Recent Publications

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  1. Title: Ionization dynamics of laser-driven H-2(+)

    Author(s): Dundas D., McCann J.F., Parker J.S., Taylor K.T.

    Journal Of Physics B-Atomic Molecular And Optical Physics, 33, No. 17, pp. 3261-3276 (SEP 14 2000)

    doi: 10.1088/0953-4075/33/17/308
    Abstract



    We set out aspects of a numerical algorithm used in solving the full-dimensionality time-dependent Schrodinger equation describing the electronic motion of the hydrogen molecular ion driven by an intense, linearly polarized laser pulse aligned along the molecular axis. This algorithm has been implemented within the fixed inter-nuclear separation approximation in a parallel computer code, a brief summary of which is given. Ionization rates are calculated and compared with results from other methods, notably the time-independent Floquet method. Our results compare very favourably with the precise predictions of the Floquet method, although there is some disagreement with other wavepacket calculations. Visualizations of the electron dynamics are also presented in which electron rescattering is observed.

  2. Title: Double ionization of helium at 390 um

    Author(s): Parker J.S., Moore L.R., Dundas D., Taylor K.T.

    Journal Of Physics B-Atomic Molecular And Optical Physics, 33, No. 20, pp. L691-L698 (OCT 28 2000)

    doi: 10.1088/0953-4075/33/20/106
    Abstract



    We present calculations of single- and double-ionization rates of helium at 390 nm, accurate to within 10%, obtained from a full-dimensional integration of the time-dependent Schrodinger equation. The theoretical results are compared with experimental data at the same wavelength. Excellent agreement is obtained, allowing for likely uncertainties in the experimental determination of laser intensity.

  3. Title: Non-linear conductance of disordered quantum wires

    Author(s): Todorov T.N.

    Journal Of Physics-Condensed Matter, 12, No. 42, pp. 8995-9006 (OCT 23 2000)

    doi: 10.1088/0953-8984/12/42/306
    Abstract



    The self-consistent electron potential in a current-carrying disordered quantum wire is spatially inhomogeneous due to the formation of resistivity dipoles across scattering centres. In this paper it is argued that these inhomogeneities in the potential result in a suppression of the differential conductance of such a wire at finite applied voltage. A semi-classical argument allows this suppression, quadratic in the voltage, to be related directly to the amount of intrinsic defect scattering in the wire. This result is then tested against numerical calculations.

  4. Title: Electromigration of vacancies in copper

    Author(s): Hoekstra J., Sutton A.P., Todorov T.N., Horsfield A.P.

    Physical Review B, 62, No. 13, pp. 8568-8571 (OCT 1 2000)

    doi: 10.1103/PhysRevB.62.8568
    Abstract



    The total current-induced force on atoms in a Cu wire containing a vacancy are calculated using the self consistent one-electron density matrix in the presence of an electric current, without separation into electron-wind and direct forces. By integrating the total current-induced force, the change in vacancy migration energy due to the current is calculated. We use the change in migration energy with current to infer an effective electromigration driving force F-e. Finally, we calculate the proportionality constant rho* between F-e and the current density in the wire.

  5. Title: Simulation study of Pd submonolayer films on Au(hkl) and Pt(hkl) and their relationship to underpotential deposition

    Author(s): Rojas M.I., Del Popolo M.G., Leiva E.P.M.

    Langmuir, 16, No. 24, pp. 9539-9546 (NOV 28 2000)

    doi: 10.1021/la990731g
    Abstract



    The structure and stability of palladium adlayers on Au(hkl) and Pt(hkl) were studied at different coverage degrees by means of Monte Carlo simulations using the interatomic potentials of the embedded atom model. In all cases the Pd films were found to grow epitaxially and pseudomorphically with the crystallographic orientation of the substrate. The differences and similarities of the adlayer with the substrate were analyzed.

  6. Title: Landau free energy curves for melting of quantum solids

    Author(s): Chakravarty C., Lynden-Bell R.M.

