bluuebunny/crossref_metadata_2025 huggingface dataset:
Limitations of ray tracing techniques in optical modeling of silicon solar cells and photodiodes
Authors: Abouelsaood A. A. , Ghannam M. Y. , Al Omar A. S.
Published: July 2002

The physical optical effects which take place when the size of the surface textures of a solar cell or a photodiode is in the micron range are studied. Both the antireflective and the light trapping properties of linear periodic grooves of triangular cross sections are treated in the framework of an exact, coupled-wave, electromagnetic analysis. The most important physical optics effects are the diffraction of light at the groove edges, the interference of light reflected by the different surfaces of the grooves, and the tunneling across the grooved surface of light incident from the silicon side at an angle greater than the critical angle. There is also the effect of the interference of the light reflected by the front and back surfaces of the silicon layer which is important only for very thin layers. As the groove period increases above the wavelength, the exact results approach the geometrical optics predictions rather slowly in an oscillatory way. Simple physical optics arguments are presented for obtaining a criterion for the accuracy of ray tracing methods in optical modeling of surface textures.
Magnetic refrigeration materials (invited)
Authors: Gschneidner K. A. , Pecharsky V. K.
Published: July 2002

Research on the magnetocaloric effect and its application for cooling near room temperature over the past few years has helped to move this phenomenon from a scientific curiosity to an emerging technology. Two of the most important advances include the demonstration which proved that it is possible to obtain significant cooling powers (600 W) at high Carnot efficiencies (60%) and with a large coefficient of performance (15) near room temperature in moderately strong magnetic fields (⩽5 T); and the discovery of the giant magnetocaloric effect in the Gd5(SixGe1−x)4 series of alloys. Also, new knowledge about the magnetocaloric effect has been gained. This includes: the relationship between the nature of the magnetic transformation(s) and the temperature dependence of the magnetocaloric effect, the entropy utilized in the magnetocaloric process, and the role of impurities on the giant magnetocaloric effect.
Structure and annealing of NiMn spin valves
Authors: Loch Christian , Maass W. , Ocker B. , Röll K.
Published: July 2002

The thermal stability of sputtered NiMn pinned spin valves (NiFe/Co/Cu/Co/NiFe/NiMn) was studied for different Co and Cu thicknesses. The coupling between NiFe and antiferromagnetic NiMn was established by an annealing process which also led to a change of magnetoresistance due to diffusion through the Cu layer. Annealing was performed under vacuum in a magnetic field to prevent degradation of the magnetic anisotropy in the antiferromagnet. High angle x-ray diffraction showed a typical shift of the (111) NiMn peak during annealing and a structural change with different Co thicknesses.
Electrically controlled elasticity utilizing piezoelectric coupling
Authors: Date Munehiro , Kutani Masayuki , Sakai Shigeru
Published: July 2002

Electrical control of elasticity was performed in piezoelectric polymer films by connecting an electric circuit parallel to the sample electrodes. A polyvinylidene fluoride film and a vinylidene fluoride tetrafluoroethylene copolymer (73/27) film were used. Electrical circuits acting as a variable negative capacitor or a variable inductor were constructed using operational amplifiers. Theoretically, when the value of the external negative capacitance is increased, the observed elastic constant increases from that at an open circuit to positive infinity at around the value of the sample, turns to negative infinity, and then increases through zero to that at a short circuit. If the inductance of the external circuit is changed around the electrical resonant frequency, the elastic constant exhibits resonance and antiresonance against frequency. Experimental observations of the characteristics of the dynamic elastic constant agreed well with these theoretical predictions. By coupling the negative capacitance, the elastic constant changed between 0.5 and 2 times the original value, and the elastic loss increased to tan δ=0.7.
Solidification of Ga–Mg–Zn in a gas-filled drop tube: Experiments and modeling
Authors: Fransaer Jan , Wagner Andrew V. , Spaepen Frans
Published: July 2002

Droplets of Ga24Mg36Zn40 were solidified in a 3 m, helium-filled drop tube. The solidified droplets contained uniformly distributed MgZn2 dendritic primary crystals, whose number was determined as a function of droplet size. A new model for the solidification in a gas-filled drop tube was developed. It allows introduction, stochastically in space and time, of multiple nuclei with interfering thermal fields. Using classical homogeneous nucleation kinetics, the model reproduces the size dependence of the number of primary crystals. The crystal-melt interfacial tension is estimated at 0.0008–0.002 J/m2. This low value is attributed to the polytetrahedral structural similarity between the melt and the MgZn2 Frank-Kasper phase. The program is available upon request.
Nd
3+
-doped fluoroaluminate glasses for a 1.3 μm amplifier
Authors: Naftaly M. , Jha A.
Published: July 2002

