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1

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

2

The hyperfine A-tensor and Zeeman g-tensor parameterize the interaction of an 'effective' electron spin with the magnetic field due to the nuclear spin and the homogeneous external magnetic field, respectively. The A- and g-tensors are the quantities of primary interest in electron paramagnetic resonance (EPR) spectroscopy. In this paper, we review our work [E.S. Kadantsev, T. Ziegler, J. Phys. Chem. A 2008, 112, 4521; E. S. Kadantsev, T. Ziegler, J. Phys. Chem. A 2009, 113, 1327] on the calculation of these EPR parameters under periodic boundary conditions (PBC) from first-principles. Our methodology is based on the Kohn-Sham DFT (KS DFT), explicit usage of Bloch basis set made up of numerical and Slater-type atomic orbitals (NAOs/STOs), and is implemented in the 'full potential' program BAND. Our implementation does not rely on the frozen core approximation. The NAOs/STOs basis is well suited for the accurate representation of the electron density near the nuclei, a prerequisite for the calculation of highly accurate hyperfine parameters. In the case of g-tensor, our implementation is based on the method of Van Lenthe et al. [E. van Lenthe, P. E. S. Wormer, A. van der Avoird, J. Chem. Phys. 1997, 107, 2488] in which the spin-orbital coupling is taken into account variationally. We demonstrate the viability of our scheme by calculating EPR parameters of paramagnetic defects in solids. We consider the A-tensor of 'normal' and 'anomalous' muonium defect in IIIA-VA semiconductors as well as the S2 anion radical in KCl host crystal lattice. PMID:20821407

Kadantsev, Eugene S; Ziegler, Tom

2010-09-05

3

Electron microscopy, spectroscopy, and first-principles calculations of Cs{sub 2}O

Oxides of cesium play a key role in ameliorating the photoelectron emission of various opto-electronic devices. However, due to their extreme reactivity, their electronic and optical properties have hardly been touched upon. With the objective of better understanding the electronic and optical properties of Cs{sub 2}O in relationship to its structure, an experimental and theoretical study of this compound was undertaken. First-principles density functional theory calculations were performed. The preferred structural motif for this compound was found to be anti-CdCl{sub 2}. Here three Cs-O-Cs molecular layers are stacked together through relatively weak van-der-Waals forces. The energy bands were also calculated. The lowest transition at 1.45eV, was found to be between the K point in the valence band to the {gamma} point in the conduction band. A direct transition at 2eV was found in the center ({gamma}) of the Brillouin zone. X-ray powder diffraction, transmission electron microscopy and selected area electron diffraction were used to analyze the synthesized material. These measurements showed good agreement with the calculated structure of this compound. Absorption measurements at 4.2K indicated two optical transitions with somewhat higher energy (indirect one at 1.65 and a direct transition at 2.2eV, respectively). Photoluminescence measurements also showed similar transitions, suggesting that the lower indirect transition is enhanced by three nearby minima at 1.5eV in the Brillouin zone.

Gemming, S. [Institut fuer Physikalische Chemie, Technische Universitaet, Dresden, 01062 Dresden (Germany)]. E-mail: gotthard.seifert@chemie.tu-dresden.de; Seifert, G. [Institut fuer Physikalische Chemie, Technische Universitaet, Dresden, 01062 Dresden (Germany); Muehle, C. [Max-Planck-Institut fuer Festkoerperforschung, Heisenbergstrasse 1, 70569 Stuttgart (Germany); Jansen, M. [Max-Planck-Institut fuer Festkoerperforschung, Heisenbergstrasse 1, 70569 Stuttgart (Germany); Albu-Yaron, A. [Department of Materials and Interfaces, Weizmann Institute of Science, Faculty of Chemistry, PO Box 26, 76100 Rehovot (Israel); Arad, T. [Department of Materials and Interfaces, Weizmann Institute of Science, Faculty of Chemistry, PO Box 26, 76100 Rehovot (Israel); Tenne, R. [Department of Materials and Interfaces, Weizmann Institute of Science, Faculty of Chemistry, PO Box 26, 76100 Rehovot (Israel)]. E-mail: reshef.tenne@weizmann.ac.il

2005-04-15

4

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

5

We investigate the impact of water, a polar solvent, on the optical absorption of prototypical silicon clusters with oxygen passivation. We approach this complex problem by assessing the contributions of three factors: chemical reactivity; thermal equilibration and dielectric screening. We find that the silanone (Si=O) functional group is not chemically stable in the presence of water and exclude this as a source of significant red shift in absorption in aqueous environments. We perform first principles molecular dynamics simulations of the solvation of an oxygenated silicon cluster with explicit water molecules at 300 K. We find a systematic 0.7 eV red shift in the absorption gap of this cluster, which we attribute to thermal strain of the molecular structure. Surprisingly, we find no observable screening impact of the solvent, in contrast with consistent blue shifts observed for similarly sized organic molecules in polar solvents. The predicted red shift is expected to be significantly smaller for larger Si quantum dots produced experimentally, guaranteeing that their vacuum optical properties are preserved even in aqueous environments.

Prendergast, D; Grossman, J; Williamson, A; Fattebert, J; Galli, G

2004-04-08

6

First-principles theoretical analysis of transition-metal doping of ZnSe quantum dots

NASA Astrophysics Data System (ADS)

We present a systematic analysis of the underlying mechanism of transition-metal doping in ZnSe nanocrystals, using first-principles density functional theory calculations. Our analysis focuses on the adsorption and surface segregation of Mn dopants on ZnSe nanocrystal surface facets. We find that the chemical potentials of the growth precursor species determine the surface structure and morphology of the nanocrystals. We report binding energies for Mn adsorption onto ZnSe surfaces and find that all the anion-rich surfaces contribute toward dopant adsorption onto ZnSe nanocrystal surface facets. Beyond a critical value of dopant surface coverage, these adsorbed dopants may induce structural transitions in low-Miller-index surface facets, resulting in morphological transitions of the ZnSe nanocrystals. In addition, the dopant binding-energy dependence on the dopant surface concentration explains the doping difficulties during nanocrystal growth. Finally, we report surface segregation energy profiles for Mn dopant segregation on low-Miller-index ZnSe nanocrystal surface facets. We find that, under conditions that render ZnSe(001)-(2 × 1) as the dominant dopable surface of ZnSe nanocrystals, Mn dopants do not have a tendency to segregate on this surface; this guarantees that the dopants remain incorporated into the core regions of the nanocrystal instead of escaping to the surface.

Singh, Tejinder; Mountziaris, T. J.; Maroudas, Dimitrios

2012-07-01

7

NASA Astrophysics Data System (ADS)

The superfield theoretical technique has been shown by the author^1 to be a natural many-body field theoretical technique for formulating nonequilibrium quantum physics. This real-time technique is especially useful for systems exhibiting pairing dynamics, such as in nonequilibrium superconductivity, spin systems, two-level atomic systems, and two-band model of semiconductors. Here it will be demonstrated that the resulting general quantum transport equations, valid for systems evolving inhomogeneously in space and time, also generalize the Semiconductor Bloch Equations (SBE) in ultrafast dynamics of optically excited semiconductors. Various well-known limiting cases in nanoelectronic and optoelectronic device physics will be discussed. Physical interpretation of general quantum transport equations in terms of interband susceptibility, parametric and nonlinear physical phenomena will be discussed.

Buot, Felix A.

2000-03-01

8

A series of transition-metal organometallic complexes with commonly occurring metal?chlorine bonding motifs were characterized using (35) Cl solid-state NMR (SSNMR) spectroscopy, (35) Cl nuclear quadrupole resonance (NQR) spectroscopy, and first-principles density functional theory (DFT) calculations of NMR interaction tensors. Static (35) Cl ultra-wideline NMR spectra were acquired in a piecewise manner at standard (9.4?T) and high (21.1?T) magnetic field strengths using the WURST-QCPMG pulse sequence. The (35) Cl electric field gradient (EFG) and chemical shielding (CS) tensor parameters were readily extracted from analytical simulations of the spectra; in particular, the quadrupolar parameters are shown to be very sensitive to structural differences, and can easily differentiate between chlorine atoms in bridging and terminal bonding environments. (35) Cl?NQR spectra were acquired for many of the complexes, which aided in resolving structurally similar, yet crystallographically distinct and magnetically inequivalent chlorine sites, and with the interpretation and assignment of (35) Cl?SSNMR spectra. (35) Cl?EFG tensors obtained from first-principles DFT calculations are consistently in good agreement with experiment, highlighting the importance of using a combined approach of theoretical and experimental methods for structural characterization. Finally, a preliminary example of a (35) Cl?SSNMR spectrum of a transition-metal species (TiCl4 ) diluted and supported on non-porous silica is presented. The combination of (35) Cl?SSNMR and (35) Cl?NQR spectroscopy and DFT calculations is shown to be a promising and simple methodology for the characterization of all manner of chlorine-containing transition-metal complexes, in pure, impure bulk and supported forms. PMID:23907813

Johnston, Karen E; O'Keefe, Christopher A; Gauvin, Régis M; Trébosc, Julien; Delevoye, Laurent; Amoureux, Jean-Paul; Popoff, Nicolas; Taoufik, Mostafa; Oudatchin, Konstantin; Schurko, Robert W

2013-08-01

9

The chemical states of organic semiconductors were investigated by total-electron-yield soft X-ray absorption spectroscopy (TEY-XAS) and first-principles calculations. The organic semiconductors, pentacene (C(22)H(14)) and pentacenequinone (C(22)H(12)O(2)), were subjected to TEY-XAS and the experimental spectra obtained were compared with the 1s core-level excited spectra of C and O atoms, calculated by a first-principles planewave pseudopotential method. Excellent agreement between the measured and the calculated spectra were obtained for both materials. Using this methodology, we examined the chemical states of the aged pentacene, and confirmed that both C-OH and C?O chemical bonds are generated by exposure to air. This result implies that not only oxygen but also humidity causes pentacene oxidation. PMID:22217144

Natsume, Yutaka; Kohno, Teiichiro; Minakata, Takashi; Konishi, Tokuzo; Gullikson, Eric M; Muramatsu, Yasuji

2012-02-02

10

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

NASA Astrophysics Data System (ADS)

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

Legut, Dominik; Friák, Martin; Šob, Mojmír

2010-06-01

11

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

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

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

2012-10-11

12

The search for technologically and economically viable storage solutions for hydrogen fuel would benefit greatly from research strategies that involve systematic property tuning of potential storage materials via atomic-level modification. Here, we use first-principles density-functional theory to investigate theoretically the structural and electronic properties of ultrathin Mg films and Mg-based alloy films and their interaction with atomic hydrogen. Additional delocalized charges are distributed over the Mg films upon alloying them with 11.1% of Al or Na atoms. These extra charges contribute to enhance the hydrogen binding strength to the films. We calculated the chemical potential of hydrogen in Mg films for different dopant species and film thickness, and we included the vibrational degrees of freedom. By comparing the chemical potential with that of free hydrogen gas at finite temperature (T) and pressure (P), we construct a hydrogenation phase diagram and identify the conditions for hydrogen absorption or desorption. The formation enthalpies of metal hydrides are greatly increased in thin films, and in stark contrast to its bulk phase, the hydride state can only be stabilized at high P and T (where the chemical potential of free H{sub 2} is very high). Metal doping increases the thermodynamic stabilities of the hydride films and thus significantly helps to reduce the required pressure condition for hydrogen absorption from H{sub 2} gas. In particular, with Na alloying, hydrogen can be absorbed and/or desorbed at experimentally accessible T and P conditions.

Yoon, Mina [ORNL; Weitering, Harm H [ORNL; Zhang, Zhenyu [ORNL

2011-01-01

13

NASA Astrophysics Data System (ADS)

Low sensitivity is a key problem in inelastic electron tunneling spectroscopy (IETS) with the scanning tunneling microscope. Using first-principles simulations, we predict different means to tune the IETS sensitivity of symmetrical functional aromatics on a Cu(111) surface. We show how the IET-spectra of phenyl-NO2 compounds can be greatly enhanced as compared to pristine phenyl. More precisely, the NO2 substituent qualifies as a sensitizer of low-frequency wagging modes, but also as a quencher of high-frequency stretching modes. At variance, the CO2 substituent is found to suppress the whole IET-activity. The head-up (non-anchoring) and head-down (anchoring) configurations of the functional group lead to minor changes in the signals, nevertheless allowing access to discriminate configurational features. It is shown how to disentangle the electronic and steric effects of the substituent in the STM junction.

Burema, S. R.; Bocquet, M.-L.

2012-08-01

14

NASA Astrophysics Data System (ADS)

abstract- Positron Annihilation Spectroscopy is a powerful experimental tool for probing defects in crystalline materials. The correct identification of defects with PAS requires the knowledge of accurate positron lifetimes for the various kinds of defects. That can be provided by numerical calculations in the framework of the Two-Component Density-Functional Theory. This method accurately treat on the same footing, the electrons and positron densities as well as the atomic structure. We have implemented this formalism within the Projector Augmented-Wave method in the ABINIT code, optimizing the electrons and positron densities self-consistently and calculating positron-induced forces accurately. That allows to properly determining the relaxed geometries of defects that trapped positron. We have applied the TC-DFT to various point defects in UO2. The use of the PAW method allows considering large super cells to simulate point defects, we have typically used cells that contain 32 UO2 unit formulas. We use the LDA+U framework in order to treat strong electronic correlations. This work is a first attempt to help for the interpretation of PAS experiments on UO2 but it seems really promising.-

Torrent, Marc; Jomard, Gerald

2011-03-01

15

Polymorphism of Li(2)MnSiO(4) was inspected by (6)Li MAS NMR spectroscopy. The detected isotropic shifts and spinning-sideband patterns were successfully reproduced by first-principles calculations and offered an insight into structural differences among the polymorphs. The approach for predicting isotropic shifts was also tested on several other Li-containing paramagnetic structures. PMID:20372695

Mali, Gregor; Meden, Anton; Dominko, Robert

2010-04-06

16

NASA Astrophysics Data System (ADS)

We present a first-principles calculation of the theoretical shear strength of the Al?3(112bar) tilt boundary. Here we replace the usual ? surface (defined by rigid atomic translations parallel to the boundary) by a ``generalized ? surface'' that includes complex cooperative atomic motions occurring at the grain boundary during shear. Transition-state-finding techniques are well suited for calculations of generalized ?-surface cross sections. The transition states, saddles on the ? surface, occur during grain-boundary sliding and at grain-boundary dislocations.

Hamilton, J. C.; Foiles, S. M.

2002-02-01

17

NASA Astrophysics Data System (ADS)

In some circumstances, the mechanical and optical properties of multiphase brittle materials strongly depend on the level of residual micromechanical stresses that arise upon cooling due to thermal and elastic mismatch between the constituent phases. Here we study the residual internal stress in a partially crystallized oxyfluoride glass, best known as photothermorefractive (PTR) glass. This material is composed of a glass matrix with embedded nanosize sodium fluoride (NaF) crystals. Using both the Selsing model and solid-state nuclear magnetic resonance in combination with first principles calculations we found that the crystals are under a tensile stress field of approximately 610-800 MPa. For this stress level the estimated critical crystal diameter for spontaneous cracking is about 2300-1900 nm, which greatly exceeds the observed diameters of 7-35 nm. Hence no spontaneous cracking is expected for the PTR glasses. First principles calculations indicate that the stress induced change of the refractive index of the NaF crystals is about -0.08%, which agrees with the observed refractive index changes.

Zwanziger, J. W.; Werner-Zwanziger, U.; Zanotto, E. D.; Rotari, E.; Glebova, L. N.; Glebov, L. B.; Schneider, J. F.

2006-04-01

18

We perform first-principles computational tensile and compressive tests (FPCTT and FPCCT) to investigate the intrinsic bonding and mechanical properties of a ?-TiAl intermetallic compound (L 1(0) structure) using a first-principles total energy method. We found that the stress-strain relations and the corresponding theoretical tensile strengths exhibit strong anisotropy in the [001], [100] and [110] crystalline directions, originating from the structural anisotropy of ?-TiAl. Thus, ?-TiAl is a representative intermetallic compound that includes three totally different stress-strain modes. We demonstrate that all the structure transitions in the FPCTT and FPCCT result from the breakage or formation of bonds, and this can be generalized to all the structural transitions. Furthermore, based on the calculations we qualitatively show that the Ti-Al bond should be stronger than the Ti-Ti bond in ?-TiAl. Our results provide a useful reference for understanding the intrinsic bonding and mechanical properties of ?-TiAl as a high-temperature structural material. PMID:21825422

Zhou, Hong-Bo; Zhang, Ying; Liu, Yue-Lin; Kohyama, Masanori; Yin, Peng-Gang; Lu, Guang-Hong

2009-03-30

19

By relativistic first-principles photoemission calculations for the topological insulator Bi2Te3, we study how the spin texture of the Dirac state manifests itself in circular dichroism. On one hand, there are significant modifications of the initial state's spin texture, which are explained by final-state effects and the symmetry of the photoemission setup. On the other hand, a highly symmetric setup allows us to draw conclusions about the detailed Dirac state's spin texture. Our study supports that circular dichroism in angular distribution successfully complements spin- and angle-resolved photoelectron spectroscopy from topological insulators. PMID:22861884

Mirhosseini, H; Henk, J

2012-07-20

20

NASA Astrophysics Data System (ADS)

A theoretical study of vibrational spectral diffusion and hydrogen bond dynamics in aqueous ionic solutions is presented from first principles without employing any empirical potential models. The present calculations are based on ab initio molecular dynamics for trajectory generation and wavelet analysis of the simulated trajectories for time dependent frequency calculations. Results are obtained for two different deuterated aqueous solutions: the first one is a relatively dilute solution of a single Cl- ion and the second one is a concentrated solution of NaCl (~3M) dissolved in liquid D2O. It is found that the frequencies of OD bonds in the anion hydration shell, i.e., those which are hydrogen bonded to the chloride ion, have a higher stretch frequency than those in the bulk water. Also, on average, the frequencies of hydration shell OD modes are found to increase with increase in the anion-water hydrogen bond distance. On the dynamical side, when the vibrational spectral diffusion is calculated exclusively for the hydration shell water molecules in the first solution, the dynamics reveals three time scales: a short-time relaxation (~200 fs) corresponding to the dynamics of intact ion-water hydrogen bonds, a slower relaxation (~3 ps) corresponding to the lifetimes of chloride ion-water hydrogen bonds, and another longer-time constant (~20 ps) corresponding to the escape dynamics of water from the anion hydration shell. Existence of such three time scales for hydration shell water molecules was also reported earlier for water containing a single iodide ion using classical molecular dynamics [B. Nigro et al., J. Phys. Chem. A 110, 11237 (2006)]. Hence, the present study confirms the basic results of this earlier work using a different methodology. However, when the vibrational spectral diffusion is calculated over all the OD modes, only two time scales of ~150 fs and ~2.7 ps are found without the slowest component of ~20 ps. This is likely because of the very small weight that the hydration shell water molecules carry to the overall spectral diffusion in the solution containing a single ion. For the concentrated solution also, the slowest component of ~20 ps is not found in the spectral diffusion of all water molecules because a distinct separation between the hydration shell and bulk water in terms of their stretch frequencies does not hold at this high concentration regime. The present first principles results are compared with those of the available experiments and classical simulations.

Mallik, Bhabani S.; Semparithi, A.; Chandra, Amalendu

2008-11-01

21

NASA Astrophysics Data System (ADS)

We have studied structural and electronic properties of a Ge(111) surface covered with a monatomic Pb layer [Pb/Ge(111)-?] by means of core-level photoelectron spectroscopy, angle-resolved photoelectron spectroscopy (ARPES), and a first-principles band structure calculation. There has been a controversy about the surface structure of Pb/Ge(111)-? between a close-packed model with a coverage of 4/3 monolayers and a trimer model with a coverage of 1 monolayer. This problem has been examined by analyzing the line shape of a Pb 5d core-level spectrum and comparing the experimental band structure with those calculated for two models. The line shape of the core-level spectrum agrees with a close-packed model. The valence band structure observed by ARPES has been well reproduced by the calculation employing the close-packed model. The close-packed model therefore describes correctly the surface structure of Pb/Ge(111)-?. The scanning-tunneling microscopy (STM) image simulated for the close-packed model is in good agreement with the experimental filled-state STM image, in which three protrusions per unit cell were observed.

Yaji, Koichiro; Hatta, Shinichiro; Aruga, Tetsuya; Okuyama, Hiroshi

2012-12-01

22

NASA Astrophysics Data System (ADS)

The adsorption and vibrational frequency of CO on defective and undefective titanium dioxide surfaces is examined applying first-principles molecular dynamics simulations. In particular, the vibrational frequencies are obtained beyond the harmonic approximation, through the time correlation functions of the atomic trajectories. In agreement with experiments, at low CO coverages we find an upshift in the vibration frequency with respect to the free CO molecule, of 45 and 35 cm-1 on the stoichiometric rutile (110) and anatase (101) faces, respectively. A band falling 8 cm-1 below the frequency corresponding to the perfect face is observed for the reduced rutile (110) surface in the low vacancy concentration limit, where the adsorption is favored on Ti4 + sites. At a higher density of defects, adsorption on Ti3 + sites becomes more stable, accompanied by a downshift in the stretching band. In the case of anatase (101), we analyze the effect of subsurface oxygen vacancies, which have been shown to be predominant in this material. Interestingly, we find that the adsorption of CO on five coordinate Ti atoms placed over subsurface vacancies is favored with respect to other Ti4 + sites (7.25 against 6.95 kcal/mol), exhibiting a vibrational redshift of 20 cm-1. These results provide the basis to quantitatively assess the degree of reduction of rutile and anatase surfaces via IR spectroscopy, and at the same time allow for the assignment of characteristic bands in the CO spectra on TiO2 whose origin has remained ambiguous.

Lustemberg, Pablo G.; Scherlis, Damián A.

2013-03-01

23

Theoretical methods for ultrafast spectroscopy.

Time-resolved spectroscopy in the femtosecond and attosecond time domain is a tool to unravel the dynamics of nuclear and electronic motion in molecular systems. Theoretical insight into the underlying physical processes is ideally gained by solving the time-dependent Schrödinger equation. In this work, methods currently used to solve this equation are reviewed in a compact presentation. These methods involve numerical representations of wavefunctions and operators, the calculation of time evolution operators, the setting up of the Hamiltonian operators and the types of coordinates to be used hereto. The advantages and disadvantages of some methods are discussed. PMID:23606322

Marquardt, Roberto

2013-04-18

24

NASA Astrophysics Data System (ADS)

Using real-time spectroscopic ellipsometry, we directly observed a reversible lattice and electronic structure evolution in SrCoOx (x=2.5-3) epitaxial thin films. Drastically different electronic ground states, which are extremely susceptible to the oxygen content x, are found in the two topotactic phases: i.e., the brownmillerite SrCoO2.5 and the perovskite SrCoO3. First-principles calculations confirmed substantial differences in the electronic structure, including a metal-insulator transition, which originate from the modification in the Co valence states and crystallographic structures. More interestingly, the two phases can be reversibly controlled by changing the ambient pressure at greatly reduced temperatures. Our finding provides an important pathway to understanding the novel oxygen-content-dependent phase transition uniquely found in multivalent transition metal oxides.

Choi, Woo Seok; Jeen, Hyoungjeen; Lee, Jun Hee; Seo, S. S. Ambrose; Cooper, Valentino R.; Rabe, Karin M.; Lee, Ho Nyung

2013-08-01

25

Using real-time spectroscopic ellipsometry, we directly observed a reversible lattice and electronic structure evolution in SrCoO_{x} (x=2.5-3) epitaxial thin films. Drastically different electronic ground states, which are extremely susceptible to the oxygen content x, are found in the two topotactic phases: i.e., the brownmillerite SrCoO_{2.5} and the perovskite SrCoO_{3}. First-principles calculations confirmed substantial differences in the electronic structure, including a metal-insulator transition, which originate from the modification in the Co valence states and crystallographic structures. More interestingly, the two phases can be reversibly controlled by changing the ambient pressure at greatly reduced temperatures. Our finding provides an important pathway to understanding the novel oxygen-content-dependent phase transition uniquely found in multivalent transition metal oxides. PMID:24033069

Choi, Woo Seok; Jeen, Hyoungjeen; Lee, Jun Hee; Seo, S S Ambrose; Cooper, Valentino R; Rabe, Karin M; Lee, Ho Nyung

2013-08-27

26

Exciton dispersion from first principles

NASA Astrophysics Data System (ADS)

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

Gatti, Matteo; Sottile, Francesco

2013-10-01

27

Oxygen incorporation in the Ti2AlC MAX phase and TiC was investigated in the electron microscope using spatially resolved fine-structure electron energy-loss spectroscopy analysis. Corresponding fine structures were calculated within the full-potential-linearized augmented plane-wave framework. In the calculations, oxygen was substituted for aluminum and carbon in Ti2AlC as well as for carbon in TiC, in concentrations of 3.1, 6.2, and 12.5at%

P. O. Å. Persson; J. Rosén; D. R. McKenzie; M. M. M. Bilek

2009-01-01

28

First-Principles Theories of Piezoelectric Materials

NASA Astrophysics Data System (ADS)

Piezoelectrics have long been studied using parameterized models fit to experimental data, starting with the work of Devonshire in 1954 [1]. Much has been learned using such approaches, but they can also miss major phenomena if the materials properties are not well understood, as is exemplified by the realization that low-symmetry monoclinic phases are common around morphotropic phase boundaries, which was missed completed by low-order Devonshire models, and can only appear in higher-order models [2]. In the last 15 years, a new approach has developed using first-principles computations, based on fundamental physics, with no essential experimental input other than the desired chemistry (nuclear charges). First-principles theory laid the framework for a basic understanding of the origins of ferroelectric behavior [3-7] and piezoelectric properties [8-11]. The range of properties accessible to theory continues to expand as does the accuracy of the predictions. We are moving towards the ability to design materials of desired properties computationally. Here, we review some of the fundamental developments of our understanding of piezoelectric material behavior and the ability to predict a wide range of properties using theoretical methods. This is not meant as a review of the literature. Comprehensive reviews of the literature of theoretical studies of ferroelectrics are given by Resta [12] and Rabe and Ghosez [13].

Cohen, R. E.

29

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

30

NASA Astrophysics Data System (ADS)

A morphotropic phase boundary (MPB) in a solid solution of (1- x)Na1/2Bi1/2TiO3- xBaTiO3 (0? x?1.0) (NBT-BT) is directly demonstrated by first-principle calculations. The results show that in the NBT-BT system, a structural transition from rhombohedral to tetragonal occurs when x is changed from 0.06 to 0.12, and the MPB appears at compositions of around 0.05? x?0.07. It can be verified by the stronger hybridization orbital of p-d based on the analysis of the partial density of states (PDOS) in the Ti-O atoms. The theoretical predictions agree well with experimental observations which indicate that the best ferroelectric properties can be attained in the vicinity of around 0.05? x?0.07 in the MPB of the NBT-BT system. Another tetragonal-cubic phase transition which needs further experimental verification is also found from the system with compositions of 0.475< x<0.65.

Deng, Yang; Wang, Ru-Zhi; Xu, Li-Chun; Fang, Hui; Yang, Xiaodong; Yan, Hui; Chu, Paul K.

2011-09-01

31

First-Principles Calculations of Complex Metal-Oxide Materials

NASA Astrophysics Data System (ADS)

The application of first-principles methods to the study of complex-structured oxides, primarily spinels and pyrochlores, is reviewed. The primary focus is on the crystal structure and structural energetics, and on the magnetic ordering when present. Results are presented for the structure and magnetic exchange interactions of a wide range of systems. The first-principles results for phonon frequencies and eigenvectors are seen to compare well to values from infrared and Raman spectroscopy. The first-principles investigation of magnetostructural coupling is discussed. The first-principles results presented can provide valuable information and insight into the physics of these systems, especially in the case of magnetic and/or structural frustration. Challenges and prospects for future research are identified.

Rabe, Karin M.

2010-04-01

32

First principles studies on periodic TS-1 models at Ti content corresponding to 1.35% and 2.7% in weight of TiO(2) are presented. The problem of Ti preferential siting is addressed by using realistic models corresponding to the TS-1 unit cell [TiSi(95)O(192)] and adopting for the first time a periodic DFT approach, thus providing an energy scale for Ti in the different crystallographic sites in nondefective TS-1. The structure with Ti in site T3 is the most stable, followed by T4 (+0.3 kcal/mol); the less stable structure, corresponding to Ti in T1, is 5.6 kcal/mol higher in energy. The work has been extended to investigate models with two Ti's per unit cell [Ti(2)Si(94)O(192)] (2.7%). The possible existence of Ti-O-Ti bridges, formed by two corner-sharing TiO(4) tetrahedra, is discussed. By using cluster models cut from the optimized periodic DFT structures, both vibrational (DFT) and electronic excitation spectra (TDDFT) have been calculated and favorably compared with the experimental data available on TS-1. Interesting features emerged from excitation spectra: (i) Isolated tetrahedral Ti sites show a Beer-Lambert behavior, with absorption intensity proportional to concentration. Such a behavior is gradually lost when two Ti's occupy sites close to each other. (ii) The UV-vis absorption in the 200-250 nm region can be associated with transitions from occupied states delocalized on the framework oxygens to empty d states localized on Ti. Such extended-states-to-local-states transitions may help the interpretation of the photovoltaic activity recently detected in Ti zeolites. PMID:19785451

Gamba, Aldo; Tabacchi, Gloria; Fois, Ettore

2009-12-31

33

NASA Astrophysics Data System (ADS)

First principles studies on periodic TS-1 models at Ti content corresponding to 1.35% and 2.7% in weight of TiO2 are presented. The problem of Ti preferential siting is addressed by using realistic models corresponding to the TS-1 unit cell [TiSi95O192] and adopting for the first time a periodic DFT approach, thus providing an energy scale for Ti in the different crystallographic sites in nondefective TS-1. The structure with Ti in site T3 is the most stable, followed by T4 (+0.3 kcal/mol); the less stable structure, corresponding to Ti in T1, is 5.6 kcal/mol higher in energy. The work has been extended to investigate models with two Ti's per unit cell [Ti2Si94O192] (2.7%). The possible existence of Ti-O-Ti bridges, formed by two corner-sharing TiO4 tetrahedra, is discussed. By using cluster models cut from the optimized periodic DFT structures, both vibrational (DFT) and electronic excitation spectra (TDDFT) have been calculated and favorably compared with the experimental data available on TS-1. Interesting features emerged from excitation spectra: (i) Isolated tetrahedral Ti sites show a Beer-Lambert behavior, with absorption intensity proportional to concentration. Such a behavior is gradually lost when two Ti's occupy sites close to each other. (ii) The UV-vis absorption in the 200-250 nm region can be associated with transitions from occupied states delocalized on the framework oxygens to empty d states localized on Ti. Such extended-states-to-local-states transitions may help the interpretation of the photovoltaic activity recently detected in Ti zeolites.

Gamba, Aldo; Tabacchi, Gloria; Fois, Ettore

2009-09-01

34

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

35

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

36

Methods for First-Principles Alloy Thermodynamics

NASA Astrophysics Data System (ADS)

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

van de Walle, Axel

2013-10-01

37

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

38

Transversity from First Principles in QCD

Transversity observables, such as the T-odd Sivers single-spin asymmetry measured in deep inelastic lepton scattering on polarized protons and the distributions which are measured in deeply virtual Compton scattering, provide important constraints on the fundamental quark and gluon structure of the proton. In this talk I discuss the challenge of computing these observables from first principles; i.e.; quantum chromodynamics, itself. A key step is the determination of the frame-independent light-front wavefunctions (LFWFs) of hadrons - the QCD eigensolutions which are analogs of the Schroedinger wavefunctions of atomic physics. The lensing effects of initial-state and final-state interactions, acting on LFWFs with different orbital angular momentum, lead to T-odd transversity observables such as the Sivers, Collins, and Boer-Mulders distributions. The lensing effect also leads to leading-twist phenomena which break leading-twist factorization such as the breakdown of the Lam-Tung relation in Drell-Yan reactions. A similar rescattering mechanism also leads to diffractive deep inelastic scattering, as well as nuclear shadowing and non-universal antishadowing. It is thus important to distinguish 'static' structure functions, the probability distributions computed the target hadron's light-front wavefunctions, versus 'dynamical' structure functions which include the effects of initial- and final-state rescattering. I also discuss related effects such as the J = 0 fixed pole contribution which appears in the real part of the virtual Compton amplitude. AdS/QCD, together with 'Light-Front Holography', provides a simple Lorentz-invariant color-confining approximation to QCD which is successful in accounting for light-quark meson and baryon spectroscopy as well as hadronic LFWFs.

Brodsky, Stanley J.; /SLAC /Southern Denmark U., CP3-Origins

2012-02-16

39

Atomistic models of hydrogenated amorphous silicon nitride from first principles

We present a theoretical study of hydrogenated amorphous silicon nitride (a-SiNx:H) , with equal concentrations of Si and N atoms (x=1) , for two considerably different densities (2.0 and 3.0g\\/cm3 ). Densities and hydrogen concentration were chosen according to experimental data. Using first-principles molecular-dynamics within density-functional theory the models were generated by cooling from the liquid. Where both models have

K. Jarolimek; R. A. de Groot; G. A. de Wijs; M. Zeman

2010-01-01

40

First principles study of magnetism in nanographenes

Magnetism in nanographenes [also known as polycyclic aromatic hydrocarbons (PAHs)] is studied with first principles density functional calculations. We find that an antiferromagnetic (AFM) phase appears as the PAH reaches a certain size. This AFM phase in PAHs has the same origin as the one in infinitely long zigzag-edged graphene nanoribbons, namely, from the localized electronic state at the zigzag

De-En Jiang; Bobby G. Sumpter; Sheng Dai

2007-01-01

41

Physics of Life from First Principles

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

Michail Zak

2007-01-01

42

Thermodynamics of glasses: a first principle computation

We propose a first principle computation of the thermodynamics of simple fragile glasses starting from the two body interatomic potential. A replica for- mulation translates this problem into that of a gas of interacting molecules, each molecule being built of m atoms, and having a gyration radius (related to the cage size) which vanishes at zero temperature. We use a

Marc Mezard; Giorgio Parisi; Aldo Moro

1999-01-01

43

Thermodynamics of glasses: a first principles computation

We propose a first principles computation of the thermodynamics of simple fragile glasses starting from the two-body interatomic potential. A replica formulation translates this problem into that of a gas of interacting molecules, each molecule being build of m atoms, and having a gyration radius (related to the cage size) which vanishes at zero temperature. We use a small cage

Marc Mézard; Giorgio Parisi

1999-01-01

44

Giant magnetoresistance calculated from first principles.

National Technical Information Service (NTIS)

The Layer Korringa Kohn Rostoker-Coherent Potential Approximation technique was used to calculate the low temperature Giant Magnetoresistance from first principles for Co(vert bar)Cu and permalloy(vert bar)Cu superlattices. Our calculations predict large ...

W. H. Butler J. M. MacLaren X. G. Zhang

1994-01-01

45

Terry and Fourth Amendment First Principles

Thirty years after the Supreme Court's landmark decision in Terry v. Ohio and almost seven-times-thirty years after the adoption of the Fourth Amendment, many lawyers, scholars, and judges still fail to grasp the basic insights of the case and the first principles of the Amendment. Chief Justice Warren's opinion for the Court in Terry must bear some of the blame

Akhil Reed Amar

1998-01-01

46

Computational Materials Science from First Principles

We review recent developments in atomistic computer simulations of matter incorporating both quantum and classical statistical mechanical elements. These methods treat the electronic and geometric structure of solids, liquids and molecules on an equal footing and require no a priori knowledge (i.e. they are based on first principles, the basic laws of quantum and classical physics, not on experimental information).

D. Hohl

1994-01-01

47

First-principles theory of ultrathin magnetic films

NASA Astrophysics Data System (ADS)

We report on a set of systematic first-principles electronic structure investigations of the magnetic spin moments, the magnetic spin configurations, and the magnetic coupling of ultrathin magnetic films on (001)- and (111)-oriented noble-metal substrates and on the Fe(001) substrate. Magnetism is found for 3d-, 4d-, and 5d-transition-metal monolayers on noble-metal substrates. For V, Cr, and Mn on (001) substrates a c(2 × 2) antiferromagnetic superstructure has the lowest energy, and Fe, Co, Ni are ferromagnetic. On (111) substrates, for Cr the energy minimum is found for a 120° non-collinear magnetic configuration in a (icons/Journals/Common/sqrt3" ALT="sqrt3" ALIGN="TOP"/> × icons/Journals/Common/sqrt3" ALT="sqrt3" ALIGN="TOP"/>)R30° unit cell, and for Mn a row-wise antiferromagnetic structure is found. On Fe(001), V and Cr monolayers prefer the layered antiferromagnetic coupling, and Fe, Co, and Ni monolayers favour the ferromagnetic coupling to Fe(001). The magnetic structure of Mn on Fe(001) is a difficult case: at least two competing magnetic states are found within an energy of 7 meV. The Cr/Fe(001) system is discussed in more detail as the surface-alloy formation is investigated, and this system is used as a test case to compare theoretical and experimental scanning tunnelling spectroscopy (STS) results. The possibility of resolving magnetic structures by STS is explored. The results are based on the local spin-density approximation and the generalized gradient approximation to the density functional theory. The calculations are carried out with the full-potential linearized augmented-plane-wave method in film geometry.

Asada, T.; Bihlmayer, G.; Handschuh, S.; Heinze, S.; Kurz, Ph; Blügel, S.

1999-12-01

48

Ferroelectric Phase Transitions from First Principles

NASA Astrophysics Data System (ADS)

For a deeper understanding of structural phase transitions in perovskite-structure oxides, first-principles calculations offer valuable access to microscopic information. With recent advances in algorithms and computational capabilities, structural energetics has been largely met, and high-accuracy density-functional studies for a wide range of perovskite compounds have been presented in the literature. The practical application of these methods to temperature-driven structural transitions involves the construction of an effective Hamiltonian with parameters determined from first-principles calculations. The lattice Wannier function method(K. M. Rabe and U. V. Waghmare, Phys. Rev. B52), 13236 (1995). offers a systematic approach for the construction of first-principles effective Hamiltonians applicable to complex structural transitions involving multiple unstable modes at arbitrary points in the Brillouin zone. The parameters appearing in the LWF effective Hamiltonians for ferroelectric PbTiO3 and antiferroelectric PbZrO3 are obtained from density-functional-theory linear response calculations of phonon frequencies, Z and ?_?, and total-energy calculations using the conjugate-gradients method with optimized pseudopotentials and a plane-wave basis set. The finite-temperature behavior of the model systems is studied using mean field theory and Monte Carlo simulation to yield values of the transition temperature, latent heat and various distribution functions for comparison with experiment. The role of strain coupling in producing the observed transition behavior is investigated. Recent work on the extension to mixed systems will be illustrated by results on Pb_1-xGe_xTe. The use of first-principles effective Hamiltonians to study the temperature dependence of dielectric and piezoelectric response, as well as phonon and domain wall dynamics, will be discussed.

Rabe, Karin M.

