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1

Spectroscopy of organic semiconductors from first principles

NASA Astrophysics Data System (ADS)

Advances in organic optoelectronic materials rely on an accurate understanding their spectroscopy, motivating the development of predictive theoretical methods that accurately describe the excited states of organic semiconductors. In this work, we use density functional theory and many-body perturbation theory (GW/BSE) to compute the electronic and optical properties of two well-studied organic semiconductors, pentacene and PTCDA. We carefully compare our calculations of the bulk density of states with available photoemission spectra, accounting for the role of finite temperature and surface effects in experiment, and examining the influence of our main approximations -- e.g. the GW starting point and the application of the generalized plasmon-pole model -- on the predicted electronic structure. Moreover, our predictions for the nature of the exciton and its binding energy are discussed and compared against optical absorption data. We acknowledge DOE, NSF, and BASF for financial support and NERSC for computational resources.

Sharifzadeh, Sahar; Biller, Ariel; Kronik, Leeor; Neaton, Jeffery

2011-03-01

2

Doppler effects in a left-handed material: a first-principle theoretical study

The Doppler effects for the reflected wave from a moving media are systemically analyzed in this paper. The theoretical formula for the Doppler shift in the left-handed material, which is described by Drude's dispersion model, is presented. This formula is examined by first-principles numerical experiments, which are in agreement with the theoretical results.

Sanshui Xiao; Min Qiu

2005-09-01

3

-photon and single-photon action spectroscopy: Case study of the proton-bound dimethyl ether dimer Xiaohu Li,1 David multiple-photon dissociation pro- cesses. We consider here a case study of a proton bound diether: Me2O 2HInsights from first principles molecular dynamics studies toward infrared multiple

Iyengar, Srinivasan S.

4

A theoretical study of blue phosphorene nanoribbons based on first-principles calculations

NASA Astrophysics Data System (ADS)

Based on first-principles calculations, we present a quantum confinement mechanism for the band gaps of blue phosphorene nanoribbons (BPNRs) as a function of their widths. The BPNRs considered have either armchair or zigzag shaped edges on both sides with hydrogen saturation. Both the two types of nanoribbons are shown to be indirect semiconductors. An enhanced energy gap of around 1 eV can be realized when the ribbon's width decreases to ˜10 Å. The underlying physics is ascribed to the quantum confinement effect. More importantly, the parameters to describe quantum confinement are obtained by fitting the calculated band gaps with respect to their widths. The results show that the quantum confinement in armchair nanoribbons is stronger than that in zigzag ones. This study provides an efficient approach to tune the band gap in BPNRs.

Xie, Jiafeng; Si, M. S.; Yang, D. Z.; Zhang, Z. Y.; Xue, D. S.

2014-08-01

5

First-principles theoretical study of hydrolysis of stepped and kinked Ga-terminated GaN surfaces

We have investigated the initial stage of hydrolysis process of Ga-terminated GaN surfaces by using first-principles theoretical calculations. We found that the activation barrier of H2O dissociation at the kinked site of the Ga-terminated GaN surface is about 0.8 eV, which is significantly lower than that at the stepped site of about 1.2 eV. This is consistent with the experimental observation where a step-terrace structure is observed after the etching process of Ga-terminated GaN surfaces with catalyst-referred etching method. Detailed analysis on the nature of the chemical interaction uring the hydrolysis processes will be discussed. PMID:23679914

2013-01-01

6

Phase stability, elasticity, and theoretical strength of polonium from first principles

NASA Astrophysics Data System (ADS)

Employing full-potential linearized augmented plane-wave method, we investigate the stability of Po in its ground-state simple cubic structure (?-Po) with respect to the trigonal spiral structure exhibited by Se and Te and to the displacive phase transformations into either tetragonal or trigonal phases. The origin of the phase stability of ?-Po is analyzed with the help of densities of states, electronic band structures, and total energies of competing higher-energy structures corresponding to selected stationary points of the total energy. The electronic structures and total energies are calculated both within the generalized gradient approximation and local-density approximation (LDA) to the exchange-correlation energy as well as with and without inclusion of the spin-orbit (SO) coupling. The total energies are displayed in contour plots as functions of selected structural parameters and atomic volume. It turns out that the LDA calculation with SO interaction incorporated provides best agreement with existing experimental data and that the simple cubic structure of ?-Po is stabilized by relativistic effects of core electrons. High elastic anisotropy of ?-Po is explained as a consequence of its simple cubic structure and is compared with elastic properties of other crystal structures. Finally, an uniaxial tensile test for loading along the [001] and [111] directions is simulated; the corresponding theoretical tensile strengths calculated within the LDA+SO approach amount to 4.2 GPa and 4.7 GPa, respectively, which are the lowest values predicted in an element so far. According to Pugh and Frantsevich criteria, ?-Po is predicted to be ductile. Also a positive value of the Cauchy pressure confirms the metallic type of interatomic bonding.

Legut, Dominik; Friák, Martin; Šob, Mojmír

2010-06-01

7

First-principles study on core-level spectroscopy of arginine in gas and solid phases.

First-principles simulations have been performed for near-edge X-ray absorption fine-structure (NEXAFS) spectra of neutral arginine at different K-edges in the solid phase as well as X-ray photoelectron spectra (XPS) of neutral, deprotonated, and protonated arginines in the gas phase. Influences of the intra- and intermolecular hydrogen bonds (HBs) and different charge states have been carefully examined to obtain useful structure-property relationships. Our calculations show a noticeable difference in the NEXAFS/XPS spectra of the canonical and zwitterionic species that can be used for unambiguously identifying the dominant form in the gas phase. It is found that the deprotonation/protonation always results in red/blue shifts of several electronvolts for the core binding energies (BEs) at all edges. The normal hydrogen bond Y-H···X (X, Y = N, O) can cause a blue/red shift of ca. 1 eV to the core BEs of the proton acceptor X/donor Y, while the weak C-H···Y hydrogen bond may also lead to a weak red shift (less than 1 eV) of the C1s BEs. Moreover, the influence of intermolecular interactions in the solid state is reflected as a broadening in the ?* region of the NEXAFS spectra at each edge, while in the ?* region, these interactions lead to a strengthening or weakening of individual transitions from different carbons, although no evident visual change is found in the resolved total spectra. Our results provide a better understanding of the influences of the intra- and intermolecular forces on the electronic structure of arginine. PMID:23016930

Li, Hongbao; Hua, Weijie; Lin, Zijing; Luo, Yi

2012-10-25

8

We present a step-by-step theoretical protocol based on the first-principles methods to reveal the insight into the origin of the high photocatalytic activity achieved by the mixed-phase TiO2, consisting of anatase and rutile. The interfacial geometries, density of states, charge densities, optical absorption spectrum, electrostatic potential, and band offsets have been calculated. The most stable mixed-phase structures have been identified, the interfacial tensile strain-dependent electronic structures have been observed, and the energy level diagram of band alignment has been given. We find that the geometrical reconstruction around the interfacial area has a negligible influence on the light absorption of the heterojunction and the interfacial sites seem not to dominantly contribute to the band-edge states. For the most stable heterojunction, the calculated valence-band maximum and conduction-band minimum of rutile, respectively, lie 0.52 and 0.22 eV above those of anatase, which agrees well with the experimental measurements and other theoretical predications. The good match of band energies to reaction requirements, large driving force for the charge immigration across the interface, and the difference of electrostatic potentials around the interface successfully explain the high photocatalytic activity achieved by the mixed-phase TiO2. PMID:24964379

Ju, Ming-Gang; Sun, Guangxu; Wang, Jiajun; Meng, Qiangqiang; Liang, WanZhen

2014-08-13

9

1 Oxidized GaN(0001) Surfaces studied by Scanning Tunneling Microscopy and Spectroscopy Abstract Oxidized Ga-polar GaN surfaces have been studied both experimentally and theoretically. For in tunneling spectroscopy revealed a surface band gap with size close to that of GaN, indicating that any

Feenstra, Randall

10

First-principles study on stability and photoelectron spectroscopy of Ga(n)As2(n = 1-9) clusters.

The stability and photoelectron spectroscopy of the Ga(n)As2(n=1-9) clusters have been studied by using first-principles based on density functional theory (DFT). Our calculations reveal that the stabilities of the Ga(n)As2(n=1-9) clusters tend to increase with the increase of the number of total atoms. The calculated second-order difference values of the binding energy show a certain law of even-odd alternation, and the value of the even-numbered clusters is much larger than those of the odd-numbered ones. The energy gap Egap also shows a certain law of even-odd alternation, i.e. the Egap of the even-numbered clusters is much larger than the odd-numbered ones. The Egap of the clusters is between 0.2 eV and 0.6 eV, it will provide a reference for GaAs defect level research. The Ga(n)As2(n=1-9) clusters are potential to detect and emit THz radiation due to their ground-state vibration frequency are in THz range. PMID:24084481

Ma, Deming; Qiao, Hongbo; Shi, Wei; Li, Enling; Ma, Youheng; Wang, Wei

2014-01-24

11

First-principles study on stability and photoelectron spectroscopy of GanAs2(n = 1-9) clusters

NASA Astrophysics Data System (ADS)

The stability and photoelectron spectroscopy of the GanAs2(n = 1-9) clusters have been studied by using first-principles based on density functional theory (DFT). Our calculations reveal that the stabilities of the GanAs2(n = 1-9) clusters tend to increase with the increase of the number of total atoms. The calculated second-order difference values of the binding energy show a certain law of even-odd alternation, and the value of the even-numbered clusters is much larger than those of the odd-numbered ones. The energy gap Egap also shows a certain law of even-odd alternation, i.e. the Egap of the even-numbered clusters is much larger than the odd-numbered ones. The Egap of the clusters is between 0.2 eV and 0.6 eV, it will provide a reference for GaAs defect level research. The GanAs2(n = 1-9) clusters are potential to detect and emit THz radiation due to their ground-state vibration frequency are in THz range.

Ma, Deming; Qiao, Hongbo; Shi, Wei; Li, Enling; Ma, Youheng; Wang, Wei

2014-01-01

12

NASA Astrophysics Data System (ADS)

Recently, inelastic x-ray scattering measurements on single crystals of PrFeAsOy (y˜0.2) have reported that phonons related with Fe-Fe and Fe-As bondings are significantly more softened than those obtained by the first principle calculations [1]. However, it is noted that any previous calculations do not include the magnetic degree of freedom. Therefore, we performed the phonon structure calculations by taking into account the magnetic structure in mother compounds. The magnetic calculations are in better agreement with the observed softening. We show the results and clarify the reason. [3pt] [1] T. Fukuda et al, J. Phys. Soc. Jpn. 77, 103715 (2008).

Nakamura, Hiroki; Machida, Masahiko; Baron, Alfred; Fukuda, Tatsuo; Shamoto, Shinich

2009-03-01

13

Theoretical insight into hydrogen adsorption onto graphene: a first-principles B3LYP-D3 study.

This work investigates hydrogen adsorption onto various graphene flakes such as coronene and coronene-like as suitable models of graphene within the framework of the DFT-B3LYP method. The non-local van der Waals (vdW) density functional (B3LYP-D3) method is used for both structural geometry optimization and total energy estimations. Calculations were carried out for a hydrogen molecule above a coronene surface with both conventional and vdW corrected DFT to investigate how these approaches perform in the case of hydrogen adsorption on a graphene surface. Our first-principles results within the B3LYP-D3/def2-TZVPP model show that hydrogen physisorbs on a coronene surface with an adsorption energy of -5.013 (kJ mol(-1)) which is in good agreement with the experimental value. The influence of the basis set and graphene flake size were also evaluated and the results indicate that these slightly affect the adsorption properties. We found also that it is crucial to use non-local dispersion interactions to get accurate results for hydrogen adsorption on a graphene surface. Furthermore, the co-adsorption of H2 molecules onto the graphene surface was investigated. The results obtained at the B3LYP-D3/def2-TZVP level show that H2 molecules can be physisorbed on both sides of the graphene layer with adsorption properties similar to those for a single surface. Finally, we showed that H2 molecules might be bound to the graphene surface via a bilayer adsorption scheme with weak adsorption energy. Charge population and electron density analysis confirm the weak binding nature of the system under consideration. PMID:25490973

Darvish Ganji, M; Hosseini-Khah, S M; Amini-Tabar, Z

2015-01-28

14

NASA Astrophysics Data System (ADS)

Metal-Mqx (M = Al, Ga, Zn, Be, and Ca, x = 2 or 3) complexes play a key role in organic spintronics and organic optoelectronics. However, the accurate structure determination of these complexes has been a challenge for a long time. Here, we report the structure of Mn-Gaq3 investigated by using first-principle density functional theory (DFT) calculations and extended X-ray absorption fine structure (EXAFS) spectroscopy. First, the structures of Mn-Gaq3 were predicted by first-principle DFT calculations. Then, all reasonable structures achieved from the calculations were used to fit the EXAFS spectra. By this method, the structure of Mn-Gaq3 is well obtained. We believe this method is also applicable to other metal-Mqx films.

Fang, Shaojie; Pang, Zhiyong; Du, Yonghua; Zheng, Lirong; Zhang, Xijian; Wang, Fenggong; Yuan, Huimin; Han, Shenghao

2012-12-01

15

NASA Astrophysics Data System (ADS)

As a representative lanthanide endohedral metallofullerene, Gd@C82 has attracted a widespread attention among theorists and experimentalists ever since its first synthesis. Through comprehensive comparisons and discussions, as well as references to the latest high precision experiments, we evaluated the performance of different computational methods. Our results showed that the appropriate choice of the exchange-correlation functionals is the decisive factor to accurately predict both geometric and electronic structures for Gd@C82. The electronic structure of the ground state and energy gap between the septet ground state and the nonet low-lying state obtained from pure density functional methods, such as PBE and PW91, are in good agreement with current experiment. Unlike pure functionals, the popularly used hybrid functionals in previous studies, such as B3LYP, could infer the qualitative correct ground state only when small basis set for C atoms is employed. Furthermore, we also highlighted that other geometric structures of Gd@C82 with the Gd staying at different positions are either not stable or with higher energies. This work should provide some useful references for various theoretical methodologies in further density functional studies on Gd@C82 and its derivatives in the future.

Dai, Xing; Gao, Yang; Xin, Minsi; Wang, Zhigang; Zhou, Ruhong

2014-12-01

16

NASA Astrophysics Data System (ADS)

A thorough study of the vibrational dynamics of xenotime YPO4 and its isomorphs, pretulite ScPO4 single crystals, with tetragonal zircon structure, was carried out by means of an integrated approach consisting of both polarized micro-Raman scattering spectroscopy and first-principles calculations. Polarized Raman scattering experiments were performed at room temperature either in the 180° or in the 90° geometry on both samples under different crystal orientations set by a micromanipulator. All the 12 independent components of the polarizability tensor, expected on the basis of the group theory for both YPO4 and ScPO4, were selected in turn and assigned in symmetry. In particular, it was possible to unambiguously identify a B1g mode peaked at 316 cm-1 in YPO4 and two B1g modes peaked at 350 and 677 cm-1 in ScPO4, respectively, all of them never reported before. These observations were corroborated by the results of first-principles calculations, based on density functional theory in the generalized gradient approximation, performed to determine Raman and infrared vibrational modes. In fact, an excellent agreement between computational and experimental results has been found.

Giarola, M.; Sanson, A.; Rahman, A.; Mariotto, G.; Bettinelli, M.; Speghini, A.; Cazzanelli, E.

2011-06-01

17

NASA Astrophysics Data System (ADS)

We have studied Zn(CN)2 at high pressure using Raman spectroscopy and report Grüneisen parameters of the soft phonons. The phonon frequencies and eigenvectors obtained from ab initio calculations are used for the assignment of the observed phonon spectra. Out of the 11 zone-center optical modes, 6 modes exhibit negative Grüneisen parameter. The calculations suggest that the soft phonons correspond to the librational and translational modes of C=N rigid unit, with librational modes contributing more to thermal expansion. A rapid disordering of the lattice is found above 1.6GPa from x-ray diffraction.

Ravindran, T. R.; Arora, A. K.; Chandra, Sharat; Valsakumar, M. C.; Chandra Shekar, N. V.

2007-08-01

18

NASA Astrophysics Data System (ADS)

Layered materials, such as graphite/graphene, boron nitride, transition metal dichalcogenides, represent materials in which reduced size, dimensionality, and symmetry play critical roles in their physical properties. Here, we report on a comprehensive investigation of the phonon properties in the topological insulator Bi2Te3 and Bi2Se3 two-dimensional (2D) crystals, with the combination of Raman spectroscopy, first-principles calculations, and group theory analysis. Low frequency (<30 c m-1) interlayer vibrational modes are revealed in few-quintuple-layer (QL) B i2T e3/B i2S e3 2D crystals, which are absent in the bulk crystal as a result of different symmetries. The experimentally observed interlayer shear and breathing mode frequencies both show blueshifts, with decreasing thickness in few-QL Bi2Te3 (down to 2QL) and Bi2Se3 (down to 1QL), in agreement with first-principles calculations and a linear chain model, from which the interlayer coupling force constants can be estimated. Besides, an intense ultralow (<12 c m-1) frequency peak is observed in 2-4QL Bi2Te3 , which is tentatively attributed to a substrate-induced interface mode supported by a linear chain model analysis. The high frequency Raman peaks exhibit frequency shifts and broadening from 3D to 2D as a result of the phonon confinement effect. Our studies shed light on a general understanding of the influence of dimensionality and crystal symmetry on the phonon properties in layered materials.

Zhao, Yanyuan; Luo, Xin; Zhang, Jun; Wu, Junxiong; Bai, Xuxu; Wang, Meixiao; Jia, Jinfeng; Peng, Hailin; Liu, Zhongfan; Quek, Su Ying; Xiong, Qihua

2014-12-01

19

Towards first-principles electrochemistry

This doctoral dissertation presents a comprehensive computational approach to describe quantum mechanical systems embedded in complex ionic media, primarily focusing on the first-principles representation of catalytic ...

Dabo, Ismaila

2008-01-01

20

A DFT-based molecular model for imidazolium-silica-based nanoparticle networks (INNs) is presented. The INNs were synthesized and characterized by using small-angle X-ray scattering (SAXS), NMR spectroscopy, and theoretical ab initio calculations. (11)B and (31)P HETCOR CP MAS experiments were recorded. Calculated (19)F?NMR spectroscopy results, combined with the calculated anion-imidazolium (IM) distances, predicted the IM chain density in the INN, which was also confirmed from thermogravimetric analysis/mass spectrometry results. The presence of water molecules trapped between the nanoparticles is also suggested. First considerations on possible ?-? stacking between the IM rings are presented. The predicted electronic properties confirm the photoluminescence emissions in the correct spectral domain. PMID:25241702

Neouze, Marie-Alexandra; Kronstein, Martin; Litschauer, Marco; Puchberger, Michael; Coelho, Cristina; Bonhomme, Christian; Gervais, Christel; Tielens, Frederik

2014-11-10

21

Exciton dispersion from first principles

NASA Astrophysics Data System (ADS)

We present a scheme to calculate exciton dispersions in real materials that is based on the first-principles many-body Bethe-Salpeter equation. We assess its high level of accuracy by comparing our results for LiF with recent inelastic x-ray scattering experimental data on a wide range of energy and momentum transfer. We show its great analysis power by investigating the role of the different electron-hole interactions that determine the exciton band structure and the peculiar “exciton revival” at large momentum transfer. Our calculations for solid argon are a prediction and a suggestion for future experiments. These results demonstrate that the first-principles Bethe-Salpeter equation is able to describe the dispersion of localized and delocalized excitons on equal footing and represent a key step for the ab initio study of the exciton mobility.

Gatti, Matteo; Sottile, Francesco

2013-10-01

22

Morse potential derived from first principles

NASA Astrophysics Data System (ADS)

We show that a direct connection can be drawn, based on fundamental quantum principles, between the Morse potential, extensively used as an empirical description for the atomic interaction in diatomic molecules, and the harmonic potential. This is conceptually achieved here through a non-additive translation operator, whose action leads to a perfect equivalence between the quantum harmonic oscillator in deformed space and the quantum Morse oscillator in regular space. In this way, our theoretical approach provides a distinctive first-principle rationale for anharmonicity, therefore revealing a possible quantum origin for several related properties as, for example, the dissociation energy of diatomic molecules and the deformation of cubic metals.

Costa Filho, Raimundo N.; Alencar, Geová; Skagerstam, Bo-Sture; Andrade, José S., Jr.

2013-01-01

23

First principles quantum Monte Carlo

Present quantum Monte Carlo codes use statistical techniques adapted to find the amplitude of a quantum system or the associated eigenvalues. Thus, they do not use a true physical random source. It is demonstrated that, in fact, quantum probability admits a description based on a specific class of random process at least for the single particle case. Then a first principle Monte Carlo code that exactly simulates quantum dynamics can be constructed. The subtle question concerning how to map random choices in amplitude interferences is explained. Possible advantages of this code in simulating single hit experiments are discussed.

J. M. A. Figueiredo

2006-12-07

24

Polymer actuators from first principles

NASA Astrophysics Data System (ADS)

We investigate the structure and stability of novel molecular architectures based on the actuation of flexible calixarene hinges and conductive oligothiophenes. When oxidized the oligothiophenes drive the actuation via ?-stacking. We investigate from first principles the components of this actuator, paying particular attention to the structure of the hinge, the energetics of ?-stacking in charged oligothiophens, and environmental effects (i.e. solvation and counterions). Since ?-stacking occurs in an oxidized state, the latter effects are of particular importance in screening long range Coulomb interactions and the concentration of the charge.

Singh-Miller, Nicholas; Scherlis, Damian; Marzari, Nicola

2007-03-01

25

The polymorphism of resorcinol has been complementary studied by combining Raman, time-domain terahertz, and inelastic neutron scattering spectroscopy with modern solid-state density functional theory (DFT) calculations. The spectral differences, emerging from the temperature-induced structural phase transition, have been successfully interpreted with an emphasis on the low-wavenumber range. The given interpretation is based on the plane-wave DFT computations, providing an excellent overall reproduction of both wavenumbers and intensities and revealing the source of the observed spectral differences. The performance of the generalized gradient approximation (GGA) functionals in prediction of the structural parameters and the vibrational spectra of the normal-pressure polymorphs of resorcinol has been extensively examined. The results show that the standard Perdew, Burke, and Ernzerhof (PBE) approach along with its "hard" revised form tends to be superior if compared to the "soft" GGA approximation. PMID:25564699

Dru?bicki, Kacper; Mikuli, Edward; Pa?ka, Norbert; Zalewski, S?awomir; Ossowska-Chru?ciel, Miros?awa D

2015-01-29

26

NASA Astrophysics Data System (ADS)

The electronic structure in the new transition-metal carbide Ti4SiC3 has been investigated by bulk-sensitive soft x-ray emission spectroscopy and compared to the well-studied Ti3SiC2 and TiC systems. The measured high-resolution Ti L , C K , and Si L x-ray emission spectra are discussed with ab initio calculations based on density-functional theory including core-to-valence dipole matrix elements. The detailed investigations of the Ti-C and Ti-Si chemical bonds provide increased understanding of the physical properties of these nanolaminates. A strongly modified spectral shape is detected for the intercalated Si monolayers due to Si 3p hybridization with the Ti 3d orbitals. As a result of relaxation of the crystal structure and the charge-transfer from Ti (and Si) to C, the strength of the Ti-C covalent bond is increased. The differences between the electronic and crystal structures of Ti4SiC3 and Ti3SiC2 are discussed in relation to the number of Si layers per Ti layer in the two systems and the corresponding change of materials properties.

Magnuson, M.; Mattesini, M.; Wilhelmsson, O.; Emmerlich, J.; Palmquist, J.-P.; Li, S.; Ahuja, R.; Hultman, L.; Eriksson, O.; Jansson, U.

2006-11-01

27

Introductory Training on Theoretical Spectroscopy

. Experimental techniques measuring electronic excitations have seen rapid progress (high spatial resolution, in particular valence electron excitations. The aim is not to learn technical details of theoretical methods electron density 10h40 - 11h10 Caffeine Break 11h10 - 11h40 Green's Functions theory: description through

Botti, Silvana

28

Hydrogen donors in SnO2 studied by infrared spectroscopy and first-principles calculations W. M; revised manuscript received 8 October 2010; published 3 November 2010 Hydrogen is a potentially important source of n-type conductivity in oxide materials. We have investigated hydrogen in tin oxide SnO2

McCluskey, Matthew

29

Theoretical aspects of light meson spectroscopy

In this pedagogical review the authors discuss the theoretical understanding of light hadron spectroscopy in terms of QCD and the quark model. They begin with a summary of the known and surmised properties of QCD and confinement. Following this they review the nonrelativistic quark potential model for q{anti q} mesons and discuss the quarkonium spectrum and methods for identifying q{anti q} states. Finally, they review theoretical expectations for non-q{anti q} states (glueballs, hybrids and multiquark systems) and the status of experimental candidates for these states.

Barnes, T. [Oak Ridge National Lab., TN (United States). Computational and Theoretical Physics Group]|[Univ. of Tennessee, Knoxville, TN (United States). Dept. of Physics and Astronomy

1995-12-31

30

Methods for First-Principles Alloy Thermodynamics

NASA Astrophysics Data System (ADS)

Traditional first-principles calculations excel at providing formation energies at absolute zero, but obtaining thermodynamic information at nonzero temperatures requires suitable sampling of all the excited states visited in thermodynamic equilibrium, which would be computationally prohibitive via brute-force quantum mechanical calculations alone. In the context of solid-state alloys, this issue can be addressed via the coarse-graining concept and the cluster expansion formalism. This process generates simple, effective Hamiltonians that accurately reproduce quantum mechanical calculation results and that can be used to efficiently sample configurational, vibrational, and electronic excitations and enable the prediction of thermodynamic properties at nonzero temperatures. Vibrational and electronic degrees of freedom are formally eliminated from the problem by writing the system's partition function in a nested form in which the inner sums can be readily evaluated to yield an effective Hamiltonian. The remaining outermost sum corresponds to atomic configurations and can be handled via Monte Carlo sampling driven by the resulting effective Hamiltonian, thereby delivering thermodynamic properties at nonzero temperatures. This article describes these techniques and their implementation in the alloy theoretic automated toolkit, an open-source software package. The methods are illustrated by applications to various alloy systems.

van de Walle, Axel

2013-11-01

31

First Principles Quantitative Modeling of Molecular Devices

NASA Astrophysics Data System (ADS)

In this thesis, we report theoretical investigations of nonlinear and nonequilibrium quantum electronic transport properties of molecular transport junctions from atomistic first principles. The aim is to seek not only qualitative but also quantitative understanding of the corresponding experimental data. At present, the challenges to quantitative theoretical work in molecular electronics include two most important questions: (i) what is the proper atomic model for the experimental devices? (ii) how to accurately determine quantum transport properties without any phenomenological parameters? Our research is centered on these questions. We have systematically calculated atomic structures of the molecular transport junctions by performing total energy structural relaxation using density functional theory (DFT). Our quantum transport calculations were carried out by implementing DFT within the framework of Keldysh non-equilibrium Green's functions (NEGF). The calculated data are directly compared with the corresponding experimental measurements. Our general conclusion is that quantitative comparison with experimental data can be made if the device contacts are correctly determined. We calculated properties of nonequilibrium spin injection from Ni contacts to octane-thiolate films which form a molecular spintronic system. The first principles results allow us to establish a clear physical picture of how spins are injected from the Ni contacts through the Ni-molecule linkage to the molecule, why tunnel magnetoresistance is rapidly reduced by the applied bias in an asymmetric manner, and to what extent ab initio transport theory can make quantitative comparisons to the corresponding experimental data. We found that extremely careful sampling of the two-dimensional Brillouin zone of the Ni surface is crucial for accurate results in such a spintronic system. We investigated the role of contact formation and its resulting structures to quantum transport in several molecular wires and show that interface contacts critically control charge conduction. It was found, for Au/BDT/Au junctions, the H atom in -SH groups energetically prefers to be non-dissociative after the contact formation, which was supported by comparison between computed and measured break-down forces and bonding energies. The H-non-dissociated (HND) junctions give equilibrium conductances from0.054G0 (equilibrium structure) to 0.020G0 (stretched structure) which is within a factor of 2-5 of the measured data. On the other hand, for all H-dissociated contact structures - which were the assumed structures in the literature, the conductance is at least more than an order of magnitude larger that the experimental value. The HND-model significantly narrows down the theory/experiment discrepancy. Finally, a by-product of this work is a comprehensive pseudopotential and atomic orbital basis set database that has been carefully calibrated and can be used by the DFT community at large.

Ning, Zhanyu

32

Proca equations derived from first principles

Gersten has shown how Maxwell equations can be derived from first principles, similar to those which have been used to obtain the Dirac relativistic electron equation. We show how Proca equations can be also deduced from first principles, similar to those which have been used to find Dirac and Maxwell equations. Contrary to Maxwell equations, it is necessary to introduce a potential in order to transform a second order differential equation, as the Klein-Gordon equation, into a first order differential equation, like Proca equations.

Michel Gondran

2009-01-21

33

Physics of Life from First Principles

The objective of this work is to extend the First Principles of Newtonian mechanics to include modeling of behavior of Livings. One of the most fundamental problems associated with modeling life is to understand a mechanism of progressive evolution of complexity typical for living systems. It has been recently recognized that the evolution of living systems is progressive in a

Michail Zak

2007-01-01

34

FIRST PRINCIPLES CALCULATIONS OF TOKAMAK ENERGY TRANSPORT

Institute for Fusion Studies, University of Texas, Austin, Texas, United States of America G.W. HAMMETT, M, California, United States of America Abstract FIRST PRINCIPLES CALCULATIONS OF TOKAMAK ENERGY TRANSPORT States of America A. BONDESON Institute for Electromagnetic Theory, Euratom-NFRIFusion Association

Hammett, Greg

35

Theoretical aspects of photoacoustic spectroscopy with light scattering samples

A theoretical model for photoacoustic spectroscopy with light scattering samples is discussed. Some special cases are derived and the nature of the measured signal and the underlying physics are analyzed. Comparisons with transmission and diffuse reflectance spectroscopy are made.

Per Helander

1983-01-01

36

Interface Structure Prediction from First-Principles

Information about the atomic structures at solid–solid interfaces is crucial for understanding and predicting the performance of materials. Due to the complexity of the interfaces, it is very challenging to resolve their atomic structures using either experimental techniques or computer simulations. In this paper, we present an efficient first-principles computational method for interface structure prediction based on an adaptive genetic algorithm. This approach significantly reduces the computational cost, while retaining the accuracy of first-principles prediction. The method is applied to the investigation of both stoichiometric and nonstoichiometric SrTiO3 ?3(112)[1?10] grain boundaries with unit cell containing up to 200 atoms. Several novel low-energy structures are discovered, which provide fresh insights into the structure and stability of the grain boundaries.

Zhao, Xin; Shu, Qiang; Nguyen, Manh Cuong; Wang, Yangang; Ji, Min; Xiang, Hongjun; Ho, Kai-Ming; Gong, Xingao; Wang, Cai-Zhuang

2014-05-08

37

First principles materials design for semiconductor spintronics

Materials design of new functional diluted magnetic semiconductors (DMSs) is presented based on first principles calculations. The stability of the ferromagnetic state in ZnO-, ZnS-, ZnSe-, ZnTe-, GaAs- and GaN-based DMSs is investigated systematically and it is suggested that V- or Cr-doped ZnO, ZnS, ZnSe and ZnTe are candidates for high-TC ferromagnetic DMSs. V-, Cr- or Mn-doped GaAs and GaN

K. Sato; H. Katayama-Yoshida

2002-01-01

38

First principles calculations for lithiated manganese oxides.

First principles calculations within the local-spin-density-functional theory (LSDFF) framework are presented of densities of electronic states for MnO, LiMnO{sub 2} in the monoclinic and orthorhombic structures, cubic LiMn{sub 2}O{sub 4} spinel and {lambda}-MnO{sub 2} (delithiated spinel), all in antiferromagnetic spin configurations. The changes in energy spectra as the Mn oxidation state varies between 2+ and 4+ are illustrated. Preliminary calculations for Co-doped LiMnO{sub 2} are presented, and the destabilization of a monoclinic relative to a rhombohedral structure is discussed.

Prasad, R.

1998-12-23

39

Theoretical study of single-molecule spectroscopy and vibrational spectroscopy in condensed phases

In this thesis, theoretical models and computer simulations are employed to study several problems of single-molecule spectroscopy and vibrational spectroscopy in condensed phases. The first part of the thesis concentrates ...

Yang, Shilong, 1975-

2005-01-01

40

Iron diffusion from first principles calculations

NASA Astrophysics Data System (ADS)

The cores of Earth and other terrestrial planets are made up largely of iron1 and it is therefore very important to understand iron's physical properties. Chemical diffusion is one such property and is central to many processes, such as crystal growth, and viscosity. Debate still surrounds the explanation for the seismologically observed anisotropy of the inner core2, and hypotheses include convection3, anisotropic growth4 and dendritic growth5, all of which depend on diffusion. In addition to this, the main deformation mechanism at the inner-outer core boundary is believed to be diffusion creep6. It is clear, therefore, that to gain a comprehensive understanding of the core, a thorough understanding of diffusion is necessary. The extremely high pressures and temperatures of the Earth's core make experiments at these conditions a challenge. Low-temperature and low-pressure experimental data must be extrapolated across a very wide gap to reach the relevant conditions, resulting in very poorly constrained values for diffusivity and viscosity. In addition to these dangers of extrapolation, preliminary results show that magnetisation plays a major role in the activation energies for diffusion at low pressures therefore creating a break down in homologous scaling to high pressures. First principles calculations provide a means of investigating diffusivity at core conditions, have already been shown to be in very good agreement with experiments7, and will certainly provide a better estimate for diffusivity than extrapolation. Here, we present first principles simulations of self-diffusion in solid iron for the FCC, BCC and HCP structures at core conditions in addition to low-temperature and low-pressure calculations relevant to experimental data. 1. Birch, F. Density and composition of mantle and core. Journal of Geophysical Research 69, 4377-4388 (1964). 2. Irving, J. C. E. & Deuss, A. Hemispherical structure in inner core velocity anisotropy. Journal of Geophysical Research 116, B04307 (2011). 3. Buffett, B. A. Onset and orientation of convection in the inner core. Geophysical Journal International 179, 711-719 (2009). 4. Bergman, M. Measurements of electric anisotropy due to solidification texturing and the implications for the Earth's inner core. Nature 389, 60-63 (1997). 5. Deguen, R. & Cardin, P. Thermochemical convection in Earth's inner core. Geophysical Journal International 187, 1101-1118 (2011). 6. Reaman, D. M., Daehn, G. S. & Panero, W. R. Predictive mechanism for anisotropy development in the Earth's inner core. Earth and Planetary Science Letters 312, 437-442 (2011). 7. Ammann, M. W., Brodholt, J. P., Wookey, J. & Dobson, D. P. First-principles constraints on diffusion in lower-mantle minerals and a weak D'' layer. Nature 465, 462-5 (2010).

Wann, E.; Ammann, M. W.; Vocadlo, L.; Wood, I. G.; Lord, O. T.; Brodholt, J. P.; Dobson, D. P.

2013-12-01

41

First principles study of magnetism in nanographenes

Magnetism in nanographenes (also know as polycyclic aromatic hydrocarbons, or PAHs) are studied with first principles density functional calculations. We find that an antiferromagnetic (AFM) phase appears as the PAH reaches a certain size. This AFM phase in PAHs has the same origin as the one in infinitely long zigzag-edged graphene nanoribbons, namely, from the localized electronic state at the zigzag edge. The smallest PAH still having an AFM ground state is identified. With increased length of the zigzag edge, PAHs approach an infinitely long ribbon in terms of (1) the energetic ordering and difference among the AFM, ferromagnetic (FM), and nonmagnetic (NM) phases and (2) the average local magnetic moment at the zigzag edges. These PAHs serve as ideal targets for chemical synthesis of nanographenes that possess magnetic properties. Moreover, our calculations support the interpretation that experimentally observed magnetism in activated carbon fibers originates from the zigzag edges of the nanographenes.

Jiang, Deen [ORNL; Sumpter, Bobby G [ORNL; Dai, Sheng [ORNL

2007-01-01

42

Numerical inductance calculations based on first principles.

A method of calculating inductances based on first principles is presented, which has the advantage over the more popular simulators in that fundamental formulas are explicitly used so that a deeper understanding of the inductance calculation is obtained with no need for explicit discretization of the inductor. It also has the advantage over the traditional method of formulas or table lookups in that it can be used for a wider range of configurations. It relies on the use of fast computers with a sophisticated mathematical computing language such as Mathematica to perform the required integration numerically so that the researcher can focus on the physics of the inductance calculation and not on the numerical integration. PMID:25402467

Shatz, Lisa F; Christensen, Craig W

2014-01-01

43

Cumulene molecular wire conductance from first principles

NASA Astrophysics Data System (ADS)

We present first principles calculations of current-voltage characteristics (IVC) and conductance of Au(111):S2 -cumulene- S2:Au(111) molecular wire junctions with realistic contacts. The transport properties are calculated using full self-consistent ab initio nonequilibrium Green’s function density-functional theory methods under external bias. The conductance of the cumulene wires shows oscillatory behavior depending on the number of carbon atoms (double bonds). Among all conjugated oligomers, we find that cumulene wires with odd number of carbon atoms yield the highest conductance with metalliclike ballistic transport behavior. The reason is the high density of states in broad lowest unoccupied molecular orbital levels spanning the Fermi level of the electrodes. The transmission spectrum and the conductance depend only weakly on applied bias, and the IVC is nearly linear over a bias region of ±1V . Cumulene wires are therefore potential candidates for metallic connections in nanoelectronic applications.

Prasongkit, J.; Grigoriev, A.; Wendin, G.; Ahuja, Rajeev

2010-03-01

44

First principles: Systems and their analysis

This paper is intended to challenge systems professionals to think about systems -- not at the process level but at the foundational level: first principles. System principles at the concept level, and what one understands about them, determine what one practices at the process level -- that is, how one defines ``systems engineering``. When Kant, Kepler, Newton, Einstein, and the others were deriving the natural laws, where was the comparable basic work in the natural order of things: systems? Is our profession one of simply employing some fairly good empirical procedures? Is there a legitimate place for a ``First Law of Systems`` alongside The First Law of Thermodynamics? Who would do this research? Who would fund it? Is now the time? Why should we care?

Woods, T.W.

1993-04-01

45

First principles: Systems and their analysis

This paper is intended to challenge systems professionals to think about systems -- not at the process level but at the foundational level: first principles. System principles at the concept level, and what one understands about them, determine what one practices at the process level -- that is, how one defines systems engineering''. When Kant, Kepler, Newton, Einstein, and the others were deriving the natural laws, where was the comparable basic work in the natural order of things: systems Is our profession one of simply employing some fairly good empirical procedures Is there a legitimate place for a First Law of Systems'' alongside The First Law of Thermodynamics Who would do this research Who would fund it Is now the time Why should we care

Woods, T.W.

1993-04-01

46

A popular method for predicting pK(a) values for organic molecules in aqueous solution is to establish empirical linear least-squares fits between calculated deprotonation energies and known experimental pK(a) values. In virtually all such calculations, the empirically observed slope of the pK(a) vs. ?E fit is significantly less than the theoretical value, 1/(2.303RT) (which is 0.73 mol/kcal at room temperature). In our own continuum solvation calculations (Zhang et al., J Phys Chem A 2010, 114, 432), the empirical slope for carboxylic acids was only 0.23 mol/kcal, despite the excellent fit to the experimental pK(a) values. There has been much speculation about the reason for this phenomenon. Although the ?E - pK(a) relation neglects entropic effects, these are expected to largely cancel. The most likely cause for the strange behavior of the fitted slope is explicit solute-solvent (water) interactions, especially involving the ions, which cannot be described accurately by continuum solvation models. We used our previously developed pK(a) protocol (OLYP/6-311+G(d,p)//3-21G(d) with the COSMO solvation model) to investigate the effect of adding one or two explicit water molecules to the system. The slopes for organic acids (especially carboxylic acids) are much closer to the theoretical value when explicit water molecules are added to both the neutral molecule and the anion. However, explicit water molecules have almost no effect on the slopes for organic bases. Adding explicit water molecules to the ions only produces intermediate results. Unfortunately, linear fits involving explicit water molecules have much larger errors than with continuum solvation models alone and are also much more expensive. Consequently, they are not suitable for large-scale pK(a) calculations. The results compared with literature values showed that our predicted pK(a) s are more accurate. PMID:22173939

Zhang, Shuming

2012-02-15

47

Charge separation in nanoscale photovoltaic materials: recent insights from first-scale photovoltaic materials; in particular recent theoretical/computational work based on first principles electron and hole in so-called excitonic photovoltaic cells. Emphasis is placed on theoretical results

Wu, Zhigang

48

Thermodynamics of magnetic systems from first principles

NASA Astrophysics Data System (ADS)

Density functional calculations have proven to be a useful tool in the study of ground state properties of many materials. The investigation of finite temperature magnetism on the other hand has to rely usually on the usage of empirical models that allow the large number of evaluations of the system's Hamiltonian that are required to obtain the phase space sampling needed to obtain the free energy, specific heat, magnetization, susceptibility, and other quantities as function of temperature. We have demonstrated a solution to this problem that harnesses the computational power of today's large massively parallel computers by combining a classical Monte-Carlo calculations with our first principles multiple scattering electronic structure code (LSMS) for constrained magnetic states. Here we will present recent advances in our method that improve the convergence as well as applications to 3d element based ferromagnets. This research was performed at Oak Ridge National Lab and sponsored in parts by the Center for Nanophase Material Sciences, Scientific User Facilities Division, the Center for Defect Physics, an Energy Frontier Research Center funded by the US DOE Office of Basic Energy Sciences and the Division of Materials Science and Engineering, Office of Basic Energy Science of

Eisenbach, Markus; Brown, Gregory; Rusanu, Aurelian; Nicholson, Don M.

2012-02-01

49

First-principles study of point defects in thorium carbide

NASA Astrophysics Data System (ADS)

Thorium-based materials are currently being investigated in relation with their potential utilization in Generation-IV reactors as nuclear fuels. One of the most important issues to be studied is their behavior under irradiation. A first approach to this goal is the study of point defects. By means of first-principles calculations within the framework of density functional theory, we study the stability and formation energies of vacancies, interstitials and Frenkel pairs in thorium carbide. We find that C isolated vacancies are the most likely defects, while C interstitials are energetically favored as compared to Th ones. These kind of results for ThC, to the best authors' knowledge, have not been obtained previously, neither experimentally, nor theoretically. For this reason, we compare with results on other compounds with the same NaCl-type structure.

Pérez Daroca, D.; Jaroszewicz, S.; Llois, A. M.; Mosca, H. O.

2014-11-01

50

First-principles modeling of Li-air battery materials

NASA Astrophysics Data System (ADS)

Of the many possible battery chemistries, the so-called "Li-air" system is noteworthy in that its theoretical capacity (˜5 kWh/kg, including mass of oxygen) exceeds that of any electrochemical system. Perhaps more importantly, the simplified composition of its air cathode -- involving only the inlet of oxygen from the atmosphere -- has the potential to provide cost benefits in comparison to the Li-ion systems of today. Although the first rechargeable Li-air battery was demonstrated by Abraham and Jiang 14 years ago, its performance in many dimensions remains poor, and relatively little computational work has been done to elucidate performance-limiting phenomena. This talk will introduce the basic properties and main performance issues associated with Li-air batteries. Opportunities for first-principles modeling to assist in overcoming these obstacles will be highlighted.

Radin, Maxwell; Siegel, Donald

2011-03-01

51

Theoretical Calculations of Atomic Data for Spectroscopy

Several different approximations and techniques have been developed for the calculation of atomic structure, ionization, and excitation of atoms and ions. These techniques have been used to compute large amounts of spectroscopic data of various levels of accuracy. This paper presents a review of these theoretical methods to help non-experts in atomic physics to better understand the qualities and limitations of various data sources and assess how reliable are spectral models based on those data.

Manuel A. Bautista

2000-06-20

52

Theoretical Calculations of Atomic Data for Spectroscopy

NASA Technical Reports Server (NTRS)

Several different approximations and techniques have been developed for the calculation of atomic structure, ionization, and excitation of atoms and ions. These techniques have been used to compute large amounts of spectroscopic data of various levels of accuracy. This paper presents a review of these theoretical methods to help non-experts in atomic physics to better understand the qualities and limitations of various data sources and assess how reliable are spectral models based on those data.

Bautista, Manuel A.

2000-01-01

53

We carry out the first-principles calculations to investigate the electronic properties of bilayer graphene. The ultrasoft pseudopotential density functional methods within the local density approximation (LDA) are applied. The band structures and density of states of bilayer graphene are investigated. Intrinsic defects, including Stone–Wales (SW) defect and atom vacancy, are considered, and proper theoretical analysis about its electronic properties is

Ting Li; Xianqiong Tang; Zhi Liu; Ping Zhang

2011-01-01

54

First principles study of point defects in SnS.

Photovoltaic cells based on SnS as the absorber layer show promise for efficient solar devices containing non-toxic materials that are abundant enough for large scale production. The efficiency of SnS cells has been increasing steadily, but various loss mechanisms in the device, related to the presence of defects in the material, have so far limited it far below its maximal theoretical value. In this work we perform first principles, density-functional-theory calculations to examine the behavior and nature of both intrinsic and extrinsic defects in the SnS absorber layer. We focus on the elements known to exist in the environment of SnS-based photovoltaic devices during growth. In what concerns intrinsic defects, our calculations support the current understanding of the role of the Sn vacancy (VSn) acceptor defect, namely that it is responsible for the p-type conductivity in SnS. We also present calculations for extrinsic defects and make extensive comparison to experimental expectations. Our detailed treatment of electrostatic correction terms for charged defects provides theoretical predictions on both the high-frequency and low-frequency dielectric tensors of SnS. PMID:25363023

Malone, Brad D; Gali, Adam; Kaxiras, Efthimios

2014-12-21

55

First-principles Prediction for Mechanical and Optical Properties of Al3BC3

NASA Astrophysics Data System (ADS)

Using the first-principles technique, we systematically investigate the elastic, electronic, mechanical and optical properties of Al3BC3. The calculated structural parameters of Al3BC3 are in agreement with the experimental results. Electronic structure calculations indicate that Al3BC3 has a larger indirect band gap. Based on the first-principles model of intrinsic hardness, the theoretical hardness of Al3BC3 is calculated to be 14.7 GPa, indicating a potential hard material. The analyses of electronic structure, charge density distribution and Mulliken overlap population provide further understanding of the hardness and C—B bonding properties of Al3BC3.

Qiu, Ping-Yi

2014-06-01

56

A LANGUAGE FOR SPECIFYING INFORMATIONAL GRAPHICS FROM FIRST PRINCIPLES

A LANGUAGE FOR SPECIFYING INFORMATIONAL GRAPHICS FROM FIRST PRINCIPLES Stuart M. Shieber Division principles on which (a subset of) informational graphics are built. The functionality we aim to provide the fact that they are built on the basis of the same first principles from which more standard

Shieber, Stuart

57

Designing Interactive Learning Environments: An Approach from First Principles

ERIC Educational Resources Information Center

Purpose: Today's technology supports the design of more and more sophisticated interactive learning environments. This paper aims to argue that such design should develop from first principles. Design/methodology/approach: In the paper by first principles is meant: learning theory and principles of course design. These principles are briefly…

Scott, Bernard; Cong, Chunyu

2007-01-01

58

NASA Astrophysics Data System (ADS)

The electronic structure of Bi(110) thin films as a function of film thickness is investigated by first-principles calculations, angle-resolved photoemission spectroscopy, and scanning tunneling microscopy. Energy minimization in the calculation reveals significant atomic relaxation and rebonding at the surface. The calculated surface energy for the relaxed structures indicates that films consisting of odd numbers of atomic layers are inherently unstable and tend to bifurcate into film domains consisting of neighboring even numbers of atomic layers. This theoretical trend agrees with experimental observations. The results can be explained by the presence of unsaturated pz dangling bonds on the surfaces of films of odd-numbered atomic layers only. These pz dangling bonds form a Dirac-cone feature near the Fermi level at the M ¯ point as a consequence of the interplay of mirror symmetry and spin-orbit coupling. Films consisting of even numbers of atomic layers exhibit a band gap at M ¯ instead.

Bian, G.; Wang, X.; Miller, T.; Chiang, T.-C.; Kowalczyk, P. J.; Mahapatra, O.; Brown, S. A.

2014-11-01

59

NMR characterization of hydrocarbon adsorption on calcite surfaces: A first principles study.

The electronic and coordination environment of minerals surfaces, as calcite, are very difficult to characterize experimentally. This is mainly due to the fact that there are relatively few spectroscopic techniques able to detect Ca(2+). Since calcite is a major constituent of sedimentary rocks in oil reservoir, a more detailed characterization of the interaction between hydrocarbon molecules and mineral surfaces is highly desirable. Here we perform a first principles study on the adsorption of hydrocarbon molecules on calcite surface (CaCO3 (101¯4)). The simulations were based on Density Functional Theory with Solid State Nuclear Magnetic Resonance (SS-NMR) calculations. The Gauge-Including Projector Augmented Wave method was used to compute mainly SS-NMR parameters for (43)Ca, (13)C, and (17)O in calcite surface. It was possible to assign the peaks in the theoretical NMR spectra for all structures studied. Besides showing different chemical shifts for atoms located on different environments (bulk and surface) for calcite, the results also display changes on the chemical shift, mainly for Ca sites, when the hydrocarbon molecules are present. Even though the interaction of the benzene molecule with the calcite surface is weak, there is a clearly distinguishable displacement of the signal of the Ca sites over which the hydrocarbon molecule is located. A similar effect is also observed for hexane adsorption. Through NMR spectroscopy, we show that aromatic and alkane hydrocarbon molecules adsorbed on carbonate surfaces can be differentiated. PMID:25429955

Bevilaqua, Rochele C A; Rigo, Vagner A; Veríssimo-Alves, Marcos; Miranda, Caetano R

2014-11-28

60

NMR characterization of hydrocarbon adsorption on calcite surfaces: A first principles study

NASA Astrophysics Data System (ADS)

The electronic and coordination environment of minerals surfaces, as calcite, are very difficult to characterize experimentally. This is mainly due to the fact that there are relatively few spectroscopic techniques able to detect Ca2+. Since calcite is a major constituent of sedimentary rocks in oil reservoir, a more detailed characterization of the interaction between hydrocarbon molecules and mineral surfaces is highly desirable. Here we perform a first principles study on the adsorption of hydrocarbon molecules on calcite surface (CaCO3 ( {10bar 14} )). The simulations were based on Density Functional Theory with Solid State Nuclear Magnetic Resonance (SS-NMR) calculations. The Gauge-Including Projector Augmented Wave method was used to compute mainly SS-NMR parameters for 43Ca, 13C, and 17O in calcite surface. It was possible to assign the peaks in the theoretical NMR spectra for all structures studied. Besides showing different chemical shifts for atoms located on different environments (bulk and surface) for calcite, the results also display changes on the chemical shift, mainly for Ca sites, when the hydrocarbon molecules are present. Even though the interaction of the benzene molecule with the calcite surface is weak, there is a clearly distinguishable displacement of the signal of the Ca sites over which the hydrocarbon molecule is located. A similar effect is also observed for hexane adsorption. Through NMR spectroscopy, we show that aromatic and alkane hydrocarbon molecules adsorbed on carbonate surfaces can be differentiated.

Bevilaqua, Rochele C. A.; Rigo, Vagner A.; Veríssimo-Alves, Marcos; Miranda, Caetano R.

2014-11-01

61

First principles characterization of silicate sites in clay surfaces.

Aluminosilicate clays like Montmorillonite (MMT) and Muscovite Mica (MT) have siloxane cavities on the basal plane. The hydroxyl groups localized in these cavities and van der Waals (vdW) forces contribute significantly to adsorption processes. However, the basal sites are found to be difficult to characterize experimentally. Here, (001) surfaces of MMT and MT clays were investigated using first-principles calculations to understand how these silicate surface sites are influenced by hydroxyl groups and the effective role of inner layer vdW interactions. Based on density-functional theory (DFT) within the generalized gradient approximation (GGA), different types of exchange-correlation functionals were tested to check the effect of vdW dispersion correction. Noncontact atomic force microscopy (nc-AFM), X-ray absorption spectroscopy (XAS) in the near-edge region and solid-state nuclear magnetic resonance (SS-NMR) spectroscopy were simulated. In both clays, the oxygen surface sites are directly affected by the intralayer interaction through hydroxyl groups. Our results indicated that the chemical environment of the hydroxyl groups is distinct in the MMT and MT structures. The vdW correction was essential for a better description of the surface oxygen sites and correctly describes the similarity between both clays. Particularly, the bulk apical oxygen sites in the MT structure are less influenced by vdW interaction. Compared to MMT, the silicon surface sites of MT are more sensitive to the intralayer changes in Si-Oapical-Al and with less effect of the hydroxyl groups. These results provide a clear understanding of influence of the siloxane cavity on the oxygen and silicon surface sites in aluminosilicates. PMID:25592132

Alvim, Raphael S; Miranda, Caetano R

2015-02-01

62

Predicted boron-carbide compounds: a first-principles study.

