Title: A reactive force field simulation of liquid-liquid phase transitions in phosphorus
Author(s): Ballone P., Jones R.O.
Journal Of Chemical Physics, 121, No. 16, pp. 8147-8157 (OCT 22 2004)
A force field model of phosphorus has been developed based on density functional (DF) computations and experimental results, covering low energy forms of local tetrahedral symmetry and more compact (simple cubic) structures that arise with increasing pressure. Rules tailored to DF data for the addition, deletion, and exchange of covalent bonds allow the system to adapt the bonding configuration to the thermodynamic state. Monte Carlo simulations in the N-P-T ensemble show that the molecular (P-4) liquid phase, stable at low pressure P and relatively low temperature T, transforms to a polymeric (gel) state on increasing either P or T. These phase changes are observed in recent experiments at similar thermodynamic conditions, as shown by the close agreement of computed and measured structure factors in the molecular and polymer phases. The polymeric phase obtained by increasing pressure has a dominant simple cubic character, while the polymer obtained by raising T at moderate pressure is tetrahedral. Comparison with DF results suggests that the latter is a semiconductor, while the cubic form is metallic. The simulations show that the T-induced polymerization is due to the entropy of the configuration of covalent bonds, as in the polymerization transition in sulfur. The transition observed with increasing P is the continuation at high T of the black P to arsenic (A17) structure observed in the solid state, and also corresponds to a semiconductor to metal transition. (C) 2004 American Institute of Physics.
Title: Density functional and Monte Carlo studies of sulfur. I. Structure and bonding in S-n rings and chains (n=2-18) (vol 118, pg 9257, 2003)
Author(s): Jones R.O., Ballone P.
Journal Of Chemical Physics, 121, No. 15, Art. No. 7535 (OCT 15 2004)
Title: Relaxation to a perpetually pulsating equilibrium
Author(s): Lynden-Bell D., Lynden-Bell R.M.
Journal Of Statistical Physics, 117, No. 1-2, pp. 199-209 (OCT 2004)
Paper in honour of Freeman Dyson on the occasion of his 80th birthday. Normal N-body systems relax to equilibrium distributions in which classical kinetic energy components are 1/2kT, but, when inter-particle forces are an inverse cubic repulsion together with a linear (simple harmonic) attraction, the system pulsates for ever. In spite of this pulsation in scale, r(t), other degrees of freedom relax to an ever-changing Maxwellian distribution. With a new time, tau, defined so that r(2)d/dt = d/dtau it is shown that the remaining degrees of freedom evolve with an unchanging reduced Hamiltonian. The distribution predicted by equilibrium statistical mechanics applied to the reduced Hamiltonian is an ever-pulsating Maxwellian in which the temperature pulsates like r(-2). Numerical simulation with 1000 particles demonstrate a rapid relaxation to this pulsating equilibrium.
Title: Beyond Ehrenfest: correlated non-adiabatic molecular dynamics
Author(s): Horsfield A.P., Bowler D.R., Fisher A.J., Todorov T.N., Sanchez C.G.
Journal Of Physics-Condensed Matter, 16, No. 46, pp. 8251-8266 (NOV 24 2004)
A method for introducing correlations between electrons and ions that is computationally affordable is described. The central assumption is that the ionic wavefunctions are narrow, which makes possible a moment expansion for the full density matrix. To make the problem tractable we reduce the remaining many-electron problem to a single-electron problem by performing a trace over all electronic degrees of freedom except one. This introduces both one- and two-electron quantities into the equations of motion. Quantities depending on more than one electron are removed by making a Hartree-Fock approximation. Using the first-moment approximation, we perform a number of tight binding simulations of the effect of an electric current on a mobile atom. The classical contribution to the ionic kinetic energy exhibits cooling and is independent of the bias. The quantum contribution exhibits strong heating, with the heating rate proportional to the bias. However, increased scattering of electrons with increasing ionic kinetic energy is not observed. This effect requires the introduction of the second moment.
Title: Transport in nanoscale systems: the microcanonical versus grand-canonical picture
Author(s): Di Ventra M., Todorov T.N.