A Nd3+-doped 1.3 μm fiber amplifier has the potential to be an efficient, high-gain device. A major problem to overcome is the long wavelength of the gain spectrum. In the majority of Nd3+-doped glasses the gain is shifted out of the second telecom window as a result of long-wavelength emission and signal excited-state absorption (ESA). In this article we present new fluoroaluminate glasses developed as hosts for the 1.3 μm Nd3+-doped fiber amplifier. Nd3+ emission peaks below 1320 nm were demonstrated in bulk glasses. Gain in the 1310–1320 nm region was measured in unclad fibers, with evidence of significantly reduced ESA. Another serious problem in amplifier design lies in overcoming the competing amplified spontaneous emission (ASE) at 1050 nm. We discuss ASE filtering using in-fiber Bragg gratings and present results supporting its feasibility. The article also examines thermal and viscous properties of matched core and cladding glasses in relation to preform and fiber fabrication.
Effects of thickness on the piezoelectric and dielectric properties of lead zirconate titanate thin films
Authors: Lian L. , Sottos N. R.
Published: July 2002

Lead zirconate titanate (PZT) thin films with a Zr/Ti ratio of 52/48 were deposited on platinized silicon substrates by a sol-gel method and crystallized with preferred (111) or (100) orientation. Both the piezoelectric properties (d33) and the field-induced strains of the films with different thickness and preferred orientation were measured by a laser Doppler heterodyne interferometer. The weak-field dielectric constants and dissipation factors were also measured as a function of film thickness for comparison with other work reported in the literature. The field-forced polarization switching (P-E) behavior was examined by Sawyer–Tower measurement. Both the piezoelectric properties and the dielectric constants of the PZT thin films were found to be strongly dependent on the film thickness. An increase in the piezoelectric constants and dielectric constants with increasing film thickness was observed for both films with preferred (111) and (100) orientations. Films with (100) preferred orientation were found to have higher piezoelectric constants and lower dielectric dissipation factors than films with (111) preferred orientation. A finite element analysis was carried out to evaluate the existing methods for determination of the d33 coefficient of ferroelectric thin films. The measured changes in properties with thickness were correlated with the residual stress state in the films.
Fluids in narrow pores: Adsorption, capillary condensation, and critical points
Authors: Evans R. , Marconi U. Marini Bettolo , Tarazona P.
Published: July 2002

By means of a density functional approach the phase equilibria of a simple fluid confined by two adsorbing walls have been investigated as a function of wall separation H and chemical potential μ for temperature T corresponding to both partial and complete wetting situations. For large values of H and small undersaturations Δμ ≡ μsat−μ, we recover the macroscopic formulas for the undersaturation at which a first- order phase transition (capillary condensation) from dilute ‘‘gas’’ to a dense ‘‘liquid’’ occurs in a single, infinitely long slit. For smaller H we compute the lines of coexistence between gas and liquid in the (Δμ, 1/H) plane at fixed values of T. The adsorption Γ(Δμ), at fixed T and H, is characterized by a loop. At the first order transition Γ jumps discontinuously by a finite amount; however metastable states exist and these could give rise to hysteresis of the adsorption isotherms obtained for the single slit. The loop disappears at a capillary critical point (Δμc, 1/Hc) at each T. For HΔμc, condensation can no longer occur and no metastable states are present. The location of the critical points is described and for a complete wetting situation we find that these lie outside the bulk two phase region. Our theory provides a simple explanation of phase transitions observed in earlier computer simulations and mean-field lattice gas calculations for confined fluids and suggests that measurements of the forces between plates, either by simulation or in real fluids, should provide rather direct information about capillary condensation and, possibly, capillary critical points. The relevance of our results for adsorption experiments on mesoporous solids is discussed briefly.
Equilibrium theory of polymer liquids: Linear chains
Authors: Curro John G. , Schweizer Kenneth S.
Published: July 2002

An equilibrium theory of polymer melts, developed previously by us for polymer rings, is generalized to include linear polymer chains. This theory is based on the reference interaction site model (RISM) integral equation approach developed by Chandler and co-workers for molecular liquids. We are able to construct a tractable formalism for the high polymer problem by employing the fact that a polymer molecule in a melt is ideal. This leads to a set of coupled, nonlinear integral equations for the intermolecular radial distribution functions. A simple optimized perturbative scheme is developed for long linear chains based on the relative unimportance of end effects. To lowest order, the theory reduces to a single nonlinear integral equation. Chain end corrections to the site-averaged intermolecular correlation functions vanish according to N−2, where N is the degree of polymerization. Numerical techniques were used to compute the radial distribution function and structure factor for hard core linear Gaussian chains of 16 000 units over a range of densities. Calculation of higher order corrections for end effects is possible with our formalism.
Free energy of ionic hydration: Analysis of a thermodynamic integration technique to evaluate free energy differences by molecular dynamics simulations
Authors: Straatsma T. P. , Berendsen H. J. C.
Published: July 2002