    Journal Of Chemical Physics, 113, No. 20, pp. 9239-9247 (NOV 22 2000)

    doi: 10.1063/1.1316105
    Abstract



    Path integral Monte Carlo simulations are used to study quantum Lennard-Jones solids and to examine the dependence of the melting temperature on the degree of quantum delocalization. To determine the solid-liquid coexistence conditions and metastability limits, an umbrella sampling strategy is adopted to generate Landau free energy curves. Simulation results for the melting point dependence on the particle mass are shown to be reasonable in comparison with experimental data on the melting curves of solid H-2 and D-2. Both the enthalpy and entropy of melting are shown to decrease with increasing strength of quantum effects. For solid molecular hydrogen and its isotopomers, zero-point energy effects are shown to be important even at melting temperatures. (C) 2000 American Institute of Physics. [S0021-9606(00)71342-0].

  7. Title: An embedded atom approach to underpotential deposition phenomena (vol 421, pg 59, 1999)

    Author(s): Rojas M.I., Sanchez C.G., Del Popolo M.G., Leiva E.P.M.

    Surface Science, 453, No. 1-3, pp. 225-228 (MAY 10 2000)

    doi: 10.1016/S0039-6028(00)00240-5
    Abstract



  8. Title: Catalytic-reactions of living polymers: Density functional study of reactivity of phenol and phenoxides with the cyclic tetramer of polycarbonate

    Author(s): Ballone P., Montanari B., Jones R.O.

    Journal Of Physical Chemistry A, 104, No. 12, pp. 2793-2798 (MAR 30 2000)

    doi: 10.1021/jp993734a
    Abstract



    The reactivity of phenol, lithium phenoxide (LiOPh), and sodium phenoxide (NaOPh) with the cyclic tetramer of bisphenol A polycarbonate (BPA-PC) has been investigated using density functional calculations. The potential energy of the system is computed using a suitable reaction coordinate and relaxing all other degrees of freedom by Car-Parrinello molecular dynamics. Both LiOPh and NaOPh catalyze ring opening with small energy barriers (Delta E 4.0, 2.5 kcal/mol, respectively) to a chain with a phenyl carbonate at one end and a phenoxide at the other, a "living polymer". The barrier is large for phenol (Delta E > 40 kcal/mol), but the total energy differences between the reactants and the chain are very small in all three molecules. We discuss the balance between changes in entropy and energy, and we compute the vibrational properties of metastable intermediate species.

  9. Title: Stochastic motion of solitary excitations on the classical Heisenberg chain

    Author(s): Meister M., Mertens F.G.

    Journal Of Physics A-Mathematical And General, 33, No. 11, pp. 2195-2207 (MAR 24 2000)

    doi: 10.1088/0305-4470/33/11/303
    Abstract



    We study stochastic motion of solitary excitations on a classical, discrete, isotropic, ferromagnetic Heisenberg spin chain with nearest-neighbour exchange interactions. Gaussian white noise is coupled to the spins in a way that allows for the noise to be interpreted as a stochastic magnetic field. The noise translates into a collective stochastic force affecting a solitary excitation as a whole. The position of a solitary excitation has to be calculated from the noisy spin configuration, i.e. the position is defined as a function of the spin components. Two examples of such definitions are given, because we want to investigate the dependence of the results on the choice of definition. Using these definitions, we calculate the variance of the position as a function of time and determine the variance from simulations as well. The calculations require knowledge of the shape of the solitary wave. We approximate the shape with that of soliton solutions of the continuum Heisenberg chain, restricting our considerations to solitary waves of large width, in which case this approximation is good. The calculations yield a linear dependence of the variance on time, the slope being determined by parameters describing the shape of the soliton. The two definitions of the position we use provide different results for this slope. The origin of this difference is discussed. With both definitions very good agreement is found between the results of the simulations and the corresponding theoretical results, for not too large time scales.

  10. Title: Constitutional and thermal point defects in B2 NiAl

    Author(s): Korzhavyi P.A., Ruban A.V., Lozovoi A.Y., Vekilov Y.K., Abrikosov I.A., Johansson B.

