Spectroscopy of organic semiconductors from first principles
NASA Astrophysics Data System (ADS)
Sharifzadeh, Sahar; Biller, Ariel; Kronik, Leeor; Neaton, Jeffery
2011-03-01
Advances in organic optoelectronic materials rely on an accurate understanding their spectroscopy, motivating the development of predictive theoretical methods that accurately describe the excited states of organic semiconductors. In this work, we use density functional theory and many-body perturbation theory (GW/BSE) to compute the electronic and optical properties of two well-studied organic semiconductors, pentacene and PTCDA. We carefully compare our calculations of the bulk density of states with available photoemission spectra, accounting for the role of finite temperature and surface effects in experiment, and examining the influence of our main approximations -- e.g. the GW starting point and the application of the generalized plasmon-pole model -- on the predicted electronic structure. Moreover, our predictions for the nature of the exciton and its binding energy are discussed and compared against optical absorption data. We acknowledge DOE, NSF, and BASF for financial support and NERSC for computational resources.
Infrared Spectroscopy of Functionalized Graphene Sheets from First Principle Calculations
NASA Astrophysics Data System (ADS)
Zhang, Cui; Dabbs, Daniel; Aksay, Ilhan; Car, Roberto; Selloni, Annabella
2014-03-01
Detailed characterization of the structure of functionalized graphene sheets (FGSs) is an important and challenging task which could help to improve the performance of FGS materials for technological applications. We present here first principles calculations for the infrared (IR) spectra of different FGS models aimed at identifying the IR signatures of different functional groups and defect sites on FGSs. We found that vacancies and edges have significant effects on the IR frequencies of the functional groups on FGSs. In particular, hydroxyl groups close to vacancies have higher stretching and lower bending frequencies in comparison to hydroxyls in defect free regions of FGSs. More interestingly, the OH vibrations of carboxyl groups at edges exhibit unique features in the high frequency IR bands, which originate from the interactions with neighboring groups and the relative orientation of the carboxyl with respect to the FGS plane. Our results are supported by experimental IR measurements on FGS powders.
NASA Astrophysics Data System (ADS)
Nguyen, Ngoc Linh; Borghi, Giovanni; Ferretti, Andrea; Dabo, Ismaila; Marzari, Nicola
2015-04-01
The determination of spectral properties from first principles can provide powerful connections between microscopic theoretical predictions and experimental data, but requires complex electronic-structure formulations that fall outside the domain of applicability of common approaches, such as density-functional theory. We show here that Koopmans-compliant functionals, constructed to enforce piecewise linearity and the correct discontinuity derivative in energy functionals with respect to fractional occupation—i.e., with respect to charged excitations—provide molecular photoemission spectra and momentum maps of Dyson orbitals that are in excellent agreement with experimental ultraviolet photoemission spectroscopy and orbital tomography data. These results highlight the role of Koopmans-compliant functionals as accurate and inexpensive quasiparticle approximations to the spectral potential.
Roy, Tapta Kanchan; Kopysov, Vladimir; Nagornova, Natalia S; Rizzo, Thomas R; Boyarkin, Oleg V; Gerber, R Benny
2015-05-18
Calculated structures of the two most stable conformers of a protonated decapeptide gramicidin S in the gas phase have been validated by comparing the vibrational spectra, calculated from first- principles and measured in a wide spectral range using infrared (IR)-UV double resonance cold ion spectroscopy. All the 522 vibrational modes of each conformer were calculated quantum mechanically and compared with the experiment without any recourse to an empirical scaling. The study demonstrates that first-principles calculations, when accounting for vibrational anharmonicity, can reproduce high-resolution experimental spectra well enough for validating structures of molecules as large as of 200 atoms. The validated accurate structures of the peptide may serve as templates for in silico drug design and absolute calibration of ion mobility measurements. PMID:25721337
Kishi, Hiroki; Miyazawa, Miki; Matsushima, Naoki; Yamauchi, Jun [Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Yokohama-shi, Kanagawa-ken 223-8522 (Japan)
2014-02-21
We investigate the X-ray photoelectron spectroscopy (XPS) binding energies of As 3d in Si for various defects in neutral and charged states by first-principles calculation. It is found that the complexes of a substitutional As and a vacancy in charged and neutral states explain the experimentally observed unknown peak very well.
Inelastic Electron Tunneling Spectroscopy in Molecular Electronic Devices from First-Principles
NASA Astrophysics Data System (ADS)
Ji, Tao
In this thesis, we present the first-principle calculations of inelastic electron tunneling spectroscopy(IETS) in single molecular break junctions. In a two-probe electrode-molecule-electrode setup, density functional theory(DFT) is used for the construction of the Hamiltonian and the Keldysh non-equilibrium Green's function(NEGF) technique will be employed for determining the electron density in non-equilibrium system conditions. Total energy functional, atomic forces and Hessian matrix can be obtained in the DFT-NEGF formalism and self-consistent Born approximation(SCBA) is used to integrate the molecular vibrations (phonons) into the framework once the phonon spectra and eigenvectors are calculated from the dynamic matrix. Geometry optimization schemes will also be discussed as an indispensable part of the formalism as the equilibrium condition is crucial to correctly calculate the phonon properties of the system. To overcome the numerical difficulties, especially the large computational time demand of the electron-phonon coupling problem, we develop a numerical approximation for the electron self-energy due to phonons and the error is controlled within numerical precision. Besides, a direct IETS second order I-V derivative expression is derived to reduce the error of numerical differentiation under reasonable assumptions. These two approximations greatly reduce the computation requirement and make the calculation feasible within current numerical capability. As the application of the DFT-NEGF-SCBA formalism, we calculate the IETS of the gold-octanedithiol(ODT) molecular junction. The I-V curve, conductance and IETS from ab-inito calculations are compared directly to experiments. A microscopic understanding of the electron-phonon coupling mechanism in the molecular tunneling junctions is explained in this example. In addition, comparisons of the hydrogen-dissociative and hydrogen-non-dissociative ODT junctions as well as the different charge transfer behaviors are presented to show the effects of thiol formation in the ODT molecular junction.
X-ray photoelectron spectroscopy and first principles calculation of BCN nanotubes.
Kim, Shin Young; Park, Jeunghee; Choi, Hyun Chul; Ahn, Jae Pyung; Hou, Jin Qiang; Kang, Hong Seok
2007-02-14
Multiwalled boron carbonitride (BCN) nanotubes with two different structures were synthesized via thermal chemical vapor deposition; one has 10% C atoms homogeneously doped into BN nanotubes (B0.45C0.1N0.45 NTs), and the other has BN layers sheathed with 5-nm-thick C outerlayers (BN-C NTs). The electronic structures of the B, C, and N atoms were thoroughly probed by synchrotron X-ray photoelectron spectroscopy and the X-ray absorption near-edge structure method. The B0.45C0.1N0.45 NTs contain a significant amount of B-C and C-N bonding with a pyridine-like structure (hole structure), which reduces the pi bonding states of the B and N atoms. From the XPS valence band spectrum, the band gap was estimated to be about 2.8 eV. In the BN-C NTs, the C and BN domains are separated without forming the pyridine-like structure. Using the first principles method, we investigated the relative stabilities and electronic structures of the various isomers of the double-walled (12,0)@(20,0) BCN NTs. The C-outerlayer BN nanotube structure is the most stable isomer, when there exist no defects in the tubes with B/N = 1.0 (i.e., graphite-like structure). In addition, a reasonable model, which is characterized by the motives consisted of three pyridine-like rings around a hollow site, is presented for the local structure of C atoms in the B0.45N0.45C0.1 NTs. A considerable decrease of the band gap due to the 10% C doping was predicted, which was consistent with the experimental results. PMID:17243688
A theoretical study of blue phosphorene nanoribbons based on first-principles calculations
Xie, Jiafeng; Si, M. S., E-mail: sims@lzu.edu.cn; Yang, D. Z.; Zhang, Z. Y.; Xue, D. S. [Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000 (China)
2014-08-21
Based on first-principles calculations, we present a quantum confinement mechanism for the band gaps of blue phosphorene nanoribbons (BPNRs) as a function of their widths. The BPNRs considered have either armchair or zigzag shaped edges on both sides with hydrogen saturation. Both the two types of nanoribbons are shown to be indirect semiconductors. An enhanced energy gap of around 1?eV can be realized when the ribbon's width decreases to ?10?Å. The underlying physics is ascribed to the quantum confinement effect. More importantly, the parameters to describe quantum confinement are obtained by fitting the calculated band gaps with respect to their widths. The results show that the quantum confinement in armchair nanoribbons is stronger than that in zigzag ones. This study provides an efficient approach to tune the band gap in BPNRs.
Jiang, Jun; Mukamel, Shaul
2011-01-01
We report a first principles study of two dimensional electronic spectroscopy of aromatic side chain transitions in the 32-residue ?-amyloid (A?9–40) fibrils in the near ultraviolet (250–300 nm). An efficient exciton Hamiltonian with electrostatic fluctuations (EHEF) algorithm is used to compute the electronic excitations in the presence of environmental fluctuations. The through-space inter- and intra-molecular interactions are calculated with high level quantum mechanics (QM) approaches, and interfaced with molecular mechanics (MM) simulations. Distinct two dimensional near ultraviolet (2DNUV) spectroscopic signatures are identified for different aromatic transitions, and the couplings between them. 2DNUV signals associated with the transition couplings are shown to be very sensitive to the change of residue-residue interactions induced by residue mutations. Our simulations suggest that 2DNUV spectra could provide a useful local probe for the structure and kinetics of fibrils. PMID:21132201
T. Hashimoto; H. Moriwake
2008-01-01
We have studied the formation energy of oxygen vacancy and its effect on spontaneous polarizations of Bi4Ti3O12 (BT) by first-principles theoretical calculations. We have found that neutral and +2 charged oxygen vacancies prefer sites in the Bi2O2 layers, while +1 charged oxygen vacancies are not stable. The spontaneous polarizations of the perfect-crystal BT in both a and c directions are
NASA Astrophysics Data System (ADS)
Nakamura, Hisao; Asai, Yoshihiro; Hihath, Josh; Tao, Nongjian
2011-03-01
The concept of a single molecular diode was first proposed by Aviram and Ratner, and there have been many studies of synthesis D- ? -A or p - n di-block molecules and measurements of the current-voltage (I - V) characteristics for relating molecular junctions. Recently, the I - V measurement in a symmetric tetraphenyl junction and non-symmetric dipyrimidinyl -diphenyl diblock junction was performed, and clear rectification was found in the latter system, which resembles the p - n junction by the covalent connection between electron-deficient bypyrimidinyl and electron-rich biphenyl moieties, though an applied bias is much lower than the resonant level. In this presentation, we performed the first principles calculations of electron transport for the above tetraphenyl and dipyrimidinyl -diphenyl diblock junctions by the self-consistent nonequilibrium Green's function theory with the use of our HiRUNE program module. We carried out the systematic analysis of the rectification behavior and identified the change of electron-pathway in the bridge molecule relating to p - n junction based on the first principles data. The relation between the rectifying action and molecular conformation, particularly, the torsion of diblock, will be discussed.
NASA Astrophysics Data System (ADS)
Pokrovski, Gleb S.; Roux, Jacques; Ferlat, Guillaume; Jonchiere, Romain; Seitsonen, Ari P.; Vuilleumier, Rodolphe; Hazemann, Jean-Louis
2013-04-01
The molecular structure and stability of species formed by silver in aqueous saline solutions typical of hydrothermal settings were quantified using in situ X-ray absorption spectroscopy (XAS) measurements, quantum-chemical modeling of near-edge absorption spectra (XANES) and extended fine structure spectra (EXAFS), and first-principles molecular dynamics (FPMD). Results show that in nitrate-bearing acidic solutions to at least 200 °C, silver speciation is dominated by the hydrated Ag+ cation surrounded by 4-6 water molecules in its nearest coordination shell with mean Ag-O distances of 2.32 ± 0.02 Å. In NaCl-bearing acidic aqueous solutions of total Cl concentration from 0.7 to 5.9 mol/kg H2O (m) at temperatures from 200 to 450 °C and pressures to 750 bar, the dominant species are the di-chloride complex AgCl2- with Ag-Cl distances of 2.40 ± 0.02 Å and Cl-Ag-Cl angle of 160 ± 10°, and the tri-chloride complex AgCl32- of a triangular structure and mean Ag-Cl distances of 2.60 ± 0.05 Å. With increasing temperature, the contribution of the tri-chloride species decreases from ˜50% of total dissolved Ag in the most concentrated solution (5.9m Cl) at 200 °C to less than 10-20% at supercritical temperatures for all investigated solutions, so that AgCl2- becomes by far the dominant Ag-bearing species at conditions typical of hydrothermal-magmatic fluids. Both di- and tri-chloride species exhibit outer-sphere interactions with the solvent as shown by the detection, using FPMD modeling, of H2O, Cl-, and Na+ at distances of 3-4 Å from the silver atom. The species fractions derived from XAS and FPMD analyses, and total AgCl(s) solubilities, measured in situ in this work from the absorption edge height of XAS spectra, are in accord with thermodynamic predictions using the stability constants of AgCl2- and AgCl32- from Akinfiev and Zotov (2001) and Zotov et al. (1995), respectively, which are based on extensive previous AgCl(s) solubility measurements. These data are thus recommended for chemical equilibrium calculations in mineral-fluid systems above 200 °C. In contrast, our data disagree with SUPCRT-based datasets for Ag-Cl species, which predict large fractions of high-order chloride species, AgCl32- and AgCl43- in high-temperature saline fluids. Comparisons of the structural and stability data of Ag-Cl species derived in this study with those of their Au and Cu analogs suggest that molecular-level differences amongst the chloride complexes such as geometry, dipole moment, distances, and resulting outer-sphere interactions with the solvent may account, at least partly, for the observed partitioning of Au, Ag and Cu in vapor-brine and fluid-melt systems. In hydrothermal environments dominated by fluid-rock interactions, the contrasting affinity of these metals for sulfur ligands and the differences both in chemistry and stability of their main solid phases (Ag sulfides, Cu-Fe sulfides, and native Au) largely control the concentration and distribution of these metals in their economic deposits.
Theoretical insight into hydrogen adsorption onto graphene: a first-principles B3LYP-D3 study.
Darvish Ganji, M; Hosseini-Khah, S M; Amini-Tabar, Z
2015-01-28
This work investigates hydrogen adsorption onto various graphene flakes such as coronene and coronene-like as suitable models of graphene within the framework of the DFT-B3LYP method. The non-local van der Waals (vdW) density functional (B3LYP-D3) method is used for both structural geometry optimization and total energy estimations. Calculations were carried out for a hydrogen molecule above a coronene surface with both conventional and vdW corrected DFT to investigate how these approaches perform in the case of hydrogen adsorption on a graphene surface. Our first-principles results within the B3LYP-D3/def2-TZVPP model show that hydrogen physisorbs on a coronene surface with an adsorption energy of -5.013 (kJ mol(-1)) which is in good agreement with the experimental value. The influence of the basis set and graphene flake size were also evaluated and the results indicate that these slightly affect the adsorption properties. We found also that it is crucial to use non-local dispersion interactions to get accurate results for hydrogen adsorption on a graphene surface. Furthermore, the co-adsorption of H2 molecules onto the graphene surface was investigated. The results obtained at the B3LYP-D3/def2-TZVP level show that H2 molecules can be physisorbed on both sides of the graphene layer with adsorption properties similar to those for a single surface. Finally, we showed that H2 molecules might be bound to the graphene surface via a bilayer adsorption scheme with weak adsorption energy. Charge population and electron density analysis confirm the weak binding nature of the system under consideration. PMID:25490973
Zwier, Timothy S.
Theoretical modeling of the OH stretch infrared spectrum of carboxylic acid dimers based on first, Wisconsin 53706-1396 Received 20 September 2002; accepted 28 October 2002 Carboxylic acid dimers serve, and OH bend internal coordinates for the formic acid and benzoic acid dimers. These are then projected
NASA Astrophysics Data System (ADS)
Barman, S.; Rajesh, C.; Das, G. P.; Majumder, C.
2009-12-01
Here we report a systematic theoretical study of the structure and electronic properties of Snn-1Pb and Pbn-1Sn (n = 2-13) clusters and compare these results with pure Snn and Pbn to understand the influence of the dopant elements. The calculations were carried out using the density functional theory with generalized gradient approximation for the exchange-correlation potential. Extensive search based on large number of initial configurations has been carried out to locate the stable isomers of Snn-1Pb and Pbn-1Sn (n = 2-13) clusters. The relative stability of Snn-1Pb and Pbn-1Sn (n = 2-13) clusters is analyzed based on the calculated binding energies and second difference in energy. The stability analysis of these clusters suggests that, while the substitution of Sn by Pb lowers the stability of Snn clusters, presence of Sn enhances the stability of the Pbn clusters. The results suggest that while for Snn-1Pb, n=4, 7, 10, 12 clusters are more stable than their respective neighbors, Pbn-1Sn clusters with n = 4, 7 and 9 are found to be more stable. Based on the fragmentation pattern it is seen that for Snn-1Pb and Pbn-1Sn clusters favor monomer evaporation of the Pb atom up to n =11 and n =12, respectively. Unlike this trend, the Sn11Pb undergoes fission type fragment into Sn5Pb and Sn6 clusters. A comparison between our theoretical results and surface induced dissociation experiment shows good agreement, which gives confidence on the prediction of the ground state geometries.
Reeves, Kyle G; Kanai, Yosuke
2014-07-14
Oxidation state is a powerful concept that is widely used in chemistry and materials physics, although the concept itself is arguably ill-defined quantum mechanically. In this work, we present impartial comparison of four, well-recognized theoretical approaches based on Lowdin atomic orbital projection, Bader decomposition, maximally localized Wannier function, and occupation matrix diagonalization, for assessing how well transition metal oxidation states can be characterized. Here, we study a representative molecular complex, tris(bipyridine)ruthenium. We also consider the influence of water solvation through first-principles molecular dynamics as well as the improved electronic structure description for strongly correlated d-electrons by including Hubbard correction in density functional theory calculations. PMID:25028017
Chen, Jie-Jie; Wang, Wei-Kang; Li, Wen-Wei; Pei, Dan-Ni; Yu, Han-Qing
2015-06-17
Titania modified with nanosized metallic clusters is found to substantially enhance its photocatalytic capacity for renewable energy generation and environmental purification, but the underlying mechanism, especially the roles of crystal surface in noble-metal-loaded TiO2, remain unclear. In this work, such roles in the Pt-loaded anatase TiO2 for the photocatalytic conversion of nitrobenzene (NB), a model pollutant, are explored by first-principle calculations. The theoretical calculations reveal that the Pt-TiO2 complex has a higher catalytic activity toward NB conversion than pure Pt clusters, and the (001) facets of TiO2 in this complex tend to accumulate more positively charged holes and thus have a higher photocatalytic activity than the (101) facets. Furthermore, the thermodynamic and kinetic results also show that the Pt cluster loaded on the (001) surface of anatase TiO2 is favored for NB conversion in the photooxidation pathway. This work deepens our fundamental understanding on the evolution of molecule-photocatalyst interface and provides implications for designing and preparing photocatalysts. PMID:26013255
Wang Jingyang [International Center for Young Scientists (ICYS), National Institute for Materials Science, Tsukuba, Ibaraki 305-0044 (Japan); High-Performance Ceramic Division, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016 (China); Zhou Yanchun; Lin Zhijun [High-Performance Ceramic Division, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016 (China); Ohno, Takahisa [National Institute for Materials Science, Tsukuba, Ibaraki 305-0047 (Japan)
2008-03-01
The theoretical elastic stiffness of Y{sub 3}Si{sub 5}N{sub 9}O, the only Y-Si-O-N quaternary crystal that contains a framework of corner-sharing SiN{sub 4} and/or SiON{sub 3} tetrahedron in three dimensions, was investigated using the first-principles total energy calculations. The full set of second order elastic coefficients, polycrystalline bulk and shear moduli, and anisotropic elastic moduli were reported and further compared with those of Y{sub 2}O{sub 3} and {beta}-Si{sub 3}N{sub 4}. The equation of state and compressibility of Y{sub 3}Si{sub 5}N{sub 9}O were investigated at pressures up to 50 GPa. The crystal structure is stable up to 50 GPa and exhibits anisotropic compressibility under hydrostatic pressure. The relatively softer YN{sub 6} and/or YN{sub 5}O polyhedra are more prone to distort or deform than the SiN{sub 4} and/or SiON{sub 3} tetrahedra. Therefore, although the crystal structure of Y{sub 3}Si{sub 5}N{sub 9}O contains a Si-N-O framework similar to that in {beta}-Si{sub 3}N{sub 4}, it displays elastic stiffness between those of Y{sub 2}O{sub 3} and {beta}-Si{sub 3}N{sub 4}.
Karre, Rajamallu; Niranjan, Manish K; Dey, Suhash R
2015-05-01
High alloyed ?-phase stabilized titanium alloys are known to provide comparable Young's modulus as that to the human bones (~30 GPa) but is marred by its high density. In the present study the low titanium alloyed compositions of binary Ti-Nb and ternary Ti-Nb-Zr alloy systems, having stable ?-phase with low Young's modulus are identified using first principles density functional framework. The theoretical results suggest that the addition of Nb in Ti and Zr in Ti-Nb increases the stability of the ?-phase. The ?-phase in binary Ti-Nb alloys is found to be fully stabilized from 22 at.% of Nb onwards. The calculated Young's moduli of binary ?-Ti-Nb alloy system are found to be lower than that of pure titanium (116 GPa). For Ti-25(at.%)Nb composition the calculated Young's modulus comes out to be ~80 GPa. In ternary Ti-Nb-Zr alloy system, the Young's modulus of Ti-25(at.%)Nb-6.25(at.%)Zr composition is calculated to be ~50 GPa. Furthermore, the directional Young's moduli of these two selected binary (Ti-25(at.%)Nb) and ternary alloy (Ti-25(at.%)Nb-6.25(at.%)Zr) compositions are found to be nearly isotropic in all crystallographic directions. PMID:25746245
NASA Astrophysics Data System (ADS)
Hirayama, Naomi; Iida, Tsutomu; Funashima, Hiroki; Morioka, Shunsuke; Sakamoto, Mariko; Nishio, Keishi; Kogo, Yasuo; Takanashi, Yoshifumi; Hamada, Noriaki
2015-07-01
We theoretically investigate the impurity doping effects on the structural parameters such as lattice constant, atomic positions, and site preferences of impurity dopants for Al-doped magnesium silicide (Mg2Si) crystal using the first-principles calculation methods. We present comparison between several codes: ABCAP, Quantum Espresso, and Machikaneyama2002 (Akai KKR), which are based on the full-potential linearized augmented plane-wave method, the pseudopotential method, and KKR/GGA Green’s function method, respectively. As a result, any codes used in the present study exhibit qualitative consistency both in the dependence of the lattice constants on the doping concentration and the energetic preference of the Al atom for the following sites; substitutional Si and Mg sites, and interstitial 4b site; in particular, ABCAP, which is based on the all-electron full-potential method, and Quantum Espresso, which is a code of the pseudopotential method, produce closely-resemble calculation results. We also discuss the effects of local atomic displacement owing to the presence of impurities to the structural parameters of a bulk. Using the analytical method considering the local atomic displacement, moreover, we evaluate the formation energy of Na- and B-doped systems as examples of p-type doping in order to examine the possilbility of realizing p-type Mg2Si.
NASA Astrophysics Data System (ADS)
Zhao, Yanyuan; Luo, Xin; Zhang, Jun; Wu, Junxiong; Bai, Xuxu; Wang, Meixiao; Jia, Jinfeng; Peng, Hailin; Liu, Zhongfan; Quek, Su Ying; Xiong, Qihua
2014-12-01
Layered materials, such as graphite/graphene, boron nitride, transition metal dichalcogenides, represent materials in which reduced size, dimensionality, and symmetry play critical roles in their physical properties. Here, we report on a comprehensive investigation of the phonon properties in the topological insulator Bi2Te3 and Bi2Se3 two-dimensional (2D) crystals, with the combination of Raman spectroscopy, first-principles calculations, and group theory analysis. Low frequency (<30 c m-1) interlayer vibrational modes are revealed in few-quintuple-layer (QL) B i2T e3/B i2S e3 2D crystals, which are absent in the bulk crystal as a result of different symmetries. The experimentally observed interlayer shear and breathing mode frequencies both show blueshifts, with decreasing thickness in few-QL Bi2Te3 (down to 2QL) and Bi2Se3 (down to 1QL), in agreement with first-principles calculations and a linear chain model, from which the interlayer coupling force constants can be estimated. Besides, an intense ultralow (<12 c m-1) frequency peak is observed in 2-4QL Bi2Te3 , which is tentatively attributed to a substrate-induced interface mode supported by a linear chain model analysis. The high frequency Raman peaks exhibit frequency shifts and broadening from 3D to 2D as a result of the phonon confinement effect. Our studies shed light on a general understanding of the influence of dimensionality and crystal symmetry on the phonon properties in layered materials.
Towards first-principles electrochemistry
Dabo, Ismaila
2008-01-01
This doctoral dissertation presents a comprehensive computational approach to describe quantum mechanical systems embedded in complex ionic media, primarily focusing on the first-principles representation of catalytic ...
Theoretical investigations on KCl xBr 1-x, KCl xI 1-x and KBr xI 1-x: A first-principles study
NASA Astrophysics Data System (ADS)
Amrani, B.; Kazempoor, A.; Khosravizadeh, Sh.; El Haj Hassan, F.; Akbarzadeh, H.
2008-08-01
Using first-principles total energy calculations within the full-potential linearized augmented plane wave (FP-LAPW) method, we have investigated the structural, electronic and thermodynamic properties of potassium halides (KCl xBr 1-x, KCl xI 1-x and KBr xI 1-x), with x concentrations varying from 0% up to 100%. The effect of composition on lattice constants, bulk modulus, band gap and dielectric function was investigated. Deviations of the lattice constants from Vegard's law and the bulk modulus from linear concentration dependence (LCD) were observed for the three alloys. The microscopic origins of the gap bowing were explained by using the approach of Zunger and coworkers. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ? Hm as well as the phase diagram.
NASA Astrophysics Data System (ADS)
Guo, Weifu; Mosenfelder, Jed L.; Goddard, William A., III; Eiler, John M.
2009-12-01
Phosphoric acid digestion has been used for oxygen- and carbon-isotope analysis of carbonate minerals since 1950, and was recently established as a method for carbonate 'clumped isotope' analysis. The CO 2 recovered from this reaction has an oxygen isotope composition substantially different from reactant carbonate, by an amount that varies with temperature of reaction and carbonate chemistry. Here, we present a theoretical model of the kinetic isotope effects associated with phosphoric acid digestion of carbonates, based on structural arguments that the key step in the reaction is disproportionation of H 2CO 3 reaction intermediary. We test that model against previous experimental constraints on the magnitudes and temperature dependences of these oxygen isotope fractionations, and against new experimental determinations of the fractionation of 13C- 18O-containing isotopologues ('clumped' isotopic species). Our model predicts that the isotope fractionations associated with phosphoric acid digestion of carbonates at 25 °C are 10.72‰, 0.220‰, 0.137‰, 0.593‰ for, respectively, 18O/ 16O ratios (1000 ln ??) and three indices that measure proportions of multiply-substituted isotopologues (?47?,?48?,?49?). We also predict that oxygen isotope fractionations follow the mass dependence exponent, ? of 0.5281 (where ?=?18?). These predictions compare favorably to independent experimental constraints for phosphoric acid digestion of calcite, including our new data for fractionations of 13C- 18O bonds (the measured change in ? 47 = 0.23‰) during phosphoric acid digestion of calcite at 25 °C. We have also attempted to evaluate the effect of carbonate cation compositions on phosphoric acid digestion fractionations using cluster models in which disproportionating H 2CO 3 interacts with adjacent cations. These models underestimate the magnitude of isotope fractionations and so must be regarded as unsucsessful, but do reproduce the general trend of variations and temperature dependences of oxygen isotope acid digestion fractionations among different carbonate minerals. We suggest these results present a useful starting point for future, more sophisticated models of the reacting carbonate/acid interface. Examinations of these theoretical predictions and available experimental data suggest cation radius is the most important factor governing the variations of isotope fractionation among different carbonate minerals. We predict a negative correlation between acid digestion fractionation of oxygen isotopes and of 13C- 18O doubly-substituted isotopologues, and use this relationship to estimate the acid digestion fractionation of ?47? for different carbonate minerals. Combined with previous theoretical evaluations of 13C- 18O clumping effects in carbonate minerals, this enables us to predict the temperature calibration relationship for different carbonate clumped isotope thermometers (witherite, calcite, aragonite, dolomite and magnesite), and to compare these predictions with available experimental determinations. The success of our models in capturing several of the features of isotope fractionation during acid digestion supports our hypothesis that phosphoric acid digestion of carbonate minerals involves disproportionation of transition state structures containing H 2CO 3.
Unfolding first-principles band structures.
Ku, Wei; Berlijn, Tom; Lee, Chi-Cheng
2010-05-28
A general method is presented to unfold band structures of first-principles supercell calculations with proper spectral weight, allowing easier visualization of the electronic structure and the degree of broken translational symmetry. The resulting unfolded band structures contain additional rich information from the Kohn-Sham orbitals, and absorb the structure factor that makes them ideal for a direct comparison with angle resolved photoemission spectroscopy experiments. With negligible computational expense via the use of Wannier functions, this simple method has great practical value in the studies of a wide range of materials containing impurities, vacancies, lattice distortions, or spontaneous long-range orders. PMID:20867120
First principles quantum Monte Carlo
J. M. A. Figueiredo
2006-12-07
Present quantum Monte Carlo codes use statistical techniques adapted to find the amplitude of a quantum system or the associated eigenvalues. Thus, they do not use a true physical random source. It is demonstrated that, in fact, quantum probability admits a description based on a specific class of random process at least for the single particle case. Then a first principle Monte Carlo code that exactly simulates quantum dynamics can be constructed. The subtle question concerning how to map random choices in amplitude interferences is explained. Possible advantages of this code in simulating single hit experiments are discussed.
NASA Astrophysics Data System (ADS)
Lin, Wenzhi; Takeuchi, Noboru; Wang, Kangkang; Chinchore, Abhijit; Shi, Meng; Smith, Arthur; Albrithen, Hamad
2011-03-01
Iron/gallium nitride bi-layer structures have potential use for spintronic applications. Therefore, we have carried out an investigation of the initial phase of sub-monolayer iron growth on GaN(0001) pseudo- 1 × 1-1 +1/12 surface. To begin with, we verified an atomically smooth GaN growth surface with the assistance of in situ reflection high energy electron diffraction. STM shows smooth terraces separated by single and double height bilayer atomic steps. About 0.4 ML iron was deposited on the smooth GaN, and the subsequent STM images reveal Fe islands with a height of ~ 2 AA growing in a two-dimensional step-flow mode outward from the GaN step edges of the pseudo-1x1-1+1/12 surface. A clear 6 × 6 structure is observed for the islands. First principles theoretical calculations are being carried out in order to interpret the experimental results. This work is supported by the U.S. Department of Energy (Grant No. DE-FG02-06ER46317).
NASA Astrophysics Data System (ADS)
Gamba, Aldo; Tabacchi, Gloria; Fois, Ettore
2009-09-01
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.
Chamber Clearing First Principles Modeling
Loosmore, G
2009-06-09
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.
NASA Astrophysics Data System (ADS)
Chang, Yunhee; Kim, Howon; Lee, Eui-Sup; Jang, Won-Jun; Kim, Yong-Hyun; Kahng, Se-Jong
2015-03-01
To microscopically understand the mechanisms of electron-induced NO dissociations, we performed first-principles density-functional theory (DFT) calculations for NO-CoTPP on Au(111). We explain the scanning tunneling microscopy (STM) results that the dissociations of NO were induced by both positive and negative voltage pulses with threshold voltages, +0.68 V and 0.74 V, respectively, at 0.1 nA tunneling current, showing power law relations between tunneling current and dissociation yield. To evaluate first-principles thermodynamics of the NO dissociation, we considered not only adsorption-desorption energetics, zero-point energy, and vibrational free energy at experiment temperature from first-principles, but also the chemical potential of NO gas at the cryogenic ultra-high vacuum condition. Using first-principles thermodynamics for the NO dissociation, we argue that the dissociations are induced with inelastic electron tunneling through molecular orbital resonances.
First-principles study of quasi-one-dimensional ?-Na 0.33V 2O 5
C. Ma; R. J. Xiao; H. X. Yang; Z. A. Li; H. R. Zhang; C. Y. Liang; J. Q. Li
2006-01-01
The electronic structure of ?-Na0.33V2O5 has been evaluated using the first-principle density functional theory approach. All energy bands near the Fermi surface (FS) disperse principally along the b-axis direction indicating the quasi-one-dimensionality of this system. The theoretical simulation of the optical property yields reasonable explanations for the notable features revealed in the measurements of the optical spectroscopy. Superconductivity appearing under
Theoretical aspects of light meson spectroscopy
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
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.
Novel magneto-electric multiferroics from first-principles calculations
NASA Astrophysics Data System (ADS)
Varignon, Julien; Bristowe, Nicholas C.; Bousquet, Éric; Ghosez, Philippe
2015-03-01
Interest in first-principles calculations within the multiferroic community has been rapidly on the rise over the last decade. Initially considered as a powerful support to explain experimentally observed behaviours, the trend has evolved and, nowadays, density functional theory calculations have become also an essential predicting tool for identifying original rules to achieve multiferroism and design new magneto-electric compounds. This chapter aims at highlighting the key advances in the field of multiferroics, to which first-principles methods have contributed significantly. The essential theoretical developments that made this research possible are also briefly presented.
Methods for First-Principles Alloy Thermodynamics
NASA Astrophysics Data System (ADS)
van de Walle, Axel
2013-11-01
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.
Kim, Gunwoo; Griffin, John M; Blanc, Frédéric; Haile, Sossina M; Grey, Clare P
2015-03-25
(17)O NMR spectroscopy combined with first-principles calculations was employed to understand the local structure and dynamics of the phosphate ions and protons in the paraelectric phase of the proton conductor CsH2PO4. For the room-temperature structure, the results confirm that one proton (H1) is localized in an asymmetric H-bond (between O1 donor and O2 acceptor oxygen atoms), whereas the H2 proton undergoes rapid exchange between two sites in a hydrogen bond with a symmetric double potential well at a rate ?10(7) Hz. Variable-temperature (17)O NMR spectra recorded from 22 to 214 °C were interpreted by considering different models for the rotation of the phosphate anions. At least two distinct rate constants for rotations about four pseudo C3 axes of the phosphate ion were required in order to achieve good agreement with the experimental data. An activation energy of 0.21 ± 0.06 eV was observed for rotation about the P-O1 axis, with a higher activation energy of 0.50 ± 0.07 eV being obtained for rotation about the P-O2, P-O3(d), and P-O3(a) axes, with the superscripts denoting, respectively, dynamic donor and acceptor oxygen atoms of the H-bond. The higher activation energy of the second process is most likely associated with the cost of breaking an O1-H1 bond. The activation energy of this process is slightly lower than that obtained from the (1)H exchange process (0.70 ± 0.07 eV) (Kim, G.; Blanc, F.; Hu, Y.-Y.; Grey, C. P. J. Phys. Chem. C 2013, 117, 6504-6515) associated with the translational motion of the protons. The relationship between proton jumps and phosphate rotation was analyzed in detail by considering uncorrelated motion, motion of individual PO4 ions and the four connected/H-bonded protons, and concerted motions of adjacent phosphate units, mediated by proton hops. We conclude that, while phosphate rotations aid proton motion, not all phosphate rotations result in proton jumps. PMID:25732257
Unfolding First-Principles Band Structures
Wei Ku; Tom Berlijn; Chi-Cheng Lee
2010-01-01
A general method is presented to unfold band structures of first-principles supercell calculations with proper spectral weight, allowing easier visualization of the electronic structure and the degree of broken translational symmetry. The resulting unfolded band structures contain additional rich information from the Kohn-Sham orbitals, and absorb the structure factor that makes them ideal for a direct comparison with angle resolved
Unfolding first-principles band structures
Wei Ku; T. Berlijn; C.-C. Lee
2011-01-01
A general method [1] is presented to unfold band structures of first-principles supercell calculations with proper spectral weight, allowing easier visualization of the electronic structure and the degree of broken translational symmetry. The resulting unfolded band structures contain additional rich information from the Kohn-Sham orbitals, and absorb the structure factor that makes them ideal for a direct comparison with angle
First-principles prediction of superplastic transition-metal alloys
NASA Astrophysics Data System (ADS)
Souvatzis, P.; Katsnelson, M. I.; Simak, S.; Ahuja, R.; Eriksson, O.; Mohn, P.
2004-07-01
Superplastic transition metal alloys and compounds are predicted from first principles calculations. Provided a suitable tuning of the alloying is done, materials with vanishingly low shear modulus C' have recently been identified among the 3d , 4d , and 5d elements if the valence electron average number is close to 4.24 (i.e., Ti-Ta-Nb-V-Zr-O and Ti-Nb-Ta-Zr-O alloys). The vanishingly low C' elastic constant of these bcc alloys is, according to the joint experimental and theoretical studies [T. Saito , Science 300, 464 (2003)], the crucial material parameter that is responsible for the superplasticity. We predict here, using first principles calculations, that superplastic alloys should also be found for alloys with drastically different valence electron concentrations, i.e., for W-Re-, W-Tc-, Mo-Re-, Mo-Tc-, and Fe-Co-based alloys.
Interface Structure Prediction from First-Principles
Zhao, Xin; Shu, Qiang; Nguyen, Manh Cuong; Wang, Yangang; Ji, Min; Xiang, Hongjun; Ho, Kai-Ming; Gong, Xingao; Wang, Cai-Zhuang
2014-05-08
Information about the atomic structures at solid–solid interfaces is crucial for understanding and predicting the performance of materials. Due to the complexity of the interfaces, it is very challenging to resolve their atomic structures using either experimental techniques or computer simulations. In this paper, we present an efficient first-principles computational method for interface structure prediction based on an adaptive genetic algorithm. This approach significantly reduces the computational cost, while retaining the accuracy of first-principles prediction. The method is applied to the investigation of both stoichiometric and nonstoichiometric SrTiO3 ?3(112)[1?10] grain boundaries with unit cell containing up to 200 atoms. Several novel low-energy structures are discovered, which provide fresh insights into the structure and stability of the grain boundaries.
Theoretical study of single-molecule spectroscopy and vibrational spectroscopy in condensed phases
Yang, Shilong, 1975-
2005-01-01
In this thesis, theoretical models and computer simulations are employed to study several problems of single-molecule spectroscopy and vibrational spectroscopy in condensed phases. The first part of the thesis concentrates ...
First principles materials design for semiconductor spintronics
K. Sato; H. Katayama-Yoshida
2002-01-01
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
Giant magnetoresistance calculated from first principles
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
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.
Origin of Spinel Nanocheckerboards via First Principles
NASA Astrophysics Data System (ADS)
Kornbluth, Mordechai; Marianetti, Chris A.
2015-06-01
Self-organizing nanocheckerboards have been experimentally fabricated in Mn-based spinels but have not yet been explained with first principles. Using density-functional theory, we explain the phase diagram of the ZnMnxGa2 -xO4 system and the origin of nanocheckerboards. We predict total phase separation at zero temperature and then show the combination of kinetics, thermodynamics, and Jahn-Teller physics that generates the system's observed behavior. We find that the {011 } surfaces are strongly preferred energetically, which mandates checkerboard ordering by purely geometrical considerations.
Aqueous solvation of methane from first principles
Lorenzo Rossato; Francesco Rossetto; Pier Luigi Silvestrelli
2012-01-25
Structural, dynamical, bonding, and electronic properties of water molecules around a soluted methane molecule are studied from first principles. The results are compatible with experiments and qualitatively support the conclusions of recent classical Molecular Dynamics simulations concerning the controversial issue on the presence of "immobilized" water molecules around hydrophobic groups: the hydrophobic solute slightly reduces (by a less than 2 factor) the mobility of many surrounding water molecules rather than immobilizing just the few ones which are closest to methane, similarly to what obtained by previous first-principles simulations of soluted methanol. Moreover, the rotational slowing down is compatible with that one predicted on the basis of the excluded volume fraction, which leads to a slower Hydrogen bond-exchange rate. The analysis of simulations performed at different temperatures suggests that the target temperature of the soluted system must be carefully chosen, in order to avoid artificial slowing-down effects. By generating maximally-localized Wannier functions, a detailed description of the polarization effects in both solute and solvent molecules is obtained, which better characterizes the solvation process.
Gubler, Joël; Finkelmann, Arndt R; Reiher, Markus
2013-12-16
[Fe] hydrogenase is a hydrogen activating enzyme that features a monoiron active site, which can be well characterized by Mössbauer spectroscopy. Mössbauer spectra have been measured of the CO and CN(-) inhibited species as well as under turnover conditions [Shima, S. et al., J. Am. Chem. Soc., 2005, 127, 10430]. This study presents calculated Mössbauer parameters for various active-site models of [Fe] hydrogenase to provide structural information about the species observed in experiment. Because theoretical Mössbauer spectroscopy requires the parametrization of observables from first-principles calculations (i.e., electric-field gradients and contact densities) to the experimental observables (i.e., quadrupole splittings and isomer shifts), nonrelativistic and relativistic density functional theory methods are parametrized against a reference set of Fe complexes specifically selected for the application to the Fe center in [Fe] hydrogenase. With this methodology, the measured parameters for the CO and CN(-) inhibited complexes can be reproduced. Evidence for the protonation states of the hydroxyl group in close proximity to the active site and for the thiolate ligand, which could participate in proton transfer, is obtained. The unknown resting state measured in the presence of the substrate and under pure H2 atmosphere is identified to be a water-coordinated complex. Consistent with previous assignments based on infrared and X-ray absorption near-edge spectroscopy, all measured Mössbauer data can be reproduced with the active site's iron atom being in oxidation state +2. PMID:24328345
Iron diffusion from first principles calculations
NASA Astrophysics Data System (ADS)
Wann, E.; Ammann, M. W.; Vocadlo, L.; Wood, I. G.; Lord, O. T.; Brodholt, J. P.; Dobson, D. P.
2013-12-01
The cores of Earth and other terrestrial planets are made up largely of iron1 and it is therefore very important to understand iron's physical properties. Chemical diffusion is one such property and is central to many processes, such as crystal growth, and viscosity. Debate still surrounds the explanation for the seismologically observed anisotropy of the inner core2, and hypotheses include convection3, anisotropic growth4 and dendritic growth5, all of which depend on diffusion. In addition to this, the main deformation mechanism at the inner-outer core boundary is believed to be diffusion creep6. It is clear, therefore, that to gain a comprehensive understanding of the core, a thorough understanding of diffusion is necessary. The extremely high pressures and temperatures of the Earth's core make experiments at these conditions a challenge. Low-temperature and low-pressure experimental data must be extrapolated across a very wide gap to reach the relevant conditions, resulting in very poorly constrained values for diffusivity and viscosity. In addition to these dangers of extrapolation, preliminary results show that magnetisation plays a major role in the activation energies for diffusion at low pressures therefore creating a break down in homologous scaling to high pressures. First principles calculations provide a means of investigating diffusivity at core conditions, have already been shown to be in very good agreement with experiments7, and will certainly provide a better estimate for diffusivity than extrapolation. Here, we present first principles simulations of self-diffusion in solid iron for the FCC, BCC and HCP structures at core conditions in addition to low-temperature and low-pressure calculations relevant to experimental data. 1. Birch, F. Density and composition of mantle and core. Journal of Geophysical Research 69, 4377-4388 (1964). 2. Irving, J. C. E. & Deuss, A. Hemispherical structure in inner core velocity anisotropy. Journal of Geophysical Research 116, B04307 (2011). 3. Buffett, B. A. Onset and orientation of convection in the inner core. Geophysical Journal International 179, 711-719 (2009). 4. Bergman, M. Measurements of electric anisotropy due to solidification texturing and the implications for the Earth's inner core. Nature 389, 60-63 (1997). 5. Deguen, R. & Cardin, P. Thermochemical convection in Earth's inner core. Geophysical Journal International 187, 1101-1118 (2011). 6. Reaman, D. M., Daehn, G. S. & Panero, W. R. Predictive mechanism for anisotropy development in the Earth's inner core. Earth and Planetary Science Letters 312, 437-442 (2011). 7. Ammann, M. W., Brodholt, J. P., Wookey, J. & Dobson, D. P. First-principles constraints on diffusion in lower-mantle minerals and a weak D'' layer. Nature 465, 462-5 (2010).
Ferroelectricity in the Dion-Jacobson CsBiNb2O7 from first principles
NASA Astrophysics Data System (ADS)
Fennie, Craig J.; Rabe, Karin M.
2006-06-01
We have studied ferroelectricity in Dion-Jacobson CsBiNb2O7 from first principles. Using group-theoretical analysis and first-principles density functional calculations of the total energy and phonons, we perform a systematic study of the energy surface around a paraelectric prototypic phase. Our results suggest that CsBiNb2O7 is a ferroelectric with a polarization of Ps=40?Ccm-2. We propose further experiments to clarify this point.
First principles potential for the cytosine dimer.
Manukyan, Artür; Tekin, Adem
2015-06-14
We developed a new first principles potential for the cytosine dimer. The ab initio calculations were performed with a DFT-SAPT combination of the symmetry-adapted perturbation method and density functional theory, and fitted to a model site-site functional form. The model potential was used to predict cluster structures up to cytosine hexamers. The global cluster structure optimizations showed that the new potential is able to reproduce some of the 2D filament structures. Moreover many new non-planar cytosine cluster structures were also discovered. Interaction energies of these clusters were compared with B3LYP-D, MP2, SCS-MP2, SCS-MI-MP2 and AMBER. It has been shown that the model agrees well with all ab initio methods, especially for the cytosine hexamer. The model potential outperforms the AMBER force field and therefore it can be exploited to study more challenging larger systems. PMID:25971940
First principles approach to ionicity of fragments
NASA Astrophysics Data System (ADS)
Pilania, Ghanshyam; Liu, Xiang-Yang; Valone, Steven M.
2015-02-01
We develop a first principles approach towards the ionicity of fragments. In contrast to the bond ionicity, the fragment ionicity refers to an electronic property of the constituents of a larger system, which may vary from a single atom to a functional group or a unit cell to a crystal. The fragment ionicity is quantitatively defined in terms of the coefficients of contributing charge states in a superposition of valence configurations of the system. Utilizing the constrained density functional theory-based computations, a practical method to compute the fragment ionicity from valence electron charge densities, suitably decomposed according to the Fragment Hamiltonian (FH) model prescription for those electron densities, is presented for the first time. The adopted approach is illustrated using BeO, MgO and CaO diatomic molecules as simple examples. The results are compared and discussed with respect to the bond ionicity scales of Phillips and Pauling.
Numerical Inductance Calculations Based on First Principles
Shatz, Lisa F.; Christensen, Craig W.
2014-01-01
A method of calculating inductances based on first principles is presented, which has the advantage over the more popular simulators in that fundamental formulas are explicitly used so that a deeper understanding of the inductance calculation is obtained with no need for explicit discretization of the inductor. It also has the advantage over the traditional method of formulas or table lookups in that it can be used for a wider range of configurations. It relies on the use of fast computers with a sophisticated mathematical computing language such as Mathematica to perform the required integration numerically so that the researcher can focus on the physics of the inductance calculation and not on the numerical integration. PMID:25402467
First-principles simulations of thiophene oligomers
NASA Astrophysics Data System (ADS)
Scherlis, Damian; Marzari, Nicola
2003-03-01
Conducting polymers, extensively investigated for their use in electronic and nanotechnology applications, have recently gained prominence for their possible use as molecular actuators in mechanical and bioengineering devices. We have focused our efforts on thiophene-based compounds, a class of materials that can be designed for high stress generation and large linear displacement (actuation strain), ideally outperforming mammalian muscle. Key features for the development of these materials are the microscopic binding properties of thiophene and thiophene oligomers stacks, where applied electric fields lead to oxidation and enhanced pi-pi bonding. We have completed the structural studies of neutral and charged oligothiophene dimers, in the search for efficient dimerization mechanisms. A comparison between different density-functional and quantum-chemistry approaches is critically presented, as are solvation effects, described in this work with a combination of first-principles molecular dynamics and a QM/MM approach for the solvating medium.
Accurate Thermal Conductivities from First Principles
NASA Astrophysics Data System (ADS)
Carbogno, Christian
2015-03-01
In spite of significant research efforts, a first-principles determination of the thermal conductivity at high temperatures has remained elusive. On the one hand, Boltzmann transport techniques that include anharmonic effects in the nuclear dynamics only perturbatively become inaccurate or inapplicable under such conditions. On the other hand, non-equilibrium molecular dynamics (MD) methods suffer from enormous finite-size artifacts in the computationally feasible supercells, which prevent an accurate extrapolation to the bulk limit of the thermal conductivity. In this work, we overcome this limitation by performing ab initio MD simulations in thermodynamic equilibrium that account for all orders of anharmonicity. The thermal conductivity is then assessed from the auto-correlation function of the heat flux using the Green-Kubo formalism. Foremost, we discuss the fundamental theory underlying a first-principles definition of the heat flux using the virial theorem. We validate our approach and in particular the techniques developed to overcome finite time and size effects, e.g., by inspecting silicon, the thermal conductivity of which is particularly challenging to converge. Furthermore, we use this framework to investigate the thermal conductivity of ZrO2, which is known for its high degree of anharmonicity. Our calculations shed light on the heat resistance mechanism active in this material, which eventually allows us to discuss how the thermal conductivity can be controlled by doping and co-doping. This work has been performed in collaboration with R. Ramprasad (University of Connecticut), C. G. Levi and C. G. Van de Walle (University of California Santa Barbara).
NASA Astrophysics Data System (ADS)
Rajesh, Chinagandham; Majumder, Chiranjib
2008-01-01
A systematic theoretical study of the PbnM (M =C, Al, In, Mg, Sr, Ba, and Pb; n =8, 10, 12, and 14) clusters have been investigated to explore the effect of impurity atoms on the structure and electronic properties of lead clusters. The calculations were carried out using the density functional theory with generalized gradient approximation for exchange-correlation potential. Extensive search based on large numbers of initial configurations has been carried out to locate the stable isomers of PbnM clusters. The results revealed that the location of the impurity atom depends on the nature of interaction between the impurity atom and the host cluster and the size of the impurity atom. Whereas, the impurity atoms smaller than Pb favor to occupy the endohedral position, the larger atoms form exohedral capping of the host cluster. The stability of these clusters has been analyzed based on the average binding energy, interaction energy of the impurity atoms, and the energy gap between the highest occupied and lowest unoccupied energy levels (HLG). Based on the energetics, it is found that p-p interaction dominates over the s-p interaction and smaller size atoms interact more strongly. The stability analysis of these clusters suggests that, while the substitution of Pb by C or Al enhances the stability of the Pbn clusters, Mg lowers the stability. Further investigations of the stability of PbnM clusters reveal that the interplay between the atomic and electronic structure is crucial to understand the stability of these clusters. The energy gap analysis reveals that, while the substitution of Mg atom widens the HLG, all other elements reduce the gap of the PbnM clusters.
Theoretical and experimental investigation of polarization spectroscopy
Hanna, Sherif Fayez
2001-01-01
, Sweden. Polarization spectroscopy saturation curves in the co-propagating beam geometry from the excitation of OH A ²[]?-X²[] (0,0) at the Q?(8) line for sub-atmospheric pressures have been fitted to the proposed model. The model proposed in this work...
