Recent Publications

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  1. Title: Hydrogen bonding and collective proton modes in clusters and periodic layers of squaric acid: A density functional study

    Author(s): Rovira C., Novoa J.J., Ballone P.

    Journal Of Chemical Physics, 115, No. 14, pp. 6406-6417 (OCT 8 2001)

    doi: 10.1063/1.1402166

    Hydrogen bonding in clusters and extended layers of squaric acid molecules has been investigated by density functional computations. Equilibrium geometries, harmonic vibrational frequencies, and energy barriers for proton transfer along hydrogen bonds have been determined using the Car-Parrinello method. The results provide crucial parameters for a first principles modeling of the potential energy surface, and highlight the role of collective modes in the low-energy proton dynamics. The importance of quantum effects in condensed squaric acid systems has been investigated, and shown to be negligible for the lowest-energy collective proton modes. This information provides a quantitative basis for improved atomistic models of the order-disorder and displacive transitions undergone by squaric acid crystals as a function of temperature and pressure. (C) 2001 American Institute of Physics.

  2. Title: Time-dependent tight binding

    Author(s): Todorov T.N.

    Journal Of Physics-Condensed Matter, 13, No. 45, pp. 10125-10148 (NOV 12 2001)

    doi: 10.1088/0953-8984/13/45/302

    Starting from a Lagrangian mean-field theory, a set of time-dependent tight-binding equations is derived to describe dynamically and self-consistently an interacting system of quantum electrons and classical nuclei. These equations conserve norm, total energy and total momentum. A comparison with other tight-binding models is made. A previous tight-binding result for forces on atoms in the presence of electrical current flow is generalized to the time-dependent domain and is taken beyond the limit of local charge neutrality.

  3. Title: Using simulation to study solvation in water

    Author(s): Lynden-Bell R.M., Rasaiah J.C., Noworyta J.P.

    Pure And Applied Chemistry, 73, No. 11, pp. 1721-1731 (NOV 2001)

    doi: 10.1351/pac200173111721

    Simulations of simple solutes (charged and uncharged spheres) in model water have been performed in order to elucidate aspects of solvation in water at ambient and supercritical states. The variation of solvation entropy as a function of solute charge has been used to investigate hydrophobic and hydrophilic ordering and the structure-making and structure-breaking effects of ions. Simulations with model solvents, which differ from water in certain features, have been used to try to identify the particular properties of water that are associated with these phenomena.

  4. Title: Density functional calculations for polymers and clusters - progress and limitations

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

    Computational Materials Science, 22, No. 1-2, pp. 1-6 (NOV 2001)

    doi: 10.1016/S0927-0256(01)00155-0

    The applicability of density functional (DF) methods has progressed greatly since the first workshop of this series ten years ago. Applications that show both the successes and the limitations can be found in the fields of: (a) the structures of the isomers of atomic clusters. and (b) the structure of organic molecules and polymers, and their reactions with additional molecules. We shall review some of the results and the lessons to be learned from them. (C) 2001 Elsevier Science B.V. All rights reserved.

  5. Title: Interplay between proton ordering and ferroelectric polarization in H-bonded KDP-type crystals

    Author(s): Koval S., Kohanoff J., Migoni R.L., Bussmann-Holder A.

    Computational Materials Science, 22, No. 1-2, pp. 87-93 (NOV 2001)

    doi: 10.1016/S0927-0256(01)00172-0

    The origin of ferroelectricity in KH2PO4 (KDP) is studied by first-principles electronic structure calculations. In the low-temperature phase, the collective off-centre ordering of the protons is accompanied by an electronic charge delocalization from the near and localization at the far oxygen within the O-H...O bonds. Electrostatic forces. then, push the K+ ions towards off-centre positions, and induce a macroscopic polarization. The analysis of the correlation between different geometrical and electronic quantities, in connection with experimental data. supports the idea that the role of tunnelling in isotopic effects is irrelevant. Instead, geometrical quantum effects appear to play a central role. (C) 2001 Elsevier Science B.V. All rights reserved.