    Physical Review B, 61, No. 9, pp. 6003-6018 (MAR 1 2000)

    doi: 10.1103/PhysRevB.61.6003
    Abstract



    The formation energies of point defects and the interaction energies of various defect pairs in NiAl are calculated from first principles within an order N, locally self-consistent Green's-function method in conjunction with multipole electrostatic corrections to the atomic sphere approximation. The theory correctly reproduces the ground state for the off-stoichiometric NiAl alloys. The constitutional defects (antisite Ni atoms and Ni vacancies in Ni-rich and Al-rich NiAl, respectively) are shown to form ordered structures in the ground state, in which they tend to avoid each other at the shortest distance on their sublattice. The dominant thermal defects in Ni-rich and stoichiometric NiAl are calculated to be triple defects. In Al-rich alloys another type of thermal defect dominates, where two Ni vacancies are replaced by one antisite Al atom. As a result, the vacancy concentration decreases with temperature in this region. The effective defect formation enthalpies for different concentration regions of NiAl are also obtained.

  11. Title: Current-induced forces in atomic-scale conductors

    Author(s): Todorov T.N., Hoekstra J., Sutton A.P.

    Philosophical Magazine B-Physics Of Condensed Matter Statistical Mechanics Electronic Optical And Magnetic Properties, 80, No. 3, pp. 421-455 (MAR 2000)

    doi: 10.1080/13642810008208601
    Abstract



    We present a self-consistent tight-binding formalism to calculate the forces on individual atoms due to the flow of electrical current in atomic-scale conductors. Simultaneously with the forces, the method yields the local current density and the local potential in the presence of current flow, allowing a direct comparison between these quantities. The method is applicable to structures of arbitrary atomic geometry and can be used to model current-induced mechanical effects in realistic nanoscale junctions and wires. The formalism is implemented within a simple Is tight-binding model and is applied to two model structures; atomic chains and a nanoscale wire containing a vacancy.

  12. Title: Concertedness and solvent effects in multiple proton transfer reactions: The formic acid dimer in solution

    Author(s): Kohanoff J., Koval S., Estrin D.A., Laria D., Abashkin Y.

    Journal Of Chemical Physics, 112, No. 21, pp. 9498-9508 (JUN 1 2000)

    doi: 10.1063/1.481585
    Abstract



    The issue of multiple proton transfer (PT) reactions in solution is addressed by performing molecular dynamics simulations for a formic acid dimer embedded in a water cluster. The reactant species is treated quantum mechanically, within a density functional approach, while the solvent is represented by a classical model. By constraining different distances within the dimer we analyze the PT process in a variety of situations representative of more complex environments. Free energy profiles are presented, and analyzed in terms of typical solvated configurations extracted from the simulations. A decrease in the PT barrier height upon solvation is rationalized in terms of a transition state which is more polarized than the stable states. The dynamics of the double PT process is studied in a low-barrier case and correlated with solvent polarization fluctuations. Cooperative effects in the motion of the two protons are observed in two different situations: when the solvent polarization does not favor the transfer of one of the two protons and when the motion of the two protons is not synchronized. This body of observations is correlated with local structural and dynamical properties of the solvent in the vicinity of the reactant. (C) 2000 American Institute of Physics. [S0021-9606(00)51121-0].

  13. Title: Squeezing lubrication films: Layering transition for curved solid surfaces with long-range elasticity

    Author(s): Persson B.N.J., Ballone P.

    Journal Of Chemical Physics, 112, No. 21, pp. 9524-9542 (JUN 1 2000)

    doi: 10.1063/1.481589
    Abstract



    The properties of an atomic lubricant confined between two approaching solids are investigated by a model that accounts for the curvature and elastic properties of the solid surfaces. Well defined atomic layers develop in the lubricant film when the width of the film is of the order of a few atomic diameters. An external squeezing-pressure induces discontinuous, thermally activated changes in the number n of lubricant layers. The precise mechanism for these layering transitions depends on n, and on the lubricant-surface pinning barriers. Thus, in the absence of sliding, unpinned or weakly pinned incommensurate lubricant layers give rise to fast and complete layering transitions. Strongly pinned incommensurate and commensurate layers give rise to sluggish and incomplete transformations, resulting in trapped islands. In particular, for commensurate layers it is often not possible to squeeze out the last few lubricant layers. However, lateral sliding of the two solid surfaces breaks down the pinned structures, greatly enhancing the rate of the layering transitions. In the case of sliding, an important parameter is the barrier for sliding one lubricant layer with respect to the others. When this barrier is larger than the lubricant-surface pinning barrier, the lubricant film tends to move like a rigid body with respect to the solid surface. In the opposite case, slip events may occur both within the lubricant film and at the lubricant-solid interface, making the squeeze-out process much more complex. In some of the simulations we observe an intermediate phase, forming immediately before the layering transition. This transient structure has a lower 2D density than the initial phase, and allows the system to release elastic energy, which is the driving force for the phase transformation. (C) 2000 American Institute of Physics. [S0021-9606(00)70421-1].

