bluuebunny/crossref_metadata_2025 huggingface dataset:
Differential geometry of group lattices
Authors: Dimakis Aristophanes , Müller-Hoissen Folkert
Published: March 2003

In a series of publications we developed “differential geometry” on discrete sets based on concepts of noncommutative geometry. In particular, it turned out that first-order differential calculi (over the algebra of functions) on a discrete set are in bijective correspondence with digraph structures where the vertices are given by the elements of the set. A particular class of digraphs are Cayley graphs, also known as group lattices. They are determined by a discrete group G and a finite subset S. There is a distinguished subclass of “bicovariant” Cayley graphs with the property ad(S)S⊂S. We explore the properties of differential calculi which arise from Cayley graphs via the above correspondence. The first-order calculi extend to higher orders and then allow us to introduce further differential geometric structures. Furthermore, we explore the properties of “discrete” vector fields which describe deterministic flows on group lattices. A Lie derivative with respect to a discrete vector field and an inner product with forms is defined. The Lie–Cartan identity then holds on all forms for a certain subclass of discrete vector fields. We develop elements of gauge theory and construct an analog of the lattice gauge theory (Yang–Mills) action on an arbitrary group lattice. Also linear connections are considered and a simple geometric interpretation of the torsion is established. By taking a quotient with respect to some subgroup of the discrete group, generalized differential calculi associated with so-called Schreier diagrams are obtained.
Relativistic nuclear magnetic resonance chemical shifts of heavy nuclei with pseudopotentials and the zeroth-order regular approximation
Authors: Yates Jonathan R. , Pickard Chris J. , Payne Mike C. , Mauri Francesco
Published: March 2003

We present a method for the first principles density functional calculation of relativistic all-electron nuclear magnetic resonance chemical shifts using pseudopotentials. The method is based on the gauge including projector augmented wave approach of Pickard and Mauri [Phys. Rev. B 63, 245101 (2001)]. Relativistic effects are included at the level of the scalar-relativistic zeroth-order regular approximation. The method allows chemical shifts of large, low symmetry structures containing heavy elements to be calculated efficiently. We demonstrate its success for a range of Se and Te containing molecules.
Temperature dependence of the critical current of YBCO–STO–LCMO heterostructures near Tc
Authors: Khokhlov V. A. , Prokhorov A. Yu. , Drobotko V. F. , Levchenko G. G. , Mikheenko P. N. , Chakalov R. , Muirhead C.
Published: March 2003

The complex differential susceptibility of a YBCO film and YBCO–STO–LCMO structures with different thicknesses of the STO layer (0, 2, and 7 nm) is investigated. It is shown that for a YBCO–LCMO structure the temperature of the superconducting transition Tonset and the critical current density jc are observed to decrease in comparison with a pure YBCO film. In a YBCO–STO–LCMO structure, on the contrary, Tonset increases, while jc increases for the structure with the 2 nm thickness of STO and decreases for that with 7 nm of STO. It is conjectured that there is a change of the vortex pinning mechanism as the temperature is lowered.
Observation of the strain-driven charge-ordered state in a La0.7Ca0.3MnO3−δ thin film with oxygen deficiency
Authors: Prokhorov V. G. , Komashko V. A. , Kaminsky G. G. , Svetchnikov V. L. , Zandbergen H. W. , Lee Y. P. , Park J. S. , Kim K. W.
Published: March 2003

The magnetic and transport properties of La0.7Ca0.3MnO3−δ films with an oxygen deficiency (δ≃0.1) and a La0.9Ca0.1MnO3 film with the stoichiometric oxygen content are investigated in a wide temperature range. It is shown that the charge-ordered insulating (COI) state is observed for a La0.7Ca0.3MnO2.9 film with thickness d⩽30 nm, which manifests mainly a cubic crystal structure with an anomalously small lattice parameter for this composition. An increase in the film thickness (d≃60 nm) leads to a structural transition from the lattice-strained cubic to the relaxed rhombohedral phase and is accompanied by a shift of the Curie point (TC) to lower temperature and a frustration of the COI state. The magnetic and transport properties of the La0.7Ca0.3MnO2.9 film with d≃60 nm are similar to those exhibited by the optimally oxygen-doped La0.9Ca0.1MnO3 film. It is concluded that the formation of the COI state in the La0.7Ca0.3MnO3−δ compound is governed by a compression of the crystal lattice rather than accumulation of oxygen vacancies, the low doping of the substituted divalent ions, or electronic phase separation.
Point-contact spectroscopy of the relaxation dynamics of two-level systems upon structural changes in Ni–Nb glasses
Authors: Balkashin O. P. , Yanson I. K. , Halbritter A. , Mihály G.
Published: March 2003