  6. Title: Intermolecular potentials for simulations of liquid imidazolium salts

    Author(s): Hanke C.G., Price S.L., Lynden-Bell R.M.

    Molecular Physics, 99, No. 10, pp. 801-809 (MAY 2001)

    doi: 10.1080/00268970010018981

    Intermolecular potentials suitable for molecular dynamics or Monte Carlo simulations have been developed for dimethyl imidazolium and methyl ethyl imidazolium ions. The predicted crystal structures were compared with experimental crystal structures for chloride and PF6- salts and found to be satisfactory when the dominant electrostatic interactions were modelled by either an accurate distributed multipole description or a simplified atomic point charge model. A further simplification of using united atoms in place of methyl or methylene groups on the side chains gave a much less satisfactory reproduction of the crystal structures. Liquid dimethyl imidazolium chloride and dimethyl imidazolium PF6- were simulated using the explicit atom and united atom potentials. The local structure showed a strong preference for the chloride ions to be located in certain regions around the cation, and a similar, but less strong localization of the larger PF6-. Significant differences in density and diffusion rates were found when the explicit atom model was replaced by the cheaper united atom model, showing that the latter potential is significantly poorer for modelling both the static solid and dynamic liquid simulations.

  7. Title: A dielectric continuum molecular dynamics method

    Author(s): Marchi M., Borgis D., Levy N., Ballone P.

    Journal Of Chemical Physics, 114, No. 10, pp. 4377-4385 (MAR 8 2001)

    doi: 10.1063/1.1348028

    We introduce a novel method to simulate hydrated macromolecules with a dielectric continuum representation of the surrounding solvent. In our approach, the interaction between the solvent and the molecular degrees of freedom is described by means of a polarization density free energy functional which is minimum at electrostatic equilibrium. After a pseudospectral expansion of the polarization and a discretization of the functional, we construct the equations of motion for the system based on a Car-Parrinello technique. In the limit of the adiabatic evolution of the polarization field variables, our method provides the solution of the dielectric continuum problem "on the fly," while the molecular coordinates are propagated. In this first study, we show how our dielectric continuum molecular dynamics method can be successfully applied to hydrated biomolecules, with low cost compared to free energy simulations with explicit solvent. To our knowledge, this is the first time that stable and conservative molecular dynamic simulations of solutes can be performed for a dielectric continuum model of the solvent. (C) 2001 American Institute of Physics.

  8. Title: Symmetry breaking of the triiodide ion in acetonitrile solution

    Author(s): Margulis C.J., Coker D.F., Lynden-Bell R.M.

    Chemical Physics Letters, 341, No. 5-6, pp. 557-560 (JUN 29 2001)

    doi: 10.1016/S0009-2614(01)00548-6

    The triiodide ion shows evidence of symmetry breaking due to spontaneous polarisation and distortion when dissolved in hydrogen-bonding solvents, but in polar solvents such as acetonitrile with no hydrogen bonds the symmetry appears to be D-infinityh. In this Letter we use Monte Carlo simulations to show that there is a symmetry breaking in model triiodide ion in acetonitrile solution but the barrier between the two equivalent forms is less than kT. This result is contrasted with our previous work on aqueous solutions. The instantaneous vibrational frequencies of the triiodide ion in acetonitrile solution were determined. In agreement with experiment we find that the mean value of the antisymmetric mode is less shifted to the blue and the spread of frequencies is less than in aqueous solution. (C) 2001 Elsevier Science B,V. All rights reserved.