  14. Title: Structural and vibrational properties of trehalose: A density functional study

    Author(s): Ballone P., Marchi M., Branca C., Magazu S.

    Journal Of Physical Chemistry B, 104, No. 26, pp. 6313-6317 (JUL 6 2000)

    doi: 10.1021/jp994346b
    Abstract



    We apply density functional theory in the gradient-corrected local-density approximation to the determination of structural properties and harmonic vibrational modes of trehalose in the gas phase and in the monohydrate crystal. We analyze the local conformation and the relative strength of intra- and intermolecular hydrogen bonds, and we discuss the effects of the crystal environment on the molecular geometry and vibrational frequencies. The density functional results are used to assess the quality of a recent molecular mechanics model. This simplified scheme provides a fairly good description of ground-state geometries, while vibrational properties are only in qualitative agreement with those computed by the density functional method.

  15. Title: Surface stoichiometry and the initial oxidation of NiAl(110)

    Author(s): Lozovoi A.Y., Alavi A., Finnis M.W.

    Physical Review Letters, 85, No. 3, pp. 610-613 (JUL 17 2000)

    doi: 10.1103/PhysRevLett.85.610
    Abstract



    Selective oxidation of the surface of an ordered alloy requires redistribution of the atomic species in the vicinity of the surface. This process can be understood in terms of the formation and movements of point defects in the compound. On the basis of nb initio density-functional calculation we found both the creation of exchange defects near the NiAl surface and segregation of Ni vacancies to the top layer to be extremely favorable in the presence of oxygen. Scenarios for the initial oxidation of NiAl are suggested which demonstrate the appearance of an additional energy barrier on the Ni-rich side compared to the Al-rich side. The expulsion of Ni from the oxide layer as it forms is the driving force for its stability.

  16. Title: Bain transformation in CuxPd1-x (x similar to 0.5) alloys: An embedded-atom study

    Author(s): Donato M.G., Ballone P., Giaquinta P.V.

    Physical Review B, 61, No. 1, pp. 24-27 (JAN 1 2000)

    doi: 10.1103/PhysRevB.61.24
    Abstract



    We investigate the B2 to random-fee structural transformation in CuxPd1-x alloys as a function of concentration around x = 0.5. The system is modeled by the embedded atom method (EAM), and its free energy is computed by Monte Carlo simulation in the isothermal-isobaric ensemble. Our results show that the temperature stability range for the B2 phase is estimated correctly by the EAM model, while the x dependence of the transformation temperature around the stoichiometric composition (x = 0.5) is not well reproduced.

  17. Title: Comment on "Zero temperature phases of the electron gas" - Ortiz et al. reply

    Author(s): Ortiz G., Harris M., Ballone P.

    Physical Review Letters, 84, No. 8, Art. No. 1843 (FEB 21 2000)

    doi: 10.1103/PhysRevLett.84.1843
    Abstract



  18. Title: The behaviour of liquid alkanes near interfaces

    Author(s): Smith P., Lynden-Bell R.M., Smith W.

    Molecular Physics, 98, No. 4, pp. 255-260 (FEB 20 2000)

    doi: 10.1080/002689700162685
    Abstract



    Simulations of thick films of liquid alkanes supported on a wax-like substrate were carried out at a number of temperatures in order to investigate the structure and dynamics of molecules near the solid-liquid and the liquid-vapour interfaces. Films of butane, octane and a mixture were investigated. Near the solid surface the liquids were found to be structured and molecular diffusion slowed. However, there was no evidence of a frozen layer at this interface even near the bulk freezing temperature. The mixed liquid showed considerable segregation at both interfaces with preferential absorption of butane at the liquid-vapour interface and octane at the liquid-solid interface.

  19. Title: Optimal basis set for electronic structure calculations in periodic systems

    Author(s): Scandolo S., Kohanoff J.