First Principles Studies of Complex Oxides for Energy Applications
NASA Astrophysics Data System (ADS)
Rebola, Alejandro Federico
In this thesis, I present results and analyses of first principles calculations based on Density Functional Theory (DFT) to examine the structural, electronic and lattice properties of two complex oxide systems: the misfit-layered thermoelectric Ca3Co4O9 (CCO) and a polar interface containing the solid oxide electrolyte LaGaO3 (LGO). In Chapter 1, I present a general introduction and the motivation for the study of these systems. This is followed by a brief summary of the fundamental ideas in DFT and the different theoretical methods employed in my calculations in Chapter 2. In Chapter 3, I present results and discussion related to the electronic structure of CCO calculated by means of increasing order Fibonacci approximants. In Chapter 4, I focus on the lattice and thermal properties of this material, in particular, the calculation of the thermal conductivity by combining first principles results with the Boltzmann transport equation. In Chapter 5, I propose and analyze the computational design of a polar interface in order to increase the ionic conductivity of LGO. Finally, in Chapter 6 I present a summary and the conclusions of my studies.
First principles studies of multiferroic materials.
Picozzi, Silvia; Ederer, Claude
2009-07-29
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 BiFeO(3) 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 BiFeO(3), 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 HoMnO(3)), 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. PMID:21828545
First Principle Modeling of Energetic Storm Particles
NASA Astrophysics Data System (ADS)
Shiota, Daikou; Kataoka, Ryuho; Sugiyama, Tooru; Kusano, Kanya
The origin and cause of solar energetic particles (SEPs) has been one of the most important topics in the field of space weather research. Energetic storm particle (ESP) event is a subset of SEP events, which is characterized as proton flux enhancement of the relatively low energy range (¡10 MeV) associated with interplanetary shock arrivals at the Earth. Here we perform the first principle modeling of ESP and quantitatively compare the results with in-situ observations, using hybrid plasma simulation. In this paper we focus on the coronal mass ejection event on 13 Dec 2006. As the input parameters for the simulation, fundamental shock parameters of the plasma beta, Alfven mach, and shock angle are given from a global solar wind simulation of Kataoka et al. (2009). As a result, it is found that proton flux and spectral index of ESP event can be quantitatively reproduced by the interlocked simulation, which is driven by the realistic shock parameters without injection particles. We suggest that ESPs can be originated from thermal solar wind plasma accelerated by the passage of interplanetary shocks.
THERMODYNAMIC MODELING AND FIRST-PRINCIPLES CALCULATIONS
Turchi, P; Abrikosov, I; Burton, B; Fries, S; Grimvall, G; Kaufman, L; Korzhavyi, P; Manga, R; Ohno, M; Pisch, A; Scott, A; Zhang, W
2005-12-15
The increased application of quantum mechanical-based methodologies to the study of alloy stability has required a re-assessment of the field. The focus is mainly on inorganic materials in the solid state. In a first part, after a brief overview of the so-called ab initio methods with their approximations, constraints, and limitations, recommendations are made for a good usage of first-principles codes with a set of qualifiers. Examples are given to illustrate the power and the limitations of ab initio codes. However, despite the ''success'' of these methodologies, thermodynamics of complex multi-component alloys, as used in engineering applications, requires a more versatile approach presently afforded within CALPHAD. Hence, in a second part, the links that presently exist between ab initio methodologies, experiments, and CALPHAD approach are examined with illustrations. Finally, the issues of dynamical instability and of the role of lattice vibrations that still constitute the subject of ample discussions within the CALPHAD community are revisited in the light of the current knowledge with a set of recommendations.
Safeguards First Principle Initiative (SFPI) Cost Model
Mary Alice Price
2010-07-11
The Nevada Test Site (NTS) began operating Material Control and Accountability (MC&A) under the Safeguards First Principle Initiative (SFPI), a risk-based and cost-effective program, in December 2006. The NTS SFPI Comprehensive Assessment of Safeguards Systems (COMPASS) Model is made up of specific elements (MC&A plan, graded safeguards, accounting systems, measurements, containment, surveillance, physical inventories, shipper/receiver differences, assessments/performance tests) and various sub-elements, which are each assigned effectiveness and contribution factors that when weighted and rated reflect the health of the MC&A program. The MC&A Cost Model, using an Excel workbook, calculates budget and/or actual costs using these same elements/sub-elements resulting in total costs and effectiveness costs per element/sub-element. These calculations allow management to identify how costs are distributed for each element/sub-element. The Cost Model, as part of the SFPI program review process, enables management to determine if spending is appropriate for each element/sub-element.
Principles of silicon surface chemistry from first principles
Doren, D.J. [Univ. of Delaware Newark, DE (United States)
1996-10-01
First principles theoretical studies of dissociative adsorption of H{sub 2}, H{sub 2}O, SiH{sub 4} and other species on Si(100)-2x1 demonstrate some common principles that permit qualitative understanding of the mechanisms of reactive adsorption on Si. The structures of transition states and the interactions among surface sites can also be understood in terms of correlations between surface structure and local electron density. For example, the transition states for dissociative adsorption involve buckled surface dimers, which present both electrophilic and nucleophilic reaction sites and allow efficient addition across the dimer. A surface Diels-Alder reaction will also be described, in which symmetric addition to an unbuckled surface dimer is allowed by orbital symmetry. The Diets-Alder product establishes novel reactive surface sites that may be useful for subsequent surface modification. This work has been done in collaboration with Sharmila Pai, Robert Konecny and Anita Robinson Brown.
A metallic superhard boron carbide: first-principles calculations.
Ma, Mengdong; Yang, Bingchao; Li, Zihe; Hu, Meng; Wang, Qianqian; Cui, Lin; Yu, Dongli; He, Julong
2015-04-21
A monoclinic BC3 phase (denoted M-BC3) has been predicted using first principles calculations. The M-BC3 structure is formed by alternately stacking sequences of metallic BC-layers and insulating C atom layers, thus, the structure exhibits two-dimensional conductivity. Its stability has been confirmed by our calculations of the total energy, elastic constants, and phonon frequencies. The pressure of phase transition from graphite-like BC3 to M-BC3 is calculated to be 9.3 GPa, and the theoretical Vickers hardness of M-BC3 is 43.8 GPa, this value indicates that the compound is a potentially superhard material. By comparing Raman spectral calculations of M-BC3 and previously proposed structures with the experimental data, we speculate that the experimentally synthesized BC3 crystal may simultaneously contain M-BC3 and Pmma-b phases. PMID:25772428
First-principles study of decagonal quasicrystals
NASA Astrophysics Data System (ADS)
Al-Lehyani, Ibrahim Hamdan
This thesis studies the energetics of decagonal quasicrystals using ab-initio methods. First, we extend the generalized pseudopotential theory (GPT) to treat ternary transition metal (TM) aluminides. The GPT interactions are decomposed to pair and many-body interactions that allow efficient calculations of total energies. In aluminum-rich systems treated at the pair-potential level, one practical limitation is a transition-metal over-binding that creates an unrealistic TM-TM attraction at short separations. An additional potential term is added for systems with short TM atom separations, formally folding repulsive contributions of the three- and higher-body interactions into the pair potentials. To do this, we have performed numerical ab-initio total-energy calculations using an Al-Co-Ni compound in a particular quasicrystalline approximant structure. The results allow us to fit a short-ranged, many-body correction of the form a( r0/r)b to the GPT pair potentials for Co-Co, Co-Ni, and Ni-Ni interactions. We employ the corrected potentials to predict the structure of a decagonal quasicrystal from first-principles considerations. The resulting structure obeys a nearly deterministic decoration of tiles on a hierarchy of length scales related by powers of tau, the golden mean. Second, an investigation of matching rules in Al-Co-Cu quasicrystals using a form of tile Hamiltonian (TH) reveals several results. Phason flips that replace a star-hexagon pair with a pair of boats lower the energy. In Penrose tilings, quasiperiodicity is forced by arrow matching rules on rhombus edges. The edge orientation in Al-Co-Cu is due to Co/Cu chemical ordering. Tile edges meet in vertices with 72° or 144° angles. We find strong interactions between edge orientations at 72° vertices that force a type of matching rule. Comparisons between the ab-initio methods and pair potentials are presented. Lastly, we explore the applicability of the locally self-consistent multiple scattering method (LSMS) in the energy calculations of our quasicrystal models. This is an O(N) all-electron method, which makes calculations both faster and more accurate, in principle, than other available ab-initio methods.
Buannic, Lucienne; Blanc, Frédéric; Middlemiss, Derek S; Grey, Clare P
2012-09-01
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
Ethanol adsorbed on ice: A first-principles study C. Thierfelder*
Schmidt, Wolf Gero
of ethanol molecules on the ice Ih basal plane. Apart from smooth surfaces, also substrate modelsV for the liquid- vapor interface24 and 0.59 eV for the ice surface.6 Using the adsorption of ethanol on the ice IhEthanol adsorbed on ice: A first-principles study C. Thierfelder* Centre of Theoretical Chemistry
First Principles Simulations of Ice Surfaces , C. Thierfelder, W. G. Schmidt, P. Schwerdtfeger
Schmidt, Wolf Gero
of hexagonal ice Ih is investigated from first principles density functional calculations. Surface formation-melting process. For a review of early work on the ice Ih(0001) basal plane, see, e.g., [2]. In recent years, combined experimental and theoretical works have found the ice Ih(0001) surface to be fully bilayer
Theoretical Modeling of Various Spectroscopies for Cuprates and Topological Insulators
NASA Astrophysics Data System (ADS)
Basak, Susmita
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
First principles study of O defects in CdSe
NASA Astrophysics Data System (ADS)
T-Thienprasert, J.; Limpijumnong, S.; Du, M.-H.; Singh, D. J.
2012-08-01
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.
Theoretical aspects of resonant X-ray emission spectroscopy
A. Kotani
2004-01-01
We report recent theoretical topics for resonant X-ray emission spectroscopy (RXES) in d and f electron systems: (1) RXES in high Tc cuprates, (2) effects of electric quadrupole excitation, and (3) magnetic circular dichroism (MCD) in RXES for ferromagnetic systems. In high Tc cuprates, RXES with Cu 1s excitation detects a 6eV charge transfer excitation whose intensity is affected by
First principles study of biomineral hydroxyapatite
NASA Astrophysics Data System (ADS)
Slepko, Alexander
2010-03-01
Hydroxyapatite (HA) [Ca10(PO4)6(OH)2] is one of the most abundant materials in mammal bone. It crystallizes within the spaces between the tropocollagen chains and strengthens the bone tissue. The mineral content of human bone increases with age reaching a maximum value from which it starts to decrease leading to diseases such as osteomalacia. Therefore, an emergent application of this study is bone repair and the production of synthetic bone. Despite its importance, little is known about the growth of HA crystallites in bones. Nor is it well understood how the HA attaches to protein chains and interacts with the surrounding aqueous solution. Using density functional theory (DFT) we calculate the theoretical ground state structure, electronic and vibration properties of hexagonal HA. We find several low energy structures and analyze the energy barriers for spontaneous phase transitions. We calculate the phonon density of states and study the surface energetics for different orientations. We identify the surfaces with highest reactivity using the frontier orbital approach and analyze interactions between these surfaces and water molecules/amino acids.
First principles characterization of silicate sites in clay surfaces.
Alvim, Raphael S; Miranda, Caetano R
2015-02-21
Aluminosilicate clays like Montmorillonite (MMT) and Muscovite Mica (MT) have siloxane cavities on the basal plane. The hydroxyl groups localized in these cavities and van der Waals (vdW) forces contribute significantly to adsorption processes. However, the basal sites are found to be difficult to characterize experimentally. Here, (001) surfaces of MMT and MT clays were investigated using first-principles calculations to understand how these silicate surface sites are influenced by hydroxyl groups and the effective role of inner layer vdW interactions. Based on density-functional theory (DFT) within the generalized gradient approximation (GGA), different types of exchange-correlation functionals were tested to check the effect of vdW dispersion correction. Noncontact atomic force microscopy (nc-AFM), X-ray absorption spectroscopy (XAS) in the near-edge region and solid-state nuclear magnetic resonance (SS-NMR) spectroscopy were simulated. In both clays, the oxygen surface sites are directly affected by the intralayer interaction through hydroxyl groups. Our results indicated that the chemical environment of the hydroxyl groups is distinct in the MMT and MT structures. The vdW correction was essential for a better description of the surface oxygen sites and correctly describes the similarity between both clays. Particularly, the bulk apical oxygen sites in the MT structure are less influenced by vdW interaction. Compared to MMT, the silicon surface sites of MT are more sensitive to the intralayer changes in Si-Oapical-Al and with less effect of the hydroxyl groups. These results provide a clear understanding of influence of the siloxane cavity on the oxygen and silicon surface sites in aluminosilicates. PMID:25592132
NMR characterization of hydrocarbon adsorption on calcite surfaces: A first principles study
NASA Astrophysics Data System (ADS)
Bevilaqua, Rochele C. A.; Rigo, Vagner A.; Veríssimo-Alves, Marcos; Miranda, Caetano R.
2014-11-01
The electronic and coordination environment of minerals surfaces, as calcite, are very difficult to characterize experimentally. This is mainly due to the fact that there are relatively few spectroscopic techniques able to detect Ca2+. Since calcite is a major constituent of sedimentary rocks in oil reservoir, a more detailed characterization of the interaction between hydrocarbon molecules and mineral surfaces is highly desirable. Here we perform a first principles study on the adsorption of hydrocarbon molecules on calcite surface (CaCO3 ( {10bar 14} )). The simulations were based on Density Functional Theory with Solid State Nuclear Magnetic Resonance (SS-NMR) calculations. The Gauge-Including Projector Augmented Wave method was used to compute mainly SS-NMR parameters for 43Ca, 13C, and 17O in calcite surface. It was possible to assign the peaks in the theoretical NMR spectra for all structures studied. Besides showing different chemical shifts for atoms located on different environments (bulk and surface) for calcite, the results also display changes on the chemical shift, mainly for Ca sites, when the hydrocarbon molecules are present. Even though the interaction of the benzene molecule with the calcite surface is weak, there is a clearly distinguishable displacement of the signal of the Ca sites over which the hydrocarbon molecule is located. A similar effect is also observed for hexane adsorption. Through NMR spectroscopy, we show that aromatic and alkane hydrocarbon molecules adsorbed on carbonate surfaces can be differentiated.
First principles molecular dynamics without self-consistent field optimization
Souvatzis, Petros, E-mail: petros.souvatsiz@fysik.uu.se [Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Box 516, SE-75120 Uppsala (Sweden)] [Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Box 516, SE-75120 Uppsala (Sweden); Niklasson, Anders M. N., E-mail: amn@lanl.gov [Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)] [Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)
2014-01-28
We present a first principles molecular dynamics approach that is based on time-reversible extended Lagrangian Born-Oppenheimer molecular dynamics [A. M. N. Niklasson, Phys. Rev. Lett. 100, 123004 (2008)] in the limit of vanishing self-consistent field optimization. The optimization-free dynamics keeps the computational cost to a minimum and typically provides molecular trajectories that closely follow the exact Born-Oppenheimer potential energy surface. Only one single diagonalization and Hamiltonian (or Fockian) construction are required in each integration time step. The proposed dynamics is derived for a general free-energy potential surface valid at finite electronic temperatures within hybrid density functional theory. Even in the event of irregular functional behavior that may cause a dynamical instability, the optimization-free limit represents a natural starting guess for force calculations that may require a more elaborate iterative electronic ground state optimization. Our optimization-free dynamics thus represents a flexible theoretical framework for a broad and general class of ab initio molecular dynamics simulations.
First Principles Structure Calculations Using the General Potential Lapw Method
NASA Astrophysics Data System (ADS)
Wei, Su-Huai
We have developed a completely general first principles self-consistent full-potential linearized-augmented-plane -wave (LAPW) method program within the density functional formalism to calculate electronic band structure, total energy, pressure and other quantities. No symmetry assumptions are used for the crystal structure. Shape unrestricted charge densities and potentials are calculated inside muffin -tin (MT) spheres as well as in the interstitial regions. All contributions to the Hamiltonian matrix elements are completely taken into account. The core states are treated fully relativistically using the spherical part of the potential only. Scalar relativistic effects are included for the band-states, and spin-orbit coupling is included using a second variation procedure. Both core states and valence states are treated self-consistently, the frozen core approximation is not required. The fast Fourier transformation method is used wherever it is applicable, and this greatly improves the efficiency. This state-of-the-art program has been tested extensively to check the accuracy and convergence properties by comparing calculated electronic band structures, ground state properties, equations of state and cohesive energies for bulk W and GaAs with other theoretical calculations and experimental results. It has been successfully applied to calculate and predict structural and metal-insulator phase transitions for close-packed crystal BaSe and BaTe and the geometric structure of the d-band metal W(001) surface. The results are in generally good agreement with experiment.
Methane adsorption on graphene from first principles including dispersion interaction
Schmidt, Wolf Gero
. Among others, it was investigated by heat capacity experiments [4], adsorption isotherms [5], NuclearMethane adsorption on graphene from first principles including dispersion interaction C functional calculations Graphene Alkanes Physical adsorption The methanegraphene interaction is studied
Inelastic transport In molecular junctions from first principles
Kim, Sejoong, Ph. D. Massachusetts Institute of Technology
2012-01-01
This work is dedicated to development of a first-principle approach to study electron-vibration interactions on quantum transport properties. In the first part we discuss a general implementation for inelastic transport ...
Ballistic Transport in Nanostructures from First-Principles Simulations
Marzari, Nicola
We developed and implemented a first-principles based theory of the Landauer ballistic conductance, to determine the transport properties of nanostructures and molecular-electronics devices. Our approach starts from a ...
Time-dependent first-principles approaches to PV materials
Miyamoto, Yoshiyuki [Nanosystem Research Institute, AIST, Central 2, 1-1-1, Umezono, Tsukuba, 305-8568 (Japan)
2013-12-10
Computational scheme for designing photovoltaic (PV) materials is presented. First-principles electron dynamics of photo-excitation and subsequent electron-hole splitting is performed based on the time-dependent density functional theory. Photo-induced enhancement of dipole moment was observed in a polar crystal and a donor-acceptor molecular pair. These experiences will pave a way to design PV material from first-principles simulations.
Theoretical Study of the Vibrational Spectroscopy of the Ethyl Radical
NASA Astrophysics Data System (ADS)
Tabor, Daniel P.; Sibert, Edwin. L. Sibert, Iii
2013-06-01
The rich spectroscopy of the ethyl radical has attracted the attention of several experimental and theoretical investigations. The purpose of these studies was to elucidate the signatures of hyperconjugation, torsion, inversion, and Fermi coupling in the molecular spectra. Due to the number of degrees of freedom in the system, previous theoretical studies have implemented reduced-dimensional models. Our ultimate goal is a full-dimensional theoretical treatment of the vibrations using both Van Vleck and variational approaches. The methods will be combined with the potential that we have calculated using the CCSD(T) method on the cc-pVTZ basis set. In this talk we will discuss our initial work, which builds up from these reduced-dimensional models. Our calculations use coordinates that exploit the system's G_{12} PI symmetry in a simple fashion. By systematically adding more degrees of freedom to our model, we can determine the effects of specific couplings on the spectroscopy. T. Häber, A. C. Blair, D. J. Nesbitt and M. D. Schuder J. Chem. Phys. {124}, 054316, (2006). G .E. Douberly, unpublished. R. S. Bhatta, A. Gao and D. S. Perry J. Mol. Struct.: THEOCHEM {941}, 22, (2010).
Xiao-Wei Sun; Qi-Feng Chen; Xiang-Rong Chen; Ling-Cang Cai; Fu-Qian Jing
2011-01-01
The authors have presented a detailed investigation on the phase stabilities and electronic properties of ideal stoichiometric platinum carbide (PtC) in the rock-salt (RS) and zinc-blende (ZB) structures under high pressure. Theoretical calculations are performed using the first-principles pseudopotential density functional method, in which we employ the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof form and local density approximation (LDA)
First-principles study on the electronic structure of bismuth transition-metal oxides
T. Shishidou; N. Mikamo; Y. Uratani; F. Ishii; T. Oguchi
2004-01-01
The electronic structure, magnetic and electric properties, and lattice stability of multiferroic BiMnO3 as a typical system in perovskite Bi transition-metal oxides (BiMO3) are studied from first principles. It is demonstrated theoretically for the first time that the orbital ordering within the Mn eg orbitals is actually realized in BiMnO3, being consistent with crystallographic data, and plays a crucial role
Francesco Buonocore
2010-01-01
In this paper, we investigate the adsorption mechanisms at the interface between carbon nanotubes and metal electrodes that can influence the Schottky barrier (SB). We developed a theoretical model based on the first-principles density functional theory for the interaction of an armchair single-wall carbon nanotube (SWNT) with either Au(111) or Pd(111) surface. We considered the side-wall contact by modelling the
Transport and first-principles study of novel thermoelectric materials
NASA Astrophysics Data System (ADS)
Chi, Hang
Thermoelectric materials can recover waste industrial heat and convert it to electricity as well as provide efficient local cooling of electronic devices. The efficiency of such environmentally responsible and exceptionally reliable solid state energy conversion is determined by the dimensionless figure-of-merit ZT = alpha2 sigmaT/kappa, where alpha is the Seebeck coefficient, sigma is the electrical conductivity, kappa is the thermal conductivity, and T is the absolute temperature. The goal of the thesis is to (i) illustrate the physics to achieve high ZT of advanced thermoelectric materials and (ii) explore fundamental structure and transport properties in novel condensed matter systems, via an approach combining comprehensive experimental techniques and state-of-the-art first-principles simulation methods. Thermo-galvanomagnetic transport coefficients are derived from Onsager's reciprocal relations and evaluated via solving Boltzmann transport equation using Fermi-Dirac statistics, under the relaxation time approximation. Such understanding provides insights on enhancing ZT through two physically intuitive and very effective routes: (i) improving power factor PF = alpha2sigma; and (ii) reducing thermal conductivity kappa, as demonstrated in the cases of Mg2Si1-xSnx solid solution and Ge/Te double substituted skutterudites CoSb3(1-x)Ge1.5x Te1.5x, respectively. Motivated by recent theoretical predictions of enhanced thermoelectric performance in highly mismatched alloys, ZnTe:N molecular beam epitaxy (MBE) films deposited on GaAs (100) substrates are carefully examined, which leads to a surprising discovery of significant phonon-drag thermopower (reaching 1-2 mV/K-1) at ~13 K. Further systematic study in Bi2Te3 MBE thin films grown on sapphire (0001) and/or BaF2 (111) substrates, reveal that the peak of phonon drag can be tuned by the choice of substrates with different Debye temperatures. Moreover, the detailed transport and structure studies of Bi2-xTl xTe3 single crystals demonstrate that thallium doping leads to a bulk insulating state for such a topological insulator, which opens an avenue for further investigations of transport phenomena related to surface states. Finally, using the combined theoretical and experimental approaches, a new layered transition metal dichalcogenide type of ground state of Cu 2Se is proposed, which exhibits extraordinary weak anti-localization type of magnetoresistance at liquid helium temperatures.
Nanoflare Statistics from First Principles: Fractal Geometry and Temperature Synthesis
NASA Astrophysics Data System (ADS)
Aschwanden, Markus J.; Parnell, Clare E.
2002-06-01
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).
Interface Enhancement of Gilbert Damping from First Principles
NASA Astrophysics Data System (ADS)
Liu, Yi; Yuan, Zhe; Wesselink, R. J. H.; Starikov, Anton A.; Kelly, Paul J.
2014-11-01
The enhancement of Gilbert damping observed for Ni80Fe20 (Py) films in contact with the nonmagnetic metals Cu, Pd, Ta, and Pt is quantitatively reproduced using first-principles scattering calculations. The "spin-pumping" theory that qualitatively explains its dependence on the Py thickness is generalized to include a number of extra factors known to be important for spin transport through interfaces. Determining the parameters in this theory from first principles shows that interface spin flipping makes an essential contribution to the damping enhancement. Without it, a much shorter spin-flip diffusion length for Pt would be needed than the value we calculate independently.
First-principles modeling of hard and soft matter
NASA Astrophysics Data System (ADS)
Car, Roberto
2013-03-01
Electronic and atomistic processes are key to bio-inspired functional materials and nanocatalysts for energy applications. This talk will review recent simulation studies and discuss the challenges that first-principles quantum mechanical approaches face when addressing these issues. Electronic and atomistic processes are key to bio-inspired functional materials and nanocatalysts for energy applications. This talk will review recent simulation studies and discuss the challenges that first-principles quantum mechanical approaches face when addressing these issues. Supported by DOE-DE-FG02-06ER-46344, DOE-DE-SC0008626, DOE-DE-SC0005180, and NSF-CHE-0956500.
First-principles study of multiferroic oxides Tamio OGUCHI
Katsumoto, Shingo
on double-perovskite bis- muth transition-metal oxides. 1 Introduction Any condensed matter system reveals) order of the microscopic de- grees of freedom, magnetic moments, electric dipoles and atomicFirst-principles study of multiferroic oxides Tamio OGUCHI Department of Quantum Matter, ADSM
Magnetoelectric Phase Control in Epitaxial Oxides from First Principles
Craig Fennie; Karin Rabe
2007-01-01
We propose a design strategy - based on the coupling of spins, optical phonons, and strain - for systems in which magnetic (electric) phase control can be achieved by an applied electric (magnetic) field. Using first-principles density-functional theory calculations, we present a realization of this strategy for the magnetic perovskite EuTiO3.
FIRST-PRINCIPLES MOLECULAR DYNAMICS Roberto Car1
Giannozzi, Paolo
1.4 FIRST-PRINCIPLES MOLECULAR DYNAMICS Roberto Car1 , Filippo de Angelis2 , Paolo Giannozzi3. Car et al. elements such as hydrogen; classical or ab initio path integral approaches can vibrational level in the system considered. In the following, we will describe the combined approach of Car
COMMUNICATION First Principles Prediction of Protein Folding Rates
Goddard III, William A.
COMMUNICATION First Principles Prediction of Protein Folding Rates Derek A. Debe and William A studies have demonstrated that many small, single-domain proteins fold via simple two-state kinetics. We. # 1999 Academic Press Keywords: protein folding; kinetics; diffusion; fold topology; nucleation
Force field development from first principles for materials design
NASA Astrophysics Data System (ADS)
Chan, Maria; Kinaci, Alper; Narayanan, Badri; Sen, Fatih; Gray, Stephen; Davis, Michael; Sankaranaryanan, Subramanian
2015-03-01
The ability to perform accurate calculations efficiently is crucial for computational materials design. In this talk, we will discuss a stream-lined approach to force field development using first principles density functional theory training data and machine learning algorithms. We will also discuss the validation of this approach on precious metal nanoparticles.
IEEE TRANSACTION MANUSCRIPT 1 First Principles Simulation of Spaceborne
Kerekes, John
part to sea level rise and affect global climate change [4]. Hence, accurate knowledge of surfaceIEEE TRANSACTION MANUSCRIPT 1 First Principles Simulation of Spaceborne Micropulse Photon mission, this paper establishes a framework that simulates the perfor- mance of a spaceborne micropulse
First Principles Vulnerability Assessment James A. Kupsch Barton P. Miller
Miller, Barton P.
First Principles Vulnerability Assessment James A. Kupsch Barton P. Miller University of Wisconsin.Cesar,Elisa.Heymann}@uab.es September 2009 Abstract Vulnerability assessment is a key part of deploying secure software. As part Vulnerability Assessment (FPVA). FPVA is a primarily analyst-centric (manual) approach to assessment, whose aim
Recent Progress in First Principles O(N) Methods
D. R. Bowler; M. J. Gillan
2000-01-01
We describe recent progress in developing practical first principles methods for which the computer effort is proportional to the number of atoms: linear scaling or O(N) methods. It is shown that the locality property of the density matrix gives a general framework for constructing such methods. We then outline some of the main technical problems which must be solved in
Understanding nano-materials from first principles Leeor Kronik
Adler, Joan
Understanding nano-materials from first principles Leeor Kronik Department of Materials and Interfaces, Weizmann Institute of science, Rehovoth 76100 Nano-sized materials often exhibit exciting new traditional solid-state physics models are applicable to the intermediate nano- size range. As a result, first
Zhu, G.; Lewandowski, A.
2012-11-01
A new analytical method -- First-principle OPTical Intercept Calculation (FirstOPTIC) -- is presented here for optical evaluation of trough collectors. It employs first-principle optical treatment of collector optical error sources and derives analytical mathematical formulae to calculate the intercept factor of a trough collector. A suite of MATLAB code is developed for FirstOPTIC and validated against theoretical/numerical solutions and ray-tracing results. It is shown that FirstOPTIC can provide fast and accurate calculation of intercept factors of trough collectors. The method makes it possible to carry out fast evaluation of trough collectors for design purposes. The FirstOPTIC techniques and analysis may be naturally extended to other types of CSP technologies such as linear-Fresnel collectors and central-receiver towers.
First Principle Approach to Modeling of Small Scale Helicopter
Budiyono, A; Lesmana, H
2008-01-01
The establishment of global helicopter linear model is very precious and useful for the design of the linear control laws, since it is never afforded in the published literatures. In the first principle approach, the mathematical model was developed using basic helicopter theory accounting for particular characteristic of the miniature helicopter. No formal system identification procedures are required for the proposed model structure. The relevant published literatures however did not present the linear models required for the design of linear control laws. The paper presents a step by step development of linear model for small scale helicopter based on first-principle approach. Beyond the previous work in literatures, the calculation of the stability derivatives is presented in detail. A computer program is used to solve the equilibrium conditions and then calculate the change in aerodynamics forces and moments due to the change in each degree of freedom and control input. The detail derivation allows the c...
Accurate atomistic first-principles calculations of electronic stopping
NASA Astrophysics Data System (ADS)
Schleife, André; Kanai, Yosuke; Correa, Alfredo A.
2015-01-01
We show that atomistic first-principles calculations based on real-time propagation within time-dependent density functional theory are capable of accurately describing electronic stopping of light projectile atoms in metal hosts over a wide range of projectile velocities. In particular, we employ a plane-wave pseudopotential scheme to solve time-dependent Kohn-Sham equations for representative systems of H and He projectiles in crystalline aluminum. This approach to simulate nonadiabatic electron-ion interaction provides an accurate framework that allows for quantitative comparison with experiment without introducing ad hoc parameters such as effective charges, or assumptions about the dielectric function. Our work clearly shows that this atomistic first-principles description of electronic stopping is able to disentangle contributions due to tightly bound semicore electrons and geometric aspects of the stopping geometry (channeling versus off-channeling) in a wide range of projectile velocities.
Modeling Chemical Reactions with First-Principle Molecular Dynamics
Artur Michalak; Tom Ziegler
Density functional theory (DFT)-based molecular dynamics (MD) has established itself as a valuable and powerful tool in studies\\u000a of chemical reactions. Thanks to the rapid increase in power of modern computers, ab initio MD has nowadays become practical. Within the Car-Parinello approach, first-principle MD is already quite popular methodology\\u000a in molecular modeling. MD reveals the dynamical effects at finite temperatures
Predictions of the properties of water from first principles.
Bukowski, Robert; Szalewicz, Krzysztof; Groenenboom, Gerrit C; van der Avoird, Ad
2007-03-01
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
Predictions of the Properties of Water from First Principles
NASA Astrophysics Data System (ADS)
Bukowski, Robert; Szalewicz, Krzysztof; Groenenboom, Gerrit C.; van der Avoird, Ad
2007-03-01
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.
Equilibrium Se isotope fractionation parameters: A first-principles study
Xuefang Li; Yun Liu
2011-01-01
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
Isotope effects in N2O photolysis from first principles
J. A. Schmidt; M. S. Johnson; R. Schinke
2011-01-01
For the first time, accurate first principles potential energy surfaces allow N2O cross sections and isotopic fractionation spectra to be derived that are in agreement with all available experimental data, extending our knowledge to a much broader range of conditions. Absorption spectra of rare N- and O-isotopologues (15N14N16O, 14N15N16O, 15N216O, 14N217O and 14N218O) calculated using wavepacket propagation are compared to
NASA Astrophysics Data System (ADS)
Dal Corso, Andrea
2015-07-01
We present a first-principle investigation of the fully relativistic electronic surface states and resonances of clean Pt(111) and Ir(111) and compare them with those of Au(111). Our calculations are based on a recently introduced fully relativistic projector augmented-wave (PAW) approach that includes spin-orbit coupling and allows us to access both the relativistic energy splittings and the spin polarization of the surface states. The maps of the electronic structure of the two surfaces are critically discussed in comparison with previous calculations and with some of the available angle-resolved photoelectron spectroscopy data.
Solution-based thermodynamic modeling of the NiAlMo system using first-principles calculations
Chen, Long-Qing
, the binary NiMo phase diagram resulting from the model by Lu et al. [11] is different from those by Frisk Keywords: CALPHAD First-principles calculation NiAlMo phase diagram ATAT a b s t r a c t A solution for the configurations with the lowest energies of the N phase, the Alloy Theoretic Automated Toolkit (ATAT) was employed
NASA Astrophysics Data System (ADS)
Chandel, Surjeet Kumar; Kumar, Arun; Bharti, Ankush; Sharma, Raman
2015-05-01
Using first principles density functional theoretical calculations, the present paper reports a systematic study of phonon dispersion curves in pristine carbon (CNT) and silicon nanotubes (SiNT) having chirality (6,6) in the armchair configuration. Some of the phonon modes are found to have negative frequencies which leads to instability of the systems under study. The number of phonon branches has been found to be thrice as much as the number of atoms. The frequency of the higher optical bands varies from 1690 to 1957 cm-1 for CNT(6,6) while it is 596 to 658 cm-1 for SiNT.
Pascal, Tod A; Boesenberg, Ulrike; Kostecki, Robert; Richardson, Thomas J; Weng, Tsu-Chien; Sokaras, Dimosthenis; Nordlund, Dennis; McDermott, Eamon; Moewes, Alexander; Cabana, Jordi; Prendergast, David
2014-01-21
We elucidate the role of room-temperature-induced instantaneous structural distortions in the Li K-edge X-ray absorption spectra (XAS) of crystalline LiF, Li2SO4, Li2O, Li3N, and Li2CO3 using high resolution X-ray Raman spectroscopy (XRS) measurements and first-principles density functional theory calculations within the eXcited electron and Core Hole approach. Based on thermodynamic sampling via ab initio molecular dynamics simulations, we find calculated XAS in much better agreement with experiment than those computed using the rigid crystal structure alone. We show that local instantaneous distortion of the atomic lattice perturbs the symmetry of the Li 1s core-excited-state electronic structure, broadening spectral line-shapes and, in some cases, producing additional spectral features. The excellent agreement with high-resolution XRS measurements validates the accuracy of our first-principles approach to simulating XAS, and provides both accurate benchmarks for model compounds and a predictive theoretical capability for identification and characterization of multi-component systems, such as lithium-ion batteries, under working conditions. PMID:25669363
NASA Astrophysics Data System (ADS)
Pascal, Tod A.; Boesenberg, Ulrike; Kostecki, Robert; Richardson, Thomas J.; Weng, Tsu-Chien; Sokaras, Dimosthenis; Nordlund, Dennis; McDermott, Eamon; Moewes, Alexander; Cabana, Jordi; Prendergast, David
2014-01-01
We elucidate the role of room-temperature-induced instantaneous structural distortions in the Li K-edge X-ray absorption spectra (XAS) of crystalline LiF, Li2SO4, Li2O, Li3N, and Li2CO3 using high resolution X-ray Raman spectroscopy (XRS) measurements and first-principles density functional theory calculations within the eXcited electron and Core Hole approach. Based on thermodynamic sampling via ab initio molecular dynamics simulations, we find calculated XAS in much better agreement with experiment than those computed using the rigid crystal structure alone. We show that local instantaneous distortion of the atomic lattice perturbs the symmetry of the Li 1s core-excited-state electronic structure, broadening spectral line-shapes and, in some cases, producing additional spectral features. The excellent agreement with high-resolution XRS measurements validates the accuracy of our first-principles approach to simulating XAS, and provides both accurate benchmarks for model compounds and a predictive theoretical capability for identification and characterization of multi-component systems, such as lithium-ion batteries, under working conditions.
Pascal, Tod A.; Prendergast, David, E-mail: dgprendergast@lbl.gov [The Molecular Foundry, Materials Science Division, Lawrence Berkeley National Laboratory (LBNL), Berkeley, California 94720 (United States)] [The Molecular Foundry, Materials Science Division, Lawrence Berkeley National Laboratory (LBNL), Berkeley, California 94720 (United States); Boesenberg, Ulrike; Kostecki, Robert; Richardson, Thomas J. [Environmental Energy Technologies Division, LBNL, Berkeley, California 94720 (United States)] [Environmental Energy Technologies Division, LBNL, Berkeley, California 94720 (United States); Weng, Tsu-Chien; Sokaras, Dimosthenis; Nordlund, Dennis [Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford, California 94720 (United States)] [Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford, California 94720 (United States); McDermott, Eamon; Moewes, Alexander [University of Saskatchewan, Department of Physics and Engineering Physics, Saskatoon, Saskatchewan S7N 5E2 (Canada)] [University of Saskatchewan, Department of Physics and Engineering Physics, Saskatoon, Saskatchewan S7N 5E2 (Canada); Cabana, Jordi [Environmental Energy Technologies Division, LBNL, Berkeley, California 94720 (United States) [Environmental Energy Technologies Division, LBNL, Berkeley, California 94720 (United States); Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60605 (United States)
2014-01-21
We elucidate the role of room-temperature-induced instantaneous structural distortions in the Li K-edge X-ray absorption spectra (XAS) of crystalline LiF, Li{sub 2}SO{sub 4}, Li{sub 2}O, Li{sub 3}N, and Li{sub 2}CO{sub 3} using high resolution X-ray Raman spectroscopy (XRS) measurements and first-principles density functional theory calculations within the eXcited electron and Core Hole approach. Based on thermodynamic sampling via ab initio molecular dynamics simulations, we find calculated XAS in much better agreement with experiment than those computed using the rigid crystal structure alone. We show that local instantaneous distortion of the atomic lattice perturbs the symmetry of the Li 1s core-excited-state electronic structure, broadening spectral line-shapes and, in some cases, producing additional spectral features. The excellent agreement with high-resolution XRS measurements validates the accuracy of our first-principles approach to simulating XAS, and provides both accurate benchmarks for model compounds and a predictive theoretical capability for identification and characterization of multi-component systems, such as lithium-ion batteries, under working conditions.
Collective modes in light nuclei from first principles.
Dytrych, T; Launey, K D; Draayer, J P; Maris, P; Vary, J P; Saule, E; Catalyurek, U; Sosonkina, M; Langr, D; Caprio, M A
2013-12-20
Results for ab initio no-core shell model calculations in a symmetry-adapted SU(3)-based coupling scheme demonstrate that collective modes in light nuclei emerge from first principles. The low-lying states of 6Li, 8Be, and 6He are shown to exhibit orderly patterns that favor spatial configurations with strong quadrupole deformation and complementary low intrinsic spin values, a picture that is consistent with the nuclear symplectic model. The results also suggest a pragmatic path forward to accommodate deformation-driven collective features in ab initio analyses when they dominate the nuclear landscape. PMID:24483740
First-principles studies of dilute magnetic ferroelectrics
NASA Astrophysics Data System (ADS)
Weston, Leigh; Cui, Xiangyuan; Ringer, Simon P.; Stampfl, Catherine
2014-03-01
Using first-principles density functional calculations, we have investigated the magnetic properties of dilute magnetic ferroelectrics (DMF), where a nominally non-magnetic ferroelectric host is doped with a small concentration of magnetic impurities. We find that DMFs may exhibit simultaneous electrical and magnetic polarization, consistent with recent experimental observations. A possible mechanism for magnetoelectric coupling was explored, and it was found that through a strong spin-lattice coupling, electric field induced switching of magnetization may be possible. Thus, DMFs may provide a route to achieving a single phase, room temperature multiferroic with strong magnetoelectric coupling.
First-Principles Approach to Lattice-Mediated Magnetoelectric Effects
NASA Astrophysics Data System (ADS)
Iñiguez, Jorge
2009-03-01
I will present a microscopic theory of the magnetoelectric response of an insulator, and derive from it an analytical expression for the lattice-mediated part of the effect. As I will show, such a result provides us with distinct hints at strategies to increase the magnitude of the response, as well as with a convenient method for performing first-principles calculations. I will illustrate the usefulness of the proposed approach with applications to Cr2O3, a model magnetoelectric crystal, and BiFeO3 and related compounds, the best studied, and arguably most technologically promising, family of multiferroics. Ref.: J. Iñiguez, Phys. Rev. Lett. 101, 117201 (2008).
First principles calculations of shock compressed fluid helium.
Militzer, B
2006-10-27
The properties of hot dense helium at megabar pressures are studied with two first principles computer simulation techniques: path integral Monte Carlo simulation and density functional molecular dynamics. The simulations predict that the compressibility of helium is substantially increased by electronic excitations that are present in the hot fluid at thermodynamic equilibrium. A maximum compression ratio of 5.24(4)-fold the initial density was predicted for 360 GPa and 150,000 K. This result distinguishes helium from deuterium, for which simulations predicted a maximum compression ratio of 4.3(1). Hugoniot curves for statically precompressed samples are also discussed. PMID:17155480
The intrinsic electrical breakdown strength of insulators from first principles
NASA Astrophysics Data System (ADS)
Sun, Y.; Boggs, S. A.; Ramprasad, R.
2012-09-01
A first principles quantum-mechanical method for estimating intrinsic breakdown strength of insulating materials has been implemented based on an average electron model which assumes that the breakdown occurs when the average electron energy gain from the electric field exceeds the average energy loss to phonons. The approach is based on density functional perturbation theory and on the direct integration of electronic scattering probabilities over all possible final states, with no adjustable parameters. The computed intrinsic breakdown field for several prototypical materials compares favorably with available experimental data. This model also provides physical insight into the material properties that affect breakdown.
Carbon-rich icosahedral boron carbide designed from first principles
NASA Astrophysics Data System (ADS)
Jay, Antoine; Vast, Nathalie; Sjakste, Jelena; Duparc, Olivier Hardouin
2014-07-01
The carbon-rich boron-carbide (B11C)C-C has been designed from first principles within the density functional theory. With respect to the most common boron carbide at 20% carbon concentration B4C, the structural modification consists in removing boron atoms from the chains linking (B11C) icosahedra. With C-C instead of C-B-C chains, the formation of vacancies is shown to be hindered, leading to enhanced mechanical strength with respect to B4C. The phonon frequencies and elastic constants turn out to prove the stability of the carbon-rich phase, and important fingerprints for its characterization have been identified.
First principles energetic calculations of sapphire (0001) and (1{bar 1}02) surfaces
Guo, J.; Ellis, D.E.; Lam, D.J.
1992-03-01
Various crystallographic faces of sapphire, the undoped single crystal {alpha}-Al{sub 2}O{sub 3}, have been widely used as substrates fro depositing thin films of metals, semiconductors, and insulators for basic scientific studies and for microelectronic applications. Epitaxial TiO{sub 2} and VO{sub 2} thin films were successfully grown on (0001), (1102) and (1120) surfaces of sapphire substrates by the Metal Organic Chemical Vapor Deposition (MOCVD) technique in our experimental program. The present paper is one of our theoretical efforts in parallel with the experimental program to gain microscopic understandings of the effects of substrate structure on the epitaxial relationship and the overlayer lattice structure of thin film deposition. The first principles energetic calculations on the complicated surfaces like sapphire can now be carried out because of the recent development in our computer programs and availability of high speed super computers. The phase first principles means that the calculations are based only on the know physical laws and approximations, and are free of any adjustable parameters. A complete discussion of the theoretical formalism has been presented in our previous works.
First principles energetic calculations of sapphire (0001) and (1 1 02) surfaces
Guo, J.; Ellis, D.E.; Lam, D.J.
1992-03-01
Various crystallographic faces of sapphire, the undoped single crystal {alpha}-Al{sub 2}O{sub 3}, have been widely used as substrates fro depositing thin films of metals, semiconductors, and insulators for basic scientific studies and for microelectronic applications. Epitaxial TiO{sub 2} and VO{sub 2} thin films were successfully grown on (0001), (1102) and (1120) surfaces of sapphire substrates by the Metal Organic Chemical Vapor Deposition (MOCVD) technique in our experimental program. The present paper is one of our theoretical efforts in parallel with the experimental program to gain microscopic understandings of the effects of substrate structure on the epitaxial relationship and the overlayer lattice structure of thin film deposition. The first principles energetic calculations on the complicated surfaces like sapphire can now be carried out because of the recent development in our computer programs and availability of high speed super computers. The phase first principles means that the calculations are based only on the know physical laws and approximations, and are free of any adjustable parameters. A complete discussion of the theoretical formalism has been presented in our previous works.
First principles simulations of hydrogen and helium at high pressures
NASA Astrophysics Data System (ADS)
Morales, Miguel Angel
In this dissertation, a study of hydrogen and helium at high pressure is presented using state-of-the-art first-principles simulations using Quantum Monte Carlo (QMC) and Density Functional Theory (DFT). The Coupled Electron Ion Monte Carlo method, a novel QMC-based first-principles simulation method, is extensively used in order to produce the most accurate study of hydrogen at high pressure to date. The main motivation for this work comes from the study of giant planets, like Jupiter and Saturn. These planets are mainly composed of mixture of hydrogen and helium and the thermodynamic properties of the mixture is a basic ingredient to models of their evolution and interior structure. One of the main goals of our work is to produce an accurate parametrization of the free energy of hydrogen and helium from first principles simulations that can replace the Saumon-Chabrier-Van Horn model equations of state. We present a detail study of molecular dissociation in liquid hydrogen. We observe clear evidence of the plasma phase transition at low temperatures, through regions of (PARTP/PARTp)=0 and from discontinuities in the electronic conductivity. Both QMC-based and DFT-based simulation methods agree in the reported transition, although Monte Carlo methods predict higher transition pressures. Using the size of the discontinuity in the electronic conductivity, we estimate the critical point of the transition at temperatures slightly below 2000 K. We calculate the melting curve of solid molecular hydrogen up to pressures of 200 GPa using free energy integrations in the liquid and solid phases, and determine its intersection with the plasma phase transition. We also study the equation of state of hydrogen, helium and hydrogen-helium mixtures at pressures beyond 200 GPa. We find very good agreement between both simulation methodologies for pressures beyond 600 GPa. From the equation of state calculations, an analysis of the miscibility properties of the mixture is performed. We find that the temperatures for the dcmixing of helium and metallic hydrogen are sufficiently high to cross the planetary adiabat of Saturn at pressures around 5 Mbar. Helium is partially miscible throughout a significant portion of the interior of Saturn, and to a lesser extent in Jupiter. These results are directly relevant to models of the interior structure and evolution of Jovian planets.
First-Principles Informed Thermodynamics of CRUD Deposition
NASA Astrophysics Data System (ADS)
O'Brien, Christopher John
The recent emphasis in the United States on developing abundant domestic sources of energy, together with an increasing awareness of the environmental hazards of fossil fuels, has led to a fresh look at the challenges of nuclear energy within the science and engineering community. One of these challenges is controlling the precipitation of porous oxide deposits onto the nuclear fuel rod cladding from the primary coolant during operation of pressurized light-water reactors (PWRs). These deposits, called CRUD (an acronym for Chalk River Unidentified Deposits), are a major concern to reactor operation because they reduce fuel lifetime and efficiency by reducing heat transfer to the coolant, promote corrosion, and depress neutron flux. This dissertation provides fundamental insights into the process by which CRUD is formed in PWRs by providing a framework linking the results of first-principles calculations to experimental data. The technique developed to facilitate the investigation is referred to as Density Functional Theory (DFT) referenced semi-empirical thermodynamics; It links 0K first-principles calculations with high temperature thermodynamics by redefining the reference chemical potentials of the constituent elements. The technique permits aqueous chemistry to be incorporated into thermodynamic calculations and allows for the prediction of temperature and pressure dependent free energies of materials that are experimentally inaccessible or have not yet been measured. The ability to extend accurate first-principles calculations to high temperatures and aqueous environments allows the stability of crystal surfaces, calculated with DFT techniques, to be predicted at conditions representative of an operating PWR. Accurate values of surface energies are used in fulfilling the principal goal of this dissertation, which is to investigate the aqueous thermodynamics of formation of nickel oxide (NiO) and nickel ferrite (NiFe 2O4) crystallites as representative CRUD components. Specifically, this dissertation investigates the thermodynamics of the homogeneous nucleation of crystallites and proposes a mechanism for their agglomeration. This work also contributes to a more complete understanding of CRUD by generating improved thermodynamic data to enable modeling of the incorporation of boron into CRUD through the formation of Bonaccordite (Ni2FeBO5) which has formed in PWRs operating at high-power conditions.
Hydrogen storage in LiH: A first principle study
Banger, Suman, E-mail: sumanphy28@gmail.com; Nayak, Vikas, E-mail: sumanphy28@gmail.com; Verma, U. P., E-mail: sumanphy28@gmail.com [School of Studies in Physics, Jiwaji University, Gwalior-474011 (India)
2014-04-24
First principles calculations have been performed on the Lithium hydride (LiH) using the full potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory. We have extended our calculations for LiH+2H and LiH+6H in NaCl structure. The structural stability of three compounds have been studied. It is found that LiH with 6 added Hydrogen atoms is most stable. The obtained results for LiH are in good agreement with reported experimental data. Electronic structures of three compounds are also studied. Out of three the energy band gap in LiH is ?3.0 eV and LiH+2H and LiH+6H are metallic.
Carbon-rich icosahedral boron carbide designed from first principles
Jay, Antoine; Vast, Nathalie; Sjakste, Jelena; Duparc, Olivier Hardouin [Ecole Polytechnique, Laboratoire des Solides Irradiés, CEA-DSM-IRAMIS, CNRS UMR 7642, F-91120 Palaiseau (France)
2014-07-21
The carbon-rich boron-carbide (B{sub 11}C)C-C has been designed from first principles within the density functional theory. With respect to the most common boron carbide at 20% carbon concentration B{sub 4}C, the structural modification consists in removing boron atoms from the chains linking (B{sub 11}C) icosahedra. With C-C instead of C-B-C chains, the formation of vacancies is shown to be hindered, leading to enhanced mechanical strength with respect to B{sub 4}C. The phonon frequencies and elastic constants turn out to prove the stability of the carbon-rich phase, and important fingerprints for its characterization have been identified.
Auger recombination in sodium-iodide scintillators from first principles
NASA Astrophysics Data System (ADS)
McAllister, Andrew; Åberg, Daniel; Schleife, André; Kioupakis, Emmanouil
2015-04-01
Scintillator radiation detectors suffer from low energy resolution that has been attributed to non-linear light yield response to the energy of the incident gamma rays. Auger recombination is a key non-radiative recombination channel that scales with the third power of the excitation density and may play a role in the non-proportionality problem of scintillators. In this work, we study direct and phonon-assisted Auger recombination in NaI using first-principles calculations. Our results show that phonon-assisted Auger recombination, mediated primarily by short-range phonon scattering, dominates at room temperature. We discuss our findings in light of the much larger values obtained by numerical fits to z-scan experiments.
