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1

A first-principles theoretical approach to heterogeneous nanocatalysis.

A theoretical approach to heterogeneous catalysis by sub-nanometre supported metal clusters and alloys is presented and discussed. Its goal is to perform a computational sampling of the reaction paths in nanocatalysis via a global search in the phase space of structures and stoichiometry combined with filtering which takes into account the given experimental conditions (catalytically relevant temperature and reactant pressure), and corresponds to an incremental exploration of the disconnectivity diagram of the system. The approach is implemented and applied to the study of propylene partial oxidation by Ag(3) supported on MgO(100). First-principles density-functional theory calculations coupled with a Reactive Global Optimization algorithm are performed, finding that: (1) the presence of an oxide support drastically changes the potential energy landscape of the system with respect to the gas phase, favoring configurations which interact positively with the electrostatic field generated by the surface; (2) the reaction energy barriers for the various mechanisms are crucial in the competition between thermodynamically and kinetically favored reaction products; (3) a topological database of structures and saddle points is produced which has general validity and can serve for future studies or for deriving general trends; (4) the MgO(100) surface captures some major features of the effect of an oxide support and appears to be a good model of a simple oxide substrate; (5) strong cooperative effects are found in the co-adsorption of O(2) and other ligands on small metal clusters. The proposed approach appears as a viable route to advance the role of predictive computational science in the field of heterogeneous nanocatalysis. PMID:22057595

Negreiros, Fabio R; Aprà, Edoardo; Barcaro, Giovanni; Sementa, Luca; Vajda, Stefan; Fortunelli, Alessandro

2012-02-21

2

A first-principles theoretical approach to heterogeneous nanocatalysis

NASA Astrophysics Data System (ADS)

A theoretical approach to heterogeneous catalysis by sub-nanometre supported metal clusters and alloys is presented and discussed. Its goal is to perform a computational sampling of the reaction paths in nanocatalysis via a global search in the phase space of structures and stoichiometry combined with filtering which takes into account the given experimental conditions (catalytically relevant temperature and reactant pressure), and corresponds to an incremental exploration of the disconnectivity diagram of the system. The approach is implemented and applied to the study of propylene partial oxidation by Ag3 supported on MgO(100). First-principles density-functional theory calculations coupled with a Reactive Global Optimization algorithm are performed, finding that: (1) the presence of an oxide support drastically changes the potential energy landscape of the system with respect to the gas phase, favoring configurations which interact positively with the electrostatic field generated by the surface; (2) the reaction energy barriers for the various mechanisms are crucial in the competition between thermodynamically and kinetically favored reaction products; (3) a topological database of structures and saddle points is produced which has general validity and can serve for future studies or for deriving general trends; (4) the MgO(100) surface captures some major features of the effect of an oxide support and appears to be a good model of a simple oxide substrate; (5) strong cooperative effects are found in the co-adsorption of O2 and other ligands on small metal clusters. The proposed approach appears as a viable route to advance the role of predictive computational science in the field of heterogeneous nanocatalysis.

Negreiros, Fabio R.; Aprà, Edoardo; Barcaro, Giovanni; Sementa, Luca; Vajda, Stefan; Fortunelli, Alessandro

2012-02-01

3

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

4

Exploiting periodic first-principles calculations in NMR spectroscopy of disordered solids.

Much of the information contained within solid-state nuclear magnetic resonance (NMR) spectra remains unexploited because of the challenges in obtaining high-resolution spectra and the difficulty in assigning those spectra. Recent advances that enable researchers to accurately and efficiently determine NMR parameters in periodic systems have revolutionized the application of density functional theory (DFT) calculations in solid-state NMR spectroscopy. These advances are particularly useful for experimentalists. The use of first-principles calculations aids in both the interpretation and assignment of the complex spectral line shapes observed for solids. Furthermore, calculations provide a method for evaluating potential structural models against experimental data for materials with poorly characterized structures. Determining the structure of well-ordered, periodic crystalline solids can be straightforward using methods that exploit Bragg diffraction. However, the deviations from periodicity, such as compositional, positional, or temporal disorder, often produce the physical properties (such as ferroelectricity or ionic conductivity) that may be of commercial interest. With its sensitivity to the atomic-scale environment, NMR provides a potentially useful tool for studying disordered materials, and the combination of experiment with first-principles calculations offers a particularly attractive approach. In this Account, we discuss some of the issues associated with the practical implementation of first-principles calculations of NMR parameters in solids. We then use two key examples to illustrate the structural insights that researchers can obtain when applying such calculations to disordered inorganic materials. First, we describe an investigation of cation disorder in Y2Ti(2-x)Sn(x)O7 pyrochlore ceramics using (89)Y and (119)Sn NMR. Researchers have proposed that these materials could serve as host phases for the encapsulation of lanthanide- and actinide-bearing radioactive waste. In a second example, we discuss how (17)O NMR can be used to probe the dynamic disorder of H in hydroxyl-humite minerals (nMg2SiO4·Mg(OH)2), and how (19)F NMR can be used to understand F substitution in these systems. The combination of first-principles calculations and multinuclear NMR spectroscopy facilitates the investigation of local structure, disorder, and dynamics in solids. We expect that applications will undoubtedly become more widespread with further advances in computational and experimental methods. Insight into the atomic-scale environment is a crucial first step in understanding the structure-property relationships in solids, and it enables the efficient design of future materials for a range of end uses. PMID:23402741

Ashbrook, Sharon E; Dawson, Daniel M

2013-09-17

5

The surface characteristics of hydroxyapatite (HA) are probed using a combination of NMR spectroscopy and first principles calculations. The NMR spectrum is taken from a bone sample and the first principles calculations are performed using a plane-wave density functional approach within the pseudopotential approximation. The computational work focuses on the (100) and (200) surfaces, which exhibit a representative range of phosphate, hydroxyl and cation bonding geometries. The shielding tensors for the 31P, 1H and 17O nuclei are calculated from the relaxed surface structures using an extension of the projector augmented-wave method. The calculated 31P chemical shifts for the surface slab are found to be significantly different from the bulk crystal and are consistent with the NMR data from bone and also synthetically prepared nanocrystalline samples of HA. Rotational relaxations of the surface phosphate ions and the sub-surface displacement of other nearby ions are identified as causing the main differences. The investigation points to further calculations of other crystallographic surfaces and highlights the potential of using NMR with ab initio modelling to fully describe the surface structure and chemistry of HA, which is essential for understanding its reactivity with the surrounding organic matrix. PMID:18183321

Chappell, Helen; Duer, Melinda; Groom, Nicholas; Pickard, Chris; Bristowe, Paul

2008-01-28

6

We investigate the X-ray photoelectron spectroscopy (XPS) binding energies of As 3d in Si for various defects in neutral and charged states by first-principles calculation. It is found that the complexes of a substitutional As and a vacancy in charged and neutral states explain the experimentally observed unknown peak very well.

Kishi, Hiroki; Miyazawa, Miki; Matsushima, Naoki; Yamauchi, Jun [Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Yokohama-shi, Kanagawa-ken 223-8522 (Japan)

2014-02-21

7

-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.

8

Theoretical studies of aluminum and aluminide alloys using CALPHAD and first-principles approach

NASA Astrophysics Data System (ADS)

Heat-treatable aluminum alloys have been widely used in the automobile and aerospace industries as structural materials due to their light weight and high strength. To study the age-hardening process in heat-treatable aluminum alloys, the Gibbs energies of the strengthening metastable phases, e.g. theta ' and theta?, are critical. However, those data are not included in the existing thermodynamic databases for aluminum alloys due to the semi-empirical nature of the CALPHAD approach. In the present study, the thermodynamics of the Al-Cu system, the pivotal age-hardening system, is remodeled using a combined CALPHAD and first-principles approach. The formation enthalpies and vibrational formation entropies of the stable and metastable phases in the Al-Cu system are provided by first-principles calculations. Special Quasirandom Structures (SQS's) are applied to model the substitutionally random fee and bee alloys. SQS's for binary bee alloys are developed and tested in the present study. Finally, a self-consistent thermodynamic description of the Al-Cu system including the two metastable theta? and theta' phases is obtained. During welding of heat-treatable aluminum alloys, a detrimental phenomenon called constitutional liquation, i.e. the local eutectic melting of second-phase particles in a matrix at temperatures above the eutectic temperature but below the solidus of the alloy, may occur in the heat-affected zone (HAZ). In the present study, diffusion code DICTRA coupled with realistic thermodynamic and kinetic databases is used to simulate the constitutional liquation in the model Al-Cu system. The simulated results are in quantitative agreement with experiments. The critical heating rate to avoid constitutional liquation is also determined through computer simulations. Besides the heat-treatable aluminum alloys, intermetallic compounds based on transition metal aluminides, e.g. NiAl and FeAl, are also promising candidates for the next-generation of high-temperature structural materials for aerospace applications due to their high melting temperature and good oxidation resistance. Many important properties of B2 aluminides are governed by the existences of point defects. In the present study, Special Quasirandom Structures (SQS's) are developed to model non-stoichiometric B2 compounds containing large concentrations of constitutional point defects. The SQS's are then applied to study B2 NiAl. The first-principles SQS results provide formation enthalpies, equilibrium lattice parameters and elastic constants of B2 NiAl which agree satisfactorily with the existing experimental data in the literature. It is unambiguously shown that, at T = 0K and zero pressure, Ni vacancies and antisite Ni atoms are the energetically favorable point defects in Al-rich and Ni-rich B2 NiAl, respectively. Remarkably, it is predicted that high defect concentrations can lead to structural instability of B2 NiAl, which explains well the martensitic transformation observed in this compound at high Ni concentrations.

Jiang, Chao

9

The ability of nonlinear electronic spectroscopy to track folding/unfolding processes of proteins in solution by monitoring aromatic interactions is investigated by first-principles simulations of two-dimensional (2D) electronic spectra of a model peptide. A dominant reaction pathway approach is employed to determine the unfolding pathway of a tetrapeptide, which connects the initial folded configuration with stacked aromatic side chains and the final unfolded state with distant noninteracting aromatic residues. The ?-stacking and excitonic coupling effects are included through ab initio simulations based on multiconfigurational methods within a hybrid quantum mechanics/molecular mechanics scheme. It is shown that linear absorption spectroscopy in the ultraviolet (UV) region is unable to resolve the unstacking dynamics characterized by the three-step process: T-shaped?twisted offset stacking?unstacking. Conversely, pump-probe spectroscopy can be used to resolve aromatic interactions by probing in the visible region, the excited-state absorptions (ESAs) that involve charge-transfer states. 2D?UV spectroscopy offers the highest sensitivity to the unfolding process, by providing the disentanglement of ESA signals belonging to different aromatic chromophores and high correlation between the conformational dynamics and the quartic splitting. PMID:25145908

Nenov, Artur; A Beccara, Silvio; Rivalta, Ivan; Cerullo, Giulio; Mukamel, Shaul; Garavelli, Marco

2014-10-20

10

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

11

NASA Astrophysics Data System (ADS)

The molecular structure and stability of species formed by silver in aqueous saline solutions typical of hydrothermal settings were quantified using in situ X-ray absorption spectroscopy (XAS) measurements, quantum-chemical modeling of near-edge absorption spectra (XANES) and extended fine structure spectra (EXAFS), and first-principles molecular dynamics (FPMD). Results show that in nitrate-bearing acidic solutions to at least 200 °C, silver speciation is dominated by the hydrated Ag+ cation surrounded by 4-6 water molecules in its nearest coordination shell with mean Ag-O distances of 2.32 ± 0.02 Å. In NaCl-bearing acidic aqueous solutions of total Cl concentration from 0.7 to 5.9 mol/kg H2O (m) at temperatures from 200 to 450 °C and pressures to 750 bar, the dominant species are the di-chloride complex AgCl2- with Ag-Cl distances of 2.40 ± 0.02 Å and Cl-Ag-Cl angle of 160 ± 10°, and the tri-chloride complex AgCl32- of a triangular structure and mean Ag-Cl distances of 2.60 ± 0.05 Å. With increasing temperature, the contribution of the tri-chloride species decreases from ˜50% of total dissolved Ag in the most concentrated solution (5.9m Cl) at 200 °C to less than 10-20% at supercritical temperatures for all investigated solutions, so that AgCl2- becomes by far the dominant Ag-bearing species at conditions typical of hydrothermal-magmatic fluids. Both di- and tri-chloride species exhibit outer-sphere interactions with the solvent as shown by the detection, using FPMD modeling, of H2O, Cl-, and Na+ at distances of 3-4 Å from the silver atom. The species fractions derived from XAS and FPMD analyses, and total AgCl(s) solubilities, measured in situ in this work from the absorption edge height of XAS spectra, are in accord with thermodynamic predictions using the stability constants of AgCl2- and AgCl32- from Akinfiev and Zotov (2001) and Zotov et al. (1995), respectively, which are based on extensive previous AgCl(s) solubility measurements. These data are thus recommended for chemical equilibrium calculations in mineral-fluid systems above 200 °C. In contrast, our data disagree with SUPCRT-based datasets for Ag-Cl species, which predict large fractions of high-order chloride species, AgCl32- and AgCl43- in high-temperature saline fluids. Comparisons of the structural and stability data of Ag-Cl species derived in this study with those of their Au and Cu analogs suggest that molecular-level differences amongst the chloride complexes such as geometry, dipole moment, distances, and resulting outer-sphere interactions with the solvent may account, at least partly, for the observed partitioning of Au, Ag and Cu in vapor-brine and fluid-melt systems. In hydrothermal environments dominated by fluid-rock interactions, the contrasting affinity of these metals for sulfur ligands and the differences both in chemistry and stability of their main solid phases (Ag sulfides, Cu-Fe sulfides, and native Au) largely control the concentration and distribution of these metals in their economic deposits.

Pokrovski, Gleb S.; Roux, Jacques; Ferlat, Guillaume; Jonchiere, Romain; Seitsonen, Ari P.; Vuilleumier, Rodolphe; Hazemann, Jean-Louis

2013-04-01

12

NASA Astrophysics Data System (ADS)

The vibrational dynamics of germanium dioxide in the rutile structure has been investigated by using polarized micro-Raman scattering spectroscopy coupled with first-principles calculations. Raman spectra were carried out in backscattering geometry at room temperature from micro-crystalline samples either unoriented or oriented by means of a micromanipulator, which enabled successful detection and identification of all the Raman active modes expected on the basis of the group theory. In particular, the Eg mode, incorrectly assigned or not detected in the literature, has been definitively observed by us and unambiguously identified at 525 \\text{cm}-1 under excitation by certain laser lines, thus revealing an unusual resonance phenomenon. First-principles calculations within the framework of the density functional theory allow quantifying both wave number and intensity of the Raman vibrational spectra. The excellent agreement between calculated and experimental data corroborates the reliability of our findings.

Sanson, A.; Pokrovski, G. S.; Giarola, M.; Mariotto, G.

2015-01-01

13

Theoretical modeling of the OH stretch infrared spectrum of carboxylic acid dimers based on first, Wisconsin 53706-1396 Received 20 September 2002; accepted 28 October 2002 Carboxylic acid dimers serve, and OH bend internal coordinates for the formic acid and benzoic acid dimers. These are then projected

Zwier, Timothy S.

14

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

15

Oxidation state is a powerful concept that is widely used in chemistry and materials physics, although the concept itself is arguably ill-defined quantum mechanically. In this work, we present impartial comparison of four, well-recognized theoretical approaches based on Lowdin atomic orbital projection, Bader decomposition, maximally localized Wannier function, and occupation matrix diagonalization, for assessing how well transition metal oxidation states can be characterized. Here, we study a representative molecular complex, tris(bipyridine)ruthenium. We also consider the influence of water solvation through first-principles molecular dynamics as well as the improved electronic structure description for strongly correlated d-electrons by including Hubbard correction in density functional theory calculations.

Reeves, Kyle G. [Department of Chemistry, University of North Carolina at Chapel Hill, North Carolina 27599-3290 (United States); Kanai, Yosuke, E-mail: ykanai@unc.edu [Department of Chemistry, University of North Carolina at Chapel Hill, North Carolina 27599-3290 (United States); Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, California, 94550 (United States)

2014-07-14

16

High alloyed ?-phase stabilized titanium alloys are known to provide comparable Young's modulus as that to the human bones (~30GPa) but is marred by its high density. In the present study the low titanium alloyed compositions of binary Ti-Nb and ternary Ti-Nb-Zr alloy systems, having stable ?-phase with low Young's modulus are identified using first principles density functional framework. The theoretical results suggest that the addition of Nb in Ti and Zr in Ti-Nb increases the stability of the ?-phase. The ?-phase in binary Ti-Nb alloys is found to be fully stabilized from 22at.% of Nb onwards. The calculated Young's moduli of binary ?-Ti-Nb alloy system are found to be lower than that of pure titanium (116GPa). For Ti-25(at.%)Nb composition the calculated Young's modulus comes out to be ~80GPa. In ternary Ti-Nb-Zr alloy system, the Young's modulus of Ti-25(at.%)Nb-6.25(at.%)Zr composition is calculated to be ~50GPa. Furthermore, the directional Young's moduli of these two selected binary (Ti-25(at.%)Nb) and ternary alloy (Ti-25(at.%)Nb-6.25(at.%)Zr) compositions are found to be nearly isotropic in all crystallographic directions. PMID:25746245

Karre, Rajamallu; Niranjan, Manish K; Dey, Suhash R

2015-05-01

17

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

18

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

19

First-Principles Molecular Dynamics

Ab initio or first-principles methods have emerged in the last two decades as a powerful tool to probe the properties of matter at the microscopic scale. These approaches are used to derive macroscopic observables under the controlled condition of a \\

Roberto Car; Filippo de Angelis; Paolo Giannozzi; Nicola Marzari

2005-01-01

20

Theoretical Spectroscopy Lectures Theory and Codes

Theoretical Spectroscopy Lectures Theory and Codes 411, 6-9 February 2011 Electronic excitations and frameworks, which are able to describe electronic excitations and spectroscopy, have become more and more Â· Introduction to Spectroscopy (Theory - 1/2 h) Â· Microscopic-Macroscopic connection (Theory - 1 h) Â· DFT

Botti, Silvana

21

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

22

The correlation between the local structure and the electric-field gradient at the Sc site in ScO6 polyhedra in Sc2O3, Ba3Sc4O9 and BaSc2O4 was investigated by means of (45)Sc NMR spectroscopy and DFT calculations. The electric-field gradient at the nucleus as determined experimentally and by calculations is compared using the quadrupolar coupling constant, CQ, around chemically and crystallographically distinct Sc sites in Sc-containing compounds. With CQ as the NMR parameter, the results obtained from the DFT calculation were in good agreement with the NMR measurements. The increase in the CQ values with the standard deviation of the O-O bond length surrounding Sc indicates that CQ is affected by the distribution of the six nearest neighboring O atoms around Sc. This study suggests that CQ plays an important role as an indicator of the local structure around ions, and that a combined complementary approach using both NMR spectroscopy and DFT calculation can be used along with diffraction techniques to provide a detailed understanding of crystal structures. PMID:24834902

Oikawa, Itaru; Takamura, Hitoshi

2014-07-01

23

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

24

NASA Astrophysics Data System (ADS)

BiFeO3 (BFO) is a multi-functional material with high ferroelectric and magnetic ordering temperature. Here we have investigated the electronic structure of (001) oriented 100nm rhombohedral BFO thin films using high energy X-ray photoelectron spectroscopy (XPS). By making use of the energy dependence of the relative cross sections for different states, we were able to selectively probe the elemental contributions to the valence band . At high energies, states with high main quantum number will have a higher relative probability for photo-ionization, i.e., the Bi 6s and 6p contributions in the valence region are enhanced relative to the Fe 3d and O 2p. We find that the Bi 6p states hybridize strongly with the valence band dominated by the Fe 3d and O 2p states, resulting in a splitting of the 3d states due to bonding and anti-bonding combinations with the Bi 6p. Our results thus suggest that a previously relatively ignored electronic interaction needs to be considered for BFO and related Bi-TMOs. Ab initio calculations indicate the importance of screened Coulomb correlations to describe Bi and Fe electronic states.

Knut, R.; Faleev, S.; Mazumdar, Dipanjan; Mryasov, O.; Gupta, Arunava; Karis, O.

2012-02-01

25

Chamber Clearing First Principles Modeling

LIFE fusion is designed to generate 37.5 MJ of energy per shot, at 13.3 Hz, for a total average fusion power of 500 MW. The energy from each shot is partitioned among neutrons ({approx}78%), x-rays ({approx}12%), and ions ({approx}10%). First wall heating is dominated by x-rays and debris because the neutron mean free path is much longer than the wall thickness. Ion implantation in the first wall also causes damage such as blistering if not prevented. To moderate the peak-pulse heating, the LIFE fusion chamber is filled with a gas (such as xenon) to reduce the peak-pulse heat load. The debris ions and majority of the x-rays stop in the gas, which re-radiates this energy over a longer timescale (allowing time for heat conduction to cool the first wall sufficiently to avoid damage). After a shot, because of the x-ray and ion deposition, the chamber fill gas is hot and turbulent and contains debris ions. The debris needs to be removed. The ions increase the gas density, may cluster or form aerosols, and can interfere with the propagation of the laser beams to the target for the next shot. Moreover, the tritium and high-Z hohlraum debris needs to be recovered for reuse. Additionally, the cryogenic target needs to survive transport through the gas mixture to the chamber center. Hence, it will be necessary to clear the chamber of the hot contaminated gas mixture and refill it with a cool, clean gas between shots. The refilling process may create density gradients that could interfere with beam propagation, so the fluid dynamics must be studied carefully. This paper describes an analytic modeling effort to study the clearing and refilling process for the LIFE fusion chamber. The models used here are derived from first principles and balances of mass and energy, with the intent of providing a first estimate of clearing rates, clearing times, fractional removal of ions, equilibrated chamber temperatures, and equilibrated ion concentrations for the chamber. These can be used to scope the overall problem and provide input to further studies using fluid dynamics and other more sophisticated tools.

Loosmore, G

2009-06-09

26

First-principles studies of boron nanostructures

NASA Astrophysics Data System (ADS)

Boron is an 'electron deficient' element which has a rather fascinating chemical versatility. In the solid state, the elemental boron has neither a pure covalent nor a pure metallic character. As a result, its vast structural dimensionality and peculiar bonding features hold a unique place among other elements in the periodic table. In order to understand and properly describe these unusual bonding features, a detailed and systematic theoretical study is needed. In this work, I will show that some of the qualitative features of boron nanostructures, including clusters, sheets and nanotubes can easily be extracted from the results of first principles calculations based on density functional theory. Specifically, the size-dependent evolution of topological structures and bonding characteristics of boron clusters, Bn will be discussed. Based on the scenario observed in the boron clusters, the unique properties of boron sheets and boron nanotubes will be described. Moreover, the ballistic electron transport in single-walled boron nanotube relative to that of single-walled carbon nanotubes will be considered. It is expected that the theoretical results obtained in the present thesis will initiate further studies on boron nanostructures, which will be helpful in understanding, designing and realizing boron-based nanoscale devices.

Lau, Kah Chun

27

First-principles study of metal-ceramic interfaces

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

Donald Jason Siegel

2001-01-01

28

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

29

(17)O NMR spectroscopy combined with first-principles calculations was employed to understand the local structure and dynamics of the phosphate ions and protons in the paraelectric phase of the proton conductor CsH2PO4. For the room-temperature structure, the results confirm that one proton (H1) is localized in an asymmetric H-bond (between O1 donor and O2 acceptor oxygen atoms), whereas the H2 proton undergoes rapid exchange between two sites in a hydrogen bond with a symmetric double potential well at a rate ?10(7) Hz. Variable-temperature (17)O NMR spectra recorded from 22 to 214 °C were interpreted by considering different models for the rotation of the phosphate anions. At least two distinct rate constants for rotations about four pseudo C3 axes of the phosphate ion were required in order to achieve good agreement with the experimental data. An activation energy of 0.21 ± 0.06 eV was observed for rotation about the P-O1 axis, with a higher activation energy of 0.50 ± 0.07 eV being obtained for rotation about the P-O2, P-O3(d), and P-O3(a) axes, with the superscripts denoting, respectively, dynamic donor and acceptor oxygen atoms of the H-bond. The higher activation energy of the second process is most likely associated with the cost of breaking an O1-H1 bond. The activation energy of this process is slightly lower than that obtained from the (1)H exchange process (0.70 ± 0.07 eV) ( Kim, G.; Blanc, F.; Hu, Y.-Y.; Grey, C. P. J. Phys. Chem. C 2013 , 117 , 6504 - 6515 ) associated with the translational motion of the protons. The relationship between proton jumps and phosphate rotation was analyzed in detail by considering uncorrelated motion, motion of individual PO4 ions and the four connected/H-bonded protons, and concerted motions of adjacent phosphate units, mediated by proton hops. We conclude that, while phosphate rotations aid proton motion, not all phosphate rotations result in proton jumps. PMID:25732257

Kim, Gunwoo; Griffin, John M; Blanc, Frédéric; Haile, Sossina M; Grey, Clare P

2015-03-25

30

A first principle study of terahertz (THz) spectra of acephate

Using first principles calculations based on the density functional method we have studied THz spectra of molecular and solid forms of acephate. The computations are preformed using different functionals for both isolated-molecule as well as solid-state periodic system. The computed infrared spectra are compared with experimental data using FTIR techniques and terahertz time-domain spectroscopy (THz-TDS) over a wide range of

Y. Zhang; X.-H. Peng; Y. Chen; J. Chen; A. Curioni; W. Andreoni; S. K. Nayak; X.-C. Zhang

2008-01-01

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

Constructing optimal local pseudopotentials from first principles

Local pseudopotential (LPP) is an important component of the orbital free density functional theory (OF-DFT), which is a promising large scale simulation method that can still maintain information of electron state in materials. Up to date, LPP is usually extracted from the solid state DFT calculations. It is unclear how to assess its transferability while applying to a much different chemical environment. Here we reveal a fundamental relation between the first principles norm-conserving PP (NCPP) and the LPP. Using the optimized effective potential method developed for exchange functional, we demonstrate that the LPP can be constructed optimally from the NCPP for a large number of elements. Our theory also reveals that the existence of an LPP is intrinsic to the elements, irrespective to the parameters used for the construction. Our method provides a unified method in constructing and assessing LPP in the framework of first principles pseudopotentials.

Mi, Wenhui; Ma, Yanming; Miao, Maosheng

2015-01-01

33

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

34

First-principles calculation of elastic moduli of early-late transition metal alloys

NASA Astrophysics Data System (ADS)

Motivated by interest in the elastic properties of high-strength amorphous metals, we examine the elastic properties of select crystalline phases. Using first-principles methods, we calculate elastic moduli in various chemical systems containing transition metals, specifically early (Ta,W) and late (Co,Ni). Theoretically predicted alloy elastic properties are verified for Ni-Ta by comparison with experimental measurements using resonant ultrasound spectroscopy. Comparison of our computed elastic moduli with effective medium theories shows that alloying leads to enhancement of bulk moduli relative to averages of the pure elements and considerable deviation of predicted and computed shear moduli. Specifically, we find an enhancement of bulk modulus relative to effective medium theory and propose a candidate system for high-strength, ductile amorphous alloys. Trends in the elastic properties of chemical systems are analyzed using force constants, electronic densities of state, and crystal overlap Hamilton populations. We interpret our findings in terms of the electronic structure of the alloys.

Huhn, William Paul; Widom, Michael; Cheung, Andrew M.; Shiflet, Gary J.; Poon, S. Joseph; Lewandowski, John

2014-03-01

35

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

36

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

37

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

38

GPU Based Acceleration of First Principles Calculations

NASA Astrophysics Data System (ADS)

The saturation of the acceleration using the silicon devices has required the parallel computing using multiple CPU's (central processing units). The parallel computing has been widely used in the field of the high-performance computing. On the other hand, graphics processing units (GPU's) were designed to accelerate graphic applications in 1978. NVIDIA Co. began to provide CUDA for C-language users to manipulate the GPU's in 2007. They applied it to computational fluid dynamics, medical real time simulation and astronomical N-body problem among others. This is the GPGPU (general-purpose computation on GPU's), which is faster in operation than CPU in the fields of linear algebras, FFT, and others. We have experienced that one- dimensional CUFFT ver1.1 (GPU-FFT) is eight times faster than FFTW for single-precision case. We implement the GPU-FFT into our in-house first principles planewave code, in which the hot spot is the FFT routine. We will present the performance of the implementation.

Tomono, Hidekazu; Iitaka, Toshiaki; Tsumuraya, Kazuo

2009-03-01

39

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

40

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

Wu, Zhigang

41

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

42

Theoretical standards in x-ray spectroscopies

We propose to extend our state-of-the-art, ab initio XAFS (X-ray absorption fine structure) codes, FEFF. Our current work has been highly successful in achieving accurate, user-friendly XAFS standards, exceeding the performance of both tabulated standards and other codes by a considerable margin. We now propose to add the capability to treat more complex materials. This includes multiple-scattering, polarization dependence, an approximate treatment of XANES (x-ray absorption near edge structure), and other improvements. We also plan to adapt FEFF to other spectroscopies, e.g. photoelectron diffraction (PD) and diffraction anomalous fine structure (DAFS).

Not Available

1992-01-01

43

First Principles Study on Mechanical Properties of Superhard ?-Ga Boron

NASA Astrophysics Data System (ADS)

The mechanical properties and intrinsic hardness of the ?-Ga boron phase (?-Ga-B) are studied by using the combination of first-principles calculations and a semiempirical macroscopic hardness model. It is found that ?-Ga-B is mechanically stable and possesses higher bulk/shear modulus as compared with ?-B28, a newly discovered high-pressure boron phase. The theoretical hardness of ?-Ga-B is estimated to be 45 GPa, which is much higher than 38 GPa for ?-B28. The results strongly indicate that ?-Ga-B is a potential superhard boron phase. To further obtain insight into the superhard nature of ?-Ga-B, we simulate stress—strain curves under tensile and shear deformation. Meanwhile, the microscopic mechanism driving the tensile and shear deformation modes in ?-Ga-B is discussed in detail.

Xu, Yuan-Hui; Liu, Hui-Yun; Hao, Xian-Feng; Chen, Rong-Na; Gao, Fa-Ming

2015-02-01

44

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

45

Engineering graphene by oxidation: a first-principles study

NASA Astrophysics Data System (ADS)

Graphene epoxide, with oxygen atoms lining up on pristine graphene sheets, is investigated theoretically here using first-principles calculations. Two distinct phases—metastable clamped and stable unzipped structures—are observed consistent with experimental observations. In the clamped structure, oxygen atoms form a regular lattice on the graphene sheet. In the unzipped phase, an epoxy group breaks the lower sp2 bond and modifies the mechanical and electronic properties of graphene remarkably. The foldable epoxy ring structure reduces its Young's modulus by 42.4%, while leaving the tensile strength almost unchanged. The perturbation of epoxidation on the band structures depends on the density and symmetry of oxidation. These results pave the way for oxidation-based engineering of graphene-related materials.

Xu, Zhiping; Xue, Kun

2010-01-01

46

First-Principles Investigation on Boron Nanostructures

NASA Astrophysics Data System (ADS)

First-principles calculations based on density functional theory are employed to study and predict the properties of boron and Mg boride nanostructures. For boron nanostructures, two-dimensional boron sheets are found to be metallic and made of mixtures of triangles and hexagons which benefit from the balance of two-center bonding and three-center bonding. This unusual bonding in boron sheets results in a self-doping picture where adding atoms to the hexagon centers does not change the number of bonding states but merely increases the electron count. Boron sheets can be either flat or buckled depending on the ratio between hexagons and triangles. Formed by stacking two identical boron sheets, double-layered boron sheets can form interlayer bonds, and the most stable one is semiconducting. Built from single-layered boron sheets, single-walled boron nanotubes have smaller curvature energies than carbon nanotubes and undergo a metal-to-semiconductor transition once the diameter is smaller than ˜20 A. Optimal double-walled boron nanotubes with inter-walled bonds formed are metallic and always more stable than single-walled ones. For Mg boride nanostructures, certain Mg boride sheets prefer to curve themselves into nanotubes, which is explained via Mg-Mg interactions governed by the charge state of Mg. In addition, optimal Mg boride sheet structures are explored with a genetic algorithm. Phase diagrams for Mg boride sheet structures are constructed and stable phases under boron-rich environments are identified. Curvature effects on the phase diagram of Mg boride nanotubes are also discussed. As a natural extension to boron sheets, layered boron crystals based on boron sheets are then presented and are shown to be stable under high pressure. Finally, this thesis ends with an investigation of hydrogen-storage properties of pristine and metal doped boron nanostructures.

Tang, Hui

2011-12-01

47

First-principles, predictive engineering models provide a sound theoretical basis for quantifying the inherent light energy utilization capabilities and performance limitations of candidate commercial photocatalytic oxidation reactor configurations. In particular, these models provide insight into the similarities and differences between photoreactors based on structured honeycombed monoliths, and those based on reticulated foams or other random catalyst supports.For honeycombed monoliths, a deterministic

Gregory B Raupp; Alessio Alexiadis; Rajnish Changrani

2001-01-01

48

First-principles Theory of Surface Thermodynamics

NASA Astrophysics Data System (ADS)

Describing the interaction of gas-phase species with surfaces, their dissociation, adsorption and diffusion, possible reaction and desorption is a formidable task, especially when we consider the important influence of temperature and pressure.( C. Stampfl, M.V. Ganduglia-Pirovano, K. Reuter, and M. Scheffler, to appear in Surf. Sci. 500 (2001).) Recent advances in theoretical methodologies are approaching such problems through the combination of density functional theory (DFT) calculations with statistial or phenomenological macroscopic theories whereby insight into the physical mechanisms is obtained and also the bridging of length and time scales. In this talk I will present a DFT based description for the surface thermodynamics and kinetics of adparticles at surfaces.(C. Stampfl, H.J. Kreuzer, S.H. Payne, H. Pfnür, and M. Scheffler, Phys. Rev. Lett.(1999); Appl. Phys. A (1999).) The theoretical approach is, in principle, applicable to a variety of problems, such as adsorbate phase transitions, partition functions of bulk and surface alloys, segregation, as well as adsorption/desorption. Total energy calculations are used to determine the geometries, energetics, and vibrational properties of adsorbate covered surfaces, from which many-body interaction parameters are extracted and a lattice-gas Hamiltonian constructed. The accuracy of the Hamiltonian can be improved and controlled by the number and order of interactions included. It can then be used in various statistial or phenomenological approaches; traditionally, parameters entering such models are just adjusted to fit experimental data. Applications of the approach to calculate, e.g., temperature-programmed desorption, heat of adsorption, for the O/Ru(0001) (using transfer matrix and rate equations) and Xe/Pt(111) systems (using Kinetic Monte Carlo) will be presented. note

Stampfl, Catherine

2001-03-01

49

First Principles Calculations for X-ray Resonant Spectra and Elastic Properties

In this thesis, we discuss applications of first principles methods to x-ray resonant spectra and elastic properties calculation. We start with brief reviews about theoretical background of first principles methods, such as density functional theory, local density approximation (LDA), LDA+U, and the linear augmented plane wave (LAPW) method to solve Kohn-Sham equations. After that we discuss x-ray resonant scattering (XRMS), x-ray magnetic circular dichroism (XMCD) and the branching problem in the heavy rare earths Ledges. In the last chapter we discuss the elastic properties of the second hardest material AlMgB{sub 14}.

