bluuebunny/crossref_metadata_2025 huggingface dataset:
Orbital and shakeup operator renormalizations in electron propagator theory
Authors: Ortiz J. V.
Published: July 2002

Two renormalizations of the electron propagator are introduced. The first adds ground-state correlation corrections to superoperator Hamiltonian matrix elements between shakeup (two-hole–one-particle) operators. The second replaces Hartree–Fock orbitals in the reference determinant with approximate Brueckner orbitals generated with coupled-cluster theory. Formulas for superoperator matrix elements that apply to both bases are presented. Calculations on electron detachment energies of molecules and anions show the predictive superiority of the method that adopts both improvements.
Inverted vibrational distributions from N2 recombination at Ru(001): Evidence for a metastable molecular chemisorption well
Authors: Murphy M. J. , Skelly J. F. , Hodgson A. , Hammer B.
Published: July 2002

We have measured translational and internal state distributions for N2 desorbed from a Ru(001) surface following NH3 cracking at 900 K. Nitrogen is formed with a vibrational population inversion, P(v=1)/P(v=0)=1.4, but a subthermal rotational energy release, Trot(v=0)=630 K. The translational energy distributions show a peak at low energy with a tail extending up to ∼2 eV and a mean energy release of 0.62 eV for N2(v=0) and 0.61 eV for (v=1). The product state distributions indicate a preferential energy release into the N2 stretching coordinate with a relatively weak N2–surface repulsion. Density functional calculations for N2 dissociation on Ru(001) and Cu(111) have been performed to compare the shape of the potentials in the N2 stretching (d) and translational (Z) coordinates. These reveal a sharp curvature of the surface for Ru, the energy release occurring close to the surface over a narrow range of Z. We suggest that this behavior is the result of the presence of a metastable molecular state, bound close to the surface with a short N2 bond, as predicted by Mortensen et al. [J. Catalysis, 169, 85 (1997)]. We contrast the dynamics on Ru with that observed for N recombination on Cu(111) [Murphy et al., J. Chem. Phys. 109, 3619 (1998)], where the potential energy surface shows no evidence for a molecular chemisorption well. Detailed balance arguments predict that N2 dissociation on Ru(001) is highly activated, S(E) increasing by nine orders of magnitude between 0.1 and 2 eV translational energy. The vibrational population inversion implies that vibration promotes dissociation more efficiently than translational excitation, sticking having a vibrational efficacy of 1.3. The predicted S(E) are consistent with reports of a very low sticking probability (S<10−9) on Ru(001) at thermal energies but in disagreement with recent molecular beam adsorption measurements.
Dynamic properties of double-stranded DNA by normal mode analysis
Authors: Matsumoto Atsushi , Go Nobuhiro
Published: July 2002

We have carried out normal mode analysis for four B-DNA molecules with different lengths and sequences. By focusing our attention on motions of the helical axes of the molecules and comparing them with those of the ideal rod obeying vibrational theory, we have shown that DNA molecules behave like isotropic and homogeneous elastic rods. Then, we have calculated dynamic constants representing their rigidities for bending and twisting. In the analysis of their twisting motions, we have shown that sliding and shifting motions between adjacent bases are important for the motions.
Comparison of surface hopping and mean field approaches for model proton transfer reactions
Authors: Fang Jian-Yun , Hammes-Schiffer Sharon
Published: July 2002

This paper presents a comparison of surface hopping and mean field approaches for simulating proton transfer reactions. In these mixed quantum/classical simulations, the transferring proton(s) are treated quantum mechanically, while the remaining nuclei are treated classically. The surface hopping method used for these calculations is the molecular dynamics with quantum transitions (MDQT) method based on Tully’s fewest switches algorithm. In addition, this paper describes a modified MDQT method (denoted MDQT*) that eliminates classically forbidden transitions to promote consistency between the quantum probabilities and the fraction of trajectories in each adiabatic state. The MDQT, MDQT*, mean field, and fully quantum dynamical methods are applied to one-dimensional model single and double proton transfer reactions. Both the MDQT and MDQT* calculations agree remarkably well with the fully quantum dynamical calculations, while the mean field calculations exhibit qualitatively incorrect behavior.
Total energy partitioning within a one-electron formalism: A Hamilton population study of surface–CO interaction in the c(2×2)-CO/ Ni(100) chemisorption system
Authors: Glassey Wingfield V. , Papoian Garegin A. , Hoffmann Roald
Published: July 2002