By using developed particle swarm optimization algorithm on crystal structural prediction, we have explored the possible crystal structures of B-C system. Their structures, stability, elastic properties, electronic structure, and chemical bonding have been investigated by first-principles calculations with density functional theory. The results show that all the predicted structures are mechanically and dynamically stable. An analysis of calculated enthalpy with pressure indicates that increasing of boron content will increase the stability of boron carbides under low pressure. Moreover, the boron carbides with rich carbon content become more stable under high pressure. The negative formation energy of predicted B5C indicates its high stability. The density of states of B5C show that it is p-type semiconducting. The calculated theoretical Vickers hardnesses of B-C exceed 40 GPa except B4C, BC, and BC4, indicating they are potential superhard materials. An analysis of Debye temperature and electronic localization function provides further understanding chemical and physical properties of boron carbide. PMID:24929411

Wang, De Yu; Yan, Qian; Wang, Bing; Wang, Yuan Xu; Yang, Jueming; Yang, Gui

2014-06-14

63

First principles Tafel kinetics of methanol oxidation on Pt(111)

NASA Astrophysics Data System (ADS)

Electrocatalytic methanol oxidation is of fundamental importance in electrochemistry and also a key reaction in direct methanol fuel cell. To resolve the kinetics at the atomic level, this work investigates the potential-dependent reaction kinetics of methanol oxidation on Pt(111) using the first principles periodic continuum solvation model based on modified-Poisson-Boltzmann equation (CM-MPB), focusing on the initial dehydrogenation elementary steps. A theoretical model to predict Tafel kinetics (current vs potential) is established by considering that the rate-determining step of methanol oxidation (to CO) is the first Csbnd H bond breaking (CH3OH(aq) ? CH2OH* + H*) according to the computed free energy profile. The first Csbnd H bond breaking reaction needs to overcome a large entropy loss during methanol approaching to the surface and replacing the adsorbed water molecules. While no apparent charge transfer is involved in this elementary step, the charge transfer coefficient of the reaction is calculated to be 0.36, an unconventional value for charge transfer reactions, and the Tafel slope is deduced to be 166 mV. The results show that the metal/adsorbate interaction and the solvation environment play important roles on influencing the Tafel kinetics. The knowledge learned from the potential-dependent kinetics of methanol oxidation can be applied in general for understanding the electrocatalytic reactions of organic molecules at the solid-liquid interface.

Fang, Ya-Hui; Liu, Zhi-Pan

2015-01-01

64

Predicted boron-carbide compounds: A first-principles study

NASA Astrophysics Data System (ADS)

By using developed particle swarm optimization algorithm on crystal structural prediction, we have explored the possible crystal structures of B-C system. Their structures, stability, elastic properties, electronic structure, and chemical bonding have been investigated by first-principles calculations with density functional theory. The results show that all the predicted structures are mechanically and dynamically stable. An analysis of calculated enthalpy with pressure indicates that increasing of boron content will increase the stability of boron carbides under low pressure. Moreover, the boron carbides with rich carbon content become more stable under high pressure. The negative formation energy of predicted B5C indicates its high stability. The density of states of B5C show that it is p-type semiconducting. The calculated theoretical Vickers hardnesses of B-C exceed 40 GPa except B4C, BC, and BC4, indicating they are potential superhard materials. An analysis of Debye temperature and electronic localization function provides further understanding chemical and physical properties of boron carbide.

Wang, De Yu; Yan, Qian; Wang, Bing; Wang, Yuan Xu; Yang, Jueming; Yang, Gui

2014-06-01

65

First-principles structural design of superhard materials.

We reported a developed methodology to design superhard materials for given chemical systems under external conditions (here, pressure). The new approach is based on the CALYPSO algorithm and requires only the chemical compositions to predict the hardness vs. energy map, from which the energetically preferable superhard structures are readily accessible. In contrast to the traditional ground state structure prediction method where the total energy was solely used as the fitness function, here we adopted hardness as the fitness function in combination with the first-principles calculation to construct the hardness vs. energy map by seeking a proper balance between hardness and energy for a better mechanical description of given chemical systems. To allow a universal calculation on the hardness for the predicted structure, we have improved the earlier hardness model based on bond strength by applying the Laplacian matrix to account for the highly anisotropic and molecular systems. We benchmarked our approach in typical superhard systems, such as elemental carbon, binary B-N, and ternary B-C-N compounds. Nearly all the experimentally known and most of the earlier theoretical superhard structures have been successfully reproduced. The results suggested that our approach is reliable and can be widely applied into design of new superhard materials. PMID:23534621

Zhang, Xinxin; Wang, Yanchao; Lv, Jian; Zhu, Chunye; Li, Qian; Zhang, Miao; Li, Quan; Ma, Yanming

2013-03-21

66

First-principles structural design of superhard materials

NASA Astrophysics Data System (ADS)

We reported a developed methodology to design superhard materials for given chemical systems under external conditions (here, pressure). The new approach is based on the CALYPSO algorithm and requires only the chemical compositions to predict the hardness vs. energy map, from which the energetically preferable superhard structures are readily accessible. In contrast to the traditional ground state structure prediction method where the total energy was solely used as the fitness function, here we adopted hardness as the fitness function in combination with the first-principles calculation to construct the hardness vs. energy map by seeking a proper balance between hardness and energy for a better mechanical description of given chemical systems. To allow a universal calculation on the hardness for the predicted structure, we have improved the earlier hardness model based on bond strength by applying the Laplacian matrix to account for the highly anisotropic and molecular systems. We benchmarked our approach in typical superhard systems, such as elemental carbon, binary B-N, and ternary B-C-N compounds. Nearly all the experimentally known and most of the earlier theoretical superhard structures have been successfully reproduced. The results suggested that our approach is reliable and can be widely applied into design of new superhard materials.

Zhang, Xinxin; Wang, Yanchao; Lv, Jian; Zhu, Chunye; Li, Qian; Zhang, Miao; Li, Quan; Ma, Yanming

2013-03-01

67

First-principles simulations at constant electric polarization

NASA Astrophysics Data System (ADS)

We develop a formalism to perform first-principles calculations for insulators at fixed electric polarization. As shown by Sai, Rabe, and Vanderbilt (SRV),ootnotetextN. Sai, K.M. Rabe, and D. Vanderbilt, Phys. Rev. B 66, 104108 (2002). such an approach allows one to map out the energy landscape as a function of polarization, providing a powerful tool for the theoretical investigation of polar materials. While the SRV method is only approximate because the effect of electric field is described using low-order Taylor expansions, our method is exact because we use the finite-fields approach of Souza, 'Iñiguez, and Vanderbilt.ootnotetextI. Souza, J. 'Iñiguez, and D. Vanderbilt, Phys. Rev. Lett. 89, 117602 (2002). We apply our method both to systems where the ionic contribution to the polarization dominates, and to systems where this is not the case. We show that the SRV method gives rather accurate results in the former case as expected, while the present exact method provides substantial improvements in the latter case.

Dieguez, Oswaldo; Vanderbilt, David

2005-03-01

68

DNA translocation through graphene nanopores: a first-principles study

NASA Astrophysics Data System (ADS)

With first-principles transport simulation, a biosensor device built from a graphene nanoribbon containing a nanopore is designed for DNA sequencing. The four DNA nucleobases can be distinguished from one another by detecting the transverse-currents of this device. To investigate the transport properties and mechanisms of such a device, we examine the motion effects of nucleobases. The analysis of the transmission spectra and frontier orbital energy shows that the transverse-currents variation of the device strongly results from the long-range interaction between nucleobases and the device. This interaction makes transverse-currents ultra-sensitive to the molecule inside the pore. By rotating the nucleotides inside the pore, the transverse-currents of the device vary along with the changes of molecular orientation. Due to the long-range interaction, when nucleobases chain translocates through nanopore of the device, the influences of adjacent nucleobases on transverse-currents cannot be ignored. These novel effects of nucleobases on the transport capacity of the device provide some theoretical guidance for the design of graphene-based nanopore sensor devices.

Peng, Shenglin; Yang, Zhixiong; Ni, Xiang; Zhang, Hua; Ouyang, Jun; Fangping, Ouyang

2014-03-01

69

First-principles theory of multipolar order in actinide dioxides

NASA Astrophysics Data System (ADS)

Magnetic phase transitions that involve multipolar degrees of freedom have been widely studied during the last couple of decades, challenging the common approximation which assumes that the physical properties of a magnetic material could be effectively described by purely dipolar degrees of freedom. Due to the complexity of the problem and to the large number of competing interactions involved, the simple (fcc) crystal structure of the actinide dioxides made them the ideal playground system for such theoretical and experimental studies. In the present paper, we summarize our recent attempts to provide an ab initio description of the ordered phases of UO2, NpO2, and AmO2 by means of state-of-the-art LDA+U first-principles calculations. This systematic analysis of the electronic structures is here naturally connected to the local crystalline fields of the 5f states in the actinide dioxide series. Related to these we find that the mechanisms which lead to the experimentally observed insulating ground states work in distinctly different ways for each compound. xml:lang="fr"

Magnani, Nicola; Suzuki, Michi-To; Oppeneer, Peter M.

2014-08-01

70

First principles molecular dynamics without self-consistent field optimization

We present a first principles molecular dynamics approach that is based on time-reversible extended Lagrangian Born-Oppenheimer molecular dynamics [A. M. N. Niklasson, Phys. Rev. Lett. 100, 123004 (2008)] in the limit of vanishing self-consistent field optimization. The optimization-free dynamics keeps the computational cost to a minimum and typically provides molecular trajectories that closely follow the exact Born-Oppenheimer potential energy surface. Only one single diagonalization and Hamiltonian (or Fockian) construction are required in each integration time step. The proposed dynamics is derived for a general free-energy potential surface valid at finite electronic temperatures within hybrid density functional theory. Even in the event of irregular functional behavior that may cause a dynamical instability, the optimization-free limit represents a natural starting guess for force calculations that may require a more elaborate iterative electronic ground state optimization. Our optimization-free dynamics thus represents a flexible theoretical framework for a broad and general class of ab initio molecular dynamics simulations.

Souvatzis, Petros, E-mail: petros.souvatsiz@fysik.uu.se [Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Box 516, SE-75120 Uppsala (Sweden)] [Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Box 516, SE-75120 Uppsala (Sweden); Niklasson, Anders M. N., E-mail: amn@lanl.gov [Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)] [Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

2014-01-28

71

A first-principles approach to finite temperature elastic constants.

A first-principles approach to calculating the elastic stiffness coefficients at finite temperatures was proposed. It is based on the assumption that the temperature dependence of elastic stiffness coefficients mainly results from volume change as a function of temperature; it combines the first-principles calculations of elastic constants at 0 K and the first-principles phonon theory of thermal expansion. Its applications to elastic constants of Al, Cu, Ni, Mo, Ta, NiAl, and Ni?Al from 0 K up to their respective melting points show excellent agreement between the predicted values and existing experimental measurements. PMID:21393743

Wang, Y; Wang, J J; Zhang, H; Manga, V R; Shang, S L; Chen, L-Q; Liu, Z-K

2010-06-01

72

Wavelength modulation photoacoustic spectroscopy: Theoretical description and experimental results

A theoretical description of photoacoustic spectroscopy generated by wavelength modulation of a semiconductor laser source is reported for a Lorentzian absorption line. This model describes the first- and second-harmonic photoacoustic signals produced by a current-modulated semiconductor laser. Combined intensity- and wavelength-modulation is considered with arbitrary phase shift. Experimental results obtained when probing a CO2 absorption line with a 2-?m distributed

Stéphane Schilt; Luc Thévenaz

2006-01-01

73

First Principles Modeling for Research and Design of New Materials

First principles computation can be used to investigate an design materials in ways that can not be achieved with experimental means. We show how computations can be used to rapidly capture the essential physics that ...

Ceder, Gerbrand

74

Design of water-splitting photocatalysts by first principles computations

This thesis focuses on the design of novel inorganic water-splitting photocatalysts for solar applications using first principles computations. Water-splitting photocatalysts are materials that can photo-catalyze the ...

Wu, Yabi

2014-01-01

75

Ionization potentials of semiconductors from first-principles

NASA Astrophysics Data System (ADS)

The ionization potential is the key to determine the absolute positions of valence and conduction bands of a semiconductor with respect to the vacuum level, which play a crucial role in physical and chemical properties of surfaces and interfaces. In spite of its far-reaching significance, theoretical determination of ionization potentials has not attained as much attention as that of band gaps. In this work, a set of prototypical semiconductors are considered to establish the performance of the state-of-the-art first-principles approaches. We have shown that in general Kohn-Sham density functional theory with local density approximation or generalized gradient approximation (LDA/GGA) significantly underestimates the ionization potentials of semiconductors. When the quasi-particle correction from many-body perturbation theory in the GW approximation is taken into account, the agreement between theory and experiment can be greatly improved. We have made a critical comparison between two GW correction schemes, one taking into account the GW correction to the valence band maximum (VBM) of the bulk system, and the other based on the assumption that the LDA/GGA gives correct band gap center (BGC). Our study shows that the VBM scheme is better founded theoretically and leads to closer agreement with experiment practically than the BGC scheme. For semiconductors with shallow semicore states, for which the band gaps from the GW approach also exhibit significant errors, there is still significant discrepancy between GW and experiment, indicating the necessity to go beyond the standard GW approach for these materials.

Jiang, Hong; Shen, Yu-Chen

2013-10-01

76

First-principles study of metal-ceramic interfaces

NASA Astrophysics Data System (ADS)

Despite their use in a variety of applications, the fundamental properties of metal-ceramic interfaces are still poorly understood. Historically, this has been due to experimental complications associated with the study of a buried interface, and to theoretical difficulties caused by complex interfacial bonding interactions. However, the advent of first-principles techniques based on Density Functional Theory (DFT) has led to new opportunities for highly-accurate computer simulations of the atomic and electronic structure of interfaces. In this study we present a series of DFT calculations on a broad class of Al/ceramic interface systems, with the goal of explaining and predicting the nature of metal-ceramic adhesion. Since the strength of interfacial bonding, as quantified by the ideal work of adhesion Wad , plays a crucial role in determining the mechanical properties of an interface, we have systematically calculated Wad for 20 different interface geometries including oxide (alpha-Al 2O3), carbide (WC, VC), and nitride (VN) ceramic compounds with the goal of identifying trends. In order to identify the optimal atomic structures we consider the effects of several stacking sequences, interface terminations (both polar and non-polar), and allow for full atomic relaxations. For each system we use a variety of methods to carefully analyze the interfacial electronic structure in order to determine the nature of the metal-ceramic bonding. Although we find significant differences in both the interfacial bonding and the inherent electronic structures of the bulk ceramics, our calculations reveal a trend in Wad with respect to the summed surface energies of the interfaced materials (sigmaAl + sigmaceramic). Specifically, we find that surfaces having larger sigma's adhere at interfaces more strongly than those with small sigma's. We argue that this behavior is consistent with the intuitive notion of the surface energy being a measure of surface reactivity, and hence it could be used as a guide in the design of interfacial properties.

Siegel, Donald Jason

2001-07-01

77

Theoretical Study of the Vibrational Spectroscopy of the Ethyl Radical

NASA Astrophysics Data System (ADS)

The rich spectroscopy of the ethyl radical has attracted the attention of several experimental and theoretical investigations. The purpose of these studies was to elucidate the signatures of hyperconjugation, torsion, inversion, and Fermi coupling in the molecular spectra. Due to the number of degrees of freedom in the system, previous theoretical studies have implemented reduced-dimensional models. Our ultimate goal is a full-dimensional theoretical treatment of the vibrations using both Van Vleck and variational approaches. The methods will be combined with the potential that we have calculated using the CCSD(T) method on the cc-pVTZ basis set. In this talk we will discuss our initial work, which builds up from these reduced-dimensional models. Our calculations use coordinates that exploit the system's G_{12} PI symmetry in a simple fashion. By systematically adding more degrees of freedom to our model, we can determine the effects of specific couplings on the spectroscopy. T. Häber, A. C. Blair, D. J. Nesbitt and M. D. Schuder J. Chem. Phys. {124}, 054316, (2006). G .E. Douberly, unpublished. R. S. Bhatta, A. Gao and D. S. Perry J. Mol. Struct.: THEOCHEM {941}, 22, (2010).

Tabor, Daniel P.; Sibert, Edwin. L. Sibert, Iii

2013-06-01

78

Self-trapping and diffusion of hydrogen in Nb and Ta from first principles

Interstitial hydrogen in bcc Nb and Ta is studied theoretically, using first-principles density-functional calculations. The effect of self-trapping is investigated in some detail, and our calculated energies, forces, and displacements for hydrogen at tetrahedral sites are all found to be in good agreement with experiments. The local motion of H and D is treated quantum mechanically by mapping out potential

Per G. Sundell; Göran Wahnström

2004-01-01

79

Transport and first-principles study of novel thermoelectric materials

NASA Astrophysics Data System (ADS)

Thermoelectric materials can recover waste industrial heat and convert it to electricity as well as provide efficient local cooling of electronic devices. The efficiency of such environmentally responsible and exceptionally reliable solid state energy conversion is determined by the dimensionless figure-of-merit ZT = alpha2 sigmaT/kappa, where alpha is the Seebeck coefficient, sigma is the electrical conductivity, kappa is the thermal conductivity, and T is the absolute temperature. The goal of the thesis is to (i) illustrate the physics to achieve high ZT of advanced thermoelectric materials and (ii) explore fundamental structure and transport properties in novel condensed matter systems, via an approach combining comprehensive experimental techniques and state-of-the-art first-principles simulation methods. Thermo-galvanomagnetic transport coefficients are derived from Onsager's reciprocal relations and evaluated via solving Boltzmann transport equation using Fermi-Dirac statistics, under the relaxation time approximation. Such understanding provides insights on enhancing ZT through two physically intuitive and very effective routes: (i) improving power factor PF = alpha2sigma; and (ii) reducing thermal conductivity kappa, as demonstrated in the cases of Mg2Si1-xSnx solid solution and Ge/Te double substituted skutterudites CoSb3(1-x)Ge1.5x Te1.5x, respectively. Motivated by recent theoretical predictions of enhanced thermoelectric performance in highly mismatched alloys, ZnTe:N molecular beam epitaxy (MBE) films deposited on GaAs (100) substrates are carefully examined, which leads to a surprising discovery of significant phonon-drag thermopower (reaching 1-2 mV/K-1) at ~13 K. Further systematic study in Bi2Te3 MBE thin films grown on sapphire (0001) and/or BaF2 (111) substrates, reveal that the peak of phonon drag can be tuned by the choice of substrates with different Debye temperatures. Moreover, the detailed transport and structure studies of Bi2-xTl xTe3 single crystals demonstrate that thallium doping leads to a bulk insulating state for such a topological insulator, which opens an avenue for further investigations of transport phenomena related to surface states. Finally, using the combined theoretical and experimental approaches, a new layered transition metal dichalcogenide type of ground state of Cu 2Se is proposed, which exhibits extraordinary weak anti-localization type of magnetoresistance at liquid helium temperatures.

Chi, Hang

80

Interface enhancement of Gilbert damping from first principles.

The enhancement of Gilbert damping observed for Ni_{80}Fe_{20} (Py) films in contact with the nonmagnetic metals Cu, Pd, Ta, and Pt is quantitatively reproduced using first-principles scattering calculations. The "spin-pumping" theory that qualitatively explains its dependence on the Py thickness is generalized to include a number of extra factors known to be important for spin transport through interfaces. Determining the parameters in this theory from first principles shows that interface spin flipping makes an essential contribution to the damping enhancement. Without it, a much shorter spin-flip diffusion length for Pt would be needed than the value we calculate independently. PMID:25432053

Liu, Yi; Yuan, Zhe; Wesselink, R J H; Starikov, Anton A; Kelly, Paul J

2014-11-14

81

Interface Enhancement of Gilbert Damping from First Principles

NASA Astrophysics Data System (ADS)

The enhancement of Gilbert damping observed for Ni80Fe20 (Py) films in contact with the nonmagnetic metals Cu, Pd, Ta, and Pt is quantitatively reproduced using first-principles scattering calculations. The "spin-pumping" theory that qualitatively explains its dependence on the Py thickness is generalized to include a number of extra factors known to be important for spin transport through interfaces. Determining the parameters in this theory from first principles shows that interface spin flipping makes an essential contribution to the damping enhancement. Without it, a much shorter spin-flip diffusion length for Pt would be needed than the value we calculate independently.

Liu, Yi; Yuan, Zhe; Wesselink, R. J. H.; Starikov, Anton A.; Kelly, Paul J.

2014-11-01

82

First-principles study of hydrogen in perfect tungsten crystal

Tungsten-based materials are used as the first wall materials in ITER. Hydrogen impurities were introduced via bombarding with the reaction plasma, which are important for the behavior and stability of the tungsten wall. Using the first-principles density functional theory and planewave pseudopotential technique, we have simulated the behaviors of hydrogen atoms inside the perfect tungsten bcc lattice. The binding energies

Jingcheng Xu; Jijun Zhao

2009-01-01

83

Understanding nano-materials from first principles Leeor Kronik

Understanding nano-materials from first principles Leeor Kronik Department of Materials and Interfaces, Weizmann Institute of science, Rehovoth 76100 Nano-sized materials often exhibit exciting new traditional solid-state physics models are applicable to the intermediate nano- size range. As a result, first

Adler, Joan

84

Towards first principles modeling of electrochemical electrode-electrolyte interfaces

NASA Astrophysics Data System (ADS)

We present a mini-perspective on the development of first principles modeling of electrochemical interfaces. We show that none of the existing methods deal with all the thermodynamic constraints that the electrochemical environment imposes on the structure of the interface. We present two directions forward to make the description more realistic and correct.

Nielsen, Malte; Björketun, Mårten E.; Hansen, Martin H.; Rossmeisl, Jan

2015-01-01

85

COMMUNICATION First Principles Prediction of Protein Folding Rates

COMMUNICATION First Principles Prediction of Protein Folding Rates Derek A. Debe and William A studies have demonstrated that many small, single-domain proteins fold via simple two-state kinetics. We. # 1999 Academic Press Keywords: protein folding; kinetics; diffusion; fold topology; nucleation

Goddard III, William A.

86

Atomistic and first principles studies of Si nanoparticles under pressure

NASA Astrophysics Data System (ADS)

In this talk, we will discuss the structural, optical and electronic properties of silicon nanoparticles under high pressure, obtained using a combination of classical molecular dynamics and first principles density functional theory calculations. The results will be corroborated with experimental findings.

Chan, Maria; Hannah, Daniel; Schaller, Richard

2013-03-01

87

Diagnosis: Reasoning from first principles and experiential knowledge

NASA Technical Reports Server (NTRS)

Completeness, efficiency and autonomy are requirements for suture diagnostic reasoning systems. Methods for automating diagnostic reasoning systems include diagnosis from first principles (i.e., reasoning from a thorough description of structure and behavior) and diagnosis from experiential knowledge (i.e., reasoning from a set of examples obtained from experts). However, implementation of either as a single reasoning method fails to meet these requirements. The approach of combining reasoning from first principles and reasoning from experiential knowledge does address the requirements discussed above and can possibly ease some of the difficulties associated with knowledge acquisition by allowing developers to systematically enumerate a portion of the knowledge necessary to build the diagnosis program. The ability to enumerate knowledge systematically facilitates defining the program's scope, completeness, and competence and assists in bounding, controlling, and guiding the knowledge acquisition process.

Williams, Linda J. F.; Lawler, Dennis G.

1987-01-01

88

Accurate atomistic first-principles calculations of electronic stopping

NASA Astrophysics Data System (ADS)

We show that atomistic first-principles calculations based on real-time propagation within time-dependent density functional theory are capable of accurately describing electronic stopping of light projectile atoms in metal hosts over a wide range of projectile velocities. In particular, we employ a plane-wave pseudopotential scheme to solve time-dependent Kohn-Sham equations for representative systems of H and He projectiles in crystalline aluminum. This approach to simulate nonadiabatic electron-ion interaction provides an accurate framework that allows for quantitative comparison with experiment without introducing ad hoc parameters such as effective charges, or assumptions about the dielectric function. Our work clearly shows that this atomistic first-principles description of electronic stopping is able to disentangle contributions due to tightly bound semicore electrons and geometric aspects of the stopping geometry (channeling versus off-channeling) in a wide range of projectile velocities.

Schleife, André; Kanai, Yosuke; Correa, Alfredo A.

2015-01-01

89

The first-principles design of ductile refractory alloys

The purpose of this work is to predict elastic and thermodynamic properties of chromium-based alloys based on first-principles calculations and to demonstrate an appropriate computational approach to develop new materials for high-temperature applications in energy systems. In this study, Poisson ratio is used as a screening parameter to identify ductilizing additives to the refractory alloys. The results predict that elements

Michael C. Gao; Ömer N. Dogan; Paul King; Anthony D. Rollett; Michael Widom

2008-01-01

90

The first-principles design of ductile refractory alloys

The purpose of this work is to predict elastic and thermodynamic properties of chromium-based alloys based on first-principles\\u000a calculations and to demonstrate an appropriate computational approach to develop new materials for high-temperature applications\\u000a in energy systems. In this study, Poisson ratio is used as a screening parameter to identify ductilizing additives to the\\u000a refractory alloys. The results predict that elements

Michael C. Gao; Ömer N. Do?an; Paul King; Anthony D. Rollett; Michael Widom

2008-01-01

91

First-principles calculations of dynamic permeability in porous media

By starting from the linearized Navier-Stokes equation for the fluid and the elastic wave equation for the solid frame of a porous medium, the first-principles definition of the frequency-dependent permeability kappa(omega) and the recipe for its calculation are derived through the application of the homogenization procedure. It is shown systematically that, in the limit of wavelength much larger than the

Min-Yao Zhou; Ping Sheng

1989-01-01

92

Understanding and Predicting Thiolated Gold Nanoclusters from First Principles

This is an exciting time for studying thiolated gold nanoclusters. Single crystal structures of Au{sub 102}(SR){sub 44} and Au{sub 25}(SR){sub 18}{sup -} (-SR being an organothiolate group) bring both surprises and excitement in this field. First principles density functional theory (DFT) simulations turn out to be an important tool to understand and predict thiolated gold nanoclusters. In this review, I summarize the progresses made by us and others in applying first principles DFT to thiolated gold nanoclusters, as inspired by the recent experiments. First, I will give some experimental background on synthesis of thiolated gold nanoclusters, followed by a description of the recent experimental breakthroughs. Then I will introduce the superatom complex concept as a way to understand the electronic structure of thiolated gold nanoclusters or smaller nanoparticles. Next, I will describe in detail how first principles DFT is used to understand the Au-thiolate interface, predict structures for Au{sub 38}(SR){sub 24}, screen good dopants for the Au{sub 25}(SR){sub 18}{sup -} cluster, design the smallest magic thiolated gold cluster, and demonstrate the need for the trimer protecting motif. I will conclude with a grand challenge: the real time monitoring of nucleation of thiolated gold nanoclusters.

Jiang, Deen [ORNL

2010-01-01

93

Modeling the interface of Li metal and Li solid electrolytes from first principles Nicholas Lepley battery electrolytes. Simplified theoretical models often fail to agree with experimental observations of the stability of electrode electrolyte interfaces. An example of this disagreement is the thiophosphate

Holzwarth, Natalie

94

NASA Astrophysics Data System (ADS)

In spite of the strong relevance of electrochemical energy conversion and storage, the atomistic modeling of structures and processes in electrochemical systems from first principles is hampered by severe problems. Among others, these problems are associated with the theoretical description of the electrode potential, the characterization of interfaces, the proper treatment of liquid electrolytes, changes in the bulk structure of battery electrodes, and limitations of the functionals used in first-principles electronic structure calculations. We will illustrate these obstacles, but also indicate strategies to overcome them.

Hörmann, Nicolas G.; Jäckle, Markus; Gossenberger, Florian; Roman, Tanglaw; Forster-Tonigold, Katrin; Naderian, Maryam; Sakong, Sung; Groß, Axel

2015-02-01

95

Mechanical and Vibrational Properties of ZnS with Wurtzite Structure: A First-Principles Study

NASA Astrophysics Data System (ADS)

We perform a first-principles study of the mechanical and vibrational properties of ZnS with a wurtzite structure. The calculated elastic constants by using a pseudopotential plane-wave method agree well with the experimental data and with the previous theoretical works. Based on the elastic constants and their related parameters, the crystal mechanical stability is discussed. Calculations of the zone-center optical-mode frequencies including longitudinal-optical/transverse-optical splitting, by using the density functional perturbation theory, are reported. All optical modes are identified, especially B1 modes, and agree with Raman measurements.

Yu, You; Chen, Chun-Lin; Zhao, Guo-Dong; Zheng, Xiao-Lin; Zhu, Xing-Hua

2014-10-01

96

NASA Astrophysics Data System (ADS)

We theoretically investigate the electronic structure and spin polarization properties of Na-doped meridianal tris(8-hydroxyquinoline) aluminum (Alq3) by first principles calculations. It is found that the spin density is distributed mainly in the Alq3 part in the Alq3:Na complex. Electron charge transfer takes place from the Na atom to the Alq3 molecule, which induces asymmetric changing of the molecule bond lengths, thus the spin density distribution becomes asymmetric. Spin polarization of the complex originates from the preferable filling of the spin-split nitrogen and carbon p-orbitals because of the different bond length changes of the Alq3 molecule upon Na doping.

Ren, Jun-Feng; Yuan, Xiao-Bo; Hu, Gui-Chao

2014-04-01

97

NASA Astrophysics Data System (ADS)

We carry out the first-principles calculations to investigate the electronic properties of bilayer graphene. The ultrasoft pseudopotential density functional methods within the local density approximation (LDA) are applied. The band structures and density of states of bilayer graphene are investigated. Intrinsic defects, including Stone-Wales (SW) defect and atom vacancy, are considered, and proper theoretical analysis about its electronic properties is given. Finally, we arrive at the point that Stone-Wales defect can generate a small energy gap in bilayer graphene, and the vacancy defect can bring in a magnetic moment of bilayer graphene. The results may be valuable for the application of bilayer graphene in electronic devices.

Li, Ting; Tang, Xianqiong; Liu, Zhi; Zhang, Ping

2011-07-01

98

First-principles study of thermoelectric properties of CuI

NASA Astrophysics Data System (ADS)

Theoretical investigations of the thermoelectric properties of CuI have been carried out employing first-principles calculations followed by the calculations of transport coefficients based on Boltzmann transport theory. Among the three different phases of CuI, viz. zinc-blende, wurtzite and rock salt, the thermoelectric power factor is found to be the maximum for the rock salt phase. We have analysed the variations of Seebeck coefficients and thermoelectric power factors on the basis of calculated electronic structures near the valence band maxima of these phases.

Yadav, Manoj K.; Sanyal, Biplab

2014-03-01

99

We elucidate the role of room-temperature-induced instantaneous structural distortions in the Li K-edge X-ray absorption spectra (XAS) of crystalline LiF, Li{sub 2}SO{sub 4}, Li{sub 2}O, Li{sub 3}N, and Li{sub 2}CO{sub 3} using high resolution X-ray Raman spectroscopy (XRS) measurements and first-principles density functional theory calculations within the eXcited electron and Core Hole approach. Based on thermodynamic sampling via ab initio molecular dynamics simulations, we find calculated XAS in much better agreement with experiment than those computed using the rigid crystal structure alone. We show that local instantaneous distortion of the atomic lattice perturbs the symmetry of the Li 1s core-excited-state electronic structure, broadening spectral line-shapes and, in some cases, producing additional spectral features. The excellent agreement with high-resolution XRS measurements validates the accuracy of our first-principles approach to simulating XAS, and provides both accurate benchmarks for model compounds and a predictive theoretical capability for identification and characterization of multi-component systems, such as lithium-ion batteries, under working conditions.

Pascal, Tod A.; Prendergast, David, E-mail: dgprendergast@lbl.gov [The Molecular Foundry, Materials Science Division, Lawrence Berkeley National Laboratory (LBNL), Berkeley, California 94720 (United States)] [The Molecular Foundry, Materials Science Division, Lawrence Berkeley National Laboratory (LBNL), Berkeley, California 94720 (United States); Boesenberg, Ulrike; Kostecki, Robert; Richardson, Thomas J. [Environmental Energy Technologies Division, LBNL, Berkeley, California 94720 (United States)] [Environmental Energy Technologies Division, LBNL, Berkeley, California 94720 (United States); Weng, Tsu-Chien; Sokaras, Dimosthenis; Nordlund, Dennis [Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford, California 94720 (United States)] [Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford, California 94720 (United States); McDermott, Eamon; Moewes, Alexander [University of Saskatchewan, Department of Physics and Engineering Physics, Saskatoon, Saskatchewan S7N 5E2 (Canada)] [University of Saskatchewan, Department of Physics and Engineering Physics, Saskatoon, Saskatchewan S7N 5E2 (Canada); Cabana, Jordi [Environmental Energy Technologies Division, LBNL, Berkeley, California 94720 (United States) [Environmental Energy Technologies Division, LBNL, Berkeley, California 94720 (United States); Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60605 (United States)

2014-01-21

100

Collective modes in light nuclei from first principles.

Results for ab initio no-core shell model calculations in a symmetry-adapted SU(3)-based coupling scheme demonstrate that collective modes in light nuclei emerge from first principles. The low-lying states of 6Li, 8Be, and 6He are shown to exhibit orderly patterns that favor spatial configurations with strong quadrupole deformation and complementary low intrinsic spin values, a picture that is consistent with the nuclear symplectic model. The results also suggest a pragmatic path forward to accommodate deformation-driven collective features in ab initio analyses when they dominate the nuclear landscape. PMID:24483740

Dytrych, T; Launey, K D; Draayer, J P; Maris, P; Vary, J P; Saule, E; Catalyurek, U; Sosonkina, M; Langr, D; Caprio, M A

2013-12-20

101

Hybrid perovskites for photovoltaics: Insights from first principles

NASA Astrophysics Data System (ADS)

The methylammonium lead iodide perovskites at the core of recently proposed solar cells with exceptionally large quantum conversion efficiency are studied by first-principles methods. Large absorption coefficients (0.03-0.04 nm-1 for wavelength ˜500 nm) and small effective masses suited for both n-type and p-type transport are obtained as a consequence of their peculiar structural and electronic characteristics. In particular, the presence of a direct gap between highly dispersed Pb(6s)-I(5p) valence bands and Pb(6p) conduction bands is the key ingredient at the basis of their excellent performance in photovoltaic applications.

Filippetti, A.; Mattoni, A.

2014-03-01

102

Value of first principles and phenomenological modeling in mineral processing

There is confusion in naming the several models developed in Mineral Processing. The authors often hear of empirical, first principle, mechanistic and phenomenological models. The objective of this paper is to clarify and distinguish between these models, based on a philosophical and linguistic analysis. A state of the art review for mathematical modeling in Mineral Processing is also made. The advantage of considering Mineral Processing as a series of unit operations was recognized by Gaudin a long time ago. He divided the area into four unit operations: (1) comminution, (2) classification, (3) concentration and (4) dewatering.

Concha, F. [Univ. of Concepcion (Chile). Dept. of Metallurgical Engineering

1995-12-31

103

Comparative study of Ti and Ni clusters from first principles

Icosahedral clusters in Ti and Ni are studied with first-principles density functional calculations. We find significant distortion on the Ti icosahedron caused by the strong interaction between surface atoms on the icosahedron but not between the center atom and surface atoms, whereas no such distortion is observed on Ni clusters. In addition, distortion becomes more severe when atoms are added to the Ti13 cluster resulting in short bonds. Such distorted icosahedra having short bonds are essentially to explain the structure factor of Ti liquid obtained in experiment.

Lee, B; Lee, G W

2007-08-20

104

Large impurity effects in rubrene crystals: First-principles calculations

Carrier mobilities of rubrene films are among the highest values reported for any organic semiconductor. Here, we probe with first-principles calculations the sensitivity of rubrene crystals on impurities. We find that isolated oxygen impurities create distinct peaks in the electronic density of states consistent with observations of defect levels in rubrene and that increased O content changes the position and shape of rubrene energy bands significantly. We also establish a dual role of hydrogen as individual H species and H impurity pairs create and annihilate deep carrier traps, respectively. The results are relevant to the performance and reliability of rubrene-based devices.

Tsetseris, L. [Aristotle University of Thessaloniki, Greece; Pantelides, Sokrates T. [Vanderbilt University

2008-01-01

105

First principles pseudopotential calculations on aluminum and aluminum alloys

Recent advances in computational techniques have led to the possibility of performing first principles calculations of the energetics of alloy formation on systems involving several hundred atoms. This includes impurity concentrations in the 1% range as well as realistic models of disordered materials (including liquids), vacancies, and grain boundaries. The new techniques involve the use of soft, fully nonlocal pseudopotentials, iterative diagonalization, and parallel computing algorithms. This approach has been pioneered by Car and Parrinello. Here the authors give a review of recent results using parallel and serial algorithms on metallic systems including liquid aluminum and liquid sodium, and also new results on vacancies in aluminum and on aluminum-magnesium alloys.

Davenport, J.W.; Chetty, N.; Marr, R.B.; Narasimhan, S.; Pasciak, J.E.; Peierls, R.F.; Weinert, M.

1993-12-31

106

First-Principles Informed Thermodynamics of CRUD Deposition

NASA Astrophysics Data System (ADS)

The recent emphasis in the United States on developing abundant domestic sources of energy, together with an increasing awareness of the environmental hazards of fossil fuels, has led to a fresh look at the challenges of nuclear energy within the science and engineering community. One of these challenges is controlling the precipitation of porous oxide deposits onto the nuclear fuel rod cladding from the primary coolant during operation of pressurized light-water reactors (PWRs). These deposits, called CRUD (an acronym for Chalk River Unidentified Deposits), are a major concern to reactor operation because they reduce fuel lifetime and efficiency by reducing heat transfer to the coolant, promote corrosion, and depress neutron flux. This dissertation provides fundamental insights into the process by which CRUD is formed in PWRs by providing a framework linking the results of first-principles calculations to experimental data. The technique developed to facilitate the investigation is referred to as Density Functional Theory (DFT) referenced semi-empirical thermodynamics; It links 0K first-principles calculations with high temperature thermodynamics by redefining the reference chemical potentials of the constituent elements. The technique permits aqueous chemistry to be incorporated into thermodynamic calculations and allows for the prediction of temperature and pressure dependent free energies of materials that are experimentally inaccessible or have not yet been measured. The ability to extend accurate first-principles calculations to high temperatures and aqueous environments allows the stability of crystal surfaces, calculated with DFT techniques, to be predicted at conditions representative of an operating PWR. Accurate values of surface energies are used in fulfilling the principal goal of this dissertation, which is to investigate the aqueous thermodynamics of formation of nickel oxide (NiO) and nickel ferrite (NiFe 2O4) crystallites as representative CRUD components. Specifically, this dissertation investigates the thermodynamics of the homogeneous nucleation of crystallites and proposes a mechanism for their agglomeration. This work also contributes to a more complete understanding of CRUD by generating improved thermodynamic data to enable modeling of the incorporation of boron into CRUD through the formation of Bonaccordite (Ni2FeBO5) which has formed in PWRs operating at high-power conditions.

O'Brien, Christopher John

107

Dirac cone in ?-graphdiyne: a first-principles study.

: We investigate the Dirac cone in ?-graphdiyne, which is a predicted flat one-atom-thick allotrope of carbon using first-principles calculations. ?-graphdiyne is derived from graphene where two acetylenic linkages (-C ?C-) are inserted into the single bonds (-C-C-). Thus, ?-graphdiyne possesses a larger lattice constant which subsequently affects its electronic properties. Band structures show that ?-graphdiyne exhibits similar Dirac points and cone to graphene. Further, the tight-binding method is used to exploit the linear dispersion in the vicinity of Dirac points. Thanks to the larger lattice constant, ?-graphdiyne yields a lower Fermi velocity, which might make itself an ideal material to serve the anomalous integer quantum Hall effect. PMID:24206912

Niu, Xiaoning; Mao, Xingze; Yang, Dezheng; Zhang, Zhiya; Si, Mingsu; Xue, Desheng

2013-01-01

108

Dirac cone in ?-graphdiyne: a first-principles study

We investigate the Dirac cone in ?-graphdiyne, which is a predicted flat one-atom-thick allotrope of carbon using first-principles calculations. ?-graphdiyne is derived from graphene where two acetylenic linkages (-C ?C-) are inserted into the single bonds (-C-C-). Thus, ?-graphdiyne possesses a larger lattice constant which subsequently affects its electronic properties. Band structures show that ?-graphdiyne exhibits similar Dirac points and cone to graphene. Further, the tight-binding method is used to exploit the linear dispersion in the vicinity of Dirac points. Thanks to the larger lattice constant, ?-graphdiyne yields a lower Fermi velocity, which might make itself an ideal material to serve the anomalous integer quantum Hall effect. PMID:24206912

2013-01-01

109

First-principles study of low compressibility osmium borides

NASA Astrophysics Data System (ADS)

Using first-principles total energy calculations we investigate the structural, elastic, and electronic properties of OsB2 and OsB, respectively. The calculated equilibrium structural parameters of OsB2 are in agreement with the available experimental results. The calculations indicate that OsB in tungsten carbide is more energetically stable under the ambient condition than the metastable cesium chloride phase of OsB. Results of bulk modulus show that they are potential low compressible materials. The hardness of OsB2 is estimated by employing a semiempirical theory. The results indicate that OsB2 is an ultraincompressible material, but not a superhard material. The method designing superhard materials is different from one creating ultraincompressible materials.

Gou, Huiyang; Hou, Li; Zhang, Jingwu; Li, Hui; Sun, Guifang; Gao, Faming

2006-05-01

110

Accurate First Principles Model Potentials for Intermolecular Interactions

NASA Astrophysics Data System (ADS)

The general effective fragment potential (EFP) method provides model potentials for any molecule that is derived from first principles, with no empirically fitted parameters. The EFP method has been interfaced with most currently used ab initio single-reference and multireference quantum mechanics (QM) methods, ranging from Hartree-Fock and coupled cluster theory to multireference perturbation theory. The most recent innovations in the EFP model have been to make the computationally expensive charge transfer term much more efficient and to interface the general EFP dispersion and exchange repulsion interactions with QM methods. Following a summary of the method and its implementation in generally available computer programs, these most recent new developments are discussed.

Gordon, Mark S.; Smith, Quentin A.; Xu, Peng; Slipchenko, Lyudmila V.

2013-04-01

111

Accurate first principles model potentials for intermolecular interactions.

The general effective fragment potential (EFP) method provides model potentials for any molecule that is derived from first principles, with no empirically fitted parameters. The EFP method has been interfaced with most currently used ab initio single-reference and multireference quantum mechanics (QM) methods, ranging from Hartree-Fock and coupled cluster theory to multireference perturbation theory. The most recent innovations in the EFP model have been to make the computationally expensive charge transfer term much more efficient and to interface the general EFP dispersion and exchange repulsion interactions with QM methods. Following a summary of the method and its implementation in generally available computer programs, these most recent new developments are discussed. PMID:23561011

Gordon, Mark S; Smith, Quentin A; Xu, Peng; Slipchenko, Lyudmila V

2013-01-01

112

Hydrogen storage in LiH: A first principle study

NASA Astrophysics Data System (ADS)

First principles calculations have been performed on the Lithium hydride (LiH) using the full potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory. We have extended our calculations for LiH+2H and LiH+6H in NaCl structure. The structural stability of three compounds have been studied. It is found that LiH with 6 added Hydrogen atoms is most stable. The obtained results for LiH are in good agreement with reported experimental data. Electronic structures of three compounds are also studied. Out of three the energy band gap in LiH is ˜3.0 eV and LiH+2H and LiH+6H are metallic.

Banger, Suman; Nayak, Vikas; Verma, U. P.

2014-04-01

113

First principles modeling of panchromatic dyes for solar cells applications.

NASA Astrophysics Data System (ADS)

The state-of-the-art dye in Gr"atzel solar cells, N719, exhibits a total solar-to-electric conversion efficiency of 11.2%. However, it severely lacks absorption in the red and the near infrared regions of the electromagnetic spectrum, which represent more than 70% of the solar radiation spectrum. Using calculations from first principles in the time-dependent domain, we have studied the electronic and optical response of a novel class of panchromatic sensitizers that can harvest solar energy efficiently across the visible and near infrared regions, which have been recently synthesized [A. El-Shafei, M. Hussain, A. Atiq, A. Islam, and L. Han, J. Mater. Chem. 22, 24048 (2012)]. Our calculations show that, by tuning the properties of antenna groups, one can achieve a substantial improvement of the optical properties.

di Felice, Rosa; Calzolari, Arrigo; Dong, Rui; Buongiorno Nardelli, Marco

2013-03-01

114

Softening of hydroxyapatite by vacancies: a first principles investigation.

First-principles plane-wave calculations were performed to investigate the influences of intrinsic vacancy on the stability and elastic properties of hydroxyapatite (HA). Five types of vacancies, i.e. H, O, OH, Ca, and Ca(2+) vacancy, were considered. Formation energies were evaluated and compared with experimental measurements. It was shown that HA with a Ca(2+) vacancy is the most stable one among the considered systems. Elastic constants were estimated via curves of total energy against strain. Bulk, shear and Young's moduli, and Poisson's ratio are also evaluated to compare with the experimental value. The elastic properties of HA are significantly affected by the vacancy. Vacancy can soften HA via reducing its elastic moduli. The HA with Ca(2+) vacancy is the softest one among the considered systems. Electronic structures of HA with vacancy are also analyzed to explain the softening mechanisms. PMID:23827549

Sun, J P; Song, Y; Wen, G W; Wang, Y; Yang, R

2013-04-01

115

First principle study of manganese doped cadmium sulphide sheet

First-principle electronic structure calculations for cadmium sulphide (CdS) sheet in hexagonal phase, with Manganese substitution and addition, as well as including the Cd defects, are investigated. The lattice constants calculated for CdS sheet agrees fairly well with results reported for thin films experimentally. The calculations of total spin density of states and partial density of states in different cases shows substantial magnetic dipole moments acquired by the sheet. A magnetic dipole moment 5.00612 ?{sub B} and band gap of the order 1 eV are found when cadmium atom is replaced by Manganese. The magnetism acquired by the sheet makes it functionally important candidate in many applications.

Kumar, Sanjeev, E-mail: drskumar11@gmail.com [Department of Physics, St. Bede's College, Shimla-171002 (India); Kumar, Ashok; Ahluwalia, P. K. [Department of Physics, Himachal Pradesh University, Shimla-171005 (India)

2014-04-24

116

First-principles simulations of electrostatic interactions between dust grains

NASA Astrophysics Data System (ADS)

We investigated the electrostatic interaction between two identical dust grains of an infinite mass immersed in homogeneous plasma by employing first-principles N-body simulations combined with the Ewald method. We specifically tested the possibility of an attractive force due to overlapping Debye spheres (ODSs), as was suggested by Resendes et al. [Phys. Lett. A 239, 181-186 (1998)]. Our simulation results demonstrate that the electrostatic interaction is repulsive and even stronger than the standard Yukawa potential. We showed that the measured electric field acting on the grain is highly consistent with a model electrostatic potential around a single isolated grain that takes into account a correction due to the orbital motion limited theory. Our result is qualitatively consistent with the counterargument suggested by Markes and Williams [Phys. Lett. A 278, 152-158 (2000)], indicating the absence of the ODS attractive force.

Itou, H.; Amano, T.; Hoshino, M.

2014-12-01

117

Hydrogen storage in LiH: A first principle study

First principles calculations have been performed on the Lithium hydride (LiH) using the full potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory. We have extended our calculations for LiH+2H and LiH+6H in NaCl structure. The structural stability of three compounds have been studied. It is found that LiH with 6 added Hydrogen atoms is most stable. The obtained results for LiH are in good agreement with reported experimental data. Electronic structures of three compounds are also studied. Out of three the energy band gap in LiH is ?3.0 eV and LiH+2H and LiH+6H are metallic.

Banger, Suman, E-mail: sumanphy28@gmail.com; Nayak, Vikas, E-mail: sumanphy28@gmail.com; Verma, U. P., E-mail: sumanphy28@gmail.com [School of Studies in Physics, Jiwaji University, Gwalior-474011 (India)

2014-04-24

118

First-principles studies of native defects in olivine phosphates

NASA Astrophysics Data System (ADS)

Olivine phosphates LiMPO4 (M=Mn, Fe, Co, Ni) are promising candidates for rechargeable Li-ion battery electrodes because of their energy storage capacity and electrochemical and thermal stability. It is known that native defects have strong effects on the performance of olivine phosphates. Yet, the formation and migration of these defects are not fully understood, and we expect that once such understanding has been established, one can envisage a solution for improving the materials' performance. In this talk, we present our first-principles density-functional theory studies of native point defects and defect complexes in LiMPO4, and discuss the implications of these defects on the performance of the materials. Our results also provide guidelines for obtaining different native defects in experiments.

Hoang, Khang; Johannes, Michelle

2011-03-01

119

Preferential functionalization on zigzag graphene nanoribbons: first-principles calculations.

We investigate the functionalization of functional groups to graphene nanoribbons with zigzag and armchair edges using first-principles calculations. We find that the formation energy for the configuration of the functional groups functionalized to the zigzag edge is ~0.2 eV per functional group lower than that to the armchair edge. The formation energy difference arises from a structural deformation on the armchair edge by the functionalization whereas there is no structural deformation on the zigzag edge. Selective functionalization on the zigzag edge takes place at a condition of the temperature and the pressure of ~25 °C and 10(-5) atm. Our findings show that selective functionalization can offer the opportunity for an approach to the separation of zigzag graphene nanoribbons with their solubility change. PMID:21403278

Lee, Hoonkyung

2010-09-01

120

First-principles molecular dynamics simulations of the H2O/Cu(111) interface.

A first-principles theoretical study of the water-Cu(111) interface based on density functional calculations is reported. Using differently sized surface models: p(2 × 2), p(4 × 4) and p(4 × 5), we found out that the adsorption energy of a H(2)O monomer does not significantly change with the surface model though the adsorption geometry is sensitive to the choice of the super-cell surface and, also, to the coverage. Molecular dynamics simulations on the Born-Oppenheimer surface of liquid water on a Cu(111) surface reveal that H(2)O in the first solvent layer adsorbs O-down and that the H-bond network is weaker upon adsorption on the Cu. Furthermore, absolute electrochemical potentials are presented and compared to the potential of zero charge obtained experimentally and theoretically. PMID:22006214

Nadler, Roger; Fernandez Sanz, Javier

2012-06-01

121

Large-Scale First-Principles Molecular Dynamics simulations on the BlueGene/L Platform

Large-Scale First-Principles Molecular Dynamics simulations on the BlueGene/L Platform using. Keywords Electronic structure. Molecular Dynamics. Ab initio simulations. First-principles simulations that the Qbox code supports unprecedented large-scale First-Principles Molecular Dynamics (FPMD) applications

Franchetti, Franz

122

Carbon impurity dissolution and migration in bcc Fe-Cr: First-principles calculations

NASA Astrophysics Data System (ADS)

First-principles density-functional theory calculations for C solution enthalpies, Hsol , and diffusion activation enthalpies, Hdiff , in body-centered-cubic Fe and Cr are presented. The results for C in Fe compare well with experiments, provided that the effect of magnetic disordering is accounted for. Likewise, in Cr, the calculated Hsol and Hdiff agree well with available experiments. In both materials, the deviation between calculated enthalpies and critically assessed experimental enthalpies are less than 0.05 eV. Further, first-principles calculations for the interaction energies between a solute (e.g., a Cr atom in bcc Fe) and an interstitial C atom are presented. The results are in conflict with those inferred from internal friction (IF) experiments in disordered Fe-Cr-C alloys. A simple model of C relaxation in disordered Fe-Cr is used to compare theoretical and experimental IF curves directly. The results suggest that a more extensive study of the energetic, thermodynamic, and kinetic aspects of C migration in Fe-Cr is needed.

Sandberg, Nils; Henriksson, Krister O. E.; Wallenius, Jan

2008-09-01

123

First-principles study of ground-state properties of U2Mo.

By means of first-principles calculations, we have systematically investigated the structural, elastic, vibrational, thermal and electronic properties of the ground-state phase for the intermetallic compound U2Mo. Our results reveal that the previously synthesized I4/mmm structure of U2Mo is a metastable phase and unstable, neither thermodynamically nor vibrationally at the ground state. In combination with the evolutionary structural searches, our first-principles calculations suggest a new ground-state Pmmn phase, which has been confirmed theoretically to be stable, both thermodynamically and vibrationally. Moreover, through the DFT + D technique we have discussed the influence of van der Waals interactions on the structural, elastic and vibrational properties, revealing a weak effect in pure U and Mo solids and U2Mo alloy. The analysis of the electronic band structures evidences its electronic stabilities with the appearance of a deep valley in the density of states at the Fermi level. Moreover, we have investigated further the temperature-dependent structural, thermal expansion and elastic properties of our proposed Pmmn ground-state phase. These results are expected to stimulate further experimental investigations of the ground-state phase of U2Mo. PMID:25380409

Wang, Xin; Cheng, Xiyue; Zhang, Yuting; Li, Ronghan; Xing, Weiwei; Zhang, Pengcheng; Chen, Xing-Qiu

2014-12-28

124

NASA Astrophysics Data System (ADS)

Despite well documented first-principles theoretical determination of the low migration energy (0.06 eV) of a single He in tungsten, fully quantum mechanical calculations on the migration of a He pair still present a challenge due to the complexity of its trajectory. By identifying the six most stable configurations of the He pair in W and decomposing its motion into rotational, translational, and rotational-translational routines, we are able to determine its migration barrier and trajectory. Our density functional theory calculations demonstrate a He pair has three modes of motion: a close or open circular two-dimensional motion in (100) plane with an energy barrier of 0.30 eV, a snaking motion along [001] direction with a barrier of 0.30 eV, and a twisted-ladder motion along [010] direction with the two He swinging in the plane (100) and a barrier of 0.31 eV. The graceful associative movements of a He pair are related to the chemical-bonding-like He-He interaction being much stronger than its migration barrier in W. The excellent agreement with available experimental measurements (0.24-0.32 eV) on He migration makes our first-principles result a solid input to obtain accurate He-W interatomic potentials in molecular dynamics simulations.