Yongbin Lee

2006-05-01

50

Electronic and vibrational properties of nickel sulfides from first principles

NASA Astrophysics Data System (ADS)

We report the results of first-principles calculations (generalized gradient approximation-Perdew Wang 1991) on the electronic and vibrational properties of several nickel sulfides that are observed on Ni-based anodes in solid oxide fuel cells (SOFCs) upon exposure to H2S contaminated fuels: heazlewoodite Ni3S2, millerite NiS, polydymite Ni3S4, and pyrite NiS2. The optimized lattice parameters of these sulfides are within 1% of the values determined from x-ray diffraction. The electronic structure analysis indicates that all Ni-S bonds are strongly covalent. Furthermore, it is found that the nickel d orbitals shift downward in energy, whereas the sulfur p orbitals shift upward with increasing sulfur content; this is consistent with the decrease in conductivity and catalytic activity of sulfur-contaminated Ni-based electrodes (or degradation in SOFC performance). In addition, we systematically analyze the classifications of the vibrational modes at the ? point from the crystal symmetry and calculate the corresponding vibrational frequencies from the optimized lattice constants. This information is vital to the identification with in situ vibrational spectroscopy of the nickel sulfides formed on Ni-based electrodes under the conditions for SOFC operation. Finally, the effect of thermal expansion on frequency calculations for the Ni3S2 system is also briefly examined.

Wang, Jeng-Han; Cheng, Zhe; Brédas, Jean-Luc; Liu, Meilin

2007-12-01

51

Spectroscopy and photophysics of 9-methylalloxazine. Experimental and theoretical study

We present a systematic study of spectroscopy and photophysics of 9-methylalloxazine using experimental and theoretical approaches. Fluorescence lifetimes and quantum yields were measured in a range of solvents. The singlet state decay is dominated by non-radiative processes. The non-radiative relaxation rate and the radiative rate undergo notable changes when going from non-polar solvents to polar and hydrogen-donor solvents, due to

Ewa Sikorska; Igor V Khmelinskii; Wies?aw Pruka?a; Siân L Williams; David R Worrall; Jose L Bourdelande; Aneta Bednarek; Jacek Koput; Marek Sikorski

2004-01-01

52

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

53

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

54

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

55

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

56

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

57

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

58

Phonon Thermal Transport in Thermoelectric Materials from First Principles

NASA Astrophysics Data System (ADS)

Breakthroughs in nanoscience and materials fabrication technology have led to the creation of materials with very low lattice thermal conductivity [1, 2], a requirement for high thermoelectric efficiency. There is now an unprecedented need for quantitative, predictive theoretical approaches to provide fundamental understanding of lattice thermal transport in thermoelectric materials and insight into the design and development of new materials for enhanced thermoelectric applications. In this talk, I will describe our atomistic first principles approach for calculating lattice thermal conductivity of materials [3-6], which combines a complete solution of the Boltzmann transport equation for phonons with harmonic and anharmonic interatomic forces determined from density functional theory. I will present an overview of this theoretical approach along with some of our recent calculated results for a range of test materials such as Si, Ge and III-V compounds. I will also discuss results for thermoelectric alloys such as SixGe1-x and Mg2SixSn1-x and nanoparticle embedded in alloy thermoelectric (NEAT) materials. Finally, I will discuss insights gained from this effort such as the importance of anharmonic coupling of acoustic and optic phonon modes.[4pt] [1] Bed Poudel et al., Science 320, 634 (2008);[0pt] [2] D. T. Morelli et al., Phys. Rev. Lett. 101, 035901 (2008);[0pt] [3] D. A. Broido et al, Appl. Phys. Lett., 91, 231922 (2007);[0pt] [4] A. Kundu et al, Phys. Rev. B, 84, 125426 (2011);[0pt] [5] L. Lindsay et al., Phys. Rev. Lett. 109, 095901 (2012).[0pt] [6] W. Li et al, submitted (2012).

Broido, David

2013-03-01

59

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

60

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

61

Ballistic Transport in Nanostructures from First-Principles Simulations

We developed and implemented a first-principles based theory of the Landauer ballistic conductance, to determine the transport properties of nanostructures and molecular-electronics devices. Our approach starts from a ...

Marzari, Nicola

62

Time-dependent first-principles approaches to PV materials

Computational scheme for designing photovoltaic (PV) materials is presented. First-principles electron dynamics of photo-excitation and subsequent electron-hole splitting is performed based on the time-dependent density functional theory. Photo-induced enhancement of dipole moment was observed in a polar crystal and a donor-acceptor molecular pair. These experiences will pave a way to design PV material from first-principles simulations.

Miyamoto, Yoshiyuki [Nanosystem Research Institute, AIST, Central 2, 1-1-1, Umezono, Tsukuba, 305-8568 (Japan)

2013-12-10

63

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

64

The authors have presented a detailed investigation on the phase stabilities and electronic properties of ideal stoichiometric platinum carbide (PtC) in the rock-salt (RS) and zinc-blende (ZB) structures under high pressure. Theoretical calculations are performed using the first-principles pseudopotential density functional method, in which we employ the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof form and local density approximation (LDA)

Xiao-Wei Sun; Qi-Feng Chen; Xiang-Rong Chen; Ling-Cang Cai; Fu-Qian Jing

2011-01-01

65

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

66

Simulating quantum effects in ferroelectrics from first-principles

NASA Astrophysics Data System (ADS)

We have developed/adopted first-principles-based effective Hamiltonian approaches, within classical Monte Carlo (CMC) and path integral quantum Monte Carlo (PI-QMC) methods, to simulate quantum effects in the incipient ferroelectric KTaO3, both in bulk and thin films. A combined experimental and theoretical (using the above methods) study was also carried out to reveal low temperature properties of BaZrO3. Our findings on KTaO3 bulk reveal that zero-point motion of ions suppresses the paraelectric-to-ferroelectric transition and results in the saturation of the dielectric response to a plateau with a height of 4000 below 10 K, in very good agreement with experiments. Interestingly, our PI-QMC simulations reveal the formation of needle-like correlated dipoles along the [100] directions with a correlation length of ?16 A at low-temperature, as consistent with previous Raman and neutron scattering. Furthermore, very little is known about ferroelectric thin films and specifically about thin films of incipient ferroelectrics. Recently. Haeni et, al. [Nature, 430:758, 2004] observed room temperature ferroelectricity in strained SrTiO3 film. Therefore, the Pertsev phase diagram was determined within CMC and PI-QMC for 28-A-thick KTaO 3 thin films. Films are epitaxially grown along [001] directions and are under different electrical boundary conditions. Our findings predict four different phases, i.e. paraelectric, c-phase, r-phase, and aa-phase. CMC results indicate a phase transition at 460 K in a stress-free film under perfect electrical short-circuit conditions. i.e., zero depolarizing field. While this transition is dramatically reduced to 60 K for the same film but experiencing an incomplete screening of 96%. Moreover, PI-QMC indicates that under this 96% screening, quantum effects completely wash out the r-phase in favor of c-phase in case of films that are under compressive strains, while it barely survives at very low temperature for tensile misfit strains ranging between zero and 0.2%. Also regardless of the applied strain, predictions of both CMC and PI-QMC overlap above 140 K. Finally, some possible mechanisms for the experimental anomalies observed in BaZrO3 bulk are discussed. Further theoretical works are needed in these regards, and in particular the development of numerical tools able to simulate dielectric loss and AC fields are desired.

Akbarzadeh, Alireza

67

Reliability evaluation of thermophysical properties from first-principles calculations.

Thermophysical properties, such as heat capacity, bulk modulus and thermal expansion, are of great importance for many technological applications and are traditionally determined experimentally. With the rapid development of computational methods, however, first-principles computed temperature-dependent data are nowadays accessible. We evaluate various computational realizations of such data in comparison to the experimental scatter. The work is focussed on the impact of different first-principles codes (QUANTUM ESPRESSO and VASP), pseudopotentials (ultrasoft and projector augmented wave) as well as phonon determination methods (linear response and direct force constant method) on these properties. Based on the analysis of data for two pure elements, Cr and Ni, consequences for the reliability of temperature-dependent first-principles data in computational thermodynamics are discussed. PMID:25071092

Palumbo, Mauro; Fries, Suzana G; Dal Corso, Andrea; Kürmann, Fritz; Hickel, Tilmann; Neugebauer, Jürg

2014-08-20

68

Advances in First-Principles Studies of Transducer Materials

We have used first-principles linear response calculations and molecular dynamics to study the relaxor ferroelectric Pb(Mg1\\/3Nb2\\/3)O3-xPbTiO3 (PMN-x PT). First-principles calculations for ordered PMN show a low-symmetry monoclinic ground state. A new set of phase transitions to lower symmetry rhombohedral and monoclinic structures is predicted for PT at high pressures, showing predicted piezoelectric response as high as is seen in the

A. Asthagiri; Z. Wu; N. Choudhury; R. E. Cohen

2006-01-01

69

Static Dielectric Permittivity of Ice from First Principles

NASA Astrophysics Data System (ADS)

The static permittivity of ice is computed from first principles as a function of the electric field, together with the generalized Kirkwood factor. The molecular dipole in ice is unambiguously obtained by an original method combining a slab approach and Berry phase calculations, and the fluctuations of the polarization are sampled by Monte Carlo runs using first-principles model Hamiltonians for different proton configurations. Common approximations in the exchange-correlation functionals overestimate the dielectric permittivity and enhance ferroelectric configurations and the Kirkwood factor, whereas dielectric saturation effects compare well with experiment.

Bonnet, Nicéphore; Marzari, Nicola

2014-12-01

70

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.

71

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

72

First-principles derived potentials to study glass transitions

NASA Astrophysics Data System (ADS)

Using first-principles results, we have developed potentials within the embedded atom method to carry out molecular dynamics dynamics simulations in bulk metallic glasses. The potentials are fit to ab-initio forces in a large number of liquid and glass configurations. These potentials are used to study trends in the glass transition temperature as a function of composition

Fuentes-Cabrera, Miguel; Nicholson, Don M.; Widom, Mike; Wang, Yang; Mihalkovic, Marek

2003-03-01

73

A First Principles Study of Substitutional Copper in Aluminium

This study aims to gain insight into the early stages of precipitation hardening by determining the energetics of atom replacement by first principles computations. Density functional theory calculations using the ABINIT code were performed to determine cohesive energies and the equilibrium lattice constants of aluminium and copper for the conventional face centred cubic structure of 4 atoms. These were extended

A. E. Smith; S. Homolya

2004-01-01

74

Recent progress in first-principles studies of magnetoelectric multiferroics

Materials that combine magnetic and ferroelectric properties have generated increasing interest over the last few years, due to both their diverse properties and their potential utility in new types of magnetoelectric device applications. In this review we discuss recent progress in the study of such magnetoelectric multiferroics which has been achieved using computational first-principles methods based on density functional theory.

Claude Ederer; Nicola A. Spaldin

2005-01-01

75

A new analytical method -- First-principle OPTical Intercept Calculation (FirstOPTIC) -- is presented here for optical evaluation of trough collectors. It employs first-principle optical treatment of collector optical error sources and derives analytical mathematical formulae to calculate the intercept factor of a trough collector. A suite of MATLAB code is developed for FirstOPTIC and validated against theoretical/numerical solutions and ray-tracing results. It is shown that FirstOPTIC can provide fast and accurate calculation of intercept factors of trough collectors. The method makes it possible to carry out fast evaluation of trough collectors for design purposes. The FirstOPTIC techniques and analysis may be naturally extended to other types of CSP technologies such as linear-Fresnel collectors and central-receiver towers.

Zhu, G.; Lewandowski, A.

2012-11-01

76

Graphene spintronics: Spin injection and proximity effects from first principles

NASA Astrophysics Data System (ADS)

Ferromagnet/graphene (F/Gr) junctions are important building blocks for graphene spintronics. While simple models of spin injection are very successful for macroscopic metallic junctions, they reveal many deficiencies in describing F/Gr junctions. First-principles methods are key to assess such Gr-based junctions, but the computational cost is often too high. We focus on Ni(111)/Gr junctions and include van der Waals interactions from first principles, crucial for their correct description. We formulate a computationally inexpensive model to examine the nonuniformity and bias dependence of spin injection and elucidate proximity effects using spin polarization maps. Our results could extend the applicability of simple spin injection models in F/Gr junctions.

Lazi?, P.; Sipahi, G. M.; Kawakami, R. K.; Žuti?, Igor

2014-08-01

77

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

78

First Principle Approach to Modeling of Small Scale Helicopter

The establishment of global helicopter linear model is very precious and useful for the design of the linear control laws, since it is never afforded in the published literatures. In the first principle approach, the mathematical model was developed using basic helicopter theory accounting for particular characteristic of the miniature helicopter. No formal system identification procedures are required for the proposed model structure. The relevant published literatures however did not present the linear models required for the design of linear control laws. The paper presents a step by step development of linear model for small scale helicopter based on first-principle approach. Beyond the previous work in literatures, the calculation of the stability derivatives is presented in detail. A computer program is used to solve the equilibrium conditions and then calculate the change in aerodynamics forces and moments due to the change in each degree of freedom and control input. The detail derivation allows the c...

Budiyono, A; Lesmana, H

2008-01-01

79

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

80

Growth Mechanisms of Metal Nanoparticles via First Principles

NASA Astrophysics Data System (ADS)

Despite the important applications of nanoparticles (NPs), their mechanisms of growth still remain elusive. Herein we elucidate the growth mechanisms of silver NPs via first-principle calculations to elucidate anisotropic growth and shape selectivity by a symmetry-break mechanism. The capping agent (citrate) can play a multifunctional action: under certain conditions, it blocks NP growth via an electrostatic or a van der Waals stabilization mechanism, and under others, it induces NP growth.

Mpourmpakis, Giannis; Vlachos, Dionisios G.

2009-04-01

81

Asset-Liability Management Under the Safety-First Principle

Under the safety-first principle (Roy in Econometrica 20:431–449, 1952), one investment goal in asset-liability (AL) management is to minimize an upper bound of the ruin probability which measures\\u000a the likelihood of the final surplus being less than a given target level. We derive solutions to the safety-first AL management\\u000a problem under both continuous-time and multiperiod-time settings via investigating the relationship

M. C. Chiu; D. Li

2009-01-01

82

Can silicon behave like graphene? A first-principles study

The electronic properties of two-dimensional hexagonal silicon (silicene) are investigated using first-principles simulations. Though silicene is predicted to be a gapless semiconductor, due to the sp2-hybridization of its atomic orbitals, the weak overlapping between 3pz orbitals of neighbor Si atoms leads to a very reactive surface, resulting in a more energetically stable semiconducting surface upon the adsorption of foreign chemical

M. Houssa; G. Pourtois; V. V. Afanas'ev; A. Stesmans

2010-01-01

83

First-principles studies of RDX crystals under compression

Using the plane-wave pseudopotential technique (CASTEP program) within the generalized gradient approximation (GGA), we have performed first principles calculations to examine RDX crystals under hydrostatic and uniaxial strain compression. Crystal structures were calculated at ambient conditions for the orthorhombic structure and a vibrational analysis was performed. Second order elastic constants such as C11, C22, and C33 were calculated by applying

Jijun Zhao; Warren Perger

2005-01-01

84

First-principles study of vacancy formation in hydroxyapatite

First-principles plane-wave calculations were performed for hydroxyapatite (HAp) in order to investigate the electronic structure and vacancy formation mechanisms. The HAp unit cell contains PO4 tetrahedra and OH groups formed by covalent P-O and H-O bonds. Ca ions play a role for connecting the PO4 tetrahedra and OH groups in an ionic character of bonding. It is found that OH-

Katsuyuki Matsunaga; Akihide Kuwabara

2007-01-01

85

Growth Mechanisms of Metal Nanoparticles via First Principles

Despite the important applications of nanoparticles (NPs), their mechanisms of growth still remain elusive. Herein we elucidate the growth mechanisms of silver NPs via first-principle calculations to elucidate anisotropic growth and shape selectivity by a symmetry-break mechanism. The capping agent (citrate) can play a multifunctional action: under certain conditions, it blocks NP growth via an electrostatic or a van der Waals stabilization mechanism, and under others, it induces NP growth.

Mpourmpakis, Giannis; Vlachos, Dionisios G. [Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 (United States)

2009-04-17

86

Evolutionary approach for determining first-principles hamiltonians

NASA Astrophysics Data System (ADS)

Modern condensed-matter theory from first principles is highly successful when applied to materials of given structure-type or restricted unit-cell size. But this approach is limited where large cells or searches over millions of structure types become necessary. To treat these with first-principles accuracy, one 'coarse-grains' the many-particle Schrödinger equation into 'model hamiltonians' whose variables are configurational order parameters (atomic positions, spin and so on), connected by a few 'interaction parameters' obtained from a microscopic theory. But to construct a truly quantitative model hamiltonian, one must know just which types of interaction parameters to use, from possibly 106-108 alternative selections. Here we show how genetic algorithms, mimicking biological evolution ('survival of the fittest'), can be used to distil reliable model hamiltonian parameters from a database of first-principles calculations. We demonstrate this for a classic dilemma in solid-state physics, structural inorganic chemistry and metallurgy: how to predict the stable crystal structure of a compound given only its composition. The selection of leading parameters based on a genetic algorithm is general and easily applied to construct any other type of complex model hamiltonian from direct quantum-mechanical results.

Hart, Gus L. W.; Blum, Volker; Walorski, Michael J.; Zunger, Alex

2005-05-01

87

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

88

On the magnetic structure of (Fe1-xMnx)Pt: A first-principles study

NASA Astrophysics Data System (ADS)

This study investigates the composition dependence of magnetic structures of (Fe1-xMnx)Pt using first-principles method. A series of possible magnetic configurations are considered in the calculation to compare the relative stabilities after structural optimization, based on which, together with the comparison between theoretical and experimental magnetic properties, the results largely support the experimental finding in powders. The experimentally found stability of ferrimagnetic state in films with a wide composition range of x ? 0.5 is not fully supported by this study. The composition-dependent magnetic structures are discussed according to the exchange interactions.

Luo, Hu-Bin; Xia, Wei-Xing; Du, Juan; Zhang, Jian; Liu, J. Ping; Yan, Aru

2015-03-01

89

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

90

Structural and thermodynamic properties of MgB 2 from first-principles calculations

NASA Astrophysics Data System (ADS)

A first-principles plane wave method with the relativistic analytic pseudopotential of Hartwigsen, Goedecker and Hutter (HGH) scheme in the frame of local density approximation is performed to calculate the lattice parameters and the equation of states (EOS) of superconducting MgB 2. Our calculations show that the ratio c/ a of about 1.134 is the most stable structure for MgB 2, as is consistent with experiment and other theoretical results. Also, the isothermal and isobaric properties are discussed from energy-volume curves using a quasi-harmonic Debey model.

Guo, Hua-Zhong; Chen, Xiang-Rong; Cai, Ling-Cang; Zhu, Jun; Gao, Jie

2005-06-01

91

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

92

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

93

First principle study of unzipped boron nitride nanotubes

NASA Astrophysics Data System (ADS)

Systematic first principle calculations have been used to explain the dangling bonds behaviour in the rolling up of a boron nitride nanoribbon (BNNR) to construct a single-walled boron nitride nanotube (BNNT). We found in armchair BNNR two degenerate dangling bonds split and move up to higher energies due to symmetry breaking of system. While in zigzag BNNR changing the topology of system does not affect on metallic features of the band structure, but in unzipped BNNT case a metallic-semimetallic phase transition occurs. Considering the width dependent electronic properties of hydrogen passivated armchair BNNRs, exhibit zigzag behaviour of energy gap in agreement with previous results.

Azadi, Sam; Moradian, Rostam

2010-01-01

94

First-principles investigation of Nitrosyl formation in zirconia

We report first-principles calculations aimed at understanding the properties of nitrogen in ZrO{sub 2}. We find that interstitial N occurs covalently bonded to O in the form of NO units, in contrast to previous expectations of a N substitutional for O. This reveals a different chemistry for N in ZrO{sub 2} and perhaps other highly stable oxide species. This leads to a natural oxygen vacancy formation mechanism in ZrO{sub 2} in the presence of nitrogen.

Yu, Z.G. [IHPC, Singapore; Zhang, J. [IHPC, Singapore; Singh, David J [ORNL; Wu, Ping [IHPC, Singapore

2012-01-01

95

Spin Crossover in Ferropericlase from First-Principles Molecular Dynamics

NASA Astrophysics Data System (ADS)

Ferropericlase, (Mg,Fe)O, is the second-most abundant mineral of Earth's lower mantle. With increasing pressure, the Fe ions in the material begin to collapse from a magnetic to nonmagnetic spin state. We present a finite-temperature first-principles phase diagram of this spin crossover, finding a broad pressure range with coexisting magnetic and nonmagnetic ions due to favorable enthalpy of mixing of the two. Furthermore, we find the electrical conductivity of the mineral to reach semimetallic values inside Earth.

Holmström, E.; Stixrude, L.

2015-03-01

96

First-principles theory of multipolar order in neptunium dioxide

NASA Astrophysics Data System (ADS)

We provide a first-principles, materials-specific theory of multipolar order and superexchange in NpO2 by means of a noncollinear local-density approximation +U (LDA+U) method. Our calculations offer a precise microscopic description of the triple- q antiferro ordered phase in the absence of any dipolar moment. We find that, while the most common nondipolar degrees of freedom (e.g., electric quadrupoles and magnetic octupoles) are active in the ordered phase, both the usually neglected higher-order multipoles (electric hexadecapoles and magnetic triakontadipoles) have at least an equally significant effect.

Suzuki, M.-T.; Magnani, N.; Oppeneer, P. M.

2010-12-01

97

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

98

First-principles study of nitrobenzene adsorption on graphene

NASA Astrophysics Data System (ADS)

Based on first-principles calculations, the properties of nitrobenzene adsorption on graphene have been investigated. The results show that nitrobenzene prefers to be parallel to the plane of graphene with nitro closer to graphene than phenyl. Due to molecular adsorption, there is a doping band near Fermi energy. The adsorbed molecules will become perpendicular to the plane of graphene under a electrostatic field, and the doping band varies conspicuously with respect to Fermi energy. In addition, a 5000 steps MD is performed at T = 300 K to test the stability of nitrobenzene molecules adsorbed erectly on graphene under electric field.

Dai, Zhenhong; Zhao, Yinchang

2014-06-01

99

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

100

Superconductivity of compressed solid argon from first principles

NASA Astrophysics Data System (ADS)

We present first-principles calculations on the superconductivity of solid argon under high pressure. Solid argon is found to take the double hexagonal close-packed structure in pressure range from 420 to 690 GPa, where an insulator-to-metal transition occurs at around 590 GPa. The crystal structure transforms into the hexagonal close-packed structure at 690 GPa and into the face-centered cubic structure at 2300 GPa. The superconducting critical temperature is gradually increased with the successive phase transitions and reaches the maximum value of 12 K at 2600 GPa due to the enhancement of the Fermi surface nesting.

Ishikawa, Takahiro; Asano, Masamichi; Suzuki, Naoshi; Shimizu, Katsuya

2015-02-01

101

NASA Astrophysics Data System (ADS)

We elucidate the role of room-temperature-induced instantaneous structural distortions in the Li K-edge X-ray absorption spectra (XAS) of crystalline LiF, Li2SO4, Li2O, Li3N, and Li2CO3 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.; Boesenberg, Ulrike; Kostecki, Robert; Richardson, Thomas J.; Weng, Tsu-Chien; Sokaras, Dimosthenis; Nordlund, Dennis; McDermott, Eamon; Moewes, Alexander; Cabana, Jordi; Prendergast, David

2014-01-01

102

We elucidate the role of room-temperature-induced instantaneous structural distortions in the Li K-edge X-ray absorption spectra (XAS) of crystalline LiF, Li2SO4, Li2O, Li3N, and Li2CO3 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. PMID:25669363

Pascal, Tod A; Boesenberg, Ulrike; Kostecki, Robert; Richardson, Thomas J; Weng, Tsu-Chien; Sokaras, Dimosthenis; Nordlund, Dennis; McDermott, Eamon; Moewes, Alexander; Cabana, Jordi; Prendergast, David

2014-01-21

103

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

104

First principles energetic calculations of sapphire (0001) and (1 1 02) surfaces

Various crystallographic faces of sapphire, the undoped single crystal {alpha}-Al{sub 2}O{sub 3}, have been widely used as substrates fro depositing thin films of metals, semiconductors, and insulators for basic scientific studies and for microelectronic applications. Epitaxial TiO{sub 2} and VO{sub 2} thin films were successfully grown on (0001), (1102) and (1120) surfaces of sapphire substrates by the Metal Organic Chemical Vapor Deposition (MOCVD) technique in our experimental program. The present paper is one of our theoretical efforts in parallel with the experimental program to gain microscopic understandings of the effects of substrate structure on the epitaxial relationship and the overlayer lattice structure of thin film deposition. The first principles energetic calculations on the complicated surfaces like sapphire can now be carried out because of the recent development in our computer programs and availability of high speed super computers. The phase first principles means that the calculations are based only on the know physical laws and approximations, and are free of any adjustable parameters. A complete discussion of the theoretical formalism has been presented in our previous works.

Guo, J.; Ellis, D.E.; Lam, D.J.

1992-03-01

105

First principle study of manganese doped cadmium sulphide sheet

NASA Astrophysics Data System (ADS)

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 ?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; Kumar, Ashok; Ahluwalia, P. K.

2014-04-01

106

Carbon-rich icosahedral boron carbide designed from first principles

The carbon-rich boron-carbide (B{sub 11}C)C-C has been designed from first principles within the density functional theory. With respect to the most common boron carbide at 20% carbon concentration B{sub 4}C, the structural modification consists in removing boron atoms from the chains linking (B{sub 11}C) icosahedra. With C-C instead of C-B-C chains, the formation of vacancies is shown to be hindered, leading to enhanced mechanical strength with respect to B{sub 4}C. The phonon frequencies and elastic constants turn out to prove the stability of the carbon-rich phase, and important fingerprints for its characterization have been identified.

Jay, Antoine; Vast, Nathalie; Sjakste, Jelena; Duparc, Olivier Hardouin [Ecole Polytechnique, Laboratoire des Solides Irradiés, CEA-DSM-IRAMIS, CNRS UMR 7642, F-91120 Palaiseau (France)

2014-07-21

107

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

108

First principles study of oxygen vacancy defects in tantalum pentoxide

NASA Astrophysics Data System (ADS)

First principles total energy calculations were performed to characterize oxygen vacancy defects in tantalum pentoxide (Ta2O5). A simplified version of the crystalline orthorhombic phase of Ta2O5 was used in this study. Results indicate that O vacancies in Ta2O5 can be broadly classified based on their location in the lattice. One type of vacancy that occupies the "in-plane" sites displays deep or midgap occupied states and shallow unoccupied states, while a second type occupying "cap" sites results in shallow occupied states. For a wide range of Fermi levels or chemical potentials, the neutral and +2 charged states of the in-plane type vacancy and the +2 charge state of the cap type vacancy are found to be most stable.

Ramprasad, R.

2003-11-01

109

Thermoelastic properties of ? -iron from first-principles

NASA Astrophysics Data System (ADS)

We calculate the thermomechanical properties of ? -iron, and in particular its isothermal and adiabatic elastic constants, using first-principles total-energy and lattice-dynamics calculations, minimizing the quasiharmonic vibrational free energy under finite strain deformations. Particular care is made in the fitting procedure for the static and temperature-dependent contributions to the free energy, in discussing error propagation for the two contributions separately, and in the verification and validation of pseudopotential and all-electron calculations. We find that the zero-temperature mechanical properties are sensitive to the details of the calculation strategy employed, and common semilocal exchange-correlation functionals provide only fair to good agreement with experimental elastic constants, while their temperature dependence is in excellent agreement with experiments in a wide range of temperature almost up to the Curie transition.

Dragoni, Daniele; Ceresoli, Davide; Marzari, Nicola

2015-03-01

110

First Principles Study of Phase Transformations in Polyvinylidene Fluoride

NASA Astrophysics Data System (ADS)

Polyvinylidene fluoride (PVDF) with a small concentration of chlorotrifluoroethylene (CTFE) has been observed to store very high energy [1] as compared to currently used polymers. Chain rotations within the PVDF crystal have been proposed as the reason behind the high energy storage. However, in a recent work, we suggested [2] that the ultra-high energy storage in P(VDF-CTFE) is due to an electric-field induced phase transition from the non-polar ? to the polar ?-PVDF. We have now determined a low-energy relaxation path from ?- to ?-PVDF which confirms this suggestion. Our first-principles calculations reveal an intricate relationship between the kinetics of the chain rotation and the phase transformatio [4pt] [1] B. Chun, Science 313, 334 (2006).[0pt] [2] V. Ranjan, Phys. Rev. Lett. 99, 047801 (2007).

Ranjan, V.; Buongiorno Nardelli, Marco; Bernholc, J.

2010-03-01

111

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

112

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

113

Ferroelectric Transitions at Ferroelectric Domain Walls Found from First Principles

NASA Astrophysics Data System (ADS)

We present a first-principles study of model domain walls (DWs) in prototypic ferroelectric PbTiO3. At high temperature the DW structure is somewhat trivial, with atoms occupying high-symmetry positions. However, upon cooling the DW undergoes a symmetry-breaking transition characterized by a giant dielectric anomaly and the onset of a large and switchable polarization. Our results thus corroborate previous arguments for the occurrence of ferroic orders at structural DWs, providing a detailed atomistic picture of a temperature-driven DW-confined transformation. Beyond its relevance to the field of ferroelectrics, our results highlight the interest of these DWs in the broader areas of low-dimensional physics and phase transitions in strongly fluctuating systems.

Wojde?, Jacek C.; Íñiguez, Jorge

2014-06-01

114

Spin-transfer torques in antiferromagnetic metals from first principles.

In spite of the absence of a macroscopic magnetic moment, an antiferromagnet is spin-polarized on an atomic scale. The electric current passing through a conducting antiferromagnet is polarized as well, leading to spin-transfer torques when the order parameter is textured, such as in antiferromagnetic noncollinear spin valves and domain walls. We report a first principles study on the electronic transport properties of antiferromagnetic systems. The current-induced spin torques acting on the magnetic moments are comparable with those in conventional ferromagnetic materials, leading to measurable angular resistances and current-induced magnetization dynamics. In contrast to ferromagnets, spin torques in antiferromagnets are very nonlocal. The torques acting far away from the center of an antiferromagnetic domain wall should facilitate current-induced domain wall motion. PMID:18643438

Xu, Yuan; Wang, Shuai; Xia, Ke

2008-06-01

115

Water wires in aqueous solutions from first-principles calculations.

We elucidate the concept of water wires in aqueous solutions in view of their structural and dynamical properties by means of first-principles molecular dynamics simulations. We employ a specific set of hydroxyquinoline derivatives (heteroaromatic fluorescent dyes) as probe molecules that provide a well-defined initial and final coordinate for possible water wires by means of their photoacid and photobase functionalities. Besides the geometric structure of the hydrogen bond network connecting these functional sites, we focus on the dependence of the length of the resulting water wire on the initial/final coordinates determined by the chromophore. Special attention is devoted to the persistence of the wires on the picosecond time scale and their capability of shifting the nature of the proton transfer process from a concerted to a stepwise mechanism. Our results shed light on the long debate on whether water wires represent characteristic structural motifs or transient phenomena. PMID:25714490

Bekçio?lu, Gül; Allolio, Christoph; Sebastiani, Daniel

2015-03-12

116

Electronic Stopping Power in LiF from First Principles

Using time-dependent density-functional theory we calculate from first principles the rate of energy transfer from a moving proton or antiproton to the electrons of an insulating material, LiF. The behavior of the electronic stopping power versus projectile velocity displays an effective threshold velocity of {approx}0.2 a.u. for the proton, consistent with recent experimental observations, and also for the antiproton. The calculated proton/antiproton stopping-power ratio is {approx}2.4 at velocities slightly above the threshold (v{approx}0.4 a.u.), as compared to the experimental value of 2.1. The projectile energy loss mechanism is observed to be extremely local.

Pruneda, J. M. [Instituto de Ciencia de Materiales de Barcelona (ICMAB-CSIC) Campus de Bellaterra, 08193 Barcelona (Spain); Department of Physics, University of California, Berkeley, California 94720 (United States); Sanchez-Portal, D. [Donostia International Physics Center, Paseo Manuel de Lardizabal 4, 20018 San Sebastian (Spain); Centro de Fisica de Materiales, Centro Mixto CSIC-UPV/EHU, Apartado 1072, 20080 San Sebastian (Spain); Arnau, A.; Juaristi, J. I. [Donostia International Physics Center, Paseo Manuel de Lardizabal 4, 20018 San Sebastian (Spain); Centro Mixto CSIC-UPV/EHU, Facultad de Quimica, Apartado 1072, 20080 San Sebastian (Spain); Departamento de Fisica de Materiales, Facultad de Quimica, Apartado 1072, 20080 San Sebastian (Spain); Artacho, Emilio [Donostia International Physics Center, Paseo Manuel de Lardizabal 4, 20018 San Sebastian (Spain); Department of Earth Sciences, Downing Street, University of Cambridge, Cambridge CB2 3EQ (United Kingdom)

2007-12-07

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

Vibration-rotation transition dipoles from first principles

NASA Astrophysics Data System (ADS)

The use of ab initio methods to calculate line positions and associated transition intensities for the infrared spectrum of small molecules has recently become common. The first principles calculation of transition dipoles, upon which the intensity is based, relies on three distinct steps: the quantum chemical calculation of the dipole moment surface at a grid of geometries, the accurate representation of this surface using an appropriate functional form and the wave functions used to represent the initial and final states, which in turn depend on the accuracy of the potential energy surface used to generate them. Each of these stages is discussed with a view to obtaining the highest possible accuracy. The prospect of computed transition intensities displacing measured ones as the primary source of such information is considered.

