Title: Dynamic tunnelling ionization of H-+(2) in intense fields
Author(s): Peng L.Y., Dundas D., McCann J.F., Taylor K.T., Williams I.D.
Journal Of Physics B-Atomic Molecular And Optical Physics, 36, No. 18, pp. L295-L302 (SEP 28 2003)
Intense-field ionization of the hydrogen molecular ion by linearly polarized light is modelled by direct solution of the fixed-nuclei time-dependent Schrodinger equation and compared with recent experiments. Parallel transitions are calculated using algorithms which exploit massively parallel computers. We identify and calculate dynamic tunnelling ionization resonances that depend on laser wavelength and intensity, and molecular bond length. Results for lambda similar to 1064 nm are consistent with static tunnelling ionization. At shorter wavelengths lambda similar to 790 nm large dynamic corrections are observed. The results agree very well with recent experimental measurements of the ion spectra. Our results reproduce the single peak resonance and provide accurate ionization rate estimates at high intensities. At lower intensities our results confirm a double peak in the ionization rate as the bond length varies.
Title: Temperature and solvent dependence of vibrational relaxation of tri-iodide: A simulation study
Author(s): Zhang F.S., Lynden-Bell R.M.
Journal Of Chemical Physics, 119, No. 12, pp. 6119-6131 (SEP 22 2003)
Simulations of a flexible model of the tri-iodide ion have been used to study the effects of temperature and solvent on the vibrational energy relaxation rate and the pure dephasing rate of the two stretching modes. These rates were determined in ethanol and 2-methyltetrahydrofuran at 100 K, 200 K, and 300 K, covering the range between glass and mobile liquid states for both solvents. In agreement with experiment the temperature dependence is small. In ethanol the symmetry of the tri-iodide ion is broken at the three temperatures studied, while in methyltetrahydrofuran symmetry is retained except at the lowest temperature. In order to examine different contributions to the relaxation processes, perturbation theory was applied to simulations with rigid ions. Although the large amplitude of the solvent-induced geometry changes suggests that perturbation theory is at best qualitative, reasonable agreement was obtained between the results from the flexible simulation and perturbation theory in 2-methyltetrahydrofuran. Both solvent-induced curvature and solvent force terms were found to contribute to the solvent frequency shifts and the pure dephasing rates. In ethanolic solutions the time correlation functions of the forces decay rapidly, but the curvature term probes a slow dynamical process. At least in our model, the lines would not be completely motionally narrowed in solution in ethanol. Comparisons are made with experiment and previous work. (C) 2003 American Institute of Physics.
Title: QMC computations for homogeneous and inhomogeneous jellium systems.
Author(s): Ballone P.
Abstracts Of Papers Of The American Chemical Society, 226, pp. U338-U339 (SEP 2003)Abstract
Title: The basis for the formation of stable metal clusters on an electrode surface
Author(s): Del Popolo M.G., Leiva E.P.M., Mariscal M., Schmickler W.
Nanotechnology, 14, No. 9, pp. 1009-1013 (SEP 2003)
Metal nanoclusters can be produced cheaply and precisely in an electrochemical environment. Experimentally this method works in some systems, but not in others, and the unusual stability of the clusters has remained a mystery. We have simulated the deposition of the clusters using classical molecular dynamics and studied their stability by grand-canonical Monte Carlo simulations. We find that electrochemically stable clusters occur only in those cases where the two metals involved form stable alloys.
Title: Density functional and Monte Carlo studies of sulfur. II. Equilibrium polymerization of the liquid phase
Author(s): Ballone P., Jones R.O.
Journal Of Chemical Physics, 119, No. 16, pp. 8704-8715 (OCT 22 2003)
The equilibrium polymerization of sulfur is investigated by Monte Carlo simulations. The potential energy model is based on density functional results for the cohesive energy, structural, and vibrational properties as well as reactivity of sulfur rings and chains [Part I, J. Chem. Phys. 118, 9257 (2003)]. Liquid samples of 2048 atoms are simulated at temperatures 450less than or equal toTless than or equal to850 K and P=0 starting from monodisperse S-8 molecular compositions. Thermally activated bond breaking processes lead to an equilibrium population of unsaturated atoms that can change the local pattern of covalent bonds and allow the system to approach equilibrium. The concentration of unsaturated atoms and the kinetics of bond interchanges is determined by the energy DeltaE(b) required to break a covalent bond. Equilibrium with respect to the bond distribution is achieved for 15less than or equal toDeltaE(b)less than or equal to21 kcal/mol over a wide temperature range (Tgreater than or equal to450 K), within which polymerization occurs readily, with entropy from the bond distribution overcompensating the increase in enthalpy. There is a maximum in the polymerized fraction at temperature T-max that depends on DeltaE(b). This fraction decreases at higher temperature because broken bonds and short chains proliferate and, for Tless than or equal toT(max), because entropy is less important than enthalpy. The molecular size distribution is described well by a Zimm-Schulz function, plus an isolated peak for S-8. Large molecules are almost exclusively open chains. Rings tend to have fewer than 24 atoms, and only S-8 is present in significant concentrations at all T. The T dependence of the density and the dependence of polymerization fraction and degree on DeltaE(b) give estimates of the polymerization temperature T-f=450+/-20 K. (C) 2003 American Institute of Physics.