Thermodynamics of Magnetic Systems from First Principles: WL-LSMS
Eisenbach, Markus [ORNL; Zhou, Chenggang [ORNL; Nicholson, Don M [ORNL; Brown, Greg [ORNL; Larkin, Jeffrey M [ORNL; Schulthess, Thomas C [ORNL
2010-01-01
Density Functional calculations have proven to be a powerful tool to study the ground state of many materials. For finite temperatures the situation is less ideal and one is often forced to rely on models with parameters either fitted to zero temperature first principles calculations or experimental results. This approach is especially unsatisfacory in inhomogeneous systems, nano particles, or other systems where the model parameters could vary significantly from one site to another. Here we describe a possible solution to this problem by combining classical Monte Carlo calculations the Wang-Landau method in this case with a firs principles electronic structure calculation, specifically our locally selfconsistent multiple scallering code (LSMS). The combined code shows superb scaling behavior on massively parallel computers. The code sustained 1.836 Petaflop/s on 223232 cores of the Cray XT5 jaguar system at Oak Ridge.
First-principles simulation of molecular dissociation-recombination equilibrium
Kylaenpaeae, Ilkka; Rantala, Tapio T. [Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere (Finland)
2011-09-14
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.
General microscopic model of magnetoelastic coupling from first principles
NASA Astrophysics Data System (ADS)
Lu, X. Z.; Wu, Xifan; Xiang, H. J.
2015-03-01
Magnetoelastic coupling, i.e., the change of crystal lattice induced by a spin order, is not only scientifically interesting, but also technically important. In this work, we propose a general microscopic model from first-principles calculations to describe the magnetoelastic coupling and provide a way to construct the microscopic model from density functional theory calculations. Based on this model, we reveal that there exists a previously unexpected contribution to the electric polarization induced by the spin order in multiferroics due to the combined effects of magnetoelastic coupling and piezoelectric effect. Interestingly and surprisingly, we find that this lattice-deformation contribution to the polarization is even larger than that from the pure electronic and ion-displacement contributions in BiFe O3 . This model of magnetoelastic coupling can be generally applied to investigate the other magnetoelastic phenomena.
Thermoelectric properties of titanium dioxide polymorphs from first principles
NASA Astrophysics Data System (ADS)
Bayerl, Dylan; Kioupakis, Emmanouil
2014-03-01
Titanium oxides are promising materials for high-temperature thermoelectrics because of their high Seebeck coefficients, thermal stability, and natural abundance. We use first-principles calculations to investigate the thermoelectric transport properties of several titanium dioxide polymorphs. Our methodology is based on density functional and many-body perturbation theory within the GW approximation. The maximally localized Wannier function method is employed to interpolate the GW bands in the Brillouin zone. We use the Boltzmann transport formalism within the constant relaxation time approximation to determine the temperature and carrier-density dependence of the Seebeck coefficient, electron mobility, and electron thermal conductivity from the calculated electronic band structures. We demonstrate agreement with experimentally measured transport parameters and enhanced power factor at high temperature in certain heavily doped phases. This research was supported as part of CSTEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. Computational resources were provided by the DOE NERSC facility.
Ziegler Natta heterogeneous catalysis by first principles computer experiments
NASA Astrophysics Data System (ADS)
Boero, M.; Parrinello, M.; Terakura, K.
1999-09-01
In this work we present a first attempt to study the polymerization process of ethylene in a realistic Ziegler-Natta heterogeneous system by means of first principles molecular dynamics. In particular, we simulate, in a very unbiased way, both the deposition of the catalyst TiCl 4 on the (110) active surface of a solid MgCl 2 support and the polymer chain formation. By using a constrained molecular dynamics approach, we work out the energetics and the reaction pathway of the polymerization process as it occurs in a laboratory or an industrial plant. The good agreement of the results of our simulations with the available experimental data indicates that these kinds of simulations can be used as a skilful approach to study the details of the reaction mechanism which are not accessible to experimental probes. This offers a tool to improve the production and/or to design reactants and products for practical use.
First-Principles Studies of Single-Molecule Photovoltaics
NASA Astrophysics Data System (ADS)
Doak, Peter; Segalman, R. A.; Tilley, T. D.; Neaton, J. B.
2009-03-01
Organic photovoltaics consist of electron donor and acceptor polymers or molecules blended together, and are promising inexpensive, lightweight alternatives to conventional silicon solar cells. However, many of the physical processes responsible for their poor efficiencies are not well understood. Here, using first-principles calculations based on density functional theory, including self-energy corrections within the GW approximation and a discussion of excitonic effects, we examine the relationship between molecular structure and electronic level alignment at a covalent donor-acceptor interface. We consider small asymmetric molecules subdivided into discrete covalently linked moieties based on thiophene, tetrafluorobenzene, pyridine, and durene. Excited states of each of these moieties, as well as their covalently-linked combinations, are computed and discussed in the context of their ability to absorb photons and separate charge. Work supported in part by the DOE Helios SERC. Computational resources provided by NERSC.
First principles modeling of panchromatic dyes for solar cells applications.
NASA Astrophysics Data System (ADS)
di Felice, Rosa; Calzolari, Arrigo; Dong, Rui; Buongiorno Nardelli, Marco
2013-03-01
The state-of-the-art dye in Grätzel 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.
First Principles Study of Phase Transformations in Polyvinylidene Fluoride
NASA Astrophysics Data System (ADS)
Ranjan, V.; Buongiorno Nardelli, Marco; Bernholc, J.
2010-03-01
Polyvinylidene fluoride (PVDF) with a small concentration of chlorotrifluoroethylene (CTFE) has been observed to store very high energy [1] as compared to currently used polymers. Chain rotations within the PVDF crystal have been proposed as the reason behind the high energy storage. However, in a recent work, we suggested [2] that the ultra-high energy storage in P(VDF-CTFE) is due to an electric-field induced phase transition from the non-polar ? to the polar ?-PVDF. We have now determined a low-energy relaxation path from ?- to ?-PVDF which confirms this suggestion. Our first-principles calculations reveal an intricate relationship between the kinetics of the chain rotation and the phase transformatio [4pt] [1] B. Chun, Science 313, 334 (2006).[0pt] [2] V. Ranjan, Phys. Rev. Lett. 99, 047801 (2007).
Tomoko K. Shimizu; Jaehoon Jung; Hiroyuki S. Kato; Yousoo Kim; Maki Kawai
2010-01-01
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
Integration (Multi-Variable Included) From First Principles
NSDL National Science Digital Library
Spunde, Walter
Integration has traditionally been so closely linked to the interpretation as an area and to the techniques of anti-differentiation as to appear inseparable from them. While largely a consequence of the fact that, in pre-Personal-Computer times, anti-differentiation was the key to effective integration and that line and surface integrals were generally intractable using that technique, the advent of computer algebra systems and easy large scale numerical computation seems not to have had much effect on the way integration is presented in standard texts. On the one hand, it is not clear what sorts of skills are required for a novice to handle computer algebra effectively and on the other hand most available software does not provide the data structures and tools for dealing conveniently with numerical integration from first principles in the general case. We focus on the latter. In this article we examine an approach to the principles of integration based on computer manipulation of multi-dimensional arrays for the coordinate grids, referring to the area interpretation as only one among several possibilities and presenting integration as the solution to non-trivial anti-differentiation problems. Underlying the implementation of the approach is the mathematical notation of Iversons J, an array-processing, functional, computer language. We suggest that the mathematical foundations of the topic existence of, and convergence to, the limit should be postponed till after students can effectively compute and manipulate the approximations that are used to define integrals from first principles. Target Audience: 2-4 Year College Faculty/Administrators, Engineers
First principles studies of silicon as a negative electrode material
NASA Astrophysics Data System (ADS)
Chevrier, Vincent L.
Batteries with higher volumetric and specific energy capacities are needed. Silicon is a promising candidate to replace graphite as the negative electrode material in Li-ion batteries. Silicon alloys with lithium, meaning its structure changes significantly during lithiation. Unlike other lithium alloys, lithiated silicon is amorphous when created electrochemically at room temperature. However, when lithiated at 415°C, crystalline Li-Si phases are experimentally found. This thesis focused on the study of the Li-Si crystalline phases and the lithiation of amorphous LixSi using first-principles calculations. A novel protocol to model the lithiation of amorphous silicon was developed, yielding results in good agreement with experiment. This represents the first time the lithiation of an amorphous alloy material has been modeled using first-principles calculations. Density functional theory calculations yielded formation energies for the crystalline and amorphous structures, from which potential-composition curves were calculated and compared to experiment. Good agreement with experiment was found, providing validation of the calculation methods and proposed protocol. Charge transfer studies and calculations of electronic densities of states for crystalline and amorphous structures were also completed. These confirmed the understanding of Li-Si structures as Zintl phases and quantified the charge transferred from Li to Si atoms. Phonon studies were completed for the crystalline Li-Si phases and helped explain their stability as a function of temperature. The phonon studies revealed that the Li15Si4 phase is unstable with respect to the other crystalline phases at elevated temperature, in agreement with experiment. Finally, experimental thermal studies of lithiated Si were used to obtain activation energies of the various crystallization events that occur when heating lithiated Si.
Large-Scale First-Principles Molecular Dynamics simulations on the BlueGene/L Platform
Franchetti, Franz
Large-Scale First-Principles Molecular Dynamics simulations on the BlueGene/L Platform using. Keywords Electronic structure. Molecular Dynamics. Ab initio simulations. First-principles simulations that the Qbox code supports unprecedented large-scale First-Principles Molecular Dynamics (FPMD) applications
Niu, J. G. [School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083 (China); Department of Mechanical Engineering, Hebei University, Baoding 071000 (China); Zhan, Q., E-mail: qzhan@mater.ustb.edu.cn [School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083 (China); Geng, W. T., E-mail: geng@ustb.edu.cn [School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083 (China); Psi Quantum Materials LLC, Laiwu 271100 (China)
2014-06-15
Despite well documented first-principles theoretical determination of the low migration energy (0.06 eV) of a single He in tungsten, fully quantum mechanical calculations on the migration of a He pair still present a challenge due to the complexity of its trajectory. By identifying the six most stable configurations of the He pair in W and decomposing its motion into rotational, translational, and rotational-translational routines, we are able to determine its migration barrier and trajectory. Our density functional theory calculations demonstrate a He pair has three modes of motion: a close or open circular two-dimensional motion in (100) plane with an energy barrier of 0.30 eV, a snaking motion along [001] direction with a barrier of 0.30 eV, and a twisted-ladder motion along [010] direction with the two He swinging in the plane (100) and a barrier of 0.31 eV. The graceful associative movements of a He pair are related to the chemical-bonding-like He-He interaction being much stronger than its migration barrier in W. The excellent agreement with available experimental measurements (0.24–0.32 eV) on He migration makes our first-principles result a solid input to obtain accurate He-W interatomic potentials in molecular dynamics simulations.
Exact results and open questions in first principle functional RG
Le Doussal, Pierre [LPTENS CNRS UMR 8549 24, Rue Lhomond, 75231 Paris Cedex 05 (France)], E-mail: ledou@lpt.ens.fr
2010-01-15
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.
TOPICAL REVIEW: First principles studies of multiferroic materials
NASA Astrophysics Data System (ADS)
Picozzi, Silvia; Ederer, Claude
2009-07-01
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.
First Principles Studies of ABO3 Perovskite Surfaces and Nanostructures
NASA Astrophysics Data System (ADS)
Pilania, Ghanshyam
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.
First Principles XPS Calculation for the B Defects in SiC
NASA Astrophysics Data System (ADS)
Matsushima, Naoki; Yamauchi, Jun
We investigate the core-level X-ray photoelectron spectroscopy (XPS) spectra of various defects containing boron in silicon carbide (SiC) using a first principles calculation with careful evaluation of the local potential boundary condition. It is found that a 512-atom cubic supercell for 3C-SiC and a 576-atom hexagonal supercell for 4H-SiC are required for convergence of the XPS binding energy to within 0.1 eV. The XPS binding energies of SiC have considerable site dependence owing to the electronegativity difference between Si and C. The difference of XPS binding energies between the Si and C substitutional sites is 3.4 eV.
NASA Astrophysics Data System (ADS)
Kanagaprabha, S.; Rajeswarapalanichamy, R.; Sudhapriyanga, G.; Murugan, A.; Santhosh, M.; Iyakutti, K.
2015-06-01
The structural and mechanical properties of CrH and MnH are investigated using first principles calculation based on density functional theory as implemented in VASP code with generalized gradient approximation. The calculated ground state properties are in good agreement with previous experimental and other theoretical results. A structural phase transition from NaCl to NiAs phase at a pressure of 76 GPa is predicted for both CrH and MnH.
Bruno Reynard
2009-01-01
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
First principles studies of the graphene-phenol interactions.
Hernández, José M Galicia; Anota, Ernesto Chigo; de la Cruz, María T Romero; Melchor, Minerva González; Cocoletzi, Gregorio Hernández
2012-08-01
Studies of the interaction between phenol and intrinsic graphene, as well as phenol and aluminum doped graphene layer are performed using first principles total energy calculations within the periodic density functional theory. A 4x4 periodic structure is used to explore the adsorption of a phenol molecule on the intrinsic graphene and on aluminum doped graphene layer. The electron-ion interactions are modeled using ultra-soft pseudo-potentials, and the exchange-correlation energies are treated according to the generalized gradient approximation (GGA) with the PBE parameterization. We consider different molecule orientations: parallel and perpendicular to the graphene layer to relax the atomic structure. To explain the optimized atomic geometry we determine binding energies for all cases and the density of states (DOS) and partial DOS for the most relevant configurations. Results indicate that the direct interaction of oxygen with aluminum yields the ground state geometry with the phenol molecule adsorbed on the graphene layer. Binding energies and DOS structures also demonstrate that the ground state configuration is that where the O and Al atoms interact with a separation distance of 1.97 Å. PMID:22415370
"Postural first" principle when balance is challenged in elderly people.
Lion, Alexis; Spada, Rosario S; Bosser, Gilles; Gauchard, Gérome C; Anello, Guido; Bosco, Paolo; Calabrese, Santa; Iero, Antonella; Stella, Giuseppe; Elia, Maurizio; Perrin, Philippe P
2014-08-01
Human cognitive processing limits can lead to difficulties in performing two tasks simultaneously. This study aimed to evaluate the effect of cognitive load on both simple and complex postural tasks. Postural control was evaluated in 128 noninstitutionalized elderly people (mean age = 73.6 ± 5.6 years) using a force platform on a firm support in control condition (CC) and mental counting condition (MCC) with eyes open (EO) and eyes closed (EC). Then, the same tests were performed on a foam support. Sway path traveled and area covered by the center of foot pressure were recorded, low values indicating efficient balance. On firm support, sway path was higher in MCC than in CC both in EO and EC conditions (p < 0.001). On foam support, sway path was higher in CC than in MCC in EC condition (p < 0.001), area being higher in CC than in MCC both in EO (p < 0.05) and EC (p < 0.001) conditions. The results indicate that cognitive load alters balance control in a simple postural task (i.e. on firm support), which is highlighted by an increase of energetic expenditure (i.e. increase of the sway path covered) to balance. Awareness may not be increased and the attentional demand may be shared between balance and mental task. Conversely, cognitive load does not perturb the realization of a new complex postural task. This result showed that postural control is prioritized ("postural first" principle) when seriously challenged. PMID:24205810
Anisotropic intrinsic lattice thermal conductivity of phosphorene from first principles.
Qin, Guangzhao; Yan, Qing-Bo; Qin, Zhenzhen; Yue, Sheng-Ying; Hu, Ming; Su, Gang
2015-02-21
Phosphorene, the single layer counterpart of black phosphorus, is a novel two-dimensional semiconductor with high carrier mobility and a large fundamental direct band gap, which has attracted tremendous interest recently. Its potential applications in nano-electronics and thermoelectrics call for fundamental study of the phonon transport. Here, we calculate the intrinsic lattice thermal conductivity of phosphorene by solving the phonon Boltzmann transport equation (BTE) based on first-principles calculations. The thermal conductivity of phosphorene at 300 K is 30.15 W m(-1) K(-1) (zigzag) and 13.65 W m(-1) K(-1) (armchair), showing an obvious anisotropy along different directions. The calculated thermal conductivity fits perfectly to the inverse relationship with temperature when the temperature is higher than Debye temperature (?D = 278.66 K). In comparison to graphene, the minor contribution around 5% of the ZA mode is responsible for the low thermal conductivity of phosphorene. In addition, the representative mean free path (MFP), a critical size for phonon transport, is also obtained. PMID:25594447
Coarse graining approach to First principles modeling of structural materials
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
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.
Mechanical properties of graphyne monolayers: a first-principles study.
Peng, Qing; Ji, Wei; De, Suvranu
2012-10-14
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
First principles study of hydrogen behaviors in hexagonal tungsten carbide
NASA Astrophysics Data System (ADS)
Kong, Xiang-Shan; You, Yu-Wei; Liu, C. S.; Fang, Q. F.; Chen, Jun-Ling; Luo, G.-N.
2011-11-01
Understanding the behaviors of hydrogen in hexagonal tungsten carbide (WC) is of particular interest for fusion reactor design due to the presence of WC in the divertor of fusion reactors. Here, we have used first principles calculations to study the hydrogen behavior in WC. It is found that the most stable interstitial site for the hydrogen atom is the projection of the octahedral interstitial site on tungsten basal plane, followed by the site near the projection of the octahedral interstitial site on carbon basal plane. The binding energy between two interstitial hydrogen atoms is negative, suggesting that hydrogen itself is not capable of trapping another hydrogen atoms to form hydrogen molecule. The calculated results on the interaction between hydrogen and vacancy indicate that hydrogen atom is preferably trapped by vacancy defects and hydrogen molecule can not be formed in mono-vacancy. In addition, the hydrogen atom bound to carbon is only found in tungsten vacancy. We also study the migrations of hydrogen in WC and find that the interstitial hydrogen atom prefers to diffuse along the c-axis. Our studies provide some explanations for the results of the thermal desorption process of energetic hydrogen ion implanted into WC.
Selenium adsorption on Mo(110): A first-principles investigation
NASA Astrophysics Data System (ADS)
Roma, Guido; Chiodo, Letizia
2013-06-01
Selenium adsorption on molybdenum surfaces is a relevant process in the production of thin-film solar cells, in particular as far as the formation of the layered compound MoSe2 is concerned. In this paper we investigate the energetics of Se adsorption on the (110) surface of molybdenum using first-principles calculations in the two limiting cases of low and high coverage, and we establish a comparison with the more extensively investigated case of sulfur adsorption at submonolayer coverage. The studied system provides the opportunity for testing the most crucial approximations, namely, the choice of the exchange-correlation functional and the pseudopotential generation. We find that semicore states of molybdenum have an influence on calculated surface energies and, to a lesser extent, on adsorption energies. We compare some more or less popular semilocal exchange-correlation functionals, including one recently proposed as an improvement for quasi-two-dimensional systems. The results show that the preferred adsorption site changes with coverage and suggest a strong variation of the adsorption energy with coverage.
Equilibrium Se isotope fractionation parameters: A first-principles study
NASA Astrophysics Data System (ADS)
Li, Xuefang; Liu, Yun
2011-04-01
Several important equilibrium Se isotope fractionation parameters are investigated by first-principles calculations, involving dominant inorganic and organic Se-bearing species in gaseous, aqueous and condensed phases. Because anharmonic effects are found to be negligible for Se isotope fractionation calculation, the Bigeleisen-Mayer equation method is used without corrections beyond harmonic approximation. All calculations are made at B3LYP/6-311 + G(d,p) level, with a frequency scaling factor of 1.05. Solvation effects are carefully evaluated by the explicit solvent model (i.e. the "water droplet" method). A number of conformers are used for aqueous complexes in order to reduce the possible error coming from different configurations. Redox state is found to be an important factor controlling equilibrium Se isotope fractionations. Our results suggest a trend of heavy Se isotopes enrichment as SeO 42- > SeO 32- > HSeO 3- > SeO 2 > selenoamino acids > alkylselenides > Se(0) or H 2Se > HSe -. The Se(- II) species regardless of organic and inorganic forms can enrich extremely light Se isotopes comparing with other species. Equilibrium Se isotope fractionation factors provided in this study suggest Se isotopes can be used as a tracer of redox conditions and also useful to study Se cycling.
Isotope effects in N2O photolysis from first principles
NASA Astrophysics Data System (ADS)
Schmidt, J. A.; Johnson, M. S.; Schinke, R.
2011-09-01
For the first time, accurate first principles potential energy surfaces allow N2O cross sections and isotopic fractionation spectra to be derived that are in agreement with all available experimental data, extending our knowledge to a much broader range of conditions. Absorption spectra of rare N- and O-isotopologues (15N14N16O, 14N15N16O, 15N216O, 14N217O and 14N218O) calculated using wavepacket propagation are compared to the most abundant isotopologue (14N216O). The fractionation constants as a function of wavelength and temperature are in excellent agreement with experimental data. The study shows that excitations from the 3rd excited bending state, (0,3,0), and the first combination state, (1,1,0), are important for explaining the isotope effect at wavelengths longer than 210 nm. Only a small amount of the mass independent oxygen isotope anomaly observed in atmospheric N2O samples can be explained as arising from photolysis.
First-principles study of water on Cu (110) surface
NASA Astrophysics Data System (ADS)
Ren, Jun; Meng, Sheng
2009-03-01
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.
First-principles Simulations and the Criticality of Calving Glaciers
NASA Astrophysics Data System (ADS)
Vallot, D.; Åström, J. A.; Schäfer, M.; Welty, E.; O'Neel, S.; Bartholomaus, T. C.; Liu, Y.; Riikilä, T.; Zwinger, T.; Timonen, J.; Moore, J.
2014-12-01
The algoritm of a first principles calving-simulation computer-code is outlined and demonstrated. The code is particle-based and uses Newtonian dynamics to simulate ice-fracture, motion and calving. The code can simulate real-size glacier but is only able to simualte individual calving events within a few tens of minutes in duration. The code couples to the Elmer/Ice ice flow-simulation code: Elmer is employed to produce various glacier geomteries, which are then tested for stability using the particle code. In this way it is possible to pin-point the location of calving fronts. The particle simulation code and field observations are engaged to investigate the criticality of calving glaciers. The calving mass and inter-event waiting times both have power-law distributions with the same critical exponents as found for Abelian sand-pile models. This indicate that calving glaciers share characteristics with Self-Organized Critical systems (SOC). This would explain why many glacier found in nature may become unstable as a result of even minor changes in their environment. An SOC calving glacier at the critical point will display so large fluctuations in calving rate that it will render the concept 'average calving rate' more or less useless. I.e. 'average calving rate' will depend on measurement time and always have fluctuaions in the range of 100% more or less independent of the averaging time.
First-Principles Study of Back Contact Effects on CdTe Thin Film Solar Cells
Du, Mao-Hua [ORNL
2009-01-01
Forming a chemically stable low-resistance back contact for CdTe thin-film solar cells is critically important to the cell performance. This paper reports theoretical study of the effects of the back-contact material, Sb{sub 2}Te{sub 3}, on the performance of the CdTe solar cells. First-principles calculations show that Sb impurities in p-type CdTe are donors and can diffuse with low diffusion barrier. There properties are clearly detrimental to the solar-cell performance. The Sb segregation into the grain boundaries may be required to explain the good efficiencies for the CdTe solar cells with Sb{sub 2}Te{sub 3} back contacts.
Supercapacitors composed of graphene and boron nitride layers: A first-principles study
Özçelik, V Ongun; Ciraci, S
2012-01-01
We propose a model for nanoscale supercapacitor consisting of two-dimensional insulating BN layers placed between two commensurate and metallic graphene layers. First-principles Density Functional calculations of structure optimized total energy and self-consistent field potential performed on these nanoscale capacitors for different levels of charging and different number of BN layers mark the values of capacitance per unit mass, which are larger than those measured values for the supercapacitors made from other carbon based materials. Our theoretical study also compares results obtained for the present nanoscale capacitor with those of classical Helmholtz model and reveals crucial differences. Our model allows the fabrication of series/parallel mixed combinations consisting of epitaxially grown, sequential and multiple graphene/BN sheets.
Experimental and first principle studies on electronic structure of BaTiO3
NASA Astrophysics Data System (ADS)
Sagdeo, Archna; Ghosh, Haranath; Chakrabarti, Aparna; Kamal, C.; Ganguli, Tapas; Phase, D. M.; Deb, S. K.
2014-04-01
We have carried out photoemission experiments to obtain valence band spectra of various crystallographic symmetries of BaTiO3 system which arise as a function of temperature. We also present results of a detailed first principle study of these symmetries of BaTiO3 using generalized gradient approximation for the exchange-correlation potential. Here we present theoretical results of density of states obtained from DFT based simulations to compare with the experimental valence band spectra. Further, we also perform calculations using post density functional approaches like GGA + U method as well as non-local hybrid exchange-correlation potentials like PBE0, B3LYP, HSE in order to understand the extent of effect of correlation on band gaps of different available crystallographic symmetries (5 in number) of BaTiO3.
Gettering mechanism of transition metals in silicon calculated from first principles
NASA Astrophysics Data System (ADS)
Matsukawa, Kazuhito; Hattori, Nobuyoshi; Maegawa, Shigeto; Shirai, Koun; Katayama-Yoshida, Hiroshi
2006-04-01
The gettering efficiency for 3d transition-metal impurities (Fe, Ni, Cu) were studied theoretically by calculation of the binding energy of the complex formed between TM and an electronic dopant atom. In this study the B dopant in silicon was used as the gettering center. The binding energy of FeB, NiB and CuB were calculated by the first principle method, and the results were 0.64, 0.44 and 0.57, respectively. We succeeded to reproduce some of experimental facts, and it can simply be said that Ni is difficult to be gettered by ionized B, and it was found that the gettering of Fe is more efficient than of Cu when B is used as the gettering center.
NASA Astrophysics Data System (ADS)
Li, Yang; Lian, Fang; Chen, Ning; Hao, Zhen-jia; Chou, Kuo-chih
2015-05-01
A first-principles method is applied to comparatively study the stability of lithium metal oxides with layered or spinel structures to predict the most energetically favorable structure for different compositions. The binding and reaction energies of the real or virtual layered LiMO2 and spinel LiM2O4 (M = Sc-Cu, Y-Ag, Mg-Sr, and Al-In) are calculated. The effect of element M on the structural stability, especially in the case of multiple-cation compounds, is discussed herein. The calculation results indicate that the phase stability depends on both the binding and reaction energies. The oxidation state of element M also plays a role in determining the dominant structure, i.e., layered or spinel phase. Moreover, calculation-based theoretical predictions of the phase stability of the doped materials agree with the previously reported experimental data.
Electronic and optical properties of RESn3 (RE=Pr & Nd) intermetallics: A first principles study
NASA Astrophysics Data System (ADS)
Pagare, G.; Abraham, Jisha A.; Sanyal, S. P.
2015-06-01
A theoretical study of structural, electronic and optical properties of RESn3 (RE = Pr & Nd) intermetallics have been investigated systematically using first principles density functional theory. The calculations are carried out within the PBE-GGA and LSDA for the exchange correlation potential. The ground state properties such as lattice parameter (a0), bulk modulus (B) and its pressure derivative (B') are calculated and the calculated lattice parameters show well agreement with the experimental results. We first time predict elastic constants for these compounds. From energy dispersion curves, it is found that these compounds are metallic in nature. The linear optical response of these compounds are also studied and the higher value of static dielectric constant shows the possibility to use them as good dielectric materials.
Experimental and first principle studies on electronic structure of BaTiO{sub 3}
Sagdeo, Archna, E-mail: archnaj@rrcat.gov.in; Ghosh, Haranath, E-mail: archnaj@rrcat.gov.in; Chakrabarti, Aparna, E-mail: archnaj@rrcat.gov.in; Kamal, C., E-mail: archnaj@rrcat.gov.in; Ganguli, Tapas, E-mail: archnaj@rrcat.gov.in; Deb, S. K. [Indus Synchrotrons Utilization Division, Raja Ramanna Centre for Advanced Technology, Indore 452013 (India); Phase, D. M. [IUC-DAEF, University Campus, Khandwa Road, Indore-452017 (India)
2014-04-24
We have carried out photoemission experiments to obtain valence band spectra of various crystallographic symmetries of BaTiO{sub 3} system which arise as a function of temperature. We also present results of a detailed first principle study of these symmetries of BaTiO{sub 3} using generalized gradient approximation for the exchange-correlation potential. Here we present theoretical results of density of states obtained from DFT based simulations to compare with the experimental valence band spectra. Further, we also perform calculations using post density functional approaches like GGA + U method as well as non-local hybrid exchange-correlation potentials like PBE0, B3LYP, HSE in order to understand the extent of effect of correlation on band gaps of different available crystallographic symmetries (5 in number) of BaTiO{sub 3}.
First-principles investigations of the physical properties of binary uranium silicide alloys
NASA Astrophysics Data System (ADS)
Yang, Jin; Long, Jianping; Yang, Lijun; Li, Dongmei
2013-11-01
The structural, elastic properties and the Debye temperature of binary Uranium Silicide (U-Si) alloys are investigated by using the first-principles plane-wave pseudopotential density function theory within the generalized gradient approximation (GGA). The ground states properties are found to agree with the available experimental data. The mechanical properties like shear modulus, Young's modulus, Poisson's ratio ? and ratio B/G are also calculated. Finally, The averaged sound velocity (vm), the longitudinal sound velocity (vl), transverse sound velocity (vt) and the Debye temperature (?D) are obtained. However, the theoretical values are slightly different from few existed experiment data because the latter was obtained at room temperature while the former one at 0 K.
First-principles prediction of charge mobility in carbon and organic nanomaterials.
Xi, Jinyang; Long, Mengqiu; Tang, Ling; Wang, Dong; Shuai, Zhigang
2012-08-01
We summarize our recent progresses in developing first-principles methods for predicting the intrinsic charge mobility in carbon and organic nanomaterials, within the framework of Boltzmann transport theory and relaxation time approximation. The electron-phonon couplings are described by Bardeen and Shockley's deformation potential theory, namely delocalized electrons scattered by longitudinal acoustic phonons as modeled by uniform lattice dilation. We have applied such methodology to calculating the charge carrier mobilities of graphene and graphdiyne, both sheets and nanoribbons, as well as closely packed organic crystals. The intrinsic charge carrier mobilities for graphene sheet and naphthalene are calculated to be 3 × 10(5) and ?60 cm(2) V(-1) s(-1) respectively at room temperature, in reasonable agreement with previous studies. We also present some new theoretical results for the recently discovered organic electronic materials, diacene-fused thienothiophenes, for which the charge carrier mobilities are predicted to be around 100 cm(2) V(-1) s(-1). PMID:22695470
Theoretical Study of the Spectroscopy of TiO
NASA Astrophysics Data System (ADS)
Langhoff, Stephen R.
1997-05-01
Theoretical electronic transition moments are presented for all of the dipole-allowed transitions connecting the f 1?, c 1?, b 1?, d 1?+, and a 1? singlet states and the C 3?, B 3?, A 3?, E 3?, and X 3? triplet states of TiO. The transition moments connecting the C 3? state with lower states were determined at the complete active space self-consistent field (CASSCF) level, and the transition moments connecting the other states were determined at both the CASSCF and the internally contracted multireference configuration-interaction (IC-MRCI) level. The CASSCF active space can be made sufficiently large that the effect of electron correlation on the transition moments is included quantitatively. The theoretical radiative lifetimes for the short-lived states are generally within 10% of the recent laser fluorescence measurements of Hedgecock, Naulin, & Costes. For the v = 0 level of the E 3? state, we obtain a radiative lifetime of 4259 ns, consistent with their determination of ?v=0 > 2000 ns, but much longer than the lifetime of 770 +/- 40 ns reported by Simard & Hackett. Except for the ?-bands, the theoretical oscillator strengths are less than those currently employed in molecular opacity calculations.
First principles modeling of nonlinear incidence rates in seasonal epidemics.
Ponciano, José M; Capistrán, Marcos A
2011-02-01
In this paper we used a general stochastic processes framework to derive from first principles the incidence rate function that characterizes epidemic models. We investigate a particular case, the Liu-Hethcote-van den Driessche's (LHD) incidence rate function, which results from modeling the number of successful transmission encounters as a pure birth process. This derivation also takes into account heterogeneity in the population with regard to the per individual transmission probability. We adjusted a deterministic SIRS model with both the classical and the LHD incidence rate functions to time series of the number of children infected with syncytial respiratory virus in Banjul, Gambia and Turku, Finland. We also adjusted a deterministic SEIR model with both incidence rate functions to the famous measles data sets from the UK cities of London and Birmingham. Two lines of evidence supported our conclusion that the model with the LHD incidence rate may very well be a better description of the seasonal epidemic processes studied here. First, our model was repeatedly selected as best according to two different information criteria and two different likelihood formulations. The second line of evidence is qualitative in nature: contrary to what the SIRS model with classical incidence rate predicts, the solution of the deterministic SIRS model with LHD incidence rate will reach either the disease free equilibrium or the endemic equilibrium depending on the initial conditions. These findings along with computer intensive simulations of the models' Poincaré map with environmental stochasticity contributed to attain a clear separation of the roles of the environmental forcing and the mechanics of the disease transmission in shaping seasonal epidemics dynamics. PMID:21379320
First-principles investigation of hydrous post-perovskite
NASA Astrophysics Data System (ADS)
Townsend, Joshua P.; Tsuchiya, Jun; Bina, Craig R.; Jacobsen, Steven D.
2015-07-01
A stable, hydrogen-defect structure of post-perovskite (hy-ppv, Mg1-xSiH2xO3) has been determined by first-principles calculations of the vibrational and elastic properties up to 150 GPa. Among three potential hy-ppv structures analyzed, one was found to be stable at pressures relevant to the lower-mantle D? region. Hydrogen has a pronounced effect on the elastic properties of post-perovskite due to magnesium defects associated with hydration, including a reduction of the zero-pressure bulk (K0) and shear (G0) moduli by 5% and 8%, respectively, for a structure containing ?1 wt.% H2O. However, with increasing pressure the moduli of hy-ppv increase significantly relative to ppv, resulting in a structure that is only 1% slower in bulk compressional velocity and 2.5% slower in shear-wave velocity than ppv at 120 GPa. In contrast, the reduction of certain anisotropic elastic constants (Cij) in hy-ppv increases with pressure (notably, C55, C66, and C23), indicating that hydration generally increases elastic anisotropy in hy-ppv at D? pressures. Calculated infrared absorption spectra show two O-H stretching bands at ?3500 cm-1 that shift with pressure to lower wavenumber by about 2 cm-1/GPa. At 120 GPa the hydrogen bonds in hy-ppv are still asymmetric. The stability of a hy-ppv structure containing 1-2 wt.% H2O at D? pressures implies that post-perovskite may be a host for recycled or primordial hydrogen near the Earth's core-mantle boundary.
First-principles studies of Ni-Ta intermetallic compounds
Zhou Yi [Department of Materials Science and Engineering, College of Materials, and Research Center of Materials Design and Applications, Xiamen University, Xiamen 361005 (China); Wen Bin, E-mail: wenbin@ysu.edu.cn [State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004 (China); Ma Yunqing [Department of Materials Science and Engineering, College of Materials, and Research Center of Materials Design and Applications, Xiamen University, Xiamen 361005 (China); Melnik, Roderick [M2NeT Lab, Wilfrid Laurier University, Waterloo,75 University Ave. West, Ontario, N2L 3C5 (Canada); MIT Department, University of Jyvaeskylae, Jyvaeskylae (Finland); Liu Xingjun, E-mail: lxj@xmu.edu.cn [Department of Materials Science and Engineering, College of Materials, and Research Center of Materials Design and Applications, Xiamen University, Xiamen 361005 (China)
2012-03-15
The structural properties, heats of formation, elastic properties, and electronic structures of Ni-Ta intermetallic compounds are investigated in detail based on density functional theory. Our results indicate that all Ni-Ta intermetallic compounds calculated here are mechanically stable except for P21/m-Ni{sub 3}Ta and hc-NiTa{sub 2}. Furthermore, we found that Pmmn-Ni{sub 3}Ta is the ground state stable phase of Ni{sub 3}Ta polymorphs. The polycrystalline elastic modulus has been deduced by using the Voigt-Reuss-Hill approximation. All Ni-Ta intermetallic compounds in our study, except for NiTa, are ductile materials by corresponding G/K values and poisson's ratio. The calculated heats of formation demonstrated that Ni{sub 2}Ta are thermodynamically unstable. Our results also indicated that all Ni-Ta intermetallic compounds analyzed here are conductors. The density of state demonstrated the structure stability increases with the Ta concentration. - Graphical abstract: Mechanical properties and formation heats of Ni-Ta intermetallic compounds are discussed in detail in this paper. Highlights: Black-Right-Pointing-Pointer Ni-Ta intermetallic compounds are investigated by first principle calculations. Black-Right-Pointing-Pointer P21/m-Ni{sub 3}Ta and hc-NiTa{sub 2} are mechanically unstable phases. Black-Right-Pointing-Pointer Pmmn-Ni{sub 3}Ta is ground stable phase of Ni{sub 3}Ta polymorphs. Black-Right-Pointing-Pointer All Ni-Ta intermetallic compounds are conducting materials.
Risk reduction and the privatization option: First principles
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
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.
First-principles study of codoping in lanthanum bromide
NASA Astrophysics Data System (ADS)
Erhart, Paul; Sadigh, Babak; Schleife, André; Åberg, Daniel
2015-04-01
Codoping of Ce-doped LaBr3 with Ba, Ca, or Sr improves the energy resolution that can be achieved by radiation detectors based on these materials. Here, we present a mechanism that rationalizes this enhancement on the basis of first-principles electronic structure calculations and point defect thermodynamics. It is shown that incorporation of Sr creates neutral VBr-SrLa complexes that can temporarily trap electrons. As a result, Auger quenching of free carriers is reduced, allowing for a more linear, albeit slower, scintillation light yield response. Experimental Stokes shifts can be related to different CeLa-SrLa-VBr triple complex configurations. Codoping with other alkaline as well as alkaline-earth metals is considered as well. Alkaline elements are found to have extremely small solubilities on the order of 0.1 ppm and below at 1000 K. Among the alkaline-earth metals the lighter dopant atoms prefer interstitial-like positions and create strong scattering centers, which has a detrimental impact on carrier mobilities. Only the heavier alkaline-earth elements (Ca, Sr, Ba) combine matching ionic radii with sufficiently high solubilities. This provides a rationale for the experimental finding that improved scintillator performance is exclusively achieved using Sr, Ca, or Ba. The present mechanism demonstrates that codoping of wide-gap materials can provide an efficient means for managing charge carrier populations under out-of-equilibrium conditions. In the present case dopants are introduced that manipulate not only the concentrations but also the electronic properties of intrinsic defects without introducing additional gap levels. This leads to the availability of shallow electron traps that can temporarily localize charge carriers, effectively deactivating carrier-carrier recombination channels. The principles of this mechanism are therefore not specific to the material considered here but can be adapted for controlling charge carrier populations and recombination in other wide-gap materials.
First principles evaluation of the photocatalytic properties of cuprous oxide
NASA Astrophysics Data System (ADS)
Bendavid, Leah Isseroff
Cuprous oxide (Cu2O) is a semiconductor attractive for use as a photocatalyst in renewable fuel production, but has thus far exhibited low efficiencies in solar energy technologies. A thorough understanding of its photocatalytically relevant properties is needed to develop improved cuprous oxide-based photocatalysts. This dissertation uses first principles calculations founded in quantum mechanics to study the physical, optical, electronic, and chemical properties of cuprous oxide and to optimize its performance in solar energy applications. The key properties that affect efficiency include electronic excitations, the band gap, band edge positions, charge transport, defect trap states, catalyst stability, and surface chemistry. The band gap of Cu 2O, which defines the efficiency of solar energy absorption, is first calculated with hybrid density functional theory (DFT) followed by a single GW perturbation. We also design methods to calculate optical excitations using embedded correlated wavefunction theory. The low-index surfaces are characterized using DFT+U, where we identify the (111) surface as the most stable. This surface is employed in the derivation of the band edges of Cu2O, which demonstrate that Cu2O can provide the thermodynamic overpotential needed to drive water splitting and the reduction of CO2 to methanol. We also identify the adsorption mechanisms of weakly physisorbed CO2 and the more strongly adsorbed H2O on the Cu2O(111) surface. Effective charge transport is needed so that photoexcited carriers can reach the surface active sites prior to recombination. We study electron and hole transport in Cu2O using the small polaron model, and show that its localized description is inappropriate for carrier transport, which is better modeled using band theory. We then use an approach founded in band theory to analyze the cause of intrinsic trap states, which promote carrier recombination. We conclude that doping with Li can prevent trap state formation and thus reduce recombination. Finally, because Cu2O is known to be photocathodically unstable, we consider a suggested method of stabilizing Cu2O via deposition on a ZnO substrate. We evaluate the properties of the Cu2O(111)/ZnO(101¯0) interface, revealing that it is weakly bound. The ZnO substrate reduces the band gap of the Cu2O coating.
First-principles calculations of conductivity in transparent semiconducting oxides
NASA Astrophysics Data System (ADS)
Varley, Joel Basile
2011-12-01
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.
Electron Exchange and Conduction in Nontronite from First-Principles
Alexandrov, Vitali Y.; Neumann, Anke; Scherer, Michelle; Rosso, Kevin M.
2013-01-11
Fe-bearing clay minerals serve as an important source and sink for electrons in redox reactions in various subsurface geochemical environments, and electron transfer (ET) properties of the Fe2+/Fe3+ redox couple play a decisive role in a variety of physicochemical processes involving clays. Here, we apply first-principles calculations using both periodic GGA+U planewave and Hartree-Fock molecular-cluster frameworks in conjuction with small polaron hopping approach and Marcus electron transfer theory to examine electron exchange mobilities in an Fe-rich smectite, taking nontronite as a case study. GGA+U calculations of the activation barrier for small-polaron migration provide rates of electron hopping that agree very well with values deduced from variable temperature Mössbauer data (M. V. Schaefer, et. al., Environ. Sci. Technol. 45, 540, (2011)), indicating a surprisingly fast electron mobility at room temperature. Based on molecular cluster calculations, we show that the state with tetrahedral Fe2+ ion in the nontronite lattice is about 0.9 eV higher than the one with octahedral Fe2+. Also, evaluation of the ET rates for the Fe2+/Fe3+ electron hopping in tetrahedral (TS) and octahedral sheets (OS), as well as across the sheets (TS–OS) shows that the dominant contribution to the bulk electronic conductivity should come from the ET within the OS. Deprotonation of structural OH groups mediating ET between the Fe ions in the OS is found to decrease the internal reorganization energy and to increase the magnitude of the electronic coupling matrix element, whereas protonation (to OH2 groups) has the opposite effect. Overall, our calculations suggest that the major factors affecting ET rates are the nature and structure of the nearest-neighbor local environment and the degree of covalency of the bonds between Fe and ligands mediating electron hops. The generally higher reorganization energy and weaker electronic coupling found in Fe-bearing clay minerals leads to electron mobilities much lower than in iron oxides.
First Principles Modeling of Bimolecular Reactions with Diffusion
NASA Astrophysics Data System (ADS)
Hansen, S. K.; Scher, H.; Berkowitz, B.
2013-12-01
We consider three approaches to modeling A + B ? C irreversible reactions in natural media: 1) a discretized diffusion-reaction equation (DRE), 2) a particle tracking (PT) scheme in which reaction occurs if and only if an A and B particle pair are within a fixed distance, r (the "reaction radius"), and 3) a PT scheme using an alternative to the fixed reaction radius: a collocation probability distribution derived directly from first principles. Each approach has advantages. In some cases a discretized DRE may be the most computationally efficient method. For PT simulations, robust codes exist based on use of a fixed reaction radius. And finally, collocation probabilities may be derived directly from the Fick's Law constant, D, which is a well-established property for most species. In each approach, a single parameter governs the 'promiscuity' of the reaction (i.e. the thermodynamic favorability of reaction, predicated on the particles being locally well mixed). For the DRE, fixed-reaction-radius PT, and collocation-based PT, these parameters are, respectively: a second-order decay rate, r, and D. We established a number of new results enhancing these approaches and relating them to each other (and to nature). In particular, a thought experiment concerning a simple system in which the predictions of each approach can be computed analytically was used to derive formulas establishing a universal one-to-one correspondence among each of the governing parameters. We thus showed the conditions for equivalence of the three approaches, and grounded both the DRE approach and the fixed-radius PT approach in the Fick's Law D. We further showed that the existing collocation-based PT theory is based on a probability distribution that is only correct for infinitesimally small times, but which can be modified to be accurate for larger times by means of continuous time random walk analysis and first-passage probability distributions. Finally, we employed a novel mathematical approach to adapt this into a workable collocation-based particle tracking technique.
NASA Astrophysics Data System (ADS)
Muthu, D. V. S.; Teredesai, Pallavi; Saha, S.; Suchitra, Waghmare, U. V.; Sood, A. K.; Rao, C. N. R.
2015-06-01
We report high-pressure Raman-scattering studies on single-crystal Re O3 up to 26.9 GPa at room temperature, complemented by first-principles density functional calculations to assign the modes and to develop understanding of the subtle features of the low-pressure phase transition. The pressure (P ) dependence of phonon frequencies (? ) reveals three phase transitions at 0.6, 3, and 12.5 GPa with characteristic splitting and changes in the slope of ? (P ). Our first-principles theoretical analysis confirms the role of the rotational modes of Re O6 ,M3, to the lowest pressure structural transition, and shows that the transition from the P m 3 m to the I m 3 structure is a weak first-order transition, originating from the strong anharmonic coupling of the M3 modes with the acoustic modes (strain).
Shock Hugoniot of osmium up to 800 GPa from first principles calculations.
Joshi, K D; Gupta, Satish C; Banerjee, S
2009-10-14
First principles total energy calculations on hcp, ? (a three atom simple hexagonal), ? (bcc) and fcc phases of osmium have been performed as a function of hydrostatic compression employing the FP-LAPW method. The comparison of total energies of these phases up to a maximum compression V/V(0) = 0.58 (pressure?700 GPa) shows that the hcp structure remains stable up to this compression. The 300 K isotherm is determined after adding finite temperature thermal contributions to the total energy calculated as a function of volume at 0 K. From the theoretically determined isotherm, we have derived the shock Hugoniot of this metal and determined the shock parameters C(0) and s to be 4.48 km s(-1) and 1.32, respectively. Employing the theoretically calculated Gruneisen parameter in the differential form of the Lindemann melting rule, we have determined the variation of melting point of the osmium with pressure. The theoretically derived melting curve and the temperature rise along the Hugoniot predict the shock melting of osmium at ?447 GPa with a corresponding temperature of ?9203 K. PMID:21693986
First-principles theory, coarse-grained models, and simulations of ferroelectrics.
Waghmare, Umesh V
2014-11-18
CONSPECTUS: A ferroelectric crystal exhibits macroscopic electric dipole or polarization arising from spontaneous ordering of its atomic-scale dipoles that breaks inversion symmetry. Changes in applied pressure or electric field generate changes in electric polarization in a ferroelectric, defining its piezoelectric and dielectric properties, respectively, which make it useful as an electromechanical sensor and actuator in a number of applications. In addition, a characteristic of a ferroelectric is the presence of domains or states with different symmetry equivalent orientations of spontaneous polarization that are switchable with large enough applied electric field, a nonlinear property that makes it useful for applications in nonvolatile memory devices. Central to these properties of a ferroelectric are the phase transitions it undergoes as a function of temperature that involve lowering of the symmetry of its high temperature centrosymmetric paraelectric phase. Ferroelectricity arises from a delicate balance between short and long-range interatomic interactions, and hence the resulting properties are quite sensitive to chemistry, strains, and electric charges associated with its interface with substrate and electrodes. First-principles density functional theoretical (DFT) calculations have been very effective in capturing this and predicting material and environment specific properties of ferroelectrics, leading to fundamental insights into origins of ferroelectricity in oxides and chalcogenides uncovering a precise picture of electronic hybridization, topology, and mechanisms. However, use of DFT in molecular dynamics for detailed prediction of ferroelectric phase transitions and associated temperature dependent properties has been limited due to large length and time scales of the processes involved. To this end, it is quite appealing to start with input from DFT calculations and construct material-specific models that are realistic yet simple for use in large-scale simulations while capturing the relevant microscopic interactions quantitatively. In this Account, we first summarize the insights obtained into chemical mechanisms of ferroelectricity using first-principles DFT calculations. We then discuss the principles of construction of first-principles model Hamiltonians for ferroelectric phase transitions in perovskite oxides, which involve coarse-graining in time domain by integrating out high frequency phonons. Molecular dynamics simulations of the resulting model are shown to give quantitative predictions of material-specific ferroelectric transition behavior in bulk as well as nanoscale ferroelectric structures. A free energy landscape obtained through coarse-graining in real-space provides deeper understanding of ferroelectric transitions, domains, and states with inhomogeneous order and points out the key role of microscopic coupling between phonons and strain. We conclude with a discussion of the multiscale modeling strategy elucidated here and its application to other materials such as shape memory alloys. PMID:25361389
First-Principles Investigations of Defects in Minerals
NASA Astrophysics Data System (ADS)
Verma, Ashok K.
2011-07-01
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.
First-principles models of equilibrium tellurium isotope fractionation
NASA Astrophysics Data System (ADS)
Haghnegahdar, M. A.; Schauble, E. A.; Fornadel, A. P.; Spry, P. G.
2013-12-01
In this study, equilibrium mass-dependent isotopic fractionation among representative Te-bearing species is estimated with first-principles thermodynamic calculations. Tellurium is a group 16 element (along with O, S, and Se) with eight stable isotopes ranging in mass from 120Te to 130Te, and six commonly-occurring oxidation states: -II, -I, 0, +II, +IV, and +VI. In its reduced form, Te(-II), tellurium has a unique crystal-chemical role as a bond partner for gold and silver in epithermal and orogenic gold deposits, which likely form when oxidized Te species (e.g., H2TeO3, TeO32-) or perhaps polytellurides (e.g., Te22-) interact with precious metals in hydrothermal solution. Te(IV) is the most common oxidation state at the Earth's surface, including surface outcrops of telluride ore deposits, where tellurite and tellurate minerals form by oxidation. In the ocean, dissolved tellurium tends to be scavenged by particulate matter. Te(VI) is more abundant than Te(IV) in the ocean water (1), even though it is thought to be less stable thermodynamically. This variety of valence states in natural systems and range of isotopic masses suggest that tellurium could exhibit geochemically useful isotope abundance variations. Tellurium isotope fractionations were determined for representative molecules and crystals of varying complexity and chemistry. Gas-phase calculations are combined with supermolecular cluster models of aqueous and solid species. These in turn are compared with plane-wave density functional theory calculations with periodic boundary conditions. In general, heavyTe/lightTe is predicted to be higher for more oxidized species, and lower for reduced species, with 130Te/125Te fractionations as large as 4‰ at 100?C between coexisting Te(IV) and Te(-II) or Te(0) compounds. This is a much larger fractionation than has been observed in naturally occurring redox pairs (i.e., Te (0) vs. Te(IV) species) so far, suggesting that disequilibrium processes may control oxidation and reduction. Se- and S-isotope redox systems also show kinetically-controlled isotopic disequilibrium, and may serve as useful analogues. Among Te(-II) species, fractionations are smaller (< 1‰ at 100?C). (Au,Ag)Te2 minerals (calaverite, krennerite) and (Au,Ag)2Te minerals (petzite, hessite) are expected to have similar isotopic compositions to vapor-phase H2Te, and there appears to be little discernable effect from Ag,Au solid solution. Altaite (PbTe) will have somewhat lower 130Te/125Te. Calculated fractionation factors for gas-phase species like H2Te and TeF6, based on hybrid density functional theory (B3LYP), agree well with earlier estimates (2). For telluride-bearing crystals, cluster models are in qualitative agreement with periodic boundary condition calculations, especially when larger basis sets (e.g., aug-cc-PVDZ-PP) are used; however, cluster models tend to predict higher 130Te/125Te for reasons that are not yet clear. References: 1. Hein et al. (2003) GCA 67:6. 2. Smithers et al. (1968) Can. J. Chem. 46:4.