Tennyson, Jonathan

2014-04-01

119

First-principles simulation of molecular dissociation-recombination equilibrium

For the first time, the equilibrium composition of chemical dissociation-recombination reaction is simulated from first-principles, only. Furthermore, beyond the conventional ab initio Born-Oppenheimer quantum chemistry the effects from the thermal and quantum equilibrium dynamics of nuclei are consistently included, as well as, the nonadiabatic coupling between the electrons and the nuclei. This has been accomplished by the path integral Monte Carlo simulations for full NVT quantum statistics of the H{sub 3}{sup +} ion. The molecular total energy, partition function, free energy, entropy, and heat capacity are evaluated in a large temperature range: from below room temperature to temperatures relevant for planetary atmospheric physics. Temperature and density dependent reaction balance of the molecular ion and its fragments above 4000 K is presented, and also the density dependence of thermal ionization above 10 000 K is demonstrated.

Kylaenpaeae, Ilkka; Rantala, Tapio T. [Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere (Finland)

2011-09-14

120

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

121

Integration (Multi-Variable Included) From First Principles

NSDL National Science Digital Library

Integration has traditionally been so closely linked to the interpretation as an area and to the techniques of anti-differentiation as to appear inseparable from them. While largely a consequence of the fact that, in pre-Personal-Computer times, anti-differentiation was the key to effective integration and that line and surface integrals were generally intractable using that technique, the advent of computer algebra systems and easy large scale numerical computation seems not to have had much effect on the way integration is presented in standard texts. On the one hand, it is not clear what sorts of skills are required for a novice to handle computer algebra effectively and on the other hand most available software does not provide the data structures and tools for dealing conveniently with numerical integration from first principles in the general case. We focus on the latter. In this article we examine an approach to the principles of integration based on computer manipulation of multi-dimensional arrays for the coordinate grids, referring to the area interpretation as only one among several possibilities and presenting integration as the solution to non-trivial anti-differentiation problems. Underlying the implementation of the approach is the mathematical notation of Iversons J, an array-processing, functional, computer language. We suggest that the mathematical foundations of the topic existence of, and convergence to, the limit should be postponed till after students can effectively compute and manipulate the approximations that are used to define integrals from first principles. Target Audience: 2-4 Year College Faculty/Administrators, Engineers

Spunde, Walter

122

Transition metal-aromatic hydrocarbon complexes were generated in a supersonic jet and studied by zero electron kinetic energy (ZEKE) photoelectron spectroscopy and theoretical calculations. The target metal complexes were identified using time-of-flight mass spectrometry, and their ionization thresholds were located via photoionization efficiency spectroscopy. ZEKE spectroscopy was used to measure the ionization energies and vibrational frequencies of the metal complexes. Their

Bradford Raymond Sohnlein

2007-01-01

123

NASA Astrophysics Data System (ADS)

We report a complete structural study of CoF2 under pressure. Its crystal structure and vibrational and electronic properties have been studied both theoretically and experimentally using first-principles density functional theory (DFT) methods, x-ray diffraction, x-ray absorption at Co K-edge experiments, Raman spectroscopy, and optical absorption in the 0-80 GPa range. We have determined the structural phase-transition sequence in CoF2 and corresponding transition pressures. The results are similar to other transition-metal difluorides such as FeF2 but different to ZnF2 and MgF2, despite that the Co2+ size (ionic radius) is similar to Zn2+ and Mg2+. We found that the complete phase-transition sequence is tetragonal rutile (P42/mnm) ? CaCl2 type (orthorhombic Pnnm) ? distorted PdF2 (orthorhombic Pbca)+PdF2 (cubic Pa3¯) in coexistence ? fluorite (cubic Fm3¯m) ? cotunnite (orthorhombic Pnma). It was observed that the structural phase transition to the fluorite at 15 GPa involves a drastic change of coordination from sixfold octahedral to eightfold cubic with important modifications in the vibrational and electronic properties. We show that the stabilization of this high-pressure cubic phase is possible under nonhydrostatic conditions since ideal hydrostaticity would stabilize the distorted-fluorite structure (tetragonal I4/mmm) instead. Although the first rutile ? CaCl2-type second-order phase transition is subtle by Raman spectroscopy, it was possible to define it through the broadening of the Eg Raman mode which is split in the CaCl2-type phase. First-principles DFT calculations are in fair agreement with the experimental Raman mode frequencies, thus providing an accurate description for all vibrational modes and elastic properties of CoF2 as a function of pressure.

Barreda-Argüeso, J. A.; López-Moreno, S.; Sanz-Ortiz, M. N.; Aguado, F.; Valiente, R.; González, J.; Rodríguez, F.; Romero, A. H.; Muñoz, A.; Nataf, L.; Baudelet, F.

2013-12-01

124

First-principles calculations of twin-boundary and stacking-fault energies in magnesium

First-principles calculations of twin-boundary and stacking-fault energies in magnesium Y. Wang and stacking faults in metal magnesium have been calculated using first-principles supercell approach. Four. Keywords: Magnesium; Interfaces; Twinning; First-principles calculation Magnesium alloys are increasingly

Chen, Long-Qing

125

Exact results and open questions in first principle functional RG

Some aspects of the functional RG (FRG) approach to pinned elastic manifolds (of internal dimension d) at finite temperature T > 0 are reviewed and reexamined in this much expanded version of Le Doussal (2006) . The particle limit d = 0 provides a test for the theory: there the FRG is equivalent to the decaying Burgers equation, with viscosity {nu} {approx} T-both being formally irrelevant. An outstanding question in FRG, i.e. how temperature regularizes the otherwise singular flow of T = 0 FRG, maps to the viscous layer regularization of inertial range Burgers turbulence (i.e. to the construction of the inviscid limit). Analogy between Kolmogorov scaling and FRG cumulant scaling is discussed. First, multi-loop FRG corrections are examined and the direct loop expansion at T > 0 is shown to fail already in d = 0, a hierarchy of ERG equations being then required (introduced in Balents and Le Doussal (2005) ). Next we prove that the FRG function R(u) and higher cumulants defined from the field theory can be obtained for any d from moments of a renormalized potential defined in an sliding harmonic well. This allows to measure the fixed point function R(u) in numerics and experiments. In d = 0 the beta function (of the inviscid limit) is obtained from first principles to four loop. For Sinai model (uncorrelated Burgers initial velocities) the ERG hierarchy can be solved and the exact function R(u) is obtained. Connections to exact solutions for the statistics of shocks in Burgers and to ballistic aggregation are detailed. A relation is established between the size distribution of shocks and the one for droplets. A droplet solution to the ERG functional hierarchy is found for any d, and the form of R(u) in the thermal boundary layer is related to droplet probabilities. These being known for the d = 0 Sinai model the function R(u) is obtained there at any T. Consistency of the {epsilon}=4-d expansion in one and two loop FRG is studied from first principles, and connected to shock and droplet relations which could be tested in numerics.

Le Doussal, Pierre [LPTENS CNRS UMR 8549 24, Rue Lhomond, 75231 Paris Cedex 05 (France)], E-mail: ledou@lpt.ens.fr

2010-01-15

126

Phase transition of Sr on Si (0 0 1): First principles prediction and experiment

NASA Astrophysics Data System (ADS)

The ability to understand and predict the phase diagrams of surface phases from first principles can be valuable for developing processes for growth of epitaxial structures. In the growth of epitaxial oxides on Si (0 0 1), a submonolayer phase of Sr plays a key role. The physical structure for this phase, which has 2 × 3 symmetry and occurs at 1/6 monolayer Sr coverage, was recently elucidated using both first principles theory and diffraction experiments [J.W. Reiner, K.F. Garrity, F.J. Walker, S. Ismail-Beigi, C.H. Ahn, Role of strontium in oxide epitaxy on silicon (0 0 1), Phys. Rev. Lett. 101 (10) (2008) 105503.]. Our approach to understanding the broader Sr/Si phase diagram combines density functional theory with a thermodynamic analysis of the phase equilibrium between a Sr lattice gas and the 2 × 3 structure. We use reflection high energy electron diffraction (RHEED) to experimentally determine the phase diagram, finding good agreement with theoretical predictions.

Garrity, Kevin F.; Padmore, Myrtle-Rose; Segal, Yaron; Reiner, J. W.; Walker, F. J.; Ahn, C. H.; Ismail-Beigi, S.

2010-05-01

127

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

128

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. [School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083 (China); Department of Mechanical Engineering, Hebei University, Baoding 071000 (China); Zhan, Q., E-mail: qzhan@mater.ustb.edu.cn [School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083 (China); Geng, W. T., E-mail: geng@ustb.edu.cn [School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083 (China); Psi Quantum Materials LLC, Laiwu 271100 (China)

2014-06-15

129

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

130

Experimental and first-principles study of ferromagnetism in Mn-doped zinc stannate nanowires

Room temperature ferromagnetism was observed in Mn-doped zinc stannate (ZTO:Mn) nanowires, which were prepared by chemical vapor transport. Structural and magnetic properties and Mn chemical states of ZTO:Mn nanowires were investigated by X-ray diffraction, superconducting quantum interference device (SQUID) magnetometry and X-ray photoelectron spectroscopy. Manganese predominantly existed as Mn{sup 2+} and substituted for Zn (Mn{sub Zn}) in ZTO:Mn. This conclusion was supported by first-principles calculations. Mn{sub Zn} in ZTO:Mn had a lower formation energy than that of Mn substituted for Sn (Mn{sub Sn}). The nearest neighbor Mn{sub Zn} in ZTO stabilized ferromagnetic coupling. This observation supported the experimental results.

Deng Rui; Zhou Hang; Qin Jieming; Wan Yuchun; Jiang Dayong; Liang Qingcheng [School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022 (China); Li Yongfeng [Key Laboratory of Physics and Technology for Advanced Batteries, College of Physics, Jilin University, Changchun 130012 (China); Wu, Tom [Physical Sciences and Engineering Division, Solar and Photovoltaics Engineering Research Center, King Abdullah University of Science and Technology, Thuwal 23955-6900 (Saudi Arabia); Yao Bin [State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012 (China); Liu Lei [State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, No.3888 Dongnanhu Road, Changchun 130033 (China)

2013-07-21

131

Experimental and first-principles study of ferromagnetism in Mn-doped zinc stannate nanowires

NASA Astrophysics Data System (ADS)

Room temperature ferromagnetism was observed in Mn-doped zinc stannate (ZTO:Mn) nanowires, which were prepared by chemical vapor transport. Structural and magnetic properties and Mn chemical states of ZTO:Mn nanowires were investigated by X-ray diffraction, superconducting quantum interference device (SQUID) magnetometry and X-ray photoelectron spectroscopy. Manganese predominantly existed as Mn2+ and substituted for Zn (MnZn) in ZTO:Mn. This conclusion was supported by first-principles calculations. MnZn in ZTO:Mn had a lower formation energy than that of Mn substituted for Sn (MnSn). The nearest neighbor MnZn in ZTO stabilized ferromagnetic coupling. This observation supported the experimental results.

Deng, Rui; Zhou, Hang; Li, Yong-Feng; Wu, Tom; Yao, Bin; Qin, Jie-Ming; Wan, Yu-Chun; Jiang, Da-Yong; Liang, Qing-Cheng; Liu, Lei

2013-07-01

132

Cobalt (hydro)oxide electrodes under electrochemical conditions: a first principle study

NASA Astrophysics Data System (ADS)

There is currently much interest in photoelectrochemical water splitting as a promising pathway towards sustainable energy production. A major issue of such photoelectrochemical devices is the limited efficiency of the anode, where the oxygen evolution reaction (OER) takes place. Cobalt (hydro)oxides, particularly Co3O4 and Co(OH)2, have emerged as promising candidates for use as OER anode materials. Interestingly, recent in-situ Raman spectroscopy studies have shown that Co3O4 electrodes undergo progressive oxidation and transform into oxyhydroxide, CoO(OH), under electrochemical working conditions. (Journal of the American Chemical Society 133, 5587 (2011))Using first principle electronic structure calculations, we provide insight into these findings by presenting results on the structural, thermodynamic, and electronic properties of cobalt oxide, hydroxide and oxydroxide CoO(OH), and on their relative stabilities when in contact with water under external voltage.

Chen, Jia; Selloni, Annabella

2013-03-01

133

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

134

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

135

First principles studies of the graphene-phenol interactions.

Studies of the interaction between phenol and intrinsic graphene, as well as phenol and aluminum doped graphene layer are performed using first principles total energy calculations within the periodic density functional theory. A 4x4 periodic structure is used to explore the adsorption of a phenol molecule on the intrinsic graphene and on aluminum doped graphene layer. The electron-ion interactions are modeled using ultra-soft pseudo-potentials, and the exchange-correlation energies are treated according to the generalized gradient approximation (GGA) with the PBE parameterization. We consider different molecule orientations: parallel and perpendicular to the graphene layer to relax the atomic structure. To explain the optimized atomic geometry we determine binding energies for all cases and the density of states (DOS) and partial DOS for the most relevant configurations. Results indicate that the direct interaction of oxygen with aluminum yields the ground state geometry with the phenol molecule adsorbed on the graphene layer. Binding energies and DOS structures also demonstrate that the ground state configuration is that where the O and Al atoms interact with a separation distance of 1.97 Å. PMID:22415370

Hernández, José M Galicia; Anota, Ernesto Chigo; de la Cruz, María T Romero; Melchor, Minerva González; Cocoletzi, Gregorio Hernández

2012-08-01

136

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

137

A First-principles Molecular Dynamics Investigation of Superionic Conductivity

NASA Astrophysics Data System (ADS)

Superionic materials---solids with liquid-like transport properties---have found widespread use in a variety of applications in fuel cells, switches, sensors, and batteries. However, reasons for fast-ion conduction in such materials, as well as the specific atomistic mechanisms involved, remain ill understood. Our work uses first-principles molecular dynamics to illuminate the mechanisms, pathways, and motivations for superionic conductivity in two materials representing different classes of ion conductors: ?-AgI, an archetypal Type-I superionic; and CsHSO4, an anhydrous solid-state electrolyte candidate for hydrogen fuel cells. For ?-AgI, we trace common pathways for silver ion conduction and discuss how a chemical signature in the electronic structure relates to enhanced silver ion mobility. We also characterize the dynamical lattice structure in the superionic phase and present the likely motivations for its existence. For CsHSO4, we isolate the dominant atomistic mechanisms involved in superprotonic conduction and discuss the effect of correlated diffusive events in enhancing proton transport. We also offer a detailed description of the dynamics of the hydrogen bond network topology in the course of proton diffusion and discuss the relevance of atomistic processes with competing timescales in facilitating proton transport.

Wood, Brandon; Marzari, Nicola

2007-03-01

138

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

139

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

140

Safeguards First Principles Initiative at the Nevada Test Site

The Material Control and Accountability (MC&A) program at the Nevada Test Site (NTS) was selected as a test bed for the Safeguards First Principles Initiative (SFPI). The implementation of the SFPI is evaluated using the system effectiveness model and the program is managed under an approved MC&A Plan. The effectiveness model consists of an evaluation of the critical elements necessary to detect, deter, and/or prevent the theft or diversion of Special Nuclear Material (SNM). The modeled results indicate that the MC&A program established under this variance is still effective, without creating unacceptable risk. Extensive performance testing is conducted through the duration of the pilot to ensure the protection system is effective and no material is at an unacceptable risk. The pilot was conducted from January 1, 2007, through May 30, 2007. This paper will discuss the following activities in association with SFPI: 1. Development of Timeline 2. Crosswalk of DOE Order and SFPI 3. Peer Review 4. Deviation 5. MC&A Plan and Procedure changes 6. Changes implemented at NTS 7. Training 8. Performance Test

Geneva Johnson

2007-07-08

141

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

142

First principles NEXAFS simulations of N-donor Uranyl complexes

NASA Astrophysics Data System (ADS)

The synthesis and study of soft-donor uranyl complexes can provide new insights into the coordination chemistry of non-aqueous [UO]2^+ Recently, the tunable N-donor ligand 2,6-Bis(2-benzimidazyl)pyridine (BBP) was employed to produce novel uranyl complexes in which the [UO]2^+ cation is ligated by anionic and covalent groups with discrete chemical differences. In this work we investigate the electronic structure of the three such uranyl-BBP complexes via near-edge X-ray absorption fine structure (NEXAFS) experiments and simulations using the eXcited electron and Core-Hole (XCH) approach [1]. The evolution of the structural as well as electronic properties across the three complexes is studied systematically. Computed N K-edge and O K-edge NEXAFS spectra are compared with experiment and spectral features assigned to specific electronic transitions in these complexes. Studying the variations in spectral features arising from N K-edge absorption provides a clear picture of ligand-uranyl bonding in these systems. References: [1] D. Prendergast and G. Galli, X-ray absorption spectra of water from first-principles calculations, Phys. Rev. Lett., 215502 (2006).

Pemmaraju, C. D.; Duan, R.; Copping, R.; Jeon, B.; Teat, S. J.; Janousch, M.; Tyliszczak, T.; Canning, A.; Grønbech-Jensen, N.; Shuh, D. K.; Prendergast, D.

2013-03-01

143

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

144

Electronic Structure and Ionicity of Actinide Oxides from First Principles

The ground-state electronic structures of the actinide oxides AO, A{sub 2}O{sub 3}, and AO{sub 2} (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 AO{sub 2} and A{sub 2}O{sub 3} is found to follow the stoichiometry, namely, corresponding to A{sup 4+} ions in the dioxide and A{sup 3+} ions in the sesquioxides. In contrast, the A{sup 2+} 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, Leon [ORNL; Svane, Axel [University of Aarhus, Denmark; Szotek, Zdzislawa [Daresbury Laboratory, UK; Temmerman, Walter M [Daresbury Laboratory, UK; Stocks, George Malcolm [ORNL

2010-01-01

145

First-principles study of liquid and amorphous metals

NASA Astrophysics Data System (ADS)

Computer simulations using state of the art First-Principles ab-initio methods enable us to probe the structural features of novel materials like liquid metals and metallic glass forming alloys, both in their supercooled liquid state as well as in their quenched amorphous forms where available. The ab-initio nature of the calculations enable us to capture the chemical identity realistically at the atomistic level without any free parameters. The results show that even though elemental liquid metals like face-centered cubic (FCC) Cu and body-centered cubic (BCC) Fe (and W) have similar atomic structure at high temperature, which is also similar to jammed packing of hard-spheres, they differ quite appreciably even with slight supercooling. This difference enables us to further supercool Fe and W to a much greater degree than Cu. The origin of this difference between elemental metals with different crystalline ground states can be understood based on concepts of geometric frustration. Further, the role played by atoms of different sizes in controlling the geometric frustration in glass forming alloys has been investigated. Studies of Silicon in its supercooled regime have been made to investigate the existence of a possible structural transition. Attempts to clarify if the structural transition could be a thermodynamic phase transition have been made and changes in electronic properties accompanying this structural change have been studied.

Ganesh, Panchapakesan

146

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

147

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

148

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

149

Isotopic fractionation between two phases can be calculated if the vibrational properties of isotopic end-members are fully characterized. We assessed a theoretical approach based on first-principles density-functional density (DFT) prediction of vibrational frequencies by comparing with spectroscopic data on isotopically substituted brucite, a model mineral for which detailed experimental data on isotopic effects and brucite–water D\\/H partitioning exist. The deviation

Bruno Reynard

2009-01-01

150

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

151

First-principle study on the chromium doping effect on the crystal structure of metallic VO 2

A theoretical study on the chromium (Cr) doping effect on the crystal structure of metallic VO2 has been performed by first principles calculation based on local density approximation (LDA). The results are consistent with the experimental observations: the substitution impurity of Cr on cation site (CrV) defects are created by Cr-doping and the Cr-doping results in the shrink of the

Mei Pan; Hongmei Zhong; Zhifeng Li; Xiaoshuang Chen; Wei Lu

2004-01-01

152

First-principles study of back-contact effects on CdTe thin-film solar cells

Forming a chemically stable low-resistance back contact for CdTe thin-film solar cells is critically important to the cell performance. This paper reports theoretical study of the effects of the back-contact material, Sb2Te3 , on the performance of the CdTe solar cells. First-principles calculations show that Sb impurities in p -type CdTe are donors and can diffuse with low diffusion barrier.

Mao-Hua Du; Mao-Hua

2009-01-01

153

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

154

First-Principles Based Model of Spin-state Phase Transition

NASA Astrophysics Data System (ADS)

The nature of spin-state phase transition is investigated with [Fe(C4H4N2){Pt(CN)4}] that is a novel 3D Hofmann—like compound. The bistability of this system is obtained by the first-principles calculation. It is demonstrated that thermal expansion is the intrinsic force involved in spin-state transition. Based on these results, we suggest a thermal exciting bistable model of spin-state transition with a temperature dependent crystal-field splitting (CFS). Experimental evidence of spin-state phase transition coincides with our theoretical model. This model approaches something fundamental in the mechanism leading to the transition, and it is important in developing new and practical controllable quantum devices.

Wang, Xue-Li; Wang, Chuan-Hui; Tian, Zhao-Ming; Yin, Shi-Yan; Yuan, Song-Liu

2010-10-01

155

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

156

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

157

First-principles calculations of elastic constants of c-BN

NASA Astrophysics Data System (ADS)

The lattice parameters, elastic constants, bulk modulus and its pressure derivative of c-BN are calculated using a first-principles plane wave method with the relativistic analytic pseudopotential of the Hartwigen, Goedecker and Hutter (HGH)-type and the pseudopotential of Troullier-Martins (TM)-type in the frame of local density approximation, and the dependences of bulk modulus on temperature and on pressure are investigated by the quasi-harmonic Debye model. The Debye temperature and the thermal expansion coefficient of c-BN are also calculated. The results obtained are well consistent with the available experimental data and other theoretical results. At zero pressure, the thermal expansion coefficients are about 6.0×10 -6 K -1 at T=1724 K and 6.5×10 -6 K -1 at T=1972 K, respectively, consistent with the experimental data.

Hao, Yan-Jun; Chen, Xiang-Rong; Cui, Hong-Ling; Bai, Yu-Lin

2006-06-01

158

Formation and annealing behaviors of qubit centers in 4H-SiC from first principles

Inspired by finding that the nitrogen-vacancy center in diamond is a qubit candidate, similar defects in silicon carbide (SiC) have drawn considerable interest. However, the generation and annealing behaviors of these defects remain unclear. Using first-principles calculations, we describe the equilibrium concentrations and annealing mechanisms based on the diffusion of silicon vacancies. The formation energies and energy barriers along different migration paths, which are responsible for the formation rates, stability, and concentrations of these defects, are investigated. The effects on these processes of charge states, annealing temperature, and crystal orientation are also discussed. These theoretical results are expected to be useful in achieving controllable generation of these defects in experiments.

Wang, Xiaopeng; Zhao, Mingwen, E-mail: zmw@sdu.edu.cn; Bu, Hongxia; He, Xiujie; Wang, Aizhu [School of Physics and State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100 (China); Zhang, Hongyu [Department of Physics, East China University of Science and Technology, Shanghai 200237 (China)

2013-11-21

159

Sulfur dioxide adsorbed on graphene and heteroatom-doped graphene: a first-principles study

NASA Astrophysics Data System (ADS)

The adsorption of sulfur dioxide (SO2) on intrinsic graphene and heteroatom-doped (B, N, Al, Si, Cr, Mn, Ag, Au, and Pt) graphene samples was theoretically studied using first-principles approach based on density functional theory to exploit their potential applications as SO2 gas sensors. The structural and electronic properties of the graphene-molecule adsorption adducts are strongly dependent on the dopants. SO2 molecule is adsorbed weakly on intrinsic graphene, and B-, N-doped graphene; in general, strong chemisorption is observed on Al-, Si-, Cr-, Mn-, Ag-, Au-, and Pt-doped graphene. The adsorption mechanisms are discussed from charge transfers and density of states. This work reveals that the sensitivity of graphene-based chemical gas sensors for SO2 can be drastically improved by introducing appropriate dopant, and Cr, as well as Mn, may be the best choices among all the dopants.

Shao, Li; Chen, Guangde; Ye, Honggang; Wu, Yelong; Qiao, Zhijuan; Zhu, Youzhang; Niu, Haibo

2013-02-01

160

Stability of the hcp Ruthenium at high pressures from first principles

The method of calculation of the elastic constants up to third order from the energy-strain relation under pressure for the hcp crystals is given and described in details. The method is applied to the hcp phase of Ruthenium. Elastic constants, lattice dynamics, and electronic structure are investigated in the pressure interval of 0–600?GPa by means of first principles calculations. The obtained parameters are in very good agreement with available experimental and theoretical data. No preconditions for phase transformation driven by mechanical or dynamical instabilities for hcp Ru were found in the investigated pressure range. The reason of stability at such high pressures is explained in the context of electronic structure peculiarities.

Lugovskoy, A. V., E-mail: andrey.lugovskoy@gmail.com; Belov, M. P.; Vekilov, Yu. Kh [Department of Theoretical Physics and Quantum Technologies, National Research Technological University MISiS, Leninskii pr. 4, Moscow 119049 (Russian Federation); Materials Modeling and Development Laboratory, National Research Technological University MISiS, Leninskii pr. 4, Moscow 119049 (Russian Federation); Krasilnikov, O. M. [Department of Theoretical Physics and Quantum Technologies, National Research Technological University MISiS, Leninskii pr. 4, Moscow 119049 (Russian Federation)

2014-09-14

161

Solute/impurity diffusivities in bcc Fe: A first-principles study

NASA Astrophysics Data System (ADS)

Chinese low activation martensitic steel (CLAM) has been designed with decreased W content and increased Ta content to improve performance. We performed first-principles calculations to investigate the diffusion properties of solute element (Cr, W, Mn, V, Ta) and C diffusion with a nearby solute element inside bcc Fe. The self-diffusion coefficients and solute diffusion coefficients in Fe host were derived using the nine-frequency model. A relatively lower diffusivity was observed for W in paramagnetic state, implying enriched W concentration inside Fe host. The solute atom interacts strongly with C impurity, depending on the interatomic distance. According to our calculations, formation of Ta carbide precipitates is energetically preferred by trapping C impurity around Ta atom. Our theoretical results are helpful for investigating the evolution of microstructure of steels for engineering applications.

Zhang, Chong; Fu, Jie; Li, Ruihuan; Zhang, Pengbo; Zhao, Jijun; Dong, Chuang

2014-12-01

162

Electron field emission in nanostructures: A first-principles study

NASA Astrophysics Data System (ADS)

The objective of this work was to study electron field emission from several nanostructures using a first-principles framework. The systems studied were carbon nanowires, graphene nanoribbons, and nanotubes of varying composition. These particular structures were chosen because they have recently been identified as showing novel physical phenomena, as well as having tremendous industrial applications. We examined the field emission under a variety of conditions, including laser illumination and the presence of adsorbates. The goal was to explore how these conditions affect the field emission performance. In addition to the calculations, this dissertation has presented computational developments by the author that allowed these demanding calculations to be performed. There are many possible choices for basis when performing an electronic structure calculation. Examples are plane waves, atomic orbitals, and real-space grids. The best choice of basis depends on the structure of the system being analyzed and the physical processes involved (e.g., laser illumination). For this reason, it was important to conduct rigorous tests of basis set performance, in terms of accuracy and computational efficiency. There are no existing benchmark calculations for field emission, but transport calculations for nanostructures are similar, and so provide a useful reference for evaluating the performance of various basis sets. Based on the results, for the purposes of studying a non-periodic nanostructure under field emission conditions, we decided to use a real-space grid basis which incorporates the Lagrange function approach. Once a basis was chosen, in this case a real-space grid, the issue of boundary conditions arose. The problem is that with a non-periodic system, field emitted electron density can experience non-physical reflections from the boundaries of the calculation volume, leading to inaccuracies. To prevent this issue, we used complex absorbing potentials (CAPs) to absorb electron density before it could reach the boundaries and reflect. The CAPs were zero in the region of the emitting structure and in regions were measurements were made. Still, we wanted to make sure that complex potentials were an accurate and efficient solution to the boundary problem. To evaluate this, we once again turned to benchmarks from transport calculations. The results showed CAPs to be an extremely accurate and efficient computational tool, and were incorporated into all of this dissertation's calculations. Our calculations show that adsorbate atoms significantly increase the field emission current of carbon nanotubes. Adsorbate atoms also cause strong differences and nonlinearity in the Fowler-Nordheim plot. The source of the enhanced current is electronic states introduced by the adsorbate atom. We have investigated the emission of electrons from nanostructures induced by short intense laser pulses in the presence of a weaker uniform static field. Based on the results of our simulations, two important qualitative features of this process have been determined: (1) a significant enhancement of the emission when a laser pulse is applied, and (2) the field emission current has a peak of some duration and the position of this peak correlates with the time of the pulse arrival. These two features suggest the possibility of using short laser pulses for making few-electron emitters of nanoscale size [212]. Such emitters could have many desirable properties, especially very high spatial and time resolutions. The field emission from nanotubes of various composition has been studied. The calculations predict that the GaN, SiC, and Si nanotubes are particularly good field emitters. The highest-current nanotube, Si, is predicted to produce a current an order of magnitude higher than BN or C nanotubes. The calculations predict that carbon nanotubes with various adsorbates can be used as spin-polarized current sources. These are the first first-principles calculations to show spin-polarized field emission for carbon nanotubes with iron adsorbates. Also, there i

Driscoll, Joseph Andrew

163

First Principles Explanation of the Positive Seebeck Coefficient of Lithium

NASA Astrophysics Data System (ADS)

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.

Xu, Bin; Verstraete, Matthieu J.

2014-05-01

164

Vibrational spectra of vitreous germania from first-principles

NASA Astrophysics Data System (ADS)

We report on a first-principles investigation of the structural and vibrational properties of vitreous germania (v-GeO2) . Our work focuses on a periodic model structure of 168 atoms, but three smaller models are also studied for comparison. We first carry out a detailed structural analysis both in real and reciprocal spaces. Our study comprises the partial pair correlation functions, the angular distributions, the total neutron correlation function, the neutron and x -ray total structure factors, and the Faber-Ziman and Bhatia-Thornthon partial structure factors. We find overall good agreement with available experimental data. We then obtain the vibrational frequencies and eigenmodes. We analyze the vibrational density of states in terms of Ge and O motions, and further in terms of rocking, bending, and stretching contributions. The inelastic neutron spectrum is found to differ only marginally from the vibrational density of states. Using a methodology based on the application of finite electric fields, we derive dynamical Born charge tensors and Raman coupling tensors. For the infrared spectra, we calculate the real and imaginary parts of the dielectric function, including the high-frequency and static dielectric constants. The Raman spectra are shown to be sensitive to the medium-range structure and support an average Ge-O-Ge angle of 135°. We identify the shoulder X2 as a signature of breathing O vibrations in three-membered rings. Four-membered rings are found to contribute to the main Raman peak. We advance an interpretation for the shoulder X1 in terms of delocalized bond-bending modes. We derive bond polarizability parameters from the calculated Raman coupling tensors and demonstrate their level of reliability in reproducing the spectra. The calculated vibrational spectra all show good agreement with the respective experimental spectra.

Giacomazzi, Luigi; Umari, P.; Pasquarello, Alfredo

2006-10-01

165

First-principles study of polyacetylene derivatives bearing nitroxide radicals

NASA Astrophysics Data System (ADS)

Electrodes made of organic polymers bearing redox-active radical pendant groups have attractive features for use in rechargeable batteries. Electronic structure and electrochemical properties of cathode- and anode-active organic polymers are investigated here by means of first-principles calculations performed in the framework of the density functional theory. We consider organic radical polymers (ORPs) that consist of trans-polyacetylene derivatives bearing a variety of nitroxide radicals. A number of neutral and charged supercells are utilized to compute the ionization potentials and electron affinities as well as the one-electron states of these ORPs. By revealing the polyacetylene-derived highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) as well as the radical-derived singly occupied molecular orbital (SOMO), the variation of the SOMO energy within the HOMO-LUMO gap is determined in the course of the oxidization or reduction of ORPs. Our results indicate that the ionization potential I and electron affinity A of polyacetylene would act as a lower or upper bound in the variation of the electrochemical potential of cathode- or anode-active ORPs in the course of battery discharge or charge owing to pinning of the radical-derived SOMO to the polyacetylene-derived HOMO or LUMO. Accordingly, it is anticipated that the electrochemical “window” [-I,-A] of the polymeric backbone of ORPs will impose certain limitations in accomplishing a high charge/discharge voltage range in a totally organic rechargeable battery with positive and negative electrodes made of cathode- and anode-active ORPs, respectively. On the other hand, our findings suggest that one could, in principle, take advantage of using two different (conducting) polymeric backbones in the anode and cathode with adjusted HOMO and LUMO offsets once the electron transfer is accomplished to take place through the conducting backbones.

Bilgiç, Beyza; K?l?ç, Çetin; Esat, Burak

2011-09-01

166

First-principles studies of Ni-Ta intermetallic compounds

The structural properties, heats of formation, elastic properties, and electronic structures of Ni-Ta intermetallic compounds are investigated in detail based on density functional theory. Our results indicate that all Ni-Ta intermetallic compounds calculated here are mechanically stable except for P21/m-Ni{sub 3}Ta and hc-NiTa{sub 2}. Furthermore, we found that Pmmn-Ni{sub 3}Ta is the ground state stable phase of Ni{sub 3}Ta polymorphs. The polycrystalline elastic modulus has been deduced by using the Voigt-Reuss-Hill approximation. All Ni-Ta intermetallic compounds in our study, except for NiTa, are ductile materials by corresponding G/K values and poisson's ratio. The calculated heats of formation demonstrated that Ni{sub 2}Ta are thermodynamically unstable. Our results also indicated that all Ni-Ta intermetallic compounds analyzed here are conductors. The density of state demonstrated the structure stability increases with the Ta concentration. - Graphical abstract: Mechanical properties and formation heats of Ni-Ta intermetallic compounds are discussed in detail in this paper. Highlights: Black-Right-Pointing-Pointer Ni-Ta intermetallic compounds are investigated by first principle calculations. Black-Right-Pointing-Pointer P21/m-Ni{sub 3}Ta and hc-NiTa{sub 2} are mechanically unstable phases. Black-Right-Pointing-Pointer Pmmn-Ni{sub 3}Ta is ground stable phase of Ni{sub 3}Ta polymorphs. Black-Right-Pointing-Pointer All Ni-Ta intermetallic compounds are conducting materials.