Niu, J. G.; Zhan, Q.; Geng, W. T.

2014-06-01

125

First principles study of structural, elastic, and electronic properties of the cubic perovskitetype BaHfO$_3$ has been performed using the plane wave ultrasoft pseudo-potential method based on density functional theory with revised Perdew-Burke-Ernzerhof exchange-correlation functional of the generalized gradient approximation (GGA-RPBE). The calculated equilibrium lattice constant of this compound is in good agreement with the available experimental and theoretical data reported

Hongsheng Zhao; Aimin Chang; Yunlan Wang

2008-01-01

126

First Principles Studies of ABO3 Perovskite Surfaces and Nanostructures

NASA Astrophysics Data System (ADS)

Perovskite-type complex oxides, with general formula ABO 3, constitute one of the most prominent classes of metal oxides which finds key applications in diverse technological fields. In recent years, properties of perovskites at reduced dimensions have aroused considerable interest. However, a complete atomic-level understanding of various phenomena is yet to emerge. To fully exploit the materials opportunities provided by nano-structured perovskites, it is important to characterize and understand their bulk and near-surface electronic structure along with the electric, magnetic, elastic and chemical properties of these materials in the nano-regime, where surface and interface effects naturally play a dominant role. In this thesis, state-of-the-art first principles computations are employed to systematically study properties of one- and two-dimensional perovskite systems which are of direct technological significance. Specifically, our bifocal study targets (1) polarization behavior and dielectric response of ABO3 ferroelectric nanowires, and (2) oxygen chemistry relevant for catalytic properties of ABO3 surfaces. In the first strand, we identify presence of novel closure or vortex-like polarization domains in PbTIO3 and BaTiO3 ferroelectric nanowires and explore ways to control the polarization configurations by means of strain and surface chemistry in these prototypical model systems. The intrinsic tendency towards vortex polarization at reduced dimensions and the underlying driving forces are discussed and previously unknown strain induced phase transitions are identified. Furthermore, to compute the dielectric permittivity of nanostructures, a new multiscale model is developed and applied to the PbTiO3 nanowires with conventional and vortex-like polarization configurations. The second part of the work undertaken in this thesis is comprised of a number of ab initio surface studies, targeted to investigate the effects of surface terminations, prevailing chemical environment and processing conditions on the surface relaxations, local electronic structure and chemical reactivity. By combining our first principles computations with an in-house developed kMC simulation approach, we describe the thermodynamics, steady-state kinetics and the long-time and large-length scale behavior of the catalytically active (001) MnO2-terminated LaMnO3 surface in contact with an oxygen reservoir, as a function of temperature and partial pressure of oxygen. The results obtained are in excellent agreement with available experimental data in the literature.

Pilania, Ghanshyam

127

First-principle calculation of core level binding energies of LixPOyNz solid electrolyte

NASA Astrophysics Data System (ADS)

We present first-principle calculations of core-level binding energies for the study of insulating, bulk phase, compounds, based on the Slater-Janak transition state model. Those calculations were performed in order to find a reliable model of the amorphous LixPOyNz solid electrolyte which is able to reproduce its electronic properties gathered from X-ray photoemission spectroscopy (XPS) experiments. As a starting point, Li2PO2N models were investigated. These models, proposed by Du et al. on the basis of thermodynamics and vibrational properties, were the first structural models of LixPOyNz. Thanks to chemical and structural modifications applied to Li2PO2N structures, which allow to demonstrate the relevance of our computational approach, we raise an issue concerning the possibility of encountering a non-bridging kind of nitrogen atoms (=N-) in LixPOyNz compounds.

Guille, Émilie; Vallverdu, Germain; Baraille, Isabelle

2014-12-01

128

First-principle calculation of core level binding energies of LixPOyNz solid electrolyte.

We present first-principle calculations of core-level binding energies for the study of insulating, bulk phase, compounds, based on the Slater-Janak transition state model. Those calculations were performed in order to find a reliable model of the amorphous LixPOyNz solid electrolyte which is able to reproduce its electronic properties gathered from X-ray photoemission spectroscopy (XPS) experiments. As a starting point, Li2PO2N models were investigated. These models, proposed by Du et al. on the basis of thermodynamics and vibrational properties, were the first structural models of LixPOyNz. Thanks to chemical and structural modifications applied to Li2PO2N structures, which allow to demonstrate the relevance of our computational approach, we raise an issue concerning the possibility of encountering a non-bridging kind of nitrogen atoms (=N(-)) in LixPOyNz compounds. PMID:25554171

Guille, Émilie; Vallverdu, Germain; Baraille, Isabelle

2014-12-28

129

Cavity ring-down spectroscopy and theoretical calculations of the S1,,1

was prepared with a pulsed discharge slit nozzle and detected by cavity ring-down spectroscopy. A numberCavity ring-down spectroscopy and theoretical calculations of the S1,,1 B3u...]S0,,1 Ag of the trapped species with the solid lattice. This shortcoming was overcome by the application of cavity ring

130

Electronic structure and ionicity of actinide oxides from first principles

NASA Astrophysics Data System (ADS)

The ground-state electronic structures of the actinide oxides AO , A2O3 , and AO2 ( A=U , Np, Pu, Am, Cm, Bk, and Cf) are determined from first-principles calculations, using the self-interaction corrected local spin-density approximation. Emphasis is put on the degree of f -electron localization, which for AO2 and A2O3 is found to follow the stoichiometry, namely, corresponding to A4+ ions in the dioxide and A3+ ions in the sesquioxides. In contrast, the A2+ ionic configuration is not favorable in the monoxides, which therefore become metallic. The energetics of the oxidation and reduction in the actinide dioxides is discussed, and it is found that the dioxide is the most stable oxide for the actinides from Np onward. Our study reveals a strong link between preferred oxidation number and degree of localization which is confirmed by comparing to the ground-state configurations of the corresponding lanthanide oxides. The ionic nature of the actinide oxides emerges from the fact that only those compounds will form where the calculated ground-state valency agrees with the nominal valency expected from a simple charge counting.

Petit, L.; Svane, A.; Szotek, Z.; Temmerman, W. M.; Stocks, G. M.

2010-01-01

131

Interaction between ferroelectric nanodots: a first-principles-based study

NASA Astrophysics Data System (ADS)

We have performed Monte-Carlo simulations within a first- principles-based effective Hamiltonian approach to investigate the consequences (if any) of the interaction between nanodots made of lead titanium zirconate. We found that two dots under open circuit electrical boundary conditions weakly interact with each other, implying that they both adopt a vortex structure for their dipoles that is very similar to the one occurring in a single isolated dot. The situation is dramatically different if the first dot is polarized (e.g., by being under an external field) while the second dot is still under open circuit condition. In that case, the dots interact stronger, with this interaction causing the development of a relatively small spontaneous polarization in the second dot and, more importantly, being able to influence the direction of the toroid moment in this second dot -- which is of technological importance. We further show how this last interaction, and its consequences, depend on the geometry and shape of the two dots, and reveal that all the striking features can be well understood within a multipole expansion of the field produced by the lattice polarization vortex in the free dot. This work is supported by DOE grant DE- FG02-05ER46188, by ONR grants N00014-01-1-0365, N00014-04-1- 0413 and N00014-01-1-0600, and by NSF grants DMR-0404335 and DMR-9983678.

Prosandeev, S.; Ponomareva, I.; Naumov, I.; Kornev, I.; Bellaiche, L.

2006-03-01

132

Transport Properties of Nanoscale Materials by First-principles Calculations

NASA Astrophysics Data System (ADS)

Molecular devices are potential candidates for the next step towards nanoelectronic technology. Our group has covered a wide range of nanoscale wires, which have potential application in molecular electronics using first-principles calculations and nonequilibrium Green's function formalism [1]. Our target materials are supramolecular enamel wires (covered wires) [2], connection between organic molecules and metal electrodes, self-assembled nanowires on silicon surface [3], porphyrin [4], phthalocyanine, metallocene [5], fused-ring thiophene molecules, length dependence of conductance in alkanedithiols and so on. Namely, we have investigated a relationship of the energy levels of delocalized frontier orbitals (HOMO and LUMO) and Fermi level of metal electrodes and estimate the electronic transport properties through atomic and molecular wires using Green's function approach. References [1] http://www-lab.imr.edu/˜mizuseki/nanowire.html [2] R. V. Belosludov, A. A. Farajian, H. Baba, H. Mizuseki, and Y. Kawazoe, Jpn. J. Appl. Phys., 44, 2823 (2005). [3] R. V. Belosludov, A. A. Farajian, H. Mizuseki, K. Miki, and Y. Kawazoe, Phys. Rev. B, 75, 113411 (2007). [4] S.-U. Lee, R. V. Belosludov, H. Mizuseki, and Y. Kawazoe, Small 4 (2008) 962. [5] S.-U Lee, R. V. Belosludov, H. Mizuseki, and Y. Kawazoe, J. Phys. Chem. C. 111 (2007) 15397.

Mizuseki, Hiroshi; Belosludov, Rodion V.; Lee, S.-U.; Kawazoe, Yoshiyuki

2009-03-01

133

First-Principles Study on Crystal Phase Superlattice Nanowires Heterostructures

NASA Astrophysics Data System (ADS)

We perform a first-principles density functional theory study on structural and electronic properties of a sery of crystal-phase heterostructure atomic-scale superlattice (SL) nanowires (NW) from GaN material, i.e. GaN wurtzite(WZ) /zincblende (ZB) material interface. The effects of surface/interface relaxation and surface stress which are absent in atomistic models are carefully taken into account. Structural properties, energy bands and electronic properties for a class of hexagonal wires with various period of SL structure and diameter size are discussed. Pseudo hydrogen atoms, i.e. hydrogen with partial charges, are used to passivate the dangling surface bonds, which remove the localized in-gap surface states and suppress the surface reconstructions. With this passivation procedure the band structure show the type II for all wires. While the electrical aspects of these SL nanowires are explored through density functional theory, their subsequent band structures are used to determine the thermoelectric properties via the Boltzmann transport theory. Finally, the thermoelectric propertys dependence on temperature is unveiled.

Tuoc, Vu Ngoc; Doan Huan, Tran; Viet Minh, Nguyen; Lien, Le Thi Hong

2014-09-01

134

Mechanical properties of graphyne monolayers: a first-principles study.

We investigated the mechanical properties of graphyne monolayers using first-principles calculations based on the Density Functional Theory. Graphyne has a relatively low in-plane Young's modulus (162 N m(-1)) and a large Poisson ratio (0.429) compared to graphene. It can sustain large nonlinear elastic deformations up to an ultimate strain of 0.2 followed by strain softening until failure. The single bond is more vulnerable to rupture than the triple bond and aromatic bond, although it has a shorter bond length (0.19 Å shorter) than the aromatic bond. A rigorous continuum description of the elastic response is formulated by expanding the elastic strain energy density in a Taylor series in strain truncated after the fifth-order term. We obtained a total of fourteen nonzero independent elastic constants which are components of tensors up to the tenth order. Pressure effects on the second-order elastic constants, in-plane Young's modulus, and Poisson ratio are predicted. This study implies that graphyne-based surface acoustic wave sensors and waveguides may be synthesized by introducing precisely controlled local strains on graphyne monolayers. PMID:22941420

Peng, Qing; Ji, Wei; De, Suvranu

2012-10-14

135

First-principles investigation of Ag-doped gold nanoclusters.

Gold nanoclusters have the tunable optical absorption property, and are promising for cancer cell imaging, photothermal therapy and radiotherapy. First-principle is a very powerful tool for design of novel materials. In the present work, structural properties, band gap engineering and tunable optical properties of Ag-doped gold clusters have been calculated using density functional theory. The electronic structure of a stable Au(20) cluster can be modulated by incorporating Ag, and the HOMO-LUMO gap of Au(20-) (n)Ag(n) clusters is modulated due to the incorporation of Ag electronic states in the HOMO and LUMO. Furthermore, the results of the imaginary part of the dielectric function indicate that the optical transition of gold clusters is concentration-dependent and the optical transition between HOMO and LUMO shifts to the low energy range as the Ag atom increases. These calculated results are helpful for the design of gold cluster-based biomaterials, and will be of interest in the fields of radiation medicine, biophysics and nanoscience. PMID:21686162

Zhang, Xiao-Dong; Guo, Mei-Li; Wu, Di; Liu, Pei-Xun; Sun, Yuan-Ming; Zhang, Liang-An; She, Yi; Liu, Qing-Fen; Fan, Fei-Yue

2011-01-01

136

First-principles modelling of magnesium titanium hydrides.

Mixing Mg with Ti leads to a hydride Mg(x)Ti((1 - x))H(2) with markedly improved (de)hydrogenation properties for x ? 0.8, as compared to MgH(2). Optically thin films of Mg(x)Ti((1 - x))H(2) have a black appearance, which is remarkable for a hydride material. In this paper we study the structure and stability of Mg(x)Ti((1 - x))H(2), x = 0-1 by first-principles calculations at the level of density functional theory. We give evidence for a fluorite to rutile phase transition at a critical composition x(c) = 0.8-0.9, which correlates with the experimentally observed sharp decrease in (de)hydrogenation rates at this composition. The densities of states of Mg(x)Ti((1 - x))H(2) have a peak at the Fermi level, composed of Ti d states. Disorder in the positions of the Ti atoms easily destroys the metallic plasma, however, which suppresses the optical reflection. Interband transitions result in a featureless optical absorption over a large energy range, causing the black appearance of Mg(x)Ti((1 - x))H(2). PMID:21386386

Er, Süleyman; van Setten, Michiel J; de Wijs, Gilles A; Brocks, Geert

2010-02-24

137

First principle active neutron coincidence counting measurements of uranium oxide

NASA Astrophysics Data System (ADS)

Uranium is present in most nuclear fuel cycle facilities ranging from uranium mines, enrichment plants, fuel fabrication facilities, nuclear reactors, and reprocessing plants. The isotopic, chemical, and geometric composition of uranium can vary significantly between these facilities, depending on the application and type of facility. Examples of this variation are: enrichments varying from depleted (~0.2 wt% 235U) to high enriched (>20 wt% 235U); compositions consisting of U3O8, UO2, UF6, metallic, and ceramic forms; geometries ranging from plates, cans, and rods; and masses which can range from a 500 kg fuel assembly down to a few grams fuel pellet. Since 235U is a fissile material, it is routinely safeguarded in these facilities. Current techniques for quantifying the 235U mass in a sample include neutron coincidence counting. One of the main disadvantages of this technique is that it requires a known standard of representative geometry and composition for calibration, which opens up a pathway for potential erroneous declarations by the State and reduces the effectiveness of safeguards. In order to address this weakness, the authors have developed a neutron coincidence counting technique which uses the first principle point-model developed by Boehnel instead of the "known standard" method. This technique was primarily tested through simulations of 1000 g U3O8 samples using the Monte Carlo N-Particle eXtended (MCNPX) code. The results of these simulations showed good agreement between the simulated and exact 235U sample masses.

Goddard, Braden; Charlton, William; Peerani, Paolo

2014-03-01

138

Coarse graining approach to First principles modeling of structural materials

Classical Molecular Dynamic (MD) simulations characterizing extended defects typically require millions of atoms. First principles calculations employed to understand these defect systems at an electronic level cannot, and should not deal with such large numbers of atoms. We present an e cient coarse graining (CG) approach to calculate local electronic properties of large MD-generated structures from the rst principles. We used the Locally Self-consistent Multiple Scattering (LSMS) method for two types of iron defect structures 1) screw-dislocation dipoles and 2) radiation cascades. The multiple scattering equations are solved at fewer sites using the CG. The atomic positions were determined by MD with an embedded atom force eld. The local moments in the neighborhood of the defect cores are calculated with rst-principles based on full local structure information, while atoms in the rest of the system are modeled by representative atoms with approximated properties. This CG approach reduces computational costs signi cantly and makes large-scale structures amenable to rst principles study. Work is sponsored by the USDoE, O ce of Basic Energy Sciences, Center for Defect Physics, an Energy Frontier Research Center. This research used resources of the Oak Ridge Leadership Computing Facility at the ORNL, which is supported by the O ce of Science of the USDoE under Contract No. DE-AC05-00OR22725.

Odbadrakh, Khorgolkhuu [ORNL; Nicholson, Don M [ORNL; Rusanu, Aurelian [ORNL; Samolyuk, German D [ORNL; Wang, Yang [Pittsburgh Supercomputing Center; Stoller, Roger E [ORNL; Zhang, X.-G. [Oak Ridge National Laboratory (ORNL); Stocks, George Malcolm [ORNL

2013-01-01

139

Thermal conductivity of silicene from first-principles

Silicene, as a graphene-like two-dimensional material, now receives exceptional attention of a wide community of scientists and engineers beyond graphene. Despite extensive study on its electric property, little research has been done to accurately calculate the phonon transport of silicene so far. In this paper, thermal conductivity of monolayer silicene is predicted from first-principles method. At 300?K, the thermal conductivity of monolayer silicene is found to be 9.4?W/mK and much smaller than bulk silicon. The contributions from in-plane and out-of-plane vibrations to thermal conductivity are quantified, and the out-of-plane vibration contributes less than 10% of the overall thermal conductivity, which is different from the results of the similar studies on graphene. The difference is explained by the presence of small buckling, which breaks the reflectional symmetry of the structure. The flexural modes are thus not purely out-of-plane vibration and have strong scattering with other modes.

Xie, Han; Bao, Hua, E-mail: hum@ghi.rwth-aachen.de, E-mail: hua.bao@sjtu.edu.cn [University of Michigan–Shanghai Jiao Tong University Joint Institute, Shanghai Jiao Tong University, Shanghai 200240 (China); Hu, Ming, E-mail: hum@ghi.rwth-aachen.de, E-mail: hua.bao@sjtu.edu.cn [Institute of Mineral Engineering, Division of Materials Science and Engineering, Faculty of Georesources and Materials Engineering, RWTH Aachen University, Aachen 52064 (Germany); Aachen Institute for Advanced Study in Computational Engineering Science (AICES), RWTH Aachen University, Aachen 52062 (Germany)

2014-03-31

140

First-principles modeling of magnetic excitations in Mn12

NASA Astrophysics Data System (ADS)

We have developed a fully microscopic theory of magnetic properties of the prototype molecular magnet Mn12. First, the intramolecular magnetic properties have been studied by means of first-principles density functional based methods, with local correlation effects being taken into account within the local density approximation plus U (LDA+U) approach. Using the magnetic force theorem, we have calculated the interatomic isotropic and anisotropic exchange interactions and full tensors of single-ion anisotropy for each Mn ion. Dzyaloshinskii-Moriya (DM) interaction parameters turned out to be unusually large, reflecting a low symmetry of magnetic pairs in molecules, in comparison with bulk crystals. Based on these results we predict a distortion of ferrimagnetic ordering due to DM interactions. Further, we use an exact diagonalization approach allowing one to work with as large a Hilbert space dimension as 108 without any particular symmetry (the case of the constructed magnetic model). Based on the computational results for the excitation spectrum, we propose a distinct interpretation of the experimental inelastic neutron scattering spectra.

Mazurenko, V. V.; Kvashnin, Y. O.; Jin, Fengping; De Raedt, H. A.; Lichtenstein, A. I.; Katsnelson, M. I.

2014-06-01

141

First-principles prediction of disordering tendencies in complex oxides

The disordering tendencies of a series of zirconate (A{sub 2}Zr{sub 2}O{sub 7}) , hafnate (A{sub 2}Hf{sub 2}O{sub 7}), titanate (A{sub 2}Ti{sub 2}O{sub 7}), and stannate (A{sub 2} Sn{sub 2}O{sub 7}) pyrochlores are predicted in this study using first-principles total energy calculations. To model the disordered (A{sub 1/2}B{sub 1/2})(O{sub 7/8}/V{sub 1/8}){sub 2} fluorite structure, we have developed an 88-atom two-sublattice special quasirandom structure (SQS) that closely reproduces the most important near-neighbor intra-sublattice and inter-sublattice pair correlation functions of the random alloy. From the calculated disordering energies, the order-disorder transition temperatures of those pyrochlores are further predicted and our results agree well with the existing experimental phase diagrams. It is clearly demonstrated that both size and electronic effects play an important role in determining the disordering tendencies of pyrochlore compounds.

Jiang, Chao [Los Alamos National Laboratory; Stanek, Christopher R [Los Alamos National Laboratory; Sickafus, Kurt E [Los Alamos National Laboratory; Uberuaga, Blas P [Los Alamos National Laboratory

2008-01-01

142

First-principles study of hydrogen storage materials

NASA Astrophysics Data System (ADS)

In this thesis, we use first-principles calculations to study the structural, electronic, and thermal properties of several complex hydrides. We investigate structural and electronic properties of Na-Li alanates. Although Na alanate can reversibly store H with Ti catalyst, its weight capacity needs to be improved. This can be accomplished by partial replacement of Na with lighter elements. We explore the structures of possible Na-Li alloy alanates, and study their phase stability. We also study the structural and thermal properties of Li/Mg/Li-Mg Amides/Imides. Current experimental results give a disordered model about the structure of Li-Mg Imide, in which the positions of Li and Mg are not specified. In addition the model gives a controversial composition stoichiometry. We try to resolve this controversy by searching for low-energy ordered phases. In the last part, we study the structural, energetic, and electronic properties of the La-Mg-Pd-H system. This quaternary system is another example of hydrogenation-induced metal-nonmetal transition without major reconstruction of metal host structure, and it is also with partial reversible H capacity. Experiment gives partially disordered H occupancy on two Wyckoff positions. Our calculation explains the structural and bonding characteristics observed in experiment.

Ma, Zhu

2008-10-01

143

Equilibrium Se isotope fractionation parameters: A first-principles study

NASA Astrophysics Data System (ADS)

Several important equilibrium Se isotope fractionation parameters are investigated by first-principles calculations, involving dominant inorganic and organic Se-bearing species in gaseous, aqueous and condensed phases. Because anharmonic effects are found to be negligible for Se isotope fractionation calculation, the Bigeleisen-Mayer equation method is used without corrections beyond harmonic approximation. All calculations are made at B3LYP/6-311 + G(d,p) level, with a frequency scaling factor of 1.05. Solvation effects are carefully evaluated by the explicit solvent model (i.e. the "water droplet" method). A number of conformers are used for aqueous complexes in order to reduce the possible error coming from different configurations. Redox state is found to be an important factor controlling equilibrium Se isotope fractionations. Our results suggest a trend of heavy Se isotopes enrichment as SeO 42- > SeO 32- > HSeO 3- > SeO 2 > selenoamino acids > alkylselenides > Se(0) or H 2Se > HSe -. The Se(- II) species regardless of organic and inorganic forms can enrich extremely light Se isotopes comparing with other species. Equilibrium Se isotope fractionation factors provided in this study suggest Se isotopes can be used as a tracer of redox conditions and also useful to study Se cycling.

Li, Xuefang; Liu, Yun

2011-04-01

144

First-Principles Investigation of Ag-Doped Gold Nanoclusters

Gold nanoclusters have the tunable optical absorption property, and are promising for cancer cell imaging, photothermal therapy and radiotherapy. First-principle is a very powerful tool for design of novel materials. In the present work, structural properties, band gap engineering and tunable optical properties of Ag-doped gold clusters have been calculated using density functional theory. The electronic structure of a stable Au20 cluster can be modulated by incorporating Ag, and the HOMO–LUMO gap of Au20?nAgn clusters is modulated due to the incorporation of Ag electronic states in the HOMO and LUMO. Furthermore, the results of the imaginary part of the dielectric function indicate that the optical transition of gold clusters is concentration-dependent and the optical transition between HOMO and LUMO shifts to the low energy range as the Ag atom increases. These calculated results are helpful for the design of gold cluster-based biomaterials, and will be of interest in the fields of radiation medicine, biophysics and nanoscience. PMID:21686162

Zhang, Xiao-Dong; Guo, Mei-Li; Wu, Di; Liu, Pei-Xun; Sun, Yuan-Ming; Zhang, Liang-An; She, Yi; Liu, Qing-Fen; Fan, Fei-Yue

2011-01-01

145

First principles study of lithium insertion in bulk silicon

NASA Astrophysics Data System (ADS)

Si is an important anode material for the next generation of Li ion batteries. Here the energetics and dynamics of Li atoms in bulk Si have been studied at different Li concentrations on the basis of first principles calculations. It is found that Li prefers to occupy an interstitial site as a shallow donor rather than a substitutional site. The most stable position is the tetrahedral (Td) site. The diffusion of a Li atom in the Si lattice is through a Td-Hex-Td trajectory, where the Hex site is the hexagonal transition site with an energy barrier of 0.58 eV. We have also systematically studied the local structural transition of a LixSi alloy with x varying from 0 to 0.25. At low doping concentration (x = 0-0.125), Li atoms prefer to be separated from each other, resulting in a homogeneous doping distribution. Starting from x = 0.125, Li atoms tend to form clusters induced by a lattice distortion with frequent breaking and reforming of Si-Si bonds. When x >= 0.1875, Li atoms will break some Si-Si bonds permanently, which results in dangling bonds. These dangling bonds create negatively charged zones, which is the main driving force for Li atom clustering at high doping concentration.

Wan, Wenhui; Zhang, Qianfan; Cui, Yi; Wang, Enge

2010-10-01

146

Anisotropic intrinsic lattice thermal conductivity of phosphorene from first principles.

Phosphorene, the single layer counterpart of black phosphorus, is a novel two-dimensional semiconductor with high carrier mobility and a large fundamental direct band gap, which has attracted tremendous interest recently. Its potential applications in nano-electronics and thermoelectrics call for fundamental study of the phonon transport. Here, we calculate the intrinsic lattice thermal conductivity of phosphorene by solving the phonon Boltzmann transport equation (BTE) based on first-principles calculations. The thermal conductivity of phosphorene at 300 K is 30.15 W m(-1) K(-1) (zigzag) and 13.65 W m(-1) K(-1) (armchair), showing an obvious anisotropy along different directions. The calculated thermal conductivity fits perfectly to the inverse relationship with temperature when the temperature is higher than Debye temperature (?D = 278.66 K). In comparison to graphene, the minor contribution around 5% of the ZA mode is responsible for the low thermal conductivity of phosphorene. In addition, the representative mean free path (MFP), a critical size for phonon transport, is also obtained. PMID:25594447

Qin, Guangzhao; Yan, Qing-Bo; Qin, Zhenzhen; Yue, Sheng-Ying; Hu, Ming; Su, Gang

2015-02-01

147

Theoretical X-ray Spectroscopy of Iron Complexes.

??This thesis discusses both theoretical developments for the calculation of X-ray spectra and applications of quantum-chemical methods to the calculation and interpretation of experimental X-ray… (more)

Atkins, Andrew J.

2013-01-01

148

Theoretical Modeling for the X-ray Spectroscopy of Iron-bearing MgSiO3 under High Pressure

NASA Astrophysics Data System (ADS)

The behaviors of iron (Fe) in MgSiO3 perovskite, including valence state, spin state, and chemical environments, at high pressures are of fundamental importance for more detailed understanding the properties of the Earth's lower mantle. The pressure induced spin transition of Fe-bearing MgO and MgSiO3 are detected often by using high-resolution K-edge X-ray emission spectroscopy (XES) [1,2,3] and confirmed by theoretical simulations. [4,5] Since the Fe K-edge XES is associated to the 3p orbital, which is far from the valence orbitals (3d and 4s), it provides no information about its coordination environments. However, the Fe L-edge XES and X-ray absorption spectroscopy (XAS) can directly present the distribution and intensity of Fe-3d character. To identify both the spin states and the coordination environments of iron-bearing MgSiO3, we systematically investigate the L-edge XAS, XES and X-ray photoelectron (XPS) spectroscopy of Fe2+- and Fe3+-bearing MgSiO3 under high pressure by using the first-principles density functional method combined with the slater-transition method. Our results show that Fe2+ and Fe3+ can be distinguished easily by taking the XPS spectra. The spin transition of Fe2+ and Fe3+ can also be clearly certified by XAS and XES. Interestingly, the broadness of L-edge XES of Fe changes depending on the iron position, meaning that its coordination environment might also be distinguishable by using high-resolution XES measurements. Research supported by the Ehime University G-COE program and KAKENHI. [1] James Badro, Guillaume Fiquet, FranÇois Guyot, Jean-Pascal Rueff, Viktor V. Struzhkin, György VankÓ, and Giulio Monaco. Science 300, 789 (2003), [2] James Badro, Jean-Pascal Rueff, György VankÓ, Giulio Monaco, Guillaume Fiquet, and FranÇois Guyot, Science 305, 383 (2004), [3] Jung-Fu Lin, Viktor V. Struzhkin, Steven D. Jacobsen, Michael Y. Hu, Paul Chow, Jennifer Kung, Haozhe Liu, Ho-kwang Mao, and Gussell J. Hemley, Nature 436, 377 (2005). [4] Taku Tsuchiya, Renata M. Wentzcovitch, Cesar R.S. da Silva, and Stefano de Gironcoli, Phys. Rev. Lett. 96, 198501 (2006). [4] Han Hsu, Peter Blaha, Matteo Cococcioni, and Renata M. Wentzcovitch, Phys. Rev. Lett. 106, 118501 (2011).

Wang, X.; Tsuchiya, T.

2012-12-01

149

Experimental and first principle studies on electronic structure of BaTiO{sub 3}

We have carried out photoemission experiments to obtain valence band spectra of various crystallographic symmetries of BaTiO{sub 3} system which arise as a function of temperature. We also present results of a detailed first principle study of these symmetries of BaTiO{sub 3} using generalized gradient approximation for the exchange-correlation potential. Here we present theoretical results of density of states obtained from DFT based simulations to compare with the experimental valence band spectra. Further, we also perform calculations using post density functional approaches like GGA + U method as well as non-local hybrid exchange-correlation potentials like PBE0, B3LYP, HSE in order to understand the extent of effect of correlation on band gaps of different available crystallographic symmetries (5 in number) of BaTiO{sub 3}.

Sagdeo, Archna, E-mail: archnaj@rrcat.gov.in; Ghosh, Haranath, E-mail: archnaj@rrcat.gov.in; Chakrabarti, Aparna, E-mail: archnaj@rrcat.gov.in; Kamal, C., E-mail: archnaj@rrcat.gov.in; Ganguli, Tapas, E-mail: archnaj@rrcat.gov.in; Deb, S. K. [Indus Synchrotrons Utilization Division, Raja Ramanna Centre for Advanced Technology, Indore 452013 (India); Phase, D. M. [IUC-DAEF, University Campus, Khandwa Road, Indore-452017 (India)

2014-04-24

150

NASA Astrophysics Data System (ADS)

We estimate the anomalous Hall effect of selected Heusler alloys from first principles. An emphasis is put on the effect of the native disorder which is often present in the stoichiometric samples. Such disorder can strongly influence both the magnetic and transport properties of these alloys. We employ a recently developed fully relativistic Kubo-St?eda approach adapted to disordered multisublattice systems in which the chemical disorder is described in terms of the coherent potential approximation. As case studies we choose half-metallic Heusler alloys Co2CrAl and Co2MnAl, as well as the spin gapless semiconductor alloy Mn2CoAl for which experimental and theoretical studies appeared recently. We demonstrate that proper inclusion of disorder significantly improves agreement between the experiment and theory.

Kudrnovský, J.; Drchal, V.; Turek, I.

2013-07-01

151

Isotope dependence of the vibrational lifetimes of light impurities in Si from first principles

NASA Astrophysics Data System (ADS)

The vibrational lifetimes of a range of H-related defects and interstitial O (Oi) in Si, including isotopic substitutions, are calculated from first principles as a function of temperature. The theoretical approach is explained in detail. The vibrational lifetimes of highest-frequency local vibrational modes of HBC+ , D2* , HD* , DH* , HBC+ , DBC+ , HV•VH , DV•VH , DV•VD , IH2 , ID2 , and various O and Si isotopic combinations of Oi are predicted and the decay channels analyzed. We show that the complete vibrational spectrum of the defects must be known in order to predict vibrational lifetimes. We also show that the “frequency-gap law” is not always valid for high-frequency local vibrational modes.

West, D.; Estreicher, S. K.

2007-02-01

152

Structural instabilities and mechanical properties of U2Mo from first principles calculations.

We perform detailed first principles calculations of the structural parameters at zero pressure and high pressure, the elastic properties, phonon dispersion relation, and ideal strengths of U2Mo with the C11b structure. In contrast to previous theoretical studies, we show that the I4/mmm structure is indeed a mechanically and dynamically unstable phase, which is confirmed by the negative elastic constant C66 as well as the imaginary phonon modes observed along the ?1-N-P line. The calculations of ideal strengths for U2Mo are performed along the [100], [001], and [110] directions for tension and on (001)[010] and (010)[100] slip systems for shear load. The ideal shear strength is about 8.1 GPa, much smaller than a tension of 18-28 GPa, which indicates that the ductile U2Mo alloy will fail by shear rather than by tension. PMID:25560204

Liu, Ben-Qiong; Duan, Xiao-Xi; Sun, Guang-Ai; Yang, Jin-Wen; Gao, Tao

2015-01-28

153

Structural, elastic and thermal properties of curium mono pnictides: A first principles study

NASA Astrophysics Data System (ADS)

We study theoretically the pressure induced solid-solid phase transitions and elastic properties of curium monopnictides (CmX; X=N, P and As) by using first principles calculations based on density functional theory (DFT) within the local spin density approximation (LSDA). We predict the first order structural phase transition from their ambient NaCl(B1) type structure to CsCl(B2) type structure at 111.80, 36.81 and 22.08 GPa for CmN, CmP and CmAs respectively. The calculated structural parameters like lattice constant (a0), bulk modulus (B) and pressure derivative of bulk modulus (B') are reported in B1 and B2 phases and compared with the available results. We present first time the elastic constants for curium monopnictides.

Devi, Hansa; Pagare, Gitanjali; Chouhan, Sunil S.; Sanyal, Sankar P.

2013-06-01

154

Equation of state of Al for compressed and expanded states from first-principles calculations

NASA Astrophysics Data System (ADS)

Results of theoretical calculations of equation of state and critical temperature of Al are presented using a three-term EOS model. In this model cold (0 K) term is calculated from first-principles method near normal conditions. Cold curve is extrapolated to ultrahigh pressures using Thomas- Fermi-Dirac model and to expanded states using a soft sphere function. Electron thermal term is calculated using Thomas-Fermi statistical model. Ion-thermal term is calculated using the modified Cowan model. In expanded state, the adjustable parameters of the modified Cowan model are tuned using quantum molecular dynamics (QMD) results. In compressed state, P-? and Us-Up Hugoniots derived using our results show good agreement with the reported experimental results. In expanded state, the estimated critical temperature shows good agreement with the reported results and pressure versus internal energy along isochores show reasonably good agreement with the reported experimental results.

Mishra, Vinayak; Chaturvedi, Shashank

2012-07-01

155

The electronic structure and hydrogen storage capability of Yttrium-doped grapheme has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom prefers the hollow site of the hexagonal ring with a binding energy of 1.40 eV. Doping by Y makes the system metallic and magnetic with a magnetic moment of 2.11 ?{sub B}. Y decorated graphene can adsorb up to four hydrogen molecules with an average binding energy of 0.415 eV. All the hydrogen atoms are physisorbed with an average desorption temperature of 530.44 K. The Y atoms can be placed only in alternate hexagons, which imply a wt% of 6.17, close to the DoE criterion for hydrogen storage materials. Thus, this system is potential hydrogen storage medium with 100% recycling capability.

Desnavi, Sameerah, E-mail: sameerah-desnavi@zhcet.ac.in [Department of Electronic Engineering, ZHCET, Aligarh Muslim University, Aligarh-202002 (India); Chakraborty, Brahmananda; Ramaniah, Lavanya M. [High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai-400085 (India)

2014-04-24

156

First-principles elastic stiffness of LaPO4 monazite

NASA Astrophysics Data System (ADS)

In this letter, the full set of elastic coefficients of LaPO4 monazite is presented based on the first-principles plane-wave pseudopotential total energy method. Mechanical parameters (bulk modulus, shear modulus, Young's moduli, and Poisson's ratio) are also presented and compared with experimental results for polycrystalline monazite. The responses of electronic structure and chemical bonds to a series of {010}?001? shear strains are examined in order to study the mechanism of low shear strain resistance. The results show that small shear moduli originate from the inhomogeneous strengths of atomic bonds. For example, the weak La-O bonds accommodate the shear strain locally, while the PO4 tetrahedra are almost rigid. The theoretical elastic stiffness may be useful to understand the deformation mechanisms of LaPO4 monazite.

Wang, Jingyang; Zhou, Yanchun; Lin, Zhijun

2005-08-01

157

Supercapacitors composed of graphene and boron nitride layers: A first-principles study

We propose a model for nanoscale supercapacitor consisting of two-dimensional insulating BN layers placed between two commensurate and metallic graphene layers. First-principles Density Functional calculations of structure optimized total energy and self-consistent field potential performed on these nanoscale capacitors for different levels of charging and different number of BN layers mark the values of capacitance per unit mass, which are larger than those measured values for the supercapacitors made from other carbon based materials. Our theoretical study also compares results obtained for the present nanoscale capacitor with those of classical Helmholtz model and reveals crucial differences. Our model allows the fabrication of series/parallel mixed combinations consisting of epitaxially grown, sequential and multiple graphene/BN sheets.

Özçelik, V Ongun; Ciraci, S

2012-01-01

158

Theoretical study of the spectroscopy of Al2(+)

NASA Technical Reports Server (NTRS)

The electronic states of Al2(+) below about 40,000/cm are studied using a CASSCF/MRCI approach in a large Gaussian basis set. The computed spectroscopic constants, excitation energies, Einstein coefficients, and radiative lifetimes should give insight into the spectroscopy of this ion.

Rosi, Marzio; Bauschlicher, Charles W., Jr.; Langhoff, Stephen R.

1991-01-01

159

First principles explanation of the positive Seebeck coefficient of lithium.

Lithium is one of the simplest metals, with negative charge carriers and a close reproduction of free-electron dispersion. Experimentally, however, Li is one of a handful of elemental solids (along with Cu, Ag, and Au) where the sign of the Seebeck coefficient (S) is opposite to that of the carrier. This counterintuitive behavior still lacks a satisfactory interpretation. We calculate S fully from first principles, within the framework of Allen's formulation of Boltzmann transport theory. Here it is crucial to avoid the constant relaxation time approximation, which gives a sign for S which is necessarily that of the carriers. Our calculated S are in excellent agreement with experimental data, up to the melting point. In comparison with another alkali metal, Na, we demonstrate that within the simplest nontrivial model for the energy dependency of the electron lifetimes, the rapidly increasing density of states (DOS) across the Fermi energy is related to the sign of S in Li. The exceptional energy dependence of the DOS is beyond the free-electron model, as the dispersion is distorted by the Brillouin zone edge; this has a stronger effect in Li than other alkali metals. The electron lifetime dependency on energy is central, but the details of the electron-phonon interaction are found to be less important, contrary to what has been believed for several decades. Band engineering combined with the mechanism exposed here may open the door to new "ambipolar" thermoelectric materials, with a tunable sign for the thermopower even if either n- or p-type doping is impossible. PMID:24877957

Xu, Bin; Verstraete, Matthieu J

2014-05-16

160

A digitally reconstructed radiograph algorithm calculated from first principles

Purpose: To develop an algorithm for computing realistic digitally reconstructed radiographs (DRRs) that match real cone-beam CT (CBCT) projections with no artificial adjustments. Methods: The authors used measured attenuation data from cone-beam CT projection radiographs of different materials to obtain a function to convert CT number to linear attenuation coefficient (LAC). The effects of scatter, beam hardening, and veiling glare were first removed from the attenuation data. Using this conversion function the authors calculated the line integral of LAC through a CT along rays connecting the radiation source and detector pixels with a ray-tracing algorithm, producing raw DRRs. The effects of scatter, beam hardening, and veiling glare were then included in the DRRs through postprocessing. Results: The authors compared actual CBCT projections to DRRs produced with all corrections (scatter, beam hardening, and veiling glare) and to uncorrected DRRs. Algorithm accuracy was assessed through visual comparison of projections and DRRs, pixel intensity comparisons, intensity histogram comparisons, and correlation plots of DRR-to-projection pixel intensities. In general, the fully corrected algorithm provided a small but nontrivial improvement in accuracy over the uncorrected algorithm. The authors also investigated both measurement- and computation-based methods for determining the beam hardening correction, and found the computation-based method to be superior, as it accounted for nonuniform bowtie filter thickness. The authors benchmarked the algorithm for speed and found that it produced DRRs in about 0.35 s for full detector and CT resolution at a ray step-size of 0.5 mm. Conclusions: The authors have demonstrated a DRR algorithm calculated from first principles that accounts for scatter, beam hardening, and veiling glare in order to produce accurate DRRs. The algorithm is computationally efficient, making it a good candidate for iterative CT reconstruction techniques that require a data fidelity term based on the matching of DRRs and projections. PMID:23298093

Staub, David; Murphy, Martin J.

2013-01-01

161

First-principles calculations of dynamic permeability in porous media

NASA Astrophysics Data System (ADS)

By starting from the linearized Navier-Stokes equation for the fluid and the elastic wave equation for the solid frame of a porous medium, the first-principles definition of the frequency-dependent permeability ?(?) and the recipe for its calculation are derived through the application of the homogenization procedure. It is shown systematically that, in the limit of wavelength much larger than the typical pore size, the fluid may be regarded as incompressible in the permeability calculation, and the solid-frame displacement acts as an additional pressure source term for the fluid flow. The physics underlying the generic asymptotic frequency dependence of ?(?) is introduced through its analytic solution in the case of a cylindrical tube. To calculate ?(?) for periodic porous media models, we have formulated a finite-element approach for the numerical solution of the incompressible fluid equations at low and intermediate frequencies and of the Laplace equation at high frequencies. The numerical results for the sinusoidally modulated tube, the fused-spherical-bead lattice, and the fused-diamond lattice indicate a large range of values for the static permeability ?0 as well as the other asymptotic parameters such as the tortuosity ? and the surface length parameter ? as defined by Johnson et al. However, despite their variability, almost all the numerical data on periodic models are shown to satisfy the approximate scaling relation ?(?)~=?0f(?/?0), where ?0 is a characteristic frequency particular to the medium, and f is a universal function independent of microstructures. We advance arguments that delineate the physical reason for this scaling behavior as well as the condition for its validity. The scaling prediction is then generalized to the case of random porous media through both numerical simulations and the critical-path argument. Our theory gives a simple explanation to the observed correlations in sedimentary rocks, and the scaling prediction is supported by experimental ?(?) measurements on fused glass beads and crushed-glass samples.

Zhou, Min-Yao; Sheng, Ping

1989-06-01

162

A digitally reconstructed radiograph algorithm calculated from first principles

Purpose: To develop an algorithm for computing realistic digitally reconstructed radiographs (DRRs) that match real cone-beam CT (CBCT) projections with no artificial adjustments. Methods: The authors used measured attenuation data from cone-beam CT projection radiographs of different materials to obtain a function to convert CT number to linear attenuation coefficient (LAC). The effects of scatter, beam hardening, and veiling glare were first removed from the attenuation data. Using this conversion function the authors calculated the line integral of LAC through a CT along rays connecting the radiation source and detector pixels with a ray-tracing algorithm, producing raw DRRs. The effects of scatter, beam hardening, and veiling glare were then included in the DRRs through postprocessing. Results: The authors compared actual CBCT projections to DRRs produced with all corrections (scatter, beam hardening, and veiling glare) and to uncorrected DRRs. Algorithm accuracy was assessed through visual comparison of projections and DRRs, pixel intensity comparisons, intensity histogram comparisons, and correlation plots of DRR-to-projection pixel intensities. In general, the fully corrected algorithm provided a small but nontrivial improvement in accuracy over the uncorrected algorithm. The authors also investigated both measurement- and computation-based methods for determining the beam hardening correction, and found the computation-based method to be superior, as it accounted for nonuniform bowtie filter thickness. The authors benchmarked the algorithm for speed and found that it produced DRRs in about 0.35 s for full detector and CT resolution at a ray step-size of 0.5 mm. Conclusions: The authors have demonstrated a DRR algorithm calculated from first principles that accounts for scatter, beam hardening, and veiling glare in order to produce accurate DRRs. The algorithm is computationally efficient, making it a good candidate for iterative CT reconstruction techniques that require a data fidelity term based on the matching of DRRs and projections.

Staub, David; Murphy, Martin J. [Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia 23298 (United States)

2013-01-15

163

Theoretical analysis of a quartz-enhanced photoacoustic spectroscopy sensor

Quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors are based on a recent approach to photoacoustic detection which\\u000a employs a quartz tuning fork as an acoustic transducer. These sensors enable detection of trace gases for air quality monitoring,\\u000a industrial process control, and medical diagnostics. To detect a trace gas, modulated laser radiation is directed between\\u000a the tines of a tuning fork. The optical energy

N. Petra; J. Zweck; A. A. Kosterev; S. E. Minkoff; D. Thomazy

2009-01-01

164

Doping of rhenium disulfide monolayers: a systematic first principles study.

The absence of a direct-to-indirect band gap transition in ReS2 when going from the monolayer to bulk makes it special among the other semiconducting transition metal dichalcogenides. The functionalization of this promising layered material emerges as a necessity for the next generation technological applications. Here, the structural, electronic, and magnetic properties of substitutionally doped ReS2 monolayers at either the S or Re site were systematically studied by using first principles density functional calculations. We found that substitutional doping of ReS2 depends sensitively on the growth conditions of ReS2. Among the large number of non-metallic atoms, namely H, B, C, Se, Te, F, Br, Cl, As, P, and N, we identified the most promising candidates for n-type and p-type doping of ReS2. While Cl is an ideal candidate for n-type doping, P appears to be the most promising candidate for p-type doping of the ReS2 monolayer. We also investigated the doping of ReS2 with metal atoms, namely Mo, W, Ti, V, Cr, Co, Fe, Mn, Ni, Cu, Nb, Zn, Ru, Os and Pt. Mo, Nb, Ti, and V atoms are found to be easily incorporated in a single layer of ReS2 as substitutional impurities at the Re site for all growth conditions considered in this work. Tuning chemical potentials of dopant atoms energetically makes it possible to dope ReS2 with Fe, Co, Cr, Mn, W, Ru, and Os at the Re site. We observe a robust trend for the magnetic moments when substituting a Re atom with metal atoms such that depending on the electronic configuration of dopant atoms, the net magnetic moment of the doped ReS2 becomes either 0 or 1 ?B. Among the metallic dopants, Mo is the best candidate for p-type doping of ReS2 owing to its favorable energetics and promising electronic properties. PMID:25001566

Çak?r, Deniz; Sahin, Hasan; Peeters, François M

2014-08-21

165

First principles evaluation of the photocatalytic properties of cuprous oxide

NASA Astrophysics Data System (ADS)

Cuprous oxide (Cu2O) is a semiconductor attractive for use as a photocatalyst in renewable fuel production, but has thus far exhibited low efficiencies in solar energy technologies. A thorough understanding of its photocatalytically relevant properties is needed to develop improved cuprous oxide-based photocatalysts. This dissertation uses first principles calculations founded in quantum mechanics to study the physical, optical, electronic, and chemical properties of cuprous oxide and to optimize its performance in solar energy applications. The key properties that affect efficiency include electronic excitations, the band gap, band edge positions, charge transport, defect trap states, catalyst stability, and surface chemistry. The band gap of Cu 2O, which defines the efficiency of solar energy absorption, is first calculated with hybrid density functional theory (DFT) followed by a single GW perturbation. We also design methods to calculate optical excitations using embedded correlated wavefunction theory. The low-index surfaces are characterized using DFT+U, where we identify the (111) surface as the most stable. This surface is employed in the derivation of the band edges of Cu2O, which demonstrate that Cu2O can provide the thermodynamic overpotential needed to drive water splitting and the reduction of CO2 to methanol. We also identify the adsorption mechanisms of weakly physisorbed CO2 and the more strongly adsorbed H2O on the Cu2O(111) surface. Effective charge transport is needed so that photoexcited carriers can reach the surface active sites prior to recombination. We study electron and hole transport in Cu2O using the small polaron model, and show that its localized description is inappropriate for carrier transport, which is better modeled using band theory. We then use an approach founded in band theory to analyze the cause of intrinsic trap states, which promote carrier recombination. We conclude that doping with Li can prevent trap state formation and thus reduce recombination. Finally, because Cu2O is known to be photocathodically unstable, we consider a suggested method of stabilizing Cu2O via deposition on a ZnO substrate. We evaluate the properties of the Cu2O(111)/ZnO(101¯0) interface, revealing that it is weakly bound. The ZnO substrate reduces the band gap of the Cu2O coating.

Bendavid, Leah Isseroff

166

Photoexcitation and Photochemical Stability of Organic Photovoltaic Materials from First Principles

NASA Astrophysics Data System (ADS)

The development of high efficiency organic photovoltaics (OPV) has recently become enabled by the synthesis of new conjugated polymers with low band gap that allow light absorption over a broader range of the spectrum. Stability of these new polymers, a key requirement for commercialization, has not yet received sufficient attention. Here, we report first-principles theoretical modeling of photo-induced degradation of OPV polymers carried out using ab-initio density functional theory (DFT). We report photooxidation routes and reaction products for reactive species including superoxide oxygen anions and hydroxyl groups interacting with the standard workhorse OPV polymer, poly(3-hexyl-thiophene) (P3HT). We discuss theoretical issues and challenges affecting the modeling such reactions in OPV polymers. We also discuss the application of theoretical methods to low-band-gap polymers, and in particular, the effect of the chemical substitution on the photoexcitation properties of these new polymers. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Deparment of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Sai, Na; Leung, Kevin

2013-03-01

167

First principle optoelectronic studies of visible light sensitive CZT

NASA Astrophysics Data System (ADS)

The most attractive optical material in solar cells applications, CdZnTe (CZT), has been extensively studied both, experimentally and theoretically. The results obtained by various theoretical models are way too far from the experiments and hence, they fail to demonstrate the true theoretical picture of the compound. In the present study the electronic band gap, band gap bowing and optical properties of Cd1-xZnxTe (0 ? x ? 1) are revisited with an effective theoretical model to provide accurate theoretical foundations to the experimental results. The calculations are performed with the full potential linearized augmented plane wave (FPLAPW) method with the generalized gradient approximation + modified Becke-Johnson (GGAmBJ) potential. The calculated results are in agreement with the experimental results. The band gap increases non-lineally with the concentration. The optical properties of the compound like absorption coefficient, refractive index, reflectivity, energy loss function and oscillator strength are also calculated. The number of effective electrons in optical excitations decreases with the increase in concentration of Zn.

Khan, Imad; Aliabad, H. A. Rahnamaye; Ahmad, Waqar; Ali, Zahid; Ahmad, Iftikhar

2013-11-01

168

Experimental and theoretical investigation on the vibrational spectroscopy of l-theanine

In this work, experimental and theoretical investigations on vibrational spectroscopy of l-theanine were presented. FT-IR and Raman spectra of l-theanine powder sample were recorded and corresponding theoretical calculations were performed based on Density Functional Theory (DFT) at B3LYP level using 6-31++G(d,p) and 6-311++G(d,p) basis sets combined with the Polarized Continuum Model (PCM) with water as the solvent. The experimental vibrational

Yongjian Chen; Gangqin Xi; Rong Chen; Yongzeng Li; Shangyuan Feng; Jinping Lei; Hongxing Lin

169

Molecular species with electron affinities (EAs) larger than that of the chlorine atom (3.6131 eV) are superhalogens. The corresponding negative ions, namely, superhalogen anions, are intrinsically very stable with high electron binding energies (EBEs), and widely exist as building blocks of bulk materials and ionic liquids. The most common superhalogen anions proposed and confirmed to date are either ionic salts or compact inorganic species. Herein we report a new class of superhalogen species, a series of tetracoordinated organoboron anions [BL4]– (L = phenyl (1), 4-fluorophenyl (2), 1-imidazolyl (3), L4 = H(pyrazolyl)3 (4)) with bulky organic ligands covalently bound to the central B atom. Negative ion photoelectron spectroscopy (NIPES) reveals all of these anions possessing EBEs higher than that of Cl- with the adiabatic / vertical detachment energy (ADE / VDE) of 4.44/4.8 (1), 4.78/5.2 (2), 5.08/5.4 (3), and 4.59/4.9 eV (4), respectively. First-principles calculations confirmed high EBEs of [BL4]– and predicted that these anions are thermodynamically stable against fragmentation. The unraveled superhalogen nature of these species provides a molecular basis to explain the wide-range applications of tetraphenylborate (TPB) (1) and trispyrazolylborate (Tp) (4) in many areas spanning from industrial waste treatment to soft material synthesis and organometallic chemistry

Zhang, Jian; Yang, Ping; Sun, Zhenrong; Wang, Xue B.

2014-09-18

170

NASA Astrophysics Data System (ADS)

The electronic structures and structural properties of body-centered cubic Ti-Mo alloys were studied by first-principles calculations. The special quasirandom structures (SQS) model was adopted to emulate the solid solution state of the alloys. The valence band electronic structures of Ti-Mo and Ti-Mo-Fe alloys were measured by hard x-ray photoelectron spectroscopy. The structural parameters and valence band photoelectron spectra were calculated using first-principles calculations. The results obtained with the SQS models showed better agreement with the experimental results than those obtained using the conventional ordered structure models. This indicates that the SQS model is effective for predicting the various properties of solid solution alloys by means of first-principles calculations.