1997-03-01

49

First-principles investigation of uranium monochalcogenides

We present first-principles investigation of the electronic structure and\\u000amagnetic properties of uranium monochalcogenides: US, USe, UTe. The\\u000acalculations were performed by using recently developed LDA+U+SO method in\\u000awhich both Coulomb and spin-orbit interactions have been taken into account in\\u000arotationally invariant form. We discuss the problem of choice of the Coulomb\\u000ainteraction value. The calculated [111] easy axes agree

A. O. Shorikov; J. E. Medvedeva; A. I. Poteryaev; V. V. Mazurenko; V. I. Anisimov

2009-01-01

50

Epitaxial strain effects from first principles

Epitaxial strain can substantially enhance the spontaneous polarizations and Curie temperatures of ferroelectric thin films compared to the corresponding bulk materials. In this work we use first principles calculations to calculate the effect of epitaxial strain on the spontaneous polarization of the ferroelectrics BaTiO3, PbTiO3, and LiNbO3, and the multiferroic material BiFeO3. We show that the epitaxial strain dependence of

Claude Ederer; Nicola Spaldin

2006-01-01

51

Positron lifetimes in solids from first principles calculations

We present a first principles method for calculating positron lifetimes in solids, based on self-consistent calculations using the Linear Muffin-Tin Orbital method. Local density approximations are used for both electron-electron and electron-positron interactions. Results are presented for a variety of elemental metals and vacancies to demonstrate the reliability of this approach. Theoretical calculations of positron lifetimes can be used to interpret experimental data. As an examples of this, we interpret our experimental lifetime data for the oxide superconductor Ba{sub 1-x}K{sub x}BiO{sub 3} using calculations based on this method. 12 refs., 3 figs.

Sterne, P.A.; O'Brien, J.C.; Howell, R.H. (Lawrence Livermore National Lab., CA (United States)); Kaiser, J.H. (Texas Univ., Arlington, TX (United States). Dept. of Physics)

1991-08-07

52

Hydrogenated BN monolayers: A first principles study

NASA Astrophysics Data System (ADS)

In the present contribution we apply first principles calculations to investigate the electronic structures and stability of BN hydrogenated monolayers which include a substitutional carbon atom. For comparison, additional C hydrogenated structures are considered. The obtained results demonstrate that BN chair-like monolayers are more stable than boat-like configurations. It is found that the most stable structures present bond angles quite similar to the characteristic one observed for sp3 hybridization. Moreover, a net magnetic moment arises from the introduction of a substitutional carbon impurity. In addition, the results indicate that carbon substitutionals can induce a remarkable reduction of the work function.

Azevedo, S.; Kaschny, J. R.

2013-09-01

53

Giant magnetoresistance calculated from first principles

The Layer Korringa Kohn Rostoker-Coherent Potential Approximation technique was used to calculate the low temperature Giant Magnetoresistance from first principles for Co{vert_bar}Cu and permalloy{vert_bar}Cu superlattices. Our calculations predict large giant magnetoresistance ratios for Co{vert_bar}Cu and extremely large ratios for permalloy{vert_bar}Cu for current perpendicular to the layers. Mechanisms such as spin-orbit coupling which mix spin channels are expected to greatly reduce the GMR effect for permalloy{vert_bar}Cu.

Butler, W.H. [Oak Ridge National Lab., TN (United States); MacLaren, J.M. [Tulane Univ., New Orleans, LA (United States). Dept. of Physics; Zhang, X.G. [Univ. of Kentucky, Lexington, KY (United States). Center for Computational Sciences

1994-09-01

54

First principles materials design for semiconductor spintronics

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

K. Sato; H. Katayama-Yoshida

2002-01-01

55

NASA Astrophysics Data System (ADS)

La(Mg0.5Ti0.5)O3 (LMT) ceramics were prepared by either the solid-state reaction (LMT)SS or the citric-acid chemical method (LMT)CA. A combination of Raman scattering, infrared reflectivity, and first-principles calculations was carried out to elucidate the correlation between lattice dynamics and the dielectric properties of these materials. Twelve Raman-active phonons are observed in both samples, displaying similar frequency positions. Interestingly, the Ag phonon (g11 mode) of (LMT)SS at about 717 cm - 1 involving the oxygen octahedron breathing vibrations demonstrates a narrower linewidth, suggesting its better crystallinity. Furthermore, an infrared-active u2 phonon band due to the vibrations of OI and OII layers, which possesses the largest oscillator strength, exhibits stronger intensity for (LMT)SS, as compared with those for (LMT)CA. Additionally, the Q × f values (the product of dielectric Q values and measurement frequency) of (LMT)SS estimated from either microwave cavity or infrared spectroscopic measurements are larger than those of (LMT)CA. These results indicate that the better coherence of lattice vibrations in (LMT)SS leads to its higher Q × f value, providing evidence for a strong connection between optical spectroscopic behavior and microwave dielectric characteristics in these materials.

Liu, Hsiang-Lin; Hsueh, Hung-Chung; Lin, I.-Nan; Yang, Ming-Ti; Lee, Wei-Chung; Chen, Yi-Chun; Chia, Chia-Ta; Cheng, Hsiu-Fung

2011-06-01

56

First principles study of magnetism in nanographenes

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

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

2007-01-01

57

First principles study of magnetism in nanographenes

NASA Astrophysics Data System (ADS)

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

Jiang, De-En; Sumpter, Bobby G.; Dai, Sheng

2007-09-01

58

First principles materials design for semiconductor spintronics

NASA Astrophysics Data System (ADS)

Materials design of new functional diluted magnetic semiconductors (DMSs) is presented based on first principles calculations. The stability of the ferromagnetic state in ZnO-, ZnS-, ZnSe-, ZnTe-, GaAs- and GaN-based DMSs is investigated systematically and it is suggested that V- or Cr-doped ZnO, ZnS, ZnSe and ZnTe are candidates for high-TC ferromagnetic DMSs. V-, Cr- or Mn-doped GaAs and GaN are also candidates for high-TC ferromagnets. It is also shown that Fe-, Co- or Ni-doped ZnO is ferromagnetic. In particular, the carrier-induced ferromagnetism in ZnO-based DMSs is investigated and it is found that their magnetic states are controllable by changing the carrier density. The origin of the ferromagnetism in the DMSs is also discussed.

Sato, K.; Katayama-Yoshida, H.

2002-04-01

59

First-principles modeling of energetic materials

NASA Astrophysics Data System (ADS)

The prediction of properties of energetic materials using atomic-scale simulation techniques is one of the promising areas of energetic materials (EM) research. One of the challenges is to understand the initial response of EM to shock loading based on fundamental atomic-scale properties of EM crystals. We report the results of first-principles density-functional calculations of static and thermodynamic properties of PETN, HMX and RDX molecular crystals including properties of different crystalline phases and their equations of states (EOS). The EOS are extended beyond simple isotropic constitutive relationships to include materials response upon uniaxial compressions and high pressures up to 100 GPa. The predictions of the theory are compared with recent experimental results.

Conroy, Mike; Oleynik, Ivan; White, Carter

2006-03-01

60

Cumulene molecular wire conductance from first principles

NASA Astrophysics Data System (ADS)

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

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

2010-03-01

61

Electrostatics of superlattices by first principles

NASA Astrophysics Data System (ADS)

A complete theory of epitaxial perovskite superlattices requires an understanding of both epitaxial strain effects and of electrostatic boundary conditions. Here, focusing on the latter issue, we have carried out first-principles calculations of the nonlinear dielectric properties of short-period BaTiO3/SrTiO3 and PbTiO3/SrTiO3 superlattices having the in-plane lattice constant of SrTiO3. In particular, we have calculated the layer polarizations pj as defined using the Wannier-based method of Wu, Di'eguez, Rabe and Vanderbilt for each neutral BaO, SrO, PbO, or TiO2 layer, and modeled pj as a function of displacement field D (which is uniform throughout the superlattice), the chemical identity of the layer itself, and the chemical identity of its near neighbors. We then test our expectation that the dependence on the identity of neighboring layers should decay rapidly with distance. If we apply a cut-off to the range of this interlayer interaction, we arrive at a model description that allows us to predict pj(D) for each layer, and thus the overall P(D) (and trivially, also P vs. electric field and related quantities) for a superlattice of arbitrary layer sequence. X. Wu, O. Di'eguez, K. Rabe and D. Vanderbilt, Phys. Rev. Lett. 97, 107602 (2006).

Wu, Xifan; Diéguez, Oswaldo; Stengel, Massimiliano; Rabe, Karin; Vanderbilt, David

2007-03-01

62

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

63

First-Principles Calculations of Graphene Nanomesh

NASA Astrophysics Data System (ADS)

Graphene has recently attracted intensive attentions owing to its remarkable structural and electronic properties and its significant potential for applications in electronic and optoelectronic devices for size miniaturization and fast electron transportation. However, bulk graphene is a semi-metal with zero bandgap Eg, and opening a sizable Eg is critical for building operational graphene-based transistors. Recently, a new scheme of opening bandgap through punching nanoscale holes in graphene sheet, the graphene nanomesh, was proposed and verified experimentally [1]. However, the mechanism leading to the bandgap opening remains unknown. We have carried out first-principles calculations based on density functional theory (DFT) to study the bandgap opening mechanism and Eg as functions of structural parameters, including the hole size, the hole shape, and the hole-hole distances. Our results suggest that the bandgap opening is a result of quantum confinement at nanomesh necks, while the value of Eg depends not only on the width of nanomesh necks, but also on the chirality of the hole edge. This work was supported by the start-up research funds from Colorado School of Mines. [4pt] [1] J. Bai, X. Zhong, S. Jiang, Y. Huang, and X. Duan, Nature Nanotech. 5, 190 (2010).

Oswald, William; Wu, Zhigang

2011-03-01

64

Towards Experimental Accuracy from the First Principles

NASA Astrophysics Data System (ADS)

Producing ab initio ro-vibrational energy levels of small, gas-phase molecules with an accuracy of 0.10 cm^{-1} would constitute a significant step forward in theoretical spectroscopy and would place calculated line positions considerably closer to typical experimental accuracy. Such an accuracy has been recently achieved for the H_3^+ molecular ion for line positions up to 17 000 cm ^{-1}. However, since H_3^+ is a two-electron system, the electronic structure methods used in this study are not applicable to larger molecules. A major breakthrough was reported in ref., where an accuracy of 0.10 cm^{-1} was achieved ab initio for seven water isotopologues. Calculated vibrational and rotational energy levels up to 15 000 cm^{-1} and J=25 resulted in a standard deviation of 0.08 cm^{-1} with respect to accurate reference data. As far as line intensities are concerned, we have already achieved for water a typical accuracy of 1% which supersedes average experimental accuracy. Our results are being actively extended along two major directions. First, there are clear indications that our results for water can be improved to an accuracy of the order of 0.01 cm^{-1} by further, detailed ab initio studies. Such level of accuracy would already be competitive with experimental results in some situations. A second, major, direction of study is the extension of such a 0.1 cm^{-1} accuracy to molecules containg more electrons or more than one non-hydrogen atom, or both. As examples of such developments we will present new results for CO, HCN and H_2S, as well as preliminary results for NH_3 and CH_4. O.L. Polyansky, A. Alijah, N.F. Zobov, I.I. Mizus, R. Ovsyannikov, J. Tennyson, L. Lodi, T. Szidarovszky and A.G. Csaszar, Phil. Trans. Royal Soc. London A, {370}, 5014-5027 (2012). O.L. Polyansky, R.I. Ovsyannikov, A.A. Kyuberis, L. Lodi, J. Tennyson and N.F. Zobov, J. Phys. Chem. A, (in press). L. Lodi, J. Tennyson and O.L. Polyansky, J. Chem. Phys. {135}, 034113 (2011).

Polyansky, O. L.; Lodi, L.; Tennyson, J.; Zobov, N. F.

2013-06-01

65

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

66

Infrared spectroscopy is a powerful technique to unravel the structure and dynamics of molecular systems of ever increasing complexity. For isolated molecules in the gas phase theoretical approaches that directly rely on solving the Schrödinger equation, either approximately or quasi-exactly, are well established. A distinctly different approach to compute infrared spectra can be based on advanced molecular dynamics, itself being based on classical Newtonian dynamics, in conjunction with concurrent first principles electronic structure calculations. At variance with traditional methods, which are formulated in terms of the Schrödinger representation of quantum mechanics, the molecular dynamics approach stems from Heisenberg's representation and thus relies on computing thermal expectation values of time-correlation functions. Crucial in addition to generating the spectra themselves is their decomposition in terms of modes, which can be assigned to correlated atomic motion. This ab initio molecular dynamics route to compute infrared spectra, and its recent extension to quasiclassical techniques relying on approximate path integral dynamics, is covered in the review part of this Perspective. The usefulness of this unconventional approach, which can be generalized beyond infrared spectroscopy, is demonstrated in detail by applying the full machinery in computing and assigning the infrared spectra of protonated methane and its isotopologues. This particular molecule is often considered to be the most prominent member of the class of floppy or fluxional molecules. CH5(+) has been a longstanding challenge for theoretical infrared spectroscopy because it undergoes intricate large-amplitude motion, which is also reviewed. Molecular dynamics based infrared spectroscopy is general and can be applied to diverse systems such as molecular complexes in the gas phase, chromophores in biomolecular environments, and solute-solvent systems in the liquid phase. PMID:23666315

Ivanov, Sergei D; Witt, Alexander; Marx, Dominik

2013-05-13

67

First-principles study of hydrogen in perfect tungsten crystal

NASA Astrophysics Data System (ADS)

Tungsten-based materials are used as the first wall materials in ITER. Hydrogen impurities were introduced via bombarding with the reaction plasma, which are important for the behavior and stability of the tungsten wall. Using the first-principles density functional theory and planewave pseudopotential technique, we have simulated the behaviors of hydrogen atoms inside the perfect tungsten bcc lattice. The binding energies for different interstitial sites were compared to determine the optimal trapping site for the hydrogen atom inside the tungsten lattice. The diffusion barriers for hydrogen atom between nearby trapping sites and the interaction between two interstitial hydrogen atoms were also calculated. The implication of our theoretical results on the hydrogen diffusion and accumulation behavior was discussed.

Xu, Jingcheng; Zhao, Jijun

2009-09-01

68

Optimized Materials From First Principles Simulations: Are We There Yet?

In the past thirty years, the use of scientific computing has become pervasive in all disciplines: collection and interpretation of most experimental data is carried out using computers, and physical models in computable form, with various degrees of complexity and sophistication, are utilized in all fields of science. However, full prediction of physical and chemical phenomena based on the basic laws of Nature, using computer simulations, is a revolution still in the making, and it involves some formidable theoretical and computational challenges. We illustrate the progress and successes obtained in recent years in predicting fundamental properties of materials in condensed phases and at the nanoscale, using ab-initio, quantum simulations. We also discuss open issues related to the validation of the approximate, first principles theories used in large scale simulations, and the resulting complex interplay between computation and experiment. Finally, we describe some applications, with focus on nanostructures and liquids, both at ambient and under extreme conditions.

Galli, G; Gygi, F

2005-07-26

69

First-principles modeling of Li-air battery materials

NASA Astrophysics Data System (ADS)

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

Radin, Maxwell; Siegel, Donald

2011-03-01

70

Theoretical Calculations of Atomic Data for Spectroscopy

NASA Astrophysics Data System (ADS)

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.

2000-10-01

71

First principles studies of semiconductor epitaxial growth

NASA Astrophysics Data System (ADS)

This thesis conducts investigations mainly on the structures, energetics, and recations of semiconductor as well as oxide surfaces using first principles cluster model approach. The first part of the research work addresses the issues in the epitaxial growth of Hgsb{1-x}Cdsb{x}Te (MCT) materials. Hg divalent compounds were studied thoroughly using a variety of quantum chemical methods in order to understand the energetics of Hg precursors for growth. The (001) growth surfaces were then examined in detail using cluster model calculations. Based on these results, a novel metal-organic molecular beam epitaxial (MOMBE) growth strategy with favorable energetics for growing MCT using Hsb2C=CH-CHsb2-Hg-Cequiv C-CHsb3 is proposed. It is hoped that with this new growth strategy, the Hg vacancy and p-doping problems that currently exist in growth can be avoided. The second part of the thesis discusses the molecular beam epitaxial (MBE) growth of cubic GaN on the (001) surface using various N sources. Surface reconstructions and the interactions of gas-phase atomic and molecular nitrogens with the surface were elucidated using cluster models. Using these results an energy phase diagram for the growth of GaN has been constructed. It suggests that excited state molecular Nsb2\\ (sp3Sigmasbsp{u}{+}) is the most favorable of all N species for growth of high quality GaN because it can undergo a dissociative chemisorption process. Ground state atomic N\\ (sp4S) is also good for growth. The doublet excited states N\\ (sp2D and sp2P) might cause surface N abstraction, leading to N vacancies in the material. Finally, a Fe(OH)sb3(Hsb2O)sb3 GVB cluster model of crystalline alpha-Fesb2Osb3 was developed. This simple model can describe the local geometry and bonding of Fe in the bulk oxide. Using quantum mechanical calculations, the orientation of the oleic imidazoline (OI) molecule bonding to the oxide surface has been determined. OI class of molecules are used extensively for corrosion inhibitor in oil field pipeline applications. It is found in this work that OI can make very strong bonding to the Fe of the iron oxide. In aqueous environments they can replace water on the pipe surface to form a protective layer to prevent corrosion.

Tsai, Bao-Liang

72

Shell Model in a First Principles Approach.

National Technical Information Service (NTIS)

We develop and apply an ab-initio approach to nuclear structure. Starting with the NN interaction, that fits two-body scattering and bound state data, and adding a theoretical NNN potential, we evaluate nuclear properties in a no-core approach. For presen...

P. Navratil

2004-01-01

73

Residential Care: Back to First Principles.

ERIC Educational Resources Information Center

|Residential care must be redefined, free from jargon and rhetoric. Over the past 20 years, the social welfare approach, which encompasses the medical model, has dominated legislative and practical thinking about residential care. This theoretical thinking reached its culmination in the concept of the therapeutic community. The therapeutic…

Burns, David A.

74

Hydrated BaSn(1-x)Y(x)O(3-x/2) is a protonic conductor that, unlike many other related perovskites, shows high conductivity even at high substitution levels. A joint multinuclear NMR spectroscopy and density functional theory (total energy and GIPAW NMR calculations) investigation of BaSn(1-x)Y(x)O(3-x/2) (0.10 ? x ? 0.50) was performed to investigate cation ordering and the location of the oxygen vacancies in the dry material. The DFT energetics show that Y doping on the Sn site is favored over doping on the Ba site. The (119)Sn chemical shifts are sensitive to the number of neighboring Sn and Y cations, an experimental observation that is supported by the GIPAW calculations and that allows clustering to be monitored: Y substitution on the Sn sublattice is close to random up to x = 0.20, while at higher substitution levels, Y-O-Y linkages are avoided, leading, at x = 0.50, to strict Y-O-Sn alternation of B-site cations. These results are confirmed by the absence of a "Y-O-Y" (17)O resonance and supported by the (17)O NMR shift calculations. Although resonances due to six-coordinate Y cations were observed by (89)Y NMR, the agreement between the experimental and calculated shifts was poor. Five-coordinate Sn and Y sites (i.e., sites next to the vacancy) were observed by (119)Sn and (89)Y NMR, respectively, these sites disappearing on hydration. More five-coordinated Sn than five-coordinated Y sites are seen, even at x = 0.50, which is ascribed to the presence of residual Sn-O-Sn defects in the cation-ordered material and their ability to accommodate O vacancies. High-temperature (119)Sn NMR reveals that the O ions are mobile above 400 °C, oxygen mobility being required to hydrate these materials. The high protonic mobility, even in the high Y-content materials, is ascribed to the Y-O-Sn cation ordering, which prevents proton trapping on the more basic Y-O-Y sites. PMID:22691062

Buannic, Lucienne; Blanc, Frédéric; Middlemiss, Derek S; Grey, Clare P

2012-08-23

75

Ammonia Synthesis from First-Principles Calculations

NASA Astrophysics Data System (ADS)

The rate of ammonia synthesis over a nanoparticle ruthenium catalyst can be calculated directly on the basis of a quantum chemical treatment of the problem using density functional theory. We compared the results to measured rates over a ruthenium catalyst supported on magnesium aluminum spinel. When the size distribution of ruthenium particles measured by transmission electron microscopy was used as the link between the catalyst material and the theoretical treatment, the calculated rate was within a factor of 3 to 20 of the experimental rate. This offers hope for computer-based methods in the search for catalysts.

Honkala, K.; Hellman, A.; Remediakis, I. N.; Logadottir, A.; Carlsson, A.; Dahl, S.; Christensen, C. H.; Nørskov, J. K.

2005-01-01

76

First principles study of hydroxyapatite surface

NASA Astrophysics Data System (ADS)

The biomineral hydroxyapatite (HA) [Ca10(PO4)6(OH)2] is the main mineral constituent of mammal bone. We report a theoretical investigation of the HA surface. We identify the low energy surface orientations and stoichiometry under a variety of chemical environments. The surface most stable in the physiologically relevant OH-rich environment is the OH-terminated (1000) surface. We calculate the work function of HA and relate it to the surface composition. For the lowest energy OH-terminated surface we find the work function of 5.1 eV, in close agreement with the experimentally reported range of 4.7 eV-5.1 eV [V. S. Bystrov, E. Paramonova, Y. Dekhtyar, A. Katashev, A. Karlov, N. Polyaka, A. V. Bystrova, A. Patmalnieks, and A. L. Kholkin, J. Phys.: Condens. Matter 23, 065302 (2011)].

Slepko, Alexander; Demkov, Alexander A.

2013-07-01

77

National Technical Information Service (NTIS)

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...

S. Susan

1993-01-01

78

High Temperature Thermal Conductivity from First Principles

NASA Astrophysics Data System (ADS)

In spite of significant research efforts, little is yet known about the atomistic details and mechanisms that underlie peculiarly low (or high) thermal conductivities, especially at elevated pressures and temperatures. Under such extreme conditions, systematic experimental measurements are hard to perform; conventional theoretical approaches typically fail to capture significant physical aspects of the problem, since these methods are either inherently limited to (a) low temperatures and/or (b) to perfect crystals. A recently developed ab initio simulation strategy [1] allows to overcome the latter limitation, but the assessment of the high temperature regime remains an unsolved challenge. Within this work, we present efficient strategies to overcome this serious restriction and show their applicability for zirconia based ceramics - a material typically used in high temperature applications, for instance in thermal barrier coatings [2].[0pt] [1] T. M. Gibbons, and S. K. Estreicher, Phys. Rev. Lett. 102, 255502 (2009).[0pt] [2] D. R. Clarke, and C. G. Levi, Annu. Rev. Mat. Res. 33, 383 (2003).

Carbogno, Christian; Ramprasad, Ramamurthy; Scheffler, Matthias

2011-03-01

79

Evolution of cooperative strategies from first principles.

One of the greatest challenges in the modern biological and social sciences is to understand the evolution of cooperative behaviour. General outlines of the answer to this puzzle are currently emerging as a result of developments in the theories of kin selection, reciprocity, multilevel selection and cultural group selection. The main conceptual tool used in probing the logical coherence of proposed explanations has been game theory, including both analytical models and agent-based simulations. The game-theoretic approach yields clear-cut results but assumes, as a rule, a simple structure of payoffs and a small set of possible strategies. Here we propose a more stringent test of the theory by developing a computer model with a considerably extended spectrum of possible strategies. In our model, agents are endowed with a limited set of receptors, a set of elementary actions and a neural net in between. Behavioural strategies are not predetermined; instead, the process of evolution constructs and reconstructs them from elementary actions. Two new strategies of cooperative attack and defence emerge in simulations, as well as the well-known dove, hawk and bourgeois strategies. Our results indicate that cooperative strategies can evolve even under such minimalist assumptions, provided that agents are capable of perceiving heritable external markers of other agents. PMID:16625195

Burtsev, Mikhail; Turchin, Peter

2006-04-20

80

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

81

First principles study of O defects in CdSe

NASA Astrophysics Data System (ADS)

Recently, the vibrational signatures related to oxygen defects in oxygen-doped CdSe were measured using ultrahigh resolution Fourier transform infrared (FTIR) spectroscopy by Chen et al.(2008) [1]. They observed two absorption bands centered at ˜1991.77 and 2001.3 cm-1, which they attributed to the LVMs of OCd, in the samples grown with the addition of CdO and excess Se. For the samples claimed to be grown with even more excess Se, three high-frequency modes (1094.11, 1107.45, and 1126.33) were observed and assigned to the LVMs of OSe-VCd complex. In this work, we explicitly calculated the vibrational signatures of OCd and OSe-VCd complex defects based on first principles approach. The calculated vibrational frequencies of OCd and OSe-VCd complex are inconsistent with the frequencies observed by Chen et al., indicating that their observed frequencies are from other defects. Potential defects that could explain the experimentally observed modes are suggested.

T-Thienprasert, J.; Limpijumnong, S.; Du, M.-H.; Singh, D. J.

2012-08-01

82

First principles molecular dynamics study of filled ice hydrogen hydrate.

We investigated structural changes, phase diagram, and vibrational properties of hydrogen hydrate in filled-ice phase C(2) by using first principles molecular dynamics simulation. It was found that the experimentally reported "cubic" structure is unstable at low temperature and/or high pressure: The "cubic" structure reflects the symmetry at high (room) temperature where the hydrogen bond network is disordered and the hydrogen molecules are orientationally disordered due to thermal rotation. In this sense, the "cubic" symmetry would definitely be lowered at low temperature where the hydrogen bond network and the hydrogen molecules are expected to be ordered. At room temperature and below 30 GPa, it is the thermal effects that play an essential role in stabilizing the structure in "cubic" symmetry. Above 60 GPa, the hydrogen bonds in the framework would be symmetrized and the hydrogen bond order-disorder transition would disappear. These results also suggest the phase behavior of other filled-ice hydrates. In the case of rare gas hydrate, there would be no guest molecules' rotation-nonrotation transition since the guest molecules keep their spherical symmetry at any temperature. On the contrary methane hydrate MH-III would show complex transitions due to the lower symmetry of the guest molecule. These results would encourage further experimental studies, especially nuclear magnetic resonance spectroscopy and neutron scattering, on the phases of filled-ice hydrates at high pressures and/or low temperatures. PMID:22938248

Zhang, Jingyun; Kuo, Jer-Lai; Iitaka, Toshiaki

2012-08-28

83

First principles molecular dynamics study of filled ice hydrogen hydrate

NASA Astrophysics Data System (ADS)

We investigated structural changes, phase diagram, and vibrational properties of hydrogen hydrate in filled-ice phase C2 by using first principles molecular dynamics simulation. It was found that the experimentally reported ``cubic'' structure is unstable at low temperature and/or high pressure: The ``cubic'' structure reflects the symmetry at high (room) temperature where the hydrogen bond network is disordered and the hydrogen molecules are orientationally disordered due to thermal rotation. In this sense, the ``cubic'' symmetry would definitely be lowered at low temperature where the hydrogen bond network and the hydrogen molecules are expected to be ordered. At room temperature and below 30 GPa, it is the thermal effects that play an essential role in stabilizing the structure in ``cubic'' symmetry. Above 60 GPa, the hydrogen bonds in the framework would be symmetrized and the hydrogen bond order-disorder transition would disappear. These results also suggest the phase behavior of other filled-ice hydrates. In the case of rare gas hydrate, there would be no guest molecules' rotation-nonrotation transition since the guest molecules keep their spherical symmetry at any temperature. On the contrary methane hydrate MH-III would show complex transitions due to the lower symmetry of the guest molecule. These results would encourage further experimental studies, especially nuclear magnetic resonance spectroscopy and neutron scattering, on the phases of filled-ice hydrates at high pressures and/or low temperatures.

Zhang, Jingyun; Kuo, Jer-Lai; Iitaka, Toshiaki

2012-08-01

84

First-principles structural design of superhard materials.

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

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

2013-03-21

85

First-Principles Calculations for Insulators at Constant Polarization

NASA Astrophysics Data System (ADS)

We develop an exact formalism for performing first-principles calculations for insulators at fixed electric polarization. As shown by Sai, Rabe, and Vanderbilt (SRV) [Phys. Rev. BPRBMDO0163-1829 66, 104108 (2002)10.1103/PhysRevB.66.104108], who designed an approximate method to tackle the same problem, such an approach allows one to map out the energy landscape as a function of polarization, providing a powerful tool for the theoretical investigation of polar materials. We apply our method to a system in which the ionic contribution to the polarization dominates (a broken-inversion-symmetry perovskite), one in which this is not the case (a III-V semiconductor), and one in which an additional degree of freedom plays an important role (a ferroelectric phase of KNO3). We find that while the SRV method gives rather accurate results in the first case, the present approach provides important improvements to the physical description in the latter cases.

Diéguez, Oswaldo; Vanderbilt, David

2006-02-01

86

First-principles simulations at constant electric polarization

NASA Astrophysics Data System (ADS)

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

Dieguez, Oswaldo; Vanderbilt, David

2005-03-01

87

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

88

Thermal Conductivity of Aluminum Oxide from First Principles

NASA Astrophysics Data System (ADS)

Alumina (Al2O3) is a well-known ceramic material. First-principles study of lattice thermal conductivity can assist our understanding in extreme conditions that are difficult to achieve experimentally, as well as analyze the fundamental difference between other materials. We combine density functional theory and the Peierls--Boltzmann transport theory to predict the temperature and pressure dependencies of lattice thermal conductivity of the corundum phase. We use a real space super cell method to extract second force constants and third order lattice anharmonicity tensors. These are then used to directly evaluate the phonon scattering rates due to lattice anharmonicity. Our preliminary results show that at a density of 4.23 g/cm^3 Al2O3 has thermal conductivities of 14.8Wm-1K-1 at 300K and 5.31Wm-1K-1 at 1000K. Moreover, we calculated the thermodynamic properties such as thermal expansion coefficient, bulk modulus and heat capacity, which are in excellent agreement with available measurements and previous theoretical calculations.

Ntam, Moses; Dong, Jianjun; Xu, Bin

2011-03-01

89

Theoretical Modeling of Various Spectroscopies for Cuprates and Topological Insulators

NASA Astrophysics Data System (ADS)

Spectroscopies resolved highly in momentum, energy and/or spatial dimensions are playing an important role in unraveling key properties of wide classes of novel materials. However, spectroscopies do not usually provide a direct map of the underlying electronic spectrum, but act as a complex 'filter' to produce a 'mapping' of the underlying energy levels, Fermi surfaces (FSs) and excitation spectra. The connection between the electronic spectrum and the measured spectra is described as a generalized 'matrix element effect'. The nature of the matrix element involved differs greatly between different spectroscopies. For example, in angle-resolved photoemission (ARPES) an incoming photon knocks out an electron from the sample and the energy and momentum of the photoemitted electron is measured. This is quite different from what happens in K-edge resonant inelastic X-ray scattering (RIXS), where an X-ray photon is scattered after inducing electronic transitions near the Fermi energy through an indirect second order process, or in Compton scattering where the incident X-ray photon is scattered inelastically from an electron transferring energy and momentum to the scattering electron. For any given spectroscopy, the matrix element is, in general, a complex function of the phase space of the experiment, e.g. energy/polarization of the incoming photon and the energy/momentum/spin of the photoemitted electron in the case of ARPES. The matrix element can enhance or suppress signals from specific states, or merge signals of groups of states, making a good understanding of the matrix element effects important for not only a robust interpretation of the spectra, but also for ascertaining optimal regions of the experimental phase space for zooming in on states of the greatest interest. In this thesis I discuss a comprehensive scheme for modeling various highly resolved spectroscopies of the cuprates and topological insulators (TIs) where effects of matrix element, crystal structure, strong electron correlations (for cuprates) and spin-orbit coupling (for TIs) are included realistically in material-specific detail. Turning to the cuprates, in order to obtain a realistic description of various spectroscopies, one must include not only the effects of the matrix elements and the complexity of the crystal structure, but also of strong electronic correlations beyond the local density approximation (LDA)-based conventional picture, so that the physics of kinks, pseudogaps and superconductivity can be taken into account properly. In this connection, a self-consistent, intermediate coupling scheme informed by material-specific, first-principles band structures has been developed, where electron correlation effects beyond the LDA are incorporated via appropriate self-energy corrections to the electron and hole one-particle Green's functions. Here the antiferromagnetic (AFM) order is used as the simplest model of a competing order. A number of salient features of the resulting electronic spectrum and its energy, momentum and doping dependencies are in accord with experimental observations in electron as well as hole doped cuprates. This scheme thus provides a reasonable basis for undertaking a comprehensive, beyond-LDA level of modeling of various spectroscopies. The specific topics considered here are: (i) Origin of high-energy kink or the waterfall effect found in ARPES; (ii) Identification of the three energy scales observed in RIXS spectra as the pseudogap, charge transfer gap, and Mott gap; (iii) Evolution of the electron momentum densities with holedoping as seen in Compton scattering experiments. For three dimensional topological insulators, the ARPES and scanning tunneling microscopy (STM) spectra has been analyzed using a tight-binding model as well as a k · p model. The spin-orbit coupling, which is essential to produce the characteristic features of the surface states of a TI, is included realistically in the above models. In our generalized k · p model Dresselhaus spin-orbit coupling term extends up to fifth order to reproduce the c

Basak, Susmita

90

First-Principles Simulations of Violent Space-Weather Events.

National Technical Information Service (NTIS)

We have developed a first-principles model for simulating the initiation and propagation of magnetic eruptions originating in the Sun's atmosphere. The ejecta from these violent events adversely affect military and civilian facilities and operations on Ea...

C. R. DeVore S. K. Antiochos

2008-01-01

91

Atomistic models of hydrogenated amorphous silicon nitride from first principles

NASA Astrophysics Data System (ADS)

We present a theoretical study of hydrogenated amorphous silicon nitride (a-SiNx:H) , with equal concentrations of Si and N atoms (x=1) , for two considerably different densities (2.0 and 3.0g/cm3 ). Densities and hydrogen concentration were chosen according to experimental data. Using first-principles molecular-dynamics within density-functional theory the models were generated by cooling from the liquid. Where both models have a short-range order resembling that of crystalline Si3N4 because of their different densities and hydrogen concentrations they show marked differences at longer length scales. The low-density nitride forms a percolating network of voids with the internal surfaces passivated by hydrogen. Although some voids are still present for the high-density nitride, this material has a much denser and uniform space filling. The structure factors reveal some tendency for the nonstoichiometric high-density nitride to phase separate into nitrogen rich and poor areas. For our slowest cooling rate (0.023 K/fs) we obtain models with a modest number of defect states, where the low (high) density nitride favors undercoordinated (overcoordinated) defects. Analysis of the structural defects and electronic density of states shows that there is no direct one-to-one correspondence between the structural defects and states in the gap. There are several structural defects that do not contribute to in-gap states and there are in-gap states that do only have little to no contributions from (atoms in) structural defects. Finally an estimation of the size and cooling rate effects on the amorphous network is reported.

Jarolimek, K.; de Groot, R. A.; de Wijs, G. A.; Zeman, M.

2010-11-01

92

First Principles Simulations of THz Spectra of Acephate: Insight Into the Phonon Signatures.

NASA Astrophysics Data System (ADS)

Acephate is an insecticide that kills insects by disrupting nervous system functions. THz spectroscopy offers a unique tool for detecting trace amount of these materials. Using a combination of solid state first principles simulations and gas phase quantum mechanical modeling we have studied phonon spectra of acephate compound. This talk will present a detailed vibrational spectra analysis over a wide range of frequency and our computational data will be compared with available experimental results.

Zhang, Yiming; Peng, Xihong; Chen, Yunqing; Nayak, Saroj; Zhang, X.-C.

2007-03-01

93

Oxygen vacancies in N doped anatese TiO2: Experiment and first principles calculations

We have determined the electronic and atomic structure of N doped TiO{sub 2} using a combination of hard x-ray photoelectron spectroscopy and first-principles density functional theory calculations. Our results reveal that N doping of TiO{sub 2} leads to the formation of oxygen vacancies and the combination of both N impurity and oxygen vacancies accounts for the observed visible light catalytic behavior of N doped TiO{sub 2}.

Rumaiz, A.; Woicik, J.C.; Cockayne, E.; Lin, H.Y.; Hassnain, J.; Shah, S.I.

2009-12-31

94

Oxygen vacancies in N doped anatase TiO2: Experiment and first-principles calculations

NASA Astrophysics Data System (ADS)

We have determined the electronic and atomic structure of N doped TiO2 using a combination of hard x-ray photoelectron spectroscopy and first-principles density functional theory calculations. Our results reveal that N doping of TiO2 leads to the formation of oxygen vacancies and the combination of both N impurity and oxygen vacancies accounts for the observed visible light catalytic behavior of N doped TiO2.

Rumaiz, Abdul K.; Woicik, J. C.; Cockayne, E.; Lin, H. Y.; Jaffari, G. Hassnain; Shah, S. I.

2009-12-01

95

Nanoflare Statistics from First Principles: Fractal Geometry and Temperature Synthesis

NASA Astrophysics Data System (ADS)

We derive universal scaling laws for the physical parameters of flarelike processes in a low-? plasma, quantified in terms of spatial length scales l, area A, volume V, electron density ne, electron temperature Te, total emission measure M, and thermal energy E. The relations are specified as functions of two independent input parameters, the power index a of the length distribution, N(l)~l-a, and the fractal Haussdorff dimension D between length scales l and flare areas, A(l)~lD. For values that are consistent with the data, i.e., a=2.5+/-0.2 and D=1.5+/-0.2, and assuming the RTV scaling law, we predict an energy distribution N(E)~E-? with a power-law coefficient of ?=1.54+/-0.11. As an observational test, we perform statistics of nanoflares in a quiet-Sun region covering a comprehensive temperature range of Te~1-4 MK. We detected nanoflare events in extreme-ultraviolet (EUV) with the 171 and 195 Å filters from the Transition Region and Coronal Explorer (TRACE), as well as in soft X-rays with the AlMg filter from the Yohkoh soft X-ray telescope (SXT), in a cospatial field of view and cotemporal time interval. The obtained frequency distributions of thermal energies of nanoflares detected in each wave band separately were found to have power-law slopes of ?~1.86+/-0.07 at 171 Å (Te~0.7-1.1 MK), ?~1.81+/-0.10 at 195 Å (Te~1.0-1.5 MK), and ?~1.57+/-0.15 in the AlMg filter (Te~1.8-4.0 MK), consistent with earlier studies in each wavelength. We synthesize the temperature-biased frequency distributions from each wavelength and find a corrected power-law slope of ?~1.54+/-0.03, consistent with our theoretical prediction derived from first principles. This analysis, supported by numerical simulations, clearly demonstrates that previously determined distributions of nanoflares detected in EUV bands produced a too steep power-law distribution of energies with slopes of ?~2.0-2.3 mainly because of this temperature bias. The temperature-synthesized distributions of thermal nanoflare energies are also found to be more consistent with distributions of nonthermal flare energies determined in hard X-rays (?~1.4-1.6) and with theoretical avalanche models (?~1.4-1.5).

Aschwanden, Markus J.; Parnell, Clare E.

2002-06-01

96

Magnetic ordering of rare-earth compounds: first-principles studies

We report a systematic theoretical study on the magnetic ordering in heavy rare-earth compounds with face-centered cubic structure. Based on first-principles total energy calculations of Gd monopnictides, we deduced the exchange interaction parameters of these systems from fitting the total energies of different magnetic configurations to those computed from the Heisenberg model. Then we demonstrated the formation of different magnetic

Chun-Gang Duan; R. F. Sabirianov; L. Liu; W. N. Mei; P. A. Dowben; E. Y. Tsymbal

2006-01-01

97

Electron correlation effects in the MAX phase Cr2AlC from first-principles

With the help of first-principles GGA+U calculations, the origin of the deviation between the experimental and theoretical data for Cr2AlC was revealed. The structural, electrical and elastic properties of Cr2AlC were well described by considering the Cr 3d on-site Coulomb energy. The temperature effect on the bulk moduli of Cr2AlC was also studied within the quasiharmonic Debye model. Based on

Y. L. Du; Z. M. Sun; H. Hashimoto; M. W. Barsoum

2011-01-01

98

A first-principle computation of the thermodynamics of glasses

We propose a first-principle computation of the equilibrium thermodynamics of simple fragile glasses starting from the two-body interatomic potential. A replica formulation translates this problem into that of a gas of interacting molecules, each molecule being built of m atoms, and having a gyration radius (related to the cage size) which vanishes at zero temperature. We use a small cage

Marc Mézard; Giorgio Parisi

1999-01-01

99

Fermions in d = 1 + 2 dimensions from first principles

In this work we construct states describing planar electrons ('spin' (1/2) particles with well defined parity) in d = 1 + 2 from first principles and show that they satisfy Dirac equation, which turns out to be the covariant form of the eigenvalue equation for spatial inversion (parity) just like in d = 1 + 3.