Title: A simulation study of water-dialkylimidazolium ionic liquid mixtures
Author(s): Hanke C.G., Lynden-Bell R.M.
Journal Of Physical Chemistry B, 107, No. 39, pp. 10873-10878 (OCT 2 2003)
Molecular dynamics simulations of mixtures of 1,3-dialkyl imidazolium ionic liquids and water have been performed in order to investigate the microscopic physical properties as a function of composition. The behavior of two ionic liquids has been compared, namely 1,3-dimethylimidazolium chloride and 1,3-dimethylimidazolium hexafluorophosphate. The hexafluorophosphate ionic liquids are known to be more hydrophobic as solvents than the corresponding chloride liquids. This is manifested in our simulations by differences in the signs of both the excess volumes and the enthalpies of mixing. However, there is surprisingly little difference in the microscopic properties. In both liquids, water molecules tend to be isolated from each other in mixtures with more ions than water molecules. When the molar proportion of water molecules reaches 75%, a percolating network of waters is found as well as some isolated molecules and small clusters. In all cases, molecular motion becomes faster as the proportion of water increases, although there are some differences in the details of the translational and rotational motion.
Title: Tools for analysing configuration interaction wavefunctions
Author(s): Delaney P., Greer J.C.
Computational Materials Science, 28, No. 2, pp. 240-249 (OCT 2003)
The configuration interaction (CI) approach to quantum chemical calculations is a well-established means of calculating accurately the solution to the Schrodinger equation for many-electron systems. It represents the many-body electron wavefunction as a sum of spin-projected Slater determinants of orthogonal one-body spin-orbitals. The CI wavefunction becomes the exact solution of the Schrodinger equation as the length of the expansion becomes infinite, however, it is a difficult quantity to visualise and analyse for many-electron problems. We describe a method for efficiently calculating the spin-averaged one- and two-body reduced density matrices rho(psi)((r) over bar; (r) over bar' ) and Gamma(psi)((r) over bar (1), (r) over bar (2); (r) over bar'(1), (r) over bar'(2)) of an arbitrary CI wavefunction Psi. These low-dimensional functions are helpful tools for analysing many-body wavefunctions; we illustrate this for the case of the electron-electron cusp. From rho and Gamma one can calculate the matrix elements of any one- or two-body spin-free operator (O) over cap. For example, if (O) over cap is an applied electric field, this field can be included into the CI Hamiltonian and polarisation or gating effects may be studied for finite electron systems. (C) 2003 Elsevier B.V. All rights reserved.
Title: Dissociative ionization of molecules in intense laser fields
Author(s): Dundas D., Meharg K.J., McCann J.F., Taylor K.T.
European Physical Journal D, 26, No. 1, pp. 51-57 (OCT 2003)
Accurate and efficient grid based techniques for the solution of the time-dependent Schrodinger equation for few-electron diatomic molecules irradiated by intense, ultrashort laser pulses are described. These are based on hybrid finite-difference, Lagrange mesh techniques. The methods are applied in three scenarios, namely H-2(+) with fixed internuclear separation, H-2(+) with vibrating nuclei and H-2 with fixed internuclear separation and illustrative results presented.
Title: Laser-driven helium at 780 nm
Author(s): Taylor K.T., Parker J.S., Meharg K.J., Dundas D.
European Physical Journal D, 26, No. 1, pp. 67-71 (OCT 2003)
We briefly review the methods under development at Queen's University Belfast to solve the full-dimensionality time-dependent Schrodinger equation for helium in intense laser fields. We set out the computational challenges involved in performing calculations that handle Ti:sapphire laser light at its fundamental wavelength (similar to780 nm) in comparison to those encountered for 390 nm light. We remark upon the very considerable importance of accurate and reliable calculations at 780 nm and present results for single-ionization of helium at this wavelength.
Title: Diffusion of intrinsic localized modes by attractor hopping
Author(s): Meister M., Vazquez L.
Journal Of Physics A-Mathematical And General, 36, No. 47, pp. 11779-11790 (NOV 28 2003)
Propagating intrinsic localized modes exist in the damped-driven discrete sine-Gordon chain as attractors of the dynamics. The equations of motion of the system are augmented with Gaussian white noise in order to model the effects of temperature on the system. The noise induces random transitions between attracting configurations corresponding to opposite signs of the propagation velocity of the mode, which leads to a diffusive motion of the excitation. The Heun method is used to numerically generate the stochastic time-evolution of the configuration. We also present a theoretical model for the diffusion which contains two parameters, a transition probability Theta and a delay time tau(A). The mean value and the variance of the position of the intrinsic localized mode, obtained from simulations, can be fitted well with the predictions of our model, Theta and tau(A) being used as parameters in the fit. After a transition period following the switching on of the noise, the, variance shows a linear behaviour as a function of time and the mean value remains constant. An increase in the strength of the noise lowers the variance, leads to an increase in Theta, a decrease in tau(A) and reduces the average distance a mode travels during the transition period.
