Synchrotron-based far-infrared spectroscopy of nickel tungstate
NASA Astrophysics Data System (ADS)
Kalinko, A.; Kuzmin, A.; Roy, P.; Evarestov, R. A.
2016-07-01
Monoclinic antiferromagnetic NiWO4 was studied by far-infrared (30-600 cm-1) absorption spectroscopy in the temperature range of 5-300 K using the synchrotron radiation from SOLEIL source. Two isomorphous CoWO4 and ZnWO4 tungstates were investigated for comparison. The phonon contributions in the far-infrared range of tungstates were interpreted using the first-principles spin-polarized linear combination of atomic orbital calculations. No contributions from magnetic excitations were found in NiWO4 and CoWO4 below their Neel temperatures down to 5 K.
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
Uranyl tungstate and zirconium tungstate in salt melts
Kryukova, A.I.; Bragina, R.A.; Kazantsev, G.N.; Korshunov, I.A.
1988-05-01
The article discusses the preparation, properties, and behavior of uranyl tungstate and zirconium tungstate in salt melts. Procedures for their preparation are presented. The radiographic and IR spectroscopic characteristics, the thermal stability, and solubility and stability in chloride-tungstate melts of different composition have been studied.
Description of charge conjugation from first principles
Lujan-Peschard, C.; Napsuciale, M.
2006-09-25
We construct the charge conjugation operator as a unitary automorphism in the spinor space ((1/2), 0) + (0 (1/2)) from first principles. We calculate its eigenspinors and derive the equation of motion they satisfy. The mapping associated to charge conjugation is constructed from parity eigenstates which are considered as particle and antiparticle.
Rediscovering First Principles through Online Learning.
ERIC Educational Resources Information Center
Kidney, Gary W.; Puckett, Edmond G.
2003-01-01
Describes an evaluation of Web-based instruction at the University of Houston-Clear Lake (Texas) that showed that the design team had been distracted from many first principles of instructional design by the creative chaos on the Web and discusses how self-reflection and role definitions allowed the team to overcome these disappointments and…
First principle thousand atom quantum dot calculations
Wang, Lin-Wang; Li, Jingbo
2004-03-30
A charge patching method and an idealized surface passivation are used to calculate the single electronic states of IV-IV, III-V, II-VI semiconductor quantum dots up to a thousand atoms. This approach scales linearly and has a 1000 fold speed-up compared to direct first principle methods with a cost of eigen energy error of about 20 meV. The calculated quantum dot band gaps are parametrized for future references.
Electron-phonon interactions from first principles
NASA Astrophysics Data System (ADS)
Giustino, Feliciano
2017-01-01
This article reviews the theory of electron-phonon interactions in solids from the point of view of ab initio calculations. While the electron-phonon interaction has been studied for almost a century, predictive nonempirical calculations have become feasible only during the past two decades. Today it is possible to calculate from first principles many materials properties related to the electron-phonon interaction, including the critical temperature of conventional superconductors, the carrier mobility in semiconductors, the temperature dependence of optical spectra in direct and indirect-gap semiconductors, the relaxation rates of photoexcited carriers, the electron mass renormalization in angle-resolved photoelectron spectra, and the nonadiabatic corrections to phonon dispersion relations. In this article a review of the theoretical and computational framework underlying modern electron-phonon calculations from first principles as well as landmark investigations of the electron-phonon interaction in real materials is given. The first part of the article summarizes the elementary theory of electron-phonon interactions and their calculations based on density-functional theory. The second part discusses a general field-theoretic formulation of the electron-phonon problem and establishes the connection with practical first-principles calculations. The third part reviews a number of recent investigations of electron-phonon interactions in the areas of vibrational spectroscopy, photoelectron spectroscopy, optical spectroscopy, transport, and superconductivity.
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
First principles studies on anatase surfaces
NASA Astrophysics Data System (ADS)
Selcuk, Sencer
TiO2 is one of the most widely studied metal oxides from both the fundamental and the technological points of view. A variety of applications have already been developed in the fields of energy production, environmental remediation, and electronics. Still, it is considered to have a high potential for further improvement and continues to be of great interest. This thesis describes our theoretical studies on the structural and electronic properties of anatase surfaces, and their (photo)chemical behavior. Recently much attention has been focused on anatase crystals synthesized by hydrofluoric acid assisted methods. These crystals exhibit a high percentage of {001} facets, generally considered to be highly reactive. We used first principles methods to investigate the structure of these facets, which is not yet well understood. Our results suggest that (001) surfaces exhibit the bulk-terminated structure when in contact with concentrated HF solutions. However, 1x4-reconstructed surfaces, as observed in UHV, become always more stable at the typical temperatures used to clean the as-prepared crystals in experiments. Since the reconstructed surfaces are only weakly reactive, we predict that synthetic anatase crystals with dominant {001} facets should not exhibit enhanced photocatalytic activity. Understanding how defects in solids interact with external electric fields is important for technological applications such as memristor devices. We studied the influence of an external electric field on the formation energies and diffusion barriers of the surface and the subsurface oxygen vacancies at the anatase (101) surface from first principles. Our results show that the applied field can have a significant influence on the relative stabilities of these defects, whereas the effect on the subsurface-to-surface defect migration is found to be relatively minor. Charge carriers play a key role in the transport properties and the surface chemistry of TiO2. Understanding their
Phonon-phonon interactions: First principles theory
Gibbons, T. M.; Bebek, M. B.; Kang, By.; Stanley, C. M.; Estreicher, S. K.
2015-08-28
We present the details of a method to perform molecular-dynamics (MD) simulations without thermostat and with very small temperature fluctuations ±ΔT starting with MD step 1. It involves preparing the supercell at the time t = 0 in physically correct microstates using the eigenvectors of the dynamical matrix. Each initial microstate corresponds to a different distribution of kinetic and potential energies for each vibrational mode (the total energy of each microstate is the same). Averaging the MD runs over many initial microstates further reduces ΔT. The electronic states are obtained using first-principles theory (density-functional theory in periodic supercells). Three applications are discussed: the lifetime and decay of vibrational excitations, the isotope dependence of thermal conductivities, and the flow of heat at an interface.
Primordial Black Holes from First Principles (Overview)
NASA Astrophysics Data System (ADS)
Lam, Casey; Bloomfield, Jolyon; Moss, Zander; Russell, Megan; Face, Stephen; Guth, Alan
2017-01-01
Given a power spectrum from inflation, our goal is to calculate, from first principles, the number density and mass spectrum of primordial black holes that form in the early universe. Previously, these have been calculated using the Press- Schechter formalism and some demonstrably dubious rules of thumb regarding predictions of black hole collapse. Instead, we use Monte Carlo integration methods to sample field configurations from a power spectrum combined with numerical relativity simulations to obtain a more accurate picture of primordial black hole formation. We demonstrate how this can be applied for both Gaussian perturbations and the more interesting (for primordial black holes) theory of hybrid inflation. One of the tools that we employ is a variant of the BBKS formalism for computing the statistics of density peaks in the early universe. We discuss the issue of overcounting due to subpeaks that can arise from this approach (the ``cloud-in-cloud'' problem). MIT UROP Office- Paul E. Gray (1954) Endowed Fund.