A scheme for total electronic energy partitioning within the framework of a one-electron theory of the extended Hückel-type is presented, with a view to extending and augmenting the capabilities of existing theoretical electronic structure analysis tools, specifically overlap population analysis. A total electronic energy partitioning is developed first for molecular and subsequently extended materials. In constructing the partitioning, we define molecular orbital Hamilton populations (MOHP’s) for discrete systems, and Crystal Orbital Hamilton Populations (COHP’s) for extended systems. The various energy partitionings and overlap population analyses are exemplified and contrasted for HX (X=F,Cl,Br), ethane, and a [PtH4]2− polymer. The utility of energy partitioning is demonstrated by effecting a COHP partitioning of the surface–CO interaction for the c(2×2)-CO/Ni(100) chemisorption system. Aspects of the surface–CO interaction less amenable to overlap population analysis are addressed, specifically the role of energetically low-lying filled CO orbitals and the relative contributions of surface s, p, and d bands to surface–CO interaction. Hamilton population analysis leads to a CO (4σ, 5σ)–metal forward donation, metal–CO(2π*) backdonation model for the surface–CO interaction. The metal σ contribution to surface–CO bonding is described as sp dominated metal spd hybrid–CO bonding, modifying slightly the metal d–CO σ bonding model proposed by Blyholder. The metal d-2π* backdonation of the Blyholder model remains. The role of the CO(1π) orbitals is also discussed in the context of CO orbital mixing on binding CO to the Ni(100) surface.
A new kinetic model for three-dimensional heterogeneous nucleation
Authors: Liu X. Y.
Published: July 2002

Despite the relevance to most aspects of crystallization, a comprehensive understanding of the kinetics of heterogeneous nucleation has not been well established yet. In this paper, a new kinetic model based on the “steady-state” approach will be put forward to describe both heterogeneous and homogeneous nucleation. As a key point in this model, the effect of foreign particles on both the nucleation barrier, the “chain reaction” process and the transport of structural units is taken into account. Ranging from low to high supersaturations, heterogeneous nucleation plays a more comprehensive role in nucleation than we expected, depending on the size of foreign particles and the interaction and structural matching between foreign particles and the nucleating phase. It follows that genuine homogeneous nucleation may only be possible at very high supersaturations, and can become easier when convection is eliminated. The results have been verified by the CaCO3 nucleation experiments. This general model allows us to describe many unexpected nucleation phenomena for the first time, and will have significant impact on the control of nucleation, understanding of crystallization in microgravity, the measurement of the crystal–fluid interfacial tension, and crystallization in general.
An experimental and theoretical study of the spin–orbit interaction for CO+(A 2Π3/2,1/2, v+=0–41) and O2+(X 2Π3/2,1/2g, v+=0–38)
Authors: Fedorov D. G. , Evans M. , Song Y. , Gordon M. S. , Ng C. Y.
Published: July 2002

Accurate spin–orbit splitting constants (Av+) for the vibrational levels v+=0–41 of CO+(A 2Π3/2,1/2) have been determined in a rotationally resolved pulsed field ionization photoelectron study. A change in slope is observed in the v+ dependence for Av+ at v+≈19–20. This observation is attributed to perturbation of the CO+(A 2Π) potential by the CO+(B 2Σ+) state. Theoretical Av+ values for CO+(A 2Π3/2,1/2, v+=0–41) have also been obtained using a newly developed ab initio computational routine for spin–orbit coupling calculations. The theoretical Av+ predictions computed using this routine are found to be in agreement with the experimental Av+ values for CO+(A 2Π3/2,1/2, v+=0–41). Similar Av+ calculations obtained for O2+(X 2Π3/2,1/2g, v+=0–38) are also in accord with the recent experimental Av+ values reported by Song et al. [J. Chem. Phys. 111, 1905 (1999)].
Structural analogy between aqueous and oily magnetic fluids
Authors: Dubois E. , Cabuil V. , Boué F. , Perzynski R.
Published: July 2002

In this paper, we compare the structure and the phase behavior of two kinds of magnetic fluids, also called ferrofluids. They are constituted of the same maghemite particles, the diameters of which lie around 8 nm, dispersed either in water or in cyclohexane. Both systems are constructed to get the same interparticle interactions and differ only through the nature of the repulsion. Repulsion is either electrostatic, due to the charges of citrate molecules adsorbed on the particles surface in water, or steric, due to the alkyl chains of adsorbed surfactants in cyclohexane. Small angle neutron scattering (SANS) experiments show that both systems are highly repulsive and that the structure factors are very similar. This is confirmed by stability measurements: the samples are stable if temperature is decreased and if a magnetic field is applied. If the repulsion is decreased by the addition of electrolyte in water or bad solvent in cyclohexane, a gas–liquid-like transition is observed in both systems. However, the standard electrostatic potential (Derjaguin–Landau–Verwey–Overbeek potential) fails to describe the electrostatic repulsion in the aqueous ferrofluid while the behavior of this system is very similar to the behavior of the sterically stabilized ferrofluid. This underestimate of the electrostatic repulsion is probably due to the finite size effects of the trivalent ions. The striking similarities in the structure and the behavior of both kinds of dispersions, despite their chemical differences, seems to be related to the presence, in both cases, of the adsorbed surface species which ensure the repulsion between particles. Moreover, this repulsion may be described by an effective Yukawa potential very similar in range and intensity in both systems.
Joint local- and normal-mode studies of the overtone spectra of the methyl halides: CH3F, CH3Cl, CH3Br, CD3Br, and CH3I
Authors: Law M. M.
Published: July 2002