Title: Surface alloying and mixing at the Mn/Fe(001) interface: Real-time photoelectron spectroscopy and modified embedded atom simulations
Author(s): Torelli P., Sirotti F., Ballone P.
Physical Review B, 68, No. 20, Art. No. 205413 (NOV 2003)
Structural and magnetic properties of thin Mn films on the Fe(001) surface have been investigated by a combination of photoelectron spectroscopy and computer simulation in the temperature range 300 Kless than or equal toTless than or equal to750 K. Room-temperature as deposited Mn overlayers are found to be ferromagnetic up to 2.5-monolayer (ML) coverage, with a magnetic moment parallel to that of the iron substrate. The Mn atomic moment decreases with increasing coverage, and thicker samples (4-ML and 4.5-ML coverage) are antiferromagnetic. Photoemission measurements performed while the system temperature is rising at constant rate (dT/dtsimilar to0.5 K/s) detect the first signs of Mn-Fe interdiffusion at T=450 K, and reveal a broad temperature range (610 Kless than or equal toTless than or equal to680 K) in which the interface appears to be stable. Interdiffusion resumes at Tgreater than or equal to680 K. Molecular dynamics and Monte Carlo simulations allow us to attribute the stability plateau at 610 Kless than or equal toTless than or equal to680 K to the formation of a single-layer MnFe surface alloy with a 2x2 unit cell and a checkerboard distribution of Mn and Fe atoms. X-ray-absorption spectroscopy and analysis of the dichroic signal show that the alloy has a ferromagnetic spin structure, collinear with that of the substrate. The magnetic moments of Mn and Fe atoms in the alloy are estimated to be 0.8mu(B) and 1.1mu(B), respectively.
Title: Point defects in NiAl: The effect of lattice vibrations
Author(s): Lozovoi A.Y., Mishin Y.
Physical Review B, 68, No. 18, Art. No. 184113 (NOV 2003)
We investigate the effect of atomic vibrations on point defect free energies and equilibrium concentrations in the B2 NiAl compound using the quasiharmonic approximation in combination with a recently developed embedded-atom potential. The entropy term appears to be the dominant contribution to the Gibbs free energies of point defects. Vibrational entropies of the main defect complexes: triple-Ni defect, exchange defect, divacancy, and even of the interbranch-Al defect turn out to be positive in the whole range of temperatures studied here (0-1700 K). This leads to an increase in the concentrations of all four types of point defects in Ni-rich and stoichiometric NiAl. On the Al-rich side, the effect of lattice vibrations is to shift the minimum on the vacancy concentration versus temperature curve towards lower temperatures. The effect of zero-point vibrations is shown to be too small to affect the type of constitutional defects in NiAl. The constitutional defects remain nickel antisite atoms on the Ni-rich side and nickel vacancies on the Al-rich side.
Title: Solvent-induced symmetry breaking
Author(s): Zhang F.S., Lynden-Bell R.M.
Physical Review Letters, 90, No. 18, Art. No. 185505 (MAY 9 2003)
Symmetry breaking can be induced in a number of ways including interactions with a solvent. An example is the triiodide ion which is centrosymmetric in the gas phase. Molecular dynamics simulations of the triiodide ion in solution have been used to investigate the extent of symmetry breaking in a variety of solvents. We find that the triiodide ion loses its symmetry in water, ethanol, and methanol which form hydrogen bonds with the ion. This results in a localization of charge at one end of the ion and breaking of the geometric symmetry. The extent of symmetry breaking increases as the temperature is lowered. Correlation times for interconversion are reported and the energetics of symmetry breaking are presented. Analogies are made with second-order phase transitions.
Title: Density functional and Monte Carlo studies of sulfur. I. Structure and bonding in S-n rings and chains (n=2-18)
Author(s): Jones R.O., Ballone P.
Journal Of Chemical Physics, 118, No. 20, pp. 9257-9265 (MAY 22 2003)
Density functional calculations have been performed for ring isomers of sulfur with up to 18 atoms, and for chains with up to ten atoms. There are many isomers of both types, and the calculations predict the existence of new forms. Larger rings and chains are very flexible, with numerous local energy minima. Apart from a small, but consistent overestimate in the bond lengths, the results reproduce experimental structures where known. Calculations are also performed on the energy surfaces of S-8 rings, on the interaction between a pair of such rings, and the reaction between one S-8 ring and the triplet diradical S-8 chain. The results for potential energies, vibrational frequencies, and reaction mechanisms in sulfur rings and chains provide essential ingredients for Monte Carlo simulations of the liquid-liquid phase transition. The results of these simulations will be presented in Part II. (C) 2003 American Institute of Physics.
