bluuebunny/crossref_metadata_2025 huggingface dataset:
Micromagnetics simulation of deep-submicron supermalloy disks
Authors: Dao N. , Whittenburg S. L. , Cowburn R. P.
Published: July 2002

The results of recent micromagnetic simulations of deep submicron supermalloy disks are presented. A recent experimental measurement of the hysteresis and magnetic domain structure in supermalloy disks with diameters ranging from 55 to 500 nm and thickness ranging from 6 to 15 nm has been reported. Our micromagnetic simulations show remarkable agreement with the experimental hysteresis loops. The simulation results show that for thin or small diameter disks a single magnetic domain exists with all spins aligned. The hysteresis loop represents free rotation of these spins. For larger diameter disks or as the thickness increases the hysteresis loops change shape due to the appearance of a single vortex state appearing at low applied fields.
The nonlinear dynamics of the modulational instability of drift waves and the associated zonal flows
Authors: Lashmore-Davies C. N. , McCarthy D. R. , Thyagaraja A.
Published: July 2002

The linear and nonlinear dynamics of zonal flows and their interactions with drift wave turbulence is considered in the simple but illuminating generalized Charney–Hasegawa–Mima model due to Smolyakov et al. [Phys. Plasmas 7, 1349 (2000)]. Two positive definite, exact, integral invariants associated with the full generalized Charney–Hasegawa–Mima system are derived. For an initial monochromatic drift wave pump with small but finite amplitude, a modulational instability can occur, characterized by growing zonal flow and sideband perturbations (i.e., a four-wave interaction). The pump threshold for instability is readily satisfied, depending on the zonal flow wave number. The fully nonlinear Charney–Hasegawa–Mima equations are solved with a numerical scheme which is validated by demonstrating the conservation of the two exact invariants. The simulations show that the validity of the four-wave model is limited to approximately three instability growth times. The radial structure of the zonal flow can be “jet-like” or highly oscillatory depending upon the ratio of the system size to the density scale length and initial conditions. It is found that zonal flows can be dramatically reduced if the most unstable zonal flow wave number does not fit into the system.
Coherent response of a biased double-well superlattice subjected to an ultrashort interband excitation
Authors: Hernández-Cabrera A. , Aceituno P. , Vasko F. T.
Published: July 2002

The temporal evolution of electrons in a biased double-well superlattice, subjected to an ultrashort interband excitation, is theoretically examined. Both the temporal oscillations of the induced current and the evolution of the photoexcited electron concentration are considered for different parameters of the structure subjected to a δ-pulse excitation. Due to the mixing of the Wannier–Stark-ladder branches, the induced response depends on the applied bias voltage in a complicated way. In particular, long-time beats appear in the vicinities of intra- and inter-cell anti-crossings of the energy spectrum. These dependencies are examined within the framework of the Kane model, invoking the parabolic approximation for electron and heavy hole states. Numerical calculations of the carrier coherent dynamics are performed for the case of a δ-pulse excitation and with a phenomenologically introduced damping factor.
The Sc+NO→ScO+N reaction: Rotational state distribution in ScOX 2Σ+(v″=0)
Authors: Luc P. , Vetter R.
Published: July 2002

The Sc+NO→ScO+N reaction has been investigated in a beam-gas arrangement, with characterization of ScO products by cw laser-induced fluorescence: absorption versus laser frequency over the A 2Π(v′=1)–X 2Σ+(v″=0) band and fluorescence over the A 2Π(v′=1)–X 2Σ+(v″=1) one. It leads to the direct determination of the nascent rotational state distribution in the X 2Σ+(v″=0) level of ScO. This distribution is close to a Prior statistical one, with a well-characterized weak “surprisal,” indicating that a momentum constraint takes place during the reaction process. In the frame of this statistical distribution, a new accurate value for the dissociation energy of ScO is proposed: D00(ScO)=(6.92±0.01) eV. Spectroscopic data are reported for the A 2Π(v′=1)–X 2Σ+(v=0) band, up to N=98.
A polynomial expansion of the quantum propagator, the Green’s function, and the spectral density operator
Authors: Vijay Amrendra , Metiu Horia
Published: July 2002

One of the methods for calculating time propagators in quantum mechanics uses an expansion of e−iĤt/ℏ in a sum of orthogonal polynomial. Equations involving Chebychev, Legendre, Laguerre, and Hermite polynomials have been used so far. We propose a new formula, in which the propagator is expressed as a sum in which each term is a Gegenbauer polynomial multiplied with a Bessel function. The equations used in previous work can be obtained from ours by giving specific values to a parameter. The expression allows analytic continuation from imaginary to real time, transforming thus results obtained by evaluating thermal averages into results pertaining to the time evolution of the system. Starting from the expression for the time propagator we derive equations for the Green’s function and the density of states. To perform computations one needs to calculate how the polynomial in the Hamiltonian operator acts on a wave function. The high order polynomials can be obtained from the lower ordered ones through a three term recursion relation; this saves storage and computer time. As a numerical test, we have computed the bound state spectrum of the Morse oscillator and the transmission coefficient for tunneling through an Eckart barrier. We have also studied the evolution of a Gaussian wave packet in a Morse potential well.
Static nonlinear optical susceptibilities: Testing approximation schemes against exact results
Authors: Del Freo Luca , Terenziani Francesca , Painelli Anna
Published: July 2002