Carrillo-Ruiz, Ma. Georgina; Napsuciale, Mauro [Instituto de Fisica, Universidad de Guanajuato, Lomas del Basque 103, Fracc. Lomas del Campestre, C.P. 37150, Leon Gto. (Mexico)

2006-09-25

100

First-Principles Study of Cu Doped Zigzag Graphene Nanoribbons

We use first principles method to study the electronic properties of zigzag graphene nanoribbons (ZGNRs) substitutionally doped with Cu. We found that unlike pristine nanoribbons the electronic properties are much influenced by doping sites rather than ribbon widths. Cu doped narrow ZGNRs are semiconducting while broader ones are metallic. We predict that Cu doping at one edge is energetically more

Neeraj K. Jaiswal; Pankaj Srivastava

2011-01-01

101

First Principles and Doctrines in the Novels of Charles Dickens

The thesis is that Charles Dickens proposes principles so fundamental that they become an artistically essential reference in his novels. This metaphysics, consisting of first principles and doctrines on Nature, Christian Theology, Good and Evil, Appearance and Reality, among others, is established mainly in the early novels, almost always quite explicitly and with great emphasis. Despite this emphasis the role

Michael Walker

1987-01-01

102

Predictions of the properties of water from first principles

A force field for water has been developed entirely from first principles, without any fitting to experimental data. It contains both pairwise and many-body interactions. This force field predicts the properties of the water dimer and of liquid water in excellent agreement with experiments, a previously elusive objective. Precise knowledge of the intermolecular interactions in water will facilitate a better

R. Bukowski; K. Szalewicz; G. C. Groenenboom; A. van der Avoird

2007-01-01

103

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

Agus Budiyono; T. Sudiyanto; H. Lesmana

2008-01-01

104

Intrinsic buckling strength of graphene: First-principles density functional theory calculations

NASA Astrophysics Data System (ADS)

How graphene, an atomically thin two-dimensional crystal, explores the third spatial dimension by buckling under compression is not yet understood. Knowledge of graphene’s buckling strength, the load at which it transforms from planar to buckled form, is a key to ensure mechanical stability of graphene-based nanoelectronic and nanocomposite devices. Here, we establish using first-principles theoretical analysis that graphene has an intrinsic rigidity against buckling, and it manifests in a weakly linear component in the dispersion of graphene’s flexural acoustic mode, which is believed to be quadratic. Contrary to the expectation from the elastic plate theory, we predict within continuum analysis that a graphene monolayer of macroscopic size buckles at a nonzero critical compressive strain at T=0K , and demonstrate it numerically from first principles. The origin of this rigidity is traced to the coupling between structural and electronic degrees of freedom arising from curvature-induced overlap between ? orbitals in graphene.

Kumar, Sandeep; Hembram, K. P. S. S.; Waghmare, Umesh V.

2010-09-01

105

First-principles approaches to simulate lithiation in silicon electrodes

NASA Astrophysics Data System (ADS)

Silicon is viewed as an excellent electrode material for lithium batteries due to its high lithium storage capacity. Various Si nanostructures, such as Si nanowires, have performed well as lithium battery anodes and have opened up exciting opportunities for the use of Si in energy storage devices. The mechanism of lithium insertion and the interaction between Li and the Si electrode must be understood at the atomic level; this understanding can be achieved by first-principles simulation. Here, first-principles computations of lithiation in silicon electrodes are reviewed. The review focuses on three aspects: the various properties of bulk Li–Si compounds with different Li concentrations, the electronic structure of Si nanowires and Li insertion behavior in Si nanowires, and the dynamic lithiation process at the Li/Si interface. Potential study directions in this research field and difficulties that the field still faces are discussed at the end.

Zhang, Qianfan; Cui, Yi; Wang, Enge

2013-10-01

106

Electrical properties of improper ferroelectrics from first principles

NASA Astrophysics Data System (ADS)

We study the interplay of structural and polar distortions in hexagonal YMnO3 and short-period PbTiO3/SrTiO3 (PTO/STO) superlattices by means of first-principles calculations at constrained electric displacement field D. We find that in YMnO3 the tilts of the oxygen polyhedra produce a robustly polar ground state, which persists at any choice of the electrical boundary conditions. Conversely, in PTO/STO the antiferrodistortive instabilities alone do not break inversion symmetry, and open-circuit boundary conditions restore a nonpolar state. We suggest that this qualitative difference naturally provides a route to rationalizing the concept of “improper ferroelectricity” from the point of view of first-principles theory. We discuss the implications of our arguments for the design of novel multiferroic materials with enhanced functionalities and for the symmetry analysis of the phase transitions.

Stengel, Massimiliano; Fennie, Craig J.; Ghosez, Philippe

2012-09-01

107

First-principles modeling of electrostatically doped perovskite systems.

Macroscopically, confined electron gases at polar oxide interfaces are rationalized within the simple "polar catastrophe" model. At the microscopic level, however, many other effects such as electric fields, structural distortions and quantum-mechanical interactions enter into play. Here, we show how to bridge the gap between these two length scales, by combining the accuracy of first-principles methods with the conceptual simplicity of model Hamiltonian approaches. To demonstrate our strategy, we address the equilibrium distribution of the compensating free carriers at polar LaAlO(3)/SrTiO(3) interfaces. Remarkably, a model including only calculated bulk properties of SrTiO(3) and no adjustable parameters accurately reproduces our full first-principles results. Our strategy provides a unified description of charge compensation mechanisms in SrTiO(3)-based systems. PMID:21517406

Stengel, Massimiliano

2011-03-29

108

First principles modeling of stability mechanism of nonstoichiometric uranium dioxide

To understand the stability mechanism of defects in the nonstoichiometric uranium dioxides, first-principles calculations have been performed by PAW-LSDA+U method for various defects clusters formed from interstitial oxygen atoms and the lattice vacancies. Calculations revealed that the cuboctahedron cluster embedded into the crystal UO2 with one O-atom at the center is the most stable configuration among all known clusters including

Ying Chen; Hua Y. Geng; Yasunori Kaneta; Motoyasu Kinoshita; Shuichi Iwata

2010-01-01

109

NMR shifts for polycyclic aromatic hydrocarbons from first-principles

We present first-principles, density-functional theory calculations of the NMR chemical shifts for polycyclic aromatic hydrocarbons, starting with benzene and increasing sizes up to the one- and two-dimensional infinite limits of graphene ribbons and sheets. Our calculations are performed using a combination of the recently developed theory of orbital magnetization in solids, and a novel approach to NMR calculations where chemical

Timo Thonhauser; Davide Ceresoli; Nicola N. Marzari

2009-01-01

110

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

111

High-pressure phases of titanium: First-principles calculations

NASA Astrophysics Data System (ADS)

We investigate through first-principles calculations the controversial observation of the high-pressure orthorhombic ( ? and ? ) phases of titanium. Our calculations predict the transition sequence ?-?-? under pressure, and reveal that the ? phase is elastically unstable under isotropic compression. We attribute its observation to nonhydrostatic stresses present in the diamond-anvil cell experiments. We find the ? phase to be stable in the 102-112GPa pressure range, with the upper limit of this pressure range increasing under nonhydrostatic conditions.

Verma, A. K.; Modak, P.; Rao, R. S.; Godwal, B. K.; Jeanloz, R.

2007-01-01

112

First-principles investigation of technetium carbides and nitrides

Phase stabilities and mechanical properties of ideal stoichiometric technetium monocarbide (TcC) and technetium mononitride (TcN) in the tungsten carbide (WC), nickel arsenide (NiAs), rocksalt (NaCl), and zinc-blende (ZnS) structures, respectively, have been studied systematically by first-principles calculations. It is found that both TcC and TcN in two hexagonal phases (WC and NiAs) are not only elastically stable but also hard

Yongcheng Liang; Chun Li; Wanlin Guo; Wenqing Zhang

2009-01-01

113

ABINIT: First-principles approach to material and nanosystem properties

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

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

2009-01-01

114

First principles study of ``111'' type iron arsenide superconductors

NASA Astrophysics Data System (ADS)

First principles calculations are performed using Tight-binding LMTO method with Local density approximation (LDA) and Atomic sphere approximation (ASA) to understand the electronic properties of of iron arsenide compounds. The estimated superconducting transition temperature (Tc) for LiFeAs and NaFeAs are ~18K, ~26K respectively and is in agreement with the experimental value. The pressure dependence of superconducting transition temperature is also investigated.

Murugan, A.; Kanagaprabha, S.; Palanichamy, R. Rajeswara

2013-06-01

115

Finite Element Method In First-principles Calculation

We propose a finite element implementation for the first-principles calculation based on the Density Functional Theory (DFT).\\u000a The atomic-scale simulation based on the DFT plays an important roll to predict various material properties such as the physical\\u000a strength. Such simulation seems contribute much to design of new materials of useful functions without loborious compricated\\u000a experiments. Practical complex atomic systems, such

Yoshinori Shiihara; Osamu Kuwazuru; Nobuhiro Yoshikawa

116

Predictions of the properties of water from first principles.

A force field for water has been developed entirely from first principles, without any fitting to experimental data. It contains both pairwise and many-body interactions. This force field predicts the properties of the water dimer and of liquid water in excellent agreement with experiments, a previously elusive objective. Precise knowledge of the intermolecular interactions in water will facilitate a better understanding of this ubiquitous substance. PMID:17332406

Bukowski, Robert; Szalewicz, Krzysztof; Groenenboom, Gerrit C; van der Avoird, Ad

2007-03-01

117

First principles simulations of metallic alloys and liquids

In recent years it has become possible to calculate the structural and thermodynamic properties of materials from first principles. The words first principles'' refer to the fact that there are no adjustable parameters in these theories. The calculator need only specify the atomic constituents -- the rest is handled by the computer. In this sense the calculations are unbiased'' and can be used to search for new materials with interesting new properties. The search for new alloys by traditional techniques (based largely on trial and error) has become much too costly. Being both accurate and cheaper, guidance from computational and analytical theory is needed to narrow the search. In addition, it is important to extend these calculations to nonzero temperature --including the liquid phase. First principles calculations are nearly all based on the local density approximation (LDA). This is a technique for solving the quantum mechanical motion of the electrons in solids. It resembles the Hartree Fock approximation in the sense that a single particle Schrodinger equation must be solved and the resulting wave functions used to construct the electronic charge density. As an example of this method, we discuss platinum-titanium alloys. 10 refs., 2 figs.

Davenport, J.W.; Fernando, G.W.; Qian, G.X.; Watson, R.E.; Weinert, M.

1991-01-01

118

Phonon dispersion measurements and first-principles calculations for the Au(110) surface

We have measured the dispersion of surface phonons on the (1{times}2) reconstructed Au(110) surface along the high-symmetry directions using momentum resolved electron-energy-loss spectroscopy. The frequencies of surface modes at high-symmetry points of the surface Brillouin zone have been calculated with force constants determined from first-principles self-consistent total-energy calculations and are in excellent agreement with the experimental data. Glue-model'' calculations which predicted an experimentally not observed dipole active mode peeling off the top of the bulk phonon band seem to overestimate the bonding strength of atoms near the surface.

Voigtlander, B.; Lehwald, S.; Ibach, H. (Institut fur Grenzflachenforschung und Vakuumphysik, Kernforschungsanlage Julich GmbH, Postfach 1913, 5170 Julich, Federal Repubic of Germany (DE)); Bohnen, K.P. (Kernforschungszentrum Karlsruhe, Institut fur Nukleare Festkorperphysik, Postfach 3640, D-7500 Karlsruhe 1, Federal Republic of Germany); Ho, K.M. (Ames Laboratory, United States Department of Energy and Department of Physics, Iowa State University, Ames, Iowa 50011)

1989-10-15

119

First principles calculation on the adsorption of water on lithium-montmorillonite (Li-MMT)

NASA Astrophysics Data System (ADS)

The interaction of water molecules and lithium-montmorillonite (Li-MMT) is theoretically investigated using density functional theory (DFT) based first principles calculation. The mechanism of water adsorption at two different water concentrations on Li-MMT as well as their structural and electronic properties are investigated. It is found that the adsorption stability in Li-MMT is higher in higher water concentration. It is also found that an adsorbed water molecule on Li-MMT causes the Li to protrude from the MMT surface, so it is expected that Li may be mobile on H2O/Li-MMT.

Dewi Kencana Wungu, Triati; Kemal Agusta, Mohammad; Gandaryus Saputro, Adhitya; Kresno Dipojono, Hermawan; Kasai, Hideaki

2012-11-01

120

First-principles studies of electrical resistivity of iron under pressure.

We investigate the temperature and pressure dependences of the electrical resistivity, thermal conductivity and thermal diffusivity for bcc and hcp Fe using the low-order variational approximation and theoretical transport spectral functions calculated from the first-principles linear response linear-muffin-tin-orbital method in the generalized gradient approximation. The calculated values for the electrical resistivity show a strong increase with temperature and decrease with pressure, and are in agreement with high-temperature shock data. We also discuss the behavior of the electrical resistivity for the bcc?hcp phase transition. PMID:21411882

Sha, Xianwei; Cohen, R E

2011-02-02

121

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

122

Crystal structure and stability of magnesium borohydride from first principles

NASA Astrophysics Data System (ADS)

We present a structure model for constructing Mg(BH4)2 . The first-principles calculations reveal that some stable ground-state crystal structures of Mg(BH4)2 have the same or slightly lower energy than the I-4m2 structure predicted in a paper by Ozolins [Phys. Rev. Lett. 100, 135501 (2008)]. The possible mechanism for the anomalous stability originates from the fact that the charge transfer between different atoms is more homogeneous. The structure model can be anticipated to have potential application for predicting new stable structures of this group of homologous materials.

Zhou, Xiang-Feng; Qian, Quang-Rui; Zhou, Jian; Xu, Bo; Tian, Yongjun; Wang, Hui-Tian

2009-06-01

123

Variational Formulation of the First Principle of Continuum Thermodynamics

NASA Astrophysics Data System (ADS)

The First Principle of Continuum Thermodynamics is formulated as a variational condition whose test fields are piecewise constant virtual temperatures. Lagrange multipliers theorem is applied to relax the constraint of piecewise constancy of test fields. This provides the existence of square summable vector fields of heat flow through the body fulfilling a virtual thermal work principle, analogous to the virtual work principle in Mechanics. The issue of compatibility of thermal gradients is dealt with and expressed by the complementary variational condition. Primal, complementary and mixed variational inequalities leading to computational methods in heat-conduction boundary-value problems are briefly discussed.

Romano, Giovanni; Diaco, Marina; Barretta, Raffaele

2010-03-01

124

Direct first-principles chemical potential calculations of liquids

NASA Astrophysics Data System (ADS)

We propose a scheme that drastically improves the efficiency of Widom's particle insertion method by efficiently sampling cavities while calculating the integrals providing the chemical potentials of a physical system. This idea enables us to calculate chemical potentials of liquids directly from first-principles without the help of any reference system, which is necessary in the commonly used thermodynamic integration method. As an example, we apply our scheme, combined with the density functional formalism, to the calculation of the chemical potential of liquid copper. The calculated chemical potential is further used to locate the melting temperature. The calculated results closely agree with experiments.

Hong, Qi-Jun; van de Walle, Axel

2012-09-01

125

First Principles Simulation of a Ceramic /Metal Interface with Misfit

NASA Astrophysics Data System (ADS)

The relaxed atomic structure of a model ceramic/metal interface, \\{222\\}MgO/Cu, is simulated, including lattice constant mismatch, using first principles local-density functional theory plane wave pseudopotential methods. The 399-atom computational unit cell contains 36 O and 49 Cu atoms per layer in accordance with the 7/6 ratio of MgO to Cu lattice constants. The atomic layers on both sides of the interface warp to optimize the local bonding. The interface adhesive energy is calculated. The interface electronic structure is found to vary appreciably with the local environment.

Benedek, R.; Alavi, A.; Seidman, D. N.; Yang, L. H.; Muller, D. A.; Woodward, C.

2000-04-01

126

First-principles study of lithium intercalated bilayer graphene

NASA Astrophysics Data System (ADS)

Lithium intercalated bilayer graphene has been investigated using first-principles density functional theory calculations. Results show that there exist AB and AA stacking sequences for bilayer graphene in which the latter is more favorable for the Li storage and the former will evolve into the latter with the intercalation of Li ions. The relationship between the interlayer distance of two graphene sheets and the intercalated capacity of Li ions is discussed. It is found that structural defect is identified to store Li ions more favorably than pristine bilayer graphene and an isolated C atom vacancy in bilayer graphene can capture three Li ions between two graphene sheets.

Zhou, JingJing; Zhou, WeiWei; Guan, ChunMei; Shen, JingQin; Ouyang, ChuYing; Lei, MinSheng; Shi, SiQi; Tang, WeiHua

2012-08-01

127

Stability of graphene oxide phases from first-principles calculations

NASA Astrophysics Data System (ADS)

We determine the energy diagram of graphene oxides (GOs) as a function of oxygen and hydrogen chemical potentials by systematic first-principles calculations. The diagram reveals that thermodynamically stable GOs can exist only in stringent growth conditions in the form of hydroxyl, epoxy, or mixed hydroxyl/epoxy phases. There is no mixed phase with sp2 carbon because of substrate relaxation, which is unique to two-dimensional system with little coupling in the third direction such as graphene. The mixed phase observed experimentally is interpreted instead, in terms of a kinetic stability of nonequilibrium grown GOs against phase separation.

Wang, Lu; Sun, Y. Y.; Lee, Kyuho; West, D.; Chen, Z. F.; Zhao, J. J.; Zhang, S. B.

2010-10-01

128

First-principles study of graphene - carbon nanotube contacts

NASA Astrophysics Data System (ADS)

The electron transport properties of carbon nanotube -- graphene junctions are investigated with first-principles total energy and electron transport calculations. By combining the advantageous material properties of graphene and nanotubes one can create all carbon hybrid architectures with properties that are particularly well suited to applications. The p-type Schottky barrier height is calculated in model junctions with (8,0) and (10,0) nanotubes in a top-contact configuration. Results indicate a lower barrier in carbon nanotube -- graphene junctions than in other carbon nanotue -- metal systems.

Cook, Brandon; Varga, Kalman

2012-02-01

129

First-Principles Study of Impurities in TlBr

TlBr is a promising semiconductor material for room-temperature radiation detection. Material purification has been the driver for the recent improvement in the TlBr detector performance, mainly reflected by the significant increase in the carrier mobility-lifetime product. This suggests that impurities have significant impact on the carrier transport in TlBr. In this paper, first-principles calculations are used to study the properties of a number of commonly observed impurities in TlBr. The impurity-induced gap states are presented and their effects on the carrier trapping are discussed.

Du, Mao-Hua [ORNL

2012-01-01

130

First-principles study of manganese-stabilized hafnia

NASA Astrophysics Data System (ADS)

We present a first-principles study of the electronic and magnetic properties of cubic hafnium dioxide stabilized by Mn. We find this material to be ferromagnetic and half-metallic, with the Mn-impurity electronic states lying in the band gap of hafnia for a wide range of manganese concentration. Our ab initio calculations, within the local spin-density approximation, demonstrate that Mn-doped hafnia may be ferromagnetic at 700 K while its high-TC ferromagnetism is robust to the oxygen vacancy defects and to how the Mn impurities are distributed over the cation sublattice.

Maznichenko, I. V.; Ostanin, S.; Ernst, A.; Mertig, I.

2009-04-01

131

Thermopower switching by magnetic field: First-principles calculations

NASA Astrophysics Data System (ADS)

We present first-principles studies of the thermopower of the organometallic V4Bz5 molecule attached between Co electrodes with noncollinear magnetization directions. Different regimes in the formation of the noncollinear magnetic state of the molecule lead to a remarkable nonmonotonous dependence of the thermopower on the angle between the magnetizations of the electrodes. This complex behavior is explained by the resonant properties of the electron transmission. Consequently, the nanocontacts can be utilized for local heating or cooling controlled by the external magnetic field.

Maslyuk, Volodymyr V.; Achilles, Steven; Sandratskii, Leonid; Brandbyge, Mads; Mertig, Ingrid

2013-08-01

132

Geometrically induced melting variation in gallium clusters from first principles

NASA Astrophysics Data System (ADS)

First-principles Born-Oppenheimer molecular dynamics simulations of gallium clusters reproduce experimental specific heats to a high degree of accuracy, with no experimental input or fitting. Remaining systematic shifts of the melting temperature highlight important limitations of density-functional approximations. An analysis of the structural changes observed as a function of finite temperature provides evidence for the assignment of a melting temperature to a specific peak in the specific heat. The structural basis for features in the specific heat curves, in particular, the existence of multiple peaks, is demonstrated.

Steenbergen, K. G.; Gaston, N.

2013-10-01

133

Magnetism in Mn doped yttrium nitride: First-principles calculations

NASA Astrophysics Data System (ADS)

The magnetism and aggregation trends in 25% and 12.5% Mn doped yttrium nitride (YN) have been investigated using the first-principles calculations. We demonstrate that the ferromagnetic delta doping is the ground state in higher concentration Mn doped YN, and the ferromagnetic homogeneous half-delta doping is the ground state in lower concentration Mn half-delta-doped YN. The latter shows ferromagnetic stabilization energy of 424.8 meV/Mn-Mn pair, which is substantially higher than many other compound semiconductor based magnetic semiconductors. Finally, we anticipate the potential spintronics application of Mn doped YN.

Jia, Xingtao; Yang, Wei; Qin, Minghui

2008-12-01

134

NASA Astrophysics Data System (ADS)

We report a combined experimental and theoretical study of the unoccupied electronic states of the neutral molecular organic materials TTF (tetrathiafulvalene) and TCNQ (7,7,8,8-tetracyano-p-quinodimethane) and of the one-dimensional metallic charge transfer salt TTF-TCNQ. The experimental density of states (DOS) is obtained by x-ray absorption near edge spectroscopy (XANES) with synchrotron light and the predicted DOS by means of first-principles density functional theory calculations. Most of the experimentally derived element-specific XANES features can be associated to molecular orbitals of defined symmetry. Because of the planar geometry of the TTF and TCNQ molecules and the polarization of the synchrotron light, the energy dependent ? or ? character of the orbitals can be inferred from angular dependent XANES measurements. The present work represents the state of the art analysis of the XANES spectra of this type of materials and points out the need for additional work in order to elucidate the governing selection rules in the excitation process.

Fraxedas, J.; Lee, Y. J.; Jiménez, I.; Gago, R.; Nieminen, R. M.; Ordejón, P.; Canadell, E.

2003-11-01

135

First principles calculations are used to show that O chemical shielding tensors, sigma, are a sensitive indicator of local structure in transition metal ABO3 perovskites, due to their strong dependence on covalent O(2p)-B(nd) interactions.footnotetextPechkis et al., JCP 131, 184511 (2009); references therein. This indicates that ^17O NMR spectroscopy, coupled with first principles calculations, can be an especially useful tool to

Daniel L. Pechkis; Eric J. Walter; Henry Krakauer

2010-01-01

136

NMR computations for carbon nanotubes from first principles: Present status and future directions

NASA Astrophysics Data System (ADS)

NMR (nuclear magnetic resonance) is a versatile experimental tool to study the properties of molecules and solids. It has been proposed that reliable computational data of the 13C NMR chemical shifts of different types of carbon nanotubes may be used to guide experimental characterization by NMR. Within the last few years this field has become quite active. After outlining the background for first-principles calculations, as well as early model calculations, we focus on recent first-principles theoretical studies performed toward this end. Studies on finite and infinite SWNT systems have indicated that 13C NMR may be used to determine the diameter distribution of the tubes in a bulk sample. The Knight shift of metallic tubes has been examined. NICS (nucleus independent chemical shifts) have yielded information about the aromaticity of various systems, and the NMR chemical shifts of small molecules trapped in nanotubes have been calculated. Work on SWNTs functionalized with NR groups has suggested that 13C NMR may be used to determine which nanotube carbons are derivatized, and perhaps even yield information about the diameter of the tubes. It has also been found that 13C NMR may be useful to quantify the degree of fluorination. Studies on Stone-Wales defects have indicated that well-resolved NMR signals may arise from atoms in the defect site. Shielding tensor data is also discussed. The theoretical progress made in this field shows that a wealth of information is contained within the NMR chemical shifts of carbon nanotubes.

Zurek, Eva; Autschbach, Jochen

137

First principles calculations are performed for the interpretation of the L?,? x-ray absorption spectrum of calcium oxide and calcium fluoride. The first principles calculations are based on configuration interaction (CI) calculations using fully relativistic molecular spinors. The first principles results are compared to experimental data and also to calculations based on a semi-empirical crystal field multiplet model and also on a multichannel multiple scattering method. We show that the CI calculations show good agreement with experiment, both for bulk and for surface experiments. The remaining differences with experiment and between the theoretical models are discussed in detail. PMID:21427477

Miedema, P S; Ikeno, H; de Groot, F M F

2011-03-22

138

NASA Astrophysics Data System (ADS)

First principles calculations are performed for the interpretation of the L2, 3 x-ray absorption spectrum of calcium oxide and calcium fluoride. The first principles calculations are based on configuration interaction (CI) calculations using fully relativistic molecular spinors. The first principles results are compared to experimental data and also to calculations based on a semi-empirical crystal field multiplet model and also on a multichannel multiple scattering method. We show that the CI calculations show good agreement with experiment, both for bulk and for surface experiments. The remaining differences with experiment and between the theoretical models are discussed in detail.

Miedema, P. S.; Ikeno, H.; de Groot, F. M. F.

2011-04-01

139

Understanding the State of the Inner Core from First Principles

NASA Astrophysics Data System (ADS)

Earth's inner core plays a vital role in the dynamics of our planet and is itself strongly exposed to dynamic processes as evidenced by a complex pattern of elastic structure. To gain deeper insight into the nature of these processes we rely on a characterization of the physical properties of the inner core that are governed by the material physics of its main constituent, iron. We will discuss the thermal state and elasticity of the inner core, based on predictions of material properties of iron under inner core conditions, obtained from first principles computational methods. We will show that aggregate seismic properties of the inner core, including its high Poissons ratio, can be explained by temperature and compression effects on the elasticity of pure iron alone. We use single crystal anisotropy of hcp iron to develop a speculative model of texture in the inner core that can explain major aspects of inner core anisotropy.

Steinle-Neumann, G.; Stixrude, L.; Cohen, R. E.

2002-05-01

140

Hydration of alkali ions from first principles molecular dynamics revisited.

Structural and dynamical properties of the hydration of Li(+), Na(+), and K(+) in liquid water at ambient conditions were studied by first principles molecular dynamics. Our simulations successfully captured the different hydration behavior shown by the three alkali ions as observed in experiments. The present analyses of the dependence of the self-diffusion coefficient and rotational correlation time of water on the ion concentration suggest that Li(+) (K(+)) is certainly categorized as a structure maker (breaker), whereas Na(+) acts as a weak structure breaker. An analysis of the relevant electronic structures, based on maximally localized Wannier functions, revealed that the dipole moment of H(2)O molecules in the first solvation shell of Na(+) and K(+) decreases by about 0.1 D compared to that in the bulk, due to a contraction of the oxygen lone pair orbital pointing toward the metal ion. PMID:17249878

Ikeda, Takashi; Boero, Mauro; Terakura, Kiyoyuki

2007-01-21

141

Redox potential of liquid water: A first-principles theory

NASA Astrophysics Data System (ADS)

A first-principles molecular dynamic method is proposed to calculate the absolute redox potentials of liquid water. The key of the method is the evaluation of the difference between the vacuum level and the average electrostatic potential inside liquid water, which employs an average over both time and space. By avoiding the explicit use of the Kohn-Sham level, such as the position of the valence band maximum, as the reference energy for the excited electrons, we are able to calculate water redox potentials accurately. The results using the PBE functional are in good agreement with experiment. We attribute the success of the method to the accurate charge density given by density functional calculation under the local or semi-local approximations. This establishes the validity to apply these effective and efficient approximations to study both the energetics and dynamics of the redox processes at more complex systems such as solid/solution interfaces.

Lucking, Michael; Sun, Yiyang; West, Damien; Zhang, Shengbai

2013-03-01

142

High-pressure behavior of phosphorus from first principles calculations

NASA Astrophysics Data System (ADS)

High-pressure structural behavior, electronic structure, and vibrational properties of P have been investigated by means of first principles calculations. We perform an ab initio search for the P-IV phase by analyzing the phonon dispersions of the simple cubic (sc) structure as a function of pressure. In particular, we use the dynamical instability of the sc structure above the transition into the simple hexagonal structure to extract a possible candidate structure for P-IV. Additionally, in contrast to general expectations, we show that the body-centered cubic (bcc) P-V phase cannot be the end member of the high-pressure structural sequence of P because it becomes dynamically unstable at high compression. We propose that at ultrahigh pressure above 280GPa , the bcc structure transforms first to the IM7 structure and then to the hexagonal close-packed structure.

Mikhaylushkin, A. S.; Simak, S. I.; Johansson, B.; Häussermann, U.

2007-09-01

143

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

144

NMR shifts for polycyclic aromatic hydrocarbons from first-principles

NASA Astrophysics Data System (ADS)

We present first-principles, density-functional theory calculations of the NMR chemical shifts for polycyclic aromatic hydrocarbons, starting with benzene and increasing sizes up to the one- and two-dimensional infinite limits of graphene ribbons and sheets. Our calculations are performed using a combination of the recently developed theory of orbital magnetization in solids, and a novel approach to NMR calculations where chemical shifts are obtained from the derivative of the orbital magnetization with respect to a microscopic, localized magnetic dipole. Using these methods we study on equal footing the 1H and 13 shifts in benzene, pyrene, coronene, in naphthalene, anthracene, naphthacene, and pentacene, and finally in graphene, graphite, and an infinite graphene ribbon. Our results show very good agreement with experiments and allow us to characterize the trends for the chemical shifts as a function of system size.

Thonhauser, T.; Ceresoli, Davide; Marzari, Nicola

145

First-principles modeling of magnetic misfit interfaces

NASA Astrophysics Data System (ADS)

We investigate the structural and magnetic properties of interfaces with large lattice mismatch, choosing Pt/Co and Au/Co as prototypes. For our first-principles calculations, we reduce the lattice mismatch to 0.2% by constructing Moiré supercells. Our results show that the roughness and atomic density, and thus the magnetic properties, depend strongly on the substrate and thickness of the Co slab. An increasing thickness leads to the formation of a Co transition layer at the interface, especially for Pt/Co due to strong Pt-Co interaction. A Moiré supercell with a transition layer is found to reproduce the main experimental findings and thus turns out to be the appropriate model for simulating magnetic misfit interfaces.

Grytsyuk, Sergiy; Schwingenschlögl, Udo

2013-10-01

146

Testing the Calibration of Classification Models from First Principles

The accurate assessment of the calibration of classification models is severely limited by the fact that there is no easily available gold standard against which to compare a model’s outputs. The usual procedures group expected and observed probabilities, and then perform a ?2 goodness-of-fit test. We propose an entirely new approach to calibration testing that can be derived directly from the first principles of statistical hypothesis testing. The null hypothesis is that the model outputs are correct, i.e., that they are good estimates of the true unknown class membership probabilities. Our test calculates a p-value by checking how (im)probable the observed class labels are under the null hypothesis. We demonstrate by experiments that our proposed test performs comparable to, and sometimes even better than, the Hosmer-Lemeshow goodness-of-fit test, the de facto standard in calibration assessment.

Dreiseitl, Stephan; Osl, Melanie

2012-01-01

147

First Principles Study of Piezoelectricty in Improper Ferroelectrics

NASA Astrophysics Data System (ADS)

Piezoelectric materials are key components of many important technologies, and discovering materials with improved piezoelectric responses is a major goal of materials science. In particular, finding new mechanisms for piezoelectricity which allow for high piezoelectric coefficients, especially in lead-free materials, could have great technological impact. Recently, there has been a renewed interest in improper ferroelectrics, which are materials where a non-zero polarization is induced indirectly by the coupling of the polar distortion to non-polar unstable modes, frequently oxygen octahedral rotations. This mechanism for creating a polarization may also offer the possibility of increased coupling to strain, leading to high piezoelectric coefficients. Here, we use first principles density functional theory to investigate the mechanism of the piezoelectric response of Ca3Ti2O7, an improper ferroelectric which we find to have large piezoelectric coefficients.

Garrity, Kevin; Rabe, Karin

2012-02-01

148

Simulated doping of Si from first principles using pseudoatoms

NASA Astrophysics Data System (ADS)

Semiconductor doping is a process of fundamental importance to semiconductor physics and solid-state electronics, but cannot be explicitly simulated from first principles due to the huge system size needed for most doping scenarios. We examine the efficacy of the simulation of doping in silicon by the inclusion of “pseudoatoms” with fractional nuclear charge, introduced via specially constructed pseudopotentials. These provide a net charge carrier concentration matching an arbitrarily chosen doping level, at no increase of the computational cost. By extending this approach to consider minute deviations from the integer charge, we demonstrate that the electron Fermi level can be set to any value within the forbidden gap, at minimal perturbation of the electronic structure. Beyond the bulk scenario, we successfully simulate the development of the space-charge region in a heavily doped p-n junction and the doping dependence of the work function of the hydrogen-passivated (semiconducting) Si(111) surface.

Sinai, Ofer; Kronik, Leeor

2013-06-01

149

First-principles molecular reactive dynamics of energetic materials

NASA Astrophysics Data System (ADS)

The understanding of initiation chemistry of shock-compressed energetic materials on the atomic scale is of fundamental importance for developing a predictive theory of initiation of detonation. We have performed first-principles density-functional modeling of reactive molecular collisions of PETN and RDX molecules aimed at elucidating the first chemical events that trigger the chemistry behind the shock wave front. We will discuss fundamental mechanisms responsible for the transformation of mechanical energy from the shock wave into molecular degrees of freedom that result in excitation of a reaction mode and eventual bond breaking. We will also discuss the stereochemistry of initial reaction events, unimolecular, bi-molecular or multi-molecular nature of initial reactive events and the relationship of the simulated reactive collisions with a non-equilibrium shock wave environment.

Landerville, Aaron; Oleynik, Ivan; White, Carter

2007-03-01

150

First-principles calculation of current density in molecular devices

NASA Astrophysics Data System (ADS)

Based on the single-particle nonequilibrium Green's function (NEGF) technique coupled with the density-functional theory (DFT), we investigate the current density distribution of a molecular device Al-C60-Al from first principles. Due to the presence of nonlocal pseudopotential, the conventional definition of current density is not suitable to describe the correct current density profile inside the molecular device. By using the new definition of current density, which includes the contribution due to the nonlocal potential, our numerical results show that the new definition of current density J(r) conserves the current. In addition, the current obtained from the current density calculated inside the molecular device equals to that calculated from the Landauer-Büttiker formula. Finally, for the molecular device Al-C60-Al, loop currents were found, which confirms the result obtained from the tight-binding approach.

Zhang, Lei; Wang, Bin; Wang, Jian

2011-09-01

151

Lattice dynamics of anharmonic solids from first principles

NASA Astrophysics Data System (ADS)

An accurate and easily extendable method to deal with lattice dynamics of solids is offered. It is based on first-principles molecular dynamics simulations and provides a consistent way to extract the best possible harmonic--or higher order--potential energy surface at finite temperatures. It is designed to work even for strongly anharmonic systems where the traditional quasiharmonic approximation fails. The accuracy and convergence of the method are controlled in a straightforward way. Excellent agreement of the calculated phonon dispersion relations at finite temperature with experimental results for bcc Li and bcc Zr is demonstrated. In addition to that the bcc-hcp phase diagram for Zr is calculated with high accuracy. arXiv:1103.5590v3 [cond-mat.mtrl-sci

Hellman, Olle; Abrikosov, Igor; Simak, Sergei

2012-02-01

152

Surface energies of AlN allotropes from first principles.

In this letter we present first-principles calculations of the surface energies of rock-salt (B1), zinc-blende (B3) and wurtzite (B4) AlN allotropes. Of several low-index facets, the highest energies are obtained for monoatomic surfaces (i.e. of only either Al or N atoms): [Formula: see text] and [Formula: see text]. The difference between Al- and N-terminated surfaces in these cases is less then 20 meV/?(2). The stoichiometric facets have energies lower by 100 meV/?(2) or more. The obtained trends could be rationalized by a simple nearest-neighbour broken-bond model. PMID:23471418

Holec, David; Mayrhofer, Paul H

2012-11-01

153

First-principles methodology for quantum transport in multiterminal junctions.

We present a generalized approach for computing electron conductance and I-V characteristics in multiterminal junctions from first-principles. Within the framework of Keldysh theory, electron transmission is evaluated employing an O(N) method for electronic-structure calculations. The nonequilibrium Green function for the nonequilibrium electron density of the multiterminal junction is computed self-consistently by solving Poisson equation after applying a realistic bias. We illustrate the suitability of the method on two examples of four-terminal systems, a radialene molecule connected to carbon chains and two crossed-carbon chains brought together closer and closer. We describe charge density, potential profile, and transmission of electrons between any two terminals. Finally, we discuss the applicability of this technique to study complex electronic devices. PMID:19894925

Saha, Kamal K; Lu, Wenchang; Bernholc, J; Meunier, Vincent

2009-10-28

154

The first-principles calculation of molecular conduction

NASA Astrophysics Data System (ADS)

We used the self-consistent method-based density functional theory (DFT) and non-equilibrium Green’s function (NEGF) to simulate molecular transport. Our numerical calculations for the organic molecular measurement made by Reichert et al. (Phys. Rev. Lett., 2002, 88: 176804) and for the alkanedithiols measurement made by Xu et al. (Science, 2003, 301: 1221) met the related experimental values quite well. This means that the first-principles calculations based on DFT and NEGF can well explain the conduction measurements of some large molecules. The numerical study reveals the fact that molecular conduction does not obey the classic law; in stead it illustrates the quantum behavior. We designed active molecular transistors controlled by the gate bias with high working frequency. They may be the next generation electronic devices.

Chen, Hao

2009-09-01

155

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

156

First-principles investigation of technetium carbides and nitrides

NASA Astrophysics Data System (ADS)

Phase stabilities and mechanical properties of ideal stoichiometric technetium monocarbide (TcC) and technetium mononitride (TcN) in the tungsten carbide (WC), nickel arsenide (NiAs), rocksalt (NaCl), and zinc-blende (ZnS) structures, respectively, have been studied systematically by first-principles calculations. It is found that both TcC and TcN in two hexagonal phases (WC and NiAs) are not only elastically stable but also hard and ultrastiff materials. Remarkably, for the two hexagonal TcC phases, both bulk moduli and linear incompressibilities along the c axis exceed that of c BN and even rival with diamond. Their hardness can also match the known hard materials such as WC. The combination of good metallicity, strong stiffness, and high hardness suggests that the materials may find applications as hard conductors and cutting tools.

Liang, Yongcheng; Li, Chun; Guo, Wanlin; Zhang, Wenqing

2009-01-01

157

First-principles GW calculations for DNA and RNA nucleobases

NASA Astrophysics Data System (ADS)

On the basis of first-principles GW calculations, we study the quasiparticle properties of the guanine, adenine, cytosine, thymine, and uracil DNA and RNA nucleobases. Beyond standard G0W0 calculations, starting from Kohn-Sham eigenstates obtained with (semi)local functionals, a simple self-consistency on the eigenvalues allows us to obtain vertical ionization energies and electron affinities within an average 0.11 and 0.18 eV error, respectively, as compared to state-of-the-art coupled-cluster and multiconfigurational perturbative quantum chemistry approaches. Further, GW calculations predict the correct ?-character of the highest occupied state, due to several level crossings between density functional and GW calculations. Our study is based on a recent Gaussian-basis implementation of GW calculations with explicit treatment of dynamical screening through contour deformation techniques.