Title: Reactions and polymerization in polycarbonates - How can theory help our understanding?
Author(s): Jones R.O., Ballone P., Akola J.
Abstracts Of Papers Of The American Chemical Society, 225, Art. No. U646 (MAR 2003)Abstract
Title: Why are aromatic compounds more soluble than aliphatic compounds in dimethylimidazolium ionic liquids? A simulation study
Author(s): Hanke C.G., Johansson A., Harper J.B., Lynden-Bell R.M.
Chemical Physics Letters, 374, No. 1-2, pp. 85-90 (JUN 4 2003)
Molecular dynamics simulations of solutions of benzene in dimethylimidazolium chloride and dimethylimidazolium hexafluorophosphate have been performed with a view to answering the question posed in the title. The difference between the chemical potential of a normal model of benzene and one with no charges was found to depend on the solvent but is at least 4 k(B)T. This difference is sufficient to account for the observed solubility differences. There are substantial changes in the local structure around benzene with and without charges. (C) 2003 Elsevier Science B.V. All rights reserved.
Title: Atomistic molecular dynamics Simulations of chemical force microscopy
Author(s): Patrick D.L., Flanagan J.F., Kohl P., Lynden-Bell R.M.
Journal Of The American Chemical Society, 125, No. 22, pp. 6762-6773 (JUN 4 2003)
Chemical force microscopy and related force measurement techniques have emerged as powerful tools for studying fundamental interactions central to understanding adhesion and tribology at the molecular scale. However, detailed interpretation of these interactions requires knowledge of chemical and physical processes occurring in the region of the tip-sample junction that experiments cannot provide, such as atomic-scale motions and distribution of forces. In an effort to address some of these open issues, atomistic molecular dynamics simulations were performed modeling a chemical force microscope stylus covered with a planar C12 alkylthiolate self-assembled monolayer (SAM) interacting with a solid wall. A complete loading - unloading sequence was simulated under conditions of near-constant equilibrium, approximating the case of infinitely slow tip motion. In the absence of the solid wall, the stylus film existed in a fluid state with structural and dynamic properties similar to those of the analogous planar SAM at an elevated temperature. When the wall was brought into contact with the stylus and pressed against it, a series of reversible changes occurred culminating with solidification of the SAM film at the largest compressive force. During loading, the chemical composition of the contact changed, as much of the film's interior was exposed to the wall. At all tip heights, the distribution of forces within the contact zone was uneven and subject to large local fluctuations. Analysis using the Johnson-Kendall-Roberts, Derjaguin-Muller-Toporov, and Hertz contacts mechanics models revealed significant deviations from the simulation results, with the JKR model providing best overall agreement. Some of the discrepancies found would be overlooked in an actual experiment, where, unlike the simulations, contact area is not separately known, possibly producing a misleading or incorrect interpretation of experimental results. These shortcomings may be improved upon by using a model that correctly accounts for the finite thickness of the compliant components and nonlinear elastic effects.
Title: Dominique Levesque
Author(s): Hansen J.P., Lynden-Bell R.
Molecular Physics, 101, No. 11, pp. 1555-1557 (JUN 10 2003)
Title: A simulation study of vibrational relaxation of I-3(-) in liquids
Author(s): Zhang F.S., Lynden-Bell R.M.
Molecular Physics, 101, No. 11, pp. 1641-1649 (JUN 10 2003)
The temperature dependence of the vibrational relaxation of a flexible model of triiodide in a Lennard-Jones solvent (xenon) has been studied using equilibrium molecular dynamics simulations. The internal dynamics of the ion is calculated from a previously published semi-empirical valence bond model with a limited number of basis states. Vibrational decorrelation rates of the symmetric and antisymmetric stretching modes were found from the time correlation functions of the normal coordinate velocities and the vibrational energy relaxation rates from the time correlation functions of the kinetic energy in each mode. The vibrational dephasing rates and the energy relaxation rates decrease slowly as the temperature is lowered and do not show a discontinuity when the fluid solidifies, although the reorientational diffusion rates change rapidly at low temperatures. In order to interpret the results, perturbation theory expressions for the relaxation rates were evaluated for simulations of a rigid model of the ion and found to agree well with the direct observations. These showed that, unusually, both the solvent force and its derivative, the solvent potential curvature, contribute to the dephasing of the symmetric mode. The relevant fluctuation correlation times are very short, which may explain the insensitivity of the vibrational relaxation to the state of the solvent.
Title: Adsorption of HF and HCl molecules on ice at 190 and 235 K from molecular dynamics simulations: Free energy profiles and residence times
Author(s): Toubin C., Picaud S., Hoang P.N.M., Girardet C., Lynden-Bell R.M., Hynes J.T.