The reliability of the approximations commonly adopted in the calculation of static optical (hyper) polarizabilities is tested against exact results obtained for an interesting toy-model. The model accounts for the principal features of typical nonlinear organic materials with mobile electrons strongly coupled to molecular vibrations. The approximations introduced in sum over states and finite field schemes are analyzed in detail. Both the Born–Oppenheimer and the clamped nucleus approximations turn out to be safe for molecules, whereas for donor–acceptor charge transfer complexes deviations from adiabaticity are expected. In the regime of low vibrational frequency, static susceptibilities are strongly dominated by the successive derivatives of the potential energy and large vibrational contributions to hyperpolarizabilities are found. In this regime anharmonic corrections to hyperpolarizabilities are very large, and the harmonic approximation, exact for the linear polarizability, turns out totally inadequate for nonlinear responses. With increasing phonon frequency the role of vibrations smoothly decreases, until, in the antiadiabatic (infinite vibrational frequency) regime, vibrations do not contribute anymore to static susceptibilities, and the purely electronic responses are regained.
Nanometric inversion domains in conventional molecular-beam-epitaxy GaN thin films observed by atomic-resolution high-voltage electron microscopy
Authors: Iwamoto C. , Shen X. Q. , Okumura H. , Matuhata H. , Ikuhara Y.
Published: July 2002

GaN films grown on sapphire substrates by conventional molecular-beam epitaxy were investigated by means of atomic-resolution high-voltage electron microscopy (ARHVEM). The atomic positions of Ga and N could be directly discriminated by ARHVEM to determine the polarity in GaN. It was revealed that N polarity GaN films possessed a high density of nanometric inversion domains (IDs) with Ga polarity. The ID boundary was constructed by an inversion and a c/2 translation, and formed fourfold and eightfold coordination along the boundary.
Collective many-body resonances in condensed phase nonlinear spectroscopy
Authors: Tortschanoff Andreas , Mukamel Shaul
Published: July 2002

The optical response of assemblies of electronic and vibrational chromphores may show two types of collective resonances induced by either direct short-range coupling (multiple quantum coherence) or by long-range macroscopic local field and cascading processes. Using a unified approach for both types of resonances, we demonstrate how specific signatures in line shapes, phase profiles, and density dependence may be used to distinguish between the two. New high harmonic resonances at combinations and multiples of optical frequencies of the single exciton transitions are predicted in the k1+k2−k3 four wave mixing signal for several model systems.
Interaction of a vortex ring with a diffusion flame
Authors: Safta C. , Enachescu S. , Madnia C. K.
Published: July 2002

The interaction between a laminar vortex ring and a flat diffusion flame is investigated via direct numerical simulations. The diffusion flame is generated by a “spark” and is implemented as initial condition for the simulations. The chemistry is modeled by an Arrhenius, single step, irreversible reaction. A heat-releasing flame sheet is also considered in addition to the finite rate flames. The vortex ring is generated by an axisymmetric discharge of fuel-inert species mixture that enters a quiescent medium with the same composition and temperature. Two stages are identified during the finite rate diffusion flame–vortex ring interaction. The first stage corresponds to the head-on collision between the flame and the vortex ring, and lasts until the flame is quenched near the centerline. The unsteady effects are dominant and examination of the terms in the temperature transport equation reveals that, depending on the relative strengths of the vortex ring and the flame, convection and/or diffusion terms are responsible for the local flame extinction. The extinction patterns obtained from the flame–vortex ring interaction during this stage, are compared with the results from unsteady counterflow diffusion flame simulations. The second stage corresponds to the passage of the ring through the flame and its interaction with the flame from the oxidizer side. During this stage, the vortex ring loses its strength and, in addition to the unsteady effects, curvature effects can also become important.
Mono 1000: A simple and efficient 2.45 GHz electron cyclotron resonance ion source using a new magnetic structure concept
Authors: Jardin P. , Barue C. , Canet C. , Dupuis M. , Flambard J. L. , Gaubert G. , Lecesne N. , Lehérissier P. , Lemagnen F. , Leroy R. , Pacquet J.-Y. , Pellemoine F. , Rataud J.-P. , Laurent M. G. Saint , Villari A. C. C.
Published: July 2002

The production of singly charged atomic and molecular ions with a new 2.45 GHz electron cyclotron resonance ion source has been studied. The ion source Mono 1000 uses a new magnetic confinement structure. The elements Ne, Ar, and Kr are ionized with efficiencies close to 100%, while 45% has been achieved for He. In the case of the molecules SO2 and SF6, more than 90% overall efficiency has been observed with more than 40% of sulfur atoms leaving the source under the form S+. A total extracted yield of 4×1012 singly charged fulleren (C60) ions per second has also been observed.
Dipole formation by two interacting shielded monopoles in a stratified fluid
Authors: Beckers M. , Clercx H. J. H. , van Heijst G. J. F. , Verzicco R.
Published: July 2002