Sahara, Ryoji; Emura, Satoshi; Seiichiro, Ii; Ueda, Shigenori; Tsuchiya, Koichi

2014-06-01

171

NASA Astrophysics Data System (ADS)

Using multi-ion interatomic potentials derived from first-principles generalized pseudopotential theory, we have been studying point defects and dislocations in bcc transition metals, with molybdenum (Mo) as a prototype. For point defects in Mo, the calculated vacancy formation and activation energies are in excellent agreement with experimental results. The energetics of six self-interstitial configurations in Mo have also been investigated. The <110> split dumb-bell is found to have the lowest formation energy, as is experimentally observed, but the corresponding migration energy is calculated to be 3 15 times higher than previous theoretical estimates. The atomic structure and energetics of <1l1> screw dislocations in Mo are now being investigated. We have found that the ‘easy’ core configuration has a lower formation energy than the ‘hard’ one, consistent with previous theoretical studies. The former has a distinctive threefold symmetry with a spread-out of the dislocation core along the <112> directions, an effect which is driven by the strong angular forces present in these metals.

Xu, Wei; Moriarty, John A.

1996-08-01

172

Using multi-ion interatomic potentials derived from first-principles generalized pseudopotential theory, we have been studying point defects and dislocations in bcc transition metals, with molybdenum (Mo) as a prototype. For point defects in Mo, the calculated vacancy formation and activation energies are in excellent agreement with experimental results. The energetics of six self-interstitial configurations in Mo have also been investigated. The <110> split dumb-bell is found to have the lowest formation energy, as is experimentally observed, but the corresponding migration energy is calculated to be 3--15 times higher than previous theoretical estimates. The atomic structure and energetics of <111> screw dislocations in Mo are now being investigated. We have found that the ``easy`` core configuration has a lower formation energy than the ``hard`` one, consistent with previous theoretical studies. The former has a distinctive 3-fold symmetry with a spread out of the dislocation core along the <112> directions, an effect which is driven by the strong angular forces present in these metals.

Xu, W; Moriarty, J.A.

1996-01-19

173

NASA Astrophysics Data System (ADS)

Using multi-ion interatomic potentials derived from first-principles generalized pseudopotential theory, we are studying point defects and dislocations in molybdenum (Mo). For point defects, the calculated vacancy formation and activation energies are in excellent agreement with experimental results. In ascending order, the sequence of energetically stable self- interstitials is: <110> split dumb-bell, tetrahedral site, crowdion, <111> split dumb-bell, <100> split dumb-bell, and octahedral site. In addition, the migration paths for the <110> dumb-bell interstitial have been studied. The migration energy is found to be one order of magnitude higher than previous theoretical estimates obtained using a Finnis-Sinclair potential for Mo. The energetics and core structure of <111> screw dislocations in Mo are being investigated. We have found that the ``easy" core configuration has lower formation energy than the ``hard" one, consistent with previous theoretical studies. The differential displacement method has been used to elaborate the details of both dislocation configurations, revealing some new and interesting features. Calculation of the Peierls barrier is in progress and extention of the work to tantalum (Ta) is planned.

Xu, Wei; Moriarty, J. A.

1996-03-01

174

A first principles method to simulate electron mobilities in 2D materials

NASA Astrophysics Data System (ADS)

We examine the predictive capabilities of first-principles theoretical methods to calculate the phonon- and impurity-limited electron mobilities for a number of technologically relevant two-dimensional materials in comparison to experiment. The studied systems include perfect graphene, graphane, germanane and MoS2, as well as graphene with vacancies, and hydrogen, gold, and platinum adsorbates. We find good agreement with experiments for the mobilities of graphene (? = 2 × 105 cm2 V-1s-1) and graphane (? = 166 cm2 V-1s-1) at room temperature. For monolayer MoS2 we obtain ? = 225 cm2 V-1s-1. This value is higher than what is observed experimentally (0.5-200 cm2 V-1s-1) but is on the same order of magnitude as other recent theoretical results. For bulk MoS2 we obtain ? = 48 cm2 V-1s-1. We obtain a very high mobility of 18 200 cm2 V-1s-1 for single-layer germanane. The calculated reduction in mobility from the different impurities compares well to measurements where experimental data are available, demonstrating that the proposed method has good predictive capabilities and can be very useful for validation and materials design.

Restrepo, Oscar D.; Krymowski, Kevin E.; Goldberger, Joshua; Windl, Wolfgang

2014-10-01

175

First principles effective electronic couplings for hole transfer in natural and size-expanded DNA

Hole transfer processes between base pairs in natural DNA and size-expanded DNA (xDNA) are studied and compared, by means of an accurate first principles evaluation of the effective electronic couplings (also known as transfer integrals), in order to assess the effect of the base augmentation on the efficiency of charge transport through double-stranded DNA. According to our results, the size expansion increases the average electronic coupling, and thus the CT rate, with potential implications in molecular biology and in the implementation of molecular nanoelectronics. Our analysis shows that the effect of the nucleobase expansion on the charge-transfer (CT) rate is sensitive to the sequence of base pairs. Furthermore, we find that conformational variability is an important factor for the modulation of the CT rate. From a theoretical point of view, this work offers a contribution to the CT chemistry in ?-stacked arrays. Indeed, we compare our methodology against other standard computational frameworks that have been adopted to tackle the problem of CT in DNA, and unravel basic principles that should be accounted for in selecting an appropriate theoretical level. PMID:19537767

Migliore, Agostino; Corni, Stefano; Varsano, Daniele; Klein, Michael L.; Di Felice, Rosa

2009-01-01

176

A first-principles measure for the twinnability of FCC metals

NASA Astrophysics Data System (ADS)

Twinnability is the property describing the ease with which a metal plastically deforms by twinning relative to deforming by dislocation-mediated slip. In this paper a theoretical measure for twinnability in face-centered-cubic (fcc) metals is obtained through homogenization of a recently introduced criterion for deformation twinning (DT) at a crack tip in a single crystal. The DT criterion quantifies the competition between slip and twinning at the crack tip as a function of crack orientation and applied loading. The twinnability of bulk material is obtained by constructing a representative volume element of the material as a polycrystal containing a distribution of microcracks and integrating the DT criterion over all possible grain and microcrack orientations. The resulting integral expression depends weakly on Poisson's ratio and significantly on three interfacial energies: the stacking-fault energy, the unstable-stacking energy and the unstable-twinning energy. All these four quantities can be computed from first principles. The weak dependence on Poisson's ratio is exploited to derive a simple and accurate closed-form approximation for twinnability which clarifies its dependence on the remaining material parameters. To validate the new measure, the twinnability of eight pure fcc metals is computed using parameters obtained from quantum-mechanical tight-binding calculations. The ranking of these materials according to their theoretical twinnability agrees with the available experimental evidence, including the low incidence of DT in Al, and predicts that Pd should twin as easily as Cu.

Tadmor, E. B.; Bernstein, N.

2004-11-01

177

First-principles elastic properties of (alpha)-Pu

Density-functional electronic structure calculations have been used to investigate the ambient pressure and low temperature elastic properties of the ground-state {alpha} phase of plutonium metal. The electronic structure and correlation effects are modeled within a fully relativistic anti-ferromagnetic treatment with a generalized gradient approximation for the electron exchange and correlation functionals. The 13 independent elastic constants, for the monoclinic {alpha}-Pu system, are calculated for the observed geometry. A comparison of the results with measured data from resonant ultrasound spectroscopy for a cast sample is made.

Soderlind, P; Klepeis, J E

2008-11-04

178

-molecular assemblies in the brain of Alzheimer's disease patients. In fact, amyloid fibrils are found in more than 20Why Are Diphenylalanine-Based Peptide Nanostructures so Rigid? Insights from First Principles from first principles, using density functional theory with dispersive corrections. The calculation

Hod, Oded

179

Density limit in a first principles model of a magnetized plasma

Density limit in a first principles model of a magnetized plasma in the DebyeÂHÂ¨uckel approximation first principles model" Dipartimento di Matematica, Universit`a degli Studi di Milano, Milano, Italy page 382 of the first scientific paper [3] of Bohr), a plasma can present a diamagnetic behavior (and

Carati, Andrea

180

Mechanisms of Li+ diffusion in crystalline -and -Li3PO4 electrolytes from first principles

Mechanisms of Li+ diffusion in crystalline - and -Li3PO4 electrolytes from first principles Yaojun electrolytes for use in batteries and related technologies. We have used first-principles modeling techniques diffusion mechanisms in idealized crystals of the electrolyte material Li3PO4 in both the and crystalline

Holzwarth, Natalie

181

First principles simulations of Li ion migration in materials related to LiPON electrolytes

First principles simulations of Li ion migration in materials related to LiPON electrolytes Y. A electrolytes, we have carried out first principles calculations of several related crystalline materials. Sim state film electrolyte LiPON, developed at Oak Ridge National Laboratory,[1, 2] is the most widely used

Holzwarth, Natalie

182

Spin-orbital excitations in correlated materials: a theoretical spectroscopy approach

Spin-orbital excitations in correlated materials: a theoretical spectroscopy approach M.Gatti (LSI. The goal of this thesis is to address the role played by the spin and orbital degrees of freedom in the determination of the electronic and optical properties of strongly correlated materials. Prototypical compounds

Botti, Silvana

183

Vibrational Sum-Frequency Spectroscopy of Alkane/Water Interfaces: Experiment and Theoretical, 2002 The vibrational spectra of interfacial water at a series of alkane/water interfaces have been to characterize the water-water and alkane-water interactions present at these hydrophobic/aqueous interfaces

Richmond, Geraldine L.

184

Design and Characterization of Photoelectrodes from First Principles

Although significant performance improvements have been realized since the first demonstration of sunlight-driven water splitting in 1972, mainstream adoption of photoelectrochemical (PEC) cells remains limited by an absence of cost-effective electrodes that show simultaneously high conversion efficiency and good durability. Here we outline current and future efforts to use advanced theoretical techniques to guide the development of a durable, high-performance PEC electrode material. Working in close collaboration with experimental synthesis and characterization teams, we use a twofold approach focusing on: (1) rational design of novel high-performance electrode materials; and (2) characterization and optimization of the electrode-electrolyte interface.

Ogitsu, T; Wood, B; Choi, W; Huda, M; Wei, S

2012-05-11

185

First-principle study of the physics properties of DO3-Mg3Nd compound under high pressure

NASA Astrophysics Data System (ADS)

A theoretical investigation on the structural, phonon, elastic and thermodynamic properties of the DO3-Mg3Nd compound has been conducted through the first-principles calculations within the frame of the density functional theory (DFT) and the density functional perturbation theory (DFPT) in the VASP and PHONONPY code. We calculate the phonon spectrum and phonon density of states under different pressures and find that the DO3-Mg3Nd compound keeps dynamically stable up to 100 GPa. The elastic constants and thermodynamic quantities as a function of pressures and temperatures are also reported and discussed.

Zhang, Xudong; Ying, Caihong; Jiang, Wei; Shi, Haifeng

2014-09-01

186

We present calculated second-harmonic-generation (SHG) spectra of the Si(001) surface based on a first-principles description of eigenvalues and eigenvectors using ab initio pseudopotentials. We also present SHG spectra for Ge-covered Si(001). The theoretical results explain all essential features of recent experimental SHG spectra of the Si(001)-(2x1) surface with low coverages of hydrogen and/or germanium, which alter the E{sub 1} resonance in contrasting ways. The strong adatom specificity of the spectra results from redistribution of the adatom-related electronic states on the surface.

Gavrilenko, V. I.; Wu, R. Q.; Downer, M. C.; Ekerdt, J. G.; Lim, D.; Parkinson, P.

2001-04-15

187

NASA Astrophysics Data System (ADS)

The structural, electronic and mechanical properties of binary B2 - type CsCl structured intermetallic compounds of Ag (ReAg, Re= Y, La, Pr and Er) have been studies systematically by means of first principles density functional theory within generalized gradient approximation. Ground state properties such as lattice constant (a0), bulk modulus (B) and its pressure derivative (B') are obtained. The present results are in good agreement with the experimental and other theoretical calculation available. Amongst all the Intermetallics is found ErAg to be most ductile due to the presence of strong metallic bonding.

Sahu, A.; Makode, Chandrabhan; Pataiya, J.; Sanyal, Sankar P.

2013-02-01

188

The electronic, structural and mechanical properties of ZrH and ZrH{sub 2} are investigated by means of first principles calculation based on density functional theory as implemented in VASP code with generalized gradient approximation. The calculated ground state properties are in good agreement with previous experimental and other theoretical results. Among the six crystallographic structures considered for ZrH, ZB phase is found to be the most stable phase, whereas ZrH{sub 2} is energetically stable in tetragonal structure at ambient condition. A structural phase transition from ZB?NaCl at a pressure 10 GPa is predicted for ZrH.

Kanagaprabha, S. [Department of Physics, Kamaraj College, Tuticorin, Tamil nadu-628003 (India); Rajeswarapalanichamy, R., E-mail: rrpalanichamy@gmail.com; Sudhapriyanga, G., E-mail: rrpalanichamy@gmail.com; Murugan, A., E-mail: rrpalanichamy@gmail.com; Santhosh, M., E-mail: rrpalanichamy@gmail.com [Department of Physics, N.M.S.S.V.N College, Madurai, Tamilnadu-625019 (India); Iyakutti, K. [Department of Physics and Nanotechnology, SRM University, Chennai, Tamilnadu-603203 (India)

2014-04-24

189

NASA Astrophysics Data System (ADS)

Ce, Cu co-doped ZnO (Zn1-2xCexCuxO: x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05) nanocrystals were synthesized by a microwave combustion method. These nanocrystals were investigated by using X-ray diffraction (XRD), UV-visible diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The stability and magnetic properties of Ce and Cu co-doped ZnO were probed by first principle calculations. XRD results revealed that all the compositions are single crystalline. hexagonal wurtzite structure. The optical band gap of pure ZnO was found to be 3.22 eV, and it decreased from 3.15 to 3.10 eV with an increase in the concentration of Cu and Ce content. The morphologies of Ce and Cu co-doped ZnO samples confirmed the formation of nanocrystals with an average grain size ranging from 70 to 150 nm. The magnetization measurement results affirmed the antiferro and ferromagnetic state for Ce and Cu co-doped ZnO samples and this is in agreement with the first principles theoretical calculations.

Mary, J. Arul; Vijaya, J. Judith; Bououdina, M.; Kennedy, L. John; Dai, J. H.; Song, Y.

2015-02-01

190

First-Principles Studies of Earth-Abundant Tetrahedrite Thermoelectrics

NASA Astrophysics Data System (ADS)

Recent experiments have shown inexpensive and naturally occuring tetrahedrite-based materials that exhibit a thermoelectric figure of merit near unity. These compounds are typically of the form Cu12-xMxSb4S13, where M is a transition metal, such as Zn or Fe, for a wide range of x. Using density-functional theory calculations, the ternary phase diagram and various defect formation energies are calculated. Furthermore, the electronic structure, phonon spectrum and thermoelectric properties are investigated. We observe metallic behavior and strong lattice anharmonicity of stoichiometric Cu12Sb4S13. In addition, doping with transitional metals Zn or Fe increases both resistivity and anharmonicity. The theoretical calculations are in good agreement with experimental measurements.

Xia, Yi; Zhou, Fei; Ozolins, Vidvuds

2013-03-01

191

High-pressure elastic properties of major materials of Earth's mantle from first principles

NASA Astrophysics Data System (ADS)

The elasticity of materials is important for our understanding of processes ranging from brittle failure, to flexure, to the propagation of elastic waves. Seismologically revealed structure of the Earth's mantle, including the radial (one-dimensional) profile, lateral heterogeneity, and anisotropy are determined largely by the elasticity of the materials that make up this region. Despite its importance to geophysics, our knowledge of the elasticity of potentially relevant mineral phases at conditions typical of the Earth's mantle is still limited: Measuring the elastic constants at elevated pressure-temperature conditions in the laboratory remains a major challenge. Over the past several years, another approach has been developed based on first-principles quantum mechanical theory. First-principles calculations provide the ideal complement to the laboratory approach because they require no input from experiment; that is, there are no free parameters in the theory. Such calculations have true predictive power and can supply critical information including that which is difficult to measure experimentally. A review of high-pressure theoretical studies of major mantle phases shows a wide diversity of elastic behavior among important tetrahedrally and octahedrally coordinated Mg and Ca silicates and Mg, Ca, Al, and Si oxides. This is particularly apparent in the acoustic anisotropy, which is essential for understanding the relationship between seismically observed anisotropy and mantle flow. The acoustic anisotropy of the phases studied varies from zero to more than 50% and is found to depend on pressure strongly, and in some cases nonmonotonically. For example, the anisotropy in MgO decreases with pressure up to 15 GPa before increasing upon further compression, reaching 50% at a pressure of 130 GPa. Compression also has a strong effect on the elasticity through pressure-induced phase transitions in several systems. For example, the transition from stishovite to CaCl2 structure in silica is accompanied by a discontinuous change in the shear (S) wave velocity that is so large (60%) that it may be observable seismologically. Unifying patterns emerge as well: Eulerian finite strain theory is found to provide a good description of the pressure dependence of the elastic constants for most phases. This is in contrast to an evaluation of Birch's law, which shows that this systematic accounts only roughly for the effect of pressure, composition, and structure on the longitudinal (P) wave velocity. The growing body of theoretical work now allows a detailed comparison with seismological observations. The athermal elastic wave velocities of most important mantle phases are found to be higher than the seismic wave velocities of the mantle by amounts that are consistent with the anticipated effects of temperature and iron content on the P and S wave velocities of the phases studied. An examination of future directions focuses on strategies for extending first-principles studies to more challenging but geophysically relevant situations such as solid solutions, high-temperature conditions, and mineral composites.

Karki, Bijaya B.; Stixrude, Lars; Wentzcovitch, Renata M.

2001-11-01

192

Electronic and optical properties of Au-doped Cu2O: A first principles investigation

NASA Astrophysics Data System (ADS)

The Cu2O and Au-doped Cu2O films are prepared on MgO (001) substrates by pulsed laser deposition. The X-ray photoelectron spectroscopy proves that the films are of Au-doped Cu2O. The optical absorption edge decreases by 1.6% after Au doping. The electronic and optical properties of pure and Au-doped cuprite Cu2O films are investigated by the first principles. The calculated results indicate that Cu2O is a direct band-gap semiconductor. The scissors operation of 1.64 eV has been carried out. After correcting, the band gaps for pure and Au doped Cu2O are about 2.17 eV and 2.02 eV, respectively, decreasing by 6.9%. All of the optical spectra are closely related to the dielectric function. The optical spectrum red shift corresponding to the decreasing of the band gap, and the additional absorption, are observed in the visible region for Au doped Cu2O film. The experimental results are generally in agreement with the calculated results. These results indicate that Au doping could become one of the more important factors influencing the photovoltaic activity of Cu2O film.

Jiang, Zhong-Qian; Yao, Gang; An, Xin-You; Fu, Ya-Jun; Cao, Lin-Hong; Wu, Wei-Dong; Wang, Xue-Min

2014-05-01

193

In this work the growth of a graphene monolayer on copper substrate, as typically achieved via chemical vapor deposition of propene (C3H6), was investigated by first-principles and kinetic Monte Carlo calculations. A comparison between calculated C1s core-level binding energies and electron spectroscopy measurements showed that graphene nucleates from isolated carbon atoms adsorbed on surface defects or sub-superficial layers upon hydrocarbon fragmentation. In this respect, ab initio nudged elastic band simulations yield the energetic barriers characterizing the diffusion of elemental carbon on the Cu(111) surface and atomic carbon uptake by the growing graphene film. Our calculations highlight a strong interaction between the growing film edges and the copper substrate, indicative of the importance of the grain boundaries in the epitaxy process. Furthermore, we used activation energies to compute the reaction rates for the different mechanisms occurring at the carbon-copper interface via harmonic transition state theory. Finally, we simulated the long-time system growth evolution through a kinetic Monte Carlo approach for different temperatures and coverage. Our ab initio and Monte Carlo simulations of the out-of-equilibrium system point towards a growth model strikingly different from that of standard film growth. Graphene growth on copper turns out to be a catalytic, thermally-activated process that nucleates from carbon monomers, proceeds by adsorption of carbon atoms, and is not self-limiting. Furthermore, graphene growth seems to be more effective at carbon supersaturation of the surface-a clear fingerprint of a large activation barrier for C attachment. Our growth model and computational results are in good agreement with recent X-ray photoelectron spectroscopy experimental measurements. PMID:24939464

Taioli, Simone

2014-07-01

194

Rare-earth vs. heavy metal pigments and their colors from first principles.

Many inorganic pigments contain heavy metals hazardous to health and environment. Much attention has been devoted to the quest for nontoxic alternatives based on rare-earth elements. However, the computation of colors from first principles is a challenge to electronic structure methods, especially for materials with localized f-orbitals. Here, starting from atomic positions only, we compute the colors of the red pigment cerium fluorosulfide as well as mercury sulfide (classic vermilion). Our methodology uses many-body theories to compute the optical absorption combined with an intermediate length-scale modelization to assess how coloration depends on film thickness, pigment concentration, and granularity. We introduce a quantitative criterion for the performance of a pigment. While for mercury sulfide, this criterion is satisfied because of large transition matrix elements between wide bands, cerium fluorosulfide presents an alternative paradigm: the bright red color is shown to stem from the combined effect of the quasi-2D and the localized nature of states. Our work shows the power of modern computational methods, with implications for the theoretical design of materials with specific optical properties. PMID:23302689

Tomczak, Jan M; Pourovskii, Leonid V; Vaugier, Loig; Georges, Antoine; Biermann, Silke

2013-01-15

195

Review of high pressure phases of calcium by first-principles calculations

NASA Astrophysics Data System (ADS)

We review high pressure phases of calcium which have obtained by recent experimental and first-principles studies. In this study, we investigated the face-centered cubic (fcc) structure, the body-centered cubic (bcc) structure, the simple cubic (sc) structure, a tetragonal P43212 [Ishikawa T et al. 2008 Phys. Rev. B 77 020101(R)], an orthorhombic Cmca [Ishikawa T et al. 2008 Phys. Rev. B 77 020101(R)], an orthorhombic Cmcm [Teweldeberhan A M and Bonev S A 2008 Phys. Rev. B 78 140101(R)], an orthorhombic Pnma [Yao Y et al. 2008 Phys. Rev. B 78 054506] and a tetragonal I4/mcm(00) [Arapan S et al. 2008 Proc. Natl. Acad. Sci. USA 105 20627]. We compared the enthalpies among the structures up to 200 GPa and theoretically determined the phase diagram of calcium. The sequence of the structural transitions is fcc (0- 3.5 GPa) ? bcc (3.5 - 35.7 GPa) ? Cmcm (35.7- 52GPa) ? P43212 (52-109 GPa) ? Cmca (109-117.4GPa) ? Pnma (117.4-134.6GPa) ? I4/mcm(00) (134.6 GPa -). The sc phase is experimentally observed in the pressure range from 32 to 113 GPa but, in our calculation, there is no pressure region where the sc phase is the most stable. In addition, we found that the enthalpy of the hexagonal close-packed (hcp) structure is lower than that of I4/mcm(00) above 495 GPa.

Ishikawa, T.; Nagara, H.; Suzuki, N.; Tsuchiya, J.; Tsuchiya, T.

2010-03-01

196

First-principles equations of state and elastic properties of seven metals

NASA Astrophysics Data System (ADS)

We present a systematic comparison of first-principles zero-temperature equations of state and elastic constants of seven metals (aluminum, titanium, copper, tantalum, tungsten, platinum, and gold) with the most recent diamond-anvil-cell (DAC) experimental data, for pressures up to 150 GPa. Our calculations were performed within density functional theory, testing both the local density approximation (LDA) and the generalized gradient approximation (GGA) to the exchange-correlation term, and using several types of pseudopotentials. The obtained pressure-volume relationships show good agreement with DAC data: the difference between ab initio pressure and experiment is at most 5 GPa at 100-150 GPa except for Au and Pt. The equilibrium volumes V0 and bulk moduli K0 are determined within 1.5% and 6% of DAC data respectively. Experimental results are better reproduced with GGA for Al, Ti, Cu, Ta and W, but with LDA for Pt and Au, in agreement with previous theoretical studies. The predicted elastic constants are within 10% of experiment. For tantalum we have also calculated phonon spectra under pressure. They are in excellent agreement with experimental data: especially they accurately reproduced the inflexion on the longitudinal branch in the ? -H direction, which is typical of the VB column (V, Nb, Ta).

Bercegeay, C.; Bernard, S.

2005-12-01

197

Calculation of the Vibrational Stark Effect Using a First-Principles QM/MM Approach.

The proper description of the electric environment of condensed phases is a critical challenge for force field methods. To test and validate the ability of the CHARMM additive force field to describe the electric environment in aqueous solution combined QM/MM calculations have been used to calculate the vibrational Stark effect (VSE). We utilized a first principles methodology using correlated electronic structure techniques to compute the Stark shift between the gas phase and solvent environments and between two different solvent environments of three VSE probes containing acetonitrile or fluorine functionalities which have been well-characterized experimentally. Reasonable agreement with the experimentally determined Stark shifts is obtained when the MM atoms are described by the CHARMM additive force field, though it is essential to employ an anharmonic correction in the frequency calculation. In addition, the electric field created by the solvent is computed along the CN bond and a theoretical Stark tuning rate is determined for acetonitrile and shown to be in satisfactory agreement with experiment. PMID:21423871

Ringer, Ashley L; Mackerell, Alexander D

2011-02-21

198

High pressure phase transformation in yttrium sulfide(YS): A first principle study

First principles calculations have been carried out to analyze structural, elastic and dynamic stability, of YS under hydrostatic compression. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of compression suggests the B1?B2 transition at ? 49 GPa. Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state. The single crystal elastic constants derived from the energy strain method agree well with the experimental values. The activation barrier between B1 and B2 phases calculated at transition point is ? 17/mRy/formula unit. Our lattice dynamic calculations show that at ambient condition, the B1 phase is lattice dynamically stable and frequencies of phonon modes in different high symmetry directions of Brillouin zone agrees well with experimental values. The B2 phase also is dynamical stable at ambient condition as well as at ? 49 GPa, supporting our static lattice calculation.

Sahoo, B. D., E-mail: bdsahoo@barc.gov.in; Joshi, K. D., E-mail: bdsahoo@barc.gov.in; Gupta, Satish C., E-mail: bdsahoo@barc.gov.in [Applied Physics Division, Bhabha Atomic Research Centre, Mumbai-400085 (India)

2014-04-24

199

First principles DFT study of dye-sensitized CdS quantum dots

Dye-sensitized quantum dots (QDs) are considered promising candidates for dye-sensitized solar cells. In order to maximize their efficiency, detailed theoretical studies are important. Here, we report a first principles density functional theory (DFT) investigation of experimentally realized dye - sensitized QD / ligand systems, viz., Cd{sub 16}S{sub 16}, capped with acetate molecules and a coumarin dye. The hybrid B3LYP functional and a 6?311+G(d,p)/LANL2dz basis set are used to study the geometric, energetic and electronic properties of these clusters. There is significant structural rearrangement in all the clusters studied - on the surface for the bare QD, and in the positions of the acetate / dye ligands for the ligated QDs. The density of states (DOS) of the bare QD shows states in the band gap, which disappear on surface passivation with the acetate molecules. Interestingly, in the dye-sensitised QD, the HOMO is found to be localized mainly on the dye molecule, while the LUMO is on the QD, as required for photo-induced electron injection from the dye to the QD.

Jain, Kalpna; Singh, Kh. S. [Department of Physics, D. J. College, Baraut -250611, U.P. (India); Kishor, Shyam, E-mail: shyam387@gmail.com [Department of Chemistry, J. V. College, Baraut -250611, U.P. (India); Josefesson, Ida; Odelius, Michael [Fysikum, Albanova University Center, Stockholm University, S-106 91 Stockholm (Sweden); Ramaniah, Lavanya M. [High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai-400085 (India)

2014-04-24

200

Thermoelectric properties of AgSbTe{sub 2} from first-principles calculations

The structural, electronic, and transport properties of AgSbTe{sub 2} are studied by using full-relativistic first-principles electronic structure calculation and semiclassical description of transport parameters. The results indicate that, within various exchange-correlation functionals, the cubic Fd3{sup ¯}m and trigonal R3{sup ¯}m structures of AgSbTe{sub 2} are more stable than two other considered structures. The computed Seebeck coefficients at different values of the band gap and carrier concentration are accurately compared with the available experimental data to speculate a band gap of about 0.1–0.35?eV for AgSbTe{sub 2} compound, in agreement with our calculated electronic structure within the hybrid HSE (Heyd-Scuseria-Ernzerhof) functional. By calculating the semiclassical Seebeck coefficient, electrical conductivity, and electronic part of thermal conductivity, we present the theoretical upper limit of the thermoelectric figure of merit of AgSbTe{sub 2} as a function of temperature and carrier concentration.

Rezaei, Nafiseh; Akbarzadeh, Hadi [Department of Physics, Isfahan University of Technology, 84156-83111 Isfahan (Iran, Islamic Republic of); Hashemifar, S. Javad, E-mail: hashemifar@cc.iut.ac.ir [Department of Physics, Isfahan University of Technology, 84156-83111 Isfahan (Iran, Islamic Republic of); Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, 84156-83111 Isfahan (Iran, Islamic Republic of)

2014-09-14

201

Thermoelectric properties of AgSbTe2 from first-principles calculations

NASA Astrophysics Data System (ADS)

The structural, electronic, and transport properties of AgSbTe2 are studied by using full-relativistic first-principles electronic structure calculation and semiclassical description of transport parameters. The results indicate that, within various exchange-correlation functionals, the cubic F d 3 ¯ m and trigonal R 3 ¯ m structures of AgSbTe2 are more stable than two other considered structures. The computed Seebeck coefficients at different values of the band gap and carrier concentration are accurately compared with the available experimental data to speculate a band gap of about 0.1-0.35 eV for AgSbTe2 compound, in agreement with our calculated electronic structure within the hybrid HSE (Heyd-Scuseria-Ernzerhof) functional. By calculating the semiclassical Seebeck coefficient, electrical conductivity, and electronic part of thermal conductivity, we present the theoretical upper limit of the thermoelectric figure of merit of AgSbTe2 as a function of temperature and carrier concentration.

Rezaei, Nafiseh; Hashemifar, S. Javad; Akbarzadeh, Hadi

2014-09-01

202

Rare-earth vs. heavy metal pigments and their colors from first principles

Many inorganic pigments contain heavy metals hazardous to health and environment. Much attention has been devoted to the quest for nontoxic alternatives based on rare-earth elements. However, the computation of colors from first principles is a challenge to electronic structure methods, especially for materials with localized f-orbitals. Here, starting from atomic positions only, we compute the colors of the red pigment cerium fluorosulfide as well as mercury sulfide (classic vermilion). Our methodology uses many-body theories to compute the optical absorption combined with an intermediate length-scale modelization to assess how coloration depends on film thickness, pigment concentration, and granularity. We introduce a quantitative criterion for the performance of a pigment. While for mercury sulfide, this criterion is satisfied because of large transition matrix elements between wide bands, cerium fluorosulfide presents an alternative paradigm: the bright red color is shown to stem from the combined effect of the quasi-2D and the localized nature of states. Our work shows the power of modern computational methods, with implications for the theoretical design of materials with specific optical properties. PMID:23302689

Tomczak, Jan M.; Pourovskii, Leonid V.; Vaugier, Loig; Georges, Antoine; Biermann, Silke

2013-01-01

203

The structure and mechanical properties of tantalum mononitride (TaN) are investigated at high pressure from first-principles using the plane wave pseudopotential method within the local density approximation. Three stable phases were considered, i.e., two hexagonal phases (? and ?) and a cubic ? phase. The obtained equilibrium structure parameters and ground state mechanical properties are in excellent agreement with the experimental and other theoretical results. A full elastic tensor and crystal anisotropy of the ultra-incompressible TaN in three stable phases are determined in the wide pressure range. Results indicated that the elastic properties of TaN in three phases are strongly pressure dependent. And the hexagonal ?-TaN is the most ultraincompressible among the consider phases, which suggests that the ? phase of TaN is a potential candidate structure to be one of the ultraincompressible and hard materials. By the elastic stability criteria, it is predicted that ?-TaN is not stable above 53.9 GPa. In addition, the calculated B/G ratio indicated that the ? and ? phases possess brittle nature in the range of pressure from 0 to 100?GPa. While ? phase is brittleness at low pressure (below 8.2?GPa) and is strongly prone to ductility at high pressure (above 8.2?GPa). The calculated elastic anisotropic factors for three phases of TaN suggest that they are elastically highly anisotropic and strongly dependent on the propagation direction. PMID:23185097

Chang, Jing; Zhao, Guo-Ping; Zhou, Xiao-Lin; Liu, Ke; Lu, Lai-Yu

2012-01-01

204

Organic coverage of the silicon (100) surface: first-principles calculations

NASA Astrophysics Data System (ADS)

Interfacing semiconductor surfaces with organic molecule adsorbates is one of the most challenging aspects of the modern surface and interface engineering. Controlled and periodic surface coverage can have important implications in lots of technological applications, such as molecular sensing, molecular electronics, etc. One of the widely investigated surfaces is the silicon <100>. Such a surface shows a periodic arrangement of silicon dimers (induced by reconstruction) whose bonding has been extensively debated. It turns out that its properties are similar to those of a double carbon-carbon bond, and it is therefore suitable for attaching organic molecules, especially those containing a double bond. In this study we theoretically investigate from first principles the adsorption of ethylene, cyclopentene and a class of its derivatives on the Si <100> surface, discussing the implications in tailoring the surface properties, such as the electron affinity and work function. Each molecular adsorbate induces a dipole layer on the surface, whose magnitude depends on the considered molecular species. Our findings demonstrate that, for this class of systems, it is not enough the knowledge of the isolated molecule properties for predicting the properties of the surface-adsorbate complex.

Cantele, Giovanni; Borriello, Ivo; Ninno, Domenico

2008-03-01

205

The structure and mechanical properties of tantalum mononitride (TaN) are investigated at high pressure from first-principles using the plane wave pseudopotential method within the local density approximation. Three stable phases were considered, i.e., two hexagonal phases (? and ?) and a cubic ? phase. The obtained equilibrium structure parameters and ground state mechanical properties are in excellent agreement with the experimental and other theoretical results. A full elastic tensor and crystal anisotropy of the ultra-incompressible TaN in three stable phases are determined in the wide pressure range. Results indicated that the elastic properties of TaN in three phases are strongly pressure dependent. And the hexagonal ?-TaN is the most ultraincompressible among the consider phases, which suggests that the ? phase of TaN is a potential candidate structure to be one of the ultraincompressible and hard materials. By the elastic stability criteria, it is predicted that ?-TaN is not stable above 53.9 GPa. In addition, the calculated B/G ratio indicated that the ? and ? phases possess brittle nature in the range of pressure from 0 to 100?GPa. While ? phase is brittleness at low pressure (below 8.2?GPa) and is strongly prone to ductility at high pressure (above 8.2?GPa). The calculated elastic anisotropic factors for three phases of TaN suggest that they are elastically highly anisotropic and strongly dependent on the propagation direction. PMID:23185097

Chang, Jing; Zhao, Guo-Ping; Zhou, Xiao-Lin; Liu, Ke; Lu, Lai-Yu

2012-10-15

206

NASA Astrophysics Data System (ADS)

First principles calculations have been carried out to analyze structural, elastic, and dynamic stability of yttrium sulphide (YS) under hydrostatic compression. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of compression suggests the B1 ? B2 transition at ˜49 GPa (the same transition occurs at ˜48 GPa at 300 K). Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state. The single crystal elastic constants derived from the energy strain method agree well with the experimental values. The activation barrier between B1 and B2 phases calculated at transition point is ˜17/mRy/f.u. Our lattice dynamic calculations show that at ambient condition, the B1 phase is lattice dynamically stable, and frequencies of phonon modes in different high symmetry directions of Brillouin zone agrees well with experimental values. The B2 phase also is dynamical stable at ambient condition as well as at ˜49 GPa, supporting our static lattice calculation. The effect of temperature on volume and bulk modulus of the YS in B1 phase has also been examined. The superconducting temperature of ˜2.78 K determine at zero pressure agrees well with experimental data. The effect of pressure is found to suppress the superconducting nature of this material.

Sahoo, B. D.; Joshi, K. D.; Gupta, Satish C.

2014-03-01

207

Models of Giant Planet Interiors Derived from First-Principles Simulation

NASA Astrophysics Data System (ADS)

Our understanding of the interior of giant planets is based on the accurate characterization of hydrogen and helium at megabar pressures and temperatures of several thousands of Kelvin. Theoretical method including first-principles computer simulations have been the preferred tool to study these dense fluids because laboratory experiments cannot yet probe deep into Jupiter's interior despite great progress in shock wave measurements with precompressed samples. Results from an extensive set of density-functional molecular dynamics simulations will be presented [J. Vorberger et al., ``Hydrogen-Helium Mixtures in the Interiors of Giant Planets,'' Phys. Rev. B 75 (2006) 024206]. A new and more accurate equation of state (EOS) will be derived that spans the interior of giant planets. Differences from the widely used Saumon-Chabrier-Van Horn (SCVH) EOS will be analyzed. An updated model for the interior of Jupiter will be introduced. Estimates for the heavy element enrichment as well as for the size of Jupiter's core will be discussed and compared with previous models based on the SCVH EOS. This work is supported by NASA grants PGG04-0000-0116 and NAG5-13775 as well as NSF grant 0507321.

Militzer, Burkhard; Vorberger, Jan; Hubbard, William

2008-03-01

208

First-principles study on the stability of intermediate compounds of LiBH4

NASA Astrophysics Data System (ADS)

We report the results of the first-principles calculation on the intermediate compounds of LiBH4 . The stability of LiB3H8 and Li2BnHn(n=5-12) has been examined with the ultrasoft pseudopotential method based on the density-functional theory. Theoretical prediction has suggested that monoclinic Li2B12H12 is the most stable among the candidate materials. We propose the following hydriding (dehydriding) process of LiBH4 via this intermediate compound: LiBH4?(1)/(12)Li2B12H12+(5)/(6)LiH+(13)/(12)H2?LiH+B+(3)/(2)H2 . The hydrogen content and enthalpy of the first reaction are estimated to be 10 mass% and 56kJ/mol H2 , respectively, and those of the second reaction are 4mass% and 125kJ/mol H2 . They are in good agreement with experimental results of the thermal desorption spectra of LiBH4 . Our calculation has predicted that the bending modes for the ? -phonon frequencies of monoclinic Li2B12H12 are lower than that of LiBH4 , while stretching modes are higher. These results are very useful for the experimental search and identification of possible intermediate compounds.

Ohba, Nobuko; Miwa, Kazutoshi; Aoki, Masakazu; Noritake, Tatsuo; Towata, Shin-Ichi; Nakamori, Yuko; Orimo, Shin-Ichi; Züttel, Andreas

2006-08-01

209

High-pressure phases of lithium borohydride LiBH4: A first-principles study

NASA Astrophysics Data System (ADS)

High-pressure phase transformations in LiBH4 were theoretically investigated using first-principles density functional methods. A series of pressure-induced structural transformations are predicted in LiBH4, as Pnma (phase II) ? I41/acd (phase III) ? NaCl type (phase V) ? NiAs type (phase VI) ? polymeric forms. The calculated pressures for the II ? III transition and the III ? V transition are 0.9 and 27 GPa, respectively, and both agree very well with recent experimental observations. A B1-B8 transformation becomes more favored at higher pressure, and this results in a distorted NiAs structure of LiBH4. Denoted as phase VI, the distorted NiAs structure is the lowest enthalpy phase of LiBH4 above 60 GPa and confirmed to be dynamically stable by phonon calculations. The ionic character and band gap of the phase VI decreases with increasing pressure. At still higher pressures, the extended structures formed by polymeric BH4 layers intercalated by Li?+ cations may exist, and these represent the metallic forms of LiBH4.

Yao, Yansun; Klug, Dennis D.

2012-08-01

210

NASA Astrophysics Data System (ADS)

We apply first-principles density-functional theory (DFT) calculations, ab-initio molecular dynamics, and the Kubo-Greenwood formula to predict electrical conductivity in Ta2Ox (0 ? x ? 5) as a function of composition, phase, and temperature, where additional focus is given to various oxidation states of the O monovacancy (VOn; n = 0,1+,2+). In the crystalline phase, our DFT calculations suggest that VO0 prefers equatorial O sites, while VO1+ and VO2+ are energetically preferred in the O cap sites of TaO7 polyhedra. Our calculations of DC conductivity at 300 K agree well with experimental measurements taken on Ta2Ox thin films (0.18 ? x ? 4.72) and bulk Ta2O5 powder-sintered pellets, although simulation accuracy can be improved for the most insulating, stoichiometric compositions. Our conductivity calculations and further interrogation of the O-deficient Ta2O5 electronic structure provide further theoretical basis to substantiate VO0 as a donor dopant in Ta2O5. Furthermore, this dopant-like behavior is specific to the neutral case and not observed in either the 1+ or 2+ oxidation states, which suggests that reduction and oxidation reactions may effectively act as donor activation and deactivation mechanisms, respectively, for VOn in Ta2O5.

Bondi, Robert J.; Desjarlais, Michael P.; Thompson, Aidan P.; Brennecka, Geoff L.; Marinella, Matthew J.

2013-11-01

211

Electrical contacts to monolayer black phosphorus: A first-principles investigation

NASA Astrophysics Data System (ADS)

We report first-principles theoretical investigations of possible metal contacts to monolayer black phosphorus (BP). By analyzing lattice geometry, five metal surfaces are found to have minimal lattice mismatch with BP: Cu(111), Zn(0001), In(110), Ta(110), and Nb(110). Further studies indicate Ta and Nb bond strongly with monolayer BP causing substantial bond distortions, but the combined Ta-BP and Nb-BP form good metal surfaces to contact a second layer BP. By analyzing the geometry, bonding, electronic structure, charge transfer, potential, and band bending, it is concluded that Cu(111) is the best candidate to form excellent Ohmic contact to monolayer BP. The other four metal surfaces or combined surfaces also provide viable structures to form metal/BP contacts, but they have Schottky character. Finally, the band bending property in the current-in-plane (CIP) structure where metal/BP is connected to a freestanding monolayer BP, is investigated. By both work function estimates and direct calculations of the two-probe CIP structure, we find that the freestanding BP channel is n type.

Gong, Kui; Zhang, Lei; Ji, Wei; Guo, Hong

2014-09-01

212

Water confined in nanotubes and between graphene sheets: A first principle study

Water confined at the nanoscale has been the focus of numerous experimental and theoretical investigations in recent years, y yet there is no consensus on such basic properties et as diffusion and the nature of hydrogen bonding (HB) under confinement. Unraveling these properties is important to understand fluid flow and transport at the nanoscale, and to shed light on the solvation of biomolecules. Here we report on a first principle, computational study focusing on water confined between prototypical non polar substrate, i.e. , single wall carbon nanotubes and graphene sheets, 1 to 2.5 nm apart. The results of our molecular dynamics simulations show the presence of a thin, interfacial liquid layer ({approx} 5 Angstroms) whose microscopic structure and thickness are independent of the distance between confining layers. The prop properties of the hydrogen bonded network are very similar to those of the bulk outside the interfacial region, even in the case of strong confinement , confinement. Our findings indicate that the perturbation induced by the presence of confining media is extremely local in liquid water, and we propose that many of the effects attributed to novel phases under confinement are determined by subtle electronic structure rearrangements occurring at the interface with the confining medium.

Cicero, G; Grossman, J C; Schwegler, E; Gygi, F; Galli, G

2008-10-17

213

Water solubility in calcium aluminosilicate glasses investigated by first principles techniques

NASA Astrophysics Data System (ADS)

First-principles techniques have been employed to study the reactivity of water into a calcium aluminosilicate glass. In addition to the well known hydrolysis reactions Si-O-Si+H2O?Si-OH+Si-OH and Si-O-Al+H2O?Si-OH+Al-OH, a peculiar mechanism is found, leading to the formation of an AlO 3-H 2O entity and the breaking of Al-O-Si bond. In the glass bulk, most of the hydrolysis reactions are endothermic. Only a few regular sites are found reactive ( i.e. in association with an exothermic reaction), and in that case, the hydrolysis reaction leads to a decrease of the local disorder in the amorphous vitreous network. Afterwards, we suggest that ionic charge compensators transform into network modifiers when hydrolysis occurs, according to a global process firstly suggested by Burnham in 1975. Our theoretical computations provide a more general model of the first hydrolysis steps that could help to understand experimental data and water speciation in glasses.

Bouyer, Frédéric; Geneste, Grégory; Ispas, Simona; Kob, Walter; Ganster, Patrick

2010-12-01

214

First-principles calculation on Î²-SiC(111)/Î±-WC(0001) interface

NASA Astrophysics Data System (ADS)

The ?-WC(0001) surface and ?-SiC(111)/?-WC(0001) interface were studied by first-principles calculation based on density functional theory. It is demonstrated that the ?-WC(0001) surface models with more than nine atom-layers exhibit bulk-like interior, wherein the surface relaxations localized within the top three layers are well converged. Twenty-four specific geometry models of SiC/WC interface structures with different terminations and stacking sites were chosen. The calculated work of adhesion and interface energy suggest that the most stable interface structure has the C-C bonding across the interface, yielding the largest work of adhesion and the lowest interface energy. Moreover, the top-site stacking sequence is preferable for the C/C-terminated interface. The effects of the interface on the electronic structures of the C/C-terminated interfaces are mainly localized within the first and second layers of the interface. Calculations of the work of adhesion and interface energy provide theoretical evidence that the mechanical failure may initiate at the interface or in SiC but not in WC.

Jin, Na; Yang, Yanqing; Li, Jian; Luo, Xian; Huang, Bin; Sun, Qing; Guo, Pengfei

2014-06-01

215

First-principles studies of hydrogen interaction with ultrathin Mg and Mg-based alloy films

NASA Astrophysics Data System (ADS)

The search for technologically and economically viable storage solutions for hydrogen fuel would benefit greatly from research strategies that involve systematic property tuning of potential storage materials via atomic-level modification. Here, we use first-principles density-functional theory to investigate theoretically the structural and electronic properties of ultrathin Mg films and Mg-based alloy films and their interaction with atomic hydrogen. Additional delocalized charges are distributed over the Mg films upon alloying them with 11.1% of Al or Na atoms. These extra charges contribute to enhance the hydrogen binding strength to the films. We calculated the chemical potential of hydrogen in Mg films for different dopant species and film thickness, and we included the vibrational degrees of freedom. By comparing the chemical potential with that of free hydrogen gas at finite temperature (T) and pressure (P), we construct a hydrogenation phase diagram and identify the conditions for hydrogen absorption or desorption. The formation enthalpies of metal hydrides are greatly increased in thin films, and in stark contrast to its bulk phase, the hydride state can only be stabilized at high P and T (where the chemical potential of free H2 is very high). Metal doping increases the thermodynamic stabilities of the hydride films and thus significantly helps to reduce the required pressure condition for hydrogen absorption from H2 gas. In particular, with Na alloying, hydrogen can be absorbed and/or desorbed at experimentally accessible T and P conditions.

Yoon, Mina; Weitering, Hanno H.; Zhang, Zhenyu

2011-01-01

216

First principles calculation of thermodynamic properties of NaAlSi ternary

NASA Astrophysics Data System (ADS)

PbFCl-type NaAlSi ternary is a corrosion compound found in aluminum, which is used as a sealing material in sodium sulfur battery. To understand and control the corrosion process, it is important to predict its quantitative properties. In this study, a first-principles calculation has been carried out to calculate its equilibrium lattice parameters, bulk modulus and pressure derivative of bulk modulus by both all-electron full-potential linear augmented plane wave scheme and pseudopotential plane wave scheme within the generalized gradient approximation. The theoretical results show good agreement with the available experimental data. The thermodynamic properties, including the specific heat capacity and entropy with pressure up to 9 GPa, have been investigated for the first time by coupling of density functional perturbation theory and quasiharmonic approximation. The volume and linear thermal expansion coefficients were estimated and the results show that the linear thermal expansion on c-axis is nearly twice as large as that on a-axis within the calculated temperature.

Qin, Jining; Lu, Weijie; Zhang, Di; Fan, Tongxiang

2012-01-01

217

Water solubility in calcium aluminosilicate glasses investigated by first principles techniques

First-principles techniques have been employed to study the reactivity of water into a calcium aluminosilicate glass. In addition to the well known hydrolysis reactions Si-O-Si+H{sub 2}O{yields}Si-OH+Si-OH and Si-O-Al+H{sub 2}O{yields}Si-OH+Al-OH, a peculiar mechanism is found, leading to the formation of an AlO{sub 3}-H{sub 2}O entity and the breaking of Al-O-Si bond. In the glass bulk, most of the hydrolysis reactions are endothermic. Only a few regular sites are found reactive (i.e. in association with an exothermic reaction), and in that case, the hydrolysis reaction leads to a decrease of the local disorder in the amorphous vitreous network. Afterwards, we suggest that ionic charge compensators transform into network modifiers when hydrolysis occurs, according to a global process firstly suggested by Burnham in 1975. Our theoretical computations provide a more general model of the first hydrolysis steps that could help to understand experimental data and water speciation in glasses. -- Graphical Abstract: Reactivity within glass bulk: structures obtained after hydrolyses reactions (endothermic and exothermic processes) and mechanisms involving Si-OH, Al-OH, Si-OH-Al groups within aluminosilicates glasses (through ab initio molecular dynamics): formation of the Si-OH-Al entity coupled with an H exchange-Frederic Bouyer and Gregory Geneste. Display Omitted

Bouyer, Frederic, E-mail: frederic.bouyer@cea.f [Laboratoire d'etude du Comportement a Long Terme, Commissariat a l'Energie Atomique et aux Energies Alternatives, Centre de Marcoule, SECM - LCLT, BP 17171, 30207 Bagnols-sur-Ceze Cedex (France); Geneste, Gregory [CEA, DAM, DIF, F-91297 Arpajon (France); Ispas, Simona; Kob, Walter [Laboratoire des Colloides, Verres et Nanomateriaux, UMR CNRS-UM2 5587, Universite Montpellier II, Place Eugene Bataillon, 34095 Montpellier Cedex 5 (France); Ganster, Patrick [Centre SMS, UMR CNRS 5146, Ecole des Mines de Saint-Etienne, 158 cours Fauriel, 42023 Saint-Etienne Cedex 2 (France)

2010-12-15

218

First principles calculations have been carried out to analyze structural, elastic, and dynamic stability of yttrium sulphide (YS) under hydrostatic compression. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of compression suggests the B1 ? B2 transition at ?49?GPa (the same transition occurs at ?48?GPa at 300?K). Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state. The single crystal elastic constants derived from the energy strain method agree well with the experimental values. The activation barrier between B1 and B2 phases calculated at transition point is ?17/mRy/f.u. Our lattice dynamic calculations show that at ambient condition, the B1 phase is lattice dynamically stable, and frequencies of phonon modes in different high symmetry directions of Brillouin zone agrees well with experimental values. The B2 phase also is dynamical stable at ambient condition as well as at ?49?GPa, supporting our static lattice calculation. The effect of temperature on volume and bulk modulus of the YS in B1 phase has also been examined. The superconducting temperature of ?2.78?K determine at zero pressure agrees well with experimental data. The effect of pressure is found to suppress the superconducting nature of this material.

Sahoo, B. D., E-mail: bdsahoo@barc.gov.in; Joshi, K. D.; Gupta, Satish C. [Applied Physics Division, Bhabha Atomic Research Centre, Mumbai 400085 (India)

2014-03-28

219

Thermodynamic ground state of MgB{sub 6} predicted from first principles structure search methods

Crystalline structures of magnesium hexaboride, MgB{sub 6}, were investigated using unbiased structure searching methods combined with first principles density functional calculations. An orthorhombic Cmcm structure was predicted as the thermodynamic ground state of MgB{sub 6}. The energy of the Cmcm structure is significantly lower than the theoretical MgB{sub 6} models previously considered based on a primitive cubic arrangement of boron octahedra. The Cmcm structure is stable against the decomposition to elemental magnesium and boron solids at atmospheric pressure and high pressures up to 18.3 GPa. A unique feature of the predicted Cmcm structure is that the boron atoms are clustered into two forms: localized B{sub 6} octahedra and extended B{sub ?} ribbons. Within the boron ribbons, the electrons are delocalized and this leads to a metallic ground state with vanished electric dipoles. The present prediction is in contrast to the previous proposal that the crystalline MgB{sub 6} maintains a semiconducting state with permanent dipole moments. MgB{sub 6} is estimated to have much weaker electron-phonon coupling compared with that of MgB{sub 2}, and therefore it is not expected to be able to sustain superconductivity at high temperatures.