The point-contact spectra (the energy dependence of the second derivatives of the current–voltage characteristics) of bimetal contacts of the amorphous alloy Ni59Nb41 with silver are investigated in the frequency range 103–5×109 Hz. At low energies the spectral feature due to the scattering of conduction electrons on two-level tunneling systems in point contacts of the quenched alloy is clearly observed in the microwave region. This confirms the conclusion reached previously by the authors for homocontacts [O. P. Balkashin et al., Solid State Commun. 118, 623 (2001)] that rapidly relaxing two-level systems with a characteristic relaxation frequency Γ>1011 Hz are present in metallic glasses. After a high-temperature annealing of the amorphous alloy (for 1 hour at T=800 °C) and its crystallization, the intensity of the spectral feature in the region of low voltages across the contact increases substantially. A sharp frequency dispersion of the amplitude of this feature is observed; it vanishes completely for measurements at frequencies below ∼5×109 Hz. The data attest to the formation of qualitatively new, slowly relaxing two-level systems in the annealed Ni–Nb amorphous alloys. An estimate of the characteristic relaxation frequency of the two-level systems from the frequency dispersion of the measured signal gives a value Γ/2π≈0.7×109 Hz. The possible causes of the formation of two-level systems in point contacts are discussed.
On the nature of the contributions to the entropy of Pauling ice
Authors: Lokotosh T. V. , Gorun O. M.
Published: March 2003

A calculation is presented for the entropy and correlation functions of Pauling ice—a model in which the energy of the microstates can take only two values: zero or infinity. The central point of the proposed approach is the use of a canonical expansion of the thermodynamic functions in irreducible multiparticle correlations. Reduction rules are formulated which establish the relation between the correlation functions of orders 1, …, k−1 and the kth-order correlation function of the system. The latter is calculated on the basis of the assumption of equal probabilities of all allowed configurations of a compact group of k particles and the normalization condition. The order of approximation is determined by the number of particles k for which the correlations between states is taken into account. It is shown that with increasing k the values of the entropy of two-dimensional “square ice” converge nonmonotonically to its exact value obtained by Lieb. The best agreement corresponds to those approximations in which the group of k particles has the lattice symmetry and contains closed loops of hydrogen bonds. The method can be extended to arbitrary lattice systems.
Theoretical and experimental study of the chemisorption of 1,3 disilabutane on the Si(100) surface
Authors: Valente G. , Stoldt C. R. , Maboudian R. , Carraro C.
Published: March 2003

The adsorption of 1,3 disilabutane on Si(100)-(2×1) is studied both computationally and experimentally. First, the possible adsorption species are calculated through density functional theory using the Becke three parameter Lee–Yang–Parr functional. The Si9H12 cluster is adopted to simulate the Si(100) dimer. Frequency calculations are also performed to find the harmonic frequencies and infrared intensities of the calculated species. Adsorption experiments are then performed on the Si(100)-(2×1) surface. The surface is subsequently characterized using high-resolution electron energy loss spectroscopy. By comparing calculated and experimental spectra, it is found that the most probable adsorption product is the C2H9Si2 species bonded to the surface through a silicon–silicon bond. To confirm this finding, the transition state of this reaction is calculated and compared with other possible adsorption paths. It is found that the chemisorption reaction leading to silicon–silicon bonded C2H9Si2 is the most probable reaction with an activation energy of about 11 kcal/mol.
Glassy states of clusters with pairwise atomic interactions
Authors: Berry R. Stephen , Smirnov Boris M.
Published: March 2003

The glassy state of argon is examined from the standpoint that this state of a system is an ensemble of frozen excited configurations. The basis is experimental data for deposition of an argon stream on a very cold substrate; the measurements provide the dependence of the saturated vapor pressure over this system. The immobile state prepared by such deposition is an amorphous argon, and the activation energy for its transition to the crystalline state is found from a theoretical model. A criterion is established for the minimum cooling rate of liquid rare gases to form the glassy state. An analogous glassy state for clusters is analyzed, and a minimum rate of cooling for its formation is also developed. The glassy state of a bulk system of atoms bound by pair interactions, thermodynamically unstable, relaxes to equilibrium by diffusion of voids to the boundary; the corresponding phase change of a cluster can be thought of at least as easily in terms of diffusion of atoms to sites in its lowest-energy, ordered configuration. A common feature of these small systems is the way their counterparts of the glassy state form from fast cooling of liquid aggregate states to become frozen liquid states.
Addition of water and methanol to Al3O3− studied by mass spectrometry and anion photoelectron spectroscopy
Authors: Akin F. Ahu , Jarrold Caroline Chick
Published: March 2003

The 4.66 eV photoelectron spectra of Al3O3−, Al3O3−⋅solvent and Al3O3−⋅(solvent)2 (solvent=H2O, D2O, and CH3OH) have been obtained and analyzed in the context of existing and preliminary new density functional theory calculations. The structures and vibrational frequencies of the two isomers of Al3O3− and Al3O3 proposed by Ghanty and Davidson [J. Phys. Chem. A 103, 8985 (1999)] agree well with structural information extracted from the Al3O3− spectra using Franck–Condon simulations. Photoelectron spectra of Al3O3−⋅solvent complexes [EA=2.5(1) eV] are broad and congested, and hydroxide formation, multiple structural isomers, and anion photodissociation are suggested as possible sources of this. The photoelectron spectra of Al3O3−⋅(solvent)2 complex spectra [EA=3.05(10) eV] show two distinct electronic transitions, several of which exhibit partially-resolved vibrational structure that are similar to the two electronic bands attributed to the bare rectangular structural isomer of Al3O3−. Possible adsorption scenarios are suggested, with hydroxide formation being the most consistent with trends observed in both the photoelectron and mass spectra.
Poiseuille advection of chemical reaction fronts: Eikonal approximation
Authors: Spangler Robert S. , Edwards Boyd F.
Published: March 2003