The interaction between two shielded monopolar vortices has been investigated experimentally in a nonrotating linearly stratified fluid and by full three-dimensional (3D) numerical simulations. The characteristic Reynolds and Froude numbers in the experiments are approximately Re≈500–1000 and F≈0.2–0.4. In a first set of numerical simulations the flow is initialized with the experimentally obtained two-dimensional horizontal velocity field, which is measured in the horizontal symmetry plane of the vortices, and the lacking initial data such as the density perturbation and the full 3D vorticity field are provided by the so-called diffusion model [Beckers et al., J. Fluid Mech. 433, 1 (2001)]. A comparison of the experimental and numerical data shows that using the diffusion model to supply the full 3D initial data is reliable for the Reynolds and Froude numbers considered in present experiments. Conjecturing a wider range of applicability, a second set of numerical simulations has been performed using initial conditions for the flow field and the density perturbation entirely based on the diffusion model. These numerical simulations enable an investigation of the role of Reynolds numbers (up to Re=10 000) and Froude numbers (F=0.30, 0.65, and 1.0) which are outside the experimentally accessible range. These simulations provide a better understanding of the dipole formation process, the influence of the vortex shields and the density perturbation in this process, and of the 3D structure of the dipoles. Compactness of the dipole, entrainment of irrotational fluid from the rear, and tail formation behind the dipole have been discussed. Additionally, experimental and numerical results are reported on the interaction between two shielded monopoles of the same sign in a linearly stratified fluid. It is found that for the full range of Reynolds and Froude numbers mentioned above the presence of the vortex shields prevents the merging of these monopoles.
Molecular hydrodynamic theory of nonresonant Raman spectra in liquids: Third-order spectra
Authors: Denny R. Aldrin , Reichman David R.
Published: July 2002

A fully microscopic molecular hydrodynamic theory for the third-order Raman spectrum of an atomic liquid (Xe) is presented. The spectrum is obtained from a mode-coupling theory by projecting the dynamics onto bilinear pairs of fluctuating density variables. For the densities and temperatures studied, semiquantitative agreement is obtained in comparison with molecular dynamics simulation on all time scales. The theory is contrasted with previous molecular hydrodynamic theories of depolarized light scattering spectra. Extensions of the approach outlined here to both classical molecular and quantum liquids are presented. The theory presented here represents the first microscopic solvation theory which quantitatively captures both the inertial and long-time dynamics as well as the crossover between these regimes.
Solving integral equations for binary and ternary systems
Authors: Nader Lotfollahi Mohammad , Modarress Hamid
Published: July 2002

Solving integral equations is an effective approach to obtain the radial distribution function (RDF) of multicomponent mixtures. In this work, by extending Gillan’s approach [M. J. Gillan, Mol. Phys. 38(6), 1781 (1979)], the integral equation was solved by numerical method and was applied to both binary and ternary mixtures. The Lennard-Jones (LJ) potential function was used to express the pair molecular interactions in calculating the RDF and chemical potential. This allowed a comparison with available simulation data, on the RDF and the chemical potential, since the simulation data have been reported for the LJ potential function. The RDF and the chemical potential results indicated good agreement with the simulation data. The calculations were extended to the ternary system and the RDFs for carbon dioxide–octane–naphthalene were obtained. The numerical method used in solving integral equation was rapidly convergent and not sensitive to the first estimation. The method proposed in this work can be easily extended to more than the three-component systems.
General parameterization of Auger recombination in crystalline silicon
Authors: Kerr Mark J. , Cuevas Andres
Published: July 2002

A parameterization for band-to-band Auger recombination in silicon at 300 K is proposed. This general parameterization accurately fits the available experimental lifetime data for arbitrary injection level and arbitrary dopant density, for both n-type and p-type dopants. We confirm that Auger recombination is enhanced above the traditional free-particle rate at both low injection and high injection conditions. Further, the rate of enhancement is shown to be less for highly injected intrinsic silicon than for lowly injected doped silicon, consistent with the theory of Coulomb-enhanced Auger recombination. Variations on the parameterization are discussed.
Quantum beats in the S1 dynamics of glyoxal
Authors: Chang Chushuan , Chen I-Chia
Published: July 2002

Fluorescence of transition Ã 1Au−X̃ 1Ag of trans-glyoxal in a supersonic jet is recorded with laser excitation in the wavelength region 393–417 nm. Full rotational analysis is performed for nine bands in a region 395–410 nm. Fluorescence decays with quantum beats from several rovibrational levels in Ã 1Au of glyoxal are detected. As lines in Fourier-transform spectra of those decay curves split on application of an external magnetic field, this oscillatory behavior results from coherent excitation of mixed S1 and T1 states. On excitation with high energy, those lines become broadened as the lifetime of the T1 state decreases because of coupling to a dissociation continuum. An abrupt increase of linewidth in transformed spectra at 25 350 cm−1 (394.5 nm) indicates that this position corresponds to a threshold for dissociation. For excitation energy greater than 25 450 cm−1, most observed decay curves of fluorescence emission display biexponential behavior. The decay of the fast component of the biexponential curve corresponds to intersystem crossing and the slow component to decay of those eigenstates. For energies exceeding 25 550 cm−1, the slow component diminishes, to yield nearly single exponential decay, indicating that the rate of dissociation is greater than that of intersystem crossing. Results of these quantum-beat experiments imply a new dissociation pathway from the triplet surface.
The molecular properties of chlorosyl fluoride, FClO, as determined from the ground-state rotational spectrum
Authors: Müller Holger S. P. , Cohen Edward A.
Published: July 2002