The thermodynamic integration technique to evaluate free energy differences by molecular dynamics simulations is analyzed. The hydration of the ions Na+ , K+ , Ca++ , F−, Cl−, and Br− is used as the process to illustrate the potential utility of the method. A neon–water system is used as a reference system. The parameters that influence the performance and accuracy of the thermodynamic integration, in which the potential interaction parameters are gradually and continuously changed, are studied. These parameters include the total simulation time, the magnitude of the time step for the numerical integration of the equations of motion, the system size, and the cutoff radii for the intermolecular interactions. Fast convergence is found for the Gibbs free energy difference between Ne and Na+ with respect to total simulation time. The time step and system size are relatively unimportant. The use of cutoff radii, for the ion–water but especially unfortunately also the water–water intermolecular interactions, seriously influences the results obtained. A simple correction for the use of cutoff radii cannot be made. Results are compared to experimental values.
Critical evaluation of the application of direct energy transfer in probing the morphology of porous solids
Authors: Levitz P. , Drake J. M. , Klafter J.
Published: July 2002

The decay of an initially excited donor due to the presence of acceptor molecules embedded in porous silica gels has been studied. The time evolution of the decay process has been related to the geometrical restrictions imposed on the participating molecules by the porous structure of the silicas. Fractal and regular geometrical shape models have been confronted with experimental findings. We conclude that for silica gels simple pore geometries, i.e., spheres and cylinders, account well for the relaxation behavior. The decay patterns exhibit temporal crossovers typical of dynamics in restricted geometries.
Is coupled cluster singles and doubles (CCSD) more computationally intensive than quadratic configuration interaction (QCISD)?
Authors: Scuseria Gustavo E. , Schaefer Henry F.
Published: July 2002

It is shown that the recently proposed QCI method including all single and double substitutions has essentially the same computational requirements as the more complete CCSD approach. If properly formulated, the CCSD equations contain at most quadratic terms in the excitation amplitudes.
Attractive, weakly attractive, and repulsive near-critical systems
Authors: Debenedetti Pablo G. , Mohamed Rahoma S.
Published: July 2002

Mixtures composed of trace amounts of a solute in the vicinity of a solvent’s critical point exhibit one of three types of behavior: repulsive, weakly attractive, or attractive. Each regime is characterized by the signs of the diverging solute partial molar volume (and enthalpy), and of the correlations between solute and solvent concentration fluctuations at infinite dilution. Attractive behavior, in which the former quantity is negative and the latter positive, underlies all of the actual or proposed applications of supercritical fluids to processes ranging from precipitation polymerization to activated carbon regeneration. The presence of trace amounts of a solute within a solvent characterized by long-ranged density fluctuations gives rise to a cooperative phenomenon which, for attractive systems, involves the formation of a large solvent-rich region around each solute molecule. Likewise, for repulsive behavior, the cooperative phenomenon involves the formation of a large solvent-deficient region around each solute molecule. The calculated volume, enthalpy, and entropy changes per solvent molecule are very similar to the corresponding condensation (attractive behavior) and evaporation (repulsive behavior) effects for the pure solvent at subcritical temperatures. This suggests a qualitative description of near-critical cooperative phenomena as induced phase transitions.
The electron density distribution in ice Ih determined by single-crystal x-ray diffractometry
Authors: Goto Akira , Hondoh Takeo , Mae Shinji
Published: July 2002

Precise structure analysis of ice Ih was carried out by single-crystal x-ray diffractometry. The reliability factor R was as small as 0.007 for the refined structure based on the half-hydrogen model. Both of the O–H distances at 243 K, those oblique and parallel to the c axis, were determined to be 0.85(2) and 0.82(3) Å, respectively, which are significantly shorter than those determined by the neutron diffraction method. These discrepancies were attributed to the difference between the electron and protonic density distribution along the O–H---O bond. Fourier synthesis and difference synthesis were computed for clarifying the electron density distribution relating to the hydrogen bonding.
A
b i
n
i
t
i
o study of intermolecular potential of H2O trimer
Authors: Chal/asiński G. , Szczȩśniak M. M. , Cieplak P. , Scheiner S.
Published: July 2002

Nonadditive contribution to the interaction energy in water trimer is analyzed in terms of Heitler–London exchange, SCF deformation, induction and dispersion nonadditivities. Nonadditivity originates mainly from the SCF deformation effect which is due to electric polarization. However, polarization does not serve as a universal mechanism for nonadditivity in water. In the double-donor configuration, for example, the Heitler–London exchange contribution is the most important and polarization yields the wrong sign. Correlation effects do not contribute significantly to the nonadditivity. A detailed analysis of the pair potential is also provided. The present two-body potential and its components are compared to the existing ab initio potentials (MCY) as well as to empirical ones (RWK2,TIP,SPC). The ways to improve these potentials are suggested.
The use of midbond functions for ab initio calculations of the asymmetric potentials of He–Ne and He–Ar
Authors: Tao Fu-Ming
Published: July 2002