Faber, Carina; Attaccalite, Claudio; Olevano, V.; Runge, E.; Blase, X.

2011-03-01

158

First-principles investigation of transient dynamics of molecular devices

NASA Astrophysics Data System (ADS)

Based on the nonequilibrium Green's function (NEGF) and time-dependent density-functional theory (TDDFT), we propose a formalism to study the time-dependent transport behavior of molecular devices from first principles. While this approach is equivalent to the time-dependent wave-function approach within TDDFT, it has the advantage that the scattering states and bound states are treated on equal footing. Furthermore, it is much easier to implement our approach numerically. Different from the time-dependent wave-function [?(t,E)] approach, our formalism is in the time space [Gr(t,t')], making this method superior in the time-dependent transport problem with many subbands in the transverse direction. For the purpose of numerical implementation on molecular devices, a computational tractable numerical scheme is discussed in detail. We have applied our formalism to calculate the transient current of two molecular devices Al-1,4-dimethylbenzene-Al and Al-benzene-Al from first principles. In the calculation, we have gone beyond the wideband limit and used the adiabatic local density approximation that was used within TDDFT. It is known that when the wideband limit is abandoned, the boundary condition of the transport problem is non-Markovian, resulting in a memory term in the effective Hamiltonian of the scattering region. To overcome the computational complexity due to the memory term, we have employed a fast algorithm to speed up the calculation and reduced the CPU time from the scaling N3 to N2log22(N) for the steplike pulse, where N is the number of time steps in the time evolution of the Green's function. To ensure the accuracy of our method, we have done a benchmark transient calculation on an atomic junction using a time-dependent wave-function approach within TDDFT in momentum space, which agrees very well with the result from our method.

Zhang, Lei; Xing, Yanxia; Wang, Jian

2012-10-01

159

First-principles studies for understanding diverse high-Tc

NASA Astrophysics Data System (ADS)

In this talk, I survey results and insights gained from first-principles calculations on materials that exhibit superconducting behavior at temperatures higher than those characteristic of conventional BCS superconductors. These range from highly correlated cuprate Mott insulators as represented by the bismuth-strontium-calcium-copper-oxides (BSCCOs) to border-line itinerant-Mott systems such as the recently discovered 1111 and 122 pnictides. ultimate goal of our studies is to correlate Tc with specific material composition using detailed first-principles calculations in conjunction with many-body physics techniques via the critical step of constructing real-materials model Hamiltonians. By manipulating impurity doping, which plays a crucial role in the phase diagrams of high Tc materials, we hope to find guidance for designing candidate systems with Tc higher than ones currently known. BSCCO material, density functional calculations using a good generalized-gradient approximation (GGA) yield structural information that is correlated to the experimentally observed (STM) super-modulation and impurity peak in the high energy regime (˜1 eV), even though the Kohn-Sham bands from such functionals fail to have a band gap. For FeAs-based high-Tc systems, DFT band-structure calculations provide a very good starting point for constructing model Hamiltonians for studies of spin fluctuation and electron pairing mechanisms. Fermi sheets that have been constructed using Wannier transformed Kohn-Sham states have provided critical information for understanding this family of superconducting materials. Analysis of the details of magnetic ordering, density of states, and 2D vs. 3D features in both the 1111 and 122 materials have been valuable in understanding sometimes perplexing experimental findings. Effects of Co impurities have been studied and fully analyzed as well., I will discuss persistent challenges related to calculations on the structure of the non-magnetic state Ba1Fe2As2 system. Both further examination of the underlying physics and development of new approximate functionals are needed.

Cheng, Hai-Ping

2011-03-01

160

Theoretical calculations of electron energy loss near edge structures (ELNES) of lattice imperfections, particularly a Ni(111)/ZrO?(111) heterointerface and an Al?O? stacking fault on the {1100} plane, are performed using a first principles pseudopotential method. The present calculation can qualitatively reproduce spectral features as well as chemical shifts in experiment by employing a special pseudopotential designed for the excited atom with a core-hole. From the calculation, spectral changes observed in O-K ELNES from a Ni/ZrO? interface can be attributable to interfacial oxygen-Ni interactions. In the O-K ELNES of Al?O? stacking faults, theoretical calculation suggests that the spectral feature reflects coordination environment and chemical bonding. Powerful combinations of ELNES with a pseudopotential method used to investigate the atomic and electronic structures of lattice imperfections are demonstrated. PMID:21803589

Mizoguchi, Teruyasu; Matsunaga, Katsuyuki; Tochigi, Eita; Ikuhara, Yuichi

2011-07-14

161

NASA Astrophysics Data System (ADS)

Spectroscopy forms the bridge between theory and experiment in the analysis of structure, properties, and reactivity of functional molecules and molecular aggregates. Our knowledge on the basic working principles of systems such as photosynthetic units strongly relies on spectroscopic information, which is interpreted in terms of molecular or submolecular building blocks. To choose such entities as the essential ingredients in a quantum chemical framework is thus a promising route to the theoretical spectroscopy of complex systems. This work describes developments of chromophore-specific quantum chemical methods, which focus on relevant substructures without sacrificing the view on the entire system. A subsystem density-functional theory approach is analyzed that employs a real-space partitioning of the electron density for the description of complex aggregates in terms of simple fragments. This approach can be used as a chromophore-specific embedding method and allows for efficient and accurate analyses of environmental effects. However, it fails for phenomena caused by a collective response of an aggregate of molecules. The limitations of this embedding scheme can be overcome by a general subsystem approach to time-dependent density functional theory, which easily relates to phenomenological theories such as excitonic coupling models. Resonance Raman spectroscopy can be used to probe local excited states in larger molecules and is thus intrinsically chromophore-specific. It is shown that well-known approximations for resonance Raman calculations can efficiently be used with time-dependent density-functional theory methods to study photochemical and photophysical processes in large molecules such as artificial photosynthetic systems. Intensity-driven approaches to resonance Raman calculations can exploit the selectivity observed in experiments for an iterative determination of high-intensity spectral features. Applications of such schemes to biochemical building blocks are discussed.

Neugebauer, Johannes

2010-04-01

162

First-principles studies of ferroelectric surfaces, alloys, and domain walls

NASA Astrophysics Data System (ADS)

This thesis is organized as four main parts that follow a short introductory Chapter. The first part consists of Chapters 2 and 3. In Chapter 2, we introduce the theoretical methods we used in our calculations. In Chapter 3, we describe the work on bulk perovkite compounds by R. D. King-Smith and D. Vanderbilt. This will serve as an introduction to the rest of the thesis. It is closely based on the paper "First-principles investigation of ferroelectricity in perovkite compounds" [43] by these two authors, and includes some tables and figures that are reproduced here with the permission of the authors. In the second part (Chapter 4) the ferroelectric surfaces of BaTiO 3 and SrTiO3 are studied using density functional theory with Vanderbilt ultrasoft-pseudopotentials. The third part (Chapters 5 and 6) is a study of alloys PZT PbZr1-xTix O3, BZN BaZr1-xTix O3, and PZN PbZn1/3Nb2/3 O3, using the same method. This work was done in collaboration with L. Bellaiche and D. Vanderbilt. In the last part (Chapter 7) we present a first-principles study of 180° ferroelectric domain walls in tetragonal barium titanate. The theory is based on an effective Hamiltonian that has previously been determined from first-principles calculations. Statistical properties are investigated using Monte Carlo simulations. This work was done in collaboration with W. Zhong and D. Vanderbilt.

Padilla, Jorge L.

163

Phonons in Bi2S3 nanostructures: Raman scattering and first-principles studies

NASA Astrophysics Data System (ADS)

Bi2S3 has shown promise in thermoelectric and optoelectronic applications as well as biological and chemical sensors. We present here a comprehensive study on the lattice dynamics of Bi2S3 nanostructures probed by micro-Raman scattering spectroscopy and first-principles calculations. Bi2S3 nanowires are synthesized using a physical vapor transport method via a vapor-liquid-solid mechanism on silicon substrates. Oriented Bi2S3 nanosheets are also obtained on mica substrates. The structure of the nanowires is determined to be orthorhombic with a growth orientation of [110] by x-ray diffraction and high-resolution transmission electron microscopy. A Raman scattering study is conducted for as-prepared Bi2S3 nanostructures, in which 33, 38, 46, and 53 cm-1 phonon modes are observed for the first time. We find several modes to be very sensitive to excitation wavelength and power. First-principles calculations of orthorhombic Bi2S3 predict a series of Raman modes, in good agreement with our experiments. Phonon-dispersion curves of Bi2S3 are also presented, and the effect of Born effective charges on the longitudinal-optical-transverse-optical splitting at the zone center is taken into account.

Zhao, Yanyuan; Chua, Kun Ting Eddie; Gan, Chee Kwan; Zhang, Jun; Peng, Bo; Peng, Zeping; Xiong, Qihua

2011-11-01

164

First Principles Studies of ABO3 Perovskite Surfaces and Nanostructures

NASA Astrophysics Data System (ADS)

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

Pilania, Ghanshyam

165

TOPICAL REVIEW: First principles studies of multiferroic materials

NASA Astrophysics Data System (ADS)

Multiferroics, materials where spontaneous long-range magnetic and dipolar orders coexist, represent an attractive class of compounds, which combine rich and fascinating fundamental physics with a technologically appealing potential for applications in the general area of spintronics. Ab initio calculations have significantly contributed to recent progress in this area, by elucidating different mechanisms for multiferroicity and providing essential information on various compounds where these effects are manifestly at play. In particular, here we present examples of density-functional theory investigations for two main classes of materials: (a) multiferroics where ferroelectricity is driven by hybridization or purely structural effects, with BiFeO3 as the prototype material, and (b) multiferroics where ferroelectricity is driven by correlation effects and is strongly linked to electronic degrees of freedom such as spin-, charge-, or orbital-ordering, with rare-earth manganites as prototypes. As for the first class of multiferroics, first principles calculations are shown to provide an accurate qualitative and quantitative description of the physics in BiFeO3, ranging from the prediction of large ferroelectric polarization and weak ferromagnetism, over the effect of epitaxial strain, to the identification of possible scenarios for coupling between ferroelectric and magnetic order. For the second class of multiferroics, ab initio calculations have shown that, in those cases where spin-ordering breaks inversion symmetry (e.g. in antiferromagnetic E-type HoMnO3), the magnetically induced ferroelectric polarization can be as large as a few µC cm-2. The examples presented point the way to several possible avenues for future research: on the technological side, first principles simulations can contribute to a rational materials design, aimed at identifying spintronic materials that exhibit ferromagnetism and ferroelectricity at or above room temperature. On the fundamental side, ab initio approaches can be used to explore new mechanisms for ferroelectricity by exploiting electronic correlations that are at play in transition metal oxides, and by suggesting ways to maximize the strength of these effects as well as the corresponding ordering temperatures.

Picozzi, Silvia; Ederer, Claude

2009-07-01

166

Absolute Diffusion Rates in Minerals from First Principles

NASA Astrophysics Data System (ADS)

Absolute diffusion rates in minerals depend on vacancy concentrations, the enthalpies and entropies of migration, and the ionic jump attempt frequencies. It is now possible to use ab initio (or first principles) methods to calculate all these parts, and therefore, predict diffusion rates accurately for minerals under conditions which are difficult to obtain experimentally. Techniques such as the Climbing-Image Nudged-Elastic-Band, make it straightforward to find transition states (i.e., the saddle point over which the migrating ion passes). The use of harmonic theories, such as that of Vineyard, allow us to calculate pre-factors (attempt frequencies and migration entropies). And computers are now capable of handling sufficiently large systems (i.e., several hundred atoms) that all parts can be calculated from first principles. We have used these methods to calculate absolute diffusion rates of all three major cation species, Mg, Si, and O, in perovskite, post-perovskite and periclase. Comparison of our results with experimental results (where they exist), show excellent agreement. The largest uncertainty is the vacancy concentration. When vacancy concentrations are constrained extrinsically in the experiments and, therefore, can be accurately determined, our results agree particularly well with the experiments. Results on post-perovskite (where there is no experimental data) show that Si and Mg diffusion is highly anisotropic. Although we may expect slow diffusion across the (010) layers, the surprising results is that the anisotropy is mostly caused by extremely fast diffusion in the [100] direction, which is four orders of magnitude faster than Mg and Si diffusion in perovskite. This means that post-perovskite deforming under dislocation creep could be up to four orders of magnitude weaker than perovskite. This will have a number of important consequences for the dynamics and behaviour of the lower-most mantle. We have also calculated Fe diffusion in these phases. In both periclase and perovskite we find that the diffusion of Fe depends strongly on spin state, with low spin Fe diffusing significantly slower than high spin Fe. Although this will strongly affect the rheology of Fe-bearing periclase, its affect on the mantle will depend on whether periclase is interconnected or not. Results from these studies, together with on-going calculations on the diffusion of hydrogen in perovskite, will be presented.

Brodholt, John; Ammann, Michael; Dobson, David

2010-05-01

167

First Principles Models for Ferroelectricity and Piezoelectricity in Solid Solutions

NASA Astrophysics Data System (ADS)

The largest piezoelectric coefficients are observed not in pure compounds, but in solid solutions such as Pb(Zr_1-xTi_x)O3 and Pb(A_1/3Nb_2/3)O_3--PbTiO_3, (A = Zn, Mg)(S.-E. Park and T. R. Strout, J. Appl. Phys.) 82, 1804 (1997).. In this talk, we describe our first-principles approach to modeling ferroelectricity and piezoelectricity in solid solutions(E. Cockayne and K. M. Rabe, Phys. Rev. B) 56, 7947 (1997)., and apply it to Pb_1-xGe_xTe. By exploiting the microscopic information available from first principles calculations, we can identify factors that enhance piezoelectricity. For Pb_1-xGe_xTe, detailed ab initio calculations on various ordered supercells show significant differences in the low-temperature piezoelectric response of different ordered configurations, even at fixed composition. We have identified the following trends: (1) The highest piezoelectricity is often associated with the greatest deviation of local structure from average structure. Local relaxation occurs due to size mismatch between the substituting ions. Locally, this relaxation can have an effect similar to applying a large positive or negative local pressure to the corresponding pure system, driving it into a regime of large response. (2) As the strength of instability of the high symmetry prototype structure decreases, the piezoelectric response increases. Fitting of the ab initio results to double-well potentials explains the latter trend, showing that in the limit of marginal instability piezoelectricity diverges. Our results suggest the possibility of tuning ferroelectric and piezoelectric properties through controlling configurations. Typically, the piezoelectric response of a ferroelectric is sharply peaked near the paraelectric-ferroelectric transition temperature. In solid solutions, it becomes possible to obtain a large response over a range of temperatures, which is desirable for applications. We present an effective Hamiltonian approach to modeling the temperature dependence of the polarization and piezoelectricity in Pb_1-xGe_xTe.

Cockayne, Eric

1998-03-01

168

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

169

NASA Astrophysics Data System (ADS)

Localized basis ab initio molecular dynamics simulation within the density functional framework has been used to generate realistic configurations of amorphous silicon carbide (a-SiC). Our approach consists of constructing a set of smart initial configurations that conform to essential geometrical and structural aspects of the materials obtained from experimental data, which is subsequently driven via a first-principles force field to obtain the best solution in a reduced solution space. A combination of a priori information (primarily structural and topological) along with the ab initio optimization of the total energy makes it possible to model a large system size (1000 atoms) without compromising the quantum mechanical accuracy of the force field to describe the complex bonding chemistry of Si and C. The structural, electronic and vibrational properties of the models have been studied and compared to existing theoretical models and available data from experiments. We demonstrate that the approach is capable of producing large, realistic configurations of a-SiC from first-principles simulation that display its excellent structural and electronic properties. Our study reveals the presence of predominant short range order in the material originating from heteronuclear Si-C bonds with a coordination defect concentration as small as 5% and a chemical disorder parameter of about 8%.

Atta-Fynn, Raymond; Biswas, Parthapratim

2009-07-01

170

Phonon spectrum, thermal expansion and heat capacity of UO2 from first-principles

NASA Astrophysics Data System (ADS)

We report first-principles calculations of the phonon dispersion spectrum, thermal expansion, and heat capacity of uranium dioxide. The so-called direct method, based on the quasiharmonic approximation, is used to calculate the phonon frequencies within a density functional framework for the electronic structure. The phonon dispersions calculated at the theoretical equilibrium volume agree well with experimental dispersions. The computed phonon density of states (DOSs) compare reasonably well with measured data, as do also the calculated frequencies of the Raman and infrared active modes including the LO/TO splitting. To study the pressure dependence of the phonon frequencies we calculate phonon dispersions for several lattice constants. Our computed phonon spectra demonstrate the opening of a gap between the optical and acoustic modes induced by pressure. Taking into account the phonon contribution to the total free energy of UO2 its thermal expansion coefficient and heat capacity have been computed from first-principles. Both quantities are in good agreement with available experimental data for temperatures up to about 500 K.

Yun, Younsuk; Legut, Dominik; Oppeneer, Peter M.

2012-07-01

171

Adaptive kinetic Monte Carlo for first-principles accelerated dynamics

NASA Astrophysics Data System (ADS)

The adaptive kinetic Monte Carlo method uses minimum-mode following saddle point searches and harmonic transition state theory to model rare-event, state-to-state dynamics in chemical and material systems. The dynamical events can be complex, involve many atoms, and are not constrained to a grid-relaxing many of the limitations of regular kinetic Monte Carlo. By focusing on low energy processes and asserting a minimum probability of finding any saddle, a confidence level is used to describe the completeness of the calculated event table for each state visited. This confidence level provides a dynamic criterion to decide when sufficient saddle point searches have been completed. The method has been made efficient enough to work with forces and energies from density functional theory calculations. Finding saddle points in parallel reduces the simulation time when many computers are available. Even more important is the recycling of calculated reaction mechanisms from previous states along the dynamics. For systems with localized reactions, the work required to update the event table from state to state does not increase with system size. When the reaction barriers are high with respect to the thermal energy, first-principles simulations over long time scales are possible.

Xu, Lijun; Henkelman, Graeme

2008-09-01

172

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

173

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

174

Stress dependent defect energetics in Tungsten from first-principles

NASA Astrophysics Data System (ADS)

Tungsten (W) is an important material for high temperature applications due to its refractory nature. However, like all transition metals from the VI-A group, W suffers from low-temperature brittleness and lack of ductility, which poses serious questions for its use as a structural material. Tungsten's mechanical properties can be enhanced by alloying with elements with d-electrons, such as Re, which has resulted in successful commercial alloys. In this work, we obtain the formation and migration energetics of Re solute atoms in terms of their interaction with vacancies and dislocations. To explore the influence of external stresses on Re transport properties, we examine the role of hydrostatic and shear deformation on the vacancy formation energy (VFE) and migration energy barrier (Em) in BCC W from first-principles calculations by developing a pseudopotential with 6s2, 6p0, 5d4, and 5f0 electronic states for the valence electrons. We find that under hydrostatic deformation, increase or decrease of vacancy formation energy depends on the type of deformation -- tensile or compressive, while for shear deformation it decreases irrespective of the magnitude of applied deformation. On the other hand, migration energy barrier always decreases under hydrostatic deformation, but shows path-length dependent behavior under shear deformation. This talk will discuss the underlying principles and possible routes for enhancing mechanical strength from a physics perspective.

Hossain, Md.; Marian, Jaime

2013-03-01

175

First-principles calculations of uranium diffusion in uranium dioxide

NASA Astrophysics Data System (ADS)

The present work reports first-principles DFT +U calculations of uranium self-diffusion in uranium dioxide (UO2), with a focus on comparing calculated activation energies to those determined from experiments. To calculate activation energies, we initially formulate a point defect model for UO2±x that is valid for small deviations from stoichiometry. We investigate five migration mechanisms and calculate the corresponding migration barriers using both the LDA +U and GGA +U approximations. These energy barriers are calculated using the occupation matrix control scheme that allows one to avoid the metastable states that exist in the DFT +U approximation. The lowest migration barrier is obtained for a vacancy mechanism along the <110> direction. This mechanism involves significant contribution from the oxygen sublattice, with several oxygen atoms being displaced from their original position. The <110> vacancy diffusion mechanism is predicted to have lower activation energy than any of the interstitial mechanisms and comparison to experimental data for stoichiometric UO2 also confirms this mechanism.

Dorado, Boris; Andersson, David A.; Stanek, Christopher R.; Bertolus, Marjorie; Uberuaga, Blas P.; Martin, Guillaume; Freyss, Michel; Garcia, Philippe

2012-07-01

176

Equilibrium Se isotope fractionation parameters: A first-principles study

NASA Astrophysics Data System (ADS)

Several important equilibrium Se isotope fractionation parameters are investigated by first-principles calculations, involving dominant inorganic and organic Se-bearing species in gaseous, aqueous and condensed phases. Because anharmonic effects are found to be negligible for Se isotope fractionation calculation, the Bigeleisen-Mayer equation method is used without corrections beyond harmonic approximation. All calculations are made at B3LYP/6-311 + G(d,p) level, with a frequency scaling factor of 1.05. Solvation effects are carefully evaluated by the explicit solvent model (i.e. the “water droplet” method). A number of conformers are used for aqueous complexes in order to reduce the possible error coming from different configurations.Redox state is found to be an important factor controlling equilibrium Se isotope fractionations. Our results suggest a trend of heavy Se isotopes enrichment as SeO42- > SeO32- > HSeO3- > SeO2 > selenoamino acids > alkylselenides > Se(0) or H2Se > HSe-. The Se(- II) species regardless of organic and inorganic forms can enrich extremely light Se isotopes comparing with other species. Equilibrium Se isotope fractionation factors provided in this study suggest Se isotopes can be used as a tracer of redox conditions and also useful to study Se cycling.

Li, Xuefang; Liu, Yun

2011-04-01

177

First-principles pressure-temperature phase diagrams in metals

Using interatomic potentials derived from first-principles generalized pseudopotential theory, finite-temperature phase transitions in both simple and transition metals can be studied through a combination of analytic statistical methods and molecular-dynamics simulation. In the prototype simple metal-Mg, where volume and pair forces adequately describe the energetics, a complete and accurate phase diagram has thereby been obtained to 60 GPa. A rapidly temperature-dependent hcp-bcc phase line is predicted which ends in a triple point on the melting curve near 4 GPa. In central transition metals such as Mo or Fe, on the other hand, the energetics are complicated by d-state interactions which give rise to both many-body angular forces and enhanced electron-thermal contributions. We have made a detailed study of these phenomena and their impact on melting in the prototype case of Mo and a full melting curve to 2 Mbar has been obtained. In the case of Fe, we are examining the high-pressure phase diagram and the question of whether or not there exists a high-pressure, high-temperature solid bcc phase, as has been speculated. To date, we have shown that the bcc structure is both thermodynamically and mechanically unstable at high pressure and zero temperature, with a large and increasing bcc-hcp energy difference under compression.

Moriarty, J.A.

1993-07-01

178

First principles calculation of finite temperature magnetism in Ni

NASA Astrophysics Data System (ADS)

We harnesses the computational power of massively parallel computers to calculate finite temperature magnetic properties by combining classical Monte-Carlo calculations with our first principles multiple scattering electronic structure code (LSMS) for constrained magnetic states. Our previous calculations of Fe and Fe3C [J. Appl. Phys. 109, 07E138 (2011)] only considered fluctuations in the local moment directions. Recent advances, both in the understanding of the Wang-Landau method used in our calculations [Phys. Rev. E 84, 065702(R) (2011)] and more powerful computing resources have enabled us to investigate Ni where the fluctuation in the magnitude of the local magnetic moments is of importance equal to their directional fluctuations. Here we will present our recent results for Ni that axpands our method to an even wider class of 3d element based ferromagnets. This research was sponsored by the Offices of Basic Energy Science (M.E. and D.M.N) and the Office of Advanced Computing Research (J.Y. and Y.W.L) of the US Department of Energy. This research used resources of the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory, which is supported by the Office of Science of the Department of Energy under contract DE-AC05-00OR22725.

Eisenbach, Markus; Yin, Junqi; Nicholson, Don M.; Li, Ying Wai

2013-03-01

179

First-principles study of water on Cu (110) surface

NASA Astrophysics Data System (ADS)

The persistent demand for cheaper and high efficient catalysts in industrial chemical synthesis, such as ammonia, and in novel energy applications, hydrogen generation and purification in fuel cells motivated us to study the fundamental interaction involved in water-Cu system, with an intension to examine Cu as a possible competitive candidate for cheaper catalysts. Water structure and dissociation kinetics on a model open metal surface: Cu (110), have been investigated in detail based on first-principles electronic structure calculations. We revealed that in both monomer and overlayer forms, water adsorbs molecularly, with a high tendency for diffusion and/or desorption rather than dissociation on clean surfaces at low temperature. With the increase of the water coverage on the Cu (110) surface, the H-bond pattern lowers the dissociation barrier efficiently. More importantly, if the water molecule is dissociated, the hydrogen atoms can diffuse freely along the [110] direction, which is very useful in the hydrogen collection. In addition, we extended to study water on other noble metal (110) surfaces. The result confirms that Cu (110) is the borderline between intact and dissociative adsorption, differing in energy by only 0.08 eV. This may lead to promising applications in hydrogen generation and fuel cells.

Ren, Jun; Meng, Sheng

2009-03-01

180

First-principles simulations of exciton diffusion in organic semiconductors

NASA Astrophysics Data System (ADS)

Exciton diffusion is of great importance to the performance of organic optoelectronic devices, including organic photovoltaics and solid-state lighting. The ability to control exciton diffusion in organic semiconductors is crucial to the design of efficient optoelectronic devices. However, such ability can only be achieved through a fundamental understanding of exciton diffusion mechanism. We have proposed a first-principles based frame work that can predict exciton dynamics in organic semiconductors.The framework is based on time-dependent density functional theory to provide the energy and many-body wave functions of excitons. Nonadiabatic ab initio molecular dynamics is used to calculate phonon-assisted transition rates between localized exciton states. Using Monte Carlo simulations, we determine exciton diffusion length, lifetime, diffusivity, and harvesting efficiency in poly(3-hexylthiophene) polymers at different temperatures, and the results agree very well with corresponding experimental values. We find that exciton diffusion is primarily determined by the density of states of low-energy excitons; a widely speculated diffusion mechanism has been confirmed and elucidatedby the simulations. Some general guidelines for designing more efficient organic solar cells can be gleaned from the simulation results

Zhang, Xu; Li, Zi; Lu, Gang

2013-03-01

181

Phases of vanadium under pressure investigated from first principles

NASA Astrophysics Data System (ADS)

The existence and stability under pressure of three phases of vanadium are calculated from first principles. The phases are one body-centered cubic (bcc) and two rhombohedral phases (rhu and rhl), which have greater and lower ? values than the primitive bcc rhombohedral cell. The bcc phase is shown in two ways to become unstable at 0.65 Mbar, in agreement with an observed phase transition, but in disagreement with previous higher estimates. The rh phases exist when the bcc phase is unstable, but the bcc phase stabilizes again at 3 Mbar, and the rh phases disappear. At 1.15 Mbar the rhu phase becomes and remains unstable and the rhl phase becomes the ground state up to 3 Mbar, where the rhl phase disappears and the bcc phase takes over. The theory gives four phase transitions among the phases over the pressure range from 0 to 3 Mbar two of them are bcc to rhu—a low pressure (0.3 Mbar) thermodynamic transition found from Gibbs free energies being equal (not observed) and a higher pressure (0.65 Mbar) instability transition when the bcc phase becomes unstable (observed).

Qiu, S. L.; Marcus, P. M.

2008-07-01

182

Phases of vanadium under pressure investigated from first principles.

The existence and stability under pressure of three phases of vanadium are calculated from first principles. The phases are one body-centered cubic (bcc) and two rhombohedral phases (rh(u) and rh(l)), which have greater and lower ? values than the primitive bcc rhombohedral cell. The bcc phase is shown in two ways to become unstable at 0.65 Mbar, in agreement with an observed phase transition, but in disagreement with previous higher estimates. The rh phases exist when the bcc phase is unstable, but the bcc phase stabilizes again at 3 Mbar, and the rh phases disappear. At 1.15 Mbar the rh(u) phase becomes and remains unstable and the rh(l) phase becomes the ground state up to 3 Mbar, where the rh(l) phase disappears and the bcc phase takes over. The theory gives four phase transitions among the phases over the pressure range from 0 to 3 Mbar; two of them are bcc to rh(u)-a low pressure (0.3 Mbar) thermodynamic transition found from Gibbs free energies being equal (not observed) and a higher pressure (0.65 Mbar) instability transition when the bcc phase becomes unstable (observed). PMID:21694379

Qiu, S L; Marcus, P M

2008-06-04

183

Vacancy formation process in carbon nanotubes: first-principles approach.

The electronic and structural properties of a single-walled carbon nanotube (SWNT) under mechanical deformation are studied using first-principles calculations based on the density functional theory. A force is applied over one particular C-atom with enough strength to break the chemical bonds between the atom and its nearest neighbors, leading to a final configuration represented by one tube with a vacancy and an isolated C-atom inside the tube. Our investigation demonstrates that there is a tendency that the first bond to break is the one most parallel possible to the tube axis and, after, the remaining two other bonds are broken. The analysis of the electronic charge densities, just before and after the bonds breaking, helps to elucidate how the vacancy is formed on an atom-by-atom basis. In particular, for tubes with a diameter around 11 angstroms, it is shown that the chemical bonds start to break only when the externally applied force is of the order of 14 nN and it is independent of the chirality. The formation energies for the vacancies created using this process are almost independent of the chirality, otherwise the bonds broken and the reconstruction are dependent. PMID:15792439

Rossato, Jussane; Baierle, R J; Fazzio, A; Mota, R

2005-01-01

184

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

185

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

186

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

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

Peng, Qing; Ji, Wei; De, Suvranu

2012-10-14

187

First Principles Calculations of o2 Diffusion in SIO2

NASA Astrophysics Data System (ADS)

In thermal oxide growth, oxygen molecules must first diffuse through the oxide layer in order to reach the interface to promote further growth. Therefore, it is important to understand the diffusion at a molecular level. In this work we study the oxygen diffusion using first principles total energy calculations based on the spin-density functional theory. We study initially the possible interstitial positions for an oxygen molecule, both for a triplet as well as a singlet spin configuration, and obtain that the triplet is always lower in energy. We also study the possible orientations of the molecules within the interstitial sites, and obtain configurations of lowest energy that differ from previous results. We also study the energy barrier for a molecule to hop between these interstitial sites. Again, we consider both the triplet and singlet configurations, and obtain that the energy barriers are very different, which indicates that spin-polarized calculations are fundamental to understand the properties of this system. Our results indicate that the oxygen molecules do not interact strongly with the lattice throughout the hopping process, and therefore we do not expect it to be incorporated into the SiO2 network during the diffusion process, as has been previously suggested.

Orellana, W.; da Silva, Antônio J. R.; Fazzio, A.

2001-03-01

188

Structural properties of plutonium from first-principles theory

First-principles theory is shown to account for the unique low-temperature crystal structure of plutonium metal ({alpha}-Pu). Also the observed, and debated, upturn of the equilibrium volume between neptunium and plutonium is reproduced and found to be a consequence of the different crystal structures for these two metals. Thus it is shown that density-functional theory is able to accurately describe bonding properties of 5f electrons in an outstandingly complex system, where also relativistic effects are large. The electronic structure for {alpha}-Pu and for plutonium in competing close-packed crystal structures are also presented. Moreover, an explanation for the occurrence of the highly complex {alpha}-Pu structure is given. The mechanism is described in terms of a Peierls distortion in conjunction with a narrow 5f-band width. The energy gained from the splitting of the 5f bands outweighs the electrostatic energy which favors the high symmetry structures found for most other metals. At lower volumes we predict that plutonium should become bcc. {copyright} {ital 1997} {ital The American Physical Society}

Soederlind, P. [Physics Department, Lawrence Livermore National Laboratory, Livermore, California 94550 (United States); Wills, J.M. [Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States); Johansson, B.; Eriksson, O. [Condensed Matter Theory Group, Physics Department, Uppsala University, Box 530 (Sweden)

1997-01-01

189

Efficient First-principles Wang-Landau Calculations

NASA Astrophysics Data System (ADS)

The Wang-Landau (WL) method of finding the density of states g(E) contributing to the partition function Z(kT) is useful for determining thermodynamic properties from first-principles energy calculations for magnetic systems. Since DFT calculations require significant computer resources, it is important to make the convergence of the WL method to a self-consistent g(E) as efficient as possible. We present approaches for making accurate initial estimates of g(E) based on similar Hamiltonians or estimates of g(E) for a different number of atoms. These approaches can include workstation-based calculations using classical Heisenberg Hamiltonians based on exchange parameters calculated from initial data from DFT WL calculations. In addition, we annunciate serveral insights we have gained into the convergence of the WL method. For instance, the minimum curvature of the calculated g(E) is limited by the update parameter and the maximum energy step of the Markov chain. This material is based upon work supported as part of the Center for Defect Physics, an Energy Frontier Research Center funded by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences. This research used resources of the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory, which is supported by the Office of Science of the Department of Energy.

Brown, G.; Rusanu, A.; Odbadrakh, Kh.; Eisenbach, M.; Nicholson, D. M.

2011-03-01

190

First-principles thermodynamic modeling of lanthanum chromate perovskites

NASA Astrophysics Data System (ADS)

Tendencies toward local atomic ordering in (A,A')(B,B')O3-? mixed composition perovskites are modeled to explore their influence on thermodynamic, transport, and electronic properties. In particular, dopants and defects within lanthanum chromate perovskites are studied under various simulated redox environments. (La1-x,Srx)(Cr1-y,Fey)O3-? (LSCF) and (La1-x,Srx)(Cr1-y,Ruy)O3-? (LSCR) are modeled using a cluster expansion statistical thermodynamics method built upon a density functional theory database of structural energies. The cluster expansions are utilized in lattice Monte Carlo simulations to compute the ordering of Sr and Fe(Ru) dopant and oxygen vacancies (Vac). Reduction processes are modeled via the introduction of oxygen vacancies, effectively forcing excess electronic charge onto remaining atoms. LSCR shows increasingly extended Ru-Vac associates and short-range Ru-Ru and Ru-Vac interactions upon reduction; LSCF shows long-range Fe-Fe and Fe-Vac interaction ordering, inhibiting mobility. First principles density functional calculations suggest that Ru-Vac associates significantly decrease the activation energy of Ru-Cr swaps in reduced LSCR. These results are discussed in view of experimentally observed extrusion of metallic Ru from LSCR nanoparticles under reducing conditions at elevated temperature.

Dalach, P.; Ellis, D. E.; van de Walle, A.

2012-01-01

191

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

192

Phonon relaxation times extracted from first principles thermal conductivity calculations

NASA Astrophysics Data System (ADS)

The lattice thermal conductivity of semiconductors, ?L, is a key component in assessing a material's utility for thermoelectric applications. Calculations of ?L commonly employ phonon relaxation times, ?ph. Over the past few decades, a variety of mathematical forms have been used for these?phs to represent the phonon-phonon scattering [1], which dominates the behavior of ?L around and above room temperature. However, these forms have typically been obtained in a low frequency/low temperature approximation where umklapp scattering is weak and outside the thermal regime of interest for thermoelectrics. Recently we have developed a first principles approach that accurately calculates ?L using no adjustable parameters [2]. In this presentation, we use our ab initio results for Si, Ge and diamond to examine the accuracy of the different models for ?ph, and we identify alternative models. [1] See for example, M. Asen-Palmer et al., Phys. Rev. B 56, 9431 (1997), and references therein. [2] D. A. Broido, M. Malorny, G. Birner, N. Mingo and D. A. Stewart, Appl. Phys. Lett. 91, 231922 (2007).

Broido, D. A.; Ward, A.

2009-03-01

193

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

194

First Principles Study of Substituents in KTaO_3

NASA Astrophysics Data System (ADS)

The structural properties, energetics, and dynamics of Ca^2+ and Mn^2+ substituents in KTaO3 are investigated from first principles. It is found that Ca substitutes for both K and Ta ions. Oxygen vacancies bind to isolated Ca ions residing at Ta-sites, causing off-center Ca displacement and forming large dipoles. There is also evidence that oppositely charged defects may cluster together. Our calculations predict that the activation energy for dipole reorientation via oxygen vacancy hopping within the first neighbor shell of Ta-substituting Ca or Mn exceeds 2 eV. On the other hand, Mn^2+ substituting at the K-site displaces off center along the (100) direction, also forming a dipole. This dipole can reorient via Mn hopping motion with an activation energy of ~ 0.18 eV, in reasonable agreement with experiments. We argue that, in general, metal ion hopping at the A-site, not oxygen vacancy hopping, is responsible for the small activation energies found in experiments.

Leung, Kevin

2001-03-01

195

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

196

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.

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

197

NASA Astrophysics Data System (ADS)

The electronic structure of the (Zn, Co)O diluted magnetic semiconductors (DMSs) were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure was obtained with the local spin-density approximation. The x-ray absorption spectra (XAS), x-ray magnetic circular dichroism (XMCD), and x-ray linear dichroism (XLD) spectra at the Co, Zn, and O K and Zn, Co L2,3 edges were investigated theoretically from first principles. The origin of the XMCD and XLD spectra in these compounds was examined. The orientation dependence of the XAS at the Co and Zn K edges were investigated by calculating the XAS spectra for the <001> and <110> magnetization axis. The calculated results are compared with available experimental data.

Antonov, V. N.; Bekenov, L. V.; Jepsen, O.; Mazur, D. V.; Germash, L. P.

2012-04-01

198

The mechanical stability, elastic, and thermodynamic properties of the anti-perovskite superconductors MNNi3 (M = Zn, Mg, Al) are investigated by means of the first-principles calculations. The calculated structural parameters and elastic properties of MNNi3 are in good agreement with the experimental and the other theoretical results. From the elastic constants under high pressure, we predict that ZnNNi3, MgNNi3, and AlNNi3

Zhai Hong-Cun; Li Xiao-Feng; Du Jun-Yi; Ji Guang-Fu

2012-01-01

199

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

200

First-principles study of III-V electrode interfaces for photoelectrochemical hydrogen production

NASA Astrophysics Data System (ADS)

Photoelectrochemical (PEC) cells promise clean, sustainable production of hydrogen fuel using water and sunlight. However, combining solar conversion efficiency with durability in electrolyte solution has proven difficult, in part because the complex chemistry active at the electrode-electrolyte interface remains poorly understood. We use first-principles molecular dynamics simulations and model density-functional calculations to study the structure, stability, and chemical activity of GaP/InP semiconductor electrodes in contact with water. We find that a local bond-topological model is able to capture much of the basic surface chemistry. Interpretation of our results points to the particular importance of surface-adsorbed oxygen in determining the available reaction pathways for photocorrosion and water dissociation. Electronic signatures of the local bond topologies are compared to data from X-ray absorption and emission spectroscopy for insight into actual electrode structure.