The pure rotational spectrum of chlorosyl fluoride was studied by microwave Fourier transform spectroscopy and conventional millimeter and submillimeter absorption spectroscopy. More than 100 rotational transitions for each isotopomer were observed involving J⩽73, 61 and Ka⩽19, 16 for F35ClO and F37ClO, respectively. The analysis yielded precise rotational, centrifugal distortion, Cl35,37 nuclear quadrupole, and F19 and Cl35,37 nuclear spin–rotation coupling constants. The spin–rotation constants were used to derive nuclear magnetic shielding values. All nonzero elements of the Cl quadrupole tensor were obtained, permitting its diagonalization. The dipole moment was determined by Stark measurements in the millimeter region. The ground-state average structure rz and an estimate of the equilibrium structure re were calculated. The properties derived for FClO were compared with those of ClF3 and other related molecules. Structural parameters, harmonic force constants, the dipole moment, and the nuclear quadrupole coupling constants were also evaluated by means of quantum-chemical calculations. The results are in good agreement with experiment.
Electronic excitation spectra of Cl2O, ClOOCl, and F2O: A SAC-CI study
Authors: Tomasello P. , Ehara M. , Nakatsuji H.
Published: July 2002

The SAC-CI method was applied to the electronic excitation spectra of Cl2O, ClOOCl, and F2O. The ensuing theoretical spectra have well reproduced the experimental spectra for Cl2O and Cl2O2 and detailed characterizations of the peaks and structures were given by calculating the oscillator strength, second moment, and dipole moment for each state. For the UV region of Cl2O, the observed peaks were assigned to the valence excited states in agreement with the previous theoretical studies. For the VUV region, in the present study we have proposed the first theoretical assignments. Both valence and Rydberg excited states were calculated in this energy region, some of which resulted different from the experimental assignments. For Cl2O2, the broad band of the experimental spectrum was mainly attributed to one intense peak due to the 3B state. The theoretical spectrum above 6 eV was proposed, which so far has not been addressed and clarified before. The excitation spectrum of F2O up to about 13.0 eV was calculated and compared with the previous theoretical study. The shape and main features of the calculated spectrum was found to be very similar to that of Cl2O, and the valence and Rydberg excited states were clearly discriminated.
Viscosity dependence of geminate recombination efficiency after bimolecular charge separation
Authors: Neufeld A. A. , Burshtein A. I. , Angulo G. , Grampp G.
Published: July 2002

The efficiency of geminate recombination after bimolecular ionization of an excited electron donor (or acceptor) is studied. For ions starting from inside or outside the recombination layer, the recombination efficiency has the opposite viscosity dependence. As a result the latter becomes nonmonotonous, provided the kinetic controlled ionization gives way to a diffusional one, creating the more remote ions the higher the solvent viscosity. This effect, first discovered experimentally, receives its explanation here, which is conceivable only on the basis of the Integral Encounter Theory of remote electron transfer in the liquid state.
Microwave rotational spectra of the Ar3–NH3 van der Waals tetramer
Authors: van Wijngaarden Jennifer , Jäger Wolfgang
Published: July 2002

Rotational spectra of the Ar3–NH3 van der Waals tetramer were recorded between 4 and 17 GHz using a pulsed jet Fourier transform microwave spectrometer. Five isotopomers, namely Ar3–NH3, Ar3–15NH3, Ar3–ND3, Ar3–ND2H, and Ar3–NDH2, were studied and the spectra observed are characteristic of oblate symmetric tops. The transitions were assigned to three different K progressions, K=0, K=3, and K=6 of the ground internal rotor state of the complex. A small inversion tunneling splitting was observed for each of the deuterium containing isotopomers. For Ar3–NH3 and Ar3–15NH3, one of these inversion components has a spin statistical weight of zero in the ground state and is, therefore, not observed. The N14 quadrupole hyperfine structure was resolved and included in the fit of the spectroscopic constants for each of the N14 containing isotopomers. The resulting spectroscopic constants were used to estimate the structure of the complex. Furthermore, the N14 nuclear quadruple coupling constants and the inversion tunneling splittings observed for the various isotopomers contain information about the dynamics of the ammonia subunit within the Ar3–NH3 complex.
Nonadiabatic quantum reactive scattering calculations for the O(1D)+H2, D2, and HD reactions on the lowest three potential energy surfaces
Authors: Takayanagi Toshiyuki
Published: July 2002

Time-independent three-dimensional quantum reactive scattering calculations including the effect of electronically nonadiabatic coupling have been carried out for the O(1D)+H2, D2, and HD reactions using the recent ab initio versions of the lowest three potential energy surfaces (1 1A′, 2 1A′, and 1 1A″) of Dobbyn and Knowles. The hyperspherical close-coupling technique has been used and the calculations have been carried out only for zero total angular momentum (J=0). We present total reaction probabilities, the effect of initial rotational excitation, and cumulative reaction probabilities. We found that electronically nonadiabatic transitions are very important for these reactions similar to previous nonadiabatic wave packet calculations using the same surfaces but found isotopic substitution does not largely affect the nonadiabatic reaction dynamics. We also calculated the OH/OD isotopic branching fraction for the O(1D)+HD reaction and found that the OD+H production channel is dominant over the OH+D channel in the energy range considered.
Formation of nanoclusters under radiation pressure in solution: A Brownian dynamics simulation study
Authors: Jose Prasanth P. , Bagchi Biman
Published: July 2002