The van der Waals interaction potentials of the asymmetric pairs of noble-gas atoms, He–Ne and He–Ar, are calculated by complete fourth-order Mo/ller–Plesset perturbation theory (MP4) with an extensive investigation of midbond functions. Calculations with several extended basis sets, with and without midbond functions, consistently demonstrate the effective role of midbond functions in accurate calculations of intermolecular energies and potentials. A moderately polarized basis set, augmented with a simple set of midbond functions (3s3p2d), is capable of recovering 92%–95% of the experimental well depth and reproducing the potential minimum within 0.1 a0. We attribute the remaining errors mainly to the deficiency of the MP4 theory. The present study shows that the use of midbond functions effectively recovers the intersystem correlation energy, which is normally obtained in a purely nucleus-centered basis set by use of diffuse low (first and second) polarization functions and high polarization functions. We therefore propose the use of nucleus-centered compact polarization functions and midbond functions to maintain the balanced treatment of the intra- and intersystem correlations for efficient and reliable calculations. The study also shows that the calculated interaction energy is highly insensitive to the displacement of midbond functions and thus the geometric midpoint of the van der Waals bond is a convenient and suitable choice for the position of midbond functions.
Photoelectron spectra of the hydrated iodine anion from molecular dynamics simulations
Authors: Dang Liem X. , Garrett Bruce C.
Published: July 2002

In this paper, we present the first calculations, based on molecular dynamics techniques, of vertical electron binding energies for the ionic clusters I−(H2O)n, (n=1–15). In these studies, we employ the polarizable water model developed recently by Dang [J. Chem. Phys. 97, 2659 (1992)]. We construct the ion–water potential so that the successive binding energies for the ionic clusters, the hydration enthalpy, and the structural properties of the aqueous ionic solution agree with the results obtained from experiments. The simulated vertical electron binding energies compare well with recent data from photoelectron spectroscopy experiments by Markovich, Giniger, Levin, and Cheshnovsky [J. Chem. Phys. 95, 9416 (1991)]. Interestingly, we obtain coordination numbers of 4 to 5 for the ionic clusters, I−(H2O)n, for n≥6. This result is smaller than the coordination number, based on the energetic properties predicted by Markovich et al. Possible reasons for this discrepancy are discussed in the paper. Furthermore, our simulations place the iodine anion on the surface of the water clusters. This study demonstrates the usefulness of the molecular dynamics technique and provides a detailed picture of the ion solvation in clusters.
Path integral simulations of mixed para-D2 and ortho-D2 clusters: The orientational effects
Authors: Buch V.
Published: July 2002

A recently suggested computational scheme [Buch and Devlin, J. Chem. Phys. 98, 4195 (1993)] is extended to describe orientational phenomena in systems containing assemblies of odd-J and even-J hydrogen and deuterium molecules. Nuclear symmetry and rotational effects are incorporated in the path integral Monte Carlo algorithm. The scheme is employed in a study of the (para-D2)3(ortho-D2)10 and (para-D2)13 clusters in the 1–3 K temperature range. In the (para-D2)3(ortho-D2)10 cluster at 1 K, the three J=1 molecules form a ‘‘cluster within cluster,’’ held together by the anisotropic quadrupole–quadrupole interactions. At 3 K the three molecule J=1 cluster is dissolved within the (D2)13 cluster. The orientational structure of the (para-D2)13 cluster at 1 K is composed of two distinct distorted-T pair configurations of neighboring J=1 molecules. Upon heating to 3 K the orientational structure changes, and a clear separation between the two pair geometries is no longer obtained. Moreover at 3 K thermal distortion of the cluster structure from icosahedral relaxes the extent of orientational frustration of the system, and thus contributes to the lowering of the rotational energy.
Semiclassical molecular dynamics for strongly coupled Coulomb systems
Authors: Klakow D. , Toepffer C. , Reinhard P.-G.
Published: July 2002

Based on the time-dependent variational principle for a Hartree wave function, a semiclassical approximation for a strongly correlated plasma is derived. This procedure maps the quantum dynamical problem to a classical one with one additional degree of freedom per particle and is considered the natural extension of classical molecular dynamics. As a test case, a single electron in a Coulomb potential is studied. For the full many-body problem the pair distribution function, the velocity autocorrelation function, the conductivity, and the diffusion constants are calculated for a plasma. We also consider liquid molecular hydrogen and lithium as a further test case which is sensitive to the treatment of the Pauli exclusion principle.
Rubidium ion hydration in ambient and supercritical water
Authors: Fulton J. L. , Pfund D. M. , Wallen S. L. , Newville M. , Stern E. A. , Ma Yanjun
Published: July 2002

X-ray absorption fine structure (XAFS) measurements and analyses are presented for Rb+ in supercritical water solutions. The structure of the first hydration shell at ambient conditions is compared to that in the supercritical region at a temperature of 424 °C and pressures from 382 to 633 bar. For all reported studies, RbBr at a concentration of 0.5 molal was used. XAFS results show that there is a well-defined hydration shell around the cation even at 424 °C but at these high temperatures the extent of hydration of the Rb cation is reduced by about 40%. A slight contraction of this first shell distance by about 0.10 Å is also observed under supercritical conditions. The reduction in the number of water-ion bonds is analogous to the reduction in the amount of water–water hydrogen bonding that has been observed by others under supercritical conditions. The reduction in waters-of-hydration under supercritical conditions may also be in part due to formation of contact-ion pairs.
The zero-order regular approximation for relativistic effects: The effect of spin–orbit coupling in closed shell molecules
Authors: van Lenthe E. , Snijders J. G. , Baerends E. J.
Published: July 2002