Journal Of Physics-Condensed Matter, 16, No. 45, pp. 8025-8034 (NOV 17 2004)
We analyse a picture of transport in which two large but finite charged electrodes discharge across a nanoscale junction. We identify a functional whose minimization, within the space of all bound many-body wavefunctions, defines an instantaneous steady state. We also discuss factors that favour the onset of steady-state conduction in such systems, make a connection with the notion of entropy, and suggest a novel source of steady-state noise. Finally, we prove that the true many-body total current in this closed system is given exactly by the one-electron total current, obtained from time-dependent density-functional theory.
Title: Monte Carlo simulation of cluster growth in surface defects induced by the tip of a scanning tunnelling microscope
Author(s): Luque N.B., Del Popolo M.G., Leiva E.P.M.
Surface Science, 571, No. 1-3, pp. L319-L324 (NOV 1 2004)
First steps are taken to model the electrochemical deposition of metals in nanometer-sized cavities. In the present work, the electrochemical deposition of Cu atoms in nanometer-sized holes dug on Au(111) is investigated through Monte Carlo simulations using the embedded atom method to represent particle interactions. By sweeping the chemical potential of Cu, a cluster is allowed to grow within the hole rising four atomic layers above the surface. Its lateral extension remains confined to the area defined by the borders of the original defect. (C) 2004 Elsevier B.V. All rights reserved.
Title: Monte Carlo simulation of properties of monolayers and metal islands adsorbed on metallic (111) surfaces
Author(s): Rojas M.L., Del Popolo M.G., Leiva E.P.M.
Langmuir, 20, No. 10, pp. 4279-4288 (MAY 11 2004)
To obtain the surface stress changes Deltasigma due to the adsorption of metal monolayers onto metallic surfaces, a new model derived from thermodynamic considerations is presented. Such a model is based on continuum Monte Carlo simulations with embedded atom method potentials in the canonical ensemble, and it is extended to consider the behavior on different islands adsorbed onto (111) substrate surfaces. Homoepitaxial and heteroepitaxial systems are studied. Pseudomorphic growth is not observed for small metal islands with considerable positive misfit with the substrate. Instead, the islands become compressed upon increase of their size. A simple model is proposed to interpolate between the misfits of atoms in small islands and the pseudomorphic behavior of the monolayer.
Title: A Maxwell relation for current-induced forces
Author(s): Sutton A.P., Todorov T.N.
Molecular Physics, 102, No. 9-10, pp. 919-925 (MAY 10 2004)
A Maxwell relation is presented involving current-induced forces. It provides a new physical picture of the origin of current-induced forces and in the small-voltage limit it enables the identification of a simple thermodynamic potential which drives electromigration. The question of whether current-induced forces are conservative or non-conservative is discussed briefly in the light of these insights.
Title: Multiple proton translocation in biomolecular systems: concerted to stepwise transition in a simple model
Author(s): Kohanoff J.J., Cachau R.E.
Molecular Physics, 102, No. 9-10, pp. 1007-1014 (MAY 10 2004)
In this paper we study a simple model potential energy surface (PES) useful for describing multiple proton translocation mechanisms. The approach presented is relevant to the study of more complex biomolecular systems like enzymes. In this model, at low temperatures, proton tunnelling favours a concerted proton transport mechanism, while at higher temperatures there is a crossover from concerted to stepwise mechanisms; the crossover temperature depends on the energetic features of the PES. We illustrate these ideas by calculating the crossover temperature using energies taken from ab initio calculations on specific systems. Interestingly, typical crossover temperatures lie around room temperature; thus both concerted and stepwise reaction mechanisms should play an important role in biological systems, and one can be easily turned into another by external means such as modifying the temperature or the pH, thus establishing a general mechanism for modulation of the biomolecular function by external effectors.
Title: Field-evaporation from first-principles
Author(s): Sanchez C.G., Lozovoi A.Y., Alavi A.