When radiation is scattered by a medium, a part of its momentum is transferred to the target particles. This is purely a mechanical force which comes into effect when radiation is not coherently interacting. This force is known in literature as radiation pressure. Recent experimental studies have demonstrated the feasibility of using radiation pressure of a laser beam as a tool for cluster formation in solution. In this paper we describe the Brownian dynamics simulation of solute molecules under the perturbation induced by laser radiation. Here the force field generated by a laser beam in the fundamental mode is modeled as that of a two-dimensional harmonic oscillator. The radial distribution function of the perturbed system gives indication of high inhomogeneities in the solute distribution. An explicit analysis of the nature of these clusters is carried out by calculating the density–density correlation functions in the plane perpendicular to beam direction g(rxy); and along the direction of beam g(z), they give an average picture of shell structure formation in the different directions. The relaxation time of the first shell structure calculated from the van Hove correlation function is found to be relatively large in the perturbed solution. This is the signature of formation of stable nanoclusters in the presence of the radiation field. Our study on the dynamics of solute molecules during the cluster formation and dissolution gives the duration of collective relaxation, far away from the equilibrium to an equilibrium distribution. This relaxation time is found to be large for a perturbed solution.
Centroid-based methods for calculating quantum reaction rate constants: Centroid sampling versus centroid dynamics
Authors: Shi Qiang , Geva Eitan
Published: July 2002

A new method was recently introduced for calculating quantum mechanical rate constants from centroid molecular dynamics (CMD) simulations [E. Geva, Q. Shi, and G. A. Voth, J. Chem. Phys. 115, 9209 (2001)]. This new method is based on a formulation of the reaction rate constant in terms of the position-flux correlation function, which can be approximated in a well defined way via CMD. In the present paper, we consider two different approximated versions of this new method, which enhance its computational feasibility. The first approximation is based on propagating initial states which are sampled from the initial centroid distribution, on the classical potential surface. The second approximation is equivalent to a classical-like calculation of the reaction rate constant on the centroid potential, and has two distinct advantages: (1) it bypasses the problem of inefficient sampling which limits the applicability of the full CMD method at very low temperatures; (2) it has a well defined TST limit which is directly related to path-integral quantum transition state theory (PI-QTST). The approximations are tested on a model consisting of a symmetric double-well bilinearly coupled to a harmonic bath. Both approximations are quite successful in reproducing the results obtained via full CMD, and the second approximation is shown to provide a good estimate to the exact high-friction rate constants at very low temperatures.
Creation and annihilation of 360° domain walls in magnetic tunnel junctions with exchange-biased artificial ferrimagnet
Authors: Boeve H. , Esparbe L. , Gieres G. , Bär L. , Wecker J. , Brückl H.
Published: July 2002

In state-of-the-art read heads, exchange-biased artificial ferrimagnet hard layers are extensively used. In this paper, we analyze different magnetic tunnel junction stacks with respect to the formation and annihilation of 360° walls in their hard subsystem, visualized using magnetic force microscopy. The existence of 360° walls in a magnetic tunnel junction has a microscopic origin, i.e., the local competition between different coupling mechanisms in the layered system, and can qualitatively be interpreted from a strong but reversible reduction in minor loop tunnel magnetoresistance. Therefore, the magnetic history of a tunnel junction is of great importance in the interpretation of minor loop measurements.
Potential energy surface and lower bound states of HCCH-Cl−
Authors: Meuwly Markus , Wolynec P. Peter , Bieske Evan J.
Published: July 2002

Lower lying rovibrational energy levels of the acetylene-Cl− anion complex are calculated using an ab initio potential energy surface (PES) computed at MP2/aug-cc-pVTZ level. The PES is characterized by a deep minimum in the linear configuration (De=3760 cm−1, Re=3.97 Å) and a purely repulsive interaction for a T-shaped configuration. Rovibrational calculations are performed on PESs that are adiabatically corrected for the influence of the local mode vibration of the intermediate proton (hydrogen-bonded C-H stretch mode). The adiabatic correction leads to an enhanced intermolecular interaction with a deepening of the potential well and shortening of the equilibrium intermolecular separation. These effects becomes more pronounced as additional quanta are added to the hydrogen-bonded νCH stretch mode. Band energies and rotational constants are calculated for states likely to be observable in spectroscopic searches. The predicted complexation-induced frequency shift for the acetylene ν3 vibration is −446 cm−1.
Aging and memory properties of topologically frustrated magnets
Authors: Dupuis V. , Vincent E. , Hammann J. , Greedan J. E. , Wills A. S.
Published: July 2002