Journal Of Chemical Physics, 118, No. 21, pp. 9814-9823 (JUN 1 2003)
Constrained molecular dynamics simulations are carried out to compare the adsorption/incorporation mechanism of two (nonionizable) hydrogen halide acid molecules HF and HCl on/in ice at typical stratospheric temperatures (190 and 235 K). From the determination of the free energy profiles, it is shown that the free energy barrier to molecular HF incorporation is higher than that for molecular HCl. This difference is interpreted as resulting from the formation of two strong H bonds between HF and its water neighbors, while a single, more flexible, H bond with water favors the penetration for HCl. (C) 2003 American Institute of Physics.
Title: Dissipative discrete breathers: Periodic, quasiperiodic, chaotic, and mobile
Author(s): Martinez P.J., Meister M., Floria L.M., Falo F.
Chaos, 13, No. 2, pp. 610-623 (JUN 2003)
The properties of discrete breathers in dissipative one-dimensional lattices of nonlinear oscillators subject to periodic driving forces are reviewed. We focus on oscillobreathers in the Frenkel-Kontorova chain and rotobreathers in a ladder of Josephson junctions. Both types of exponentially localized solutions are easily obtained numerically using adiabatic continuation from the anticontinuous limit. Linear stability (Floquet) analysis allows the characterization of different types of bifurcations experienced by periodic discrete breathers. Some of these bifurcations produce nonperiodic localized solutions, namely, quasiperiodic and chaotic discrete breathers, which are generally impossible as exact solutions in Hamiltonian systems. Within a certain range of parameters, propagating breathers occur as attractors of the dissipative dynamics. General features of these excitations are discussed and the Peierls-Nabarro barrier is addressed. Numerical scattering experiments with mobile breathers reveal the existence of two-breather bound states and allow a first glimpse at the intricate phenomenology of these special multibreather configurations. (C) 2003 American Institute of Physics.
Title: Evaluation of diffusion mechanisms in NiAl by embedded-atom and first-principles calculations
Author(s): Mishin Y., Lozovoi A.Y., Alavi A.
Physical Review B, 67, No. 1, Art. No. 014201 (JAN 1 2003)
The energetics of Ni vacancy jumps in the intermetallic compound NiAl are studied by combining embedded-atom and first-principles calculations. The embedded-atom potential used in this work is fit to both experimental and first-principles data and provides an accurate description of point defect energies and vacancy jump barriers in NiAl. Some of the embedded-atom results reported here, are independently verified by plane-wave pseudopotential calculations. The results suggest that the atomic configuration produced by a nearest-neighbor jump of a Ni vacancy is mechanically unstable. Because of this instability, the vacancy implements two sequential nearest-neighbor jumps as one collective, two-atom transition. Such collective jumps initiate and complete six-jump vacancy cycles of a Ni vacancy, which are shown to occur by either four or three vacancy jumps. Next-nearest-neighbor vacancy jumps are shown to have diffusion rates comparable to experimental ones at the stoichiometric composition, suggesting that this is an important diffusion mechanism in NiAl.
Title: Laser-driven helium, H-2(+) and H-2
Author(s): Taylor K.T., Parker J.S., Dundas D., Meharg K.J., Moore L.R., McCann J.F.
Journal Of Modern Optics, 50, No. 3-4, pp. 401-422 (JAN 2003)
We review work carried out in recent years at Belfast devoted to handling the dynamics of laser-driven few-electron atomic and molecular systems in full dimensionality with the goal of bringing quantitative discipline to the field. The design and application of quantitatively accurate computational methods are discussed. Electron-electron correlations induced by intense external fields are observed by calculating the position and momentum space distributions of the doubly ionizing two-electron wave packets and the application of the techniques of scientific visualization analysis to such studies is emphasized. Agreement of results obtained with those from recent laboratory experiments is demonstrated. Work in hand and plans for future calculations are outlined.
Title: Inelastic current-voltage spectroscopy of atomic wires
Author(s): Montgomery M.J., Hoekstra J., Todorov T.N., Sutton A.P.
Journal Of Physics-Condensed Matter, 15, No. 4, pp. 731-742 (FEB 5 2003)
A tight-binding model is developed to describe the electron-phonon coupling in atomic wires under an applied voltage and to model, their inelastic current-voltage spectroscopy. Particular longitudinal phonons are found to have greatly enhanced coupling to the electronic states of the system. This leads to a large drop in differential conductance at threshold energies associated with these phonons. It is found that with increasing tension these energies decrease, while the size of the conductance drops increases, in agreement with experiment.
Title: Pure vibrational dephasing of triiodide in liquids and glasses
Author(s): Zhang F.S., Lynden-Bell R.M.