First-principles elastic properties of (alpha)-Pu
Soderlind, P; Klepeis, J E
2008-11-04
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.
NASA Astrophysics Data System (ADS)
Hermann, A.; Schmidt, W. G.; Bechstedt, F.
2005-04-01
The modification of the semiconductor-surface optical response is investigated for the prototypical system of 9,10-phenanthrenequinone adsorbed on Si(001). By means of first-principles calculations we reproduce and explain recently measured reflectance-anisotropy data. The detailed analysis of the optical signal shows that intramolecular ?-?* transitions as well as adsorption-modified Si bulk states contribute to the surface spectrum. The calculations illustrate the sensitivity of optical spectroscopy to molecular adsorption, but demonstrate clearly that a naive interpretation of the spectrum in terms of gas-phase molecular data fails.
Xu, W; Moriarty, J.A.
1996-01-19
Using multi-ion interatomic potentials derived from first-principles generalized pseudopotential theory, we have been studying point defects and dislocations in bcc transition metals, with molybdenum (Mo) as a prototype. For point defects in Mo, the calculated vacancy formation and activation energies are in excellent agreement with experimental results. The energetics of six self-interstitial configurations in Mo have also been investigated. The <110> split dumb-bell is found to have the lowest formation energy, as is experimentally observed, but the corresponding migration energy is calculated to be 3--15 times higher than previous theoretical estimates. The atomic structure and energetics of <111> screw dislocations in Mo are now being investigated. We have found that the ``easy`` core configuration has a lower formation energy than the ``hard`` one, consistent with previous theoretical studies. The former has a distinctive 3-fold symmetry with a spread out of the dislocation core along the <112> directions, an effect which is driven by the strong angular forces present in these metals.
First-principle study of thermoelectric properties of impurity-doped magnesium silicide Mg2Si
NASA Astrophysics Data System (ADS)
Funashima, Hiroki
2014-03-01
The electronic structure and the thermoelectric properties of Mg2Si doped with several dopants, Al, Bi, Sb, and Zn, are theoretically examined using a first-principles calculation method. Mg2Si is a promising thermoelectric material that is functional in the temperature range from 500 to 800 K. Therefore, it is expected to be useful for recovering waste heat from exhaust gas in automotive applications, incinerators, and boilers. Moreover, this material has several desirable attributes with respect to cost and environmental protection: it is cheap, nontoxic, and composed of elements abundant on Earth. These advantages are important for practical usage in thermoelectric applications. Impurity doping is a well-established way to improve the thermoelectric performance of Mg2Si. Undoped Mg2Si crystals have n-type conductivity, but they can be doped with both n- and p-type impurities. A fundamental understanding of the relationship between impurity doping and the thermoelectric properties of Mg2Si will allow us to provide theoretical guidelines for further development of this material. As an effort toward this goal, we present here the band structure of Mg2Si using the full-potential linearized augmented plane-wave (FLAPW) method based on LDA/DFT and the conductivity.
Marc Blanchard; Franck Poitrasson; Merlin Méheut; Michele Lazzeri; Francesco Mauri; Etienne Balan
2009-01-01
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
Ceder, Gerbrand
of first-principles energy calculations, a cluster expansion technique, and Monte Carlo simulations. Our), and near-zero temperature coefficient of the resonant frequency (TCf) are desirable properties enhances microwave properties in ceramics. Some theoretical investigation into the ordering strength
NASA Astrophysics Data System (ADS)
Blanchard, Marc; Poitrasson, Franck; Méheut, Merlin; Lazzeri, Michele; Mauri, Francesco; Balan, Etienne
2009-11-01
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.
Design and Characterization of Photoelectrodes from First Principles
Ogitsu, T; Wood, B; Choi, W; Huda, M; Wei, S
2012-05-11
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.
NASA Astrophysics Data System (ADS)
Sun, Xiao-Wei; Chen, Qi-Feng; Chen, Xiang-Rong; Cai, Ling-Cang; Jing, Fu-Qian
2011-11-01
The authors have presented a detailed investigation on the phase stabilities and electronic properties of ideal stoichiometric platinum carbide (PtC) in the rock-salt (RS) and zinc-blende (ZB) structures under high pressure. Theoretical calculations are performed using the first-principles pseudopotential density functional method, in which we employ the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof form and local density approximation (LDA) of Ceperly and Adler parameterized by Perdew and Zunger together with plane-wave basis sets for expanding the periodic electron density. Through a series of tests, such as the total energy as a function of volume, the Gibbs free energy as a function of pressure, the P-V equation of states, the elastic stabilities, and the electronic band structures of PtC with ZB and RS phases, we have confirmed that the recently synthesized compound PtC is crystallized in the ZB structure at zero pressure and that the RS structure is a high-pressure phase; the phase transition studied from the usual condition of equal enthalpies occurs at the pressures of 46.6 and 46.5 GPa for GGA and LDA calculations, respectively. Our conclusions are consistent with the theoretical prediction obtained from the full-potential linearized augmented plane-wave method, but are reversed with the DAC experimental results and other pseudopotential plane-wave theoretical results. Therefore, the experimental observation of the RS structure in PtC remains a puzzle, and our study indicates that further experimental and theoretical investigations need to be carried out to find the cause of the stability of the PtC.
Learning Biped Locomotion from First Principles on a Simulated Humanoid Robot
Fernandez, Thomas
Learning Biped Locomotion from First Principles on a Simulated Humanoid Robot using Linear Genetic://www.frt.fy.chalmers.se/cs/index.html Abstract. We describe the first instance of an approach for control programming of humanoid robots, based in this concept is to evolve control pro- grams from first principles on a simulated robot, transfer the resulting
Length dependence of electron transport through molecular wires--a first principles perspective.
Khoo, Khoong Hong; Chen, Yifeng; Li, Suchun; Quek, Su Ying
2015-01-01
One-dimensional wires constitute a fundamental building block in nanoscale electronics. However, truly one-dimensional metallic wires do not exist due to Peierls distortion. Molecular wires come close to being stable one-dimensional wires, but are typically semiconductors, with charge transport occurring via tunneling or thermally-activated hopping. In this review, we discuss electron transport through molecular wires, from a theoretical, quantum mechanical perspective based on first principles. We focus specifically on the off-resonant tunneling regime, applicable to shorter molecular wires (first principles theory are discussed, starting with the computational workhorse - density functional theory (DFT), and moving on to many-electron GW methods as well as GW-inspired DFT + Sigma calculations. These different levels of theory are applied in two major computational frameworks - complex band structure (CBS) calculations to estimate the tunneling decay constant, beta, and Landauer-Buttiker transport calculations that consider explicitly the effects of contact geometry, and compute the transmission spectra directly. In general, for the same level of theory, the Landauer-Buttiker calculations give more quantitative values of beta than the CBS calculations. However, the CBS calculations have a long history and are particularly useful for quick estimates of beta. Comparing different levels of theory, it is clear that GW and DFT + Sigma calculations give significantly improved agreement with experiment compared to DFT, especially for the conductance values. Quantitative agreement can also be obtained for the Seebeck coefficient - another independent probe of electron transport. This excellent agreement provides confirmative evidence of off-resonant tunneling in the systems under investigation. Calculations show that the tunneling decay constant beta is a robust quantity that does not depend on details of the contact geometry, provided that the same contact geometry is used for all molecular lengths considered. However, because conductance is sensitive to contact geometry, values of beta obtained by considering conductance values where the contact geometry is changing with the molecular junction length can be quite different. Experimentally measured values of beta in general compare well with beta obtained using DFT + Sigma and GW transport calculations, while discrepancies can be attributed to changes in the experimental contact geometries with molecular length. This review also summarizes experimental and theoretical efforts towards finding perfect molecular wires with high conductance and small beta values. PMID:25407785
Sum frequency generation spectra of ice surfaces from first principles simulations
NASA Astrophysics Data System (ADS)
Wan, Quan; Gygi, Francois; Galli, Giulia
2014-03-01
The morphology and structure of ice surfaces play an important role in a variety of chemical and physical processes. Yet a detailed knowledge of the structural properties of ice surfaces at the molecular level is still lacking. Sum frequency generation (SFG) spectroscopy is a promising technique to address this problem, due to its high interface sensitivity. Here we present first principles simulations of SFG spectra of ice surfaces obtained by ab initio molecular dynamics, combined with density functional perturbation theory (DFPT), as implemented in the Qbox Code. We computed SFG signals from classical time correlation functions of the system's dipole moment and polarizability tensor, evaluated by using maximally localized Wannier functions and DFPT. By projecting the total SFG intensities onto each molecules, we analyzed the stretching band region of the SFG spectra and identified intra- and inter-bilayer modes. This analysis was then used to shed light on whether the ice surface is proton ordered or disordered. In addition to ice, we will also discuss simulations of SFG signals obtained for solid-water interfaces using the same ab initio method. Work supported by DOE/BES DE-SC0008938.
First-principles simulations of charge storage at electrochemical interfaces in supercapacitors
NASA Astrophysics Data System (ADS)
Wood, Brandon
2015-03-01
Supercapacitors store charge via polarization at the electrode-electrolyte interface. Many models of interfacial charge storage focus on the formation of the electric double layer (EDL) in the electrolyte, but it is often assumed that in the electrode, a shift in the Fermi level is the only notable response to interface polarization. In reality, the presence of the interface impacts the fundamental properties of both the electrode and the electrolyte, often in complex and nontrivial ways that are difficult to capture using simple models. I will discuss how including an applied bias potential in first-principles simulations allows one to directly simulate the process of charge storage at the electrode-electrolyte interface, and thereby to unravel the interplay between the electrode and the electrolyte. I will show how these more complex treatments lead to improved descriptions of intrinsic quantum and EDL capacitance contributions in graphene-based supercapacitors, which can be used to suggest engineering strategies for improved electrode materials. I will also discuss how combining theory with in operando X-ray spectroscopy can give insights into nanoscale chemical changes and mesoscale morphological changes in electrodes during charging. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344.
"First-principles" kinetic Monte Carlo simulations revisited: CO oxidation over RuO2 (110).
Hess, Franziska; Farkas, Attila; Seitsonen, Ari P; Over, Herbert
2012-03-15
First principles-based kinetic Monte Carlo (kMC) simulations are performed for the CO oxidation on RuO(2) (110) under steady-state reaction conditions. The simulations include a set of elementary reaction steps with activation energies taken from three different ab initio density functional theory studies. Critical comparison of the simulation results reveals that already small variations in the activation energies lead to distinctly different reaction scenarios on the surface, even to the point where the dominating elementary reaction step is substituted by another one. For a critical assessment of the chosen energy parameters, it is not sufficient to compare kMC simulations only to experimental turnover frequency (TOF) as a function of the reactant feed ratio. More appropriate benchmarks for kMC simulations are the actual distribution of reactants on the catalyst's surface during steady-state reaction, as determined by in situ infrared spectroscopy and in situ scanning tunneling microscopy, and the temperature dependence of TOF in the from of Arrhenius plots. PMID:22253041
Neutral and charged excitations in carbon fullerenes from first-principles many-body theories
NASA Astrophysics Data System (ADS)
Tiago, Murilo L.; Kent, P. R. C.; Hood, Randolph Q.; Reboredo, Fernando A.
2008-08-01
We investigate the accuracy of first-principles many-body theories at the nanoscale by comparing the low-energy excitations of the carbon fullerenes C20, C24, C50, C60, C70, and C80 with experiment. Properties are calculated via the GW-Bethe-Salpeter equation and diffusion quantum Monte Carlo methods. We critically compare these theories and assess their accuracy against available photoabsorption and photoelectron spectroscopy 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. These results establish the validity of many-body theories as viable alternative to density-functional theory in describing electronic properties of confined carbon nanostructures. We find a correlation between energy gap and stability of fullerenes. We also find that the electron affinity of fullerenes is very high and size independent, which explains their tendency to form compounds with electron-donor cations.
Kanagaprabha, S. [Department of Physics, Kamaraj College, Tuticorin, Tamil nadu-628003 (India); Rajeswarapalanichamy, R., E-mail: rrpalanichamy@gmail.com; Sudhapriyanga, G., E-mail: rrpalanichamy@gmail.com; Murugan, A., E-mail: rrpalanichamy@gmail.com; Santhosh, M., E-mail: rrpalanichamy@gmail.com [Department of Physics, N.M.S.S.V.N College, Madurai, Tamilnadu-625019 (India); Iyakutti, K. [Department of Physics and Nanotechnology, SRM University, Chennai, Tamilnadu-603203 (India)
2014-04-24
The electronic, structural and mechanical properties of ZrH and ZrH{sub 2} are investigated by means of first principles calculation based on density functional theory as implemented in VASP code with generalized gradient approximation. The calculated ground state properties are in good agreement with previous experimental and other theoretical results. Among the six crystallographic structures considered for ZrH, ZB phase is found to be the most stable phase, whereas ZrH{sub 2} is energetically stable in tetragonal structure at ambient condition. A structural phase transition from ZB?NaCl at a pressure 10 GPa is predicted for ZrH.
Rohlfing, Michael; Tiago, M.L.; Louie, Steven G.
2000-03-20
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).
NASA Astrophysics Data System (ADS)
Groh, Sébastien; Alam, Masud
2015-06-01
An approach to derive, from first-principles data, accurate and reliable potentials in the modified embedded-atom method in view of modeling the mechanical behavior of metals is presented in this work and applied to the optimization of a potential representative of lithium (Li). Although the theoretical background of the modified embedded-atom method was considered in this work, the proposed method is general and it can be applied to any other functional form. The main feature of the method is to introduce several path transformations in the material database that are critical for plastic and failure behavior. As part of the potential validation, path transformations different from the ones used for the parameterization procedure are considered. Applied in the case of Li, the material database was enriched with the generalized stacking fault energy curve along the??<1?1?1>??-direction on the {1?1?0}-plane, and with the traction-separation behavior of a {1?0?0}-surface. The path transformations used to enrich the material database were initially derived from first-principles calculations. For validation, the generalized stacking fault energy curves along the??<1?1?1>??-direction on the {1?1?2}- and {1?2?3}-planes were considered for plasticity, while traction-separation behavior of {1?1?0} and {1?1?1}-planes were considered for failure behavior. As part of the validation procedure, the predictions made in the MEAM framework were validated by first-principles data. The final potential accurately reproduced basic equilibrium properties, elastic constants, surface energies in agreement with first-principles predictions, and transition energy between different crystal structures. Furthermore, generalized stacking fault energy curves along the??<1?1?1>??-direction on the {1?1?0}, {1?1?2}, and {1?2?3}-planes, and tensile cohesive stress, characteristic length of fracture, and work of separation of a {1?0?0}, {1?1?0}, and {1?1?1} surfaces obtained in the MEAM framework compared well with first-principles predictions. It also predicts good elastic constants for a crystal structure different than the one used for the fitting of the potential and the other four path transformations. The potential was tested for failure behavior using a full atomistic setup, and in addition of being qualitatively correct, the stress intensity factor for different crack orientations was found to be in agreement with the theory of Rice (1992 J. Mech. Phys. Solids 40 239–71) within an error of 10%. Finally, the optimized Li-MEAM potential is expected to be transferable to different local environments encountered in atomistic simulations of lattice defects.
Electronic and optical properties of GaSb:N from first principles
NASA Astrophysics Data System (ADS)
Jadaun, Priyamvada; Nair, Hari; Lordi, Vincenzo; Bank, Seth; Banerjee, Sanjay
2014-03-01
We present an ab-initio study of dilute nitride III-Vs, focusing on dilute nitride GaSb (GaSb:N). GaSb:N displays promise towards realization of optoelectronic devices accessing the mid-infrared wavelength regime. Theoretical and experimental results on its electronic and optical properties are however few. To address this, we present a first principles, density functional theory study using the hybrid HSE06 exchange-correlation functional of GaSb doped with 1.6% nitrogen. We conduct a comparative study on GaAs:N, also with 1.6% nitrogen mole fraction, and find that GaSb:N has a smaller band gap and displays more band gap bowing than GaAs:N. In addition we examine the orbital character of the bands, finding the lowest conduction band to be quasi-delocalized, with a large N-3s contribution. At high concentrations, the N atoms interact via the host matrix, forming a dispersive band of their own which governs optoelectronic properties and dominates band gap bowing. While this band drives the optical and electronic properties of GaSb:N, its physics is not captured by traditional models for dilute-nitrides. We thus propose that a complete theory of dilute-nitrides should incorporate orbital character examination, especially at high N concentrations. Texas Advanced Computing Center (TACC), U.S. Department of Energy, Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344.
Thermodynamic ground state of MgB{sub 6} predicted from first principles structure search methods
Wang, Hui [State Key Lab of Superhard Materials, Jilin University, Changchun 130012 (China) [State Key Lab of Superhard Materials, Jilin University, Changchun 130012 (China); Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada); LeBlanc, K. A. [Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada)] [Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada); Gao, Bo [State Key Lab of Superhard Materials, Jilin University, Changchun 130012 (China)] [State Key Lab of Superhard Materials, Jilin University, Changchun 130012 (China); Yao, Yansun, E-mail: yansun.yao@usask.ca [Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada) [Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada); Canadian Light Source, Saskatoon, Saskatchewan S7N 0X4 (Canada)
2014-01-28
Crystalline structures of magnesium hexaboride, MgB{sub 6}, were investigated using unbiased structure searching methods combined with first principles density functional calculations. An orthorhombic Cmcm structure was predicted as the thermodynamic ground state of MgB{sub 6}. The energy of the Cmcm structure is significantly lower than the theoretical MgB{sub 6} models previously considered based on a primitive cubic arrangement of boron octahedra. The Cmcm structure is stable against the decomposition to elemental magnesium and boron solids at atmospheric pressure and high pressures up to 18.3 GPa. A unique feature of the predicted Cmcm structure is that the boron atoms are clustered into two forms: localized B{sub 6} octahedra and extended B{sub ?} ribbons. Within the boron ribbons, the electrons are delocalized and this leads to a metallic ground state with vanished electric dipoles. The present prediction is in contrast to the previous proposal that the crystalline MgB{sub 6} maintains a semiconducting state with permanent dipole moments. MgB{sub 6} is estimated to have much weaker electron-phonon coupling compared with that of MgB{sub 2}, and therefore it is not expected to be able to sustain superconductivity at high temperatures.
Susan, S.
1993-04-30
Theoretical electronic structure techniques are used to analyze widely different systems from Si clusters to transition metal solids and surfaces. For the Si clusters, first principles density functional methods are used to investigate Si{sub N} for N=2-8. Goal is to understand the different types of bonding that can occur in such small clusters where the atomic coordination differs substantially from tetrahedral bonding; such uncoordinated structures can test approximate models of Si surfaces. For the transition metal systems, non-self-consistent electronic structure methods are used to understand the driving force for surface relaxations. In-depth analysis of results is presented and physical basis of surface relaxation within the theory is discussed. Limitations inherent in calculations of metal surface relaxation are addressed. Finally, in an effort to understand approximate methods, a novel non-self- consistent density functional electronic structure method is developed that is about 1000 times faster than more sophisticated methods; this method is tested for various systems including diatomics, mixed clusters, surfaces, and bulk lattices.
Analyzing Variability in Short-Channel Quantum Transport from Atomistic First Principles
NASA Astrophysics Data System (ADS)
Shi, Qing; Guo, Hong; Zhu, Yu; Liu, Lei
2015-06-01
Effects of disorder scattering critically influence quantum-transport properties of nanostructures both fundamentally and practically. In this work, we report a theoretical analysis of the important issue of device-to-device quantum-transport variability (DDV) induced by random configurations of discrete dopants. Instead of calculating many impurity configurations by brute force, which is practically impossible to accomplish from first principles, here we use a state-of-the-art atomistic technique where the configurational average is carried out analytically, thereby, DDV can be predicted for any impurity concentration. The DDV we quantitatively analyze is the off-state tunnel conductance variability in Si nanosized field-effect transistor channels with channel lengths ranging from 6.5 to 15.2 nm doped with different concentrations of boron impurity atoms. The variability is predicted by varying the doping concentration, channel length, and the doping positions. We find that doping away from the source or drain contacts of the channel very significantly reduces variability, and doping close to the source or drain produces a nonintuitive outcome of increasing variability. The physics is understood by analyzing the microscopic details of the potential profile in the tunnel barrier. Finally, we organize the ab initio data by a Wentzel-Kramers-Brillouin model.
High pressure phase transformation in yttrium sulfide(YS): A first principle study
Sahoo, B. D., E-mail: bdsahoo@barc.gov.in; Joshi, K. D., E-mail: bdsahoo@barc.gov.in; Gupta, Satish C., E-mail: bdsahoo@barc.gov.in [Applied Physics Division, Bhabha Atomic Research Centre, Mumbai-400085 (India)
2014-04-24
First principles calculations have been carried out to analyze structural, elastic and dynamic stability, of YS under hydrostatic compression. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of compression suggests the B1?B2 transition at ? 49 GPa. Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state. The single crystal elastic constants derived from the energy strain method agree well with the experimental values. The activation barrier between B1 and B2 phases calculated at transition point is ? 17/mRy/formula unit. Our lattice dynamic calculations show that at ambient condition, the B1 phase is lattice dynamically stable and frequencies of phonon modes in different high symmetry directions of Brillouin zone agrees well with experimental values. The B2 phase also is dynamical stable at ambient condition as well as at ? 49 GPa, supporting our static lattice calculation.
Sahoo, B. D., E-mail: bdsahoo@barc.gov.in; Joshi, K. D.; Gupta, Satish C. [Applied Physics Division, Bhabha Atomic Research Centre, Mumbai 400085 (India)
2014-03-28
First principles calculations have been carried out to analyze structural, elastic, and dynamic stability of yttrium sulphide (YS) under hydrostatic compression. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of compression suggests the B1 ? B2 transition at ?49?GPa (the same transition occurs at ?48?GPa at 300?K). Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state. The single crystal elastic constants derived from the energy strain method agree well with the experimental values. The activation barrier between B1 and B2 phases calculated at transition point is ?17/mRy/f.u. Our lattice dynamic calculations show that at ambient condition, the B1 phase is lattice dynamically stable, and frequencies of phonon modes in different high symmetry directions of Brillouin zone agrees well with experimental values. The B2 phase also is dynamical stable at ambient condition as well as at ?49?GPa, supporting our static lattice calculation. The effect of temperature on volume and bulk modulus of the YS in B1 phase has also been examined. The superconducting temperature of ?2.78?K determine at zero pressure agrees well with experimental data. The effect of pressure is found to suppress the superconducting nature of this material.
First-principles calculations of CsMX3(M = Sn, Pb; X = Cl, Br, I)
NASA Astrophysics Data System (ADS)
Yuan, Ye; Xu, Run; Xu, Haitao; Wang, Linjun
2013-12-01
CsMX3(M = Sn, Pb; X = Cl, Br, I) are strong candidates for the fast high energy irradiation detectors, ionic conductors, and optoelectronic devices. There are many experimental and theoretical investigations devoted to the study of perovskites ABX3 (A is a cation with different valence, B is a transition metal and X is oxides, halides or chlorides). But there is no systematic study of CsMX3 using HSE approximation particularly. In this paper, the band structures, density of states and optical properties of CsMX3(M = Sn, Pb; X = Cl, Br, I) have been studied by first-principles calculations using both the hybrid functional (HSE) approximation and the PBE-GGA approximation. The results of both approximations are compared with the experimental values. The results of HSE are closer to the experimental values. The changes of properties have been founded by comparing the band structures, density of states and optical properties of this series of thin film materials respectively. The trend of impact on these properties by replace elements has also been found. Our results provide a basis for the design of specific performance thin film materials.
Thermoelectric properties of AgSbTe2 from first-principles calculations
NASA Astrophysics Data System (ADS)
Rezaei, Nafiseh; Hashemifar, S. Javad; Akbarzadeh, Hadi
2014-09-01
The structural, electronic, and transport properties of AgSbTe2 are studied by using full-relativistic first-principles electronic structure calculation and semiclassical description of transport parameters. The results indicate that, within various exchange-correlation functionals, the cubic F d 3 ¯ m and trigonal R 3 ¯ m structures of AgSbTe2 are more stable than two other considered structures. The computed Seebeck coefficients at different values of the band gap and carrier concentration are accurately compared with the available experimental data to speculate a band gap of about 0.1-0.35 eV for AgSbTe2 compound, in agreement with our calculated electronic structure within the hybrid HSE (Heyd-Scuseria-Ernzerhof) functional. By calculating the semiclassical Seebeck coefficient, electrical conductivity, and electronic part of thermal conductivity, we present the theoretical upper limit of the thermoelectric figure of merit of AgSbTe2 as a function of temperature and carrier concentration.
First principles DFT study of dye-sensitized CdS quantum dots
Jain, Kalpna; Singh, Kh. S. [Department of Physics, D. J. College, Baraut -250611, U.P. (India); Kishor, Shyam, E-mail: shyam387@gmail.com [Department of Chemistry, J. V. College, Baraut -250611, U.P. (India); Josefesson, Ida; Odelius, Michael [Fysikum, Albanova University Center, Stockholm University, S-106 91 Stockholm (Sweden); Ramaniah, Lavanya M. [High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai-400085 (India)
2014-04-24
Dye-sensitized quantum dots (QDs) are considered promising candidates for dye-sensitized solar cells. In order to maximize their efficiency, detailed theoretical studies are important. Here, we report a first principles density functional theory (DFT) investigation of experimentally realized dye - sensitized QD / ligand systems, viz., Cd{sub 16}S{sub 16}, capped with acetate molecules and a coumarin dye. The hybrid B3LYP functional and a 6?311+G(d,p)/LANL2dz basis set are used to study the geometric, energetic and electronic properties of these clusters. There is significant structural rearrangement in all the clusters studied - on the surface for the bare QD, and in the positions of the acetate / dye ligands for the ligated QDs. The density of states (DOS) of the bare QD shows states in the band gap, which disappear on surface passivation with the acetate molecules. Interestingly, in the dye-sensitised QD, the HOMO is found to be localized mainly on the dye molecule, while the LUMO is on the QD, as required for photo-induced electron injection from the dye to the QD.
Two-dimensional arsenic monolayer sheet predicted from first-principles
NASA Astrophysics Data System (ADS)
Pu, Chun-Ying; Ye, Xiao-Tao; Jiang, Hua-Long; Zhang, Fei-Wu; Lu, Zhi-Wen; He, Jun-Bao; Zhou, Da-Wei
2015-03-01
Using first-principles calculations, we investigate the two-dimensional arsenic nanosheet isolated from bulk gray arsenic. Its dynamical stability is confirmed by phonon calculations and molecular dynamics analyzing. The arsenic sheet is an indirect band gap semiconductor with a band gap of 2.21 eV in the hybrid HSE06 functional calculations. The valence band maximum (VBM) and the conduction band minimum (CBM) are mainly occupied by the 4p orbitals of arsenic atoms, which is consistent with the partial charge densities of VBM and CBM. The charge density of the VBM G point has the character of a ? bond, which originates from p orbitals. Furthermore, tensile and compressive strains are applied in the armchair and zigzag directions, related to the tensile deformations of zigzag and armchair nanotubes, respectively. We find that the ultimate strain in zigzag deformation is 0.13, smaller than 0.18 of armchair deformation. The limit compressive stresses of single-layer arsenic along armchair and zigzag directions are ?4.83 GPa and ?4.76 GPa with corresponding strains of ?0.15 and ?0.14, respectively. Projected supported by the Henan Joint Funds of the National Natural Science Foundation of China (Grant Nos. U1304612 and U1404608), the National Natural Science Foundation of China (Grant Nos. 51374132 and 11404175), the Special Fund for Theoretical Physics of China (Grant No. 11247222), and Nanyang Normal University Science Foundation, China (Grant Nos. ZX2012018 and ZX2013019).
Rare-earth vs. heavy metal pigments and their colors from first principles.
Tomczak, Jan M; Pourovskii, Leonid V; Vaugier, Loig; Georges, Antoine; Biermann, Silke
2013-01-15
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
Rare-earth vs. heavy metal pigments and their colors from first principles
Tomczak, Jan M.; Pourovskii, Leonid V.; Vaugier, Loig; Georges, Antoine; Biermann, Silke
2013-01-01
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
Electrical contacts to monolayer black phosphorus: A first-principles investigation
NASA Astrophysics Data System (ADS)
Gong, Kui; Zhang, Lei; Ji, Wei; Guo, Hong
2014-09-01
We report first-principles theoretical investigations of possible metal contacts to monolayer black phosphorus (BP). By analyzing lattice geometry, five metal surfaces are found to have minimal lattice mismatch with BP: Cu(111), Zn(0001), In(110), Ta(110), and Nb(110). Further studies indicate Ta and Nb bond strongly with monolayer BP causing substantial bond distortions, but the combined Ta-BP and Nb-BP form good metal surfaces to contact a second layer BP. By analyzing the geometry, bonding, electronic structure, charge transfer, potential, and band bending, it is concluded that Cu(111) is the best candidate to form excellent Ohmic contact to monolayer BP. The other four metal surfaces or combined surfaces also provide viable structures to form metal/BP contacts, but they have Schottky character. Finally, the band bending property in the current-in-plane (CIP) structure where metal/BP is connected to a freestanding monolayer BP, is investigated. By both work function estimates and direct calculations of the two-probe CIP structure, we find that the freestanding BP channel is n type.
First-principles calculation on dilute magnetic alloys in zinc blend crystal structure
NASA Astrophysics Data System (ADS)
Ullah, Hamid; Inayat, Kalsoom; . Khan, S. A.; Mohammad, S.; Ali, A.; Alahmed, Z. A.; Reshak, A. H.
2015-07-01
Ab-initio calculations are performed to investigate the structural, electronic and magnetic properties of spin-polarized diluted magnetic alloys in zinc blende structure. The first-principles study is carried out on Mn doped III-V semiconductors. The calculated band structures, electronic properties and magnetic properties of Ga1-xMnxX (X=P, As) compounds reveal that Ga0.75Mn0.25P is half metallic turned to be metallic with increasing x to 0.5 and 0.75, whereas substitute P by As cause to maintain the half-metallicity nature in both of Ga0.75Mn0.25As and Ga0.5Mn0.5As and tune Ga0.25Mn0.75As to be metallic. Calculated total magnetic moments and the robustness of half-metallicity of Ga0.75Mn0.25P, Ga0.75Mn0.25As and Ga0.5Mn0.5As with respect to the variation in lattice parameters are also discussed. The predicted theoretical evidence shows that some Mn-doped III-V semiconductors can be effectively used in spintronic devices.
First-principles calculation for electron dynamics in dielectrics induced by intense laser pulses
NASA Astrophysics Data System (ADS)
Sato, Shunsuke A.; Yabana, Kazuhiro; Shinohara, Yasushi; Lee, Kyung-Min; Otobe, Tomohito; Bertsch, George F.
2015-03-01
We have been developing a theoretical and computational method to describe electron dynamics in crystalline solids irradiated by laser pulses. Our method is based on the time-dependent density functional theory (TDDFT), which enables us to treat quantum electron dynamics in the first principles level. By solving the time-dependent Kohn-Sham equation in real-time, it is possible to describe nonperturbative nonlinear electron dynamics induced by intense laser fields. We further combine the electron dynamics calculation in the TDDFT with the Maxwell's equation for electromagnetic fields so as to achieve simultaneous descriptions of both micro-meter-scale laser-field propagation and nanometer-scale electron dynamics induced by the fields. We call it ``Maxwell + TDDFT multiscale simulation.'' As an application of the method, we show calculated transferred energy distribution from a laser pulse to a bulk SiO2 sample. We evaluated the laser damage threshold and the ablation depth from the distribution. We found that the calculation nicely reproduced measured results of both threshold and depth.
First principles DFT study of dye-sensitized CdS quantum dots
NASA Astrophysics Data System (ADS)
Jain, Kalpna; Kishor, Shyam; Singh, Kh. S.; Josefesson, Ida; Odelius, Michael; Ramaniah, Lavanya M.
2014-04-01
Dye-sensitized quantum dots (QDs) are considered promising candidates for dye-sensitized solar cells. In order to maximize their efficiency, detailed theoretical studies are important. Here, we report a first principles density functional theory (DFT) investigation of experimentally realized dye - sensitized QD / ligand systems, viz., Cd16S16, capped with acetate molecules and a coumarin dye. The hybrid B3LYP functional and a 6-311+G(d,p)/LANL2dz basis set are used to study the geometric, energetic and electronic properties of these clusters. There is significant structural rearrangement in all the clusters studied - on the surface for the bare QD, and in the positions of the acetate / dye ligands for the ligated QDs. The density of states (DOS) of the bare QD shows states in the band gap, which disappear on surface passivation with the acetate molecules. Interestingly, in the dye-sensitised QD, the HOMO is found to be localized mainly on the dye molecule, while the LUMO is on the QD, as required for photo-induced electron injection from the dye to the QD.
First-principles prediction of superconductivity in LiBSi1-xAlx
NASA Astrophysics Data System (ADS)
Miao, Rende; Yang, Jun; Bai, Zhong; Can, Dan; Zhang, Xi; Jiang, Min; Liu, Cuicui; Wu, Fangping; Ma, Shuyun
2015-02-01
Electronic structure, lattice dynamics and superconducting properties for theoretically devised superconductor LiBSi1-x Alx are obtained by first-principles calculations. We assume that Lithium Boron Silicon (LiBSi) has the same crystal structure as that of Lithium borocarbide (LiBC). The pristine LiBSi is predicted to be a zero-gap semiconductor. Hole doping of LiBSi through partial substitution of Si by SiAl atoms can produce a semiconductor-metal transition and develop superconductivity. To assess the thermodynamic stability of LiBSi1-xAlx, the formation energies are calculated using the supercell method. For LiBSi0.75 Al0.25 and LiBSi0.875Al0.125, the obtained formation energies are -5.9 and -6.1 eV, respectively, indicating that LiBSi1-xAlx is energetically favorable at least in the range of 0 ? x ? 0.25. Phonon spectra and superconducting properties are obtained within the virtual-crystal approximation (VCA) treatment. The results show that LiBSi1-xAlx is dynamically stable approximately in the range of 0 ? x ? 0.35. For LiBSi0.8Al0.2, the obtained electron-phonon coupling constant ? is 0.86 and superconducting transition temperature TC is predicted to be in the range of 11-13 K (0.14 ? ?* ? 0.1).
First principle study of elastic and thermodynamic properties of FeB4 under high pressure
NASA Astrophysics Data System (ADS)
Zhang, Xinyu; Qin, Jiaqian; Ning, Jinliang; Sun, Xiaowei; Li, Xinting; Ma, Mingzhen; Liu, Riping
2013-11-01
The elastic properties, elastic anisotropy, and thermodynamic properties of the lately synthesized orthorhombic FeB4 at high pressures are investigated using first-principles density functional calculations. The calculated equilibrium parameters are in good agreement with the available experimental and theoretical data. The obtained normalized volume dependence of high pressure is consistent with the previous experimental data investigated using high-pressure synchrotron x-ray diffraction. The complete elastic tensors and crystal anisotropies of the FeB4 are also determined in the pressure range of 0-100 GPa. By the elastic stability criteria and vibrational frequencies, it is predicted that the orthorhombic FeB4 is stable up to 100 GPa. In addition, the calculated B/G ratio reveals that FeB4 possesses brittle nature in the range of pressure from 0 to 100 GPa. The calculated elastic anisotropic factors suggest that FeB4 is elastically anisotropic. By using quasi-harmonic Debye model, the compressibility, bulk modulus, the coefficient of thermal expansion, the heat capacity, and the Grüneisen parameter of FeB4 are successfully obtained in the present work.
Theoretical investigation of nuclear quadrupole interactions in DNA at first-principles level
NASA Astrophysics Data System (ADS)
Mahato, Dip N.; Dubey, Archana; Pink, R. H.; Scheicher, R. H.; Badu, S. R.; Nagamine, K.; Torikai, E.; Saha, H. P.; Chow, Lee; Huang, M. B.; Das, T. P.
2008-01-01
We have studied the nuclear quadrupole interactions (NQI) of the 14N, 17O and 2H nuclei in the nucleobases cytosine, adenine, guanine and thymine in the free state as well as when they are bonded to the sugar ring in DNA, simulated through a CH3 group attached to the nucleobases. The nucleobase uracil, which replaces thymine in RNA, has also been studied. Our results show that there are substantial indirect effects of the bonding with the sugar group in the nucleic acids on the NQI parameters e 2 qQ/h and ?. It is hoped that measurements of these NQI parameters in DNA will be available in the future to compare with our predictions. Our results provide the conclusion that for any property dependent on the electronic structures of the nucleic acids, the effects of the bonding between the nucleobases and the nucleic acid backbones have to be included.
Theoretical investigation of nuclear quadrupole interactions in DNA at first-principles level
NASA Astrophysics Data System (ADS)
Mahato, Dip N.; Dubey, Archana; Pink, R. H.; Scheicher, R. H.; Badu, S. R.; Nagamine, K.; Torikai, E.; Saha, H. P.; Chow, Lee; Huang, M. B.; Das, T. P.
We have studied the nuclear quadrupole interactions (NQI) of the 14N, 17O and 2H nuclei in the nucleobases cytosine, adenine, guanine and thymine in the free state as well as when they are bonded to the sugar ring in DNA, simulated through a CH3 group attached to the nucleobases. The nucleobase uracil, which replaces thymine in RNA, has also been studied. Our results show that there are substantial indirect effects of the bonding with the sugar group in the nucleic acids on the NQI parameters e 2 qQ/h and ?. It is hoped that measurements of these NQI parameters in DNA will be available in the future to compare with our predictions. Our results provide the conclusion that for any property dependent on the electronic structures of the nucleic acids, the effects of the bonding between the nucleobases and the nucleic acid backbones have to be included.
Adsorption of carbon on Pd clusters of nanometer size: A first-principles theoretical study
Konstantin M. Neyman; Chan Inntam; Alexei B. Gordienko; Ilya V. Yudanov; Notker Rösch
2005-01-01
Adsorbed atomic C species can be formed in the course of surface reactions and commonly decorate metal catalysts. We studied computationally C adsorption on Pd nanoclusters using an all-electron scalar relativistic density functional method. The metal particles under investigation, Pd55, Pd79, Pd85, Pd116, Pd140, and Pd146, were chosen as fragments of bulk Pd in the form of three-dimensional octahedral or
W. Guo; J. Granger; D. M. Sigman
2010-01-01
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., Deltadelta18O: Deltadelta15N) of
NASA Astrophysics Data System (ADS)
Guo, W.; Granger, J.; Sigman, D. M.
2010-12-01
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.
NASA Astrophysics Data System (ADS)
Wu, Xifan
Density-functional theory (DFT) has provided a powerful tool for studying solid-state physics problems accurately from a first-principles approach. The applications of DFT have undergone rapid development in recent decades in a broad area in condensed-matter physics. In my thesis work, I am focusing on the study of the problems that are related to the ground-state properties of insulating solids in the framework of DFT. First, using the methods of density-functional perturbation theory, it is possible to compute the first derivatives of the electronic wavefunctions with respect to atomic-displacement, electric-field, and strain perturbations; and from these, tensors of second energy derivatives including both 'diagonal responses' and 'off-diagonal' ones. We developed a systematic approach for computing all of these quantities in a unified fashion, and for combining the information thus obtained to compute such properties as clamped-strain vs. free-stress dielectric tensors, elastic constants under different electrical boundary conditions, and piezoelectric coupling factors. Next, using first-principles density-functional methods, we investigate PbTiO3 superlattices in which the polarization alternates from up to down along the growth direction, and in which the large polarization charges at the 180° head-to-head and tail-to-tail domain walls are compensated by heterovalent substitution. We show that it is theoretically possible to construct insulating superlattice structures of this kind, and investigate their novel properties. In the third part, I will present our work in which we have extended a method that maps out the energy as a function of polarization E (P) in insulating solids by including the strain relaxation. I will show by examples that in ferroelectric materials, the strain relaxation is crucial for the real application of the E(P) mapping method. The maximally localized Wannier function (WF) method of Marzari and Vanderbilt (Phys. Rev. B 56, 12847 (1997)) generates WFs and WF centers directly related to electronic polarization. In the last part, in superlattices made of perovskites, we defined a new concept of "layer polarization" (LP) based on the charges and locations of both WF centers and ionic centers in different layers in the direction normal to the plane. It is shown by examples that LP is a powerful tool for analyzing local properties associated with interfaces, structural distortions, etc.
Qiu, Bo
The mean free paths (MFPs) of energy carriers are of critical importance to the nano-engineering of better thermoelectric materials. Despite significant progress in the first-principles–based understanding of the spectral ...
Thermal conductivity of half-Heusler compounds from first-principles calculations
Shiomi, Junichiro
We demonstrate successful application of first-principles-based thermal conductivity calculation on half-Heusler compounds that are promising, environmentally friendly thermoelectric materials. Taking the case of a p-type ...
First-principles investigation of Li intercalation kinetics in phospho-olivines
Malik, Rahul
2013-01-01
This thesis focuses broadly on characterizing and understanding the Li intercalation mechanism in phospho-olivines, namely LiFePO? and Li(Fe,Mn)PO?, using first-principles calculations. Currently Li-ion battery technology ...
First-principles study of thermal transport in FeSB[subscript 2
Liao, Bolin
We study the thermal transport properties of FeSb[subscript 2], a promising thermoelectric material for cooling applications at cryogenic temperatures. A first-principles formalism based on density functional theory and ...
Lin, Xi, 1973-
2003-01-01
In this thesis, the chemistry of sulfur oxides on transition metals is studied extensively via first-principles density functional theory (DFT) computations, focusing on the chemical reactivity and selectivity in sulfur ...
Mochrie, Simon
Available online at www.sciencedirect.com Protein engineering and design: from first principles with protein engineering to study protein folding. She is a Wellcome Trust Senior Research Fellow. Lynne Regan
Kim, Sejoong
We present a first-principles approach for inelastic quantum transport calculations based on maximally localized Wannier functions. Electronic-structure properties are obtained from density-functional theory in a plane-wave ...
cond-mat* First-principles prediction of a decagonal quasicrystal containing Boron
Widom, Michael
First-principles prediction of a decagonal quasicrystal containing Boron: July 2, 2004) We interpret experimentally known B-Mg-Ru crystals as quasicrystal a-fold rotational symmetry. We created a se- ries of hypothetical quasicrystal approximant structures
Phase diagram and electrochemical properties of mixed olivines from first-principles calculations
Malik, Rahul
Using first-principles calculations, we study the effect of cation substitution on the transition-metal sublattice in phospho-olivines, with special attention given to the Li[subscript x](Fe[subscript 1?y]Mn[subscript ...
Aqueous systems from first-principles : structure, dynamics and electron-transfer reactions
Sit, Patrick Hoi Land
2006-01-01
In this thesis, we show for the first time how it is possible to calculated fully from first-principles the diabatic free-energy surfaces of electron-transfer reactions. The excitation energy corresponding to the transfer ...
Ceder, Gerbrand
The idea of first-principles methods is to determine the properties of materials by solving the basic equations of quantum mechanics and statistical mechanics. With such an approach, one can, in principle, predict the ...
First principles high throughput screening of oxynitrides for water-splitting photocatalysts
Wu, Yabi
2013-01-01
In this paper, we present a first principles high throughput screening system to search for new water-splitting photocatalysts. We use the approach to screen through nitrides and oxynitrides. Most of the known photocatalytic ...
First-principles Study on the Magnetic Interaction in ZnO-based Dilute Magnetic Semiconductors
Masayuki Toyoda; Hisazumi Akai; Kazunori Sato; Hiroshi Katayama-Yoshida
2008-01-01
Using first-principles calculations, we investigate the electronic structures and magnetic properties of dilute magnetic semiconductors (DMS). The electronic structures are calculated by using Korringa-Kohn-Rostoker method combined with the coherent potential approximation. Since the d electrons of the magnetic impurity in DMS are strongly localized, we apply self-interaction correction to the local density approximation for the exchange-correlation energy. From the first-principles
NASA Astrophysics Data System (ADS)
Takahashi, Masae; Ishikawa, Yoichi; Ito, Hiromasa
2013-03-01
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 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 The research was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant No. 22550003).
M. G. Brik; K. Ogasawara
2007-01-01
Systematic analysis of the energy level scheme and ground state absorption of the Cr4+ ion in Li2CaSiO4 crystal was performed using the exchange charge model of the crystal field [B.Z. Malkin, in: A.A. Kaplyanskii, B.M. Macfarlane (Eds.), Spectroscopy of Solids Containing Rare-earth Ions, North-Holland, Amsterdam, 1987, pp. 33–50] and recently developed first-principles approach to the analysis of the absorption spectra
First-principles study of electronic structure and optical properties of Sr(Ti,Zr)O3
NASA Astrophysics Data System (ADS)
Celik, Gulden; Cabuk, Suleyman
2013-03-01
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.
Vibrational signatures of OTe and OTe-VCd in CdTe: A first principles study
Thienprasert, J.T. [Kasetsart University, Thailand; Limpijumnong, Sukit [Kasetsart University, Thailand; Janotti, Anderson [University of California, Santa Barbara; Van de Walle, C.G. [University of California, Santa Barbara; Zhang, Lijun [ORNL; Du, Mao-Hua [ORNL; Singh, David J [ORNL
2010-01-01
Chen et al. [Phys. Rev. Lett. 96 (2006) 035508] experimentally observed vibrational signatures related to defects in oxygen-doped CdTe using ultrahigh resolution Fourier transform infrared (FTIR) spectroscopy. They observed an absorption peak at 350 cm{sup -1}. In addition, for samples grown under certain conditions, they observed two higher frequency peaks (1097 and 1108 cm{sup -1}) at low temperature that merged into one at room-temperature. They attributed the low-frequency peak (350 cm{sup -1}) to the vibration of O{sub Te} and the two higher frequency peaks to the vibrational modes of a O{sub Te}-V{sub Cd} complex. Subsequently, they reported similar modes around 1100 cm{sup -1} in O-doped CdSe [Phys. Rev. Lett. 101 (2008) 195502] which were attributed to an O{sub Se}-V{sub Cd} complex. We employed first-principles DFT calculations to calculate the vibrational modes of O{sub Te} and O{sub Te}-V{sub Cd} complex in CdTe. Our calculations show that the 350 cm{sup -1} mode is consistent with O{sub Te}. However, the frequencies of the modes around 1100 cm{sup -1} are more than twice the expected frequencies for O{sub Te}-V{sub Cd} complexes in CdTe (or O{sub Se}-V{sub Cd} in CdSe), indicating that the O{sub Te}-V{sub Cd} complex cannot be the cause of the observed 1100 cm{sup -1} modes. A search for a new defect model is in order.
Valentina Cervetto; Jan Helbing; Jens Bredenbeck; Peter Hamm
2004-01-01
In this study we focus on the differences and analogies of two experimental implementations of two-dimensional infrared (2D-IR) spectroscopy: double-resonance or dynamic hole burning 2D-IR spectroscopy and pulsed Fourier transform or heterodyne detected photon echo spectroscopy. A comparison is done theoretically as well as experimentally by contrasting data obtained from both methods. As an example we have studied the strongly
ERIC Educational Resources Information Center
Cappelletti, John
1989-01-01
Details the following thoughts on directing for the theater: select the play, read the play, read the play again, ask questions, discover the spine, form a concept, cast the play, listen to the actors, make it move, make it spontaneous, make it real, look for obstacles, make it relevant, and make it important. (PRA)
Meng, Lingyan; Huang, Tengxiang; Wang, Xiang; Chen, Shu; Yang, Zhilin; Ren, Bin
2015-06-01
The optimal gold-coated atomic force microscopy (AFM) tip-substrate system for tip-enhanced Raman spectroscopy (TERS) was designed theoretically and demonstrated experimentally. By optimizing the tip, excitation laser, and the substrate, the TERS enhancement factor can be tuned to as high as 9 orders of magnitude, and the spatial resolution could be down to 5 nm. Preliminary experimental results for AFM tips coated with gold layer of different thicknesses reveal that the maximum enhancement can be achieved when the thickness is about 60-80 nm, which is in good agreement with the theoretical prediction. Our results not only provide a deep understanding of the underlying physical mechanism of AFM tip-based TERS, but also guide the rational construction of a working AFM-TERS system with a high efficiency. PMID:26072752
Edge reconstructions of hexagonal boron nitride nanoribbons: A first-principles study
NASA Astrophysics Data System (ADS)
Hu, Ting; Han, Yang; Dong, Jinming
2013-12-01
The edge reconstructions of hexagonal boron nitride nanoribbons (BNNRs) and their stabilities have been investigated by the first-principles calculations of both their binding and edge energies. It is found from our calculations that the binding energy we have used is a reliable and useful quantity for judging the stabilities of different edge reconstructions of the hetero-elemental BNNRs instead of the conventional edge energy one, especially for those BNNR's edges with unequal number of B and N atoms. In addition, other four main results have been obtained: (1) the armchair BNNR is the most stable edge structure and the characteristic ac-48 edge reconstruction for the BNNRs is predicted to be the second most stable edge in all the discussed BNNR edge structures. But, its zigzag edge is less stable. (2) The zigzag-like ac-56 type reconstructions are more stable than the pristine zigzag structures, which is different from that of the graphene nanoribbon (GNR), being less stable than the zigzag GNR. (3) The stabilities of BNNR's ac-677 and ac-678 type edge reconstructions lie between its ac-56-B and zz-57-B edges. (4) The zz-57-B(-N) edge reconstruction lowers the edge energy by a small quantity, which is also different from that of GNR. Moreover, the zz-57-B edge structure is more stable than the zz-57-N one, indicating that B-rich edge is easier to be reconstructed while the N-rich edge is more stable. Our theoretical calculations suggest the many possible reconstructed edge structures for the bare BNNRs, which is important for BNNR's application in future nanoelectronics and spintronics.
Prediction of new high pressure structural sequence in thorium carbide: A first principles study
NASA Astrophysics Data System (ADS)
Sahoo, B. D.; Joshi, K. D.; Gupta, Satish C.
2015-05-01
In the present work, we report the detailed electronic band structure calculations on thorium monocarbide. The comparison of enthalpies, derived for various phases using evolutionary structure search method in conjunction with first principles total energy calculations at several hydrostatic compressions, yielded a high pressure structural sequence of NaCl type (B1) ? Pnma ? Cmcm ? CsCl type (B2) at hydrostatic pressures of ˜19 GPa, 36 GPa, and 200 GPa, respectively. However, the two high pressure experimental studies by Gerward et al. [J. Appl. Crystallogr. 19, 308 (1986); J. Less-Common Met. 161, L11 (1990)] one up to 36 GPa and other up to 50 GPa, on substoichiometric thorium carbide samples with carbon deficiency of ˜20%, do not report any structural transition. The discrepancy between theory and experiment could be due to the non-stoichiometry of thorium carbide samples used in the experiment. Further, in order to substantiate the results of our static lattice calculations, we have determined the phonon dispersion relations for these structures from lattice dynamic calculations. The theoretically calculated phonon spectrum reveal that the B1 phase fails dynamically at ˜33.8 GPa whereas the Pnma phase appears as dynamically stable structure around the B1 to Pnma transition pressure. Similarly, the Cmcm structure also displays dynamic stability in the regime of its structural stability. The B2 phase becomes dynamically stable much below the Cmcm to B2 transition pressure. Additionally, we have derived various thermophysical properties such as zero pressure equilibrium volume, bulk modulus, its pressure derivative, Debye temperature, thermal expansion coefficient and Gruneisen parameter at 300 K and compared these with available experimental data. Further, the behavior of zero pressure bulk modulus, heat capacity and Helmholtz free energy has been examined as a function temperature and compared with the experimental data of Danan [J. Nucl. Mater. 57, 280 (1975)].