  9. Title: Surface energy and the early stages of oxidation of NiAl(110)

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

    Computer Physics Communications, 137, No. 1, pp. 174-194 (JUN 1 2001)

    doi: 10.1016/S0010-4655(01)00178-3

    We have studied the (110) surface of NiAl, an ordered alloy of B2 structure, using a plane-wave pseudopotential method. The clean surface and several oxidized surfaces were investigated, with oxygen coverages up to 1.5 ML (1 ML = 1 O-atom per surface metal atom). In order to compare the energies of the oxidized structures, which comprise different numbers of metal and oxygen atoms, one has to take account of the chemical potentials of the Ni, Al and O. To this end, for each system, we have applied simple analytic models to study their surface energy as a function of temperature, alloy stoichiometry (assumed to be near 50-50) and oxygen partial pressure. The calculations predict how, at oxygen pressures just above the threshold for decomposing NiAl, the clean surface should be coated by an alumina layer, with the consequent depletion of Ni near the surface. Varying stoichiometry has the relatively minor effect of shifting the crossovers in oxygen pressure at which different oxidized surfaces become stable. (C) 2001 Elsevier Science B.V. All rights reserved.

  10. Title: Is the hydrophobic effect unique to water? The relation between solvation properties and network structure in water and modified water models

    Author(s): Bergman D.L., Lynden-Bell R.M.

    Molecular Physics, 99, No. 12, pp. 1011-1021 (JUN 2001)

    doi: 10.1080/00268970110041632

    The properties of water as a solvent are related to the structure of its liquid phase which in turn depends on the intermolecular potential. In order to explore this relationship we investigate the properties of liquids formed by a number of modified water models. Changing the molecular geometry changes short-range molecular correlations and the network of hydrogen bonds. The solubility and anomalously low entropy of non-polar solutes is only slightly reduced. Reducing the hydrogen-bond strength increases the solubility and removes the low entropy of solution of uncharged spheres. We conclude that the hydrophobic effect depends on the existence of hydrogen bonds and other strong intermolecular interactions but not on the presence of a three-dimensional network.

  11. Title: Quantum electronics - Nanotubes go ballistic

    Author(s): White C.T., Todorov T.N.

    Nature, 411, No. 6838, pp. 649-651 (JUN 2001)

    doi: 10.1038/35079720

  12. Title: Ab initio simulation of charged slabs at constant chemical potential

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

    Journal Of Chemical Physics, 115, No. 4, pp. 1661-1669 (JUL 22 2001)

    doi: 10.1063/1.1379327

    We present a practical scheme for performing ab initio supercell calculations of charged slabs at constant electron chemical potential mu, rather than at constant number of electrons N-e. To this end, we define the chemical potential relative to a plane (or "reference electrode") at a finite distance from the slab (the distance should reflect the particular geometry of the situation being modeled). To avoid a net charge in the supercell, and thus make possible a standard supercell calculation, we restore the electroneutrality of the periodically repeated unit by means of a compensating charge, whose contribution to the total energy and potential is subtracted afterwards. The "constant mu" mode enables one to perform supercell calculation on slabs, where the slab is kept at a fixed potential relative to the reference electrode. We expect this to be useful in modeling many experimental situations, especially in electro-chemistry. (C) 2001 American Institute of Physics.

  13. Title: Fixed-node diffusion Monte Carlo computations for closed-shell jellium spheres

    Author(s): Sottile F., Ballone P.

    Physical Review B, 6404, No. 4, Art. No. 045105 (JUL 15 2001)

    doi: 10.1103/PhysRevB.64.045105

    Fixed-node diffusion Monte Carlo computations are used to determine the ground state energy and electron density for jellium spheres with up to N = 106 electrons and background densities corresponding to the electron gas parameter 1 less than or equal to r(s)less than or equal to5.62. We analyze the density and size dependence of the surface energy, and we extrapolate our data to the thermodynamic limit. The results agree well with the predictions of density functional computations using the local density approximation. In the case of N = 20, we extend our computation to higher densities and identify a transition between atomic- and jelliumlike nodal structures occurring at the background density corresponding to r(s)=0.13. In this case the local density approximation is unable to reproduce the changes in the correlation energy due to the discontinuous transition in the ground state nodal structure. We discuss the relevance of our results for nonlocal approximations to density functional theory.