The infrared spectra of CH3F, CH3Cl, CH3Br, and CH3I have been investigated in the range 2500–16 500 cm−1 and that of CD3Br in the range 2000–7000 cm−1. New experimental data are reported for CH3F and CD3Br and these data together with existing literature data for CH3Cl, CH3Br, and CH3I have been analyzed in terms of a hybrid local-mode/normal-mode model. Local-mode basis functions have been used for the C–H(D) stretching vibrations and normal-mode basis functions to describe the C–H(D) bending vibrations. This joint approach, which also takes account of Fermi resonances, enabled the determination of stretch/bend anharmonicity and Fermi-resonance parameter sets which reproduce the observed data over wide energy ranges and compare much more favorably with literature ab initio results than previous parameter sets. The analyses have lead to a new vibrational assignment in CH3Cl and two corrected band centers in CH3Br and CH3I and predictions of unassigned bands raise several challenges to experiment.
Density-matrix representation of nonadiabatic couplings in time-dependent density functional (TDDFT) theories
Authors: Chernyak Vladimir , Mukamel Shaul
Published: July 2002

Closed expressions for nonadiabatic coupling between the ground and an excited electronic state of a molecule are derived by representing the time-dependent density functional (TDDFT) equations in a form of classical dynamics for the Kohn-Sham (KS) single-electron density matrix. Applicability of Krylov-space-type fast algorithms to nonadiabatic TDDFT as well as the representivity of the time-dependent charge density of a driven system are discussed.
Accurate potential energy and transition dipole moment curves for several electronic states of CO+
Authors: Okada Kazutoshi , Iwata Suehiro
Published: July 2002

Accurate calculations were performed for several doublet and quartet states of CO+ using the multireference configuration interaction method. With a single set of molecular orbitals the accurate transition dipole moments were evaluated. The calculated lifetimes of the vibronic states of the A 2Π and B 2Σ+ states were in excellent agreement with the available experimental data. The calculated vibrational spacings and rotational constants up to v=40 of the X 2Σ+ state were compared with the latest experimental data, and quantitative agreement was attained.
Femtosecond pump–probe ionization of small NaI–Sn clusters, S:H2O,NH3: A tool to probe the structure of the cluster
Authors: Grégoire G. , Mons M. , Dimicoli I. , Dedonder-Lardeux C. , Jouvet C. , Martrenchard S. , Solgadi D.
Published: July 2002

Femtosecond pump–probe ionization experiments on small NaI–Sn=1–7 clusters, S:NH3,H2O, are reported. The excitation of the clusters by the pump laser induces the NaI bond breaking within the cluster as well as evaporation of solvent molecules both in the excited and ionic states. Depending on the detection scheme used, namely one-photon or resonant two-photon ionization, the resulting Na+–Sn time-dependent signals are not similar. This reflects the major role of the ionization process in the nature of the dynamics probed. Furthermore, a solvent-selective behavior evidenced on the Na+–Sn signals has been related to different cluster structures according to the solvent nature: NaI seems to be embedded within ammonia clusters while it sticks on the surface of water clusters, when they are larger than four molecules.
Anharmonic force field and vibrational frequencies of tetrafluoromethane (CF4) and tetrafluorosilane (SiF4)
Authors: Wang Xiao-Gang , Sibert Edwin L. , Martin Jan M. L.
Published: July 2002

Accurate quartic anharmonic force fields for CF4 and SiF4 have been calculated using the CCSD(T) method and basis sets of spdf quality. Based on the ab initio force field with a minor empirical adjustment, the vibrational energy levels of these two molecules and their isotopomers are calculated by means of high order Canonical Van Vleck Perturbation Theory (CVPT) based on curvilinear coordinates. The calculated energies agree very well with the experimental data. The full quadratic force field of CF4 is further refined to the experimental data. The symmetrization of the Cartesian basis for arbitrary combination bands of Td group molecules is discussed using the circular promotion operator for the doubly degenerate modes, together with tabulated vector coupling coefficients. The extraction of the spectroscopic constants from our second order transformed Hamiltonian in curvilinear coordinates is discussed, and compared to a similar procedure in rectilinear coordinates.
Quantum mechanical integral cross sections and rate constants for the F+HD reactions
Authors: Zhang Dong H. , Lee Soo-Y. , Baer Michael
Published: July 2002

In this article we report on the first accurate quantum mechanical temperature-dependent rate constants for the two possible products of the (F+HD) system and on the corresponding intramolecular kinetic isotope effects. The calculations were done for the Stark–Werner and the Hartke–Stark–Werner potential energy surfaces. It was found that the two surfaces yield significantly different rate constants for both products but similar molecular kinetic isotope effects. These isotope effects are about two times larger than the experimental ones, at the lowest measured temperature region (160–200 K) but become rather close to them at ∼400 K. The F+HD is known to exhibit, at the low energy region, various kinds of isotope effects. In the present study we revealed a new isotope effect related to the dependence of (integral) cross sections on the initial rotational states ji at intermediate energies. Whereas the cross sections to form DF are only mildly dependent on ji (at most 20% for j0=4) a very large effect—which at some energies (∼0.1 eV) enlarges the integral cross sections almost three times—is obtained for HF.
Alkalization of aluminum clusters
Authors: Rao B. K. , Jena P.
Published: July 2002