Bondi, Robert J., E-mail: rjbondi@sandia.gov; Desjarlais, Michael P.; Thompson, Aidan P.; Brennecka, Geoff L.; Marinella, Matthew J. [Sandia National Laboratories, Albuquerque, New Mexico 87185 (United States)
2013-11-28
We apply first-principles density-functional theory (DFT) calculations, ab-initio molecular dynamics, and the Kubo-Greenwood formula to predict electrical conductivity in Ta{sub 2}O{sub x} (0???x???5) as a function of composition, phase, and temperature, where additional focus is given to various oxidation states of the O monovacancy (V{sub O}{sup n}; n?=?0,1+,2+). In the crystalline phase, our DFT calculations suggest that V{sub O}{sup 0} prefers equatorial O sites, while V{sub O}{sup 1+} and V{sub O}{sup 2+} are energetically preferred in the O cap sites of TaO{sub 7} polyhedra. Our calculations of DC conductivity at 300?K agree well with experimental measurements taken on Ta{sub 2}O{sub x} thin films (0.18???x???4.72) and bulk Ta{sub 2}O{sub 5} powder-sintered pellets, although simulation accuracy can be improved for the most insulating, stoichiometric compositions. Our conductivity calculations and further interrogation of the O-deficient Ta{sub 2}O{sub 5} electronic structure provide further theoretical basis to substantiate V{sub O}{sup 0} as a donor dopant in Ta{sub 2}O{sub 5}. Furthermore, this dopant-like behavior is specific to the neutral case and not observed in either the 1+ or 2+ oxidation states, which suggests that reduction and oxidation reactions may effectively act as donor activation and deactivation mechanisms, respectively, for V{sub O}{sup n} in Ta{sub 2}O{sub 5}.
Li, Zhenyu; Mukamel, Shaul
2008-01-01
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. PMID:17941619
NASA Astrophysics Data System (ADS)
Astapenko, Valerie; Bagatur'yants, Alexander; Chernishova, Irina; Deminsky, Maxim; Eletskii, Alexander; Kirillov, Igor; Knizhnik, Andrei; Potapkin, Boris; Rykova, Elena; Umanskii, Stanislaw; Zaitsevskii, Andrei; Strelkova, Marina; Sukhanov, Leonid; Safonov, Andrei; Cotzas, George M.; Dean, Anthony; Michael, J. Darryl; Midha, Vikas; Smith, David J.; Sommerer, Timothy J.; Varatharajan, Bala; Tentner, Adrian
2007-04-01
Recent progress in several related research areas such as first-principles electronic-structure calculations of atoms and diatomic molecules, theory of elementary processes, kinetics, and numerical engineering, and also continued exponential growth in computational resources enhanced by recent advances in massively parallel computing have opened the possibility of directly designing kinetics mechanisms to describe chemical processes and light emission in such complex media as nonequilibrium plasmas and reacting gases. It is important that plasma and combustion kinetics can be described in the framework of this direct approach to a sufficiently high accuracy, which makes it an independent predictive research tool complementary to experimental techniques. This paper demonstrates the capabilities of the first-principles based approach to develop kinetic mechanisms. Two examples are discussed in detail: (1) the mechanism of hydrocarbon fuel combustion at high temperatures and (2) light emission in non-thermal glow discharge plasma of metal halides; special attention is paid to a comparison of the results obtained at every level of system description with the appropriate experimental data. In house software tools that can be used in such multilevel theoretical works are discussed as well.
Astapenko, Valerie; Bagatur'yants, Alexander; Chernishova, Irina; Deminsky, Maxim; Eletskii, Alexander; Knizhnik, Andrei; Potapkin, Boris; Rykova, Elena; Umanskii, Stanislaw; Zaitsevskii, Andrei; Safonov, Andrei [Kinetics Technologies, Moscow (Russian Federation); Kirillov, Igor; Strelkova, Marina; Sukhanov, Leonid [RRC Kurchatov Institute, Moscow (Russian Federation); Cotzas, George M.; Dean, Anthony; Michael, J. Darryl; Midha, Vikas; Smith, David J.; Sommerer, Timothy J. [GE Global Research, Niskayuna, New York (United States)] (and others)
2007-04-06
Recent progress in several related research areas such as first-principles electronic-structure calculations of atoms and diatomic molecules, theory of elementary processes, kinetics, and numerical engineering, and also continued exponential growth in computational resources enhanced by recent advances in massively parallel computing have opened the possibility of directly designing kinetics mechanisms to describe chemical processes and light emission in such complex media as nonequilibrium plasmas and reacting gases. It is important that plasma and combustion kinetics can be described in the framework of this direct approach to a sufficiently high accuracy, which makes it an independent predictive research tool complementary to experimental techniques. This paper demonstrates the capabilities of the first-principles based approach to develop kinetic mechanisms. Two examples are discussed in detail: (1) the mechanism of hydrocarbon fuel combustion at high temperatures and (2) light emission in non-thermal glow discharge plasma of metal halides; special attention is paid to a comparison of the results obtained at every level of system description with the appropriate experimental data. In house software tools that can be used in such multilevel theoretical works are discussed as well.
NASA Astrophysics Data System (ADS)
Schweflinghaus, Benedikt; dos Santos Dias, Manuel; Costa, Antonio T.; Lounis, Samir
2014-06-01
Access to magnetic excitation spectra of single atoms deposited on surfaces is nowadays possible by means of low-temperature inelastic scanning tunneling spectroscopy. We present a first-principles method for the calculation of inelastic tunneling spectra utilizing the Korringa-Kohn-Rostoker Green function method combined with time-dependent density functional theory and many-body perturbation theory. The key quantity is the electron self-energy describing the coupling of the electrons to the spin excitation within the adsorbate. By investigating Cr, Mn, Fe, and Co adatoms on a Cu(111) substrate, we spin-characterize the spectra and demonstrate that their shapes are altered by the magnetization of the adatoms, of the tip and the orbital decay into vacuum. Our method also predicts spectral features more complex than the steps obtained by simpler models for the adsorbate (e.g., localized spin models).
V. Ozolins; C. Wolverton; Alex Zunger
1998-01-01
The classic metallurgical systems-noble-metal alloys-that have formed the benchmark for various alloy theories are revisited. First-principles fully relaxed general-potential linearized augmented plane-wave (LAPW) total energies of a few ordered structures are used as input to a mixed-space cluster expansion calculation to study the phase stability, thermodynamic properties, and bond lengths in Cu-Au, Ag-Au, Cu-Ag, and Ni-Au alloys. (i) Our theoretical
NASA Astrophysics Data System (ADS)
Qiu, Bo; Tian, Zhiting; Vallabhaneni, Ajit; Liao, Bolin; Mendoza, Jonathan M.; Restrepo, Oscar D.; Ruan, Xiulin; Chen, Gang
2015-03-01
The mean free paths (MFPs) of energy carriers are of critical importance to the nano-engineering of better thermoelectric materials. Despite significant progress in the first-principles–based understanding of the spectral distribution of phonon MFPs in recent years, the spectral distribution of electron MFPs remains unclear. In this work, we compute the energy-dependent electron scatterings and MFPs in silicon from first principles. The electrical conductivity accumulation with respect to electron MFPs is compared to that of the phonon thermal conductivity accumulation to illustrate the quantitative impact of nanostructuring on electron and phonon transport. By combining all electron and phonon transport properties from first principles, we predict the thermoelectric properties of the bulk and nanostructured silicon, and find that silicon with 20 nm nanograins can result in a higher than five times enhancement in their thermoelectric figure of merit as the grain boundaries scatter phonons more significantly than that of electrons due to their disparate MFP distributions.
First principle chemical kinetics in zeolites: the methanol-to-olefin process as a case study.
Van Speybroeck, Veronique; De Wispelaere, Kristof; Van der Mynsbrugge, Jeroen; Vandichel, Matthias; Hemelsoet, Karen; Waroquier, Michel
2014-11-01
To optimally design next generation catalysts a thorough understanding of the chemical phenomena at the molecular scale is a prerequisite. Apart from qualitative knowledge on the reaction mechanism, it is also essential to be able to predict accurate rate constants. Molecular modeling has become a ubiquitous tool within the field of heterogeneous catalysis. Herein, we review current computational procedures to determine chemical kinetics from first principles, thus by using no experimental input and by modeling the catalyst and reacting species at the molecular level. Therefore, we use the methanol-to-olefin (MTO) process as a case study to illustrate the various theoretical concepts. This process is a showcase example where rational design of the catalyst was for a long time performed on the basis of trial and error, due to insufficient knowledge of the mechanism. For theoreticians the MTO process is particularly challenging as the catalyst has an inherent supramolecular nature, for which not only the Brønsted acidic site is important but also organic species, trapped in the zeolite pores, must be essentially present during active catalyst operation. All these aspects give rise to specific challenges for theoretical modeling. It is shown that present computational techniques have matured to a level where accurate enthalpy barriers and rate constants can be predicted for reactions occurring at a single active site. The comparison with experimental data such as apparent kinetic data for well-defined elementary reactions has become feasible as current computational techniques also allow predicting adsorption enthalpies with reasonable accuracy. Real catalysts are truly heterogeneous in a space- and time-like manner. Future theory developments should focus on extending our view towards phenomena occurring at longer length and time scales and integrating information from various scales towards a unified understanding of the catalyst. Within this respect molecular dynamics methods complemented with additional techniques to simulate rare events are now gradually making their entrance within zeolite catalysis. Recent applications have already given a flavor of the benefit of such techniques to simulate chemical reactions in complex molecular environments. PMID:25054453
NASA Astrophysics Data System (ADS)
Liu, Qi-Jun; Liu, Zheng-Tang; Feng, Li-Ping; Tian, Hao
2012-12-01
On the plane-wave ultrasoft pseudopotential technique based on the first-principles density functional theory (DFT), we calculated the structural, elastic, electronic and optical properties of the seven different phases of SrZrO3. The obtained ground-state properties are in good agreement with previous experiments and calculations, which indicate that the most stable phase is orthorhombic Pnma structure. Seven phases of SrZrO3 are mechanically stable with cubic, tetragonal and orthorhombic structures. The mechanical and thermodynamic properties have been obtained by using the Voigt-Reuss-Hill approach and Debye-Grüneisen model. The electronic structures and optical properties are obtained and compared with the available experimental and theoretical data.
NASA Astrophysics Data System (ADS)
Zhang, W. X.; He, C.; Li, T.; Gong, S. B.; Zhao, L.; Tao, J. Y.
2015-06-01
In this paper, the electronic and magnetic properties of electronic and magnetic properties of armchair graphane/graphene heterostructure nanoribbons (AGA/GNRs) have been systematically investigated by first-principles calculations based on density functional theory. The calculated results indicate that 13-armchair graphane nanoribbon (13-AGANR), 13-armchair graphene nanoribbons (13-AGNR) and hybrid armchair graphane/graphene nanoribbons (AGA13-x/GxNRs) are all direct semiconductors (13, 13-x and x are denoted as the nanoribbons' widths). The band structures near the Fermi level of AGA13-x/GxNRs are mainly determined by the graphene section and the atomic charge transfers in the interface of AGA13-x/GxNRs are stronger. AGA7/G6NR with DB defects at AGANR edge obviously affect the magnetic properties. These diverse and tunable electronic and magnetic properties can be a theoretical guidance for the design of novel nanoelectronic devices.
NASA Astrophysics Data System (ADS)
Jain, Richa Naja; Chakraborty, Brahmananda; Ramaniah, Lavanya M.
2015-06-01
The electronic structure and hydrogen storage capability of Yttrium-doped BNNTs has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom prefers the hollow site in the center of the hexagonal ring with a binding energy of 0.8048eV. Decorating by Y makes the system half-metallic and magnetic with a magnetic moment of 1.0µB. Y decorated Boron-Nitride (8,0) nanotube can adsorb up to five hydrogen molecules whose average binding energy is computed as 0.5044eV. All the hydrogen molecules are adsorbed with an average desorption temperature of 644.708 K. Taking that the Y atoms can be placed only in alternate hexagons, the implied wt% comes out to be 5.31%, a relatively acceptable value for hydrogen storage materials. Thus, this system can serve as potential hydrogen storage medium.
NASA Astrophysics Data System (ADS)
Lakel, S.; Okbi, F.; Ibrir, M.; Almi, K.
2015-03-01
We have performed first-principles calculations to investigate the behavior under hydrostatic pressure of the structural, elastic and lattice dynamics properties of aluminum phosphide crystal (AlP), in both zinc-blende (B3) and nickel arsenide (B8) phases. Our calculated structural and electronic properties are in good agreement with previous theoretical and experimental results. The elastic constants, bulk modulus (B), shear modulus (G), and Young's modulus (E), Born effective charge and static dielectric constant ?0, were calculated with the generalized gradient approximations and the density functional perturbation theory (DFPT). Our results in the pressure behavior of the elastic and dielectric properties of both phases are compared and contrasted with the common III-V materials. The Born effective charge ZB decreases linearly with pressure increasing, while the static dielectric constant decreases quadratically with the increase of pressure.
First-principles study of 180? domain walls in BaTiO3: Mixed Bloch-Néel-Ising character
NASA Astrophysics Data System (ADS)
Li, Menglei; Gu, Yijia; Wang, Yi; Chen, Long-Qing; Duan, Wenhui
2014-08-01
The 180? ferroelectric domain walls (FDWs) have long been regarded as purely Ising type in ferroelectrics, but recent theoretical works suggested that they can also have Néel- and/or Bloch-like rotations. Using a combination of first-principles calculations with phase-field simulations, we studied the 180? FDWs on different crystallographic planes in prototypical ferroelectric perovskite BaTiO3. The polarization profiles of 180? FDWs on (100) and (410) planes revealed that the (100)- and (410)-FDWs both exhibit Néel-like character besides their intrinsic Ising character, while the (410)-FDW also simultaneously shows a Bloch-like oblique of ˜6 nm, as a consequence of the deviation of polarization gradient from the high symmetry direction. Due to the existence of the Néel-like component of polarization, 180? FDWs in BaTiO3 exhibit a multilayer charge redistribution and thus may strongly trap charged defects.
NASA Astrophysics Data System (ADS)
Gouda, Mohammed K.; Nakamura, Koichi; A. H. Gepreel, Mohamed
2015-06-01
Theoretical deformation response of hypothetical ?-titanium alloys was investigated using first-principles calculation technique under periodic boundary conditions. Simulation was carried out on hypothetical 54-atom supercell of Ti-X (X = Cr, Mn, Fe, Zr, Nb, Mo, Al, and Sn) binary alloys. The results showed that the strength of Ti increases by alloying, except for Cr. The most effective alloying elements are Nb, Zr, and Mo in the current simulation. The mechanism of bond breaking was revealed by studying the local structure around the alloying element atom with respect to volume change. Moreover, the effect of alloying elements on bulk modulus and admissible strain was investigated. It was found that Zr, Nb, and Mo have a significant effect to enhance the admissible strain of Ti without change in bulk modulus.
NASA Astrophysics Data System (ADS)
Song, Chi; Li, Dong-Dong; Xu, Yi-Chun; Pan, Bi-Cai; Liu, Chang-Song; Wang, Zhi-Guang
2014-05-01
The corrosion of steels in liquid metal lead (Pb) and bismuth (Bi) is a critical challenge in the development of accelerator driven systems (ADS). Using a first-principles method with a slab model, we theoretically investigate the interaction between the Pb (Bi) atom and the iron (Fe) (100) surface to assess the fundamental corrosion properties. Our investigation demonstrates that both Pb and Bi atoms favorably adsorb on the (100) surface. Such an adsorption decreases the energy required for the dissociation of an Fe atom from the surface, enhancing the dissolution tendency significantly. The segregation of six common alloying elements (Cr, Al, Mn, Ni, Nb, and Si) to the surface and their impacts on the corrosion properties are also considered. The present results reveal that Si seems to have a relatively good performance to stabilize the surface and alleviate the dissolving trend caused by Pb and Bi.
Predicting Raman Spectra of Aqueous Silica and Alumina Species in Solution From First Principles
NASA Astrophysics Data System (ADS)
Hunt, J. D.; Schauble, E. A.; Manning, C. E.
2006-12-01
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.
Buck, D.R.
2000-09-12
Theoretical simulations and ultrafast pump-probe laser spectroscopy experiments were used to study photosynthetic pigment-protein complexes and antennae found in green sulfur bacteria such as Prosthecochloris aestuarii, Chloroflexus aurantiacus, and Chlorobium tepidum. The work focused on understanding structure-function relationships in energy transfer processes in these complexes through experiments and trying to model that data as we tested our theoretical assumptions with calculations. Theoretical exciton calculations on tubular pigment aggregates yield electronic absorption spectra that are superimpositions of linear J-aggregate spectra. The electronic spectroscopy of BChl c/d/e antennae in light harvesting chlorosomes from Chloroflexus aurantiacus differs considerably from J-aggregate spectra. Strong symmetry breaking is needed if we hope to simulate the absorption spectra of the BChl c antenna. The theory for simulating absorption difference spectra in strongly coupled photosynthetic antenna is described, first for a relatively simple heterodimer, then for the general N-pigment system. The theory is applied to the Fenna-Matthews-Olson (FMO) BChl a protein trimers from Prosthecochloris aestuarii and then compared with experimental low-temperature absorption difference spectra of FMO trimers from Chlorobium tepidum. Circular dichroism spectra of the FMO trimer are unusually sensitive to diagonal energy disorder. Substantial differences occur between CD spectra in exciton simulations performed with and without realistic inhomogeneous distribution functions for the input pigment diagonal energies. Anisotropic absorption difference spectroscopy measurements are less consistent with 21-pigment trimer simulations than 7-pigment monomer simulations which assume that the laser-prepared states are localized within a subunit of the trimer. Experimental anisotropies from real samples likely arise from statistical averaging over states with diagonal energies shifted by in homogeneous broadening and as such, are quite sensitive to diagonal energy disorder. The experimental anisotropies exhibit strong oscillations with {approximately}220 fs period for certain wavelengths in one-color absorption difference experiments. The oscillations only appear when the laser pulse spectrum overlaps both of the lowest-energy groups of exciton levels clustered near 815 and 825 nm. Results suggest that the oscillations stem from quantum beating between exciton levels, rather than from coherent nuclear motion.
NASA Astrophysics Data System (ADS)
Brik, M. G.; Ogasawara, K.
2007-11-01
Systematic analysis of the energy level scheme and ground state absorption of the Cr4+ ion in Li2CaSiO4 crystal was performed using the exchange charge model of the crystal field [B.Z. Malkin, in: A.A. Kaplyanskii, B.M. Macfarlane (Eds.), Spectroscopy of Solids Containing Rare-earth Ions, North-Holland, Amsterdam, 1987, pp. 33-50] and recently developed first-principles approach to the analysis of the absorption spectra of impurity ions in crystals based on the discrete variational multielectron (DVME) method [K. Ogasawara, T. Iwata, Y. Koyama, T. Ishii, I. Tanaka, H. Adachi, Phys. Rev. B 64 (2001) 115413]. Using the former method, the values of parameters of crystal field acting on the Cr4+ ion valence electrons were calculated using the Li2CaSiO4 crystal structure data. Energy levels of the Cr4+ ion obtained after diagonalizing the crystal field Hamiltonian are in good agreement with those obtained from the experimental spectra. The latter method is based on the numerical solution of the Dirac equation; therefore, all relativistic effects are automatically considered. As a result, energy level scheme of Cr4+ and its absorption spectra in both polarizations were calculated, assigned and compared with experimental data; energy of the lowest charge transfer transition was evaluated and compared with theoretical predictions for the CrO44- complex available in the literature. The main features of the experimental spectra shape are reproduced well by the calculations. By performing analysis of the molecular orbitals (MO) population, it was shown that the covalent effects play an important role in formation of the spectral properties of Cr4+ ion in the considered crystal.
NASA Astrophysics Data System (ADS)
Rey, M.; Nikitin, A. V.; Tyuterev, V.
2014-06-01
Knowledge of near infrared intensities of rovibrational transitions of polyatomic molecules is essential for the modeling of various planetary atmospheres, brown dwarfs and for other astrophysical applications 1,2,3. For example, to analyze exoplanets, atmospheric models have been developed, thus making the need to provide accurate spectroscopic data. Consequently, the spectral characterization of such planetary objects relies on the necessity of having adequate and reliable molecular data in extreme conditions (temperature, optical path length, pressure). On the other hand, in the modeling of astrophysical opacities, millions of lines are generally involved and the line-by-line extraction is clearly not feasible in laboratory measurements. It is thus suggested that this large amount of data could be interpreted only by reliable theoretical predictions. There exists essentially two theoretical approaches for the computation and prediction of spectra. The first one is based on empirically-fitted effective spectroscopic models. Another way for computing energies, line positions and intensities is based on global variational calculations using ab initio surfaces. They do not yet reach the spectroscopic accuracy stricto sensu but implicitly account for all intramolecular interactions including resonance couplings in a wide spectral range. The final aim of this work is to provide reliable predictions which could be quantitatively accurate with respect to the precision of available observations and as complete as possible. All this thus requires extensive first-principles quantum mechanical calculations essentially based on three necessary ingredients which are (i) accurate intramolecular potential energy surface and dipole moment surface components well-defined in a large range of vibrational displacements and (ii) efficient computational methods combined with suitable choices of coordinates to account for molecular symmetry properties and to achieve a good numerical convergence. Because high-resolution ab initio spectra predictions for systems with N>4 atoms is a very challenging task, the major issue is to minimize the cost of computations and the loss of accuracy during calculations. To this end, a truncation-reduction technique for the Hamiltonian operator as well as an extraction-compression procedure for the basis set functions will be introduced and discussed in detail. We will give a review on the recent progress in computational methods as well as on existing experimental and theoretical databases 4,5,6,7,8,9. This presentation will be focused on highly symmetric molecules such as methane and phosphine, with the corresponding applications at low-T in relation with Titan's atmosphere and at high-T with the production of theoretical line lists for astrophysical opacity calculations10. The study of isotopic H?D and 12C?13C substitutions will be also addressed and carried out by means of symmetry and coordinate transformations11. Finally we hope this work will help refining studies of currently available analyses which are not yet finalized. The modeling of non-LTE emissions accounting for contribution of many fundamental and hot bands could also be possible. Support from PNP (French CNRS national planetology program) is acknowledged.
First-principles study of Ti-doped sodium alanate surfaces Jorge igueza
Yildirim, Taner
First-principles study of Ti-doped sodium alanate surfaces Jorge Íñigueza NIST Center for Neutron-principles calculations of thick slabs of Ti-doped sodium alanate NaAlH4 , which allows one to study the system energetics as the dopant progresses from the surface to the bulk. Our calculations predict that Ti stays on the surface
First-Principles Study of Phonon Anomalies and the Bcc-Hcp Martensitic Phase Transition
Yin Chen
1986-01-01
First-principles total energy calculations have been performed for transition metal Zr in the (omega), bcc, hcp, fcc structures and for Ba in the bcc, fcc structures. From our calculations we obtained equilibrium lattice constants, cohesive energies, bulk moduli and energy differ- ence between the bcc and the hcp phases which are in good agree - ment with experiment. The total
Thermal Conductivity of Periclase (MgO) from First Principles Stephen Stackhouse*
Stixrude, Lars
Thermal Conductivity of Periclase (MgO) from First Principles Stephen Stackhouse* Department thermal conductivity k of periclase (MgO) up to conditions representative of the Earth's core, 83.10.Rs Thermal conductivity is central to our understanding of planetary evolution as it sets
Magnetic and electric phase control in epitaxial EuTiO(3) from first principles.
Fennie, Craig J; Rabe, Karin M
2006-12-31
We propose a design strategy--based on the coupling of spins, optical phonons, and strain--for systems in which magnetic (electric) phase control can be achieved by an applied electric (magnetic) field. Using first-principles density-functional theory calculations, we present a realization of this strategy for the magnetic perovskite EuTiO(3). PMID:17280465
Magnetic and Electric Phase Control in Epitaxial EuTiO3 from First Principles
Craig J. Fennie; Karin M. Rabe
2006-01-01
We propose a design strategy---based on the coupling of spins, optical phonons, and strain---for systems in which magnetic (electric) phase control can be achieved by an applied electric (magnetic) field. Using first-principles density-functional theory calculations, we present a realization of this strategy for the magnetic perovskite EuTiO3.
Militzer, Burkhard
FPEOS: A First-Principles Equation of State Table of Deuterium for Inertial Confinement Fusion) Understanding and designing inertial confinement fusion (ICF) implosions through radiation- hydrodynamics. To minimize the drive energy for ignition, the imploding shell of DT- fuel needs to be kept as cold
First principle study of cobalt impurity in bcc Fe with Cu precipitates
Medvedeva, Julia E.
DOI 10.1007/s10853-012-6884-2 #12;on hardening in ferrite. Cobalt addition was experimentally foundFirst principle study of cobalt impurity in bcc Fe with Cu precipitates N. I. Medvedeva · A. S 2012 Ó Springer Science+Business Media, LLC 2012 Abstract The addition of cobalt was experimentally
First-principles atomic cluster study of boron interactions in Ni3Al
Gayle S. Painter; C. L. Fu; F. W. Averill
1997-01-01
First-principles atomic cluster calculations have been carried out in the local density approximation to understand the segregation behavior and strengthening effects of boron in Ni3Al. The binding energy of boron is calculated in lattice fragment clusters representing the perfect crystal, as well as various defect sites. The agreement between trends in energetics determined for small clusters and periodic supercells indicates
First-principles atomic cluster study of boron interactions in Ni3Al
Gayle S. Painter; C. L. Fu; F. W. Averill
1997-01-01
First-principles atomic cluster calculations have been carried out in the local density approximation to understand the segregation behavior and strengthening effects of boron in NiâAl. The binding energy of boron is calculated in lattice fragment clusters representing the perfect crystal, as well as various defect sites. The agreement between trends in energetics determined for small clusters and periodic supercells indicates
Robert Bos; Xavier Bombois; Paul M. J. Van den Hof
2005-01-01
First principles models of complex industrial processes are often derived using finite element or finite difference methods. One of the advantages of these models is that the states in the model have a clear physical interpretation. Using such models we can attempt to monitor or control selected physical quantities, even though they may not be directly measurable. Unfortunately the CPU
Robert Bos; Xavier Bombois
2009-01-01
First principles models are commonly obtained using finite element or finite difference methods. One of the advantages of these models is that the states in the model have a clear physical interpretation. This makes of them perfect candidates for the monitoring of the states of the system. Unfortunately, the CPU time associated with each evaluation of these complex models is
First-Principles Study of Secondary Slip in Zirconium Nermine Chaari and Emmanuel Clouet
Paris-Sud XI, Université de
First-Principles Study of Secondary Slip in Zirconium Nermine Chaari and Emmanuel Clouet CEA, DEN, as found in iridium [8]. Cross slip is also observed in hexagonal close-packed (hcp) metals. In zirconium secondary slip in hcp metals. We fo- cus mainly on zirconium, although we checked that the FIG. 1. Hexagonal
First-Principles Modeling of Multiferroic RMn2O5 Guang-Can Guo,1
Vanderbilt, David
First-Principles Modeling of Multiferroic RMn2O5 Kun Cao,1 Guang-Can Guo,1 David Vanderbilt,2, 77.80.Àe RMn2O5 (R ¼ Tb, Dy, Ho, Y, etc.) belong to a very special class of multiferroics because in the view of fundamental physics, but also they have potential important applications in future
Wiese, Kay Jörg
2012-01-01
avalanches. We introduce an efficient and systematic method to compute the statistics of avalanche sizesPHYSICAL REVIEW E 85, 061102 (2012) First-principles derivation of static avalanche statistics is described by the Brownian force model (BFM), the static version of the so-called Alessandro
Surface Energy Engineering of Cu Surface by Strain: First-Principles Calculations
NASA Astrophysics Data System (ADS)
He, L.; Liu, Y. W.; Tong, W. J.; Lin, J. G.; Wang, X. F.
2013-12-01
Surface energies of strained Cu surfaces were studied systematically using first-principles methods. Results showed that the strain-stabilization of Cu surface was anisotropic and strongly related to the strain distribution. This strain-induced approach could be used as an effective way to engineer the surface energies of metals.
First principles studies of the potential-induced lifting of the Au(100) surface reconstruction
First principles studies of the potential-induced lifting of the Au(100) surface reconstruction and electrode potential by using capacitance measurements, we calculate lifting of the reconstruction to occur, is customarily observed under ultra high vacuum (UHV) conditions on the low-index faces of various noble metals
ENERGETIC MATERIALS AT HIGH COMPRESSION: FIRST-PRINCIPLES DENSITY FUNCTIONAL THEORY AND REACTIVE
Oleynik, Ivan
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 are extended to high pressures up to 50 GPa. The anisotropic characteristics of PETN upon uniaxial compression
Drabold, David
of Wannier-like functions without using eigenstates. In this paper, we investigate such functions within for truncated Wannier functions. S0163-1829 98 01611-7 I. INTRODUCTION One of the most exciting developmentsOrder-N projection method for first-principles computations of electronic quantities and Wannier
First-Principles Prediction of a Decagonal Quasicrystal Containing Boron M. Mihalkovic* and M. Widom
Widom, Michael
First-Principles Prediction of a Decagonal Quasicrystal Containing Boron M. Mihalkovic* and M as quasicrystal approximants whose deter- ministic decoration of tiles by atoms can be extended quasiperiodically45 forms a metastable decagonal quasicrystal that may be thermodynamically stable at high
cond-mat/0405298 First-principles prediction of a decagonal quasicrystal containing Boron
Widom, Michael
cond-mat/0405298 First-principles prediction of a decagonal quasicrystal containing Boron M, 2004) We interpret experimentally known B-Mg-Ru crystals as quasicrystal approximants. These ap45 forms a decagonal quasicrystal that is metastable at low temperatures and may be thermodynamically
M. W. Conroy; I. I. Oleynik; S. V. Zybin; C. T. White
2008-01-01
First-principles density functional theory (DFT) calculations have been used to obtain the constitutive relationships of pentaerythritol tetranitrate (PETN-I), a crystalline energetic material. The isotropic equation of state (EOS) for hydrostatic compression has been extended to include uniaxial compressions in the , , , , , , and crystallographic directions up to a compression ratio of V\\/V0=0.70 . DFT predicts equilibrium
Bennett, Joseph W. [Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854 (United States)] [Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854 (United States); Rabe, Karin M., E-mail: rabe@physics.rutgers.edu [Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854 (United States)
2012-11-15
In this concept paper, the development of strategies for the integration of first-principles methods with crystallographic database mining for the discovery and design of novel ferroelectric materials is discussed, drawing on the results and experience derived from exploratory investigations on three different systems: (1) the double perovskite Sr(Sb{sub 1/2}Mn{sub 1/2})O{sub 3} as a candidate semiconducting ferroelectric; (2) polar derivatives of schafarzikite MSb{sub 2}O{sub 4}; and (3) ferroelectric semiconductors with formula M{sub 2}P{sub 2}(S,Se){sub 6}. A variety of avenues for further research and investigation are suggested, including automated structure type classification, low-symmetry improper ferroelectrics, and high-throughput first-principles searches for additional representatives of structural families with desirable functional properties. - Graphical abstract: Integration of first-principles methods with crystallographic database mining, for the discovery and design of novel ferroelectric materials, could potentially lead to new classes of multifunctional materials. Highlights: Black-Right-Pointing-Pointer Integration of first-principles methods and database mining. Black-Right-Pointing-Pointer Minor structural families with desirable functional properties. Black-Right-Pointing-Pointer Survey of polar entries in the Inorganic Crystal Structural Database.
Ordered magnesium-lithium alloys: First-principles predictions Richard H. Taylor
Hart, Gus
lithium at certain concentrations.11,12 Binary ordered phases have not been conclusively identified in the HT results and by phases present in experimental phase diagrams Fig. 2 . 0 20 40 60 80 100 -70 -60Ordered magnesium-lithium alloys: First-principles predictions Richard H. Taylor Department
The New Face of Rhodium Alloys: Revealing Ordered Structures from First Principles
Hart, Gus
Alloy Phase Diagrams; ASM International: Materials Park, OH, 1990. (6) Tsai, M.-H.; Hass, K. C. Phys. Re-mail: stefano@duke.edu Abstract: The experimental and computational data on rhodium binary alloys is sparseThe New Face of Rhodium Alloys: Revealing Ordered Structures from First Principles Ohad Levy
Mark David Asta
1993-01-01
In this dissertation it is shown how quantum and statistical mechanical computational techniques can be combined in order to make possible the calculation of thermodynamic properties for solid-state binary substitutional alloy phases from first principles, i.e., from a knowledge of only basic crystallographic information and the atomic numbers of the alloy constituents. The framework which is discussed here for performing
ccsd00001055 First principles study of the solubility of Zr in Al
ccsd00001055 (version 1) : 22 Jan 2004 First principles study of the solubility of Zr in Al (Dated: January 22, 2004) The experimental solubility limit of Zr in Al is well-known. Al3Zr has a stable structure DO23 and a metastable one L12 . Consequently there is a metastable solubility limit for which only
Curtarolo, Stefano
Calculation of solubility in titanium alloys from first principles Roman V. Chepulskii, Stefano solubility in binary alloys based on the statistical-thermodynamic theory of dilute lattice gas. The model dependence determined by a ``low-solubility formation enthalpy". This quantity, directly obtainable from
Computing surface dipoles and potentials of self-assembled monolayers from first principles
Shapira, Yoram
Computing surface dipoles and potentials of self-assembled monolayers from first principles Amir reserved. Keywords: Density functional theory; Dipole layer; Work function; Self-assembled monolayers 1 principles calculations of surface dipoles and potentials in general, and surface-adsorbed self- assembled
First-principles studies of NiTa intermetallic compounds , Bin Wen b,n
Melnik, Roderick
properties, and electronic structures of NiTa intermetallic compounds are investigated in detail basedFirst-principles studies of NiTa intermetallic compounds Yi Zhou a , Bin Wen b,n , Yunqing Ma Available online 10 January 2012 Keywords: Intermetallics Miscellaneous Crystal chemistry of intermetallics
First Principles Hartree-Fock Cluster Study Of Very Dilute Transition Metal And Rare-Earth
Chow, Lee
First Principles Hartree-Fock Cluster Study Of Very Dilute Transition Metal And Rare-Earth Ion INTRODUCTION There is currently great interest in the doping of silicon with transition metal and rare-earth temperature with potential applications for spintronics2 . As far as the rare-earth ion Er3+ is concerned
First-principles study on lithium borohydride LiB H4
Kazutoshi Miwa; Nobuko Ohba; Shin-Ichi Towata; Yuko Nakamori; Shin-Ichi Orimo
2004-01-01
First-principles calculations have been performed on lithium borohydride LiB H4 using the ultrasoft pseudopotential method, which is a potential candidate for hydrogen storage materials due to its extremely large gravimetric capacity of 18 mass % hydrogen. We focus on an orthorhombic phase observed at ambient conditions and predict its fundamental properties; the structural properties, electronic properties, dielectric properties, vibrational properties,
First-principles codes for computational crystallography in the Quantum-ESPRESSO package
Giannozzi, Paolo
, Switzerland Received August 6, 2004; accepted December 5, 2004 Ab-initio calculations / Molecular dynamics calculations (phonons, elastic constants, di- electric and Raman tensors, etc.) and high-temperature mo; Laasonen et al., 1993; Giannozzi, de Angelis, Car, 2004); FPMD (First-Principles Molecular Dynamics
Hafnium binary alloys from experiments and first principles Ohad Levy a,b
Hart, Gus
Hafnium binary alloys from experiments and first principles Ohad Levy a,b , Gus L.W. Hart c February 2010 Abstract Despite the increasing importance of hafnium in numerous technological applications binary systems believed to be phase-separating. We performed a comprehen- sive study of hafnium binary
First Principles Modeling of Electrolyte Materials in All-Solid-State Batteries
NASA Astrophysics Data System (ADS)
Holzwarth, N. A. W.
This contribution reviews the series of approximations that comprise "first principles" modeling techniques which we have used in recent years to simulate ideal models of solid electrolytes, some of which are of technological interest in the development of all-solid-state battery technologies.
-carbide in Alloy Steels: First-principles Jae Hoon Jang a
Cambridge, University of
that the silicon can enhance the formation of carbide; the mechanism of this effect is not understood-carbide in Alloy Steels: First-principles Assessment Jae Hoon Jang a In Gee Kim a H. K. D. H. Bhadeshia a,b aGraduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang
Interfacial water: A first principles molecular dynamics study of a nanoscale water film on salt
Alavi, Ali
Interfacial water: A first principles molecular dynamics study of a nanoscale water film on salt Li Density functional theory DFT molecular dynamics simulations of a thin 15 Å water film on NaCl 001 have interfacial water system. The interaction of the water film with the surface orders the water molecules
First principles study of Li diffusion in I-Li_{2}NiO_{2} structure
Ceder, Gerbrand
First principles computations have been used to study Li mobility in the orthorhombic Li2NiO2 structure with the Immm space group (I-Li2NiO2). Understanding Li mobility in I-Li2NiO2 structure other than the conventional ...
A First-Principles Approach to Studying the Thermal Stability of Oxide Cathode Materials
Ceder, Gerbrand
the thermal stability of rechargeable lithium batteries, it is important to understand the thermal degradationA First-Principles Approach to Studying the Thermal Stability of Oxide Cathode Materials L. Wang, TVember 15, 2006 We present a new method for predicting the thermodynamics of thermal degradation of charged
Mechanisms of Li+ diffusion in crystalline -and -Li3PO4 electrolytes from first principles
Holzwarth, Natalie
electrolytes for use in batteries and related technologies. We have used first-principles modeling techniques results in a thermal activation energy EA=Em+Ef /2. The calculated values of EA are in excellent agreement I. INTRODUCTION In batteries and related technologies, electrolyte materials must be permeable to Li
Copyright 2006 by Rich Janow Page 1 A First-Principles Model for Estimating Atmospheric Nuclear
Janow, Rich
a simple, first principles model for nuclear weapon effects pertinent to Command, Control 26, 2006 Page 2 REFERENCES 1. Glasstone, S. and P. Dolan, eds., 'The Effects of Nuclear Weapons', 3rd edition, USDOD/USDOE (1977). 2. Brode, H., 'A Review of Nuclear Weapons Effects', Ann. Rev. Nuc. Sci. 18
Ordered Structures in Rhenium Binary Alloys from First-Principles Calculations
Hart, Gus
Ordered Structures in Rhenium Binary Alloys from First-Principles Calculations Ohad Levy,, Michal-mail: stefano@duke.edu Abstract: Rhenium is an important alloying agent in catalytic materials and superalloys to a comprehensive screening of rhenium intermetallic binary alloys. This choice is motivated by the wide array
Phonon lifetimes and thermal conductivity of graphene from first-principles
Nicola Bonini; Jivtesh Garg; Nicola Marzari
2010-01-01
Carbon nanostructures, such as graphene and carbon nanotubes, are particularly promising materials for thermal management applications because of their very high thermal conductivity. A clear understanding of the transport properties of these materials is a key step in view of their possible integration into future devices. Here, we present a first-principles study of the thermal transport properties of graphene. We
Multipole moments of water molecules in clusters and ice Ih from first principles calculations
Enrique R. Batista; Sotiris S. Xantheas; Hannes Jónsson
1999-01-01
We have calculated molecular multipole moments for water molecules in clusters and in ice Ih by partitioning the charge density obtained from first principles calculations. Various schemes for dividing the electronic charge density among the water molecules were used. They include Bader's zero flux surfaces and Voronoi partitioning schemes. A comparison was also made with an induction model including dipole,
FT-Raman, surface-enhanced Raman spectroscopy and theoretical investigations of diclofenac sodium
NASA Astrophysics Data System (ADS)
Iliescu, T.; Baia, M.; Kiefer, W.
2004-03-01
Raman and surface-enhanced Raman (SER) spectroscopies have been applied to the vibrational characterization of diclofenac sodium (DCF-Na). Theoretical calculations (DFT and ab initio) of two DCF-Na conformers have been performed to find the optimized structure and computed vibrational wavenumbers of the most stable one. SER spectra in silver colloid at different pH values have been also recorded and analyzed. Good SER spectra have been obtained in acidic and neutral environments, proving the chemisorption of the DCF-Na molecule on the silver surface. In the investigated pH range the carboxylate anion has been bonded to the silver surface through the lone pair oxygen electrons. The phenyl rings' orientation with respect to the silver surface changed on passing from acidic to neutral pH from a tilted close to flat to a more perpendicular one.
Nuclear Quantum Effects in Ice Phases and Water from First Principles Calculations
NASA Astrophysics Data System (ADS)
Pamuk, Betul
Despite the simplicity of the molecule, condensed phases of water show many physical anomalies, some of which are still unexplained to date. This thesis focuses on one striking anomaly that has been largely neglected and never explained. When hydrogen (1H) is replaced by deuterium (2 D), zero point fluctuations of the heavy isotope causes ice to expand, whereas in normal isotope effect, heavy isotope causes volume contraction. Furthermore, in a normal isotope effect, the shift in volume should decrease with increasing temperature, while, in ice, the volume shift increases with increasing temperature and persists up to the melting temperature and also exists in liquid water. In this dissertation, nuclear quantum effects on structural and cohesive properties of different ice polymorphs are investigated. We show that the anomalous isotope effect is well described by first principles density functional theory with van der Waals (vdW-DF) functionals within the quasi-harmonic approximation. Our theoretical modeling explains how the competition between the intra- and inter-molecular bonding of ice leads to an anomalous isotope effect in the volume and bulk modulus of ice. In addition, we predict a normal isotope effect when 16O is replaced by 18O, which is experimentally confirmed. Furthermore, the transition from proton disordered hexagonal phase, ice Ih to proton ordered hexagonal phase, ice XI occurs with a temperature difference between 1H and 2D of 6K, in good agreement with experimental value of 4K. We explain, for first time for that this temperature difference is entirely due to the zero point energy. In the second half of this thesis, we expand our study to the other ice phases: ice Ic, ice IX, ice II, ice VIII, clathrate hydrates, and low and high density amorphous ices. We employ the methodology that we have developed to investigate the isotope effect in structures with different configurations. We show that there is a transition from anomalous isotope effect to normal isotope effect in these structures as the density increases. We analyse the bonding mechanism of these structures and make links to the most important anomalies of liquid water.
Wei, Wei; Zhu, Yong; Lin, Cheng; Tian, Li; Xu, Zu-Wen; Nong, Jin-Peng
2014-04-01
It is quite urgent to need a flexible photodetector in the ultraviolet-visible-near infrared region for building a miniaturization broadband spectrometer. In the present paper, one kind of flexible black silicon doped with sulfur and fluorine was proposed and the optical absorption spectrum was investigated in broadband region. Firstly, the electronic structure, band structure and the optical absorption properties of the flexible black silicon doped with sulfur and fluoride were calculated using the first-principles pseudo potential calculations based on density-functional theory. Then, the absorption spectrum model of the flexible black silicon was built based on both the first-principles and finite domain time difference method. The results show that the cut-off wavelength has a red shift as the band gap of doped material becomes narrower. The higher the doping concentration is, the higher the optical absorption coefficient is obtained. The absorption coefficient of flexible black silicon doped with 50% sulfur is 8.3 times higher than that of 1.5% sulfur doping sample at the wavelength of 1 500 nm while the ratio turns to be 3 times when doped with 50% and 1.5% fluoride. The black silicon with small-size surface microstructure has the highest absorptance in the near-infrared region at the same doping concentration of 50%. Finally, a sample of flexible black silicon was fabricated by the femtosecond laser auto scanning system. The test results indicate that the absorptance of the sample is higher than 95% both in the ultraviolet and visible region and is fluctuated from 70% to 80% in the near-infrared region. It shows that as a novel light-absorbing material in broadband region the flexible black silicon doped with Sulfur and Fluorine has an potential application in exploring miniaturization broadband spectroscopy. PMID:25007598
Krcmar, M; Morris, James R
2014-04-01
Martensitic phase transformations in TiPd2 and TiPd alloys are studied employing density-functional, first-principles calculations. We examine the transformation of tetragonal C11b TiPd2 to the low-temperature orthorhombic phase (C11b ? oI6), and the transformation of cubic B2 TiPd under orthorhombic (B2?B19) and subsequent monoclinic transformations (B19?B19') as the system is cooled. We employ a theoretical approach based on a phenomenological Landau theory of the structural phase transitions and a mean-field approximation for the free energy, utilizing first-principles calculations to obtain the deformation energy as a function of strains and to deduce parameters for constructing the free energy. The predicted transition temperature for the TiPd2 C11b ? oI6 transition is in good agreement with reported experimental results. To investigate the TiPd B2?B19 transformation, we employ both the Cauchy-Born rule and a soft-mode-based approach, and elucidate the importance of the coupling between lattice distortion and atomic displacements (i.e. shuffling) in the formation of the final structure. The calculated B2?B19 transition temperature for TiPd alloy agrees well with the experimental results. We also find that there exists a very small but finite (0.0005?eV/atom) energy barrier of B19 TiPd under monoclinic deformation for B19?B19' structural phase transformation. PMID:24625683
A First Principles Molecular Dynamics Study Of Calcium Ion In Water
Lightstone, F; Schwegler, E; Allesch, M; Gygi, F; Galli, G
2005-01-28
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+}.
NASA Astrophysics Data System (ADS)
Zhou, Fei; Nielson, Weston; Xia, Yi; Ozoli?š, Vidvuds
2014-10-01
First-principles prediction of lattice thermal conductivity ?L of strongly anharmonic crystals is a long-standing challenge in solid-state physics. Making use of recent advances in information science, we propose a systematic and rigorous approach to this problem, compressive sensing lattice dynamics. Compressive sensing is used to select the physically important terms in the lattice dynamics model and determine their values in one shot. Nonintuitively, high accuracy is achieved when the model is trained on first-principles forces in quasirandom atomic configurations. The method is demonstrated for Si, NaCl, and Cu12Sb4S13 , an earth-abundant thermoelectric with strong phonon-phonon interactions that limit the room-temperature ?L to values near the amorphous limit.
Bartolomei, Massimiliano; Hernández, Marta I; Campos-Martínez, José; Pirani, Fernando; Giorgi, Giacomo; Yamashita, Koichi
2013-01-01
Graphynes are novel two-dimensional carbon-based materials that -due to their nanoweb-like structure- have been proposed as molecular filters, especially for water purification technologies. In this work we carry out first principles electronic structure calculations at the MP2C level of theory to assess the interaction between water and graphyne, graphdiyne and graphtriyne pores. The computed penetration barriers suggest that water transport is unfeasible through graphyne while being unimpeded for graphtriyne. Nevertheless, for graphdiyne, which presents a pore size almost matching that of water, a low barrier is found which in turn disappears if an active hydrogen bond with an additional water molecule on the opposite side of the opening is taken into account. These results support the possibility of using graphtriyne as an efficient membrane for water filtration but, in contrast with previous determinations, they do not exclude graphdiyne. In fact, the related first principles penetration barrier leads to ...
NASA Astrophysics Data System (ADS)
Johnston, Karen; Nieminen, Risto M.
2007-03-01
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.
Zhou, Fei; Nielson, Weston; Xia, Yi; Ozoli?š, Vidvuds
2014-10-31
First-principles prediction of lattice thermal conductivity ?(L) of strongly anharmonic crystals is a long-standing challenge in solid-state physics. Making use of recent advances in information science, we propose a systematic and rigorous approach to this problem, compressive sensing lattice dynamics. Compressive sensing is used to select the physically important terms in the lattice dynamics model and determine their values in one shot. Nonintuitively, high accuracy is achieved when the model is trained on first-principles forces in quasirandom atomic configurations. The method is demonstrated for Si, NaCl, and Cu(12)Sb(4)S(13), an earth-abundant thermoelectric with strong phonon-phonon interactions that limit the room-temperature ?(L) to values near the amorphous limit. PMID:25396378
Zhou, Fei [Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Nielson, Weston [Univ. of California, Los Angeles, CA (United States); Xia, Yi [Univ. of California, Los Angeles, CA (United States); Ozoli?š, Vidvuds [Univ. of California, Los Angeles, CA (United States)
2014-10-01
First-principles prediction of lattice thermal conductivity ?_{L} of strongly anharmonic crystals is a long-standing challenge in solid-state physics. Making use of recent advances in information science, we propose a systematic and rigorous approach to this problem, compressive sensing lattice dynamics. Compressive sensing is used to select the physically important terms in the lattice dynamics model and determine their values in one shot. Nonintuitively, high accuracy is achieved when the model is trained on first-principles forces in quasirandom atomic configurations. The method is demonstrated for Si, NaCl, and Cu_{12}Sb_{4}S_{13}, an earth-abundant thermoelectric with strong phonon-phonon interactions that limit the room-temperature ?_{L} to values near the amorphous limit.
First-Principles Studies of the Excited States of Chromophore Monomers and Dimers
NASA Astrophysics Data System (ADS)
Hamed, Samia; Sharifzadeh, Sahar; Neaton, Jeffrey
2015-03-01
Elucidation of the energy transfer mechanism in natural photosynthetic systems remains an exciting challenge. Through the careful analysis of excited states on individual chromophores and dimers - and the predictive first-principles methods used to compute them - we are building towards an understanding of the nature of excitation transfer among arrays of chromophores embedded in protein environments. Excitation energies, transition dipoles, and natural transition orbitals for the important low-lying singlet and triplet states of experimentally-relevant chromophores are obtained from first-principles time-dependent density functional theory (TDDFT) and many body perturbation theory. The effect of the Tamm-Dancoff approximation and the performance of several exchange-correlation functionals, including an optimally-tuned range-separated hybrid, are evaluated with TDDFT, and compared to MBPT calculations and experiments. This work has been supported by the DOE; computational resources have been provided by NERSC.
Amritendu Roy; Rajendra Prasad; Sushil Auluck; Ashish Garg
2009-01-01
In this study, we present the results of our first-principles calculations of\\u000athe band structure, density of states and the Born effective charge tensors for\\u000athe ferroelectric (ground state B1a1) and paraelectric (I4\\/mmm) phases of\\u000abismuth titanate. The calculations are done using the generalized gradient\\u000aapproximation (GGA) as well as the local density approximation (LDA) of the\\u000adensity functional theory.
Amritendu Roy; Rajendra Prasad; Sushil Auluck; Ashish Garg
2010-01-01
In this study, we present the results of our first-principles calculations of the band structure, density of states and the Born effective charge tensors for the ferroelectric (ground state B 1a 1) and paraelectric (I4\\/mmm) phases of bismuth titanate. The calculations are done using the generalized gradient approximation (GGA) as well as the local density approximation (LDA) of the density
X. M. Duan; C. Stampfl
2009-01-01
We investigate the structural and electronic properties and formation energies of vacancy, interstitial, and antisite defects, as well as complex formation, in wurtzite InN using first-principles calculations. The N interstitial, which forms a split-interstitial configuration with a N2 -like bonding, has the lowest formation energy under N-rich conditions in p -type material, where it is a triple donor. We find
Y. Uratani; T. Shishidou; F. Ishii; T. Oguchi
2006-01-01
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
Magnetic and Electric Phase Control in Epitaxial EuTiO3 from First Principles
Craig J. Fennie; Karin M. Rabe
2006-01-01
We propose a design strategy - based on the coupling of spins, optical\\u000aphonons, and strain - for systems in which magnetic (electric) phase control\\u000acan be achieved by an applied electric (magnetic) field. Using first-principles\\u000adensity-functional theory calculations, we present a realization of this\\u000astrategy for the magnetic perovskite EuTiO$_3$.