First-principles determination of magnetic properties
NASA Astrophysics Data System (ADS)
Wu, Ruqian; Yang, Zongxian; Hong, Jisang
2003-02-01
First-principles density functional theory calculations have achieved great success in the exciting field of low-dimension magnetism, in explaining new phenomena observed in experiments as well as in predicting novel properties and materials. As known, spin-orbit coupling (SOC) plays an extremely important role in various magnetic properties such as magnetic anisotropy, magnetostriction, magneto-optical effects and spin-dynamics. Using the full potential linearized augmented plane wave approach, we have carried out extensive investigations for the effects of SOC in various materials. Results of selected examples, such as structure and magnetic properties of Ni/Cu(001), magnetism and magnetic anisotropy in magnetic Co/Cu(001) thin films, wires and clusters, magnetostriction in FeGa alloys and magneto-optical effects in Fe/Cr superlattices, are discussed.
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.
Anisotropic Spin Hall Effect from First Principles
NASA Astrophysics Data System (ADS)
Freimuth, Frank; Blügel, Stefan; Mokrousov, Yuriy
2011-03-01
We present first principles calculations of the intrinsic non-dissipative spin Hall conductivity (SHC) for 3 d , 4 d and 5 d transition metals focusing in particular on the anisotropy of the SHC in nonmagnetic hcp metals and in antiferromagnetic Cr. For the metals of this study we generally find large anisotropies. We derive the general relation between the SHC vector and the direction of spin-polarization and discuss its consequences for hcp metals. Especially, it is predicted that for systems where the SHC changes sign due to the anisotropy the spin Hall effect may be tuned such that the spin polarization is parallel either to the electric field or to the spin current. Additionally, we describe our computational method [2,3] emphasizing the Wannier interpolation technique and the definition of the conserved spin current. This work is supported by the DFG Project MO 1731/3-1 and HGF-YIG grant VH-NG-513.
Intrinsic ferroelectric switching from first principles
NASA Astrophysics Data System (ADS)
Liu, Shi; Grinberg, Ilya; Rappe, Andrew M.
2016-06-01
The existence of domain walls, which separate regions of different polarization, can influence the dielectric, piezoelectric, pyroelectric and electronic properties of ferroelectric materials. In particular, domain-wall motion is crucial for polarization switching, which is characterized by the hysteresis loop that is a signature feature of ferroelectric materials. Experimentally, the observed dynamics of polarization switching and domain-wall motion are usually explained as the behaviour of an elastic interface pinned by a random potential that is generated by defects, which appear to be strongly sample-dependent and affected by various elastic, microstructural and other extrinsic effects. Theoretically, connecting the zero-kelvin, first-principles-based, microscopic quantities of a sample with finite-temperature, macroscopic properties such as the coercive field is critical for material design and device performance; and the lack of such a connection has prevented the use of techniques based on ab initio calculations for high-throughput computational materials discovery. Here we use molecular dynamics simulations of 90° domain walls (separating domains with orthogonal polarization directions) in the ferroelectric material PbTiO3 to provide microscopic insights that enable the construction of a simple, universal, nucleation-and-growth-based analytical model that quantifies the dynamics of many types of domain walls in various ferroelectrics. We then predict the temperature and frequency dependence of hysteresis loops and coercive fields at finite temperatures from first principles. We find that, even in the absence of defects, the intrinsic temperature and field dependence of the domain-wall velocity can be described with a nonlinear creep-like region and a depinning-like region. Our model enables quantitative estimation of coercive fields, which agree well with experimental results for ceramics and thin films. This agreement between model and experiment suggests
Intrinsic ferroelectric switching from first principles.
Liu, Shi; Grinberg, Ilya; Rappe, Andrew M
2016-06-16
The existence of domain walls, which separate regions of different polarization, can influence the dielectric, piezoelectric, pyroelectric and electronic properties of ferroelectric materials. In particular, domain-wall motion is crucial for polarization switching, which is characterized by the hysteresis loop that is a signature feature of ferroelectric materials. Experimentally, the observed dynamics of polarization switching and domain-wall motion are usually explained as the behaviour of an elastic interface pinned by a random potential that is generated by defects, which appear to be strongly sample-dependent and affected by various elastic, microstructural and other extrinsic effects. Theoretically, connecting the zero-kelvin, first-principles-based, microscopic quantities of a sample with finite-temperature, macroscopic properties such as the coercive field is critical for material design and device performance; and the lack of such a connection has prevented the use of techniques based on ab initio calculations for high-throughput computational materials discovery. Here we use molecular dynamics simulations of 90° domain walls (separating domains with orthogonal polarization directions) in the ferroelectric material PbTiO3 to provide microscopic insights that enable the construction of a simple, universal, nucleation-and-growth-based analytical model that quantifies the dynamics of many types of domain walls in various ferroelectrics. We then predict the temperature and frequency dependence of hysteresis loops and coercive fields at finite temperatures from first principles. We find that, even in the absence of defects, the intrinsic temperature and field dependence of the domain-wall velocity can be described with a nonlinear creep-like region and a depinning-like region. Our model enables quantitative estimation of coercive fields, which agree well with experimental results for ceramics and thin films. This agreement between model and experiment suggests
Dipole strength from first principles calculations
NASA Astrophysics Data System (ADS)
Miorelli, Mirko; Bacca, Sonia; Barnea, Nir; Hagen, Gaute; Jansen, Gustav R.; Papenbrock, Thomas; Orlandini, Giuseppina
2016-09-01
The electric dipole polarizability quantifies the low-energy behavior of the dipole strength. It is related to the proton and neutron distributions of the nucleus, and thereby can be used to constrain the neutron equation of state and the physics of neutron stars. Only recently however, new developments in ab initio methods finally allowed first principles studies of the dipole strength in medium-mass nuclei. Using the Lorentz integral transform coupled cluster method with the newly developed chiral interaction NNLOsat we study the low energy behavior of the dipole strength in 4He, 16O and 22O. For the exotic 22O we observe large contributions to the dipole strength at very low energy, indicating the presence of a pygmy dipole resonance, in agreement with what experimentally found by Leistenschneider et al.. We then study correlations between the electric dipole polarizability and the charge radius in 16O and 40Ca using a variety of realistic Hamiltonians, showing the importance of three-nucleon forces. We aknowledge NRC and NSERC.
High Pressure Hydrogen from First Principles
NASA Astrophysics Data System (ADS)
Morales, M. A.