Wood, Brandon; Ogitsu, Tadashi; Choi, Wooni; Schwegler, Eric

2012-02-01

201

We show in this paper that in the presence of Zn ions a peculiar structural aggregation pattern of ?-amyloid peptides in which metal ions are sequentially coordinated to either three or four histidines of nearby peptides is favored. To stabilize this configuration a deprotonated imidazole ring from one of the histidines forms a bridge connecting two adjacent Zn ions. Though present in zeolite imidazolate frameworks, remarkably in biological compounds this peculiar Zn-imidazolate-Zn topology is only found in enzymes belonging to the Cu,Zn-superoxide dismutase family in the form of an imidazolate bridging Cu and Zn. The results we present are obtained by combining X-ray absorption spectroscopy experimental data with detailed first-principle molecular dynamics simulations. PMID:22170501

Giannozzi, Paolo; Jansen, Karl; La Penna, Giovanni; Minicozzi, Velia; Morante, Silvia; Rossi, Giancarlo; Stellato, Francesco

2011-12-15

202

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

203

NASA Astrophysics Data System (ADS)

Nuclear magnetic resonance (NMR) spectroscopy is one of the most important experimental probes of local atomistic structure, chemical ordering, and dynamics. Recently, NMR has increasingly been used to study complex ferroelectric perovskite alloys, where spectra can be difficult to interpret. First-principles calculations of NMR spectra can greatly assist in this task. In this work, oxygen, titanium, and niobium NMR chemical shielding tensors, ? , were calculated with first-principles methods for ferroelectric transition metal prototypical ABO3 perovskites [SrTiO3, BaTiO 3, PbTiO3 and PbZrO3] and A(B,B')O3 perovskite alloys Pb(Zr1/2Ti1/2)O3 (PZT) and Pb(Mg1/3Nb2/3)O3 (PMN). The principal findings are 1) a large anisotropy between deshielded sigma xx(O) ? sigmayy(O) and shielded sigma zz(O) components; 2) a nearly linear dependence on nearest-distance transition-metal/oxygen bond length, rs, was found for both isotropic deltaiso(O) and axial deltaax(O) chemical shifts ( d?=? reference- ? ), across all the systems studied, with deltaiso(O) varying by ? 400 ppm; 3) the demonstration that the anisotropy and linear variation arise from large paramagnetic contributions to sigmaxx(O) and sigmayy(O), due to virtual transitions between O(2p) and unoccupied B(nd) states. Using these results, an argument against Ti clustering in PZT, as conjectured from recent 17O NMR magic-angle-spinning measurements, is made. The linear dependence of the chemical shifts on rs provides a scale for determining transition-metal/oxygen bond lengths from experimental 17O NMR spectra. As such, it can be used to assess the degree of local tetragonality in perovskite solid solutions for piezoelectric applications. Results for transition metal atoms show less structural sensitivity, compared to 17O NMR, in homovalent B-site materials, but could be more useful in heterovalent B-site perovskite alloys. This work shows that both 17O and B-site NMR spectroscopy, coupled with first principles calculations, can be an especially useful probe of local structure in complex perovskite alloys.

Pechkis, Daniel Lawrence

204

First-principles studies of magnetic complex oxide heterointerfaces

NASA Astrophysics Data System (ADS)

Despite the technological advancements driven by conventional semiconductors, continued improvements in nanoelectronics will require new materials with greater functionality. Perovskite-structured transition metal oxides with ABO3 stoichiometry are leading candidates that display amyriad of useful phenomena: ferroelectricity, magnetism, and superconductivity. Since these properties arise from correlated electronic interactions, field-tuning techniques make possible ultra-fast phase transitions between dramatically different states. Unfortunately, the integration of these materials into microelectronics has not yet occurred because of a fundamental lack in understanding how to predict and control these phase transitions at oxide--oxide heterointerfaces. The exceedingly difficult challenge of identifying the microscopic origins of interface electronic behavior is crucial to the functional design and discovery of next generation electronic materials. This dissertation focuses on developing that understanding at magnetic perovskite oxide heterointerfaces using first-principles (parameter free) density functional calculations. New ideas for oxide-oxide superlattice design emerge by considering the interfaces as entirely new complex materials: the interfacial electronic and magnetic structure in artificial geometries is genuinely different from those of the parent bulk materials due to changes in symmetry- and size-dependent properties. By isolating the role of the interacting electron-, orbital-, and spin-lattice degrees of freedom at the interfaces, I identify that the primary interaction governing the ground state derives from latent instabilities present in the bulk phases. The heteroepitaxial structural constraints enhance these modes to re-normalize the low energy electronic structure. To develop insight into the role of thin film thickness and strain effects, I explore how the electronic and magnetic structures of single component films respond to the elastic constraints, in particular, whether ultra-thin layers of SrRuO3 are susceptible to a metal-insulator transition and if strained LaCoO3 films support reversible magnetic spin state transitions. I then examine how the interface between two dissimilar materials---a polarizable dielectric SrTiO3 and a ferromagneticmetal SrRuO 3---responds to an external electric field; I find a spin-dependent screening effect at the heterointerface that manifests as an interfacial magnetoelectric effect and makes possible electric-field control of magnetization. I then explore how the orbital degree of freedom in the electronically degenerate and magnetic SrFeO3 is modified by geometric confinement and changes in chemical bonding at a heterointerface with SrTiO3. I find lattice instabilities are enhanced in the superlattice, and their condensation leads to an electronic phase transition. By isolating the chemical effects at the heterointerface, I identify an additional route to control octahedral rotation patterns pervasive in perovskite oxides films through structural coherency. This study suggests a complementary strain-free avenue for functional thin film design. The materials understanding obtained from these first-principles calculations, when leveraged with new synthesis techniques, offers to have substantial impact on the search and control of new functionalities in oxide heterostructures.

Rondinelli, James M.

205

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

206

Elasticity of Mantle Pyroxenes from Experiments and First Principles Calculations

NASA Astrophysics Data System (ADS)

Experimental and first principles studies have been conducted on mantle pyroxenes with compositions of MgSiO3 (orthopyroxene, clinopyroxene and high-pressure clinopyroxene) and CaMgSi2O6 (diopside). Experimental data include those from static compression at room temperature using diamond anvil cell up to 30GPa, ultrasonic velocities and density measurements up to 17GPa 1273K using combined ultrasonic and X-radiation techniques in multi-anvil apparatus. Bulk and shear moduli as well as their pressure and temperature derivatives are derived by fitting the measured velocities and densities to finite strain equations, with which the pressure on the sample can be directly determined. Density functional theory (DFT) calculations are carried out to obtain complementary information, such as enthalpies and elastic constants, in order to understand the structural behavior and physical properties derived from experiments. In particular, the anomalous behavior of velocities in MgSiO3 orthopyroxene within 9-14 GPa observed in our ultrasonic measurements are explored by DFT calculations of the single crystal elastic constants as a function of pressure, and the velocity softening has been confirmed to be caused by softening of certain shear elastic constants. A subsequent compression study in diamond anvil cell on natural orthopyroxene exhibits nonlinear compressibility with increasing pressure, suggesting similar elastic softening in the same pressure range. In fact, DFT calculations suggest that clinopyroxene and orthopyroxene of MgSiO3 both undergo elastic softening with increasing pressure, presumably due to the flexibility of the silicate chain structures. In addition to an updated view about the structural instabilities and their corresponding mechanisms, the seismic signatures of these pyroxenes at mantle depths, such as compressional and shear velocities as well as their anisotropies will all be presented.

Li, B.; Liu, W.; Ma, M.; Kung, J.

2011-12-01

207

Structure and elasticity of hydrous ringwoodite: A first principle investigation

NASA Astrophysics Data System (ADS)

First principle calculations were performed to investigate structural, IR, and elastic properties of hydrous ringwoodite and their evolution with pressure up to 36 GPa. Hydrogen defects are introduced by creating Mg- or Si-vacancies Mg1.875H0.25SiO4, Mg1.75H0.5SiO4 and Mg2Si0.875H0.5O4. Energy considerations imply that the Mg-vacancy coupled substitution will be the easiest to form, but, in the Earth, both vacancies will participate in the process. Calculated IR spectra, when compared with reported observations, suggest that both types of defects are abundant in synthetic samples. We find that (d ln VS/d ln VP) for lateral variations in the H content of ringwoodite will be quite small, suggesting that this quantity will be a sensitive metric for identifying the presence of dissolved water in the transition zone. The calculated bulk modulus decreases linearly with increasing water content with dK/d(CO)=-7.1(GPa/wt%) at room pressure, decreasing to -6.0 (GPa/wt%) at 20 GPa. The shear modulus similarly demonstrates a decrease with increased water content given, averaged over the substitution models, by dG/d(CO)=-3.0(GPa/wt%) at room pressure, decreasing to -1.8 (GPa/wt%) at 20 GPa. Over this pressured range, the water induce variation of d ln(VS)/d ln(VP) is 0.62 at 0 GPa to 0.2 at 20 GPa.

Li, Li; Brodholt, John; Alfè, Dario

2009-12-01

208

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

209

Oxidation of Al doped Au clusters: A first principles study

Using first principles method we report the oxidation of Al doped Au clusters. This work is divided into two parts: (i) the equilibrium structures and stability of Al doped Au{sub n-1} clusters (n=2-7,21) and (ii) the interaction of O{sub 2} with stable clusters. The calculations are performed using the plane wave pseudopotential approach under the density functional theory and generalized gradient approximation for the exchange and correlation functional. The optimized geometries of Au{sub n-1}Al clusters indicate that the substitution of Au by Al results an early onset of three-dimensional structures from tetramer onwards. This is different from the results of transition metal doped Au clusters, where the planar conformation of Au clusters retains up to heptamer. The stability of Au{sub n-1}Al clusters has been analyzed based on the binding energy, second difference in energy, and the energy gaps between the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. Based on the energetics, the Au{sub 3}Al and Au{sub 5}Al clusters are found to have extraordinary stability. The oxidation mechanism of Al doped Au clusters have been studied by the interaction of O{sub 2} with Al, Au, AuAl, Au{sub 3}Al, and Au{sub 20}Al clusters. It is found that the oxidation of Au{sub n-1}Al clusters undergoes via dissociative mechanism, albeit significant charge transfer from Al to Au. Moreover, the O{sub 2} molecule prefers to attach at the Al site rather than at the Au site.

Rajesh, Chinagandham [RMC, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085 (India); Majumder, Chiranjib [Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085 (India)

2009-06-21

210

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.

2011-01-01

211

Evolutionary approach for determining first-principles model Hamiltonians

NASA Astrophysics Data System (ADS)

The ability to perform accurate solid-state calculations based completely on first principles (for relatively small unit cells) has made it possible to develop model Hamiltonians that can be rapidly ``searched'' for optimal target properties---i.e., true materials-by-design. Recent applications include ferroelectric properties and band-gap engineering. The most difficult step in ``training'' such model Hamiltonians is making choices for the number and types of parameters in the model that insure the predictive power of the model. Based on an evolutionary approach, we have developed an algorithm for selecting the types and number of terms in a Cluster Expansion model for a binary alloy. This approach removes much of the tedium of constructing the model and robustly finds the best possible set of parameters. The approach is general and can be applied to a wide variety of other models as well. I illustrate the success of the new approach first on systems where the best parameter set is known analytically, and second, as applied to several recent ``real-world'' examples, including (1) the role of long-period-superlattices in the Cu-Pd system, (2) predicting configuration-dependent bulk-moduli in transition- metal carbides and nitrides, (3) predicting optimal superlattice stacking/orientations to engineer desired band-gaps in MgO-ZnO wide gap alloys. Gus L. W. Hart, V. Blum, M. J. Walorski, and A. Zunger, Nature Materials 4 391 (2005) V. Blum, Gus L. W. Hart, M. J. Walorski, and A. Zunger, Phys. Rev. B 72, 165111 (2005).

Hart, Gus L. W.

2006-03-01

212

First principle kinetic studies of zeolite-catalyzed methylation reactions.

Methylations of ethene, propene, and butene by methanol over the acidic microporous H-ZSM-5 catalyst are studied by means of state of the art computational techniques, to derive Arrhenius plots and rate constants from first principles that can directly be compared with the experimental data. For these key elementary reactions in the methanol to hydrocarbons (MTH) process, direct kinetic data became available only recently [J. Catal.2005, 224, 115-123; J. Catal.2005, 234, 385-400]. At 350 °C, apparent activation energies of 103, 69, and 45 kJ/mol and rate constants of 2.6 × 10(-4), 4.5 × 10(-3), and 1.3 × 10(-2) mol/(g h mbar) for ethene, propene, and butene were derived, giving following relative ratios for methylation k(ethene)/k(propene)/k(butene) = 1:17:50. In this work, rate constants including pre-exponential factors are calculated which give very good agreement with the experimental data: apparent activation energies of 94, 62, and 37 kJ/mol for ethene, propene, and butene are found, and relative ratios of methylation k(ethene)/k(propene)/k(butene) = 1:23:763. The entropies of gas phase alkenes are underestimated in the harmonic oscillator approximation due to the occurrence of internal rotations. These low vibrational modes were substituted by manually constructed partition functions. Overall, the absolute reaction rates can be calculated with near chemical accuracy, and qualitative trends are very well reproduced. In addition, the proposed scheme is computationally very efficient and constitutes significant progress in kinetic modeling of reactions in heterogeneous catalysis. PMID:21182253

Van Speybroeck, Veronique; Van der Mynsbrugge, Jeroen; Vandichel, Matthias; Hemelsoet, Karen; Lesthaeghe, David; Ghysels, An; Marin, Guy B; Waroquier, Michel

2010-12-23

213

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

214

Fp Shell Spectroscopy: Numerical Calculations and Theoretical Aspects.

National Technical Information Service (NTIS)

The fp shell spectroscopy is reviewed and the fsup(n) model is introduced. It is shown that the two-body Hamiltonian monopolar terms play a very important part in the behavior of these spectra, and that realistic interactions do not reproduce them. The de...

E. A. Pasquini

1976-01-01

215

First-principles calculations of the structural and dynamical properties of ferroelectric YMnO3

NASA Astrophysics Data System (ADS)

We report the structural and dynamical properties of the ground state ferroelectric P63cm structure of hexagonal YMnO3. The lattice parameters, atomic positions, local magnetic moment of Mn atoms, band gap, and ?-point phonons are calculated within the local spin-density approximation plus Hubbard U term and the B1-WC hybrid functional. Our results are discussed in comparison to theoretical and experimental values available in the literature. Based on our first-principles calculations, we propose a reassignment of certain modes. We also discuss the relation between the phonon modes of the ferroelectric P63cm phase and those of the paraelectric P63/mmc phase that condense at the phase transition.

Prikockyt?, Alina; Bilc, Daniel; Hermet, Patrick; Dubourdieu, Catherine; Ghosez, Philippe

2011-12-01

216

NASA Astrophysics Data System (ADS)

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

Kudrnovský, J.; Drchal, V.; Turek, I.

2013-07-01

217

Structural, electronic, and dynamical properties of Pca21-TiO2 by first principles

NASA Astrophysics Data System (ADS)

First-principles calculations of the structural, electronic, and mechanical properties of the modified fluorite structure of TiO2 with Pca21 symmetry are obtained using the plane-wave pseudopotential density functional theory. The results indicate that Pca21-TiO2 is a semiconductor with an indirect band gap. The calculated static dielectric constants are larger than those of anatase and brookite, but they are much smaller than those of rutile. The calculated bulk modulus using the equation of state is in good agreement with that calculated from elastic constants. The calculated bulk modulus is in agreement with a recent theoretical and experimental report, which confirms that the experimentally claimed structure (cubic fluorite phase) can be Pca21-TiO2.

Abbasnejad, M.; Mohammadizadeh, M. R.; Maezono, R.

2012-03-01

218

Order-disorder phase boundary in Ice VII-VIII investigated by first principles

NASA Astrophysics Data System (ADS)

Phase boundaries among the various forms of ice are difficult to determine experimentally because of large hysteresis involved, especially at the lowest temperatures. Theoretically, there are also great challenges, including the order-disorder (OD) phenomenon. The ice VII-VIII boundary, a typical OD boundary, has been reasonably well constrained experimentally and is an ideal study case. We present a first principles quasiharmonic study consisting in the complete statistical sampling of molecular orientations within a 16-molecules supercell. Our calculation accounts well for several important aspects: equation of state of ice VII, negative Clapeyron slope of the phase boundary, and the isotope effect. We will discuss also some factors to be improved, including XC functionals. Research was supported by NSF grants EAR 0810272, EAR 0635990, ATM 0428774 (VLab), EAR 0757903.

Wentzcovitch, Renata; Umemoto, Koichiro; de Gironcoli, Stefano; Baroni, Stefano

2010-03-01

219

Order-disorder phase boundary in ice VII-VIII investigated by first principles

NASA Astrophysics Data System (ADS)

Phase boundaries among the various forms of ice are difficult to determine experimentally because of large hysteresis involved, especially at the lowest temperatures. Theoretically, there are also great challenges, including the order-disorder (OD) phenomenon. The ice VII-VIII boundary, a typical OD boundary, has been reasonably well constrained experimentally and is an ideal study case. We present a first principles quasiharmonic study consisting in the complete statistical sampling of molecular orientations within a 16 molecules supercell. This supercell size accounts well for several aspects of this transition, including the Clapeyron slope and the isotopic effect. Research supported by NSF grants ATM 0428774 (VLab) and EAR 0757903. Computations were performed at the Minnesota Supercomputing Institute.

Wentzcovitch, R. M.; Umemoto, K.; Baroni, S.; de Gironcoli, S.

2009-12-01

220

Effect of strain on thermoelectric properties of SrTiO3: First-principles calculations

NASA Astrophysics Data System (ADS)

The electronic structures of strained SrTiO3 were investigated by using first-principles calculations, and the anisotropic thermoelectric properties of n-type SrTiO3 under biaxial strain were calculated on the base of the semi-classical Boltzmann transport theory. It was theoretically found that the in-plane and out-of-plane power factors of n-type SrTiO3 can be increased under compressive and tensile strains, respectively, and such dependence can be explained by the strain-induced redistribution of electrons. To further optimize the thermoelectric performance of n-type SrTiO3, the maximum power factors and the corresponding optimal n-type doping levels were evaluated.

Zou, Daifeng; Liu, Yunya; Xie, Shuhong; Lin, Jianguo; Li, Jiangyu

2013-10-01

221

First-principles study of electron transport in Si atom wires under finite bias voltage

NASA Astrophysics Data System (ADS)

We have theoretically analyzed electron transport in wires consisting of one to three Si atoms at a finite bias voltage using a first-principles method. The electronic states and transport properties are calculated in the framework of density functional theory using the Lippmann-Schwinger equation in the Laue representation. We analyzed the transport properties of Si wires between metallic electrodes and elucidated the effects of metallic contacts on a Si atom wire, the characteristics of conduction channels, and their dependence on the bias voltage. The conduction channels are analyzed using eigenchannel decompositions, and it is found that the three channels contributing to the transport are almost open in the bias window under a finite bias voltage.

Kusaka, Hiroyuki; Kobayashi, Nobuhiko

2012-01-01

222

Pressure-induced phase transition for ScVO4: A first-principles study

NASA Astrophysics Data System (ADS)

We theoretically investigated the structural stability and electronic properties of ScVO4 by the first-principles pseudopotential method. The tetragonal zircon-type and scheelite-type structures, LaTaO4-type structure of ScVO4 have been considered. The calculations indicate that the LaTaO4-type phase is not stable in the pressure 0-100 GPa, and the structural phase transformation from zircon to scheelite-type structure occurs at 5.4 GPa. The band structure shows that zircon-type structure at zero pressure and scheelite-type structure at transition pressure have direct gaps of 2.58 eV and 2.35 eV, respectively. The detailed volume changes during the phase transition were analyzed.

Sheng, Shu-Fang

2013-10-01

223

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. 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

224

First-principles study of semiconductor and metal surfaces

NASA Astrophysics Data System (ADS)

In this dissertation, we study the electronic and geometric structure of semiconductors and metal surfaces based on quantum mechanical first-principles calculations. We determine the geometry of vacancy defects of hydrogen adsorbed on a Pd(111) surface by treating the motion of a hydrogen atom, in addition to electrons, quantum mechanically. The calculated ground state wave function has high probability density in the hcp site located at the center of the vacancy instead of the fcc sites where the potential is minimum and hydrogen atoms on a Pd(111) surface normally adsorb. The geometry of quantum mechanically determined divacancy provides a simple and clear explanation for the scanning tunneling microscopy (STM) images of these defects that appear as three-lobed objects as observed in recent experiments [Mitsui, et al, Nature 422, 705 (2003)]. We employ the same principle to successfully elucidate the STM images of larger size vacancy defects. Our model also provides a compelling argument to explain the unusual recent experimental result that aggregates of three or more hydrogen vacancies are much more active in adsorption of hydrogen molecules while two-vacancy defects are never inactive. The InAs (110) surfaces appear lower than GaSb in STM images. This height difference is caused primarily by differences in the electronic structure of the two materials according to our calculations in a good agreement with measurements. In contrast, local variations in the apparent height of (110) surface atoms at InSb- or GaAs-like interfaces arise primarily from geometric distortions associated with local differences in bond length. The arsenic atoms adsorb preferably at the bridge sites between the dimerized Sb atoms on Sb-terminating (001) surfaces. Indium atoms, on the other hand, have somewhat equal probabilities at a few different sites on Ga-terminating (001) surfaces. Our calculated energies for atomic intermixing indicate that anion exchanges are exothermic for As atoms on Ga-terminating (001) interfaces but endothermic for In atoms on Sb-terminating (001) interfaces. This difference may explain why GaAs interfaces are typically more disordered than InSb interfaces in these heterostructures.

Kim, Sungho

225

First-Principle Derivation of Entropy Production in Transport Phenomena

NASA Astrophysics Data System (ADS)

The linear response framework was established by Kubo a half century ago, but no clear explanation of irreversibility namely entropy production has been given in this scheme. This has been now solved. The serious puzzle up to now is the following. Even using the linear response density matrix ?lr = ?0 + ?1(t), it has been difficult to derive the entropy production. Surprisingly, the correct entropy production is given by the second-order term ?2(t) as . It is shown to agree with the ordinary expression J·E/T = ?E2/T in the case of electric conduction for a static electric field E, where ? denotes the electric conductivity expressed by the famous canonical current-current time correlation functions in equilibrium. The present article gives a review of the derivation of entropy production (M.S., Physica A 390(2011)1904-1916) based on the first-principle of using the projected density matrix ?2(t) or more generally ?even(t), while the previous standard argument is due to the thermodynamic energy balance. This new derivation clarifies conceptually the physics of irreversibility in transport phenomena. In general, the transport phenomena are described by the odd part ?odd(t) of the density matrix and the entropy production (namely irreversibility) is described by the even part ?even(t). These are related to each other through the coupled equations as , where "q" denotes "even" (symmetric) or "odd" (antisymmetric), Pq is the projection operator to the "q" part of ?(t), and Script H1(t) denotes the partial Hamiltonian due to the external force such as in electric conduction. The concept of a stationary temperature Tst in steady states with current (say electric current) is also proposed by using the projected and symmetry-separated von Newmann equation introduced by the present author. The entropy production of the relevant steady state depends on this stationary temperature. A mechanical formulation of thermal conduction is given by introducing a thermal field ET and its conjugate "heat" operator for a local internal energy hj of the thermal particle j.

Suzuki, Masuo

2011-05-01

226

Diffusion in Post-Perovskite from First Principles

NASA Astrophysics Data System (ADS)

Diffusion controls many physical and chemical processes in the solid Earth. While experiments are limited to conditions of the shallow lower mantle, ab initio calculations allow us to probe conditions at any pressure and temperature. By applying harmonic transition state theory (Vineyard-theory [1]), we have calculated self-diffusion coefficients of MgO and MgSiO3 perovskite and post-perovskite at conditions of Earth's lower mantle from first principles. Our analysis shows that magnesium and oxygen in MgSiO3 perovskite migrate via simple single jumps to nearest neighbour sites. For silicon we propose diffusion via a six-jump cycle on the silicon-magnesium sublattice. We compared our absolute diffusion rates with experimental data of magnesium and oxygen diffusion in MgO at temperatures between 1873 K and 2273 K and at pressures ranging from 7 GPa to 35 GPa. Our calculated diffusion rates of magnesium, silicon and oxygen in MgSiO3 perovskite have also been compared with experiments at 25 GPa with temperatures ranging from 1259 K to 2273K. All our diffusion rates are in excellent agreement with all the available experimental data. The six-jump cycle mechanism works also in MgSiO3 post-perovskite allowing both cations to cross the SiO6-octahedra-layers. Nevertheless, we find that cation diffusion in MgSiO3 post-perovskite is very anisotropic. This is not due to slow diffusion across the SiO6-octahedra-layers (which is similar to diffusion in perovskite), but because of very fast diffusion along the a-axis. Therefore, if deformation takes place via diffusion-controlled mechanism, we would expect post-perovskite to be weaker than perovskite. There is strong evidence that lower mantle deforms in the diffusion-creep regime. However, how a material with strongly anisotopic diffusion deforms under low stresses is not clear. Although it is generally believed that diffusion creep does not produce a lattice preferred orientation (LPO), this may not be true for a diffusionally very anisotropic material. If so, we would expect a significant history-dependent rheology in D’’, with the development of fast diffusion creep as grains become increasingly oriented. This may lead to localised shear-zones that separate large regions undergoing little deformation. REFERENCES: [1] G. H. Vineyard (1957), J. Phys. Chem. Solids 3 121-127

Ammann, M. W.; Brodholt, J. P.; Dobson, D.

2009-12-01

227

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

228

First principle optoelectronic studies of visible light sensitive CZT

NASA Astrophysics Data System (ADS)

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

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

2013-11-01

229

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

230

First principles calculations of interlayer exchange coupling in bcc Fe/Cu/Fe structures

The authors report on theoretical calculations of interlayer exchange coupling between two Fe layers separated by a modified Cu spacer. These calculations were motivated by experimental investigations of similar structures by the SFU group. The multilayer structures of interest have the general form: Fe/Cu(k)/Fe and Fe/Cu(m)/X(1)/Cu(n)/Fe where X indicates one AL (atomic layer) of foreign atoms X (Cr, Ag, or Fe) and k, m, n represent the number of atomic layers of Cu. The purpose of the experimental and theoretical work was to determine the effect of modifying the pure Cu spacer by replacing the central Cu atomic layer with the atomic layer of foreign atoms X. The first principles calculation were performed using the Layer Korringa-Kohn-Rostoker (LKKR) method. The theoretical thickness dependence of the exchange coupling between two semi-infinite Fe layers was calculated for pure Cu spacer thicknesses in the range of 0 < k < 16. The effect of the foreign atoms X on the exchange coupling was investigated using the structure with 9 AL Cu spacer as a reference sample. The calculated changes in the exchange coupling are in qualitative agreement with experiment.

Kowalewski, M.; Heninrich, B. [Simon Fraser Univ., Burnaby, British Columbia (Canada); Schulthess, T.C.; Butler, W.H. [Oak Ridge National Lab., TN (United States)

1998-01-01

231

First principles calculations of interlayer exchange coupling in bcc Fe/Cu/Fe structures

The authors report on theoretical calculations of interlayer exchange coupling between two Fe layers separated by a modified Cu spacer. These calculations were motivated by experimental investigations of similar structures by the SFU group. The multilayer structures of interest have the general form: Fe/Cu(k)Fe and Fe/Cu(m)/X(1)/Cu(n)/Fe where X indicates one AL (atomic layer) of foreign atoms X (Cr, Ag or Fe) and k, m, n represent the number of atomic layers of Cu. The purpose of the experimental and theoretical work was to determine the effect of modifying the pure Cu spacer by replacing the central Cu atomic layer with the atomic layer of foreign atoms X. The first principles calculation were performed using the Layer Korringa-Kohn-Rostoker (LKKR) method. The theoretical thickness dependence of the exchange coupling between two-semi-infinite Fe layers was calculated for pure Cu spacer thicknesses in the range of 0

Kowalewski, M.; Heinrich, B. [Simon Fraser Univ., Burnaby, British Columbia (Canada); Schulthess, T.C.; Butler, W.H. [Oak Ridge National Lab., TN (United States)

1998-07-01

232

Theoretical aspects of double resonance phenomena in Mössbauer spectroscopy

NASA Astrophysics Data System (ADS)

We study theoretically radio-frequency/gamma-regime double resonance. In calculating the time dependent Mössbauer absorption Floquet states are used, which incorporate both the rf field andthe static splitting of nuclear levels. A novel concise expression results that encompasses all known cases of direct electromagnetic rf excitation. Methods for solving the theory numerically are discussed. Our computer programs are faster andmore flexible than the earlier matrix continued fraction programs. A comparison of time averaged andtime averaged andtime dependent absorption elucidates the role of transverse andlongitudinal rf fields.

Tittonen, I.; Javanainen, J.; Lippmaa, M.; Katila, T.

1993-03-01

233

First-Principles Investigations of Defects in Minerals

NASA Astrophysics Data System (ADS)

The ideal crystal has an infinite 3-dimensional repetition of identical units which may be atoms or molecules. But real crystals are limited in size and they have disorder in stacking which as called defects. Basically three types of defects exist in solids: 1) point defects, 2) line defects, and 3) surface defects. Common point defects are vacant lattice sites, interstitial atoms and impurities and these are known to influence strongly many solid-state transport properties such as diffusion, electrical conduction, creep, etc. In thermal equilibrium point defects concentrations are determined by their formation enthalpies and their movement by their migration barriers. Line and surface defects are though absent from the ideal crystal in thermal equilibrium due to higher energy costs but they are invariably present in all real crystals. Line defects include edge-, screw- and mixed-dislocations and their presence is essential in explaining the mechanical strength and deformation of real crystals. Surface defects may arise at the boundary between two grains, or small crystals, within a larger crystal. A wide variety of grain boundaries can form in a polycrystal depending on factors such growth conditions and thermal treatment. In this talk we will present our first-principles density functional theory based defect studies of SiO2 polymorphs (stishovite, CaCl2-, ?-PbO2-, and pyrite-type), Mg2SiO4 polymorphs (forsterite, wadsleyite and ringwoodite) and MgO [1-3]. Briefly, several native point defects including vacancies, interstitials, and their complexes were studied in silica polymorphs upto 200 GPa. Their values increase by a factor of 2 over the entire pressure range studied with large differences in some cases between different phases. The Schottky defects are energetically most favorable at zero pressure whereas O-Frenkel pairs become systematically more favorable at pressures higher than 20 GPa. The geometric and electronic structures of defects and migrating ions vary largely among different types of defects. In particular, the O-defects introduce localized electronic states. For Mg2SiO4 polymorphs native and protonic point defects were investigated upto 30 GPa. The Mg2+-Frenkel defects in forsterite and MgO pseudo-Schottky defects in wadsleyite and ringwoodite are energetically most favorable. Mg migration is easiest in forsterite and ringwoodite whereas Si migration is easiest in wadsleyite. Protons show substantially effect on structural transition pressures and PV equations-of-states. In our work on MgO, we showed that the point defect formation is easier in grain boundary interfacial regions than in bulk and pressure increasingly stabilizes interfacial vacancies relative to bulk thereby causing as enhancement in the vacancy concentrations. Symmetric tilt grain boundaries show structural phase transitions to asymmetric tilt grain boundaries under pressure.

Verma, Ashok K.

2011-07-01

234

Strain sensitivity and superconducting properties of Nb3Sn from first principles calculations.

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, T(c), has been calculated by means of the Allen-Dynes modification of the McMillan formula. As a result, the characteristic bell-shaped T(c) 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 T(c)(?) 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. PMID:23478497

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

2013-03-11

235

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

236

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

237

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

238

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

239

In the present study, the inclusion processes of ?-carotene, astaxanthin, lycopene, and norbixin (NOR) into the ?-cyclodextrin (?-CD) cavity were investigated by means of Raman spectroscopy and quantum mechanics calculations. The Raman ?(1) band assigned to C?C stretching was sensitive to the host-guest interaction and in general undergoes a blue shift (3-13 cm(-1)) after inclusion takes place, which is the consequence of the localization of single and double bonds. This is supported by the molecular modeling prediction, which inclusion complexes show the ?(1) band blue shifted by 1-8 cm(-1). The calculated complexation energies was small for most of derivatives and was found to be -11.1 kcal mol(-1) for inclusion of AST and +0.27 kcal mol(-1) for NOR. The stability order was qualitatively correlated to topological parameters accounting for the opening angle of the chain. This means that after inclusion the guest molecules assume a slightly more extended conformation, which enhances the host-guest contact, improving the interaction energy. The results discussed here clearly demonstrate the matrix effect on the carotenes' spectroscopic profile and should contribute to fully characterize the raw samples. PMID:21728366

de Oliveira, Vanessa E; Almeida, Eduardo W C; Castro, Harlem V; Edwards, Howell G M; Dos Santos, Hélio F; de Oliveira, Luiz Fernando C

2011-07-14

240

Magnetoelectric coupling in layered perovskites from first principles

NASA Astrophysics Data System (ADS)

The rational design of a multiferroic with a large polarization and a strong coupling between the polarization and the magnetization remains a challenge. Recognizing the limitations of bulk materials, we attempt to design a strongly coupled multiferroic by focusing on artificial layered materials. In particular, strained Sr-Ti-O layered perovskites have recently been shown to have ferroelectric lattice instabilities that can be controlled by altering the effective dimensionality of the layered system. We use a combination of density-functional theory and group theoretical methods to investigate the interplay of magnetization with ferroelectricity when a layer of magnetic transition metal ions are introduced into this highly tunable dielectric superlattice.

Birol, Turan; Fennie, Craig J.

2011-03-01

241

Optical activity of selenium: A nearly first-principles calculation

NASA Astrophysics Data System (ADS)

For optical activity, we have derived a band-theoretic formula and evaluated it within the self-energy-corrected local-density-approximation approach using a plane-wave basis. For optically uniaxial, trigonal selenium, we compute (i) ?, the optical rotatory power for propagation along the optic axis, (ii) g11, a gyration tensor component, (iii) d11 for second-harmonic generation, and (iv) ?e and ?o, the two dielectric constants. These comprehensive results are the first reported in the literature for any crystal, and they agree with some, but not all, available measurements which are often not consistent with each other.

Zhong, Hua; Levine, Zachary H.; Allan, Douglas C.; Wilkins, John W.

1992-07-01

242

Magnetic nanoscale laminates with tunable exchange coupling from first principles

NASA Astrophysics Data System (ADS)

The Mn+1AXn (MAX) phases are nanolaminated compounds with a unique combination of metallic and ceramic properties, not yet including magnetism. We carry out a systematic theoretical study of potential magnetic MAX phases and predict the existence of stable magnetic (Cr1-xMnx)2AlC alloys. We show that in this system ferromagnetically ordered Mn layers are exchange coupled via nearly nonmagnetic Cr layers, forming an inherent structure of atomic-thin magnetic multilayers, and that the degree of disorder between Cr and Mn in the alloy can be used to tune the sign and magnitude of the coupling.

Dahlqvist, M.; Alling, B.; Abrikosov, I. A.; Rosen, J.

2011-12-01

243

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

244

First-principles study of luminescence in Ce-doped inorganic scintillators

NASA Astrophysics Data System (ADS)

Luminescence in Ce-doped materials corresponds to a transition from an excited state where the lowest Ce 5d level is filled [often called the (Ce3+)* state] to the ground state where a single 4f level is filled. We have performed theoretical calculations based on density functional theory to calculate the ground-state band structure of Ce-doped materials as well as the (Ce3+)* excited state. The excited-state calculations used a constrained occupancy approach by setting the occupation of the Ce 4f states to zero and allowing the first excited state above them to be filled. These calculations were performed on a set of Ce-doped materials that are known from experiment to be scintillators or nonscintillators to relate theoretically calculable parameters to measured scintillator performance. From these studies, we developed a set of criteria based on calculated parameters that are necessary characteristics for bright Ce-activated scintillators. Applying these criteria to about 100 new materials, we developed a list of candidate materials for new bright Ce-activated scintillators. After synthesis in powder form, one of these new materials (Ba2YCl7:Ce) was found to be a bright scintillator. This approach, involving first-principles calculations of modest computing requirements, was designed as a systematic, high-throughput method to aid in the discovery of new bright scintillator materials by prioritization and down-selection on the large number of potential new materials.

Canning, A.; Chaudhry, A.; Boutchko, R.; Grønbech-Jensen, N.

2011-03-01

245

First principles Investigations of the Conductance of Stretched Molecules

NASA Astrophysics Data System (ADS)

A novel experimental setup developed at Arizona State University examines the molecular conductance across a variety of gap lengths by lowering a gold-plated AFM tip into a monolayer deposited on a gold substrate [1]. Theoretical investigations into these systems have revealed interesting trends in the conductance of these molecules as they are stretched. We investigate this system using a variety of theoretical models, such as DFT and Hartree-Fock calculations of the Hamiltonian (and a variety of basis sets), which is implemented into a Landauer formula based rapid transfer matrix method with charge self-consistency [2]. Here we solve a self-consistent potential, which obviates the need to parameterize the voltage. Conduction across the molecule occurs in multiple channels; gold states couple with varying strengths to the orbitals of the molecule. We will report the effects of strain across the molecule, and distortion of the molecule, on the conductive nature of the coupling. * Work supported by the Office of Naval Research [1] B. Xu and N. J. Tao, Science 301, 1221 (2003). [2] T. Usuki, M. Saito, M. Takatsu, R.A. Kiehl, and N. Yokoyama, Phys. Rev. B 52, 8244 (1995).

Speyer, Gil; Akis, Richard; Ferry, David K.; Li, Jun; Sankey, Otto F.

2004-03-01

246

Lattice dynamics and ferroelectric instability of PZT from first principles

NASA Astrophysics Data System (ADS)

The lattice dynamics of the prototypical cubic perovskite structure has long been considered central to the understanding of the ferroelectric and piezoelectric behavior of perovskite oxides. In solid solutions such as PZT, compositional disorder greatly complicates the theoretical study of the lattice dynamics. The virtual crystal approximation (VCA) is an easily implemented approach to the calculation of interatomic force constants (IFC) in solid solutions; however, as shown by comparison with full ab-initio calculations for ordered alloy configurations, it provides a poor description of the lattice dynamics of PZT. An alternative ab-initio based approach, proposed in [1], is to transfer the IFC's computed for the endpoint compounds to the solid solution. We will present results of the application of this approach to PZT and discuss the implications for effective Hamiltonian simulations of finite-temperature behavior. 1. Ph. Ghosez, E. Cockayne, U. V. Waghmare and K. M. Rabe, Phys. Rev. B60, 836 (1999).

Bungaro, Claudia; Rabe, Karin M.

2001-03-01

247

Combining First Principles with Grey-Box Approaches for Modelling a Water Gas Heater System

This paper presents two different approaches for parameter identification in hybrid models (HM). The hybrid model consists in the parallel or cascade connection of two blocks: an approximate first principles model (FPM) and an unknown block model. The first principles model is constructed based in the balance equations of the system that could have some unknown parts. The unknown parts

J. A. Vieira; A. M. Mota

2005-01-01

248

NASA Astrophysics Data System (ADS)

Scanning tunneling microscopy and scanning tunneling spectroscopy combined with first-principles calculations have been applied to investigate the (111) surface of a naturally grown Fe3O4 single crystal. The commonly observed surface is determined as a layer of Fe cations at tetrahedral sites, known as the Fetet1 termination. A surface terminated with Fe cations at octahedral sites, another proposed termination in previous studies, is found only when the surface was prepared under oxygen-poor conditions. Scanning tunneling spectra at room temperature and at 77 K indicate that the (111) surface undergoes a metal-insulator transition.

Shimizu, Tomoko K.; Jung, Jaehoon; Kato, Hiroyuki S.; Kim, Yousoo; Kawai, Maki

2010-06-01

249

In addition to equilibrium isotopic fractionation factors experimentally derived, theoretical predictions are needed for interpreting isotopic compositions measured on natural samples because they allow exploring more easily a broader range of temperature and composition. For iron isotopes, only aqueous species were studied by first-principles methods and the combination of these data with those obtained by different methods for minerals leads

Marc Blanchard; Franck Poitrasson; Merlin Méheut; Michele Lazzeri; Francesco Mauri; Etienne Balan

2009-01-01

250

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.

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

2012-01-01

251

First-principles study of excitonic effects in Raman intensities

NASA Astrophysics Data System (ADS)

The ab initio prediction of Raman intensities for bulk solids usually relies on the hypothesis that the frequency of the incident laser light is much smaller than the band gap. However, when the photon frequency is a sizable fraction of the energy gap, or higher, resonance effects appear. In the case of silicon, when excitonic effects are neglected, the response of the solid to light increases by nearly three orders of magnitude in the range of frequencies between the static limit and the gap. When excitonic effects are taken into account, an additional tenfold increase in the intensity is observed. We include these effects using a finite-difference scheme applied on the dielectric function obtained by solving the Bethe-Salpeter equation. Our results for the Raman susceptibility of silicon show stronger agreement with experimental data compared with previous theoretical studies. For the sampling of the Brillouin zone, a double-grid technique is proposed, resulting in a significant reduction in computational effort.