Molecular Physics, 102, No. 9-10, pp. 1045-1055 (MAY 10 2004)
Under the application of a strong electric field, atoms from a metal surface can rupture their bonds and escape, leading to 'field-evaporation'. We present a first-principles description of this phenomenon, taking as an example the evaporation of Al adatoms from an Al(111) surface. The 'charged-plane' method [Lozovoi, A. Y., and ALAVI, A., 2003, Phys. Rev. B, 68, 246416.] has been implemented in the context of a localized-basis code (SIESTA). This enables appreciable fields to be stably and efficiently applied to surfaces, represented using slab geometries. We quantify details of the evaporation process as a function of the applied field strength. The field at which the zero-temperature barrier disappears (evaporation field) is predicted and possible scenarios of the evaporation of surface atoms are discussed. Results are compared to the 'image-hump' model for this process. The field dependence of the barrier is described by this model surprisingly well, despite the potential energy surface not being satisfactorily reproduced.
Title: Field configured assembly: Programmed manipulation and self-assembly at the mesoscale
Author(s): O'Riordan A., Delaney P., Redmond G.
Nano Letters, 4, No. 5, pp. 761-765 (MAY 2004)
Field configured assembly is a programmable force field method that permits rapid, "hands-free" manipulation, assembly, and integration of mesoscale objects and devices. In this method, electric fields, configured by specific addressing of receptor and counter electrode sites pre-patterned at a silicon chip substrate, drive the field assisted transport, positioning, and localization of mesoscale devices at selected receptor locations. Using this approach, we demonstrate field configured deterministic and stochastic self-assembly of model mesoscale devices, i.e., 50 mum diameter, 670 nm emitting GaAs-based light emitting diodes, at targeted receptor sites on a silicon chip. The versatility of the field configured assembly method suggests that it is applicable to self-assembly of a wide variety of functionally integrated nanoscale and mesoscale systems.
Title: Power dissipation in nanoscale conductors: classical, semi-classical and quantum dynamics
Author(s): Horsfield A.P., Bowler D.R., Fisher A.J., Todorov T.N., Montgomery M.J.
Journal Of Physics-Condensed Matter, 16, No. 21, pp. 3609-3622 (JUN 2 2004)
Modelling Joule heating is a difficult problem because of the need to introduce correct correlations between the motions of the ions and the electrons. In this paper we analyse three different models of current induced heating (a purely classical model, a fully quantum model and a hybrid model in which the electrons are treated quantum mechanically and the atoms are treated classically). We find that all three models allow for both heating and cooling processes in the presence of a current, and furthermore the purely classical and purely quantum models show remarkable agreement in the limit of high biases. However, the hybrid model in the Ehrenfest approximation tends to suppress heating. Analysis of the equations of motion reveals that this is a consequence of two things: the electrons are being treated as a continuous fluid and the atoms cannot undergo quantum fluctuations. A means for correcting this is suggested.
Title: Molecular dynamics simulation of silicon sputtering: sensitivity to the choice of potential
Author(s): Thijsse B.J., Klaver T.P.C., Haddeman E.F.C.
Applied Surface Science, 231-2, pp. 29-38 (JUN 15 2004)
Molecular dynamics simulations of Si(0 0 1) targets subject to uninterrupted Ar bombardment of various energies and angles of incidence were performed, in order to study the atomic processes that take place during the transient and steady state sputtering regimes. Silicon interactions were described with the Modified Embedded Atom Method potential and the Stillinger-Weber potential. Detailed information is extracted and briefly discussed. It is found that these potentials lead to significant differences in sputter behavior. By testing these potentials (and two others) against newly calculated DFT data for Si energies in a wide variety of atomic environments, we demonstrate that they all have severe shortcomings. Therefore, the Modified Embedded Atom Method potential was re-fitted to the DFT data, leading to a new parametrization and a greatly improved fit. However, a spurious 'amorphous phase' energy minimum was detected during molecular dynamics simulations at room temperature. Roads to dealing with this problem are discussed. (C) 2004 Elsevier B.V. All rights reserved.
Title: A discrete time-dependent method for metastable atoms and molecules in intense fields
Author(s): Peng L.Y., McCann J.F., Dundas D., Taylor K.T., Williams I.D.