Modern Physics Letters A, 18, No. 2-6, pp. 406-409 (FEB 28 2003)
A molecular dynamics model is developed to investigate the pure vibrational dephasing of 1 3 in solvent at temperatures ranging from room temperature to 100 K. The triiodide ion is described by a valence bond model which responds to the field of the classical solvent molecules. We find that the pure dephasing rate does not change much with temperature. This result can then be interpreted in terms of of Kubo lineshape theory.
Title: Catalytic role of metal oxides in gold-based catalysts: A first principles study of CO oxidation on TiO2 supported Au
Author(s): Liu Z.P., Gong X.Q., Kohanoff J., Sanchez C., Hu P.
Physical Review Letters, 91, No. 26, Art. No. 266102 (DEC 31 2003)
CO oxidation on TiO2 supported Au has been studied using density functional theory calculations. Important catalytic roles of the oxide have been identified: (i) CO oxidation occurs at the interface between Au and the oxide with a very small barrier; and (ii) O-2 adsorption at the interface is the key step in the reaction. The physical origin of the oxide promotion effect has been further investigated: The oxide enhances electron transfer from the Au to the antibonding states of O-2, giving rise to (i) strong ionic bonding between the adsorbed O-2, Au, and the Ti cation; and (ii) a significant activation of O-2 towards CO oxidation.
Title: Electron-phonon interaction in atomic-scale conductors: Einstein oscillators versus full phonon modes
Author(s): Montgomery M.J., Todorov T.N.
Journal Of Physics-Condensed Matter, 15, No. 50, pp. 8781-8795 (DEC 24 2003)
Two extreme pictures of electron-phonon interactions in nanoscale conductors are compared: one in which the vibrations are treated as independent Einstein atomic oscillators, and one in which electrons are allowed to couple to the full, extended phonon modes of the conductor. It is shown that, under a broad range of conditions, the full-mode picture and the Einstein picture produce essentially the same net power at any given atom in the nanojunction. The two pictures begin to differ significantly in the limit of low lattice temperature and low applied voltages, where electron-phonon scattering is controlled by the detailed phonon energy spectrum. As an illustration of the behaviour in this limit, we study the competition between trapped vibrational modes and extended modes in shaping the inelastic current-voltage characteristics of one-dimensional atomic wires.
Title: Reconstruction of charged surfaces: General trends and a case study of Pt(110) and Au(110)
Author(s): Lozovoi A.Y., Alavi A.
Physical Review B, 68, No. 24, Art. No. 245416 (DEC 2003)
The stability of missing-row reconstructions of (110) surfaces with respect to surface charging has been investigated using ab initio theory, taking Pt and Au as representative systems. A thermodynamic formulation is derived to compare the relative stability of charged surfaces either in constant-potential or constant-charge mode. By generalizing Koopmans' theorem to charged metallic surfaces, we obtain an expression for the surface (excess) energy as a function of charge (or potential) in terms of the neutral surface energy, work function, and the position of the image plane. A surface is shown to reconstruct in constant-charge mode if and only if it reconstructs in constant-potential mode. We next address the question of whether a positive (negative) surface charge can lift (induce) the reconstruction, as suggested in the literature. This turns out not to be the case. Instead the following consistent picture arises: at small surface charges, the effect of the charge follows the difference of the work functions; i.e., positive charge favors a surface having a smaller work function and vice versa. Larger charges, either positive or negative, tend to stabilize the reconstructed surface or, more generally, the 1xr reconstruction with larger r. The latter essentially results in that the 1x2 reconstruction in either Pt or Au is never lifted in our study, although the 1x3 surface in Au eventually becomes more stable.
Title: On the reasons for stepwise changes in the tunneling current across metallic nanogaps
Author(s): Narambuena C.F., Del Popolo M.G., Leiva E.P.M.
Nano Letters, 3, No. 12, pp. 1633-1637 (DEC 2003)
Tao and co-workers (Li, C. Z.; He, H. X.; Tao, N. J. AppL Phys. Lett. 2000, 77, 3995-3997) have shown experimentally that the size of the nanogap between two electrodes changes with increments that are very often close to 0.5 Angstrom. We employ a grand canonical Monte Carlo simulation using embedded atom potentials to analyze some aspects of this phenomenon. Although the simulations do not deliver a single distance but a log-normal-like distribution of distance increments that presents a maximum close to 0.5 Angstrom, the present results provide insight for understanding why the changes in the distances in the nanogaps are considerably smaller that the distances between lattice planes.
Title: Gas-liquid interfaces of room temperature ionic liquids
Author(s): Lynden-Bell R.M.
Molecular Physics, 101, No. 16, pp. 2625-2633 (AUG 20 2003)
The structure of the gas-liquid surface of dimethylimidazolium chloride has been studied using atomistic simulation. We find that there is a region of enhanced density immediately below the interface in which the cations are oriented with their planes perpendicular to the surface and their dipoles in the surface plane. There is negligible segregation of cations and anions. The temperature dependence of the surface tension is predicted to be anomalously low or be reversed in sign. The vapour-liquid interfaces between mixtures of water and dimethylimidazolium chloride show similar regions of enhanced density and preferential orientation of the cations. Water molecules also show preferential orientation in the interface region and are preferentially adsorbed on the vapour side of the interface. The surface tension decreases with increase in the mole fraction of water.