  14. Title: A simple model of atomic interactions in noble metals based explicitly on electronic structure

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

    Philosophical Magazine A-Physics Of Condensed Matter Structure Defects And Mechanical Properties, 81, No. 7, pp. 1833-1848 (JUL 2001)

    doi: 10.1080/01418610108216639

    A total energy tight-binding model with a basis of just one s state per atom is introduced. It is argued that this simplest of all tight-binding models provides a surprisingly good description of the structural stability and elastic constants of noble metals. By assuming inverse power scaling laws for the hopping integrals and the repulsive pair potential, it is shown that the density matrix in a perfect primitive crystal is independent of volume, and structural energy differences and equations of state are then derived analytically. The model is most likely to be of use when one wishes to consider explicitly and self-consistently the electronic and atomic structures of a generic metallic system, with the minium of computation expense. The relationship to the free-electron jellium model is described. The applicability of the model to other metals is also considered briefly.

  15. Title: A Monte Carlo study of symmetry breaking of I-3(-) in aqueous solution using a multistate diabatic Hamiltonian

    Author(s): Margulis C.J., Coker D.F., Lynden-Bell R.M.

    Journal Of Chemical Physics, 114, No. 1, pp. 367-376 (JAN 1 2001)

    doi: 10.1063/1.1328757

    A model for the description of the electronic ground state of the triiodide; ion in solution is developed. It is based on the "diatomics in molecules" technique and is parametrized from experimental data. The solvent molecules are treated by classical intermolecular potentials. The solvent-ion interaction, which depends on the instantaneous positions of the solvent molecules, enters into the Hamiltonian matrix elements as a spatially varying external electrostatic potential. We use the model to investigate the distribution of the bond lengths of a linear triiodide ion in water at 300 K using Monte Carlo calculations. We find that under these conditions the molecule is significantly distorted with considerable redistribution of charge and bond lengths of 2.95 Angstrom and 3.38 Angstrom. The free energy barrier to switching bond lengths at room temperature is quite high (of the order of 10 kT) so that the distortion is predicted to have a long lifetime. The distribution of instantaneous vibrational frequencies is investigated and shows that the solvent has a greater effect on the frequency of the antisymmetric stretch than on that of the symmetric stretch vibration. (C) 2001 American Institute of Physics.

  16. Title: Porosity formation in axi-symmetric castings produced by counter-pressure casting method

    Author(s): Katzarov I.H., Arsov Y.B., Stoyanov P., Zeuner T., Buehrig-Polaczek A., Sahm P.R.

    International Journal Of Heat And Mass Transfer, 44, No. 1, pp. 111-119 (JAN 2001)

    doi: 10.1016/S0017-9310(00)00085-5

    We propose a method for simultaneous treatment of heat and mass transfer processes and porosity formation of castings produced by Counter Pressure Casting (CPC) method. The method enables us to account for the influence of the CPC parameters on the mechanical properties of the casting. Numerical results and comparison with experimental data are given for an axis-symmetric casting (hemisphere with constant wall thickness). (C) 2000 Elsevier Science Ltd, All rights reserved.

  17. Title: Double-electron above threshold ionization of helium

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

    Journal Of Physics B-Atomic Molecular And Optical Physics, 34, No. 3, pp. L69-L78 (FEB 14 2001)

    doi: 10.1088/0953-4075/34/3/103

    We present calculations of intense-field multiphoton ionization processes in helium at XUV wavelengths. The calculations are obtained from a full-dimensional integration of the two-electron time-dependent Schrodinger equation. A momentum-space analysis of the ionizing two-electron wavepacket reveals the existence of double-electron above threshold ionization (DATI). In momentum-space two distinct forms of DAITI are resolved, namely non-sequential and sequential. In non-sequential DATI correlated electrons resonantly absorb and share energy in integer units of h omega (laser).

  18. Title: The status of the low-temperature phase diagram of hydrogen at the turn of the century

    Author(s): Kohanoff J.

    Journal Of Low Temperature Physics, 122, No. 3-4, pp. 297-311 (FEB 2001)

    doi: 10.1023/A:1004896716926

    Hydrogen. is the simplest element in nature. This simplicity in the atomic state is often assumed to hold also for its condensed phases. Nevertheless; experiments carried out during the past 15 years of the XXth century have shown that this picture is not necessarily a faithful one. Several different low-temperature solid phases have been identified, in contrast with the simplicity idea. These exhibit outstanding features like pressure-independent phonon bands, large isotope effects, and strong infrared activity. In this paper I will give an overview of the current understanding of the low-temperature region of the phase diagram of hydrogen, as emerges from a fruitful cooperative action between diamond anvil cell experiments and first-principles theoretical calculations.