Journal Of Chemical Physics, 120, No. 21, pp. 10046-10055 (JUN 1 2004)
The full-dimensional time-dependent Schrodinger equation for the electronic dynamics of single-electron systems in intense external fields is solved directly using a discrete method. Our approach combines the finite-difference and Lagrange mesh methods. The method is applied to calculate the quasienergies and ionization probabilities of atomic and molecular systems in intense static and dynamic electric fields. The gauge invariance and accuracy of the method is established. Applications to multiphoton ionization of positronium, the hydrogen atom and the hydrogen molecular ion are presented. At very high laser intensity, above the saturation threshold, we extend the method using a scaling technique to estimate the quasienergies of metastable states of the hydrogen molecular ion. The results are in good agreement with recent experiments. (C) 2004 American Institute of Physics.
Title: Accurate and efficient non-adiabatic quantum molecular dynamics approach for laser-matter interactions
Author(s): Dundas D.
Journal Of Physics B-Atomic Molecular And Optical Physics, 37, No. 14, pp. 2883-2901 (JUL 28 2004)
A non-adiabatic quantum molecular dynamics approach for treating the interaction of matter with intense, short-duration laser pulses is developed. This approach, which is parallelized to run on massively-parallel supercomputers, is shown to be both accurate and efficient. Illustrative results are presented for harmonic generation occurring in diatomic molecules using linearly polarized laser pulses.
Title: Correlated electron transport in molecular electronics
Author(s): Delaney P., Greer J.C.
Physical Review Letters, 93, No. 3, Art. No. 036805 (JUL 16 2004)
Theoretical and experimental values to date for the resistances of single molecules commonly disagree by orders of magnitude. By reformulating the transport problem using boundary conditions suitable for correlated many-electron systems, we approach electron transport across molecules from a new standpoint. Application of our correlated formalism to benzene-dithiol gives current-voltage characteristics close to experimental observations. The method can solve the open system quantum many-body problem accurately, treats spin exactly, and is valid beyond the linear response regime.
Title: QSAR studies of multidentate nitrogen ligands used in lanthanide and actinide extraction processes
Author(s): Drew M.G.B., Hudson M.J., Youngs T.G.A.
Journal Of Alloys And Compounds, 374, No. 1-2, pp. 408-415 (JUL 14 2004)
Quantitative structure activity relationships (QSARs) have been developed to optimise the choice of nitrogen heterocyclic molecules that can be used to separate the minor actinides such as americium(III) from europium(III) in the aqueous PUREX raffinate of nuclear waste. Experimental data on distribution coefficients and separation factors (SFs) for 47 such ligands have been obtained and show SF values ranging from 0.61 to 100. The ligands were divided into a training set of 36 molecules to develop the QSAR and a test set of 11 molecules to validate the QSAR. Over 1500 molecular descriptors were calculated for each heterocycle and the Genetic Algorithm was used to select the most appropriate for use in multiple regression equations. Equations were developed fitting the separation factors to 6-8 molecular descriptors which gave r(2) values of >0.8 for the training set and values of >0.7 for the test set, thus showing good predictive quality. The descriptors used in the equations were primarily electronic and steric. These equations can be used to predict the separation factors of nitrogen heterocycles not yet synthesised and/or tested and hence obtain the most efficient ligands for lanthanide and actinide separation. (C) 2003 Elsevier B.V. All rights reserved.
Title: C-60 as a Faraday cage
Author(s): Delaney P., Greer J.C.
Applied Physics Letters, 84, No. 3, pp. 431-433 (JAN 19 2004)
Endohedral fullerenes have been proposed for a number of technological uses, for example, as a nanoscale switch, memory bit and as qubits for quantum computation. For these technology applications, it is important to know the ease with which the endohedral atom can be manipulated using an applied electric field. We find that the Buckminsterfullerene (C-60) acts effectively as a small Faraday cage, with only 25% of the field penetrating the interior of the molecule. Thus influencing the atom is difficult, but as a qubit the endohedral atom should be well shielded from environmental electrical noise. We also predict how the field penetration should increase with the fullerene radius. (C) 2004 American Institute of Physics.
