bluuebunny/crossref_metadata_2025 huggingface dataset:
Modification of the Dillmann–Meier theory of homogeneous nucleation
Authors: Ford I. J. , Laaksonen A. , Kulmala M.
Published: July 2002

We describe a modification of the Dillmann–Meier theory of homogeneous nucleation which removes an inconsistency within the original model. The predicted nucleation rates of the corrected model are much larger than those of the original model. We also point out that predictions of the model are independent of one of the input parameters.
Efficient Fock matrix diagonalization by a Krylov-space method
Authors: Pollard W. Thomas , Friesner Richard A.
Published: July 2002

The solution of the Hartree–Fock equations involves the iterative construction of the Fock matrix based on approximate molecular orbitals and the diagonalization of that Fock matrix to obtain new approximations to those orbitals. A significant portion of this work is unnecessary, however, because the occupied molecular orbitals, which are required to construct the Fock matrix, represent a small fraction of the total number of orbitals that are obtained in the diagonalization, and furthermore, typically change little in each iteration. In this paper we introduce a new method which significantly accelerates diagonalization of the Fock matrix by avoiding the unnecessary calculation of the virtual orbitals. Using the occupied orbitals from the previous iteration as an initial guess, accurate updated orbitals are obtained through a combination of diagonalization in the subspace spanned by the occupied orbitals and the mixing of virtual orbital character into the occupied orbitals using a single-vector Lanczos algorithm. Calculations are presented which demonstrate up to 15-fold acceleration of the Fock matrix diagonalizations in a typical problem of 430 orbitals.
Inhomogeneous broadening of optical spectra in mixed crystals: Basic model and its application to Sm2+ in SrFClxBr1−x
Authors: Jaaniso R. , Hagemann H. , Bill H.
Published: July 2002

We have developed a model to describe the inhomogeneous broadening of optical spectra in the substitutionally disordered crystals. The comparison with the experimental f–f fluorescence spectra of SrFClxBr1−x:Sm2+ (0≤x≤1) allowed to establish, in a very detailed manner, the relationship between the inhomogeneous spectral distribution and the crystal structure around the Sm2+ impurity.
A structure adapted multipole method for electrostatic interactions in protein dynamics
Authors: Niedermeier Christoph , Tavan Paul
Published: July 2002

We present an algorithm for rapid approximate evaluation of electrostatic interactions in molecular dynamics simulations of proteins. Traditional algorithms require computational work of the order O(N2) for a system of N particles. Truncation methods which try to avoid that effort entail untolerably large errors in forces, energies and other observables. Hierarchical multipole expansion algorithms, which can account for the electrostatics to numerical accuracy, scale with O(N log N) or even with O(N) if they become augmented by a sophisticated scheme for summing up forces. To further reduce the computational effort we propose an algorithm that also uses a hierarchical multipole scheme but considers only the first two multipole moments (i.e., charges and dipoles). Our strategy is based on the consideration that numerical accuracy may not be necessary to reproduce protein dynamics with sufficient correctness. As opposed to previous methods, our scheme for hierarchical decomposition is adjusted to structural and dynamical features of the particular protein considered rather than chosen rigidly as a cubic grid. As compared to truncation methods we manage to reduce errors in the computation of electrostatic forces by a factor of 10 with only marginal additional effort.
Density functional based studies of transition states and barriers for hydrogen exchange and abstraction reactions
Authors: Porezag Dirk , Pederson Mark R.
Published: July 2002

The overbinding that is inherent in existing local approximations to the density functional formalism has limited the usefulness of the local density approximation (LDA) for describing phenomena that are mediated by reaction barriers. Since the generalized gradient approximation (GGA) significantly decreases the overbinding, prospects for density functional based reaction dynamics are promising. Using both LDA and GGA functionals, we determined the transition state properties for four different reactions; H2+H→H+H2, CH4+H→CH3+H2,H+CH4→CH4 +H, and CH4+CH3 →CH3+CH4. Although we find that GGA still underestimates reaction barriers, our results show that this functional leads to significant improvements of the calculated reaction barriers and energetics.
Analytical first and second energy derivatives of the generalized conductorlike screening model for free energy of solvation
Authors: Truong Thanh N. , Stefanovich Eugene V.
Published: July 2002

We present analytical expressions for the first and second energy derivatives of our recently proposed generalized conductorlike screening model (GCOSMO) for free energy of solvation of solute in an arbitrary shape cavity. An application to study hydration effects on structure and stability of glycine zwitterion in aqueous solution is also presented. These calculations were carried out at the Hartree–Fock, second-order Mo/ller–Plesset perturbation theory and different nonlocal density functional theory levels using the 6-31G(d,p) basis set. We found that our quantum mechanical GCOSMO solvation model costs from 10% to 40% extra cpu time per one Berny optimization step compared to the gas-phase calculations for different levels of theory. For the glycine system, the optimized zwitterionic structure in aqueous solution agrees very well with experimental crystal structure and the enthalpy change for transfering glycine from the gas phase to the aqueous solution is also in excellent agreement with experimental data. The ‘‘single point’’ approach, which has been used in the past, yields erroneous results. The efficiency and accuracy of our GCOSMO solvation model indicate that this model can be a practical tool for studying structure and activity of moderately large biological systems in solutions.
Order–disorder transition and ordering kinetics in binary diblock copolymer mixtures of styrene and isoprene
Authors: Floudas G. , Vlassopoulos D. , Pitsikalis M. , Hadjichristidis N. , Stamm M.
Published: July 2002