Wang, Hui [State Key Lab of Superhard Materials, Jilin University, Changchun 130012 (China) [State Key Lab of Superhard Materials, Jilin University, Changchun 130012 (China); Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada); LeBlanc, K. A. [Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada)] [Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada); Gao, Bo [State Key Lab of Superhard Materials, Jilin University, Changchun 130012 (China)] [State Key Lab of Superhard Materials, Jilin University, Changchun 130012 (China); Yao, Yansun, E-mail: yansun.yao@usask.ca [Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada) [Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada); Canadian Light Source, Saskatoon, Saskatchewan S7N 0X4 (Canada)

2014-01-28

220

First principle computation of biomolecule-ligand interaction

NASA Astrophysics Data System (ADS)

We have determined the calibration constants for the SDFT prediction of 57Fe Mossbauer parameters. The calibration constants are basis set and exchange-correlation functional specific. The basis set is 6-311G* whereas the exchange-correlation functional are UB3LYP, UBLYP, UBPW91, and UMPW1PW91. This method is used to predict the binding conformation of a dioxygen molecule to the P intermediate of methane monooxygenase whose experimental binding structure is presently unknown. When O2 binds to the diiron cluster in a mu-1,2-peroxo fashion, the associated theoretical Mossbauer parameters are in good agreement with the experimental values. An efficient method for locating minimum energy crossing points is introduced and tested on the phenyl cation. An agreement of the present method and previous works is satisfactory. The convergence rate obeys a logarithmic law and is verified on the phenyl cation. Due to its rapid convergence rate, the method is suitable for a large molecular system. As an application of the new methodology, the crossing points of the cation of [Fe(ptz)6](BF4) 2 were studied in order to identify the geometrical parameters of the spin crossing points between S=0 ? S=1, and S=1 ? S=2. The calculation shows that the transition from a singlet ground state to the triplet intermediate state is accompanied by almost 0.3A bond length elongation of the axial ligands. We have implemented an approximation scheme that allows one to study protein system such as a ligand-protein binding conformation and a protein active site geometry optimization. The scheme, named E-MFCC, is a significant improvement over the previous MFCC approximation originally put forth by Zhang et al. The geometry optimization of some small test systems utilizing the E-MFCC scheme introduces an error on the order of 10-2A as compared to the all-atom calculation.

Chachiyo, Teepanis

221

NASA Astrophysics Data System (ADS)

The discovery of fullerenes and carbon nanotubes has been very significant to the field of nanotechnology by providing an abundance of stable, highly symmetric, non-reactive, and relatively large molecules that can, in principle, be manipulated one at a time. At the present stage, a theoretical effort should be carried out in order to find and understand novel phenomena in molecule-based nanostructures which could serve as a basis for fabricating useful molecular devices. In this thesis we investigate from first-principles the transport properties of molecular devices: fullerene and carbon nanotube systems. We begin with charge transport in carbon nanotubes with oxygen, and find that the interaction between oxygen molecules and carbon nanotubes significantly modifies the electronic structure near the Fermi level for both zigzag and armchair tubes. The subtle difference of the adsorption sites of oxygen and the distance between oxygen and nanotubes can cause totally different results of their transport properties. Then we investigate current flow from the point of view of current density distribution in molecular devices, for current density gives local information of nonequilibrium transport, thereby providing useful and vivid insight to transport properties of molecular electronics. It has been found when an intrinsic carbon nanotube is doped with either a boron or a nitrogen atoms through a replacement of a carbon atom, the local physical properties around the impurity atoms (boron or nitrogen) undergo a significant change, resulting in a dramatic change of the local current distribution. It is suggested that there appears a chiral current flow in the B- and N-doped armchair nanotubes near the impurity. As for a gated C 60 molecular device, the current distribution and the total current flow are both obviously affected by the gate voltage, which indicates the importance of the gate voltage in such a molecular device. Finally, we discuss the contact effects on transport properties of the molecular devices. We study the effects of the contact geometry as well as the electrode material and find that different orientations of C 60 connected to Au(111) leads can cause significant changes in the current-voltage (I-V) characteristics of such C60 molecular devices. On the other hand, the electrode material is crucial to obtain low resistance ohmic contacts. Our first-principles calculations of transport suggest that Ti has higher affinity for carbide formation. So the choice of proper electrode materials will play an important role in the design of nanoscale devices.

Liu, Yi

222

Adsorption of carbon on Pd clusters of nanometer size: A first-principles theoretical study

NASA Astrophysics Data System (ADS)

Adsorbed atomic C species can be formed in the course of surface reactions and commonly decorate metal catalysts. We studied computationally C adsorption on Pd nanoclusters using an all-electron scalar relativistic density functional method. The metal particles under investigation, Pd55, Pd79, Pd85, Pd116, Pd140, and Pd146, were chosen as fragments of bulk Pd in the form of three-dimensional octahedral or cuboctahedral crystallites, exposing (111) and (100) facets as well as edge sites. These cluster models are shown to yield size-converged adsorption energies. We examined which surface sites of these clusters are preferentially occupied by adsorbed C. According to calculations, surface C atoms form strongly adsorbed carbide species (with adsorption energies of more than 600kJmol-1) bearing a significant negative charge. Surface sites allowing high, fourfold coordination of carbon are overall favored. To avoid effects of adsorbate-adsorbate interaction in the cluster models for carbon species in the vicinity of cluster edges, we reduced the local symmetry of selected adsorption complexes on the nanoclusters by lowering the global symmetry of the nanocluster models from point group Oh to D4h. On (111) facets, threefold hollow sites in the center are energetically preferred; adsorbed C is calculated to be slightly less stable when displaced to the facet borders.

Neyman, Konstantin M.; Inntam, Chan; Gordienko, Alexei B.; Yudanov, Ilya V.; Rösch, Notker

2005-05-01

223

NASA Astrophysics Data System (ADS)

Diffusion of point defects on crystalline surfaces and in their bulk is an important and ubiquitous phenomenon affecting film quality, electronic properties and device functionality. A complete understanding of these diffusion processes enables one to predict and then control those processes. Such understanding includes knowledge of the structural, energetic and electronic properties of these native and non-native point defect diffusion processes. Direct experimental observation of the phenomenon is difficult and microscopic theories of diffusion mechanisms and pathways abound. Thus, knowing the nature of diffusion processes, of specific point defects in given materials, has been a challenging task for analytical theory as well as experiment. The recent advances in computing technology have been a catalyst for the rise of a third mode of investigation. The advent of tremendous computing power, breakthroughs in algorithmic development in computational applications of electronic density functional theory now enables direct computation of the diffusion process. This thesis demonstrates such a method applied to several different examples of point defect diffusion on the (001) surface of gallium arsenide (GaAs) and the bulk of cadmium telluride (CdTe) and cadmium sulfide (CdS). All results presented in this work are ab initio, total-energy pseudopotential calculations within the local density approximation to density-functional theory. Single particle wavefunctions were expanded in a plane-wave basis and reciprocal space k-point sampling was achieved by Monkhorst-Pack generated k-point grids. Both surface and bulk computations employed a supercell approach using periodic boundary conditions. Ga adatom adsorption and diffusion processes were studied on two reconstructions of the GaAs(001) surface including the c(4x4) and c(4x4)-heterodimer surface reconstructions. On the GaAs(001)- c(4x4) surface reconstruction, two distinct sets of minima and transition sites were discovered for a Ga adatom relaxing from heights of 3 and 0.5 A from the surface. These two sets show significant differences in the interaction of the Ga adatom with surface As dimers and an electronic signature of the differences in this interaction was identified. The energetic barriers to diffusion were computed between various adsorption sites. Diffusion profiles for native Cd and S, adatom and vacancy, and non-native interstitial adatoms of Te, Cu and Cl were investigated in bulk wurtzite CdS. The interstitial diffusion paths considered in this work were chosen parallel to c-axis as it represents the path encountered by defects diffusing from the CdTe layer. Because of the lattice mismatch between zinc-blende CdTe and hexagonal wurtzite CdS, the c-axis in CdS is normal to the CdTe interface. The global minimum and maximum energy positions in the bulk unit cell vary for different diffusing species. This results in a significant variation, in the bonding configurations and associated strain energies of different extrema positions along the diffusion paths for various defects. The diffusion barriers range from a low of 0.42 eV for an S interstitial to a high of 2.18 eV for a S vacancy. The computed 0.66 eV barrier for a Cu interstitial is in good agreement with experimental values in the range of 0.58 - 0.96 eV reported in the literature. There exists an electronic signature in the local density of states for the s- and d-states of the Cu interstitial at the global maximum and global minimum energy position. The work presented in this thesis is an investigation into diffusion processes for semiconductor bulk and surfaces. The work provides information about these processes at a level of control unavailable experimentally giving an elaborate description into physical and electronic properties associated with diffusion at its most basic level. Not only does this work provide information about GaAs, CdTe and CdS, it is intended to contribute to a foundation of knowledge that can be extended to other systems to expand our overall understanding into the diffusion proc

Roehl, Jason L.

224

Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights 27 May 2009; available online 23 June 2009 Abstract Phosphoric acid digestion has been used with phosphoric acid digestion of carbonates at 25 Â°C are 10.72&, 0.220&, 0.137&, 0.593& for, respectively, 18 O

Goddard III, William A.

225

NASA Astrophysics Data System (ADS)

Ab initio calculation on B2-cadmium rare earth (RE), CdRE (RE=La, Ce and Pr) intermetallics has been performed at T=0 K with respect to their structural, electronic and thermal properties. The structural and electronic properties are derived using self -consistent tight binding linear muffin tin orbital method at ambient and at high pressure. Other properties like lattice parameter, bulk modulus, density of states, electronic specific heat coefficient, cohesive energy, heat of formation, Debye temperature and Grüneisen constant for CdRE are also estimated. The RE -f effect can be seen in CdPr in terms of variation in the density of states and opens a possibility of structural instability. A pressure induced variation of Debye temperature is also presented for three cadmium rare earth intermetallics.

Srivastava, Vipul; Khan, Afroj A.; Rajagopalan, M.; Sanyal, Sankar P.

2012-01-01

226

Modeling Precipitate Microstructure Evolution in Alloys With First-Principles Energetic Information

Modeling Precipitate Microstructure Evolution in Alloys With First- Principles Energetic energetic contributions to the driving force for microstructure evolution. This is particularly true for Al, and coherency strain energies. The incorporation of these energetic properties, obtained from atomistics

Chen, Long-Qing

227

First-principles investigation of Li intercalation kinetics in phospho-olivines

This thesis focuses broadly on characterizing and understanding the Li intercalation mechanism in phospho-olivines, namely LiFePO? and Li(Fe,Mn)PO?, using first-principles calculations. Currently Li-ion battery technology ...

Malik, Rahul

2013-01-01

228

In this thesis, the chemistry of sulfur oxides on transition metals is studied extensively via first-principles density functional theory (DFT) computations, focusing on the chemical reactivity and selectivity in sulfur ...

Lin, Xi, 1973-

2003-01-01

229

Thermal conductivity of half-Heusler compounds from first-principles calculations

We demonstrate successful application of first-principles-based thermal conductivity calculation on half-Heusler compounds that are promising, environmentally friendly thermoelectric materials. Taking the case of a p-type ...

Shiomi, Junichiro

230

Aqueous systems from first-principles : structure, dynamics and electron-transfer reactions

In this thesis, we show for the first time how it is possible to calculated fully from first-principles the diabatic free-energy surfaces of electron-transfer reactions. The excitation energy corresponding to the transfer ...

Sit, Patrick Hoi Land

2006-01-01

231

In this paper, thermal conductivity of crystalline GaAs is calculated using first-principles lattice dynamics. The harmonic and cubic force constants are obtained by fitting them to the force-displacement data from density ...

Luo, Tengfei

232

Thermal conductivity from first-principles in bulk, disordered, and nanostructured materials

Thermal conductivity is an important transport property that plays a vital role in applications such as high efficiency thermoelectric devices as well as in thermal management of electronics. We present a first-principles ...

Garg, Jivtesh

2011-01-01

233

Lattice thermal conductivity of Bi, Sb, and Bi-Sb alloy from first principles

Using first principles, we calculate the lattice thermal conductivity of Bi, Sb, and Bi-Sb alloys, which are of great importance for thermoelectric and thermomagnetic cooling applications. Our calculation reveals that the ...

Lee, Sangyeop

234

First-principles Study on the Magnetic Interaction in ZnO-based Dilute Magnetic Semiconductors

Using first-principles calculations, we investigate the electronic structures and magnetic properties of dilute magnetic semiconductors (DMS). The electronic structures are calculated by using Korringa-Kohn-Rostoker method combined with the coherent potential approximation. Since the d electrons of the magnetic impurity in DMS are strongly localized, we apply self-interaction correction to the local density approximation for the exchange-correlation energy. From the first-principles

Masayuki Toyoda; Hisazumi Akai; Kazunori Sato; Hiroshi Katayama-Yoshida

2008-01-01

235

We apply first-principles density-functional theory (DFT) calculations, ab-initio molecular dynamics, and the Kubo-Greenwood formula to predict electrical conductivity in Ta{sub 2}O{sub x} (0???x???5) as a function of composition, phase, and temperature, where additional focus is given to various oxidation states of the O monovacancy (V{sub O}{sup n}; n?=?0,1+,2+). In the crystalline phase, our DFT calculations suggest that V{sub O}{sup 0} prefers equatorial O sites, while V{sub O}{sup 1+} and V{sub O}{sup 2+} are energetically preferred in the O cap sites of TaO{sub 7} polyhedra. Our calculations of DC conductivity at 300?K agree well with experimental measurements taken on Ta{sub 2}O{sub x} thin films (0.18???x???4.72) and bulk Ta{sub 2}O{sub 5} powder-sintered pellets, although simulation accuracy can be improved for the most insulating, stoichiometric compositions. Our conductivity calculations and further interrogation of the O-deficient Ta{sub 2}O{sub 5} electronic structure provide further theoretical basis to substantiate V{sub O}{sup 0} as a donor dopant in Ta{sub 2}O{sub 5}. Furthermore, this dopant-like behavior is specific to the neutral case and not observed in either the 1+ or 2+ oxidation states, which suggests that reduction and oxidation reactions may effectively act as donor activation and deactivation mechanisms, respectively, for V{sub O}{sup n} in Ta{sub 2}O{sub 5}.

Bondi, Robert J., E-mail: rjbondi@sandia.gov; Desjarlais, Michael P.; Thompson, Aidan P.; Brennecka, Geoff L.; Marinella, Matthew J. [Sandia National Laboratories, Albuquerque, New Mexico 87185 (United States)

2013-11-28

236

NASA Astrophysics Data System (ADS)

Spinel phase aluminum oxynitride solid solution (?-alon, with formula of Al(8+x)/3O4-xNx) exists in the narrow Al2O3-rich region of Al2O3-AlN systems. The first-principles calculations were developed to investigate the composition-dependent bonding and hardness of ?-alon. Six supercell model for Al(8+x)/3O4-xNx (x = 0, 0.25, 0.44, 0.63, 0.81, and 1) was constructed to perform our calculations with high accuracy. It was found that the lattice constant increases with increasing composition of nitrogen in ?-alon. The bond lengths of AlIV-O, AlVI-O, AlIV-N, and AlVI-N all increase with the expansion of crystal structure. The well-known Mulliken overlap populations were calculated to estimate the bonding and hardness. As the content of nitrogen substitute increases, the Al-N bonds present more covalent characteristic, while the Al-O bonds present more ionic characteristic. The AlIV-N is the hardest bond in ?-alon. The theoretical hardness of ?-alon could be slightly enhanced from 17.16 GPa to 17.97 GPa by increasing content of nitrogen in full solubility range. The contribution ratio, CH?, was proposed to quantify the contribution of bonds to hardness of ?-alon. The Al-O bonds are found to contribute more to the hardness. The Al-N bonds are the main influencing factor to enhance the hardness of ?-alon. These calculated results provide the basis for understanding the composition-dependent bonding and hardness of ?-alon.

Tu, Bingtian; Wang, Hao; Liu, Xiao; Khan, Shahzad A.; Wang, Weimin; Fu, Zhengyi

2014-06-01

237

First-principles calculations of electric field gradients in complex perovskites

NASA Astrophysics Data System (ADS)

Various experimental and theoretical work indicate that the local structure and chemical ordering play a crucial role in the different physical behaviors of lead-based complex ferroelectrics with the ABO 3 perovskite structure. First-principles linearized augmented plane wave (LAPW) with the local orbital extension method within local density approximation (LDA) are performed on structural models of Pb(Zr1/2Ti1/2 )O3 (PZT), Pb(Sc1/2Ta1/2)O3 (PST), Pb(Sc2/3W1/3)O3 (PSW), and Pb(Mg 1/3Nb2/3)O3 (PMN) to calculate electric field gradients (EFGs). In order to simulate these disordered alloys, various structural models were constructed with different imposed chemical orderings and symmetries. Calculations were carried out as a function of B-site chemical ordering, applied strain, and imposed symmetry. Large changes in the EFGs are seen in PZT as the electric polarization rotates between the tetragonal and rhombohedral directions. The onset of polarization rotation in monoclinic Cm symmetry strongly correlates with the shearing of the TiO6 octahedron, and there is a sharp change in slope in plots of Ti EFGs versus octahedral distortion index. The same changes in EFGs and the BO6 shearing corresponding to the change of off-centering direction are also seen in PST. In PSW and PMN, the calculated B cation EFGs showed more sensitivity to the surrounding nearest B neighboring environments. Calculated B atom EFGs in all alloys are considerably larger than those inferred from the NMR measurements. Based on comparisons with experiments, the calculated results are interpreted in terms of static and dynamic structural models of these materials.

Mao, Dandan

238

First Principles Calculations of Hydrogen-Helium Mixtures and the Structure of Jupiter

NASA Astrophysics Data System (ADS)

Most of the over two hundred recently discovered extrasolar planets are gas giants that are primarily composed of dense fluid hydrogen and helium at megabar pressures and temperature of thousands of degrees Kelvin. The characterization of hydrogen-helium mixtures at such extreme conditions has posed a challenge to experimental and theoretical methods. Great progress has been made with recent shock wave experiments and the megabar regime has been probed. However, the temperatures were much higher and the densities were significantly lower than those present in giant planets because planetary interiors are characterized by isentropes, which rise much slower in the P-T plane than shock Hugoniot curves. In this talk, results from an extensive set of density-functional molecular dynamics simulations will be presented [J. Vorberger et al., Phys. Rev. B 75 (2006) 024206] and an equation of state (EOS) of hydrogen-helium mixtures that spans for Jupiter's interior is derived. Going beyond the commonly assumed linear mixing approximation, the interaction effects of dense hydrogen and helium are analyzed. It will be discussed how helium affects the molecular-to-metallic transition in hydrogen and why the presence of helium stabilizes the molecular phase. Based on this first-principles EOS, an updated model for the interior of Jupiter will be introduced. Our interior model updates the suite of models that were based on the widely used Saumon-Chabrier-Van Horn (SCVH) EOS. Deviations from SCVH are up to about 5 percent depending on the pressure, and thus affect interior models at the same level. Unlike SCVH, the computed DFT-EOS does not predict any first-order thermodynamic discontinuities associated with pressure-dissociation and metallization of hydrogen. Finally, the size of a rocky core in Jupiter and the heavy elements enrichment in its mantle will be estimated. It will be descussed whether the planet was form by core-accretion.

Militzer, Burkhard

2008-04-01

239

First-principles calculation of the indentation strength of FeB4

NASA Astrophysics Data System (ADS)

A recent experiment [H. Y. Gou et al., Phys. Rev. Lett. 111, 157002 (2013), 10.1103/PhysRevLett.111.157002] reported a measured Vickers hardness of over 60 GPa for FeB4, placing it as a superhard material far above all other similar ultrahard transition-metal borides, such as ReB2,WB4, and especially CrB4, which has the same crystalline structure as that of FeB4 but a much lower measured Vickers hardness of around 23 GPa. This result, however, is contradicted by theoretical calculations that predict a smaller shear modulus for FeB4 than that of CrB4, indicating that FeB4 is softer than CrB4. Here we report first-principles calculations that aim to understand the stress response of FeB4 under indentation loadings and examine whether there exists a strain-stiffening effect that might enhance the indentation strength. Our results show that there is no strain stiffening in FeB4; instead, the normal pressure beneath the indenter drives a lateral structural expansion which further stretches and weakens the boron-boron and boron-iron bonds in addition to that caused by the shear deformation in Vickers indentation tests. This effect leads to a considerably reduced strength of FeB4, producing an ideal (i.e., an upper bound) indentation strength of 17 GPa that is lower than that (27 GPa) predicted for CrB4. The present results suggest that FeB4 is unlikely to be superhard and further experimental investigation is needed to clarify this issue.

Li, Bing; Sun, Hong; Chen, Changfeng

2014-07-01

240

Thermal physics of the lead chalcogenides PbS, PbSe, and PbTe from first principles

NASA Astrophysics Data System (ADS)

The lead chalcogenides represent an important family of functional materials, in particular due to the benchmark high-temperature thermoelectric performance of PbTe. A number of recent investigations, experimental and theoretical, have aimed to gather insight into their unique lattice dynamics and electronic structure. However, the majority of first-principles modeling has been performed at fixed temperatures, and there has been no comprehensive and systematic computational study of the effect of temperature on the material properties. We report a comparative lattice-dynamics study of the temperature dependence of the properties of PbS, PbSe, and PbTe, focusing particularly on those relevant to thermoelectric performance, viz. phonon frequencies, lattice thermal conductivity, and electronic band structure. Calculations are performed within the quasiharmonic approximation, with the inclusion of phonon-phonon interactions from many-body perturbation theory, which are used to compute phonon lifetimes and predict the lattice thermal conductivity. The results are critically compared against experimental data and other calculations, and add insight to ongoing research on the PbX compounds in relation to the off-centering of Pb at high temperatures, which is shown to be related to phonon softening. The agreement with experiment suggests that this method could serve as a straightforward, powerful, and generally applicable means of investigating the temperature dependence of material properties from first principles.

Skelton, Jonathan M.; Parker, Stephen C.; Togo, Atsushi; Tanaka, Isao; Walsh, Aron

2014-05-01

241

NASA Astrophysics Data System (ADS)

Recent progress in several related research areas such as first-principles electronic-structure calculations of atoms and diatomic molecules, theory of elementary processes, kinetics, and numerical engineering, and also continued exponential growth in computational resources enhanced by recent advances in massively parallel computing have opened the possibility of directly designing kinetics mechanisms to describe chemical processes and light emission in such complex media as nonequilibrium plasmas and reacting gases. It is important that plasma and combustion kinetics can be described in the framework of this direct approach to a sufficiently high accuracy, which makes it an independent predictive research tool complementary to experimental techniques. This paper demonstrates the capabilities of the first-principles based approach to develop kinetic mechanisms. Two examples are discussed in detail: (1) the mechanism of hydrocarbon fuel combustion at high temperatures and (2) light emission in non-thermal glow discharge plasma of metal halides; special attention is paid to a comparison of the results obtained at every level of system description with the appropriate experimental data. In house software tools that can be used in such multilevel theoretical works are discussed as well.

Astapenko, Valerie; Bagatur'yants, Alexander; Chernishova, Irina; Deminsky, Maxim; Eletskii, Alexander; Kirillov, Igor; Knizhnik, Andrei; Potapkin, Boris; Rykova, Elena; Umanskii, Stanislaw; Zaitsevskii, Andrei; Strelkova, Marina; Sukhanov, Leonid; Safonov, Andrei; Cotzas, George M.; Dean, Anthony; Michael, J. Darryl; Midha, Vikas; Smith, David J.; Sommerer, Timothy J.; Varatharajan, Bala; Tentner, Adrian

2007-04-01

242

First-principles study of the electronic structures of icosahedral TiN (N=13,19,43,55) clusters.

We have studied the electronic structures of icosahedral Ti(N) clusters (N=13, 19, 43, and 55) by using a real-space first-principles cluster method with generalized gradient approximation for exchange-correlation potential. The hexagonal close-packed and fcc close-packed clusters have been studied additionally for comparisons. It is found that the icosahedral structures are the most stable ones except for Ti(43), where fcc close-packed structure is favorable in energy. We present and discuss the variation of bond length, the features of the highest occupied molecular orbitals and the lowest unoccupied molecular orbital, the evolution of density of states, and the magnetic moment in detail. The results are in good agreement with the predictions from the collision-induced dissociation and size-selected anion photoelectron spectroscopy experiments. PMID:15267771

Wang, Shan-Ying; Yu, Jing-Zhi; Mizuseki, Hiroshi; Yan, Jia-An; Kawazoe, Yoshiyuki; Wang, Chong-Yu

2004-05-01

243

NASA Astrophysics Data System (ADS)

Access to magnetic excitation spectra of single atoms deposited on surfaces is nowadays possible by means of low-temperature inelastic scanning tunneling spectroscopy. We present a first-principles method for the calculation of inelastic tunneling spectra utilizing the Korringa-Kohn-Rostoker Green function method combined with time-dependent density functional theory and many-body perturbation theory. The key quantity is the electron self-energy describing the coupling of the electrons to the spin excitation within the adsorbate. By investigating Cr, Mn, Fe, and Co adatoms on a Cu(111) substrate, we spin-characterize the spectra and demonstrate that their shapes are altered by the magnetization of the adatoms, of the tip and the orbital decay into vacuum. Our method also predicts spectral features more complex than the steps obtained by simpler models for the adsorbate (e.g., localized spin models).

Schweflinghaus, Benedikt; dos Santos Dias, Manuel; Costa, Antonio T.; Lounis, Samir

2014-06-01

244

We present results of gas phase cluster and liquid water simulations from the recently determined VRT(ASP-W)III water dimer potential energy surface. VRT(ASP-W)III is shown to not only be a model of high ''spectroscopic'' accuracy for the water dimer, but also makes accurate predictions of vibrational ground-state properties for clusters up through the hexamer. Results of ambient liquid water simulations from VRT(ASP-W)III are compared to those from ab initio Molecular Dynamics, other potentials of ''spectroscopic'' accuracy, and to experiment. The results herein represent the first time that a ''spectroscopic'' potential surface is able to correctly model condensed phase properties of water.

Goldman, N; Leforestier, C; Saykally, R J

2004-05-25

245

Point defects in ZnO: an approach from first principles

NASA Astrophysics Data System (ADS)

Recent first-principles studies of point defects in ZnO are reviewed with a focus on native defects. Key properties of defects, such as formation energies, donor and acceptor levels, optical transition energies, migration energies and atomic and electronic structure, have been evaluated using various approaches including the local density approximation (LDA) and generalized gradient approximation (GGA) to DFT, LDA+U/GGA+U, hybrid Hartree-Fock density functionals, sX and GW approximation. Results significantly depend on the approximation to exchange correlation, the simulation models for defects and the post-processes to correct shortcomings of the approximation and models. The choice of a proper approach is, therefore, crucial for reliable theoretical predictions. First-principles studies have provided an insight into the energetics and atomic and electronic structures of native point defects and impurities and defect-induced properties of ZnO. Native defects that are relevant to the n-type conductivity and the non-stoichiometry toward the O-deficient side in reduced ZnO have been debated. It is suggested that the O vacancy is responsible for the non-stoichiometry because of its low formation energy under O-poor chemical potential conditions. However, the O vacancy is a very deep donor and cannot be a major source of carrier electrons. The Zn interstitial and anti-site are shallow donors, but these defects are unlikely to form at a high concentration in n-type ZnO under thermal equilibrium. Therefore, the n-type conductivity is attributed to other sources such as residual impurities including H impurities with several atomic configurations, a metastable shallow donor state of the O vacancy, and defect complexes involving the Zn interstitial. Among the native acceptor-type defects, the Zn vacancy is dominant. It is a deep acceptor and cannot produce a high concentration of holes. The O interstitial and anti-site are high in formation energy and/or are electrically inactive and, hence, are unlikely to play essential roles in electrical properties. Overall defect energetics suggests a preference for the native donor-type defects over acceptor-type defects in ZnO. The O vacancy, Zn interstitial and Zn anti-site have very low formation energies when the Fermi level is low. Therefore, these defects are expected to be sources of a strong hole compensation in p-type ZnO. For the n-type doping, the compensation of carrier electrons by the native acceptor-type defects can be mostly suppressed when O-poor chemical potential conditions, i.e. low O partial pressure conditions, are chosen during crystal growth and/or doping.

Oba, Fumiyasu; Choi, Minseok; Togo, Atsushi; Tanaka, Isao

2011-06-01

246

NASA Astrophysics Data System (ADS)

In the course of my PhD I have worked on a broad range of problems using simulations from first principles: from catalysis and chemical reactions at surfaces and on nanostructures, characterization of carbon-based systems and devices, and surface and interface physics. My research activities focused on the application of ab-initio electronic structure techniques to the theoretical study of important aspects of the physics and chemistry of materials for energy and environmental applications and nano-electronic devices. A common theme of my research is the computational study of chemical reactions of environmentally important molecules (CO, CO2) using high performance simulations. In particular, my principal aim was to design novel nano-structured functional catalytic surfaces and interfaces for environmentally relevant remediation and recycling reactions, with particular attention to the management of carbon dioxide. We have studied the carbon-mediated partial sequestration and selective oxidation of carbon monoxide (CO), both in the presence and absence of hydrogen, on graphitic edges. Using first-principles calculations we have studied several reactions of CO with carbon nanostructures, where the active sites can be regenerated by the deposition of carbon decomposed from the reactant (CO) to make the reactions self-sustained. Using statistical mechanics, we have also studied the conditions under which the conversion of CO to graphene and carbon dioxide is thermodynamically favorable, both in the presence and in the absence of hydrogen. These results are a first step toward the development of processes for the carbon-mediated partial sequestration and selective oxidation of CO in a hydrogen atmosphere. We have elucidated the atomic scale mechanisms of activation and reduction of carbon dioxide on specifically designed catalytic surfaces via the rational manipulation of the surface properties that can be achieved by combining transition metal thin films on oxide substrates. We have analyzed the mechanisms of the molecular reactions on the class of catalytic surfaces so designed in an effort to optimize materials parameters in the search of optimal catalytic materials. All these studies are likely to bring new perspectives and substantial advancement in the field of high-performance simulations in catalysis and the characterization of nanostructures for energy and environmental applications. Moving to novel materials for electronics applications, I have studied the structural and vibrational properties of mono and bi-layer graphene. I have characterized the lattice thermal conductivity of ideal monolayer and bi-layer graphene, demonstrating that their behavior is similar to that observed in graphite and indicating that the intra-layer coupling does not affect significantly the thermal conductance. I have also calculated the electron-phonon interaction in monolayer graphene and obtained electron scattering rates associated with all phonon modes and the intrinsic resistivity/mobility of monolayer graphene is estimated as a function of temperature. On another project, I have worked on ab initio molecular dynamic studies of novel Phase Change Materials (PCM) for memory and 3D-integration. We characterized high-temperature, sodium | nickel chloride, rechargeable batteries. These batteries are under consideration for hybrid drive systems in transportation applications. As part of our activities to improve performance and reliability of these batteries, we developed an engineering transport model of the component electrochemical cell. To support that model, we have proposed a reaction kinetics expression for the REDOX (reduction-oxidation) reaction at the porous positive electrode. We validate the kinetics expression with electrochemical measurements. A methodology based on the transistor body effect is used to estimate inversion oxide thicknesses (Tinv) in high-kappa/metal gate, undoped, ultra-thin body SOI FINFETs. The extracted Tinvs are compared to independent capacitance voltage (CV) measurements.

Paul, Sujata

247

A first principles approach to understanding the physics of precursory accelerating seismicity

NASA Astrophysics Data System (ADS)

Observational studies indicate that many large earthquakes are preceded by accelerating seismic release rates (Accelerated Seismic Deformation - ASD), characterized by a cumulative Benioff strain following a power law time to failure relation of the form S(t) = K + A(tf t)^m, (1) where tf is the failure time of the large event and m of the order of 0.2 - 0.4. More recent theoretical studies relate the behaviour of seismicity prior to a large earthquake to the excitation in proximity of a spinodal instability and show that the power-law activation associated with the spinodal instability is essentially identical to the power-law acceleration of Benioff strain observed prior to earthquakes, with m = 0.25. In the present work, we discuss a theoretical framework in which we derive the time-to-failure power-law from basic principles. We use energy conservation in a faulted crustal volume undergoing stress loading and we assume that the fault system obeys a fractal / hierarchical distribution law. Furthermore, we assume that the precursory seismic activation extends over a broad area around the impending failure and rapidly converges to the rupture zone as a function of the time-to-failure. By considering the analytic conditions near the time of failure, we derive the time-to-failure power-law and show that the critical exponent m is a function of the fractal dimension and that the cumulative precursory crustal deformation (A) is a function of the energy supplied to the system and the size of the rupture. The fractallity of the fault system is a necessary condition for the appearance of power-law acceleration in the seismic release rates. We note that this approach is based on first principles and gives a clear interpretation of the empirical parameters involved in equation (1). On the basis of these results, it is possible to explain a set of empirical laws derived by other researchers (e.g. Papazachos et al., Bull. Seism. Soc. Am., 92, 570-580, 2002), in terms of a plausible physical framework. Furthermore, by considering the relationship of the instantaneous Benioff strain rate with respect to the mean Benioff strain rate, it is possible to construct approximate analytical expressions to estimate the magnitude and time of failure of the impending earthquake. Examples and applications of this technique to observations of accelerating seismicity in Greece and abroad, will also be presented and discussed. The work of FV was supported by the grant INTAS 99-1102

Vallianatos, F.; Tzanis, A.

2003-04-01

248

Theoretical Development of M-shell Spectroscopy for Z-Pinch Plasma Diagnostics

NASA Astrophysics Data System (ADS)

Tungsten wire explosions are being intensively studied at Sandia National Laboratories. Any available x-ray spectral data accumulated in Z experiments with appropriate theoretical modeling can lead to better understanding of plasma evolution during a wire explosion. The present work focuses on the theoretical development of M-shell spectroscopy of W ions in the spectral range from 4 up to 8 Å. The majority of line emissions in this spectral region is composed of 31 ? 41', 51? transitions. Atomic data were calculated using Cowan and MBPT codes for all isoelectronic sequences contributing into this spectral range. The non-LTE kinetic model was developed based on these atomic data. The sensitivity of this model to the number of included ions, configurations and levels, the electron density, ionization balance, and electron distribution function is discussed. The complete modeling of this spectrum allows a detailed diagnostic of a hotter plasma core in z-pinch experiments involving heavy ions.

Shlyaptseva, Alla S.; Hamasha, Safeia M.; Hansen, Stephanie B.; Ouart, Nicholas D.; Safronova, Ulyana I.

2002-12-01

249

NASA Astrophysics Data System (ADS)

To study temperature dependent elastic constants, a new computational method is proposed by combining continuum elasticity theory and first principles calculations. A Gibbs free energy function with one variable with respect to strain at given temperature and pressure was derived; hence, the minimization of the Gibbs free energy with respect to temperature and lattice parameters can be put into effective operation by using first principles. Therefore, with this new theory, anisotropic thermal expansion and temperature dependent elastic constants can be obtained for crystals with arbitrary symmetry. In addition, we apply our method to hexagonal beryllium, hexagonal diamond, and cubic diamond to illustrate its general applicability.

Shao, Tianjiao; Wen, Bin; Melnik, Roderick; Yao, Shan; Kawazoe, Yoshiyuki; Tian, Yongjun

2012-04-01

250

NASA Astrophysics Data System (ADS)

The corrosion of steels in liquid metal lead (Pb) and bismuth (Bi) is a critical challenge in the development of accelerator driven systems (ADS). Using a first-principles method with a slab model, we theoretically investigate the interaction between the Pb (Bi) atom and the iron (Fe) (100) surface to assess the fundamental corrosion properties. Our investigation demonstrates that both Pb and Bi atoms favorably adsorb on the (100) surface. Such an adsorption decreases the energy required for the dissociation of an Fe atom from the surface, enhancing the dissolution tendency significantly. The segregation of six common alloying elements (Cr, Al, Mn, Ni, Nb, and Si) to the surface and their impacts on the corrosion properties are also considered. The present results reveal that Si seems to have a relatively good performance to stabilize the surface and alleviate the dissolving trend caused by Pb and Bi.

Song, Chi; Li, Dong-Dong; Xu, Yi-Chun; Pan, Bi-Cai; Liu, Chang-Song; Wang, Zhi-Guang

2014-05-01

251

NASA Astrophysics Data System (ADS)

In this work we present the structural properties of lithium tetrasilicate glass (LS4) obtained by combined classical and Car-Parrinello molecular dynamics simulations. The computed features are compared with corresponding experimental results as well as to those obtained for sodium tetrasilicate glass (NS4) models generated using the same approach, and containing the some percentage of alkali oxide. The ab initio description improves the comparison of the structural characteristics with experimental data, in particular concerning the Li-O, Si-Li and Si-Si bond lengths. The influence of the network modifier nature on the relevant structural parameters is further examined in terms of their correlations to the first-principles calculated NMR parameters of our glassy models. For both glasses, 29Si MAS NMR measurements are reported and the obtained experimental spectra are compared to the theoretical ones.

Ispas, S.; Charpentier, T.; Mauri, F.; Neuville, D. R.

2010-02-01

252

NASA Astrophysics Data System (ADS)

On the plane-wave ultrasoft pseudopotential technique based on the first-principles density functional theory (DFT), we calculated the structural, elastic, electronic and optical properties of the seven different phases of SrZrO3. The obtained ground-state properties are in good agreement with previous experiments and calculations, which indicate that the most stable phase is orthorhombic Pnma structure. Seven phases of SrZrO3 are mechanically stable with cubic, tetragonal and orthorhombic structures. The mechanical and thermodynamic properties have been obtained by using the Voigt-Reuss-Hill approach and Debye-Grüneisen model. The electronic structures and optical properties are obtained and compared with the available experimental and theoretical data.

Liu, Qi-Jun; Liu, Zheng-Tang; Feng, Li-Ping; Tian, Hao

2012-12-01

253

NASA Astrophysics Data System (ADS)

We describe the geometric, energetic and electronic structure of the CuAu(1 0 0) surface using a combination of scanning tunneling microscopy (STM) and first-principles calculations based on density-functional theory (DFT). Our calculations show that the half-half Cu-Au termination is the one with lower surface energy, in agreement with experiments. The surface atomic rippling proposed by experimental data is also well reproduced by the calculations. STM images with atomic resolution display interesting voltage dependence, showing both types of atoms in the surface unit cell for lower voltages but just one type for higher voltages. Comparisons with theoretically simulated STM images and cross-sectional density of states profiles allows for an unambiguous assignment of Au atoms as the one appearing in higher-voltage images, thus providing chemical identification at the surface.

Dias, L. G.; Leitão, A. A.; Achete, C. A.; Blum, R.-P.; Niehus, H.; Capaz, Rodrigo B.

2007-12-01

254

NASA Astrophysics Data System (ADS)

The magnetization behaviour of the ferromagnetic shape memory Heusler Ni2MnGa alloy under applied magnetic fields is studied using first-principles and Monte Carlo (MC) calculations. Calculations were carried out for single-crystal and polycrystalline structures with magnetic domains. In the multi-domain approach, the stochastic competition between the magnetic anisotropy field and the external magnetic field is taken into account by introducing a probability factor. By constructing a complex Hamiltonian model with ab initio input parameters, we can predict the temperature dependence of the magnetization in Heusler alloys for low and high magnetic fields by means of MC simulations. The theoretical iso-field magnetization curves are in good agreement with experimental data.

Sokolovskiy, V. V.; Pavlukhina, O.; Buchelnikov, V. D.; Entel, P.

2014-10-01

255

NASA Astrophysics Data System (ADS)

We study the influence of the tetragonalization occurring during the martensitic phase transition on the exchange interactions in Ni2MnGa Heusler alloy using first-principles calculations in conjunction with the frozen-magnon approximation. We show that the tetragonalization alters only the exchange constants characterizing the Mn-Mn interactions. Calculated Curie temperatures within the random-phase approximation are found to agree with experimental data. Moreover, we study the temperature dependence of the magnetization and the small deviation from the experimental data exactly at the temperature of the phase transition is discussed. Obtained results agree with previous theoretical results using Liechtenstein's formula to calculate the exchange constants and the Monte Carlo simulation technique to estimate the Curie temperature.

Galanakis, I.; ?a??o?lu, E.

2011-06-01

256

Structural and elastic properties of TiN and AlN compounds: first-principles study

NASA Astrophysics Data System (ADS)

First-principles calculations of the lattice constants, bulk modulus, pressure derivatives of the bulk modulus and elastic constants of AlN and TiN compounds in rock-salt (B1) and wurtzite (B4) structures are presented. We have used the fullpotential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT) in the generalized gradient approximation (GGA) for the exchange-correlation functional. Moreover, the elastic properties of cubic TiN and hexagonal AlN, including elastic constants, bulk and shear moduli are determined and compared with previous experimental and theoretical data. Our results show that the structural transition at 0 K from wurtzite to rock-salt phase occurs at 10 GPa and -26 GPa for AlN and TiN, respectively. These results are consistent with those of other studies found in the literature.

Fodil, Meriem; Mounir, Amine; Ameri, Mohammed; Baltache, Hadj; Bouhafs, Bachir; Al-Douri, Y.; Ameri, Ibrahim

2014-06-01

257

Prediction of MAX phases, VN +1SiCN (N=1,2), from first-principles theory

NASA Astrophysics Data System (ADS)

We have investigated the phase stability of two MAX phases, V3SiC2 and V2SiC, by means of first-principles total-energy calculations within the generalized-gradient approximation and the projector-augmented wave method. The theoretical bulk modulus of V3SiC2 is 219GPa, which is ˜17% larger than that of Ti3SiC2 (187GPa ). The total-energy calculations show that V2SiC is stable with a formation energy of about 0.27eV?f.u. and that V3SiC2 is metastable (only 0.02eV?f.u. is required to stabilize this phase from its competing phases). We suggest that both these two MAX compounds should be possible to synthesize as stable (or metastable) phases using, e.g., thin-film deposition.

Fang, C. M.; Ahuja, R.; Eriksson, O.

2007-01-01

258

Predicting Raman Spectra of Aqueous Silica and Alumina Species in Solution From First Principles

NASA Astrophysics Data System (ADS)

Dissolved silica and alumina play an important role in lithospheric fluid chemistry. Silica concentrations in aqueous fluids vary over the range of crustal temperatures and pressures enough to allow for significant mass transport of silica via fluid-rock interaction. The polymerization of silica, and the possible incorporation of alumina into the polymer structure, could afford crystal-like or melt-like sites to otherwise insoluble elements such as titanium, leading to enhanced mobility. Raman spectroscopy in a hydrothermal diamond anvil cell (HDAC) has been used to study silica polymerization at elevated pressure and temperature [Ref. 1, 2], but Raman spectra of expected solutes are not fully understood. We calculated Raman spectra of H4SiO4 monomers, H6Si2O7 dimers, and H6SiAlO_7^- dimers, from first principles using hybrid density functional theory (B3LYP). These spectra take into account the variation in bridging angle (Si-O-Si and Si-O-Al angles) that the dimers will have at a given temperature by calculating a potential energy surface of the dimer as the bridging angle varies, and using a Boltzmann distribution at that temperature to determine relative populations at each geometry. Solution effects can be incorporated by using a polarizable continuum model (PCM), and a potential energy surface has been constructed for the silica dimer using a PCM. The bridging angle variation explains the broadness of the 630 cm^-^1 silica dimer peak observed in HDAC experiments [Ref. 1, 2] at high temperatures. The silica-alumina dimer bridging angle is shown to be stiffer than the silica dimer bridging angle, which results in a much narrower main peak. The synthetic spectrum obtained for the silica-alumina dimer suggests that there may be a higher ratio of complexed alumina to free alumina in solution at highly basic pH than previously estimated [Ref. 3]. References: 1. Zotov, N. and H. Keppler, Chemical Geology, 2002. 184: p. 71-82. 2. Zotov, N. and H. Keppler, American Mineralogist, 2000. 85: p. 600-603. 3. Gout, R., et al., Journal of Solution Chemistry, 2000. 29: p. 1173-1186.

Hunt, J. D.; Schauble, E. A.; Manning, C. E.

2006-12-01

259

Cesium stability in a typical mica structure in dry and wet environments from first-principles

NASA Astrophysics Data System (ADS)

Cesium ion stability in a typical mica structure in various environments of solid salts (XCl; X = Cs, K, and Na), metals (X) and saltwaters (XCl aqueous liquids) was investigated using first-principles density-functional theory (DFT) with the Perdew-Burke-Ernzerhof (PBE) functional as well as a van der Waals (vdW) corrected functional (vdW-DFC09x). We specifically examined interlayer ion-exchange in bulk phlogopite-type mica, which is expected to produce a well-defined benchmark in a thermodynamic equilibrium state. In general, theoretical models have well reproduced the experimental and theoretical data found in the literature from the viewpoints of structure, heat capacity, and entropy. The vdW-DFC09x lattice parameters of the mica appear to be better reproducible than the PBE parameters are. However, the vdW correction calculations of the thermodynamic properties with the harmonic approximation using the phonon frequencies showed poor results in some cases, whereas the PBE calculations yielded robust and reasonable results in terms of structure and thermodynamic properties. The isotope effect of the 137Cs atom appears to be confined in thermodynamic properties such as entropy, heat capacity, and ion-exchange energy, although the theoretical infrared spectra showed a small redshift ca. 1 cm-1 in the far-infrared region of 50-75 cm-1. The calculated RDF and the coordination number for X-O (i.e., X-H2O) for the saltwater model indicated that the Cs, K, and Na ions with respective hydrated radii of 0.323, 0.284, and 0.238 nm were surrounded, respectively, by 6.5, 4.5, and 4.0 of H2O molecules in a water solution. Ion-exchange energy values based on free-energy calculations around ambient temperatures derived using the PBE functional and a harmonic approximation suggest that the cesium ion in mica interlayer phlogopite is stable in an environment consisting of KCl, NaCl, K, and Na solids, and in NaCl saltwater as well. However, it can be exchanged competitively by potassium ion in the KCl saltwater environment. The theoretical ion-selectivity order of the mica interlayer site is Cs > K > Na (PBE, vdW-DFC09x) in the solid environment, whereas the order of K ? Cs > Na (PBE) or K > Cs ? Na (vdW-DFC09x) is suggested in the saltwater environment. This selectivity-order difference between Cs and K underscores the importance of investigating the physical and chemical states of the counterphase as well as the host materials, and suggests that catching and releasing of the Cs atom in micaceous soil is possible through solvent control from a thermodynamic perspective.

Suehara, Shigeru; Yamada, Hirohisa

2013-05-01

260

NASA Astrophysics Data System (ADS)

Knowledge of near infrared intensities of rovibrational transitions of polyatomic molecules is essential for the modeling of various planetary atmospheres, brown dwarfs and for other astrophysical applications 1,2,3. For example, to analyze exoplanets, atmospheric models have been developed, thus making the need to provide accurate spectroscopic data. Consequently, the spectral characterization of such planetary objects relies on the necessity of having adequate and reliable molecular data in extreme conditions (temperature, optical path length, pressure). On the other hand, in the modeling of astrophysical opacities, millions of lines are generally involved and the line-by-line extraction is clearly not feasible in laboratory measurements. It is thus suggested that this large amount of data could be interpreted only by reliable theoretical predictions. There exists essentially two theoretical approaches for the computation and prediction of spectra. The first one is based on empirically-fitted effective spectroscopic models. Another way for computing energies, line positions and intensities is based on global variational calculations using ab initio surfaces. They do not yet reach the spectroscopic accuracy stricto sensu but implicitly account for all intramolecular interactions including resonance couplings in a wide spectral range. The final aim of this work is to provide reliable predictions which could be quantitatively accurate with respect to the precision of available observations and as complete as possible. All this thus requires extensive first-principles quantum mechanical calculations essentially based on three necessary ingredients which are (i) accurate intramolecular potential energy surface and dipole moment surface components well-defined in a large range of vibrational displacements and (ii) efficient computational methods combined with suitable choices of coordinates to account for molecular symmetry properties and to achieve a good numerical convergence. Because high-resolution ab initio spectra predictions for systems with N>4 atoms is a very challenging task, the major issue is to minimize the cost of computations and the loss of accuracy during calculations. To this end, a truncation-reduction technique for the Hamiltonian operator as well as an extraction-compression procedure for the basis set functions will be introduced and discussed in detail. We will give a review on the recent progress in computational methods as well as on existing experimental and theoretical databases 4,5,6,7,8,9. This presentation will be focused on highly symmetric molecules such as methane and phosphine, with the corresponding applications at low-T in relation with Titan's atmosphere and at high-T with the production of theoretical line lists for astrophysical opacity calculations10. The study of isotopic H?D and 12C?13C substitutions will be also addressed and carried out by means of symmetry and coordinate transformations11. Finally we hope this work will help refining studies of currently available analyses which are not yet finalized. The modeling of non-LTE emissions accounting for contribution of many fundamental and hot bands could also be possible. Support from PNP (French CNRS national planetology program) is acknowledged.

Rey, M.; Nikitin, A. V.; Tyuterev, V.

2014-06-01

261

Application of first-principles calculations to the design of rechargeable Li-batteries

Rechargeable Li batteries consist of an anode, electrolyte, and cathode. The cathode is typically an oxide that intercalates Li at very low chemical potential ensuring a large open-cell voltage for the battery. We show how first-principles pseudopotential calculations can be used to predict the intercalation voltage for these materials. By means of a series of computational experiments on virtual structures,

G. Ceder; M. K. Aydinol; A. F. Kohan

1997-01-01

262

Effect of dopants on grain boundary decohesion of Ni: A first-principles study

First-principles density functional theory (DFT) calculations were used to determine decohesion properties of ?5(012) grain boundary of Ni with dopants B, C, S, Cr, and Hf. The relative stability of sites was evaluated and cleavage energies were calculated. Electronic structure was used to understand these properties in terms of changes in bonding with addition of dopants. It was found that

Suchismita Sanyal; Umesh V. Waghmare; P. R. Subramanian; Michael F. X. Gigliotti

2008-01-01

263

In this concept paper, the development of strategies for the integration of first-principles methods with crystallographic database mining for the discovery and design of novel ferroelectric materials is discussed, drawing on the results and experience derived from exploratory investigations on three different systems: (1) the double perovskite Sr(Sb{sub 1/2}Mn{sub 1/2})O{sub 3} as a candidate semiconducting ferroelectric; (2) polar derivatives of schafarzikite MSb{sub 2}O{sub 4}; and (3) ferroelectric semiconductors with formula M{sub 2}P{sub 2}(S,Se){sub 6}. A variety of avenues for further research and investigation are suggested, including automated structure type classification, low-symmetry improper ferroelectrics, and high-throughput first-principles searches for additional representatives of structural families with desirable functional properties. - Graphical abstract: Integration of first-principles methods with crystallographic database mining, for the discovery and design of novel ferroelectric materials, could potentially lead to new classes of multifunctional materials. Highlights: Black-Right-Pointing-Pointer Integration of first-principles methods and database mining. Black-Right-Pointing-Pointer Minor structural families with desirable functional properties. Black-Right-Pointing-Pointer Survey of polar entries in the Inorganic Crystal Structural Database.

Bennett, Joseph W. [Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854 (United States)] [Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854 (United States); Rabe, Karin M., E-mail: rabe@physics.rutgers.edu [Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854 (United States)

2012-11-15

264

Prediction of a new layered phase of nitrogen from first-principles simulations

A new phase of nonmolecular nitrogen has been suggested by first-principles density functional theory simulations at high pressures. The phase has zigzag chains similar to the BP, A7 and LB phases, but the zigzag chains in the PP phase are connected by two atoms with double bonds with Pmna symmetry, which we designated as the PP phase. Properties of PP

X. L. Wang; Z. He; Y. M. Ma; T. Cui; Z. M. Liu; B. B. Liu; J. F. Li; G. T. Zou

2007-01-01

265

We apply first-principles calculations to study structural and electronic properties of a semiconductor carbon nanotube bent to a large angle. The geometry optimization results in fourfold carbon rings at the bend region. These fourfold rings, seldomly seen in carbon structures, result from the collapse of carbon hexagons at the most stressed region of the bend. Part of the atoms at

Mario S. C. Mazzoni; Helio Chacham

2000-01-01

266

First-principles study of hydrogen storage over Ni and Rh doped BN sheets

Absorption of hydrogen molecules on Nickel and Rhodium-doped hexagonal boron nitride (BN) sheet is investigated by using the first principle method. The most stable site for the Ni atom was the on top side of nitrogen atom, while Rh atoms deservers a hollow site over the hexagonal BN sheet. The first hydrogen molecule was absorbed dissociatively over Rh atom, and

Natarajan Sathiyamoorthy Venkataramanan; Mohammad Khazaei; Ryoji Sahara; Hiroshi Mizuseki; Yoshiyuki Kawazoe

2009-01-01

267

Employing a first-principles method, we have studied the stability, diffusivity, and permeation properties of hydrogen (H) and its isotopes in bcc vanadium (V). A single H atom is found to favor the tetrahedral interstitial site (TIS) in V. The charge density distribution exhibits a strong interaction between H and its neighbor V atoms. Analysis of DOS and Bader charge reveals

Jian Luo; Hong-Bo Zhou; Yue-Lin Liu; Li-Jiang Gui; Shuo Jin; Ying Zhang; Guang-Hong Lu

2011-01-01

268

Defect and solute properties in dilute Fe-Cr-Ni austenitic alloys from first principles

We present results of an extensive set of first-principles density functional theory calculations of point defect formation, binding, and clustering energies in austenitic Fe with dilute concentrations of Cr and Ni solutes. A large number of possible collinear magnetic structures were investigated as appropriate reference states for austenite. We found that the antiferromagnetic single- and double-layer structures with tetragonal relaxation

T. P. C. Klaver; D. J. Hepburn; G. J. Ackland

2012-01-01

269

Diffusion of hydrogen in bcc tungsten studied with first principle calculations

First principle calculations were used to study the hydrogen migration properties in bulk bcc tungsten. Hydrogen has low solubility in tungsten and occupies the tetrahedral interstitial site with an energy difference of 0.38 eV compared to the octahedral interstitial site. The hydrogen diffusion coefficient was evaluated using the harmonic transition state theory and was found to agree with the experimental

K. Heinola; T. Ahlgren

2010-01-01

270

First principles study of segregation to the Sigma5(310) grain boundary of cubic zirconia

While yttrium and impurity segregation at interfaces of yttria-stabilized zirconia (YSZ) has been intensively studied experimentally, the mechanisms governing the propensity for segregation are still not fully understood. The segregation energetics of yttrium and aluminum, another common segregant at interfaces of YSZ, were studied by means of first principles calculations based on density functional theory. Site-dependent formation energies were calculated

A. G. Marinopoulos

2011-01-01

271

Turtles All The Way Down: A Clean-Slate, Ground-Up, First-Principles

Turtles All The Way Down: A Clean-Slate, Ground-Up, First-Principles Approach to Secure Systems theory. Categories and Subject Descriptors D.4.6 [Operating Systems]: Security and Protection; K.6.5 [Management of Computing and Information Sys- tems]: Security and Protection; H.1 [Information Sys- tems

Peisert, Sean

272

Calculation of solubility in titanium alloys from first principles Roman V. Chepulskii, Stefano solubility in binary alloys based on the statistical-thermodynamic theory of dilute lattice gas. The model dependence determined by a ``low-solubility formation enthalpy". This quantity, directly obtainable from

Curtarolo, Stefano

273

First principles simulations of Li ion migration in materials related to LiPON electrolytes a

First principles simulations of Li ion migration in materials related to LiPON electrolytes, USA Â· Comments on solid electrolytes Â· Overview of LiPON family Â· Computational methods Â· Simulations-UniversitÂ¨at Bochum, Germany 216th ECS Meeting, Vienna 1 #12;Solid vs liquid electrolytes in Li ion batteries Solid

Holzwarth, Natalie

274

Super hard cubic phases of period VI transition metal nitrides: First principles investigation

Super hard cubic phases of period VI transition metal nitrides: First principles investigation S July 2008 Keywords: Coatings Elastic properties Hardness Nitrides We report a systematic studyN in rocksalt phase has a bulk modulus of 380 GPa making them candidates for super hardness. Based on the bulk

Khare, Sanjay V.