Title: Macroscopic and microscopic properties of solutions of aromatic compounds in an ionic liquid
Author(s): Harper J.B., Lynden-Bell R.M.
Molecular Physics, 102, No. 1, pp. 85-94 (JAN 10 2004)
Molecular dynamics simulations of binary mixtures of benzene, 1,3,5-trifluorobenzene and hexafluorobenzene with dimethylimidazolium hexafluorophosphate were carried out to examine their macroscopic and microscopic properties. The energies and volumes of mixing of these mixtures correlate well with observed microscopic properties including coordination number about the aromatic compound. The local ordering of the ions about an aromatic molecule was found to depend on the quadrupole moment of the aromatic species and to remain qualitatively the same on varying the mole fraction of the aromatic species. Interaction energies showed the most significant interactions to be between the aromatic molecule and the ions located about its equator. These findings have implications for the practical use of ionic liquids as solvents for chemical processes.
Title: Bound states of breathers in the Frenkel-Kontorova model
Author(s): Meister M., Floria L.M.
European Physical Journal B, 37, No. 2, pp. 213-221 (JAN 2004)
In the dissipative, driven standard Frenkel-Kontorova model propagating breathers exist as attractors of the dynamics. In collisions, these excitations interact through the phonons they emit. A possible result of a two-breather collision is a bound state of two breathers. After looking at phonons and breather collisions, we present phenomenological results on breather bound states obtained from lattice dynamics simulations. In particular, we find that bound states can be characterised by the distance between the two breathers they comprise and their propagation velocity. Contrary to the single breather case, several values of the propagation velocity are easily accessible to bound states at fixed model parameters. The results are interpreted on the basis of the observed phonon spectra. The latter can easily be explained as Doppler-shifted combination frequencies of breather harmonics and a discreteness-induced perturbation frequency.
Title: On the structure and dynamics of ionic liquids
Author(s): Del Popolo M.G., Voth G.A.
Journal Of Physical Chemistry B, 108, No. 5, pp. 1744-1752 (FEB 5 2004)
The structure and dynamics of the ionic liquid 1-ethyl-3-methylimidazolium nitrate is studied by molecular dynamics simulations. We find long-range spatial correlations between the ions and a three-dimensional local structure that reflects the asymmetry of the cations. The main contribution to the configurational energy comes from the electrostatic interactions which leads to charge-ordering effects. Radial screening and three-dimensional distribution of charge are also analyzed. The motion of a single ion is studied via velocity and reorientational correlation functions. It is found that ions "rattle" in a long-lived cage, while the orientational structure relaxes on a time scale longer than 200 ps. As in a supercooled liquid, the mean square displacements reveal a subdiffusive dynamics. In addition, the presence of dynamic heterogeneities can be detected by analyzing the non-Gaussian behavior of the van Hove correlation function and the spatial arrangement of the most mobile ions. The short-time collective dynamics is also studied through the electric current time correlation function.
Title: Bismuth embrittlement of copper is an atomic size effect
Author(s): Schweinfest R., Paxton A.T., Finnis M.W.
Nature, 432, No. 7020, pp. 1008-1011 (DEC 23 2004)
Embrittlement by the segregation of impurity elements to grain boundaries is one of a small number of phenomena that can lead to metallurgical failure by fast fracture(1). Here we settle a question that has been debated for over a hundred years(2): how can minute traces of bismuth in copper cause this ductile metal to fail in a brittle manner? Three hypotheses for Bi embrittlement of Cu exist: two assign an electronic effect to either a strengthening(3) or weakening(4) of bonds, the third postulates a simple atomic size effect(5). Here we report first principles quantum mechanical calculations that allow us to reject the electronic hypotheses, while supporting a size effect. We show that upon segregation to the grain boundary, the large Bi atoms weaken the interatomic bonding by pushing apart the Cu atoms at the interface. The resolution of the mechanism underlying grain boundary weakening should be relevant for all cases of embrittlement by oversize impurities.