An eikonal equation including fluid advection is derived from the cubic reaction-diffusion-advection equation, and is used to investigate the speeds and shapes of chemical reaction fronts subject to Poiseuille flow between parallel plates. Although the eikonal equation is usually regarded as valid when the front thickness is small compared to the radius of curvature of the front and to the size of the system, it is also found to be valid when the reaction front is thick with respect to the gap width. This new regime of applicability of the eikonal equation is consistent with its derivation, which requires only that the reaction front curvature and the fluid velocity vary negligibly across the front. The front distortion and the front speed increase with increasing η, defined as the ratio of the gap half-width to the reaction front thickness. Analytical limits of the front distortion and front velocity for small and large η are compared with general numerical results.
Wave packet dynamics along bifurcating reaction paths
Authors: Lasorne B. , Dive G. , Lauvergnat D. , Desouter-Lecomte M.
Published: March 2003

The problem of bifurcating reaction paths is revisited by wave packet (WP) dynamics. The pitchfork model connecting five stationary points—a reactive, two transition structures and two enantiomeric products—is characterized by a Valley Ridge inflection point (VRI) where WP could leave the standard intrinsic reaction path. We question the role of such a VRI point to determine whether the mechanism is sequential or concerted. WP simulations on two-dimensional minimum energy surfaces are carried out in the benchmark case of the methoxy radical isomerization H3CO→H2COH. The ab initio potential energy surface (PES) is fitted to an analytical model which is bent to analyze the incidence of geometrical parameters on the WP behavior. For each of these generated PES, the WP width in the entrance valley is the main factor which conditions the behavior on the unstable ridge. The WP evolution is also analyzed in terms of nonadiabatic transitions among adiabatic channels along the reaction coordinate. Finally, the location of VRI points according to an invariant definition is discussed.
Origin of scaling behavior of protein packing density: A sequential Monte Carlo study of compact long chain polymers
Authors: Zhang Jinfeng , Chen Rong , Tang Chao , Liang Jie
Published: March 2003

Single domain proteins are thought to be tightly packed. The introduction of voids by mutations is often regarded as destabilizing. In this study we show that packing density for single domain proteins decreases with chain length. We find that the radius of gyration provides a poor description of protein packing but the alpha contact number we introduce here characterize proteins well. We further demonstrate that protein-like scaling relationship between packing density and chain length is observed in off-lattice self-avoiding walks. A key problem in studying compact chain polymers is the attrition problem: It is difficult to generate independent samples of compact long self-avoiding walks. We develop an algorithm based on the framework of sequential Monte Carlo and succeed in generating populations of compact long chain off-lattice polymers up to length N=2000. Results based on analysis of these chain polymers suggest that maintaining high packing density is only characteristic of short chain proteins. We found that the scaling behavior of packing density with chain length of proteins is a generic feature of random polymers satisfying loose constraint in compactness. We conclude that proteins are not optimized by evolution to eliminate packing voids.
Characterization of magnetically accelerated flyer plates
Authors: Lemke R. W. , Knudson M. D. , Hall C. A. , Haill T. A. , Desjarlais P. M. , Asay J. R. , Mehlhorn T. A.
Published: March 2003

The intense magnetic field generated by the 20 megaampere Z machine [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories is being used as a pressure source for material science studies. An application we have studied in great detail involves using the intense magnetic field to accelerate flyer plates (small metal disks) to very high velocities (>20 km/s) for use in shock loading experiments. We have used highly accurate velocity interferometry measurements (error ∼1%) in conjunction with one-dimensional magnetohydrodynamic (MHD) simulation to elucidate details of the flyer dynamics. One-dimensional MHD simulations are able to produce experimental results with a high degree of accuracy, thereby revealing otherwise unobtainable, but useful information about magnetically accelerated flyers on Z. Comparisons of simulation results with time-resolved measurements of velocity from a shock loading experiment involving a 925 μm aluminum flyer are presented. Results show that Joule heating related to magnetic diffusion constrains the minimum possible initial thickness of a flyer.
Two-fluid theory of acoustic-gravity waves in a plasma
Authors: Wahlberg C. , Revenchuk S. M.
Published: March 2003

The dispersion relation for acoustic-gravity waves in a plasma atmosphere with different electron and ion temperatures is derived from a two-fluid description of the plasma. The electrons are assumed to satisfy an isothermal equation of state and the ions an adiabatic equation of state. This model together with the quasi-neutrality condition is shown to reproduce the one-fluid hydrodynamic dispersion relation for acoustic-gravity waves, provided that both the acoustic cut-off frequency and the Brunt–Väisälä frequency are properly expressed in terms of the ion-sound velocity, including the contribution from the ion temperature.
Darboux covariant equations of von Neumann type and their generalizations
Authors: Cieśliński Jan L. , Czachor Marek , Ustinov Nikolai V.
Published: March 2003

Lax pairs with operator valued coefficients, which are explicitly connected by means of an additional condition, are considered. This condition is proved to be covariant with respect to the Darboux transformation of a general form. Nonlinear equations arising from the compatibility condition of the Lax pairs in the matrix case include, in particular, Nahm equations, and Volterra, Bogoyavlenskii and Toda lattices. The examples of other one-, two- and multi-field lattice equations are also presented.
Wave function confinement via transfer matrix methods
Authors: Olson Jeffrey D. , Mace Jonathan Lee
Published: March 2003