Gillet, Yannick; Giantomassi, Matteo; Gonze, Xavier

2013-09-01

252

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

253

NASA Astrophysics Data System (ADS)

In addition to equilibrium isotopic fractionation factors experimentally derived, theoretical predictions are needed for interpreting isotopic compositions measured on natural samples because they allow exploring more easily a broader range of temperature and composition. For iron isotopes, only aqueous species were studied by first-principles methods and the combination of these data with those obtained by different methods for minerals leads to discrepancies between theoretical and experimental isotopic fractionation factors. In this paper, equilibrium iron isotope fractionation factors for the common minerals pyrite, hematite, and siderite were determined as a function of temperature, using first-principles methods based on the density functional theory (DFT). In these minerals belonging to the sulfide, oxide and carbonate class, iron is present under two different oxidation states and is involved in contrasted types of interatomic bonds. Equilibrium fractionation factors calculated between hematite and siderite compare well with the one estimated from experimental data (ln ?57Fe/ 54Fe = 4.59 ± 0.30‰ and 5.46 ± 0.63‰ at 20 °C for theoretical and experimental data, respectively) while those for Fe(III) aq-hematite and Fe(II) aq-siderite are significantly higher that experimental values. This suggests that the absolute values of the reduced partition functions ( ?-factors) of aqueous species are not accurate enough to be combined with those calculated for minerals. When compared to previous predictions derived from Mössbauer or INRXS data [Polyakov V. B., Clayton R. N., Horita J. and Mineev S. D. (2007) Equilibrium iron isotope fractionation factors of minerals: reevaluation from the data of nuclear inelastic resonant X-ray scattering and Mössbauer spectroscopy. Geochim. Cosmochim. Acta71, 3833-3846], our iron ?-factors are in good agreement for siderite and hematite while a discrepancy is observed for pyrite. However, the detailed investigation of the structural, electronic and vibrational properties of pyrite as well as the study of sulfur isotope fractionation between pyrite and two other sulfides (sphalerite and galena) indicate that DFT-derived ?-factors of pyrite are as accurate as for hematite and siderite. We thus suggest that experimental vibrational density of states of pyrite should be re-examined.

Blanchard, Marc; Poitrasson, Franck; Méheut, Merlin; Lazzeri, Michele; Mauri, Francesco; Balan, Etienne

2009-11-01

254

The generalized stacking fault (GSF) energy surfaces have received considerable attention due to their close relation to the mechanical properties of solids. We present a detailed study of the GSF energy surfaces of silicon within the framework of density functional theory. We have calculated the GSF energy surfaces for the shuffle and glide set of the (111) plane, and that

Yu-Min Juan; Efthimios Kaxiras

1996-01-01

255

Surface phase stability diagram for Pd deposits on Ni(110): A first-principles theoretical study

NASA Astrophysics Data System (ADS)

Different surface structures of Pd deposits on Ni(110), from alloys to ordered phases, in a range between 0.125 monolayer and near 4 monolayers, have been studied using density functional calculations. The main feature of these deposits is the stress exerted by the Ni substrate on the Pd atoms, due to the size difference between the two metals. The surface phase stability diagram has been constructed, allowing to find the most stable surface structures at a given coverage. At low coverage (<=0.7 monolayer), the disordered alloy phase is found to be stable. Then, at higher coverage, ordered surface structures are stable, and a mixing of some of them can coexist in some ranges. Between 0.7 and 1 monolayer, the surface presents an equilibrium between the alloy phase (containing 70% of Pd atoms) and a 1 monolayer phase. In the 1-4 monolayers range, the equilibrium is set up between the two extreme phases: 1 monolayer and 4 monolayers: These deposits show a peculiar structure, in which a periodical Pd vacancy at the Ni-Pd interface ensures the release of the stress and induces a heteroepitaxial dislocation. The stability of these structures versus the coverage is discussed as a competition between surface alloying and surface stress. Finally, the electronic structure of these phases is studied and appears to be closely linked to the surface strain of the Pd deposit, even for the alloy phases. These bimetallic surfaces reveal a wide range of local density of states, simply by changing the Pd coverage. Then, the possibility to tune the specific catalytic properties of these deposits is discussed.

Filhol, J.-S.; Simon, D.; Sautet, P.

2001-08-01

256

NASA Astrophysics Data System (ADS)

Coupled fractionations of N and O isotopes during biological nitrate reduction provide important constraints on the marine nitrogen cycle at present and in the geologic past. Recent laboratory experiments with mono-cultures of nitrate-assimilative algae and plankton, and denitrifying bacteria demonstrate that N and O isotopic compositions of the residual nitrate co-vary linearly with a constant ratio (i.e., ??18O: ??15N) of ~1 or ~0.6 [1]. These systematic variations have been inferred to derive from the kinetic isotope fractionations associated with nitrate reductases. The isotope fractionation mechanisms at the enzymatic level, however, remain elusive. Here we present models of isotope fractionations accompanying the nitrate reduction (NO3-?NO2-) by three functional types of nitrate reductases, using techniques from ab initio, transition state and statistical thermodynamic theory. We consider three types of nitrate reductases: eukNR (eukaryotic assimilatory nitrate reductase), NAR (prokaryotic respiratory nitrate reductase) and Nap (prokaryotic periplasmic nitrate reductase). All are penta- or hexa-coordinated molybdo-enzymes, but bear considerable differences in protein geometry among functional types. Our models, based on the simplified structures of their active sites, predict N and O isotope effects (15? and 18?) ranging from 32.7 to 36.6‰ and from 33.5 to 34.8‰, respectively, at 300K with 18?:15? ratios of 0.9-1.1. The predicted amplitudes of N and O isotope fractionations are in the range measured for eukNR in vitro (~27‰, Karsh et al. in prep), and also correspond to the upper amplitudes observed for denitrifiers in vivo (~25‰, [1]). Moreover, the computed 18?:15? ratios corroborate the consistent relationships of ~1 observed experimentally for eukNR and the respiratory NAR. These findings indicate the enzymatic reduction is likely the rate-limiting step in most biological nitrate reductions. In addition, the predicted similarity of 18?:15? ratios among different nitrate reductases suggests that the nitrate isotope fractionations by nitrate reductases are governed by the kinetics of the O-N bond cleavage, which incurs negligible differences from variations in surrounding moieties at the active sites. However, our model similarly predicts a 15? of 36.6‰ and 18?:15? of 0.9 for the auxiliary Nap, although it exhibits a 15? of ~15‰ and 18?:15? of ~0.6 in vivo [1]. This discrepancy is suspected to arise from slower binding and release of NO3- from Nap, which could be partially rate-determining in this enzymatic catalysis, or from the assumptions of our modeled enzyme structures. By extending our above models to include the multiply-substituted (clumped) isotopologues, we predict that isotope fractionations during biological nitrate reduction decrease the proportion of 15N-18O bonds in the residual nitrate relative to their expected equilibrium abundances (~0.02‰ decrease for every 1‰ kinetic enrichment in nitrate ?15N). Future quantification of 15N-18O clumped isotope anomalies in natural nitrate may provide additional constraints on the nitrogen cycle in the ocean. Reference: [1] Granger et al. (2010) GCA, 74: 1030-1040.

Guo, W.; Granger, J.; Sigman, D. M.

2010-12-01

257

First-principles theoretical studies of half-metallic ferromagnetism in CrTe

NASA Astrophysics Data System (ADS)

Using full-potential linear augmented plane-wave method (FP-LAPW) we have studied the stability and electronic properties of the chalcogenide CrTe in three competing structures: rocksalt (RS), zinc blende (ZB), and the NiAs-type (NA) hexagonal. Although the ground state is of NA structure, RS and ZB are interesting in that these fcc-based structures, which can perhaps be grown on various semiconductor substrates, exhibit half-metallic (HM) phases above some critical values of the lattice parameter. We find that the NA structure is not HM at its equilibrium volume while both ZB and RS structures are. The RS structure is more stable than the ZB with an energy that is lower by 0.25 eV/atom. While confirming previous results on the HM phase in ZB structure, we provide hitherto unreported results on the HM RS phase, with a gap in the minority channel and a magnetic moment of 4.0?B/f.u. A comparison of total energies for the ferromagnetic (FM), nonmagnetic, and antiferromagnetic (AFM) configurations shows the lowest energy configuration to be FM for CrTe in all the three structures. The exchange interactions in the RS and ZB structures are studied for a wide range of the lattice parameter using the linear-response method and a mapping of the total energy to the classical Heisenberg model. These linear-response calculations are performed in the linear muffin-tin orbitals (LMTOs) basis, using the atomic sphere approximation (ASA). We have verified that the results of the electronic structure obtained via the LMTO-ASA method under local-density approximation (LDA) and LDA+U schemes are in close agreement with those obtained via the more accurate FP-LAPW method. The results show that the exchange interactions in the RS structure are much more short ranged than in the ZB structure. Hence, for the RS structure the exchange interactions are also studied by using a nearest- and next-nearest-neighbor (J1-J2) model and the energy differences between FM and two AFM states. These J1-J2 model results are obtained by using both the FP-LAPW and LMTO-ASA methods and compared with the linear-response results. The calculated Curie temperatures for the RS phase are consistently higher than those for the ZB phase.

Liu, Y.; Bose, S. K.; Kudrnovský, J.

2010-09-01

258

Modeling the reactive ion etching process for the CoO(001) surface via first principles calculations

NASA Astrophysics Data System (ADS)

Using first principles calculations with local density approximation (LDA) and LDA+U methods, we present a detailed theoretical study of reactive gas combinations (O2, N2, CH4, and CHF3) for the etching processes of CoO(001) surfaces. The calculation results show that the best possible gas combinations for the etching process contain CH4. Despite differences in the intermediate state total energies predicted by the two methods, the favorable results in the final state lead to the same reaction products.

Ozawa, Nobuki; Roman, Tanglaw; David, Melanie; Kishi, Hirofumi; Kasai, Hideaki

2008-09-01

259

NASA Astrophysics Data System (ADS)

We present a theoretical study of the transport properties of a CrAs/GaAs/CrAs trilayer. The theory was based on a first principles method for calculating the electronic structure, in combination with a Kubo-Landauer approach for calculating the transport properties in a current perpendicular to the plane geometry. We have also investigated the electronic structure and the magnetic properties of this trilayer, with special focus on electronic and magnetic properties at the CrAs/GaAs interface. Finally, we have studied the effects of chemical disorder on the transport properties, in particular the influence of As antisites at both the Cr and Ga sites.

Bengone, O.; Eriksson, O.; Fransson, J.; Turek, I.; Kudrnovský, J.; Drchal, V.

2004-07-01

260

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

261

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

262

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

We have investigated the phase stability of two MAX phases, V3SiC2 and V2SiC, by means of first-principles total-energy calculations within the generalized-gradient approximation and the projector-augmented wave method. The theoretical bulk modulus of V3SiC2 is 219 GPa, which is ?17% larger than that of Ti3SiC2(187 GPa). The total-energy calculations show that V2SiC is stable with a formation energy of about

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

2007-01-01

263

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

264

First principle computation of biomolecule-ligand interaction

NASA Astrophysics Data System (ADS)

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

Chachiyo, Teepanis

265

First-principles study of alloying effect of transition metals on He in titanium ditritide

NASA Astrophysics Data System (ADS)

Due to its inert reactivity with almost elements, 3He produced from tritium decay has extremely detrimental effects on the tritide. To refrain from this 3He-induced damage, an efficient way is to increase the stability of 3He in metal tritide by alloying. Using a first-principles discrete variational method in two cluster models, one for a low 3He concentration and the other for a high 3He concentration, the authors study the alloying effect of 3d and 4d transition metals on the stability of 3He in TiT2 system. It is found that the preferring and metastable sites of 3He are affected by 3He concentration: 3He prefers to stay at original tetrahedral interstitial site when 3He concentration is low but moves to octahedral site when 3He concentration is high enough. A criterion of alloying effect is proposed, according to which Nb, Y, Zr, Pd, Ru, Tc, Rh, Cr, Mo and Ag are suggested to be the beneficial alloying elements for increasing the stability of 3He in the alloyed TiT2 with a low 3He concentration and Y, Nb, Mo, Zr, Cr, Tc, Ru, Rh and Cu for that with a high 3He concentration. Our results of alloying effect are supported by the positron annihilation spectroscopy (PAS) measurements for He-implanted Ti, TiMoYAl and TiZrYAl films.

Wu, Y. X.; Yang, R.; Zheng, H.; Wang, Y. M.

2006-08-01

266

A first principles study of the acetylene-water interaction

We present an extensive study of the stationary points on the acetylene-water (AW) ground-state potential energy surface (PES) aimed in establishing accurate energetics for the two different bonding scenarios that are considered. Those include arrangements in which water acts either as a proton acceptor from one of the acetylene hydrogen atoms or a proton donor to the triple bond. We used a hierarchy of theoretical methods to account for electron correlation [MP2 (second-order Moller-Plesset), MP4 (fourth-order Moller-Plesset), and CCSD(T) (coupled-cluster single double triple)] coupled with a series of increasing size augmented correlation consistent basis sets (aug-cc-pVnZ, n=2,3,4). We furthermore examined the effect of corrections due to basis set superposition error (BSSE). We found that those have a large effect in altering the qualitative features of the PES of the complex. They are responsible for producing a structure of higher (C{sub 2v}) symmetry for the global minimum. Zero-point energy (ZPE) corrections were found to increase the stability of the C{sub 2v} arrangement. For the global (water acceptor) minimum of C{sub 2v} symmetry our best estimates are {delta}E{sub e}=-2.87 kcal/mol ({delta}E{sub 0}=-2.04 kcal/mol) and a van der Waals distance of R{sub e}=2.190 Aa. The water donor arrangement lies 0.3 kcal/mol (0.5 kcal/mol including ZPE corrections) above the global minimum. The barrier for its isomerization to the global minimum is E{sub e}=0.18 kcal/mol; however, inclusion of BSSE- and ZPE-corrections destabilize the water donor arrangement suggesting that it can readily convert to the global minimum. We therefore conclude that there exists only one minimum on the PES in accordance with previous experimental observations. To this end, vibrational averaging and to a lesser extend proper description of intermolecular interactions (BSSE) were found to have a large effect in altering the qualitative features of the ground-state PES of the acetylene-water complex. (c) 2000 American Institute of Physics.

Tzeli, Demeter [Laboratory of Physical Chemistry, Department of Chemistry, National and Kapodistrain University of Athens, P.O. Box 64 004, 157 10 Zografou, Athens, (Greece); Mavridis, Aristides [Laboratory of Physical Chemistry, Department of Chemistry, National and Kapodistrain University of Athens, P.O. Box 64 004, 157 10 Zografou, Athens, (Greece); Xantheas, Sotiris S. [Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 906 Battelle Boulevard, P.O. Box 999, MS K8-91, Richland, Washington 99352 (United States)

2000-04-08

267

First-principles studies of electronic, optical, and vibrational properties of LaVO4 polymorph

NASA Astrophysics Data System (ADS)

First-principles calculations of electronic, optical, and vibrational properties of LaVO4 polymorph were performed with the density functional theory plane-wave pseudopotential method. The results of the electronic structure reveal that the different coordinated structure for monoclinic LaVO4 leads to an indirect band gap, while tetragonal LaVO4 has a direct band gap. Besides, the analysis of the electronic structure shows ionic nature in La-O bonds and covalent nature in V-O bonds. From further study in chemical bonding behavior, we find that the V-O covalent bonds have four types: ? bonding, ? bonding, ?* antibonding, and ?* antibonding states. Various optical properties, including the dielectric function, reflectivity, absorption coefficient, refractive index, and the energy-loss spectrum as functions of the photon energy were calculated. Our calculations indicate that monoclinic LaVO4 has excellent dielectric properties along [0 0 1] direction. In the optical-frequency (?-->?) contributed from electrons the optical properties of tetragonal LaVO4 show the isotropy, while the diagonal components of static dielectric tensors ?(0) of tetragonal LaVO4 have the Vxx=Vyy?Vzz relation by adding the lattice vibration contribution (?-->0) to the electronic dielectric tensor. The vibrational spectra of LaVO4 polymorph have also been calculated from first principles by the linear response method. The calculated frequencies are in good agreement with the experimental data available for these crystals obtained by the methods of infrared and Raman spectroscopies. The vibrational spectra of monoclinic and tetragonal LaVO4 crystal exhibit three groups of frequencies: the low-frequency (<240 cm-1), middle-frequency (270-450 cm-1), and high-frequency region (850-970 cm-1), according to the vibration dominated by translation of La atoms, the bending vibration of O-V-O bonds, and stretching vibration of O-V-O bonds, respectively. Our studies report on microstructure of LaVO4 polymorph, and provide useful information for the potential application of this material.

Sun, Liming; Zhao, Xian; Li, Yanlu; Li, Pan; Sun, Honggang; Cheng, Xiufeng; Fan, Weiliu

2010-11-01

268

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.

2013-08-01

269

NASA Astrophysics Data System (ADS)

Theoretical studies of nanoscale systems, such as functionalized carbon nanotubes (CNTs) and graphene, present major challenges to computational methods employed in quantum chemistry and condensed matter physics. For this reason, accurate numerical calculations for functionalized CNTs and graphene require a direct quantum mechanical approach. This thesis presents first principles calculations using density functional theory (DFT) to study the structural and electronic properties of functionalized and doped graphene and CNTs. The calculations are performed using density-functional pseudopotential computational methods combined with the generalized gradient approximation (GGA) for the exchange-correlation functional. The structural optimization of graphene and CNTs is carried out by energy and force minimization. We investigate the chemical functionalization of graphene and CNTs with carboxyl (COOH) groups. The binding energies, equilibrium geometries, and the charge transfer of graphene sheets and CNTs are examined. We find that the attachment of COOH groups induces substantial structural changes in graphene and CNTs. Our calculations show that the binding of the COOH group is significantly stronger in the presence of surface defects. The properties of graphene and CNTs doped with boron (B) and nitrogen (N) atoms are also studied. The calculations confirm that B doping increases and N doping decreases the binding energy of COOH groups to both defect-free and defective graphene and CNTs. The interactions between B and N atoms doping graphene are examined. The B-B and N-N interactions are found to be repulsive, while the B-N interaction is found to be attractive.

Al-Aqtash, Nabil

270

Neutral and charged excitations in carbon fullerenes from first-principles many-body theories

We use first-principles many-body theories to investigate the low energy excitations of the carbon fullerenes C_20, C_24, C_50, C_60, C_70, and C_80. Properties are calculated via the GW-Bethe-Salpeter Equation (GW-BSE) and diffusion Quantum Monte Carlo (QMC) methods. At a lower level of theoretical complexity, we also calculate these properties using static and time-dependent density-functional theory. We critically compare these theories and assess their accuracy against available experimental data. The first ionization potentials are consistently well reproduced and are similar for all the fullerenes and methods studied. The electron affinities and first triplet excitation energies show substantial method and geometry dependence. Compared to available experiment, GW-BSE underestimates excitation energies by approximately 0.3 eV while QMC overestimates them by approximately 0.5 eV. We show the GW-BSE errors result primarily from a systematic overestimation of the electron affinities, while the QMC errors likely result from nodal error in both ground and excited state calculations.

Tiago, Murilo L [ORNL; Kent, Paul R [ORNL; Hood, Randolph Q. [Lawrence Livermore National Laboratory (LLNL); Reboredo, Fernando A [ORNL

2008-01-01

271

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

NASA Astrophysics Data System (ADS)

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

Bercegeay, C.; Bernard, S.

2005-12-01

272

First-principles study of the behavior of O, N and C impurities in vanadium solids

NASA Astrophysics Data System (ADS)

Vanadium alloys are promising candidate for the structural materials of first-wall in future fusion reactor. In realistic vanadium alloys, there always exist some impurities (e.g. oxygen, nitrogen and carbon). To understand the microscopic behavior of these impurities, we investigated energetic and diffusion of O, N and C impurities as well as O-O/N-N/C-C interactions in pure vanadium using first-principles calculations. The O, N and C atoms prefer to occupy an octahedral interstitial site, and exhibit high diffusion barrier with 1.23 eV, 1.48 eV and 1.14 eV via diffusing between two neighboring octahedral interstitial sites, respectively. Such high barriers indicate that these impurities are hard to diffuse inside bulk vanadium. The corresponding diffusion coefficients as function of temperature were estimated using the Arrhenius diffusion equation. Our theoretical results provide the fundamental parameters for understanding the impurity effects in early stage of irradiation damage.

Li, Ruihuan; Zhang, Pengbo; Li, Xiaoqing; Zhang, Chong; Zhao, Jijun

2013-04-01

273

First-principles study of oxygen activation of nanoporous carbon for styrene catalysis

NASA Astrophysics Data System (ADS)

Dehydrogenation of ethylbenzene (ET) to styrene is an economically important and scientifically most puzzling chemical reaction. The industrial process uses iron oxide catalysts to enhance the conversion. However, recent theoretical work showed that the composition of iron oxide surfaces may be very different to what was hitherto believed, i.e. they are oxygen (if not OH) terminated.[1] Subsequently performed experimental work showed that indeed ET does not get in contact with iron, and that the active catalyst may be a carbon material that forms on the metal oxide during the catalyst' induction period.[2] We therefore studied the chemical activity of various motifs expected to be present in nanoporous carbon [3] by first-principles calculations. The calculations show that vacancies and edges are very reactive, e.g., they dissociate O2 without a barrier. Oxidation of the single vacancy leads to CO-formation while the O-atoms bound into larger vacancies or at edges may form active sites for ET dehydrogenation. [1] X-G Wang et al., Phys. Rev. Lett. 81, 1038 (1998). [2] G. Mestl et al., Angew. Chem. Int. Ed 40, 2066 (2001). [3] V. Petkov et al., Phil. Mag. B 79, 1519 (1999).

Carlsson, Johan M.; Scheffler, Matthias

2003-03-01

274

What about U on surfaces? Extended Hubbard models for adatom systems from first principles.

Electronic correlations together with dimensional constraints lead to some of the most fascinating properties known in condensed matter physics. As possible candidates where these conditions are realized, semiconductor (111) surfaces and adatom systems on surfaces have been under investigation for quite some time. However, state-of-the-art theoretical studies on these materials that include many-body effects beyond the band picture are rare. First principles estimates of inter-electronic Coulomb interactions for the correlated states are missing entirely, and usually these interactions are treated as adjustable parameters. In this work, we report on calculations of the interaction parameters for the group IV surface-adatom systems in the ?-phase series of Si(111):C, Si, Sn, Pb. For all systems investigated, the inter-electronic Coulomb interactions are indeed large compared to the kinetic energies of the states in question. Moreover, our study reveals that intersite interactions cannot be disregarded. We explicitly construct an extended Hubbard model for the series of group IV surface-adatom systems on silicon, which can be used for further many-body calculations. PMID:23400014

Hansmann, Philipp; Vaugier, Loïg; Jiang, Hong; Biermann, Silke

2013-02-12

275

NASA Astrophysics Data System (ADS)

We compute the lattice-dynamical and thermal equation of state properties of ferromagnetic bcc iron using the first-principles linear response linear-muffin-tin-orbital method in the generalized-gradient approximation. The calculated phonon dispersion and phonon density of states, both at ambient and high pressures, show good agreement with inelastic neutron scattering data. We find the free energy as a function of volume and temperature, including both electronic excitations and phonon contributions, and we have derived various thermodynamic properties at high pressure and temperature. The thermal equation of state at ambient temperature agrees well with diamond-anvil-cell measurements. We have performed detailed investigations on the behavior of various thermal equation of state parameters, such as the bulk modulus K , the thermal expansivity ? , the Anderson-Grüneisen parameter ?T , the Grüneisen ratio ? , and the heat capacity CV as function of temperature and pressure. A detailed comparison has been made with available experimental measurements, as well as results from similar theoretical studies on nonmagnetic bcc tantalum.

Sha, Xianwei; Cohen, R. E.

2006-03-01

276

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

NASA Astrophysics Data System (ADS)

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

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

2011-01-01

277

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.

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

2013-01-01

278

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

2013-01-09

279

NASA Astrophysics Data System (ADS)

Hundreds of half Heusler (HH) and full Heusler (FH) compounds have been synthesized, and they exhibit a multitude of properties. However, we are unaware of any Heusler compounds showing ferroelectricity (FE), or for which the piezoelectricity (PzE) has been measured. Determining these polar properties would be of theoretical interest as well as having practical importance for the design of new functional materials. In this ab initio study, we search a large set of HH and FH compounds, both known and hypothetical, for FE/PzE. We screen the zone-center phonons, computed with first-principles density-functional-theory methods, for unstable polar modes that would drive a distortion to a ferroelectric phase, and calculate PzE coefficients of compounds in the F43m space group, which includes all HH and many FH, using density-functional perturbation theory. Preliminary results from our calculations confirm that the Heusler compounds are very robust against FE instabilities. However, we found several HH compounds having e14 coefficients in the range of 0.5-1.0 C/m^2, comparable to that of some well-known piezoelectric materials such as ZnO. We also investigate the effects of epitaxial constraints on these properties, both for bulk materials and for superlattices built of Heusler materials.

Roy, Anindya; Bennett, Joseph; Rabe, Karin M.; Vanderbilt, David

2011-03-01

280

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

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

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

2008-10-17

281

O2 dissociation on nitrogen doped carbon nanotubes (10, 0) from first principles simulation

NASA Astrophysics Data System (ADS)

Reducing the amount of precious platinum (Pt) loading by identifying non-precious metal catalyst is essential for large-scale applications of fuel cells, which provide a cleaning energy technology. Recent experimental, theoretical, and simulation works accelerate the advance in the research area of doped carbon nanotubes acting as an alternate non-precious metal catalyst for dioxygen reduction in the fuel cells. First principles spin polarized density functional theory(DFT) simulations have been performed to understand O2 dissociation on nitrogen doped carbon nanotubes. We have studied nitrogen substitutional doping of carbon nanotubes (CNTs) for dioxygen adsorption, reduction, and dissociation. The calculated results show that nitrogen prefers to stay at the open-edge of short CNTs. Two O2 chemisorption sites are found, the carbon-nitrogen complex (Pauling site) and carbon-carbon long bridge (long bridge) sites. The spin polarized DFT calculations using the nudged elastic band (NEB) method show that O2 dissociation at the Pauling site has a reaction energy barrier of about 0.55 eV. The unique open-edge structure and charge redistribution are crucial to the novel properties of nitrogen-doped CNTs as a new non-precious metal catalyst for fuel cells.

Yang, Shizhong; Zhao, Guang-Lin; Khosravi, Ebrahim

2011-03-01

282

First-principles investigation of solute-hydrogen interaction in a ?-Ti solid solution

NASA Astrophysics Data System (ADS)

In this paper, a first-principles method is used to calculate the interaction energy between substitutional solute atoms and hydrogen in ?-Ti. The results show that simple metal (SM) solute atoms are repulsive to H and therefore are detraps for H, whereas transition metal (TM) solute atoms, with smaller sizes than that of the host atoms, attract H and provide traps for H. The relationship between the interaction energy and lattice distortion as well as the electronic structure is investigated. The SM-H and TM-H interactions are dominated by different factors. The repulsive interaction between SM atoms and H is mainly due to the hybridization between the electrons of SM atoms and H when they are close to each other. The interaction between the TM solutes and H is attributable to the atomic size effect, and can be described satisfactorily by Matsumoto's strain field relaxation model. From the solute-H interaction energy and available measured terminal solubility of hydrogen (TSH), the relationship between the solute trapping of hydrogen and TSH in ?-Ti is discussed. No coherent relationship is found between the theoretical hydrogen trapping effect and the experimental TSH in ?-Ti alloys.

Hu, Q. M.; Xu, D. S.; Yang, R.; Li, D.; Wu, W. T.

2002-08-01

283

Tunneling properties versus electronic structures in Si/SiO2/Si junctions from first principles

NASA Astrophysics Data System (ADS)

Using first-principles calculations, we study tunneling properties and electronic structures of Si(001)/SiO2/Si(001) junctions in a wide energy range covering the local energy gap in the SiO2 regions. We show that the tunneling spectra T(E) as functions of energy E have overall similarity to the projected densities of states (PDOS) at the centers of the SiO2 regions, but T(E) and PDOS have significant difference in their dependencies on the SiO2 thickness. From the energy dependencies of T(E) and PDOS, distinctive energy ranges are recognized in the valence and conduction bands, reflecting the local electronic structures in the SiO2 region induced from the Si regions. From the difference in the SiO2-thickness dependencies of T(E) and PDOS and from eigenchannel analysis, we find that the tunneling wave function inside the SiO2 region decreases with a decay rate which itself decreases as the tunneling distance increases, resulting in a smaller averaged decay rate per length for a thicker SiO2 region. These results provide a rich picture for the SiO2 barrier in the aspects of tunneling and local electronic structures, and a theoretical framework generally applicable to other tunneling barriers.

Ko, Eunjung; Lee, Kwang-Ryeol; Choi, Hyoung Joon

2013-07-01

284

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

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

2012-01-01

285

Phase transition and thermodynamic properties of CaF2 via first principles

NASA Astrophysics Data System (ADS)

The structural stabilities, phase transitions and thermodynamic properties of CaF2 under high pressure and temperature are investigated by first-principles calculations based on the plane-wave pseudopotential density functional theory method within the generalized gradient approximation (GGA). The calculated lattice parameters of CaF2 under zero pressure and zero temperature are in good agreement with the existing experimental data and other theoretical data. Our results demonstrate that the sequence of the pressure-induced phase transition of CaF2 is the fluorite structure (Fm3m), PbCl2-type structure (Pnma) and Ni2In-type structure (P63/mmc), and the transition pressures are obtained. The temperature-dependent volume and thermodynamic properties of Fm3m phase CaF2 at 0 GPa are presented. The thermodynamic properties of CaF2 in Fm3m, Pnma and P63/mmc phases at 300 K are predicted using the corrected and uncorrected quasi-harmonic approximation model. The variations of the thermal expansion ? and heat capacity CV with pressure P and temperature T of CaF2 in the three phases are systematically obtained.

Qi, Yuan-Yuan; Cheng, Yan; Liu, Min; Chen, Xiang-Rong; Cai, Ling-Cang

2013-10-01

286

First-principles calculation on the Curie temperature of Gd3NiSi2

NASA Astrophysics Data System (ADS)

The electronic structure and magnetic properties for Gd3NiSi2 have been studied theoretically from a first-principles density functional calculation. The energy band structure is calculated in a local spin density approximation (LSDA), and in a LSDA+Hubbard U approach (LSDA+U), respectively. For Gd atoms, in the LSDA+U approximation, seven spin-up 4f bands are fully occupied and situated at the bottom of Si s states, while the spin-down 4f hole levels are completely unoccupied and well above the Fermi level. The calculated magnetic moments for the three Gd sites vary from 7.13 to 7.16 µB, leading to a total magnetization of 21.5 µB per formula unit including the small induced moments at Ni and Si atoms. The exchange coupling parameters for the nearest Gd-Gd pairs (JGd-Gd) are 0.16 mRyd, 0.14 mRyd and 0.19 mRyd in the three Gd sub-lattices, respectively. The inter-site distance dependence of JGd-Gd shows a RKKY-like oscillation. The estimated Curie temperature is about 251 K from the calculated exchange coupling parameters based on the mean-field approximation, in good agreement with the experimental value (TCexp. = 215 K).

Liu, X. B.; Altounian, Z.

2009-10-01

287

Water solubility in calcium aluminosilicate glasses investigated by first principles techniques

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

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

2010-12-15

288

Water solubility in calcium aluminosilicate glasses investigated by first principles techniques

NASA Astrophysics Data System (ADS)

First-principles techniques have been employed to study the reactivity of water into a calcium aluminosilicate glass. In addition to the well known hydrolysis reactions Si-O-Si+H2O?Si-OH+Si-OH and Si-O-Al+H2O?Si-OH+Al-OH, a peculiar mechanism is found, leading to the formation of an AlO3-H2O entity and the breaking of Al-O-Si bond. In the glass bulk, most of the hydrolysis reactions are endothermic. Only a few regular sites are found reactive (i.e. in association with an exothermic reaction), and in that case, the hydrolysis reaction leads to a decrease of the local disorder in the amorphous vitreous network. Afterwards, we suggest that ionic charge compensators transform into network modifiers when hydrolysis occurs, according to a global process firstly suggested by Burnham in 1975. Our theoretical computations provide a more general model of the first hydrolysis steps that could help to understand experimental data and water speciation in glasses.

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

2010-12-01

289

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

NASA Astrophysics Data System (ADS)

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

Yao, Yansun; Klug, Dennis D.

2012-08-01

290

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

We have studied by the first-principles total energy method the off-center instability of a substitutional Li impurity in KCl. We report here the results of super-cell calculations of the energy associated with displacing the Li along [Formula: see text], [Formula: see text] and [Formula: see text] directions relative to the K vacancy. To understand the influence of relaxations, we performed three levels of relaxations-only first nearest neighbors of the vacancy, first and second nearest neighbors, and full relaxation. Our calculations show that the relaxation significantly affects the energy surface and that the relaxation of the nearest neighbors to the vacancy accounts for most of the trend. The position of minimum energy for the Li was found to be along the [Formula: see text] direction. The calculated well along [Formula: see text] is 102 meV deep relative to the on-center energy and lies about 0.86 Å off-center. Minima along adjacent [Formula: see text] directions are separated by a saddle-point barrier of 44 meV along the [Formula: see text] direction. Our calculated results are in agreement with prior model calculations and we believe our calculated potential surfaces are the best theoretical result so far. PMID:21715802

Gao, Xing; Daw, Murray S

2008-12-15

291

First-principles study of lattice thermal conductivity of MgSiO3 perovskite

NASA Astrophysics Data System (ADS)

The Earth's lower mantle is composed of mostly magnesium silicate (MgSiO3) with significant amounts of magnesium oxide (MgO), and with about 8-10% Fe2+ and/or Fe3+ distributed in these phases. Though the thermal conductivity of MgO crystals at high temperature and pressure conditions has been studied both experimentally and theoretically, only one experimental study has measured the thermal conductivity of MgSiO3 perovskite, and only at ambient pressure and temperature. The large uncertainty in current estimates of lower mantle thermal conductivity is a direct consequence of a long extrapolation in both pressure and temperature of measured ? of lower mantle minerals at relatively low temperatures and pressures. Ab initio computational methods provide a complimentary approach to determining the phonon thermal conductivity of lower mantle materials as a function of temperature, pressure, and composition, and can enhance physical understanding of lattice phonon contribution to thermal conductivity. In this talk, we will present our recent first-principles calculation of lattice thermal conductivity of MgSiO3 perovskite. Our methods combine first-principles calculations of lattice dynamics, quantum phonon-phonon scattering theory, and the kinetic Peierls-Boltzmann phonon transport theory. This method has been well-established for the B1-structured MgO, with good agreement with independent experimental work. However, it is significantly more challenging to accurately compute all the phonon-phonon scattering rates in the 20-atom orthorhombic unit-cell of MgSiO3 perovskite crystals than in the 2-atom cubic unit-cell of MgO crystals. To adopt this method to the study of perovskite, we further improved the numerical efficiency of our algorithm and used the TeraGrid Ranger supercomputer to directly calculate the lattice thermal conductivity at 36 pressure-temperature (p-T) conditions. Using the calculated results of single crystal MgSiO3 perovskite as the starting point, we then quantitatively accounted for the phonon scattering rates due to Mg/Fe substitution to evaluate the thermal conductivity of Fe-bearing (Mg,Fe)SiO3 solid solutions within the vibrational virtual crystal approximation.

Dong, J.; Tang, X.; Ntam, M. C.; Kavner, A.

2011-12-01

292

NASA Astrophysics Data System (ADS)

The molecular structure of three ladder oligo(p-aniline)s, 5,11-diethyl-6,12-dimethylindolo[3,2-b]carbazole (DIMER 2P), 14-ethyl-5,8-dihydro-diindolo[3,2-b:2',3'-h]carbazole (TRIMER 2P), and 5,8,14-triethyl-diindolo[3,2-b:2',3'-h]carbazole (TRIMER 3P) were investigated by first principles calculations at the Hartree-Fock (HF/6-31G*) and density functional theory (DFT/B3LYP/6-31G*) levels. It is found that the agreement between theoretical and x-ray geometrical parameters is good and rather similar for both theoretical methods. The nature and the energy of the first two singlet-singlet electronic transitions have been obtained by Zerner intermediate neglect of differential overlap/spectroscopy semiempirical calculations performed on the HF/6-31G* and DFT/B3LYP/6-31G* optimized geometries, as well as time-dependent density functional theory (TDDFT) calculations performed on the DFT/B3LYP/6-31G* optimized structures. For all the compounds and for all the theoretical approaches, it is observed that the S1<--S0 electronic transition (??*) is weakly allowed and polarized along the short axis (y) of the molecule. On the other hand, the S2<--S0 electronic transition of each oligomer possesses a much larger oscillator strength and is polarized along the long (x) molecular axis. It is found that TDDFT calculations provide the best overall agreement between the energies and the corresponding optical transitions obtained from the absorption bands (0-0 peaks) measured in dichloromethane as well as providing a good evaluation of the bathochromic shifts caused by the increase in the conjugation length or by the presence of extra alkyl chains on the nitrogen atoms in TRIMER 3P compared to TRIMER 2P.

Belletête, Michel; Durocher, Gilles; Hamel, Sébastien; Côté, Michel; Wakim, Salem; Leclerc, Mario

2005-03-01

293

NASA Astrophysics Data System (ADS)

In situ X-ray spectroscopy will reveal fundamental details of elecltrochemistry in working cells, provided that the data is interpretable. To this end, we are developing first-principles methods to simulate core-level absorption spectra of molecules, condensed phases, and interfaces with explicit inclusion of dynamics. We validate this approach by application to various lithium compounds that may be present in the solid electrolyte interphase (SEI) and to lithiation of graphite in the anode. Our calculations reveal that instantaneous broken symmetry about the x-ray excited atom may be evident in the resulting spectroscopy and highlights both dynamical and static disorder in these materials. Furthermore, we observe complex anisotropic interactions upon charge transfer between lithium and graphite that contradict a simplistic view of intercalation in terms of complete electron transfer and the rigid band approximation.

Prendergast, David; Pascal, Tod; Li, Xin; Guo, Jinghua; Luo, Yi

2013-03-01

294

First-principles study of electric polarization in piezoelectric and magnetoelectric materials

First-principles calculations based on the density-functional theory (DFT) have proven to be extremely useful in the study of properties of matter. Not only do they provide a sufficient accuracy to reproduce experimental results, but also they make it possible to predict materials with enhanced or even new properties. Often first-principles calculations become a cheap alternative to real experiments or even

Andrei Malashevich

2009-01-01

295

Combining First Principles Models and Neural Networks for Generic Model Control

$EVWUDFW Generic Model Control (GMC) is a control algorithm capable of using non-linear process model directly. In GMC, mostly, first-principles models derived from dynamic mass, energy and momentum balances are used. When the process is not perfectly known, the unknown parts of first principles models can be represented by black-box models, e.g. by neural networks. This paper is devoted to

Janos Abonyi; Janos Madar; Ferenc Szeifert

296

First-principles study of electronic structure and optical properties of Sr(Ti,Zr)O3

NASA Astrophysics Data System (ADS)

Electronic and optical properties of Sr(Ti,Zr)O3 crystals in the cubic ( Pm-3m) and tetragonal ( I4/mcm) phase were calculated by the first-principles calculations using the density functional theory and the local density approximation. The band structure of cubic and tetragonal phases show an indirect band gap at (R-?) point and at (M-?) point in the Brillouin zone, respectively. The linear photon-energy dependent dielectric functions and some optical properties such as the absorption coefficient, energy-loss function and reflectivity are calculated for both phases. The optical properties of tetragonal phase of Sr(Ti,Zr)O3 were investigated by theoretical methods for the first time. We have also made some comparisons with the available related experimental and theoretical data.