In this paper we will calculate the effect of spin–orbit coupling on properties of closed shell molecules, using the zero-order regular approximation to the Dirac equation. Results are obtained using density functionals including density gradient corrections. Close agreement with experiment is obtained for the calculated molecular properties of a number of heavy element diatomic molecules.
Heterogeneous two-dimensional nucleation and growth kinetics
Authors: Liu X. Y. , Maiwa K. , Tsukamoto K.
Published: July 2002

A kinetic model is developed to describe the growth of crystals under the influence of foreign particles in terms of heterogeneous two-dimensional nucleation. In the context of this model, the free energy barrier of two-dimensional nucleation in the presence of foreign particles and the kinetics for the nucleation and growth are examined theoretically. It follows that the contact angle, size and density of adsorbed foreign particles play a crucial role in controling the 2D nucleation barrier and growth kinetics. Based on our model, many crucial experimental findings, such as dust-induced surface roughening and the various kinetics of dislocation-free growth, are properly interpreted. The promotion effect of foreign particles on crystal growth is also analyzed from the view of designing additives. To our knowledge, this is the first systematic consideration of effects of foreign particles on the 2D nucleation process and growth kinetics, and the model generally covers both heterogeneous and conventional homogeneous 2D nucleation growth.
Photoelectron and photofragment velocity map imaging of state-selected molecular oxygen dissociation/ionization dynamics
Authors: Parker D. H. , Eppink A. T. J. B.
Published: July 2002

A substantial improvement in the photofragment imaging technique is illustrated in a study of molecular oxygen photodynamics. In this method, labeled velocity map imaging, electrostatic ion lenses are shown to allow mapping of all particles with the same initial velocity vector onto the same point on a 2D detector, irrespective of their position of creation in the ionization volume. This leads to a dramatic increase in image resolution. Velocity map imaging of photoelectrons from molecular ionization is also demonstrated and applied along with O+ imaging to identify the processes leading to O+ formation when using (2+1) resonantly enhanced multiphoton ionization (REMPI) detection for O2. Oxygen molecules prepared in the (v=2, N=2) level of the 3dπ(3Σ1g−) Rydberg state by two-photon excitation at 11.02 eV are excited by a third photon to an energy near v=24 of ground-state O2+ (equivalent to one-photon excitation at 75 nm). All energetically accessible excited oxygen atoms and an extensive range of vibrationally excited O2+ ions result, with the primary dissociation/ionization events taking place at the third-photon level. Competition between dissociation into excited atoms and formation of O2+ is gauged by comparing images for e− and O+ products. Trends in the photoelectron and O+ fragment angular distributions are discussed for each active channel.
Analytical energy gradients for local second-order Mo/ller–Plesset perturbation theory
Authors: El Azhary Adel , Rauhut Guntram , Pulay Peter , Werner Hans-Joachim
Published: July 2002

Based on the orbital invariant formulation of Mo/ller–Plesset (MP) perturbation theory, analytical energy gradients have been formulated and implemented for local second order MP (LMP2) calculations. The geometry-dependent truncation terms of the LMP2 energy have to be taken into account. This leads to a set of coupled-perturbed localization (CPL) equations which must be solved together with the coupled-perturbed Hartree–Fock (CPHF) equations. In analogy to the conventional non-local theory, the repeated solution of these equations for each degree of freedom can be avoided by using the z-vector method of Handy and Schaefer. Explicit equations are presented for the Pipek–Mezey localization. Test calculations on smaller organic molecules demonstrate that the local approximations introduce only minor changes of computed equilibrium structures.
Electron capture by large helium droplets
Authors: Henne Ulrich , Toennies J. Peter
Published: July 2002

The attachment of electrons to large helium droplets containing up to 108 atoms produced in supercritical liquid free jet expansions has been investigated in a crossed beam scattering experiment. Negative cluster ions were formed in collisions with electrons from a nearly monoenergetic (δE≈0.25 eV) electron beam with energies Eel=1.0–100 eV and were subsequently size selected by electrostatic deflection. Depending on the droplet size up to seven distinct resonancelike maxima in the negative-ion signal with peak widths ΔE1/2 proportional to the droplet radius could be resolved. The lowest energy peak at Eel=1.8–2.3 eV depending on size, is attributed to a direct localization of the electron which subsequently creates a bubble inside the He droplet. The sharp additional peaks at energies above 20 eV are explained by the thresholds for single or successive electronic excitations of the droplets which result in a zero-kinetic-energy electron which then also localizes in an internal bubble.
Collision induced desorption and dissociation of O2 chemisorbed on Ag(001)
Authors: Vattuone L. , Gambardella P. , Burghaus U. , Cemič F. , Cupolillo A. , Valbusa U. , Rocca M.
Published: July 2002

