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| | | | | | | | | > Title: Computational Verification of Two Universal Relations for Simple Ionic Liquids. Kinetic Properties of a Model 2:1 Molten Salt Author(s): Armstrong J.A, Ballone P. Journal of Physical Chemistry B, 115, No. 17, pp. 4927-4938 (April 8 2011)
Title: Is the pinning of ordinary dislocations in γ-TiAl intrinsic or extrinsic in nature? A combined atomistic and kinetic Monte Carlo approach. Author(s): Katzarov I.H., Paxton A.T. Acta Materialia, 59, No. 3, pp. 1281-1290 (2011) doi:
We address the question of the observed pinning of 1/2 <110] ordinary screw dislocations in γ-TiAl which leads to the characteristic trailing of dipoles in the microstructure. While it has been proposed that these may be variously intrinsic or extrinsic in nature, we are able to rule out the former mechanism. We do this by means of very large scale, three dimensional atomistic simulations using the quantum mechanical bond order potential. We find that the kink-pair formation energy is large: 6eV, while the single kink migration energy is conversely very small: 0.13eV. Using these, and other atomistically derived data, we make kinetic Monte Carlo simulations at realistic time and length scales to simulate dislocation mobility as a function of stress and temperature. In the temperature range of the stress anomaly in γ-TiAl, we determine whether one or several of the pinning and unzipping processes associated with generation of jogs are observed during our simulations. We conclude that the pinning of ordinary dislocations and anomalous mechanical behaviour in γ-TiAl must be attributed to a combination of extrinsic obstacles and extensive cross-slip in a crystal containing impurities. Title: A tight binding model for water Author(s): Paxton A.T., Kohanoff J.J. Journal of Chemical Physics, 134, No. 4, Art. No. 044130 (2011) doi:
We demonstrate for the first time a tight binding model for water incorporating polarizable oxygen atoms. A novel aspect is that we adopt a "ground up" approach in that properties of the monomer and dimer only are fitted. Subsequently we make predictions of the structure and properties of hexamer clusters, ice-XI and liquid water. A particular feature, missing in current tight binding and semiempirical hamiltonians, is that we reproduce the almost two-fold increase in molecular dipole moment as clusters are built up towards the limit of bulk liquid. We concentrate on properties of liquid water, particularly dielectric constant and self diffusion coefficient, which are very well rendered in comparison with experiment. Finally we comment on the question of the contrasting densities of water and ice which is central to an understanding of the subtleties of the hydrogen bond. Title: Effect of hydrophobic nanopatches within an ionic surface on the structure of liquids Author(s): Youngs T.G.A., Hardacre C., Physical Chemistry Chemical Physics, 13, No. 2, pp. 582-585 (2011)
The structures of liquid water and isopropanol have been studied as a function of the size of a hydrophobic patch present in a model hydrophilic surface via molecular dynamics simulations. A significant anisotropy extending into the first few solvent layers is found over the patch which suggests implications for many real-world systems in which nanoscale heterogeneity is found. Title: Excess Electron Localization in Solvated DNA Bases Author(s): Smyth M., Kohanoff J.J, Physical Review Letters, 106, pp. 238108- (June 10 2011) doi:
We present a first-principles molecular dynamics study of an excess electron in condensed phase models of solvated DNA bases. Calculations on increasingly large microsolvated clusters taken from liquid phase simulations show that adiabatic electron affinities increase systematically upon solvation, as for optimized gas-phase geometries. Dynamical simulations after vertical attachment indicate that the excess electron, which is initially found delocalized, localizes around the nucleobases within a 15 fs time scale. This transition requires small rearrangements in the geometry of the bases. Title: Electron detachment from negative ions in a short laser pulse Author(s): Shearer S. F. C. , Smyth M., and Gribakin G. F. Physical Review A, 84, pp. 033409- (12 September 2011) doi:
We present an efficient and accurate method to study electron detachment from negative ions by a few-cycle linearly polarized laser pulse. The adiabatic saddle-point method of Gribakin and Kuchiev [ Phys. Rev. A 55 3760 (1997)] is adapted to calculate the transition amplitude for a short laser pulse. Its application to a pulse with N optical cycles produces 2(N+1) saddle points in complex time, which form a characteristic “smile.” Numerical calculations are performed for H− in a 5-cycle pulse with frequency 0.0043 a.u. and intensities of 1010, 5×1010, and 1011 W/cm2, and for various carrier-envelope phases. We determine the spectrum of the photoelectrons as a function of both energy and emission angle, as well as the angle-integrated energy spectra and total detachment probabilities. Our calculations show that the dominant contribution to the transition amplitude is given by 5–6 central saddle points, which correspond to the strongest part of the pulse. We examine the dependence of the photoelectron angular distributions on the carrier-envelope phase and show that measuring such distributions can provide a way of determining this phase. Title: Nonconservative current-induced forces: A physical interpretation
Author(s): Todorov T.N., Dundas D., Paxton A.T., Horsfield A.P. Beilstein Journal of Nanotechnology, 2, pp. 727-733 (2011) doi:
We give a physical interpretation of the recently demonstrated nonconservative nature of interatomic forces in current-carrying nanostructures. We start from the analytical expression for the curl of these forces, and evaluate it for a point defect in a current-carrying system. We obtain a general definition of the capacity of electrical current flow to exert a nonconservative force, and thus do net work around closed paths, by a formal noninvasive test procedure. Second, we show that the gain in atomic kinetic energy over time, generated by nonconservative current-induced forces, is equivalent to the uncompensated stimulated emission of directional phonons. This connection with electronâphonon interactions quantifies explicitly the intuitive notion that nonconservative forces work by angular momentum transfer.
Title: Screening of pairs of ions dissolved in ionic liquids Author(s): Lynden-Bell R.M. Physical Chemistry Chemical Physics, 12, No. 8, pp. 1733-1740 (2010) doi:
The properties of pairs of solute ions in the ionic liquid, dimethylimidazolium chloride or [dmim][Cl], are studied as a function of their separation. The potential of mean force curves show that there is only a small stabilisation of ion pairs with opposite charges followed by a barrier to separation. Ion pairs with the same charge are also stabilised by solvent screening due to induced polarisation of the solvent. In both cases screening is essentially complete outside the first shell, but is large even when there is no solvent between the ions. Charge distributions in the solvation shells around ion pairs are shown and the results interpreted with the aid of a simple model of two ions in an spheroidal cavity in a conducting medium. The actual or Madelung potential experienced by the second solute ion is found to decay rapidly with distance and does not show the oscillations found in the Poisson potential around a single solute ion. We conclude that ionic liquids provide very effective electrostatic screening between solute molecules. Title: Microscopic Origin of Channeled Flow in Lamellar Titanium Aluminide Author(s): Katzarov I.H., Paxton A.T. Physical Review Letters, 104, No. 22, Art. No. 225502 (2010) doi:
We employ a quantum mechanical bond order potential in an atomistic simulation of channeled flow. We show that the original hypothesis that this is achieved by a cooperative deployment of slip and twinning is correct, first because a twin is able to “protect” a 60° ordinary dislocation from becoming sessile, and second because the two processes are found to be activated by Peierls stresses of similar magnitude. In addition we show an explicit demonstration of the lateral growth of a twin, again at a similar level of stress. Thus these simultaneous processes are shown to be capable of channeling deformation into the observed state of plane strain in so-called “A”-oriented mechanical testing of titanium aluminide superalloy. Title: Spin-Polarization Mechanisms of the Nitrogen-Vacancy Center in Diamond Author(s): Delaney P., Greer J.C., Larsson J.A. Nano Letters, 10, No. 2, pp. 610-614 (2010) doi:
The nitrogen-vacancy (NV) center in diamond has shown great promise for quantum information due to the ease of initializing the qubit and of reading out its state. Here we show the leading mechanism for these effects gives results opposite from experiment; instead both must rely on new physics. Furthermore, NV centers fabricated in nanometer-sized diamond clusters are stable, motivating a bottom-up qubit approach, with the possibility of quite different optical properties to bulk.
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