The order–disorder transition (ODT) and the ordering kinetics have been studied in two symmetric diblock copolymers of styrene and isoprene and their binary mixtures with rheology. The binary mixtures formed a single microdomain (with lamellar morphology) composed of short and long chains as confirmed by small angle x-ray scattering (SAXS). The order–disorder transition temperature (TODT), obtained from the discontinuity in the storage modulus G′, varies linearly with the number average degree of polymerization. For shallow quenches, the ordering kinetics proceed via nucleation and growth. The characteristic time of this process scales with N̄−1/3, where N̄ is the Ginzburg parameter, in agreement with the theoretical predictions [G. H. Fredrickson and K. Binder, J. Chem. Phys. 91, 7265 (1989)]. Our results indicate that the ordering kinetics of symmetric diblock copolymers near the ODT are fluctuation controlled.
Infrared spectroscopy of OH stretching vibrations of hydrogen-bonded tropolone-(H2O)n (n=1–3) and tropolone-(CH3OH)n (n=1 and 2) clusters
Authors: Mitsuzuka Akira , Fujii Asuka , Ebata Takayuki , Mikami Naohiko
Published: July 2002

Infrared spectra of jet-cooled tropolone-(H2O)n (n=1–3) and tropolone-(CH3OH)n (n=1 and 2) clusters were observed in the OH stretching region by using infrared-ultraviolet double resonance techniques. Size separated electronic spectra of these clusters were also observed with hole-burning spectroscopy in which the infrared laser was used as hole light. Both the infrared and hole-burning spectra of the tropolone-methanol clusters were found to be quite similar to those of the corresponding tropolone-water clusters, indicating that a similar structure is expected for both the clusters. Structure of the n=1 and 2 clusters of tropolone-water and -methanol is discussed. The infrared (IR) spectra suggest that the intramolecular hydrogen bond of tropolone OH is not destroyed in tropolone-(H2O)n (n≤2) and -CH3OH, while the intermolecular hydrogen bond dominates in tropolone-(H2O)3 and -(CH3OH)2. The transformation of the intramolecular to intermolecular hydrogen bond induced by the solvation is also discussed.
Intensity perturbations from vibrational resonance coupling in isotropic Raman spectra of neat liquids and solutions
Authors: Daniel Douglas C. , McHale Jeanne L.
Published: July 2002

The influence of vibrational resonance coupling (VRC) on the intensity of isotropic Raman spectra is investigated theoretically for neat liquids and binary mixtures. Experimental studies of the effect are presented for overlapping Raman bands in binary liquid solutions (DMSO/CHCl3 and DMSO/CDCl3) perturbed by hydrogen bonding between the two components. The vibrational resonance coupling perturbation leads to a nonlinear density dependence of the intensity, and is strongly dependent on the presence of short range order. An estimate of the typical magnitude of the intensity perturbation due to VRC is made by assuming that the coupling derives from the interaction of vibrational transition dipoles. It is concluded that the intensity perturbations are ordinarily quite small except in the case of low frequency vibrations having large transition dipoles, or in the case of very strong orientational correlations. For VRC between vibrations localized on molecules of different species, the role of the frequency matching is explored theoretically and experimentally, and it is found that the frequency mismatch must to be small compared to thermal energy kT for appreciable intensity perturbations to be observed.
Application of free energy expansions to mesoscopic dynamics of copolymer melts using a Gaussian chain molecular model
Authors: Maurits N. M. , Fraaije J. G. E. M.
Published: July 2002

The present paper deals with some mathematical aspects of generalized time-dependent Ginzburg–Landau theories for the numerical simulation of mesoscale phase separation kinetics of copolymer melts. We shortly discuss the underlying theory and introduce an expansion of the external potential, to be used in the dynamics algorithm, which is similar to free-energy expansions. This expansion is valid for both compressible and incompressible multicomponent copolymer melts using a Gaussian chain model. The expansion is similar to the well-known random phase approximation (RPA) but differs in some important aspects. Also, the application of RPA like free energy expansions to dynamics is new. Our derivation leads to simple expressions for the vertex coefficients, which enables us to numerically calculate their full wave vector dependence, without assuming an ordered morphology. We find that our fourth-order vertex is negative for some wave vectors which has important consequences for the simulation of mesoscopic dynamics. We propose a fitting procedure for the vertex coefficients to overcome the computationally expensive calculation of the linear and bilinear expansion terms in the expansion. This procedure provides analytically derived parameters for a gradient free energy expansion, which allows for a whole new class of phase-separation models to be defined.
A first-principles simulation of the semiconductor/water interface
Authors: Ursenbach C. P. , Calhoun August , Voth Gregory A.
Published: July 2002