Steven T. Fiorino; Richard J. Bartell; Matthew J. Krizo; Gregory L. Caylor; Kenneth P. Moore; Thomas R. Harris; Salvatore J. Cusumano
2008-01-01
The Air Force Institute of Technology Center for Directed Energy (AFIT\\/CDE) has developed a first principles atmospheric propagation and characterization model called the Laser Environmental Effects Definition and Reference or LEEDR. This package enables the creation of profiles of temperature, pressure, water vapor content, optical turbulence, and atmospheric particulates and hydrometeors as they relate to line-by-line layer extinction coefficient magnitude
First-principles study of the anisotropic thermal expansion of wurtzite ZnS
S. Q. Wang
2006-01-01
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
A first-principles approach to total-dose hardness assurance
Fleetwood
1995-01-01
A first-principles approach to radiation hardness assurance was described that provides the technical background to the present US and European total-dose radiation hardness assurance test methods for MOS technologies, TM 1019.4 and BS 22900. These test methods could not have been developed otherwise, as their existence depends not on a wealth of empirical comparisons of IC data from ground and
First-principles study of He point-defects in HCP rare-earth metals
Li, Yang; Chen, Ru; Peng, SM; Long, XG; Wu, Z.; Gao, Fei; Zu, Xiaotao
2011-05-01
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.
Xiaohui Song; Sheng Liu; Han Yan; Zhiyin Gan
2009-01-01
The first-principles approach of Car–Parrinello molecular dynamics is used to study the chemisorption of a single oxygen atom on the outer surface of zigzag single-wall carbon nanotubes (SWCNT) under various axial strains and bending deformation. The effect of mechanical deformation on adsorption of oxygen atom on carbon nanotubes (CNTs) is demonstrated by linking the chemical reactivity and structural deformation. The
Fullerene-like CS x : A first-principles study of synthetic growth
C. Goyenola; G. K. Gueorguiev; S. Stafström; L. Hultman
2011-01-01
Fullerene-Like (FL) Sulpho-Carbide (CSx) compounds have been addressed by first principles calculations. Geometry optimization and cohesive energy results are presented for the relative stability of precursor species such as C2S, CS2, and C2S2 in isolated form. The energy cost for structural defects, arising from the substitution of C by S is also reported. Similar to previously synthesized FL-CNx and FL-CPx
First-principles study of spin-transfer torques in layered systems with noncollinear magnetization
Shuai Wang; Yuan Xu; Ke Xia
2008-01-01
An efficient first-principles method was developed to calculate spin-transfer torques in layered system with noncollinear magnetization. The complete scattering wave function is determined by matching the wave function in the scattering region with the Bloch states in the leads. The spin-transfer torques are obtained with the aid of the scattering wave function. We applied our method to the ferromagnetic spin
First Principles Modeling of the Growth of Crystalline Oxides on Silicon: The First Two Monolayers
G. Malcolm Stocks; Nassrin Moghadam; R. E. McKee; F. J. Walker; M. F. Chisholm; J. Hafner; G. Kresse
2000-01-01
We present the results of first principles modeling of the hetroepitaxy of alkaline earth oxides on Si. Specifically we calculate the atomistic and electronic structure of the first two layers of growth appropriate to the recent experiments of McKee et al.( R.A. McKee et al., Phys. Rev. Letters, 81), 3014 (1998). in which SrTiO3 was successfully grown on the (100)
First-Principles Calculations of Magnetic Properties of MnBi doped with Co
NASA Astrophysics Data System (ADS)
Nandadasa, Chandani N.; Dikshith, Vivek; Kim, Sungho; Kim, Seong-Gon; Park, Jihoon; Hong, Yang-Ki
2014-03-01
First principles total-energy calculations were performed to investigate the magnetic and electronic properties of MnBi doped with Co. We used Density Functional Theory (DFT) within the generalized gradient approximation (GGA) with Projector Augmented Wave (PAW) potentials. We found that when MnBi was doped with Co,the magnetization increased as the concentration of Co increased. We also calculated magnetic anisotropy energy (MAE) and magnetic anisotropy constant (Ku) of MnBi before and after doping Co.
First-principles study of the electronic structure and optical properties of UO 2
Qiuyun Chen; Xinchun Lai; Tao Tang; Bin Bai; Mingfu Chu; Yongbin Zhang; Shiyong Tan
2010-01-01
We present first-principles calculations of the electronic structure and optical properties for UO2 based on density-functional theory using the generalized gradient approximation (GGA). Hubbard U correction is employed to treat the strong correlation 5f electrons. The calculated lattice parameters and band gap are in good agreement with the experimental data. Furthermore, the dielectric function and the optical properties, such as
First principles predictions of intrinsic defects in aluminum arsenide, AlAs : numerical supplement.
Schultz, Peter Andrew
2012-04-01
This Report presents numerical tables summarizing properties of intrinsic defects in aluminum arsenide, AlAs, as computed by density functional theory. This Report serves as a numerical supplement to the results published in: P.A. Schultz, 'First principles predictions of intrinsic defects in Aluminum Arsenide, AlAs', Materials Research Society Symposia Proceedings 1370 (2011; SAND2011-2436C), and intended for use as reference tables for a defect physics package in device models.
Hubbard-U calculations from first principle Wannier functions for Cu
Ilan Schnell; Gerd Czycholl; R. C. Albers
2002-01-01
The local-density approximation is used to determine interaction matrix elements from first principle Wannier functions. We start from a standard LMTO-ASA calculation which yields Bloch functions in terms of linearized muffin-tin orbitals. From these, maximally localized Wannier functions are constructed, using a method proposed by Marzari and Vanderbilt. We have developed methods to calculate the matrix elements of various operators
First-principles Wannier functions and effective lattice fermion models for narrow-band compounds
I. V. Solovyev
2006-01-01
We propose a systematic procedure for constructing effective lattice fermion models for narrow-band compounds on the basis of first-principles electronic-structure calculations. The method is illustrated for the series of transition-metal (TM) oxides: SrVO3 , YTiO3 , V2O3 , and Y2Mo2O7 , whose low-energy properties are linked exclusively to the electronic structure of an isolated t2g band. The method consists of
First-principles calculation of temperature-composition phase diagrams of semiconductor alloys
S.-H. Wei; L. G. Ferreira; Alex Zunger
1990-01-01
The three-dimensional spin-1\\/2 Ising model with multiple-site interactions provides a natural framework for describing the temperature-composition phase diagram of substitutional binary alloys. We have carried out a ``first-principles'' approach to this problem in the following way: (i) The total energy of an A1-xBx alloy in any given substitutional arrangement of A and B on a given lattice is expanded in
First principles studies of the geometric and electronic structure of nanoalloy Ag27Cu7
M. Alcantara Ortigoza; T. S. Rahman
2007-01-01
We present first-principles calculations of the structure and electronic density of states (DOS) for the perfect core-shell Ag27Cu7 nanocluster. Our results show an expansion of 0.4 A in the diameter of the cluster compared with previous results^*. From the projected DOS we conclude that the 34-atom cluster has only 2 non-equivalent Cu atoms (core) and 4 non-equivalent Ag atoms (shell),
First-principles electronic-structure approach for phase diagrams of binary alloys
R. Podloucky; H. J. F. Jansen; X. Q. Guo; A. J. Freeman
1988-01-01
A combined statistical-mechanical and electronic-structure approach for the first-principles calculation of binary-alloy phase diagrams is presented. The grand partition function, constructed from volume-dependent internal energies obtained from local-density total-energy supercell calculations, permits the determination of the entropy and thus, in principle, all thermodynamic quantities. Illustrative results of first calculations for the Al-Li system show: (i) structural properties versus concentration in
Application of first-principles methods to binary and ternary alloy phase diagram predictions
G. Rubin; A. Finel
1995-01-01
First-principles energy calculations, using the LMTO ASA method, have been performed for seven binary alloys (Ti-Al, Ti-Mo, Ti-Nb, Ti-W, Al-Mo, Al-Nb and Al-W). For the Ti-Mo system, our results lead to stability of the B32 ordered compounds, in contradiction with the experimental phase diagrams, which display a miscibility gap for the BCC lattice. Meanwhile, recent unpublished neutron diffraction measurements have
Amit Jain; Vijay Kumar; Marcel Sluiter; Yoshiyuki Kawazoe
2006-01-01
We report the results of first principles calculations on neutral magnesium oxide clusters (MgO)nn?13, using the parallelized all-electron Tohoku University Mixed-Basis Orbitals (TOMBO) program and local density approximation for the exchange-correlation energy. It uses both plane waves and atomic orbitals as the basis functions. Various isomers are studied for each size to find the lowest energy structures and to identify
Trapping of multiple hydrogen atoms in a tungsten monovacancy from first principles
Ohsawa, Kazuhito; Yagi, Masatoshi [Research Institute for Applied Mechanics, Kyushu University, Kazuga-koen 6-1, Kasuga-shi, Fukuoka 816-8580 (Japan); Goto, Junya; Yamakami, Masahiro [Interdisciplinary Graduate School of Engineering Science, Kyushu University, Kazuga-koen 6-1, Kasuga-shi, Fukuoka 816-8580 (Japan); Yamaguchi, Masatake [Center for Computational Science and e-Systems, Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1195 (Japan)
2010-11-01
The configuration of multiple hydrogen atoms trapped in a tungsten monovacancy is investigated using first-principles calculations. Unlike previous computational studies, which have reported that hydrogen in bcc metal monovacancies occupies octahedral interstitial sites, it is found that the stable sites shift toward tetrahedral interstitial sites as the number of hydrogen atoms increases. As a result, a maximum of twelve hydrogen atoms can become trapped in a tungsten monovacancy.
First-principles theory of surface magnetocrystalline anisotropy and the diatomic-pair model
Ding-Sheng Wang; Ruqian Wu; A. J. Freeman
1993-01-01
The state-tracking method proposed recently is employed for the first-principles determination of the magnetocrystalline-anisotropy (MCA) energy. A close relationship of the MCA energy to the band structure is found for transition-metal monolayers that show the change of sign of the MCA with respect to the band filling (atomic species) to be determined mainly by the spin-orbit coupling within the spin-down
Shyue Ping Ong; Anubhav Jain; Geoffroy Hautier; Byoungwoo Kang; Gerbrand Ceder
2010-01-01
We present an analysis of the thermal reduction of delithiated LiMnPO4 and LiFePO4 based on the quarternary phase diagrams as calculated from first principles. Our results confirm the recent experimental findings that MnPO4 decomposes at a much lower temperature than FePO4, thereby potentially posing larger safety issues for LiMnPO4 cathodes. We find that while substantial oxygen is released as MnPO4
NASA Astrophysics Data System (ADS)
Liu, X. X.; Liu, L. Z.; Wu, X. L.; Chu, Paul K.
2015-07-01
The defect states and optical absorption enhancement induced by twin boundaries in silicon are investigated by first-principle calculation. The defect states in the forbidden bands are identified and based on the established electronic structures, the dielectric functions and absorption coefficients are derived. An important result of our calculations is that visible light absorption by the twinning configuration is enhanced significantly, indicating that twinning structures possibly play an important role in silicon-based photovoltaic devices.
First-principles envelope-function theory for lattice-matched semiconductor heterostructures
Bradley A. Foreman
2005-01-01
In this paper a multi-band envelope-function Hamiltonian for lattice-matched\\u000asemiconductor heterostructures is derived from first-principles norm-conserving\\u000apseudopotentials. The theory is applicable to isovalent or heterovalent\\u000aheterostructures with macroscopically neutral interfaces and no spontaneous\\u000abulk polarization. The key assumption -- proved in earlier numerical studies --\\u000ais that the heterostructure can be treated as a weak perturbation with respect\\u000ato some
First-principles envelope-function theory for lattice-matched semiconductor heterostructures
Bradley A. Foreman
2005-01-01
In this paper a multiband envelope-function Hamiltonian for lattice-matched semiconductor heterostructures is derived from first-principles self-consistent norm-conserving pseudopotentials. The theory is applicable to isovalent or heterovalent heterostructures with macroscopically neutral interfaces and no spontaneous bulk polarization. The key assumption---proved in earlier numerical studies---is that the heterostructure can be treated as a weak perturbation with respect to some periodic reference crystal,
K. Nakamura; Dzung Viet Dao; Bui Thanh Tung; T. Toriyama; S. Sugiyama
2009-01-01
We have simulated the electronic states and the piezoresistive effect response to mechanical strain in single-crystal silicon nanowires (SiNWs) with hydrogen termination by using first-principles calculations of model structures with various wire orientations. Based on our original idea for a small amount of carrier occupation, the carrier conductivity along the wire axis has been calculated in terms of band carrier
Bonding nature of metal\\/oxide incoherent interfaces by first-principles calculations
Katsuyuki Matsunaga; Takeo Sasaki; Naoya Shibata; Teruyasu Mizoguchi; Takahisa Yamamoto; Yuichi Ikuhara
2006-01-01
A bonding mechanism of large-mismatched metal\\/oxide heterointerfaces, classified as incoherent interfaces, is investigated by first-principles calculations. As a model system, incoherent Ni\\/ZrO2(111) interfaces are selected, and the interfacial bonding characters and their relevance to the interface strength are analyzed. It is found that the chemical bonds of the interfacial atomic pairs are strongly dependent on the atomic configurations in the
Wojciech Kami?ski; Rubén Pérez
2010-01-01
We present an extensive first-principles study of the interaction between a silicon oxide nanoasperity and a sexithiophene\\u000a monolayer in order to investigate the individual molecular processes responsible for the energy dissipation during atomic\\u000a force microscope (AFM) operation. Our approach includes not only ground-state calculations of the tip–sample interaction,\\u000a but an extensive set of molecular dynamics simulations at room temperature to
Lattice stability of aluminum-rare earth binary systems: A first-principles approach
Michael C. Gao; Anthony D. Rollett; Michael Widom
2007-01-01
studied via first-principles density-functional theory at low-temperature limit. The calculated phase stabilities at T=0 K are in very good agreement with experimentally reported ones for the majority of the systems. For example, we show the Al2RE.cF24 structure is the most stable compound phase in each binary and it indeed has the highest congruent melting point in each system. In some
Lattice stability of aluminum-rare earth binary systems: A first-principles approach
Michael C. Gao; Anthony D. Rollett; Michael Widom
2007-01-01
The thermodynamics of over 330 compounds in 15 Al-RE ( RE=rare earth elements) binary systems is studied via first-principles density-functional theory at low-temperature limit. The calculated phase stabilities at T=0K are in very good agreement with experimentally reported ones for the majority of the systems. For example, we show the Al2RE.cF24 structure is the most stable compound phase in each
The ideal tensile strength of tungsten and tungsten alloys by first-principles calculations
NASA Astrophysics Data System (ADS)
Giusepponi, Simone; Celino, Massimo
2013-04-01
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.
First-principles calculation of H\\/D isotopic fractionation between hydrous minerals and water
Merlin Méheut; Michele Lazzeri; Etienne Balan; Francesco Mauri
2010-01-01
Hydrogen fractionation laws between selected hydrous minerals (brucite, kaolinite, lizardite, and gibbsite) and perfect water gas have been computed from first-principles quantum-mechanical calculations. The ?-factor of each phase was calculated using the harmonic phonon dispersion curves obtained within density functional theory. All the fractionation laws show the same shape, with a minimum between 200°C (brucite) and 500°C (gibbsite). At low
Predicted ferromagnetism in hole doped armchair nanoribbons: A first principles study
NASA Astrophysics Data System (ADS)
Chen, Xi; Ni, Jun
2013-01-01
We have investigated the magnetic properties of hexagonal AlSi monolayer and nanoribbons by first principles method. The armchair AlSi nanoribbons are predicted to show itinerant magnetism. Since the armchair AlSi nanoribbons can be regarded as armchair silicene nanoribbons with injected holes, the results suggest the importance of holes in inducing magnetism in armchair nanoribbons. We further discuss boron doped armchair graphene nanoribbons and predict magnetism. These results have potential applications on electronic or spintronic devices.
First-Principles Optical Cross-Sections of Ultrathin ZnO Nanowires
Shelly Elizondo; John Mintmire
2006-01-01
One-dimensional nanostructures such as inorganic nanowires and nanotubes represent potential materials for key components of future electronic, optoelectronic, and nanoelectromechanical systems. They will also serve as important model systems to demonstrate quantum-size effects in nanostructured materials. We examine the electronic and optical properties of ZnO nanowires with different geometrical configurations within a first-principles, all-electron self-consistent local density functional (LDF) approach.
Dimensional crossover of thermal conductance in graphene nanoribbons: a first-principles approach
Jian Wang; Xiao-Ming Wang; Yun-Fei Chen; Jian-Sheng Wang
2012-01-01
First-principles density-functional calculations are performed to investigate the thermal transport properties in graphene nanoribbons (GNRs). The dimensional crossover of thermal conductance from one to two dimensions (2D) is clearly demonstrated with increasing ribbon width. The thermal conductance of GNRs of a few nanometers width already exhibits an approximate low-temperature dependence of T1.5, like that of 2D graphene sheets which is
First-principles based semi-classical model for transport in magnetic layered structures
Butler, W.H.; Zhang, X.G. [Oak Ridge National Lab., TN (United States). Metals and Ceramics Div.] [Oak Ridge National Lab., TN (United States). Metals and Ceramics Div.; MacLaren, J.M. [Tulane Univ., New Orleans, LA (United States). Dept. of Physics] [Tulane Univ., New Orleans, LA (United States). Dept. of Physics
1998-07-01
The authors present a first principles based semiclassical model for transport in an inhomogeneous magnetic layered structure. The approach solves the Boltzmann transport equation using the band structure, Bloch wave velocities and scattering matrices, describing the reflection and transmission of Bloch waves from interfaces, derived from ab-initio local spin density electronic structure calculations. The model has been tested for thick Co and Cu films as well as for a set of Co{vert_bar}Cu{vert_bar}Co spin valves.
Diffraction of helium on MgO(100) surface calculated from first-principles.
Martinez-Casado, Ruth; Usvyat, Denis; Mallia, Giuseppe; Maschio, Lorenzo; Casassa, Silvia; Ellis, John; Schütz, Martin; Harrison, Nicholas M
2014-10-21
In this work we simulate the diffraction peak intensities of He beams scattered on the MgO(100) surface from first principles. It turns out that diffraction peak intensities are extremely sensitive to the quality of the potential describing the He-MgO surface interaction. Achieving the required accuracy in first principles calculations is very challenging indeed. The present work describes a first principles protocol able to achieve very high accuracy for reasonable computational cost. This method is based on periodic local second-order Møller-Plesset perturbation theory where systematic corrections for basis set truncation and for high-order electronic correlation are introduced using coupled cluster calculations on finite model systems mimicking the target system. For the He-MgO system the requirements with respect to the level of theory are very high; it turns out that contributions from connected quadruple excitations are non-negligible. Here we demonstrate that using this protocol, it is possible to reach the accuracy in the He-MgO potential that is required to predict the observed He diffraction peak intensities. PMID:24985572
(Un)folding of a high-temperature stable polyalanine helix from first principles
NASA Astrophysics Data System (ADS)
Blum, Volker; Rossi, Mariana; Tkatchenko, Alex; Scheffler, Matthias
2010-03-01
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).
Phonon thermal transport in strained and unstrained graphene from first principles
NASA Astrophysics Data System (ADS)
Lindsay, L.; Li, Wu; Carrete, Jesús; Mingo, Natalio; Broido, D. A.; Reinecke, T. L.
2014-04-01
A rigorous first principles Boltzmann-Peierls equation (BPE) for phonon transport approach is employed to examine the lattice thermal conductivity, ?L, of strained and unstrained graphene. First principles calculations show that the out-of-plane, flexural acoustic phonons provide the dominant contribution to ?L of graphene for all strains, temperatures, and system sizes considered, supporting a previous prediction that used an optimized Tersoff empirical interatomic potential. For the range of finite system sizes considered, we show that the ?L of graphene is relatively insensitive to strain. This provides validation for use of the BPE approach to calculate ?L for unstrained graphene, which has recently been called into question. The temperature and system size dependence of the calculated ?L of graphene is in good agreement with experimental data. The enhancement of ?L with isotopic purification is found to be relatively small due to strong anharmonic phonon-phonon scattering. This work provides insight into the nature of phonon thermal transport in graphene, and it demonstrates the power of first principles thermal transport techniques.
Structural phase transitions and fundamental band gaps of MgxZn1 xO alloys from first principles
Maznichenko, I. V. [Martin-Luther University Halle-Wittenberg; Ernst, Arthur [Max Planck Institute of Microstructure Physics; Bouhassoune, M. [Max Planck Institute of Microstructure Physics; Henk, J. [Max Planck Institute of Microstructure Physics; Daene, Markus W [ORNL; Lueders, Martin [Daresbury Laboratory, UK; Bruno, Patrick [European Synchrotron Radiation Facility (ESRF); Wolfam, Hergert [Martin-Luther University Halle-Wittenberg; Mertig, I. [Martin-Luther University Halle-Wittenberg; Szotek, Zdzislawa [Daresbury Laboratory, UK; Temmerman, Walter M [Daresbury Laboratory, UK
2009-01-01
The structural phase transitions and the fundamental band gaps of MgxZn1 xO alloys are investigated by detailed first-principles calculations in the entire range of Mg concentrations x, applying a multiple-scattering theoretical approach (Korringa-Kohn-Rostoker method). Disordered alloys are treated within the coherent-potential approximation. The calculations for various crystal phases have given rise to a phase diagram in good agreement with experiments and other theoretical approaches. The phase transition from the wurtzite to the rock-salt structure is predicted at the Mg concentration of x=0.33, which is close to the experimental value of 0.33 0.40. The size of the fundamental band gap, typically underestimated by the local-density approximation, is considerably improved by the self-interaction correction. The increase in the gap upon alloying ZnO with Mg corroborates experimental trends. Our findings are relevant for applications in optical, electrical, and, in particular, in magnetoelectric devices.
Theoretical study of core-loss electron energy-loss spectroscopy at graphene nanoribbon edges.
Fujita, N; Hasnip, P J; Probert, M I J; Yuan, J
2015-08-01
A systematic study of simulated atomic-resolution electronic energy-loss spectroscopy (EELS) for different graphene nanoribbons (GNRs) is presented. The results of ab initio studies of carbon [Formula: see text] core-loss EELS on GNRs with different ribbon edge structures and different hydrogen terminations show that theoretical core-loss EELS can distinguish key structural features at the atomic scale. In addition, the combination of polarized core-loss EELS with symmetry resolved electronic partial density of states calculations can be used to identify the origins of all the primary features in the spectra. For example, the nature of the GNR edge structure (armchair, zigzag, etc) can be identified, along with the degree of hydrogenation. Hence it is possible to use the combination of ab initio calculations with high resolution, high energy transmission core-loss EELS experiments to determine the local atomic arrangement and chemical bonding states (i.e. a structural fingerprint) in GNRs, which is essential for future practical applications of graphene. PMID:26173149
Zhang, Jian; Zhou, Bin; Sun, Zhenrong; Wang, Xue B.
2015-01-01
Proposed in theory and confirmed to exist, anion?? interactions have been recognized as new and important non-covalent binding forces. Despite extensive theoretical studies, numerous crystal structural identifications, and a plethora of solution phase investigations, intrinsic anion???interaction strengths that are free from complications of condensed phases’ environments, have not been directly measured in the gas phase. Herein we present a joint photoelectron spectroscopic and theoretical study on this subject, in which tetraoxacalix[2]arene[2]triazine 1, an electron-deficient and cavity self-tunable macrocyclic was used as a charge-neutral molecular host to probe its interactions with a series of anions with distinctly different shapes and charge states (spherical halides Cl?, Br?, I?, linear thiocyanate SCN?, trigonal planar nitrate NO3?, pyramidic iodate IO3?, and tetrahedral sulfate SO42?). The binding energies of the resultant gaseous 1:1 complexes (1•Cl?, 1•Br?, 1•I?, 1•SCN?, 1•NO3?, 1•IO3? and 1•SO42?) were directly measured experimentally, exhibiting substantial non-covalent interactions with pronounced anion specific effects. The binding strengths of Cl?, NO3?, IO3? with 1 are found to be strongest among all singly charged anions, amounting to ca. 30 kcal/mol, but only about 40% of that between 1 and SO42?. Quantum chemical calculations reveal that all anions reside in the center of the cavity of 1 with anion???binding motif in the complexes’ optimized structures, where 1 is seen to be able to self-regulate its cavity structure to accommodate anions of different geometries and three-dimensional shapes. Electron density surface and natural bond orbital charge distribution analysis further support anion???binding formation. The calculated??binding energies of the anions and 1 nicely reproduce the experimentally estimated electron binding energy increase. This work illustrates that size-selective photoelectron spectroscopy combined with theoretical calculations represent a powerful technique to probe intrinsic anion???interactions and has potential to provide quantitative guest-host molecular binding strengths and unravel fundamental insights in specific anion recognitions.
NASA Astrophysics Data System (ADS)
Yamada, A.; Nanbu, S.; Kasai, Y.; Ozima, M.
2009-12-01
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.
NASA Astrophysics Data System (ADS)
Ku, Wei
2012-02-01
This talk will discuss recently developed first-principles methods for materials with disordered impurities, and their applications to case studies of correlated oxides and Fe-based superconductors containing vacancies, substitutions and intercallants. Simplified via the use of Wannier functions, the first method [1] is to unfold the one-particle spectral function from the reduced Brillouin zone of a broken symmetry state back to the regular Brillouin zone of the normal state. This unfolding not only allows a clearer visualization of the physical effects of the broken translational symmetry, but also connects directly to the experimental spectral weight of angular resolved photo emission spectroscopy. The second method [2] is to reduce the computational expense of configuration-averaged spectral function of disordered materials by orders of magnitude, to allow inclusion of large length scale required for weakly localized states and short-range orders. This Wannier function based method is systematically improvable, beyond-mean-field, and not perturbation limited. Case studies to be discussed include dilute magnetic semiconductors [3], transition metal oxides[1,2], and Fe-based superconductors [4,5]. [4pt] [1] Wei Ku et al, PRL 104, 216401 (2010)[0pt] [2] T. Berlijn et al, PRL 106, 077005 (2011)[0pt] [3] T. S. Herng et al, PRL 105, 207201 (2010)[0pt] [4] C.-C. Lee et al, PRL 103, 267001 (2009)[0pt] [5] C.-H. Lin et al, arXiv:1107.1485
First-principles calculations of optical properties of GeC, SnC and GeSn under hydrostatic pressure
NASA Astrophysics Data System (ADS)
Sahnoun, M.; Khenata, R.; Baltache, H.; Rérat, M.; Driz, M.; Bouhafs, B.; Abbar, B.
2005-01-01
We present first-principles of the full-potential linearized augmented plane wave calculations of the effect of hydrostatic pressure on the optical properties of zinc-blende GeC, SnC and GeSn compounds. The refractive index and its variation with hydrostatic pressure are well described. An accurate calculation of linear optical functions (refraction index and its pressure derivative, and both imaginary and real parts of the dielectric function) is performed in the photon energy range up to 15 eV. The predicted optical constants agree well with the available experimental and theoretical ones.
A first-principles methodology for diffusion coefficients in metals and dilute alloys
NASA Astrophysics Data System (ADS)
Mantina, Manjeera
This work is a study exploring the extent of suitability of static first-principles calculations for studying diffusion in metallic systems. Specifically, vacancy-mediated volume diffusion in pure elements and alloys with dilute concentration of impurities is studied. A novel procedure is discovered for predicting diffusion coefficients that overcomes the shortcomings of the well-known transition state theory, by Vineyard. The procedure that evolves from Eyring's reaction rate theory yields accurate diffusivity results that include anharmonic effects within the quasi-harmonic approximation. Alongside, the procedure is straightforward in its application within the conventional harmonic approximation, from the results of static first-principles calculations. To prove the extensibility of the procedure, diffusivities have been computed for a variety of systems. Over a wide temperature range, the calculated self-diffusion and impurity diffusion coefficients using local density approximation (LDA) of density functional theory (DFT) are seen to be in excellent match with experimental data. Self-diffusion coefficients have been calculated for: (i) fcc Al, Cu, Ni and Ag (ii) bcc W and Mo (v) hcp Mg, Ti and Zn. Impurity diffusion coefficients have been computed for: (i) Mg, Si, Cu, Li, Ag, Mo and 3d transition elements in fcc Al (ii) Mo, Ta in bcc W and Nb, Ta and W in bcc Mo (iii) Sn and Cd in hcp Mg and Al in hcp Ti. It is also an observation from this work, that LDA does not require surface correction for yielding energetics of vacancy-containing system in good comparison with experiments, unlike generalized gradient approximation (GGA). It is known that first-principles' energy minimization procedures based on electronic interactions are suited for metallic systems wherein the valence electrons are freely moving. In this thesis, research has been extended to study suitability of first-principles calculations within LDA/GGA including the localization parameter U, for Al system with transition metal solutes, in which charges are known to localize around the transition metal element. U parameter is determined from matching the diffusivities of 3d transition metal impurity in aluminum with reliable experimental data. The effort yielded activation energies in systematic agreement with experiments and has proved useful in obtaining insights into the complex interactions in these systems. Besides the prediction of diffusion coefficients, this research has been helpful in understanding the physics underlying diffusion. Within the scope of observations from the systems studied, certain diffusion related aspects that have been clarified are: (i) cause for non-Arrnenius' nature of diffusion plots (ii) definitions of atom migration properties (iii) magnitude and sign of diffusion parameters enthalpy and entropy of formation and migration and characteristic vibrational frequency (iv) trends in diffusivities based on activation energy and diffusion prefactor (vi) cause for anomalous diffusion behavior of 3d transition metals in Al, and their magnetic nature (vii) contributions from electronic contributions to curvature at very high temperatures of bcc refractory elements (viii) temperature dependence of impurity diffusion correlation factors. Finally, the double-well potential of diffusion by vacancy mechanism has been calculated from first-principles. This aided calculation of entropy of migration and thus free energy of migration along with characteristic vibrational frequency. Also for the first time, temperature dependence of enthalpy of migration and thus atom jump frequency has been accurately predicted. From the broad perspective of predicting diffusion coefficients from computational methodologies, it can be stated as a result of this work that: static first-principles extend an irreplaceable contribution to the future of diffusion modeling. The procedure obviated the use of (i) redundant approximations that limit its accuracy and (ii) support from other computational techniques that restrict its extensibility due to insufficient i
First-principles prediction of a new Dirac-fermion material: silicon germanide monolayer.
Zhou, Hongcai; Zhao, Mingwen; Zhang, Xiaoming; Dong, Wenzheng; Wang, Xiaopeng; Bu, Hongxia; Wang, Aizhu
2013-10-01
From first-principles calculations, we proposed a silicon germanide (SiGe) analog of silicene. This SiGe monolayer is stable and free from imaginary frequency in the phonon spectrum. The electronic band structure near the Fermi level can be characterized by Dirac cones with the Fermi velocity comparable to that of silicene. The Ge and Si atoms in SiGe monolayer exhibit different tendencies in binding with hydrogen atoms, making sublattice-selective hydrogenation and consequently electron spin-polarization possible. PMID:23945421
First Principles Hartree–Fock Description of Lithium Insertion in Oxides
W. C. Mackrodt
1999-01-01
First principles periodic Hartree–Fock calculations are reported for theP42\\/mnm(rutile),I41\\/amd(anatase),Pbca(brookite),Pnma(ramsdellite),Pcbn(colombite),Fd3m(spinel), andImma(orthorhombic) polymorphs of TiO2, from which the predicted order of stability isThe calculated difference in energy between the rutile and anatase structures is 0.02–0.06 eV, in good agreement with a recent local density approximation (LDA) estimate of 0.033 eV and an experiment enthalpy difference of 0.05 eV. The corresponding Hartree–Fock and
First Principles Calculations of the Adsorption of Group III Metals on Si(001) at High Temperature
NASA Astrophysics Data System (ADS)
Cotzomi-Paleta, J.; Cocoletzi, Gregorio H.; Takeuchi, Noboru
First principles total energy calculations are performed to investigate the Si(001)-(3 × 4) reconstruction induced by the adsorption of group III metals (Ga, Al and In) when deposited at high temperatures. We have considered different models in our study and we have found that the pyramid-like structure proposed by Bunk et al. for In on Si(001) yields the most stable atomic configuration in all cases. We present a detailed description of this surface reconstruction, and compare the structural parameters for Al, Ga and In.
On Developing a European First Principles Geomagnetically Induced Current Forecasting System
NASA Astrophysics Data System (ADS)
Honkonen, Ilja; Viljanen, Ari; Vanhamäki, Heikki
2013-04-01
Geomagnetically induced currents (GIC) during space storms pose a risk to power transmission grids across the globe. As part of the European Risk from Geomagnetically Induced Currents (EURISGIC) EU/FP7 project the Finnish Meteorological Institute is developing a first principles based GIC forecasting and warning system. The system is given as input the solar wind plasma parameters measured by the ACE spacecraft and the final output consists of the ground electric field and GIC in a simplified model of European high-voltage power grids. We present the different steps involved in obtaining the final GIC solution and implementation of the required software components.
First-Principles Molecular Dynamics Calculations of the Equation of State for Tantalum
Ono, Shigeaki
2009-01-01
The equation of state of tantalum (Ta) has been investigated to 100 GPa and 3,000 K using the first-principles molecular dynamics method. A large volume dependence of the thermal pressure of Ta was revealed from the analysis of our data. A significant temperature dependence of the calculated effective Grüneisen parameters was confirmed at high pressures. This indicates that the conventional approach to analyze thermal properties using the Mie-Grüneisen approximation is likely to have a significant uncertainty in determining the equation of state for Ta, and that an intrinsic anharmonicity should be considered to analyze the equation of state. PMID:20057949
NASA Astrophysics Data System (ADS)
Yu, Haiping; Zheng, Jiming; Guo, Ping; Zhang, Zhiyong
2014-12-01
A carbon nanotube (CNT)/zigzag graphene nanoribbons (ZGNRs) junctions has been proposed and investigated by first-principles calculations. The results show that large spin polarization of currents would be achieved when only one edge of ZGNR is coupled to the other lead. By virtue of spatial separation of edge state in two spin channel, one of those channels is opened at certain energy range and gives rise to spin-polarized currents under a low bias. This feature is stable whenever the ZGNR lead is under the antiferromagnetic ground states or is under the ferromagnetic states. Our findings indicate that this approach is simple and efficient for spintronics design.
Nanoplasmon dynamics and field enhancement of graphene flakes by first-principles simulations
NASA Astrophysics Data System (ADS)
Yamamoto, Noriaki; Hu, Chunping; Hagiwara, Satoshi; Watanabe, Kazuyuki
2015-04-01
We investigated nanoplasmon dynamics in graphene flake (GF) monomers and dimers using real-time and real-space time-dependent density functional theory. By showing the characteristic features of dynamical polarizability, we verified that the two distinct peaks in the optical absorption spectra can be attributed to ? and ? + ? plasmons. We clearly show a significant difference between the on- and off-resonance responses of the ? plasmon by demonstrating the spatial distribution of the induced charge and the electric field. We thus reveal the mechanism of plasmon-induced field enhancement near GF edges, which shows great importance of first-principles studies on molecular nanoplasmonics.
Fullerene-like CS x: A first-principles study of synthetic growth
NASA Astrophysics Data System (ADS)
Goyenola, C.; Gueorguiev, G. K.; Stafström, S.; Hultman, L.
2011-04-01
Fullerene-Like (FL) Sulpho-Carbide (CSx) compounds have been addressed by first principles calculations. Geometry optimization and cohesive energy results are presented for the relative stability of precursor species such as C2S, CS2, and C2S2 in isolated form. The energy cost for structural defects, arising from the substitution of C by S is also reported. Similar to previously synthesized FL-CNx and FL-CPx compounds, the pentagon, the double pentagon defects as well as the Stone-Wales defects are confirmed as energetically feasible in CSx compounds.
First-principles simulations of heavy fermion cerium compounds based on the Kondo lattice.
Matsumoto, Munehisa; Han, Myung Joon; Otsuki, Junya; Savrasov, Sergey Y
2009-08-28
We propose a new framework for first-principles calculations of heavy-fermion materials. These are described in terms of the Kondo lattice Hamiltonian with the parameters extracted from a realistic density functional based calculation which is then solved using continuous-time quantum Monte Carlo method and dynamical mean field theory. As an example, we show our results for the Néel temperatures of cerium-122 compounds (CeX(2)Si(2) with X = Ru, Rh, Pd, Cu, Ag, and Au) where the general trend around the magnetic quantum critical point is successfully reproduced. Our results are organized on a universal Doniach phase diagram in a semiquantitative way. PMID:19792815
A. K. Ray; J. C. Boettger
2004-01-01
First principles linear combinations of Gaussian type orbitals-fitting\\u000afunction (LCGTO-FF) electronic structure calculations are used to study\\u000athickness dependencies in the surface energies and work functions of ultra-thin\\u000a(111) films of fcc delta-Pu, up to five layers thick. The calculations are\\u000acarried out at both scalar- and fully-relativistic (with and without spin-orbit\\u000acoupling) levels of approximation. The surface energy is
First-Principles Investigation of Reactive Molecular Dynamics in Detonating Rdx and Tatb
NASA Astrophysics Data System (ADS)
Oleynik, I. I.; Landerville, A. C.; White, C. T.
2009-12-01
Possible initial chemical reactions in detonating RDX and TATB that result from intermolecular collisions behind the shock wave front have been investigated using first-principles reactive molecular dynamics. The reaction dynamics was studied as a function of collision velocities and crystallographic orientations. Threshold collision velocities of chemical initiation and products of decomposition were obtained for each orientation, and compared to calculated bond dissociation energies to rationalize the outcome chemical events. Reaction timescales were also determined and used to understand whether these initial chemical events are largely driven by reaction dynamics or temperature.
Gerra, G; Tagantsev, A K; Setter, N
2007-05-18
We present an approach to the size effect problem in ferroelectric-electrode systems which combines first-principles calculations and phenomenological theory. The parameters of the model can be extracted from calculations on ultrathin films, while experimentally verifiable predictions can be made on thick films. We illustrate the approach for the case of SrRuO3/BaTiO3/SrRuO3 heterostructures with asymmetric interfaces. This enables us to provide a quantitative description of a number of manifestations of such asymmetry in films of technologically meaningful thickness. PMID:17677739
NASA Astrophysics Data System (ADS)
Yang, Q.; Cao, J. X.; Zhou, Y. C.; Zhang, Y.; Ma, Y.; Lou, X. J.
2013-09-01
Comprehensive first-principle calculations indicate that the oxygen vacancy (VO) configuration of a prototypical perovskite oxide PbTiO3 (PTO) can be tuned by strain engineering. Remarkably, we found that VO located along the c axis (Vc), normally resulting in a polarization-harming tail-to-tail domain configuration in strain-free PTO (Vcud), can be readily tuned to a polarization-harmless head-to-tail domain configuration (Vcsw) by applying an ab-biaxial compressive strain of 3%. Also, VO located in the ab-plane (Vab), another type of polarization-harmless vacancy, can be stabilized by applying a compressive strain of 2%.
Electron-phonon superconductivity in non-centrosymmetric LaNiC2: first principles calculations
Subedi, Alaska P [ORNL; Singh, David J [ORNL
2009-01-01
We report first principles calculations of the electronic structure and electron-phonon coupling in the non-centrosymmetric superconductor LaNiC2. These show that the material is a conventional electron-phonon superconductor with intermediate coupling. There are large contributions to the coupling by two low frequency C non-bond-stretching modes, one of which has strong Kohn anomalies. Since LaNiC2 lacks inversion symmetry, the pairing is of dominant $s$-wave type with some mixture of $p$-wave character. This will give exponential decay in the specific heat and can at the same time break time-reversal symmetry.
First principle calculations of hexyl thiolate monolayer on Au(1 1 1)
NASA Astrophysics Data System (ADS)
Wu, Taiquan; Cao, Dan; Wang, Xinyan; Jiao, Zhiwei; Chen, Miaogen; Luo, Honglei; Zhu, Ping
2015-03-01
The first-principle technique has been employed to determine the structure of hexyl thiolate molecular chains, monolayers and the adsorption system. CASTEP calculation shows that hexyl thiolate monolayer is a self-assembly system. And the molecular orientation of the hexyl thiolate on the surface is not symmetrical, they have the simplex structure. The electron density confirms the result. Hexyl thiolate monolayer is adsorbed on the Au(1 1 1)-(?3 × ?3)R30° surface in the bridge site with the angle between the Ssbnd C6 bond and the surface is 65°. The structural parameters in the adsorption system are the same to those in the monolayer.
Liquid–liquid phase transition in compressed hydrogen from first-principles simulations
Scandolo, Sandro
2003-01-01
The properties of compressed liquid hydrogen, the most abundant fluid in the universe, have been investigated by means of first-principles molecular dynamics at pressures between 75 and 175 GPa and temperatures closer to the freezing line than so far reported in shock-wave experiments. Evidence for a liquid–liquid transition between a molecular and a dissociated phase is provided. The transition is accompanied by a 6% increase in density and by metallization. This finding has important implications for our understanding of the interiors of giant planets and supports predictions of a quantum fluid state at low temperatures. PMID:12626753
First-principles study of hydrogen diffusion in transition metal palladium
NASA Astrophysics Data System (ADS)
Sarantuya, Nasantogtokh; Cui, Xin; Wang, Zhi Ping
2015-05-01
By adopting the first-principles total energy calculations based on density functional theory (DFT), the diffuse pattern and path of hydrogen in bulk palladium are investigated by calculating the system energy of hydrogen atom occupying different positions in palladium crystal lattice. The results indicate that the most stable position of hydrogen atom in palladium crystal lattice locates at the octahedral interstice, and the tetrahedral interstice is the second stable site. Hydrogen diffusion along the indirect octahedral-tetrahedral-octahedral (O-T-O) path is energetically most favorable in transition metal palladium, and the activation energy is 0.5245 eV.
Carrier compensation in semi-insulating CdTe: First-principles calculations
Du, Mao-Hua [ORNL; Singh, David J [ORNL
2008-01-01
Carrier compensation in semi-insulating CdTe has been attributed to the compensation of surplus shallow acceptors by deep donors, usually assumed to be Te antisites. However, our first-principles calculations show that intrinsic defects should not have a significant effect on the carrier compensation due either to lack of deep levels near midgap or to low defect concentration. We demonstrate that an extrinsic defect, OTe-H complex, may play an important role in the carrier compensation in CdTe because of its amphoteric character and reasonably high concentration. Our findings have important consequences for improving device performance in CdTe-based radiation detectors and solar cells.
Phonon transport in perovskite SrTiO3 from first principles
Feng, Lei; Shiomi, Junichiro
2015-01-01
We investigate phonon transport in perovskite strontium titanate (SrTiO3) which is stable above its phase transition temperature (~105 K) by using first-principles molecular dynamics and anharmonic lattice dynamics. Unlike conventional ground-state-based perturbation methods that give imaginary phonon frequencies, the current calculation reproduces stable phonon dispersion relations observed in experiments. We find the contribution of optical phonons to overall lattice thermal conductivity is larger than 60%, markedly different from the usual picture with dominant contribution from acoustic phonons. The mode- and pseudopotential-dependence analysis suggests the strong attenuation of acoustic phonons transport originated from strong anharmonic coupling with the transversely-polarized ferroelectric modes.
Structural phase transition and elastic properties of hafnium dihydride: A first principles study
Santhosh, M., E-mail: rrpalanichamy@gmail.com; Rajeswarapalanichamy, R., E-mail: rrpalanichamy@gmail.com; Sudhapriyanga, G.; Murugan, A.; Chinthia, A. Jemmy [Department of Physics, N.M.S.S.V.N College, Madurai, Tamil Nadu-625019 (India); Kanagaprabha, S. [Department of Physics, Kamaraj College, Tuticorin, Tamil Nadu-628003 (India); Iyakutti, K. [Department of Physics and Nanotechnology, SRM University, Chennai, Tamil Nadu-603203 (India)
2014-04-24
The structural and elastic properties of Hafnium dihydride (HfH{sub 2}) are investigated by first principles calculation based on density functional theory using Vienna ab-initio simulation package (VASP). The calculated lattice parameters are in good agreement with the available results. A pressure induced structural phase transition from CaF{sub 2} to FeS{sub 2} phase is observed in HfH{sub 2} at 10.75 GPa. The calculated elastic constants indicate that this hydride is mechanically stable at ambient condition.
Valence-band mixing in first-principles envelope-function theory
Bradley A. Foreman
2007-01-01
This paper presents a numerical implementation of a first-principles envelope-function theory derived recently by the author [B. A. Foreman, Phys. Rev. B 72, 165345 (2005)]. The examples studied deal with the valence subband structure of GaAs\\/AlAs , GaAs\\/Al0.2Ga0.8As , and In0.53Ga0.47As\\/InP (001) superlattices calculated using the local-density approximation to density-functional theory and norm-conserving pseudopotentials without spin-orbit coupling. The heterostructure Hamiltonian
NASA Astrophysics Data System (ADS)
Zhang, Cui; Pham, Tuan Anh; Gygi, François; Galli, Giulia
2013-05-01
We present first principles molecular dynamics simulations of the chloride anion in liquid water performed using gradient-corrected and hybrid density functionals. We show that it is necessary to use hybrid functionals both for the generation of molecular dynamics trajectories and for the calculation of electronic states in order to obtain a qualitatively correct description of the electronic properties of the solution. In particular, it is only with hybrid functionals that the highest occupied molecular orbital of the anion is found above the valence band maximum of water, consistent with photoelectron detachment measurements. Similar results were obtained using many body perturbation theory within the G0W0 approximation.
First principles study of photoelectron spectra of Cu{sub {ital n}}{sup {minus}} clusters
Massobrio, C.; Pasquarello, A.; Car, R. [Institut de Physique Experimentale, Ecole Polytechnique Federale de Lausanne, PHB-Ecublens, CH-1015 Lausanne (Switzerland)] [Institut de Physique Experimentale, Ecole Polytechnique Federale de Lausanne, PHB-Ecublens, CH-1015 Lausanne (Switzerland); [Institut Romand de Recherche Numerique en Physique des Materiaux (IRRMA), IN-Ecublens, CH-1015 Lausanne (Switzerland); [Department of Condensed Matter Physics, University of Geneva, CH-1211 Geneva (Switzerland)
1995-09-11
We have determined equilibrium geometries and electronic properties of neutral and anionic Cu{sub {ital n}} ({ital n}=2,9) clusters by means of first principles calculations in which {ital s} and {ital d} electrons are treated on equal footing. We find that the calculated electronic density of states is inadequate to interpret photoelectron spectra of Cu{sub {ital n}}{sup {minus}} clusters. We obtain good agreement between calculated excitation energies and experimental spectra when we include final states effects.
NASA Astrophysics Data System (ADS)
Lee, Hyung-June; Kim, Gunn; Kwon, Young-Kyun
2013-08-01
Using first-principles calculations, we investigate the electronic structures and binding properties of nicotine and caffeine adsorbed on single-walled carbon nanotubes to determine whether CNTs are appropriate for filtering or sensing nicotine and caffeine molecules. We find that caffeine adsorbs more strongly than nicotine. The different binding characteristics are discussed by analyzing the modification of the electronic structure of the molecule-adsorbed CNTs. We also calculate the quantum conductance of the CNTs in the presence of nicotine or caffeine adsorbates and demonstrate that the influence of caffeine is stronger than nicotine on the conductance of the host CNT.
First-principles study of the infrared spectra of the ice Ih (0001) surface.
Pham, T Anh; Huang, P; Schwegler, E; Galli, G
2012-09-20
We present a study of the infrared (IR) spectra of the (0001) deuterated ice surface based on first-principles molecular dynamics simulations. The computed spectra show a good agreement with available experimental IR measurements. We identified the bonding configurations associated with specific features in the spectra, allowing us to provide a detailed interpretation of IR signals. We computed the spectra of several proton ordered and disordered models of the (0001) surface of ice, and we found that IR spectra do not appear to be a sensitive probe of the microscopic arrangement of protons at ice surfaces. PMID:22913799
First principle study of elastic and thermodynamic properties of FeB{sub 4} under high pressure
Zhang, Xinyu, E-mail: xyzhang@ysu.edu.cn, E-mail: jiaqianqin@gmail.com, E-mail: riping@ysu.edu.cn; Ning, Jinliang; Sun, Xiaowei; Li, Xinting; Ma, Mingzhen; Liu, Riping, E-mail: xyzhang@ysu.edu.cn, E-mail: jiaqianqin@gmail.com, E-mail: riping@ysu.edu.cn [State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004 (China); Qin, Jiaqian, E-mail: xyzhang@ysu.edu.cn, E-mail: jiaqianqin@gmail.com, E-mail: riping@ysu.edu.cn [Metallurgy and Materials Science Research Institute, Chulalongkorn University, Bangkok 10330 (Thailand); State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004 (China)
2013-11-14
The elastic properties, elastic anisotropy, and thermodynamic properties of the lately synthesized orthorhombic FeB{sub 4} at high pressures are investigated using first-principles density functional calculations. The calculated equilibrium parameters are in good agreement with the available experimental and theoretical data. The obtained normalized volume dependence of high pressure is consistent with the previous experimental data investigated using high-pressure synchrotron x-ray diffraction. The complete elastic tensors and crystal anisotropies of the FeB{sub 4} are also determined in the pressure range of 0–100?GPa. By the elastic stability criteria and vibrational frequencies, it is predicted that the orthorhombic FeB{sub 4} is stable up to 100 GPa. In addition, the calculated B/G ratio reveals that FeB{sub 4} possesses brittle nature in the range of pressure from 0 to 100?GPa. The calculated elastic anisotropic factors suggest that FeB{sub 4} is elastically anisotropic. By using quasi-harmonic Debye model, the compressibility, bulk modulus, the coefficient of thermal expansion, the heat capacity, and the Grüneisen parameter of FeB{sub 4} are successfully obtained in the present work.
NASA Astrophysics Data System (ADS)
Rebola, Alejandro; Klie, Robert; Zapol, Peter; Ogut, Serdar
2013-03-01
The misfit-layered oxide Ca3Co4O9 (CCO) has recently been the subject of many experimental and some theoretical investigations due to its remarkable thermoelectric properties. CCO is composed of two incommensurate subsystems, a distorted rocksalt-type Ca2CoO3 layer sandwiched between hexagonal CoO2 layers. Taking into account that the composition ratio between these subsystems is very close to the golden mean, which is the limit of the sequence of the ratios of consecutive Fibonacci numbers F (n) , we model CCO from first principles[1] by using rational approximants of composition [Ca2CoO3]2 F (n)[CoO2]2 F (n + 1). In the present study, we use 3/2 and 5/3 rational approximants and PBE+U computations to calculate the ab initio phonon dispersion curves, related thermal properties, as well as ab initio electronic transport properties such as DC conductivity and thermopower within the relaxation time approximation by applying the Boltzmann transport theory. Results are compared with available experimental data and potential routes for increasing the thermopower of CCO are discussed.
NASA Astrophysics Data System (ADS)
Wang, Jingyang; Zhou, Yanchun; Lin, Zhijun; Liao, Ting
2005-08-01
We have investigated the electronic structure, chemical bonding, and equations of state of Zr2Al3C5 by means of the ab initio pseudopotential total energy method. The chemical bonding displays layered characteristics and is similar to that of nanolaminate ternary aluminum carbides Ti2AlC and Ti3AlC2 . Zr2Al3C5 could be fundamentally described as strong covalent bonding among Al-C-Zr-C-Zr-C-Al atomic chains being interleaved and mirrored by AlC2 blocks. The interplanar cohesion between covalent atomic chains and AlC2 blocks is very weak based on first-principles cohesion energy calculations. Inspired by the structure-property relationship of Ti2AlC and Ti3AlC2 , it is expected that Zr2Al3C5 will have easy machinability, damage tolerance, and oxidation resistance besides the merits of refractory ZrC. Zr2Al3C5 has a theoretical bulk modulus of 160GPa and illustrates elastic anisotropy under pressure below 20GPa .