Equilibrium geometries, binding energies, ionization potentials, and electron affinities of neutral and charged Aln clusters (n⩽8) decorated with alkali atoms such as Li and K have been calculated using molecular orbital theory based on density functional formalism and generalized gradient approximation. While the electron affinities and the ionization potentials depend on size, no clear signatures of shell closings are found in this size range. Similar studies on Al5Xm (X=Li, K, 1⩽m⩽4) also fail to provide any indication consistent with shell closings. On the other hand, the ionization potentials and electron affinities of aluminum clusters decrease with the addition of alkali atoms. The results are in good agreement with available experimental data.
Existence theorem for gauge algebra
Authors: Batalin I. A. , Vilkovisky G. A.
Published: July 2002

A general gauge action is defined by postulating a minimum of its properties necessary for the existence of loop expansion in the quantum theory. The structure of the general gauge algebra is derived from these postulates. The proof of existence of the structure functions, lacking in previous works, is given. The question of uniqueness is also completely cleared up. Some unsolved problems are discussed in connection with the covariant-quantization conjecture.
Singularities of the continuation of fields and validity of Rayleigh’s hypothesis
Authors: Maystre D. , Cadilhac M.
Published: July 2002

To formulate general results concerning the validity of the Rayleigh hypothesis, we first introduce a definition of the foci and antifoci of an analytic curve. Then, we state two lemmas on the properties of an analytic or harmonic function satisfying given conditions on an analytic curve. This allows us to predict the behavior of the analytic continuation of the field in electrostatics. The use of a conformal mapping permits the generalization of this method in electromagnetics and acoustics. As a consequence, we are able to predict the limit of validity of the Rayleigh hypothesis.
On asymptotic expansions of twisted products
Authors: Estrada Ricardo , Gracia-Bondía José M. , Várilly Joseph C.
Published: July 2002

The series development of the quantum-mechanical twisted product is studied. The series is shown to make sense as a moment asymptotic expansion of the integral formula for the twisted product, either pointwise or in the distributional sense depending on the nature of the factors. A condition is given that ensures convergence and is stronger than previously known results. Possible applications are examined.
The equations for isentropic motion of inviscid fluid in terms of wave function
Authors: Rylov Yu. A.
Published: July 2002

It is shown that by change of variables, the equations of motion of the inviscid fluid can transform into a differential equation for a wave function ψ, consisting of two complex components. A unit spin vector s can be formed by means of the wave function. The fluid flow vorticity is determined only by the spin vector. The dynamical equation for the ψ reduces to a linear equation with constant coefficients, if the flow is potential (s=const) and the state equation has some special form. Use of the wave function ψ reduces the nonlinearity connected with the convective term (v∇)v. It means that the velocity expansion over powers of ε (ε≪1) associates with the wave function expansion over powers of ε2.
Path integral solution for a particle confined in a region
Authors: Chetouani L. , Chouchaoui A. , Hammann T. F.
Published: July 2002

The propagator relative to a particle constrained to move in a finite region of space is calculated in the framework of path integrals. This region of the three-dimensional space is delimited through a sector of opening angle α, and also through the action of two attractive harmonic potentials, one being central and located in the 0xy plan, and the other directed along the z axis, with respective pulsations ω and ω0. It is shown that for α=π/2 and π the propagator is the sum of propagators evaluated on classical paths. The important case of the edge (α=2π) is considered.
Bound states in quantum waveguides of a slowly decaying curvature
Authors: Exner Pavel
Published: July 2002

Using two simple tricks, the results of our previous papers concerning bound states of Dirichlet Laplacians in curved tubes in Rν, ν=2, 3 are improved. If the tube is smooth and thin enough, the bound states are shown to exist if the tube curvature (and torsion for ν=3) decays with respect to the arclength parameter s as ‖s‖−1/2−ε. A sufficient condition is found and it is proved that it is necessary at the same time, provided the curvature (torsion) decays as ‖s‖−1−ε and the tube is only mildly bent.
The vacuum energy density for spherical and cylindrical universes
Authors: Elizalde E.
Published: July 2002

The vacuum energy density (Casimir energy) corresponding to a massless scalar quantum field living in different universes (mainly no-boundary ones), in several dimensions, is calculated. Hawking’s zeta function regularization procedure supplemented with binomial expansion is shown to be a rigorous and well suited method for performing the analysis. It is compared with other more involved techniques. The principal-part prescription is used to deal with the poles that eventually appear. Results of the analysis are the absence of poles at four dimensions (4D) (for a 4D Riemann sphere and for a 4D cylinder of 3D Riemann spherical section), the total coincidence of the results corresponding to a 3D and a 4D cylinder (the first after pole subtraction), and the fact that the vacuum energy density for cylinders is (in absolute value) over an order of magnitude smaller than for spheres of the same dimension.
Wavelike exact solutions of R+R2+Q2 gravity
Authors: Zhytnikov V. V.
Published: July 2002

The spinorial methods are applied to the analysis of the wavelike exact solutions of the gauge theory of gravitation with dynamical torsion. The N-type plane-fronted metric-torsion-electromagnetic wave for the most general quadratic in the torsion and curvature Lagrangian with nonvanishing cosmological constant are presented. A series of non-plane-fronted exact solutions with nontrivial torsion are found also. The question about equivalence of equations of the gauge theory of gravitation and the general relativity in the vacuum torsionless case are clarified. The example of non-Einsteinian vacuum solution for exceptional choice of cosmological constant are presented.
Search for first integrals in relativistic time-dependent Hamiltonian systems
Authors: Martin J. , Bouquet S.
Published: July 2002