The overall electronic structure of a water/semiconductor interface is studied for the first time by ab initio molecular dynamics for realistic solvent configurations. The present study expands on our earlier work [J. Chem. Phys. 103, 7569 (1996)] by including a complete ab initio description of the surface and the solvent. Several statistically independent solvent configurations are generated via classical molecular dynamics as a starting point for a realistic sampling of the true solvent electronic structure via ab initio dynamics. A significant degree of mixing is observed between semiconductor electronic surface states and the bulk water orbitals. A new method is also given to facilitate the use of gradient corrected density functionals with Vanderbilt pseudopotentials.
Predissociation of H2 in the Rydberg states
Authors: Gao Hong
Published: July 2002

The predissociation of H2 in the 3pπD1Πu+, 4pσB″1Σu+, and 4pπD′1Πu+ states by the lower-lying 3pσB1Σu+ and 2pπC1Πu+ states is studied using the multichannel quantum defect theory, combined with the frame transformation and the noniterative eigenchannel R-matrix method. The predissociation line positions and widths are evaluated for the 3pπD1Πu+ (v=3), 4pσB″1Σu+ (v=1,2), and 4pπD′1Πu+ (v=1) levels. The complicated rovibrational coupling responsible for the predissociation is accounted for in a unified way.
Depletion-force kinetics in confined colloidal mixtures
Authors: Hobbie E. K. , Holter M. J.
Published: July 2002

Depletion-force kinetics in confined nearly hard-sphere mixtures are interpreted within the context of reversible two-dimensional aggregation. The cluster-size distribution function of the minority component exhibits an exponential decay in the cluster size, with a time-dependent decay constant that relaxes toward the steady-state value suggested by simulations of two-dimensional evaporation–condensation phenomena with Brownian (random-walk) trajectories. This similarity is attributed to a greatly reduced mobility of the aggregates with respect to unbound large spheres.
Nonlinear response in ionic solvation: A theoretical investigation
Authors: Hyun Jin-Kee , Ichiye Toshiko
Published: July 2002

The Born model of ionic solvation assumes that the solvent is a continuum which has a linear response and does not explicitly include nonlinear effects such as dielectric saturation and electrostriction. Nonetheless, the Born model has been surprisingly successful in estimating the solvation energies of ions in solution. Recently, we developed a simple quasicontinuum theory of ionic solvation, referred to here as the HBI model [J.-K. Hyun, C. S. Babu, and T. Ichiye, J. Phys. Chem. 99, 5187 (1995)]. The HBI model exhibits dielectric saturation, which is demonstrated in the expressions for the average orientation of the solvent molecules and the solvation energy, but does not incorporate the effects of electrostriction. In this article, new models of ionic solvation are developed that exhibit not only dielectric saturation but also electrostriction. Expressions for the radial and orientational distribution functions of the solvent molecules, and the solvation energy are derived and compared with those from the HBI and Born models to examine the characteristics of different models of ionic solvation. Comparisons are also made to molecular dynamics simulations of ions in aqueous solution, where the complex molecular effects make separation of different nonlinear effects difficult. In particular, the systematic discrepancies in the solvation energies from the different models are used to understand how different nonlinear effects contribute to the response when water is the solvent. For small monovalent ions, the success of the Born model is attributed not to the lack of dielectric saturation or electrostriction, but rather to the cancellation of one effect by the other.
Hydrophobic effects: A computer simulation study of the temperature influence in dilute O2 aqueous solutions
Authors: Fois Ettore , Gamba Aldo , Redaelli Claudio
Published: July 2002

We present a computer simulation study of the temperature dependence of the structural and dynamical properties of dilute O2 aqueous solutions. A clathrate-like solvation shell, in line with other apolar gas solutions, emerged from the present simulations. The average number of water molecules in the first hydration shell decreases with temperature, and, in the investigated temperature range (291–348 K), a net transfer of one water molecule from the hydration shell to the bulk has been detected. We have found oscillations of both water density and electrostatic charges in the neighborhood of the apolar solute, which is surrounded by shells of water at different density, and with water molecules oriented in such a way as to form shells with alternating net electrostatic charges. In the O2, first hydration shell water–water interactions are stronger and water diffusional and rotational dynamics slower than in the bulk. A hydrogen bond’s mean lifetime is affected by the apolar solute as well, being shorter in the first hydration shell. Differences between shell and bulk water properties are smoothed by increasing temperature. Suggestions for the molecular mechanism relevant to the more general problems of the hydrophobic effects are deduced from the simulations. A possible microscopic explanation for the lowering of solubility of oxygen in water with temperature is given.
Coupled-cluster methods with internal and semi-internal triply and quadruply excited clusters: CCSDt and CCSDtq approaches
Authors: Piecuch Piotr , Kucharski Stanisław A. , Bartlett Rodney J.
Published: July 2002

Extension of the closed-shell coupled-cluster (CC) theory to studies of bond breaking and general quasidegenerate situations requires the inclusion of the connected triply and quadruply excited clusters, T3 and T4, respectively. Since the complete inclusion of these clusters is expensive, we explore the possibility of incorporating dominant T3 and T4 contributions by limiting them to active orbitals. We restrict T3 and T4 clusters to internal or internal and semi-internal components using arguments originating from the multireference formalism. A hierarchy of approximations to standard CCSDT (CC singles, doubles, and triples) and CCSDTQ (CC singles, doubles, triples, and quadruples) schemes, designated as the CCSDt and CCSDtq approaches, is proposed and tested using the H2O and HF molecules at displaced nuclear geometries and C2 at the equilibrium geometry. It is demonstrated that the CCSDt and CCSDtq methods provide an excellent description of bond breaking and nondynamic correlation effects. Unlike perturbative CCSDT and CCSDTQ approaches, the CCSDt and CCSDtq approaches do not fail at large internuclear separations, in spite of using the restricted Hartree–Fock reference. All CCSDt and CCSDtq approaches are essentially n6 procedures and yet they are shown to provide reliable information about T3 and T4 components, whose standard evaluation requires expensive n8 and n10 steps.
Mobilities of NO+ drifting in helium: A molecular dynamics study
Authors: Baranowski R. , Thachuk M.
Published: July 2002