  19. Title: Computed vibrational wavenumbers in ammonium fluoride crystals

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

    Journal Of Raman Spectroscopy, 32, No. 12, pp. 996-999 (DEC 2001)

    doi: 10.1002/jrs.787

    The vibrational wavenumbers of crystalline ammonium fluoride were calculated using the density functional method. The results support the experimental conclusions that the symmetric stretching mode in NH4F lies at a higher wavenumber than the asymmetric stretch, that these wavenumbers are reversed in ND4F and that the librational wavenumbers lie at similar to560 cm(-1). Copyright (C) 2001 John Wiley Sons, Ltd.

  20. Title: Equilibrium polymerization of cyclic carbonate oligomers

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

    Journal Of Chemical Physics, 115, No. 8, pp. 3895-3905 (AUG 22 2001)

    doi: 10.1063/1.1389294

    A model of the polymerization of ring oligomers of bisphenol A polycarbonate (BPA-PC) is used to investigate the influence of dimensionality (2D or 3D), density and temperature on the size distribution of the polymer chains. The polymerization step is catalyzed by a single active particle, conserves the number and type of the chemical bonds, and occurs without a significant gain in either potential energy or configurational entropy. Monte Carlo and molecular dynamics simulations show that polymerization of cyclic oligomers occurs readily at high density and is driven by the entropy associated with the distribution of interparticle bonds. Polymerization competes at lower densities with long range diffusion, which favors small molecular species, and is prevented if the system is sufficiently dilute. Polymerization occurs in 2D via a weakly first order transition as a function of density and is characterized by low hysteresis and large fluctuations in the size of polymer chains. Polymerization occurs more readily in 3D than in 2D, and is favored by increasing temperature, as expected for an entropy-driven process. (C) 2001 American Institute of Physics.

  21. Title: Computer simulation studies of the structure and dynamics of ions and non-polar solutes in water

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

    Philosophical Transactions Of The Royal Society Of London Series A-Mathematical Physical And Engineering Sciences, 359, No. 1785, pp. 1545-1574 (AUG 15 2001)

    doi: 10.1098/rsta.2001.0865

    The mobility of simple ions such as alkali-metal and halide ions at room temperature shows two anomalies. Firstly, there are maxima in mobilities as a function of ion size for both positive and negative ions and, secondly, the maximum for negative ions occurs at a larger ionic radius than the maximum for positive ions. Theoretical treatments of this problem are reviewed and it is concluded that a molecular treatment of the system is needed to understand the results. Computer simulation using the simple point charge model (SPC/E) for water reproduced the observations and is used to discuss the application of theories. In particular, the nature of the first solvation shell is correlated with ion mobility. Simulation reveals a further anomaly, namely that if the charge is removed from a large ion, then it moves more slowly. This is interpreted as the result of formation of a solvent cage around the hydrophobic solute. The changes in local structure resulting from changes in charge and size also affect the solvation thermodynamics. Simulations show that the solvation entropy has a double maximum when viewed as a function of charge. The local minimum near zero charge is interpreted as being due to hydrophobic order, and the maxima as the result of structure breaking. This double maximum in the entropy of solvation is a signature of the hydrophobic cage effect. Comparisons are made between ion mobilities in liquid water at ambient and supercritical conditions.

  22. Title: A simulation study of films of n-hexane and n-perfluorohexane on a solid surface

    Author(s): Vasilyuk A.N., Lynden-Bell R.M.