In this paper, first integrals of one-dimensional time-dependent relativistic Hamiltonians are looked for. In a first step, potentials for which the corresponding motion equations are completely integrable are found. In a second step, a more realistic potential described by the time-dependent relativistic harmonic oscillator is studied. It appears that it is not analytically integrable. However its asymptotic evolution systematically converges towards a classical evolution for which we can exhibit a first integral.
The Kazhdan–Lusztig conjecture for 𝒲 algebras
Authors: De Vos Koos , van Driel Peter
Published: July 2002

The main result in this paper is the character formula for arbitrary irreducible highest weight modules of 𝒲 algebras. The key ingredient is the functor provided by quantum Hamiltonian reduction, which constructs the 𝒲 algebras from affine Kac–Moody (KM) algebras and in a similar fashion 𝒲 modules from KM modules. Assuming certain properties of this functor, the 𝒲 characters are subsequently derived from the Kazhdan–Lusztig conjecture for KM algebras. The result can be formulated in terms of a double coset of the Weyl group of the KM algebra: the Hasse diagrams give the embedding diagrams of the Verma modules and the Kazhdan–Lusztig polynomials give the multiplicities in the characters.
The q-Coulomb problem in configuration space
Authors: Cadavid A. C. , Finkelstein R. J.
Published: July 2002

We formulate the q-Coulomb problem in configuration space with the aid of ladder operators for the radial wave function. The highest angular momentum state corresponding to the principal quantum number n is found to be the monomial rn−1 multiplied by a q-exponential. The states of lower angular momentum are q-associated Laguerre polynomials multiplied by the same q-exponential. The state functions all lie in the complex plane and may be interpreted in the standard way. The energy levels are again given by a Balmer formula with n replaced by the basic n.
Multiple condensate solutions for the Chern–Simons–Higgs theory
Authors: Tarantello Gabriella
Published: July 2002

We study the existence of condensate solutions for the Chern-Simons-Higgs model with the choice of a potential field where both the symmetric and asymmetric vacua occur as ground states [see Hong, Kim, and Pac, Phys. Rev. Lett. 64, 2230 (1990) and Jackiw and Weinberg, ibid. 64, 2234 (1990)]. We show that if the Chern-Simons coupling parameter k is above a critical value, no such solutions can exist, while for k≳0 below this critical value there exist at least two condensate solutions carrying the same quantized energy, as well as electric and magnetic charge. This multiplicity result accounts for the two vacua states present in the model. In fact, as k→0+ it is shown that the two solutions found ‘‘bifurcate’’ from the asymmetric and symmetric vacuum states respectively.
Determination of Wigner distribution function for the d-dimensional Coulomb problem
Authors: Nouri Saeid
Published: July 2002

In this work we present a theoretical study of the d-dimensional Coulomb problem in quantum phase space. A coordinate transformation in hyperspherical space is used that maps the d-dimensional Coulomb problem into the D-dimensional harmonic oscillator and the Wigner distribution function for the d-dimensional Coulomb problem is then obtained. This exactly soluble model can shed some light on finite-size features of Wigner’s distribution, which will be a vital experience for various dynamic problems.
Real sector of the nonminimally coupled scalar field to self-dual gravity
Authors: Montesinos Merced , Morales-Técotl Hugo A. , Urrutia Luis F. , Vergara J. David
Published: July 2002

A scalar field nonminimally coupled to gravity is studied in the canonical framework, using self-dual variables. The corresponding constraints are first class and polynomial. To identify the real sector of the theory, reality conditions are implemented as second class constraints, leading to three real configurational degrees of freedom per space point. Nevertheless, this realization makes nonpolynomial some of the constraints. The original complex symplectic structure reduces to the expected real one, by using the appropriate Dirac brackets. For the sake of preserving the simplicity of the constraints, an alternative method preventing the use of Dirac brackets, is discussed. It consists of converting all second class constraints into first class by adding extra variables. This strategy is implemented for the pure gravity case.
Quantum field theory on the noncommutative plane with Eq(2) symmetry
Authors: Chaichian M. , Demichev A. , Prešnajder P.
Published: July 2002

We study properties of a scalar quantum field theory on the two-dimensional noncommutative plane with Eq(2) quantum symmetry. We start from the consideration of a firstly quantized quantum particle on the noncommutative plane. Then we define quantum fields depending on noncommutative coordinates and construct a field theoretical action using the Eq(2)-invariant measure on the noncommutative plane. With the help of the partial wave decomposition we show that this quantum field theory can be considered as a second quantization of the particle theory on the noncommutative plane and that this field theory has (contrary to the common belief) even more severe ultraviolet divergences than its counterpart on the usual commutative plane. Finally we introduce the symmetry transformations of physical states on noncommutative spaces and discuss them in detail for the case of the Eq(2) quantum group.
Giant magnetoresistance in La0.7Pb0.3MnO3 thin film
Authors: Khartsev S. I.
Published: July 2002

