadpole to a two-sheeted sphere and use the analyticity of the modular parameter with respect to the complex propagator parameter. It is shown that the naive closed string extension of the Witten open string vertex fails to give the correct modular region for this one-loop amplitude.
A new family of integrable models in (2+1) dimensions associated with Hermitian symmetric spaces
Authors: Strachan I. A. B.
Published: July 2002

In a series of papers Fordy and his collaborators have studied families of integrable models in (1+1) dimensions associated with Hermitian symmetric spaces. These models were also generalized to (2+1) dimensions. However, the generalization used is not unique, and in this paper a different generalization is considered, resulting in new families of integrable models in (2+1) dimensions.
Covariant quantum Markovian evolutions
Authors: Holevo A. S.
Published: July 2002

Quantum Markovian master equations with generally unbounded generators, having physically relevant symmetries, such as Weyl, Galilean or boost covariance, are characterized. It is proven in particular that a fully Galilean covariant zero spin Markovian evolution reduces to the free motion perturbed by a covariant stochastic process with independent stationary increments in the classical phase space. A general form of the boost covariant Markovian master equation is discussed and a formal dilation to the Langevin equation driven by quantum Boson noises is described.
Statistical solutions of the Navier–Stokes equations on the phase space of vorticity and the inviscid limits
Authors: Constantin Peter , Wu Jiahong
Published: July 2002

Using the methods of Foias [Sem. Math. Univ. Padova 48, 219–343 (1972); 49, 9–123 (1973)] and Vishik–Fursikov [Mathematical Problems of Statistical Hydromechanics (Kluwer, Dordrecht, 1988)], we prove the existence and uniqueness of both spatial and space–time statistical solutions of the Navier–Stokes equations on the phase space of vorticity. Here the initial vorticity is in Yudovich space and the initial measure has finite mean enstrophy. We show under further assumptions on the initial vorticity that the statistical solutions of the Navier–Stokes equations converge weakly and the inviscid limits are the corresponding statistical solutions of the Euler equations.
Polaron conductivity of La0.7□0.3MnO3−δ thin films in the magnetic phase transition range
Authors: Krivoruchko V. N. , Khartsev S. I.
Published: July 2002

The magnetoresistive ρ(T,H) and thermoelectric (the Seebeck coefficient) S(T,H) properties of La0.7□0.3MnO3−δ thin films (□ is a cation vacancy) grown by the magnetron deposition technique are investigated. The magnetic polaron origin of the conductivity of such systems is established in the temperature range 77 K⩽T⩽350 K in magnetic fields 0⩽H⩽10 kOe. The experimental dependences ρ(T,H) and S(T,H=0) are approximated by a universal phenomenological expression. Thermopower measurements indicate a considerable change of the mobility as well as the density of states of charge carriers in the region of magnetic phase transition.
Electromagnetic eigenfrequency of anisotropic inhomogeneous axisymmetric toroidal plasmas of arbitrary meridional cross section
Authors: Cap Ferdinand F.
Published: July 2002

General solutions of Maxwell’s equations in cylindrical coordinates contain special solutions which satisfy the electromagnetic boundary conditions on toroidal surfaces. It is possible to choose the meridional cross section of these surfaces. The method of finding these special solutions is described in this paper. The results for Eφ may be used for current-driving calculations and Er, Ez may be useful for heating investigation of tokamaks.
Coalescence of stretching vortices
Authors: Jacobs P. A. , Pullin D. I.
Published: July 2002

The contour dynamics numerical technique is used to study the coalescence of two equal uniform vortices in the presence of an externally applied stretching strain field. Plane three-dimensional stretching is found to substantially inhibit vortex coalescence when the plane of the vortex axes is initially perpendicular to the plane of the straining motion. This behavior is interpreted qualitatively in terms of a purely two-dimensional flow fully equivalent to the stretching configuration.
Stochastic modeling of unidirectional fluid transport in uniform and random packed beds
Authors: Georgiadis John G. , Catton Ivan
Published: July 2002