Celik, Gulden; Cabuk, Suleyman

2013-03-01

297

First-principles investigation of alternating current density distribution in molecular devices

NASA Astrophysics Data System (ADS)

Using the nonequilibrium Green's function (NEGF) formalism, we derive the current density formula for ac quantum transport by including the self-consistent Coulomb interaction. It is well known that the Coulomb interaction is very important in determining ac current in nanostructures. As pointed out by Büttiker that the Coulomb interaction must be included to conserve the ac current. Theoretically, the displacement current can be accounted for by including a self-consistent Hartree term in the Hamiltonian as well as the exchange and correlation term while the ac current is calculated from particle current, i.e., <Î?(t)>=q

Zhang, Lei; Wang, Bin; Wang, Jian

2012-10-01

298

First-principles Study of the Electronic Structure and Optical Properties of MgH2

NASA Astrophysics Data System (ADS)

It has been noticed that magnesium might play an interesting role in recently discovered switchable-mirror systems. For example, the films of rare earth and magnesium alloys are found to be superior to the pure rare-earth samples in maximum transparency and mirror-state reflectivity [1]. Moreover, the magnesium-rich Ni-Mg alloy films turned out to be a switchable-mirror system without rare earths [2]. In both cases, pure transparent MgH2 is reversibly formed when these alloys take up hydrogen. In order to model the optical properties of these films, we need to know the electronic and optical properties of MgH2. In this work, we investigate its bonding characteristics, band structure, and dielectric properties with first-principles theoretical methods. The stability of the crystal and the bonding are studied using density functional theory and pseudopotential methods. The excited state properties (the quasiparticle spectra) are studied by many-body perturbation theory within the so-called GW approximation in which the electronic self-energy is approximated by the full Green's function (G) times the screened Coulomb interaction (W). We will report the results for both the rutile-structured alpha-MgH2 and the low-symmetry gamma-MgH2. [1] P. van der Sluis, M. Ouwerkerk, and P. A. Duine, Appl. Phys. Lett. 70, 3356 (1997). [2] T. J. Richardson, J. L. Slack, R. D. armitage, R. Kostecki, B. Farangis, and M. D. Rubin, Appl. Phys. Lett. 78, 3047 (2001).

Alford, Ashley; Chou, Mei-Yin

2003-03-01

299

NASA Astrophysics Data System (ADS)

A weak hydrogen bond (WHB) such as CH--O is very important for the structure, function, and dynamics in a chemical and biological system WHB stretching vibration is in a terahertz (THz) frequency region Very recently, the reasonable performance of dispersion-corrected first-principles to WHB has been proven. In this lecture, we report dispersion-corrected first-principles calculation of the vibrational absorption of some organic crystals, and low-temperature THz spectral measurement, in order to clarify WHB stretching vibration. The THz frequency calculation of a WHB crystal has extremely improved by dispersion correction. Moreover, the discrepancy in frequency between an experiment and calculation and is 10 1/cm or less. Dispersion correction is especially effective for intermolecular mode. The very sharp peak appearing at 4 K is assigned to the intermolecular translational mode that corresponds to WHB stretching vibration. It is difficult to detect and control the WHB formation in a crystal because the binding energy is very small. With the help of the latest intense development of experimental and theoretical technique and its careful use, we reveal solid-state WHB stretching vibration as evidence for the WHB formation that differs in respective WHB networks

Takahashi, Masae; Ishikawa, Yoichi; Ito, Hiromasa

2013-03-01

300

By combining time-dependent density functional theory (TDDFT) and molecular dynamics (MD) simulations, we calculate the ultraviolet absorption and circular dichroism (CD) of a cyclic dipeptide, cyclo(L-Pro-D-Tyr), in the 185?300 nm region. The absorption is dominated by the phenol chromophore of tyrosine. The CD spectrum shows both phenol and amide units transitions. A crude coherent two dimensional ultraviolet spectrum (2DUV) calculated by neglecting the two-excitation states shows a cross peak between two transitions of the phenol in the tyrosine side chain. Additional cross peaks between the side chain and the backbone are observed when using a chirality-induced pulse polarization configuration.

Li, Zhenyu; Mukamel, Shaul

2008-01-01

301

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

302

Statistically averaged Raman spectra of H(2) in hydrogen clathrate are calculated by quantum-mechanical calculations and molecular dynamics simulations. The result shows that the H(2) molecules in the large cages and singly occupied small cages are loosely encaged and the vibrational modes are softened and uncoupled, while those in the doubly occupied small cages are tightly confined, the vibrational modes are strongly coupled, and the frequencies are blue-shifted relative to the free gas. This finding provides important new insights on characterizing the cage occupancy and could inspire innovative experiments for synthesizing the clathrate as a hydrogen storage medium. PMID:19807169

Wang, Jianwei; Lu, Hailong; Ripmeester, John A

2009-10-14

303

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

304

The ground state properties and equation of state of the non-oxide perovskite-type superconductor MgCNi3 are investigated by first-principles calculations based on the plane-wave basis set with the local density approximation (LDA) as well as the generalized gradient approximation (GGA) for exchange and correlation, which agree well with both theoretical calculations and experiments. Some thermodynamic properties including the heat capacity, the

Wei Zhang; Zhe Li; Xiang-Rong Chen; Ling-Cang Cai; Fu-Qian Jing

2008-01-01

305

Recently, the quaternary compounds, Cu2ZnSnS4 and Cu2ZnSnSe4, have been attracted pretty much attention because of their potential use in the field of energy harvesting applications. Several theoretical calculations have been reported about their first principles electronic, optic and transport properties. However, no lattice dynamic calculations have been published yet despite the discussions about their possible ground state crystal structures and

Tanju Gurel; Cem Sevik; Tahir Cagin

2011-01-01

306

We have performed first principles total energy calculations to investigate the adsorption of Cl, Cl2, and HCl on the Ge(001)-c(2×4) surface. Based on previous experimental and theoretical results of Cl on Si(001), we have considered two different geometries for the adsorption of a single Cl atom on Ge(001). We have found that the most stable configuration corresponds to adsorption of

A Sánchez-Castillo; Gregorio H Cocoletzi; Noboru Takeuchi

2002-01-01

307

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.

2013-01-01

308

First-Principles Investigation of 180^circ Domain Walls in BaTiO_3.

NASA Astrophysics Data System (ADS)

We present a first-principles study of 180^circ ferroelectric domain walls in tetragonal BaTiO_3. The theory is based on an effective Hamiltonian that has previously been determined from first-principles ultrasoft-pseudopotential calculations.( W. Zhong, D. Vanderbilt, and K. M. Rabe, Phys. Rev. B. 52), 6301 (1995). Statistical properties are investigated using Monte Carlo simulations. We compute the domain-wall energy, free energy, and thickness, analyze the behavior of the ferroelectric order parameter in the interior of the domain wall, and study its spatial fluctuations. An abrupt reversal of the polarization is found, unlike the gradual rotation typical of the ferromagnetic case.

Padilla, Jorge; Zhong, Weiqing; Vanderbilt, David

1996-03-01

309

Growth mechanisms of ZnO(0001) investigated using the first-principles calculation

We investigated the dynamics of zinc (Zn) and oxygen (O) adsorbed atoms (adatoms) on a Zn-polar ZnO(0001) surface using the first-principles calculation. The results of the first-principles calculation revealed that a high-quality ZnO crystalline growth condition is induced by wurtzite structure packing under a Zn-rich growth condition using a Zn-polar ZnO(0001) surface. However, it was shown that an O adatom is not sufficient to promote surface atomic diffusion. For high-quality ZnO crystal, promoting surface diffusion of adatoms using high temperature is important.

Fujiwara, Katsutoshi; Ishii, Akira [Department of Applied Mathematics and Physics, Tottori University, Koyama, Tottori 680-8552 (Japan); Abe, Tomoki; Ando, Koshi [Department of Electrical and Electronic Engineering, Tottori University, Koyama, Tottori 680-8552 (Japan)

2012-09-15

310

NASA Astrophysics Data System (ADS)

Based on first principles calculations for crystals and experimental thermochemical data for aqueous solutions and molecules, Pourbaix diagrams of alkaline earth metal systems at 298.15 K are constructed. Phonon frequencies are computed for all crystals of interests by the first principles method. Then individual contributions of zero-point energies and vibrational free energies are examined. The contribution of the zero-point energy is found to be 7-8% of the total formation free energy at 298.15 K in hydroxides, which can be ascribed to the presence of high frequency OH stretching modes. The agreements between computed and experimental Pourbaix diagrams are quite satisfactory.

Suzuki, T.; Mori, M.; Matsunaga, K.; Tanaka, I.

2010-09-01

311

NASA Astrophysics Data System (ADS)

Structural parameters, elastic, mechanical, electronic, chemical bonding, and optical properties of tetragonal HfSiO4 have been investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory. The ground-state properties obtained by minimizing the total energy are in agreement with the available experimental and theoretical data. This compound is found to be mechanically stable, and we have obtained the bulk, shear, and Young's modulus; Poisson's coefficient; and Lamé's constants. We have estimated the Debye temperature of tetragonal HfSiO4 from the acoustic velocity. Electronic and chemical bonding properties have been studied. Moreover, the complex dielectric function, refractive index, extinction coefficient, absorption coefficient, energy-loss spectrum, optical reflectivity, and complex conductivity function are calculated and analyzed.

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

2012-04-01

312

Structural, energetic and thermodynamic analyses of Ca(BH4)2·2NH3 from first principles calculations

NASA Astrophysics Data System (ADS)

Ca(BH4)2·2NH3 is a relatively new compound with potential application in hydrogen storage. Here the fundamental properties of the compound, such as electronic structure, energetic and thermodynamic properties, were comprehensively studied using first-principles calculations. Results from electronic density of states (DOS) and electron localization function (ELF) indicate the covalent bond nature of the N-H bond and the B-H bond. Charge density analyses show weak ionic interactions between the Ca atom and the NH3 complexes or the (BH4)- complexes. The calculated vibration frequencies of B-H and N-H are in good agreement with other theoretical and experimental results. Furthermore, we calculated the reaction enthalpy and reaction Gibbs free energy at a range of temperature 0-700 K. Our results are in good agreement with experimental results in literature. Possible reaction mechanism of the decomposition reaction is proposed.

Yuan, Peng-Fei; Wang, Fei; Sun, Qiang; Jia, Yu; Guo, Zheng-Xiao

2012-01-01

313

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 <100>, <110> and <111> 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

314

First-principles calculation of the linear and nonlinear optical properties of LiTaO3

NASA Astrophysics Data System (ADS)

The linear and nonlinear optical (NLO) properties of the paraelectric and ferroelectric (FE) phases of lithium tantalate crystals were calculated using a first-principles approach based on density functional theory with the generalized gradient approximation. We present our results for the structural parameters, the imaginary and real parts of the frequency-dependent linear optical response, optical functions such as the spectral reflectivity, the absorption coefficient and the electron energy-loss spectrum. A simple scissors operator is applied to adjust the energy gap from calculations to match the experimental value. In the FE phase, we also study the NLO susceptibilities and calculate the NLO susceptibility tensor. LiTaO3 displays a good NLO effect. The results are compared with the theoretical calculations and available experimental data.

Cabuk, Suleyman; Simsek, Sevket

2010-05-01

315

NASA Astrophysics Data System (ADS)

In this paper, we conduct a theoretical study of the half-metallic properties of the bulk and the (110) and (001) surfaces of rocksalt VPo with an equilibrium lattice constant of 0.59 nm. The calculations are done using the first-principles full-potential linearized augmented plane-wave method based on the density functional theory. We show that both the (110) and (001) surfaces of rocksalt VPo preserve the bulk half-metallicity, and the atomic magnetic moments at both the (110) and (001) surfaces are increased compared to those in bulk VPo. Moreover, by calculating the relaxed surface energies, we assess the surface stability, and find that both (110) and (001) surfaces are not stable in equilibrium conditions, and non-equilibrium growth techniques are required for the realization of rocksalt VPo thin films.

Khalaf Al-zyadi, Jabbar M.; Gao, G. Y.; Yao, Kai-Lun

2013-03-01

316

Using a formulation of first-principles scattering theory that includes disorder and spin-orbit coupling on an equal footing, we calculate the resistivity ?, spin-flip diffusion length l(sf), and Gilbert damping parameter ? for Ni(1-x)Fe(x) substitutional alloys as a function of x. For the technologically important Ni(80)Fe(20) alloy, Permalloy, we calculate values of ? = 3.5 ± 0.15 ?? cm, l(sf) = 5.5 ± 0.3 nm, and ? = 0.0046 ± 0.0001 compared to experimental low-temperature values in the range 4.2-4.8 ?? cm for ?, 5.0-6.0 nm for l(sf), and 0.004-0.013 for ?, indicating that the theoretical formalism captures the most important contributions to these parameters. PMID:21231490

Starikov, Anton A; Kelly, Paul J; Brataas, Arne; Tserkovnyak, Yaroslav; Bauer, Gerrit E W

2010-12-02

317

NASA Astrophysics Data System (ADS)

By means of first-principles calculations, the structural stability, mechanical properties and electronic structure of the newly synthesized incompressible Re2C, Re2N, Re3N and an analogous compound Re3C have been investigated. Our results agree well with the available experimental and theoretical data. The proposed Re3C is shown to be energetically, mechanically and dynamically stable and also incompressible. Furthermore, it is suggested that the incompressibility of these compounds is originated from the strong covalent bonding character with the hybridization of 5d orbital of Re and the 2p orbital of C or N, and a zigzag topology of interconnected bonds, e.g., Re-Re, Re-C or Re-N bonding. The physical properties of rhenium carbides and nitrides have been explored. A new compound Re3C is predicted to be stable and incompressible. The origin of their incompressibility is presented.

Miao, Naihua; Sa, Baisheng; Zhou, Jian; Sun, Zhimei; Ahuja, Rajeev

2011-12-01

318

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

319

Benchmark calculations of first principles rotational and ro-vibrational line strenghts

Benchmark first principles calculations of the pure rotational and ro-vibrational transition frequencies and line strengths are presented, using two independent program suites. Both sets of calculations were performed using the same potential energy and dipole surfaces. Our example calculations use recently calculated surfaces for H2S which have been shown to give good agreement with experimental data. The results, which show

Stuart Carter; Pavel Rosmus; Nicholas C. Handy; Steven Miller; Jonathan Tennyson; Brian T. Sutcliffe

1989-01-01

320

A structured modeling approach for dynamic hybrid fuzzy-first principles models

Hybrid fuzzy-first principles models can be attractive if a complete physical model is difficult to derive. These hybrid models consist of a framework of dynamic mass and energy balances, supplemented with fuzzy submodels describing additional equations, such as mass transformation and transfer rates. In this paper, a structured approach for designing this type of model is presented. The modeling problem

Pascal F. van Lith; Ben H. L. Betlem; Brian Roffel

2002-01-01

321

Spin-polarized first-principles LMTO-ASA calculations in real space.

National Technical Information Service (NTIS)

We have developed an approach, based on the LMTO-ASA formalism and the recursion method, which allow us to perform first principles spin-polarized self-consistent calculations of electronic structure in real space. To illustrate the method we obtain the e...

J. Duarte Junior P. R. Peduto S. Frota-Pessoa

1990-01-01

322

This paper presents a detailed analysis of the steam reforming process from first-principles calculations, supported by insight from experimental investigations. In the present work we employ recently recognised scaling relationships for adsorption energies of simple molecules adsorbed at pure metal surfaces to develop an overview of the steam reforming process catalyzed by a range of transition metal surfaces. By combining

Glenn Jones; Jon Geest Jakobsen; Signe S. Shim; Jesper Kleis; Martin P. Andersson; Jan Rossmeisl; Frank Abild-Pedersen; Thomas Bligaard; Stig Helveg; Berit Hinnemann; Jens R. Rostrup-Nielsen; Ib Chorkendorff; Jens Sehested; Jens K. Nørskov

2008-01-01

323

Chemically modified ribbon edge stimulated H2 dissociation: a first-principles computational study.

First-principles computational studies indicate that (B, N, or O)-doped graphene ribbon edges can substantially reduce the energy barrier for H2 dissociative adsorption. The low barrier is competitive with many widely used metal or metal oxide catalysts. This suggests that suitably functionalized graphene architectures are promising metal-free alternatives for low-cost catalytic processes. PMID:23632601

Liao, Ting; Sun, Chenghua; Sun, Ziqi; Du, Aijun; Smith, Sean

2013-04-30

324

Size dependence of thermal properties of armchair carbon nanotubes: A first-principles study

The authors use first-principles pseudopotential-based density functional theory calculations of phonon dispersions to determine the size dependence of thermal properties of armchair single wall carbon nanotubes (SWCNTs), such as their negative thermal expansion and specific heat. While the specific heat is found to depend rather weakly on the diameter of SWCNTs, their negative thermal expansion behavior determined within a quasiharmonic

Mousumi Upadhyay Kahaly; Umesh V. Waghmare

2007-01-01

325

First-principles Study On Dielectric Properties Of Nano-Scale Films

NASA Astrophysics Data System (ADS)

Dielectric constants of Si, GaAs, and ZnSe (001) and (111) nano-size films are studied by the first-principles calculations. We found that the dielectric constant deviates from the bulk value with decreasing the film thickness and such variation shows characteristic dependences on material kinds, polarization directions, and film orientations.

Ishikawa, Masato; Nii, Harumichi; Nakayama, Takashi

2007-04-01

326

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

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

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

1998-01-01

327

The stacking fault energy (SFE) of austenitic stainless steels has been determined using a quantum mechanical first-principles approach. We identify the electronic, magnetic and volume effects responsible for the compositional dependence of the SFE. We find that both the alloying element and the composition of the host material are important for understanding the alloying effects. Our results show that no

L. Vitos; J.-O. Nilsson; B. Johansson

2006-01-01

328

We report a systematic investigation of the charging effect on hydrogen molecule chemisorption on (3, 3), (5, 5), (5, 0), and (8, 0) carbon nanotubes by first-principles calculations. The influence of injected charge on the chemisorption energy barriers is found to be sensitive to the nanotube diameter and chirality. The calculated results also indicate that electron injection is more effective

Bin Zhou; Wanlin Guo; Chun Tang

2008-01-01

329

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

330

First-Principles Reactive Molecular Dynamics of Chemistry in Detonating Energetic Materials

We investigated the initial chemistry of shock compressed energetic materials that results from inter-molecular collisions behind the shock wave front by performing first-principles MD simulations of bimolecular collisions for PETN and RDX with different crystallographic orientations and velocities. For each orientation, we determined the threshold collision velocity for reaction, the reaction timescales, and the products of decomposition. We find that

Aaron Landerville; Ivan I. Oleynik; Mortko A. Kozhushner; Carter T. White

2008-01-01

331

FIRST-PRINCIPLES REACTIVE MOLECULAR DYNAMICS OF CHEMISTRY IN DETONATING ENERGETIC MATERIALS

We investigated the initial chemistry of shock compressed energetic materials that results from inter-molecular collisions behind the shock wave front by performing first-principles MD simulations of bimolecular collisions for PETN and RDX with different crystallographic orientations and velocities. For each orientation, we determined the threshold collision velocity for reaction, the reaction timescales, and the products of decomposition. We find that

A. Landerville; I. I. Oleynik; M. A. Kozhushner; C. T. White

2007-01-01

332

First-Principles Reactive Molecular Dynamics of Chemistry in Detonating Energetic Materials

We investigated the initial chemistry of shock compressed energetic materials that results from inter-molecular collisions behind the shock wave front by performing first-principles MD simulations of bimolecular collisions for PETN and RDX with different crystallographic orientations and velocities. For each orientation, we determined the threshold collision velocity for reaction, the reaction timescales, and the products of decomposition. We find that

A. Landerville; I. I. Oleynik; M. A. Kozhushner; C. T. White

2007-01-01

333

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

334

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

Fabrication, handling and disposal of nuclear fuel materials require comprehensive knowledge of their surface morphology and reactivity. Due to unavoidable contact with air components (even at low partial pressures), UN samples contain considerable amount of oxygen impurities affecting fuel properties. The basic properties of O atoms adsorbed on the UN(001) surface are simulated here combining the two first principles calculation

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

2009-01-01

335

First principles study of CrH and CrM2H

NASA Astrophysics Data System (ADS)

First principles calculation were performed using Tight-binding LMTO method with Local density approximation (LDA) and Atomic sphere approximation (ASA) to understand the electronic properties of CrH. A pressure induced structural phase transition from cubic to hexagonal structure of CrH is predicted. The stability of CrM2H is analyzed.

Kanagaprabha, S.; Santhosh, M.; Rajeswarapalanichamy, R.; Iyakutti, K.

2013-06-01

336

NASA Astrophysics Data System (ADS)

Based on the nonequilibrium Green's function (NEGF) coupled with density function theory (DFT), namely, NEGF-DFT quantum transport theory, we propose an efficient formalism to calculate the transient current of molecular devices under a step-like pulse from first principles. By combining NEGF-DFT with the complex absorbing potential (CAP), the computational complexity of our formalism (NEGF-DFT-CAP) is proportional to O(N) where N is the number of time steps in the time-dependent transient current calculation. Compared with the state-of-the-art algorithm of first-principles time-dependent calculation that scales with at least N2, this order N technique drastically reduces the computational burden making it possible to tackle realistic molecular devices. We have presented a detailed discussion on how to implement this scheme numerically from first principles. To check the accuracy of our method, we carry out the benchmark calculation compared with NEGF-DFT formalism and they agree well with each other. As an application of this method, we investigate the transient current of a molecular device Al-C3-Al from first principles.

Zhang, Lei; Chen, Jian; Wang, Jian

2013-05-01

337

First-principles thermodynamics of La2O3-P2O5 pseudobinary system

NASA Astrophysics Data System (ADS)

Phase stabilities in the La2O3-P2O5 pseudobinary system have been theoretically analyzed. Phonon modes of five crystals, i.e., La2O3, La3PO7, LaPO4, LaP3O9, and LaP5O14, and vibrational modes of gaseous P2O5(g) are computed from first principles in order to obtain the contribution of vibrations to the free energy. Additional dynamical contributions, i.e., rotations and translations are also taken into account for the gaseous P2O5(g). Vibrational states strongly reflect the crystal structures and bonding states. In this system, the strong P-O covalent bonds in PO4 units and the relatively weak La-O bonds are found to be the key factors determining the vibrational spectra. In the oxyphosphate, La3PO7, the two bonding states are coexisting, and the vibrational spectrum is approximately an average of La2O3 and LaPO4. On the other hand, the P2O5-rich compounds, i.e., LaP3O9 and LaP5O14, cannot be treated in the same manner. Their PO4 units form corner-sharing networks whose vibrations are strongly correlated. The networks raise the vibrational frequencies, leading to high-frequency modes up to 40 THz. The Gibbs energies using the calculated vibrational spectra are in reasonable agreement with the available data of La2O3, LaPO4, and P2O5(g), e.g., the differences between the calculated and reported values are less than 2 kJ/mol-atom (20 meV/atom) at 1500 K. The Gibbs energies and the phase stabilities of the other three compounds, La3PO7, LaP3O9, and LaP5O14, are evaluated, whose data are yet unknown so far.

Toyoura, Kazuaki; Hatada, Naoyuki; Nose, Yoshitaro; Uda, Tetsuya; Tanaka, Isao

2011-11-01

338

Incorporation of Hydrogen and Aluminum in Perovskite: A First Principles Investigation

NASA Astrophysics Data System (ADS)

The incorporation of water into nominally anhydrous silicates is associated with mechanical weakening and lowered melting points such that even small amounts of water in mantle minerals can serve as a significant volatile reservoir in the planet. Experimental attempts at water incorporation in MgSiO3-perovskite have met with varied success, with little agreement in solubility or the frequency of the OH band. Here we present first principles, pseudopotential calculations of fully relaxed structures on MgSiO3-perovskite with aluminum and hydrogen substitutions. We find three symmetrically distinct stable arrangements for the location of hydrogen in the unit cell given the coupled substitution of Si4+ for Al3+ + H+ in perovskite. These three stable hydrogen locations have OH bond lengths of 1.09Å, 0.96Å and 0.99Å at 0 GPa, and less than 2 kJ/mol difference in the energy distinguishing each arrangement. These multiple substitutions with small energy differences may explain the variable OH frequencies found for hydrated perovskite, with sensitivity to the synthesis conditions. This substitution is associated with a 2.2% softening of the equation of state for structure containing 3.1 mol% H2O and Al2O3, comparable to recently reported results for softening in SiO2-stishovite containing 2.1 mol% H2O and Al2O3 [Panero and Stixrude, 2004]. We calculate that the enthalpy of solution of Mg2++Si4+=2Al3+ is slightly less than the Si4+ for Al3+ + H+ reaction, but both solution enthalpies are generally independent of pressure between 0 and 100 GPa. Further, we confirm previous theoretical results that aluminum incorporation with oxygen vacancies is not favored (2Si4++O2-=2Al3++VO**). Assuming ideal entropy of solution, at 2000 K we find Mg-perovskite saturated in aluminum and water to contain 0.13 wt% H2O.

Panero, W. R.; Stixrude, L. P.

2004-05-01

339

NASA Astrophysics Data System (ADS)

The computational framework of this study is based on the local-spin-density approximation with first-principles full-potential linear muffin-tin orbital calculations including orbital polarization (OP) correction. We have studied the magnetic anisotropy for a series of bilayer CuAu(I)-type materials such as FeX, MnX (X=Ni,Pd,Pt), CoPt, NiPt, MnHg, and MnRh in a ferromagnetic state using experimental structural parameters to understand the microscopic origin of magnetic-anisotropy energy (MAE) in magnetic multilayers. Except for MnRh and MnHg, all these phases show perpendicular magnetization. We have analyzed our results in terms of angular momentum-, spin- and site-projected density of states, magnetic-angular-momentum-projected density of states, orbital-moment density of states, and total density of states. The orbital-moment number of states and the orbital-moment anisotropy for FeX (X=Ni,Pd,Pt) are calculated as a function of band filling to study its effect on MAE. The total and site-projected spin and orbital moments for all these systems are calculated with and without OP when the magnetization is along or perpendicular to the plane. The results are compared with available experimental as well as theoretical results. Our calculations show that OP always enhances the orbital moment in these phases and brings them closer to experimental values. The changes in MAE are analyzed in terms of exchange splitting, spin-orbit splitting, and tetragonal distortion/crystal-field splitting. The calculated MAE is found to be in good agreement with experimental values when the OP correction is included. Some of the materials considered here show large magnetic anisotropy of the order of meV. In particular we found that MnPt will have a very large MAE if it could be stabilized in a ferromagnetic configuration. Our analysis indicates that apart from large spin-orbit interaction and exchange interaction from at least one of the constituents, a large crystal-field splitting originating from the tetragonal distortion is also a necessary condition for having large magnetic anisotropy in these materials. Our calculation predicts large orbital moment in the hard axis in the case of FePt, MnRh, and MnHg against expectation.

Ravindran, P.; Kjekshus, A.; Fjellvåg, H.; James, P.; Nordström, L.; Johansson, B.; Eriksson, O.

2001-04-01

340

NASA Astrophysics Data System (ADS)

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

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

2012-09-01

341

NASA Astrophysics Data System (ADS)

Theoretical approximations to the sum frequency vibrational spectroscopy (SFVS) of the carbon tetrachloride/water interface are constructed using the quantum-corrected time correlation functions (TCF) to aid in interpretation of experimental data and to predict novel vibrational modes. Instantaneous normal mode (INM) methods are used to characterize the observed modes leading to the TCF signal, thus providing molecular resolution of the vibrational lineshapes. Detailed comparisons of the theoretical signals are made with those obtained experimentally and show excellent agreement for the spectral peaks in the O-H stretching region of water. An intermolecular mode, unique to the interface, at 848 cm-1 is also identifiable, similar to the one seen for the water/vapor interface. INM analysis reveals the resonance is due to a wagging mode (hindered rotation) that was previously identified (Perry et al 2005 J. Chem. Phys. 123 144705) as localized on a single water molecule with both hydrogens displaced normal to the interface—generally it is found that the symmetry breaking at the interface leads to hindered translations and rotations at hydrophilic/hydrophobic interfaces that assume finite vibrational frequencies due to anchoring at the aqueous interface. Additionally, examination of the real and imaginary parts of the theoretical SFVS spectra reveal the spectroscopic species attributed the resonances and possible subspecies in the O-H region; these results are consistent with extant experimental data and associated analysis.

Green, Anthony J.; Perry, Angela; Moore, Preston B.; Space, Brian

2012-03-01

342

Theoretical approximations to the sum frequency vibrational spectroscopy (SFVS) of the carbon tetrachloride/water interface are constructed using the quantum-corrected time correlation functions (TCF) to aid in interpretation of experimental data and to predict novel vibrational modes. Instantaneous normal mode (INM) methods are used to characterize the observed modes leading to the TCF signal, thus providing molecular resolution of the vibrational lineshapes. Detailed comparisons of the theoretical signals are made with those obtained experimentally and show excellent agreement for the spectral peaks in the O-H stretching region of water. An intermolecular mode, unique to the interface, at 848 cm(-1) is also identifiable, similar to the one seen for the water/vapor interface. INM analysis reveals the resonance is due to a wagging mode (hindered rotation) that was previously identified (Perry et al 2005 J. Chem. Phys. 123 144705) as localized on a single water molecule with both hydrogens displaced normal to the interface-generally it is found that the symmetry breaking at the interface leads to hindered translations and rotations at hydrophilic/hydrophobic interfaces that assume finite vibrational frequencies due to anchoring at the aqueous interface. Additionally, examination of the real and imaginary parts of the theoretical SFVS spectra reveal the spectroscopic species attributed the resonances and possible subspecies in the O-H region; these results are consistent with extant experimental data and associated analysis. PMID:22395178

Green, Anthony J; Perry, Angela; Moore, Preston B; Space, Brian

2012-03-06

343

The electronic spectrum of four different anthraquinones (1,2-dihydroxyanthraquinone, 1-aminoanthraquinone, 2-aminoanthraquinone and 1-amino-2-methylanthraquinone) in methanol solution was measured and used as reference data for theoretical color prediction. The visible part of the spectrum was modeled according to TD-DFT framework with a broad range of DFT functionals. The convoluted theoretical spectra were validated against experimental data by a direct color comparison in terms of CIE XYZ and CIE Lab tristimulus model color. It was found, that the 6-31G** basis set provides the most accurate color prediction and there is no need to extend the basis set since it does not improve the prediction of color. Although different functionals were found to give the most accurate color prediction for different anthraquinones, it is possible to apply the same DFT approach for the whole set of analyzed dyes. Especially three functionals seem to be valuable, namely mPW1LYP, B1LYP and PBE0 due to very similar spectra predictions. The major source of discrepancies between theoretical and experimental spectra comes from L values, representing the lightness, and the a parameter, depicting the position on green?magenta axis. Fortunately, the agreement between computed and observed blue?yellow axis (parameter b) is very precise in the case of studied anthraquinone dyes in methanol solution. Despite discussed shortcomings, color prediction from first principle quantum chemistry computations can lead to quite satisfactory results, expressed in terms of color space parameters. PMID:23250806

Cysewski, Piotr; Jeli?ski, Tomasz

2012-12-19

344

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

345

We report an extensive photoelectron-spectroscopy study of size-selected Cr{sub n}{sup {minus}} (n=2{endash}70) clusters. Even-odd alternations are observed for the small Cr{sub n} clusters and their electron affinities. The spectra of clusters with an even number of atoms show less density of states near the threshold, while those with odd clusters show more complex features. A dimer growth path found in a previous theoretical study is used to interpret the data and accounts well for the even-odd effects. The work provides a key contribution to the understanding of the complicated electronic structure of the Cr clusters. {copyright} {ital 1997} {ital The American Physical Society}

Wang, L. [Department of Physics, Washington State University, Richland, Washington 99352 (United States)]|[EMSL, Pacific Northwest National Laboratory, MS K2-14, Richland, Washington 99352 (United States); Wu, H. [Department of Physics, Washington State University, Richland, Washington 99352 (United States); Cheng, H. [Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195 (United States)

1997-05-01

346

NASA Astrophysics Data System (ADS)

The SAC-CI method has been applied to the theoretical spectroscopy of the inner-shell electronic processes and the photochemistry of the organic light-emitting diodes (OLED) and biological chemosensors. Wide varieties of the core-electronic processes such as core-electron ionizations, shake-up satellites, vibrational excitations, valence-Rydberg coupling, and its thermal effect have been investigated by the SAC-CI calculations. The method has also been applied to the electronic spectra and the excited-state dynamics of the polymer materials of OLED such as poly para-phenylene vinylene and fluorene-thiophene. The photochemistry of the biological chemosensor has been elucidated in particular for the photo-induced electron transfer mechanism of the acridine-type fluorescent probe.

Ehara, Masahiro; Nakatsuji, Hiroshi

347

We report a combined theoretical and microwave spectroscopy study of the internal dynamics of the benzene dimer, a benchmark system for dispersion forces. Although the extensive ab initio calculations and experimental work on the equilibrium geometry of this dimer have converged to a tilted T-shaped structure, the rich internal dynamics due to low barriers for internal rotation have remained largely unexplored. We present new microwave spectroscopy data for both the normal (C6H6)2 and partially deuterated (C6D6)(C6H6) dimers. The splitting patterns obtained for both species are unraveled and understood using a reduced-dimensionality theoretical approach. The hindered sixfold rotation of the stem can explain the observed characteristic 1?:?2?:?1 tunneling splitting pattern, but only the concerted stem rotation and tilt tunneling motion, accompanied by overall rotation of the dimer, yield the correct magnitude of the splittings and their strong dependence on the dimer angular momentum J that is essential to explain the experimental data. Also the surprising observation that the splittings are reduced by 30% for the mixed (C6D6)(C)(C6H6)(S) dimer in which only the cap (C) in the T-shaped structure is deuterated, while the rotating stem (S) monomer is the same as in the homodimer, is understood using this approach. Stark shift measurements allowed us to determine the dipole moment of the benzene dimer, ? = 0.58 ± 0.051 D. The assumption that this dipole moment is the vector sum of the dipole moments induced in the monomers by the electric field of the quadrupole on the other monomer yields a calculated value of ? = 0.63 D. Furthermore, the observed Stark behavior is typical for a symmetric top, another confirmation of our analysis. PMID:23676846

Schnell, Melanie; Erlekam, Undine; Bunker, P R; von Helden, Gert; Grabow, Jens-Uwe; Meijer, Gerard; van der Avoird, Ad

2013-05-16

348

First principles analysis of quantum transport in Bi2Se3 3D topological insulators

NASA Astrophysics Data System (ADS)

By carrying out density functional theory (DFT) within the Keldysh nonequilibrium Green's function formalism (NEGF), we have investigated quantum transport properties of the Bi2Se3 topological insulator from atomistic first principles without any phenomenological parameters. Using the scattering states, our results vividly reveal the surface Dirac fermions and helical edge spin states in the momentum space. We have also determined the real-space distribution of the helical edge spin states which provide the penetration depth of the surface topological conducting channels into the bulk Bi2Se3 crystal. Our first principles calculations take into account the full non-collinear spin structure and spin-orbit interaction, the details of these technical advances within the NEGF-DFT quantum transport formalism will also be briefly discussed.

Zhao, Yonghong; Hu, Yibin; Liu, Lei; Zhu, Yu; Guo, Hong

2011-03-01

349

NASA Astrophysics Data System (ADS)

First-principles quasi-harmonic calculations play a fundamental role in mineral physics because they can predict the structure and thermodynamic properties of materials at pressure and temperature conditions that are still challenging for experiments. They also enable calculations of thermal elastic properties by providing the second derivatives of the free energies with respect to strains. However, these are demanding computations requiring hundreds of medium size jobs running on ~10^2 cores each. Here we introduce an approach that requires only calculations of static elastic constants and phonon density of states for strain-free configurations. This approach decreases the number of calculations by more than one order of magnitude. We show results for several minerals that are in very good agreement with some previous first-principles results and experimental data. Research supported by NSF under ATM-0428774 (VLab) and EAR-1019853. The computations were performed at the Minnesota Supercomputing Institute (MSI).

Wu, Z.; Wentzcovitch, R. M.

2010-12-01

350

First-principles studies of Mn-doped LiCoPO4

NASA Astrophysics Data System (ADS)

This paper investigates Mn-doped LiCoPO4 material using first-principles calculations. Results indicate that the volume change of LiMnxCo1-xPO4 to MnxCo1-xPO4 is smaller than that of undoped LiCoPO4, which is responsible for the excellent tolerance of repeated cycling in lithium ion batteries. Combining first-principles calculations with basic thermodynamics, we calculate the average intercalation voltage of Mn-doped LiCoPO4. It is shown that the redox couple Mn3+/Mn2+ can be observed with increasing Mn content. Therefore, the Mn ion displays some electrochemical activity during discharge/charge of LiMnxCo1-xPO4 due to the coexistence of Co and Mn.

Lin, Zhi-Ping; Zhao, Yan-Ming; Zhao, Yu-Jun

2011-01-01

351

Structure of the (111) surface of bismuth: LEED analysis and first-principles calculations

The surface structure of Bi(111) was investigated by low-energy electron diffraction (LEED) intensity analysis for temperatures between 140 and 313 K and by first-principles calculations. The diffraction pattern reveals a (1x1) surface structure and LEED intensity versus energy simulations confirm that the crystal is terminated with a Bi bilayer. Excellent agreement is obtained between the calculated and measured diffraction intensities in the whole temperature range. The first interlayer spacing shows no significant relaxation at any temperature while the second interlayer spacing expands slightly. The Debye temperatures deduced from the optimized atomic vibrational amplitudes for the two topmost layers are found to be significantly lower than in the bulk. The experimental results for the relaxations agree well with those of our first-principles calculation.

Moenig, H.; Wells, J.; Hofmann, Ph. [Institute for Storage Ring Facilities, University of Aarhus, 8000 Aarhus C (Denmark); Sun, J.; Pohl, K. [Department of Physics and Materials Science Program, University of New Hampshire, Durham, New Hampshire 03824 (United States); Koroteev, Yu.M. [Donostia International Physics Center (DIPC), 20018 San Sebastian, Basque Country (Spain); Institute of Strength Physics and Materials Science, Russian Academy of Sciences, 634021, Tomsk (Russian Federation); Bihlmayer, G. [Institut fuer Festkoerperforschung, Forschungszentrum Juelich, D-52425 Juelich (Germany); Chulkov, E.V. [Donostia International Physics Center (DIPC), 20018 San Sebastian, Basque Country (Spain); Departamento de Fisica de Materiales and Centro Mixto CSIC-UPV/EHU, Facultad de Ciencias Quimicas, UPV/EHU, Apdo. 1072, 20080 San Sebastian, Basque Country (Spain)

2005-08-15

352

NASA Astrophysics Data System (ADS)

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

Bennett, Joseph W.; Rabe, Karin M.

2012-11-01

353

Strain-induced ferroelectricity in orthorhombic CaTiO3 from first principles

NASA Astrophysics Data System (ADS)

First-principles calculations are used to investigate the effects of the epitaxial strain on the structure of the perovskite oxide CaTiO3 . At 1.5% tensile strain, we find an epitaxial orientation transition between the ab-ePbnm phase favored for compressive strains and the c-ePbnm phase. While no ferroelectric instability is found for compressive strains, larger tensile strains are found to stabilize a ferroelectric phase related to a polar instability identified in previous first-principles studies of the ideal cubic perovskite high-symmetry reference structure but hidden in the orthorhombic equilibrium bulk Pbnm structure. This strain-induced ferroelectric c-ePbnm phase has polarization along a ?110? direction with respect to the primitive perovskite lattice vectors of the square substrate with a magnitude of 0.46C/m2 at 4% tensile strain.