Epitaxial thin films of La0.7Pb0.3MnO3 have been grown on (100) LaAlO3 and (110) SrTiO3 substrates by dc magnetron sputtering. The magnetic-field and temperature-dependent resistivity of as-deposited and post-annealed films was examined. At H=10 kOe the maximum observed magnetoresistance ratio (R0−RH)/R0 was 62%. The bolometric response to 0.94-μm low-frequency modulated radiation in the film with the temperature coefficient of resistance d ln R/dT=14% was studied. The transport properties are described using Zhang’s spin-polaron theory. An unusual approach to stabilizing the film temperature at the point corresponding to the maximum of the magnetoresistance ratio and responsivity is discussed.
Detection of coherent structures associated with bursting events in an open-channel flow by a two-point measuring technique using two laser-Doppler velocimeters
Authors: Komori Satoru , Murakami Yasuhiro , Ueda Hiromasa
Published: July 2002

Large-scale structures in the wall region of an open-channel flow were investigated experimentally using the two-point measuring technique of Komori and Ueda [J. Fluid Mech. 152, 337 (1985)] which used two laser-Doppler velocimeters and a conditional (pattern-) averaging technique. Instantaneous streamwise and vertical velocities were measured simultaneously at two positions—a reference and a movable position—and the spatial and temporal distributions of the fluctuating velocity vectors, Reynolds stress, and turbulent kinetic energy associated with the bursting events were computed using a pattern-averaging technique based on the variable-interval time-averaging method. The results clearly show the time evolution of the coherent structure associated with the bursting events. They also supported the flow visualization results published hitherto, and gave a physical and qualitative basis for them.
Statistics of helicity fluctuations in homogeneous turbulence
Authors: Polifke Wolfgang
Published: July 2002

The statistics and dynamical significance of helicity fluctuations in turbulent flows are investigated. A random-phase or quasi-Gaussian approximation (QGA) is employed to obtain expectation values of the magnitude of helicity fluctuations in reflectionally symmetric ensembles of turbulent flows. It is shown that the QGA is compatible with Kolmogorov-type scaling arguments supplemented by elementary statistical considerations. If follows from the scaling properties of the viscous term in the helicity balance equation that in the absence of phase correlations helicity is an adiabatic invariant of fully developed turbulent flows. Possible consequences of this invariance for decaying flows are discussed. In direct numerical simulations of decaying and forced turbulence it is found that at large scales the fluctuations of helicity are well described by the QGA, and as such not strong enough to directly influence the energy transfer. The helicity fluctuations at large wave numbers, on the other hand, are repeatedly observed to deviate significantly from the QGA, indicating the presence of small-scale phase coherence. The coherence seems to be sufficiently strong to break the adiabatic invariance of helicity. The nature of the observed fluctuations suggests that the invariance properties of helicity may not be held responsible for the buildup of the correlations. In the light of these results it is questionable whether helicity fluctuations can play a fundamental role in the organization or characterization of turbulent structures.
Mixing by chaotic convection in an infinite Prandtl number fluid and implications for mantle convection
Authors: Kellogg Louise H. , Stewart Cheryl A.
Published: July 2002

Two-dimensional flow in an infinite Prandtl number constant viscosity fluid is chaotic in high-resolution numerical experiments; however, it is unknown how many modes are appropriate to describe the convection in the Earth’s mantle. It is known, for instance, that nonhomogeneous mechanical and thermal boundary conditions, such as those found in the Earth’s mantle, limit the suite of modes present and that low-order modes dominate thermal convection in the Earth’s mantle, yet the mixing consequences of limiting the spatial spectrum of thermal convection at infinite Prandtl number are unknown. Therefore the mixing which results from the nonlinear interaction of a single convection roll with a single spatial subharmonic is examined. To study mixing in this flow, the deformation history of infinitesimal strain markers and the dispersion of clusters of passive markers in the fluid are calculated. Observing the rapid scattering of clusters of markers provides a useful visualization of the mixing process. The deformation of strain markers distributed throughout the flow yields a quantitative measure of mixing by providing a statistical description of the mixing in the flow. Deformation was accomplished primarily by pure shear (exponential thinning).
Stability of propagating modons for small-amplitude perturbations
Authors: Sakuma H. , Ghil M.
Published: July 2002

A stability analysis based on Arnol’d–Lyapunov arguments in a gauge-variable formalism is applied to a generalized form of propagating modons. The method was previously applied to the special case of Stern’s modon in a quiescent background flow. The analysis presented here shows that propagating-modon solutions in the shallow-water equations are stable to small-amplitude perturbations, regardless of the sign of their translational speed, as long as this is much smaller in magnitude than the solid-rotation speed of the Earth. Sufficient conditions for stability are that the given flow be quasigeostrophic, i.e., that the Rossby number be small, and that the associated Froude number be one order of magnitude smaller than the Rossby number.
Motion of dipole vortices in a weakly inhomogeneous medium and related convective transport
Authors: Nycander J. , Isichenko M. B.
Published: July 2002