Title: A combined experimental and theoretical study of the generation of palladium clusters on Au(111) with a scanning tunnelling microscope
Author(s): Del Popolo M.G., Leiva E.P.M., Kleine H., Meier J., Stimming U., Mariscal M., Schmickler W.
Electrochimica Acta, 48, No. 9, pp. 1287-1294 (APR 20 2003)
Palladium clusters have been deposited on the surface of a Au(111) electrode with the tip of a scanning tunnelling microscope. The distance over which the tip was moved towards the surface has a decisive influence on the properties of the clusters: the larger this distance, the larger the generated clusters, and the more stable they are. These findings are supported by computer simulations, which further suggest that the larger clusters contain a sizable amount of gold, which enhances their stability. Dissolution of the clusters occurs from the edges rather than layer by layer. (C) 2003 Elsevier Science Ltd. All rights reserved.
Title: Finite element modeling of the porosity formation in castings
Author(s): Katzarov I.H.
International Journal Of Heat And Mass Transfer, 46, No. 9, pp. 1545-1552 (APR 2003)
The aim of the present paper was to contribute to understanding the origin and effects of porosity in aluminum die-castings by characterizing the distribution and geometry of the porosity. It also seeks to develop a predictive model for microporosity formation during solidification through an analysis of the alloy solidification path, the presence of gas dissolved in the molten metal and the flow of liquid metal through the mushy zone. The finite element method was used for solving porosity formation problem jointly with the problem for heat and mass transfer. (C) 2003 Elsevier Science Ltd. All rights reserved.
Title: Density Functional Theory study of alloy element interstitials in Al
Author(s): Klaver T.P.C., Chen J.H.
Journal Of Computer-Aided Materials Design, 10, No. 3, pp. 155-162 (2003)
Density Functional Theory calculations were used to study Mg, Si, Cr, Mn, Fe, Co, Ni, and Cu interstitial configurations in Al. Energies of these elements in (100) dumbbell and octahedral configurations were determined. Results show that it is energetically favourable for metal alloying element atoms to replace Al self-interstitials if the alloying atoms are smaller than the Al atoms, as expected. The system energy can thus be decreased by up to 2 eV. The difference between the energies of (100) dumbbell and octahedral configurations is only a few tenth eV for the alloys with metallic alloying elements. For Si, the difference can be up to 0.9 eV. This exceptional behavior of Si is most likely due to its angularly dependent bonding characteristics. Short ab-initio Molecular Dynamics simulations were performed on Mg and Si interstitials to allow these systems to evolve into different interstitial configurations rather than just the (100) dumbbell and octahedral configurations. For Si an alternative configuration with tetrahedral-like coordination was found. Consequences of the calculation results for radiation-induced segregation are discussed.
Title: Molecular Dynamics simulations of Cu/Ta and Ta/Cu thin film growth
Author(s): Klaver T.P.C., Thijsse B.J.
Journal Of Computer-Aided Materials Design, 10, No. 2, pp. 61-74 (2003)
Molecular Dynamics simulation results of evaporated Cu deposition on bcc Ta substrates are presented. Different substrate orientations, substrate temperatures, and initial surface roughnesses were investigated. We have also simulated the deposition of Ta on flat fcc Cu ( 111) substrates at different temperatures. Several aspects of the microstructural evolution during deposition are reported. The results are also compared with those obtained earlier for Cu deposition on beta-Ta. A connection mechanism between two different crystal structures, bcc (110) and fcc ( 111), which is significantly different from the classical misfit dislocation scenario, is reported. Also, mixing behaviour in a thermodynamically immiscible system during atom-by-atom deposition of Ta on fcc Cu ( 111) is elucidated. Finally, we show results of a Density Functional Theory verification of the accuracy of the Ta-Cu emperical potential. Results and implications are discussed.
Title: The interaction of N with atomically dispersed Ti, V, Cr, Mo, and Ni in ferritic steel
Author(s): Kamminga J.D., Klaver T.P.C., Nakata K., Thijsse B.J., Janssen G.C.A.M.