We have investigated desorption and dissociation of O2 chemisorbed on Ag(001) induced by collision with hyperthermal Xe and Ar atoms by high resolution electron energy loss spectroscopy and supersonic molecular beam technique. The cross section for both processes increases rapidly both as a function of angle of incidence and of total impact energy of the inert gas atom. While the increase with energy is expected, the increase with the angle is somewhat surprising and is sensibly larger than observed for previously investigated systems. The cross section for desorption decreases moreover with coverage. In the limit of high impact energy and high coverage its value is always larger than the one for dissociation. The branching ratio between the two processes depends thereby on energy and angle of incidence of the inert gas atom. Atomic oxygen is not removed under any impact condition, because of its larger binding energy. In order to explain the experimental results, molecular dynamics simulations have been performed using a simple model including multiple scattering. We find that the angular dependence of the cross section is determined by surface corrugation and by multiple scattering which suppresses desorption at normal incidence while the energetic threshold is determined by energy loss to the substrate.
Length and occupancy dependence of the tracer exchange in single-file systems
Authors: Rödenbeck Christian , Kärger Jörg
Published: July 2002

The tracer exchange between a finite single-file channel (in which the diffusing guest particles are not able to pass each other) and its surroundings is considered. A mean-field calculation shows that the mean intracrystalline residence time τintra is proportional to the 3rd power of the channel length L. Further, it is proportional to Θ/(1−Θ), where Θ denotes the relative occupancy. Monte Carlo simulations confirm these results to a satisfactory accuracy, especially at high occupancy. The essential mechanism of tracer exchange is considered in more detail.
The injecting energy at molecule/metal interfaces: Implications for conductance of molecular junctions from an ab initio molecular description
Authors: Yaliraki S. N. , Roitberg A. E. , Gonzalez C. , Mujica V. , Ratner Mark A.
Published: July 2002

To study the electronic transport of molecular wire circuits, we present a time-independent scattering formalism which includes an ab initio description of the molecular electronic structure. This allows us to obtain the molecule–metal coupling description at the same level of theory. The conductance of junction α, α′ xylyl dithiol and benzene-1,4-dithiol between gold electrodes is obtained and compared with available experimental data. The conductance depends dramatically on the relative position of the Fermi energy of the metal with respect to the molecular levels. We obtain an estimate for the injecting energy of the electron onto the molecule by varying the distance between the molecule and the attached gold clusters. Contrary to the standard assumption, we find that the injecting energy lies close to the molecular highest occupied molecular orbital, rather than in the middle of the gap; it is just the work function of the bulk metal. Finally, the adequacy of the widely used extended Hückel method for conductance calculations is discussed.
Proton tunneling in the benzoic acid dimer studied by high resolution ultraviolet spectroscopy
Authors: Remmers Karen , Meerts W. Leo , Ozier Irving
Published: July 2002

High resolution ultraviolet spectroscopy has been used to investigate the rotationally resolved excitation spectrum of the first singlet–singlet transition in the benzoic acid dimer. The measured spectrum consists of two overlapping components. The corresponding lines in the two components are shown to originate in different levels of the ground state potential separated by a tunneling splitting produced by concerted proton exchange between the two subunits forming the dimer. The frequency separation between the two components is equal to the difference between the tunneling splittings in the ground and the excited electronic state. This frequency separation is found to be 1107±7 MHz. From the analysis, it is estimated that the barrier for proton tunneling changes by about 20% upon electronic excitation. The structure of the dimer in the ground state is determined to be linear, while in the excited S1 state it is slightly bent (3.4°±1.7°).
Thermodynamics and kinetics of the glass transition: A generic geometric approach
Authors: Gutzow I. , Ilieva D. , Babalievski F. , Yamakov V.
Published: July 2002

A generic phenomenological theory of the glass transition is developed in the framework of a quasilinear formulation of the thermodynamics of irreversible processes. Starting from one of the basic principles of this science in its approximate form given by de Donder’s equation, after a change of variables the temperature dependence of the structural parameter ξ(T), the thermodynamic potentials ΔG̃(T), the thermodynamic functions and the time of molecular relaxation τ of vitrifying systems is constructed. In doing so, a new effect in the ΔG̃(T) course is observed. The analysis of the higher derivatives of the thermodynamic potential, and especially the nullification of the second derivative of the configurational specific heats ΔC̃p(T) of the vitrifying liquid defines glass transition temperature T̃g and leads directly to the basic dependence of glass transition kinetics: the Frenkel–Kobeko–Reiner equation. The conditions guaranteeing the fulfillment of this equation specify the temperature dependence of the activation energy U(T,ξ̃) for viscous flow and give a natural differentiation of glass formers into fragile and strong liquids. The effect of thermal prehistory on the temperature dependence of both thermodynamic functions and kinetic coefficients is established by an appropriate separation of de Donder’s equation.
Solvable Lie algebras of dimension six
Authors: Turkowski P.
Published: July 2002

Six-dimensional solvable Lie algebras over the field of real numbers that possess nilradicals of dimension four are classified into equivalence classes. This completes Mubarakzyanov’s classification of the real six-dimensional solvable Lie algebras.
Nonlinear resonant scattering and plasma instability: an integrable model
Authors: Claude C. , Latifi A. , Leon J.
Published: July 2002