Title: Quantum electronic transport in a configuration interaction basis
Author(s): Delaney P., Greer J.C.
International Journal Of Quantum Chemistry, 100, No. 6, pp. 1163-1169 (DEC 20 2004)
An overview of a many-body approach to calculation of electronic transport in molecular systems is given. The physics required to describe electronic transport through a molecule at the many-body level, without relying on commonly made assumptions such as the Landauer formalism or linear response theory, is discussed. Physically, our method relies on the incorporation of scattering boundary conditions into a many-body wavefunction and application of the maximum entropy principle to the transport region. Mathematically, this simple physical model translates into a constrained nonlinear optimization problem. A strategy for solving the constrained optimization problem is given. (C) 2004 Wiley Periodicals, Inc.
Title: Diffusion of solitons in anisotropic Heisenberg models
Author(s): Schuster C., Meister M., Mertens F.G.
European Physical Journal B, 42, No. 3, pp. 381-390 (DEC 2004)
We are interested in the thermal diffusion of a solitary wave in the anisotropic Heisenberg spin chain (HSC) with nearest-neighbor exchange interactions. The shape of the solitary wave is approximated by soliton solutions of the coutinuum HSC with on-site anisotropy, restricting ourselves to large width excitations. Temperature is simulated by white noise coupled to the, system. The noise affects the shape and position of the solitary wave and produces magnons. Using implicit collective variables we describe the former effects and neglect magnons (i.e. we use the so-called adiabatic approximation), We derive stochastic equations of motion for the collective variables which we treat, both analytically and numerically. Predictions for the mean values and the variances of the variables obtained front these equations are compared with the corresponding, results from spin dynamics simulations. For the soliton position we find reasonable agreement between spin dynamics and the results of the collective variable treatment. whereas we observe deviations for the other collective variables. The stochastic dynamics of the position shows both a standard Brownian and a super-diffusive component. These results are analogous to results for the isotropic case. previously studied by some of the authors. In the present article we discuss in particular how the anisotropy enters the stochastic equations of motion and the quantitative changes it causes to the diffusion.
Title: Liquid-liquid phase transitions in phosphorus: Insight from simulations.
Author(s): Jones R.O., Ballone P.
Abstracts Of Papers Of The American Chemical Society, 228, pp. U531-U532 (AUG 22 2004)Abstract
Title: Molecular dynamics simulation of ionic liquids: The effect of electronic polarizability
Author(s): Yan T.Y., Burnham C.J., Del Popolo M.G., Voth G.A.
Journal Of Physical Chemistry B, 108, No. 32, pp. 11877-11881 (AUG 12 2004)
An electronically polarizable model has been developed for the ionic liquid 1-ethyl-3-methylimidazolium nitrate (EMIM+/NO3-), Molecular dynamics simulation studies were then performed on both the polarizable and nonpolarizable versions of the model. Comparisons of shear viscosity and diffusion constants at 400 K show that the effects of polarizability are quite substantial and the polarizable model results are in better agreement with the experimental values.
Title: Are current-induced forces conservative?
Author(s): Di Ventra M., Chen Y.C., Todorov T.N.
Physical Review Letters, 92, No. 17, Art. No. 176803 (APR 30 2004)
The expression for the force on an ion in the presence of current can be derived from first principles without any assumption about its conservative character. However, energy functionals have been constructed that indicate that this force can be written as the derivative of a potential. On the other hand, there exist specific arguments that strongly suggest the contrary. We propose physical mechanisms that invalidate such arguments and demonstrate their existence with first-principles calculations. While our results do not constitute a formal resolution to the fundamental question of whether current-induced forces are conservative, they represent a substantial step forward in this direction.
Title: Separation of lanthanides and actinides(III) using tridentate benzimidazole, benzoxazole and benzothiazole ligands
Author(s): Drew M.G.B., Hill C., Hudson M.J., Iveson P.B., Madic C., Vaillant L., Youngs T.G.A.