The model 2d kagomé system (H3O)Fe3(SO4)2(OH)6 and the 3d pyrochlore Y2Mo2O7 are two well characterized examples of low-disordered frustrated antiferromagnets which rather than condensing into spin liquid have been found to undergo a freezing transition with spin glass-like properties. We explore more deeply the comparison of their properties with those of spin glasses, by the study of characteristic rejuvenation and memory effects in the nonstationary susceptibility. While the pyrochlore shows clear evidence for these nontrivial effects, implying temperature selective aging, that is characteristic of a wide hierarchical distribution of equilibration processes, the kagomé system does not clearly show these effects. Rather, it seems to evolve towards the same final state independently of temperature.
Homogeneous nucleation near a second phase transition and Ostwald’s step rule
Authors: Tavassoli Z. , Sear R. P.
Published: July 2002

Homogeneous nucleation of the new phase of one transition near a second phase transition is considered. The system has two phase transitions, we study the nucleation of the new phase of one of these transitions under conditions such that we are near or at the second phase transition. The second transition is an Ising-type transition and lies within the coexistence region of the first transition. The first transition can be any strongly first-order phase transition. It effects the formation of the new phase in two ways. The first is by reducing the nucleation barrier to direct nucleation. The second is by the system undergoing the second transition and transforming to a state in which the barrier to nucleation is greatly reduced. The second way occurs when the barrier to undergoing the second phase transition is less than that of the first phase transition, and is in accordance with Ostwald’s rule.
A quasiclassical trajectory study of O(1D)+HCl reactive scattering on an improved ab initio surface
Authors: Christoffel Kurt M. , Bowman Joel M.
Published: July 2002

We report quasiclassical trajectory calculations for the reaction O(1D)+HCl using a recent modification of the potential of Peterson et al. [J. Chem. Phys. 111, 7446 (1999)] based on extensive additional ab initio calculations. The cross sections and branching ratio for the OH and ClO products are examined for HCl(v=0,j=0) at relative translational energies up to 0.53 eV. Comparisons are made with previous quantum calculations of this reaction using the same potential energy surface. The effects of vibrational and rotational excitation of the diatomic target and of the orientation of the rotational angular momentum on this branching ratio are also investigated.
Anion photoelectron spectroscopy of VnOm− (n=4–15;m=0–2)
Authors: Pramann Axel , Koyasu Kiichirou , Nakajima Atsushi , Kaya Koji
Published: July 2002

The anion photoelectron (PE) spectra of small mass-selected vanadium oxide clusters VnOm− (n=4–15; m=0–2) are measured at a fixed photon energy of 4.66 eV with the aid of a magnetic bottle photoelectron spectrometer. Cluster anions are generated in a pulsed laser vaporization cluster source. The electronic structure of VnOm− clusters is investigated as a function of size n and composition m with special regard to the increasing oxidation state. The addition of one or two oxygen atoms to the vanadium cluster core induces a change of the electronic structure in the near-threshold binding energy region below 2 eV. Main spectral features are contributed from the transition metal d-derived orbitals, whereas the oxygen 2p contribution induces a hybridization between vanadium and oxygen frontier orbitals in the entire series of the investigated clusters n=4–15. Generally, electron affinities and vertical detachment energies increase with increasing cluster size revealing size-dependent discontinuities. Furthermore, relative dissociation energies for different oxygen-loss channels from the parent VnOm− clusters are determined from thermochemical cycles, and first insights into the stability patterns of neutral and negatively charged vanadium oxide clusters are provided.
Exchange potential from the common energy denominator approximation for the Kohn–Sham Green’s function: Application to (hyper)polarizabilities of molecular chains
Authors: Grüning M. , Gritsenko O. V. , Baerends E. J.
Published: July 2002

An approximate Kohn–Sham (KS) exchange potential vxσCEDA is developed, based on the common energy denominator approximation (CEDA) for the static orbital Green’s function, which preserves the essential structure of the density response function. vxσCEDA is an explicit functional of the occupied KS orbitals, which has the Slater vSσ and response vrespσCEDA potentials as its components. The latter exhibits the characteristic step structure with “diagonal” contributions from the orbital densities |ψiσ|2, as well as “off-diagonal” ones from the occupied–occupied orbital products ψiσψj(≠1)σ*. Comparison of the results of atomic and molecular ground-state CEDA calculations with those of the Krieger–Li–Iafrate (KLI), exact exchange (EXX), and Hartree–Fock (HF) methods show, that both KLI and CEDA potentials can be considered as very good analytical “closure approximations” to the exact KS exchange potential. The total CEDA and KLI energies nearly coincide with the EXX ones and the corresponding orbital energies εiσ are rather close to each other for the light atoms and small molecules considered. The CEDA, KLI, EXX–εiσ values provide the qualitatively correct order of ionizations and they give an estimate of VIPs comparable to that of the HF Koopmans’ theorem. However, the additional off-diagonal orbital structure of vxσCEDA appears to be essential for the calculated response properties of molecular chains. KLI already considerably improves the calculated (hyper)polarizabilities of the prototype hydrogen chains Hn over local density approximation (LDA) and standard generalized gradient approximations (GGAs), while the CEDA results are definitely an improvement over the KLI ones. The reasons of this success are the specific orbital structures of the CEDA and KLI response potentials, which produce in an external field an ultranonlocal field-counteracting exchange potential.
Homogeneous nucleation rates of n-propanol in nitrogen measured in a piston-expansion tube
Authors: Graßmann A. , Peters F.
Published: July 2002