275

Learning Biped Locomotion from First Principles on a Simulated Humanoid Robot

Learning Biped Locomotion from First Principles on a Simulated Humanoid Robot using Linear Genetic for- ward locomotion behavior. An application of this system with two phases of evolution could of biped gait. The traditional way of robotics locomotion control is based on derivation of an internal

Fernandez, Thomas

276

Metodi dell'Ingegneria, UniversitÃ¡ di Modena e Reggio Emilia, via Fogliani 1, I-42100 Reggio Emilia a first-principles approach Mauro Bruno,1 Maurizia Palummo,1 Andrea Marini,1 Rodolfo Del Sole,1 Valerio a single- particle approach.20 However, quantum confinement is known to strongly modify optical response

Marini, Andrea

277

Defect states in carbon nanotubes and related band structure engineering: A first-principles study

Electronic structures of faulted nanosystems are of particular technological relevance because realistic large scale synthesis of nanostructures inevitably leads to defects of one form or the other. In this work, we determine the atomic and electronic structures of carbon nanotubes (CNTs) with two of the major types of defects using first-principles pseudopotential-based density functional theory calculations: (i) substitution with other

Mousumi Upadhyay Kahaly

2009-01-01

278

Development of First Principles Capacity Fade Model for Li-Ion Cells

developed to simulate the capacity fade of Li-ion batteries. Incorporation of a continuous occurrence a first principles capacity fade model for Li-ion batteries. Darling and Newman1 made a first attempt into a lithium-ion battery model. The model explains the self-discharge process occurring in Li-ion cells

Popov, Branko N.

279

First Principles Modeling of Tunnel Magnetoresistance of Fe=MgO=Fe Trilayers Derek Waldron,1

First Principles Modeling of Tunnel Magnetoresistance of Fe=MgO=Fe Trilayers Derek Waldron,1 of Fe=MgO=Fe trilayer structures. The zero bias tunnel magnetoresistance is found to be several thousand magnetoresistance for devices without oxidization reduces monotonically to zero with a voltage scale of about 0

Gao, Hongjun

280

Thermal Conductivity of Periclase (MgO) from First Principles Stephen Stackhouse*

Thermal Conductivity of Periclase (MgO) from First Principles Stephen Stackhouse* Department thermal conductivity k of periclase (MgO) up to conditions representative of the Earth's core, 83.10.Rs Thermal conductivity is central to our understanding of planetary evolution as it sets

Stixrude, Lars

281

A first-principles model of anomalous thermal transport based on numericalsimulations is presented, with stringent comparisons to experimental datafrom the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324(1992)]. This model is based on nonlinear gyrofluid simulations, which predictthe fluctuation and thermal transport characteristics of toroidal iontemperature-gradient-driven (ITG) turbulence, and on comprehensive lineargyrokinetic ballooning calculations, which provide...

M. Kotschenreuther; W. Dorland; M. A. Beer; G. W. Hammett

1995-01-01

282

Biophysical Modeling of the Temporal Niche: From First Principles to the Evolution of Activity://www.jstor.org #12;vol. 179, no. 6 the american naturalist june 2012 Biophysical Modeling of the Temporal Niche: From of diurnal versus noc- turnal activity using a biophysical model to evaluate the preferred temporal niche

Porter, Warren P.

283

First-Principles Computation of the Vibrational Entropy of Ordered and Disordered Ni3Al

There is increasing evidence that vibrational entropy may significantly contribute to the entropy difference between the ordered and the disordered states of a compound. Through first-principles calculations, we investigate the magnitude of this vibrational entropy difference in Ni3Al, a compound where this effect is believed to be especially large. We find the vibrational entropy difference to be essentially zero and

A. van de Walle; G. Ceder; U. V. Waghmare

1998-01-01

284

Pressure dependent phase stability transformations of GaS: A first principles study

Pressure dependent phase stability transformations of GaS: A first principles study Bin Wen a principle calculations are used to determine the pressure dependent phase stability transformations for GaS polytypes at pressures up to 1000 GPa. Our results indicate that the relative stability sequence changes

Melnik, Roderick

285

NASA Astrophysics Data System (ADS)

In this paper, we propose a new active remote sensing methodology, based on laser spectroscopy, to evaluate the content of atmospheric trace gases. Its principle consists in coupling a lidar with optical correlation spectroscopy (OCS-lidar). Our theoretical and numerical studies show that OCS-lidar is a robust measurement methodology allowing trace gases environmental, agricultural, and industrial plants surveys. The novelty of this work is threefold. Firstly, we develop a new formalism to remotely evaluate the target gas concentration from optical correlation spectroscopy. Secondly, an acousto-optical programmable dispersive filter has been used to ensure that the lidar signal be spectrally correlated with the target gas of interest. It avoids using a hazardous gas reference cell, as operated in conventional OCS devices. Moreover, a clever spectral correlation is achieved since the contribution of absorption interfering species can then be minimized. Thirdly, to evaluate the performance of the OCS-lidar methodology, a numerical study of methane greenhouse gas is presented to evidence that atmospheric methane mixing ratios are retrievable over two orders of magnitude, from background level up to 100 ppb, within 100-m range resolution. Evaluation of the accuracy and the detection limit, including statistical and systematic errors assessment, are then objectively presented and discussed.

Thomas, B.; Miffre, A.; David, G.; Cariou, J.-P.; Rairoux, P.

2012-09-01

286

Experimental and theoretical investigation on the vibrational spectroscopy of L-theanine

NASA Astrophysics Data System (ADS)

In this work, experimental and theoretical investigations on vibrational spectroscopy of L-theanine were presented. FT-IR and Raman spectra of L-theanine powder sample were recorded and corresponding theoretical calculations were performed based on Density Functional Theory (DFT) at B3LYP level using 6-31++G(d,p) and 6-311++G(d,p) basis sets combined with the Polarized Continuum Model (PCM) with water as the solvent. The experimental vibrational bands were assigned based on the basis of calculations while the predicted geometric parameters were compared with those obtained in experiment, most of the bands measured were well reproduced in the calculations while the discrepancies are significant for the bands mainly related to the vibrations of protonated amino group ( NH3+) and ionized carboxyl group (COO -), which are affected by the intramolecular hydrogen bond interaction. Good agreements between the theoretical and experimental results confirm the feasibility of the DFT method combined with PCM in the study of the molecular structure and vibrational spectra of L-theanine.

Chen, Yongjian; Xi, Gangqin; Chen, Rong; Li, Yongzeng; Feng, Shangyuan; Lei, Jinping; Lin, Hongxing

2011-12-01

287

Solution-based thermodynamic modeling of the Ni-Al-Mo system using first-principles calculations

A solution-based thermodynamic description of the ternary Ni–Al–Mo system is developed here, incorporating first-principles calculations and reported modeling of the binary Ni–Al, Ni–Mo and Al–Mo systems. To search for the configurations with the lowest energies of the N phase, the Alloy Theoretic Automated Toolkit (ATAT) was employed and combined with VASP. The liquid, bcc and ?-fcc phases are modeled as random atomic solutions, and the ??-Ni3Al phase is modeled by describing the ordering within the fcc structure using two sublattices, summarized as (Al,Mo,Ni)0.75(Al,Mo,Ni)0.25. Thus, ?-fcc and ??-Ni3Al are modeled with a single Gibbs free energy function with appropriate treatment of the chemical ordering contribution. In addition, notable improvements are the following: first, the ternary effects of Mo and Al in the B2-NiAl and D0a-Ni3Mo phases, respectively, are considered; second, the N-NiAl8Mo3 phase is described as a solid solution using a three-sublattice model; third, the X-Ni14Al75Mo11 phase is treated as a stoichiometric compound. Model parameters are evaluated using first-principles calculations of zero-Kelvin formation enthalpies and reported experimental data. In comparison with the enthalpies of formation for the compounds ?-AlMo, ?-Al8Mo3 and B2-NiAl, the first-principles results indicate that the N-NiAl8Mo3 phase, which is stable at high temperatures, decomposes into other phases at low temperature. Resulting phase equilibria are summarized in the form of isothermal sections and liquidus projections. To clearly identify the relationship between the ?-fcc and ??-Ni3Al phases in the ternary Ni–Al–Mo system, the specific ?-fcc and ??-Ni3Al phase fields are plotted in x(Al)–x(Mo)–T space for a temperature range 1200–1800 K.

Zhou, S H [Ames Laboratory; Wang, Y [Pennsylvania State University; Chen, L -Q [Pennsylvania State University; Liu, Z -K [Pennsylvania State University; Napolitano, R E [Ames Laboratory

2014-09-01

288

A comparative first-principles study of martensitic phase transformations in TiPd2 and TiPd

Martensitic phase transformations in TiPd2 and TiPd alloys are studied employing density-functional, first-principles calculations. We examine the transformation of tetragonal C11b TiPd2 to the low-temperature orthorhombic phase (C11b oI6), and the transformation of cubic B2 TiPd under orthorhombic (B2 B19) and subsequent monoclinic transformations (B19 B19 ) as the system is cooled. To evaluate the transition temperature for TiPd2 we employ a theoretical approach based on a phenomenological Landau theory of the structural phase transition and a mean-field approximation for the free energy, utilizing first-principles calculations to obtain the deformation energy as a function of strains and to deduce parameters for constructing the free energy. The predicted transition temperature for the TiPd2 C11b oI6 transition temperature is in good agreement with reported experimental results. To investigate the TiPd B2 B19 transformation, we employ both the Cauchy-Born rule and a soft-mode- based approach, and elucidate on the importance of coupling of lattice distortion and atomic displacements (i.e., shuffling) in the formation of the final structure. The estimated B2 B19 transition temperature for TiPd system agrees well with the experimental results. We also find that there exists a very small but finite (0.0005 eV/atom) energy barrier of B19 TiPd under monoclinic deformation for B19 B19 structural phase transformation.

Krcmar, Maja [Grand Valley State University (GVSU), Michigan] [Grand Valley State University (GVSU), Michigan; Morris, James R [ORNL] [ORNL

2014-01-01

289

Theoretical investigation of the instrumental peak line shape in ion-scattering spectroscopy

NASA Astrophysics Data System (ADS)

The instrumental peak line shape describing the elastic scattering of a single isotopic primary ion off a single isotopic target surface has been investigated theoretically using a Bohr-screened Coulomb potential, the Thomas-Fermi-Firsov-screened Coulomb potential, and the Ziegler-Biersack-Littmark universal potential. Experimental factors such as the energy spread in the primary ion beam and the spectral resolution (channel width) have also been taken into account in this study. The calculated line shapes have been determined for 100 eV 4He scattering off 27Al and 193Nb with a scattering angle ?=143° and a ?? of +/-12°. The results show that the ion-scattering spectroscopy peaks are asymmetrical due to the variation in differential scattering cross section with energy. This asymmetry is more pronounced for Al (lighter mass), higher spectral resolution, and narrower primary ion energy distributions.

Young, Vaneica Y.; Welcome, Nicole; Hoflund, Gar B.

1993-08-01

290

FT-Raman, surface-enhanced Raman spectroscopy and theoretical investigations of diclofenac sodium

NASA Astrophysics Data System (ADS)

Raman and surface-enhanced Raman (SER) spectroscopies have been applied to the vibrational characterization of diclofenac sodium (DCF-Na). Theoretical calculations (DFT and ab initio) of two DCF-Na conformers have been performed to find the optimized structure and computed vibrational wavenumbers of the most stable one. SER spectra in silver colloid at different pH values have been also recorded and analyzed. Good SER spectra have been obtained in acidic and neutral environments, proving the chemisorption of the DCF-Na molecule on the silver surface. In the investigated pH range the carboxylate anion has been bonded to the silver surface through the lone pair oxygen electrons. The phenyl rings' orientation with respect to the silver surface changed on passing from acidic to neutral pH from a tilted close to flat to a more perpendicular one.

Iliescu, T.; Baia, M.; Kiefer, W.

2004-03-01

291

A new chiral octopolar C(3)-symmetric molecule is synthesized and its optical and chiro-optical properties are analyzed with the help of density functional theory. In all the cases, the work relates these spectroscopic properties with the phenomena of aggregation. Ultraviolet-visible absorption, infrared and Raman spectroscopies, and ECD and VCD chiro-optical spectroscopies are presented in combination with theoretical chemistry. In this regard, optical spectroscopies are insensitive to the supramolecular effects, while electronic and vibrational circular dichroism spectroscopies delivered adequate proof for the detection of aggregation. We have simulated the effect of molecular self-assembly by means of a docking process that provides a stable dimer on which all the spectroscopic properties have been theoretically calculated. The shape of this dimer provides us with a guideline to propose a helicoidal aggregate that is the responsibility of the chiro-optical spectra amplified by self-assembly. PMID:20380428

Nieto, Belen; Ramírez, Francisco J; Hennrich, Gunther; Gómez-Lor, Berta; Casado, Juan; López Navarrete, Juan T

2010-05-01

292

A First Principles Molecular Dynamics Study Of Calcium Ion In Water

In this work we report on Car-Parrinello simulations of the divalent calcium ion in water, aimed at understanding the structure of the hydration shell and at comparing theoretical results with a series of recent experiments. Our paper shows some of the progress in the investigation of aqueous solutions brought about by the advent of ab initio molecular dynamics and highlights the importance of accessing subtle details of ion-water interactions from first-principles. Calcium plays a vital role in many biological systems, including signal transduction, blood clotting and cell division. In particular, calcium ions are known to interact strongly with proteins as they tend to bind well to both negatively charged (e.g. in aspartate and glutamate) and uncharged oxygens (e.g. in main-chain carbonyls). The ability of calcium to coordinate multiple ligands (from 6 to 8 oxygen atoms) with an asymmetric coordination shell enables it to cross-link different segments of a protein and induce large conformational changes. The great biochemical importance of the calcium ion has led to a number of studies to determine its hydration shell and its preferred coordination number in water. Experimental studies have used a variety of techniques, including XRD, EXAFS, and neutron diffraction to elucidate the coordination of Ca{sup 2+} in water. The range of coordination numbers (n{sub C}) inferred by X-ray diffraction studies varies from 6 to 8, and is consistent with that reported in EXAFS experiments (8 and 7.2). A wider range of values (6 to 10) was found in early neutron diffraction studies, depending on concentration, while a more recent measurement by Badyal, et al. reports a value close to 7. In addition to experimental measurements, many theoretical studies have been carried out to investigate the solvation of Ca{sup 2+} in water and have also reported a wide range of coordination numbers. Most of the classical molecular dynamics (MD) and QM/MM simulations report n{sub C} in the range of 8 to 10; in general, n{sub C} appears to be highly sensitive to the choice of the ion-water potential used in the calculations. Even ab initio MD simulations have so far obtained conflicting values for n{sub C}. For the structure of the first salvation shell Naor, et al. found n{sub C} = 7 to 8 and a Ca{sup 2+} - oxygen average distance (r{sub Ca-O}) of 2.64 {angstrom}, while Bako, et al. found n{sub C} = 6 and r{sub Ca-O} = 2.45 {angstrom}. In view of the existing controversies, we have carried out extensive Car-Parrinello simulations of Ca{sup 2+} solvation in water, using both a rigid and a flexible water model, up to time scales of 40 ps. Our simulations show variations of coordination numbers from 6, 7 and 8 occurring over intervals of {approx} 0.3/0.4 exchanges/ps, and yielding average coordination numbers of 6.2 and 7 for flexible and rigid water models, respectively. These results are consistent with those reported in recent EXAFS and neutron diffraction experiments. In addition, our calculations show an asymmetric coordination of Ca{sup 2+} to oxygen, similar to the case of Mg{sup 2+}.

Lightstone, F; Schwegler, E; Allesch, M; Gygi, F; Galli, G

2005-01-28

293

First-principles study on elastic constants of C-S-H type minerals

NASA Astrophysics Data System (ADS)

Calcium-Silicate-Hydrate (C-S-H) is the mineral binding phase of all Portland concrete materials, and the principle source of their strength and stiffness. Despite decades of research, the elastic properties of C-S-H mineral crystals are unknown. Here we investigate two natural analogs of C-S-H, tobermorite and jennite, and characterize their mechanical properties by first-principles calculations. First, we calculate their lattice parameters and elastic constants. Second, we show that in contrast to previous suggestions, for natural tobermorite 11 Å , the mechanically weakest directions are two inclined regions that form a hinge mechanism. By studying bond length changes under deformation in tobermorite 14 Å and jennite, we show that water molecules play a major structural role in defining their elastic properties. Averaged elastic moduli obtained by first-principles calculations of tobermorite 14 Å and jennite compare well with corresponding nanoindentation experiment on C-S-H.

Shahsavari, R.; Buehler, M.; Ulm, F.

2008-12-01

294

NASA Astrophysics Data System (ADS)

In this concept paper, the development of strategies for the integration of first-principles methods with crystallographic database mining for the discovery and design of novel ferroelectric materials is discussed, drawing on the results and experience derived from exploratory investigations on three different systems: (1) the double perovskite Sr(Sb1/2Mn1/2)O3 as a candidate semiconducting ferroelectric; (2) polar derivatives of schafarzikite MSb2O4; and (3) ferroelectric semiconductors with formula M2P2(S,Se)6. A variety of avenues for further research and investigation are suggested, including automated structure type classification, low-symmetry improper ferroelectrics, and high-throughput first-principles searches for additional representatives of structural families with desirable functional properties.

Bennett, Joseph W.; Rabe, Karin M.

2012-11-01

295

First-principles prediction of lattice thermal conductivity ?_{L} of strongly anharmonic crystals is a long-standing challenge in solid-state physics. Making use of recent advances in information science, we propose a systematic and rigorous approach to this problem, compressive sensing lattice dynamics. Compressive sensing is used to select the physically important terms in the lattice dynamics model and determine their values in one shot. Nonintuitively, high accuracy is achieved when the model is trained on first-principles forces in quasirandom atomic configurations. The method is demonstrated for Si, NaCl, and Cu_{12}Sb_{4}S_{13}, an earth-abundant thermoelectric with strong phonon-phonon interactions that limit the room-temperature ?_{L} to values near the amorphous limit. PMID:25396378

Zhou, Fei; Nielson, Weston; Xia, Yi; Ozoli?š, Vidvuds

2014-10-31

296

First-principles based calculation of phonon spectrain substitutionally disordered alloys

NASA Astrophysics Data System (ADS)

A first-principles based solution to the longstanding problem of calculating the phonon spectra in substitutional disordered alloys where strong force-constant disorder plays a significantrole is provided by a combination of first-principles electronicstructure tools, physically reasonable models of force-constant in alloyenvironments, and the Itinerant Coherent-Potntial Approximation (ICPA) by Ghosh and co-workers (S. Ghosh et. al., Physical Review B 66, 214206 (2002)). Wehere present the salient features of such hybrid formalism and illustrate its capability by the computation of phonon spectrafor disordered alloys with large size mismatch of end point components. We demonstrate that the consideration of local environments insize-mismatched alloys is crucial in understanding the microscopicinterplay of forces between various pairs of chemical specie and a correctdepiction of these is important for computation of accurate phonondispersions in these systems.

Ghosh, Subhradip

2013-02-01

297

A Massive Core in Jupiter Predicted From First-Principles Simulations

Hydrogen-helium mixtures at conditions of Jupiter's interior are studied with first-principles computer simulations. The resulting equation of state (EOS) implies that Jupiter possesses a central core of 14-18 Earth masses of heavier elements, a result that supports core accretion as standard model for the formation of hydrogen-rich giant planets. Our nominal model has about 2 Earth masses of planetary ices in the H-He-rich mantle, a result that is, within modeling errors, consistent with abundances measured by the 1995 Galileo Entry Probe mission (equivalent to about 5 Earth masses of planetary ices when extrapolated to the mantle), suggesting that the composition found by the probe may be representative of the entire planet. Interior models derived from this first-principles EOS do not give a match to Jupiter's gravity moment J4 unless one invokes interior differential rotation, implying that jovian interior dynamics has an observable effect on the measured gravity field.

B. Militzer; W. B. Hubbard; J. Vorberger; I. Tamblyn; S. A. Bonev

2008-07-26

298

Large-scale first-principles molecular dynamics for electrochemical systems with O(N) methods

NASA Astrophysics Data System (ADS)

A method for large-scale first-principles molecular dynamics (MD) simulations on electrochemical systems has been developed by combining the effective screening medium (ESM) method with O(N) density functional theory (DFT). This implementation has been significantly simplified by the introduction of neutral atom potentials, which minimizes the modifications to existing DFT code. In order to demonstrate ability of this implementation, it has been applied to an electrochemical system consisting of a H-Si(111) electrode, which is a candidate anode for high-capacity Li-ion secondary batteries, and a propylene carbonate (PC) solvent to simulate how PC molecules in the vicinity of the electrode surface respond to an imposed electric field. The large-scale MD simulation clearly demonstrates that the combination of the ESM and O(N) DFT methods provides a useful tool for first-principles investigation of complicated electrochemical systems such as high-capacity batteries.

Ohwaki, Tsukuru; Otani, Minoru; Ikeshoji, Tamio; Ozaki, Taisuke

2012-04-01

299

First-principles investigation of the alloy scattering potential in dilute Si1-xCx

NASA Astrophysics Data System (ADS)

A first-principles method is applied to find the intra and intervalley n-type carrier scattering rates for substitutional carbon in silicon. The method builds on a previously developed first-principles approach with the introduction of an interpolation technique to determine the intravalley scattering rates. Intravalley scattering is found to be the dominant alloy scattering process in Si1-xCx, followed by g-type intervalley scattering. Mobility calculations show that alloy scattering due to substitutional C alone cannot account for the experimentally observed degradation of the mobility. We show that the incorporation of additional charged impurity scattering due to electrically active interstitial C complexes models this residual resistivity well.

Vaughan, M. P.; Murphy-Armando, F.; Fahy, S.

2012-04-01

300

Effect of dopants on grain boundary decohesion of Ni: A first-principles study

First-principles density functional theory (DFT) calculations were used to determine decohesion properties of Sigma5(012) grain boundary of Ni with dopants B, C, S, Cr, and Hf. The relative stability of sites was evaluated and cleavage energies were calculated. Electronic structure was used to understand these properties in terms of changes in bonding with addition of dopants. It was found that

Suchismita Sanyal; Umesh V. Waghmare; P. R. Subramanian; Michael F. X. Gigliotti

2008-01-01

301

First-principles calculations of binary Al compounds: Enthalpies of formation and elastic properties

Systematic first-principles calculations of energy vs. volume (E–V) and single crystal elastic stiffness constants (cij’s) have been performed for 50 Al binary compounds in the Al–X (X = Co, Cu, Hf, Mg, Mn, Ni, Sr, V, Ti, Y, and Zr) systems. The E–V equations of state are fitted by a four-parameter Brich–Murnaghan equation, and the cij’s are determined by an efficient strain–stress

Jiong Wang; Shun-Li Shang; Yi Wang; Zhi-Gang Mei; Yong-Feng Liang; Yong Du; Zi-Kui Liu

302

First principles predictions of intrinsic defects in aluminum arsenide, AlAs : numerical supplement.

This Report presents numerical tables summarizing properties of intrinsic defects in aluminum arsenide, AlAs, as computed by density functional theory. This Report serves as a numerical supplement to the results published in: P.A. Schultz, 'First principles predictions of intrinsic defects in Aluminum Arsenide, AlAs', Materials Research Society Symposia Proceedings 1370 (2011; SAND2011-2436C), and intended for use as reference tables for a defect physics package in device models.

Schultz, Peter Andrew

2012-04-01

303

Tunable hydrogen storage in magnesium-transition metal compounds: First-principles calculations

Magnesium dihydride (MgH2) stores 7.7wt% hydrogen but it suffers from a high thermodynamic stability and slow (de)hydrogenation kinetics. Alloying Mg with lightweight transition metals (TM) (=Sc,Ti,V,Cr) aims at improving the thermodynamic and kinetic properties. We study the structure and stability of MgxTM1-xH2 compounds, x=[0-1] , by first-principles calculations at the level of density functional theory. We find that the experimentally

Süleyman Er; Dhirendra Tiwari; Gilles A. de Wijs; Geert Brocks

2009-01-01

304

First-principles calculation of the magnetic properties of paramagnetic fcc iron

Using the disordered local moment picture of itinerant magnetism, we present calculations of the temperature and volume dependence of the magnetic moment and spin-spin correlations for fcc Fe in the paramagnetic state. These calculations are based on the parameter-free, first principles approach of local spin density functional theory and the coherent potential approximation is used to treat the disorder associated with the random orientation of the local moments.

Johnson, D.D.; Gyorffy, B.L.; Pinski, F.J.; Staunton, J.; Stocks, G.M.

1985-01-01

305

First-principles study of phenyl ethylene oligomers as current-switch

We use a self-consistent method to study the distinct current-switch of 2?-amino-4-ethynylphenyl-4?-ethynylphenyl-5?-nitro-1-benzenethiol, from the first-principles calculations. The switch behavior is in accord with the early experiment [M.A. Reed, J.H. Tour, Sci. Am. 282 (2000) 86]. To further investigate the transport mechanism of the conformational molecular switch, we calculate the switching behavior of p-terphenyl with the rotations of the middle ring

F. Jiang; Y. X. Zhou; H. Chen; H. Mizuseki; Y. Kawazoe

2006-01-01

306

First-Principles Calculation of Lead-Free Perovskite SnTiO3

NASA Astrophysics Data System (ADS)

The phonon spectra, band structure and density of states of cubic perovskite SnTiO3 were investigated using first-principles density functional theory (DFT) computation. The potential energy curves of cations displacement and the formation energy of Sn substitution to B-site were calculated to estimate the structure stability. The results indicate that perovskite SnTiO3 is a promising ferroelectric end member for lead-free piezoelectric materials and applications.

Ye, Hongjun; Zhang, Ruizhi; Wang, Dawei; Cui, Yu; Wei, Jie; Wang, Chunlei; Xu, Zhuo; Qu, Shaobo; Wei, Xiaoyong

2013-09-01

307

First-principles calculations of the dielectric properties of perovskite-type materials

We compare first-principles (FP) calculations of the ionic effective charges, phonon frequencies, and static dielectric permittivities ?s of several perovskite-type materials. Transition metal ions have anomalously large effective charges, though in the double perovskite CaAl1\\/2Nb1\\/2O3 (CAN), the effective charge of Nb is significantly lower than in the simple perovskite KNbO3, showing different Nb–O bonding chemistry. Tolerance factors, cation chemistry, and

2003-01-01

308

First-principles modeling of resistance switching in perovskite oxide material

We report a first-principles study on SrRuO3\\/SrTiO3 interface in the presence of the oxygen vacancy. While the oxygen vacancy on the side of SrTiO3 significantly lowers the Schottky barrier height, the oxygen vacancy close to the interface or inside the metallic electrode results in a Schottky barrier comparable to that of the clean interface. Based on these results, we propose

Sang Ho Jeon; Bae Ho Park; Jaichan Lee; Bora Lee; Seungwu Han

2006-01-01

309

First principles study of carbon monoxide adsorption on zirconia-supported copper

-supported copper is signiÂ®cantly stronger than CO adsorption on clean c-ZrO2 or clean Cu(1 0 0). Ã? 2001 ElsevierFirst principles study of carbon monoxide adsorption on zirconia-supported copper Eric J. Walter a of carbon monoxide on a monolayer of copper adsorbed on the (1 1 1) face of cubic zirconia. For the bulk

Rappe, Andrew M.

310

Carbon impurity dissolution and migration in bcc Fe-Cr: First-principles calculations

First-principles density-functional theory calculations for C solution enthalpies, Hsol , and diffusion activation enthalpies, Hdiff , in body-centered-cubic Fe and Cr are presented. The results for C in Fe compare well with experiments, provided that the effect of magnetic disordering is accounted for. Likewise, in Cr, the calculated Hsol and Hdiff agree well with available experiments. In both materials, the

Nils Sandberg; Krister O. E. Henriksson; Jan Wallenius

2008-01-01

311

A new and efficient scheme for first-principles calculations of phonon spectra

The authors present a new method for performing first-principles frozen phonon calculations within the framework of density-functional theory and the adiabatic approximation. In the approach the super-cell Kohn-Sham Hamiltonian is diagonalised with sufficient accuracy to compute phonon properties in the harmonic approximation in a time which is independent of the super-cell size. This method dramatically improves the efficiency of the

R. D. King-Smith; R. J. Needs

1990-01-01

312

First-principles super-cell investigation of the rattling effect in Li-doped KCl

We have studied by the first-principles total energy method the off-center instability of a substitutional Li impurity in KCl. We report here the results of super-cell calculations of the energy associated with displacing the Li along , and directions relative to the K vacancy. To understand the influence of relaxations, we performed three levels of relaxations—only first nearest neighbors of

Xing Gao; Murray S Daw

2009-01-01

313

We have studied the properties of spinel and layered cathode materials for Li ion rechargeable batteries. The analysis was done by first principle calculations, and experimental techniques to elucidate materials that can substitute the presently commercialized material, namely LiCoO 2. We have studied the influence of Ni substitution for Mn in spinel Li 2MnO4. To understand the effects of this

Jose Javier Saavedra Arias

2009-01-01

314

We report the results of a comparative study of pentaerythritol tetranitrate (PETN) at high compression using classical reactive interatomic potential ReaxFF and first-principles density functional theory (DFT). Lattice parameters of PETN I, the ground state structure at ambient conditions, is obtained by ReaxFF and two different density functional methods (plane wave and LCAO pseudopotential methods) and compared with experiment. Calculated

I. I. Oleynik; M. Conroy; S. V. Zybin; L. Zhang; A. C. van Duin; W. A. Goddard; C. T. White

2006-01-01

315

First principle study of a bimolecular thin film on Ag(1 1 1) surface

The formation of melamine–PTCDI bimolecular networks deposited on Ag(111) is studied by means of first principle calculations. Emphasis is placed on the interplay of the inter-molecular hydrogen bonds and the molecule–substrate contacts. Our simulations show rather strong distortions of the adsorbed molecules near the contact points due to the influence the hydrogen bonds. Despite this, the charge transfer from the

Michel Sassi; Vincent Oison; Jean-Marc Debierre

2008-01-01

316

Quantifying the anomalous self-diffusion in molybdenum with first-principles simulations

First-principles molecular-dynamics simulations based on a recently developed exchange-correlation functional show that self-diffusion in the refractory metal molybdenum is associated with strongly temperature-dependent activation energies for vacancy formation and migration. While static calculations of self-diffusion rates based on transition-state theory deviate systematically from experiments, with up to two orders of magnitude, the current results are accurate to within a mean

T. R. Mattsson; N. Sandberg; R. Armiento; A. E. Mattsson

2009-01-01

317

Within a first-principles framework we show how many-body effects crucially modify the electronic and optical properties of free-standing Germanium nanowires. The electron-hole binding energy and probability distribution are found to depend on both wire size and orientation. Moreover, we observe an almost complete compensation of self-energy and excitonic effects for some of the analyzed quantum wires, which we explain as

Mauro Bruno; Maurizia Palummo; Andrea Marini; Rodolfo Del Sole; Valerio Olevano; Alexandre N. Kholod; Stefano Ossicini

2005-01-01

318

Summary Using multi-ion interatomic potentials derived from first-principles generalized pseudopotential theory, we have been studying point defects and dislocations in bcc transition metals, with molybdenum (Mo) as a prototype. For point defects in Mo, the calculated vacancy formation and activation energies are in excellent agreement with experimental results. The energetics of six self-interstitial configurations in Mo have also been investigated.

Wei Xu; John A. Moriarty

1996-01-01

319

NASA Astrophysics Data System (ADS)

Inorganic scintillation phosphors (scintillators) are extensively employed as radiation detector materials in many fields of applied and fundamental research such as medical imaging, high energy physics, astrophysics, oil exploration and nuclear materials detection for homeland security and other applications. The ideal scintillator for gamma ray detection must have exceptional performance in terms of stopping power, luminosity, proportionality, speed, and cost. Recently, trivalent lanthanide dopants such as Ce and Eu have received greater attention for fast and bright scintillators as the optical 5d to 4f transition is relatively fast. However, crystal growth and production costs remain challenging for these new materials so there is still a need for new higher performing scintillators that meet the needs of the different application areas. First principles calculations can provide a useful insight into the chemical and electronic properties of such materials and hence can aid in the search for better new scintillators. In the past there has been little first-principles work done on scintillator materials in part because it means modeling f electrons in lanthanides as well as complex excited state and scattering processes. In this talk I will give an overview of the scintillation process and show how first-principles calculations can be applied to such systems to gain a better understanding of the physics involved. I will also present work on a high-throughput first principles approach to select new scintillator materials for fabrication as well as present more detailed calculations to study trapping process etc. that can limit their brightness. This work in collaboration with experimental groups has lead to the discovery of some new bright scintillators.

Canning, Andrew

2013-03-01

320

Predicted ferromagnetism in hole doped armchair nanoribbons: A first principles study

NASA Astrophysics Data System (ADS)

We have investigated the magnetic properties of hexagonal AlSi monolayer and nanoribbons by first principles method. The armchair AlSi nanoribbons are predicted to show itinerant magnetism. Since the armchair AlSi nanoribbons can be regarded as armchair silicene nanoribbons with injected holes, the results suggest the importance of holes in inducing magnetism in armchair nanoribbons. We further discuss boron doped armchair graphene nanoribbons and predict magnetism. These results have potential applications on electronic or spintronic devices.

Chen, Xi; Ni, Jun

2013-01-01

321

Comparison of Jupiter Interior Models Derived from First-Principles Simulations

Recently two groups used first-principles computer simulations to model Jupiter's interior. While both studies relied on the same simulation technique, density functional molecular dynamics, the groups derived very different conclusions. In particular estimates for the size of Jupiter's core and the metallicity of its hydrogen-helium mantle differed substantially. In this paper, we discuss the differences of the approaches and give an explanation for the differing conclusions.

B. Militzer; W. B. Hubbard

2008-07-27

322

The effects of vacancies in the mechanical properties of tungsten: A first-principles study

NASA Astrophysics Data System (ADS)

Both mechanical and structural properties of bcc crystal tungsten in presence of mono and divacancy defects has been investigated by using accurate first-principles total energy methods based on density functional theory. A model for tungsten containing a concentration of vacancies of about 2% and 4% has been developed and used to compute the maximum tensile stress required to reach elastic instability under increasing load. Moreover stress effects on the crystalline structure have been characterized in terms of structural displacements.

Giusepponi, Simone; Celino, Massimo

2015-01-01

323

The electronic structure of the PbS(-100) with vacancy defect: first-principles study

Electronic properties of both Pb and S vacancy defect in PbS(-100) have been studied using the first principles density functional theory (DFT) calculations with the plane-wave pseudopotentials. The densities of states are computed to investigate the effect of the Pb and S vacancy on the electronic structure, respectively. In the case of S vacancy defect, the Fermi energy shifted to

Zong-ling Ding; Huai-zhong Xing; Yan Huang; Xiao-shuang Chen

2008-01-01

324

Towards the first-principles design of materials with tailored magnetic properties

An important aspect in the design of materials with tailored magnetic properties is the determination of the key structural and electronic features that control those magnetic properties. Here we will address this issue by presenting first-principles results obtained within the LSDA and LDA+U approximations. In particular, we will discuss the family of molecule-based magnets M(N(CN)_2)2 (M=Mn, Co, Ni, etc.), whose

Jorge Iniguez; Taner Yildirim

2004-01-01

325

Elastocaloric Response of PbTiO3 Predicted from a First-Principles Effective Hamiltonian

NASA Astrophysics Data System (ADS)

A first-principles effective Hamiltonian is used in a molecular dynamics simulation to study the elastocaloric effect in PbTiO3. It is found that the transition temperature is a linear function of uniaxial tensile stress. A negative temperature change is calculated, when the uniaxial tensile stress is switched off, as a function of the initial temperature ?T(Tinitial). It is predicted that the formation of domain structures under uniaxial tensile stress degrades the effectiveness of the elastocaloric effect.

Barr, Jordan A.; Beckman, Scott P.; Nishimatsu, Takeshi

2015-02-01

326

First-principles free-energy calculations on condensed-matter systems: Lattice vacancy in silicon

We illustrate a method for performing first-principles free-energy calculations within the Kohn-Sham scheme. We show that the method can be used in cases in which electrons have to be decoupled from the system and illustrate it with results for the formation free energy of the Si vacancy. The results agree well with the available data from experiments and ab initio

Enrico Smargiassi; Roberto Car

1996-01-01

327

Interaction of He with Cu, V, and Ta in bcc Fe: A first-principles study

Precipitates often play key roles in improving the mechanical performance of structural materials. Using first-principles density functional theory method, we have calculated the geometry and energetics of small Xn and XnHe (X = Cu, V, and Ta) clusters in bcc Fe matrix to investigate the effect of He on X precipitation on the initial stage in neutron-irradiated Fe alloys. Both

J. X. Yan; Z. X. Tian; W. Xiao; W. T. Geng

2011-01-01

328

First-principles study of He point-defects in HCP rare-earth metals

He defect properties in Sc, Y, Gd, Tb, Dy, Ho, Er and Lu were studied using first-principles calculations based on density functional theory. The results indicate that the formation energy of an interstitial He atom is smaller than that of a substitutional He atom in all hcp rare-earth metals considered. Furthermore, the tetrahedral interstitial position is more favorable than an octahedral position for He defects. The results are compared with those from bcc and fcc metals.

Li, Yang; Chen, Ru; Peng, SM; Long, XG; Wu, Z.; Gao, Fei; Zu, Xiaotao

2011-05-01

329

Ballistic phonon thermal conductance in graphene nano-ribbon: First-principles calculations

Ballistic phonon thermal conductances for graphene nanoribbons are investigated using first-principles calculations with the density functional perturbation theory and the Landauer theory. The phonon thermal conductance per unit width for GNR is larger than that for graphene and increases with decreasing ribbon width. The normalized thermal conductances with regard to a thermal quantum for GNRs are higher than those for the single-walled carbon nanotube that have circumferential lengths corresponding to the width of GNR.

Nakamura, Jun; Tomita, Hiroki [Department of Engineering Sciences, The University of Electro-Communications (UEC-Tokyo), 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585 (Japan)

2013-12-04

330

Constraints on the phase diagram of molybdenum from first-principles free-energy calculations

NASA Astrophysics Data System (ADS)

We use first-principles techniques to reexamine the suggestion that transitions seen in high-P experiments on Mo are solid-solid transitions from the bcc structure to either the fcc or hcp structures. We confirm that in the quasiharmonic approximation the free energies of fcc and hcp structures become lower than that of bcc at P>325 GPa and T below the melting curve, as reported recently. However, we show that if anharmonic effects are fully included this is no longer true. We calculate fully anharmonic free energies of high-T crystal phases by integration of the thermal average stress with respect to strain as structures are deformed into each other, and also by thermodynamic integration from harmonic reference systems to the fully anharmonic system. Our finding that fcc is thermodynamically less stable than bcc in the relevant high-P/high-T region is supported by comparing the melting curves of the two structures calculated using the first-principles reference-coexistence technique. We present first-principles simulations based on the recently proposed Z method, which also support the stability of bcc over fcc.

Cazorla, C.; Alfè, D.; Gillan, M. J.

2012-02-01

331

(Un)folding of a high-temperature stable polyalanine helix from first principles

NASA Astrophysics Data System (ADS)

Peptides in vacuo offer a unique, well-defined testbed to match experiments directly against first-principles approaches that predict the intramolecular interactions that govern peptide and protein folding. In this respect, the polyalanine-based peptide Ac-Ala15-LysH^+ is particularly interesting, as it is experimentally known to form helices in vacuo, with stable secondary structure up to 750 K [1]. Room-temperature folding and unfolding timescales are usually not accessible by direct first-principles simulations, but this high T scale allows a rare direct first-principles view. We here use van der Waals corrected [2] density functional theory in the PBE generalized gradient approximation as implemented in the all-electron code FHI-aims [3] to show by Born-Oppenheimer ab initio molecular dynamics that Ac-Ala15-LysH^+ indeed unfolds rapidly (within a few ps) at T=800 K and 1000 K, but not at 500 K. We show that the structural stability of the ? helix at 500 K is critically linked to a correct van der Waals treatment, and that the designed LysH^+ ionic termination is essential for the observed helical secondary structure. [1] M. Kohtani et al., JACS 126, 7420 (2004). [2] A. Tkatchenko, M. Scheffler, PRL 102, 073005 (2009). [3] V. Blum et al, Comp. Phys. Comm. 180, 2175 (2009).

Blum, Volker; Rossi, Mariana; Tkatchenko, Alex; Scheffler, Matthias

2010-03-01

332

NASA Astrophysics Data System (ADS)

Experimental measurements have recently shown that Cu3SbSe3 exhibits anomalously low and nearly temperature-independent lattice thermal conductivity, whereas Cu3SbSe4 does not exhibit this anomalous behavior. To understand this strong distinction between these two seemingly similar compounds, we perform density functional theory calculations of the vibrational properties of these two semiconductors within the quasiharmonic approximation. We observe strikingly different behavior in the two compounds: almost all the acoustic-mode Grüneisen parameters are negative in Cu3SbSe4, whereas almost all are positive in Cu3SbSe3 throughout their respective Brillouin zones. The average of the square of the Grüneisen parameter for the acoustic mode in Cu3SbSe3 is larger than that of Cu3SbSe4, which theoretically confirms that Cu3SbSe3 has a stronger lattice anharmonicity than Cu3SbSe4. The soft frequency and high Grüneisen parameters in Cu3SbSe3 arise from the electrostatic repulsion between the lone s2 pair at Sb sites and the bonding charge in Sb-Se bonds. Using our first-principles-determined longitudinal and transverse acoustic-mode Grüneisen parameters, zone-boundary frequencies, and phonon group velocities, we calculate the lattice thermal conductivity using the Debye-Callaway model. The theoretical thermal conductivity is in good agreement with the experimental measurements.

Zhang, Yongsheng; Skoug, Eric; Cain, Jeffrey; Ozoli?š, Vidvuds; Morelli, Donald; Wolverton, C.

2012-02-01

333

Zirconium metal-based MAX phases Zr2AC (A = Al, Si, P and S): A first-principles study

NASA Astrophysics Data System (ADS)

We have investigated theoretical Vickers hardness, thermodynamic and optical properties of four zirconium metal-based MAX phases Zr2AC (A = Al, Si, P and S) for the first time in addition to revisiting the structural, elastic and electronic properties. First-principles calculations are employed based on density functional theory (DFT) by means of the plane-wave pseudopotential method. The theoretical Vickers hardness has been estimated via the calculation of Mulliken bond populations and electronic density of states. The thermodynamic properties such as the temperature and pressure dependent bulk modulus, Debye temperature, specific heats and volume thermal expansion coefficient of all the compounds are derived from the quasi-harmonic Debye model. Further, the optical properties, e.g., dielectric functions, indices of refraction, absorption, energy loss function, reflectivity and optical conductivity of the nanolaminates have been calculated. The results are compared with available experiments and their various implications are discussed in detail. We have also shed light on the effect of different properties of Zr2AC as the A-group atom moves from Al to S across the periodic table.

Nasir, M. T.; Hadi, M. A.; Naqib, S. H.; Parvin, F.; Islam, A. K. M. A.; Roknuzzaman, M.; Ali, M. S.

2014-11-01

334

PHYSICAL REVIEW B 89, 035120 (2014) Electronic stopping power from first-principles calculations 11100, FI-00076 AALTO, Finland 2 Materials Physics Division, University of Helsinki, P.O. Box 43, FI electronic stopping power Se of energetic ions in graphitic targets from first principles. By treating core

Krasheninnikov, Arkady V.

335

First Principles Phase Diagram Calculations for the System NaCl-KCl: the role of excess vibrational- and without an approximation for the excess vibrational entropy (SV IB). Including SV IB dramatically improvesK}Calc. Key words: NaCl-KCl; First Principles; Phase diagram calculation; Excess vibrational entropy

Burton, Benjamin P.

336

High pressure behavior of phlogopite using neutron diffraction and first principle simulations

NASA Astrophysics Data System (ADS)

Hydrous phases play an important role in the deep water cycle by transporting water into the Earth's interior. Upon, reaching their thermodynamic stability, these hydrous phases decompose and release the water. A part of the water is cycled back to the arc, thus completing the deep water cycle, the remaining water is partitioned into dense hydrous phases and nominally anhydrous phases. Hence, in order to understand the role the hydrous phases in the deep water cycle, it is important to constrain the effect of pressure, temperature, and chemistry on the thermodynamic stability of the hydrous phases. In addition, it is important to constrain the elasticity of these hydrous phases to test whether they can explain the distinct geophysical observations such as lower bulk sound velocities and elastic anisotropy. Phlogopite is a potassium bearing mica that is stable in the hydrated crust and metasomatized mantle up to pressures of ~9 GPa, i.e., base of the upper mantle. We investigated the response of the crystal structure, lattice parameters and unit-cell volume of a natural phlogopite upon compression. We conducted in situ neutron diffraction studies at high-pressures using Paris-Edinburgh press at the Spallation Neutrons and Pressure Diffractometer (SNAP), Oak Ridge National Laboratory. All the experiments were conducted at room temperatures and pressures up to 10 GPa were explored. The equation of state parameters from our experiments could be explained by a finite strain formulation with V0= 487 Å3, K0 = 49 GPa, K' = 4.1. In addition, we have used first principle simulations based on density functional theory to calculate the equation of state and elasticity. The predicted equation of state is in good agreement with the experiments, with V0= 519 Å3, K0 = 45.8 GPa and K'= 6.9. The full elastic constant tensor shows significant anisotropy with the principal elastic constants at theoretical V0: C11= 181 GPa, C22= 185 GPa, C33= 62 GPa, the shear elastic constants- C44= 14 GPa, C55=20 GPa, C66= 68 Ga, and C46 = -6 GPa; the off diagonal elastic cosntants C12= 48 GPa, C13= 12 GPa, C23 = 12 GPa, C15 = -16 GPa, C25 = -5 GPa, and C35 = -1 GPa. We also note that the shear elastic constants for phlogopite are significantly low and it also has a high VP/VS ratio (~2 km/sec). Phlogopite bearing hydrated crust could explain the low velocity layers in the top 6-8 km of the subducting slabs. __Acknowledgements__ TC and MM are supported by US NSF grant #EAR1250477 and also acknowledge computing resources (EAR130015) from XSEDE (OCI-1053575).

Chheda, T. D.; Mookherjee, M.; dos Santos, A. M.; Molaison, J.; Manthilake, G. M.; Chantel, J.; Mainprice, D.

2013-12-01

337

NASA Astrophysics Data System (ADS)

We investigated the local atomic and electronic structures around the dopants Mg and Al in a LiNiO2 -based cathode material by the combination analysis of their K shell electron energy-loss near-edge structures, x-ray absorption near-edge structures, and first-principles calculations. The occupation sites of the dopants in initial and cycled samples were examined. On the basis of the atomic structures and chemical bonding states of the models whose theoretical spectra were most consistent with the experimental spectra, we discussed the effects of Al and Mg on Li diffusion and their roles in suppressing the degradation of battery properties.

Tatsumi, Kazuyoshi; Sasano, Yusuke; Muto, Shunsuke; Yoshida, Tomoko; Sasaki, Tsuyoshi; Horibuchi, Kayo; Takeuchi, Yoji; Ukyo, Yoshio

2008-07-01

338

A first-principles methodology for diffusion coefficients in metals and dilute alloys

NASA Astrophysics Data System (ADS)

This work is a study exploring the extent of suitability of static first-principles calculations for studying diffusion in metallic systems. Specifically, vacancy-mediated volume diffusion in pure elements and alloys with dilute concentration of impurities is studied. A novel procedure is discovered for predicting diffusion coefficients that overcomes the shortcomings of the well-known transition state theory, by Vineyard. The procedure that evolves from Eyring's reaction rate theory yields accurate diffusivity results that include anharmonic effects within the quasi-harmonic approximation. Alongside, the procedure is straightforward in its application within the conventional harmonic approximation, from the results of static first-principles calculations. To prove the extensibility of the procedure, diffusivities have been computed for a variety of systems. Over a wide temperature range, the calculated self-diffusion and impurity diffusion coefficients using local density approximation (LDA) of density functional theory (DFT) are seen to be in excellent match with experimental data. Self-diffusion coefficients have been calculated for: (i) fcc Al, Cu, Ni and Ag (ii) bcc W and Mo (v) hcp Mg, Ti and Zn. Impurity diffusion coefficients have been computed for: (i) Mg, Si, Cu, Li, Ag, Mo and 3d transition elements in fcc Al (ii) Mo, Ta in bcc W and Nb, Ta and W in bcc Mo (iii) Sn and Cd in hcp Mg and Al in hcp Ti. It is also an observation from this work, that LDA does not require surface correction for yielding energetics of vacancy-containing system in good comparison with experiments, unlike generalized gradient approximation (GGA). It is known that first-principles' energy minimization procedures based on electronic interactions are suited for metallic systems wherein the valence electrons are freely moving. In this thesis, research has been extended to study suitability of first-principles calculations within LDA/GGA including the localization parameter U, for Al system with transition metal solutes, in which charges are known to localize around the transition metal element. U parameter is determined from matching the diffusivities of 3d transition metal impurity in aluminum with reliable experimental data. The effort yielded activation energies in systematic agreement with experiments and has proved useful in obtaining insights into the complex interactions in these systems. Besides the prediction of diffusion coefficients, this research has been helpful in understanding the physics underlying diffusion. Within the scope of observations from the systems studied, certain diffusion related aspects that have been clarified are: (i) cause for non-Arrnenius' nature of diffusion plots (ii) definitions of atom migration properties (iii) magnitude and sign of diffusion parameters enthalpy and entropy of formation and migration and characteristic vibrational frequency (iv) trends in diffusivities based on activation energy and diffusion prefactor (vi) cause for anomalous diffusion behavior of 3d transition metals in Al, and their magnetic nature (vii) contributions from electronic contributions to curvature at very high temperatures of bcc refractory elements (viii) temperature dependence of impurity diffusion correlation factors. Finally, the double-well potential of diffusion by vacancy mechanism has been calculated from first-principles. This aided calculation of entropy of migration and thus free energy of migration along with characteristic vibrational frequency. Also for the first time, temperature dependence of enthalpy of migration and thus atom jump frequency has been accurately predicted. From the broad perspective of predicting diffusion coefficients from computational methodologies, it can be stated as a result of this work that: static first-principles extend an irreplaceable contribution to the future of diffusion modeling. The procedure obviated the use of (i) redundant approximations that limit its accuracy and (ii) support from other computational techniques that restrict its extensibility due to insufficient i

Mantina, Manjeera

339

NASA Astrophysics Data System (ADS)

A Fortran program is developed to calculate charge carrier (electron or hole) mobility in disordered semiconductors from first-principles. The method is based on non-adiabatic ab initio molecular dynamics and static master equation, treating dynamic and static disorder on the same footing. We have applied the method to calculate the hole mobility in disordered poly(3-hexylthiophene) conjugated polymers as a function of temperature and electric field and obtained excellent agreements with experimental results. The program could be used to explore structure-mobility relation in disordered semiconducting polymers/organic semiconductors and aid rational design of these materials. Program summaryProgram title: FPMu Catalogue identifier: AEJV_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEJV_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 788 580 No. of bytes in distributed program, including test data, etc.: 8 433 024 Distribution format: tar.gz Programming language: Fortran 90 Computer: Any architecture with a Fortran 90 compiler Operating system: Linux, Windows RAM: Proportional to the system size, in our example, 1.2 GB Classification: 7.9 Nature of problem: Determine carrier mobility from first-principles in disordered semiconductors as a function of temperature, electric field and carrier concentration. Solution method: Iteratively solve master equation with carrier state energy and transition rates determined from first-principles. Restrictions: Mobility for disordered semiconductors where the carrier wave-functions are localized and the carrier transport is due to phonon-assisted hopping mechanism. Running time: Depending on the system size (about an hour for the example here).