A detailed study of a system of coupled waves is given for which an initial-boundary value problem is solved by means of the spectral transform theory. This system represents the nonlinear interaction of an electrostatic high-frequency wave with the ion acoustic wave in a two component homogeneous plasma. As a result it is understood the plasma instability as (i) a continuous secular transfer of energy from the laser beam to the acoustic wave, (ii) the evolution toward the formation of local singularities of the electrostatic wave (collapsing), (iii) a mutual trapping of the acoustic wave and the scattered Langmuir wave.
A no-hair theorem for self-gravitating nonlinear sigma models
Authors: Heusler Markus
Published: July 2002

The coupled system of gravity and mappings φ:(M,g)→(N,G) with harmonic action and additional potential is considered. For spherically symmetric manifolds (M,g) and Riemannian manifolds (N,G) it is shown that the only static, asymptotically flat solutions of the coupled Einstein-matter equations with regular event horizon and finite energy consist of the Schwarzschild metric and a constant map, being a zero of the non-negative potential.
Combinatorial formulas for Vassiliev invariants from Chern–Simons gauge theory
Authors: Labastida J. M. F. , Pérez Esther
Published: July 2002

We analyze the perturbative series expansion of the vacuum expectation value of a Wilson loop in Chern–Simons gauge theory in the temporal gauge. From the analysis emerges the notion of the kernel of a Vassiliev invariant. The kernel of a Vassiliev invariant of order n is not a knot invariant, since it depends on the regular knot projection chosen, but it differs from a Vassiliev invariant by terms that vanish on knots with n singular crossings. We conjecture that Vassiliev invariants can be reconstructed from their kernels. We present the general form of the kernel of a Vassiliev invariant and we describe the reconstruction of the full primitive Vassiliev invariants at orders two, three, and four. At orders two and three we recover known combinatorial expressions for these invariants. At order four we present new combinatorial expressions for the two primitive Vassiliev invariants present at this order.
On revolutionizing quantum field theory with Tomita’s modular theory
Authors: Borchers H. J.
Published: July 2002

In the book of Haag [Local Quantum Physics (Springer Verlag, Berlin, 1992)] about local quantum field theory the main results are obtained by the older methods of C*- and W*-algebra theory. A great advance, especially in the theory of W*-algebras, is due to Tomita’s discovery of the theory of modular Hilbert algebras [Quasi-standard von Neumann algebras, Preprint (1967)]. Because of the abstract nature of the underlying concepts, this theory became (except for some sporadic results) a technique for quantum field theory only in the beginning of the nineties. In this review the results obtained up to this point will be collected and some problems for the future will be discussed at the end. In the first section the technical tools will be presented. Then in the second section two concepts, the half-sided translations and the half-sided modular inclusions, will be explained. These concepts have revolutionized the handling of quantum field theory. Examples for which the modular groups are explicitly known are presented in the third section. One of the important results of the new theory is the proof of the PCT theorem in the theory of local observables. Questions connected with the proof are discussed in Sec. IV. Section V deals with the structure of local algebras and with questions connected with symmetry groups. In Sec. VI a theory of tensor product decompositions will be presented. In the last section problems that are closely connected with the modular theory and that should be treated in the future will be discussed.
Electron diffraction studies of mechanisms of crystal structure formation in N2 clusters
Authors: Kovalenko S. I. , Solnyshkin D. D. , Bondarenko E. A. , Verkhovtseva É. T. , Eremenko V. V.
Published: July 2002

The structure of N2 clusters is studied by the electron diffraction method over a wide range of their mean sizes N̄. It is found that, as the value of N̄ increases, the structure of the clusters is transformed in the sequence quasi-crystal (icosahedron) →α-N2 (cubic) phase with stacking faults →β-N2 (hcp) phase. The formation of the cubic phase from the icosahedral structure follows a mechanism typical of rare gases, while the transition α→β is due to an increase in the cluster temperature with the size.
Thermal transport through Luttinger liquid constriction
Authors: Krive I. V.
Published: July 2002

The heat transport through one-dimensional quantum wire is considered in the frameworks of the inhomogeneous Tomonaga–Luttinger liquid model. It is shown that even for perfect (impurity free) wire thermal transport is suppressed due to multiple scattering of plasmons on the boundaries that connect quantum wire to the leads of noninteracting electrons. In the presence of impurity inside the Luttinger liquid constriction resonant enhancement of thermal conductivity at certain conditions is predicted.
Particle response and turbulence modification in isotropic turbulence
Authors: Squires Kyle D. , Eaton John K.
Published: July 2002

The effect of turbulence on particle concentration fields and the modification of turbulence by particles has been investigated using direct numerical simulations of isotropic turbulence. The particle motion was computed using Stokes’ law of resistance and it was also assumed the particle volume fraction was negligible. For simulations in which the particles do not modify the turbulence field it was found that light particles collect preferentially in regions of low vorticity and high strain rate. For increased mass loading the particle field attenuated an increasing fraction of the turbulence energy. Examination of the spatial energy spectra showed that the fraction of turbulence kinetic energy in the high wave numbers was increased relative to the energy in the low wave numbers for increasing values of the mass loading. It was also found that the turbulence field was modified differently by light particles than by heavy particles because of the preferential collection of the light particles in low-vorticity, high-strain-rate regions. Correlation coefficients between the second invariant of the deformation tensor and pressure showed little sensitivity to increased loading while correlations between enstrophy and pressure were decreased more by the light particles than by the heavy particles for increased mass loading.
Scaling and multifractal properties of mixing in chaotic flows
Authors: Muzzio F. J. , Meneveau C. , Swanson P. D. , Ottino J. M.
Published: July 2002