A new molecular dynamics (MD) method is introduced, and used to study NO+ ions drifting in helium under the influence of a uniform electric field. Mobilities, average values of squared velocities, and self-diffusion coefficients parallel and perpendicular to the electric field are reported for two recent ab initio potential surfaces: a coupled cluster singles–doubles with perturbative treatment of triple excitations [CCSD(T)] surface [S. K. Pogrebnya et al., Int. J. Mass Spectrom. Ion Processes 149/150, 207 (1995)] and a MP4SDTQ/6-311+G(2df,p) surface [L. A. Viehland et al., Chem. Phys. 211, 1 (1996)]. Average values of angular momentum and alignment parameters are also reported and compared. In all cases, no significant differences were found in the calculated values for the two different potential surfaces. Finally, mobility values are compared with experimental measurements [J. A. de Gouw et al., J. Chem. Phys. 105, 10398 (1996)] and good agreement is obtained for both potential surfaces.
Potential energy surfaces for the CN(X 2Σ+,A 2Π)Ar system and inelastic scattering within the A state
Authors: Alexander Millard H. , Yang Xin , Dagdigian Paul J. , Berning Andreas , Werner Hans-Joachim
Published: July 2002

Adiabatic and diabatic potential energy surfaces (PES’s) for the interaction of Ar(1S0) with the CN molecule in its ground (X 2Σ+) and first excited (A 2Π) electronic states were determined using multireference configuration-interaction calculations. The three electronically adiabatic potential energy surfaces (PES’s 2A′ and 1A″) are transformed to three diabatic PES’s plus one additional PES which describes the coupling between the two diabatic PES’s of A′ symmetry which correlate asymptotically with the X 2Σ+ and A 2Π states of CN. The appropriate diabatic PES’s are then used in the study of rotationally inelastic scattering of CN(A 2Π) in collisions with Ar. Experimental state-to-state rate constants were measured with an optical–optical double resonance technique. The CN radical was prepared by 193 nm photolysis of BrCN diluted in slowly flowing argon at a total pressure of ∼0.5 Torr. Specific fine-structure Λ-doublet levels of CN(A 2Π,v=3) were prepared by excitation with a pulsed dye laser on various rotational lines in the A 2Π–X 2Σ+(3,0) band, and collisionally populated levels were probed after a short delay by laser fluorescence excitation in the B 2Σ+–A 2Π(3,3) band. State-to-state rate constants, both relative and absolute, were determined for several rotational levels with J=6.5 and 7.5. The final state distributions displayed an even–odd alternation as a function of the final angular momentum J for scattering into certain fine-structure Λ-doublet manifolds. The measured state-to-state rate constants agreed quite well with rate constants derived from quantum scattering calculations with the ab initio CN(A 2Π)–Ar PES’s. The even–odd oscillation in final state populations is ascribed to the near homonuclear character of the PES’s.
The electronic spectrum of germanium methylidyne (GeCH), the prototypical organogermanium compound
Authors: Smith Tony C. , Li Haiyang , Clouthier Dennis J. , Kingston Christopher T. , Merer Anthony J.
Published: July 2002

The Ã 2Σ+–X̃ 2Πi electronic transition of jet-cooled germanium methylidyne has been recorded in the 730–555 nm region by laser-induced fluorescence techniques. The radical was produced in a pulsed electric discharge using tetramethylgermane as the precursor. The band system consists of perpendicular and vibronically induced parallel bands with upper state frequencies of ν2′=638/497 cm−1 and ν3′=990/952 cm−1 for GeCH/GeCD. The Σ+2–2Π3/2 components of the 000 bands of GeCH and GeCD and the 201 band of GeCH have been recorded with sufficient resolution to assign the transitions of the Ge70, Ge72, Ge74, and Ge76 isotopomers. The rotational constants of the various hydrogen and germanium isotopomers were used to derive the following molecular structures: r0″(Ge–C)=1.775 79(21) Å, r0″(C–H)=1.0673(12) Å, r0′(Ge–C)=1.673 66(8) Å, and r0′(C–H)=1.0589(5) Å. The ground state germanium–carbon bond length is typical for a Ge=C double bond, but in the excited state it corresponds to a triple bond. This is a consequence of promotion of an electron from a nonbonding σ orbital to a π bonding orbital to form the …σ1π4(2Σ+) excited state. In this study we provide the first experimental value for the length of the germanium–carbon triple bond.
Ab initio computation of forces and molecular spectroscopic constants using plane waves based auxiliary field Monte Carlo with application to N2
Authors: Baer Roi
Published: July 2002