    Physical Review B, 62, No. 23, pp. 15499-15504 (DEC 15 2000)

    doi: 10.1103/PhysRevB.62.15499
    Abstract



    An efficient method for calculating the electronic structure of systems that need a very fine sampling of the Brillouin zone is presented. The method is based on the variational optimization of a single (i.e., common to all points in the Brillouin zone) basis set for the expansion of the electronic orbitals. Considerations from k.p-approximation theory help to understand the efficiency of the method. The accuracy and the convergence properties of the method as a function of the optimal basis set size are analyzed for a test calculation on a 16-atom Na supercell.

  20. Title: The structure and spectroscopy of monolayers of water on MgO: An ab initio study

    Author(s): Delle Site L., Alavi A., Lynden-Bell R.M.

    Journal Of Chemical Physics, 113, No. 8, pp. 3344-3350 (AUG 22 2000)

    doi: 10.1063/1.1287276
    Abstract



    The structure and energetics of a monolayer of water on a perfect MgO substrate is investigated by ab initio calculations. Several minima in the potential energy surface were found including both physisorbed and chemisorbed states in which one third of the water molecules were dissociated. In the more stable of the physisorbed states, the water molecules were not all parallel to the surface, but some showed hydrogen bonding with surface oxygen ions. Even in the physisorbed state, the geometry of these surface H-bonded water molecules were distorted relative to the bulk. One of the stable chemisorbed states was the same as that found in earlier work by Giordano [Phys. Rev. Lett. 81, 1271 (1998)]. Two more stable chemisorbed structures were found in which strong hydrogen bonds to the hydroxide ions in the water layer were formed. The existence of different minima appears to be the result of a subtle interplay between hydrogen bonding between adsorbed species and with the surface oxide ions. Harmonic vibrational frequencies were determined for both a chemisorbed and a physisorbed states and spectroscopic features which should discriminate between chemisorbed and physisorbed states are identified. (C) 2000 American Institute of Physics. [S0021-9606(00)30932-1].

  21. Title: Ab initio approach for electro-chemical application.

    Author(s): Lozovoi A.Y., Alavi A., Lynden-Bell R.M.

    Abstracts Of Papers Of The American Chemical Society, 220, pp. U236-U237 (AUG 20 2000)

    Abstract


  22. Title: Density functional study of carbonic acid clusters

    Author(s): Ballone P., Montanari B., Jones R.O.

    Journal Of Chemical Physics, 112, No. 15, pp. 6571-6575 (APR 15 2000)

    doi: 10.1063/1.481229
    Abstract



    Density functional calculations on carbonic acid H2CO3 are extended to clusters of up to five such units. The most stable forms are the linear, hydrogen-bonded analogs of the dimer with anti-anti orientation. We calculate structures and vibration frequencies, as well as the energy required to bend and stretch the linear isomers. Linear chains of up to similar to 20 units should be favored over ring structures, and they have a tensile strength reminiscent of chains of water molecules. We also discuss planar, nonlinear structures as well as three-dimensional isomers. (C) 2000 American Institute of Physics. [S0021-9606(00)30615-8].

  23. Title: Changes in surface stress caused by the adsorption of an epitaxial metal monolayer

    Author(s): Leiva E.P.M., Del Popolo M.G., Schmickler W.

    Chemical Physics Letters, 320, No. 5-6, pp. 393-397 (APR 14 2000)

    doi: 10.1016/S0009-2614(00)00240-2
    Abstract



    The adsorption of a metal monolayer on a foreign substrate generates a change in the surface stress. We calculate this change for a number of substrate/adsorbate systems using the embedded-atom method. The results are compared with those obtained from a continuum model. A cycle, in which the stretching of a substrate/adsorbate system is decomposed into several steps, helps in understanding the numerical results. (C) 2000 Elsevier Science B.V. All rights reserved.

  24. Title: The pathway to reorientation in ammonium fluoride

    Author(s): Alavi A., Lynden-Bell R.M., Brown R.J.C.