2014-12-01
Typical approximations employed in first-principles simulations of high-pressure hydrogen involve the neglect of nuclear quantum effects (NQE) and the approximate treatment of electronic exchange and correlation, typically through a density functional theory (DFT) formulation. In this talk I'll present a detailed analysis of the influence of these approximations on the phase diagram of high-pressure hydrogen, with the goal of identifying the predictive capabilities of current methods and, at the same time, making accurate predictions in this important regime. We use a path integral formulation combined with density functional theory, which allows us to incorporate NQEs in a direct and controllable way. In addition, we use state-of-the-art quantum Monte Carlo calculations to benchmark the accuracy of more approximate mean-field electronic structure calculations based on DFT, and we use GW and hybrid DFT to calculate the optical properties of the solid and liquid phases near metallization. We present accurate predictions of the metal-insulator transition on the solid, including structural and optical properties of the molecular phase. This work was supported by the U.S. Department of Energy at the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and by LDRD Grant No. 13-LW-004.
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.
First principles model of carbonate compaction creep
NASA Astrophysics Data System (ADS)
Keszthelyi, Daniel; Dysthe, Dag Kristian; Jamtveit, Bjørn
2016-05-01
Rocks under compressional stress conditions are subject to long-term creep deformation. From first principles we develop a simple micromechanical model of creep in rocks under compressional stress that combines microscopic fracturing and pressure solution. This model was then upscaled by a statistical mechanical approach to predict strain rate at core and reservoir scale. The model uses no fitting parameter and has few input parameters: effective stress, temperature, water saturation porosity, and material parameters. Material parameters are porosity, pore size distribution, Young's modulus, interfacial energy of wet calcite, the dissolution, and precipitation rates of calcite, and the diffusion rate of calcium carbonate, all of which are independently measurable without performing any type of deformation or creep test. Existing long-term creep experiments were used to test the model which successfully predicts the magnitude of the resulting strain rate under very different effective stress, temperature, and water saturation conditions. The model was used to predict the observed compaction of a producing chalk reservoir.
First-principles calculations of novel materials
NASA Astrophysics Data System (ADS)
Sun, Jifeng
Computational material simulation is becoming more and more important as a branch of material science. Depending on the scale of the systems, there are many simulation methods, i.e. first-principles calculation (or ab-initio), molecular dynamics, mesoscale methods and continuum methods. Among them, first-principles calculation, which involves density functional theory (DFT) and based on quantum mechanics, has become to be a reliable tool in condensed matter physics. DFT is a single-electron approximation in solving the many-body problems. Intrinsically speaking, both DFT and ab-initio belong to the first-principles calculation since the theoretical background of ab-initio is Hartree-Fock (HF) approximation and both are aimed at solving the Schrodinger equation of the many-body system using the self-consistent field (SCF) method and calculating the ground state properties. The difference is that DFT introduces parameters either from experiments or from other molecular dynamic (MD) calculations to approximate the expressions of the exchange-correlation terms. The exchange term is accurately calculated but the correlation term is neglected in HF. In this dissertation, DFT based first-principles calculations were performed for all the novel materials and interesting materials introduced. Specifically, the DFT theory together with the rationale behind related properties (e.g. electronic, optical, defect, thermoelectric, magnetic) are introduced in Chapter 2. Starting from Chapter 3 to Chapter 5, several representative materials were studied. In particular, a new semiconducting oxytelluride, Ba2TeO is studied in Chapter 3. Our calculations indicate a direct semiconducting character with a band gap value of 2.43 eV, which agrees well with the optical experiment (˜ 2.93 eV). Moreover, the optical and defects properties of Ba2TeO are also systematically investigated with a view to understanding its potential as an optoelectronic or transparent conducting material. We find
First Principles Quantitative Modeling of Molecular Devices
NASA Astrophysics Data System (ADS)
Ning, Zhanyu
In this thesis, we report theoretical investigations of nonlinear and nonequilibrium quantum electronic transport properties of molecular transport junctions from atomistic first principles. The aim is to seek not only qualitative but also quantitative understanding of the corresponding experimental data. At present, the challenges to quantitative theoretical work in molecular electronics include two most important questions: (i) what is the proper atomic model for the experimental devices? (ii) how to accurately determine quantum transport properties without any phenomenological parameters? Our research is centered on these questions. We have systematically calculated atomic structures of the molecular transport junctions by performing total energy structural relaxation using density functional theory (DFT). Our quantum transport calculations were carried out by implementing DFT within the framework of Keldysh non-equilibrium Green's functions (NEGF). The calculated data are directly compared with the corresponding experimental measurements. Our general conclusion is that quantitative comparison with experimental data can be made if the device contacts are correctly determined. We calculated properties of nonequilibrium spin injection from Ni contacts to octane-thiolate films which form a molecular spintronic system. The first principles results allow us to establish a clear physical picture of how spins are injected from the Ni contacts through the Ni-molecule linkage to the molecule, why tunnel magnetoresistance is rapidly reduced by the applied bias in an asymmetric manner, and to what extent ab initio transport theory can make quantitative comparisons to the corresponding experimental data. We found that extremely careful sampling of the two-dimensional Brillouin zone of the Ni surface is crucial for accurate results in such a spintronic system. We investigated the role of contact formation and its resulting structures to quantum transport in several molecular
Transversity from First Principles in QCD
Brodsky, Stanley J.; /SLAC /Southern Denmark U., CP3-Origins
2012-02-16
Transversity observables, such as the T-odd Sivers single-spin asymmetry measured in deep inelastic lepton scattering on polarized protons and the distributions which are measured in deeply virtual Compton scattering, provide important constraints on the fundamental quark and gluon structure of the proton. In this talk I discuss the challenge of computing these observables from first principles; i.e.; quantum chromodynamics, itself. A key step is the determination of the frame-independent light-front wavefunctions (LFWFs) of hadrons - the QCD eigensolutions which are analogs of the Schroedinger wavefunctions of atomic physics. The lensing effects of initial-state and final-state interactions, acting on LFWFs with different orbital angular momentum, lead to T-odd transversity observables such as the Sivers, Collins, and Boer-Mulders distributions. The lensing effect also leads to leading-twist phenomena which break leading-twist factorization such as the breakdown of the Lam-Tung relation in Drell-Yan reactions. A similar rescattering mechanism also leads to diffractive deep inelastic scattering, as well as nuclear shadowing and non-universal antishadowing. It is thus important to distinguish 'static' structure functions, the probability distributions computed the target hadron's light-front wavefunctions, versus 'dynamical' structure functions which include the effects of initial- and final-state rescattering. I also discuss related effects such as the J = 0 fixed pole contribution which appears in the real part of the virtual Compton amplitude. AdS/QCD, together with 'Light-Front Holography', provides a simple Lorentz-invariant color-confining approximation to QCD which is successful in accounting for light-quark meson and baryon spectroscopy as well as hadronic LFWFs.