    Molecular Physics, 99, No. 16, pp. 1407-1411 (AUG 2001)

    doi: 10.1080/00268970110061810

    The behaviour of silica-supported films of liquid n-hexane and n-perfluorohexane has been investigated by means of molecular dynamics simulations of model compounds. Thin films of both neat liquids and equimolar mixtures were studied with different surface interaction strengths at room temperature (300 K). The molecules tend to form layers parallel to the surface of the substrate. In agreement with recent experimental results for hexane, a low density region was found near the surface, provided that the interaction strength was low. There is no corresponding low density region for perfluorohexane. The study of the mixture indicates that perfluorohexane is preferentially adsorbed at both solid-liquid and liquid-vapour interfaces while the molecules of hexane are concentrated inside the film.

  23. Title: Density functional study of reactions of phenoxides with polycarbonate

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

    Journal Of Physical Chemistry A, 105, No. 13, pp. 3008-3015 (APR 5 2001)

    doi: 10.1021/jp003637d

    Density functional calculations with simulated annealing have been used to study the reactions of chains of bisphenol A polycarbonate (BPA-PC) with sodium phenoxide (NaOPh), diphenyl carbonate (DPC), and tetraphenylphosphonium phenoxide (PPh4OPh). These calculations extend our work on the reactions of LiOPh, NaOPh, and phenol with the cyclic tetramer of BPA-PC. We study, in particular, chain growth catalyzed by NaOPh and PPh4OH. The energy barriers for reactions with PPh4OPh are somewhat larger than those involving LiOPh and NaOPh, but they are significantly lower than those involving phenol (HOPh), due in part to the collective rearrangement of phenyl groups in the reacting molecules. We discuss in the Appendix the bonds between alkali metal atoms (Na in the present calculations) and other atoms (here oxygen) that are analogous to the more familiar "hydrogen bonds".

  24. Title: Relevance of heterometallic binding energy for metal underpotential deposition

    Author(s): Sanchez C.G., Leiva E.P.M., Kohanoff J.

    Langmuir, 17, No. 7, pp. 2219-2227 (APR 3 2001)

    doi: 10.1021/la001639j

    We present first-principles calculations for a number of metals adsorbed on several different metallic substrates. Some of these systems are very relevant in electrochemistry, especially in the field of underpotential deposition phenomena. The present studies reveal the existence of a relationship between the excess binding energy and the surface energy difference between substrate and adsorbate. Comparisons with experimental underpotential shifts show that excess binding energies are systematically underestimated. By analyzing experimental information on different systems, we conclude that this discrepancy between our vacuum calculations and experiments carried out in an electrolytic solution is likely to be due to anion adsorption and/or solvent effects.

  25. Title: Current-induced embrittlement of atomic wires

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

    Physical Review Letters, 86, No. 16, pp. 3606-3609 (APR 16 2001)

    doi: 10.1103/PhysRevLett.86.3606

    Recent experiments suggest that gold single-atom contacts and atomic chains break at applied voltages of 1 to 2 V. In order to understand why current flow affects these defect-free conductors, we have calculated the current-induced forces on atoms in a Au chain between two Au electrodes. These forces are not by themselves sufficient to rupture the chain. However, the current reduces the work to break the chain, which results in a dramatic increase in the probability of thermally activated spontaneous fracture of the chain. This current-induced embrittlement poses a fundamental limit to the current-carrying capacity of atomic wires.

  26. Title: Soliton diffusion on the classical, isotropic Heisenberg chain

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

    European Physical Journal B, 20, No. 3, pp. 405-417 (APR 2001)


    Full Text

    We investigate the diffusive motion of a solitary wave on a classical, isotropic, ferromagnetic Heisenberg spin chain with nearest-neighbour exchange interaction. The spins are coupled magnetically to Gaussian white noise and are subject to Gilbert damping. The noise induces a collective, stochastic time evolution of the solitary wave. Within a continuum version of the model we employ implicit collective variables to describe this stochastic behaviour. Thermally excited magnons are disregarded. We derive stochastic equations of motion for the collective variables and solve them numerically, in particular to obtain their variances as functions of time. These results are compared to data from spin dynamics simulations of a discrete chain. For some of the collective variables we find good agreement with respect to the long time behaviour, whereas for other variables the agreement is only qualitative; reasons for this are given. For shorter times we derive analytical expressions for the variances of the collective variables, which also agree well with spin dynamics.