    Chemical Physics Letters, 320, No. 5-6, pp. 487-491 (APR 14 2000)

    doi: 10.1016/S0009-2614(00)00267-0
    Abstract



    Two alternative transition states for ammonium ion rotation in crystalline NH4F have been characterised using ab initio calculations. In one all the cations rotate in phase, while in the other the rotating ions are isolated from each other by non-rotated ions. These two transition states are characterised by the cation displacement and changes in the bond lengths and bond angles within the ammonium ion. The pathway for passing over the barrier was investigated by low temperature molecular dynamics runs from each transition state. In both cases the order of events in passing from the ground state to the transition state is first cation displacement towards a neighbouring fluoride ion, secondly, lengthening of the NH bond pointing to this ion, and thirdly, rotation about this bond. (C) 2000 Elsevier Science B.V. All rights reserved.

  25. Title: Charge carrier interactions in ionic conductors: A classical molecular-dynamics and Monte Carlo study on AgI

    Author(s): Zimmer F., Ballone P., Maier J., Parrinello M.

    Journal of Chemical Physics, 112, No. 14, pp. 6416-6423 (APR 2000)

    doi: 10.1063/1.481205
    Abstract



    The equilibrium concentration of ionic and electronic charge carriers in ionic crystals as a function of temperature, concentration of dopants, and chemical environment is phenomenologically well understood as long as these point defects can be considered sufficiently dilute. However, there are cases, usually at temperatures close to the melting point, where the defects appear in higher concentrations. In these cases interactions come into play and cause anomalous increases in the conductivity or even phase transitions. Recently Hainovsky and Maier showed that for various Frenkel disordered materials this anomalous conductivity increase at high temperature can be described by a cube root term in the chemical potential of the defects. This quasi-Madelung approach does not only allow ionic conductivities and heat capacities to be computed, it also leads to a phenomenological understanding of the solid-liquid or superionic transition temperatures. In the present study we analyze this approach on the atomistic level for AgI: The defect concentrations as well as defect energies, including excess energies, are computed as a function of temperature by molecular-dynamics and Monte Carlo simulations based on a classical semiempirical potential. The simulations support the cube-root model, yield approximately the same interaction constants and show that the corrections in the chemical potential are of an energetic nature. In agreement with structural expectations, the simulations reveal that two different kinds of interstitials are present: Octahedral interstitials, which essentially determine the ionic transport at higher temperature, and tetrahedral ones, which remain substantially associated with the vacancies. It is shown how these refinements have to be introduced into the cube root. © 2000 American Institute of Physics.

  26. Title: Effect of relaxation on the oxygen K-edge electron energy-loss near-edge structure in yttria-stabilized zirconia

    Author(s): Ostanin S., Craven A.J., McComb D.W., Vlachos D., Alavi A., Finnis M.W., Paxton A.T.

    Physical Review B, 62, No. 22, pp. 14728-14735 (2000)

    doi: 10.1103/PhysRevB.62.14728
    Abstract


    Full Text

    The electron energy-loss near-edge structure (ELNES) at the oxygen K-edge has been investigated in a range of yttria-stabilized zirconia (YSZ) materials. The electronic structure of the three polymorphs of pure ZrO2 and of the doped YSZ structure close to the 33 mol %Y2O3 composition have been calculated using a full-potential linear muffin-tin orbital method (NFP-LMTO) as well as a pseudopotential based technique. Calculations of the ELNES dipole transition matrix elements in the framework of the NFP-LMTO scheme and inclusion of core hole screening within Slater's transition state theory enable the ELNES to be computed. Good agreement between the experimental and calculated ELNES is obtained for pure monoclinic ZrO2. The agreement is less good with the ideal tetragonal and cubic structures. This is because the inclusion of defects is essential in the calculation of the YSZ ELNES. If the model used contains ordered defects such as vacancies and metal Y planes, agreement between the calculated and experimental O K-edges is significantly improved. The calculations show how the five different O environments of Zr,Y,O, are connected with the features observed in the experimental spectra and demonstrate clearly the power of using ELNES to probe the stabilization mechanism in doped metal oxides.

  27. Title: Boundary lubrication: layering transition for curved solid surfaces with long-range elasticity

    Author(s): Persson B.N.J., Ballone P.