Li, Zi; Zhang, Xu; Lu, Gang

2011-12-01

340

MgO phase diagram from first principles in a wide pressure-temperature range

NASA Astrophysics Data System (ADS)

Recent laser-initiated strong shockwave measurements at Livermore provide the opportunity for verification of the MgO phase diagram at extreme pressures and temperatures. This calls for a comprehensive study of the MgO phase diagram. The phase diagram is obtained by ab initio molecular dynamics (two phase and Z method) and phonon-based thermodynamic calculations. Energies and forces are computed from first principles projector augmented wave method. The B1 transforms to B2 phase at about 490 GPa. Melting temperatures of B1 are consistent with the two-phase melting curve (Alfe, 2005). The triple point B1-B2-liquid is located at about 2.4 Mbar and 9000 K. The melting curve of the B2 phase rises rather steeply from the triple point. The Hugoniot is likely to cross the B1-B2 boundary first and then to cross the melting curve of B2, therefore, the melting curve of periclase is not relevant for the Hugoniot. MgO melts between 11.3 and 12.5 thousand K and 4.3 and 5 Mbar along the Hugoniot from the B2 phase. The two-phase melting curves of B1 computed with various semiempirical potentials are remarkably close to each other and are consistent with the B1 first principles melting curve at low pressure. This suggests the MgO melting temperatures to be in the close proximity of this determination. The search for new phases of MgO by first principles metadynamics has not produced unknown phases. Therefore, the suggested discontinuity of the Hugoniot at 170 GPa and 3000 K remains enigmatic.

Belonoshko, A. B.; Arapan, S.; Martonak, R.; Rosengren, A.

2010-02-01

341

First-principles study on the electronic and optical properties of cubic ABX3 halide perovskites

NASA Astrophysics Data System (ADS)

The electronic properties of ABX3 type compounds in the cubic phase are systematically studied using the first-principles calculations. The chemical trend of their properties as A or B or X varies is fully investigated. The optical properties of the ABX3 compounds are also investigated. Our calculations show that taking into account the spin-orbit coupling effect is crucial for predicting the accurate band gap of these halide perovskites. We predict that CH3NH3SnBr3 is a promising material for solar cells absorber with a perfect band gap and good optical absorption.

Lang, Li; Yang, Ji-Hui; Liu, Heng-Rui; Xiang, H. J.; Gong, X. G.

2014-01-01

342

First Principles Calculations of Oxygen Adsorption on the UN(001) Surface

Fabrication, handling and disposal of nuclear fuel materials require comprehensive knowledge of their surface morphology and reactivity. Due to unavoidable contact with air components (even at low partial pressures), UN samples contain considerable amount of oxygen impurities affecting fuel properties. In this study we focus on reactivity of the energetically most stable (001) substrate of uranium nitride towards the atomic oxygen as one of initial stages for further UN oxidation. The basic properties of O atoms adsorbed on the UN(001) surface are simulated here combining the two first principles calculation methods based on the plane wave basis set and that of the localized orbitals.

Zhukovskii, Yuri F.; Bocharov, Dmitry; Kotomin, Eugene Alexej; Evarestov, Robert; Bandura, A. V.

2009-01-01

343

First principles calculations of oxygen adsorption on the UN(0 0 1) surface

NASA Astrophysics Data System (ADS)

Fabrication, handling and disposal of nuclear fuel materials require comprehensive knowledge of their surface morphology and reactivity. Due to unavoidable contact with air components (even at low partial pressures), UN samples contain considerable amount of oxygen impurities affecting fuel properties. In this study we focus on reactivity of the energetically most stable (0 0 1) substrate of uranium nitride towards the atomic oxygen as one of initial stages for further UN oxidation. The basic properties of O atoms adsorbed on the UN(0 0 1) surface are simulated here combining the two first principles calculation methods based on the plane wave basis set and that of the localized orbitals.

Zhukovskii, Yu. F.; Bocharov, D.; Kotomin, E. A.; Evarestov, R. A.; Bandura, A. V.

2009-01-01

344

Effect of dopants on grain boundary decohesion of Ni: A first-principles study

NASA Astrophysics Data System (ADS)

First-principles density functional theory (DFT) calculations were used to determine decohesion properties of ?5(012) grain boundary of Ni with dopants B, C, S, Cr, and Hf. The relative stability of sites was evaluated and cleavage energies were calculated. Electronic structure was used to understand these properties in terms of changes in bonding with addition of dopants. It was found that strengthening of the Ni grain boundary results from Hf, B, and Cr doping. In contrast, the grain boundary weakens with S and C doping. These results should be useful in the design of next-generation nanostructured Ni-based alloys with improved mechanical behavior.

Sanyal, Suchismita; Waghmare, Umesh V.; Subramanian, P. R.; Gigliotti, Michael F. X.

2008-12-01

345

First-Principles Investigation of Reactive Molecular Dynamics in Detonating Rdx and Tatb

NASA Astrophysics Data System (ADS)

Possible initial chemical reactions in detonating RDX and TATB that result from intermolecular collisions behind the shock wave front have been investigated using first-principles reactive molecular dynamics. The reaction dynamics was studied as a function of collision velocities and crystallographic orientations. Threshold collision velocities of chemical initiation and products of decomposition were obtained for each orientation, and compared to calculated bond dissociation energies to rationalize the outcome chemical events. Reaction timescales were also determined and used to understand whether these initial chemical events are largely driven by reaction dynamics or temperature.

Oleynik, I. I.; Landerville, A. C.; White, C. T.

2009-12-01

346

First-Principles Molecular Dynamics Calculations of the Equation of State for Tantalum

The equation of state of tantalum (Ta) has been investigated to 100 GPa and 3,000 K using the first-principles molecular dynamics method. A large volume dependence of the thermal pressure of Ta was revealed from the analysis of our data. A significant temperature dependence of the calculated effective Grüneisen parameters was confirmed at high pressures. This indicates that the conventional approach to analyze thermal properties using the Mie-Grüneisen approximation is likely to have a significant uncertainty in determining the equation of state for Ta, and that an intrinsic anharmonicity should be considered to analyze the equation of state. PMID:20057949

Ono, Shigeaki

2009-01-01

347

First principles study of structural stability and electronic structure of CdS nanoclusters

Using first-principles density functional calculations, we have studied the structural stability of stoichiometric as well as non-stoichiometric CdS nanoclusters at ambient pressure with diameters ranging up to about 2.5 nm. Our study reveals that the relative stability of the two available structures for CdS, namely zinc blende and wurtzite, depends sensitively on the details like surface geometry and/or surface chemistry. The associated band gap also exhibits non-monotonic behavior as a function of cluster size. Our findings may shed light on reports of experimentally observed structures and associated electronic structures of CdS nanoclusters found in the literature.

Datta, Soumendu; Saha-Dasgupta, Tanusri; Sarma, D D

2008-01-01

348

First-principles study of length dependence of conductance in alkanedithiols.

Electronic transport properties of alkanedithiols are calculated by a first-principles method based on density functional theory and nonequilibrium Green's function formalism. At small bias, the I-V characteristics are linear and the resistances conform to the Magoga's exponential law. The calculated length-dependent decay constant gamma which reflects the effect of internal molecular structure is in accordance with most experiments quantitatively. Also, the calculated effective contact resistance R(0) is in good agreement with the results of repeatedly measuring molecule-electrode junctions [B. Xu and N. Tao, Science 301, 1221 (2003)]. PMID:18247978

Zhou, Y X; Jiang, F; Chen, H; Note, R; Mizuseki, H; Kawazoe, Y

2008-01-28

349

NASA Astrophysics Data System (ADS)

Using first-principles calculations, we investigate the electronic structures and binding properties of nicotine and caffeine adsorbed on single-walled carbon nanotubes to determine whether CNTs are appropriate for filtering or sensing nicotine and caffeine molecules. We find that caffeine adsorbs more strongly than nicotine. The different binding characteristics are discussed by analyzing the modification of the electronic structure of the molecule-adsorbed CNTs. We also calculate the quantum conductance of the CNTs in the presence of nicotine or caffeine adsorbates and demonstrate that the influence of caffeine is stronger than nicotine on the conductance of the host CNT.

Lee, Hyung-June; Kim, Gunn; Kwon, Young-Kyun

2013-08-01

350

Elastic and Dynamical Properties of YB4: First-Principles Study

NASA Astrophysics Data System (ADS)

We present the elastic and dynamical properties of YB4 from first-principles calculations. It is found that the optimized lattice constants and bulk modulus (182 GPa) agree well with the experimental data. The structural stability of tetragonal YB4 is confirmed by the calculated elastic constants and phonon spectra. YB4 holds a Debye temperature of 874 K and has small elastic anisotropy. The estimated hardness of YB4 is about 17 GPa, indicating that YB4 is a hard solid while not a superhard one.

Fu, Yuan-Yuan; Li, Yin-Wei; Huang, Hong-Mei

2014-11-01

351

5,6-dihydroxyindole-2-carboxylic acid (DHICA): a First Principles Density-Functional Study

We report first principles density functional calculations for 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and several reduced forms. DHICA and 5,6-dihydroxyindole (DHI) are believed to be the basic building blocks of the eumelanins. Our results show that carboxylation has a significant effect on the physical properties of the molecules. In particular, the relative stabilities and the HOMO-LUMO gaps (calculated with the $\\Delta$SCF method) of the various redox forms are strongly affected. We predict that, in contrast to DHI, the density of unpaired electrons, and hence the ESR signal, in DHICA is negligibly small.

B. J. Powell

2004-08-24

352

Dehydrogenation reaction for Na-O-H system: a first-principles study.

The crystal structures, electronic, dielectric, and vibrational properties of NaH, Na(2)O and NaOH are systematically investigated by first-principles calculations and the quasiharmonic approximation. The phonon dispersion relations and the phonon density of states of the phases and their thermodynamic functions including the heat capacity, the vibrational enthalpy, and the vibrational entropy are calculated using a direct force-constant method. Based on these results, the dehydrogenation reaction, NaH+NaOH-->H(2)+Na(2)O, is predicted to take place at 528 K, which is in agreement with the experimental observed value. PMID:17705149

Zhang, Xin-Bo; Shi, Si-Qi; Ke, Xue-Zhi; Han, Song; Shioyama, Hiroshi; Kuriyama, Nobuhiro; Kobayashi, Tetsuhiko; Xu, Qiang

2007-09-17

353

Gate-induced electron-state tuning of MoS2: first-principles calculations.

The electronic structure of electrostatically doped MoS2 thin films is investigated on the basis of first-principles total-energy calculations. We find that electron injection leads to a rapid downward shift in the energy of the unoccupied nearly free electron (NFE) state relative to other conduction bands. The NFE state finally crosses the Fermi level at an electron density of 0.81 × 10(14) cm(-2) that is attributable to the strong local electric field induced by charge accumulation near the surface. Electrons accommodated in the NFE state play an important role in determining the conducting properties of MoS2 thin films. PMID:24599313

Cuong, Nguyen Thanh; Otani, Minoru; Okada, Susumu

2014-04-01

354

Strain induced ferroelectricity in GdN: first-principles calculations.

Using first-principles density functional calculations and the generalized gradient approximation functional including the on-site Coulomb interaction of 4f orbitals, we show that ferroelectricity can be induced by appropriate epitaxial tensile strain in GdN with a simple rock-salt structure, and that the polarization is sensitive to the strain. The calculated phonon spectra of strained GdN also confirm the existence of ferroelectric polarization. In addition, the electronic structure and magnetic properties of strained GdN as a function of strain are investigated. The present work opens up the possibility of epitaxially tensioned GdN thin films as potential multiferroics. PMID:21625033

Liu, H M; Ma, C Y; Zhu, C; Liu, J-M

2011-06-22

355

NASA Astrophysics Data System (ADS)

We report first-principles calculations of the current-voltage characteristic and the conductance of carbon-based molecular wires with different length capped with sulfur ends between two metallic electrodes made of different metals. The optimized molecular structure of carbon chain in the junction is presented on the structure of polyyne. The conductance of the polyyne wires shows oscillatory behavior depending on the number of carbon atoms (triple bonds). Current rectification is found and rectification direction presents inversion with the odd and even number of carbon atoms.

Fang, Changfeng; Cui, Bin; Xu, Yuqing; Ji, Guomin; Liu, Desheng; Xie, Shijie

2011-09-01

356

First-principle path integral study of DNA under hydrodynamic flows

NASA Astrophysics Data System (ADS)

We use the worm-like chain as a first-principles model to study single molecule experiments of double stranded DNA subject to constant plug, elongational, and shear flows. The steady-state configurations of the polymer correspond to a locally defined potential and result in a path integral description of the canonical partition function. The parameters of this model are consistent with previous theory and experimental measurements. The time averaged mean extension reproduces experimental results and compares well with computationally more expensive Brownian dynamics simulations of reduced models.

Yang, Shilong; Witkoskie, James B.; Cao, Jianshu

2003-08-01

357

Adsorption of Molecules on Nitrogen-Doped Graphene: A First-Principles Study

NASA Astrophysics Data System (ADS)

We study the adsorption of NH3 and H2O molecules on pyridine-type nitrogen-doped graphene using a first-principles electronic-structure calculation. The adsorption energies of NH3 and H2O molecules on the pyridine-type defect are calculated and it is found that the adsorptions of NH3 as well as H2O molecules become energetically favorable. The pyridine-type defect in N-doped graphene is therefore expected to be highly reactive, and should be useful for reaction centers in chemical processes as well as for sensor applications.

Fujimoto, Yoshitaka; Saito, Susumu

358

First-Principles Study on Magnetic Properties of V-Doped ZnO Nanotubes

Electronic and magnetic properties of V-doped ZnO nanotubes in which one of Zn2+ ions is substituted by V2+ ions are studied by the first-principles calculations of plane wave ultra-soft pseudo-potential technology based on the spin-density function theory. The computational results reveal that spontaneous magnetization in V-doped (9,0) ZnO nanotubes can be induced without p-type or n-type doping treatment, and the

Fu-Chun Zhang; Zhi-Yong Zhang; Wei-Hu Zhang; Jun-Feng Yan; Jiang-Ni Yun

2009-01-01

359

NASA Astrophysics Data System (ADS)

A carbon nanotube (CNT)/zigzag graphene nanoribbons (ZGNRs) junctions has been proposed and investigated by first-principles calculations. The results show that large spin polarization of currents would be achieved when only one edge of ZGNR is coupled to the other lead. By virtue of spatial separation of edge state in two spin channel, one of those channels is opened at certain energy range and gives rise to spin-polarized currents under a low bias. This feature is stable whenever the ZGNR lead is under the antiferromagnetic ground states or is under the ferromagnetic states. Our findings indicate that this approach is simple and efficient for spintronics design.

Yu, Haiping; Zheng, Jiming; Guo, Ping; Zhang, Zhiyong

2014-12-01

360

First-principles study of the mobility of SrTiO3

NASA Astrophysics Data System (ADS)

We investigate the electronic and vibrational spectra of SrTiO3, as well as the coupling between them, using first-principles calculations. We compute electron-phonon scattering rates for the three lowest-energy conduction bands and use Boltzmann transport theory to calculate the room-temperature mobility of SrTiO3. The results agree with experiment and highlight the strong impact of longitudinal optical phonon scattering. Our analysis provides important insights into the key factors that determine room-temperature mobility, such as the number of conduction bands and the nature and frequencies of longitudinal phonons. Such insights provide routes to engineering materials with enhanced mobilities.

Himmetoglu, Burak; Janotti, Anderson; Peelaers, Hartwin; Alkauskas, Audrius; Van de Walle, Chris G.

2014-12-01

361

Magnetic phase diagram of the semi-Heusler alloys from first principles

NASA Astrophysics Data System (ADS)

The magnetic phase diagram of the Mn-based semi-Heusler alloys is determined at T =0 using first-principles calculations in conjunction with the frozen-magnon approximation. The authors show that the magnetism in these systems strongly depends on the number of conduction electrons, their spin polarization, and the position of the unoccupied Mn 3d states with respect to Fermi energy. Various magnetic phases are obtained depending on these characteristics. The conditions leading to diverse magnetic behavior are identified. The obtained results are in very good agreement with the available experimental data.

?a??o?lu, E.; Sandratskii, L. M.; Bruno, P.

2006-11-01

362

Structural phase transition and elastic properties of hafnium dihydride: A first principles study

The structural and elastic properties of Hafnium dihydride (HfH{sub 2}) are investigated by first principles calculation based on density functional theory using Vienna ab-initio simulation package (VASP). The calculated lattice parameters are in good agreement with the available results. A pressure induced structural phase transition from CaF{sub 2} to FeS{sub 2} phase is observed in HfH{sub 2} at 10.75 GPa. The calculated elastic constants indicate that this hydride is mechanically stable at ambient condition.

Santhosh, M., E-mail: rrpalanichamy@gmail.com; Rajeswarapalanichamy, R., E-mail: rrpalanichamy@gmail.com; Sudhapriyanga, G.; Murugan, A.; Chinthia, A. Jemmy [Department of Physics, N.M.S.S.V.N College, Madurai, Tamil Nadu-625019 (India); Kanagaprabha, S. [Department of Physics, Kamaraj College, Tuticorin, Tamil Nadu-628003 (India); Iyakutti, K. [Department of Physics and Nanotechnology, SRM University, Chennai, Tamil Nadu-603203 (India)

2014-04-24

363

Electronic and magnetic properties of CdSe nanoribbon: First-principles calculations

NASA Astrophysics Data System (ADS)

First-principles Density Functional Theory (DFT) calculations were carried out on electronic and magnetic properties of cadmium selenide nanoribbons (CdSeNRs) with both zigzag and armchair edges. All armchair nanoribbons exhibit nonmagnetic and semiconducting behavior, regardless of ribbon widths and their edge passivation status. Bare zigzag nanoribbons are found to be metallic and have non-zero net magnetic moments. The net magnetic moment of these ribbons increases as ribbon width increases. However, when zigzag edges are passivated with hydrogen, a degeneracy between the up and down spin was found, which turns ribbons into nonmagnetic ones.

Yu, Guolong; Chen, Li; Ye, Xiang

2015-01-01

364

First principles study of structural, electronic and magnetic properties of Mn2CoAs

NASA Astrophysics Data System (ADS)

We have performed first-principle calculations of the structural, electronic and magnetic properties of Mn2CoAs Heusler alloy, using full-potential linearized augmented plane wave (FP-LAPW) scheme within the GGA. Features such as the lattice constant, the bulk modulus and its pressure derivative are reported. The electronic band structures and density of states of the Mn2CoAs compound show that the spin-up electrons are metallic, but the spin-down bands have a gap of 0.48 eV, resulting in stable half-metallic ferrimagnetic behavior with a magnetic moment of 4.00 ?B.

Berri, Saadi; Ibrir, M.; Maouche, D.; Bensalem, R.

2014-06-01

365

First-Principles Prediction of Doped Graphane as a High-Temperature Electron-Phonon Superconductor

NASA Astrophysics Data System (ADS)

We predict by first-principles calculations that p-doped graphane is an electron-phonon superconductor with a critical temperature above the boiling point of liquid nitrogen. The unique strength of the chemical bonds between carbon atoms and the large density of electronic states at the Fermi energy arising from the reduced dimensionality give rise to a giant Kohn anomaly in the optical phonon dispersions and push the superconducting critical temperature above 90 K. As evidence of graphane was recently reported, and doping of related materials such as graphene, diamond, and carbon nanostructures is well established, superconducting graphane may be feasible.

Savini, G.; Ferrari, A. C.; Giustino, F.

2010-07-01

366

First-principles investigation of B- and N-doped fluorographene

NASA Astrophysics Data System (ADS)

The effect of substitutional doping of fluorographene with boron and nitrogen atoms on its electronic and magnetic properties is investigated using first-principles calculations. It is found that boron dopants can be readily incorporated in the fluorographene crystal where they act as shallow acceptors and cause hole doping, but no changes in the magnetic properties are observed. Nitrogen dopants act as deep donors and give rise to a magnetic moment, but the resulting system becomes chemically unstable. These results are opposite to what was found for substitutional doping of graphane, i.e., hydrogenated graphene, in which case B substituents induce magnetism and N dopants do not.

Leenaerts, O.; Sahin, H.; Partoens, B.; Peeters, F. M.

2013-07-01

367

Alloying InAs and InP nanowires for optoelectronic applications: A first principles study

NASA Astrophysics Data System (ADS)

The capability of nanowires to relieve the stress introduced by lattice mismatching through radial relaxation opens the possibility to search for devices for optoelectronic applications. However, there are difficulties to fabricate, and therefore to explore the properties of nanowires with narrow diameters. Here we apply first principles calculations to study the electronic and optical properties of narrow InAs1 - xPx nanowires. Our results show that the absorption threshold can be pushed to near-ultraviolet region, and suggests that arrays of these nanowires with different diameters and compositions could be used as devices acting from the mid-infrared to the near-ultraviolet region.

Toniolo, Giuliano R.; Anversa, Jonas; dos Santos, Cláudia L.; Piquini, Paulo

2014-08-01

368

First-principles study of vacancy-assisted impurity diffusion in ZnO

NASA Astrophysics Data System (ADS)

Group-III elements act as donors in ZnO when incorporated on the Zn site. Their incorporation and behavior upon annealing is governed by diffusion, which proceeds mainly through a vacancy-assisted process. We report first-principles calculations for the migration of Al, Ga, and In donors in ZnO, based on density functional theory using a hybrid functional. From the calculated migration barriers and formation energies, we determine diffusion activation energies and estimate annealing temperatures. Impurity-vacancy binding energies and migration barriers decrease from Al to In. Activation energies for vacancy-assisted diffusion are lowest for In and highest for Al.

Steiauf, Daniel; Lyons, John L.; Janotti, Anderson; Van de Walle, Chris G.

2014-09-01

369

First-principle and empirical modelling of the global-scale ionosphere

NASA Technical Reports Server (NTRS)

The SUNDIAL program offers a unique opportunity to study ionospheric behavior on a global scale. As part of this program, data pertaining to solar, interplanetary, magnetospheric, ionospheric, and thermospheric conditions are collected simultaneously from a large number of satellite and ground-based sites spread around the world. In the coming years, these data should lead to a major improvement in both empirical and first-principle ionospheric models. As a benchmark against which to compare future progress, the present state of empirical and numerical ionospheric modeling is discussed. The discussion covers the capabilities and limitations of the existing models as well as the direction of future modeling efforts.

Schunk, R. W.; Szuszczewicz, Edward P.

1988-01-01

370

Half metallic ferromagnetism in alkali metal nitrides MN (M = Rb, Cs): A first principles study

The structural, electronic and elastic properties of two alkali metal nitrides (MN: M= Rb, Cs) are investigated by the first principles calculations based on density functional theory using the Vienna ab-initio simulation package. At ambient pressure the two nitrides are stable in ferromagnetic state with CsCl structure. The calculated lattice parameters are in good agreement with the available results. The electronic structure reveals that these materials are half metallic in nature. A pressure-induced structural phase transition from CsCl to ZB phase is observed in RbN and CsN.

Murugan, A., E-mail: rrpalanichamy@gmail.com; Rajeswarapalanichamy, R., E-mail: rrpalanichamy@gmail.com; Santhosh, M., E-mail: rrpalanichamy@gmail.com; Sudhapriyanga, G., E-mail: rrpalanichamy@gmail.com [Department of Physics, N.M.S.S.V.N College, Madurai, Tamilnadu-625019 (India); Kanagaprabha, S. [Department of Physics, Kamaraj College, Tuticorin, Tamil Nadu-628003 (India)

2014-04-24

371

First principles calculation of the effect of Coulomb collisions in partially ionized gases

Coulomb collisions, at appreciable ratios (?) of the electron to the neutral particle density, influence significantly the electron kinetics in particle swarms and in plasmas of gas discharges. This paper introduces a combination of Molecular Dynamics and Monte Carlo simulation techniques, to provide a novel, approximation-free, first principles calculation method for the velocity distribution function of electrons, and related swarm characteristics, at arbitrary ?. Simulation results are presented for electrons in argon gas, for density ratios between zero and 10{sup ?1}, representing the limits of a negligible electron density and an almost complete Maxwellization of the velocity distribution function, respectively.

Donkó, Z. [Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box No. 49, H-1525 Budapest (Hungary)] [Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box No. 49, H-1525 Budapest (Hungary)

2014-04-15

372

NASA Astrophysics Data System (ADS)

Interaction between K adatoms on graphene is investigated by first-principles calculations based on density function theory and analytical analyses based on the k .p perturbation theory. The calculation shows that there is a strong repulsion between K adatoms. The main origin of this strong repulsion is not from the dipole-dipole interaction as suggested for K adatoms on graphite surface, but comes from the screened Coulomb interaction. Potassium adatom on graphene donates its s electron and becomes K+. The positively charged K adatom induces electron density oscillation on graphene which is responsible for the screened Coulomb repulsion between the K adatoms.

Liu, Xiaojie; Wang, Cai-Zhuang; Lin, Hai-Qing; Chang, Kai; Chen, Jian; Ho, Kai-Ming

2015-01-01

373

NASA Astrophysics Data System (ADS)

The effective interatomic potentials between distinct atoms in intermetallics, such as FeAl, Fe3Al, NiAl, Ni3Al, FeCr, Al3Cr, AlLi, Al3Li, and AlLi3, are obtained by inversion of first-principles cohesive energy curves based on the lattice inversion method of Chen. The obtained potentials are used to evaluate the phonon dispersions and linear thermal expansion of some intermetallic compounds, as well as the site preference of alloying element Cr in the D03-ordered Fe3Al.

Zhang, Wen-qing; Xie, Qian; Ge, Xi-jin; Chen, Nan-xian

1997-07-01

374

Oxygen reduction reactions on pure and nitrogen-doped graphene: a first-principles modeling

Based on first principles density functional theory calculations we explored energetics of oxygen reduction reaction over pristine and nitrogen-doped graphene with different amounts of nitrogen doping. The process of oxygen reduction requires one more step then same reaction catalyzed by metals. Results of calculations evidence that for the case of light doped graphene (about 4% of nitrogen) energy barrier for each step is lower than for the same process on Pt surface. In contrast to the catalysis on metal surface the maximal coverage of doped graphene is lower and depends on the corrugation of graphene. Changes of the energy barriers caused by oxygen load and corrugation are also discussed.

Boukhvalov, Danil W

2011-01-01

375

Elastic properties of Ca-based metallic glasses predicted by first-principles simulations

First-principles simulations of Ca-based metallic glass-forming alloys yield sample amorphous structures whose structures can be compared to experiment and whose properties can be analyzed. In an effort to understand and control ductility, we investigate the elastic moduli. Calculated Poisson ratios depend strongly on alloying elements in a manner that correlates with ionicity (charge transfer). Consequently, we predict that alloying Ca with Mg and Zn should result in relatively ductile glasses compared to alloying with Ag, Cu, or Al. Experimental observations validate these predictions.

Widom, M.; Sauerwine, B.; Cheung, A.M.; Poon, S.J.; Tong, P.; Louca, D.; Shiflet, G.J. (CM); (UV)

2012-07-11

376

Proposed in theory and then their existence confirmed, anion-? interactions have been recognized as new and important non-covalent binding forces. Despite extensive theoretical studies, numerous crystal structural identifications, and a plethora of solution phase investigations, anion-? interaction strengths that are free from complications of condensed-phase environments have not been directly measured in the gas phase. Herein we present a joint photoelectron spectroscopic and theoretical study on this subject, in which tetraoxacalix[2]arene[2]triazine 1, an electron-deficient and cavity self-tunable macrocyclic, was used as a charge-neutral molecular host to probe its interactions with a series of anions with distinctly different shapes and charge states (spherical halides Cl(-), Br(-), I(-), linear thiocyanate SCN(-), trigonal planar nitrate NO3(-), pyramidic iodate IO3(-), and tetrahedral sulfate SO4(2-)). The binding energies of the resultant gaseous 1?:?1 complexes (1·Cl(-), 1·Br(-), 1·I(-), 1·SCN(-), 1·NO3(-), 1·IO3(-) and 1·SO4(2-)) were directly measured experimentally, exhibiting substantial non-covalent interactions with pronounced anion-specific effects. The binding strengths of Cl(-), NO3(-), IO3(-) with 1 are found to be strongest among all singly charged anions, amounting to ca. 30 kcal mol(-1), but only about 40% of that between 1 and SO4(2-). Quantum chemical calculations reveal that all the anions reside in the center of the cavity of 1 with an anion-? binding motif in the complexes' optimized structures, where 1 is seen to be able to self-regulate its cavity structure to accommodate anions of different geometries and three-dimensional shapes. Electron density surface and charge distribution analyses further support anion-? binding formation. The calculated binding energies of the anions and 1 nicely reproduce the experimentally estimated electron binding energy increase. This work illustrates that size-selective photoelectron spectroscopy combined with theoretical calculations represents a powerful technique to probe anion-? interactions and has potential to provide quantitative guest-host molecular binding strengths and unravel fundamental insights in specific anion recognitions. PMID:25515705

Zhang, Jian; Zhou, Bin; Sun, Zhen-Rong; Wang, Xue-Bin

2015-02-01

377

Understanding the structure of water near cell membranes is crucial for characterizing water-mediated events such as molecular transport. To obtain structural information of water near a membrane, it is useful to have a surface-selective technique that can probe only interfacial water molecules. One such technique is vibrational sum-frequency generation (VSFG) spectroscopy. As model systems for studying membrane headgroup/water interactions, in this paper we consider lipid and surfactant monolayers on water. We adopt a theoretical approach combining molecular dynamics simulations and phase-sensitive VSFG to investigate water structure near these interfaces. Our simulated spectra are in qualitative agreement with experiments and reveal orientational ordering of interfacial water molecules near cationic, anionic, and zwitterionic interfaces. OH bonds of water molecules point toward an anionic interface leading to a positive VSFG peak, whereas the water hydrogen atoms point away from a cationic interface leading to a negative VSFG peak. Coexistence of these two interfacial water species is observed near interfaces between water and mixtures of cationic and anionic lipids, as indicated by the presence of both negative and positive peaks in their VSFG spectra. In the case of a zwitterionic interface, OH orientation is toward the interface on the average, resulting in a positive VSFG peak. PMID:25399167

Roy, S; Gruenbaum, S M; Skinner, J L

2014-11-14

378

NASA Astrophysics Data System (ADS)

Understanding the structure of water near cell membranes is crucial for characterizing water-mediated events such as molecular transport. To obtain structural information of water near a membrane, it is useful to have a surface-selective technique that can probe only interfacial water molecules. One such technique is vibrational sum-frequency generation (VSFG) spectroscopy. As model systems for studying membrane headgroup/water interactions, in this paper we consider lipid and surfactant monolayers on water. We adopt a theoretical approach combining molecular dynamics simulations and phase-sensitive VSFG to investigate water structure near these interfaces. Our simulated spectra are in qualitative agreement with experiments and reveal orientational ordering of interfacial water molecules near cationic, anionic, and zwitterionic interfaces. OH bonds of water molecules point toward an anionic interface leading to a positive VSFG peak, whereas the water hydrogen atoms point away from a cationic interface leading to a negative VSFG peak. Coexistence of these two interfacial water species is observed near interfaces between water and mixtures of cationic and anionic lipids, as indicated by the presence of both negative and positive peaks in their VSFG spectra. In the case of a zwitterionic interface, OH orientation is toward the interface on the average, resulting in a positive VSFG peak.

Roy, S.; Gruenbaum, S. M.; Skinner, J. L.

2014-11-01

379

Tautomerization induced by protonation of halouracils may increase their efficacy as anti-cancer drugs by altering their reactivity and hydrogen bonding characteristics, potentially inducing errors during DNA and RNA replication. The gas-phase structures of protonated complexes of five halouracils, including 5-fluorouracil, 5-chlorouracil, 5-bromouracil, 5-iodouracil, and 6-chlorouracil are examined via infrared multiple photon dissociation (IRMPD) action spectroscopy and theoretical electronic structure calculations. IRMPD action spectra were measured for each complex in the IR fingerprint region extending from ~1000 to 1900 cm(-1) using the free electron laser (FELIX). Correlations are made between the measured IRMPD action spectra and the linear IR spectra for the stable low-energy tautomeric conformations computed at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G* level of theory. Absence of an intense band(s) in the IRMPD spectrum arising from the carbonyl stretch(es) that are expected to appear near 1825 cm(-1) provides evidence that protonation induces tautomerization and preferentially stabilizes alternative, noncanonical tautomers of these halouracils where both keto functionalities are converted to hydroxyl groups upon binding of a proton. The weak, but measurable absorption, which does occur for these systems near 1835 cm(-1) suggests that in addition to the ground-state conformer, very minor populations of excited, low-energy conformers that contain keto functionalities are also present in these experiments. PMID:22821195

Crampton, K T; Rathur, A I; Nei, Y-w; Berden, G; Oomens, J; Rodgers, M T

2012-09-01

380

NASA Astrophysics Data System (ADS)

Tautomerization induced by protonation of halouracils may increase their efficacy as anti-cancer drugs by altering their reactivity and hydrogen bonding characteristics, potentially inducing errors during DNA and RNA replication. The gas-phase structures of protonated complexes of five halouracils, including 5-fluorouracil, 5-chlorouracil, 5-bromouracil, 5-iodouracil, and 6-chlorouracil are examined via infrared multiple photon dissociation (IRMPD) action spectroscopy and theoretical electronic structure calculations. IRMPD action spectra were measured for each complex in the IR fingerprint region extending from ~1000 to 1900 cm-1 using the free electron laser (FELIX). Correlations are made between the measured IRMPD action spectra and the linear IR spectra for the stable low-energy tautomeric conformations computed at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G* level of theory. Absence of an intense band(s) in the IRMPD spectrum arising from the carbonyl stretch(es) that are expected to appear near 1825 cm-1 provides evidence that protonation induces tautomerization and preferentially stabilizes alternative, noncanonical tautomers of these halouracils where both keto functionalities are converted to hydroxyl groups upon binding of a proton. The weak, but measurable absorption, which does occur for these systems near 1835 cm-1 suggests that in addition to the ground-state conformer, very minor populations of excited, low-energy conformers that contain keto functionalities are also present in these experiments.

Crampton, K. T.; Rathur, A. I.; Nei, Y.-w.; Berden, G.; Oomens, J.; Rodgers, M. T.

2012-09-01

381

First principle study of elastic and thermodynamic properties of FeB{sub 4} under high pressure

The elastic properties, elastic anisotropy, and thermodynamic properties of the lately synthesized orthorhombic FeB{sub 4} at high pressures are investigated using first-principles density functional calculations. The calculated equilibrium parameters are in good agreement with the available experimental and theoretical data. The obtained normalized volume dependence of high pressure is consistent with the previous experimental data investigated using high-pressure synchrotron x-ray diffraction. The complete elastic tensors and crystal anisotropies of the FeB{sub 4} are also determined in the pressure range of 0–100?GPa. By the elastic stability criteria and vibrational frequencies, it is predicted that the orthorhombic FeB{sub 4} is stable up to 100 GPa. In addition, the calculated B/G ratio reveals that FeB{sub 4} possesses brittle nature in the range of pressure from 0 to 100?GPa. The calculated elastic anisotropic factors suggest that FeB{sub 4} is elastically anisotropic. By using quasi-harmonic Debye model, the compressibility, bulk modulus, the coefficient of thermal expansion, the heat capacity, and the Grüneisen parameter of FeB{sub 4} are successfully obtained in the present work.

Zhang, Xinyu, E-mail: xyzhang@ysu.edu.cn, E-mail: jiaqianqin@gmail.com, E-mail: riping@ysu.edu.cn; Ning, Jinliang; Sun, Xiaowei; Li, Xinting; Ma, Mingzhen; Liu, Riping, E-mail: xyzhang@ysu.edu.cn, E-mail: jiaqianqin@gmail.com, E-mail: riping@ysu.edu.cn [State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004 (China); Qin, Jiaqian, E-mail: xyzhang@ysu.edu.cn, E-mail: jiaqianqin@gmail.com, E-mail: riping@ysu.edu.cn [Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok 10330 (Thailand); State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004 (China)

2013-11-14

382

NASA Astrophysics Data System (ADS)

Using first-principles plane-wave calculations within density functional theory, we theoretically studied the atomic structure, bonding energy and electronic properties of the perfect Mo (110)/MoSe2 (100) interface with a lattice mismatch less than 4.2%. Compared with the perfect structure, the interface is somewhat relaxed, and its atomic positions and bond lengths change slightly. The calculated interface bonding energy is about -1.2 J/m2, indicating that this interface is very stable. The MoSe2 layer on the interface has some interface states near the Fermi level, the interface states are mainly caused by Mo 4d orbitals, while the Se atom almost have no contribution. On the interface, Mo-5s and Se-4p orbitals hybridize at about -6.5 to -5.0 eV, and Mo-4d and Se-4p orbitals hybridize at about -5.0 to -1.0 eV. These hybridizations greatly improve the bonding ability of Mo and Se atom in the interface. By Bader charge analysis, we find electron redistribution near the interface which promotes the bonding of the Mo and MoSe2 layer.

Zhang, Yu; Tang, Fu-Ling; Xue, Hong-Tao; Lu, Wen-Jiang; Liu, Jiang-Fei; Huang, Min

2015-02-01

383

The structure of grain boundaries (GBs) or interfaces between nano-forms of carbon determines their evolution into 3-D forms with nano-scale architecture. Here, we present a general framework for the construction of interfaces in 2-D h-BN and graphene in terms of (a) stacking faults and (b) growth faults, using first-principles density functional theoretical analysis. Such interfaces or GBs involve deviation from their ideal hexagonal lattice structure. We show that a stacking fault involves a linkage of rhombal and octagonal rings (4?:?8), and a growth fault involves a linkage of paired pentagonal and octagonal rings (5?:?5?:?8). While a growth fault is energetically more stable than a stacking fault in graphene, the polarity of B and N leads to the reversal of their relative stability in h-BN. We show that the planar structure of these interfacing grains exhibits instability with respect to buckling (out-of-plane deformation), which results in the formation of a wrinkle at the grain boundary (GB) and rippling of the structure. Our analysis leads to prediction of new types of low-energy GBs of 2-D h-BN and graphene. Our results for electronic and vibrational signatures of these interfaces and an STM image of the most stable interface will facilitate their experimental characterization, particularly of the wrinkles forming spontaneously at these interfaces. PMID:25196681

Singh, Anjali; Waghmare, Umesh V

2014-10-21

384

We report first-principles calculations of adsorption, dissociation, penetration, and diffusion for the complete nitridation mechanism of nitrogen molecules on a pure Fe surface (bcc, ferrite phase). The mechanism of the definite reaction path was calculated by dividing the process into four steps. We investigated various reaction paths for each step including the energy barrier based on the climb image nudged elastic band (CI-NEB) method, and the complete reaction pathway was computed as the minimum energy path (MEP). The adsorption characteristics of nitrogen (N) and molecular nitrogen (N2) indicate that nitrogen atoms and molecules are energetically favorable at the hollow sites on pure Fe(100) and (110). The dissociation of the nitrogen molecule (N2) was theoretically supported by electronic structure calculations. The penetration of nitrogen from the surface to the sub-surface has a large energy barrier compared with the other steps. The activation energy calculated for nitrogen diffusion in pure bcc Fe was in good agreement with the experimental results. Finally, we confirmed the rate-determining step for the full nitridation reaction pathway. This study provides fundamental insight into the nitridation mechanism for nitrogen molecules in pure bcc Fe. PMID:23455435

Yeo, Sang Chul; Han, Sang Soo; Lee, Hyuck Mo

2013-04-14

385

NASA Astrophysics Data System (ADS)

As a candidate for hydrogen storage medium, geometric stability and hydrogen capacity of Ca-decorated graphene with topological defects are investigated using the first-principle based on density functional theory (DFT), specifically for the experimentally realizable single carbon vacancy (SV), 585 double carbon vacancy (585 DCV) and 555-777 double carbon vacancy (555-777 DCV) defects. It is found that Ca atom can be stabilized on above defective graphenes since Ca's binding energy on vacancy defect is much larger than its cohesive energy. Up to six H2 molecules can stably bind to a Ca atom on defective graphene with the average adsorption energies of 0.17-0.39 eV/H2. The hybridization of the Ca-3d orbitals with H2-?orbitals and the electrostatic interaction between the Ca cation and the induced H2 dipole both contribute to the H2 molecules binding. Double-side Ca-decorated graphene with 585 DCV and 555-777 DCV defects can theoretically reach a gravimetric capacity of 5.2 wt% hydrogen, indicating that Ca-decorated defective graphene can be used as a promising material for high density hydrogen storage.

Ma, Ling; Zhang, Jian-Min; Xu, Ke-Wei; Ji, Vincent

2014-09-01

386

First-principles calculations on mechanical and elastic properties of 2H- and 3R-WS2 under pressure

NASA Astrophysics Data System (ADS)

The structure, mechanical stability and elastic properties of 2H- and 3R-WS2 under pressure have been investigated using first-principles calculations based on density functional theory (DFT). The equilibrium lattice parameters of 2H- and 3R-WS2 at 0 GPa are consistent with experimental and other theoretical values. 2H-WS2 is more stable than 3R-WS2 when pressure is less than 5.8 GPa whereas 3R-WS2 is more stable than 2H-WS2 when pressure is higher than 5.8 GPa. According to the mechanical stability criteria, both 2H- and 3R-WS2 exhibit mechanical stability under the pressure range from 0 to 20 GPa. With the increasing pressure, the elastic moduli (E, B, G), sound velocities (vs, vp, vm) and Debye temperatures of 2H- and 3R-WS2 increase monotonously whereas volume and specific heat decrease. Large elastic anisotropies in compressibility and in shear were demonstrated for 2H- and 3R-WS2 at 0 GPa. As the pressure increases, anisotropies in compressibility and in shear become weak for 2H- and 3R-WS2. Moreover, 2H- and 3R-WS2 under pressure have higher hardness and better ductility than those at 0 GPa.

Feng, Li-ping; Wang, Zhi-qiang; Liu, Zheng-tang

2014-06-01

387

NASA Astrophysics Data System (ADS)

The carrier doping effects on the magnetic properties of defective graphene with a hydrogen chemisorbed single-atom vacancy (H-GSV) are investigated by performing extensive spin-polarized first-principles calculations. Theoretical results show that the quasi-localized pz-derived states around the Fermi level are responsible for the weakened magnetic moment (MM) and magnetic stabilized energy (MSE) of the H-GSV under carrier doping. The mechanism of reduced MSE in the carrier doped H-GSV can be well understood by the Heisenberg magnetic coupling model due to the response of these pz-derived states to the carrier doping. Within the examined range of carrier doping concentration, the total MM of H-GSV is always larger than 1.0?B with ?B representing the Bohr magneton, which is mainly contributed by the localized sp2 states of the unsaturated C atom around the vacancy. These findings of H-GSV provide fundamental insight into defective graphene and help to understand the related experimental observations.

Lei, Shu-Lai; Li, Bin; Huang, Jing; Li, Qun-Xiang; Yang, Jin-Long

2013-07-01

388

Magnetic properties of strained La2/3Sr1/3MnO3 perovskites from first principles

NASA Astrophysics Data System (ADS)

The critical temperature TC of ferromagnetic LaxSr1-xMnO3 (LSMO) can be controlled by distorting the crystal structure, as was reported by Thiele et al (2007 Phys. Rev. B 75 054408). To confirm these findings theoretically, we investigate the electronic as well as the magnetic ground state properties of La2/3Sr1/3MnO3 as a function of tetragonal lattice distortions, using a multiple-scattering Green function method. Within this approach, we calculate exchange coupling constants as well as the phase transition temperature from first principles. Comparing our findings with those for La2/3Sr1/3CoO3 (LSCO), we find that the decrease of TC is much stronger in LSMO than in LSCO. Our findings can be explained by the electronic structures and are also in accordance with the experiment. The computed decrease of TC with distortion is smaller than observed experimentally, a result that corroborates the importance of phonon contributions.

Böttcher, D.; Henk, J.

2013-04-01

389

NASA Astrophysics Data System (ADS)

It has been a notably elusive task to find a remotely sensical ansatz for a calculation of Sommerfeld's electrodynamic fine-structure constant ?QED ? 1 / 137.036 based on first principles. However, this has not prevented a number of researchers to invest considerable effort into the problem, despite the formidable challenges, and a number of attempts have been recorded in the literature. Here, we review a possible approach based on the quantum electrodynamic (QED) ? function, and on algebraic identities relating ?QED to invariant properties of "internal" symmetry groups, as well as attempts to relate the strength of the electromagnetic interaction to the natural cutoff scale for other gauge theories. Conjectures based on both classical as well as quantum-field theoretical considerations are discussed. We point out apparent strengths and weaknesses of the most prominent attempts that were recorded in the literature. This includes possible connections to scaling properties of the Einstein-Maxwell Lagrangian which describes gravitational and electromagnetic interactions on curved space-times. Alternative approaches inspired by string theory are also discussed. A conceivable variation of the fine-structure constant with time would suggest a connection of ?QED to global structures of the Universe, which in turn are largely determined by gravitational interactions.

Jentschura, U. D.; Nándori, I.

2014-12-01

390

NASA Astrophysics Data System (ADS)

It has been a notably elusive task to find a remotely sensical ansatz for a calculation of Sommerfeld's electrodynamic fine-structure constant ? QED ? 1 / 137.036 based on first principles. However, this has not prevented a number of researchers to invest considerable effort into the problem, despite the formidable challenges, and a number of attempts have been recorded in the literature. Here, we review a possible approach based on the quantum electrodynamic (QED) ? function, and on algebraic identities relating ? QED to invariant properties of "internal" symmetry groups, as well as attempts to relate the strength of the electromagnetic interaction to the natural cutoff scale for other gauge theories. Conjectures based on both classical as well as quantum-field theoretical considerations are discussed. We point out apparent strengths and weaknesses of the most prominent attempts that were recorded in the literature. This includes possible connections to scaling properties of the Einstein-Maxwell Lagrangian which describes gravitational and electromagnetic interactions on curved space-times. Alternative approaches inspired by string theory are also discussed. A conceivable variation of the fine-structure constant with time would suggest a connection of ? QED to global structures of the Universe, which in turn are largely determined by gravitational interactions.

Jentschura, U. D.; Nándori, I.

2014-12-01

391

Introduction of economically viable hydrogen cars is hindered by the need to store large amounts of hydrogen. Metal borohydrides [LiBH(4), Mg(BH(4))(2), Ca(BH(4))(2)] are attractive candidates for onboard storage because they contain high densities of hydrogen by weight and by volume. Using a set of recently developed theoretical first-principles methods, we predict currently unknown crystal structures and hydrogen storage reactions in the Li-Mg-Ca-B-H system. Hydrogen release from LiBH(4) and Mg(BH(4))(2) is predicted to proceed via intermediate Li(2)B(12)H(12) and MgB(12)H(12) phases, while for Ca borohydride two competing reaction pathways (into CaB(6) and CaH(2), and into CaB(12)H(12) and CaH(2)) are found to have nearly equal free energies. We predict two new hydrogen storage reactions that are some of the most attractive among the presently known ones. They combine high gravimetric densities (8.4 and 7.7 wt % H(2)) with low enthalpies [approximately 25 kJ/(mol H(2))] and are thermodynamically reversible at low pressures due to low vibrational entropies of the product phases containing the [B(12)H(12)](2-) anion. PMID:19072157

Ozolins, V; Majzoub, E H; Wolverton, C

2009-01-14

392

NASA Astrophysics Data System (ADS)

We have investigated the electronic structure, chemical bonding, and equations of state of Zr2Al3C5 by means of the ab initio pseudopotential total energy method. The chemical bonding displays layered characteristics and is similar to that of nanolaminate ternary aluminum carbides Ti2AlC and Ti3AlC2 . Zr2Al3C5 could be fundamentally described as strong covalent bonding among Al-C-Zr-C-Zr-C-Al atomic chains being interleaved and mirrored by AlC2 blocks. The interplanar cohesion between covalent atomic chains and AlC2 blocks is very weak based on first-principles cohesion energy calculations. Inspired by the structure-property relationship of Ti2AlC and Ti3AlC2 , it is expected that Zr2Al3C5 will have easy machinability, damage tolerance, and oxidation resistance besides the merits of refractory ZrC. Zr2Al3C5 has a theoretical bulk modulus of 160GPa and illustrates elastic anisotropy under pressure below 20GPa .

Wang, Jingyang; Zhou, Yanchun; Lin, Zhijun; Liao, Ting

2005-08-01

393

NASA Astrophysics Data System (ADS)

We perform the first-principles calculations within the framework of density functional theory to determine the electronic structure and optical properties of MgxZn1?xS bulk crystal. The results indicate that the electronic structure and optical properties of MgxZn1?xS bulk crystal are sensitive to the Mg impurity composition. In particular, the MgxZn1?xS bulk crystal displays a direct band structure and the band gap increases from 2.05 eV to 2.91 eV with Mg dopant composition value x increasing from 0 to 0.024. The S 3p electrons dominate the top of valence band, while the Zn 4s electrons and Zn 3p electrons occupy the bottom of conduction band in MgxZn1?xS bulk crystal. Moreover, the dielectric constant decreases and the optical absorption peak obviously has a blue shift. The calculated results provide important theoretical guidance for the applications of MgxZn1?xS bulk crystal in optical detectors.

Yu, Zhi-Qiang; Xu, Zhi-Mou; Wu, Xing-Hui

2014-10-01

394

NASA Astrophysics Data System (ADS)

Titanium dioxide is well known as a semiconductor material, which attracts a great deal of attention for promising applications in many fields due to its outstanding physical and chemical properties. To investigate the structural, elastic, mechanical, electronic and optical properties of various TiO2 phases systematically, we present the ultrasoft pseudopotential planewave method within local density approximation and generalized gradient approximation, as well as the norm-conserving pseudopotential within hybrid functional B3LYP by first-principles calculations on fluorite, pyrite, rutile, anatase, hollandite, brookite, columbite, cotunnite, bronze and baddeleyite TiO2 phases. The structural parameters of ten phases are calculated, which are shown to be consistent with previous theoretical and experimental data. We obtain the elastic constants of ten phases and then estimate the bulk, shear and Young’s moduli, Poisson’s coefficient and Lamé’s constants using the Voigt-Reuss-Hill approximation. The energy band structures, density of states and charge populations of ten phases were obtained and indicated there is covalency in TiO2. Moreover, the complex dielectric function, refractive index and extinction coefficient of the ten phases were calculated; this data can aid future experimental research.

Liu, Qi-Jun; Zhang, Ning-Chao; Liu, Fu-Sheng; Liu, Zheng-Tang

2014-07-01

395

NASA Astrophysics Data System (ADS)

The crystallographic structure and stability of the ?? phase relative to the ? and ? phases in Ti-x M (M=Ta, Nb, V, Mo) alloys are investigated by using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation. We show that, with increasing concentration of the alloying elements, the structure of the orthorhombic-?? phase evolutes from the hcp-? to the bcc-? phase, i.e., the lattice parameters b/a and c/a as well as the basal shuffle y decreases from those corresponding to the ? phase to those of the ? phase. The compositional ?/?? and ??/? phase boundaries are determined by comparing the total energies of the phases. The predicted ?/?? phase boundaries are about 10.2, 10.5, 11.5, 4.5 at% for Ti-V, Ti-Nb, Ti-Ta, and Ti-Mo, respectively, in reasonable agreement with experiments. The ??/? phase boundaries are higher than the experimental values, possibly due to the absence of temperature effect in the first-principles calculations. Analyzing the electronic density of states, we propose that the stability of the ?? phase is controlled by the compromise between the strength of the covalent and metallic bonds.

Li, Chun-Xia; Luo, Hu-Bin; Hu, Qing-Miao; Yang, Rui; Yin, Fu-Xing; Umezawa, Osamu; Vitos, Levente

2013-04-01

396

First principles calculations of magnetic properties of Fe and Fe3C at finite temperature

NASA Astrophysics Data System (ADS)

We demonstrate a method to investigate finite temperature magnetism from first principles that harnesses massively parallel computers to obtain the free energy, specific heat, magnetization, susceptibility, and other quantities as function of temperature by combining classical Wang-Landau Monte-Carlo calculations with a first principles electronic structure code that allows the energy calculation of constrained magnetic states. Here we will present our calculations of finite temperature properties such as specific heat, magnetization and susceptibility of Fe and Fe3C using this approach where we find the Curie temperatures to be in good agreement with experiment at 980K and 425K respectively. This work was conducted at Oak Ridge National Laboratory (ORNL), which is managed by UT-Battelle for the U.S. Department of Energy (US DOE) under contract DE-AC05-00OR22725 and sponsored in parts by the Center for Nanophase Material Sciences, Scientific User Facilities Division, the Center for Defect Physics, an Energy Frontier Research Center funded by the US DOE Office of Basic Energy Sciences and by the US DOE Office of Energy Efficiency and Renewable Energy, Industrial Technologies Program. This research used resources of the Oak Ridge Leadership Computing Facility at ORNL, which is supported by the US DOE, Office of Science.

Eisenbach, Markus; Brown, Gregory; Rusanu, Aurelian; Nicholson, Don M.

2011-03-01

397

Electronic structures and work functions of BC3 nanotubes: A first-principle study

NASA Astrophysics Data System (ADS)

The electronic structures and work functions of the single-wall BC3 nanotubes are studied by the first-principle method. In contrast to the armchair nanotubes, the zigzag nanotubes are indirect-band semiconductors. The zigzag nanotubes can be further classified into two types, depending on the wave-vector characteristics of the conduction band minima. The bandgap energies of the armchair nanotubes are always smaller than that of the BC3 sheet, and increase with the nanotube diameter. For the two types of zigzag nanotubes, the bandgap energies are always larger than that of the BC3 sheet, and decrease with the nanotube diameters. Analysis of the bond angles between the neighboring atoms on the nanotube walls shows that the mixing of the ? and ? electron orbitals exhibits opposite trends for the armchair and zigzag nanotubes. These trends are not only responsible for the above dependence of the electronic structures on the types and diameters of the nanotubes, but also give physical insight to the indirectness of the band-structures of the zigzag nanotubes. By the first-principle method, it is found that the work functions of both armchair and zigzag nanotubes scale linearly with 1/D2, D being the nanotube diameter. Moreover, it is found that the per-atom strain energies and the vacuum bandwidths of both armchair and zigzag nanotubes satisfy universal relations which also scale linearly in 1/D2.