An analytical study of the developed steady flow in saturated packed beds is presented with emphasis on the influence of the microstructure on the (ensemble) average momentum equation. The fluctuating interstitial velocity field is modeled by a spatial Ornstein–Uhlenbeck process and the functional form of the inertial (Forchheimer) drag term is correctly deduced. For Couette flow in porous media of constant porosity, the velocity profile is uniform except near the solid boundaries where boundary layers appear with thickness comparable to the bead diameter. It is proved that the random macroscopic variations of the porosity in statistically homogeneous packed beds enhance the average flow rate when the Forchheimer law holds.
Scaling of the bursting frequency in turbulent boundary layers at low Reynolds numbers
Authors: Kim John , Spalart Philippe R.
Published: July 2002

The frequency of occurrence of the bursting process near a wall is studied by examining turbulent velocity fields obtained from numerical simulations of a flat-plate boundary layer at three different Reynolds numbers. It is found that the frequency scaled by the inner variables is essentially independent of the Reynolds number.
Computation of aerodynamic coefficients for a flexible membrane airfoil in turbulent flow: A comparison with classical theory
Authors: Smith Rick , Shyy Wei
Published: July 2002

In the present paper an aeroelastic model of flexible membrane wing aerodynamics which incorporates the Reynolds-averaged Navier–Stokes equations is presented. The Reynolds stresses are prescribed by the k–ω shear-stress transport eddy-viscosity model recently proposed by Menter. The computed coefficients are compared with classical inviscid membrane airfoil theory and with a portion of the available experimental data for membrane wings. The results indicate that classical potential-based membrane airfoil theory can provide a meaningful description of membrane wing aerodynamics only for a small range of incidence angles near ideal and then only for membrane airfoils with small excess length ratios. For larger excess lengths and incidence angles viscous effects dominate the aerodynamics. The agreement of the computed results with the experimental data is mixed. The current status of the available experimental data for membrane airfoils is also reviewed.
Stability of high beta tokamak plasmas*
Authors: Strait E. J.
Published: July 2002

Stability at high beta (the ratio of plasma pressure to magnetic field pressure) is an important requirement for a compact, economically attractive fusion reactor. It is also important in present large tokamak experiments, where the best performance is now often limited by instabilities rather than by energy transport. The past decade has seen major advances in our understanding of the stability of high beta tokamak plasmas, as well as in the achievement of high values of beta. Ideal magnetohydrodynamic (MHD) theory has been remarkably successful in predicting the stability limits, and the scaling of maximum stable beta with the normalized plasma current predicted by Troyon and others has been confirmed in many experiments, yielding a limit βmax≊3.5 (%-m-T/MA) I/aB (where I is the plasma current, a is the minor radius, and B is the toroidal field). The instabilities which are predicted to limit beta have been observed experimentally, in good agreement with theoretical predictions, including long-wavelength kink modes and short-wavelength ballooning instabilities. Advances in understanding of tokamak stability have opened several paths to higher values of beta. The use of strong discharge shaping, approaching the limits of axisymmetric stability, has allowed beta values as high as 12% to be reached in agreement with Troyon scaling. Recent experimental results and ideal MHD modeling have shown that the beta limit depends on the form of the pressure and current density profiles, and modification of the current density to create a centrally peaked profile has allowed beta values up to 6I/aB to be achieved experimentally.

Recent experiments have also begun to explore both local and global access to the predicted second stable regime for ballooning modes, with the potential for very high values of β/(I/aB). Preliminary experimental investigations of wall stabilization and radio-frequency (RF) current profile control hold the promise of further improvements in beta through passive and active control of instabilities. The developing understanding of high beta stability and the application of this understanding to present experiments and future fusion devices hold the potential for production of stable, steady state plasmas at high beta with good confinement.