The exact transfer matrix approach used in studying sectionally constant potentials in one dimension is generalized to cylindrical and spherical geometries, where the potential depends only on radius. In each geometry two transfer matrices suffice to completely describe the wave function: one for handling a discontinuity in potential and one for handling a delta-function potential barrier. This method is then applied to the problem of confining a wave function in a cylindrical configuration using only a series of carefully placed delta function potential barriers. It is found that confinement can be made to increase nearly exponentially with the number of barriers if placed correctly, but that this arrangement has an exponentially sharp dependence on both barrier position and energy.
Nested wire array Z-pinch experiments operating in the current transfer mode
Authors: Bland S. N. , Lebedev S. V. , Chittenden J. P. , Jennings C. , Haines M. G.
Published: March 2003

Nested wire array Z-pinch experiments carried out on the MAGPIE generator (1.4 MA, 240 ns) [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1553 (1996)] are described. In the experiments, a high inductance connection to the inner array was used to suppress the current flowing through it. This allowed the current division expected in experiments on larger generators to be emulated, where the number of wires in the outer array acts to shield the inner array. Initially the suppression of current resulted in the wires of the inner array remaining as small discrete bodies, while the wires of the outer array ablated, prefilling the array with plasma in an identical way to a single array experiment. During implosion the outer array accelerated towards the inner array, snowploughing up the prefilled plasma, producing a rise in x-ray emission. When the outer array traveled past the inner array, a fast inductive transfer of current occurred between them, and the inner array then accelerated towards the axis where it stagnated to produce the main x-ray pulse. The risetime of the main x-ray pulse was significantly shorter than in single array experiments. The dependence of the shape of the x-ray pulse on the configuration of the outer and inner arrays was explored.
Ionization potentials and structures of small indium monoxide clusters
Authors: Janssens E. , Neukermans S. , Vanhoutte F. , Silverans R. E. , Lievens P. , Navarro-Vázquez A. , v. R. Schleyer P.
Published: March 2003

We report a combined experimental and theoretical study of the structures and ionization potentials of small InNO clusters (N=1–8). The clusters are produced using a laser vaporization cluster source, laser ionized, and mass selectively recorded by a time-of-flight mass spectrometer. Threshold photoionization spectroscopy was performed using photon energies of 4.59–5.96 eV and 6.43 eV. Adiabatic and vertical ionization potentials were compiled from the photoionization efficiency curves. Remarkably low values were obtained for In3O and In7O. Geometric and electronic structures of the InNO and InNO+ clusters were computed with density functional theory using the hybrid B3LYP functional. The bonding in these clusters is analyzed by means of Bader’s atoms in molecules method. Calculated adiabatic and vertical ionization potentials are in good agreement with the experimental values.
Comment on “One-dimensional model of the plasma flow in a Hall thruster” [Phys. Plasmas 8, 3058 (2001)]
Authors: Kundu Mrityunjay , Srinivasan R. , Deshpande S. P.
Published: March 2003

A recent paper has delineated the parameter space for the existence of equilibrium solutions for a Hall thruster, using a one-dimensional fluid model. We would like to point out that an important term is missing in the reduced equation for density. This has the effect of reducing available parameter space for solutions. Corrected equation and other comments are presented.
The three-dimensional magnetohydrodynamics of ac helicity injection in the reversed field pinch
Authors: Ebrahimi F. , Prager S. C. , Sarff J. S. , Wright J. C.
Published: March 2003

ac magnetic helicity injection (also known as oscillating field current drive, OFCD) has been proposed as a technique to sustain the plasma current in a reversed field pinch. The three-dimensional, resistive magnetohydrodynamics computation is employed to examine the full nonlinear dynamics of OFCD, including the behavior of plasma fluctuations and instabilities. The three-dimensional results are also compared with one-dimensional classical and relaxed-state modeling. In OFCD, helicity is injected by oscillating the toroidal and poloidal surface loop voltages. This technique is able to sustain the plasma current, with the edge current mainly driven directly by the OFCD-generated fields, and the core current driven by plasma fluctuations. Fluctuations increase with OFCD, although the increase is concentrated mainly in one global, nearly ideal, mode.
Renormalization of Coulomb interactions for the 1D Dirac equation
Authors: Brzeźniak Zdzisław , Jefferies Brian
Published: March 2003

The Coulomb interaction 1/|x| for the Dirac equation in one space dimension is singular in the sense that there exists a four-parameter family of self-adjoint extensions of the associated Hamiltonian operator. The purpose of this paper is to represent the dynamical group generated by some of the self-adjoint extensions as a path integral. The Feynman–Kac functional we use is constructed by a renormalization process that subtracts divergences as paths cross the isolated singularity x=0 of the interaction.
Magnetohydrodynamic study of adiabatic supersonic and subsonic expansion accelerations in spontaneous fast magnetic reconnection
Authors: Shimizu T. , Ugai M.
Published: March 2003