Journal Of Computer-Aided Materials Design, 10, No. 1, pp. 1-11 (2003)
Since long, the nitriding process has been used to improve the mechanical properties of the surface of steel. However, results from the prevailing model for the nitriding of steel are still not in perfect agreement with experimental data. It is investigated if this can be attributed to the model assumption that there is no interaction between nitrogen atoms and alloying atoms in solution in the iron matrix. To this end, the interaction of N with atomically dispersed Ti, V, Cr, Mo, and Ni alloying element atoms in ferritic steel is calculated using Density Functional Theory. Results of the Density Functional Theory calculations agree well with experimental lattice parameter, Mossbauer spectroscopy, and internal friction data taken from literature. Deficiencies in prevailing atomistic models for Fe-Me-N configurations in steel ( Me denotes the alloying element considered) are revealed and the initial stages of MexN precipitation in alloyed iron are elucidated. For all elements but Ti, the interaction with N is negligible or repulsive. Significant attractive interaction exists only if Ti is a second nearest neighbour of N (-0.22 eV). It is shown that this interaction is too small to significantly influence nitriding at typical nitriding temperatures (>500degreesC). Consequently, in modelling the nitriding of steel at typical nitriding temperatures, interaction between the atomically dispersed alloying elements and nitrogen can be disregarded. Internal friction experiments evidence nitrogen in solution in the steel bonded with higher binding energy, also for the other alloying elements. We suggest that this nitrogen is bound in Fe-Me-N accumulations consisting of more than one alloying atom and one or more nitrogen atoms. For the modelling of nitriding of steel, these interactions must be taken into account.
Title: The alpha-alumina(0001)surface: relaxations and dynamics from shell model and density functional theory
Author(s): Marmier A., Lozovoi A., Finnis M.W.
Journal Of The European Ceramic Society, 23, No. 15, pp. 2729-2735 (2003)
We have investigated the dynamics and harmonic free energy of the (0001) surface of alpha-Al2O3 with both semi-empirical and ab initio techniques. The shell models provide a convenient way to to simulate the more expensive ab initio methods and to check some of their technical limitations, in particular the quality of the vibrational Brillouin zone sampling and the harmonic approximation. We have used the supercell approach to compute the frequencies of normal modes at special k-points, which allows the free energy to be calculated at different temperatures. From the corresponding phonon eigenvectors we evaluate the mean square displacements of Al and O ions. The surface phonons Lire particularly sensitive to the modelling technique. The ab initio surface modes are more localized and of higher amplitude, but they are not anharmonic enough to account for the discrepancy observed between the measured and calculated inward relaxation of the surface aluminium atoms. (C) 2003 Elsevier Ltd. All rights reserved.
Title: The coordination chemistry of 1,2,4-triazinyl bipyridines with lanthanide(III) elements - implications for the partitioning of americium(III)
Author(s): Hudson M.J., Drew M.G.B., Foreman M.R.S., Hill C., Huet N., Madic C., Youngs T.G.A.
Dalton Transactions, No. 9, pp. 1675-1685 (2003)
It has been established that 6-(5,6-dialkyl-1,2,4-triazin-3-yl)-2,2'-bipyridines (R,hemi-BTPs) have properties which are intermediate between those of the terpyridines and the bis(1,2,4-triazin-3-yl)pyridines (BTPs). However, they resemble the terpyridines much more closely than the BTPs. It has been shown that Et, hemi-BTP when dissolved in TPH-a dodecane-like solvent-is a selective reagent for the separation of americium(III) from europium(III). Solution NMR in acetonitrile largely confirmed the crystallographic results. There was no evidence for a 1 : 3 complex cation, or for significant differences between metal(III)-N distances for the pyridine and 1,2,4-triazine rings. Intramolecular hydrogen bonding plays a crucial role in the formation of metal coordination spheres, which explains the differences between the terpyridyl, R,hemi-BTPs and the BTPs. Protonation of the R,hemi-BTPs facilitates a conformational change which is necessary for complexation.
Title: Bandstructure approach to near edge structure
Author(s): Paxton A.T., Craven A.J., Gregg J.M., McComb D.W.
Journal Of Microscopy-Oxford, 210, No. 1, pp. 35-44 (2003)
We review the current state of the art in EELS fingerprinting by computer simulation, focusing on the bandstructure approach to the problem. Currently calculations are made using a one electron theory, but we describe in principle the way to go beyond this to include final state effects. We include these effects within the one electron framework using the Slater transition state formula and assess the errors involved. Two examples are then given which illustrate the use of the one electron approximation within density functional theory. Our approach is to combine predicted atomic structure with predicted electronic structure to assist in fingerprinting of complex crystal structures.
Title: Density Functional Theory: basics, new trends and applications, in Handbook of Molecular Physics and Quantum Chemistry, ed. S. Wilson
Author(s): Kohanoff J., Gidopoulos N.
Wiley, Chichester, 2, pp. 532-568 (2003)Abstract
Title: Exchange-correlation energy and potential as approximate functionals of occupied and virtual Kohn–Sham orbitals: Application to dissociating H2
Author(s): Gruening M., Gritsenko O., Baerends E.
Journal of Chemical Physics, 118, pp. 7183- (2003)Abstract
Title: STO and GTO field-induced polarization functions for H to Kr
Author(s): Chong D., Gruening M., , Baerends E.
Journal of Computational Chemistry, 24, pp. 1582-1591 (2003)Abstract