Zhou Yi [Department of Materials Science and Engineering, College of Materials, and Research Center of Materials Design and Applications, Xiamen University, Xiamen 361005 (China); Wen Bin, E-mail: wenbin@ysu.edu.cn [State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004 (China); Ma Yunqing [Department of Materials Science and Engineering, College of Materials, and Research Center of Materials Design and Applications, Xiamen University, Xiamen 361005 (China); Melnik, Roderick [M2NeT Lab, Wilfrid Laurier University, Waterloo,75 University Ave. West, Ontario, N2L 3C5 (Canada); MIT Department, University of Jyvaeskylae, Jyvaeskylae (Finland); Liu Xingjun, E-mail: lxj@xmu.edu.cn [Department of Materials Science and Engineering, College of Materials, and Research Center of Materials Design and Applications, Xiamen University, Xiamen 361005 (China)

2012-03-15

167

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

168

First Principles Modeling of Nonlinear Incidence Rates in Seasonal Epidemics

In this paper we used a general stochastic processes framework to derive from first principles the incidence rate function that characterizes epidemic models. We investigate a particular case, the Liu-Hethcote-van den Driessche's (LHD) incidence rate function, which results from modeling the number of successful transmission encounters as a pure birth process. This derivation also takes into account heterogeneity in the population with regard to the per individual transmission probability. We adjusted a deterministic SIRS model with both the classical and the LHD incidence rate functions to time series of the number of children infected with syncytial respiratory virus in Banjul, Gambia and Turku, Finland. We also adjusted a deterministic SEIR model with both incidence rate functions to the famous measles data sets from the UK cities of London and Birmingham. Two lines of evidence supported our conclusion that the model with the LHD incidence rate may very well be a better description of the seasonal epidemic processes studied here. First, our model was repeatedly selected as best according to two different information criteria and two different likelihood formulations. The second line of evidence is qualitative in nature: contrary to what the SIRS model with classical incidence rate predicts, the solution of the deterministic SIRS model with LHD incidence rate will reach either the disease free equilibrium or the endemic equilibrium depending on the initial conditions. These findings along with computer intensive simulations of the models' Poincaré map with environmental stochasticity contributed to attain a clear separation of the roles of the environmental forcing and the mechanics of the disease transmission in shaping seasonal epidemics dynamics. PMID:21379320

2011-01-01

169

Risk reduction and the privatization option: First principles

The Department of Energy`s Office of Environmental Restoration and Waste Management (EM) faces a challenging mission. To increase efficiency, EM is undertaking a number of highly innovative initiatives--two of which are of particular importance to the present study. One is the 2006 Plan, a planning and budgeting process that seeks to convert the clean-up program from a temporally and fiscally open-ended endeavor to a strictly bounded one, with firm commitments over a decade-long horizon. The second is a major overhauling of the management and contracting practices that define the relationship between the Department and the private sector, aimed at cost reduction by increasing firms` responsibilities and profit opportunities and reducing DOE`s direct participation in management practices and decisions. The goal of this paper is to provide an independent perspective on how EM should create new management practices to deal with private sector partners that are motivated by financial incentives. It seeks to ground this perspective in real world concerns--the background of the clean-up effort, the very difficult technical challenges it faces, the very real threats to environment, health and safety that have now been juxtaposed with financial drivers, and the constraints imposed by government`s unique business practices and public responsibilities. The approach is to raise issues through application of first principles. The paper is targeted at the EM policy officer who must implement the joint visions of the 2006 plan and privatization within the context of the tradeoff between terminal risk reduction and interim risk management.

Bjornstad, D.J.; Jones, D.W.; Russell, M. [Joint Inst. for Energy and Environment, Knoxville, TN (United States); Cummings, R.C.; Valdez, G. [Georgia State Univ., Atlanta, GA (United States); Duemmer, C.L. [Hull, Duemmer and Garland (United States)

1997-06-25

170

First principles modeling of nonlinear incidence rates in seasonal epidemics.

In this paper we used a general stochastic processes framework to derive from first principles the incidence rate function that characterizes epidemic models. We investigate a particular case, the Liu-Hethcote-van den Driessche's (LHD) incidence rate function, which results from modeling the number of successful transmission encounters as a pure birth process. This derivation also takes into account heterogeneity in the population with regard to the per individual transmission probability. We adjusted a deterministic SIRS model with both the classical and the LHD incidence rate functions to time series of the number of children infected with syncytial respiratory virus in Banjul, Gambia and Turku, Finland. We also adjusted a deterministic SEIR model with both incidence rate functions to the famous measles data sets from the UK cities of London and Birmingham. Two lines of evidence supported our conclusion that the model with the LHD incidence rate may very well be a better description of the seasonal epidemic processes studied here. First, our model was repeatedly selected as best according to two different information criteria and two different likelihood formulations. The second line of evidence is qualitative in nature: contrary to what the SIRS model with classical incidence rate predicts, the solution of the deterministic SIRS model with LHD incidence rate will reach either the disease free equilibrium or the endemic equilibrium depending on the initial conditions. These findings along with computer intensive simulations of the models' Poincaré map with environmental stochasticity contributed to attain a clear separation of the roles of the environmental forcing and the mechanics of the disease transmission in shaping seasonal epidemics dynamics. PMID:21379320

Ponciano, José M; Capistrán, Marcos A

2011-02-01

171

First-principles calculations of conductivity in transparent semiconducting oxides

NASA Astrophysics Data System (ADS)

Transparent conducting oxides (TCOs) are exceptional materials that possess the unique combination of nearly metallic conductivity and optical transparency over the visible portion of the spectrum. With such features, TCOs have become critical components of many present and emerging technologies. Today these materials are already ubiquitous, appearing in windows, flat-panel displays, portable electronics, solar cells, solid-state light-emitters, and transistors. Thanks to the ever-growing list of applications that rely on TCOs, the recent surge of interest in these materials has focused on understanding the fundamental properties and doping opportunities in a variety of traditional as well as promising new TCOs. Using state-of-the-art first-principles calculations, we address several important issues in a number of technologically relevant TCOs. First, the origins of unintentional conductivity. Many TCOs exhibit high levels of n-type conductivity, even when not intentionally doped. For SnO 2, In2O3 and Ga2O3, we demonstrate that this is not due to oxygen vacancies, as is commonly assumed, but must be attributed to unintentional incorporation of impurities, with hydrogen being a prime candidate. Second, the push for higher doping levels. We suggest several donor impurities as candidate dopants with high solubility. We also investigate limitations on doping due to the formation or incorporation of compensating centers. Among intrinsic defects, cation vacancies are the most likely candidates; we also study impurities that act as acceptors. In the case of SnO2, group-V impurities are intriguing since they can act either as donors on the Sn site or acceptors on the O site. Third, the prospects for p-type doping. Here we find that none of the investigated acceptors will lead to effective hole doping. We demonstrate that the reason for this behavior is the tendency for strong localization of holes in the oxygen-derived valence bands, and relate this to the issue of polaron formation. Finally, we apply our acquired expertise to the issue of reducing the absorption edge of another wide-band-gap semiconducting oxide, TiO2, which is widely used for photocatalysis. Our conclusions resolve long-standing questions on the properties of N-doped titania, suggesting another application of acceptor doping in TCO materials.

Varley, Joel Basile

2011-12-01

172

ABINIT: First-principles approach to material and nanosystem properties

NASA Astrophysics Data System (ADS)

ABINIT [ http://www.abinit.org] allows one to study, from first-principles, systems made of electrons and nuclei (e.g. periodic solids, molecules, nanostructures, etc.), on the basis of Density-Functional Theory (DFT) and Many-Body Perturbation Theory. Beyond the computation of the total energy, charge density and electronic structure of such systems, ABINIT also implements many dynamical, dielectric, thermodynamical, mechanical, or electronic properties, at different levels of approximation. The present paper provides an exhaustive account of the capabilities of ABINIT. It should be helpful to scientists that are not familiarized with ABINIT, as well as to already regular users. First, we give a broad overview of ABINIT, including the list of the capabilities and how to access them. Then, we present in more details the recent, advanced, developments of ABINIT, with adequate references to the underlying theory, as well as the relevant input variables, tests and, if available, ABINIT tutorials. Program summaryProgram title: ABINIT Catalogue identifier: AEEU_v1_0 Distribution format: tar.gz Journal reference: Comput. Phys. Comm. Programming language: Fortran95, PERL scripts, Python scripts Computer: All systems with a Fortran95 compiler Operating system: All systems with a Fortran95 compiler Has the code been vectorized or parallelized?: Sequential, or parallel with proven speed-up up to one thousand processors. RAM: Ranges from a few Mbytes to several hundred Gbytes, depending on the input file. Classification: 7.3, 7.8 External routines: (all optional) BigDFT [1], ETSF IO [2], libxc [3], NetCDF [4], MPI [5], Wannier90 [6] Nature of problem: This package has the purpose of computing accurately material and nanostructure properties: electronic structure, bond lengths, bond angles, primitive cell size, cohesive energy, dielectric properties, vibrational properties, elastic properties, optical properties, magnetic properties, non-linear couplings, electronic and vibrational lifetimes, etc. Solution method: Software application based on Density-Functional Theory and Many-Body Perturbation Theory, pseudopotentials, with planewaves, Projector-Augmented Waves (PAW) or wavelets as basis functions. Running time: From less than one second for the simplest tests, to several weeks. The vast majority of the >600 provided tests run in less than 30 seconds. References:[1] http://inac.cea.fr/LSim/BigDFT. [2] http://etsf.eu/index.php?page=standardization. [3] http://www.tddft.org/programs/octopus/wiki/index.php/Libxc. [4] http://www.unidata.ucar.edu/software/netcdf. [5] http://en.wikipedia.org/wiki/MessagePassingInterface. [6] http://www.wannier.org.

Gonze, X.; Amadon, B.; Anglade, P.-M.; Beuken, J.-M.; Bottin, F.; Boulanger, P.; Bruneval, F.; Caliste, D.; Caracas, R.; Côté, M.; Deutsch, T.; Genovese, L.; Ghosez, Ph.; Giantomassi, M.; Goedecker, S.; Hamann, D. R.; Hermet, P.; Jollet, F.; Jomard, G.; Leroux, S.; Mancini, M.; Mazevet, S.; Oliveira, M. J. T.; Onida, G.; Pouillon, Y.; Rangel, T.; Rignanese, G.-M.; Sangalli, D.; Shaltaf, R.; Torrent, M.; Verstraete, M. J.; Zerah, G.; Zwanziger, J. W.

2009-12-01

173

NASA Astrophysics Data System (ADS)

We performed first-principles calculations of multiplet structures and the corresponding ground-state absorption and excited-state absorption spectra for ruby (Cr3+:?-Al2O3) and alexandrite (Cr3+:BeAl2O4) which included lattice relaxation. The lattice relaxation was estimated using the first-principles total energy and molecular-dynamics method of the CASTEP code. The multiplet structure and absorption spectra were calculated using the configuration-interaction method based on density-functional calculations. For both ruby and alexandrite, the theoretical absorption spectra, which were already in reasonable agreement with experimental spectra, were further improved by consideration of lattice relaxation. In the case of ruby, the peak positions and peak intensities were improved through the use of models with relaxations of 11 or more atoms. For alexandrite, the polarization dependence of the U band was significantly improved, even by a model with a relaxation of only seven atoms.

Watanabe, Shinta; Sasaki, Tomomi; Taniguchi, Rie; Ishii, Takugo; Ogasawara, Kazuyoshi

2009-02-01

174

Polarization spectroscopy using short-pulse lasers: Theoretical analysis

NASA Astrophysics Data System (ADS)

The physics of short-pulse polarization spectroscopy (PS) and the diagnostic potential for quantitative measurements of species concentration are investigated by direct numerical integration (DNI) of the time-dependent density matrix equations for a multistate system. The effects of laser power, collision rates, and Doppler broadening on the short-pulse PS signal generation process are investigated by systematically varying these parameters in the numerical calculations. It is found that the use of a short-pulse laser (laser pulse width ?L

Roy, Sukesh; Lucht, Robert P.; Reichardt, Thomas A.

2002-01-01

175

First-principles theory, coarse-grained models, and simulations of ferroelectrics.

CONSPECTUS: A ferroelectric crystal exhibits macroscopic electric dipole or polarization arising from spontaneous ordering of its atomic-scale dipoles that breaks inversion symmetry. Changes in applied pressure or electric field generate changes in electric polarization in a ferroelectric, defining its piezoelectric and dielectric properties, respectively, which make it useful as an electromechanical sensor and actuator in a number of applications. In addition, a characteristic of a ferroelectric is the presence of domains or states with different symmetry equivalent orientations of spontaneous polarization that are switchable with large enough applied electric field, a nonlinear property that makes it useful for applications in nonvolatile memory devices. Central to these properties of a ferroelectric are the phase transitions it undergoes as a function of temperature that involve lowering of the symmetry of its high temperature centrosymmetric paraelectric phase. Ferroelectricity arises from a delicate balance between short and long-range interatomic interactions, and hence the resulting properties are quite sensitive to chemistry, strains, and electric charges associated with its interface with substrate and electrodes. First-principles density functional theoretical (DFT) calculations have been very effective in capturing this and predicting material and environment specific properties of ferroelectrics, leading to fundamental insights into origins of ferroelectricity in oxides and chalcogenides uncovering a precise picture of electronic hybridization, topology, and mechanisms. However, use of DFT in molecular dynamics for detailed prediction of ferroelectric phase transitions and associated temperature dependent properties has been limited due to large length and time scales of the processes involved. To this end, it is quite appealing to start with input from DFT calculations and construct material-specific models that are realistic yet simple for use in large-scale simulations while capturing the relevant microscopic interactions quantitatively. In this Account, we first summarize the insights obtained into chemical mechanisms of ferroelectricity using first-principles DFT calculations. We then discuss the principles of construction of first-principles model Hamiltonians for ferroelectric phase transitions in perovskite oxides, which involve coarse-graining in time domain by integrating out high frequency phonons. Molecular dynamics simulations of the resulting model are shown to give quantitative predictions of material-specific ferroelectric transition behavior in bulk as well as nanoscale ferroelectric structures. A free energy landscape obtained through coarse-graining in real-space provides deeper understanding of ferroelectric transitions, domains, and states with inhomogeneous order and points out the key role of microscopic coupling between phonons and strain. We conclude with a discussion of the multiscale modeling strategy elucidated here and its application to other materials such as shape memory alloys. PMID:25361389

Waghmare, Umesh V

2014-11-18

176

First-principles elastic constants for the hcp transition metals Fe, Co, and Re at high pressure constant tensors for the hcp phases of three transition metals Co, Re, and Fe are computed as functions spectroscopy has been applied up to 25 GPa.2 We investigate the elasticity of three hexagonal transition metals

Stixrude, Lars

177

First-Principles Basin-Hopping for the Structure Determination

. In this respect not only the ground state isomer will be of importance, but potentially all energetically low in detail, using small silicon and copper clusters as illustrative benchmark systems. In order to assess. Introduction 13 I. Theoretical Background 17 2. Describing the Configurational Space of Atomic Clusters 18 2

178

First-principles models of equilibrium tellurium isotope fractionation

NASA Astrophysics Data System (ADS)

In this study, equilibrium mass-dependent isotopic fractionation among representative Te-bearing species is estimated with first-principles thermodynamic calculations. Tellurium is a group 16 element (along with O, S, and Se) with eight stable isotopes ranging in mass from 120Te to 130Te, and six commonly-occurring oxidation states: -II, -I, 0, +II, +IV, and +VI. In its reduced form, Te(-II), tellurium has a unique crystal-chemical role as a bond partner for gold and silver in epithermal and orogenic gold deposits, which likely form when oxidized Te species (e.g., H2TeO3, TeO32-) or perhaps polytellurides (e.g., Te22-) interact with precious metals in hydrothermal solution. Te(IV) is the most common oxidation state at the Earth's surface, including surface outcrops of telluride ore deposits, where tellurite and tellurate minerals form by oxidation. In the ocean, dissolved tellurium tends to be scavenged by particulate matter. Te(VI) is more abundant than Te(IV) in the ocean water (1), even though it is thought to be less stable thermodynamically. This variety of valence states in natural systems and range of isotopic masses suggest that tellurium could exhibit geochemically useful isotope abundance variations. Tellurium isotope fractionations were determined for representative molecules and crystals of varying complexity and chemistry. Gas-phase calculations are combined with supermolecular cluster models of aqueous and solid species. These in turn are compared with plane-wave density functional theory calculations with periodic boundary conditions. In general, heavyTe/lightTe is predicted to be higher for more oxidized species, and lower for reduced species, with 130Te/125Te fractionations as large as 4‰ at 100?C between coexisting Te(IV) and Te(-II) or Te(0) compounds. This is a much larger fractionation than has been observed in naturally occurring redox pairs (i.e., Te (0) vs. Te(IV) species) so far, suggesting that disequilibrium processes may control oxidation and reduction. Se- and S-isotope redox systems also show kinetically-controlled isotopic disequilibrium, and may serve as useful analogues. Among Te(-II) species, fractionations are smaller (< 1‰ at 100?C). (Au,Ag)Te2 minerals (calaverite, krennerite) and (Au,Ag)2Te minerals (petzite, hessite) are expected to have similar isotopic compositions to vapor-phase H2Te, and there appears to be little discernable effect from Ag,Au solid solution. Altaite (PbTe) will have somewhat lower 130Te/125Te. Calculated fractionation factors for gas-phase species like H2Te and TeF6, based on hybrid density functional theory (B3LYP), agree well with earlier estimates (2). For telluride-bearing crystals, cluster models are in qualitative agreement with periodic boundary condition calculations, especially when larger basis sets (e.g., aug-cc-PVDZ-PP) are used; however, cluster models tend to predict higher 130Te/125Te for reasons that are not yet clear. References: 1. Hein et al. (2003) GCA 67:6. 2. Smithers et al. (1968) Can. J. Chem. 46:4.

Haghnegahdar, M. A.; Schauble, E. A.; Fornadel, A. P.; Spry, P. G.

2013-12-01

179

Strain sensitivity and superconducting properties of Nb3Sn from first principles calculations

NASA Astrophysics Data System (ADS)

Using calculations from first principles based on density-functional theory we have studied the strain sensitivity of the A15 superconductor Nb3Sn. The Nb3Sn lattice cell was deformed in the same way as observed experimentally on multifilamentary, technological wires subject to loads applied along their axes. The phonon dispersion curves and electronic band structures along different high-symmetry directions in the Brillouin zone were calculated, at different levels of applied strain, ?, on both the compressive and the tensile side. Starting from the calculated averaged phonon frequencies and electron-phonon coupling, the superconducting characteristic critical temperature of the material, Tc, has been calculated by means of the Allen-Dynes modification of the McMillan formula. As a result, the characteristic bell-shaped Tc versus ? curve, with a maximum at zero intrinsic strain, and with a slight asymmetry between the tensile and compressive sides, has been obtained. These first-principle calculations thus show that the strain sensitivity of Nb3Sn has a microscopic and intrinsic origin, originating from shifts in the Nb3Sn critical surface. In addition, our computations show that variations of the superconducting properties of this compound are correlated to stress-induced changes in both the phononic and electronic properties. Finally, the strain function describing the strain sensitivity of Nb3Sn has been extracted from the computed Tc(?) curve, and compared to experimental data from multifilamentary, composite wires. Both curves show the expected bell-shaped behavior, but the strain sensitivity of the wire is enhanced with respect to the theoretical predictions for bulk, perfectly binary and stoichiometric Nb3Sn. An understanding of the origin of this difference might open potential pathways towards improvement of the strain tolerance in such systems.

De Marzi, G.; Morici, L.; Muzzi, L.; della Corte, A.; Buongiorno Nardelli, M.

2013-04-01

180

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

181

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 antiferromagnetic treatment with a generalized gradient approximation for the electron exchange and correlation functional. 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 recent resonant ultrasound spectroscopy for a cast sample is made.

Soderlind, P; Klepeis, J

2009-02-18

182

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

183

First-Principles Investigation of Li Intercalation Kinetics in Phospho-Olivines

NASA Astrophysics Data System (ADS)

This thesis focuses broadly on characterizing and understanding the Li intercalation mechanism in phospho-olivines, namely LiFePO 4 and Li(Fe,Mn)PO4, using first-principles calculations. Currently Li-ion battery technology is critically relied upon for the operation of electrified vehicles, but further improvements mainly in cathode performance are required to ensure widespread adoption, which in itself requires learning from existing commercial cathode chemistries. LiFePO4 is presently used in commercial Li-ion batteries, known for its rapid charge and discharge capability but with underwhelming energy density. This motivates the three central research efforts presented herein. First, we investigate the modified phase diagram and electrochemical properties of mixed olivines, such as Li(Fe,Mn)PO4, which offer improved theoretical energy density over LiFePO4 (due to the higher redox voltage associated with Mn2+/Mn3+). The Lix(Fe1-yMny)PO4 phase diagram is constructed by Monte Carlo simulation on a cluster expansion Hamiltonian parametrized by first-principles determined energies. Deviations from the equilibrium phase behavior and voltages of pure LiFePO4 and LiMnPO 4 are analyzed and discussed to good agreement with experimental observations. Second, we address why LiFePO4 exhibits superior rate performance strictly when the active particle size is brought down to the nano-scale. By considering the presence of immobile point defects residing in the 1D Li diffusion path, specifically by calculating from first principles both defect formation energies and Li migration barriers in the vicinity of likely defects, the Li diffusivity is recalculated and is found to strongly vary with particle size. At small particle sizes, the contribution from defects is small, and fast 1D Li diffusion is accessible. However, at larger particle sizes (microm scale and above) the contribution from defects is much larger. Not only is Li transport impeded, but it is also less anisotropic in agreement with experiments on large LiFePO4 single crystals. Third, we investigate why LiFePO4 can be charged and discharged rapidly despite having to undergo a first-order phase transition. Conventional wisdom dictates that a system with strong equilibrium Li segregation behavior requires both nucleation and growth in the charge and discharge process, which should impede the overall kinetics. Rather, through first-principles calculations, we determine the minimal energy required to access a non-equilibrium transformation path entirely through the solid solution. Not only does this transformation mechanism require little driving force, but it also rationalizes how a kinetically favorable but nonequilibrium path is responsible for the extremely high rate performance associated with this material. The consequences of a rapid non-equilibrium single-particle transformation mechanism on (dis)charging a multi-particle assembly, as is the case in porous electrodes, are discussed and compared to experimental observations. (Copies available exclusively from MIT Libraries, libraries.mit.edu/docs - docs mit.edu)

Malik, Rahul

184

First Principles Studies of Transport Properties for Palladium Thin-films Loaded with Hydrogen

NASA Astrophysics Data System (ADS)

First Principles methods are used to study the electronic and phonon structures of palladium hydrides (PdH). The electronic structures of Pd thin-films at various H compositions (loadings) are calculated by using both full potential linearized augmented plane wave (FLAPW) and pseudopotential plane wave (PPPW) techniques. The two methods give similar results, which closely match previous theoretical studies. The phonon structures, such as normal modes and their corresponding frequencies, are resolved using the PWSCF package [1], a density-functional perturbation theory (DFPT) method implemented in the PPPW basis. The pseudopotentials adopted in this calculation are of the norm-conversing type and have reasonable transferabiltiy. The electron-phonon coupling constants and phonon linewidths are calculated with PWSCF as well. The theoretical transport properties of PdH, such as resistivity, are then obtained at different loadings and temperatures, and compared with experimental results. The current computation on electron-phonon coupling are also compared with a few previous results in the same and a few related material systems. Reference: [1] Stefano Baroni, Stefano de Gironcoli, Andrea Dal Corso and Paolo Giannozzi, www.sissa.it/cm/PWcodes/.

Luo, Nie; Miley, George; Lipson, Andrei

2003-03-01

185

Point defects engineering in graphene/h-BN bilayer: A first principle study

NASA Astrophysics Data System (ADS)

Point defects engineering in a new type hetero bilayer consisting of graphene and hexagonal boron-nitrogen (h-BN) sheet, including vacancy, substitutional C/B/N doping and the possible combinations of the former two, was theoretically studied using first-principles calculations. The optimized geometry, formation energy, magnetic moment, and electronic property of these systems are discussed. It was found that N vacancy is more likely to form than B vacancy in graphene/h-BN bilayer and their electronic properties exhibit n-type and p-type conductivity, respectively. Divacancy of N and C in hetero bilayer shows high stability and induces direct band gap in up and down spin, respectively. Combined by N substitutional doping in graphene and B vacancy in h-BN layer, this substitution-vacancy combination shows low formation energy and changes the semiconductor property of pristine graphene/h-BN bilayer to metallic. In contrast, the graphene/h-BN bilayer with the combinated defect of C-substitution in B site and C vacancy in graphene shows half-metallic electronic property. The calculated magnetic moments are in reasonable agreement with the available theoretical analysis on atomic charge distribution. This work reveals that the electronic and magnetic properties of graphene/h-BN bilayer can be effectively tuned by above proposed point defects engineering.

Yuan, Jianmei; Wei, Zhe; Zhong, Jianxin; Huang, Yanping; Mao, Yuliang

2014-11-01

186

First-Principles Study on Electronic Structures of Protein Nanotubes

NASA Astrophysics Data System (ADS)

The electronic structures of protein nanotubes (PNTB), whichare formed by the periodical stacking of cyclo-peptide-ring (CPR)rings, are theoretically investigated in terms of the ab initiocalculations. The inter-ring H bonds among the CPRs cause theelectronic interaction when CPRs are periodically stacked. Thisinteraction has a potential to delocalize electrons and holes alongthe tube axis as if the band conduction occurs through the bridging Hbonds. The protonation of this system is also investigated. Themigrated proton (charged H* species) is expected to create theimpurity (acceptor) level in the band gap of the PNTB.

Fukasaku, Katsuhiko; Takeda, Kyozaburo; Shiraishi, Kenji

1998-11-01

187

Deriving the nuclear shell model from first principles

NASA Astrophysics Data System (ADS)

A procedure for calculating microscopically the input for standard shell-model calculations, i.e., the core and single-particle energies plus the two-body effective model-space interactions, is presented and applied to nuclei at the start of the sd-shell. Calculations with the JISP16 and Idaho ?EFT N3LO nucleon-nucleon interactions are performed and yield consistent results, which also are similar to phenomenological results in the sd-shell as well as with other theoretical calculations, utilizing other techniques. All results show only a weak A-dependence.

Barrett, B. R.; Dikmen, E.; Lisetskiy, A. F.; Maris, P.; Shirokov, A. M.; Vary, J. P.

2015-02-01

188

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

189

Kinetics of Hydrogen Abstraction Reaction Class H + H-C(sp3): First-Principles Predictions Using the Reaction Class Transition State Theory Shaowen Zhang and Thanh N. Truong* Henry Eyring Center for Theoretical Chemistry, Department of Chemistry, UniVersity of Utah, 315 S. 1400 E., Room 2020, Salt Lake City

Truong, Thanh N.

190

NASA Astrophysics Data System (ADS)

The authors have presented a detailed investigation on the phase stabilities and electronic properties of ideal stoichiometric platinum carbide (PtC) in the rock-salt (RS) and zinc-blende (ZB) structures under high pressure. Theoretical calculations are performed using the first-principles pseudopotential density functional method, in which we employ the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof form and local density approximation (LDA) of Ceperly and Adler parameterized by Perdew and Zunger together with plane-wave basis sets for expanding the periodic electron density. Through a series of tests, such as the total energy as a function of volume, the Gibbs free energy as a function of pressure, the P-V equation of states, the elastic stabilities, and the electronic band structures of PtC with ZB and RS phases, we have confirmed that the recently synthesized compound PtC is crystallized in the ZB structure at zero pressure and that the RS structure is a high-pressure phase; the phase transition studied from the usual condition of equal enthalpies occurs at the pressures of 46.6 and 46.5 GPa for GGA and LDA calculations, respectively. Our conclusions are consistent with the theoretical prediction obtained from the full-potential linearized augmented plane-wave method, but are reversed with the DAC experimental results and other pseudopotential plane-wave theoretical results. Therefore, the experimental observation of the RS structure in PtC remains a puzzle, and our study indicates that further experimental and theoretical investigations need to be carried out to find the cause of the stability of the PtC.

Sun, Xiao-Wei; Chen, Qi-Feng; Chen, Xiang-Rong; Cai, Ling-Cang; Jing, Fu-Qian

2011-11-01

191

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

192

Enthalpies of formation of magnesium compounds from first-principles calculations

Enthalpies of formation of magnesium compounds from first-principles calculations Hui Zhang t An energetics database of binary magnesium compounds has been developed from first-principles calculations. Introduction Magnesium alloys are of great importance to the industrial world. With a density of 1.741 g/cm3

Chen, Long-Qing

193

02/25/2014 27th CSP Workshop 1 First Principles Modeling of Electrolye Materials

02/25/2014 27th CSP Workshop 1 First Principles Modeling of Electrolye Materials in All-Solid-State"? Computational methods & validation Why are we interested in solid electrolytes? How can "first principles" help? Survey of known solid electrolytes Prediction of new solid electrolytes Study of electrolyte

Holzwarth, Natalie

194

Silicane and germanane: tight-binding and first-principles studies

NASA Astrophysics Data System (ADS)

We present a first-principles and tight-binding model study of silicane and germanane, the hydrogenated derivatives of two-dimensional silicene and germanene. We find that the materials are stable in freestanding form, analyse the orbital composition, and derive a tight-binding model using first-principles calculations to fit the parameters.

Zólyomi, V.; Wallbank, J. R.; Fal'ko, V. I.

2014-06-01

195

First-principles study of anhydrite, polyhalite and carnallite

NASA Astrophysics Data System (ADS)

We report density functional calculations of the structures and properties of anhydrite (CaSO4), polyhalite (K2SO4·MgSO4·2CaSO4·2H2O) and carnallite (KCl·MgCl2·6H2O). Densities of states are systematically investigated and phonon analysis using density functional perturbation theory is performed at constant equilibrium volume for anhydrite and polyhalite in order to derive their isochoric thermal properties. Thermal properties at constant atmospheric pressure are also calculated using the quasi-harmonic approximation. The computed molar entropy and isobaric heat capacity for anhydrite reproduce experimental data up to 800 K to within 3% and 10%, respectively, while further experimental work is needed to assess our theoretical predictions for polyhalite.

Weck, Philippe F.; Kim, Eunja; Jové-Colón, Carlos F.; Sassani, David C.

2014-02-01

196

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

197

Length dependence of electron transport through molecular wires--a first principles perspective.

One-dimensional wires constitute a fundamental building block in nanoscale electronics. However, truly one-dimensional metallic wires do not exist due to Peierls distortion. Molecular wires come close to being stable one-dimensional wires, but are typically semiconductors, with charge transport occurring via tunneling or thermally-activated hopping. In this review, we discuss electron transport through molecular wires, from a theoretical, quantum mechanical perspective based on first principles. We focus specifically on the off-resonant tunneling regime, applicable to shorter molecular wires (first principles theory are discussed, starting with the computational workhorse - density functional theory (DFT), and moving on to many-electron GW methods as well as GW-inspired DFT + Sigma calculations. These different levels of theory are applied in two major computational frameworks - complex band structure (CBS) calculations to estimate the tunneling decay constant, beta, and Landauer-Buttiker transport calculations that consider explicitly the effects of contact geometry, and compute the transmission spectra directly. In general, for the same level of theory, the Landauer-Buttiker calculations give more quantitative values of beta than the CBS calculations. However, the CBS calculations have a long history and are particularly useful for quick estimates of beta. Comparing different levels of theory, it is clear that GW and DFT + Sigma calculations give significantly improved agreement with experiment compared to DFT, especially for the conductance values. Quantitative agreement can also be obtained for the Seebeck coefficient - another independent probe of electron transport. This excellent agreement provides confirmative evidence of off-resonant tunneling in the systems under investigation. Calculations show that the tunneling decay constant beta is a robust quantity that does not depend on details of the contact geometry, provided that the same contact geometry is used for all molecular lengths considered. However, because conductance is sensitive to contact geometry, values of beta obtained by considering conductance values where the contact geometry is changing with the molecular junction length can be quite different. Experimentally measured values of beta in general compare well with beta obtained using DFT + Sigma and GW transport calculations, while discrepancies can be attributed to changes in the experimental contact geometries with molecular length. This review also summarizes experimental and theoretical efforts towards finding perfect molecular wires with high conductance and small beta values. PMID:25407785

Khoo, Khoong Hong; Chen, Yifeng; Li, Suchun; Quek, Su Ying

2015-01-01

198

Temperature-driven phase transitions in SrBi2Ta2O9 from first-principles calculations

NASA Astrophysics Data System (ADS)

The phase transition sequence of SrBi2Ta2O9 is investigated using a shell model with parameters fitted to first-principles calculations. We show that the complex interplay between polar and nonpolar instabilities leads to the presence of two phase transitions, corroborating the existence of an intermediate orthorhombic paraelectric phase. This phase is characterized by the rotation of the TaO6 octahedra around the a axis. We show that this phase can also be detected from the dielectric response of the material. The present approach constitutes a powerful tool for a theoretical prediction of intermediate phases, not yet observed experimentally, in other Aurivillius compounds.

Machado, R.; Sepliarsky, M.; Stachiotti, M. G.

2008-12-01

199

Experimental and theoretical studies have shown that excitonic effects play an important role in the optical properties of conjugated polymers. The optical absorption spectrum of trans-polyacetylene, for example, can be understood as completely dominated by the formation of exciton bound states. We review a recently developed first-principles method for computing the excitonic effects and optical spectrum, with no adjustable parameters. This theory is used to study the absorption spectrum of two conjugated polymers: trans-polyacetylene and poly-phenylene-vinylene(PPV).

Rohlfing, Michael; Tiago, M.L.; Louie, Steven G.

2000-03-20

200

First-principles study of electronic structure, mechanical and optical properties of V4AlC3

We calculated the mechanical properties, electronic structure, theoretical hardness and optical properties of V4AlC3 using the first-principles method. The results show that V4AlC3 shows a better performance of the resistance to shape change and against uniaxial tensions and has a slight anisotropy on elasticity. Moreover, it is more brittle than alpha-Nb4AlC3 and Ta4AlC3. The chemical bonding of V4AlC3 is a

Chenliang Li; Biao Wang; Yuanshi Li; Rui Wang

2009-01-01

201

First-principle study on the chromium doping effect on the crystal structure of metallic VO 2

NASA Astrophysics Data System (ADS)

A theoretical study on the chromium (Cr) doping effect on the crystal structure of metallic VO 2 has been performed by first principles calculation based on local density approximation (LDA). The results are consistent with the experimental observations: the substitution impurity of Cr on cation site (Cr V) defects are created by Cr-doping and the Cr-doping results in the shrink of the volume in the crystal unit cell. The substitution of Cr gives rise to two different V sbnd V distances and charge transfer from V to Cr.