The thermodynamic supersonic expansion acceleration mechanism associated with the spontaneous fast magnetic reconnection is studied by two-dimensional magnetohydrodynamic (MHD) simulations and the Rankine–Hugoniot analysis. The reconnection outflow jet can steadily exceed the Alfvén velocity measured in the upstream magnetic field region. Such a high speed jet cannot be explained by the Petschek model. According to previous studies, when supersonic (superfast) plasma jets generated by a pair of slow shocks expand in the direction normal to the jet, the jets can be further accelerated beyond the Alfvén velocity by the adiabatic supersonic expansion process. The expansion process is caused by the swelling of the plasmoid (magnetic loop). In this paper, it is theoretically shown that the sound Mach number of the reconnection jet generated by slow shocks is determined by the plasma density and beta value in the upstream magnetic field region, in which asymmetric reconnection models are also studied. Then, the theoretical prediction of the Mach number is related to the onset of the supersonic expansion acceleration process in MHD simulations. In addition, it is shown that, also when the reconnection jet is subsonic, the jet is further accelerated by the adiabatic subsonic expansion mechanism.
Theoretical investigation on the valence ionization spectra of Cl2O, ClOOCl, and F2O by correlation-based configuration interaction methods
Authors: Tomasello Pasquale , Ehara Masahiro , Nakatsuji Hiroshi
Published: March 2003

We report on theoretical valence ionization spectra of molecules relevant in the stratosphere photochemistry obtained by all-electron SAC–CI (symmetry-adapted-cluster–configuration-interaction) calculations. Vertical ionization energies and pole strengths of the one- and two-electron processes of Cl2O and F2O were calculated beyond the energy region so far explored in the few other available theoretical and experimental studies to interpret the electronic structure of these molecules. Early and recently proposed incomplete assignments of the available He I photoelectron spectra are discussed and completed at least up to 20–21 eV binding energy on firmer grounds relying on valuable and accurate results based on different basis sets and an adequate treatment of electron correlations. Our theoretical data predict satellite states of Cl2O starting already in the outer-valence region because of strong correlation effects; the (2b1−1), (8a1−1), and (6b2−1) states interact with the two-electron processes and they split into more-than-two peaks. On the other hand, Koopmans’ picture is valid for the main peaks of F2O and no prominent satellites with strong intensity were found in the outer-valence region. The (4b2−1), (6a1−1), and (1a2−1) were attributed to the second band of F2O, for which different assignments or orderings of the states have previously been proposed in some experimental and theoretical works. Differences of the valence-ionization spectra of Cl2O and F2O for the appearance of the satellites in the intermediate energy region have been discussed with the aid of the calculated ionization potentials and excitation energies. For ClOOCl, we have presented the first theoretical low-energy ionization spectrum and discussed the character of the calculated states referring to the available ionization spectra of ClO radical.
Oxide potentials from ab initio molecular dynamics: An assessment of their transferability
Authors: Aguado Andrés , Madden Paul A.
Published: March 2003

An aspherical ion model (AIM) description of interatomic interactions in alkaline earth oxides MgO, CaO, SrO, and BaO has been obtained. The parameters were determined by matching the forces predicted by the models to those calculated from an ab initio dynamics calculation in finite temperature simulations. The models were shown to be transferable in the sense that they offered equally accurate descriptions of the pure materials over very wide ranges of temperature and pressure and in phases with different coordination number. In the present paper, more wide-ranging transferability considerations are pursued. First, we check transferability between chemically related substances: we transform the model for CaO into a potential for BeO by scaling parameters according to the changes in ion sizes and polarizabilities. Second, we test transferability upon compound formation, by combining the MgO and CaO potentials in order to describe the interatomic interactions in MgO–CaO solid binary mixtures. The results show that the proposed models are able to provide a good description of interatomic interactions in both cases. The calculations demonstrate that the AIM represents the different contributions to the interionic forces faithfully and that the force-matching procedure finds physically meaningful values for the parameters therein.
Theoretical analysis of the coverage dependence of enantioselective chemisorption on a chirally templated surface
Authors: Roma F. , Zgrablich G. , Stacchiola D. , Tysoe W. T.
Published: March 2003

The effect of the formation of ensembles of chiral templates on the enantioselectivity of model heterogeneous catalysts is studied theoretically in the framework of a cooperative sequential adsorption model. Analytical solutions are presented for random adsorption onto a chirally templated surface which indicate that the surface exhibits a maximum enantioselectivity of ∼2.5, in agreement with results of enantioselective chemisorption experiments carried out in ultrahigh vacuum. It is suggested that the high enantioselectivity (≳90%) encountered in commercial catalysts could be due to correlated adsorption of the template molecules, and that these effects can be modeled using Monte Carlo calculations.
The first in situ Li7 nuclear magnetic resonance study of lithium insertion in hard-carbon anode materials for Li-ion batteries
Authors: Letellier Michel , Chevallier Frédéric , Clinard Christian , Frackowiak Elzbieta , Rouzaud Jean-Noël , Béguin François , Morcrette Mathieu , Tarascon Jean-Marie
Published: March 2003