Electronic absorption spectra from first principles
NASA Astrophysics Data System (ADS)
Hazra, Anirban
Methods for simulating electronic absorption spectra of molecules from first principles (i.e., without any experimental input, using quantum mechanics) are developed and compared. The electronic excitation and photoelectron spectra of ethylene are simulated, using the EOM-CCSD method for the electronic structure calculations. The different approaches for simulating spectra are broadly of two types---Frank-Condon (FC) approaches and vibronic coupling approaches. For treating the vibrational motion, the former use the Born-Oppenheimer or single surface approximation while the latter do not. Moreover, in our FC approaches the vibrational Hamiltonian is additively separable along normal mode coordinates, while in vibronic approaches a model Hamiltonian (obtained from ab initio electronic structure theory) provides an intricate coupling between both normal modes and electronic states. A method called vertical FC is proposed, where in accord with the short-time picture of molecular spectroscopy, the approximate excited-state potential energy surface that is used to calculate the electronic spectrum is taken to reproduce the ab initio potential at the ground-state equilibrium geometry. The potential energy surface along normal modes may be treated either in the harmonic approximation or using the full one-dimensional potential. Systems with highly anharmonic potential surfaces can be treated and expensive geometry optimizations are not required, unlike the traditional FC approach. The ultraviolet spectrum of ethylene between 6.2 and 8.7 eV is simulated using vertical FC. While FC approaches for simulation are computationally very efficient, they are not accurate when the underlying approximations are unreasonable. Then, vibronic coupling model Hamiltonians are necessary. Since these Hamiltonians have an analytic form, they are used to map the potential energy surfaces and understand their topology. Spectra are obtained by numerical diagonalization of the Hamiltonians. The
First principles investigation of substituted strontium hexaferrite
NASA Astrophysics Data System (ADS)
Dixit, Vivek
This dissertation investigates how the magnetic properties of strontium hexaferrite change upon the substitution of foreign atoms at the Fe sites. Strontium hexaferrite, SrFe12O19, is a commonly used hard magnetic material and is produced in large quantities (around 500,000 tons per year). For different applications of strontium hexaferrite, its magnetic properties can be tuned by a proper substitution of the foreign atoms. Experimental screening for a proper substitution is a cost-intensive and time-consuming process, whereas computationally it can be done more efficiently. We used the 'density functional theory' a first principles based method to study substituted strontium hexaferrite. The site occupancies of the substituted atoms were estimated by calculating the substitution energies of different configurations. The formation probabilities of configurations were used to calculate the magnetic properties of substituted strontium hexaferrite. In the first study, Al-substituted strontium hexaferrite, SrFe12-x AlxO19 with x=0.5 and x=1.0 were investigated. It was found that at the annealing temperature the non-magnetic Al +3 ions preferentially replace Fe+3 ions from the 12 k and 2a sites. We found that the magnetization decreases and the magnetic anisotropy field increases as the fraction, x of the Al atoms increases. In the second study, SrFe12-xGaxO19 and SrFe12-xInxO19 with x=0.5 and x=1.0 were investigated. In the case of SrFe12-xGaxO19, the sites where Ga+3 ions prefer to enter are: 12 k, 2a, and 4f1. For SrFe12-xInxO19, In+3 ions most likely to occupy the 12k, 4f1 , and 4f2 sites. In both cases the magnetization was found to decrease slightly as the fraction of substituted atom increases. The magnetic anisotropy field increased for SrFe12-xGaxO 19, and decreased for SrFe12-xInxO19 as the concentration of substituted atoms increased. In the third study, 23 elements (M) were screened for their possible substitution in strontium hexaferrite, SrFe12-xMxO 19
First Principles Atomistic Model for Carbon-Doped Boron Suboxide
2014-09-01
First Principles Atomistic Model for Carbon-Doped Boron Suboxide by Amol B Rahane, Jennifer S Dunn, and Vijay Kumar ARL-TR-7106...2014 First Principles Atomistic Model for Carbon-Doped Boron Suboxide Amol B Rahane Dr Vijay Kumar Foundation 1969 Sector 4 Gurgaon...Final 3. DATES COVERED (From - To) October 2013–July 2014 4. TITLE AND SUBTITLE First Principles Atomistic Model for Carbon-Doped Boron Suboxide
Designing Interactive Learning Environments: An Approach from First Principles
ERIC Educational Resources Information Center
Scott, Bernard; Cong, Chunyu
2007-01-01
Purpose: Today's technology supports the design of more and more sophisticated interactive learning environments. This paper aims to argue that such design should develop from first principles. Design/methodology/approach: In the paper by first principles is meant: learning theory and principles of course design. These principles are briefly…
Materials Databases Infrastructure Constructed by First Principles Calculations: A Review
Lin, Lianshan
2015-10-13
The First Principles calculations, especially the calculation based on High-Throughput Density Functional Theory, have been widely accepted as the major tools in atom scale materials design. The emerging super computers, along with the powerful First Principles calculations, have accumulated hundreds of thousands of crystal and compound records. The exponential growing of computational materials information urges the development of the materials databases, which not only provide unlimited storage for the daily increasing data, but still keep the efficiency in data storage, management, query, presentation and manipulation. This review covers the most cutting edge materials databases in materials design, and their hot applications such as in fuel cells. By comparing the advantages and drawbacks of these high-throughput First Principles materials databases, the optimized computational framework can be identified to fit the needs of fuel cell applications. The further development of high-throughput DFT materials database, which in essence accelerates the materials innovation, is discussed in the summary as well.
Prediction on technetium triboride from first-principles calculations
NASA Astrophysics Data System (ADS)
Miao, Xiaojia; Xing, Wandong; Meng, Fanyan; Yu, Rong
2017-02-01
Taking the Tc-B binary system as an example, here we report the first-principles prediction on new phases of technetium borides, TcB3, which has an unprecedented stoichiometry. Crystal structures, phase stability, electronic properties and mechanical properties of TcB3 have been investigated using first-principles calculations. The hexagonal P 6 bar m 2 structure (No.187) TcB3 with a high value of hardness (29 GPa) is energetically stable against decomposition into other compounds under pressures above 4 GPa, indicating that TcB3 can be synthesized above this pressure.
Fermions in d = 1 + 2 dimensions from first principles
Carrillo-Ruiz, Ma. Georgina; Napsuciale, Mauro
2006-09-25
In this work we construct states describing planar electrons ('spin' (1/2) particles with well defined parity) in d = 1 + 2 from first principles and show that they satisfy Dirac equation, which turns out to be the covariant form of the eigenvalue equation for spatial inversion (parity) just like in d = 1 + 3.
Hybrid first-principles/neural networks model for column flotation
Gupta, S.; Liu, P.H.; Svoronos, S.A.; Sharma, R.; Abdel-Khalek, N.A.; Cheng, Y.; El-Shall, H.