Pan, Mei; Zhong, Hongmei; Wang, Shao wei; Li, Zhifeng; Chen, Xiaoshuang; Lu, Wei

2004-11-01

202

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

203

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

204

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

205

First-principles studies of Al-Ni intermetallic compounds

The structural properties, heats of formation, elastic properties, and electronic structures of Al-Ni intermetallic compounds are analyzed here in detail by using density functional theory. Higher calculated absolute values of heats of formation indicate a very strong chemical interaction between Al and Ni for all Al-Ni intermetallic compounds. According to the computational single crystal elastic constants, all the Al-Ni intermetallic compounds considered here are mechanically stable. The polycrystalline elastic modulus and Poisson's ratio have been deduced by using Voigt, Reuss, and Hill (VRH) approximations, and the calculated ratio of shear modulus to bulk modulus indicated that AlNi, Al{sub 3}Ni, AlNi{sub 3} and Al{sub 3}Ni{sub 5} compounds are ductile materials, but Al{sub 4}Ni{sub 3} and Al{sub 3}Ni{sub 2} are brittle materials. With increasing Ni concentration, the bulk modulus of Al-Ni intermetallic compounds increases in a linear manner. The electronic energy band structures confirm that all Al-Ni intermetallic compounds are conductors. - Graphical abstract: Calculated bulk modulus compared to experimental and other theoretical values for the Al-Ni intermetallic compounds.

Shi Dongmin [School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023 (China); Wen Bin, E-mail: wenbin@dlut.edu.c [School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023 (China); Melnik, Roderick [M2NeT Lab, Wilfrid Laurier University, Waterloo, 75 University Ave. West, Ontario, N2L 3C5 (Canada); Yao Shan; Li Tingju [School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023 (China)

2009-10-15

206

First Principles NMR Study of Fluorapatite under Pressure

NMR is the technique of election to probe the local properties of materials. Herein we present the results of density functional theory (DFT) ab initio calculations of the NMR parameters for fluorapatite (FAp), a calcium orthophosphate mineral belonging to the apatite family, by using the GIPAW method [Pickard and Mauri, 2001]. Understanding the local effects of pressure on apatites is particularly relevant because of their important role in many solid state and biomedical applications. Apatites are open structures, which can undergo complex anisotropic deformations, and the response of NMR can elucidate the microscopic changes induced by an applied pressure. The computed NMR parameters proved to be in good agreement with the available experimental data. The structural evaluation of the material behavior under hydrostatic pressure (from ?5 to +100 kbar) indicated a shrinkage of the diameter of the apatitic channel, and a strong correlation between NMR shielding and pressure, proving the sensitivity of this technique to even small changes in the chemical environment around the nuclei. This theoretical approach allows the exploration of all the different nuclei composing the material, thus providing a very useful guidance in the interpretation of experimental results, particularly valuable for the more challenging nuclei such as 43Ca and 17O. PMID:22770669

Pavan, Barbara; Ceresoli, Davide; Tecklenburg, Mary M. J.; Fornari, Marco

2012-01-01

207

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.

208

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.

209

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

210

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

211

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

212

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

213

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

214

NASA Astrophysics Data System (ADS)

First principles calculations are performed to investigate the structural, electronic, elastic, and thermodynamic properties of the less known PrN compound for various space groups: NaCl(Fm3m(2 2 5)), CsCl(Pm3m (2 2 1)), ZB(F43m(2 1 6)), Wc(P6m2(1 8 7)), and CuAu (P4/mmm (1 2 3)). Our calculation indicates that the NaCl type structure is energetically more stable than the other phases. The calculated lattice parameters are consisted with available theoretical and experimental results. Our band structure calculations show that PrN possessess a semi-metallic character for both with and without spin polarized (SP) cases. The calculated elastic constants satisfy the mechanical stability conditions at all considered pressures and the present values are significantly higher than those of the previous results. The related mechanical properties such as Zener anisotropy factor ( A), Poisson’s ratio ( ?), Young’s modulus ( E), and shear modulus ( C) are also computed for NaCl structure. The temperature/pressure-dependent behaviours of bulk modulus, Debye temperature, heat capacity, thermal expansion coeffient, and V/ V0 ratio estimated within the quasi-harmonic Debye model.

Kocak, B.; Ciftci, Y. O.; Colakoglu, K.; Deligoz, E.

2010-10-01

215

First principle study of elastic and thermodynamic properties of FeB4 under high pressure

NASA Astrophysics Data System (ADS)

The elastic properties, elastic anisotropy, and thermodynamic properties of the lately synthesized orthorhombic FeB4 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 FeB4 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 FeB4 is stable up to 100 GPa. In addition, the calculated B/G ratio reveals that FeB4 possesses brittle nature in the range of pressure from 0 to 100 GPa. The calculated elastic anisotropic factors suggest that FeB4 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 FeB4 are successfully obtained in the present work.

Zhang, Xinyu; Qin, Jiaqian; Ning, Jinliang; Sun, Xiaowei; Li, Xinting; Ma, Mingzhen; Liu, Riping

2013-11-01

216

First-principles calculation on the Curie temperature of GdFeSi

NASA Astrophysics Data System (ADS)

Electronic structure and magnetic properties for GdFeSi have been studied theoretically from a first-principles density functional calculation. The energy band structure is calculated in a local spin density approximation (LSDA)+Hubbard U approach (LSDA+U). For Gd atoms, seven spin up 4f bands are fully occupied, while the spin down 4f hole levels are completely unoccupied and well above the Fermi level. The calculated magnetic moment of Gd is 7.20 ?B, leading to a total magnetization of 7.1 ?B per formula unit due to the small induced negative moments at Fe and Si atoms. The intersites exchange coupling parameters Jij of the Heisenberg model have been derived using a linear-response method based on the calculated band structure. The exchange coupling parameters for the nearest Gd-Gd pair (JGd-Gd) is 0.19 mRy. JGd-Gd decreases rapidly with increasing the intersites distance and shows a Ruderman-Kittel-Kasuya-Yoshida-like oscillation. The estimated Curie temperature is about 145 K from the calculated exchange coupling parameters based on the mean-field approximation, in good agreement with the experimental value (TCexp.=135 K).

Liu, X. B.; Altounian, Z.

2010-05-01

217

First Principles Study of Crystallographic Structure and Elastic Properties of Chromium Nitride

NASA Astrophysics Data System (ADS)

Chromium nitride (CrN) has extreme hardness and corrosion resistance. That is why it is used as thin surface coatings on different details. For the characterization of the properties of the deposited films the elastic constants of the CrN are required. At room temperature CrN is paramagnetic and crystallizes in face cantered cubic crystal structure of NaCl type, however below the Neel temperature the structure undergoes orthorhombic distortion and becomes antiferromagnet with fourth type of magnetic ordering. The first principles calculations, based on density functional theory with ultra-soft pseudo potentials were performed to investigate the crystallographic structure and elastic properties of chromium nitride. Simulations were performed using generalized gradient approximation exchange-correlation functional. Assumed were non-magnetic, ferromagnetic and antiferromagnetic AFM1[110] and AFM2[110] orderings. For the nonmagnetic and ferromagnetic cases the crystallographic structure of CrN remains cubic. For the experimentally observed AFM[110] structure orthorhombic distortion is evident, while for the AFM1[110] there is a tendency for a small tetragonal distortion of the cubic structure. The calculated crystal and magnetic structures, lattice parameters, atomic postitions and elastic constants were compared with the available experimental and theoretical results. The most stable structure is the experimentally observed at low temperatures AFM2[110], however the results for the elastic constants of AFM1[110] are the most suitable for the high temperature structure.

Antonov, V.; Iordanova, I.

2010-01-01

218

First-principles investigation of boron defects in nickel ferrite spinel

NASA Astrophysics Data System (ADS)

The accumulation of boron within the porous nickel ferrite (NiFe2O4, NFO) deposited on nuclear reactor fuel rods is a major technological problem with important safety and economical implications. In this work, the electronic structure of nickel ferrite spinel has been investigated using first-principles methods, and the theoretical results have been combined with experimental data to analyze B incorporation into the spinel structure of NFO. Under thermodynamic solid-solid equilibrium between NFO and atomic reservoirs of Ni and Fe, our calculations predict that the incorporation of B into the NFO structure is unfavorable. The main factors that limit B incorporation are the narrow stability domain of NFO and the precipitation of B2O3, Fe3BO5, and Ni3B2O6 compounds as secondary phases. The B incorporation energies depend sensitively on the electron chemical potential (EF) and the charge state of the defect. In n-type NFO, the most stable defect is the Ni vacancy VNi2- while in p-type material lowest the formation energy belongs to the interstitial B occupying a tetrahedrally coordinated site BT2+. Because of these limiting conditions it is more thermodynamically favorable for B to form secondary phases with Fe, Ni and O (e.g. B2O3, Fe3BO5, and Ni3B2O6) than it is to form point defects in NFO.

Rák, Zs.; O'Brien, C. J.; Brenner, D. W.

2014-09-01

219

Lattice structures and electronic properties of CIGS/CdS interface: First-principles calculations

NASA Astrophysics Data System (ADS)

Using first-principles calculations within density functional theory, we study the atomic structures and electronic properties of the perfect and defective (2VCu+InCu) CuInGaSe2/CdS interfaces theoretically, especially the interface states. We find that the local lattice structure of (2VCu+InCu) interface is somewhat disorganized. By analyzing the local density of states projected on several atomic layers of the two interfaces models, we find that for the (2VCu+InCu) interface the interface states near the Fermi level in CuInGaSe2 and CdS band gap regions are mainly composed of interfacial Se-4p, Cu-3d and S-3p orbitals, while for the perfect interface there are no clear interface states in the CuInGaSe2 region but only some interface states which are mainly composed of S-3p orbitals in the valance band of CdS region.

Tang, Fu-Ling; Liu, Ran; Xue, Hong-Tao; Lu, Wen-Jiang; Feng, Yu-Dong; Rui, Zhi-Yuan; Huang, Min

2014-07-01

220

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

221

A semiconductive superhard FeB? phase from first-principles calculations.

An oP10-FeB4 phase [H. Gou, et al., Phys. Rev. Lett., 2013, 111, 157002] was recently synthesized based on previous theoretical predictions. In this study, a high-pressure phase of FeB4 (tP10-FeB4) was proposed through first-principles calculations. The tP10-FeB4 structure, which contains two formula units per unit cell, belongs to tetragonal symmetry with the space group P42/nmc. The boron atoms in tP10-FeB4 are present as tetrahedron configurations. Enthalpies as a function of pressure indicate that this new phase is probable to achieve through a phase transition from the oP10-FeB4 phase above ?65.9 GPa. The softening of acoustic phonon at T points in the Brillouin zone may be the driving force behind the phase transition. Further analyses reveal that the tP10-FeB4 phase is a potential superhard semiconductor. PMID:25204967

Wang, Qianqian; Zhang, Qian; Hu, Meng; Ma, Mengdong; Xu, Bo; He, Julong

2014-10-28

222

First principles DFT study of dye-sensitized CdS quantum dots

NASA Astrophysics Data System (ADS)

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., Cd16S16, 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; Kishor, Shyam; Singh, Kh. S.; Josefesson, Ida; Odelius, Michael; Ramaniah, Lavanya M.

2014-04-01

223

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

224

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

225

Two-dimensional arsenic monolayer sheet predicted from first-principles

NASA Astrophysics Data System (ADS)

Using first-principles calculations, we investigate the two-dimensional arsenic nanosheet isolated from bulk gray arsenic. Its dynamical stability is confirmed by phonon calculations and molecular dynamics analyzing. The arsenic sheet is an indirect band gap semiconductor with a band gap of 2.21 eV in the hybrid HSE06 functional calculations. The valence band maximum (VBM) and the conduction band minimum (CBM) are mainly occupied by the 4p orbitals of arsenic atoms, which is consistent with the partial charge densities of VBM and CBM. The charge density of the VBM G point has the character of a ? bond, which originates from p orbitals. Furthermore, tensile and compressive strains are applied in the armchair and zigzag directions, related to the tensile deformations of zigzag and armchair nanotubes, respectively. We find that the ultimate strain in zigzag deformation is 0.13, smaller than 0.18 of armchair deformation. The limit compressive stresses of single-layer arsenic along armchair and zigzag directions are ?4.83 GPa and ?4.76 GPa with corresponding strains of ?0.15 and ?0.14, respectively. Projected supported by the Henan Joint Funds of the National Natural Science Foundation of China (Grant Nos. U1304612 and U1404608), the National Natural Science Foundation of China (Grant Nos. 51374132 and 11404175), the Special Fund for Theoretical Physics of China (Grant No. 11247222), and Nanyang Normal University Science Foundation, China (Grant Nos. ZX2012018 and ZX2013019).

Pu, Chun-Ying; Ye, Xiao-Tao; Jiang, Hua-Long; Zhang, Fei-Wu; Lu, Zhi-Wen; He, Jun-Bao; Zhou, Da-Wei

2015-03-01

226

Phase Transition and Thermodynamic Properties of Magnesium Fluoride by First Principles

NASA Astrophysics Data System (ADS)

The structural stabilities, phase transitions and thermodynamic properties of MgF2 under high pressure and temperature are investigated by first-principles calculations based on plane-wave pseudopotential density functional theory method within the local density approximation. The calculated lattice parameters of MgF2 in all four phases under zero pressure and zero temperature are in good agreement with the existing experimental data and other theoretical results. Our results demonstrate that MgF2 undergoes a series of structural phase transitions from rutile (P42/mnm)?CaCl2-type (Pnnm)?modified fluorite (Pa-3)?cotunnite (Pnam) under high pressure and the obtained transition pressures are in fairly good agreement with the experimental results. The temperature-dependent volume and thermodynamic properties of MgF2 in the rutile phase at 0 GPa are presented and the thermodynamic properties of MgF2 in the rutile, CaCl2-type, modified fluorite and cotunnite phases at 300 K are also predicted using the quasi-harmonic approximation model (QHA) and the quasi-harmonic Debye model (QHD), respectively. Moreover, the partial density of states and the electronic density of the four phases under the phase transition are also investigated.

Zhang, Tian; Cheng, Yan; Lv, Zhen-Long; Ji, Guang-Fu; Gong, Min

2014-12-01

227

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

228

First-Principles Investigation of Thermophysical Properties of Cubic ZrC Under High Pressure

NASA Astrophysics Data System (ADS)

With a motivation to understand the effect of pressure on the thermophysical properties of a transition metal carbide, zirconium carbide (ZrC) in rock salt (RS) and CsCl phases, a systematic study of the thermodynamic functions with pressure for ZrC in both phases is performed. First-principles theoretical calculations are used, based on density functional perturbation theory within the generalized gradient approximation and with a quasi-harmonic approximation. The results demonstrate that the free and internal energies are greater while the specific heat at constant volume and entropy is smaller for the RS phase of ZrC than for the CsCl phase. The pressure significantly affects the thermodynamic functions. The results also demonstrate that the effect of increasing pressure on ZrC is the same as that of decreasing temperature. The k-point convergence of the phonon spectrum at zero pressure is also analyzed, which may be useful for investigating the pressure-induced dynamic instabilities in the transition metal carbide, ZrC.

Rathod, Nikita; Gupta, Sanjeev K.; Shinde, Satyam; Jha, Prafulla K.

2013-10-01

229

Body-centered superhard BC2N phases from first principles

NASA Astrophysics Data System (ADS)

Body-centered BC2N deduced from the unit cell of the recently predicted body-centered carbon [F. J. Ribeiro , Phys. Rev. B 74, 172101 (2006)] are studied with first-principles pseudopotential density functional method. The structural, electronic, and mechanical properties are investigated for 11 possible atomic configurations of body-centered BC2N . Our results show that the sp3 -bonded body-centered BC2N phases have lower density than the previously investigated sp3 -bonded zinc-blende BC2N , wurtzite BC2N , and chalcopyrite BC2N . The struc-A and struc-B composed of the maximum numbers of C-C and B-N bonds have the lowest total energy among the investigated body-centered BC2N structures. Their calculated bulk moduli are 305 and 309GPa , respectively. The theoretical Vickers hardness of the body-centered BC2N is over 60GPa , indicating that it is a potential superhard material with the hardness comparable to cubic boron nitride.

Luo, Xiaoguang; Guo, Xiaoju; Xu, Bo; Wu, Qinghua; Hu, Qianku; Liu, Zhongyuan; He, Julong; Yu, Dongli; Tian, Yongjun; Wang, Hui-Tian

2007-09-01

230

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

231

Ordered structures in rhenium binary alloys from first-principles calculations.

Rhenium is an important alloying agent in catalytic materials and superalloys, but the experimental and computational data on its binary alloys are sparse. Only 6 out of 28 Re transition-metal systems are reported as compound-forming. Fifteen are reported as phase-separating, and seven have high-temperature disordered ? or ? phases. Comprehensive high-throughput first-principles calculations predict stable ordered structures in 20 of those 28 systems. In the known compound-forming systems, they reproduce all the known compounds and predict a few unreported ones. These results indicate the need for an extensive revision of our current understanding of Re alloys through a combination of theoretical predictions and experimental validations. The following systems are investigated: AgRe(?), AuRe(?), CdRe(?), CoRe, CrRe(?), CuRe(?), FeRe, HfRe, HgRe(?), IrRe, MnRe, MoRe, NbRe, NiRe, OsRe, PdRe, PtRe, ReRh, ReRu, ReSc, ReTa, ReTc, ReTi, ReV, ReW(?), ReY, ReZn(?), and ReZr ((?) = systems in which the ab initio method predicts that no compounds are stable). PMID:21142072

Levy, Ohad; Jahnátek, Michal; Chepulskii, Roman V; Hart, Gus L W; Curtarolo, Stefano

2011-01-12

232

Theoretical electronic structure techniques are used to analyze widely different systems from Si clusters to transition metal solids and surfaces. For the Si clusters, first principles density functional methods are used to investigate Si{sub N} for N=2-8. Goal is to understand the different types of bonding that can occur in such small clusters where the atomic coordination differs substantially from tetrahedral bonding; such uncoordinated structures can test approximate models of Si surfaces. For the transition metal systems, non-self-consistent electronic structure methods are used to understand the driving force for surface relaxations. In-depth analysis of results is presented and physical basis of surface relaxation within the theory is discussed. Limitations inherent in calculations of metal surface relaxation are addressed. Finally, in an effort to understand approximate methods, a novel non-self- consistent density functional electronic structure method is developed that is about 1000 times faster than more sophisticated methods; this method is tested for various systems including diatomics, mixed clusters, surfaces, and bulk lattices.

Susan, S.

1993-04-30

233

First principles modeling of structure and properties of multi-component amorphous steels

NASA Astrophysics Data System (ADS)

Amorphous steels (iron based metallic glasses) exhibit unique physical properties that have opened the venue to new commercial applications as well as renewed the interest in this field. To provide a realistic description of the three dimensional structures and associate the coordinated environment of atoms to physical properties, first principles quantum mechanical molecular dynamics (MD) simulations along with the pair density function (PDF) analysis of neutron and X-ray diffraction data were used by way of considering chemical effects, ionic size ratio and concentration. Direct comparison of the simulated atomic structures obtained from MD with the local atomic structures determined experimentally show a very good agreement between the two, indicating that this theoretical approach can be applied towards simulating multi-component alloys. The atomic coordinates were used to develop the building blocks of cluster-like structures that give rise to the short-range order. The diffusion rates of different atom species were modeled at several temperatures that allowed us to describe the quenching process. In addition, the bulk moduli as a function of chemical composition were simulated and showed a very good agreement with the ones obtained experimentally.

Kazimirov, Viatcheslav; Louca, Despina; Widom, Michael

2008-03-01

234

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

235

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

236

NASA Astrophysics Data System (ADS)

Using the concept of surface stress, we developed a model that is able to predict Young's modulus of nanowires as a function of nanowire diameters from the calculated properties of their surface and bulk materials. We took both equilibrium strain effect and surface stress effect into consideration to account for the geometric size influence on the elastic properties of nanowires. In this work, we combined first-principles density functional theory calculations of material properties with linear elasticity theory of clamped-end three-point bending. Furthermore, we applied this computational approach to Ag, Au, and ZnO nanowires. For both Ag and Au nanowires, our theoretical predictions agree well with the experimental data in the literature. For ZnO nanowires, our predictions are qualitatively consistent with some of experimental data for ZnO nanostructures. Consequently, we found that surface stress plays a very important role in determining Young's modulus of nanowires. Our finding suggests that the elastic properties of nanowires could be possibly engineered by altering the surface stress of their lateral surfaces.

Wang, Guofeng; Li, Xiaodong

2008-12-01

237

The idea of first-principles methods is to determine the properties of materials by solving the basic equations of quantum mechanics and statistical mechanics. With such an approach, one can, in principle, predict the ...

Ceder, Gerbrand

238

First principles high throughput screening of oxynitrides for water-splitting photocatalysts

In this paper, we present a first principles high throughput screening system to search for new water-splitting photocatalysts. We use the approach to screen through nitrides and oxynitrides. Most of the known photocatalytic ...

Wu, Yabi

2013-01-01

239

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

240

First-principle simulation of shock-wave experiments for aluminum

NASA Astrophysics Data System (ADS)

We present first-principle calculations of shock Hugoniots, release isentropes and sound speed measurements on the principle Hugoniot of aluminum. Most of the results are in good agreement with experiments and a wide-range equation of state.

Minakov, Dmitry V.; Levashov, Pavel R.; Khishchenko, Konstantin V.

2012-03-01

241

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

242

First-principles lattice dynamics, thermodynamics, and elasticity of Cr2O3 , Huazhi Fang a

First-principles lattice dynamics, thermodynamics, and elasticity of Cr2O3 Yi Wang a, , Huazhi Fang a , Chelsey L. Zacherl a , Zhigang Mei a , Shunli Shang a , Long-Qing Chen a , Paul D. Jablonski b , Zi

Chen, Long-Qing

243

Turtles All The Way Down: # A CleanSlate, GroundUp, FirstPrinciples

discuss the overriding requirement guiding our paradigm: that ``first principles'' re flects the only is the value of a nuclear re actor or a missile guidance system The economic argument fails when the value

Bishop, Matt

244

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

discuss the overriding requirement guiding our paradigm: that"first principles"re- flects the only real to sell fake Rolexes and Prada handbags [33], what is the value of a nuclear re- actor or a missile

Peisert, Sean

245

Comment on ``Band structure engineering of graphene by strain: First-principles calculations''

In their first-principles calculations of the electronic band structure of graphene under uniaxial strain, Gui, Li, and Zhong, [Phys. Rev. B 78, 075435 (2008)] have found opening of band gaps at the Fermi level. This finding is in conflict with the tight-binding description of graphene which is closed gap for small strains. In this Comment, we present first-principles calculations which

M. Farjam; H. Rafii-Tabar

2009-01-01

246

Epitaxial-strain-induced multiferroicity in SrMnO3 from first principles

In the first-principles search for new ferromagnetic-ferroelectric multiferroics, one key indicator is the softening of the lowest frequency polar phonon with ferromagnetic ordering from a paraelectric antiferromagnetic bulk state. In a first-principles survey of the phonon dispersions of a wide range of magnetic perovskites, we identified SrMnO3 as a promising candidate system. We find that a ferromagnetic-ferroelectric phase is stabilized

Jun Hee Lee; Karin M. Rabe

2010-01-01

247

ERIC Educational Resources Information Center

Details the following thoughts on directing for the theater: select the play, read the play, read the play again, ask questions, discover the spine, form a concept, cast the play, listen to the actors, make it move, make it spontaneous, make it real, look for obstacles, make it relevant, and make it important. (PRA)

Cappelletti, John

1989-01-01

248

Application of First Principles Ni-Cd and Ni-H2 Battery Models to Spacecraft Operations

NASA Technical Reports Server (NTRS)

The conclusions of the application of first principles model to spacecraft operations are: the first principles of Bi-phasic electrode presented model provides an explanation for many behaviors on voltage fading on LEO cycling.

Timmerman, Paul; Bugga, Ratnakumar; DiStefano, Salvador

1997-01-01

249

Using first-principles calculations for understanding MgB2

NASA Astrophysics Data System (ADS)

Electronic structure methods have been the key to understand the peculiar properties of the multi-band superconductor MgB2. After briefly reviewing the current state of our understanding of the material we will discuss some of the more recent observations on Al or C doped MgB2.We will demonstrate, based on first-principles band structure calculations and Eliashberg theory, that the experimentally observed decrease of the critical temperature Tc of Al and C doped MgB2 samples can be understood mainly in terms of a band filling effect due to the electron doping by Al and C. A simple scaling of the electron-phonon coupling constant ? by the variation of the density of states (DOS) as function of electron doping is sufficient to capture qualitatively the observed behavior. Using the virtual crystal approximation to account for the electron doping we calculate the change of the DOS due to doping. The expected hardening of the E2g phonon frequency at ? is also well reproduced by our calculations using this approximation. These two contributions together, the change of DOS and phonon frequency, reproduce the observed dependence of Tc on doping concentration astonishingly well not using any free parameter to fit the data.Further, we also explain the long standing open question of the experimental observation of a nearly constant ? gap as function of doping by a compensation of the effect of band filling and interband scattering. Both effects together generate a nearly constant ? gap and shift the merging point of both gaps to higher doping concentrations, resolving the discrepancy between experiment and theoretical predictions based on interband scattering only.The author acknowledges many fruitful collaborations and discussions on this exciting topic with O.K. Andersen, I.I. Mazin, A.A. Golubov, O.V. Dolgov, O. Jepsen and R.K. Kremer over the past years.

Kortus, Jens

2005-03-01

250

We expect that systematic and seamless computational upscaling and downscaling for modeling, predicting, or optimizing material and system properties and behavior with atomistic resolution will eventually be sufficiently accurate and practical that it will transform the mode of development in the materials, chemical, catalysis, and Pharma industries. However, despite truly dramatic progress in methods, software, and hardware, this goal remains elusive, particularly for systems that exhibit inherently complex chemistry under normal or extreme conditions of temperature, pressure, radiation, and others. We describe here some of the significant progress towards solving these problems via a general multiscale, multiparadigm strategy based on first-principles quantum mechanics (QM), and the development of breakthrough methods for treating reaction processes, excited electronic states, and weak bonding effects on the conformational dynamics of large-scale molecular systems. These methods have resulted directly from filling in the physical and chemical gaps in existing theoretical and computational models, within the multiscale, multiparadigm strategy. To illustrate the procedure we demonstrate the application and transferability of such methods on an ample set of challenging problems that span multiple fields, system length- and timescales, and that lay beyond the realm of existing computational or, in some case, experimental approaches, including understanding the solvation effects on the reactivity of organic and organometallic structures, predicting transmembrane protein structures, understanding carbon nanotube nucleation and growth, understanding the effects of electronic excitations in materials subjected to extreme conditions of temperature and pressure, following the dynamics and energetics of long-term conformational evolution of DNA macromolecules, and predicting the long-term mechanisms involved in enhancing the mechanical response of polymer-based hydrogels. PMID:21243466

Jaramillo-Botero, Andres; Nielsen, Robert; Abrol, Ravi; Su, Julius; Pascal, Tod; Mueller, Jonathan; Goddard, William A

2012-01-01

251

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

252

NASA Astrophysics Data System (ADS)

The structural determination, thermodynamic, mechanical, dynamic and electronic properties of 4d transitional metal diborides MB2 (M = Y-Ag) are systematically investigated by first-principles within the density functional theory (DFT). For each diboride, five structures are considered, i.e. AlB2-, ReB2-, OsB2-, MoB2- and WB2-type structures. The calculated lattice parameters are in good agreement with the previously theoretical and experimental studies. The formation enthalpy increases from YB2 to AgB2 in AlB2-type structure (similar to MoB2- and WB2-type). While the formation enthalpy decreases from YB2 to MoB2, reached minimum value to TcB2, and then increases gradually in ReB2-type structure (similar to OsB2-type), which is consistent with the results of the calculated density of states. The structural stability of these materials relates mainly on electronegative of metals, boron structure and bond characters. Among the considered structures, TcB2-ReB2 (TcB2-ReB2 represents TcB2 in ReB2-type structure, the same hereinafter) has the largest shear modulus (248 GPa), and is the hardest compound. The number of electrons transferred from metals to boron atoms and the calculated densities of states (DOS) indicate that each diboride is a complex mixture of metallic, ionic and covalent characteristics. Trends are discussed.

Ying, Chun; Zhao, Erjun; Lin, Lin; Hou, Qingyu

2014-10-01

253

First Principles Study of the Li-Bi-F Phase Diagram and Bismuth Fluoride Conversion Reactions with Lithium Citation Doe, Robert E. et al. "First Principles Study of the Li--Bi--F Phase Diagram and Bismuth matters. #12;First Principles Study of the LiÂBiÂF Phase Diagram and Bismuth Fluoride Conversion Reactions

Ceder, Gerbrand

254

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

255

We present results of gas phase cluster and liquid water simulations from the recently determined VRT(ASP-W)III water dimer potential energy surface (the third fitting of the Anisotropic Site Potential with Woermer dispersion to vibration-rotation-tunnelling data). 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 with those from ab initio molecular dynamics, other potentials of 'spectroscopic' accuracy and with experiment. The results herein represent the first time to the authors' knowledge that a 'spectroscopic' potential surface is able to correctly model condensed phase properties of water. PMID:15664895

Goldman, Nir; Leforestier, Claude; Saykally, R J

2005-02-15

256

and circular dichroism (CD) of a cyclic dipeptide, cyclo(L-Pro-D- Tyr), in the 185-300 nm region and tyrosine, i.e., cyclo(L-Pro-D-Tyr) [see Figure 1]. Its circular dichroism (CD) spectrum has been measured the vibrations, whereas visible spectra probe specific chromophores such as porphyrins or retinal. There are two

Mukamel, Shaul

257

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

258

NASA Astrophysics Data System (ADS)

Size-selected Wn clusters can be deposited firmly on a graphite (0001) surface using a novel technique, where the positive ions (of the same metal atom species) embedded on the graphite surface by ion implantation, act as anchors. The size selected metal clusters can then soft land on this anchored surface m [Hayakawa et al., 2009]. We have carried out a systematic theoretical study of the adsorption of Wn (n = 1-6) clusters on anchored graphite (0001) surface, using state-of-art spin-polarized density functional approach. In our first-principles calculations, the graphite (0001) surface has been suitably modeled as a slab separated by large vacuum layers. Wn clusters bond on clean graphite (0001) surface with a rather weak Van-der-Waals interaction. However, on the anchored graphite (0001) surface, the Wn clusters get absorbed at the defect site with a much larger adsorption energy. We report here the results of our first-principles investigation of this supported Wn cluster system, along with their reactivity trend as a function of the cluster size (n).

Barman, Sonali; Das, G. P.; Kawazoe, Y.

2013-12-01

259

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

260

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

261

NASA Astrophysics Data System (ADS)

The mean free paths (MFPs) of energy carriers are of critical importance to the nano-engineering of better thermoelectric materials. Despite significant progress in the first-principles–based understanding of the spectral distribution of phonon MFPs in recent years, the spectral distribution of electron MFPs remains unclear. In this work, we compute the energy-dependent electron scatterings and MFPs in silicon from first principles. The electrical conductivity accumulation with respect to electron MFPs is compared to that of the phonon thermal conductivity accumulation to illustrate the quantitative impact of nanostructuring on electron and phonon transport. By combining all electron and phonon transport properties from first principles, we predict the thermoelectric properties of the bulk and nanostructured silicon, and find that silicon with 20 nm nanograins can result in a higher than five times enhancement in their thermoelectric figure of merit as the grain boundaries scatter phonons more significantly than that of electrons due to their disparate MFP distributions.

Qiu, Bo; Tian, Zhiting; Vallabhaneni, Ajit; Liao, Bolin; Mendoza, Jonathan M.; Restrepo, Oscar D.; Ruan, Xiulin; Chen, Gang

2015-03-01

262

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

263

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

264

First-principles study of spin-dependent transport through graphene/BNC/graphene structure.

: First-principles study on the electronic structure and transport property of the boron nitride sheet (BNC) structure, in which a triangular graphene flake surrounded by a hexagonal boron nitride sheet, is implemented. As the graphene flake becomes small and is more isolated by the boron nitride region, the magnetic ordering of the flake increases. When the BNC structure is connected to the graphene electrodes, the spin-polarized charge-density distribution appears only at the triangular graphene flake region, and the electronic structure of the graphene electrode is not spin polarized. First-principles transport calculation reveals that the transport property of the BNC structure is spin dependent. PMID:23634806

Ota, Tadashi; Ono, Tomoya

2013-01-01

265

NASA Astrophysics Data System (ADS)

First-principles calculations of structural, electronic, optical, elastic, mechanical properties, and Born effective charges of monoclinic HfO2 are performed with the plane-wave pseudopotential technique based on the density-functional theory. The calculated structural properties are consistent with the previous theoretical and experimental results. The electronic structure reveals that monoclinic HfO2 has an indirect band gap. The analyses of density of states and Mulliken charges show mainly covalent nature in Hf-O bonds. Optical properties, including the dielectric function, refractive index, extinction coefficient, reflectivity, absorption coefficient, loss function, and optical conductivity each as a function of photon energy are calculated and show an optical anisotropy. Moreover, the independent elastic constants, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, compressibility, Lamé constant, sound velocity, Debye temperature, and Born effective charges of monoclinic HfO2 are obtained, which may help to understand monoclinic HfO2 for future work.

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

2014-04-01

266

NASA Astrophysics Data System (ADS)

The ?' phase precipitate in Mg-Gd alloy system has been investigated by means of first-principles calculation within the generalized gradient approximation. The lattice parameters are determined theoretically by structural optimization of full relaxation, and the Mg 7Gd is found to be energetically more stable compared with the Mg 15Gd from the calculated formation energy. The nine independent elastic constants are calculated, indicating the proposed Mg 15Gd structure in literature is mechanically unstable. Then the polycrystalline bulk modulus B, Young's modulus E, shear modulus G, Poisson ratio ? of Mg 7Gd are gained by the Voigt-Reuss-Hill (VRH) approximation. The ductility and plasticity, especially elastic anisotropy are discussed in details. Based on the electronic density of states and charge density distribution, the covalent bonding and metallic bonding are exhibited in Mg 7Gd compound. Last, the Debye temperature is also calculated for the investigation in the future.

Chen, Ping; Li, Dong-Lin; Yi, Jian-Xiong; Wen, Li; Tang, Bi-Yu; Peng, Li-Ming; Ding, Wen-Jiang

2009-12-01

267

NASA Astrophysics Data System (ADS)

The generalized stacking fault energies and surface energies for AgRE (RE=Sc, Tm, Dy, Tb, Ce) intermetallics with B2 structure have been investigate d using the first principle calculations. The Perdew-Burke-Ernzerhof exchange-correlation functional for the generalized-gradient-approximation is used and the electron-ion interaction is described by the full potential frozen-core projector augmented wave. The generalized stacking fault energy along <1 0 0>, <1 1 0> and <1 1 1> directions in {1 1 0} plane have been calculated. The ductility and brittleness of AgRE have been discussed based on the Rice criterion by using the ratio between surface energies and the unstable stacking fault energies. The ideal shear strength and the theoretical cleavage strength are also presented.