We show the first continuous in situ static Li7 nuclear magnetic resonance (NMR) experiment on a plastic lithium/carbon cell. The electrochemical cycling was successfully performed inside the magnet. We particularly studied the insertion/extraction of lithium in a material designed to be used as negative electrode in a secondary battery; hard carbon fibers treated to reduce the irreversible capacity (x=0.5). The reversible capacity is x=1.5 (in LixC6), higher than with graphite for which the saturating composition corresponds to the first stage intercalation compound LiC6 (x=1). Ex situ quantitative transmission electron microscopy gives a statistical description of the fibers; a core made of disorganized carbon coated by better organized pyrocarbon. Crossed with the values of capacity given by the current generator, in situ NMR indicates the chronology of the insertion. From the observed data, we propose that a Li–C metallic alloy is formed. By comparing the Knight shift with that of metal (Li0), we estimate the charge transfer from Li to C. Amongst the two types of lithium found, the first might be assigned to Li intercalating (Li+0.66) into the smallest spaces between the graphene layers. The second represents most of the reversible lithium and is assigned to be quasimetallic (Li+0.1). Its longitudinal relaxation time T1 is also near-metallic. The quality of this carbon comes from its ability to host such a species.
A new approach of the stationary axisymmetric vacuum S(A) solutions
Authors: Gariel J. , Marcilhacy G. , Santos N. O.
Published: March 2003

We revisit axisymmetric stationary vacuum solutions of the Einstein equations, like we did for the cylindrical case [J. Math. Phys. 41, 7535 (2000)]. We explicitly formulate the simplest hypothesis under which the S(A) solutions, or axisymmetric Lewis solutions can be found and demonstrate that this hypothesis leads to a linear relation between the potentials. We show that the field equations still can be associated to the motion of a classical particle in a central field, where an arbitrary harmonic χ function plays the role of time. Three classes of solutions are obtained without the need of invoking the Papapetrou class. They depend on two real parameters, and the potentials are functions of χ only. The new approach exempts the need of complex parameters. We interpret one of the parameters as related to the vorticity of the source.
Duality versus dual flatness in quantum information geometry
Authors: Ay Nihat , Tuschmann Wilderich
Published: March 2003

We investigate questions in quantum information geometry which concern the existence and nonexistence of dual and dually flat structures on stratified sets of density operators on finite-dimensional Hilbert spaces. We show that the set of density operators of a given rank admits dually flat connections for which one connection is complete if and only if this rank is maximal. We prove, moreover, that there is never a dually flat structure on the set of pure states. Thus any general theory of quantum information geometry that involves duality concepts must inevitably be based on dual structures which are nonflat.
First integrals of a generalized Darboux–Halphen system
Authors: Chakravarty S. , Halburd R.
Published: March 2003

A third-order system of nonlinear, ordinary differential equations depending on three arbitrary parameters is analyzed. The system arises in the study of SU(2)-invariant hypercomplex manifolds and is a dimensional reduction of the self-dual Yang–Mills equation. The general solution, first integrals, and the Nambu–Poisson structure of the system are explicitly derived. It is shown that the first integrals are multi-valued on the phase space even though the general solution of the system is single-valued for special choices of parameters.
Solvation dynamics of coumarin 153 in dimethylsulfoxide–water mixtures: Molecular dynamics simulations
Authors: Martins Lucimara R. , Tamashiro Alejandro , Laria Daniel , Skaf Munir S.
Published: March 2003

We present results of molecular dynamics simulations of solvation dynamics of coumarin 153 in dimethylsulfoxide (DMSO)–water mixtures of different compositions (xD=0.00, 0.25, 0.32, 0.50, 0.75, and 1.00) using an all-atom model for the solute probe. Results are reported for the global solvation responses of the simulated systems, as well as for the separate contributions from each cosolvent and the individual solute–site couplings to water and DMSO. The solvation dynamics is predominantly given by DMSO’s contribution, even at low (25%) DMSO content, because of the preferential solvation of the probe. We find that the water molecules are only mildly coupled to the charge transfer in the coumarin, resulting in a small, largely diffusive, water relaxation component. Simulation results, including solvation responses, characteristic times, and Stokes shifts are compared with recent fluorescence upconversion experimental measurements showing good agreement for the relaxation but significant differences for the shifts.
Noncommutative phase and unitarization of GLp,q(2)
Authors: Arik M. , Kaynak B. T.
Published: March 2003

In this article, imposing Hermitian conjugate relations on the two-parameter deformed quantum group GLp,q(2) is studied. This results in a noncommutative phase associated with the unitarization of the quantum group. After the achievement of the quantum group Up,q(2) with pq real via a noncommutative phase, the representation of the algebra is built by means of the action of the operators constituting the Up,q(2) matrix on states.
Picosecond absorption saturation dynamics in neutral [M(R,R′timdt)2] metal-dithiolenes
Authors: Cassano T. , Tommasi R. , Nitti L. , Aragoni M. C. , Arca M. , Denotti C. , Devillanova F. A. , Isaia F. , Lippolis V. , Lelj F. , Romaniello P.
Published: March 2003

Picosecond nonlinear absorption dynamics of neutral metal-dithiolenes belonging to the general class [M(R,R′timdt)2] (M=Ni, Pd, Pt; R,R′ timdt=monoreduced form of disubstituted imidazolidine-2,4,5-trithione) has been investigated at λ=1064 nm using the pump–probe technique. Both central metal ion M and substituents R,R′ have been found to strongly influence the temporal evolution of the absorption saturation. In particular, it has been observed that the intersystem crossing rate always increases by a factor ∼1.5 when changing the central metal from Pd to Pt. The mechanisms leading to the measured dynamics have been explained using a multilevel rate equation analysis, based on the results of relativistic time-dependent density functional calculations.
Electron paramagnetic resonance and quantum-mechanical analysis of binuclear niobium clusters in lithium–niobium phosphate glasses
Authors: Rakhimov R. R. , Turney V. J. , Jones D. E. , Dobryakov S. N. , Borisov Yu. A. , Prokof’ev A. I. , Aleksandrov A. I.
Published: March 2003