Correlated sampling within the shifted contour auxiliary field Monte Carlo method, implemented using plane waves and pseudopotentials, allows computation of electronic forces on nuclei, potential energy differences, geometric and vibrotational spectroscopic constants. This is exemplified on the N2 molecule, where it is demonstrated that it is possible to accurately compute forces, dissociation energies, bond length parameters, and harmonic frequencies.
Exact solutions of general relativity derived from 5-D ‘‘black hole’’ solutions of Kaluza–Klein theory
Authors: Liu Hongya , Wesson P. S.
Published: July 2002

It is shown that the 5-D ‘‘black hole’’ solutions of Kaluza–Klein theory can be reduced to new exact solutions of 4-D general relativity in which the matter is a spherically symmetric anisotropic fluid of radiation. This approach also provides new insights into the parameters and solutions of Kaluza–Klein theory.
Euler hierarchies and universal equations
Authors: Fairlie D. B. , Govaerts J.
Published: July 2002

Finite Euler hierarchies of field theory Lagrangians leading to universal equations of motion for new types of string and membrane theories and for classical topological field theories are constructed. The analysis uses two main ingredients. On the one hand, there exists a generic finite Euler hierarchy for one field leading to a universal equation which generalizes the Plebanski equation of self-dual four-dimensional gravity. On the other hand, specific maps are introduced between field theories which provide a ‘‘triality’’ between certain classes of arbitrary field theories, classical topological field theories and generalized string and membrane theories. The universal equations, which derive from an infinity of inequivalent Lagrangians, are generalizations of certain reductions of the Plebanski and KdV equations, and could possibly define new integrable systems, thus in particular integrable membrane theories. Some classes of solutions are constructed in the general case. The general solution to some of the universal equations is given in the simplest cases.
Maximum entropy in the finite Stieltjes and Hamburger moment problem
Authors: Frontini M. , Tagliani A.
Published: July 2002

A necessary and sufficient condition for the existence of the maximum entropy (ME) function, defined in an infinite or semiinfinite interval, is provided. The conclusions reached show that, except in a few particular cases, the necessary and sufficient conditions for the existence of maximum entropy function are identical to the conditions for the solution of the moment problem when the first M+1 moments are assigned. Even if the conclusions reached are very similar to the Hausdorff case, the specificity of the Hamburger and Stieltjes cases demands a different handling. A sufficient condition for the entropy convergence of the resulting sequence of maximum entropy estimators to the entropy of the recovering function is also provided.
q-epsilon tensor for quantum and braided spaces
Authors: Majid S.
Published: July 2002

The machinery of braided geometry introduced previously is used now to construct the ε ‘‘totally antisymmetric tensor’’ on a general braided vector space determined by R-matrices. This includes natural q-Euclidean and q-Minkowski spaces. The formalism is completely covariant under the corresponding quantum group such as SOq(4) or SOq(1,3). The Hodge * operator and differentials are also constructed in this approach.
A family of cosmological solutions in higher-dimensional Einstein gravity
Authors: Coley Alan A. , McManus Des J.
Published: July 2002

A two-parameter family of solutions to Einstein’s vacuum field equations in six dimensions is presented. The solutions are a natural extension of a one-parameter family of five-dimensional vacuum solutions found by Ponce de Leon [Gen. Relativ. Gravit. 20, 539 (1988)] and are closely related to the generalized Kasner metrics. The solution is generalized to 4+n dimensions and is briefly discussed.
Deformations of density functions in molecular quantum chemistry
Authors: Bokanowski O. , Grébert B.
Published: July 2002

We generalize the use of the local scaling transformation developed by E. S. Kryachko and E. V. Ludeña to molecules in order to deform density functions. The connection with the Jacobian problem is clearly made, and we solve that problem using a formalism introduced by J. Moser. As a consequence, we can control the density information contained in a wave function, in some sense, at the same time as we keep particular regularity and behavior assumptions in the wave function (in particular concerning the symmetries of the wave function). The principal aim of the paper is to develop a correct mathematical background for further utilization in connection with density functional theory. Theoretical implications and numerical aspects are also discussed.
The twisting connection of space–time
Authors: Monte E. M. , Maia M. D.
Published: July 2002

The compatibility between general relativity and the property that space–times are embedded manifolds is examined. It is shown that the signature of the embedding space is uniquely determined when its dimension is the smallest possible. In this case, the twisting vector transforms as a Yang–Mills potential under the signature preserving symmetry group, whose curvature is described by Ricci’s equation. The use of complex embeddings is also discussed in association with signature and topology changes.
Generalization of the Bremmer coupling series
Authors: de Hoop Maarten V.
Published: July 2002

An operator formalism is developed to expand the acoustic wave field in a multi-dimensionally smoothly varying medium, generated by a source localized in space and time, into a sum of constituents each of which can be interpreted as a wave that has traveled up and down with respect to a direction of preference a definite number of times. This expansion is a generalization of the Bremmer coupling series. The condition of smoothness of the medium relates to the width of the signature of the source in the configuration. Both the existence and the convergence (in the weak sense) of the expansion are discussed. The operator calculus involved leads to a natural generalization of the concept of slowness surface to multi-dimensionally smoothly varying media. The operator associated with the corresponding generalized vertical slowness induces the full one-way wave operator in the type of media under consideration. In addition, a wavefield decomposition operator as well as an interaction operator that couples the decomposed constituents, are derived.
Bound states and scattering in quantum waveguides coupled laterally through a boundary window
Authors: Exner P. , Šeba P. , Tater M. , Vaněk D.
Published: July 2002