Eklund, C.-J.; Fennie, C. J.; Rabe, K. M.

2009-06-01

354

The thermodynamic properties of cubic fluorite-type MgF2: first-principles prediction

NASA Astrophysics Data System (ADS)

The thermodynamic properties of MgF2 with a fluorite structure under high pressures and temperatures have been investigated in this work using the Debye model. This model combines with first-principles calculations within local density approximation (LDA) using pseudo-potentials and a plane-wave basis in the framework of density functional theory, and takes into account the phononic effects within the quasi-harmonic approximation. The effects of the temperatures and pressures on the thermodynamic properties such as the Debye temperature ?, the heat capacity CV, the entropy S and the Grüneisen parameter ? are all investigated at extended pressure and temperature ranges for the first time. On the basis of the first-principles study and the Debye model, all the thermodynamic properties of MgF2 with a fluorite structure have been predicted in the entire pressure range from 0 to 135 GPa and temperatures up to the melting temperature 1500 K.

Sun, X. W.; Liu, Z. J.; Song, T.; Quan, W. L.; Chen, Q. F.

2012-06-01

355

First Principles Optical Absorption Spectra of Organic Molecules Adsorbed on Titania Nanoparticles

NASA Astrophysics Data System (ADS)

We present results from first principles computations on passivated rutile TiO2 nanoparticles in both free-standing and dye-sensitized configurations to investigate the size dependence of their optical absorption spectra. The computations are performed using time-dependent density functional theory (TDDFT) as well as GW-Bethe-Salpeter-Equation (GWBSE) methods and compared with each other. We interpret the first principles spectra for free-standing TiO2 nanoparticles within the framework of the classical Mie-Gans theory using the bulk dielectric function of TiO2. We investigate the effects of the titania support on the absorption spectra of a particular set of perylene-diimide (PDI) derived dye molecules, namely brominated PDI (Br2C24H8N2O4) and its glycine and aspartine derivatives.

Baishya, Kopinjol; Ogut, Serdar; Mete, Ersen; Gulseren, Oguz; Ellialtioglu, Sinasi

2012-02-01

356

Stability of indium-tin-oxide and its optical properties: A first-principles study

NASA Astrophysics Data System (ADS)

Various doping sites of Sn in the In2O3 system have been modeled and simulated by the first-principles. The energy favorable site of Sn in In2O3 is found to be on the b-site, which is consistent with experiments. The doping of Sn into interstitial sites is experimentally possible due to the fluctuation in energy, and the intrinsic oxygen vacancies in In2O3 favor the formation of Sni. As Sni in doped ITO can drastically change the transparency in the visible light range, it is necessary to control the preparatory conditions to avoid Sn doping in interstitial sites for realizing ideal transparency in visible light range. By using the LDA+U approach, the band structures of In2-xSnxO3 (x?0.125) are calculated, which are in substantially better agreement with the experimental values than previous first-principles results.

Bai, L. N.; Wei, Y. P.; Lian, J. S.; Jiang, Q.

2013-03-01

357

NASA Astrophysics Data System (ADS)

We present a systematical study on the possible stable structures of C60?xSix(x = 1?12) fullerenes using first-principles calculations combined with Monte-Carlo simulations. The initial fullerenes randomly substituting with silicon atoms are firstly generated and then their total energies are calculated quickly. The ground-state structures are found by the annealing process where Si atoms exchange their positions with C atoms. The stable structures are finally obtained through first-principles calculations with high precision. For the cases with a small amount of Si atoms (x<=4), results similar to those report previously are achieved. Some new stable Si-doped fullerenes with more Si atoms are also predicated. The results show that Si atoms in the C60?xSix(x<=4) fullerenes have a trend of segregation with C atoms. The minimum-energy structure changes from a chemical unstable state to a chemical stable state when x>=8.

Fan, Bing-Bing; Shi, Chun-Yan; Zhang, Rui; Jia, Yu

2013-10-01

358

NASA Astrophysics Data System (ADS)

The adhesion of plastics to ceramics is important for many industrial and technological appliations. It is therefore essential to understand the underlying structure and bonding of the polymer and the surface. The aim of this research is to improve plastic adhesion using a multiscale approach. The first step involves the use of density functional calculations to understand the atomic-scale structure and bonding of polymers on surfaces. The plastic of interest is mainly composed of the polymer bisphenol-A-polycarbonate (BPA-PC). The BPA-PC monomer consists of two phenol groups, one propane group and a carbonic acid group. First-principles calculations of the adsorption of these molecules onto the Si(001)-(2x1) dimer surface will be presented. Finally, the incorporation of first-principles data into a coarse-graining method will be discussed.

Johnston, Karen; Nieminen, Risto M.

2007-03-01

359

First-Principles Study on the Elastic Properties of Platinum Nitride

Elastic properties of platinum nitride (PtN) are studied by first-principles calculations with the fully relativistic full potential linearized augmented plane-wave (LAPW) method, the plane-wave ultrasoft pseudopotential (PW-PP) and the projector-augmented wave (PAW) methods. The results reveal that: (1) the scalar relativistic scheme is sufficient to treat the valence electronic structure, i.e. the spin-orbit effect has little effect on the bulk

Chang-Zeng Fan; Li-Ling Sun; Yuan-Xu Wang; Zun-Jie Wei; Ri-Ping Liu; Song-Yan Zeng; Wen-Kui Wang

2005-01-01

360

We have performed systematic first-principles calculations on dicarbide, -nitride, -oxide, and -boride of platinum and osmium with the fluorite structure. It is found that only PtN2 , OsN2 , and OsO2 are mechanically stable. In particular, OsN2 has the highest bulk modulus of 360.7GPa . Both the band structure and density of states show that the new phase of OsN2

Chang-Zeng Fan; Song-Yan Zeng; Li-Xin Li; Zai-Ji Zhan; Ri-Ping Liu; Wen-Kui Wang; Ping Zhang; Yu-Gui Yao

2006-01-01

361

Equilibrium shape of NaCl crystals: A first-principles calculation

It has recently been observed that the shape of NaCl crystals in stable coexistence with the vapor, while strictly cubical at low temperature, undergoes a corner-rounding transition at T0~=650 °C. A first-principles theory of the thermal evolution of the NaCl crystal shape is presented. The theory takes proper account of the statistical mechanics of atomic-scale interface fluctuations and provides a

An-Chang Shi; Michael Wortis

1988-01-01

362

First-principles study of properties of semi-Heusler (Cu,Ni)MnSb alloys

Magnetic properties, Curie temperatures, and transport properties of semi-Heusler alloys CuxNi1-xMnSb are calculated as a function of the alloy composition from first-principles. The transition from the ferromagnetic state (NiMnSb) to the antiferromagnetic state (CuMnSb) gives rise to an abrupt change in the concentration dependence of magnetic moments and resistivity at about x = 0.7 while the Curie temperatures decrease monotonically

J. Kudrnovský; V. Drchal; I. Turek

2010-01-01

363

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

Jose Javier Saavedra Arias

2009-01-01

364

Structural phase transformations are studied in the B2-phase of the 3d and 4d transition metal aluminides (TM-Al) using the first-principles FP-LMTO method. We have found that the B2-phase in TiAl, VAl, ZrAl and NbAl is mechanically unstable at zero temperature with a negative value of the tetragonal shear constant C^'. At finite temperature, the B2-phase may be stabilizied by the

D. Nguyen-Manh; D. G. Pettifor

1996-01-01

365

First-Principles Calculation of Lead-Free Perovskite SnTiO3

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

366

Structural and Magnetic Properties of Transition Metal Nanoparticles from First Principles

Until recently, the simulation of transition metal particles in the nanometer range was only feasible with semi-empirical\\u000a approaches and classical molecular dynamics simulations. However, the close interrelation of electronic and structural properties\\u000a often leaves no alternative to a fully quantum mechanical treatment. The evolution of modern supercomputer technology nowadays\\u000a allows the simulation of nanometer-sized objects from first principles in the

Markus Ernst Gruner; Georg Rollmann; Alfred Hucht; Peter Entel

2008-01-01

367

MgO-decorated carbon nanotubes for CO2 adsorption: first principles calculations

The global greenhouse effect makes it urgent to deal with the increasing greenhouse gases. In this paper the performance of MgO-decorated carbon nanotubes for CO2 adsorption is investigated through first principles calculations. The results show that the MgO-decorated carbon nanotubes can adsorb CO2 well and are relatively insensitive to O2 and N2 at the same time. The binding energy arrives

Feng Zhu; Shan Dong; Gang Cheng

2011-01-01

368

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

369

First-principles LCGO calculation of the magneto-optical properties of nickel and iron

We report a first-principles, self-consistent, all-electron, linear combination of Gaussian orbitals calculation of a comprehensive collection of magneto-optical properties of nickel and iron based on density-functional theory. Among the many magneto-optical effects, we have studied the equatorial Kerr effect for absorption in the optical as well as soft-x-ray region, where it is called x-ray magnetic linear dichroism. In the optical

Neeraj Mainkar; Dana A. Browne; J. Callaway

1996-01-01

370

The ideal tensile strength of tungsten and tungsten alloys by first-principles calculations

NASA Astrophysics Data System (ADS)

The ideal tensile strength in the [0 0 1] direction of bcc crystal tungsten and its alloys (W-Re, W-Ta and W-V) has been investigated by using first-principles total energy method based on the density functional theory. Crystalline tungsten containing a single substitutional defect (concentration of defects about 2%) has been characterized in terms of structural and mechanical properties. The maximum tensile stress required to reach elastic instability under increasing load has been further computed.

Giusepponi, Simone; Celino, Massimo

2013-04-01

371

Coinage metal (4, 4) nanotubes, simulated by first-principles calculations

The structural stability of coinage metal nanotubes with a square cross-section has been investigated by the first-principles numerical simulations. In addition to the reported (4, 4) silver tube, it is found that the hollow (4, 4) copper and gold nanotubes can also be formed by applying an appropriate stress to an 8A\\/8B fcc wire. The stability of these coinage metal

Wei Fa; Jian Zhou; Jinming Dong; Y. Kawazoe

2011-01-01

372

First principles study of the electronic properties of twinned SiC nanowires

The electronic properties of saturated and unsaturated twinned SiC nanowires grown along [111] direction and surrounded by\\u000a {111} facets are investigated using first-principles calculations with density functional theory and generalized gradient\\u000a approximation. All the nanowires considered, including saturated and unsaturated ones, exhibit semiconducting characteristics.\\u000a The saturated nanowires have a direct band gap and the band gap decreases with increasing diameters

Zhiguo WangShengjie Wang; Shengjie Wang; Chunlai Zhang; Jingbo Li

2011-01-01

373

First principles study of the permeability of graphene to hydrogen atoms.

Using calculations from first principles and harmonic transition state theory, we investigated the permeability of a single graphene sheet to protons and hydrogen atoms. Our results show that while protons can readily pass through a graphene sheet with a low tunneling barrier, for hydrogen atoms the barriers are substantially higher. At the same time, the presence of defects in the membrane can significantly reduce the penetration barrier in a region that extends beyond the defect site itself. PMID:23986179

Miao, Meng; Nardelli, Marco Buongiorno; Wang, Qi; Liu, Yingchun

2013-08-29

374

Dependence of electronic polarization on octahedral rotations in TbMnO3 from first principles

The electronic contribution to the magnetically induced polarization in orthorhombic TbMnO3 is studied from first principles. We compare the cases in which the spin cycloid, which induces the electric polarization via the spin-orbit interaction, is in either the b-c or a-b plane. We find that the electronic contribution is negligible in the first case, but much larger, and comparable to

Andrei Malashevich; David Vanderbilt

2009-01-01

375

First Principles Study of Resonant Phonon Coupling across the LSMO \\/ STO Interface

Epitaxial interfaces permit dynamical modification of the properties of a thin film via coupling to the substrate. In particular, the coupling of phonons between two materials allows one to manipulate the atomic structure and vibrational modes near an interface. We use first principles density functional theory (DFT) to study the octahedral oxygen rotations at and across an interface between LaxSr(1-x)MnO3

Kevin F. Garrity; Yaron Segal; Carlos A. F. Vaz; Jason D. Hoffman; Fred J. Walker; Charles A. Ahn; Sohrab Ismail-Beigi

2011-01-01

376

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

377

Quantum confinement effect in Si\\/Ge core-shell nanowires: First-principles calculations

The electronic structure of Si\\/Ge core-shell nanowires along the [110] and [111] directions are studied with first-principles calculations. We identify the near-gap electronic states that are spatially separated within the core or the shell region, making it possible for a dopant to generate carriers in a different region. The confinement energies of these core and shell states provide an operational

Li Yang; Ryza N. Musin; Xiao-Qian Wang; M. Y. Chou

2008-01-01

378

First principles study of strained Si\\/Ge core-shell nanowires along [110] direction

First principles density-functional calculations were performed to study the electronic properties of Si\\/Ge core-shell nanowires along the [110] direction with the diameter of the wires up to 5 nm. It was found the band gap of the core-shell wires is smaller than that of both pure Si and Ge wires, given the same diameter. This reduced band gap is ascribed

Xihong Peng; Paul Logan

2011-01-01

379

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

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

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

1990-01-01

380

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

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

Xing Gao; Murray S Daw

2009-01-01

381

Possible ferromagnetic and ferroelectric phases are explored for bismuth transition-metal oxides with double-perovskite structure A2BB?O6 on the basis of first-principles calculations within the local spin-density approximation (LSDA) and generalized gradient approximation (GGA). It is found that a lattice instability of the cubic to a non-centrosymmetric phase always happens in the all cases of lead and bismuth perovskite oxides with the

Y. Uratani; T. Shishidou; F. Ishii; T. Oguchi

2006-01-01

382

Curie temperatures of III V diluted magnetic semiconductors calculated from first principles

Curie temperatures of the diluted magnetic semiconductors (Ga, Mn)As, (Ga, Mn)N, (Ga, Cr)As and (Ga, Cr)N are evaluated from first principles. The electronic structure is calculated in the local spin density approximation by using the Korringa-Kohn-Rostoker method combined with the coherent potential approximation to describe the substitutional and spin disorder. From the total energy differences between the ferromagnetic state and

Kazunori Sato; Peter H. Dederichs; Hiroshi Katayama-Yoshida

2003-01-01

383

Systematic first-principles study of impurity hybridization in NiAl

We have performed a systematic first-principles computational study of the effects of impurity atoms (boron, carbon, nitrogen, oxygen, silicon, phosporus, and sulfur) on the orbital hybridization and bonding properties in the intermetallic alloy NiAl using a full-potential linear muffin-tin orbital method. The matrix elements in momentum space were used to calculate real-space properties: onsite parameters, partial densities of states, and

David Djajaputra; Bernard R. Cooper

2002-01-01

384

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

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

2006-01-01

385

We investigate the functionalization of functional groups to graphene nanoribbons with zigzag and armchair edges using first-principles calculations. We find that the formation energy for the configuration of the functional groups functionalized to the zigzag edge is ~ 0.2 eV per functional group lower than that to the armchair edge. The formation energy difference arises from a structural deformation on

Hoonkyung Lee

2010-01-01

386

The current---voltage (I---V) characteristics and the transmission spectra of zigzag graphene nanoribbon with different spin-configurations are investigated by using first-principles calculations. It is shown that the I---V curves and transmission spectra strongly depend on the spin-configurations of the two sides of the ribbon. For the spin-parallel configuration structure, the curve is linear under lower bias voltage; for the spin-antiparallel configuration

An Liping; Liu Nianhua

2011-01-01

387

First-principles model of the dielectric response of ultrathin perovskite films

NASA Astrophysics Data System (ADS)

Using a recently developed first-principles model [1] for the structural energetics of ultrathin PbTiO3 films, we investigate the thickness dependence of the dielectric response of perfect single-crystal films. The dependence on field direction and on the mechanical and electrical boundary conditions are examined, and the effects of surface relaxations discussed. 1. Ph. Ghosez and K. M. Rabe, Appl. Phys. Lett. 76, 2767 (2000).

Rabe, Karin; Ghosez, Philippe

2001-03-01

388

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

389

First-principles calculations of carbon clathrates: Comparison to silicon and germanium clathrates

We employ state-of-the-art first-principles calculations based on density-functional theory and density-functional perturbation theory to investigate relevant physical properties and phase diagram of the free guest type-I (X-46) and type-II (X-34) carbon clathrates. Their properties and those of silicon and germanium diamonds, and clathrates have been computed and compared within the same approach. We briefly present and discuss their structural, cohesive,

Damien Connétable

2010-01-01

390

First principle calculations of the ?-Fe 3AlC perovskite and iron–aluminium intermetallics

We present first principle calculations of the structural, electronic, magnetic, vibrational and elastic properties of the ?-Fe3AlC perovskite, within the ab initio formalisms of the Density Functional Theory (DFT) and the linear response theory of the DFT. These properties are compared with those of the intermetallic Fe3Al-L12 isostructural phase of ?, permitting to interpret the role of the carbon element.

D. Connétable; P. Maugis

2008-01-01

391

First-principle calculation for the phonon structure on iron-based superconductors

We perform first-principle phonon calculations for three typical iron-based superconductors, i.e., LaFeAsO,BaFe2As2, and FeSe. Though those crystals have different structures, we find that the optical modes associated with Fe vibration have almost similar characters. Moreover, we examine the pressure effect on phonons in FeSe. By increasing the external pressure, the phonon mode frequency related to Fe vibration effectively rises up

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

2009-01-01

392

Magnetostriction and magnetism of rare earth intermetallic compounds: First principle study

Magnetism and magnetostriction of rare earth intermetallic compounds, GdCo2, GdFe2, NdCo2, SmCo2, and ErCo2, have been studied by using the first principles full-potential linearized augmented plane-wave method with the generalized gradient approximation. The calculated magnetostriction coefficients agree well with experiment. The itinerant electrons of transition metal elements are found to play a significant role in magnetoelastic coupling. The strong anisotropy

V. I. Gavrilenko; R. Q. Wu

2001-01-01

393

We report first principles calculations, within density functional theory, of\\u000acopper intercalated titanium diselenides, CuxTiSe2, for values of x ranging\\u000afrom 0 to 0.11. The effect of intercalation on the energy bands and densities\\u000aof states of the host material is studied in order to better understand the\\u000acause of the superconductivity that was recently observed in these structures.\\u000aWe

R. A. Jishi; H. M. Alyahyaei

2007-01-01

394

We report first-principles calculations, within density functional theory, of copper intercalated titanium diselenides, CuxTiSe2 , for values of x ranging from 0 to 0.11. The effect of intercalation on the energy bands and density of states of the host material is studied in order to better understand the cause of the superconductivity that was recently observed in these structures. We

R. A. Jishi; H. M. Alyahyaei

2008-01-01

395

Phase stability of Ti2AlC upon oxygen incorporation: A first-principles investigation

The phase stability of Ti2AlC upon oxygen incorporation has been studied by means of first-principles calculations. Recent experimental observations of this so-called MAX phase ( M=early transition metal, A=A -group element, and X=C or N) show that the characteristic nanolaminated structure is retained upon oxygen incorporation, with strong indications of O substituting for C. Therefore, a solid solution of C

Martin Dahlqvist; Björn Alling; Igor A. Abrikosov; Johanna Rosén

2010-01-01

396

Phase stability of TiAlC upon oxygen incorporation: A first-principles investigation

The phase stability of TiAlC upon oxygen incorporation has been studied by means of first-principles calculations. Recent experimental observations of this so-called MAX phase (M=early transition metal, A=A-group element, and X=C or N) show that the characteristic nanolaminated structure is retained upon oxygen incorporation, with strong indications of O substituting for C. Therefore, a solid solution of C and O

Martin Dahlqvist; Bjoern Alling; Igor A. Abrikosov; Johanna Rosen

2010-01-01

397

The explanations for the bonding nature for ZrB2 and HfB2 from electronic structure calculations based on different approaches are inconsistent and even contradictory with each other. First principles pseudopotential calculations have been performed to investigate the bonding nature, elastic property and hardness for the two compounds. The nature of chemical bonding for ZrB2 and HfB2 can be recognized as a

Xinghong Zhang; Xiaoguang Luo; Jiecai Han; Jinping Li; Wenbo Han

2008-01-01

398

Anisotropic Permittivity of Tetragonal BaTiO3: A First-Principles Study

The anisotropic nature of the permittivity of tetragonal BaTiO3 has been investigated by means of first-principles calculations using density-functional perturbation theory and constrained-force approaches. Our calculations suggest that the origin of the anisotropy is the presence of a soft mode with very shallow double-well potential surface that results in non-frozen dipole moments parallel to the a-axes of the tetragonal crystal.

Hiroki Moriwake; Craig A. J. Fisher; Akihide Kuwabara; Tamotsu Hashimoto

2011-01-01

399

Finite-temperature Properties of PSN-PT From First-principles

The first-principles derived approch of L. Bellaiche, A. Garcia and D. Vanderbilt (PRL 84, 5427 (2000)) is generalized to treat perovskite alloys with three different kinds of B-atoms. The resulting technique is used to study finite-temperature properties of disordered and (001)-ordered Pb(Sc_1\\/4Nb_1\\/4Ti_1\\/2)O3 solid solutions. The disordered alloy is predicted to undergo a phase transition from cubic paraelectric to tetragonal ferroelectric

Abdullah Al-Barakaty; Laurent Bellaiche

2002-01-01

400

First-principles investigation of Mg2THy (T=Ni, Co, Fe) complex hydrides

A first-principles plane-wave pseudopotential method based on the density functional theory is used to investigate the structural, electronic and stability properties of Mg2THy (T=Ni, Co, Fe) complex hydrides by constructing their respective cluster models. The calculation results of cohesive energy indicate that the stabilities of these hydrides gradually increase in the order of Mg2NiH4, Mg2CoH5 and Mg2FeH6, which agrees well

J. Zhang; D. W. Zhou; P. Peng; J. S. Liu

2008-01-01

401

First-principles investigation of Mg 2TH y (T=Ni, Co, Fe) complex hydrides

A first-principles plane-wave pseudopotential method based on the density functional theory is used to investigate the structural, electronic and stability properties of Mg2THy (T=Ni, Co, Fe) complex hydrides by constructing their respective cluster models. The calculation results of cohesive energy indicate that the stabilities of these hydrides gradually increase in the order of Mg2NiH4, Mg2CoH5 and Mg2FeH6, which agrees well

J. Zhang; D. W. Zhou; P. Peng; J. S. Liu

2008-01-01

402

First-principles study of electronic and magnetic properties of BiNiO3

First-principles calculations on the electronic and magnetic properties of BiNiO3 reveal that the G-type antiferromagnetic structure with the insulating ground state is more stable than other possible configurations. The hybridization of Ni-O and Bi-O leads to the reduction of the spin magnetic moment to 1.67muB in comparison with the Ni2+ d8 configuration of 2muB. The band gap of the antiferromagnetic

M. Q. Cai; G. W. Yang; Y. L. Cao; W. H. Yu; L. L. Wang; Y. G. Wang

2007-01-01

403

First-principles study of native point defects in hafnia and zirconia

A first-principles study of native point defects in hafnia (HfO2) and zirconia (ZrO2) is carried out to identify dominant defects under different oxygen chemical potentials and Fermi levels. Oxygen vacancies and oxygen interstitials in both HfO2 and ZrO2 show negative- U behavior. It is shown that HfO2 is less prone to the formation of oxygen point defects than ZrO2 under

J. X. Zheng; G. Ceder; T. Maxisch; W. K. Chim; W. K. Choi

2007-01-01

404

First-principles linear muffin-tin orbital atomic-sphere approximation calculations in real space

We have developed an approach, based on the linear muffin-tin orbital atomic-sphere approximation (LMTO-ASA) formalism and the recursion method, that allows us to perform first-principles density-functional calculations of electronic structure in real space. Using Zr2Fe as a test case, we compare our results with those obtained by using the standard reciprocal-space LMTO-ASA method. The scheme described here can be applied

Pascoal R. Peduto; Sonia Frota-Pessa; Michael S. Methfessel

1991-01-01

405

First-principles study of elastic and thermal properties of refractory carbides and nitrides

The elastic properties and thermal expansion of TiC, TiN, VC and VN are calculated by means of a precise first-principles electronic structure method. The single-crystal elastic constants are derived from the total energies of distorted structures. For the thermal expansion, the contribution of electronic states is determined within a free-electron model utilizing the density of states at the Fermi energy

W. Wolf; R. Podloucky; T. Antretter; F. D. Fischer

1999-01-01

406

First-principles study of the anisotropic thermal expansion of wurtzite ZnS

In this letter, a first-principles study of the anisotropic thermal expansion of wurtzite ZnS in the framework of the density-functional theory and the density-functional perturbation theory is reported. The compound in zinc blende structure is also studied for comparison. The curves of the linear and volume thermal expansion coefficients to temperature are presented. The volume thermal expansion coefficient of zinc

S. Q. Wang

2006-01-01

407

First-principles-based modeling of geomagnetically induced currents at mid- and low- latitudes

Recently, Pulkkinen et al. (2007, Annales Geophysicae) introduced an approach to predict geomagnetically induced current (GIC) flow in high-voltage power transmission systems based on first-principles modeling of the near-space plasma environment. Their approach that has already been implemented as an experimental real-time system providing forecasts of GIC in the North American power transmission system, however, is applicable only to high-latitude

A. Pulkkinen; N. Buzulukova; L. Rastaetter; M. Kuznetsova; A. Viljanen; R. Pirjola

2008-01-01

408

Adsorption of halocarbons with a special emphasis on ozone depleting chlorofluorocarbons (CFC) over zeolite is of major environmental importance. The current communication describes first-principles calculation performed on faujasite models to investigate the nature of CFCs including fluoro, chlorofluoro and hydrofluoro\\/chloro carbons (CF4, CF3Cl, CF2Cl2, CFCl3, CHF3, CHCl3) adsorption. The activity of the interacting CFCs was compared using reactivity index within

Abhijit Chatterjee; Takeo Ebina; Takashi Iwasaki

2003-01-01

409

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

NASA Astrophysics Data System (ADS)

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

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

2010-03-01

410

Phase transitions and magnetostructural coupling in ZnCr2O4 from first principles

NASA Astrophysics Data System (ADS)

In the spinel structure oxide ZnCr2O4, a phase transition is observed from the high-temperature cubic phase to a low-temperature low-symmetry phase, reported as tetragonal^1 or orthorhombic.^2 Building on a previous first-principles analysis of the zone-center phonons and spin-phonon coupling,^3 we construct a first-principles effective Hamiltonian to investigate this transition. The local modes included are the Cr displacements, distortions of the Zn-centered tetrahedra, and the homogeneous strain. The magnetostructural coupling of these degrees of freedom to the spins of the Cr^3+ ions is included in the effective Hamiltonian parameterization and first-principles determination using a symmetry analysis. The role of the magnetostructural coupling in the phase transition will be analyzed and discussed. 1. S. H. Lee et al., J. Phys. Cond. Matt. 19, 145259 (2007) 2. V. N. Glazkov et al., http://arxiv.org/abs/0807.0546 3. C. J. Fennie and K. M. Rabe, Phys. Rev. Lett. 96, 205505 (2006)

Eklund, Carl-Johan; Fennie, Craig J.; Rabe, Karin M.

2009-03-01

411

Coarse graining approach to First principles modeling of radiation cascade in large Fe super-cells

NASA Astrophysics Data System (ADS)

First principles techniques employed to understand systems at an atomistic level are not practical for large systems consisting of millions of atoms. We present an efficient coarse graining approach to bridge the first principles calculations of local electronic properties to classical Molecular Dynamics (MD) simulations of large structures. Local atomic magnetic moments in crystalline Fe are perturbed by radiation generated defects. The effects are most pronounced near the defect core and decay with distance. We develop a coarse grained technique based on the Locally Self-consistent Multiple Scattering (LSMS) method that exploits the near-sightedness of the electron Green function. The atomic positions were determined by MD with an embedded atom force field. The local moments in the neighborhood of the defect cores are calculated with first-principles based on full local structure information. Atoms in the rest of the system are modeled by representative atoms with approximated properties. This work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.

Odbadrakh, Khorgolkhuu; Nicholson, Don; Rusanu, Aurelian; Wang, Yang; Stoller, Roger; Zhang, Xiaoguang; Stocks, George

2012-02-01

412

NASA Astrophysics Data System (ADS)

Non nitrogen containing, organic peroxides explosives Triacetone triperoxide and diacetone diperoxide have been prepared in the laboratory in order to study various aspects of their synthesis and their experimental and theoretical spectroscopic characteristics. By using different proportions of acetone/hydrogen peroxide (Ac/H2O2), sulfuric, hydrochloric and methanosulfuric acids as catalyzers, it was possible to obtain both compounds in a rapid and simple form. Raman, IR spectroscopy, and GC-MS were used in order to determine the precursors, intermediates and final analytes. Experiments and theoretical studies using density functional theory (DFT) have been used in the elucidation step of the mechanism of the synthesis of the so called "transparent" explosives. The B3LYP functional with the 6-31G** basis set was used to carry out the electronic structure calculation of the intermediates and internal rotations and vibrations of TATP. Raman spectra of solid TATP and FTIR spectra of gas TATP, were recorded in order to assign the experimental spectra. Although full agreement with experiment was not obtained, spectral features of the main TATP bands were assigned.

Pacheco-Londoño, Leonardo C.; Peña, Álvaro J.; Primera-Pedrozo, Oliva M.; Hernández-Rivera, Samuel P.; Mina, Nairmen; García, Rafael; Chamberlain, R. Thomas; Lareau, Richard T.

2004-09-01

413

NASA Astrophysics Data System (ADS)

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

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

2012-02-01

414

Using first principles to predict bimetallic catalysts for the ammonia decomposition reaction.

The facile decomposition of ammonia to produce hydrogen is critical to its use as a hydrogen storage medium in a hydrogen economy, and although ruthenium shows good activity for catalysing this process, its expense and scarcity are prohibitive to large-scale commercialization. The need to develop alternative catalysts has been addressed here, using microkinetic modelling combined with density functional studies to identify suitable monolayer bimetallic (surface or subsurface) catalysts based on nitrogen binding energies. The Ni-Pt-Pt(111) surface, with one monolayer of Ni atoms residing on a Pt(111) substrate, was predicted to be a catalytically active surface. This was verified using temperature-programmed desorption and high-resolution electron energy loss spectroscopy experiments. The results reported here provide a framework for complex catalyst discovery. They also demonstrate the critical importance of combining theoretical and experimental approaches for identifying desirable monolayer bimetallic systems when the surface properties are not a linear function of the parent metals. PMID:20489718

Hansgen, Danielle A; Vlachos, Dionisios G; Chen, Jingguang G

2010-04-25

415

Conductivity and transparency of TiO2 from first principles

NASA Astrophysics Data System (ADS)

Titanium dioxide is a versatile material with ubiquitous applications, many of which are critically linked to either light absorption or transparency in the visible spectral range in addition to electrical conductivity. Doping is a well-known way to influence those properties in order to bring them into a desired range. Working towards a comprehensive understanding of the electronic and optical properties of TiO2 (as well as of the link between them) we review and summarize electronicstructure results that we obtained using cutting-edge theoretical spectroscopy techniques. We focus on the formation of electron and hole polarons and we elucidate the influence of doping on the optical properties of TiO2. In addition, we present new results for the reflectivity of pure TiO2.

Schleife, André; Varley, Joel B.; Janotti, Anderson; Van de Walle, Chris G.

2013-09-01

416

NASA Astrophysics Data System (ADS)

Mass-independently fractionated oxygen isotope were reported on metal particles extracted from Apollo lunar soils [1, 2], but these origins are still unknown. Since the substantial fraction of Earth-escaping O+ flux (Earth Wind, EW hereafter), comparable to the amount of the anomalous oxygen implanted on the metal particles, could reach the lunar surface [3], Ozima et al. [4] suggested that EW may be responsible to the anomalous oxygen. The purpose is to test this EW hypothesiss, we study oxygen isotopic ratios of O+ at the upper atmosphere. From quantum chemical calculations of photo-dissociation of O2, we show the results in mass-independent isotopic fractionation of oxygen, thereby in conformity with the EW hypothesis. First principles reaction dynamics simulations were performed to compute the photolysis rate for the B3?u- ? X3?g- electronic transition, for Schumann-Runge band. With the assumption of the Born-Oppenheimer approximation, we performed the wave-packet dynamics for the nuclei-motion in the potential energy curves determined by the first step calculation. Quantum chemical program package [5] was used for the first step calculation, and the quantum dynamics was carried out by our own program package. Assuming the quantum yield of the corresponding photolysis is unity, the photo-absorption cross section can be correlated with the photolysis rate. Therefore, following the time dependent approach, the autocorrelation function (A(t) = ) was numerically computed by the second step calculation. Finally, the theoretical spectrum as a function of wavelength of excitation light was estimated by the Fourier transform of the autocorrelation function A(t) [6]. Calculated absorption cross sections for C16O showed similar wavelength dependence with experiment [7], although the absolute magnitude was yet to be calibrated for a quantitative comparison. Assuming Boltzmann distribution at 1200 K, we estimated enrichment factors defined as ??(?)/?16(?) - 1 (i = 17, 18) using the above calculated cross sections. Assuming SMOW for the initial oxygen isotopic composition, the isotopic ratios of O atom dissociated from O2 are ?17O = 5.62‰, ?18O = 3.53‰, ?17O = 3.8‰, suggesting large mass-independent isotopic fractionation in photo-dissociation of CiO. Numerical values of isotopic fractionation (e.g. ?17O) can be obtained by solving photochemical reaction equations in the thermosphere conditions (>100 km) with the above estimated dissociation rates, where effective O+ pickup is likely to take place. We are currently working on the latter problem with hopes that this would test the EW hypothesis. References: [1] Ireland et al., 2006, Nature, 440:776. [2] Hashizume & Chaussidon, 2009, GCA, 73:3038. [3] Seki et al., 2001, Science, 291:1939. [4] Ozima et al., 2008, PNAS, 105:17654. [5] Werner & Knowles, http://www.molpro.net. [6] Heller, 1978, J. Chem. Phys., 68:2066. [7] Ackermann et al., 1970, Planet. Space Sci., 18:1639.

Yamada, A.; Nanbu, S.; Kasai, Y.; Ozima, M.

2009-12-01

417

a Theoretical Study of CN Spectroscopy from the IR to the VUV

NASA Astrophysics Data System (ADS)

We have carried out first principles calculations of the spectrum of CN. We have included Rydberg states, spin-orbit and non-adiabatic coupling between different electronic states, and an accurate treatment of the ro-vib-spin-electronic wavefunctions. Doublet, quartet and sextet spin states are included in our calculations. These wavefunctions are used with electric dipole and quadrapole and magnetic dipole transition moments to predict spectra. We consider both bound-bound, bound-free and free-bound transitions. When possible, comparisons are made with experimental results, both for energy levels and radiative lifetimes. Many previously uncharacterized bands are predicted.

Schwenke, David W.

2013-06-01

418

NASA Astrophysics Data System (ADS)

Sesquiterpenes, a class of biogenic volatile organic compounds, are important precursors to secondary organic aerosols (SOAs) in nature. Using density functional theory (DFT), conceptual DFT, time-dependent (TD) DFT, configuration interaction with single excitation (CIS), and Zerner's intermediate neglect of differential overlap (ZINDO) methods, the electronic structures, spectroscopy, and reactivity of sesquiterpenes were systematically investigated. Results from the CIS calculations show the best consistency in the excited energies and allow for assigning and predicting newly found sesquiterpenes. The results suggest that the first peaks in the ultraviolet-visible (UV-vis) absorption spectra for saturated and unsaturated isomers are ?-?* and ?-?* transitions, respectively. It can be deduced from the transit intensities of the isomers that an isomer with an endocyclic C = C bond presents weaker UV transition intensity than its corresponding exocyclic isomer. The electronic structures of these compounds were also analyzed by comparing published UV-spectroscopy with advanced theoretical calculations. ?-Zingiberene and longicyclene are the most and least reactive in electron-transfer reactions, respectively. No quantitative linear relationships were discovered between the changes in transit energies, DFT chemical reactivity indices of isomers, different degrees of unsaturated C = C double bonds, or the number of substituents attached to the C = C bond. The larger steric hindrance of substituents or exocyclic C = C bond is related directly to higher chemical reactivity possessed by the isomer compared to a corresponding isomer with smaller steric hindrandce or with an endo C = C bond. These results are imperative to a better understanding of SOA production mechanisms in the troposphere.

Hu, S.-X.; Yu, J.-G.; Zeng, E. Y.

2010-10-01

419

NASA Astrophysics Data System (ADS)

A mathematical formulation has been developed and computed results are presented describing the temperature profiles in gas tungsten arc welding (GTAW) arcs and, hence, the net heat flux from the welding arc to the weld pool. The formulation consists of the statement of Maxwell's equations, coupled to the Navier-Stokes equations and the differential thermal energy balance equation. The theoretical predictions for the heat flux to the workpiece are in good agreement with experimental measurements — for long arcs. The results of this work provide a fundamental basis for predicting the behavior of arc welding systems from first principles.

Choo, R. T. C.; Szekely, J.; Westhoff, R. C.

1992-06-01

420

NASA Astrophysics Data System (ADS)

A method based on first-principles calculations was used to construct temperature versus misfit strain phase diagrams for epitaxial (BaxSr1-x)TiO3 ultrathin films over the entire composition range from x=0 to x=1. The predicated phase diagrams show “topologies” that are similar to those calculated by Shirokov et al. (Phy. Rev. B. 79 (2009) 144118), but with quantitative differences that are examined and explained. The dependence of the dielectric permittivity on the misfit strain and the film composition at room temperature are also investigated and compared with available theoretical predications and experimental measurements.

Bin-Omran, S.

2012-09-01

421

Aggregation-enhanced luminescence and vibronic coupling of silole molecules from first principles

NASA Astrophysics Data System (ADS)

Aggregate formation in molecular solids usually quenches the luminescence, a piece of bad news for molecular electronic devices. However, siloles present extremely high luminescent efficiency in solid state as well as in aggregation, but have almost no luminescence in solution. By employing a first-principles calculation to study excited states and vibronic couplings, we find that it is the low-frequency twisting motions of side rings which enhance the nonradiative decay. These motions can be suppressed either by solid-state packing, by aggregation formation in polar solvents, or by increasing the solvent viscosity; thereby, the radiative decay becomes dominant, resulting in peculiar aggregate-induced emission phenomena in siloles.

Yin, Shiwei; Peng, Qian; Shuai, Z.; Fang, Weihai; Wang, Yan-Hua; Luo, Yi

2006-05-01

422

First-principles investigations of the physical properties of RCd (R=Ce, La, Pr, Nd)

NASA Astrophysics Data System (ADS)

The crystal structural, electronic, elastic and the thermodynamic properties of RCd are investigated by using the first-principles plane-wave pseudopotential density function theory within the generalized gradient approximation (GGA). The calculated equilibrium lattice parameters for RCd are in good agreement with the available experimental data. Furthermore, the optical properties, namely the dielectric function, refractive index and electron energy loss are reported for radiation up to 30 eV. Finally, the elastic properties, the bulk modulus and the Debye temperature of RCd are given for reference.

Long, Jianping

2012-12-01

423

First-Principles Electronic Structure Calculations of Zinc-Blende Chromium Monopnictides

NASA Astrophysics Data System (ADS)

Electronic band-structure of zinc-blende (zb) chromium monopnictides, CrP, CrAs, and CrSb, is studied comparatively by using first-principles density-functional calculations. Effect of spin-orbit interaction on the spin-polarization near the Fermi-level is also investigated. It is found that zb-CrAs and CrSb are predicted to be the ferromagnets exhibiting highly spin-polarized electroni