1999-03-01
A new model for phosphate column flotation is presented which for the first time relates the effects of operating variables such as frother concentration on column performance. This is a hybrid model that combines a first-principles model with artificial neural networks. The first-principles model is obtained from material balances on both phosphate particles and gangue (undesired material containing mostly silica). First-order rates of net attachment are assumed for both. Artificial neural networks relate the attachment rate constants to the operating variables. Experiments were conducted in a 6-in.-dia. (152-mm-dia.) laboratory column to provide data for neural network training and model validation. The model successfully predicts the effects of frother concentration, particle size, air flow rate and bubble diameter on grade and recovery.
First-principles modeling of electrostatically doped perovskite systems.
Stengel, Massimiliano
2011-04-01
Macroscopically, confined electron gases at polar oxide interfaces are rationalized within the simple "polar catastrophe" model. At the microscopic level, however, many other effects such as electric fields, structural distortions and quantum-mechanical interactions enter into play. Here, we show how to bridge the gap between these two length scales, by combining the accuracy of first-principles methods with the conceptual simplicity of model Hamiltonian approaches. To demonstrate our strategy, we address the equilibrium distribution of the compensating free carriers at polar LaAlO(3)/SrTiO(3) interfaces. Remarkably, a model including only calculated bulk properties of SrTiO(3) and no adjustable parameters accurately reproduces our full first-principles results. Our strategy provides a unified description of charge compensation mechanisms in SrTiO(3)-based systems.
First-principles quantum chemistry in the life sciences.
van Mourik, Tanja
2004-12-15
The area of computational quantum chemistry, which applies the principles of quantum mechanics to molecular and condensed systems, has developed drastically over the last decades, due to both increased computer power and the efficient implementation of quantum chemical methods in readily available computer programs. Because of this, accurate computational techniques can now be applied to much larger systems than before, bringing the area of biochemistry within the scope of electronic-structure quantum chemical methods. The rapid pace of progress of quantum chemistry makes it a very exciting research field; calculations that are too computationally expensive today may be feasible in a few months' time! This article reviews the current application of 'first-principles' quantum chemistry in biochemical and life sciences research, and discusses its future potential. The current capability of first-principles quantum chemistry is illustrated in a brief examination of computational studies on neurotransmitters, helical peptides, and DNA complexes.
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.
Materials Databases Infrastructure Constructed by First Principles Calculations: A Review
Lin, Lianshan
2015-10-13
The First Principles calculations, especially the calculation based on High-Throughput Density Functional Theory, have been widely accepted as the major tools in atom scale materials design. The emerging super computers, along with the powerful First Principles calculations, have accumulated hundreds of thousands of crystal and compound records. The exponential growing of computational materials information urges the development of the materials databases, which not only provide unlimited storage for the daily increasing data, but still keep the efficiency in data storage, management, query, presentation and manipulation. This review covers the most cutting edge materials databases in materials design, and their hotmore » applications such as in fuel cells. By comparing the advantages and drawbacks of these high-throughput First Principles materials databases, the optimized computational framework can be identified to fit the needs of fuel cell applications. The further development of high-throughput DFT materials database, which in essence accelerates the materials innovation, is discussed in the summary as well.« less
Evolutionary approach for determining first-principles hamiltonians.
Hart, Gus L W; Blum, Volker; Walorski, Michael J; Zunger, Alex
2005-05-01
Modern condensed-matter theory from first principles is highly successful when applied to materials of given structure-type or restricted unit-cell size. But this approach is limited where large cells or searches over millions of structure types become necessary. To treat these with first-principles accuracy, one 'coarse-grains' the many-particle Schrodinger equation into 'model hamiltonians' whose variables are configurational order parameters (atomic positions, spin and so on), connected by a few 'interaction parameters' obtained from a microscopic theory. But to construct a truly quantitative model hamiltonian, one must know just which types of interaction parameters to use, from possibly 10(6)-10(8) alternative selections. Here we show how genetic algorithms, mimicking biological evolution ('survival of the fittest'), can be used to distil reliable model hamiltonian parameters from a database of first-principles calculations. We demonstrate this for a classic dilemma in solid-state physics, structural inorganic chemistry and metallurgy: how to predict the stable crystal structure of a compound given only its composition. The selection of leading parameters based on a genetic algorithm is general and easily applied to construct any other type of complex model hamiltonian from direct quantum-mechanical results.
Value of first principles and phenomenological modeling in mineral processing
Concha, F.
1995-12-31
There is confusion in naming the several models developed in Mineral Processing. The authors often hear of empirical, first principle, mechanistic and phenomenological models. The objective of this paper is to clarify and distinguish between these models, based on a philosophical and linguistic analysis. A state of the art review for mathematical modeling in Mineral Processing is also made. The advantage of considering Mineral Processing as a series of unit operations was recognized by Gaudin a long time ago. He divided the area into four unit operations: (1) comminution, (2) classification, (3) concentration and (4) dewatering.
Transition metal doped arsenene: A first-principles study
NASA Astrophysics Data System (ADS)
Sun, Minglei; Wang, Sake; Du, Yanhui; Yu, Jin; Tang, Wencheng
2016-12-01
Using first-principles calculations, we investigate the structural, electronic, and magnetic properties of 3d transition metal (TM) atoms substitutional doping of an arsenene monolayer. Based on the binding energy, the TM-substituted arsenene systems were found to be robust. Magnetic states were obtained for Ti, V, Cr, Mn and Fe doping. More importantly, a half-metallic state resulted from Ti and Mn doping, while the spin-polarized semiconducting state occurred with V, Cr and Fe doping. Our studies demonstrated the potential applications of TM-substituted arsenene for spintronics and magnetic storage devices.
First-principles study of Frenkel pair recombination in tungsten
NASA Astrophysics Data System (ADS)
Qin, Shi-Yao; Jin, Shuo; Li, Yu-Hao; Zhou, Hong-Bo; Zhang, Ying; Lu, Guang-Hong
2017-02-01
The recombination of one Frenkel pair in tungsten has been investigated through first-principles simulation. Two different recombination types have been identified: instantaneous and thermally activated. The small recombination barriers for thermally activated recombination cases indicate that recombination can occur easily with a slightly increased temperature. For both of the two recombination types, recombination occurs through the self-interstitial atom moving towards the vacancy. The recombination process can be direct or through replacement sequences, depending on the vertical distance between the vacancy and the <1 1 1> line of self-interstitial atom pair.
First-principles study of transition metal carbides
NASA Astrophysics Data System (ADS)
Connétable, Damien
2016-12-01
This study investigates the physical properties of transition metal carbides compounds associated with the Nb-C, Ti-C, Mo-C and W-C alloys systems using first-principles calculations. The ground-state properties (lattice parameters, cohesive energies and magnetism) were analyzed and compared to the experimental and theoretical literature. The simulations are in excellent agreement with experimental findings concerning atomic positions and structures. Elastic properties, computed using a finite-differences approach, are then discussed in detail. To complete the work, their lattice dynamics properties (phonon spectra) were investigated. These results serve to establish that some structures, which are mechanically stable, are dynamically unstable.