Liu, Lili; Wu, Xiaozhi; Wang, Rui; Wu, Shaohua; Feng, Huifang

2012-11-01

268

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

269

NASA Astrophysics Data System (ADS)

First-principle calculations based on density functional theory are adopted to investigate the structural, electronic and thermodynamic properties of BiF3-type Mg3Gd compound. Our results for the equilibrium structural parameters are consistent with experimental values and other theoretical results. The single-crystal elastic constants, polycrystalline elastic moduli, Poisson's ratio and anisotropy factor are determined using the Voigt-Reuss-Hill approximation. Estimated Cauchy pressure and the G/B relationship indicate that Mg3Gd is mechanically stable and brittle. In addition, the electronic structure is studied within the generalized gradient approximation (GGA) and local spin density approximation (LSDA) along with Hubbard energy. The calculations associated with phonon properties confirm the dynamical stability of BiF3-type structure. Finally, the dependences of normalized volume, bulk modulus and thermodynamic properties of Mg3Gd are obtained via the quasi-harmonic Debye model.

Liu, Yong; Hu, Wen-Cheng; Li, De-Jiang; Zeng, Xiao-Qin; Xu, Chun-Shui; Yang, Xiang-Jie

2014-01-01

270

NASA Astrophysics Data System (ADS)

First-principles absorption spectra calculated within the time- dependent local-density approximation for Sin (n=20-28) clusters reveal that prolate and compact clusters have distinct shape signatures, but no clear size dependence for a given shape. The shape dependence and size independence of the spectra and most of the peak positions and intensities can be explained remarkably well within the classical Mie theory, developed for light absorption by metallic particles using the bulk dielectric function of Si. Moreover, the experimental spectrum of Si21 is in very good agreement with the theoretical spectrum of the prolate cluster, which is lower in energy than the compact one at this size. Idrobo, Jackson, Yang, and Ogut, Phys. Rev. B 74, 153410 (2006)

Jackson, Koblar; Idrobo, Juan C.; Ogut, Serdar; Yang, Mingli

2007-03-01

271

First-principles Study of Structural Properties of MgxZn1-xO ternary alloys

NASA Astrophysics Data System (ADS)

First-principles calculations have been carried out to investigate the structural and electronic properties of MgxZn1-xO ternary alloys. The calculations are performed using the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory(DFT). We conclude that the structural properties of these materials, in particular the composition dependence on the lattice constant and the band gap is found to be linear. The a-axis length in the lattice gradually increases, while the c-axis length decreases with the increase in Mg doping concentration, and be corresponded with the Vegard's law linear rule. The lattice parameters of the MgxZn1-xO ternary alloys are consistent with experimental data and other theoretical results. We found in the conduction band portion, the Mg 2p 2s states are moved to high energy region as the Mg content increases, so the band gap increases.

Gao, J.; Zhao, G. J.; Liang, X. X.; Song, T. L.

2015-01-01

272

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

273

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

274

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

275

Theoretical simulations and ultrafast pump-probe laser spectroscopy experiments were used to study photosynthetic pigment-protein complexes and antennae found in green sulfur bacteria such as Prosthecochloris aestuarii, Chloroflexus aurantiacus, and Chlorobium tepidum. The work focused on understanding structure-function relationships in energy transfer processes in these complexes through experiments and trying to model that data as we tested our theoretical assumptions with calculations. Theoretical exciton calculations on tubular pigment aggregates yield electronic absorption spectra that are superimpositions of linear J-aggregate spectra. The electronic spectroscopy of BChl c/d/e antennae in light harvesting chlorosomes from Chloroflexus aurantiacus differs considerably from J-aggregate spectra. Strong symmetry breaking is needed if we hope to simulate the absorption spectra of the BChl c antenna. The theory for simulating absorption difference spectra in strongly coupled photosynthetic antenna is described, first for a relatively simple heterodimer, then for the general N-pigment system. The theory is applied to the Fenna-Matthews-Olson (FMO) BChl a protein trimers from Prosthecochloris aestuarii and then compared with experimental low-temperature absorption difference spectra of FMO trimers from Chlorobium tepidum. Circular dichroism spectra of the FMO trimer are unusually sensitive to diagonal energy disorder. Substantial differences occur between CD spectra in exciton simulations performed with and without realistic inhomogeneous distribution functions for the input pigment diagonal energies. Anisotropic absorption difference spectroscopy measurements are less consistent with 21-pigment trimer simulations than 7-pigment monomer simulations which assume that the laser-prepared states are localized within a subunit of the trimer. Experimental anisotropies from real samples likely arise from statistical averaging over states with diagonal energies shifted by in homogeneous broadening and as such, are quite sensitive to diagonal energy disorder. The experimental anisotropies exhibit strong oscillations with {approximately}220 fs period for certain wavelengths in one-color absorption difference experiments. The oscillations only appear when the laser pulse spectrum overlaps both of the lowest-energy groups of exciton levels clustered near 815 and 825 nm. Results suggest that the oscillations stem from quantum beating between exciton levels, rather than from coherent nuclear motion.

Buck, D.R.

2000-09-12

276

NASA Astrophysics Data System (ADS)

We have studied the local properties of single Pt atoms adsorbed on hydroxylated TiO2(110 ) -(1 ×1 ) by combining noncontact atomic force microscopy (nc-AFM) and first-principles calculations. Room-temperature high-resolution nc-AFM images for the most frequently observed contrast modes reveal bright and elongated protrusions that can be traced back to the Pt atoms, and that are centered on the fivefold coordinated titanium rows, confined between two bridging oxygen rows. These observations are in line with the theoretical results, as the lowest energy sites for the Pt atom on the TiO2(110 ) surface are in the neighborhood of the titanium rows, and high energy barriers have to be overcome to displace the Pt atom over the bridging oxygen rows. Single Pt atoms can be distinguished from H adsorbates (OH defects) due to their characteristic shape and binding site and, because they appear as the brightest surface features in all of the contrast modes. Force spectroscopy data over the protrusion and hole imaging modes and the corresponding tip-sample forces, simulated with O and OH terminated TiO2 nanoclusters, provide an explanation for this puzzling result in terms of the intrinsic strength of the interaction with the Pt adatom and the adatom and tip apex relaxations induced by the tip-sample interaction. These imaging mechanisms can be extended to other electropositive metal dopants and support the use of nc-AFM not only to characterize their adsorption structure but also to directly probe their chemical reactivity.

Fernández-Torre, Delia; Yurtsever, Ayhan; Onoda, Jo; Abe, Masayuki; Morita, Seizo; Sugimoto, Yoshiaki; Pérez, Rubén

2015-02-01

277

NASA Astrophysics Data System (ADS)

Systematic analysis of the energy level scheme and ground state absorption of the Cr 4+ ion in Li 2CaSiO 4 crystal was performed using the exchange charge model of the crystal field [B.Z. Malkin, in: A.A. Kaplyanskii, B.M. Macfarlane (Eds.), Spectroscopy of Solids Containing Rare-earth Ions, North-Holland, Amsterdam, 1987, pp. 33-50] and recently developed first-principles approach to the analysis of the absorption spectra of impurity ions in crystals based on the discrete variational multielectron (DVME) method [K. Ogasawara, T. Iwata, Y. Koyama, T. Ishii, I. Tanaka, H. Adachi, Phys. Rev. B 64 (2001) 115413]. Using the former method, the values of parameters of crystal field acting on the Cr 4+ ion valence electrons were calculated using the Li 2CaSiO 4 crystal structure data. Energy levels of the Cr 4+ ion obtained after diagonalizing the crystal field Hamiltonian are in good agreement with those obtained from the experimental spectra. The latter method is based on the numerical solution of the Dirac equation; therefore, all relativistic effects are automatically considered. As a result, energy level scheme of Cr 4+ and its absorption spectra in both polarizations were calculated, assigned and compared with experimental data; energy of the lowest charge transfer transition was evaluated and compared with theoretical predictions for the CrO44- complex available in the literature. The main features of the experimental spectra shape are reproduced well by the calculations. By performing analysis of the molecular orbitals (MO) population, it was shown that the covalent effects play an important role in formation of the spectral properties of Cr 4+ ion in the considered crystal.

Brik, M. G.; Ogasawara, K.

2007-11-01

278

The New Face of Rhodium Alloys: Revealing Ordered Structures from First Principles

The New Face of Rhodium Alloys: Revealing Ordered Structures from First Principles Ohad Levy-mail: stefano@duke.edu Abstract: The experimental and computational data on rhodium binary alloys is sparse, and platinum), rhodium has a high catalytic potential for a variety of chemical reactions.1,2 It is used

Hart, Gus

279

Hafnium binary alloys from experiments and first principles Ohad Levy a,b

]. Hafnium is used extensively as an alloying element in nickel-, niobium- and tantalum-based superalloys titanium, tungsten and molybdenum alloys, where it forms second-phase disper- sions (with carbonHafnium binary alloys from experiments and first principles Ohad Levy a,b , Gus L.W. Hart c

Hart, Gus

280

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

281

First principles calculations of the dark current in quantum well infrared photodetectors

A first principles model of the dark current in quantum well infrared photodetectors has been derived using a quantum mechanical approach. This is based on a combined representation of the field-induced and thermionic emission components of the dark current. It is argued that the contribution of sequential tunnelling to the dark current is affected significantly by the presence of interface

Nkaepe E. I. Etteh; Paul Harrison

2002-01-01

282

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 · Comments on solid electrolytes · Overview of LiPON family · Computational methods · Simulations-Universit¨at Bochum, Germany 216th ECS Meeting, Vienna 1 #12;Solid vs liquid electrolytes in Li ion batteries Solid

Holzwarth, Natalie

283

Assessing the efficiency of first-principles basin-hopping sampling Ralf Gehrke and Karsten Reuter

into the bottlenecks and governing factors for the sampling efficiency, as well as simple rules of thumb for nearAssessing the efficiency of first-principles basin-hopping sampling Ralf Gehrke and Karsten Reuter into a local PES minimum. Rather than exploring E Rm , BH approaches concen- trate therefore on the transformed

284

First-principles computational studies of the torsional potential energy surface of the sec-butyl

to investigate the torsional potential energy surface (PES) of the sec- butyl radical. All the wave function words: sec-butyl, torsional potential energy surface, density functional theory, wave function theoryFirst-principles computational studies of the torsional potential energy surface of the sec

Wang, Yan Alexander

285

First-principles study of lattice dynamics and thermodynamics of osmium under pressure

We have performed the first-principles linear response calculations of the lattice dynamics, thermal equation of state and thermodynamical properties of hcp Os metal by using the plane-wave pseudopotential method. The thermodynamical properties are deduced from the calculated Helmholtz free energy by taking into account the electronic contribution and lattice vibrational contribution. The phonon frequencies at Gamma point are consistent with

Bo Liu; Mu Gu; Xiao-Lin Liu; Shi-Ming Huang; Chen Ni; Ze-Ren Li; Rong-Bo Wang

2010-01-01

286

Ordered magnesium-lithium alloys: First-principles predictions Richard H. Taylor

Ordered magnesium-lithium alloys: First-principles predictions Richard H. Taylor Department 2010 Magnesium-lithium Mg-Li alloys are among the lightest structural materials. Although considerable Emerging technologies increasingly depend on the pro- duction of ultralight-weight materials. Magnesium

Hart, Gus

287

ERIC Educational Resources Information Center

Research has shown that when Merrill's First Principles of Instruction are used as part of an instructional strategy, student learning increases. Several articles describe these principles of instruction, including specific methods for implementing this theory. However, because teachers and designers often have little time to design instruction,…

Gardner, Joel

2010-01-01

288

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

289

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

290

We present a plane-wave ultrasoft pseudopotential implementation of first-principle molecular dynamics, which is well suited to model large molecular systems containing transition metal centers. We describe an efficient strategy for parallelization that includes special features to deal with the augmented charge in the contest of Vanderbilt's ultrasoft pseudopotentials. We also discuss a simple approach to model molecular systems with a

P. Giannozzi; F. de Angelis; R. Car

2004-01-01

291

First principle study of cobalt impurity in bcc Fe with Cu precipitates

DOI 10.1007/s10853-012-6884-2 #12;on hardening in ferrite. Cobalt addition was experimentally foundFirst principle study of cobalt impurity in bcc Fe with Cu precipitates N. I. Medvedeva · A. S 2012 Ó Springer Science+Business Media, LLC 2012 Abstract The addition of cobalt was experimentally

Medvedeva, Julia E.

292

First principle study of cobalt impurity in bcc Fe with Cu precipitates

-012-6884-2 #12;on hardening in ferrite. Cobalt addition was experimentally found to result in alloy softeniFirst principle study of cobalt impurity in bcc Fe with Cu precipitates N. I. Medvedeva · A. S The addition of cobalt was experimentally observed to increase the strength and impact toughness of Cu

Medvedeva, Julia E.

293

Copyright 2006 by Rich Janow Page 1 A First-Principles Model for Estimating Atmospheric Nuclear

a simple, first principles model for nuclear weapon effects pertinent to Command, Control 26, 2006 Page 2 REFERENCES 1. Glasstone, S. and P. Dolan, eds., 'The Effects of Nuclear Weapons', 3rd edition, USDOD/USDOE (1977). 2. Brode, H., 'A Review of Nuclear Weapons Effects', Ann. Rev. Nuc. Sci. 18

Janow, Rich

294

ccsd00001055 First principles study of the solubility of Zr in Al

ccsd00001055 (version 1) : 22 Jan 2004 First principles study of the solubility of Zr in Al (Dated: January 22, 2004) The experimental solubility limit of Zr in Al is well-known. Al3Zr has a stable the solubility limit of Zr in Al for the stable as well as the metastable phase diagrams. The formation energies

295

FRONTIERS ARTICLE Towards the complete understanding of water by a first-principles

FRONTIERS ARTICLE Towards the complete understanding of water by a first-principles computational Nijmegen, Heijendaalseweg 135, 6525 AJ Nijmegen, The Netherlands a r t i c l e i n f o Article history phases are still not fully understood. In 2005, Science magazine included the problem of the structure

296

We present a unified framework for a first-principles calculation of the electric force acting on dielectric or metallic nanospheres suspended in a dielectric host and subject to a uniform external electric field. This framework is based on the spectral representation of the local electric field in a composite medium. The quasi-static (or \\

David J. Bergman; Xiangting Li; Kwangmoo Kim; David Stroud

2005-01-01

297

We will present our recent first principles calculation modeling work on carbon nanotubes (CNT) and copper wires for Interconnect applications. In particular we have calculated the ballistic transport properties of nanotubes based on their density of states and band structures, and compared with that of copper wires of similar dimension. By using Ohm's law and Landauer Formalism, we computed the

Yu Zhou; Yiming Zhang; Subbalakshmi Sreekala; Pulickel Ajayan; Saroj Nayak

2007-01-01

298

First-principles studies of NiTa intermetallic compounds , Bin Wen b,n

First-principles studies of NiÂTa intermetallic compounds Yi Zhou a , Bin Wen b,n , Yunqing Ma properties, and electronic structures of NiÂTa intermetallic compounds are investigated in detail based on density functional theory. Our results indicate that all NiÂTa intermetallic compounds calculated here

Melnik, Roderick

299

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

300

Adhesion of Copper and Alumina from First Principles Xiao-Gang Wang and John R. Smith

Adhesion of Copper and Alumina from First Principles Xiao-Gang Wang and John R. Smith Delphi of the Al2O3(0001)/Cu(111) interface have been studied by density-functional theory. The oxygen interfacial energies as a function of oxygen partial pressures. In particular, see Fig. 4 of Zhang et al.13

301

Neutral self-defects in a silica model: A first-principles study

We present a statistical study of silicon and oxygen neutral defects in a silica glass model. This work is performed following two complementary approaches: first-principles calculations and empirical potential molecular dynamics. We show that the defect formation energies and structures are distributed and that the energy distributions are correlated with the local stress before the defect formation. Combining defect energies

L. Martin-Samos; Y. Limoge; J.-P. Crocombette; G. Roma; N. Richard; E. Anglada; E. Artacho

2005-01-01

302

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

303

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

304

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

305

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

306

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

307

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

308

The modeling of dispersion interactions in density functional theory (DFT) is commonly performed using an energy correction that involves empirically fitted parameters for all atom pairs of the system investigated. In this study, the first-principles-derived dispersion energy from the effective fragment potential (EFP) method is implemented for the density functional theory (DFT-D(EFP)) and Hartree-Fock (HF-D(EFP)) energies. Overall, DFT-D(EFP) performs similarly to the semiempirical DFT-D corrections for the test cases investigated in this work. HF-D(EFP) tends to underestimate binding energies and overestimate intermolecular equilibrium distances, relative to coupled cluster theory, most likely due to incomplete accounting for electron correlation. Overall, this first-principles dispersion correction yields results that are in good agreement with coupled-cluster calculations at a low computational cost. PMID:25651435

Guidez, Emilie B; Gordon, Mark S

2015-03-12

309

NASA Astrophysics Data System (ADS)

The method of homogeneous deformation is combined with first-principles total-energy calculations on determining third-order elastic constants and internal relaxation for monolayer graphene. We employ density functional theory (DFT) within generalized-gradient-approximation (GGA). The elastic constants are obtained from a polynomial fitted to the calculations of strain-energy and strain-stress relations. Our results agree well with recent calculations by DFT calculations, tight-binding atomistic simulations, and experiments with an atomic force microscope. The internal relaxation displacement has also been determined from ab initio calculations. The details of internal lattice relaxation by first principles are basically consistent with the previous molecular dynamics (MD) simulation. But for tiny deformation, there is an anomalous region in which the behavior of internal relaxation is backward action. In addition, we have also demonstrated that the symmetry of the relationship between the internal displacement and the infinitesimal stains can be satisfied.

Wang, Rui; Wang, Shaofeng; Wu, Xiaozhi; Liang, Xiao

2010-08-01

310

Structures and magnetic properties of Co-Zr-B magnets studied by first-principles calculations

The structures and magnetic properties of the Co-Zr-B alloys near the Co5Zr composition were studied using adaptive genetic algorithm and first-principles calculations to guide further experimental effort on optimizing their magnetic performances. Through extensive structural searches, we constructed the contour maps of the energetics and magnetic moments of the Co-Zr-B magnet alloys as a function of composition. We found that the Co-Zr-B system exhibits the same structural motif as the "Co11Zr2" polymorphs, which plays a key role in achieving high coercivity. Boron atoms can either substitute selective cobalt atoms or occupy the interstitial sites. First-principles calculation shows that the magnetocrystalline anisotropy energies can be significantly improved through proper boron doping.

Zhao, Xin; Nguyen, Manh Cuong; Wang, Cai-Zhuang; Ho, Kai-Ming

2015-01-01

311

NASA Astrophysics Data System (ADS)

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 Cu12Sb4S13 , an earth-abundant thermoelectric with strong phonon-phonon interactions that limit the room-temperature ?L to values near the amorphous limit.

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

2014-10-01

312

First-principles based multiscale model of piezoelectric nanowires with surface effects

NASA Astrophysics Data System (ADS)

A continuum model of nanowires incorporating surface piezoelectricity is proposed which extends the electric enthalpy energy with surface terms. The corresponding equations are solved by a numerical method using finite elements technique. A methodology is introduced to compute the surface piezoelectric coefficients by first-principles calculations through the Berry phase theory. We provide the e33s, e31s, and e15s piezoelectric coefficients of (101¯0) surfaces for hexagonal wurtzite nanowires made of GaN, ZnO, and AlN. The effective piezoelectric coefficient along the axis of the nanowire is found to increase when the diameter decreases, for the three studied materials. Finally, the solution of the continuum model is compared with large-size first-principles calculations on piezoelectric nanowires.

Hoang, M.-T.; Yvonnet, J.; Mitrushchenkov, A.; Chambaud, G.

2013-01-01

313

First principles calculations of MgH2 and MgH2–Ti were presented to investigate the influence of titanium on the properties of hydrogen sorption of magnesium hydrides. The heat of formation of MgH2–Ti was enhanced compared with that of MgH2. The bonding between magnesium and hydrogen was weakened, while a stronger interaction between titanium and hydrogen exists if titanium is in solution of

Y Song; Z. X Guo; R Yang

2004-01-01

314

In this study, we present the results of our first-principles calculations of\\u000athe band structure, density of states and the Born effective charge tensors for\\u000athe ferroelectric (ground state B1a1) and paraelectric (I4\\/mmm) phases of\\u000abismuth titanate. The calculations are done using the generalized gradient\\u000aapproximation (GGA) as well as the local density approximation (LDA) of the\\u000adensity functional theory.

Amritendu Roy; Rajendra Prasad; Sushil Auluck; Ashish Garg

2009-01-01

315

In this study, we present the results of our first-principles calculations of the band structure, density of states and the Born effective charge tensors for the ferroelectric (ground state B 1a 1) and paraelectric (I4\\/mmm) phases of bismuth titanate. The calculations are done using the generalized gradient approximation (GGA) as well as the local density approximation (LDA) of the density

Amritendu Roy; Rajendra Prasad; Sushil Auluck; Ashish Garg

2010-01-01

316

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

317

Magnetic properties of C-doped Zn12O12 clusters: First-principles study

First-principles calculations have been performed to study the magnetic properties of Zn12O12 clusters doped with one or two C atoms at the O site. The results show that one C introduces a total moment of 2?B, which mainly comes from the spin-polarized C-2p states in the band gap. The ferromagnetic coupling occurs only at the (0, 1) doping configuration, in

Q. J. Wang; J. B. Wang; X. L. Zhong; Q. H. Tan; Y. C. Zhou

2011-01-01

318

First-principles study of the surfaces of zirconia A. Christensen* and Emily A. Carter

-1569 Received 24 February 1998 We have studied the surfaces of zirconia (ZrO2) by first-principles calculations-monoclinic phase transition in small ZrO2 particles. S0163-1829 98 07635-8 I. INTRODUCTION Zirconia (ZrO2 also been synthesized,6 which may allow ZrO2 to be used in, e.g., mi- croelectromechanical systems

Carter, Emily A.

319

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

320

NASA Astrophysics Data System (ADS)

Many multiscale solid state phenomena involve the transport of chemical species over large distances. This is true of crack growth in corrosive environments as well as diffusional phase transformations. Often these phenomena occur over time scales that are too long for a direct atomistic simulation. Instead continuum methods that draw on phenomenological kinetic parameters such as diffusion coefficients or thermodynamic response functions such as informed cohesive zone models are necessary. A link between first principles atomistic methods and macroscopic kinetic parameters or response functions can be made with statistical mechanical coarse-graining techniques. For multicomponent crystalline solids, this involves integrating out fast degrees of freedom to generate a coarse-grained first-principles lattice model Hamiltonian that is suited for calculating both thermodynamic and kinetic properties, the latter with the help of Green-Kubo methods. We illustrate this approach by describing (i) a first principles calculation of diffusion coefficients in non-dilute alloys, essential input for continuum simulations of diffusional phase transformations and (ii) a first principles derivation of a cohesive zone model in the presence of highly mobile impurities. Cohesive zone models are used to describe the response of a solid ahead of the crack tip in continuum simulations of crack growth. We will show that for some systems, stress induced phase transformations can occur along the cohesive zone above a critical impurity chemical potential. The stress-induced transformation is accompanied by a saturation of the cohesive zone region with impurities and leads to a dramatic reduction of critical stress for decohesion.

van der Ven, Anton

2004-03-01

321

Band Structures of Metal-Oxide Capped Graphene: A First Principles Study

We perform a first-principles calculation based on density functional theory to investigate the interface between single layer graphene and metal oxides. Our study reveals that the monolayer graphene becomes semiconducting by single crystal SiO2 and Al2O3 contact, with energy gaps to ~ 0.9 and ~ 1.8eV, respectively. We find the gap originates from the breakage of pi bond integrity, whose

Han Liu; Qing-Qing Sun; Lin Chen; Yan Xu; Shi-Jin Ding; Wei Zhang; Shi-Li Zhang

2010-01-01

322

Band structure engineering of graphene by strain: First-principles calculations

We have investigated the electronic structure of graphene under different planar strain distributions using the first-principles pseudopotential plane-wave method and the tight-binding approach. We found that graphene with a symmetrical strain distribution is always a zero band-gap semiconductor and its pseudogap decreases linearly with the strain strength in the elastic regime. However, asymmetrical strain distributions in graphene result in opening

Gui Gui; Jin Li; Jianxin Zhong

2008-01-01

323

Band-structure topologies of graphene: Spin-orbit coupling effects from first principles

The electronic band structure of graphene in the presence of spin-orbit coupling and transverse electric field is investigated from first principles using the linearized augmented plane-wave method. The spin-orbit coupling opens a gap of 24mueV (0.28 K) at the K(K') point. It is shown that the previously accepted value of 1mueV , coming from the sigma-pi mixing, is incorrect due

M. Gmitra; S. Konschuh; C. Ertler; C. Ambrosch-Draxl; J. Fabian

2009-01-01

324

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

325

First-principles calculations of hydrogen in bulk GaAs

We present the results of first-principles calculations of the properties of neutral (H0) and charged (H+ and H-) hydrogen in bulk GaAs. The equilibrium sites are determined, and the electronic properties for the equilibrium positions are studied. H+ behaves as a deep donor and prefers to stay in a high valence charge region which includes the bond center. H- behaves

L. Pavesi; P. Giannozzi

1991-01-01

326

A first-principles derived parametrization for the adiabatic bond charge model

We fit the parameters of the adiabatic bond charge model to both, phonon frequencies and atomic displacements obtained from first-principles calculations. In the case of GaAs the new version of the fit yields a much better agreement with experimental (neutron-scattering) data. In addition we show that the AlAs dispersion relations are correctly obtained within the mass approximation. The improvement of

L. Colombo; P. Giannozzi

1995-01-01

327

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

328

First-principles prediction of mechanical properties of gamma-boron

The structural and mechanical properties of gamma-B28 are investigated using first-principles density functional calculations. The single-crystal elastic constants calculations show that gamma-B28 is mechanically stable under ambient conditions. The predicted bulk and shear moduli of gamma-B28 are comparable to those of boron suboxide, suggesting that gamma-B28 can be a superhard material. We also obtained the ideal tensile strength for gamma-B28

Chao Jiang; Zhijun Lin; Jianzhong Zhang; Yusheng Zhao

2009-01-01

329

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

330

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

331

In this paper a methodology is proposed for bioreactor batch-to-batch optimization based on hybrid first principles\\/artificial neural network models. The method does not require the knowledge of the kinetics, which are modelled with neural networks. The reliability of the neural network components is monitored with a cluster-based technique during the optimization procedure. The optimization is realized numerically with a genetic

R. Oliveira; A. Cunha; J. Clemente; M. J. T. Carrondo

2004-01-01

332

First-principles approach to electrical transport in atomic-scale nanostructures

We present a first-principles numerical implementation of Landauer formalism for electrical transport in nanostructures characterized down to the atomic level. The novelty and interest of our method lie essentially on two facts. First of all, it makes use of the versatile GAUSSIAN98 code, which is widely used within the quantum chemistry community. Second, it incorporates the semi-infinite electrodes in a

J. J. Palacios; A. J. Pérez-Jiménez; E. Louis; E. Sanfabián; J. A. Vergés

2002-01-01

333

First principles total energy study of NbCr{sub 2} + V Laves phase ternary system

The C15 NbCr{sub 2} + V Laves phase ternary system is studied by using a first-principles, self-consistent, full-potential total energy method. Equilibrium lattice parameters, cohesive energies, density of states and formation energies of substitutional defects are calculated. Results of all these calculations show that in the C15 NbCr{sub 2} + V compounds, V atoms substitute Cr atoms only.

Ormeci, A. [Koc Univ., Istanbul (Turkey); Chen, S.P.; Wills, J.M.; Albers, R.C. [Los Alamos National Lab., NM (United States)

1999-04-01

334

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

335

Optical absorption spectra of intermediate-size silver clusters from first principles

Optical absorption spectra of the three lowest-energy isomers of Agn (n=10,12-20) are investigated from first principles within the time-dependent local-density approximation (TDLDA). The computed spectra are found to be generally in good agreement with the available experimental data. The analyses of the spectra indicate that the d electrons of Agn clusters in this size range have a significant (70%-80%) contribution

Kopinjol Baishya; Juan C. Idrobo; Serdar Ögüt; Mingli Yang; Koblar Jackson; Julius Jellinek

2008-01-01

336

First-principles absorption spectra of Cun (n=2-20) clusters

Optical absorption spectra for the computed ground state structures of copper clusters (Cun, n=2-20) are investigated from first principles using time-dependent density functional theory in the adiabatic local density approximation (TDLDA). The results are compared with available experimental data, existing calculations, and with results from our previous computations on silver and gold clusters. The main effects of d electrons on

Kopinjol Baishya; Juan C. Idrobo; Serdar Ögüt; Mingli Yang; Koblar A. Jackson; Julius Jellinek

2011-01-01

337

First-principles isomer-specific absorption spectra of Ag11

The atomic structures, static polarizabilities, and optical absorption spectra of eight low-energy isomers of Ag11 are investigated from first principles within static and time-dependent density functional theory. The energies of all eight fall within a range of ˜0.3eV in width. It is the spectrum of the lowest-energy isomer that exhibits the best overall agreement with the available measured spectra. The

Juan C. Idrobo; Serdar Ögüt; Karoly Nemeth; Julius Jellinek; Riccardo Ferrando

2007-01-01

338

Quasiparticle gaps and exciton Coulomb energies in Si nanoshells: First-principles calculations

Quasiparticle gaps and exciton Coulomb energies of H-passivated spherical Si nanoshells are computed using first-principles DeltaSCF method and selectively comparing to GW computations. We find that the quasiparticle gap of a nanoshell depends on both its inner radius R1 (weakly) and outer radius R2 (strongly). These dependences on R1 and R2 are mostly consistent with electrostatics of a metallic shell.

Kimberly Frey; Juan C. Idrobo; Murilo L. Tiago; Fernando A Reboredo; Serdar Ögüt

2009-01-01

339

First principles absorption spectra of Cu{sub n} (n = 2 - 20) clusters

Optical absorption spectra for the computed ground state structures of copper clusters (Cu{sub n}, n = 2-20) are investigated from first principles using time-dependent density functional theory in the adiabatic local density approximation (TDLDA). The results are compared with available experimental data, existing calculations, and with results from our previous computations on silver and gold clusters. The main effects of

K. Baishya; J. C. Idrobo; S. Ogut; M. Yang; K. A. Jackson; J. Jellinek

2011-01-01

340

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

341

First-Principles Calculations of Elastic Constants of Superconducting MgB2

NASA Astrophysics Data System (ADS)

The five independent elastic constants of superconducting MgB2 are obtained using the first-principles plane wave method with the new relativistic analytic pseudopotential of the Hartwigsen-Goedecker-Hutter (HGH) scheme in the frame of local density approximation. The dependences of bulk modulus on temperature and pressure are also obtained. It is suggested that the HGH-type pseudopotentials are successful in investigating the ground-state mechanical properties of any solids.