Electron paramagnetic resonance (EPR) spectra of Nb4+ ions in Li2O–Nb2O5–P2O5 glasses with different composition of oxide components have been investigated. The EPR spectrum shape analysis of Nb4+ (electron configuration 4d1, electron spin S=1/2) reveals the formation of triplet niobium binuclear complex (total electron spin S=1) in glasses. The amount of Nb4+ ions in glasses reversibly changes with temperature and is explained via the mechanism of electron hopping between niobium ions in clusters. The dependence of the amount of Nb4+ ions upon Li2O content has a maximal character, which implies that small amounts of Li+ ions stabilize the Nb4+ pairs, but cause their disproportionation at higher concentrations of Li+ ions in the glass. Quantum mechanical analysis of electronic and spin states of binuclear niobium clusters has been performed on model binuclear complexes, (HO)3Nb–O–Nb(OH)3, [(HO)3Nb–O–Nb(OH)3]Li+, and [(HO)3Nb–O–Nb(OH)3](Li+)2 that exhibit the reversible disproportionation reaction Nb4+–O–Nb4+⇔Nb3+–O–Nb5+. Triplet states of these complexes (total electron spin S=1) have lower energies than singlet states (S=0), and Li+ ions stabilize the binuclear niobium complex. We have found that electron spin densities on niobium ions change depending upon the shift of the bridging oxygen atom. Application of this theoretical modeling to the analysis of the experimental EPR spectrum in Li2O–Nb2O5–P2O5 glass concludes noncentrosymmetric structure of binuclear niobium complex with ∼0.1 Å offset of the bridging oxygen atom towards one Nb atom.
The self-association of acebutolol: Conductometry and light scattering
Authors: Ruso Juan M. , López-Fontán José L. , Prieto Gerardo , Sarmiento Félix
Published: March 2003

The association characteristics of an amphiphilic beta-blocker drug, acebutolol hydrochloride, in aqueous solution containing high concentrations of electrolyte and at different temperatures have been examined by static and dynamic light scattering and electrical conductivity. Time averaged light scattering measurements on aqueous solutions of acebutolol at 298.15 K in the presence of added electrolyte (0.4–1.0 mol kg−1 NaCl) have shown discontinuities which reflect the appearance of aggregates. The critical micelle concentration, aggregation numbers, effective micelle charges, and degree of micellar ionization were calculated. Dynamic light scattering has shown an increase in micellar size with increase in concentration of added electrolyte. Data have been interpreted using the DLVO theory to quantify the interaction between the drug aggregates and the colloidal stability. Critical micelle concentrations in water have been calculated from conductivity measurements over the temperature range 288.15–313.15 K. The variation in critical concentration with temperature passes through a minimum close to 294 K. Thermodynamic parameters of aggregate formation (ΔGm0,ΔHm0,ΔSm0) were obtained from a variation of the mass action model applicable to systems of low aggregation number.
Dinitrosyl formation as an intermediate stage of the reduction of NO in the presence of MoO3
Authors: Remediakis Ioannis N. , Kaxiras Efthimios , Chen Melvin , Friend Cynthia M.
Published: March 2003

We present first-principles calculations in the framework of density-functional theory and the pseudopotential approach, aiming to model the intermediate stages of the reduction of NO in the presence of MoO3(010). In particular, we study the formation of dinitrosyl, which proves to be an important intermediate stage in the catalytic reduction. We find that the replacement of an oxygen of MoO3 by NO is energetically favorable, and that the system lowers further its energy by the formation of (NO)2. Moreover, the geometry and charge distribution for the adsorbed dinitrosyl indicates a metal-oxide mediated coupling between the two nitrogen and the two oxygen atoms. We discuss the mechanisms for the dinitrosyl formation and the role of the oxide in the reaction.
Structural investigation of glycine on Cu(100) and comparison to glycine on Cu(110)
Authors: Kang J.-H. , Toomes R. L. , Polcik M. , Kittel M. , Hoeft J.-T , Efstathiou V. , Woodruff D. P. , Bradshaw A. M.
Published: March 2003

New O 1s and N 1s scanned-energy mode photoelectron diffraction (PhD) measurements and low energy electron diffraction observations from the Cu(100)(2×4)pg phase formed by deprotonated glycine, glycinate (NH2CH2COO–) have been used to determine the local structure of this adsorbed phase. The favored model involves bonding of both O atoms of the carboxylate and the N atom of the amino group in near atop sites with Cu–N and Cu–O distances of 2.05 Å. This bonding geometry is similar to that of glycinate on Cu(110), but in this case the fact that the C–C backbone is aligned along 〈100〉 straggling the more widely spaced Cu atoms rows leads to a larger offset from atop of the O atoms. A reanalysis of O 1s PhD data from the Cu(110)(3×2)pg-glycinate surface shows that the two O atoms are inequivalent, with one O being offset by 0.29 Å more than the other, leading to a twist of the molecule. The results are discussed in the light of other measurements on these surfaces and recent theoretical total energy calculations, in order to obtain models of the long-range ordered phases. These favor models for both surfaces involving only heterochiral structures in which the unit mesh contains one glycinate species with each chirality, defined by the side of the C–C backbone on which the amino group bonds to the surface.
Phase behavior of symmetric binary mixture with partially miscible components in slitlike pores: Density functional approach
Authors: Martinez Arturo , Pizio Orest , Sokołowski Stefan
Published: March 2003