First-Principles Study on Structural and Thermoelectric Properties of Al- and Sb-Doped Mg2Si
NASA Astrophysics Data System (ADS)
Hirayama, Naomi; Iida, Tsutomu; Funashima, Hiroki; Morioka, Shunsuke; Sakamoto, Mariko; Nishio, Keishi; Kogo, Yasuo; Takanashi, Yoshifumi; Hamada, Noriaki
2015-06-01
We theoretically investigate the structural and thermoelectric properties of magnesium silicide (Mg2Si) incorporating Al or Sb atoms as impurities using first-principles calculations. We optimized the structural properties through variable-cell relaxation using a pseudopotential method based on density functional theory. The result indicates that the lattice constant can be affected by the insertion of impurity atoms into the system, mainly because the ionic radii of these impurities differ from those of the matrix constituents Mg and Si. We then estimate, on the basis of the optimized structures, the site preferences of the impurity atoms using a formation energy calculation. The result shows a nontrivial concentration-dependence of the site occupation, such that Al tends to go into the Si, Mg, and interstitial sites with comparable formation energies at low doping levels (<2 at.%); it can start to substitute for the Mg sites preferentially at higher doping levels (<4 at.%). Sb, on the other hand, shows a strong preference for the Si sites at all impurity concentrations. Furthermore, we obtain the temperature-dependence of the thermoelectromotive force (Seebeck coefficient) of the Al- and Sb-doped Mg2Si using the full-potential linearized augmented-plane-wave method and the Boltzmann transport equation.
NASA Astrophysics Data System (ADS)
Liu, Qi-Jun; Zhang, Ning-Chao; Liu, Fu-Sheng; Liu, Zheng-Tang
2014-07-01
Titanium dioxide is well known as a semiconductor material, which attracts a great deal of attention for promising applications in many fields due to its outstanding physical and chemical properties. To investigate the structural, elastic, mechanical, electronic and optical properties of various TiO2 phases systematically, we present the ultrasoft pseudopotential planewave method within local density approximation and generalized gradient approximation, as well as the norm-conserving pseudopotential within hybrid functional B3LYP by first-principles calculations on fluorite, pyrite, rutile, anatase, hollandite, brookite, columbite, cotunnite, bronze and baddeleyite TiO2 phases. The structural parameters of ten phases are calculated, which are shown to be consistent with previous theoretical and experimental data. We obtain the elastic constants of ten phases and then estimate the bulk, shear and Young’s moduli, Poisson’s coefficient and Lamé’s constants using the Voigt-Reuss-Hill approximation. The energy band structures, density of states and charge populations of ten phases were obtained and indicated there is covalency in TiO2. Moreover, the complex dielectric function, refractive index and extinction coefficient of the ten phases were calculated; this data can aid future experimental research.
NASA Astrophysics Data System (ADS)
Jentschura, U. D.; Nándori, I.
2014-12-01
It has been a notably elusive task to find a remotely sensical ansatz for a calculation of Sommerfeld's electrodynamic fine-structure constant ?QED ? 1 / 137.036 based on first principles. However, this has not prevented a number of researchers to invest considerable effort into the problem, despite the formidable challenges, and a number of attempts have been recorded in the literature. Here, we review a possible approach based on the quantum electrodynamic (QED) ? function, and on algebraic identities relating ?QED to invariant properties of "internal" symmetry groups, as well as attempts to relate the strength of the electromagnetic interaction to the natural cutoff scale for other gauge theories. Conjectures based on both classical as well as quantum-field theoretical considerations are discussed. We point out apparent strengths and weaknesses of the most prominent attempts that were recorded in the literature. This includes possible connections to scaling properties of the Einstein-Maxwell Lagrangian which describes gravitational and electromagnetic interactions on curved space-times. Alternative approaches inspired by string theory are also discussed. A conceivable variation of the fine-structure constant with time would suggest a connection of ?QED to global structures of the Universe, which in turn are largely determined by gravitational interactions.
NASA Astrophysics Data System (ADS)
Jentschura, U. D.; Nándori, I.
2014-12-01
It has been a notably elusive task to find a remotely sensical ansatz for a calculation of Sommerfeld's electrodynamic fine-structure constant ? QED ? 1 / 137.036 based on first principles. However, this has not prevented a number of researchers to invest considerable effort into the problem, despite the formidable challenges, and a number of attempts have been recorded in the literature. Here, we review a possible approach based on the quantum electrodynamic (QED) ? function, and on algebraic identities relating ? QED to invariant properties of "internal" symmetry groups, as well as attempts to relate the strength of the electromagnetic interaction to the natural cutoff scale for other gauge theories. Conjectures based on both classical as well as quantum-field theoretical considerations are discussed. We point out apparent strengths and weaknesses of the most prominent attempts that were recorded in the literature. This includes possible connections to scaling properties of the Einstein-Maxwell Lagrangian which describes gravitational and electromagnetic interactions on curved space-times. Alternative approaches inspired by string theory are also discussed. A conceivable variation of the fine-structure constant with time would suggest a connection of ? QED to global structures of the Universe, which in turn are largely determined by gravitational interactions.
Magnetic properties of strained La2/3Sr1/3MnO3 perovskites from first principles
NASA Astrophysics Data System (ADS)
Böttcher, D.; Henk, J.
2013-04-01
The critical temperature TC of ferromagnetic LaxSr1-xMnO3 (LSMO) can be controlled by distorting the crystal structure, as was reported by Thiele et al (2007 Phys. Rev. B 75 054408). To confirm these findings theoretically, we investigate the electronic as well as the magnetic ground state properties of La2/3Sr1/3MnO3 as a function of tetragonal lattice distortions, using a multiple-scattering Green function method. Within this approach, we calculate exchange coupling constants as well as the phase transition temperature from first principles. Comparing our findings with those for La2/3Sr1/3CoO3 (LSCO), we find that the decrease of TC is much stronger in LSMO than in LSCO. Our findings can be explained by the electronic structures and are also in accordance with the experiment. The computed decrease of TC with distortion is smaller than observed experimentally, a result that corroborates the importance of phonon contributions.
Quantum Mechanics and First-Principles Molecular Dynamics Selection of Polymer Sensing Materials
NASA Astrophysics Data System (ADS)
Blanco, Mario; Shevade, Abhijit V.; Ryan, Margaret A.
We present two first-principles methods, density functional theory (DFT) and a molecular dynamics (MD) computer simulation protocol, as computational means for the selection of polymer sensing materials. The DFT methods can yield binding energies of polymer moieties to specific vapor bound compounds, quantities that were found useful in materials selection for sensing of organic and inorganic compounds for designing sensors for the electronic nose (ENose) that flew on the International Space Station (ISS) in 2008-2009. Similarly, we present an MD protocol that offers high consistency in the estimation of Hildebrand and Hansen solubility parameters (HSP) for vapor bound compounds and amorphous polymers. HSP are useful for fitting measured polymer sensor responses with physically rooted analytical models. We apply the method to the JPL electronic nose (ENose), an array of sensors with conducting leads connected through thin film polymers loaded with carbon black. Detection relies on a change in electric resistivity of the polymer film as function of the amount of swelling caused by the presence of the analyte chemical compound. The amount of swelling depends upon the chemical composition of the polymer and the analyte molecule. The pattern is unique and it unambiguously identifies the compound. Experimentally determined changes in relative resistivity of fifteen polymer sensor materials upon exposure to ten vapors were modeled with the first-principles HSP model.
Li, Xinru; Dai, Ying; Ma, Yandong; Huang, Baibiao
2014-07-14
p-Electron-based monolayer materials have dominated the research of Dirac fermions since the first exfoliation of graphene. In the present work, the electronic and magnetic properties of d-electron-based Dirac systems are studied by combining first-principles with mean field theory and Monte Carlo approaches. From first-principles calculations, we demonstrate that transition-metal (TM) monolayers (TM = Ti, Zr, Hf, V, Nb, or Ta), d-electron-based materials, could also hold Dirac cones and not only p-electron-based materials as known before. This may shed light on the breakthrough of new nanomaterials with d-type Dirac points. Moreover, the carrier mobility near the Dirac points of these materials can be tuned regularly by isotropic strains from -5% to 5%, without breaking the Dirac cones. However, the Dirac points would disappear under anisotropic strains, indicating that a rigorous honeycomb lattice may be the main precondition for Dirac points in TM-monolayers. Furthermore, some TM-monolayers (TM = Ti, Zr, or Hf) exhibit ferromagnetic couplings simultaneously. In addition, by mean field theory and Monte Carlo methods, it is found that Curie temperatures of TM-monolayers can be higher than 580 K even to 1180 K. Our findings significantly expand the Dirac systems. PMID:24879520
Structures and magnetic properties of Co-Zr-B magnets studied by first-principles calculations
NASA Astrophysics Data System (ADS)
Zhao, Xin; Ke, Liqin; Nguyen, Manh Cuong; Wang, Cai-Zhuang; Ho, Kai-Ming
2015-06-01
The structures and magnetic properties of Co-Zr-B alloys near the composition of Co5Zr with B at. % ?6% were studied using adaptive genetic algorithm and first-principles calculations. The energy and magnetic moment contour maps as a function of chemical composition were constructed for the Co-Zr-B magnet alloys through extensive structure searches and calculations. We found that Co-Zr-B system exhibits the same structure motif as the "Co11Zr2" polymorphs, and such motif plays a key role in achieving strong magnetic anisotropy. Boron atoms were found to be able to substitute cobalt atoms or occupy the "interruption" sites. First-principles calculations showed that the magnetocrystalline anisotropy energies of the boron-doped alloys are close to that of the high-temperature rhombohedral Co5Zr phase and larger than that of the low-temperature Co5.25Zr phase. Our calculations provide useful guidelines for further experimental optimization of the magnetic performances of these alloys.
Tunneling through Al/AlOx/Al junction: Analytical models and first-principles simulations
NASA Astrophysics Data System (ADS)
Zemanová Diešková, M.; Ferretti, A.; Bokes, P.
2013-05-01
Ultrathin AlOx layers are nowadays widely employed to make tunneling junctions and, as a common practice, experimental transport data are often rationalized in terms of analytical models invoking effective electronic and geometric properties of the oxide layer. In this paper we examine the reliability of such models by performing first-principles simulations of the transport properties of Al/AlOx/Al junctions. The band gap, effective mass, and interface width obtained from ground state density-functional calculations are used within a potential barrier model, known also as the Simmons model, and its predictions of the conductance are compared with first-principles results. We also propose an analytical expression for the conductance based on a tight-binding model of the interface oxide. We show that the success of the potential barrier model in fitting experimental transport measurements rests on its formal similarity with the tight binding model which, in contrast to the former, is directly related to the realistic electronic structure of the interface.
Phase stability analysis of the InAs/GaAs (001) wetting layer from first principles
NASA Astrophysics Data System (ADS)
Thomas, John C.; Millunchick, Joanna Mirecki; Van der Ven, Anton; Modine, Normand A.
2014-05-01
The large atomic-size mismatch between In and Ga and the large lattice-mismatch strain between InAs and GaAs make the InAs/GaAs (001) growth interface a complex alloy system, the understanding of which can enhance control of device synthesis and nanostructure self-assembly. We present a detailed first-principles analysis of the full progression of surface reconstructions observed on the InAs/GaAs(001) wetting layer during early stages of In deposition. We use systematic techniques to identify the most likely surface reconstruction prototypes of the InAs wetting layer on GaAs(001) using density functional theory (DFT) and then develop several cluster expansion Hamiltonians in order to thoroughly explore surface alloy disorder due to species substitution of In, Ga, and As at the surface. We use these results to construct a first principles 0-K surface phase diagram of the InAs wetting layer on GaAs(001) and test the sensitivity of our predicted phase diagram to DFT approximations and convergence errors. We find two alloy configurations of the (4×3) structural prototype that are likely ground-state surface reconstructions, and simulated scanning tunneling micrographs (STM) of these reconstructions indicate that they can explain prominent features of experimentally obtained STM of the InAs/GaAs (4×3) surface.
Surface of glassy GeS2: A model based on a first-principles approach
NASA Astrophysics Data System (ADS)
Ori, G.; Massobrio, C.; Bouzid, A.; Boero, M.; Coasne, B.
2014-07-01
First-principles calculations within the framework of the density functional theory are used to construct realistic models for the surface of glassy GeS2width="0.3em"/>(g-GeS2). Both calculations at T=0 K and at finite temperature (T=300 K) are considered. This allows for a comparison between the structural and electronic properties of surface and bulk g-GeS2. Although the g-GeS2 surface recovers the main tetrahedral structural motif of bulk g-GeS2, the number of fourfold coordinated Ge atoms and twofold coordinated S atoms is smaller than in the bulk. On the contrary, the surface system features a larger content of overcoordinated S atoms and threefold coordinated Ge atoms. This effect is more important for the g-GeS2 surface relaxed at 0 K. Maximally localized Wannier functions (WF) are used to inspect the nature of the chemical bonds of the structural units present at the g-GeS2 surface. We compare the ability of several charge derivation methods to capture the atomic charge variations induced by a coordination change. Our estimate for the charges allows exploiting the first-principles results as a data base to construct a reliable interatomic force field.
First-principles simulations of shock front propagation in liquid deuterium
NASA Astrophysics Data System (ADS)
Gygi, Francois; Galli, Giulia
2001-03-01
We present large-scale first-principles molecular dynamics simulations of the formation and propagation of a shock front in liquid deuterium. Molecular deuterium was subjected to supersonic impacts at velocities ranging from 10 to 30 km/s. We used Density Functional Theory in the local density approximation, and simulation cells containing 1320 deuterium atoms. The formation of a shock front was observed and its velocity was measured and compared with the results of laser-driven shock experiments [1]. The pressure and density in the compressed fluid were also computed directly from statistical averages in appropriate regions of the simulation cell, and compared with previous first-principles calculations performed at equilibrium [2]. Details of the electronic structure at the shock front, and their influence on the properties of the compressed fluid will be discussed. [1] J.W.Collins et al. Science 281, 1178 (1998). [2] G.Galli, R.Q.Hood, A.U.Hazi and F.Gygi, Phys.Rev. B61, 909 (2000).
First-Principles Investigation of Electronic Excitation Dynamics in Water under Proton Irradiation
NASA Astrophysics Data System (ADS)
Reeves, Kyle; Kanai, Yosuke
2015-03-01
A predictive and quantitative understanding of electronic excitation dynamics in water under proton irradiation is of great importance in many technological areas ranging from utilizing proton beam therapy to preventing nuclear reactor damages. Despite its importance, an atomistic description of the excitation mechanism has yet to be fully understood. Identifying how a high-energy proton dissipates its kinetic energy into the electronic excitation is crucial for predicting atomistic damages, later resulting in the formation of different chemical species. In this work, we use our new, large-scale first-principles Ehrenfest dynamics method based on real-time time-dependent density functional theory to simulate the electronic response of bulk water to a fast-moving proton. In particular, we will discuss the topological nature of the electronic excitation as a function of the proton velocity. We will employ maximally-localized functions to bridge our quantitative findings from first-principles simulations to a conceptual understanding in the field of water radiolysis.
Kotomin, E. A.; Mastrikov, Yuri A.; Rashkeev, Sergey N.; Van Uffelen, Paul
2009-09-01
Results are reported of first principles VASP supercell calculations of basic defect migration in UN nuclear fuels. The collinear interstitialcy mechanism of N migration is predicted to be energetically more favourable than direct [0 0 1] hops. It is also found that U and N vacancies have close migration energies, and O impurities accelerate migration of N vacancies nearby. These values are both in qualitative agreement with the effect of oxygen on the reduction of the activation energy for thermal creep reported in the literature, as well as in quantitative agreement with the experimental data when taking into account the uncertainties. The migration energies have been implemented in the thermal creep model of the TRANSURANUS fuel performance code. Therefore a concrete example is provided of how first principles computations can contribute directly to improve the design tools of advanced nuclear fuels, e.g. the predictions reveal a limited effect of oxygen on the thermo-mechanical performance of nitride fuels under fast breeder reactor (FBR) normal operating conditions.
First-principles theory of surface magnetocrystalline anisotropy and the diatomic-pair model
Wang, D. (Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208-3112 (United States) Institute of Physics, Academia Sinica, Beijing 100080 (China)); Wu, R. (Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208-3112 (United States)); Freeman, A.J. (Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208-3112 (United States) Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States))
1993-06-01
The state-tracking method proposed recently is employed for the first-principles determination of the magnetocrystalline-anisotropy (MCA) energy. A close relationship of the MCA energy to the band structure is found for transition-metal monolayers that show the change of sign of the MCA with respect to the band filling (atomic species) to be determined mainly by the spin-orbit coupling within the spin-down bands. The Fe monolayer with the [pi]-bonding band as the highest occupied band exhibits positive MCA (easy axis along the layer normal). However, the effect of strain on the MCA of the Fe monolayer demonstrates the effect of the spin-orbit coupling between opposite-spin states. A model for the electronic origin of the magnetic anisotropy of this two-dimensional system is presented that explains the first-principles MCA results for iron and cobalt monolayers on the basis of the bonding character between two [ital d] atoms, the band broadening due to increase in coordination, and geometry (symmetry).
NASA Astrophysics Data System (ADS)
Luo, Heng
Mixed ionic and electronic conductor oxides, in particular La1- xSrxCo yFe1-yO3-delta (LSCF), have been widely used as the cathode materials in solid oxide fuel cells for high-temperature energy applications. The focus of this thesis is primarily on constructing the instability phase diagram of Sr segregations on LSCF surfaces at the experimentally relevant temperatures and oxygen partial pressures using the first-principles density functional theory (DFT). A generic first-principles free-energy functional is developed to obtain the nonstoichiometric oxygen vacancy concentrations for the bulk and surface phases. These results agree well with the corresponding thermo-gravimetry measurements, and furthermore suggest that the oxygen vacancies are energetically stabilized at surfaces for all temperatures and oxygen partial pressures, while such surface stabilization effects become stronger at higher temperatures and lower oxygen partial pressures. Based on these nonstoichiometric oxygen vacancy predictions, we construct the free-energy phase diagrams of the Sr-segregation reaction as a function of temperature, oxygen partial pressure, and CO 2 partial pressure for both the bulk and surface LSCF phases. Our results suggest that Sr segregations strongly accumulate towards the LSCF surface phase where the oxygen vacancy nonstoichiometries are abundant. Our results also indicate that the Sr segregation reactions are significantly enhanced at high temperatures, low oxygen partial pressures, and high CO2 partial pressures. The computed reaction temperature ranges are consistent with the total reflection X-ray fluorescence (TXRF) measurements.
NASA Astrophysics Data System (ADS)
Pozhar, Liudmila A.
2010-05-01
An equilibrium two-time temperature Green's function (TTGF)-based, quantum statistical mechanical approach has been used to derive from the first principles an explicit expression for the tensor of "local" refraction indices of spatially nonuniform systems in weak external electromagnetic (EM) fields in the linear approximation with regard to the field magnitudes. Written in terms of the TTGF-based, first-principle tensorial dielectric and magnetic susceptibilities, the obtained formula for the local tensor of refraction indices (TRI) is applicable to any system, including individual nanoscale objects, such as quantum dots and wires, magnetic nanostructures, composite materials, or spatially nonuniform, bulk magnetic materials. An explicit expression for the space-time Fourier transform (STFT) of the dielectric susceptibility tensor used in TRI is derived in terms of STFTs of the charge density—charge density TTGFs, while the corresponding STFT of the magnetic susceptibility tensor also includes STFTs of the microcurrent—microcurrent TTGFs. The STFTs of the equilibrium TTGFs featuring in the susceptibilities, and thus necessary to calculate TRI, can be obtained by equilibrium quantum statistical mechanical means, modeling and simulations, or from experimental data. Two TRI regimes of significant interest for applications that can be realized in spatially inhomogeneous magnetic systems have been identified.
First-Principles Modeling of Hydrogen Storage in Metal Hydride Systems
J. Karl Johnson
2011-05-20
The objective of this project is to complement experimental efforts of MHoCE partners by using state-of-the-art theory and modeling to study the structure, thermodynamics, and kinetics of hydrogen storage materials. Specific goals include prediction of the heats of formation and other thermodynamic properties of alloys from first principles methods, identification of new alloys that can be tested experimentally, calculation of surface and energetic properties of nanoparticles, and calculation of kinetics involved with hydrogenation and dehydrogenation processes. Discovery of new metal hydrides with enhanced properties compared with existing materials is a critical need for the Metal Hydride Center of Excellence. New materials discovery can be aided by the use of first principles (ab initio) computational modeling in two ways: (1) The properties, including mechanisms, of existing materials can be better elucidated through a combined modeling/experimental approach. (2) The thermodynamic properties of novel materials that have not been made can, in many cases, be quickly screened with ab initio methods. We have used state-of-the-art computational techniques to explore millions of possible reaction conditions consisting of different element spaces, compositions, and temperatures. We have identified potentially promising single- and multi-step reactions that can be explored experimentally.
NASA Astrophysics Data System (ADS)
Lazic, Predrag; Sipahi, Guilherme; Kawakami, Roland; Zutic, Igor
2013-03-01
Recent experimental advances in graphene suggest intriguing opportunities for novel spintronic applications which could significantly exceed the state-of-the art performance of their conventional charge-based counterparts. However, for reliable operation of such spintronic devices it is important to achieve an efficient spin injection and large magnetoresistive effects. We use the first principles calculations to guide the choice of a ferromagnetic region and its relative orientation to optimize the desired effects. We propose structures which could enable uniform spin injection, one of the key factors in implementing scalable spintronic circuits. Recent experimental advances in graphene suggest intriguing opportunities for novel spintronic applications which could significantly exceed the state-of-the art performance of their conventional charge-based counterparts. However, for reliable operation of such spintronic devices it is important to achieve an efficient spin injection and large magnetoresistive effects. We use the first principles calculations to guide the choice of a ferromagnetic region and its relative orientation to optimize the desired effects. We propose structures which could enable uniform spin injection, one of the key factors in implementing scalable spintronic circuits. Supported by NSF-NRI, SRC, ONR, Croatian Ministry of Science, Education, and Sports, and CCR at SUNY UB.
Huang, Zuocai; Zhang, Lei; Pan, Wei, E-mail: panw@mail.tsinghua.edu.cn
2013-09-15
Pure zircon and scheelite LuVO{sub 4} were prepared by solid state reaction and high-pressure route, respectively. Structure, elastic constants, lattice dynamics and thermodynamics of LuVO{sub 4} polymorphs were studied by experiments and first principles calculation. Calculations here are in good agreement with the experimental results. The phonon dispersions of LuVO{sub 4} polymorphs were studied by the linear response method. The calculated phonon dispersions show that zircon and scheelite LuVO{sub 4} phases are dynamically stable. Raman-active frequencies were measured and assigned to different modes according to the calculations. The internal frequencies shift downward after phase transition from zircon to scheelite. Born effective charge tensors elements for both phases are analyzed. The finite temperature thermodynamic properties of LuVO{sub 4} polymorphs were calculated from the obtained phonon density of states by quasi-harmonic approach. - Graphical abstract: Lutetium orthovanadate polymorphs were synthesized by SSR and HP methods and their physical and chemical properties, including lattice dynamical properties, were determined by DFT calculations and experiments. Display Omitted - Highlights: • Pure zircon and scheelite LuVO{sub 4} polymorphs were synthesized by solid state reaction and high-pressure route. • Chemical and physical properties of LuVO4 polymorphs were studied by experiments and first principles calculation. • Raman-active frequencies were measured and assigned to different modes according to the calculations. • Lattice dynamics of polymorphs were discussed in details.
NASA Astrophysics Data System (ADS)
Lee, Ming-Hsien; Liu, Po-Liang; Hong, Yung-An; Chou, Yen-Ting; Hong, Jia-Yang; Siao, Yu-Jin
2013-02-01
We conduct first-principles total-energy density functional calculations to study the band structures in Ge1-xSnx infrared semiconductor alloys. The norm-conserving optimized pseudopotentials of Ge and Sn have been constructed for electronic structure calculations. The composition-bandgap relationships in Ge1-xSnx lattices are evaluated by a detailed comparison of structural models and their electronic band structures. The critical Sn composition related to the transition from indirect- to direct-gap in Ge1-xSnx alloys is estimated to be as low as x ˜ 0.016 determined from the parametric fit. Our results show that the crossover Sn concentration occurs at a lower critical Sn concentration than the values predicted from the absorption measurements. However, early results indicate that the reliability of the critical Sn concentration from such measurements is hard to establish, since the indirect gap absorption is much weaker than the direct gap absorption. We find that the direct band gap decreases exponentially with the Sn composition over the range 0
Ong, Shyue Ping
We present an analysis of the thermal reduction of delithiated LiMnPO[subscript 4] and LiFePO[subscript 4] based on the quarternary phase diagrams as calculated from first principles. Our results confirm the recent ...
Doe, Robert E.
First principles calculations have been used to explore the Li–Bi–F ternary phase diagram. Our results confirm the thermodynamic stability of previously observed phases and find no new phases in this system. Electrochemical ...
First-principles computation of mantle materials in crystalline and amorphous phases
NASA Astrophysics Data System (ADS)
Karki, Bijaya B.
2015-03-01
First-principles methods based on density functional theory are used extensively in the investigation of the behavior and properties of mantle materials over broad ranges of pressure, temperature, and composition that are relevant. A review of computational results reported during the last couple of decades shows that essentially all properties including structure, phase transition, equation of state, thermodynamics, elasticity, alloying, conductivity, defects, interfaces, diffusivity, viscosity, and melting have been calculated from first principles. Using MgO, the second most abundant oxide of Earth's mantle, as a primary example and considering many other mantle materials in their crystalline and amorphous phases, we have found that most properties are strongly pressure dependent, sometimes varying non-monotonically and anomalously, with the effects of temperature being systematically suppressed with compression. The overall agreement with the available experimental data is excellent; it is remarkable that the early-calculated results such as shear wave velocities of two key phases, MgO and MgSiO3 perovskite, were subsequently reproduced by experimentation covering almost the entire mantle pressure regime. As covered in some detail, the defect formation and migration enthalpies of key mantle materials increase with pressure. The predicted trend is that partial MgO Schottky defects are energetically most favorable in Mg-silicates but their formation enthalpies are high. So, the diffusion in the mantle is likely to be in the extrinsic regime. Preliminary results on MgO and forsterite hint that the grain boundaries can accommodate point defects (including impurities) and enhance diffusion rates at all pressures. The structures are highly distorted in the close vicinity of the defects and at the interface with excess space. Recent simulations of MgO-SiO2 binary and other silicate melts have found that the melt self-diffusion and viscosity vary by several orders of magnitudes with pressure, temperature, and composition. The predicted high compressibility and complex dynamical behavior can be associated with structural changes (involving non-bridging oxygen, oxygen tri-clusters, Si-O pentahedra, etc.) occurring on compression. We envision future prospect for massively parallel/distributed computing of unprecedented magnitude and scope in the study of relevant materials for Earth, super-Earth, and other planets. A renewed computational theme perhaps should be the first-principles simulations of large systems (with long runs) that are necessary to explore realistic (natural) compositions, polycrystalline phases, multi-component melts, crystal/melt interfaces, trace element partitioning, etc.
Nicolas Trcera; Delphine Cabaret; Stéphanie Rossano; François Farges; Anne-Marie Flank; Pierre Lagarde
2009-01-01
X-ray absorption spectroscopy at the Mg K-edge is used to obtain information on magnesium environment in minerals, silicate and alumino-silicate glasses. First-principles\\u000a XANES calculations are performed for minerals using a plane-wave density functional formalism with core-hole effects treated\\u000a in a supercell approach. The good agreement obtained between experimental and theoretical spectra provides useful information\\u000a to interpret the spectral features. With
Igor Solovyev
2011-01-01
“Realistic modeling” is a new direction of electronic structure calculations, where the main emphasis is made on the construction\\u000a of some effective low-energy model entirely within a first-principle framework. Ideally, it is a model in form, but with all\\u000a the parameters derived rigorously, on the basis of first-principles electronic structure calculations. The method is especially\\u000a suit for transition-metal oxides and
Lanphere, Marvin A.; Dalrymple, G. Brent
2000-01-01
We describe the analytical procedure for first-principles calibrations of 38Ar tracers in the U.S. Geological Survey (USGS) Laboratory in Menlo Park, California. The ages of fluence monitors determined in the USGS laboratory and those reported in the literature differ by nearly 2 percent. However, the radiogenic-40Ar content of the primary mineral standard on which these ages are based has never been verified by first-principles measurement in other laboratories.
Cohesion enhancing effect of magnesium in aluminum grain boundary: A first-principles determination
Zhang Shengjun; Freeman, Arthur J. [Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (United States); Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208 (United States); Kontsevoi, Oleg Y. [Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208 (United States); Olson, Gregory B. [Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (United States)
2012-06-04
The effect of magnesium on grain boundary cohesion in aluminum was investigated by means of first-principles calculations using the Rice-Wang model [Rice and Wang, Mater. Sci. Eng. A 107, 23 (1989)]. It is demonstrated that magnesium is a cohesion enhancer with a potency of -0.11 eV/atom. It is further determined through electronic structure and bonding character analysis that the cohesion enhancing property of magnesium is due to a charge transfer mechanism which is unusually strong and overcomes the negative result of the size effect mechanism. Consistent with experimental results, this work clarifies the controversy and establishes that Mg segregation does not contribute to stress corrosion cracking in Al alloys.
Fcc breathing instability in BaBiO3 from first principles
NASA Astrophysics Data System (ADS)
Thonhauser, T.; Rabe, K. M.
2006-06-01
We present first-principles density-functional calculations using the local density approximation to investigate the structural instability of cubic perovskite BaBiO3 . This material might exhibit charge disproportionation and some evidence thereof has been linked to the appearance of an additional fourth peak in the experimental IR spectrum. However, our results suggest that the origin of this additional peak can be understood within the picture of a simple structural instability. While the true instability consists of an oxygen-octahedra breathing distortion and a small octahedra rotation, we find that the breathing alone in a fcc-type cell doubling is sufficient to explain the fourth peak in the IR spectrum. Our results show that the oscillator strength of this particular mode is of the same order of magnitude as the other three modes, in agreement with experiment.
First principle study of transport properties of a graphene nano structure
NASA Astrophysics Data System (ADS)
Kumar, Naveen; Sharma, Munish; Sharma, Jyoti Dhar; Ahluwalia, P. K.
2013-06-01
The first principle quantum transport calculations have been performed for graphene using Tran SIESTA which calculates transport properties using nonequilibrium Green's function method in conjunction with density-functional theory. Transmission functions, electron density of states and current-voltage characteristic have been calculated for a graphene nano structure using graphene electrodes. Transmission function, density of states and projected density of states show a discrete band structure which varies with applied voltage. The value of current is very low for applied voltage between 0.0 V to 5.0 V and lies in the range of pico ampere. In the V-I characteristic current shows non-linear fluctuating pattern with increase in voltage.
First-principles calculations of the growth of InSb on GaSb(110)
NASA Astrophysics Data System (ADS)
Cocoletzi, Gregorio H.; Takeuchi, Noboru
2000-06-01
Using first-principles total-energy calculations, we have investigated the crystal growth of InSb on the (110) surface of GaSb. Upon the adsorption of one monolayer of InSb, substrate atoms move closer to their ideal positions. In particular, first layer Ga and Sb atoms shift to sites similar to the atomic positions of a second layer in the clean GaSb(110) surface. This result indicates that the GaSb substrate sees no difference whether the top layer is InSb or GaSb. At this coverage, the surface (including the InSb monolayer) is very ordered. Increasing the amount of InSb to two layers results in a disordered surface. The GaSb substrate, with a shorter lattice constant is no longer able to accommodate the InSb system in an ordered way.
First-principles study of multiferroic RbFe(MoO4)2
NASA Astrophysics Data System (ADS)
Cao, Kun; Johnson, R. D.; Giustino, Feliciano; Radaelli, Paolo G.; Guo, G.-C.; He, Lixin
2014-07-01
We have investigated the magnetic structure and ferroelectricity in RbFe(MoO4)2 via first-principles calculations. Phenomenological analyses have shown that ferroelectricity may arise due to both the triangular chirality of the magnetic structure and through coupling between the magnetic helicity and the ferroaxial structural distortion. Indeed, it was recently proposed that the structural distortion plays a key role in stabilizing the chiral magnetic structure itself. We have determined the relative contribution of the two mechanisms via ab initio calculations. Whilst the structural axiality does induce the magnetic helix by modulating the symmetric exchange interactions, the electric polarization is largely due to the in-plane spin triangular chirality, with both electronic and ionic contributions being of relativistic origin. At the microscopic level, we interpret the polarization as a secondary steric consequence of the inverse Dzyaloshinskii-Moriya mechanism and accordingly explain why the ferroaxial component of the electric polarization must be small.
First-principles study of structural and vibrational properties of SrZrO3
NASA Astrophysics Data System (ADS)
Amisi, Safari; Bousquet, Eric; Katcho, Karume; Ghosez, Philippe
2012-02-01
Using first-principles calculations, we investigate the electronic, structural, and vibrational properties of SrZrO3. We start from the high-symmetry cubic perovskite phase, for which the phonon dispersion curves are reported. We point out the coexistence of structural antiferrodistortive instabilities at the R and M zone-boundary points and a ferroelectric instability at the zone center. We show that the strong antiferrodistortive motions suppress ferroelectricity and are responsible for the orthorhombic ground state as in CaTiO3. The structural properties of possible intermediate phases and of the orthorhombic Pnma ground state are reported. For the latter, an assignment of IR and Raman zone-center phonon modes is proposed. The main features of the ferroelectric instability are also discussed, and we show that a ferroelectric ground state can even be induced in SrZrO3 by strain engineering.
First-principles calculations of phonons and Raman spectra in monoclinic CsSnCl 3
NASA Astrophysics Data System (ADS)
Huang, Ling-yi; Lambrecht, Walter R. L.
2015-02-01
Density functional perturbation theory is used to calculate the phonons at the zone center for monoclinic CsSnCl3. We report the calculated normal mode frequencies classified according to irreducible representations for both infrared-active and Raman-active modes. We also report the dielectric constants and Born-effective charges and investigate the anisotropy of the nonanalyticities near the zone center. The first-principles calculated Raman-active phonon frequencies and simulated Raman spectra are compared with experimental results from literature. We find that aside from the first-order allowed Raman modes of Ag and Bg symmetries, forbidden LO Raman modes appear prominently in the measured spectra. Several of these modes are close to or coincide with allowed modes and alter their intensity, while others appear as separate otherwise unexplained peaks. With this interpretation, good agreement is obtained with experimental mode frequencies to within a few cm-1.
On Developing a European First Principles Geomagnetically Induced Current Forecasting System
NASA Astrophysics Data System (ADS)
Honkonen, I. J.; Viljanen, A.; Vanhamäki, H.
2012-12-01
Geomagnetically induced currents (GIC) during space storms pose a risk to power transmission grids across the globe. As part of the European Risk from Geomagnetically Induced Currents (EURISGIC) EU/FP7 project the Finnish Meteorological Institute is developing a first principles based GIC forecasting and warning system. The system is given as input the solar wind plasma parameters measured by the ACE spacecraft and the final output consists of the ground electric field and GIC in a simplified model of European high-voltage power grids. We describe the different steps involved in obtaining the final GIC solution and implementation of the required software components. We also present a comparison between simulated electrojet indicators and those derived from the magnetic field measurements of the IMAGE magnetometer network. Additionally the simulated GIC are compared to natural gas pipeline observations in Mäntsälä, Finland.
Yu, Shuyin; Zeng, Qingfeng; Oganov, Artem R; Frapper, Gilles; Zhang, Litong
2015-04-21
We have performed first-principles evolutionary searches for stable Ti-N compounds and have found, in addition to the well-known rock-salt TiN, new ground states Ti3N2, Ti4N3, Ti6N5 at atmospheric pressure, and Ti2N and TiN2 at higher pressures. The latter nitrogen-rich structure contains encapsulated N2 dumbbells with a N-N distance of 1.348 Å at 60 GPa. TiN2 is predicted to be mechanically stable and quenchable. Our calculations on the mechanical properties (bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and hardness) are in excellent agreement with the available experimental data. Further analyses of the electronic density of states, crystal orbital Hamilton population and the electron localization function reveal that the hardness is enhanced by strengthening directional covalent bonds and disappearance of Ti-Ti metallic bonding. PMID:25869225
The Interface between Gd and Monolayer MoS2: A First-Principles Study
Zhang, Xuejing; Mi, Wenbo; Wang, Xiaocha; Cheng, Yingchun; Schwingenschlögl, Udo
2014-01-01
We analyze the electronic structure of interfaces between two-, four- and six-layer Gd(0001) and monolayer MoS2 by first-principles calculations. Strong chemical bonds shift the Fermi energy of MoS2 upwards into the conduction band. At the surface and interface the Gd f states shift to lower energy and new surface/interface Gd d states appear at the Fermi energy, which are strongly hybridized with the Mo 4d states and thus lead to a high spin-polarization (ferromagnetically ordered Mo magnetic moments of 0.15??B). Gd therefore is an interesting candidate for spin injection into monolayer MoS2. PMID:25482498
First-Principles Calculation of Phonon and Schottky Heat Capacities of Plutonium Dioxide
NASA Astrophysics Data System (ADS)
Nakamura, Hiroki; Machida, Masahiko; Kato, Masato
2015-05-01
Plutonium dioxide (PuO2) is a key ingredient of mixed oxide (MOX) and advanced nuclear fuels. Its thermophysical data is crucial in understanding the high-temperature behaviors of nuclear fuels. In particular, the high-temperature heat capacity is of great importance for their safety and performance analyses. Here, we evaluate the main contributions to the heat capacity of PuO2 from 0 to 1400 K through suitable first-principles calculations. Consequently, we successfully obtain a temperature dependence in good agreement with experimental measurements. This success mainly results from accurate calculations of the Schottky heat capacity caused by the excited levels of f-electrons of Pu. Our calculations resolve the mystery of why previous works failed to reproduce the measurement data. This study extends the possibility of performing simulation-based nuclear-fuel research instead of difficult measurements.
First-Principles Simulation of Hot Electron Dynamics at Silicon-Molecule Interfaces
NASA Astrophysics Data System (ADS)
Li, Lesheng; Kanai, Yosuke; Kanai Group Team
2015-03-01
Hot carrier relaxation process at an interface between semiconductor and molecular ligands is of great importance for a number of technological applications ranging from photo-electrochemical cells to quantum-dot light emitting diodes. Although a number of spectroscopic experiments suggest important role of molecular ligands at surface in the hot carrier relaxation, a quantitative understanding has not been developed. We investigate the hot electron relaxation process through synergetic use of first-principles molecular dynamics (FPMD), fewest switch surface hopping (FSSH) algorithm, and GW calculations. Using FSSH stochastic dynamics simulation based on non-adiabatic couplings from FPMD and quasi-particle energy level alignment at the interface, we investigate the role of molecular passivation at silicon (111) surface as a representative example. We will discuss how different types of molecules influence the relaxation process and elucidate important factors controlling the relaxation time scale.
NASA Astrophysics Data System (ADS)
Revard, Benjamin; Tipton, Will; Hennig, Richard
2015-03-01
Single-layer materials represent a new materials class with potentially transformative properties for applications in nanoelectronics and solar-energy harvesting. With the goal to discover novel 2D materials with unexpected compositions and structures, we have developed a grand-canonical evolutionary algorithm for two-dimensional materials. Here we present the results of applying the algorithm, coupled with first principles total energy methods, to several technologically relevant binary 2D systems, including C-Si, Sn-S and PbO. We currently use computational techniques to characterize the vibrational and electronic properties of the low energy 2D materials predicted by the algorithm and will report the findings. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1144153.
Mundy, C; Kuo, I W
2005-06-08
The field of atmospheric science is very rich in problems ranging from the molecular to the regional and global scale. These problems are often extremely complex, and although the statement of a particular atmospheric science question may be clear, finding a single, concise computational approach to address this question can be daunting. As a result, the broad scope of scientific problems that lie within the umbrella of atmospheric science require a multi-discipline approach. Of particular interest to atmospheric chemists is the role that heterogeneous chemistry plays in the important processes that take place throughout the atmosphere. The definition of heterogeneous is: consisting of dissimilar elements or parts. The chemical environment induced by the presence of the interface can be dramatically different than the corresponding gas- or condensed phase homogeneous environment and can give rise to novel chemistry. Although the importance of heterogeneous chemistry in the atmosphere has been known for decades, a challenge to both experimentalists and theorists in provide simplified models and experiments that can yield insight into the field measurements of the atmospheric process. The use of molecular modeling has been widely used to provide a particle-based picture of atmospherically relevant interfaces to deduce the novel chemistry that is taking place. Unfortunately, even with the most computationally efficient particle-based approach, it is still impossible to model the full ice-crystal in the stratosphere or the sea-salt aerosol in the troposphere. Figure 1 depicts a caricature of the actual system of interest, and highlights the region where efficient molecular modeling can be employed. Although there is seemingly a large disconnect between reality and the model, we hope to convince the reader that there is still much insight to be gained from a particle-based picture. There is a myriad of different approaches to molecular modeling that have been successfully applied to studying the complex problems put forth by atmospheric chemists. To date, the majority of the molecular models of atmospherically relevant interfaces have been comprised of two genres of molecular models. The first is based on empirical interaction potentials. The use of an empirical interaction potential suffers from at least two shortcomings. First, empirical potentials are usually fit to reproduce bulk thermodynamic states, or gas phase spectroscopic data. Thus, without the explicit inclusion of charge transfer, it is not at all obvious that empirical potentials can faithfully reproduce the structure at a solid-vapor, or liquid-vapor interface where charge rearrangement is known to occur (see section 5). One solution is the empirical inclusion of polarization effects. These models are certainly an improvement, but still cannot offer insight into charge transfer processes and are usually difficult to parameterize. The other shortcoming of empirical models is that, in general, they cannot describe bond-making/breaking events, i.e. chemistry. In order to address chemistry one has to consider an ab initio (to be referred to as first-principles throughout the remaining text) approach to molecular modeling that explicitly treats the electronic degrees of freedom. First-principles modeling also give a direct link to spectroscopic data and chemistry, but at a large computational cost. The bottle-neck associated with first-principles modeling is usually determined by the level of electronic structure theory that one chooses to study a particular problem. High-level first-principles approaches, such as MP2, provide accurate representation of the electronic degrees of freedom but are only computationally tractable when applied to small system sizes (i.e. 10s of atoms). Nevertheless, this type of modeling has been extremely useful in deducing reaction mechanisms of atmospherically relevant chemistry that will be discussed in this review (see section 4). However, to solve problems relating to heterogeneous chemistry at interfaces where the interfacial syste
First-principles study of hydrogen storage on Ti-decorated B2C sheet
NASA Astrophysics Data System (ADS)
Guo, Y. H.; Xu, B.; Xia, Y. D.; Yin, J.; Liu, Z. G.
2012-06-01
The hydrogen storage capacity of Ti decorated B2C sheet has been investigated by first-principles plane-wave calculation. It is revealed that a single Ti atom adsorbed on the B2C sheet can strongly bind up to four hydrogen molecules. The adsorption energy is in the range of -0.36--0.82 eV/H2, which is suitable for ambient temperature hydrogen storage. Considering the fact that Ti can be loaded on both sides of B2C sheet, corresponding gravimetric storage capacity of Ti/B2C system was also calculated and it can reach to about 7.0 wt%, exceeding the minimum requirement of 6.0 wt% for applications.
First-principles approach to calculating energy level alignment at aqueous semiconductor interfaces
Kharche, Neerav [Brookhaven National Lab. (BNL), Upton, NY (United States); Muckerman, James T. [Brookhaven National Lab. (BNL), Upton, NY (United States); Hybertsen, Mark S. [Brookhaven National Lab. (BNL), Upton, NY (United States)
2014-10-01
A first-principles approach is demonstrated for calculating the relationship between an aqueous semiconductor interface structure and energy level alignment. The physical interface structure is sampled using density functional theory based molecular dynamics, yielding the interface electrostatic dipole. The GW approach from many-body perturbation theory is used to place the electronic band edge energies of the semiconductor relative to the occupied 1b? energy level in water. The application to the specific cases of nonpolar (1010¯ ) facets of GaN and ZnO reveals a significant role for the structural motifs at the interface, including the degree of interface water dissociation and the dynamical fluctuations in the interface Zn-O and O-H bond orientations. These effects contribute up to 0.5 eV.
NASA Astrophysics Data System (ADS)
Das, T. P.; Pink, R. H.; Badu, S. R.; Dubey, Archana; Scheicher, R. H.; Saha, H. P.; Chow, Lee; Huang, M. B.
2009-03-01
Nuclear Quadrupole Interactions (NQI) of ^17O, ^14N and ^2H nuclei have been studied for free nucleobases and nucleobases in single strand and double strand DNA and in solid state. Our first-principles investigations were carried out using the Gaussian 2003 set of programs to implement the Hartree-Fock procedure combined with many-body effects included using many-body perturbation theory. As expected for NQI in general, many-body effects are found to be small. Results will be presented for the quadrupole coupling constants (e^2qQ) and asymmetry parameters (?) for the nucleobases in the various environments. Trends in e^2qQ and ? in the different environments will be discussed. In the case of the solid nucleobases, comparisons will be made with available experimental data [1] for ^17O nuclei.[3pt] [1] Gang Wu et al., J. Am. Chem. Soc. 124, 1768 (2002)
First-principle calculations of high-pressure phase transformations in RuC
NASA Astrophysics Data System (ADS)
Hao, Jian; Tang, Xiao; Li, Wenjing; Li, Yinwei
2014-02-01
The structural stabilities of RuC under high pressure have been studied by using first-principle calculations. Results show that RuC transforms from the ZB-type (zinc-blende-type) structure to a tetrahedral structure with space group I4mm at \\sim9.3\\ \\text{GPa} . The I4mm structure constructed by RuC5 pyramids is stable up to \\sim26\\ \\text{GPa} , above which the WC-type structure becomes energetically more favorable. An increase of the coordination number from 4 to 5 and then to 6 is observed accompanied by the phase sequence of ZB type \\to \\textit{I}4\\textit{mm} \\to WC type. Band structure calculations reveal that the ZB-type phase is a semiconductor while the I4mm and WC-type phases are metallic. Moreover, the mechanical properties of RuC in all three phases are discussed.
First-principle study of structural, elastic and electronic properties of Th monopnictides
NASA Astrophysics Data System (ADS)
Amari, S.; Méçabih, S.; Abbar, B.; Bouhafs, B.
2014-11-01
In this work, first principles calculation of structural, electronic and elastic properties of thorium monopnictides ThX (N, P, As, Sb and Bi) are presented. The calculations are performed by a developed full-potential augmented plane wave plus local orbitals (FP-L/APW+lo) method within the density functional theory (DFT). The exchange and correlation potential energies are treated according to the generalized gradient approximation (GGA) using the Perdew, Burke, Ernzerhof (PBE) parameterization, and the local density approximation (LDA). We have calculated the lattice parameters, bulk modulii and the first pressure derivatives of the bulk modulii. The elastic properties of the studied compounds are only investigated in the most stable calculated phase. We have obtained Young's modulus, shear modulus, Poisson's ratio, anisotropy factor and Kleinman parameter by the aid of the calculated elastic constants. We discuss the total and partial densities of states and charge densities.
Atomic structure and electronic properties of folded graphene nanoribbons: A first-principles study
NASA Astrophysics Data System (ADS)
Yin, Wen-Jin; Xie, Yue-E.; Liu, Li-Min; Chen, Yuan-Ping; Wang, Ru-Zhi; Wei, Xiao-Lin; Zhong, Jian-Xin; Lau, Leo
2013-05-01
Folded graphene nanoribbons (FGNRs) have attracted great attentions because of extraordinary properties and potential applications. The atomic structure, stacking sequences, and electronic structure of FGNRs are investigated by first-principle calculations. It reveals that the common configurations of all FGNRs are racket-like structures including a nanotube-like edge and two flat nanoribbons. Interestingly, the two flat nanoribbons form new stacking styles instead of the most stable AB-stacking sequences for flat zone. The final configurations of FGNRs are greatly affected by the initial interlayer distance, stacking sequences, and edge styles. The stability of folded graphene nanoribbon depends on the length, and it can only be thermodynamically stable when it reaches the critical length. The band gap of the folded zigzag graphene nanoribbons becomes about 0.17 eV, which provides a new way to open the band gap.
Unzipping carbon nanotubes into nanoribbons upon oxidation: a first-principles study.
Li, Feng; Kan, Erjun; Lu, Ruifeng; Xiao, Chuanyun; Deng, Kaiming; Su, Haibin
2012-02-21
First-principles calculations are performed to investigate the unzipping mechanism of carbon nanotubes (CNTs) into narrow graphene nanoribbons (GNRs) upon oxidation. By treating possible adsorptive structures, we found that, upon further oxidation, epoxy pairs tear the CNT up with an initial energy barrier of 0.59 eV (armchair) and 0.60 eV (zigzag), and the following steps of unzipping CNT become much easier because of the stress induced by the carbonyl pair. However, for zigzag CNTS, the unzipped edge structures of nanoribbons cannot be controlled, because of the similar stability of different oxidation process, which means zigzag CNTs should be avoided in producing high-quality GNRs. PMID:22252198
Wang, Xiao; Yang, Juan; Li, Ruoming; Jiang, Hong; Li, Yan
2015-04-15
The interaction between single-walled carbon nanotubes (SWNTs) and graphene were studied with first-principles calculations. Both SWNTs and single-layer graphene (SLG) or double-layer graphene (DLG) display more remarkable deformations with the increase of SWNT diameter, which implies a stronger interaction between SWNTs and graphene. Besides, in DLG, deformation of the upper-layer graphene is less than in SLG. Zigzag SWNTs show stronger interactions with SLG than armchair SWNTs, whereas the order is reversed for DLG, which can be interpreted by the mechanical properties of SWNTs and graphene. Density of states and band structures were also studied, and it was found that the interaction between a SWNT and graphene is not strong enough to bring about obvious influence on the electronic structures of SWNTs. PMID:25689637
First-principles study of Dirac and Dirac-like cones in phononic and photonic crystals
NASA Astrophysics Data System (ADS)
Mei, Jun; Wu, Ying; Chan, C. T.; Zhang, Zhao-Qing
2012-07-01
By using the k?·p? method, we propose a first-principles theory to study the linear dispersions in phononic and photonic crystals. The theory reveals that only those linear dispersions created by doubly degenerate states can be described by a reduced Hamiltonian that can be mapped into the Dirac Hamiltonian and possess a Berry phase of -?. Linear dispersions created by triply degenerate states cannot be mapped into the Dirac Hamiltonian and carry no Berry phase, and, therefore should be called Dirac-like cones. Our theory is capable of predicting accurately the linear slopes of Dirac and Dirac-like cones at various symmetry points in a Brillouin zone, independent of frequency and lattice structure.
Elucidation of Pathways for NO Electroreduction on Pt(111) from First Principles.