Homogeneous nucleation rates of n-propanol in nitrogen are presented. They are obtained from a piston-expansion tube involving the nucleation pulse method which generates a limited number of nuclei that grow into droplets. The detection of the droplets is achieved by a counting method based on a digital charge coupled device camera in combination with a laser light sheet. Nucleation rates between 104 and 1010 cm−3 s−1 are covered for nucleation temperatures between 242 and 273 K. The rates are plotted as isotherms versus supersaturation. Best fit curves are provided for data representation. As in a previous case dealing with n-pentanol nucleation a weak influence of the initial expansion temperature on the nucleation rate is substantiated and explained. Comparison with literature data and classical nucleation theory is provided.
Torsional path integral Monte Carlo method for the quantum simulation of large molecules
Authors: Miller Thomas F. , Clary David C.
Published: July 2002

A molecular application is introduced for calculating quantum statistical mechanical expectation values of large molecules at nonzero temperatures. The Torsional Path Integral Monte Carlo (TPIMC) technique applies an uncoupled winding number formalism to the torsional degrees of freedom in molecular systems. The internal energy of the molecules ethane, n-butane, n-octane, and enkephalin are calculated at standard temperature using the TPIMC technique and compared to the expectation values obtained using the harmonic oscillator approximation and a variational technique. All studied molecules exhibited significant quantum mechanical contributions to their internal energy expectation values according to the TPIMC technique. The harmonic oscillator approximation approach to calculating the internal energy performs well for the molecules presented in this study but is limited by its neglect of both anharmonicity effects and the potential coupling of intramolecular torsions.
Controlling structural organization of binary phase-separating fluids through mobile particles
Authors: Tang Yan-li , Ma Yu-qiang
Published: July 2002

The structural organization in binary immiscible fluids is studied in the presence of mobile particles. We show that the particles, which are included in the favorable phase due to a selective affinity and assemble therein, can drive the favorable phase to self-organize into cluster, interconnected, and network structures, depending on the particle concentration. The detailed studies enable us to confirm the physical origin of the morphology transitions: At low particle concentration, the assembly of particles would amplify the spatial heterogeneity by depleting the favorable phase in the particle-free region, where the hydrodynamic flows further accelerate the dropletlike formation of favorable phase; with increasing the particle concentration, the assembly of particles becomes energetically unfavorable due to the considerably excluded volume effect of particles and they tend to stabilize the percolated structure of the favorable phase. Furthermore, the respective crossover behaviors of the growth dynamics are also observed, corresponding to different structure formations.
C
70
adsorbed on Cu(111): Metallic character and molecular orientation
Authors: Goldoni A. , Cepek C. , Larciprete R. , Sangaletti L. , Pagliara S. , Floreano L. , Gotter R. , Verdini A. , Morgante A. , Luo Y. , Nyberg M.
Published: July 2002

We investigated the interaction of C70 with the Cu(111) surface using x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The data point to a net charge transfer from the Cu substrate to the C70 molecules directly bonded to the Cu atoms, providing a metallic character for a single layer of C70 (monolayer) adsorbed on this surface. A strong dependence of the x-ray absorption spectra from the light polarization is also observed. By comparing these experimental data with density-functional calculations of the contribution to the x-ray absorption spectral line shape of the five inequivalent carbon atoms in the C70 cage, we observed that the molecules are mainly oriented with the C5V axis perpendicular to the Cu surface.
SF
6
and its clusters in solid parahydrogen studied by infrared spectroscopy
Authors: Katsuki Hiroyuki , Momose Takamasa , Shida Tadamasa
Published: July 2002

The triply degenerate stretching vibration of sulfur hexafluoride and its clusters in solid parahydrogen at cryogenic temperatures is studied using Fourier transform infrared spectroscopy. The observed spectra are compared with theoretical spectra constructed on the basis of the intermolecular resonant dipole–dipole and the dipole–induced-dipole interactions. The absorptions due to monomer, dimer, trimer, and tetramer are discussed individually.
Precessional switching of submicrometer spin valves
Authors: Kaka S. , Russek S. E.
Published: July 2002

Small spin valves are switched using a subnanosecond field pulse applied along the magnetization hard axis. The measured probability for switching due to pulsed hard-axis fields increases as the duration is decreased from 325 to 230 ps. This indicates a large-angle precessional motion in response to the transverse applied field. The behavior is modeled with a single-domain, Landau–Lifshitz simulation. Switching in this manner requires only single-polarity transverse pulses that toggle the state of magnetic devices. This process consumes less energy than traditional quasi-static switching using long-duration field pulses along both directions.
Mesoscale structure and fractal nature of 1-propanol aqueous solution: A reverse Monte Carlo analysis of small angle neutron scattering intensity
Authors: Misawa M.
Published: July 2002

Mesoscale structures of salt-free and salt-added 1-propanol aqueous solutions have been analyzed by means of a reverse Monte Carlo (RMC) technique combined with small angle neutron scattering data. RMC analysis is carried out by using sub-million molecules and concentration-fluctuation is visualized. Effect of salt on the mesoscale structure is also visualized. Origin of fractal nature of the solution is attributed to the distribution of [gpp(r)−gpw(r)] and [gww(r)−gwp(r)]. 1-propanol-rich regions are surrounded by thin water-layers in the salt-free solution. The addition of salt shifts the thickness distributions of the water-layer to the larger value. However, the thickness distribution is scaled very well between the salt-free solution and the salt-added solution, suggesting a self-similarity of mesoscale structure of both solutions. Formation of concentration-fluctuation and salt-induced phase separation are discussed.
Metric-independent analysis of the stress-energy tensor
Authors: Segev Reuven
Published: July 2002