  27. Title: On the stability of electrochemically generated nanoclusters - A computer simulation

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

    Angewandte Chemie-International Edition, 40, No. 24, pp. 4674-4676 (2001)

    doi: 10.1002/1521-3773(20011217)40:24<4674::AID-ANIE4674>3.0.CO;2-B

  28. Title: Material effects on stress-induced defect generation in trenched silicon-on-insulator structures

    Author(s): Nevin W.A., Somasundram K., McCann P., Magee S., Paxton A.T.

    Journal Of The Electrochemical Society, 148, No. 11, pp. G649-G654 (2001)

    doi: 10.1149/1.1408636

    Full Text

    We have investigated the influence of the material properties of the silicon device layer on the generation of defects, and in particular slip dislocations, in trenched and refilled fusion-bonded silicon-on-insulator structures. A strong dependence of the ease of slip generation on the type of dopant species was observed, with the samples falling into three basic categories; heavily boron-doped silicon showed ready slip generation, arsenic and antimony-doped material was fairly resistant to slip, while silicon moderately or lightly doped with phosphorous or boron gave intermediate behavior. The observed behavior appears to be controlled by differences in the dislocation generation mechanism rather than by dislocation mobility. The introduction of an implanted buried layer at the bonding interface was found to result in an increase in slip generation in the silicon, again with a variation according to the dopant species. Here, the greatest slip occurred for both boron and antimony-implanted samples. The weakening of the implanted material may be related to the presence of a band of precipitates observed in the silicon near the bonding interface. (C) 2001 The Electrochemical Society.

  29. Title: Free energy and molecular dynamics calculations for the cubic-tetragonal phase transition in zirconia

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

    Physical Review B, 63, No. 9, Art. No. 094101 (2001)

    doi: 10.1103/PhysRevB.63.094101

    Full Text

    The high-temperature cubic-tetragonal phase transition of pure stoichiometric zirconia is studied by molecular dynamics (MD) simulations and within the framework of the Landau theory of phase transformations. The interatomic forces are calculated using an empirical, self-consistent, orthogonal tight-binding model, which includes atomic polarizabilities up to the quadrupolar level. A first set of standard MD calculations shows that, on increasing temperature, one particular vibrational frequency softens. The temperature evolution of the free-energy surfaces around the phase transition is then studied with a second set of calculations. These combine the thermodynamic integration technique with constrained MD simulations. The results seem to support the thesis of a second-order phase transition but with unusual, very anharmonic behavior above the transition temperature.

  30. Title: Linear Scaling DFT Calculations with Numerical Atomic Orbitals

    Author(s): Ordejon P., Artacho E., Cachau R., Gale J., Garcia A., Junquera J., Kohanoff J., Machado M., Sanchez-Portal D., Soler J.M., Weht R.,

    Mat. Res. Soc. Symp. Proc., 667, pp. AA9.6.1-AA9.6.12 (2001)


    Full Text

    We have recently developed a method to perform Density Functional Theory calculations in systems with a very large number of atoms, which is based on the use of numerical atomic orbitals as basis sets. The method incorporates Order-N techniques both in the calculation of the Kohn-Sham hamiltonian matrix elements and in the solution of the wave functions, which make the CPU time and memory to scale linearly with the number of atoms, allowing calculations in very large system. In this work, we present results on several test systems to show that the approach and the basis sets used with our method are able to provide an accuracy similar to that of other standard DFT techniques.

  31. Title: Shape corrections to exchange-correlation potentials by gradient-regulated seamless connection of model potentials for inner and outer region

    Author(s): Gruening M., Gritsenko O V., van Gisbergen S., Baerends J.

    Journal Chemical Physics, 114, pp. 652- (2001)


  32. Title: Density Functional Study of the Photoactive Yellow Protein’s Chromophore

    Author(s): Sergi A., Gruening M., Ferrario M., Buda F.,

    Journal of Physical Chemistry B, 105, pp. 4386 -4391 (2001)