A general equation of motion describing the propagation of dipole vortices in a weakly inhomogeneous medium is derived. The solution of this equation agrees well with previously published numerical simulations. The contribution of dipole vortices to passive transport in the direction of the external inhomogeneity is then estimated. Since they can make large, oscillatory excursions in this direction (in contrast to monopole vortices), their contribution can be quite significant.
Guiding center atoms: Three-body recombination in a strongly magnetized plasma
Authors: Glinsky Michael E. , O’Neil Thomas M.
Published: July 2002

The three-body recombination rate is calculated for an ion introduced into a magnetically confined, weakly correlated, and cryogenic pure electron plasma. The plasma is strongly magnetized in the sense that the cyclotron radius for an electron rce=(kBTe/me)1/2/Ωce is small compared to the classical distance of closest approach b=e2/kBTe, where Te is the electron temperature and Ωce=eB/mec is the electron cyclotron frequency. Since the recombination rate is controlled by a kinetic bottleneck a few kBTe below ionization, the rate may be determined by considering only the initial cascade through states of electron-ion pairs with separation of order b. These pairs may be described as guiding center atoms since the dynamics is classical and treatable with the guiding center drift approximation. In this paper, an ensemble of plasmas characterized by guiding center electrons and stationary ions is described with the BBGKY hierarchy. Under the assumption of weak electron correlation, the hierarchy is reduced to a master equation. Insight to the physics of the recombination process is obtained from the variational theory of reaction rates and from an approximate Fokker–Planck analysis. The master equation is solved numerically using a Monte Carlo simulation, and the recombination rate is determined to be 0.070(10)n2eveb5 per ion, where ne is the electron density and ve=(kBTe/me)1/2 is the thermal velocity. Also determined by the numerical simulation is the transient evolution of the distribution function from a depleted potential well about the ion to its steady state.
Nonequilibrium effects of diluent addition in a recombining argon plasma
Authors: Gordon M. H. , Kruger C. H.
Published: July 2002

Spectroscopic and calorimetric measurements have been made using a 50 kW radio frequency inductively coupled plasma torch operated at atmospheric pressure with maximum temperatures and electron densities near 8500 K and 3×1021 m−3, respectively. The plasma flowed through a water-cooled quartz test section which enabled the study of nonequilibrium effects on both a recombining pure argon plasma and a recombining argon plasma with hydrogen, nitrogen, or neon. The pure argon plasma is found to be well described by a partial equilibrium model in which the free and bound-excited electrons are in mutual equilibrium irrespective of possible departures from equilibrium with the ground state. The addition of just tenths of a percent of either atomic hydrogen or nitrogen (but not neon, in contrast) is found to significantly affect the plasma’s state of equilibrium for electron densities roughly less than 1021 m−3 because of a nearly gas-kinetic reaction between argon’s first excited state and the diluent’s ground state. This ‘‘quenching’’ reaction provides a depopulating mechanism for argon’s first excited state and thereby inhibits the establishment of partial equilibrium which then invalidates several common diagnostic methods. The extent of quenching depends on the particular diluent, the amount of diluent relative to the electron number density, and on the temperature. These experimental observations are supported by an appropriately modified argon collisional–radiative model.
Plasma structures in front of a floated emissive electrode
Authors: Ishiguro Seiji , Sato Noriyoshi
Published: July 2002

A particle simulation with plasma source is carried out on plasma structures generated by an electron emissive electrode floated in a collisionless plasma. When low-temperature, high-density thermal electrons are emitted, there appears a negative potential dip in front of the electrode, which is always accompanied by a low-frequency oscillation. On the other hand, three regimes of plasma structures appear for an electron beam injection. When a high-flux electron beam is injected, an electron sheath is generated in front of the electrode. The sheath reflects ions flowing to the electrode, providing an increase in the plasma density. When a low-flux electron beam is injected, no electron sheath is generated. When an intermediate-flux beam is injected, the electron sheath structure appears periodically in time. The lifetime of the sheath is proportional to the system length. These results of beam injection are almost consistent with those of a Q-machine experiment.
Waves of extreme form on a layer of uniform vorticity
Authors: Broadbent E. G. , Moore D. W.
Published: July 2002

Waveforms are studied on a layer of uniform vorticity bounding a uniform stream in the absence of gravitational effects. It is shown that the wave trough can develop a notch as the wave amplitude is increased, the angle of the notch being close to 90°. The results are compared briefly with those published recently by Pullin and Grimshaw [Phys. Fluids 26, 1731 (1983)].
Stochastic particle diffusion in velocity space for a bumpy torus
Authors: Chen G. L. , Owen L. W. , Batchelor D. B. , Hedrick C. L.
Published: July 2002

Nonadiabatic changes of the magnetic moment μ in the ELMO Bumpy Torus-Scale (EBT-S) have been studied both analytically and numerically. Simple forms of Δ μ and gyrophase change were obtained, permitting the changes in these quantities to be studied using an iteration mapping. The mapping results show stochastic behavior for particles having high energy and low initial μ. Otherwise, superadiabatic motion appears. The stochastic diffusion coefficient for the variation of μ was measured numerically by mapping and was also calculated from quasilinear theory. The results are shown to agree well in the stochastic region. For high-energy particles, the diffusion in μ caused by nonadiabaticity can be comparable to collisional diffusion when stochastic motion occurs for EBT-S.
The separation of viscous jets
Authors: Tanner R. I. , Lam H. , Bush M. B.
Published: July 2002