We consider a pair of parallel straight quantum waveguides coupled laterally through a window of a width l in the common boundary. We show that such a system has at least one bound state for any l≳0. We find the corresponding eigenvalues and eigenfunctions numerically using the mode-matching method, and discuss their behavior in several situations. We also discuss the scattering problem in this setup, in particular, the turbulent behavior of the probability flow associated with resonances. The level and phase-shift spacing statistics shows that in distinction to closed pseudo-integrable billiards, the present system is essentially nonchaotic. Finally, we illustrate time evolution of wave packets in the present model.
Integrable semi-discretization of the coupled modified KdV equations
Authors: Tsuchida Takayuki , Ujino Hideaki , Wadati Miki
Published: July 2002

The discrete version of the inverse scattering method proposed by Ablowitz and Ladik is extended to solve multi-component systems. The extension enables one to solve the initial value problem, which proves directly the complete integrability of a semi-discrete version of the coupled modified Korteweg–de Vries (KdV) equations and their hierarchy. It also provides a procedure to obtain conservation laws and multi-soliton solutions of the hierarchy.
Bering’s proposal for boundary contribution to the Poisson bracket
Authors: Soloviev Vladimir O.
Published: July 2002

It is shown that the Poisson bracket with boundary terms proposed by Bering can be deduced from the Poisson bracket proposed by the present author if one omits terms free of Euler–Lagrange derivatives (“annihilation principle”). This corresponds to another definition of the formal product of distributions (or, saying it in other words, to another definition of the pairing between 1-forms and 1-vectors in the formal variational calculus). We extend the formula (initially suggested by Bering for the ultralocal case with constant coefficients only) onto the general nonultralocal brackets with coefficients depending on fields and their spatial derivatives. The lack of invariance under changes of dependent variables (field redefinitions) seems to be a drawback of this proposal.
Visualization of the antiferromagnetic insulator–ferromagnetic metal phase transition in manganite Nd0.5Sr0.5MnO3
Authors: Gnatchenko S. L. , Chizhik A. B. , Shklyarevskiı̆ I. O. , Merenkov D. N. , Kamenev V. I. , Pashkevich Yu. G. , Kamenev K. V. , Balakrishnan G. , McK. Paul D.
Published: July 2002

It is found that the first-order phase transition from a nonmetallic antiferromagnetic (AFM) state to a metallic ferromagnetic (FM) state in manganite Nd0.5Sr0.5MnO3 is accompanied by a significant change in the reflectivity of the visible light. This effect is used for visualizing the AFM–FM phase transition in Nd0.5Sr0.5MnO3. The coexistence of AFM and FM phases was observed visually during spontaneous and field-induced AFM–FM transitions. In both cases, the transition occurs through nucleation and expansion of domains of the phase that is favorable from the energy point of view. However, the periodic domain structure of the intermediate magnetic state was not formed during the phase transition. A striped domain structure was formed in the AFM state while the FM phase had a uniform structure.
The effect of viscosity stratification on the stability of a free surface flow at low Reynolds number
Authors: Loewenherz D. S. , Lawrence C. J.
Published: July 2002

A zero Reynolds number approximation to the Orr–Sommerfeld equation is used to assess the effects that viscosity stratification has on the stability of a very viscous flow on an incline when surface tension is negligible. Results indicate that for a two-layer system with uniform density, the flow is always unstable when the viscosity of the upper layer is greater than that of the lower layer, regardless of the thickness of the upper layer. The wavenumber of the fastest growing mode is on the order of the inverse of the thickness of the upper layer, implying that the instability is manifested in waves having finite wavelength, whereas previous research on this topic has focused on a long wavelength approximation. It is further shown that neutral stability is independent of the angle of inclination of the underlying slope, although the growth rate of any instability is not. The results suggest that the transverse surficial ridges, which commonly occur on the surfaces of rock glacier forms, may be the product of a flow instability arising from the differing viscosities of the layers that comprise such features.
Pouring flows with separation
Authors: Vanden-Broeck Jean-Marc , Keller Joseph B.
Published: July 2002

The two-dimensional free surface potential flow of a liquid poured from a container is calculated when the liquid runs along the underside of the spout and then separates or detaches. The point of separation S can be anywhere on a straight underside, but on a curved underside it must lie between the lip and a certain point S*. When S=S*, the free surface has finite curvature at S, so the Brillouin–Villat condition is satisfied. This suggests that S* is the actual point of detachment. The calculation is done analytically for a straight underside and numerically for a curved one, with gravity omitted in both cases.
Spatiotemporal intermittency and instability of a forced turbulence
Authors: Kida Shigeo , Ohkitani Koji
Published: July 2002