New Journal Of Chemistry, 28, No. 4, pp. 462-470 (APR 2004)
The ability of new hydrophobic tridentate ligands based on 2,6-bis(benziinidazol-2-yl)pyridine, 2,6-bis(benzoxazol-2-yl)pyridine and 2,6-bis(benzothiazol-2-yl)pyridine to selectively extract americium(III) from europium(III) was measured. The most promising ligand-2,6-bis(benzoxazol-2-yl)-4-(2-decyl-1-tetradecyloxy)pyridine L-9 was found to give separation factors (SFAm/Eu) of up to 70 when used to extract cations from 0.02-0.10 M HNO3 into TPH in synergy with 2-bromodecanoic acid. Six structures of lanthanide complexes with 2,6-bis(benzoxazol-2-yl)pyridine L-6 were then determined to evaluate the types of species that are likely to be involved in the separation process. Three structural types were observed, namely [LnL(6)(NO3)(3)(H2O)2], 11-coordinate only for La, [LnL(6) (NO3)(3) (CH3CN)], 10-coordinate for Pr, Nd and Eu and [LnL(6) (NO3)(3)(H2O)], L 10-coordinate for Eu and Gd. Quantum Mechanics calculations were carried out on the tridentate ligands to elucidate the conformational preferences of the ligands in the free state and protonated and diprotonated forms and to assess the electronic properties of the ligands for comparison with other terdentate ligands used in lanthanide/actinide separation processes.
Title: Raman Spectra in Vanadate Nanotubes Revisited
Author(s): Souza Filho A.G., Ferreira O.P., Santos E.J.G., Mendes Filho J., Alves O.L.
Nano Letters, 4, No. 11, pp. 2099-2104 (23 October 2004)
In this letter we report the Raman spectra of vanadate nanotubes (VONTs). The spectra present a clear signature that can be used for probing the tubular structure. The temperature effects on the structure of dodecylamine- and Cu-intercalated VONTs were studied by changing the laser power density during the Raman measurements. We have found that low laser power densities promote the decomposition of VONTs, leading to the collapse of the tubular structure and converting the nanotubes into V2O5 oxide. The decomposition occurs through an intermediate compound that is isostructural to V2O5 xerogel. The Raman experiments in VONT-based systems should be performed at extremely low laser power densities.
Title: Theory of phonon dissipation in the conduction of stressed Au nanowires
Author(s): Stella L., Santoro G.E., Fabrizio M., Tosatti E,
Surface Science, 566, No. 1, pp. 430-435 (20 September 2004)
Recent experiments on Au break junctions [Phys. Rev. Lett. 88 (2002) 216803] have characterized the nonlinear conductance of stretched short Au nanowires. They reveal in the voltage range 10-20 meV the signatures of dissipation effects, likely due to phonons in the nanowire, reducing the conductance below the quantized value of 2e(2)/h. We present here a theory, based on a model tight-binding Hamiltonian and on non-equilibrium Green's function techniques, which accounts for the main features of the experiment. The theory helps in revealing details of the experiment which need to be addressed with a more realistic, less idealized, theoretical framework.
Title: On the solvation of L-aspartic acid
Author(s): Paxton A.T., Harper J.B.
Molecular Physics, 102, No. 9-10, pp. 953-958 (2004)
We use molecular statics and dynamics to study the stability of L-aspartic acid both in vacuo and solvated by polar and non-polar molecules using density functional theory in the generalized gradient approximation. We find that structures stable in vacuo are unstable in aqueous solution and vice versa. From our simulations we are able to come to some conclusions about the mechanism of stabilisation of zwitterions by polar protic solvents, water and methanol.
Title: SrTiO3(001)(2x1) reconstructions: First-principles calculations of surface energy and atomic structure compared with scanning tunneling microscopy images
Author(s): Johnston K., Castell M.R., Paxton A.T., Finnis M.W.