Large impurity effects in rubrene crystals: First-principles calculations
Tsetseris, L.; Pantelides, Sokrates T.
2008-01-01
Carrier mobilities of rubrene films are among the highest values reported for any organic semiconductor. Here, we probe with first-principles calculations the sensitivity of rubrene crystals on impurities. We find that isolated oxygen impurities create distinct peaks in the electronic density of states consistent with observations of defect levels in rubrene and that increased O content changes the position and shape of rubrene energy bands significantly. We also establish a dual role of hydrogen as individual H species and H impurity pairs create and annihilate deep carrier traps, respectively. The results are relevant to the performance and reliability of rubrene-based devices.
Derivation of instanton rate theory from first principles
NASA Astrophysics Data System (ADS)
Richardson, Jeremy O.
2016-03-01
Instanton rate theory is used to study tunneling events in a wide range of systems including low-temperature chemical reactions. Despite many successful applications, the method has never been obtained from first principles, relying instead on the "Im F" premise. In this paper, the same expression for the rate of barrier penetration at finite temperature is rederived from quantum scattering theory [W. H. Miller, S. D. Schwartz, and J. W. Tromp, J. Chem. Phys. 79, 4889 (1983)] using a semiclassical Green's function formalism. This justifies the instanton approach and provides a route to deriving the rate of other processes.
Hardness of covalent and ionic crystals: first-principle calculations.
Simůnek, Antonín; Vackár, Jirí
2006-03-03
A new concept, the strength of bond, and a new form expressing the hardness of covalent and ionic crystals are presented. Hardness is expressed by means of quantities inherently coupled to the atomistic structure of matter, and, therefore, hardness can be determined by first-principles calculations. Good agreement between theory and experiment is observed in the range of 2 orders of magnitude. It is shown that a lower coordination number of atoms results in higher hardness, contrary to common opinion presented in general literature.
First-Principles Calculation of forces and phonons in solid
NASA Astrophysics Data System (ADS)
Ning, Zhenhua; Shelton, William
We have developed a multiple scattering theory approach to calculate Hellmann-Feynman forces and phonons via the calculation of the force constant and dynamical matrix. To demonstrate the accuracy and validity of our approach we compare with the ELK code, which is a full potential Linear Augmented Plane Wave (FLAPW) based method. As we will show our forces and phonon dispersion curves are in good agreement with the FLAPW code. This work lays the foundation for developing a first principles approach for calculation of phonons in substitutionally disordered materials.
Diffusion in thorium carbide: A first-principles study
NASA Astrophysics Data System (ADS)
Pérez Daroca, D.; Llois, A. M.; Mosca, H. O.
2015-12-01
The prediction of the behavior of Th compounds under irradiation is an important issue for the upcoming Generation-IV nuclear reactors. The study of self-diffusion and hetero-diffusion is a central key to fulfill this goal. As a first approach, we obtained, by means of first-principles methods, migration and activation energies of Th and C atoms self-diffusion and diffusion of He atoms in ThC. We also calculate diffusion coefficients as a function of temperature.
Error propagation in first-principles kinetic Monte Carlo simulation
NASA Astrophysics Data System (ADS)
Döpking, Sandra; Matera, Sebastian
2017-04-01
First-principles kinetic Monte Carlo models allow for the modeling of catalytic surfaces with predictive quality. This comes at the price of non-negligible errors induced by the underlying approximate density functional calculation. On the example of CO oxidation on RuO2(110), we demonstrate a novel, efficient approach to global sensitivity analysis, with which we address the error propagation in these multiscale models. We find, that we can still derive the most important atomistic factors for reactivity, albeit the errors in the simulation results are sizable. The presented approach might also be applied in the hierarchical model construction or computational catalyst screening.
First-principles study of blue silicate phosphors.
Ishida, M; Imanari, Y; Isobe, T; Kuze, S; Ezuhara, T; Umeda, T; Ohno, K; Miyazaki, S
2010-09-29
First-principles calculations were performed to investigate the optical property of blue silicate phosphor, CMS:Eu. The optical absorption property is discussed based on electronic band structure and density of states. Our calculation results indicate that hybridization of the wavefunction plays an important role for nonradiative migration of electrons and holes. The calculated optical absorption spectrum could reproduce the optical features of the experimental excitation spectrum. It is also demonstrated that a practical approach using computational materials screening is effective in phosphor materials development.
Thermoelastic properties of random alloys from first-principles theory
NASA Astrophysics Data System (ADS)
Huang, L.; Vitos, L.; Kwon, S. K.; Johansson, B.; Ahuja, R.
2006-03-01
We present a first-principles description of the temperature-dependent elastic constants in random alloys. The substitutional disorder is treated using the coherent potential approximation implemented within the frameworks of exact muffin-tin orbitals theory. The temperature effects are approximated as the sum of electronic and thermal expansion contributions. Calculations on pure Nb demonstrate that this approach correctly accounts for the main temperature dependence of cubic elastic constants. When extended to Nb-Zr solid solution, the theoretical results show good agreement with experiments at temperatures ≲300K .
First-Principles Simulations of Armchair-Edge Graphene Nanostrips.
NASA Astrophysics Data System (ADS)
Li, Junwen; Mintmire, John W.; Gunlycke, Daniel; White, Carter T.
2007-03-01
We have carried out a series of first-principles, local-density functional band structure calculations of finite-width graphene nanostrips with armchair edges. A simple nearest-neighbor tight-binding model predicts that the band structures of these materials should be directly related to those of zigzag single wall carbon nanotubes, with two-thirds of the structures being small gap semiconductors and one-third of the structures being zero gap systems. The band gap in the semiconducting strips would be expected to decrease monotonically with increasing strip width. In our first-principles results, we find that in addition to the zero gap systems becoming finite gap quasimetallic systems because of symmetry breaking (as in the single-walled nanotubes), we also find that the semiconducting strips split into two families with band gaps that deviate from the simple nearest-neighbor tight binding model. Within the framework of our computational results, we compare the band structures of graphene, single-walled nanotubes, graphene nanostrips, and other carbon nanostructures. This work was supported by the US Office of Naval Research and the DoD HPCMO CHSSI program, both directly and through the US Naval Research Laboratory.