The time development of an infinitesimal disturbance and the relation to the spatiotemporal intermittency of a developed turbulence is studied by solving numerically the Navier–Stokes equation and its linearized form. A high-symmetric flow is integrated by the use of the spectral method with 3403 effective modes to realize a developed turbulent field with Reynolds number of about 186 which is in statistically stationary state. Energy transfer in the wave-number space is not uniform in time but quiescent and active periods of energy transfer and of the energy dissipation rate occur repeatedly in a large-scale eddy-turnover time. Vorticity tends to be concentrated in long thin tubelike regions and the energy dissipation field forms double peaks around a tube, which resembles a flow around a nonaxisymmetric Burgers vortex. Both the energy and the enstrophy of the disturbance field increase exponentially in time with growth rate comparable to the reciprocal of the time scale of the peak wave number of the energy dissipation spectrum. The energy spectrum of the disturbance field shows an equipartition of energy at small wave numbers including the inertial range. The disturbance vorticity field has a dipole structure at the position of a vorticity concentrated tube. The direction of this dipole structure is orthogonal to that of the energy dissipation field.
Numerical simulation of rarefied flow through a slit. Part I: Direct simulation Monte Carlo results
Authors: Wadsworth D. C. , Erwin D. A.
Published: July 2002

The pressure-driven flow of a rarefied monatomic gas through a two-dimensional slit is simulated using the direct simulation Monte Carlo technique. Of particular interest is the change in flow field structure as pressure ratio and Knudsen number are varied. Comparisons are made to quantify the limits of validity of free-molecular theory and approximate, nearly free-molecular iterative methods. Also addressed is the sensitivity of the numerical solutions to grid structure and boundary conditions. The free-molecular theory is found to predict quantitative flow field properties (e.g., centerline velocities or downstream flux profiles) reasonably well for large finite Knudsen number with the error dependent on the pressure ratio. The nearly free-molecular corrections are shown to have limited range of applicability. A previously derived parameter is found to correlate total mass flux well as a function of pressure ratio and Knudsen number over a large portion of the transitional regime.
A stability criterion for energetic particle-Alfvén modes
Authors: Betti R. , Freidberg J. P.
Published: July 2002

A stability criterion is derived for energetic particle-Alfvén modes. The criterion is valid for arbitrary aspect ratio, arbitrary β, noncircular, axisymmetric tori. The plasma is modeled by a magnetohydrodynamic (MHD) core plus a fully kinetic Vlasov species of hot particles. Electron kinetic effects are neglected. In spite of the complexity associated with the analysis of the Vlasov species, a simple but exact stability boundary is derived. The criterion is very fluidlike in nature, suggesting that accurate evaluation in realistic geometries can be accomplished with perhaps only minor modifications to any one of the existing ideal MHD stability codes.
Experimental study of millimeter wave radiation from a rotating electron beam in a rippled magnetic field
Authors: Destler W. W. , Aghamir F. M. , Boyd D. A. , Bekefi G. , Shefer R. E. , Yin Y. Z.
Published: July 2002

The generation of millimeter wave radiation from the interaction of a rotating electron beam (2 MeV, 1 kA, 5 nsec) with an azimuthally periodic wiggler magnetic field has been studied experimentally. Calculations of the effects of the wiggler magnetic field on the single particle electron orbits are presented, together with experimental measurements of the effects of the wiggler field on the electron beam. Narrow-band radiation at power levels in excess of 200 kW has been observed at 88 and 175 GHz for wiggler fields with 6.28 and 3.14 cm periods, respectively. The radiation frequency spectra for various experimental configurations are presented, and results are compared with theoretical expectations.
A general linear growth rate formula for large orbit, annular electron beams
Authors: Lawson W. , Striffler C. D.
Published: July 2002

Microwave radiation can be produced at cyclotron harmonics as a result of the synchronous interaction between a rotating E layer and an em wave. The derivation of a general growth rate expression is presented here for systems involving large orbit, thin, annular, relativistic electron beams. This growth rate formula is written in terms of parameters which contain the required information about the waveguide wall structure. This method extends previous results to systems that have empty waveguide modes which are hybrids. As an example of this latter case, growth rates for coaxial systems with dielectric liners are calculated via this formalism.
Magnetic damping of ℓ=1 diocotron mode
Authors: Roberts G. A. , Rostoker N.
Published: July 2002

Previous work has treated the diocotron mode for a low-energy beam and has found that any dissipative effect will cause this mode to grow. However, a treatment that considers the axial velocity of the beam shows that this mode will damp if the beam is relatively thin and the axial velocity of the beam is comparable with c. This damping is attributed to the perturbed magnetic field and is similar to the stabilization of sausage and kink modes in neutral plasmas. The damping rate for this case is compared with the damping observed in the University of California, Irvine (UCI) modified Betatron. Additionally, beam temperature effects are considered and these results are compared with the diocotron mode damping in the collective focusing ion accelerator.
Resistive instabilities in a flowing plasma: I. Inviscid case
Authors: Einaudi G. , Rubini F.
Published: July 2002