We investigate adsorption of a model symmetric binary mixture, exhibiting demixing in a bulk phase, in slitlike pores by using a density functional approach. Our focus is on the evaluation of the first-order phase transitions between adsorbed phases and lines delimiting mixed and demixed adsorbed phases. The scenario for phase changes is sensitive to the pore width and to the energy of adsorption. Both these parameters can qualitatively change the phase behavior of the confined fluid. In particular, for relatively wide pores and for strong wall–fluid interactions, a demixing line can precede the first-order transition. Moreover, a competition between layering type behavior and demixing within particular layers also can lead to further complications of the phase diagram, reflected by the presence of consecutive branches, describing the equilibrium between different adsorbed phases.
Molecular dynamics in grafted layers of poly(dimethylsiloxane)
Authors: Hartmann L. , Kremer F. , Pouret P. , Léger L.
Published: March 2003

Dielectric relaxation spectroscopy (10−1 Hz to 106 Hz) is employed to study the molecular dynamics of poly(dimethylsiloxane) (PDMS, Mw=1.7×105 g mol−1 and Mw=9.6×104 g mol−1) as grafted films with thicknesses d below and above the radius of gyration Rg. For the thinnest films the molecular dynamics becomes faster by up to two orders of magnitude with respect to the bulk resulting in a pronounced decrease of the Vogel temperature T0 and hence the calorimetric glass transition temperature Tg. For films with d=41 nm>Rg the molecular dynamics is comparable to that of the bulk melt. The results are interpreted in terms of a chain confinement effect and compared with findings for low molecular weight glass forming liquids contained in nanoporous materials. Crystallization effects—well known for PDMS—are observed for films of thicknesses above and below Rg.
Effects of the diffusion of subsurface O atoms on the transient yielding of CO2
Authors: D’Ajello P. C. T. , Hauser P. R. , Figueiredo W.
Published: March 2003

We investigate the catalytic reaction of CO molecules over a precovered surface of O atoms. The topmost surface is subject to a continuous flux of CO molecules and the oxidation reaction occurs through the Langmuir–Hinshelwood mechanism. The reactions only occur when a CO molecule finds an active surface O atom. The subsurface monolayers act as a reservoir of oxygen atoms, which guarantees the continuous evolution of the surface reactions. We have employed Monte Carlo simulations to monitor the CO2 yielding as a function of time. The transient profile of CO2 yielding depends on temperature, as well as on the initial distribution of O atoms on the subsurface planes. When the CO2 yielding attains a stationary state, an almost free oxygen region appears between the topmost layer and the bulk, whatever the initial distribution of oxygens we have considered. Our results are compared with some experimental observations at the oxygen-rich Ruthenium surface.
Vibronic effects in off-resonant molecular wire conduction
Authors: Troisi Alessandro , Ratner Mark A. , Nitzan Abraham
Published: March 2003

A model for the calculation of the inelastic contribution to the low-bias electron transport in molecular junctions is presented. It is an extension to the inelastic case of the Green’s function approach to the calculation of the conduction of such systems. The model is suited for the calculation in the off-resonance regime (where molecular levels are far from the Fermi energy) and in the low bias limit, a typical situation encountered in inelastic electron tunneling measurements. The presentation of a general model is followed by the introduction of several approximations that make the calculation feasible for many systems of interest. Ab initio calculations of the vibronic coupling that leads to inelastic contribution to the conductance are performed for several molecules (butadiene, biphenyl, dipyrrole, and dithiophene), representative of possible molecular wires. The role of inelastic conduction is then quantified without empirical parameters and the vibrational modes that dominate the process are identified. The situations where the inelastic mechanism is particularly relevant are considered. The limits of this approach for the resonant case are also discussed.
Adsorption structure of 2-butyne on Si(100)-(2×1)
Authors: Kim Ki-Yeo , Kim Jung-Hwan , Cho Jun-Hyung , Kleinman Leonard , Kang Heon
Published: March 2003

Adsorption of 2-butyne (CH3C≡CCH3) on a Si(100)-(2×1) surface was examined using scanning tunneling microscopy (STM), Cs+ reactive ion scattering (RIS), and density functional theory calculations. STM and RIS investigations show that 2-butyne chemisorbs on the surface as a molecule without dissociation. In STM images the adsorbed 2-butyne molecules appear as double-lobed protrusions due to two methyl groups, which provides a clue for determining the adsorption geometry of the molecule. 2-butyne binds on top of a Si dimer through di-σ bonding between the C≡C bond and the Si dimer. This is the only binding structure formed at room temperature. In contrast, acetylene is known to have several different binding geometries on Si(100)-(2×1). The exclusive formation of di-σ bonded 2-butyne is explained by the calculated adsorption energy of 2.66 eV for the di-σ species, 1.89 eV for the end-bridge species, and 0.67 eV for the r-bridge species.