First principles calculations of La2CuO4
NASA Astrophysics Data System (ADS)
Plamada, Andrei; Kozhevnikov, Anton; Haehner, Urs; Jiang, Mi; Staar, Peter; Maier, Thomas; Schulthess, Thomas
We use the DFT+DCA method for a high-end study of the electronic structure properties of La2CuO4. The parameters of a tight-binding model are created using the first-principles electronic structure calculations. The all-electron full-potential linearised augmented plane-wave method is used to solve the non-interacting band problem. Then the set of physically relevant Wannier functions is chosen as a basis for the underlying Hubbard model. The Wannier functions and the corresponding non-interacting Hamiltonian Hnm0 (k) are created using the well-established downfolding approach. The screened Coulomb interaction parameters Unm of the model are computed using the constrained random-phase approximation technique. The double counting term is assumed to be a constant multiplied by the identity operator in the correlated subspace and it is determined based on first-principles considerations. The resulting ab-initio parameterisation of the Hubbard model is solved within dynamical cluster approximation (DCA).
A high-resolution tungstate membrane label
Hainfeld, J.F.; Quaite, F.E. ); Lipka, J.J. )
1990-01-01
A new class of membrane labels was synthesized which contain a tungstate cluster (having 11 tungsten atoms) and an aliphatic organo-tin moiety with various chain lengths (C{sub 4}, C{sub 8}, C{sub 12}, C{sub 18}, C{sub 22}). These molecules were found to insert into synthetic phospholipid vesicles and biological membranes (human red blood cell membranes). The tungstate clusters can be individually visualized in the high resolution STEM or seen en mass in thin-sectioned labeled membranes in the CTEM. These new labels should provide a means for direct high-resolution imaging of lipid-phase systems.
Morphology Tuning of Strontium Tungstate Nanoparticles
Joseph, S.; George, T.; George, K. C.; Sunny, A. T.; Mathew, S.
2007-08-22
Strontium tungstate nanocrystals in two different morphologies are successfully synthesized by controlled precipitation in aqueous and in poly vinyl alcohol (PVA) medium. Structural characterizations are carried out by XRD and SEM. The average particle size calculated for the SrWO4 prepared in the two different solvents ranges 20-24 nm. The SEM pictures show that the surface morphologies of the SrWO4 nanoparticles in aqueous medium resemble mushroom and the SrWO4 nanoparticles in PVA medium resemble cauliflower. Investigations on the room temperature luminescent properties of the strontium tungstate nanoparticles prepared in aqueous and PVA medium shows strong emissions around 425 nm.
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
The Electron-Phonon Interaction from First Principles
NASA Astrophysics Data System (ADS)
Noffsinger, Jesse Dean
2011-12-01
In this thesis the ground state electronic properties, lattice dynamics, electron-phonon coupling and superconductivity of a variety materials are investigated from first principles. The first chapter provides an introduction to the material and concepts of this thesis as well as motivation for the work done herein. Additionally, an overview is given on the theoretical background governing the calculations of this work. This includes overviews of the topics of density functional theory, the pseudopotential approximation, density functional perturbation theory, and applications of these approaches to the calculations of superconductivity. In the second chapter the mechanics of actually performing calculations within the methodology of chapter one are explained. This is accomplished through a detailed description of the computer software EPW. This software has been developed to allow computationally efficient approaches for calculating the electron-phonon interaction. A description of the software package, the particular quantities which it calculates and example calculations are given. The following two chapters present the results of calculations regarding electron-phonon coupling and superconductivity in bulk carbon compounds. The occurrence or absence of superconductivity is found to be related in these compounds to Fermi surface nesting and carrier concentrations. In chapter five we investigate the role of the fluorine dopant in the recently discovered (1111) Fe-pnictide superconductors. Contrary to the results of the literature published shortly after the discovery of these compounds, the presence of the dopant is found to actually result in a net decrease in the electron concentration on the Fe-plane within the local density approximation to density functional theory. In the two chapters which follow, we investigate the limits of two dimensional superconductivity in the recent experiments on ultra-thin Pb samples. Chapter six details calculations on
First principles nuclear magnetic resonance signatures of graphene oxide.
Lu, Ning; Huang, Ying; Li, Hai-bei; Li, Zhenyu; Yang, Jinlong
2010-07-21
Nuclear magnetic resonance (NMR) has been widely used in graphene oxide (GO) structure studies. However, the detailed relationship between its spectroscopic features and the GO structural configuration remains elusive. Based on first principles (13)C chemical shift calculations using the gauge including projector augmented waves method, we provide a reliable spectrum-structure connection. The (13)C chemical shift in GO is found to be very sensitive to the atomic environment, even for the same type of oxidation groups. Factors determining the chemical shifts of epoxy and hydroxy groups have been discussed. GO structures previously reported in the literature have been checked from the NMR point of view. The energetically favorable hydroxy chain structure is not expected to be widely existed in real GO samples according to our NMR simulations. The epoxy pair species we proposed previously is also supported by chemical shift calculations.
First principles nuclear magnetic resonance signatures of graphene oxide
NASA Astrophysics Data System (ADS)
Lu, Ning; Huang, Ying; Li, Hai-bei; Li, Zhenyu; Yang, Jinlong
2010-07-01
Nuclear magnetic resonance (NMR) has been widely used in graphene oxide (GO) structure studies. However, the detailed relationship between its spectroscopic features and the GO structural configuration remains elusive. Based on first principles C13 chemical shift calculations using the gauge including projector augmented waves method, we provide a reliable spectrum-structure connection. The C13 chemical shift in GO is found to be very sensitive to the atomic environment, even for the same type of oxidation groups. Factors determining the chemical shifts of epoxy and hydroxy groups have been discussed. GO structures previously reported in the literature have been checked from the NMR point of view. The energetically favorable hydroxy chain structure is not expected to be widely existed in real GO samples according to our NMR simulations. The epoxy pair species we proposed previously is also supported by chemical shift calculations.
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.
Free-Carrier Absorption in Silicon from First Principles
NASA Astrophysics Data System (ADS)
Shi, Guangsha; Kioupakis, Emmanouil
The absorption of light by free carriers in semiconductors such as silicon results in intraband electron or hole excitations, and competes with optical transitions across the band gap. Free-carrier absorption therefore reduces the efficiency of optoelectronic devices such as solar cells because it competes with the generation of electron-hole pairs. In this work, we use first-principles calculations based on density functional theory to investigate direct and phonon-assisted free-carrier absorption in silicon. We determine the free-carrier absorption coefficient as a function of carrier concentration and temperature and compare to experiment. We also identify the dominant phonon modes that contributing to phonon-assisted free-carrier absorption processes, and analyze the results to evaluate the impact of this loss mechanism on the efficiency of silicon solar cells. This research was supported by the National Science Foundation CAREER award through Grant No. DMR-1254314. Computational resources were provided by the DOE NERSC facility.
First principle study of manganese doped cadmium sulphide sheet
Kumar, Sanjeev; Kumar, Ashok; Ahluwalia, P. K.