The properties of resistive instabilities developing in equilibrium configurations in which flows are present are studied. The linearized magnetohydrodynamics (MHD) equations are solved numerically, taking into account the effects of resistivity, adopting equilibria with magnetic and velocity fields in the ( y-z) plane, and varying the x direction with two different scale lengths. The results show that the stability properties of the system are strongly influenced by the profile of the velocity field considered. The static tearing mode is recovered in the limit of small amplitude of the velocity with respect to the Alfvén velocity and/or small values of the ratio between magnetic and velocity scale lengths. In the opposite limit we obtain a purely ideal behavior in which the Kelvin–Helmholtz instability appears to be modified by the presence of a shear magnetic field. In between those limits the effect of the velocity is to change the dependence of the growth rate on the relevant parameters and to distort the spatial profile of the perturbations with respect to the static case.
A linear growth rate fluid formulation for large orbit, annular electron layers with finite thickness
Authors: Lawson W. , Striffler C. D.
Published: July 2002

The macroscopic fluid equations are used to study the effect of beam thickness on the linear growth rates of a large orbit gyrotron. The chosen equilibrium models a rotating annular electron beam produced by a magnetic cusp. Two methods are used to calculate the growth rates. The analytic approach solves the problem to first order in the ratio of beam thickness to average beam radius and recovers the zero thickness limit. The numeric approach uses complete orthonormal systems comprised of the homogeneous wave equation solutions to calculate the growth rates for beams with arbitrary thicknesses. It is concluded that thick, tenuous beams in resonant systems must be operated near the cutoff of the electromagnetic wave. The decrease in growth rate as the wavelength decreases is attributed to the loss of synchronism in systems where the beam has a spread in its canonical angular momentum.
Variation in the amplitude of perturbations on the inner surface of an imploding shell during the coasting phase
Authors: Book David L. , Bodner Stephen E.
Published: July 2002

In some inertial confinement fusion target designs a spherical shell collapses on a void or compresses a small amount of gaseous material. There can be a period during which both the outside (driver) pressure and the inside pressure have a negligible effect on the implosion dynamics, and the motion is essentially ballistic. The changes in the aspect ratio occur mainly because of geometrical convergence. For reasonable parameters the inner surface does not begin to decelerate until shortly before convergence is complete. An approximate description of this ‘‘coasting’’ phase has been developed and applied to study the evolution of perturbations on the inner and outer surfaces of the shell in the limit where the fluid is incompressible. The two surfaces are strongly coupled as long as the shell remains thin. When the shell becomes thick compared to the inner radius, the inner and outer surface perturbations decouple. Under these conditions the surface wave action is a good adiabatic invariant, which can be used to estimate the change in the amplitude of a perturbation as a function of the shell inner radius R1. Detailed analysis confirms the adiabatic invariance argument and extends the results. The adiabatic invariant may also be applicable in the case of compressible fluids.
Absolute instability of the Gaussian wake profile
Authors: Hultgren Lennart S. , Aggarwal Arun K.
Published: July 2002

Linear parallel-flow stability theory has been used to investigate the effect of viscosity on the local absolute instability of a family of wake profiles with a Gaussian velocity distribution. The type of local instability, i.e., convective or absolute, is determined by the location of a branch-point singularity with zero group velocity of the complex dispersion relation for the instability waves. The effects of viscosity were found to be weak for values of the wake Reynolds number, based on the center-line velocity defect and the wake half-width, larger than about 400. Absolute instability occurs only for sufficiently large values of the center-line wake defect. The critical value of this parameter increases with decreasing wake Reynolds number, thereby indicating a shrinking region of absolute instability with decreasing wake Reynolds number. If backflow is not allowed, absolute instability does not occur for wake Reynolds numbers smaller than about 38.
Three-dimensional global numerical simulation of ion temperature gradient mode turbulence
Authors: Waltz R. E.
Published: July 2002

Three-dimensional global numerical simulations of ion temperature gradient mode turbulence in a sheared and curved magnetic field corresponding to tokamak geometry is obtained. The key result demonstrates that there is no significant dependence of the bulk heat transport on the edge safety factor q in agreement with analytic theories. The motivation for this question in relation to tokamak experiments is discussed.
The existence of two stages in the transition to three-dimensionality of a cylinder wake
Authors: Williamson C. H. K.
Published: July 2002

The transition to three-dimensionality in the near wake of a circular cylinder involves two successive transitions, each of which corresponds with a discontinuity in the Strouhal–Reynolds number relationship. The first discontinuity [between Reynolds numbers (Re) of 170 to 180] is associated with the inception of vortex loops, and it is hysteretic. The second discontinuity (between Re=230 to 260) corresponds with a change to a finer-scale streamwise vortex structure. At this discontinuity there is no hysteresis, and it is suggested that two modes of vortex shedding alternate in time.
Potentiostatic electroconvection in a layer and in a loop
Authors: Zaltzman T.
Published: July 2002

This paper concerns electroconvectional stability of a conduction state in an electrolyte layer flanked by cation-permselective walls (electrodialysis membranes, electrodes) under potentiostatic conditions. It is shown through a numerical finite difference solution of the linear stability problem that above a certain voltage threshold the basic conduction state becomes electroconvectionally unstable. Marginal stability curves in the voltage/wave number plane are calculated and the dependence of the critical threshold characteristics on the system’s parameters (ionic diffusivities ratio, electroconventional Péclet number) studied. Electroconvectional instability is shown to occur for an arbitrary ionic diffusivities ratio. A model problem of electroconvection in a loop under potentiostatic conditions is solved explicitly for a steady state. It is shown that above a certain voltage threshold, the quiescent conduction in the loop bifurcates into a pair of electroconvectional steady state circulations.