The stress-energy tensor of field theory is defined and analyzed in a geometric setting where a metric is not available. The stress is a linear mapping that transforms the three-form representing the flux of any given property, e.g., charge-current density, to the three-form representing the flux of energy. The example of the electromagnetic stress-energy tensor is given with the additional structure of a volume element.
Chaotic scattering, unstable periodic orbits, and fluctuations in quantum transport
Authors: Jensen R. V.
Published: July 2002

The field of quantum chaos has recently focused attention on the quantum description of chaotic scattering processes. The new physical intuition, analytical methods, and numerical tools developed in the study of the quantum behavior of classically chaotic bound systems, like quantum billiards or atoms in strong fields, has led to exciting new predictions for the scattering of electromagnetic waves in curved waveguides, electrons in mesoscopic wires, and atoms off molecules. After a brief review of recent progress in the field of quantum chaos, this paper focuses on specific results relating to ballistic electron transport in small, mesoscopic devices. Several specific geometries are suggested for experimental studies of this ‘‘game’’ of quantum pinball with explicit predictions for the fluctuations in the electrical conductivity as functions of the electron Fermi energy and of an applied magnetic field.
Multidimensional Hamiltonian chaos
Authors: Shilnikov L. P.
Published: July 2002

In this review the description of nontrivial hyperbolic sets of Hamiltonian systems having singular points (a saddle, a saddle-focus) with homoclinic orbits is given. On the basis of Melnikov’s method the criteria for the existence of homoclinic orbits to the singular points in systems close to integrable ones are pointed out.
Nonintegrable field equations and homoclinic loops of Hamiltonian systems
Authors: Eleonsky V. M. , Lerman L. M.
Published: July 2002

Connections between soliton or self-localized states of nonlinear wave equations and special objects (homo- and heteroclinic trajectories) of dynamical systems are considered.
Phase-locking effects in a system of nonlinear oscillators
Authors: Ostrovskiĭ L. A. , Soustova I. A.
Published: July 2002

Models for the excitation of collective modes in a system of nonlinear classical oscillators, initially out of phase, are discussed. The oscillators may be coupled in a dissipative or conservative manner. The analysis is based on the results of recent studies dealing with the problem of the free excitation of a coherent pulse, analogous to ‘‘superradiance’’ in two-level quantum systems. Several physical examples from the realms of electrodynamics and acoustics are discussed. The processes discussed here may be thought of as chaos–order transitions, provided that ‘‘chaos’’ is understood not as a stochastic nature of an individual oscillator but as the absence of a coherent component in their collective field.
Vortex dynamics in oscillatory chemical systems
Authors: Wu Xiao-Guang , Chee Merk-Na , Kapral Raymond
Published: July 2002

Vortex core dynamics is studied in the Brusselator both near to and far from the Hopf bifurcation line for random and pair initial conditions. Extensive simulations are carried out for a pair of counter-rotating vortices close to the Hopf bifurcation line. Provided the vortices are not so far apart that wave-front annihilation produces strong gradients between their centers, the simulation results compare favorably with theories based on the complex Ginzburg–Landau equation. Far from the Hopf line the vortex core dynamics changes character and phenomena such as periodic motion of the vortex centers arise.
Coupled trivial maps
Authors: Bunimovich L. A. , Livi R. , Martínez-Mekler G. , Ruffo S.
Published: July 2002

The first nontrivial example of coupled map lattices that admits a rigorous analysis in the whole range of the strength of space interactions is considered. This class is generated by one-dimensional maps with a globally attracting superstable periodic trajectory that are coupled by a diffusive nearest-neighbor interaction.
Fractal dimension of steady nonequilibrium flows
Authors: Hoover William G. , Posch Harald A. , Hoover Carol G.
Published: July 2002

The Kaplan–Yorke information dimension of phase-space attractors for two kinds of steady nonequilibrium many-body flows is evaluated. In both cases a set of Newtonian particles is considered which interacts with boundary particles. Time-averaged boundary temperatures are imposed by Nosé–Hoover thermostat forces. For both kinds of nonequilibrium systems, it is demonstrated numerically that external isothermal boundaries can drive the otherwise purely Newtonian flow onto a multifractal attractor with a phase-space information dimension significantly less than that of the corresponding equilibrium flow. Thus the Gibbs’ entropy of such nonequilibrium flows can diverge.
The semiclassical functional equation
Authors: Keating Jonathan
Published: July 2002

A semiclassical analog of the functional equation for the Riemann zeta function is considered. In the case of the zeta function itself, this equation forms the basis for a finite approximation to the Dirichlet series, known as the approximate functional equation. In the same way, the semiclassical functional equation can be shown to give rise to a finite approximation to the semiclassical representation of the quantum spectral determinant as a sum over classical pseudo-orbits. This finite approximation has been called the Riemann–Siegel look-alike formula. The formal nature of the derivation of this result is discussed and the fact that it appears to imply a remarkable relationship between long and short pseudo-orbits is shown.