Clayborne, Andre; Chun, Hee-Joon; Rankin, Rees B; Greeley, Jeff
2015-07-01
The mechanism of nitric oxide electroreduction on Pt(111) is investigated using a combination of first principles calculations and electrokinetic rate theories. Barriers for chemical cleavage of N?O bonds on Pt(111) are found to be inaccessibly high at room temperature, implying that explicit electrochemical steps, along with the aqueous environment, play important roles in the experimentally observed formation of ammonia. Use of explicit water models, and associated determination of potential-dependent barriers based on Bulter-Volmer kinetics, demonstrate that ammonia is produced through a series of water-assisted protonation and bond dissociation steps at modest voltages (<0.3?V). In addition, the analysis sheds light on the poorly understood formation mechanism of nitrous oxide (N2 O) at higher potentials, which suggests that N2 O is not produced through a Langmuir-Hinshelwood mechanism; rather, its formation is facilitated through an Eley-Rideal-type process. PMID:26053610
A unified electrostatic and cavitation model for first-principles molecular dynamics in solution
Damian A. Scherlis; Jean-Luc Fattebert; Francois Gygi; Matteo Cococcioni; Nicola Marzari
2005-10-06
The electrostatic continuum solvent model developed by Fattebert and Gygi is combined with a first-principles formulation of the cavitation energy based on a natural quantum-mechanical definition for the surface of a solute. Despite its simplicity, the cavitation contribution calculated by this approach is found to be in remarkable agreement with that obtained by more complex algorithms relying on a large set of parameters. Our model allows for very efficient Car-Parrinello simulations of finite or extended systems in solution, and demonstrates a level of accuracy as good as that of established quantum-chemistry continuum solvent methods. We apply this approach to the study of tetracyanoethylene dimers in dichloromethane, providing valuable structural and dynamical insights on the dimerization phenomenon.
First principles study of the aggregation of oligo and polythiophene cations in solution
Scherlis, D A; Fattebert, J; Marzari, N
2005-11-14
The stacking of positively charged (or doped) terthiophene oligomers and quaterthiophene polymers in solution is investigated applying a recently developed unified electrostatic and cavitation model for first-principles calculations in a continuum solvent. The thermodynamic and structural patterns of the dimerization are explored in different solvents, and the distinctive roles of polarity and surface tension are characterized and analyzed. Interestingly, we discover a saturation in the stabilization effect of the dielectric screening that takes place at rather small values of {epsilon}{sub 0}. Moreover, we address the interactions in trimers of terthiophene cations, with the aim of generalizing the results obtained for the dimers to the case of higher order stacks and nanoaggregates.
A unified electrostatic and cavitation model for first-principles molecular dynamics in solution
Scherlis, D A; Fattebert, J; Gygi, F; Cococcioni, M; Marzari, N
2005-11-14
The electrostatic continuum solvent model developed by Fattebert and Gygi is combined with a first-principles formulation of the cavitation energy based on a natural quantum-mechanical definition for the surface of a solute. Despite its simplicity, the cavitation contribution calculated by this approach is found to be in remarkable agreement with that obtained by more complex algorithms relying on a large set of parameters. The model allows for very efficient Car-Parrinello simulations of finite or extended systems in solution, and demonstrates a level of accuracy as good as that of established quantum-chemistry continuum solvent methods. They apply this approach to the study of tetracyanoethylene dimers in dichloromethane, providing valuable structural and dynamical insights on the dimerization phenomenon.
First principles study of the optical contrast in phase change materials.
Caravati, S; Bernasconi, M; Parrinello, M
2010-08-11
We study from first principles the optical properties of the phase change materials Ge(2)Sb(2)Te(5) (GST), GeTe and Sb(2)Te(3) in the crystalline phase and in realistic models of the amorphous phase generated by quenching from the melt in ab initio molecular dynamics simulations. The calculations reproduce the strong optical contrast between the crystalline and amorphous phases measured experimentally and exploited in optical data storage. It is demonstrated that the optical contrast is due to a change in the optical matrix elements across the phase change in all the compounds. It is concluded that the reduction of the optical matrix elements in the amorphous phases is due to angular disorder in p-bonding which dominates the amorphous network in agreement with previous proposals (Huang and Robertson 2010 Phys. Rev. B 81 081204) based on calculations on crystalline models. PMID:21399368
FIRST PRINCIPLES MODELING OF YTTRIUM-DOPED BAZRO3 SOLID ELECTROLYTE
Claudio O. Dorso; Boris V. Merinov; William A. Goddard III
2003-04-30
Ab initio Quantum mechanics calculations of the equation of states for BaZrO{sub 3} have been performed and the bulk modulus has been obtained. The value of the modulus is in good agreement with reported experimental values. Equilibrium proton positions in Y-doped BaZrO{sub 3} with dopant concentrations from 12.5 to 50% were investigated. Initial rough estimates of the transition barriers have been made. Our results suggest that the proton migration pathway may involve secondary minima with two maxima (symmetric with respect to the center of the path). In the next phase of this project the results of our quantum mechanical calculations will be used to develop a new Reactive Force Field (ReaxFF) based on first principles. This Reactive Force Field will be used for much molecular dynamics simulations or much larger systems to investigate proton migration in bulk and surface regions of fuel cells.
Cold Baryogenesis from first principles in the Two-Higgs Doublet model with Fermions
Mou, Zong-Gang; Tranberg, Anders
2015-01-01
We present a first-principles numerical computation of the baryon asymmetry in electroweak-scale baryogenesis. For the scenario of Cold Baryogenesis, we consider a one fermion-family reduced CP-violating two Higgs-doublet model, including a classical SU(2)-gauge/two-Higgs sector coupled to one quantum left-handed fermion doublet and two right-handed singlets. Separately, the C(CP) breaking of the two-Higgs potential and the C and P breaking of the gauge-fermion interactions do not provide a baryon asymmetry. Only when combined does baryogenesis occur. Through large-scale computer simulations, we compute the asymmetry for one particularly favourable scalar potential. The numerical signal is at the boundary of what is numerically discernible with the available computer resources, but we tentatively find an asymmetry of $|\\eta|\\leq 3.5\\times 10^{-7}$.
NASA Astrophysics Data System (ADS)
Lindsay, Lucas; Broido, David; Reinecke, Tom; Lindsay Collaboration
2014-03-01
We have calculated the thermal conductivities (k) of cubic III-V boron compounds using a predictive first principles approach. Boron Arsenide (BAs) is found to have a remarkable room temperature k over 2000Wm-1K-1; this is comparable to those in diamond and graphite, which are the highest bulk values known. We trace this behavior in BAs to an interplay of certain basic vibrational properties that lie outside of the conventional guidelines in searching for high k materials. We also find that cubic BN and BSb will have high k with isotopic purification. This work provides new insight into the nature of thermal transport at a quantitative level and predicts a new ultra-high k material of potential interest for passive cooling applications. This work was supported by ONR, DARPA, DOE-BES and NSF.
Wang, Zhiguo; Zhang, Chunlai; Li, Jingbo; Gao, Fei; Weber, William J.
2010-12-01
The electronic properties of hydrogen-saturated GaN nanowires with different orientations and sizes are investigated using first-principles calculations, and three types of nanowires oriented along the [001], [110] and [1-10] crystal directions are considered. The electronic properties show little dependence on orientation. The hydrogen-saturated GaN nanowires show semiconducting behavior with a direct band gap larger than that of bulk wurtzite GaN. Quantum confinement leads to a decrease in the band gap of the nanowires with increasing nanowire size. The [001]-oriented nanowires with hexagonal cross sections are energetically more favorable than the [100]- and [1-10]-oriented nanowires with triangular cross-sections.
First-Principles Studies of Hydrogen Adsorption at Pd-SiO2 Interfaces
Irokawa, Yoshihiro; Usami, Mamoru
2015-01-01
The interaction of hydrogen with Pd-SiO2 interfaces has been investigated for the first time using first-principles calculations based on density functional theory. The hydrogen-induced polarization at the Pd-SiO2 interfaces was evaluated using Pd-SiO2 interface supercells. As a result, the potential change induced by interfacial hydrogen atoms was not observed even for hydrogen concentration of ~1.3 × 1015 cm?2 at the Pd-SiO2 interface. This result implies that hydrogen does not create an electric double layer at the Pd-SiO2 interface but change the property of the SiO2 region, resulting in the hydrogen sensitivity of the devices. PMID:26110410
First-principles approach to calculating energy level alignment at aqueous semiconductor interfaces
Kharche, Neerav; Muckerman, James T.; Hybertsen, Mark S.
2014-10-01
A first-principles approach is demonstrated for calculating the relationship between an aqueous semiconductor interface structure and energy level alignment. The physical interface structure is sampled using density functional theory based molecular dynamics, yielding the interface electrostatic dipole. The GW approach from many-body perturbation theory is used to place the electronic band edge energies of the semiconductor relative to the occupied 1b? energy level in water. The application to the specific cases of nonpolar (1010¯ ) facets of GaN and ZnO reveals a significant role for the structural motifs at the interface, including the degree of interface water dissociation and themore »dynamical fluctuations in the interface Zn-O and O-H bond orientations. These effects contribute up to 0.5 eV.« less
Impact of rattlers on thermal conductivity of a thermoelectric clathrate: a first-principles study.
Tadano, Terumasa; Gohda, Yoshihiro; Tsuneyuki, Shinji
2015-03-01
We investigate the role of rattling guest atoms on the lattice thermal conductivity of a type-I clathrate Ba_{8}Ga_{16}Ge_{30} by first-principles lattice dynamics. Comparing phonon properties of filled and empty clathrates, we show that rattlers cause tenfold reductions in the relaxation time of phonons by increasing the phonon-phonon scattering probability. Contrary to the resonant scattering scenario, the reduction in the relaxation time occurs in a wide frequency range, which is crucial for explaining the unusually low thermal conductivities of clathrates. We also find that the impact of rattlers on the group velocity of phonons is secondary because the flattening of phonon dispersion occurs only in a limited phase space in the Brillouin zone. PMID:25793824
First-principles study of defect properties of zinc blende MgTe
NASA Astrophysics Data System (ADS)
Yang, Ji-Hui; Chen, Shiyou; Xiang, Hongjun; Gong, X. G.; Wei, Su-Huai
2011-06-01
We studied the general chemical trends of defect formation in MgTe using first-principles band structure methods. The formation energies and transition energy levels of intrinsic defects and extrinsic impurities and some defect complexes in zinc blende MgTe were calculated systematically using a new hybrid scheme. The limiting factors for p- and n-type doping in MgTe were investigated. Possible solutions to overcome the doping limitation of MgTe are proposed. The best p-type dopant is suggested to be N with nonequilibrium growth process and the best n-type dopant is suggested to be I with its doping complex VMg+ 4ITe.
First-principles study of defect properties of zinc blende MgTe
NASA Astrophysics Data System (ADS)
Yang, Ji-Hui; Chen, Shiyou; Xiang, Hongjun; Gong, Xingao; Wei, Su-Huai
2012-02-01
We studied the general chemical trends of defect formation in MgTe using first-principles band structure methods. The formation energies and transition energy levels of intrinsic defects and extrinsic impurities and some defect complexes in zinc blende MgTe were calculated systematically using a new hybrid scheme. The limiting factors for p- and n-type doping in MgTe were investigated. Possible solutions to overcome the doping limitation of MgTe are proposed. The best p-type dopant is suggested to be N with nonequilibrium growth process and the best n-type dopant is suggested to be I with its doping complex VMg + 4ITe.
Low-index surfaces of CoSb3 skutterudites from first principles
NASA Astrophysics Data System (ADS)
Hammerschmidt, Lukas; Quennet, Marcel; Töpfer, Kai; Paulus, Beate
2015-07-01
We have performed first-principles calculations for all the differently terminated three low-index surfaces of CoSb3 skutterudite. Surface energies, lattice structures and electronic structures have been investigated applying different functionals within the plane-wave density functional theory. Each functional consistently reveals the (110) as the most stable surface, followed by (100) and (111), respectively. For each surface plane we have studied all possible terminating surface layers and a structural comparison shows that the surface stability is mostly determined by the Sb4 subunits. Wherever complete Sb4 subunits remain intact at the surface the stability is high. For the most stable surfaces, we have calculated the surface band structure in reference to the projected bulk band continuum and identified surface states. These surface states promote the metallic character at the CoSb3 surfaces. The freedom of spin polarization leads to a magnetic configuration in the (110) case.
A first-principles study of sodium adsorption and diffusion on phosphorene.
Liu, Xiao; Wen, Yanwei; Chen, Zhengzheng; Shan, Bin; Chen, Rong
2015-07-01
The structural, electronic, electrochemical as well as diffusion properties of Na doped phosphorene have been investigated based on first-principles calculations. The strong binding energy between Na and phosphorene indicates that Na could be stabilized on the surface of phosphorene without clustering. By comparing the adsorption of Na atoms on one side and on both sides of phosphorene, it has been found that Na-Na exhibits strong repulsion at the Na-Na distance of less than 4.35 Å. The Na intercalation capacity is estimated to be 324 mA h g(-1) and the calculated discharge curve indicates quite a low Na(+)/Na voltage of phosphorene. Moreover, the diffusion energy barrier of Na atoms on the phosphorene surface at both low and high Na concentrations is as low as 40-63 meV, which implies the high mobility of Na during the charge/discharge process. PMID:26051226
First-principles study of spontaneous polarization in SrBi?Ta?O?.
Ke, Hua; Wang, Wen; Zheng, Zhenxing; Tang, Chunliu; Jia, Dechang; Lu, Zhe; Zhou, Yu
2011-01-12
We investigate the spontaneous polarization in SrBi?Ta?O? by analyzing the maximally localized Wannier functions using a first-principles method. The calculated spontaneous polarization in ferroelectric SrBi(2)Ta(2)O(9) along the polarized a axis is 23.8 µC cm( - 2), with an electronic contribution of 8.4 µC cm( - 2). The electronic contribution of each atom to the spontaneous polarization in the unit cell is quantitatively evaluated. The Bi component plays the most important role in the complex chemical bonding and the total polarization. The chemical bonding in the Ta-O octahedra and bismuth oxide slabs of ferroelectric SrBi?Ta?O? is also addressed. PMID:21406828
First-principles studies of the vibrational properties of amorphous carbon nitrides
NASA Astrophysics Data System (ADS)
Niu, Li; Wang, Xuan-Zhang; Zhu, Jia-Qi; Gao, Wei
2013-01-01
Raman spectra of amorphous carbon nitride films (a-C:N) resemble those of typical amorphous carbon (a-C), and no specific features in the spectra are shown due to N doping. The present work provides a correlation between the microstructure and vibrational properties of a-C:N films from first principles. The six periodic model structures of 64 atoms with various mass densities and nitrogen contents are generated by the liquid-quench method using Car—Parinello molecular dynamics. By using Raman coupling tensors calculated with the finite electric field method, Raman spectra are obtained. The calculated results show that the vibrations of C=N could directly contribute to the Raman spectrum. The similarity of the Raman line shapes of N-doped and N-free amorphous carbons is due to the overlapping of C=N and C=C vibration bands. In addition, the origin of characteristic Raman peaks is also given.
Magnetostriction and magnetism of rare earth intermetallic compounds: First principle study
Gavrilenko, V. I.; Wu, R. Q.
2001-06-01
Magnetism and magnetostriction of rare earth intermetallic compounds, GdCo{sub 2}, GdFe{sub 2}, NdCo{sub 2}, SmCo{sub 2}, and ErCo{sub 2}, 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 of magnetostriction in GdCo{sub 2} is explained. Contributions due to spatial anisotropic charge distribution of the incomplete 4f shells are calculated and discussed. {copyright} 2001 American Institute of Physics.
Lattice dynamics and chemical bonding in Sb2Te3 from first-principles calculations.
Wang, Bao-Tian; Souvatzis, Petros; Eriksson, Olle; Zhang, Ping
2015-05-01
Pressure effects on the lattice dynamics and the chemical bonding of the three-dimensional topological insulator, Sb2Te3, have been studied from a first-principles perspective in its rhombohedral phase. Where it is possible to compare, theory agrees with most of the measured phonon dispersions. We find that the inclusion of relativistic effects, in terms of the spin-orbit interaction, affects the vibrational features to some extend and creates large fluctuations on phonon density of state in high frequency zone. By investigations of structure and electronic structure, we analyze in detail the semiconductor to metal transition at ?2 GPa followed by an electronic topological transition at a pressure of ?4.25 GPa. PMID:25956111
Doping dependence of thermoelectric performance in Mo3Sb7: first principles calculations
Singh, David J [ORNL; Parker, David S [ORNL; Du, Mao-Hua [ORNL
2011-01-01
We study the effects of doping Mo3Sb7 with transition metals (Ni,Fe,Co,Ru) via first principles calculations, including electronic structure, lattice dynamics and Boltzmann transport. We find heavy-mass bands and large, rapidly varying density-of-states, generally favorable for high thermopower, near the band gap of this material. Transport calculations predict large Seebeck coefficients exceeding 300 $\\mu$V/K in a wide temperature range above 500 K (a range suitable for waste heat recovery), if the material can be doped into a semiconducting state. These thermopowers are much higher than those that have previously been experimentally observed; we find that performance exceeding current limits may be found at lower carrier concentration than achieved presently. We also discuss the selection of dopant and the potential thermoelectric performance of optimally doped Mo3Sb7.
Continuous wavelet transform analysis of one-dimensional quantum bound states from first principles
Handy, C.R.; Murenzi, R. [Department of Physics and Center for Theoretical Studies of Physical Systems, Clark Atlanta University, Atlanta, Georgia 30314 (United States)] [Department of Physics and Center for Theoretical Studies of Physical Systems, Clark Atlanta University, Atlanta, Georgia 30314 (United States)
1996-11-01
Over the last decade, Handy and Bessis have developed a moment-problem-based, multiscale quantization theory, the eigenvalue moment method (EMM), which has proven effective in solving singular, strongly coupled, multidimensional Schr{umlt o}dinger Hamiltonians. We extend the scope of EMM by demonstrating its essential role in the generation of wavelet transforms for one-dimensional quantum systems. Combining this with the function-wavelet reconstruction formulas currently available, we are able to recover the wave function systematically, from first principles, through a multiscale process proceeding from large spatial scales to smaller ones. This accomplishment also addresses another outstanding problem, that of reconstructing a function from its moments. For the class of problems considered, the combined EMM-wavelet analysis yields a definitive solution. {copyright} {ital 1996 The American Physical Society.}
First-principles study of direct and narrow band gap semiconducting ?-CuGaO2
NASA Astrophysics Data System (ADS)
Nguyen, Manh Cuong; Zhao, Xin; Wang, Cai-Zhuang; Ho, Kai-Ming
2015-04-01
Semiconducting oxides have attracted much attention due to their great stability in air or water and the abundance of oxygen. Recent success in synthesizing a metastable phase of CuGaO2 with direct narrow band gap opens up new applications of semiconducting oxides as absorber layer for photovoltaics. Using first-principles density functional theory calculations, we investigate the thermodynamic and mechanical stabilities as well as the structural and electronic properties of the ?-CuGaO2 phase. Our calculations show that the ?-CuGaO2 structure is dynamically and mechanically stable. The energy band gap is confirmed to be direct at the ? point of Brillouin zone. The optical absorption occurs right at the band gap edge and the density of states near the valance band maximum is large, inducing an intense absorption of light as observed in experiment.
High pressure phase transformation in thorium carbide: A first principle study
NASA Astrophysics Data System (ADS)
Sahoo, B. D.; Joshi, K. D.; Gupta, Satish C.
2013-06-01
First principles calculations using full potential linearized augmented plane wave (FP-LAPW) method have been carried out to analyze structural, electronic and elastic properties of ThC under hydrostatic compression. Our calculations carried out within the generalized gradient approximation (GGA) predict a structural phase transition from rocksalt type (B1) phase to CsCl type cubic (B2) structure at ˜ 45 GPa. The same transition is predicted to occur at ˜ 35 GPa by the calculations performed within local density approximation (LDA). Various physical quantities such as equilibrium volume, bulk modulus, pressure derivative of bulk modulus and elastic constants determined for both the phases at zero pressure are compared with data available in literature. We find that at zero pressure the B2 structure is unstable elastically also. However, it emerges as elastically stable structure before it stabilizes energetically at high pressure. Further, the activation barrier between these structures has been calculated at various pressures.
Properties of hydrofluorinated carbon- and boron nitride-based nanofilms: A first-principles study
NASA Astrophysics Data System (ADS)
Yang, Yurong; Ren, Wei; Bellaiche, L.
2014-06-01
First-principles computations are performed to investigate various properties of two different kinds of nanofilms, namely those made by hydrofluorinating graphene and those formed by hydrofluorinated BN sheets. Monolayers and bilayers are considered for these two different kinds of nanofilms. Calculations show that all these systems adopt sp3 bonds between carbon atoms or between B and N atoms due to the bindings of these atoms with H and F ions. Moreover, some of these nanofilms, especially the boron nitride-based ones, possess a spontaneous electrical polarization and piezoelectric coefficients that can be larger than those of typical ferroelectric polymers. Analysis of Wannier functions further reveals the microscopic origins of these significant polar and piezoelectric properties. In particular, it points out the importance of the orbitals of the F anion on these properties.
First-principles study of the effect of phosphorus on nickel grain boundary
Liu, Wenguan; Ren, Cuilan; Han, Han, E-mail: hanhan@sinap.ac.cn, E-mail: xuhongjie@sinap.ac.cn; Zou, Yang; Zhou, Xingtai; Huai, Ping; Xu, Hongjie, E-mail: hanhan@sinap.ac.cn, E-mail: xuhongjie@sinap.ac.cn [Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800 (China); Key Laboratory of Nuclear Radiation and Nuclear Energy Technology, Chinese Academy of Sciences, Shanghai 201800 (China); Tan, Jie [Department of Physics, Shandong University, Jinan Shandong 250100 (China)
2014-01-28
Based on first-principles quantum-mechanical calculations, the impurity-dopant effects of phosphorus on ?5(012) symmetrical tilt grain boundary in nickel have been studied. The calculated binding energy suggests that phosphorus has a strong tendency to segregate to the grain boundary. Phosphorus forms strong and covalent-like bonding with nickel, which is beneficial to the grain boundary cohesion. However, a too high phosphorus content can result in a thin and fragile zone in the grain boundary, due to the repulsion between phosphorus atoms. As the concentration of phosphorus increases, the strength of the grain boundary increases first and then decreases. Obviously, there exists an optimum concentration for phosphorus segregation, which is consistent with observed segregation behaviors of phosphorus in the grain boundary of nickel. This work is very helpful to understand the comprehensive effects of phosphorus.
Stacking fault energy, yield stress anomaly, and twinnability of Ni3Al: A first principles study
NASA Astrophysics Data System (ADS)
Liu, Li-Li; Wu, Xiao-Zhi; Wang, Rui; Li, Wei-Guo; Liu, Qing
2015-07-01
Using first principles calculations combined with the quasiharmonic approach, we study the effects of temperature on the elastic constants, generalized stacking fault energies, and generalized planar fault energies of Ni3Al. The antiphase boundary energies, complex stacking fault energies, superlattice intrinsic stacking fault energies, and twinning energies decrease slightly with temperature. Temperature dependent anomalous yield stress of Ni3Al is predicted by the energy-based criterion based on elastic anisotropy and antiphase boundary energies. It is found that p increases with temperature and this can give a more accurate description of the anomalous yield stress in Ni3Al. Furthermore, the predicted twinnablity of Ni3Al is also decreasing with temperature. Project supported by the National Natural Science Foundation of China (Grant Nos. 11104361 and 11304403) and the Fundamental Research Funds for the Central Universities, China (Grant Nos. CQDXWL2014003 and CDJZR14328801).
Yu, L.; Zunger, A.
2012-02-10
There are numerous inorganic materials that may qualify as good photovoltaic (PV) absorbers, except that the currently available selection principle - focusing on materials with a direct band gap of {approx}1.3 eV (the Shockley-Queisser criteria) - does not provide compelling design principles even for the initial material screening. Here we offer a calculable selection metric of 'spectroscopic limited maximum efficiency (SLME)' that can be used for initial screening based on intrinsic properties alone. It takes into account the band gap, the shape of absorption spectra, and the material-dependent nonradiative recombination losses. This is illustrated here via high-throughput first-principles quasiparticle calculations of SLME for {approx}260 generalized I{sub p}III{sub q}VI{sub r} chalcopyrite materials. It identifies over 20 high-SLME materials, including the best known as well as previously unrecognized PV absorbers.
First-principles study of Co3(Al,W) alloys using special quasirandom structures
Jiang, Chao [Los Alamos National Laboratory
2008-01-01
We have developed 32-atom special quasi-random structures (SQSs) to model the substitutionally random pseudo-binary A3(B0.5C0.5) alloys in L12, D019, and D03 crystal structures, respectively. First-principles SQS calculations are performed to examine the phase stability of the recently identified L12-Co3Al0.5W0.5 compound in the Co-Al-W ternary system. By computing total energy as a function of applied strain, the single-crystal elastic constants of L12-Co3Al0.5W0.5 are also predicted and our results show excellent agreement with recent experimental measurements.
Vacancy Ordering In Co3AlCx Alloys: A First Principles Study
Jiang, Chao [Los Alamos National Laboratory
2008-01-01
Ordering of structural vacancies in non-stoichiometric Co{sub 3}AlC{sub x} alloys has been studied using a combination of first-principles total energy calculations, a cluster expansion technique, and Monte-Carlo simulations. In the proximity of the experimental1y observed composition of x {approx} 0.59, our exhaustive ground state search yields two stable vacancy-ordered structures: a cubic Co{sub 3}AlC{sub 0.5} phase and a trigonal Co{sub 3}AlC{sub 0.667} phase. By performing finite-temperature Monte-Carlo simulations, the order-disorder transition temperatures of Co{sub 3}AlC{sub 0.5} and CO{sub 3}AlC{sub 0.667} are predicted to be {approx}1925K and {approx}1630K, respectively.
First-principles studies of ultrathin ferroelectric capacitors with Ru-based perovskite electrodes
NASA Astrophysics Data System (ADS)
He, Jun; Nakhmanson, Serge
2010-03-01
First-principles calculations are used to investigate the electrostatics and polarization screening effects in ultrathin PbTiO3 films capped with SrRuO3 or CaRuO3 electrodes under short-circuit boundary conditions. In accordance with previous results, we find that the SrRuO3/PbTiO3/SrRuO3 system without antiferrodistortive octahedral rotations is ``non-pathological'' with respect to the metal/ferroelectric band alignment across the interface. Such rotations, however, have to be explicitly considered to correctly determine the band alignment and polarization screening in the SrRuO3/PbTiO3/SrRuO3 nanocapacitor. (*) Present address: Vanderbilt University, Nashville, Tennessee 37235 and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
NASA Astrophysics Data System (ADS)
Gao, Qin; Yao, Sanxi; Widom, Michael
2015-03-01
Density functional theory (DFT) provides an accurate and first-principles description of solid structures and total energies. However, it is highly time-consuming to calculate structures with hundreds of atoms in the unit cell and almost not possible to calculate thousands of atoms. We apply and adapt machine learning algorithms, including compressive sensing, support vector regression and artificial neural networks to fit the DFT total energies of substitutionally disordered boron carbide. The nonparametric kernel method is also included in our models. Our fitted total energy model reproduces the DFT energies with prediction error of around 1 meV/atom. The assumptions of these machine learning models and applications of the fitted total energies will also be discussed. Financial support from McWilliams Fellowship and the ONR-MURI under the Grant No. N00014-11-1-0678 is gratefully acknowledged.
First-principles study on quantum valley Hall effects in silicene
NASA Astrophysics Data System (ADS)
Kim, Youngkuk; Jin, Hosub; Choi, Keunsu; Ihm, Jisoon
2013-03-01
Silicene is a two-dimensional honeycomb lattice of silicon atoms, similar to graphene. Based on first-principles calculations, we suggest that silicene is an ideal host for realization of quantum valley Hall effects. We show that the intrinsic buckled structure allows the formation of topological domain walls in silicene under a uniform applied electric field and valley-polarized kink states emerge on the domain walls. Peculiar behaviors of the kink states under various applied electric fields are demonstrated, and simulated scanning tunneling microscopy images are presented to show that they can be used to identify the topological domain walls as well as valley-polarized kink states. Our findings suggest that the one-dimensional domain wall may be used as an electrical wire through which valley-polarized current can flow, and silicene can be used as a valley polarizer.
Combined first-principles calculation and neural-network correction approach for heat of formation
NASA Astrophysics Data System (ADS)
Hu, LiHong; Wang, XiuJun; Wong, LaiHo; Chen, GuanHua
2003-12-01
Despite their success, the results of first-principles quantum mechanical calculations contain inherent numerical errors caused by various intrinsic approximations. We propose here a neural-network-based algorithm to greatly reduce these inherent errors. As a demonstration, this combined quantum mechanical calculation and neural-network correction approach is applied to the evaluation of standard heat of formation ?fH? for 180 small- to medium-sized organic molecules at 298 K. A dramatic reduction of numerical errors is clearly shown with systematic deviation being eliminated. For example, the root-mean-square deviation of the calculated ?fH? for the 180 molecules is reduced from 21.4 to 3.1 kcal mol-1 for B3LYP/6-311+G(d,p) and from 12.0 to 3.3 kcal mol-1 for B3LYP/6-311+G(3df,2p) before and after the neural-network correction.
Surface magnetism of Mg doped AlN: a first principle study.
Chintalapati, Sandhya; Yang, Ming; Lau, Shu Ping; Feng, Yuan Ping
2014-10-29
Using first-principles calculations, we have systematically investigated the magnetic properties of Mg-doped AlN (1 0 1 0) and (0 0 0 1) surfaces. Both the polar and non-polar surfaces are found to be magnetic and the magnetic moments are mainly due to spin polarized 2p orbitals of surface N atoms surrounding Mg. The splitting of energy levels in both cases favours charge hopping between the minority spin states of N 2p orbitals, which leads to a stable ferromagnetic ground state. However, the range of magnetic coupling and the stability of the ferromagnetic state differ between the polar and non-polar surfaces and are dependent on the nature of localization of the defect states. The ferromagnetic state in a Mg-doped reconstructed (0 0 0 1) surface is more stable than in a Mg-doped AlN (1 0 1 0) surface. PMID:25299568
First-principles study of solute-dislocation interaction in aluminum-rich alloys
NASA Astrophysics Data System (ADS)
Vannarat, S.; Sluiter, Marcel H. F.; Kawazoe, Yoshiyuki
2001-12-01
The strain dependence of the solution energy of substitutional species is derived by expanding the binary alloy energy with respect to concentration and strain to the second order. Within the approximation of small strain and concentration inhomogeneities, the energy change is found to be proportional to the solute-induced stress, and the strain dependence of the solute energy is equivalent to that from the misfit model for solute atoms. However, the solute induced stress is easily calculated from first principles, and it is computed for Ag, Cu, Fe, Li, Mg, Mn, Si, and Zn atoms in aluminum. The results are used to estimate the solute-dislocation interaction energy. The enhancement of the solute concentration near a dislocation core and the role of dislocations on precipitation of these solutes are discussed.
First-Principles Modeling of ThO2 Solid Solutions with Oxides of Trivalent Cations
NASA Astrophysics Data System (ADS)
Alexandrov, Vitaly; Asta, Mark; Gronbech-Jensen, Niels
2010-03-01
Solid solutions formed by doping ThO2 with oxides of trivalent cations, such as Y2O3 and La2O3, are suitable for solid electrolyte applications, similar to doped zirconia and ceria. ThO2 has also been gaining much attention as an alternative to UO2 in nuclear energy applications, the aforementioned trivalent cations being important fission products. In both cases the mixing energetics and short-range ordering/clustering are key to understanding structural and transport properties. Using first-principles atomistic calculations, we address intra- and intersublattice interactions for both cation and anion sublattices in ThO2-based fluorite-type solid solutions and compare the results with similar modeling studies for related trivalent-doped zirconia systems.
First-principles calculations of magnetic properties for CdCrO2 under pressure
NASA Astrophysics Data System (ADS)
Amari, S.; Méçabih, S.; Abbar, B.; Bouhafs, B.
2013-02-01
By employing the first-principles method of the full potential linear augmented plane waves plus the local orbitals (FP-L/APW+lo) within the generalized gradient approximation for the exchange and correlation potential, the structural, electronic, and magnetic properties of chalcopyrite compound CdCrO2 are investigated. In order to take into account the strong on-site Coulomb interaction, we also performed the generalized gradient approximation plus the Hubbard correlation terms. We systematically study how the exchange interactions and magnetic moments of CdCrO2 are affected by the different choice of U as well as the exchange correlation potential. We have also carried out the pressure effect on the magnetic properties.
NASA Astrophysics Data System (ADS)
Loukonen, Ville; Bork, Nicolai; Vehkamäki, Hanna
2013-05-01
The clustering process (sulfuric acid) + (base)?(sulfuric acid)1(base)1 is of fundamental importance in the atmospheric new-particle formation. Especially interesting are the collisions where a proton transfer reaction can happen, as the reaction often leads to relatively strongly bound clusters. Here, we studied the clustering process of (sulfuric acid) + (dimethylamine) ? (sulfuric acid)1(dimethylamine)1 using first-principles molecular dynamics simulations. The collision of the two molecules was simulated starting with various spatial orientations and the evolution of the cluster was followed in the NVE ensemble. The simulations suggest that the proton transfer reaction takes place regardless of the intial collision orientation. However, due to the energy released in the process, the newly-formed cluster is not able to reach the minimun energy configuration, which might affect the following growth processes.
Nanoparticle shapes by using Wulff constructions and first-principles calculations
Barmparis, Georgios D; Lodziana, Zbigniew; Lopez, Nuria
2015-01-01
Summary Background: The majority of complex and advanced materials contain nanoparticles. The properties of these materials depend crucially on the size and shape of these nanoparticles. Wulff construction offers a simple method of predicting the equilibrium shape of nanoparticles given the surface energies of the material. Results: We review the mathematical formulation and the main applications of Wulff construction during the last two decades. We then focus to three recent extensions: active sites of metal nanoparticles for heterogeneous catalysis, ligand-protected nanoparticles generated as colloidal suspensions and nanoparticles of complex metal hydrides for hydrogen storage. Conclusion: Wulff construction, in particular when linked to first-principles calculations, is a powerful tool for the analysis and prediction of the shapes of nanoparticles and tailor the properties of shape-inducing species. PMID:25821675
Lewis, J P; Sankey, O F
1995-01-01
A first principles model for calculating hydrogen bonding interactions, previously applied to water, is here applied to the more difficult problem of interactions between DNA bases. We first consider the energetics and geometry for the A-T and the G-C basepairs, comparing our results to other calculated results as well as to experiment. Next, we study the interactions of isomorphic DNA base triplet structures, which are important because of their suggested role in the recombination process. We find that energetically the third base in the triplet tends to favor a position along the dyadic axis, where it is hydrogen bonded to both bases in the duplex. Images FIGURE 1 FIGURE 2 FIGURE 3 FIGURE 4 PMID:8519960
Conductance of ferro- and antiferro-magnetic single-atom contacts: A first-principles study
Tan, Zhi-Yun [School of Physics and Mechanical and Electrical Engineering, Zunyi Normal College, Zunyi (China)] [School of Physics and Mechanical and Electrical Engineering, Zunyi Normal College, Zunyi (China); Zheng, Xiao-long; Ye, Xiang; Xie, Yi-qun [Department of Physics, Shanghai Normal University, 100 Guilin Road, Shanghai 200232 (China)] [Department of Physics, Shanghai Normal University, 100 Guilin Road, Shanghai 200232 (China); Ke, San-Huang [Department of Physics, Key Laboratory of Advanced Microstructured Materials, MOE, Tongji University, 1239 Siping Road, Shanghai 200092 (China)] [Department of Physics, Key Laboratory of Advanced Microstructured Materials, MOE, Tongji University, 1239 Siping Road, Shanghai 200092 (China)
2013-08-14
We present a first-principles study on the spin dependent conductance of five single-atom magnetic junctions consisting of a magnetic tip and an adatom adsorbed on a magnetic surface, i.e., the Co-Co/Co(001) and Ni-X/Ni(001) (X = Fe, Co, Ni, Cu) junctions. When their spin configuration changes from ferromagnetism to anti-ferromagnetism, the spin-up conductance increases while the spin-down one decreases. For the junctions with a magnetic adatom, there is nearly no spin valve effect as the decreased spin-down conductance counteracts the increased spin-up one. For the junction with a nonmagnetic adatom (Ni-Cu/Ni(001)), a spin valve effect is obtained with a variation of 22% in the total conductance. In addition, the change in spin configuration enhances the spin filter effect for the Ni-Fe/Ni(001) junction but suppresses it for the other junctions.
First-Principles Approach to Nonlinear Lattice Dynamics: Anomalous Spectra in PbTe
NASA Astrophysics Data System (ADS)
Chen, Yue; Ai, Xinyuan; Marianetti, C. A.
2014-09-01
Here we introduce a new approach to compute the finite temperature lattice dynamics from first principles via the newly developed slave mode expansion. We study PbTe where inelastic neutron scattering reveals strong signatures of nonlinearity as evidenced by anomalous features which emerge in the phonon spectra at finite temperature. Using our slave mode expansion in the classical limit, we compute the vibrational spectra and show remarkable agreement with temperature dependent inelastic neutron scattering measurements. Furthermore, we resolve an experimental controversy by showing that there are no appreciable local nor global spontaneously broken symmetries at finite temperature and that the anomalous spectral features simply arise from two anharmonic interactions. Our approach should be broadly applicable across the periodic table.
NASA Astrophysics Data System (ADS)
Lischner, Johannes; Bazhirov, Timur; MacDonald, Allan H.; Cohen, Marvin L.; Louie, Steven G.
2015-03-01
We present first-principles calculations of the coupling of quasiparticles to spin fluctuations in iron selenide and discuss which types of superconducting instabilities this coupling gives rise to. We find that strong antiferromagnetic stripe-phase spin fluctuations lead to large coupling constants for superconducting gaps with s +/- -symmetry, but these coupling constants are significantly reduced by other spin fluctuations with small wave vectors. An accurate description of this competition and an inclusion of band structure and Stoner parameter renormalization effects lead to a value of the coupling constant for an s +/- symmetric gap which can produce a superconducting transition temperature consistent with experimental measurements. This work was supported by NSF Grant No. DMR10-1006184 and by DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by the DOE at NERSC.
Valence-band mixing in first-principles envelope-function theory
Bradley A. Foreman
2007-01-01
This paper presents a numerical implementation of a first-principles\\u000aenvelope-function theory derived recently by the author [B. A. Foreman, Phys.\\u000aRev. B 72, 165345 (2005)]. The examples studied deal with the valence subband\\u000astructure of GaAs\\/AlAs, GaAs\\/Al(0.2)Ga(0.8)As, and In(0.53)Ga(0.47)As\\/InP (001)\\u000asuperlattices calculated using the local density approximation to\\u000adensity-functional theory and norm-conserving pseudopotentials without\\u000aspin-orbit coupling. The heterostructure Hamiltonian is
Phonon transport in perovskite SrTiO3 from first principles
NASA Astrophysics Data System (ADS)
Feng, Lei; Shiga, Takuma; Shiomi, Junichiro
2015-07-01
We investigate phonon transport in perovskite strontium titanate (SrTiO3), which is stable above its phase transition temperature (?105 K), by using first-principles molecular dynamics and anharmonic lattice dynamics. Unlike conventional ground-state-based perturbation methods that give imaginary phonon frequencies, the current calculation reproduces stable phonon dispersion relations observed in experiments. We find that the contribution of optical phonons to the overall lattice thermal conductivity is larger than 60%, which is markedly different from the usual picture with the dominant contribution from acoustic phonons. Mode-dependent and pseudopotential-dependent analyses suggest the strong attenuation of acoustic phonon transport originating from strong anharmonic coupling with the transversely polarized ferroelectric modes.
Correa, Alfredo A.; Bonev, Stanimir A.; Galli, Giulia
2006-01-01
At high pressure and temperature, the phase diagram of elemental carbon is poorly known. We present predictions of diamond and BC8 melting lines and their phase boundary in the solid phase, as obtained from first-principles calculations. Maxima are found in both melting lines, with a triple point located at ?850 GPa and ?7,400 K. Our results show that hot, compressed diamond is a semiconductor that undergoes metalization upon melting. In contrast, in the stability range of BC8, an insulator to metal transition is likely to occur in the solid phase. Close to the diamond/liquid and BC8/liquid boundaries, molten carbon is a low-coordinated metal retaining some covalent character in its bonding up to extreme pressures. Our results provide constraints on the carbon equation of state, which is of critical importance for devising models of Neptune, Uranus, and white dwarf stars, as well as of extrasolar carbon-rich planets. PMID:16432191
Zhang Zhuhua [Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China); Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588 (United States); Zeng Xiao Cheng [Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588 (United States); Guo Wanlin [Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China)
2010-07-15
Since electrons injected to a homogenous wire always tend to concentrate on its surface, heterogeneous coaxial structures are generally necessary to make nanocables with an insulating sheath. Here we reveal from first-principles calculations that double-walled boron-nitride nanotubes could be natural homogeneous nanocables as injected electrons prefer abnormally to concentrate on the inner semiconducting tube while the outer tube remains insulating. The ratio of extra electrons on the inner tube to total carriers in the double-walled nanotubes can be tuned widely by changing either the tube diameter or the local tube curvature through radial deformation, both attributed to the predominant band filling and weak enhancement in Coulomb interaction within the inner wall where the sublattice asymmetry is strongly attenuated by curvature effect. This exotic charge screening is universal for any form of electron-doping sources.
First-principles study on the electronic structure and optical properties of GaAs nanowires
NASA Astrophysics Data System (ADS)
Wei, Xiang-Hai; Gao, Tao; Wan, Lei; Lu, Peng-Fei
2015-10-01
The electronic structure and optical properties of wurtzite gallium arsenide (GaAs) nanowires (NWs) are investigated within the first-principles plane-wave pseudo-potential method based on the density functional theory (DFT). The calculated results show that GaAs NWs are direct band gap semiconductor and demonstrate the achievement of p-type behavior. These NWs of the electronic behavior transitions from semiconductor to metal after hydrogen-passivated. The complex dielectric function, absorption coefficient and optical reflectivity are calculated and the peak position distributions of imaginary parts of the complex dielectric function have been explained. We may conclude GaAs NWs belongs to what crystal system through the dielectric tensors. Optical reflectivity is obtained and the peak position distributions of imaginary parts of the complex dielectric function have been explained, and their absorption spectra are sensitive to the large radius of GaAs NWs.
First-principles approach to calculating energy level alignment at aqueous semiconductor interfaces
Kharche, Neerav [Brookhaven National Lab. (BNL), Upton, NY (United States); Muckerman, James T. [Brookhaven National Lab. (BNL), Upton, NY (United States); Hybertsen, Mark S. [Brookhaven National Lab. (BNL), Upton, NY (United States)
2014-10-01
A first-principles approach is demonstrated for calculating the relationship between an aqueous semiconductor interface structure and energy level alignment. The physical interface structure is sampled using density functional theory based molecular dynamics, yielding the interface electrostatic dipole. The GW approach from many-body perturbation theory is used to place the electronic band edge energies of the semiconductor relative to the occupied 1b? energy level in water. The application to the specific cases of nonpolar (1010¯ ) facets of GaN and ZnO reveals a significant role for the structural motifs at the interface, including the degree of interface water dissociation and the dynamical fluctuations in the interface Zn-O and O-H bond orientations. These effects contribute up to 0.5 eV.
First-principles prediction of shape memory behavior and ferrimagnetism in Mn2NiSn
NASA Astrophysics Data System (ADS)
Paul, Souvik; Ghosh, Subhradip
2011-05-01
Using first-principles density functional theory, we show that, in Mn2NiSn, an energy lowering phase transition from the cubic to tetragonal phase occurs which indicates a martensitic phase transition. This structural phase transition is nearly volume-conserving, implying that this alloy can exhibit shape memory behavior. The magnetic ground state is a ferrimagnetic one with antiparallel Mn spin moments. The calculated moments with different electronic structure methods in the cubic phase compare well with each other but differ from the experimental values by more than 1 ?B. The reason behind this discrepancy is explored by considering antisite disorder in our calculations, which indicates that the site ordering in this alloy can be quite complex.
Negative Poisson’s ratios in few-layer orthorhombic arsenic: First-principles calculations
NASA Astrophysics Data System (ADS)
Han, Jianwei; Xie, Jiafeng; Zhang, Zhiya; Yang, Dezheng; Si, Mingsu; Xue, Desheng
2015-04-01
Using first-principles calculations we demonstrate for the first time that few-layer orthorhombic arsenic possesses a negative Poisson’s ratio. For a single layer of arsenic, the negative Poisson’s ratio is predicted to be ??0.09. As the number of layers increases, the magnitude of the negative Poisson’s ratio increases and finally approaches a limit at four layers, becoming very close to the bulk value of ?0.13. To understand these layer-dependent negative Poisson’s ratios, we propose a rigid mechanical model in which the intra-layer bond lengths and the normal Poisson’s ratio of the in-layer plane play key roles.
Cold Baryogenesis from first principles in the Two-Higgs Doublet model with Fermions
Zong-Gang Mou; Paul M. Saffin; Anders Tranberg
2015-05-27
We present a first-principles numerical computation of the baryon asymmetry in electroweak-scale baryogenesis. For the scenario of Cold Baryogenesis, we consider a one fermion-family reduced CP-violating two Higgs-doublet model, including a classical SU(2)-gauge/two-Higgs sector coupled to one quantum left-handed fermion doublet and two right-handed singlets. Separately, the C(CP) breaking of the two-Higgs potential and the C and P breaking of the gauge-fermion interactions do not provide a baryon asymmetry. Only when combined does baryogenesis occur. Through large-scale computer simulations, we compute the asymmetry for one particularly favourable scalar potential. The numerical signal is at the boundary of what is numerically discernible with the available computer resources, but we tentatively find an asymmetry of $|\\eta|\\leq 3.5\\times 10^{-7}$.
NASA Astrophysics Data System (ADS)
Ganji, M. D.; Ahaz, B.
2010-04-01
We study the binding of molecular oxygen to a (5, 0) single walled SiC nanotube, by means of density functional calculations. The center of a hexagon of silicon and carbon atoms in sites on SiCNT surfaces is the most stable adsorption site for O2 molecule, with a binding energy of -38.22 eV and an average Si-O binding distance of 1.698 Å. We have also tested the stability of the O2-adsorbed SiCNT/CNT with ab initio molecular dynamics simulation which have been carried out at room temperature. Furthermore, the adsorption of O2 on the single walled carbon nanotubes has been investigated. Our first-principles calculations predict that the O2 adsorptive capability of silicon carbide nanotubes is much better than that of carbon nanotubes. This might have potential for gas detection and energy storage.
Jump rates for surface diffusion of large molecules from first principles
NASA Astrophysics Data System (ADS)
Shea, Patrick; Kreuzer, Hans Jürgen
2015-04-01
We apply a recently developed stochastic model for the surface diffusion of large molecules to calculate jump rates for 9,10-dithioanthracene on a Cu(111) surface. The necessary input parameters for the stochastic model are calculated from first principles using density functional theory (DFT). We find that the inclusion of van der Waals corrections to the DFT energies is critical to obtain good agreement with experimental results for the adsorption geometry and energy barrier for diffusion. The predictions for jump rates in our model are in excellent agreement with measured values and show a marked improvement over transition state theory (TST). We find that the jump rate prefactor is reduced by an order of magnitude from the TST estimate due to frictional damping resulting from energy exchange with surface phonons, as well as a rotational mode of the diffusing molecule.
First-principle calculation of solar cell efficiency under incoherent illumination
NASA Astrophysics Data System (ADS)
Sarrazin, Michaël; Herman, Aline; Deparis, Olivier
2013-07-01
Because of the temporal incoherence of sunlight, solar cells efficiency should depend on the degree of coherence of the incident light. However, numerical computation methods, which are used to optimize these devices, fundamentally consider fully coherent light. Hereafter, we show that the incoherent efficiency of solar cells can be easily analytically calculated. The incoherent efficiency is simply derived from the coherent one thanks to a convolution product with a function characterizing the incoherent light. Our approach is neither heuristic nor empiric but is deduced from first-principle, i.e. Maxwell's equations. Usually, in order to reproduce the incoherent behavior, statistical methods requiring a high number of numerical simulations are used. With our method, such approaches are not required. Our results are compared with those from previous works and good agreement is found.
Improving the electrical conductivity of carbon nanotube networks: a first-principles study.
Li, Elise Y; Marzari, Nicola
2011-12-27
We address the issue of the low electrical conductivity observed in carbon nanotube networks using first-principles calculations of the structure, stability, and ballistic transport of different nanotube junctions. We first study covalent linkers, using the nitrene-pyrazine case as a model for conductance-preserving [2 + 1] cycloadditions, and discuss the reasons for their poor performance. We then characterize the role of transition-metal adsorbates in improving mechanical coupling and electrical tunneling between the tubes. We show that the strong hybridization between the transition-metal d orbitals with the ? orbitals of the nanotube can provide an excellent electrical bridge for nanotube-nanotube junctions. This effect is maximized in the case of nitrogen-doped nanotubes, thanks to the strong mechanical coupling between the tubes mediated by a single transition metal adatom. Our results suggest effective strategies to optimize the performance of carbon nanotube networks. PMID:22059779
Detection of nucleic acids by graphene-based devices: A first-principles study
NASA Astrophysics Data System (ADS)
Zhang, Hua; Xu, Hui; Ni, Xiang; Lin Peng, Sheng; Liu, Qi; Ping OuYang, Fang
2014-04-01
Based on first-principles quantum transport calculations, we design a graphene-based biosensor device, which is composed of graphene nanoribbons electrodes and a biomolecule. It is found that when different nucleobases or poly nucleobase chains are located in the nanogap, the device presents completely different transport properties, showing different current informations. And the change of currents from 2 to 5 orders of magnitude for four different nucleobases suggests a great ability of discrimination by utilizing such a device. The physical mechanism of this phenomenon originates from their different chemical composition and structure. Moreover, we also explore the coupling effect of several neighboring bases and the size effect of the nanogap on transport properties. Our results show the possibility of rapid sequencing DNA by measuring such a transverse-current of the device, and provide a new idea for sequencing DNA.
NASA Astrophysics Data System (ADS)
Lin, Zhiping; Zhao, Yu-Jun; Zhao, Yanming; Xu, Jiantie
2014-01-01
We present a first-principles investigation for the structure, electronic properties, and average potentials of Li9M3(P2O7)3(PO4)2 (M = V, Fe, Cr) compounds. The calculated Wyckoff coordinates are in good agreement with experimental observations. All the studied compounds show semiconductor characteristics, with band gaps between 1.89 eV and 2.55 eV. It is found that the Li-ion extraction is in the order of Li1(2b), Li2(12g), and Li3(4d) based on the calculated formation enthalpies of Li vacancies. Consequently, the calculated average potentials versus the number of Li ions are in good agreement with experiment.
Structural, electronic and mechanical properties of rare earth nitride-ErN: A first principles study
NASA Astrophysics Data System (ADS)
Murugan, A.; Rajeswarapalanichamy, R.; Santhosh, M.; Priyanga, G. Sudha; Kanagaprabha, S.; Iyakutti, K.
2015-06-01
The structural, electronic and mechanical properties of rare earth nitride ErN is investigated by the first principles calculations based on density functional theory using the Vienna ab-initio simulation package. At ambient pressure ErN is stable in the ferromagnetic state with NaCl structure. The calculated lattice parameters are in good agreement with the available results. The electronic structure reveals that ErN is half metallic at normal pressure. A pressure-induced structural phase transition from NaCl (B1) to CsCl (B2) phase is observed in ErN. Ferromagnetic to non magnetic phase transition is predicted in ErN at high pressure.