A viscous jet is not usually observed to separate from a sharp edge in the manner expected theoretically. In the present paper the separation of a creeping jet emerging from a tube with a rounded exit is considered. As a separation criterion, in the absence of surface tension, we propose that the traction normal to the nozzle surface drops to zero at the separation point. Boundary-element calculations then show a behavior that agrees with experimental data and with previous finite-element computations. They also permit the Michael condition to be observed at separation, so that the discrepancy between finite-element calculations and theory is removed.
Effects of viscosity variation on the stability of a liquid film flow down heated or cooled inclined surfaces: Finite wavelength analysis
Authors: Goussis D. A. , Kelly R. E.
Published: July 2002

The effects of variable viscosity on the surface wave mode of instability of a liquid film flowing down heated or cooled inclined surfaces are examined. Stability boundaries are presented using two models of viscosity variation with temperature. Both models yield qualitatively similar results. It is shown that heating destabilizes the flow, while cooling stabilizes it. For the case of heating the critical wavenumber αc is always zero, while for the case of cooling αc can be nonzero. By variation of the governing nondimensional parameters, a reasonably complete picture is given when instability occurs first at finite wavelengths.
Decay of isotropic turbulence at low Reynolds number
Authors: Mansour N. N. , Wray A. A.
Published: July 2002

Decay of isotropic turbulence is computed using direct numerical simulations. Comparisons with experimental spectra at moderate and low Reynolds numbers (Rλ<70) show good agreement. At moderate to high Reynolds numbers (Rλ≳50), the spectra are found to collapse with Kolmogorov scaling at high wave numbers. However, at low Reynolds numbers (Rλ<50) the shape of the spectra at the Kolmogorov length scales is Reynolds number dependent. Direct simulation data from flowfields of decaying isotropic turbulence are used to compute the terms in the equation for the dissipation rate of the turbulent kinetic energy. The development of the skewness and the net destruction of the turbulence dissipation rate in the limit of low Reynolds numbers are presented. The nonlinear terms are found to remain active at surprisingly low Reynolds numbers.
On the representation of backscatter in dynamic localization models
Authors: Carati Daniele , Ghosal Sandip , Moin Parviz
Published: July 2002

The dynamic localization model is a recently developed method that allows one to compute rather than prescribe the unknown coefficients in a subgrid scale model as a function of position at each time-step. A realistic subgrid scale model should describe both the direct and reverse (backscatter) energy transfers at the local level. A previously developed dynamic localization model accounted for backscatter by means of a (deterministic) eddy viscosity that could locally assume positive as well as negative values. Here this paper presents an alternative stochastic model of backscatter in the context of the dynamic procedure. A comparative discussion of the merits of stochastic versus deterministic modeling of backscatter is presented. These models are applied to a large eddy simulation of isotropic decaying and forced turbulence. Tests are also performed with versions of the model that do not account for backscatter. The results are compared to experiments and direct numerical simulation. It is shown that the models correctly predict the energy and three-dimensional (3D) energy spectra in decaying turbulence. In the forced case the Kolmogorov 5/3 law seems better predicted by models accounting for backscatter. A relative evaluation of the various versions of the model in terms of predictive capability and cost is provided.
On the turbulent transport of a passive scalar by anisotropic turbulence
Authors: Vincent A. , Michaud G. , Meneguzzi M.
Published: July 2002

The effect of anisotropic turbulence on the vertical transport of a tracer in a fluid layer is investigated numerically. It is found that for a fixed vertical turbulence spectrum an increase in the intensity of horizontal turbulence reduces the efficiency of the vertical transport. The reduction factor is inversely proportional to the anisotropy (defined as the ratio of horizontal to vertical rms velocities) for large Peclet numbers. This effect had been overlooked in studies of turbulent particle transport in stratified media, where the reduction in vertical transport coefficients was entirely attributed to vertical mode suppression by stratification. Our study is based on Gaussian velocity fields.
Nonlinear axisymmetric and three-dimensional vorticity dynamics in a swirling jet model
Authors: Martin J. E. , Meiburg E.
Published: July 2002

The mechanisms of vorticity concentration, reorientation, and stretching are investigated in a simplified swirling jet model, consisting of a line vortex along the jet axis surrounded by a jet shear layer with both azimuthal and streamwise vorticity. Inviscid three-dimensional vortex dynamics simulations demonstrate the nonlinear interaction and competition between a centrifugal instability and Kelvin–Helmholtz instabilities feeding on both components of the base flow vorticity. Under axisymmetric flow conditions, it is found that the swirl leads to the emergence of counter-rotating vortex rings, whose circulation, in the absence of viscosity, can grow without bounds. Scaling laws are provided for the growth of these rings, which trigger a pinch-off mechanism resulting in a strong decrease of the local jet diameter. In the presence of an azimuthal disturbance, the nonlinear evolution of the flow depends strongly on the initial ratio of the azimuthal and axisymmetric perturbation amplitudes. The long term dynamics of the jet can be dominated by counter-rotating vortex rings connected by braid vortices, by like-signed rings and streamwise braid vortices, or by wavy streamwise vortices alone.