Guo, Hua-Zhong; Chen, Xiang-Rong; Zhu, Jun; Cai, Ling-Cang; Gao, Jie

2005-07-01

342

Multiscale modeling approach for calculating grain-boundary energies from first principles

A multiscale modeling approach is proposed for calculating energies of tilt-grain boundaries in covalent materials from first principles over an entire misorientation range for given tilt axes. The method uses energies from density-functional calculations for a few key structures as input into a disclination structural-units model. This approach is demonstrated by calculating energies of {l_angle}001{r_angle}-symmetrical tilt-grain boundaries in diamond. {copyright} {ital 1998} {ital The American Physical Society}

Shenderova, O.A.; Brenner, D.W. [North Carolina State University, Raleigh, North Carolina 27695-7907 (United States)] [North Carolina State University, Raleigh, North Carolina 27695-7907 (United States); Nazarov, A.A. [Institute for Metals Superplasticity Problems, Russian Academy of Sciences, 450001Ufa (Russia)] [Institute for Metals Superplasticity Problems, Russian Academy of Sciences, 450001Ufa (Russia); Romanov, A.E. [Ioffe Physico-Technical Institute, Russian Academy of Sciences, 194021St. Petersburg (Russia)] [Ioffe Physico-Technical Institute, Russian Academy of Sciences, 194021St. Petersburg (Russia); Yang, L.H. [Lawrence Livermore National Laboratory, Livermore, California 94551 (United States)] [Lawrence Livermore National Laboratory, Livermore, California 94551 (United States)

1998-02-01

343

First-principles study of ferroelectric domain walls in multiferroic bismuth ferrite

We present a first-principles density-functional study of the structural, electronic, and magnetic properties of the ferroelectric domain walls in multiferroic BiFeO3 . We find that domain walls in which the rotations of the oxygen octahedra do not change their phase when the polarization reorients are the most favorable and of these, the 109° domain wall centered around the BiO plane

Axel Lubk; S. Gemming; N. A. Spaldin

2009-01-01

344

Raman and infrared spectra of multiferroic bismuth ferrite from first principles

The entire zone-center phonon spectrum of the R3c ferroelectric antiferromagnetic phase of bismuth ferrite is computed using a first-principles approach based on density functional theory. Two phonon modes exhibiting eigendisplacement vectors that strongly overlap with the atomic distortions taking place at the ferroelectric structural phase transition are identified and give support to a transition with displacive character. Both Raman and

P. Hermet; M. Goffinet; J. Kreisel; Ph. Ghosez

2007-01-01

345

First-principles calculations of self-interstitial defect structures and diffusion paths in silicon

A first-principles pseudopotential study of neutral self-interstitial defects in silicon is reported, together with calculations for Pandey's concerted exchange mechanism for self- diffusion. The energies and structures of the fully relaxed hexagonal, tetrahedral, split- h110i, 'caged' (Clar k S J and Ackland G J 1997 Phys. Rev. B 56 47), split-h100i, and bond-centred interstitials are calculated using supercells with up

R J Needs

1999-01-01

346

Structural, electronic and magnetic properties of six 3d transition metals (TM=V, Cr, Mn, Fe, Co and Ni) linear monoatomic chains adsorbed on the (5,5) boron nitride nanotube (BNNT) at five different sites have been investigated by first-principle calculations. The results indicate all TM chains can be spontaneously adsorbed on the outer surface of the BNNT. The stable adsorption sites are

You Xie; Jian-Min Zhang; Yi-Ping Huo

2011-01-01

347

Using the first-principles calculations, we investigate the structural and electronic properties of the armchair silicene nanoribbons (ASiNRs) with a monovacancy or a divacancy. We find that either a monovacancy or a parallel oriented divacancy changes a direct semiconductor ASiNR to an indirect one, while a slanting oriented divacancy changes it to a metallic character. However, neither a monovacancy nor a

Yu-Ling Song; Yan Zhang; Jian-Min Zhang; Dao-Bang Lu; Ke-Wei Xu

2011-01-01

348

Phase-Dependent Photocatalytic Ability of TiO2: A First-Principles Study

Abstract: The electronic properties of defected TiO2 were investigated using the first-principles calculations based on density functional theory and generalized gradient approximation. Three typical defects, oxygen vacancy, titanium interstitial, and titanium vacancy, were considered in three TiO2 polymorphs, anatase, rutile, and brookite, respectively. Our calculations demonstrated that defect band is formed by removing oxygen atom from or inserting interstitial Ti

Hui Pan; Baohua Gu; Zhenyu Zhang

2009-01-01

349

First-principle study of electronic structure and optical properties of 3 CBC 4N

The 3C-BC4N, as a kind of superhard material with many potential practical applications, is studied using first-principle calculations, evaluating the structural parameters, charge transfers, bond populations, band structures, density of states, and optical properties. All the calculations are performed after geometric optimization starting from a six-atom, trigonal, and unit-cell structure. The results indicate the following: all the configurations are metastable;

M. J. Tang; D. W. He; L. He

2011-01-01

350

The effects of hydroxyl groups on Ca adsorption on rutile surfaces: a first-principles study

Hydroxyl groups on titanium surfaces have been believed to play an important role in absorbing Ca in solution, which is crucial\\u000a in the formation of bioactive calcium phosphates both in vitro and in vivo. CASTEP, a first-principles density functional\\u000a theory (DFT) code, was employed to investigate Ca adsorption on various rutile (110) surfaces in order to clarify how hydroxyl\\u000a groups

Xiong Lu; Hong-ping Zhang; Yang Leng; Liming Fang; Shuxin Qu; Bo Feng; Jie Weng; Nan Huang

2010-01-01

351

First-principles study of Ag adatoms on the Si(111)-3×3Ag surface

We have performed first-principles calculations for a model system with the 3×3 periodicity in which a Ag adatom per unit cell is adsorbed upon the Si(111)-3×3-Ag surface, whereby discussing the atomic and electronic structures of the Si(111)-21×21-(Ag+Ag) and -(Ag+Au) surfaces. It has been found that the adatom prefers to be located upon Ag triangles rather than upon Si trimers of

Hideaki Aizawa; Masaru Tsukada

1999-01-01

352

First principles determination of the phase boundaries of high-pressure polymorphs of silica

We perform first principles calculations of three ordered high-pressure polymorphs of silica to investigate their phase equilibrium: stishovite, CaCl2 and ?-PbO2. We use the density-functional linear response theory to calculate lattice dynamical properties and the quasiharmonic approximation to determine the high temperature phase boundaries. We obtain positive and almost parallel Clapeyron slopes for both the stishovite-CaCl2 and the CaCl2-?-PbO2 phase

Taku Tsuchiya; Jun Tsuchiya

2004-01-01

353

High pressure behavior of AlAs nanocrystals: the first-principle study

In this study, the first-principle density functional approach has been used to analyze the pressure-induced structural stability and phase transformation in AlAs nanocrystals. This study includes the stability analysis of AlAs nanocrystals in their B4-, B3-, B1- and B2-type phases, and we observed that the B3-type phase is the most stable. We also observed the structural transformations in AlAs nanocrystals

Anurag Srivastava; Neha Tyagi

2012-01-01

354

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

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)

C. Bercegeay; S. Bernard

2005-01-01

355

First-principle study of electronic structure and optical properties of 3CBC4N

The 3C-BC4N, as a kind of superhard material with many potential practical applications, is studied using first-principle calculations, evaluating the structural parameters, charge transfers, bond populations, band structures, density of states, and optical properties. All the calculations are performed after geometric optimization starting from a six-atom, trigonal, and unit-cell structure. The results indicate the following: all the configurations are metastable;

M. J. Tang; D. W. He; L. He

2011-01-01

356

High pressure behavior of AlAs nanocrystals: the first-principle study

In this study, the first-principle density functional approach has been used to analyze the pressure-induced structural stability and phase transformation in AlAs nanocrystals. This study includes the stability analysis of AlAs nanocrystals in their B4-, B3-, B1- and B2-type phases, and we observed that the B3-type phase is the most stable. We also observed the structural transformations in AlAs nanocrystals

Anurag Srivastava; Neha Tyagi

2011-01-01

357

A first-principles study of Group IV dimer chains on Si(100)

First-principles calculations of Pb and Sn dimer chains on Si(100) are performed up to 0.5 ML coverage, including the possibility of mixed PbSi and SnSi dimer chains. Simulated STM images for pure Pb and Sn dimer chains are found to have a better agreement with experimental observations. In order to understand whether the STM images are influenced by the dynamics

Tzu-Liang Chan; C. Z. Wang; Zhong-Yi Lu; K. M. Ho

2005-01-01

358

Topotactic valence manipulation to design strontium manganate: a first principle study

First principle periodic calculation was used to determine the feasibility of topotactic exchange and stability of bivalent metal cations inside lithium manganate, which help solid-state chemists to rational design and construction of new-nonmolecular materials. Among the bivalent metal ions strontium-incorporated lithium manganate shows better performance. The location and orientation of the metal cations are also monitored. Projected density of state

Abhijit Chatterjee; Yoshio Onodera; Takeo Ebina; Takashi Iwasaki

2002-01-01

359

A first-principles study of nitrogen- and boron-assisted platinum adsorption on carbon nanotubes

We have performed first-principles calculations to investigate the origin of adsorption of platinum on nitrogen- and boron-doped carbon nanotubes (CNTs). Our calculation results reveal that both nitrogen- and boron-doped CNTs can assist the reactivity of platinum adsorption on the CNT surface, although the detailed mechanisms are very different. For nitrogen-doped CNTs, the enhanced adsorption results from activation of the nitrogen-neighboring

Yu-Hung Li; Ting-Hsiang Hung; Chun-Wei Chen

2009-01-01

360

First-principles study of structural distortions in frustrated antiferromagnet alpha-NaMnO2

First-principles study on structural distortions of the frustrated antiferromagnet alpha-NaMnO2 has been performed by confining the distortions within ab plane of the unit cell. The calculated ratios of lattice parameters, a\\/b , for both triclinic and monoclinic phases are quite close to the experimental values. The stability of antiferromagnetic triclinic phase compared to the monoclinic phase, which cannot be interpreted

Z. W. Ouyang; B. Wang

2010-01-01

361

A detailed first-principles study of the structural and vibrational properties of crystalline silver azide under hydrostatic pressure of 0–500GPa has been performed with density functional theory in the generalized gradient approximation. The crystal structure is relaxed to allow ionic configurations, cell shape, and volume to change without any symmetry constraints. It is found that the silver azide crystal remains orthorhombic

Weihua Zhu; Heming Xiao

2007-01-01

362

We present an extensive first-principles study of the interaction between a silicon oxide nanoasperity and a sexithiophene\\u000a monolayer in order to investigate the individual molecular processes responsible for the energy dissipation during atomic\\u000a force microscope (AFM) operation. Our approach includes not only ground-state calculations of the tip–sample interaction,\\u000a but an extensive set of molecular dynamics simulations at room temperature to

Wojciech Kami?ski; Rubén Pérez

2010-01-01

363

Investigation of transient heat current from first principles using complex absorbing potential

NASA Astrophysics Data System (ADS)

We report on a first-principles investigation of transient heat current through molecular devices under steplike pulse of external and gate voltages. Using the nonequilibrium Green's function (NEGF) approach, an exact solution of transient heat current is obtained that goes beyond the wide-band limit. Combining with density-functional theory (DFT), we propose a time-dependent NEGF-DFT formalism to study the transient heat current under a steplike pulse for molecular devices from first principles. Anticipating the huge computational cost in the transient regime, we develop an algorithm to speed up the calculation using the complex absorbing potential (CAP). By adding the CAP to replace the Hamiltonian of leads, the effective self-energy of the Green's function becomes independent of energy, allowing analytic calculation of the triple integrations in the exact solution of transient heat current using the theorem of residue. With this linear scaling algorithm, the computational complexity is greatly reduced, and a first-principles calculation of transient heat current of molecular devices becomes possible. As an example, we apply our NEGF-DFT-CAP formalism for a molecular device, the Di-thiol benzene molecule connected by two semi-infinite aluminum leads, and we calculate the transient heat current under an upward gate voltage pulse. The enhancement of heat current is observed.

Yu, Zhizhou; Zhang, Lei; Xing, Yanxia; Wang, Jian

2014-09-01

364

NASA Astrophysics Data System (ADS)

We investigated the structural, elastic, and electronic properties of the cubic perovskite-type BaHfO3 using a first-principles method based on the plane-wave basis set. Analysis of the band structure shows that perovskite-type BaHfO3 is a wide gap indirect semiconductor. The band-gap is predicted to be 3.94 eV within the screened exchange local density approximation (sX-LDA). The calculated equilibrium lattice constant of this compound is in good agreement with the available experimental and theoretical data reported in the literatures. The independent elastic constants ( C11, C12, and C44), bulk modules B and its pressure derivatives B?, compressibility ?, shear modulus G, Young's modulus Y, Poisson's ratio ?, and Lamé constants ( ?,?) are obtained and analyzed in comparison with the available theoretical and experimental data for both the singlecrystalline and polycrystalline BaHfO3. The bonding-charge density calculation make it clear that the covalent bonds exist between the Hf and O atoms and the ionic bonds exist between the Ba atoms and HfO3 ionic groups in BaHfO3.

Zhao, Hongsheng; Chang, Aimin; Wang, Yunlan

2009-08-01

365

Structural phase transitions and fundamental band gaps of MgxZn1 xO alloys from first principles

The structural phase transitions and the fundamental band gaps of MgxZn1 xO alloys are investigated by detailed first-principles calculations in the entire range of Mg concentrations x, applying a multiple-scattering theoretical approach (Korringa-Kohn-Rostoker method). Disordered alloys are treated within the coherent-potential approximation. The calculations for various crystal phases have given rise to a phase diagram in good agreement with experiments and other theoretical approaches. The phase transition from the wurtzite to the rock-salt structure is predicted at the Mg concentration of x=0.33, which is close to the experimental value of 0.33 0.40. The size of the fundamental band gap, typically underestimated by the local-density approximation, is considerably improved by the self-interaction correction. The increase in the gap upon alloying ZnO with Mg corroborates experimental trends. Our findings are relevant for applications in optical, electrical, and, in particular, in magnetoelectric devices.

Maznichenko, I. V. [Martin-Luther University Halle-Wittenberg; Ernst, Arthur [Max Planck Institute of Microstructure Physics; Bouhassoune, M. [Max Planck Institute of Microstructure Physics; Henk, J. [Max Planck Institute of Microstructure Physics; Daene, Markus W [ORNL; Lueders, Martin [Daresbury Laboratory, UK; Bruno, Patrick [European Synchrotron Radiation Facility (ESRF); Wolfam, Hergert [Martin-Luther University Halle-Wittenberg; Mertig, I. [Martin-Luther University Halle-Wittenberg; Szotek, Zdzislawa [Daresbury Laboratory, UK; Temmerman, Walter M [Daresbury Laboratory, UK

2009-01-01

366

First principles analysis of graphene and its ability to maintain long-ranged interaction with H2S

NASA Astrophysics Data System (ADS)

We determine the chemical activity of (a) carbon site of pristine graphene, (b) Stone-Wales (SW) defect site, and (c) BN-sites of BN-doped graphene towards adsorption of a toxic gas H2S, through comparative analysis based on first-principles density functional theoretical calculations incorporating van der Waals (vdW) interactions. While the adsorption of H2S is weak at both C and BN sites with a binding energy of 15 k J/mol, it is significantly stronger at the Stone-Wales defect site with a much higher binding energy of 26 k J/mol. This is clearly reflected in the contrasting orientation of H2S molecule in the relaxed geometries: the sulfur atom of H2S is closer to graphene (at a distance 3.14 Å) during physisorption at C and BN sites, while the molecule's H atoms come closer to graphene (at a distance 2.84 Å) during physisorption at the Stone-Wales defect site. The origin of the stronger binding interaction between H2S and a SW defect site is attributed to two possible reasons: (i) an increase in the vdW interaction; and (ii) the lowering of both energy of the HOMO level and the total energy of the H2S molecule in attaining a stable configuration. Our findings are compared to the available theoretical results and their technological relevance is further discussed.

Hegde, Vinay I.; Shirodkar, Sharmila N.; Tit, Nacir; Waghmare, Umesh V.; Yamani, Zain H.

2014-03-01

367

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

368

NASA Astrophysics Data System (ADS)

We present a classical theoretical treatment of a two-dimensional Raman spectroscopy based on the initiation of vibrational coherence with an impulsive Raman pump and subsequent probing by two-pulse femtosecond stimulated Raman spectroscopy (FSRS). The classical model offers an intuitive picture of the molecular dynamics initiated by each laser pulse and the generation of the signal field traveling along the probe wave vector. Previous reports have assigned the observed FSRS signals to anharmonic coupling between the impulsively driven vibration and the higher-frequency vibration observed with FSRS. However, we show that the observed signals are not due to anharmonic coupling, which is shown to be a fifth-order coherent Raman process, but instead due to cascades of coherent Raman signals. Specifically, the observed vibrational sidebands are generated by parallel cascades in which a coherent anti-Stokes or Stokes Raman spectroscopy (i.e., CARS or CSRS) field generated by the coherent coupling of the impulsive pump and the Raman pump pulses participates in a third-order FSRS transition. Additional sequential cascades are discussed that will give rise to cascade artifacts at the fundamental FSRS frequencies. It is shown that the intended fifth-order FSRS signals, generated by an anharmonic coupling mechanism, will produce signals of ˜10-4 ?OD (change in the optical density). The cascading signals, however, will produce stimulated Raman signal of ˜10-2 ?OD, as has been observed experimentally. Experiments probing deuterochloroform find significant sidebands of the CCl3 bend, which has an E type symmetry, shifted from the A1 type C-D and C-Cl stretching modes, despite the fact that third-order anharmonic coupling between these modes is forbidden by symmetry. Experiments probing a 50:50 mixture of chloroform and d-chloroform find equivalent intensity signals of low-frequency CDCl3 modes as sidebands shifted from both the C-D stretch of CDCl3 and the C-H stretch of CHCl3. Such intermolecular sidebands are allowed in the cascade mechanism, but are expected to be extremely small in the fifth-order frequency modulation mechanism. Each of these observations indicates that the observed signals are due to cascading third-order Raman signals.

Mehlenbacher, Randy D.; Lyons, Brendon; Wilson, Kristina C.; Du, Yong; McCamant, David W.

2009-12-01

369

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

370

PHYSICAL REVIEW B 87, 094118 (2013) First-principles study of iron oxyfluorides and lithiation 2013) First-principles studies of iron oxyfluorides in the FeF2 rutile framework (FeOxF2-x, 0 x 1) are performed using density functional theory (DFT) in the general gradient approximation (GGA) with a Hubbard U

Ceder, Gerbrand

371

Elastic properties of olivine LixFePO4 from first principles Thomas Maxisch* and Gerband Ceder

Elastic properties of olivine LixFePO4 from first principles Thomas Maxisch* and Gerband Ceder, Massachusetts 02139, USA Received 11 March 2006; published 15 May 2006 Using first-principles pseudopotential its electrochemical performance, a better understanding of its elastic properties is needed

Ceder, Gerbrand

372

First-Principles Investigation of Phase Stability in the O2-LiCoO2 System

First-Principles Investigation of Phase Stability in the O2-LiCoO2 System D. Carlier,, A. Van der January 2, 2003. Revised Manuscript Received April 15, 2003 A first-principles investigation of the phase stability in the O2-LiCoO2 system is performed to better understand the unusual layered phases obtained upon

Ceder, Gerbrand

373

Parameter Estimation and Capacity Fade Analysis of Lithium-Ion Batteries Using First parameters of lithium-ion batteries are estimated using a first-principles electrochemical engineering model and understanding of lithium-ion batteries using physics-based first-principles models. These models are based

Subramanian, Venkat

374

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

375

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

376

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

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.

Roy, S.; Gruenbaum, S. M.; Skinner, J. L. [Theoretical Chemistry Institute and Department of Chemistry, 1101 University Ave., University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)

2014-11-14

378

Structure and properties of CaMnO3\\/SrMnO3\\/BaMnO3 superlattices from first principles

Previous theoretical and experimental studies have shown that three-component, or ``tri-color'' superlattices can exhibit intrinsic electric polarization due to inversion-symmetry breaking in the layer sequence. In ferromagnetic inversion-symmetry-breaking superlattices, controlled symmetry lowering is similarly expected to lead to interesting new and tunable properties. Here, we present results of first-principles density-functional-theory calculations for short-period CaMnO3\\/SrMnO3\\/BaMnO3 superlattices, using VASP. The ground state

Shen Li; Seongshik Oh; Karin Rabe

2008-01-01

379

NASA Astrophysics Data System (ADS)

In order to resolve a discrepancy of the magnetic moment on Fe between the experimental and calculation results, we perform first-principle electronic structure calculations for iron-based superconductors LaFeAsO1-x and LiFeAs also show similar SDW. So far, the first-principle calculations on LaFeAsO actually predicted the SDW state as a ground state. However, the predicted magnetic moment (?2 ?B) per an Fe atom is much larger than the observed one (?0.35 ?B) in experiments [2,4]. The authors suggested that the discrepancy can be resolved by expanding U into a negative U range within LSDA + U framework. In this paper, we revisit the discrepancy and clarify why the negative correction is essential in these compounds. See Ref. [5] for the details of calculation data by LSDA + negative U. In the first-principle calculation on compounds including transition metals, the total energy is frequently corrected by “LSDA + U” approach. The parameter U is theoretically re-expressed as U(?U-J), where U is the on-site Coulomb repulsion (Hubbard U) and J is the atomic-orbital intra-exchange energy (Hund’s coupling parameter) [6]. The parameter U employed in the electronic structure calculations is usually positive. The positivity promotes the localized character of d-electrons and enhances the magnetic moment in the cases of magnetically ordered compounds. Normally, this positive correction successfully works. In choosing the parameter, one can principally extend the parameter U range to a negative region. The negative case [7] is not popular, but it can occur in the following two cases [8]: (i) the Hubbard U becomes negative and (ii) the intra-exchange J is effectively larger than the Hubbard U. The case (i) has been suggested by many authors based on various theoretical considerations. Here, we note that U should be estimated once screening effects on the long-range Coulomb interaction are taken into account. In fact, small U has been reported [9]. Thus, when the screening is unusual, e.g., an overscreening occurs, the estimation of U may not simply give a positive value. In this paper, we explain why the negative U correction successfully works on these iron-based compounds. We would like to suggest that a situation, in which the on-site intra-band U becomes smaller than the inter-band U?, emerges in these compounds. Such a situation is peculiar to multi-band cases and leads to an effective intra-band attraction. Moreover, the situation can create a full-gap Cooper pair, which is consistent with several observations. The contents of this paper are as follows. Section 2 briefly explains the calculation framework and gives U dependences of the magnetic moment for two cases, i.e., the mother and doped compounds. In addition, we compare the Fermi-surfaces between U=0 and U=-1. In Section 3, we discuss unique features of the electronic structure in iron-based superconductors and explain which type of situation requires the negative U correction.

Nakamura, H.; Hayashi, N.; Nakai, N.; Okumura, M.; Machida, M.

2009-10-01

380

NASA Astrophysics Data System (ADS)

By applying non-equilibrium Green’s function (NEGF) formalism combined with first-principles density functional theory (DFT), we have investigated the electronic transport properties of the anthraquinone-based molecular switch. The molecule that comprises the switch can be converted between the hydroquinone (HQ) and anthraquinone (AQ) forms via redox reactions. The transmission spectra of these two forms are remarkably distinctive. Our results show that the current through the HQ form is significantly larger than that through the AQ form, which suggests that this system has attractive potential application in future molecular switch technology.

Zhao, P.; Liu, D. S.; Wang, P. J.; Zhang, Z.; Fang, C. F.; Ji, G. M.

2011-02-01

381

First Principles Hartree–Fock Description of Lithium Insertion in Oxides

First principles periodic Hartree–Fock calculations are reported for theP42\\/mnm(rutile),I41\\/amd(anatase),Pbca(brookite),Pnma(ramsdellite),Pcbn(colombite),Fd3m(spinel), andImma(orthorhombic) polymorphs of TiO2, from which the predicted order of stability isThe calculated difference in energy between the rutile and anatase structures is 0.02–0.06 eV, in good agreement with a recent local density approximation (LDA) estimate of 0.033 eV and an experiment enthalpy difference of 0.05 eV. The corresponding Hartree–Fock and

W. C. Mackrodt

1999-01-01

382

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

383

First principles study of electronic transport through a Cu(111) form="infix">?graphene junction

NASA Astrophysics Data System (ADS)

We report first principles investigations of the nonequilibrium transport properties of a Cu(111) form="infix">?graphene interface. The Cu(111) electrode is found to induce a transmission minimum (TM) located -0.68 eV below the Fermi level, a feature originating from the Cu-induced charge transfer resulting in n-type doped graphene with the Dirac point coinciding with the TM. An applied bias voltage shifts the n-graphene TM relative to the pure graphene TM and leads to a distinctive peak in the differential conductance indicating the doping level, a characteristic not observed in pure graphene.

Maassen, Jesse; Ji, Wei; Guo, Hong

2010-10-01

384

NASA Astrophysics Data System (ADS)

The high pressure phases, electronic structure, and optical properties of iodoform at zero temperature have been investigated by first-principles pseudopotential plane-wave calculations based on the density-functional theory. A new high pressure polar monoclinic structure with space group Cc, denoted as ? phase, has been observed after a series of simulated annealing and geometry optimizations. Our calculated enthalpies showed that the transition from ? to ? phase occurs at 40.1 GPa. Electronic structure calculated results showed that the insulator-metal transition in ? phase due to band overlap is found at about 32 GPa. In addition, the calculated absorption spectra of iodoform are consistent with the experimental results.

Bao, Gang; Duan, Defang; Tian, Fubo; Wang, Liancheng; Liu, Bingbing; Cui, Tian

2011-01-01

385

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

386

First-Principles Investigation of Mn and Co Doped Trilayer Graphene

NASA Astrophysics Data System (ADS)

First-principles calculations were performed through ABINIT to investigate trilayer graphene for spintronics materials. We studied two stacking orders for trilayer graphene: Bernal (ABA) and rhombohedra (ABC) by using interstitial and substitution transition metal Mn and Co doped trilayer graphene. We found that the ABC stacking order exhibits larger band gap than that of ABA, the Co doped ABC trilayer graphene possesses a band gap and is ferromagnetic. This results show that interstitial Co doped ABC stacking trilayer graphene has potential applications in spintronics.

Luo, Xuan

2013-03-01

387

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

NASA Astrophysics Data System (ADS)

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.; Rajeswarapalanichamy, R.; Santhosh, M.; Sudhapriyanga, G.; Kanagaprabha, S.

2014-04-01

388

First-Principles Study on ?-SiC/BNNT Core/shell Nanocable

NASA Astrophysics Data System (ADS)

In this paper, we studied the structural and electronic properties of core/shell nanocables composed of cubic silicon carbide nanowires (?-SiCNW) and boron nitride nanotubes (BNNT) using first-principles pseudopotential plane wave method within density functional theory. Our results show that the ?-SiC/BNNT heterojunction structures are metallic, which primarily originates from the contributions of the BNNTs and the surfaces of SiCNWs. The BNNTs exhibit metallic characters after the SiC nanowires are inserted. The transition of the BNNTs is attributed to the charge transfer between BNNTs and SiCNWs.

Zou, X. C.; Ouyang, J.; Wu, M. S.; Liu, G.; Lei, X. L.; Ouyang, C. Y.; Xu, B.

2013-09-01

389

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

390

First-principles prediction of mechanical properties of gamma-boron

NASA Astrophysics Data System (ADS)

The structural and mechanical properties of ?-B28 are investigated using first-principles density functional calculations. The single-crystal elastic constants calculations show that ?-B28 is mechanically stable under ambient conditions. The predicted bulk and shear moduli of ?-B28 are comparable to those of boron suboxide, suggesting that ?-B28 can be a superhard material. We also obtained the ideal tensile strength for ?-B28 through deformation from the elastic regime to structural instability. We find that the breaking of B1-B1 and B1-B2 bonds is responsible for the failure of ?-B28 under ?100? and ?010? tensile deformation, respectively.

Jiang, Chao; Lin, Zhijun; Zhang, Jianzhong; Zhao, Yusheng

2009-05-01

391

First-principles study of structurally modulated multiferroic CaMn7O12

NASA Astrophysics Data System (ADS)

We study the electronic and magnetic structures of multiferroic CaMn7O12 by first-principles calculations, based on the experimentally determined modulated crystal structure. We confirm the presence of a 3 d orbital modulation of the Mn2 (Mn3+) sites, previously inferred from the Jahn-Teller crystal distortions. Our results indicate that in the multiferroic phase the magnetic structure of the Mn3 (Mn4+) sites is anharmonically modulated via orbitally mediated coupling with the structural modulation, and that the Dzyaloshinskii-Moriya and exchange striction mechanisms contribute equally to the polarization.

Cao, Kun; Johnson, Roger D.; Perks, Natasha; Giustino, Feliciano; Radaelli, Paolo G.

2015-02-01

392

First principles FLAPW- GGA calculations have been performed to predict the structural, electronic, cohesive and magnetic properties for hexagonal WC doped with all 3d metals. The optimized lattice parameters, density of states, cohesive and formation energies have been obtained and analyzed for ternary solid solutions with nominal compositions W0.875M0.125C (where M = Sc, Ti, ..., Ni, Cu). In addition, the magnetic properties of these solid solutions have been examined, and magnetization has been established for W0.875Co0.125C.

D. V. Suetin; I. R. Shein; A. L. Ivanovskii

2009-03-31

393

Carrier compensation in semi-insulating CdTe: First-principles calculations

Carrier compensation in semi-insulating CdTe has been attributed to the compensation of surplus shallow acceptors by deep donors, usually assumed to be Te antisites. However, our first-principles calculations show that intrinsic defects should not have a significant effect on the carrier compensation due either to lack of deep levels near midgap or to low defect concentration. We demonstrate that an extrinsic defect, OTe-H complex, may play an important role in the carrier compensation in CdTe because of its amphoteric character and reasonably high concentration. Our findings have important consequences for improving device performance in CdTe-based radiation detectors and solar cells.

Du, Mao-Hua [ORNL; Singh, David J [ORNL

2008-01-01

394

NASA Astrophysics Data System (ADS)

The electronic structure and transport properties of silver (Ag) and copper (Cu) nanowires of diameters up to 1.7 nm are investigated using first principles density functional theory and the Landauer formalism in conjunction with a supercell approach. A direct comparison of the ballistic conductances, quantum capacitances, and kinetic inductances indicates that Ag and Cu nanowires show very similar performances. Compared to the electrostatic capacitance, the quantum capacitance is found to have a negligible effect on the total capacitance of the nanowire interconnect. In contrast, the overall inductance has a dominant contribution from the kinetic inductance over the magnetic inductance.

Kharche, Neerav; Manjari, Swati R.; Zhou, Yu; Geer, Robert E.; Nayak, Saroj K.

2011-03-01

395

First Principles Calculations of the Adsorption of Group III Metals on Si(001) at High Temperature

NASA Astrophysics Data System (ADS)

First principles total energy calculations are performed to investigate the Si(001)-(3 × 4) reconstruction induced by the adsorption of group III metals (Ga, Al and In) when deposited at high temperatures. We have considered different models in our study and we have found that the pyramid-like structure proposed by Bunk et al. for In on Si(001) yields the most stable atomic configuration in all cases. We present a detailed description of this surface reconstruction, and compare the structural parameters for Al, Ga and In.

Cotzomi-Paleta, J.; Cocoletzi, Gregorio H.; Takeuchi, Noboru

396

d0 Ferromagnetism in oxygen-doped CuCl: First principles study

NASA Astrophysics Data System (ADS)

Based on our first-principles all-electron and pseudopotential calculations, CuCl doped by a nonmagnetic d0 oxygen element is predicted to be ferromagnetic. O-doped (3%) CuCl shows half-metallic ferromagnetism with a net total magnetic moment of 1.0?B. Furthermore the unusual d0 ferromagnetism in O-doped CuCl can be explained in terms of O(2p)-Cu(3d) like hybridization mechanism. These results suggest that O doped CuCl may be a promising half-metallic material free from magnetic precipitate and may find application in the field of spintronics.

Adli, W.; Ferhat, M.

2014-07-01

397

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

398

Structural, Electronic, and Optical Properties of Noble Metal Clusters from First Principles

We examine low-energy isomeric forms, static polarizabilities, and optical absorption spectra of Ag\\u000a n\\u000a , n = 2–8, and Au\\u000a n\\u000a , n = 2–3, clusters using first principles computations within the static and time-dependent versions of the density functional theory. The noticeable decrease in the static polarizabilities of Ag7 and Ag8 compared to the values characteristic of Ag\\u000a n\\u000a , n = 2–6, is correlated

Serdar Ö?üt; Juan C. Idrobo; Julius Jellinek; Jinlan Wang

2006-01-01

399

Elastic and thermodynamic properties of c-BN from first-principles calculations

NASA Astrophysics Data System (ADS)

The elastic constants and thermodynamic properties of c-BN are calculated using the first-principles plane wave method with the relativistic analytic pseudopotential of the Hartwigen, Goedecker and Hutter (HGH) type in the frame of local density approximation and using the quasi-harmonic Debye model, separately. Moreover, the dependences of the normalized volume V/V0 on pressure P, as well as the bulk modulus B, the thermal expansion ?, and the heat capacity CV on pressure P and temperature T are also successfully obtained.

Hao, Yan-Jun; Cheng, Yan; Wang, Yan-Ju; Chen, Xiang-Rong

2007-01-01

400

First-Principles Calculation for the Electron Phonon Coupling Constant of Superconducting MgB2

NASA Astrophysics Data System (ADS)

We use a first-principles plane wave method with the new relativistic analytic pseudopotential of the Hartwigsen, Goedecker and Hutter (HGH) scheme and the pseudopotential of the Troullier-Martins scheme to calculate the electron-phonon coupling constant ? of superconducting MgB2. The calculated results show that there is a peak at about 300-350 cm-1 in the electron-phonon spectral functional ?2F(?), duo to the coupling of the electrons to the acoustic phonon, and the relativistic effect appears in the high frequencies zone. All the results agree well with the present and previous experimental data.

Chen, Xiang-Rong; Guo, Hua-Zhong; Cai, Ling-Cang; Gao, Jie

2005-06-01

401

NASA Astrophysics Data System (ADS)

We present first-principles calculations of the coupling of quasiparticles to spin fluctuations in iron selenide and discuss which types of superconducting instabilities this coupling gives rise to. We find that strong antiferromagnetic stripe-phase spin fluctuations lead to large coupling constants for superconducting gaps with s± symmetry, but these coupling constants are significantly reduced by other spin fluctuations with small wave vectors. An accurate description of this competition and an inclusion of band-structure and Stoner parameter renormalization effects lead to a value of the coupling constant for an s±-symmetric gap which can produce a superconducting transition temperature consistent with experimental measurements.

Lischner, Johannes; Bazhirov, Timur; MacDonald, Allan H.; Cohen, Marvin L.; Louie, Steven G.

2015-01-01

402

Phase Diagram of Pb(Zr,Ti)O{sub 3} Solid Solutions from First Principles

A first-principles-derived scheme that incorporates ferroelectric and antiferrodistortive degrees of freedom is developed to study finite-temperature properties of Pb(Zr{sub 1-x}Ti{sub x})O{sub 3} solid solution near its morphotropic phase boundary. The use of this numerical technique (i) resolves controversies about the monoclinic ground state for some Ti compositions (ii) leads to the discovery of an overlooked phase, and (iii) yields three multiphase points that are each associated with four phases. Additional neutron diffraction measurements strongly support some of these predictions.

Kornev, Igor A.; Bellaiche, L. [Physics Department, University of Arkansas, Fayetteville, Arkansas 72701 (United States); Janolin, P.-E.; Dkhil, B. [Laboratoire Structures, Proprietes et Modelisation des Solides, Ecole Centrale Paris, CNRS-UMR8580, Grande Voie des Vignes, 92295 Cha circumflex tenay-Malabry Cedex (France); Suard, E. [Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156 38042, Grenoble Cedex (France)

2006-10-13

403

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

404

Epitaxial-strain-induced multiferroicity in SrMnO3 from first principles.

First-principles calculations reveal a large spin-phonon coupling in cubic SrMnO3, with ferromagnetic ordering producing a polar instability. Through combination of this coupling with the strain-polarization coupling characteristic of perovskites, the bulk antiferromagnetic-paraelectric ground state is driven to a previously unreported multiferroic ferroelectric-ferromagnetic state by increasing epitaxial strain. This state has a computed P(s)>54???C/cm2 and magnetic T(c)>92??K. Large mixed magnetic-electric-elastic responses are predicted in the vicinity of the phase boundaries. PMID:20867057

Lee, Jun Hee; Rabe, Karin M

2010-05-21

405

Epitaxial-Strain-Induced Multiferroicity in SrMnO3 from First Principles

First-principles calculations reveal a large spin-phonon coupling in cubic SrMnO3, with ferromagnetic ordering producing a polar instability. Through combination of this coupling with the strain-polarization coupling characteristic of perovskites, the bulk antiferromagnetic-paraelectric ground state is driven to a previously unreported multiferroic ferroelectric-ferromagnetic state by increasing epitaxial strain. This state has a computed Ps>54muC\\/cm2 and magnetic Tc>92K. Large mixed magnetic-electric-elastic responses

Jun Hee Lee; Karin M. Rabe

2010-01-01

406

Epitaxial-Strain-Induced Multiferroicity in SrMnO3 from First Principles

NASA Astrophysics Data System (ADS)

First-principles calculations reveal a large spin-phonon coupling in cubic SrMnO3, with ferromagnetic ordering producing a polar instability. Through combination of this coupling with the strain-polarization coupling characteristic of perovskites, the bulk antiferromagnetic-paraelectric ground state is driven to a previously unreported multiferroic ferroelectric-ferromagnetic state by increasing epitaxial strain. This state has a computed Ps>54?C/cm2 and magnetic Tc>92K. Large mixed magnetic-electric-elastic responses are predicted in the vicinity of the phase boundaries.

Lee, Jun Hee; Rabe, Karin M.

2010-05-01

407

First-principles prediction of a new Dirac-fermion material: silicon germanide monolayer.