    Solid State Communications, 115, No. 11, pp. 599-604 (2000)

    doi: 10.1016/S0038-1098(00)00242-8
    Abstract



    The properties of an atomic lubricant confined between two approaching surfaces are investigated by molecular dynamics. In the limit of thin interfaces, the lubricant atoms form well defined layers, whose number decreases in discontinuous steps with increasing applied pressure. These transitions occur easily and completely for unpinned lubricant films, while they are sluggish and incomplete in the case of strong pinning. Before the transition, an intermediate phase arises, which facilitates the thinning of the lubricant. Lateral sliding of the surfaces enhances the thinning rate. (C) 2000 Elsevier Science Ltd. All rights reserved.

  28. Title: Relative energetics and structural properties of zirconia using a self-consistent tight-binding model

    Author(s): Fabris S., Paxton A.T., Finnis M.W.

    Physical Review B, 61, No. 10, pp. 6617-6630 (2000)

    doi: 10.1103/PhysRevB.61.6617
    Abstract


    Full Text

    We describe an empirical, self-consistent, orthogonal tight-binding model for zirconia, which allows for the polarizability of the anions at dipole and quadrupole levels and for crystal field splitting of the cation d orbitals, This is achieved by mixing the orbitals of different symmetry on a site with coupling coefficients driven by the Coulomb potentials up to octapole level. The additional forces on atoms due to the self-consistency and polarizabilities are exactly obtained by straightforward electrostatics, by analogy with the Hellmann-Feynman theorem as applied in first-principles calculations. The model correctly orders the zero temperature energies of all zirconia polymorphs. The Zr-O matrix elements of the Hamiltonian, which measure covalency, make a greater contribution than the polarizability to the energy differences between phases. Results for elastic constants of the cubic and tetragonal phases and phonon frequencies of the cubic phase are also presented and compared with some experimental data and first-principles calculations. We suggest that the model will be useful for studying finite temperature effects by means of molecular dynamics.

  29. Title: The near-edge structure in energy-loss spectroscopy: many-electron and magnetic effects in transition metal nitrides and carbides

    Author(s): Paxton A.T., van Schilfgaarde M., MacKenzie M., Craven A.J.

    Journal Of Physics-Condensed Matter, 12, No. 5, pp. 729-750 (2000)

    doi: 10.1088/0953-8984/12/5/319
    Abstract


    Full Text

    We investigate the ability of the local density approximation (LDA) in density functional theory to predict the near-edge structure in electron energy-loss spectroscopy in the dipole approximation. We include screening of the core hole within the LDA using Slater's transition state theory. We find that anion K-edge threshold energies are systematically overestimated by 4.22 +/- 0.44 eV in twelve transition metal carbides and nitrides in the rock-salt (B1) structure. When we apply this 'universal' many-electron correction to energy-loss spectra calculated within the transition state approximation to LDA, we find quantitative agreement with experiment to within one or two eV for TiC, TiN and VN. We compare our calculations to a simpler approach using a projected Mulliken density which honours the dipole selection rule, in place of the dipole matrix element itself. We find remarkably close agreement between these two approaches. Finally, we show an anomaly in the near-edge structure in CrN to be due to magnetic structure. In particular, we find that the N K edge in fact probes the magnetic moments and alignments of ther sublattice.

  30. Title: Surfactant structure around DNA in aqueous solution

    Author(s): Smith P., Lynden-Bell R.M., Smith W.

    Physical Chemistry Chemical Physics, 2, No. 6, pp. 1305-1310 (2000)

    doi: 10.1039/a909519e
    Abstract



    Two different possible structures for decanetrimethylammonium surfactants around a DNA strand in aqueous solution were investigated using molecular dynamics. The first structure had surfactant going out radially from the DNA in a 'hairy' structure and in the other the surfactants were in a micelle structure. Both systems were simulated for greater than or equal to 1 ns. Results for the radius of gyration, the density and various radial distribution functions are given. By comparison with these results neutron diffraction experiments could definitively distinguish between these structures. The 'hairy' structure showed some evidence of clumping. Although this can be interpreted as incipient micelle formation, it was also found that head groups did move into the groove, a change which is not consistent with micelle formation. In comparison the micelle structure remained stable, although the shape elongated.

  31. Title: Reply to: "The puzzling stability of monatomic gold wires is the result of small fluctuations"

    Author(s): Torres J.A., Tosatti E., Dal Corso A., Ercolessi F., Kohanoff J.J., Di Tolla F.D., Soler J.M.

    Surface Science, 463, No. 3, pp. 213-214 (2000)

    Abstract