Mixing (stretching and folding of fluid elements) in chaotic flows is an iterative process generating presumably self-similar distributions of stretching and striation thickness. This hypothesis is investigated using scaling and multifractal techniques for two prototypical time-periodic chaotic flows: one containing no detectable islands (egg-beater flow), the other involving sizable islands as well as no-slip boundaries (flow between eccentric cylinders). The simplest picture arises in the egg-beater flow. Stretching is well described by multifractal scaling if the very high tail of the distribution of stretchings is neglected. Different methods for obtaining the spectrum of fractal dimensions f(α) agree reasonably well, producing a time-independent self-similar distribution. On the other hand, in the flow between eccentric cylinders, the negative moments do not scale, and the spectrum f(α) is time-dependent (and therefore, it is not self-similar). Due to the extremely wide range of values of stretching, a very large number of points needs to be considered in order to characterize mixing in chaotic systems using a multifractal formalism; this suggests that more work is needed in order to understand finite-size effects and how asymptotic states are reached. However, for cases where multifractal scaling applies, it is possible to relate coarse-grained variables (e.g., intermaterial area density) to microscopic features of the flow (e.g., finite-time Lyapunov exponents).
An analysis of subgrid-scale interactions in numerically simulated isotropic turbulence
Authors: Domaradzki J. Andrzej , Liu Wei , Brachet Marc E.
Published: July 2002

Using a velocity field obtained in a direct numerical simulation of isotropic turbulence at moderate Reynolds number the subgrid-scale energy transfer in the spectral and the physical space representation is analyzed. The subgrid-scale transfer is found to be composed of a forward and an inverse transfer component, both being significant in dynamics of resolved scales. Energy exchanges between the resolved and unresolved scales from the vicinity of the cutoff wave number dominate the subgrid-scale processes and the energetics of the resolved scales are unaffected by the modes with wave numbers greater than twice the cutoff wave number. Correlations between the subgrid-scale transfer and the large-scale properties of the velocity field are investigated.
Ballooning instabilities in tokamaks with sheared toroidal flows
Authors: Waelbroeck F. L. , Chen L.
Published: July 2002

The stability of ballooning modes in the presence of sheared toroidal flows is investigated. The eigenmodes are shown to be related by a Fourier transformation to the nonexponentially growing Floquet solutions found by Cooper [Plasma Phys. Controlled Fusion 30, 1805 (1988)]. It is further shown that the problem cannot be reduced further than to a two-dimensional partial differential equation. Next, the generalized ballooning equation is solved analytically for a circular tokamak equilibrium with sonic flows, but with a small rotation shear compared to the sound speed. With this ordering, the centrifugal forces are comparable to the pressure gradient forces driving the instability, but coupling of the mode with the sound wave is avoided. A new stability criterion is derived that explicitly demonstrates that flow shear is stabilizing at constant centrifugal force gradient.
Theory-based transport simulations of TFTR L-mode temperature profiles
Authors: Bateman Glenn
Published: July 2002

The temperature profiles from a selection of Tokamak Fusion Test Reactor (TFTR) L-mode discharges [17th European Conference on Controlled Fusion and Plasma Heating, Amsterdam, 1990 (EPS, Petit-Lancy, Switzerland, 1990, p. 114)] are simulated with the 1 (1)/(2) -D baldur transport code [Comput. Phys. Commun. 49, 275 (1988)] using a combination of theoretically derived transport models, called the Multi-Mode Model [Comments Plasma Phys. Controlled Fusion 11, 165 (1988)]. The present version of the Multi-Mode Model consists of effective thermal diffusivities resulting from trapped electron modes and ion temperature gradient (ηi) modes, which dominate in the core of the plasma, together with resistive ballooning modes, which dominate in the periphery. Within the context of this transport model and the TFTR simulations reported here, the scaling of confinement with heating power comes from the temperature dependence of the ηi and trapped electron modes, while the scaling with current comes mostly from resistive ballooning modes.
Viscous damping of the oscillations of a rotating simple drop
Authors: Lee C. P. , Lyell M. J. , Wang T. G.
Published: July 2002

The viscous damping of the capillary oscillations of a simple drop in rotation in air is evaluated in the small viscosity limit.
Statistical theory for compressible turbulent shear flows, with the application to subgrid modeling
Authors: Yoshizawa Akira
Published: July 2002

A statistical theory for compressible turbulent shear flows subject to buoyancy effects is developed. Important correlation functions in compressible shear flows are calculated with the aid of a multiscale direct-interaction approximation. They are expressed in the gradient-diffusion form similar to the eddy-viscosity representation for the Reynolds stress in incompressible flows. The results obtained are applicable to subgrid modeling, and a Smagorinsky-type model in compressible flows is constructed.