From first-principles calculations, we proposed a silicon germanide (SiGe) analog of silicene. This SiGe monolayer is stable and free from imaginary frequency in the phonon spectrum. The electronic band structure near the Fermi level can be characterized by Dirac cones with the Fermi velocity comparable to that of silicene. The Ge and Si atoms in SiGe monolayer exhibit different tendencies in binding with hydrogen atoms, making sublattice-selective hydrogenation and consequently electron spin-polarization possible. PMID:23945421

Zhou, Hongcai; Zhao, Mingwen; Zhang, Xiaoming; Dong, Wenzheng; Wang, Xiaopeng; Bu, Hongxia; Wang, Aizhu

2013-10-01

408

The electronic and magnetic properties of functionalized silicene: a first-principles study

Based on first-principles calculations, we study the structural, electronic, and magnetic properties of two-dimensional silicene saturated with hydrogen and bromine atoms. It is found that the fully saturated silicene exhibits nonmagnetic semiconducting behavior, while half-saturation on only one side with hydrogen or bromine results in the localized and unpaired electrons of the unsaturated Si atoms, showing ferromagnetic semiconducting or half-metallic properties, respectively. Total energy calculations show that the half-hydrogenated silicene exhibits a ferromagnetic order, while the half-brominated one exhibits an antiferromagnetic behavior. PMID:22839704

2012-01-01

409

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

410

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

411

The role of Bi vacancies in the electrical conduction of BiFeO?: a first-principles approach.

We employ first-principles methods to study the mechanism controlling the electrical conduction in BiFeO3 (BFO). We find that under oxygen-rich conditions, Bi vacancies (V(Bi)) have lower defect formation energy than O vacancies (V(O)) (-0.43 eV vs. 3.35 eV), suggesting that V(Bi) are the acceptor defects and control the conductivity of BFO, making it a p-type semiconductor. In order to obtain further insight into the conduction mechanism, we calculate the effect of donor (Sn(4+)) and acceptor (Pb(2+)) impurities in BFO. Results indicate that Sn impurities prefer to substitute Fe sites to form shallow donor defects, which compensate the acceptor levels derived from V(Bi). Meanwhile, Pb atoms favour the substitution of Bi sites to form acceptor defects, reducing the overall concentration of holes (h(+)). Theoretical findings were later surveyed by current-voltage characteristics of Sn- or Pb-doped BFO nanofibers. This study is of general interest in carrier transport in charge compensation semiconductors, and of particular relevance within the context of defect-mediated conductivity in BFO. PMID:24879577

Xu, Qiang; Sobhan, Mushtaq; Yang, Qian; Anariba, Franklin; Ong, Khuong Phuong; Wu, Ping

2014-07-28

412

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

413

Our first principles density functional theory calculations within GGA + U approximation reveal that the nature of transport gaps in the zircon and scheelite phases of DyCrO4 are quite different. While in the scheelite phase the origin of the gap is more like that of the Mott-Hubbard systems, in the zircon phase the origin is not strictly a Mott-Hubbard or a charge transfer type. In the framework of the Zaanen-Sawatsky-Allen phase diagram, the DyCrO4 in its zircon phase could be placed in the intermediate regime between the charge transfer and Mott-Hubbard insulators. On the issue of ground state magnetic order in these two phases, where no consensus exists so far from experimental observations, we have performed GGA and GGA + U calculations on various possible magnetic configurations. We clearly establish from our theoretical calculations that the ferrimagnetic order, where ferromagnetic Dy and Cr sublattice are aligned antiparallel to each other, is the ground state in the zircon phase, while in the scheelite phase competing long-range antiferromagnetic orders are observed. Our estimation of various superexchange interactions indicate that competing ferro- and antiferro-magnetic interactions exist which would explain the experimental observation of metamagnetic transitions on application of a small external magnetic field in these systems. PMID:25706816

Ray, Avijeet; Maitra, Tulika

2015-03-18

414

Half metallicity in Pr 0.75Sr 0.25MnO 3 : A first principle study

NASA Astrophysics Data System (ADS)

In this communication we present a first principle study of Pr 1- xSr xMnO 3 with x=0.25. While the parent compounds of this system are antiferromagnetic insulators with different structural and magnetic ground states, the x=0.25 is in the colossal magnetoresistance regime of the Pr 1- xSr xMnO 3 phase diagram [C. Martin, A. Maignan, M. Hervieu, B. Raveau, Phys. Rev. B 60 (1999) 12191]. Our band structure calculations for the end-point compounds matches well with the existing theoretical and experimental results [C. Martin, A. Maignan, M. Hervieu, B. Raveau, Phys. Rev. B 60 (1999) 12191; Rune Sondena, P. Ravindran, Svein Stolen, Tor Grande, Michael Hanfland, Phys. Rev. B 74 (2006) 144102]. Interestingly, our calculations show that the Pr 0.75Sr 0.25MnO 3 has a half-metallic character with a huge band gap of 2.8 eV in the minority band. We believe this result would fuel further interest in some of these special compositions of colossal magnetoresistive manganites as they could be potential candidates for spintronic devices. We discuss the half-metallicity of the Pr 0.75Sr 0.25MnO 3 in the light of changes in the orbital hybridization as a result of Sr doping in PrMnO 3. Further, we highlight the importance of half-metallicity for a consolidated understanding of colossal magnetoresistance effect.

Chakraborty, Monodeep; Pal, Prabir; Sekhar, Biju Raja

2008-01-01

415

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

416

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

417

Nature of transport gap and magnetic order in zircon and scheelite type DyCrO4 from first principles

NASA Astrophysics Data System (ADS)

Our first principles density functional theory calculations within GGA + U approximation reveal that the nature of transport gaps in the zircon and scheelite phases of DyCrO4 are quite different. While in the scheelite phase the origin of the gap is more like that of the Mott–Hubbard systems, in the zircon phase the origin is not strictly a Mott–Hubbard or a charge transfer type. In the framework of the Zaanen–Sawatsky–Allen phase diagram, the DyCrO4 in its zircon phase could be placed in the intermediate regime between the charge transfer and Mott–Hubbard insulators. On the issue of ground state magnetic order in these two phases, where no consensus exists so far from experimental observations, we have performed GGA and GGA + U calculations on various possible magnetic configurations. We clearly establish from our theoretical calculations that the ferrimagnetic order, where ferromagnetic Dy and Cr sublattice are aligned antiparallel to each other, is the ground state in the zircon phase, while in the scheelite phase competing long-range antiferromagnetic orders are observed. Our estimation of various superexchange interactions indicate that competing ferro- and antiferro-magnetic interactions exist which would explain the experimental observation of metamagnetic transitions on application of a small external magnetic field in these systems.

Ray, Avijeet; Maitra, Tulika

2015-03-01

418

NASA Astrophysics Data System (ADS)

The rapid solidification processes of Ca50Mg20Cu30 liquid alloy have been simulated by first principle molecular dynamics simulation based on the density functional theory. The local structural evolution of the alloy is analysed using Honeycutt Andersen (HA) bond-type index and bond-angle distribution methods. The electronic properties of this amorphous solid are also studied. The simulated coordination numbers are very close to the theoretical values according to the efficient cluster packing model (ECP) model. The interaction between Ca-Cu atomic pairs is strongest in this alloy. The HA bond-type result shows that a large quantity of pentagonal bipyramids are formed in undercooled alloy liquid and become most common polyhedral local structures of the amorphous solid, which suppress the crystallization and increase the glass forming ability of Ca-Mg-Cu alloy. The bond-angle distribution indicates that the close packing of the three neighbor atoms and the pentagon configurations become the primary short range order (SRO) as temperature decreases. The electronic density demonstrates that the covalent bond of Ca-Mg, Ca-Cu, Mg-Cu, and Cu-Cu pairs is existed in this alloy. This chemical SRO also benefits the glass transition.

Chang, H. J.; Chen, L. F.; Zhu, X. F.

2012-10-01

419

NASA Astrophysics Data System (ADS)

In this work, the state-of-the-art infrared variable angle spectroscopic ellipsometry (IR-VASE) and first-principles molecular dynamics (FPMD) method were combined to obtain the infrared dielectric functions of MgO crystal in the spectral range 300-1000 cm-1 and for temperatures up to 1950 K. The IR-VASE can measure the infrared dielectric functions of MgO crystal at temperatures ranging from 300 to 573 K and reproduce previous infrared-reflectivity experiments. As temperature increases, it demonstrates that the amplitude of dominant absorption peak centered around 400 cm-1 reduces, the width broadens, and the position shifts to longer wavelength. Besides ellipsometry study, the FPMD method was implemented, seeking to theoretically predict the infrared spectra of MgO crystal at elevated temperatures. Comparing with experimental measurements, the FPMD method can reproduce the essential feature of ellipsometry and previous infrared-reflectivity experiments even at elevated temperatures, though with some deviations in predicting the exact position and amplitude of dominant absorption peak. On the other hand, the FPMD method can predict the temperature effect on the infrared dielectric functions of MgO crystal, e.g., redshift and broadened absorption peak with increasing temperature.

Yang, J. Y.; Zhang, W. J.; Liu, L. H.; Qiu, J.; Wang, K.; Tan, J. Y.

2014-09-01

420

The bioinformatics revolution of the last decade has been instrumental in the development of empirical potentials to quantitatively estimate protein interactions for modeling and design. Although computationally efficient, these potentials hide most of the relevant thermodynamics in 5-to-40 parameters that are fitted against a large experimental database. Here, we revisit this longstanding problem and show that a careful consideration of the change in hydrophobicity, electrostatics, and configurational entropy between the folded and unfolded state of aliphatic point mutations predicts 20-30% less false positives and yields more accurate predictions than any published empirical energy function. This significant improvement is achieved with essentially no free parameters, validating past theoretical and experimental efforts to understand the thermodynamics of protein folding. Our first principle analysis strongly suggests that both the solute-solute van der Waals interactions in the folded state and the electrostatics free energy change of exposed aliphatic mutations are almost completely compensated by similar interactions operating in the unfolded ensemble. Not surprisingly, the problem of properly accounting for the solvent contribution to the free energy of polar and charged group mutations, as well as of mutations that disrupt the protein backbone remains open. PMID:17523191

Bueno, Marta; Camacho, Carlos J; Sancho, Javier

2007-09-01

421

NASA Astrophysics Data System (ADS)

The isotopic fractionation in OCS photolysis is studied theoretically from first principles. UV absorption cross sections for OCS, OC33S, OC34S, OC36S and O13CS are calculated using the time-depedent quantum mechanical formalism and a recently developed ab-initio description of the photodissociation of OCS which takes into account the lowest four singlet and lowest four triplet electronic states. The calculated isotopic fractionations as a function of wavelength are in good agreement with recent measurements by Hattori et al. (2011) and indicate that photolysis leads to only a small enrichment of 34S in the remaining OCS. The photodissociation dynamics provide strong evidence that the photolysis quantum yield is unity at all wavelengths for atmospheric UV excitation, for all isotopologues. A simple stratospheric model is constructed taking into account the main sink reactions of OCS and it is found that overall stratospheric removal slightly favors light OCS in constrast to the findings of Leung et al. (2002). These results show, based on isotopic considerations, that OCS is an acceptable source of background stratosperic sulfate aerosol in agreement with a recent model study of of Brühl et al. (2012). The 13C isotopic fractionation due to photolysis of OCS in the upper stratosphere is significant and will leave a clear signal in the remaining OCS making it a candidate for tracing using the ACE-FTS and MIPAS data sets.

Schmidt, J. A.; Johnson, M. S.; Hattori, S.; Yoshida, N.; Nanbu, S.; Schinke, R.

2013-02-01

422

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

423

Surface of glassy GeS2: A model based on a first-principles approach

NASA Astrophysics Data System (ADS)

First-principles calculations within the framework of the density functional theory are used to construct realistic models for the surface of glassy GeS2width="0.3em"/>(g-GeS2). Both calculations at T=0 K and at finite temperature (T=300 K) are considered. This allows for a comparison between the structural and electronic properties of surface and bulk g-GeS2. Although the g-GeS2 surface recovers the main tetrahedral structural motif of bulk g-GeS2, the number of fourfold coordinated Ge atoms and twofold coordinated S atoms is smaller than in the bulk. On the contrary, the surface system features a larger content of overcoordinated S atoms and threefold coordinated Ge atoms. This effect is more important for the g-GeS2 surface relaxed at 0 K. Maximally localized Wannier functions (WF) are used to inspect the nature of the chemical bonds of the structural units present at the g-GeS2 surface. We compare the ability of several charge derivation methods to capture the atomic charge variations induced by a coordination change. Our estimate for the charges allows exploiting the first-principles results as a data base to construct a reliable interatomic force field.

Ori, G.; Massobrio, C.; Bouzid, A.; Boero, M.; Coasne, B.

2014-07-01

424

NASA Astrophysics Data System (ADS)

Results are reported of first principles VASP supercell calculations of basic defect migration in UN nuclear fuels. The collinear interstitialcy mechanism of N migration is predicted to be energetically more favourable than direct [0 0 1] hops. It is also found that U and N vacancies have close migration energies, and O impurities accelerate migration of N vacancies nearby. These values are both in qualitative agreement with the effect of oxygen on the reduction of the activation energy for thermal creep reported in the literature, as well as in quantitative agreement with the experimental data when taking into account the uncertainties. The migration energies have been implemented in the thermal creep model of the TRANSURANUS fuel performance code. Therefore a concrete example is provided of how first principles computations can contribute directly to improve the design tools of advanced nuclear fuels, e.g. the predictions reveal a limited effect of oxygen on the thermo-mechanical performance of nitride fuels under fast breeder reactor (FBR) normal operating conditions.

Kotomin, E. A.; Mastrikov, Yu. A.; Rashkeev, S. N.; Van Uffelen, P.

2009-09-01

425

We show that the Kubo formula can be used to calculate the nonlocal electrical conductivity of layered systems from first principles. We use the Layer Korringa Kohn Rostoker method to calculate the electronic structure and Green function of Co/Cu/Co trilayers within the local density approximation to density functional theory. This Green function is used to calculate the conductivity through the Kubo formula for both majority and minority spins and for alignment and anti-alignment of the Co moments on either side of the Cu spacer layer. This allows us to determine the giant magnetoresistance from first principles. We investigate three possibilities for the scattering in Co/Cu/Co: (1) equal electron lifetimes for Cu, majority spin Co, and minority spin Co, (2) equal electron lifetimes for majority and minority Co, weaker scattering in Cu and spin dependent interfacial scattering, (3) electron lifetimes for majority and minority spin cobalt proportional to their Fermi energy densities of states and spin dependent interfacial scattering.

Butler, W.H.; Zhang, X.G.; Nicholson, D.M.C. [Oak Ridge National Lab., TN (United States); MacLaren, J.M. [Tulane Univ., New Orleans, LA (United States). Dept. of Physics

1995-12-31

426

First-principles investigation of dynamical properties of molecular devices under a steplike pulse

NASA Astrophysics Data System (ADS)

We report a computationally tractable approach to first-principles investigation of time-dependent current of molecular devices under a steplike pulse. For molecular devices, all the resonant states below Fermi level contribute to the time-dependent current. Hence calculation beyond wideband limit must be carried out for a quantitative analysis of transient dynamics of molecules devices. Based on the exact nonequilibrium Green’s-function (NEGF) formalism of calculating the transient current [J. Maciejko, J. Wang, and H. Guo, Phys. Rev. B 74, 085324 (2006)10.1103/PhysRevB.74.085324], we develop two approximate schemes going beyond the wideband limit, they are all suitable for first-principles calculation using the NEGF combined with density-functional theory. Benchmark test has been done by comparing with the exact solution of a single level quantum dot system. Good agreement has been reached for two approximate schemes. As an application, we calculate the transient current using the first approximated formula with opposite voltage VL(t)=-VR(t) in two molecular structures: Al-C5-Al and Al-C60-Al . As illustrated in these examples, our formalism can be easily implemented for real molecular devices. Importantly, our new formula has captured the essential physics of dynamical properties of molecular devices and gives the correct steady state current at t=0 and t?? .

Xing, Yanxia; Wang, Bin; Wang, Jian

2010-11-01

427

First-Principles Calculations, Experimental Study, and Thermodynamic Modeling of the Al-Co-Cr System

The phase relations and thermodynamic properties of the condensed Al-Co-Cr ternary alloy system are investigated using first-principles calculations based on density functional theory (DFT) and phase-equilibria experiments that led to X-ray diffraction (XRD) and electron probe micro-analysis (EPMA) measurements. A thermodynamic description is developed by means of the calculations of phase diagrams (CALPHAD) method using experimental and computational data from the present work and the literature. Emphasis is placed on modeling the bcc-A2, B2, fcc-?, and tetragonal-? phases in the temperature range of 1173 to 1623 K. Liquid, bcc-A2 and fcc-? phases are modeled using substitutional solution descriptions. First-principles special quasirandom structures (SQS) calculations predict a large bcc-A2 (disordered)/B2 (ordered) miscibility gap, in agreement with experiments. A partitioning model is then used for the A2/B2 phase to effectively describe the order-disorder transitions. The critically assessed thermodynamic description describes all phase equilibria data well. A2/B2 transitions are also shown to agree well with previous experimental findings. PMID:25875037

Liu, Xuan L.; Gheno, Thomas; Lindahl, Bonnie B.; Lindwall, Greta; Gleeson, Brian; Liu, Zi-Kui

2015-01-01

428

Quantum Mechanics and First-Principles Molecular Dynamics Selection of Polymer Sensing Materials

NASA Astrophysics Data System (ADS)

We present two first-principles methods, density functional theory (DFT) and a molecular dynamics (MD) computer simulation protocol, as computational means for the selection of polymer sensing materials. The DFT methods can yield binding energies of polymer moieties to specific vapor bound compounds, quantities that were found useful in materials selection for sensing of organic and inorganic compounds for designing sensors for the electronic nose (ENose) that flew on the International Space Station (ISS) in 2008-2009. Similarly, we present an MD protocol that offers high consistency in the estimation of Hildebrand and Hansen solubility parameters (HSP) for vapor bound compounds and amorphous polymers. HSP are useful for fitting measured polymer sensor responses with physically rooted analytical models. We apply the method to the JPL electronic nose (ENose), an array of sensors with conducting leads connected through thin film polymers loaded with carbon black. Detection relies on a change in electric resistivity of the polymer film as function of the amount of swelling caused by the presence of the analyte chemical compound. The amount of swelling depends upon the chemical composition of the polymer and the analyte molecule. The pattern is unique and it unambiguously identifies the compound. Experimentally determined changes in relative resistivity of fifteen polymer sensor materials upon exposure to ten vapors were modeled with the first-principles HSP model.

Blanco, Mario; Shevade, Abhijit V.; Ryan, Margaret A.

429

Analysis of BCC-HCP transformation in Ti by coupling symmetry to first-principles calculations.

NASA Astrophysics Data System (ADS)

Many structural transformations involve no group-subgroup relationship and are hence beyond the purview of the classical Landau theory. We study a Ti martensitic transformation from a body-centered cubic (BCC) to a hexagonal close-packed (HCP) phase. A systematic and general methodology to describe such reconstructive transformations by coupling group symmetry arguments to first-principles calculations is presented. We describe (i) a symmetry-based four-step algorithm to enumerate transformation paths, (ii) evaluate the energy barriers along these transformation paths using full-potential first principles calculations, (iii) deduce the full set of primary and secondary order parameters (OP's) to establish the appropriate Landau-Ginsburg free-energy functionals, (iv) for each path, identify special points of the primary OP as a function of local distortions, corresponding to the end product phase. Specifically, we find a generalization of the Burger's mechanism for this transformation. Our procedure is equally applicable to other reconstructive transitions such as BCC-FCC, HCP-OMEGA and so on.

Srinivasan, S. G.; Hatch, D. M.; Lookman, T.; Saxena, A. B.; Stokes, H. T.

2001-06-01

430

Surface design of alloy protection against CO-poisoning from first principles.

Pt-based alloy catalysts are of significant importance in fuel cells due to enhanced electrode reactivity and selectivity. Designing alloy surfaces suitable for catalyst via first-principles predictions has long played a central role in identifying promising candidates. We propose surface design for polymer electrolyte fuel cell (PEFC) based on the use of thermodynamically stable alloy surfaces. Using first-principles calculation, we have explored stable Pt alloy surfaces that possess superior catalytic properties for CO-despoisoning in hydrogen-related reactions of fuel cells. The stable Pt(25)M(75) (M = Rh, Cu) alloy surfaces both exhibited weaker CO and stronger H binding compared to the pure Pt surface, which yielded a significant increase in H coverage by around one order. These modifications of molecular adsorption are attributed to the deeper band centre of surface d electrons. Understanding the adsorption properties of the stable atomic structure at surfaces will help us to design suitable alloy surfaces with high-catalytic activity and prolonged actuation in fuel cells. PMID:25078032

Yuge, Koretaka; Koyama, Yukinori; Kuwabara, Akihide; Tanaka, Isao

2014-09-01

431

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

432

First-principles molecular-dynamics study on the magnetic structure of Mn3Pt

NASA Astrophysics Data System (ADS)

A first-principles molecular-dynamics (MD) approach to the magnetic structure of itinerant magnets is proposed and applied to the magnetic structure of Mn3Pt, which is a three-dimensional frustrated magnet with an octahedral configuration of Mn local magnetic moments and exhibits two ordered phases at temperatures below the Néel temperature. The theory is formulated by incorporating the first-principles tight-binding linear muffin-tin orbital Hamiltonian into the MD approach for itinerant magnets on the basis of the functional integral method and the isothermal MD technique, and determines automatically the magnetic structures of itinerant magnets at finite temperatures. Numerical results at 25 K for a 3×3×3 fcc unit cell reveal a triangular-type noncollinear magnetic structure, which is consistent with the experimentally-proposed magnetic structure of the low-temperature-ordered phase (D-phase). With increasing temperature in the D-phase region, the amplitude and the thermal average of Mn local moments were found to decrease gradually. The stability of the experimentally-suggested magnetic structure of the higher-temperature-ordered phase (F-phase) in Mn3Pt is discussed on the basis of the nonmagnetic ground-state density of states, which shows a sharp sub-peak consisting of the Eg states of Mn at the Fermi energy.

Uchida, Takashi; Kimura, Nobuyuki; Kakehashi, Yoshiro

2013-06-01

433

Epitaxial-strain-induced multiferroicity in SrMnO3 from first principles

NASA Astrophysics Data System (ADS)

In the first-principles search for new ferromagnetic-ferroelectric multiferroics, one key indicator is the softening of the lowest frequency polar phonon with ferromagnetic ordering from a paraelectric antiferromagnetic bulk state. In a first-principles survey of the phonon dispersions of a wide range of magnetic perovskites, we identified SrMnO3 as a promising candidate system. We find that a ferromagnetic-ferroelectric phase is stabilized by both compressive and tensile epitaxial strain. For compressive strain, there is a sequence of intermediate magnetic transitions, first to C-AFM and then to A-AFM ordering, with an increasing fraction of ferromagnetically aligned nearest neighbor Mn. At each of these, the change in magnetic order is accompanied by a jump in the magnitude of the electric polarization, so, near the A-AFM-FE->FE-FM phase boundary at 3.4% and G-AFM-FE->FE-FM phase boundary at -2.9%, an applied electric field can induce a nonzero magnetization, and the jump in c-lattice constant at -2.9% strain can generate a large piezomagnetic response. The origin of the large phonon softening in SrMnO3 will be examined, which should provide guidance in identifying additional candidate systems for epitaxial-strain-induced multiferroicity.

Lee, Jun Hee; Rabe, Karin M.

2010-03-01

434

Pressure induced structural phase transition of OsB{sub 2}: First-principles calculations

Orthorhombic OsB{sub 2} was synthesized at 1000 deg. C and its compressibility was measured by using the high-pressure X-ray diffraction in a Diacell diamond anvil cell from ambient pressure to 32 GPa [R.W. Cumberland, et al. (2005)]. First-principles calculations were performed to study the possibility of the phase transition of OsB{sub 2}. An analysis of the calculated enthalpy shows that orthorhombic OsB{sub 2} can transfer to the hexagonal phase at 10.8 GPa. The calculated results with the quasi-harmonic approximation indicate that this phase transition pressure is little affected by the thermal effect. The calculated phonon band structure shows that the hexagonal P 6{sub 3}/mmc structure (high-pressure phase) is stable for OsB{sub 2}. We expect the phase transition can be further confirmed by the experimental work. - Abstract: Graphical Abstract Legend (TOC Figure): Table of Contents Figure Pressure induced structural phase transition from the orthorhombic structure to the hexagonal one for OsB{sub 2} takes place under 10.8 GPa (0 K), 10.35 GPa (300, 1000 K) by the first-principles predictions.

Ren Fengzhu [Institute of Computational Materials Science, School of Physics and Electronics, Henan University, Kaifeng 475004 (China); Wang Yuanxu, E-mail: wangyx@henu.edu.c [Institute of Computational Materials Science, School of Physics and Electronics, Henan University, Kaifeng 475004 (China); Department of Applied Physics, Hong Kong Polytechnic University (Hong Kong); Lo, V.C. [Department of Applied Physics, Hong Kong Polytechnic University (Hong Kong)

2010-04-15

435

In this paper, single pulse (SP)- and double pulse (DP)- Laser Induced Breakdown Spectroscopy (LIBS) on metallic titanium, aluminum-based alloy and copper-based alloy have been studied by spectrally resolved imaging to find out the fundamental difference in terms of fluid-dynamic and chemical aspects. To better clarify the different nature of SP- and DP-Laser Induced Plasma (LIP) a qualitative theoretical model

A. De Giacomo; M. Dell'Aglio; D. Bruno; R. Gaudiuso; O. De Pascale

2008-01-01

436

First principles calculations of optical and x-ray spectra from atomic coordinates alone

NASA Astrophysics Data System (ADS)

Theoretical calculations of various x-ray and optical spectroscopies often rely on semi-empirical or phenomenological models to account for many-body effects and thermal vibrations. Typically such models include a number of parameters which complicate fitting schemes that extract physical quantities from experimental spectra. Here we present an approach for ab initio calculations of these spectra starting from structure alone. A many-pole model of the dielectric function is introduced to calculate the self-energy and spectral function,ootnotetextJ. J. Kas et al., Phys. Rev. B 76 195116 (2007). while a density functional theory calculation of the dynamical matrix is used to calculate effects of thermal vibrations. ootnotetextFernando D. Vila et al., Phys. Rev. B 76, 014301 (2007). In addition, core-hole effects are incorporated with RPA screening. ootnotetextA. L. Ankudinov et al., Phys. Rev. B 71, 165110 (2005). This approach has been incorporated into FEFF9, a new version of the real-space multiple-scattering FEFF code for broad spectrum calculations of various optical and x-ray spectra.

Kas, J. J.; Prange, M.; Vila, F. D.; Takimoto, Y.; Rehr, J. J.

2008-03-01

437

First-principles study of structural, electronic and optical properties of ZnF2

NASA Astrophysics Data System (ADS)

The structural, electronic, and optical properties of rutile—, CaCl2-, and PdF2—ZnF2 are calculated by the plane-wave pseudopotential method within the density functional theory. The calculated equilibrium lattice constants are in reasonable agreement with the available experimental and other calculated results. The band structures show that the rutile—, CaCl2-, and PdF2—ZnF2 are all direct band insulator. The band gaps are 3.63, 3.62, and 3.36 eV, respectively. The contribution of the different bands was analyzed by the density of states. The Mulliken population analysis is performed. A mixture of covalent and weak ionic chemical bonding exists in ZnF2. Furthermore, in order to understand the optical properties of ZnF2, the dielectric function, absorption coefficient, refractive index, electronic energy loss spectroscopy, and optical reflectivity are also performed in the energy range from 0 to 30 eV. It is found that the main absorption parts locate in the UV region for ZnF2. This is the first quantitative theoretical prediction of the electronic and optical properties of ZnF2 compound, and it still awaits experimental confirmation.

Wu, Jian-Bang; Cheng, Xin-Lu; Zhang, Hong; Xiong, Zheng-Wei

2014-07-01

438

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

439

NASA Astrophysics Data System (ADS)

Since its fabrication in 2004, graphene has attracted huge attention due to its exceptional electronic properties, and is now considered as one of the most promising candidates to replace the current semiconductor technology as silicon approaches its miniaturization limit. However, the absence of an electronic band gap in pristine graphene makes it ill-suited for many electronic applications. Semiconducting character can be imparted by a variety of methods, including chemical or structural modifications. For instance, a band gap can be opened by confining the electronic wave function in one dimension by cutting graphene to form graphene nanoribbons (GNRs). To possess a band gap comparable to conventional semiconductors like silicon, GNRs are required to have a width less than 3 nm and must also display sharp edges, which remains a great experimental challenge. Recently, a breakthrough advance has been achieved with the controlled synthesis of atomically precise nanoribbons using a bottom-up approach where small aromatic molecules chemically assemble into high-quality subnanometer ribbons. This method not only allows for the synthesis of high-quality straight GNRs, but also for more complex structures like wiggle-like GNRs, called graphene nanowiggles (GNWs). In Part I of this thesis, first-principles density functional theory (DFT) calculations are carried out on a variety of GNWs to reveal their unusual electronic and magnetic properties that are absent in their individual GNRs components, such as tunable band gaps and versatile magnetic states. The relationship between the band gap and the geometry is dictated by the armchair or zigzag characters of the corresponding parallel and oblique sectors, enabling GNWs to offer a broader set of geometrical parameters to tune the electronic structures compared to GNRs. In addition, first-principles many-body Green's function calculations within the GW approximation are performed to yield a quantitative prediction of GNWs' electronic properties. The enhanced electron-electron interaction in the quasi-one-dimensional GNWs results in significant self-energy corrections to their DFT band gaps. Consequently, the quasiparticle band gaps are typically more than twice of the DFT band gaps and are within the most interesting range 0.0-3.7 eV. In Part II of this thesis, we venture beyond graphene-based systems and investigate graphene-like materials: transition metal dichalcogenides MX 2(M = Mo, W; X = S). Similar to graphite, they are also layered structures stacked by weak van der Waals (vdW) forces. Single-layer MoS2 and WS2 have been synthesized and found to show enhanced carrier charge mobilities and strong photoluminescence with direct band gaps, and thus they have been considered as replacements or complements to graphene for applications. Raman spectroscopy is often considered as one of the most popular tools to characterize them. Despite extensive experimental Raman studies on MoS 2 and WS2, it remains unclear how Raman intensities and especially intensity ratio of Raman modes E2g1 and A1g depend on the materials' thickness, due to the large spectrum of seemingly contradictory findings. In the final part of the thesis (Part III), we highlight the experimental collaboration project with Prof. Plummer's group from Louisiana State University: spin-dependent surface reconstruction of layered Fe-based superconductors CaFe2As2. Low energy electron diffraction, scanning tunneling microscopy and spectroscopy, and first-principles spin-polarized DFT are utilized to investigate the geometric, electronic, and magnetic structures of the stripe-ordered (1x2) surface of Ca(Fe1-xCox) 2As2 (x=0, 0.075). The surface is terminated with a 50% Ca layer. Compared to the bulk, the surface Ca layer has a large inward relaxation (˜ 0.5 A), and the underneath As-Fe2-As layer displays a significant buckling. First-principles calculations show that the (1x2) phase is stabilized by the bulk anti-ferromagnetic spin ordering through the spin-charge-lattice coupling. Strikingly, a superconducting gap (˜7 meV

Liang, Liangbo

440

We present an analysis of the thermal reduction of delithiated LiMnPO[subscript 4] and LiFePO[subscript 4] based on the quarternary phase diagrams as calculated from first principles. Our results confirm the recent ...

Ong, Shyue Ping

441

The structure of glassy GeS[subscript 2] is studied in the framework of density functional theory, by using a fully self-consistent first-principles molecular dynamics (FPMD) scheme. A comparative analysis is performed ...

Celino, M.

442

First principles calculations have been used to explore the Li–Bi–F ternary phase diagram. Our results confirm the thermodynamic stability of previously observed phases and find no new phases in this system. Electrochemical ...

Doe, Robert E.

443

by first-principles simulations M. Widom,1 B. Sauerwine,1 A. M. Cheung,2 S. J. Poon,3 P. Tong,3 D. Louca,3 and G. J. Shiflet2 1 Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213

Widom, Michael

2011-01-01

444

First-principles computation of mantle materials in crystalline and amorphous phases

NASA Astrophysics Data System (ADS)

First-principles methods based on density functional theory are used extensively in the investigation of the behavior and properties of mantle materials over broad ranges of pressure, temperature, and composition that are relevant. A review of computational results reported during the last couple of decades shows that essentially all properties including structure, phase transition, equation of state, thermodynamics, elasticity, alloying, conductivity, defects, interfaces, diffusivity, viscosity, and melting have been calculated from first principles. Using MgO, the second most abundant oxide of Earth's mantle, as a primary example and considering many other mantle materials in their crystalline and amorphous phases, we have found that most properties are strongly pressure dependent, sometimes varying non-monotonically and anomalously, with the effects of temperature being systematically suppressed with compression. The overall agreement with the available experimental data is excellent; it is remarkable that the early-calculated results such as shear wave velocities of two key phases, MgO and MgSiO3 perovskite, were subsequently reproduced by experimentation covering almost the entire mantle pressure regime. As covered in some detail, the defect formation and migration enthalpies of key mantle materials increase with pressure. The predicted trend is that partial MgO Schottky defects are energetically most favorable in Mg-silicates but their formation enthalpies are high. So, the diffusion in the mantle is likely to be in the extrinsic regime. Preliminary results on MgO and forsterite hint that the grain boundaries can accommodate point defects (including impurities) and enhance diffusion rates at all pressures. The structures are highly distorted in the close vicinity of the defects and at the interface with excess space. Recent simulations of MgO-SiO2 binary and other silicate melts have found that the melt self-diffusion and viscosity vary by several orders of magnitudes with pressure, temperature, and composition. The predicted high compressibility and complex dynamical behavior can be associated with structural changes (involving non-bridging oxygen, oxygen tri-clusters, Si-O pentahedra, etc.) occurring on compression. We envision future prospect for massively parallel/distributed computing of unprecedented magnitude and scope in the study of relevant materials for Earth, super-Earth, and other planets. A renewed computational theme perhaps should be the first-principles simulations of large systems (with long runs) that are necessary to explore realistic (natural) compositions, polycrystalline phases, multi-component melts, crystal/melt interfaces, trace element partitioning, etc.

Karki, Bijaya B.

2015-03-01

445

Atomic arrangements of the hydrogen-terminated Si(001) surface after wet cleaning are investigated by scanning tunneling microscopy and by first-principles calculations. The hydrogen-terminated Si(001) surface after dilute HF cleaning is atomically rough with a structure of terraces and steps. In addition, it is confirmed that 1×1 dihydride structures are formed inside the terraces. First-principles calculations reveal that the maxima of the

Katsuyoshi Endo; Kenta Arima; Kikuji Hirose; Toshihiko Kataoka; Yuzo Mori

2002-01-01

446