Su, W. S.; Chang, C. P.; Lin, M. F.; Li, T. L.

2011-07-01

398

Pure zircon and scheelite LuVO{sub 4} were prepared by solid state reaction and high-pressure route, respectively. Structure, elastic constants, lattice dynamics and thermodynamics of LuVO{sub 4} polymorphs were studied by experiments and first principles calculation. Calculations here are in good agreement with the experimental results. The phonon dispersions of LuVO{sub 4} polymorphs were studied by the linear response method. The calculated phonon dispersions show that zircon and scheelite LuVO{sub 4} phases are dynamically stable. Raman-active frequencies were measured and assigned to different modes according to the calculations. The internal frequencies shift downward after phase transition from zircon to scheelite. Born effective charge tensors elements for both phases are analyzed. The finite temperature thermodynamic properties of LuVO{sub 4} polymorphs were calculated from the obtained phonon density of states by quasi-harmonic approach. - Graphical abstract: Lutetium orthovanadate polymorphs were synthesized by SSR and HP methods and their physical and chemical properties, including lattice dynamical properties, were determined by DFT calculations and experiments. Display Omitted - Highlights: • Pure zircon and scheelite LuVO{sub 4} polymorphs were synthesized by solid state reaction and high-pressure route. • Chemical and physical properties of LuVO4 polymorphs were studied by experiments and first principles calculation. • Raman-active frequencies were measured and assigned to different modes according to the calculations. • Lattice dynamics of polymorphs were discussed in details.

Huang, Zuocai; Zhang, Lei; Pan, Wei, E-mail: panw@mail.tsinghua.edu.cn

2013-09-15

399

Temperature-dependent stability of stacking faults in Al, Cu and Ni: first-principles analysis.

We present comparative analysis of microscopic mechanisms relevant to plastic deformation of the face-centered cubic (FCC) metals Al, Cu, and Ni, through determination of the temperature-dependent free energies of intrinsic and unstable stacking faults along [1 1? 0] and [1 2? 1] on the (1?1?1) plane using first-principles density-functional-theory-based calculations. We show that vibrational contribution results in significant decrease in the free energy of barriers and intrinsic stacking faults (ISFs) of Al, Cu, and Ni with temperature, confirming an important role of thermal fluctuations in the stability of stacking faults (SFs) and deformation at elevated temperatures. In contrast to Al and Ni, the vibrational spectrum of the unstable stacking fault (USF[1 2? 1]) in Cu reveals structural instabilities, indicating that the energy barrier (?usf) along the (1?1?1)[1 2? 1] slip system in Cu, determined by typical first-principles calculations, is an overestimate, and its commonly used interpretation as the energy release rate needed for dislocation nucleation, as proposed by Rice (1992 J. Mech. Phys. Solids 40 239), should be taken with caution. PMID:25185834

Bhogra, Meha; Ramamurty, U; Waghmare, Umesh V

2014-09-24

400

A series of first principles Monte Carlo simulations in the isobaric-isothermal ensemble were carried out for liquid water at ambient conditions (T = 298 K and p = 1 atm). The Becke-Lee-Yang-Parr (BLYP) exchange and correlation energy functionals and norm-conserving Goedecker-Teter-Hutter (GTH) pseudopotentials were employed with the CP2K simulation package to examine systems consisting of 64 water molecules. The fluctuations in the system volume encountered in simulations in the isobaric-isothermal ensemble requires a reconsideration of the suitability of the typical charge density cutoff and the regular grid generation method previously used for the computation of the electrostatic energy in first principles simulations in the microcanonical or canonical ensembles. In particular, it is noted that a much higher cutoff is needed and that the most computationally efficient method of creating grids can result in poor simulations. Analysis of the simulation trajectories using a very large charge density cutoff at 1200 Ry and four different grid generation methods point to a substantially underestimated liquid density of about 0.85 g/cm{sup 3} resulting in a somewhat understructured liquid (with a value of about 2.7 for the height of the first peak in the oxygen/oxygen radial distribution function) for BLYP-GTH water at ambient conditions.

McGrath, M; Siepmann, J I; Kuo, I W; Mundy, C J; VandeVondele, J; Hutter, J; Mohamed, F; Krack, M

2004-12-02

401

Temperature-dependent stability of stacking faults in Al, Cu and Ni: first-principles analysis

NASA Astrophysics Data System (ADS)

We present comparative analysis of microscopic mechanisms relevant to plastic deformation of the face-centered cubic (FCC) metals Al, Cu, and Ni, through determination of the temperature-dependent free energies of intrinsic and unstable stacking faults along [1 \\bar{1} 0] and [1 \\bar{2} 1] on the (1?1?1) plane using first-principles density-functional-theory-based calculations. We show that vibrational contribution results in significant decrease in the free energy of barriers and intrinsic stacking faults (ISFs) of Al, Cu, and Ni with temperature, confirming an important role of thermal fluctuations in the stability of stacking faults (SFs) and deformation at elevated temperatures. In contrast to Al and Ni, the vibrational spectrum of the unstable stacking fault (USF_{[1\\,\\bar{2}\\,1]}) in Cu reveals structural instabilities, indicating that the energy barrier (?usf) along the (1?1?1)[1 \\bar{2} 1] slip system in Cu, determined by typical first-principles calculations, is an overestimate, and its commonly used interpretation as the energy release rate needed for dislocation nucleation, as proposed by Rice (1992 J. Mech. Phys. Solids 40 239), should be taken with caution.

Bhogra, Meha; Ramamurty, U.; Waghmare, Umesh V.

2014-09-01

402

First-Principles Modeling of Hydrogen Storage in Metal Hydride Systems

The objective of this project is to complement experimental efforts of MHoCE partners by using state-of-the-art theory and modeling to study the structure, thermodynamics, and kinetics of hydrogen storage materials. Specific goals include prediction of the heats of formation and other thermodynamic properties of alloys from first principles methods, identification of new alloys that can be tested experimentally, calculation of surface and energetic properties of nanoparticles, and calculation of kinetics involved with hydrogenation and dehydrogenation processes. Discovery of new metal hydrides with enhanced properties compared with existing materials is a critical need for the Metal Hydride Center of Excellence. New materials discovery can be aided by the use of first principles (ab initio) computational modeling in two ways: (1) The properties, including mechanisms, of existing materials can be better elucidated through a combined modeling/experimental approach. (2) The thermodynamic properties of novel materials that have not been made can, in many cases, be quickly screened with ab initio methods. We have used state-of-the-art computational techniques to explore millions of possible reaction conditions consisting of different element spaces, compositions, and temperatures. We have identified potentially promising single- and multi-step reactions that can be explored experimentally.

J. Karl Johnson

2011-05-20

403

Calcium borohydride is a potential candidate for onboard hydrogen storage because it has a high gravimetric capacity (11.5 wt.%) and a high volumetric hydrogen content (?130 kg m(-3)). Unfortunately, calcium borohydride suffers from the drawback of having very strongly bound hydrogen. In this study, Ca(BH?)? was predicted to form a destabilized system when it was mixed with LiBH?, NaBH?, or KBH?. The release of hydrogen from Ca(BH?)? was predicted to proceed via two competing reaction pathways (leading to CaB? and CaH? or CaB??H?? and CaH?) that were found to have almost equal free energies. Using a set of recently developed theoretical methods derived from first principles, we predicted five new hydrogen storage reactions that are among the most attractive of those presently known. These combine high gravimetric densities (>6.0 wt.% H?) with have low enthalpies [approximately 35 kJ/(mol(-1) H?)] and are thermodynamically reversible at low pressure within the target window for onboard storage that is actively being considered for hydrogen storage applications. Thus, the first-principles theoretical design of new materials for energy storage in future research appears to be possible. PMID:24092266

Guo, Yajuan; Ren, Ying; Wu, Haishun; Jia, Jianfeng

2013-12-01

404

??In this thesis we investigate the electronic and vibrational properties of several transition-metal dichalcogenide materials through first-principles calculations. First, the charge-density-wave (CDW) instability in 1T-TaSe2… (more)

[No author

2013-01-01

405

Thermodynamic properties of Al, Ni, NiAl, and Ni3Al from first-principles calculations

Thermodynamic properties of Al, Ni, NiAl, and Ni3Al from first-principles calculations Y. Wang *, Z The thermodynamic properties of Al, Ni, NiAl, and Ni3Al were studied using the first-principles approach. The 0-K of )1.6 J/mol/K for NiAl and )1.2 J/mol/K for Ni3Al. For Ni, the inclusion of thermal electronic

Chen, Long-Qing

406

NASA Astrophysics Data System (ADS)

The intriguing difference between far-infrared photoconductivity spectroscopy and absorption spectroscopy in the measurement of the magnetoplasmon frequency in GaAs quantum wells reported by Holland et al. [Phys. Rev. Lett. 93, 186804 (2004)] remains unexplained to date. This Letter provides a consistent mechanism to solve this puzzle. The mechanism is based on the electron reservoir model for the integer quantum Hall effect in graphene [Phys. Lett. A 376, 616 (2012)]. We predict sharp kinks to appear in the magnetic induction dependence of the magnetoplasmon frequency at very low temperatures such as 14 mK in the same GaAs quantum well sample used by Holland et al..

Toyoda, Tadashi; Fujita, Maho; Uchida, Tomohisa; Hiraiwa, Nobuyoshi; Fukuda, Taturo; Koizumi, Hideki; Zhang, Chao

2013-08-01

407

A first principles study of the oxidation energetics and kinetics of realgar

NASA Astrophysics Data System (ADS)

Quantum-mechanical calculations allow resolving and quantifying in detail important aspects of reaction mechanisms such as spin transitions and oxygen dissociation that can be the major rate-limiting steps in redox processes on sulfide and oxide surfaces. In addition, this knowledge can help experimentalists in setting up the framework of rate equations that can be used to describe the kinetics of, e.g., oxidation processes. The unique molecular crystal structure of realgar, As 4S 4 clusters held together by van der Waals bonds, allows for a convenient quantum-mechanical (q.m.) cluster approach to investigate the thermodynamics and kinetic pathways of oxidation. The interaction of As 4S 4 clusters with oxygen and co-adsorbed ions provides a model system for understanding the molecular-scale processes that underpin empirically-derived rate expressions, and provides clues to the oxidation mechanisms of other sulfides and oxides. Two activated processes are shown to dominate the kinetics of oxidation by molecular oxygen: (i) a paramagnetic 3O to diamagnetic 1O spin transition and (ii) oxygen dissociation on the surface, in that order. The activation energies for the spin transition and O 2 dissociation step were determined to be 1.1 eV (106 kJ/mol) and 0.9 eV (87 kJ/mol), respectively, if molecular oxygen is the only reactant on the surface. In the case of As 4S 4, q.m. calculations reveal that 3O transfers its spin to the cluster and forms a low-spin, peroxo intermediate on the surface before dissociating. The adsorption of a hydroxide ion on the surface proximate to the 3O adsorption site changes the adsorption mechanism by lowering the activation energy barriers for both the spin transition (0.30 eV/29 kJ/mol) and the O 2 dissociation step (0.72 eV/69 kJ/mol). Thus, while spin transition is rate limiting for oxidation with O 2 alone, dissociation becomes the rate-limiting step for oxidation with co-adsorption of OH -. First-principles, periodic calculations of the realgar (1¯20) surface show that the energetics and structural changes that accompany oxidation of As 4S 4 clusters on the surface are similar to those involving individual As 4S 4 clusters. Thus, assuming that an As 4S 4 cluster with an adsorbed hydroxyl group is a reasonable approximation of the surface of As 4S 4 at high pH, the theoretically calculated oxidation rate (˜1 × 10 -10 mol m -2 s -1) is of the same order as empirically-derived rates from experiments at T = 298 K, pH = 8, and similar dissolved oxygen concentrations. In addition, the co-adsorption of other anions found in alkaline waters (i.e. carbonate, bicarbonate, sulfate, and sulfite) were shown to energetically promote the oxidation of As 4S 4 (on the order of 5-40 kJ/mol depending on the co-adsorbed anion, OH -, CO32-, HCO3-, SO42-, or SO32-, and accounting for changes in the hydration of products and reactants). The effect of the co-adsorbate on the kinetics and thermodynamics of oxidation is due to each adsorbate modifying the electronic and structural environment of the other adsorption site. Activation-energy barriers due to spin transitions are rarely discussed in the literature as key factors for controlling oxidation rates of mineral surfaces, even though the magnitude of these barriers is enough to alter the kinetics significantly. The attenuation of the activation energy by co-adsorbed anions suggests the possibility of pH- or p(co-adsorbate)-dependent activation energies that can be used to refine oxidation rate laws for sulfide minerals and other, especially semiconducting minerals, such as oxides.

Renock, Devon; Becker, Udo

2010-08-01

408

NASA Astrophysics Data System (ADS)

Applying the first-principles with the generalized gradient approximation and the modified Becke and Johnson potential plus the generalized gradient approximation potential as exchange correlation potential, the electronic structures, half-metallicity and the cohesive energy for hypothetical zinc blende YC compound are calculated. Obtained results show that the zinc blende YC is typical half-metallic with a large half-metallic gap of 0.67(2) eV and magnetic moment of 1.00 ?B per molecule. Magnetic moments mainly come from the p orbital of C atom, where p-d hybridization mechanism plays a dominating role in the formation of half-metallicity. The relatively stable ferromagnetic ground state, large half-metallic gap, the robust half-metallicity with respect to the lattice constant compression, and negative cohesive energy indicate zinc blende YC would be a promising half metallic ferromagnet.

Wu, Qiao; Wang, Zhonglong; Fan, Shuaiwei; Yao, Kailun

2014-11-01

409

First-principles studies of the vibrational properties of amorphous carbon nitrides

NASA Astrophysics Data System (ADS)

Raman spectra of amorphous carbon nitride films (a-C:N) resemble those of typical amorphous carbon (a-C), and no specific features in the spectra are shown due to N doping. The present work provides a correlation between the microstructure and vibrational properties of a-C:N films from first principles. The six periodic model structures of 64 atoms with various mass densities and nitrogen contents are generated by the liquid-quench method using Car—Parinello molecular dynamics. By using Raman coupling tensors calculated with the finite electric field method, Raman spectra are obtained. The calculated results show that the vibrations of C=N could directly contribute to the Raman spectrum. The similarity of the Raman line shapes of N-doped and N-free amorphous carbons is due to the overlapping of C=N and C=C vibration bands. In addition, the origin of characteristic Raman peaks is also given.

Niu, Li; Wang, Xuan-Zhang; Zhu, Jia-Qi; Gao, Wei

2013-01-01

410

Doping dependence of thermoelectric performance in Mo3Sb7: first principles calculations

We study the effects of doping Mo3Sb7 with transition metals (Ni,Fe,Co,Ru) via first principles calculations, including electronic structure, lattice dynamics and Boltzmann transport. We find heavy-mass bands and large, rapidly varying density-of-states, generally favorable for high thermopower, near the band gap of this material. Transport calculations predict large Seebeck coefficients exceeding 300 $\\mu$V/K in a wide temperature range above 500 K (a range suitable for waste heat recovery), if the material can be doped into a semiconducting state. These thermopowers are much higher than those that have previously been experimentally observed; we find that performance exceeding current limits may be found at lower carrier concentration than achieved presently. We also discuss the selection of dopant and the potential thermoelectric performance of optimally doped Mo3Sb7.

Singh, David J [ORNL; Parker, David S [ORNL; Du, Mao-Hua [ORNL

2011-01-01

411

Prediction of a new layered phase of nitrogen from first-principles simulations

NASA Astrophysics Data System (ADS)

A new phase of nonmolecular nitrogen has been suggested by first-principles density functional theory simulations at high pressures. The phase has zigzag chains similar to the BP, A7 and LB phases, but the zigzag chains in the PP phase are connected by two atoms with double bonds with Pmna symmetry, which we designated as the PP phase. Properties of PP are presented in comparison with other polymeric phases of nitrogen discussed previously. The calculated enthalpy versus pressure reveals a similarity between the PP and CH phases. The equation of state of nitrogen in the PP, CG and ? phases shows that an ? transition to the PP phase happens at 90 GPa. The band structure and bulk modulus of PP phase are also calculated.

Wang, X. L.; He, Z.; Ma, Y. M.; Cui, T.; Liu, Z. M.; Liu, B. B.; Li, J. F.; Zou, G. T.

2007-10-01

412

First-principles calculation of the instability leading to giant inverse magnetocaloric effects

NASA Astrophysics Data System (ADS)

The structural and magnetic properties of functional Ni-Mn-Z (Z =Ga, In, Sn) Heusler alloys are studied by first-principles and Monte Carlo methods. The ab initio calculations give a basic understanding of the underlying physics which is associated with the strong competition of ferro- and antiferromagnetic interactions with increasing chemical disorder. The resulting d-electron orbital dependent magnetic ordering is the driving mechanism of magnetostructural instability which is accompanied by a drop of magnetization governing the size of the magnetocaloric effect. The thermodynamic properties are calculated by using the ab initio magnetic exchange coupling constants in finite-temperature Monte Carlo simulations, which are used to accurately reproduce the experimental entropy and adiabatic temperature changes across the magnetostructural transition.

Comtesse, D.; Gruner, M. E.; Ogura, M.; Sokolovskiy, V. V.; Buchelnikov, V. D.; Grünebohm, A.; Arróyave, R.; Singh, N.; Gottschall, T.; Gutfleisch, O.; Chernenko, V. A.; Albertini, F.; Fähler, S.; Entel, P.

2014-05-01

413

Protein-Protein Interactions from Linear-Scaling First Principles Quantum Mechanical Calculations

NASA Astrophysics Data System (ADS)

A modification of the MM-PBSA technique for calculating binding affinities of biomolecular complexes is presented. Classical molecular dynamics is used to explore the motion of the extended interface between two peptides derived from the BRC4 repeat of BRCA2 and the eukaryotic recombinase RAD51. The resulting trajectory is sampled using the linear-scaling density functional theory code, onetep, to determine from first principles, and with high computational efficiency, the relative free energies of binding of the ˜2800 atom receptor-ligand complexes. This new method provides the basis for computational interrogation of protein-protein and protein-ligand interactions, within fields ranging from chemical biological studies to small molecule binding behaviour, with both unprecedented chemical accuracy and affordable computational expense.

Cole, Daniel; Skylaris, Chris-Kriton; Rajendra, Eeson; Venkitaraman, Ashok; Payne, Mike

2010-03-01

414

Protein-protein interactions from linear-scaling first-principles quantum-mechanical calculations

NASA Astrophysics Data System (ADS)

A modification of the MM-PBSA technique for calculating binding affinities of biomolecular complexes is presented. Classical molecular dynamics is used to explore the motion of the extended interface between two peptides derived from the BRC4 repeat of BRCA2 and the eukaryotic recombinase RAD51. The resulting trajectory is sampled using the linear-scaling density functional theory code, onetep, to determine from first principles, and with high computational efficiency, the relative free energies of binding of the ~2800 atom receptor-ligand complexes. This new method provides the basis for computational interrogation of protein-protein and protein-ligand interactions within fields ranging from chemical biological studies to small-molecule binding behaviour, with both unprecedented chemical accuracy and affordable computational expense.

Cole, D. J.; Skylaris, C.-K.; Rajendra, E.; Venkitaraman, A. R.; Payne, M. C.

2010-08-01

415

The structural and electronic properties of amorphous HgCdTe from first-principles calculations

NASA Astrophysics Data System (ADS)

Amorphous mercury cadmium telluride (a-MCT) model structures, with x being 0.125 and 0.25, are obtained from first-principles calculations. We generate initial structures by computation alchemy method. It is found that most atoms in the network of amorphous structures tend to be fourfold and form tetrahedral structures, implying that the chemical ordered continuous random network with some coordination defects is the ideal structure for a-MCT. The electronic structure is also concerned. The gap is found to be 0.30 and 0.26 eV for a-Hg0.875Cd0.125Te and a-Hg0.75Cd0.25Te model structures, independent of the composition. By comparing with the properties of crystalline MCT with the same composition, we observe a blue-shift of energy band gap. The localization of tail states and its atomic origin are also discussed.

Zhao, Huxian; Chen, Xiaoshuang; Lu, Jianping; Shu, Haibo; Lu, Wei

2014-01-01

416

Properties of amorphous GaN from first-principles simulations

NASA Astrophysics Data System (ADS)

Amorphous GaN (a-GaN) models are obtained from first-principles simulations. We compare four a-GaN models generated by “melt-and-quench” and the computer alchemy method. We find that most atoms tend to be fourfold, and a chemically ordered continuous random network is the ideal structure for a-GaN albeit with some coordination defects. Where the electronic structure is concerned, the gap is predicted to be less than 1.0 eV, underestimated as usual by a density functional calculation. We observe a highly localized valence tail and a remarkably delocalized exponential conduction tail in all models generated. Based upon these results, we speculate on potential differences in n- and p-type doping. The structural origin of tail and defect states is discussed. The vibrational density of states and dielectric function are computed and seem consistent with experiment.

Cai, B.; Drabold, D. A.

2011-08-01

417

NASA Astrophysics Data System (ADS)

We studied magnetism in bilayer and multilayer zigzag graphene nanoribbons (ZGNRs) through first-principles density functional theory calculations. We found that the magnetic ground state of bilayer ZGNRs is the C-type antiferromagnetic (AFM) state, which is the AFM order between intraplane-edge carbon atoms and ferromagnetic (FM) order between interplane edge carbon atoms. In the cases of infinitely stacked multilayer ZGNRs, i.e., zigzag graphite nanoribbons, the C-type AFM state is also the most stable. By carrier doping, we found that the magnetic ground state changed from the C-AFM state to the FM state and, thus, realized two-dimensional FM surface (edge) states of graphite with a metallic conductivity.

Sawada, Keisuke; Ishii, Fumiyuki; Saito, Mineo

2014-04-01

418

The field of atmospheric science is very rich in problems ranging from the molecular to the regional and global scale. These problems are often extremely complex, and although the statement of a particular atmospheric science question may be clear, finding a single, concise computational approach to address this question can be daunting. As a result, the broad scope of scientific problems that lie within the umbrella of atmospheric science require a multi-discipline approach. Of particular interest to atmospheric chemists is the role that heterogeneous chemistry plays in the important processes that take place throughout the atmosphere. The definition of heterogeneous is: consisting of dissimilar elements or parts. The chemical environment induced by the presence of the interface can be dramatically different than the corresponding gas- or condensed phase homogeneous environment and can give rise to novel chemistry. Although the importance of heterogeneous chemistry in the atmosphere has been known for decades, a challenge to both experimentalists and theorists in provide simplified models and experiments that can yield insight into the field measurements of the atmospheric process. The use of molecular modeling has been widely used to provide a particle-based picture of atmospherically relevant interfaces to deduce the novel chemistry that is taking place. Unfortunately, even with the most computationally efficient particle-based approach, it is still impossible to model the full ice-crystal in the stratosphere or the sea-salt aerosol in the troposphere. Figure 1 depicts a caricature of the actual system of interest, and highlights the region where efficient molecular modeling can be employed. Although there is seemingly a large disconnect between reality and the model, we hope to convince the reader that there is still much insight to be gained from a particle-based picture. There is a myriad of different approaches to molecular modeling that have been successfully applied to studying the complex problems put forth by atmospheric chemists. To date, the majority of the molecular models of atmospherically relevant interfaces have been comprised of two genres of molecular models. The first is based on empirical interaction potentials. The use of an empirical interaction potential suffers from at least two shortcomings. First, empirical potentials are usually fit to reproduce bulk thermodynamic states, or gas phase spectroscopic data. Thus, without the explicit inclusion of charge transfer, it is not at all obvious that empirical potentials can faithfully reproduce the structure at a solid-vapor, or liquid-vapor interface where charge rearrangement is known to occur (see section 5). One solution is the empirical inclusion of polarization effects. These models are certainly an improvement, but still cannot offer insight into charge transfer processes and are usually difficult to parameterize. The other shortcoming of empirical models is that, in general, they cannot describe bond-making/breaking events, i.e. chemistry. In order to address chemistry one has to consider an ab initio (to be referred to as first-principles throughout the remaining text) approach to molecular modeling that explicitly treats the electronic degrees of freedom. First-principles modeling also give a direct link to spectroscopic data and chemistry, but at a large computational cost. The bottle-neck associated with first-principles modeling is usually determined by the level of electronic structure theory that one chooses to study a particular problem. High-level first-principles approaches, such as MP2, provide accurate representation of the electronic degrees of freedom but are only computationally tractable when applied to small system sizes (i.e. 10s of atoms). Nevertheless, this type of modeling has been extremely useful in deducing reaction mechanisms of atmospherically relevant chemistry that will be discussed in this review (see section 4). However, to solve problems relating to heterogeneous chemistry at interfaces where the interfacial syste

Mundy, C; Kuo, I W

2005-06-08

419

First-principles study of the critical thickness in asymmetric ferroelectric tunnel junctions

The absent critical thickness of fully relaxed asymmetric ferroelectric tunnel junctions is investigated by first-principles calculations. The results show that PbTiO{sub 3} thin film between Pt and SrRuO{sub 3} electrodes can still retain a significant and stable polarization down to thicknesses as small as 0.8 nm, quite unlike the case of symmetric ferroelectric tunnel junctions. We trace this surprising result to the generation of a large electric field by the charge transfer between the electrodes caused by their different electronic environments, which acts against the depolarization field and enhances the ferroelectricity, leading to the reduction, or even complete elimination, for the critical thickness.

Cai Mengqiu [State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083 (China); School of Physics and Microelectronics Science, Hunan University, Changsha 410082, Hunan (China); State Key Laboratory of Optoelectronic Materials and Technologies, Zhongshan University, Guangzhou 510275, Guangdong (China); Du Yong; Huang Boyun [State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083 (China)

2011-03-07

420

The Interface between Gd and Monolayer MoS2: A First-Principles Study.

We analyze the electronic structure of interfaces between two-, four- and six-layer Gd(0001) and monolayer MoS2 by first-principles calculations. Strong chemical bonds shift the Fermi energy of MoS2 upwards into the conduction band. At the surface and interface the Gd f states shift to lower energy and new surface/interface Gd d states appear at the Fermi energy, which are strongly hybridized with the Mo 4d states and thus lead to a high spin-polarization (ferromagnetically ordered Mo magnetic moments of 0.15??B). Gd therefore is an interesting candidate for spin injection into monolayer MoS2. PMID:25482498

Zhang, Xuejing; Mi, Wenbo; Wang, Xiaocha; Cheng, Yingchun; Schwingenschlögl, Udo

2014-01-01

421

Understanding Surface Photochemistry from First Principles: The Case of CO-NiO(100)

NASA Astrophysics Data System (ADS)

The excitation mechanism in the CO-NiO(100) system induced by a uv-laser pulse has been investigated from first principles. For the laser-driven process, the relevant electronically excited states are identified, and it is shown that a transition within the CO molecule is the crucial excitation step rather than substrate mediated processes. A new mechanism is proposed, in which the formation of a genuine C-Ni bond in the excited state is the driving force for photodesorption rather than electrostatic interactions, as has been found in similar systems. This results in very high velocities of CO molecules desorbing from the NiO(100) surface after electronic relaxation.

Mehdaoui, Imed; Klüner, Thorsten

2007-01-01

422

First-Principles Calculation of the Bulk Photovoltaic Effect in Bismuth Ferrite

NASA Astrophysics Data System (ADS)

We compute the bulk photovoltaic effect (BPVE) in BiFeO3 using first-principles shift current theory, finding good agreement with experimental results. Furthermore, we reconcile apparently contradictory observations: by examining the contributions of all photovoltaic response tensor components and accounting for the geometry and ferroelectric domain structure of the experimental system, we explain the apparent lack of BPVE response in striped polydomain samples that is at odds with the significant response observed in monodomain samples. We reveal that the domain-wall-driven response in striped polydomain samples is partially mitigated by the BPVE, suggesting that enhanced efficiency could be obtained in materials with cooperative rather than antagonistic interaction between the two mechanisms.

Young, Steve M.; Zheng, Fan; Rappe, Andrew M.

2012-12-01

423

First-Principles Calculation of the Bulk Photovoltaic Effect in Bismuth Ferrite

NASA Astrophysics Data System (ADS)

Bismuth ferrite is a multiferroic material with a large bulk polarization and a band gap in the visible spectrum. Significant anomalous photovoltaic effects have been observed in the material; however, the origins of this effect are unclear. While some investigations indicate that observed photovoltages and photocurrents are due to the bulk photovoltaic effect, in striped polydomain samples there is no evidence of this, and the observed response is attributed to a domain-wall-driven mechanism. We have computed the bulk photovoltaic response from first principles using shift current theory and compared it to the available experimental data, finding good agreement. By accounting for the geometry of the polydomain samples, we are able to explain the lack of observed bulk photovoltaic response. Furthermore, we show that these two mechanisms act antagonistically, suggesting that enhanced efficiency may be found in materials where these two effects interact cooperatively.

Young, Steve; Zheng, Fan; Rappe, Andrew

2013-03-01

424

First-principles study of orbital ordering in cubic fluoride KCrF3

NASA Astrophysics Data System (ADS)

Comprehensive first-principles calculations are performed to provide insight into the intriguing physical properties of the ternary cubic fluoride KCrF3. The electronic structures exhibit a prominent dependence on the effective local Coulomb interaction parameter Ueff. The ground state of the cubic phase is a ferromagnetic (FM) half-metal with Ueff equal to 0, 2, and 4 eV, whereas the insulating A-type antiferromagnetic (A-AFM) state with concomitant homogeneous orbital ordering is more robust than the FM state for Ueff exceeding 4 eV. We propose that the origin of the orbital ordering is purely electronic when the cooperative Jahn—Teller distortions are absent in cubic KCrF3.

Ming, Xing; Xiong, Liang-Bin; Xu, Huo-Xi; Du, Fei; Wang, Chun-Zhong; Chen, Gang

2014-03-01

425

First-principles calculations of enhanced ferromagnetism in ZnO codoped with cobalt and nitrogen

NASA Astrophysics Data System (ADS)

Using first-principles calculations based on density functional theory, N-codoped ZnO:Co has been demonstrated to be potentially a p-type diluted magnetic semiconductor. By investigating 13 geometrically distinct configurations, Co and N dopants are found to have a tendency toward staying close to each other with most stable -O-Co-N-Co-O- complexes. The dominant ferromagnetic interaction is due to the hybridization between N 2p and Co 3d states, which is strong enough to lead to hole-mediated ferromagnetism at room temperature. The ferromagnetic coupling strongly relies on the distance of N from Co, while it weakly depends on the direction of aligned Co ions.

Assadi, M. H. N.; Zhang, Y. B.; Li, S.

2009-02-01

426

NASA Astrophysics Data System (ADS)

Functional materials, such as piezoelectrics, ferroelectrics, and antiferroelectrics, exhibit large changes with applied fields and stresses. This behavior enables their incorporation into a wide variety of devices in technological fields such as energy conversion/storage and information processing/storage. Discovery of functional materials with improved performance or even new types of responses is thus not only a scientific challenge, but can have major impacts on society. In this talk I will review our efforts to uncover new families of functional materials using a combined crystallographic database/high-throughput first-principles approach. I will describe our work on the design and discovery of thousands of new functional materials, specifically the LiAlSi family as piezoelectrics, the LiGaGe family as ferroelectrics, and the MgSrSi family as antiferroelectrics.

Bennett, Joseph

2013-03-01

427

First-principles study of properties of Mn 2ZnMg alloy

NASA Astrophysics Data System (ADS)

We investigate the electronic structures and magnetic properties of Mn 2ZnMg compound with Hg 2CuTi-type structure using first-principles full-potential local orbital minimum basis calculations. Based on the analysis on the electronic structures, it is demonstrated that the compound is half-metallic antiferromagnet and the compound is favorable to form Hg 2CuTi-type structure instead of the conventional L2 1 one. The complicated hybridization among the p and d states dominates mainly the origin of the gap. The Fermi level ( EF) shifts slightly with the lattice parameter changed. Spin-orbit coupling hardly reduces the degree of spin polarization of the density of states at the Fermi level.

Wei, Xiao-Ping; Chu, Shi-Bing; Mao, Ge-Yong; Deng, Hong; Lei, Tao; Hu, Xian-Ru

2011-09-01

428

First-Principles Approach to Calculating Energy Level Alignment at Aqueous Semiconductor Interfaces

NASA Astrophysics Data System (ADS)

A first-principles approach is demonstrated for calculating the relationship between an aqueous semiconductor interface structure and energy level alignment. The physical interface structure is sampled using density functional theory based molecular dynamics, yielding the interface electrostatic dipole. The GW approach from many-body perturbation theory is used to place the electronic band edge energies of the semiconductor relative to the occupied 1b1 energy level in water. The application to the specific cases of nonpolar (101 ¯0) facets of GaN and ZnO reveals a significant role for the structural motifs at the interface, including the degree of interface water dissociation and the dynamical fluctuations in the interface Zn-O and O-H bond orientations. These effects contribute up to 0.5 eV.

Kharche, Neerav; Muckerman, James T.; Hybertsen, Mark S.

2014-10-01

429

First-principles study of structural and vibrational properties of SrZrO3

NASA Astrophysics Data System (ADS)

Using first-principles calculations, we investigate the electronic, structural, and vibrational properties of SrZrO3. We start from the high-symmetry cubic perovskite phase, for which the phonon dispersion curves are reported. We point out the coexistence of structural antiferrodistortive instabilities at the R and M zone-boundary points and a ferroelectric instability at the zone center. We show that the strong antiferrodistortive motions suppress ferroelectricity and are responsible for the orthorhombic ground state as in CaTiO3. The structural properties of possible intermediate phases and of the orthorhombic Pnma ground state are reported. For the latter, an assignment of IR and Raman zone-center phonon modes is proposed. The main features of the ferroelectric instability are also discussed, and we show that a ferroelectric ground state can even be induced in SrZrO3 by strain engineering.

Amisi, Safari; Bousquet, Eric; Katcho, Karume; Ghosez, Philippe

2012-02-01

430

Vacancy Ordering In Co3AlCx Alloys: A First Principles Study

Ordering of structural vacancies in non-stoichiometric Co{sub 3}AlC{sub x} alloys has been studied using a combination of first-principles total energy calculations, a cluster expansion technique, and Monte-Carlo simulations. In the proximity of the experimental1y observed composition of x {approx} 0.59, our exhaustive ground state search yields two stable vacancy-ordered structures: a cubic Co{sub 3}AlC{sub 0.5} phase and a trigonal Co{sub 3}AlC{sub 0.667} phase. By performing finite-temperature Monte-Carlo simulations, the order-disorder transition temperatures of Co{sub 3}AlC{sub 0.5} and CO{sub 3}AlC{sub 0.667} are predicted to be {approx}1925K and {approx}1630K, respectively.

Jiang, Chao [Los Alamos National Laboratory

2008-01-01

431

First Principles Calculation of Elastic Properties of Early-Late Transition Metal Alloys

NASA Astrophysics Data System (ADS)

Amorphous metals are of practical interest in applications requiring high strength materials. We choose to examine the elastic properties of crystalline phases to understand the elastic properties of amorphous solids. In this talk, we discuss our work using first principles methods to calculate elastic properties for crystalline alloys in various chemical families containing transition metals, specifically early (Ta,W) and late (Fe,Co,Rh,Ni,Cu,Zn) due to their good glass forming ability, as well as select borides. Certain Laves phases, which are known to have local chemical ordering similar to amorphous solids, are focused on. We analyze trends in the elastic properties of chemical families based on computed enthalpies of formation, elastic properties of pure elemental phases, and electronic and structural information. In particular, we use effective medium theories and enthalpies of formation to predict trends in bulk moduli. This information can be used to predict future candidate systems for high-strength amorphous metals.

Huhn, William; Widom, Michael

2013-03-01

432

Magnetoelectric coupling at the epitaxial Ni/PbTiO3 heterointerface from first principles

NASA Astrophysics Data System (ADS)

The magnetoelectric coupling at the epitaxial ferromagnetic/ferroelectric heterointerface depends strongly on the detailed bonding mechanism and the type of magnetic interaction among the interfacial atoms. First-principles density-functional calculations of the electronic structure and magnetoelectric coupling at the epitaxial Ni/PbTiO3 heterointerface are performed. Our results demonstrate that the interfacial magnetoelectric coupling in this system originates from the interface bonding and the peculiar pd?-type magnetic interaction between interfacial Ni and O atoms, which results in a large change of magnetic moments on Ni atoms near the interface. Furthermore, it is this peculiar interfacial magnetic interaction that leads to an extraordinary oscillating behavior of magnetic moments across the entire Ni slab, which may have important practical implication in multiferroic devices.

Dai, Jian-Qing; Song, Yu-Min; Zhang, Hu

2015-01-01

433

Liquid iron-sulfur alloys at outer core conditions by first-principles calculations

NASA Astrophysics Data System (ADS)

perform first-principles calculations to investigate liquid iron-sulfur alloys (Fe, Fe56S8, Fe52S12, and Fe48S16) under high-pressure and high-temperature (150-300 GPa and 4000-6000 K) conditions corresponding to the Earth's outer core. Considering only the density profile, the best match with the preliminary reference Earth model is by liquid Fe-14 wt % S (Fe50S14), assuming sulfur is the only light element. However, its bulk sound velocity is too high, in particular in the deep outer core, suggesting that another light component such as oxygen is required. An experimental check using inelastic X-ray scattering shows good agreement with the calculations. In addition, a present study demonstrates that the Birch's law does not hold for liquid iron-sulfur alloy, consistent with a previous report on pure liquid iron.

Umemoto, Koichiro; Hirose, Kei; Imada, Saori; Nakajima, Yoichi; Komabayashi, Tetsuya; Tsutsui, Satoshi; Baron, Alfred Q. R.

2014-10-01

434

First-principles search for high-pressure phases of GaAsO4

NASA Astrophysics Data System (ADS)

First-principles calculations are performed to examine the high-pressure behavior of the berlinite system GaAsO4. Structural properties of six possible GaAsO4 phases, berlinite, VCrO4, rutile, AlNbO4, BiSbO4 and a monoclinic structure with space group P21/m11, are investigated up to 25 GPa. It is shown that between 1.4 and 2.3 GPa, a monoclinic AlNbO4-type structure is energetically favored, and beyond this range the favored structure is a rutile form. We also identify an unexpected metastable form of rutile structure (rutile-II) produced as a high-pressure modification of the VCrO4-type structure.

Duan, Wenhui; Wentzcovitch, Renata M.; Chelikowsky, James R.

1999-08-01

435

Since electrons injected to a homogenous wire always tend to concentrate on its surface, heterogeneous coaxial structures are generally necessary to make nanocables with an insulating sheath. Here we reveal from first-principles calculations that double-walled boron-nitride nanotubes could be natural homogeneous nanocables as injected electrons prefer abnormally to concentrate on the inner semiconducting tube while the outer tube remains insulating. The ratio of extra electrons on the inner tube to total carriers in the double-walled nanotubes can be tuned widely by changing either the tube diameter or the local tube curvature through radial deformation, both attributed to the predominant band filling and weak enhancement in Coulomb interaction within the inner wall where the sublattice asymmetry is strongly attenuated by curvature effect. This exotic charge screening is universal for any form of electron-doping sources.

Zhang Zhuhua [Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China); Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588 (United States); Zeng Xiao Cheng [Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588 (United States); Guo Wanlin [Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China)

2010-07-15

436

First-principles study of the effect of phosphorus on nickel grain boundary

NASA Astrophysics Data System (ADS)

Based on first-principles quantum-mechanical calculations, the impurity-dopant effects of phosphorus on ?5(012) symmetrical tilt grain boundary in nickel have been studied. The calculated binding energy suggests that phosphorus has a strong tendency to segregate to the grain boundary. Phosphorus forms strong and covalent-like bonding with nickel, which is beneficial to the grain boundary cohesion. However, a too high phosphorus content can result in a thin and fragile zone in the grain boundary, due to the repulsion between phosphorus atoms. As the concentration of phosphorus increases, the strength of the grain boundary increases first and then decreases. Obviously, there exists an optimum concentration for phosphorus segregation, which is consistent with observed segregation behaviors of phosphorus in the grain boundary of nickel. This work is very helpful to understand the comprehensive effects of phosphorus.

Liu, Wenguan; Ren, Cuilan; Han, Han; Tan, Jie; Zou, Yang; Zhou, Xingtai; Huai, Ping; Xu, Hongjie

2014-01-01

437

First-principles study of properties of semi-Heusler (Cu,Ni)MnSb alloys

NASA Astrophysics Data System (ADS)

Magnetic properties, Curie temperatures, and transport properties of semi-Heusler alloys CuxNi1-xMnSb are calculated as a function of the alloy composition from first-principles. The transition from the ferromagnetic state (NiMnSb) to the antiferromagnetic state (CuMnSb) gives rise to an abrupt change in the concentration dependence of magnetic moments and resistivity at about x = 0.7 while the Curie temperatures decrease monotonically with the Cu-content. Such behaviour can be understood as due to the onset of disorder in orientations of Mn-spins at x = 0.7. A simple account of magnetic disorder as based on the uncompensated disordered local moment picture provides also a good quantitative explanation of available experimental data.

Kudrnovský, J.; Drchal, V.; Turek, I.

2010-01-01

438

NASA Astrophysics Data System (ADS)

The structural, electronic, thermoelectric and thermodynamic properties of ternary half-Heusler compound YPdSb are investigated using the first principle calculations. It is found that YPdSb is an indirect semiconductor. The calculated band gap is 0.161 eV with spin-orbital coupling including and 0.235 eV without spin-orbital coupling including, respectively. The electronic transport properties are obtained via Boltzman transport theory. The predicted Seebeck coefficient is 240 ?V/K and the thermoelectric performance can be optimized by n-type doping at room temperature. Moreover, the lattice dynamical results regarding the phonon dispersion curves, phonon density of states and thermodynamic properties are reported. Thermodynamics (heat capacity and Debye temperature) as well as mean phonon free path and the thermal conductivity in a temperature range of 0-300 K are determined.

Kong, Fanjie; Hu, Yanfei; Hou, Haijun; Liu, Yanhua; Wang, Baolin; Wang, Lili

2012-12-01

439

First-principles spectroscopic characterization of PbSe nanoparticles passivated with Fe complexes

NASA Astrophysics Data System (ADS)

Given that defining characteristics of nanoparticles -- morphology, catalytic reactivity, optical and electronic properties -- are often dictated by their surfaces, it is informative to investigate how surface chemistry and structure change as different ligands are introduced to the surface. Starting with oleate-passivated PbSe nanoparticles, we remove the oleate ligands and replace them with an organometallic complex: cyclopentadienyl iron dicarbonyl. Measured and calculated x-ray photoemission core-level shifts indicate a charge transfer between surface Pb atoms and Fe atoms. We investigate the nature of this charge transfer in more detail through analysis of x-ray absorption spectra (XAS) at the Fe L-edge. Fe XAS are calculated from first-principles using a GW-based Bethe-Salpeter approach. The spectra reveal that the extent to which pi-backbonding is possible between the Fe and associated carbonyls varies with the charge density on the Fe atom.

Gilmore, Keith; Hammack, Aaron; Sawvel, April; Rosen, Evelyn; Ogletree, D. Frank; Urban, Jeffrey; Milliron, Delia; Helms, Brett; Cohen, Bruce; Prendergast, David

2012-02-01

440

First principles investigation of Ti adsorption and migration on Si(100) surfaces

The titanium adsorption on Si(100) is investigated using first principles computer modelling methods. Two new subsurface adsorption sites are described. They are located at the edge of the cavity topped by a surface silicon dimer. The migration of the titanium from the surface to the subsurface sites is facilitated when occurring via one of these sites. The ejection of one of the silicon atoms forming the surface dimer is also investigated. The actual step of the ejection requires more energy than previously thought although, when considering the global picture of a titanium atom on the surface leading to the ejection of a silicon atom, the overall rate is compensated by the facilitated migration of the titanium to the subsurface sites. The consecutive adsorption of a second and third titanium atom is also investigated. It is shown that titanium grows evenly on the surface in normal condition, showing no intermixing of the titanium and silicon beyond the silicon layer.

Briquet, Ludovic G. V.; Wirtz, Tom; Philipp, Patrick, E-mail: philipp@lippmann.lu [Department of Science and Analysis of Materials (SAM), Centre de Recherche Public - Gabriel Lippmann, 41 rue du Brill, L-4422 Belvaux (Luxembourg)

2013-12-28

441

NASA Astrophysics Data System (ADS)

Magnesium (Mg) crystal structures are extensively explored using an evolutionary algorithm implemented in the USPEX code. Two structures with simple trigonal and tetragonal symmetries are discovered to possibly exist under high pressure. The stability of these symmetries is determined by elastic constants and phonon spectrum calculations. First-principle calculations are performed to investigate the structural, mechanical and electronic properties of different Mg structures under high pressure (up to 300 GPa). Above 190 GPa, the trigonal structure is more stable than the hexagonal close-packed (HCP) structure. Particularly, the trigonal structure can be considered a compromise between face-centered cubic (FCC) and HCP blocks. Interestingly, the tetragonal structure density is only 95% HCP structure. In addition, the tetragonal structure has strong directional bonding but is less stable than the HCP structure (up to 600 GPa). Pressure significantly changes the electronic properties of both structures although they remain metallic up to 300 GPa.

Li, Jian; Dong, Xu; Jin, Ye; Fan, Changzeng

2014-07-01

442

FIRST PRINCIPLES MODELING OF YTTRIUM-DOPED BAZRO3 SOLID ELECTROLYTE

Ab initio Quantum mechanics calculations of the equation of states for BaZrO{sub 3} have been performed and the bulk modulus has been obtained. The value of the modulus is in good agreement with reported experimental values. Equilibrium proton positions in Y-doped BaZrO{sub 3} with dopant concentrations from 12.5 to 50% were investigated. Initial rough estimates of the transition barriers have been made. Our results suggest that the proton migration pathway may involve secondary minima with two maxima (symmetric with respect to the center of the path). In the next phase of this project the results of our quantum mechanical calculations will be used to develop a new Reactive Force Field (ReaxFF) based on first principles. This Reactive Force Field will be used for much molecular dynamics simulations or much larger systems to investigate proton migration in bulk and surface regions of fuel cells.

Claudio O. Dorso; Boris V. Merinov; William A. Goddard III

2003-04-30

443

First principle study of a bimolecular thin film on Ag(1 1 1) surface

NASA Astrophysics Data System (ADS)

The formation of melamine-PTCDI bimolecular networks deposited on Ag(1 1 1) is studied by means of first principle calculations. Emphasis is placed on the interplay of the inter-molecular hydrogen bonds and the molecule-substrate contacts. Our simulations show rather strong distortions of the adsorbed molecules near the contact points due to the influence the hydrogen bonds. Despite this, the charge transfer from the substrate to a PTCDI molecule remains almost the same (0.9 e -) as obtained for an isolated PTCDI molecule. A detailed analysis of the topological features of the electronic density reveals that the charge transfer modifies the two types of hydrogen bonds in opposite ways, weakening the central bond and strengthening the two lateral ones, while roughly keeping a constant binding energy. Altogether, the influence of the substrate on the molecular network is proved to be weak.

Sassi, Michel; Oison, Vincent; Debierre, Jean-Marc

2008-09-01

444

Range parameters of heavy ions in carbon calculated with first-principles potentials

NASA Astrophysics Data System (ADS)

Interatomic potentials for Kr-C, Au-C and Pb-C diatomics have been calculated with Hartree-Fock (HF) and density-functional theory (DFT) methods in order to clarify the origins of discrepancies between available experimental data and the values predicted by Ziegler, Biersack and Littmark (ZBL) theory for range parameters of heavy ions in light targets at energies of about 1-1000 keV. Relativistic effects were taken into account in the evaluations of the potentials. Range parameters have been obtained within the framework of the standard transport theory. Good agreement between the calculated projected ranges and the experimental data in the energy range of 10-1000 keV gives use in the range-projection equations the nuclear energy losses, determined with the first-principles potentials, and the velocity proportional electronic stopping powers by Land and Brennan (LB). Considerable improvement has also been achieved in the description of the projected range stragglings.

Kuzmin, V.

2006-08-01

445

A first principles model for calculating hydrogen bonding interactions, previously applied to water, is here applied to the more difficult problem of interactions between DNA bases. We first consider the energetics and geometry for the A-T and the G-C basepairs, comparing our results to other calculated results as well as to experiment. Next, we study the interactions of isomorphic DNA base triplet structures, which are important because of their suggested role in the recombination process. We find that energetically the third base in the triplet tends to favor a position along the dyadic axis, where it is hydrogen bonded to both bases in the duplex. Images FIGURE 1 FIGURE 2 FIGURE 3 FIGURE 4 PMID:8519960

Lewis, J P; Sankey, O F

1995-01-01

446

Phonon thermal transport in Bi2Te3 from first principles

NASA Astrophysics Data System (ADS)

We present first-principles calculations of the thermal and thermal transport properties of Bi2Te3 that combine an ab initio molecular dynamics (AIMD) approach to calculate interatomic force constants (IFCs) along with a full iterative solution of the Peierls-Boltzmann transport equation for phonons. The newly developed AIMD approach allows determination of harmonic and anharmonic interatomic forces at each temperature, which is particularly appropriate for highly anharmonic materials such as Bi2Te3. The calculated phonon dispersions, heat capacity, and thermal expansion coefficient are found to be in good agreement with measured data. The lattice thermal conductivity, ?l, calculated using the AIMD approach nicely matches measured values, showing better agreement than the ?l obtained using temperature-independent IFCs. A significant contribution to ?l from optic phonon modes is found. Already at room temperature, the phonon line shapes show a notable broadening and onset of satellite peaks reflecting the underlying strong anharmonicity.

Hellman, Olle; Broido, David A.

2014-10-01

447

NASA Astrophysics Data System (ADS)

The clustering process (sulfuric acid) + (base)?(sulfuric acid)1(base)1 is of fundamental importance in the atmospheric new-particle formation. Especially interesting are the collisions where a proton transfer reaction can happen, as the reaction often leads to relatively strongly bound clusters. Here, we studied the clustering process of (sulfuric acid) + (dimethylamine) ? (sulfuric acid)1(dimethylamine)1 using first-principles molecular dynamics simulations. The collision of the two molecules was simulated starting with various spatial orientations and the evolution of the cluster was followed in the NVE ensemble. The simulations suggest that the proton transfer reaction takes place regardless of the intial collision orientation. However, due to the energy released in the process, the newly-formed cluster is not able to reach the minimun energy configuration, which might affect the following growth processes.

Loukonen, Ville; Bork, Nicolai; Vehkamäki, Hanna

2013-05-01

448

First-principles study of ternary fcc solution phases from special quasirandom structures

In the present work, ternary special quasirandom structures (SQSs) for a fcc solid solution phase are generated at different compositions, x{sub A}=x{sub B}=x{sub C}=(1/3) and x{sub A}=(1/2), x{sub B}=x{sub C}=(1/4), whose correlation functions are satisfactorily close to those of a random fcc solution. The generated SQSs are used to calculate the mixing enthalpy of the fcc phase in the Ca-Sr-Yb system. It is observed that first-principles calculations of all the binary and ternary SQSs in the Ca-Sr-Yb system exhibit very small local relaxation. It is concluded that the fcc ternary SQSs can provide valuable information about the mixing behavior of the fcc ternary solid solution phase. The SQSs presented in this work can be widely used to study the behavior of ternary fcc solid solutions.

Shin Dongwon; Wang Yi; Liu Zikui [Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 (United States); Walle, Axel van de [Engineering and Applied Science Division, California Institute of Technology, Pasadena, California 91125 (United States)

2007-10-01

449

First principles study on the ferroelectricity of the perovskite ABO3 ferroelectrics

NASA Astrophysics Data System (ADS)

In order to understand well the different ferroelectric behaviour of quantum paraelectrics and ferroelectrics and the origin of the ferroelectricity of the solid solution KTa0.5Nb0.5O3(KTN), we calculated the electronic structure of CaTiO3, BaTiO3 and KTN by first principles calculation. From total energy analysis, it is shown that, with increasing cell volume, the crystals (CaTiO3, SrTiO3) will have a ferroelectric instability. For BaTiO3, the ferroelectricity will disappear as the cell volume is decreased. From the density of states analysis, it is shown that the hybridization between B d and O p is very important for the ferroelectric stability of ABO3 perovskite ferroelectrics. This is consistent with the analysis of band structure.

Wang, Yuan-Xu; Zhong, Wei-Lie; Wang, Chun-Lei; Zhang, Pei-Lin; Su, Xuan-Tao

2002-07-01

450

First-principles study of the interaction of hydrogen molecular on Na-adsorbed graphene

NASA Astrophysics Data System (ADS)

We have performed density functional theory-based first-principles calculations to study the stability, geometrical structures, and electronic/magnetic properties of pure graphene, sodium (Na)-adsorbed graphene and also the adsorption properties of H_2 -molecular ranging from one to five molecules on their preferred structures. Using the information of binding energy of Na at different adsorption sites of varying sized graphene supercell, it has been observed that hollow position is the most preferred site for Na adsorption, and the same in 3 × 3 supercell has been used for further calculations. The band structure and density of states calculations have been performed to study the electronic/magnetic properties of Na-atom graphene. On comparing adsorption energy per H_2 -molecular in pure and Na-adsorbed graphene, we find that presence of Na atom, in general, enhances binding strength to H_2 -moleculars.

Pantha, Nurapati; Belbase, Kamal; Adhikari, Narayan Prasad

2014-07-01

451