Physical Review B, 70, No. 8, Art. No. 085415 (2004)
(1x1) and (2x1) reconstructions of the (001) SrTiO3 surface were studied using the first-principles full-potential linear muffin-tin orbital method. Surface energies were calculated as a function of TiO2 chemical potential, oxygen partial pressure and temperature. The (1x1) unreconstructed surfaces were found to be energetically stable for many of the conditions considered. Under conditions of very low oxygen partial pressure the (2x1) Ti2O3 reconstruction [Martin R. Castell, Surf. Sci. 505, 1 (2002)] is stable. The question as to why STM images of the (1x1) surfaces have not been obtained was addressed by calculating charge densities for each surface. These suggest that the (2x1) reconstructions would be easier to image than the (1x1) surfaces. The possibility that the presence of oxygen vacancies would destabilise the (1x1) surfaces was also investigated. If the (1x1) surfaces are unstable then there exists the further possibility that the (2x1) DL-TiO2 reconstruction [Natasha Erdman Nature (London) 419, 55 (2002)] is stable in a TiO2-rich environment and for p(O2)>10(-18) atm.
Title: Ab-initio investigations of pressure effects on the ferroelectric instabilities in KDP and DKDP
Author(s): Colizzi G., Kohanoff J., Lasave J., Koval S., Migoni R.L.
Ferroelectrics, 301, pp. 61-64 (2004)
Results of first-principles calculations are reported for KDP and DKDP as a junction of pressure. The global energy barrier between the two stable (oppositely polarized) equilibrium configurations, and the separation between the corresponding positions of the H-atoms in the O-H-O bridges, are analyzed in terms of a previously proposed, self-consistent double-well model. We argue that the experimentally observed similarity of the H(D) off-centering distance in KDP and DKDP at the transition temperature call be explained only by means of important geometric isotope effects.
Title: Solvent extraction and lanthanide complexation studies with new terdentate ligands containing two 1,3,5-triazine moieties
Author(s): Drew M.G.B., Hill C., Hudson M.J., Iveson P.B., Madic C., Youngs T.G.A.
Dalton Transactions, No. 2, pp. 244-251 (2004)
The extracting agent 2,6-bis(4,6-di-pivaloylamino-1,3,5-triazin-2-yl)-pyridine (L-5) in n-octanol was found, in synergy with 2-bromodecanoic acid, to give D-Am/D-Eu separation factors (SFs) between 2.4 and 3.7 when used to extract the metal ions from 0.02-0.12 M HNO3. Slightly higher SFs (4-6) were obtained in the absence of the synergist when the ligand was used to extract Am(III) and Eu(III) from 0.98 M HNO3. In order to investigate the possible nature of the extracted species crystal structures of L-5 and the complex formed between Yb(III) with 2,6-bis(4,6-di-amino-1,3,5-triazin-2-yl)-pyridine (L-4) were also determined. The structure of L-5 shows 3 methanol solvent molecules all of which form 2 or 3 hydrogen bonds with triazine nitrogen atoms, amide nitrogen or oxygen atoms, or pyridine nitrogen atoms. However, L-5 is relatively unstable in metal complexation reactions and loses amide groups to form the parent tetramine L-4. The crystal structure of Yb(L-4)(NO3)(3) shows ytterbium in a 9-coordinate environment being bonded to three donor atoms of the ligand and three bidentate nitrate ions. The solvent extraction properties of L-4 and L-5 are far inferior to those found for the 2,6-bis-(1,2,4-triazin-3-yl)-pyridines (L-1) which have SF values of ca. 140 and theoretical calculations have been made to compare the electronic properties of the ligands. The electronic charge distribution in L-4 and L-5 is similar to that found in other terdentate ligands such as terpyridine which have equally poor extraction properties and suggests that the unique properties of L-1 evolve from the presence of two adjacent nitrogen atoms in the triazine rings.
Title: Improved Description of Chemical Barriers with Generalized Gradient Approximations (GGAs) and Meta-GGAs
Author(s): Gruening M., Gritsenko O., Baerends E.,
Journal of Physical Chemistry A., 108, pp. 4459-4469 (2004)Abstract
Title: Performance of Density Functionals for Calculating Barrier Heights of Chemical Reactions Relevant to Astrophysics
Author(s): Andersson S, Gruening M.,
Journal of Physical Chemistry A, 108, pp. 7621-7636 (2004)Abstract