2014-04-24
First-principle electronic structure calculations for cadmium sulphide (CdS) sheet in hexagonal phase, with Manganese substitution and addition, as well as including the Cd defects, are investigated. The lattice constants calculated for CdS sheet agrees fairly well with results reported for thin films experimentally. The calculations of total spin density of states and partial density of states in different cases shows substantial magnetic dipole moments acquired by the sheet. A magnetic dipole moment 5.00612 μ{sub B} and band gap of the order 1 eV are found when cadmium atom is replaced by Manganese. The magnetism acquired by the sheet makes it functionally important candidate in many applications.
Carbon-rich icosahedral boron carbide designed from first principles
Jay, Antoine; Vast, Nathalie; Sjakste, Jelena; Duparc, Olivier Hardouin
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
McAllister, Andrew; Åberg, Daniel; Schleife, André; Kioupakis, Emmanouil
2015-04-06
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.
First-principles modeling hydrogenation of bilayered boron nitride
NASA Astrophysics Data System (ADS)
Jing, Wang; Peng, Zhang; Xiang-Mei, Duan
2016-05-01
We have investigated the structural and electronic characteristics of hydrogenated boron-nitride bilayer (H-BNBN-H) using first-principles calculations. The results show that hydrogenation can significantly reduce the energy gap of the BN-BN into the visible-light region. Interestingly, the electric field induced by the interface dipoles helps to promote the formation of well-separated electron-hole pairs, as demonstrated by the charge distribution of the VBM and CBM. Moreover, the applied bias voltage on the vertical direction of the bilayer could modulate the band gap, resulting in transition from semiconductor to metal. We conclude that H-BNBN-H could improve the solar energy conversion efficiency, which may provide a new way for tuning the electronic devices to meet different environments and demands. Project supported by the National Natural Science Foundation of China (Grant No. 11574167).
Point defects in thorium nitride: A first-principles study
NASA Astrophysics Data System (ADS)
Pérez Daroca, D.; Llois, A. M.; Mosca, H. O.
2016-11-01
Thorium and its compounds (carbides and nitrides) are being investigated as possible materials to be used as nuclear fuels for Generation-IV reactors. As a first step in the research of these materials under irradiation, we study the formation energies and stability of point defects in thorium nitride by means of first-principles calculations within the framework of density functional theory. We focus on vacancies, interstitials, Frenkel pairs and Schottky defects. We found that N and Th vacancies have almost the same formation energy and that the most energetically favorable defects of all studied in this work are N interstitials. These kind of results for ThN, to the best authors' knowledge, have not been obtained previously, neither experimentally, nor theoretically.
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.
Stability of hydrogenated graphene: a first-principles study
Yi, Ding; Yang, Liu; Xie, Shijie; ...
2015-02-10
In order to explain the disagreement between present theoretical and experimental investigations on the stability of hydrogenated graphene, we have systematically studied hydrogenated graphene with different configurations from the consideration of single-side and double-side adsorption using first-principles calculations. Both binding energy and formation energy are calculated to characterize the stability of the system. It is found that single-side hydrogenated graphene is always unstable. However, for double-side hydrogenation, some configurations are stable due to the increased carbon–carbon sp3 hybridization compared to single-side hydrogenation. Furthermore, it is found that the system is energetically favorable when an equal number of hydrogen atoms aremore » adsorbed on each side of the graphene.« less
First-principles studies of native defects in olivine phosphates
NASA Astrophysics Data System (ADS)
Hoang, Khang; Johannes, Michelle
2011-03-01
Olivine phosphates Li M PO4 (M = Mn, Fe, Co, Ni) are promising candidates for rechargeable Li-ion battery electrodes because of their energy storage capacity and electrochemical and thermal stability. It is known that native defects have strong effects on the performance of olivine phosphates. Yet, the formation and migration of these defects are not fully understood, and we expect that once such understanding has been established, one can envisage a solution for improving the materials' performance. In this talk, we present our first-principles density-functional theory studies of native point defects and defect complexes in Li M PO4 , and discuss the implications of these defects on the performance of the materials. Our results also provide guidelines for obtaining different native defects in experiments.
Electronic Stopping Power in LiF from First Principles
Pruneda, J. M.; Sanchez-Portal, D.; Artacho, Emilio
2007-12-07
Using time-dependent density-functional theory we calculate from first principles the rate of energy transfer from a moving proton or antiproton to the electrons of an insulating material, LiF. The behavior of the electronic stopping power versus projectile velocity displays an effective threshold velocity of {approx}0.2 a.u. for the proton, consistent with recent experimental observations, and also for the antiproton. The calculated proton/antiproton stopping-power ratio is {approx}2.4 at velocities slightly above the threshold (v{approx}0.4 a.u.), as compared to the experimental value of 2.1. The projectile energy loss mechanism is observed to be extremely local.
First-principles study of optical excitations in alphaquartz
Chang, Eric K.; Rohlfing, Michael; Louie, Steven G.
1999-06-15
The properties of silicon dioxide have been studied extensively over the years. However, there still remain major unanswered questions regarding the nature of the optical spectrum and the role of excitonic effects in this technologically important material. In this work, we present an ab initio study of the optical absorption spectrum of alpha-quartz, using a newly developed first-principles method which includes self-energy and electron-hole interaction effects. The quasiparticle band structure is computed within the GW approximation to obtain a quantitative description of the single-particle excitations. The Bethe-Salpeter equation for the electron-hole excitations is solved to obtain the optical spectrum and to understand the spatial extent and physical properties of the excitons. The theoretical absorption spectrum is found to be in excellent agreement with the measured spectrum. We show that excitonic effects are crucial in the frequency range up to 5 eV above the absorption threshold.
Electromagnetic response of C12 : A first-principles calculation
Lovato, A.; Gandolfi, S.; Carlson, J.; ...
2016-08-15
Here, the longitudinal and transverse electromagnetic response functions ofmore » $$^{12}$$C are computed in a ``first-principles'' Green's function Monte Carlo calculation, based on realistic two- and three-nucleon interactions and associated one- and two-body currents. We find excellent agreement between theory and experiment and, in particular, no evidence for the quenching of measured versus calculated longitudinal response. This is further corroborated by a re-analysis of the Coulomb sum rule, in which the contributions from the low-lying $$J^\\pi\\,$$=$$\\, 2^+$$, $0^+$ (Hoyle), and $4^+$ states in $$^{12}$$C are accounted for explicitly in evaluating the total inelastic strength.« less
Vibrational and thermophysical properties of PETN from first principles
NASA Astrophysics Data System (ADS)
Gonzalez, Joseph M.; Landerville, Aaron C.; Oleynik, Ivan I.
2017-01-01
Thermophysical properties are urgently sought as input for meso- and continuum-scale modeling of energetic materials (EMs). However, experimental data are often limited as they cover a narrow region of specific pressures and temperatures. Such modeling of EMs can be greatly improved by inclusion of thermophysical properties over a wide range of pressures and temperatures, provided such data could be reliably obtained from theory. We demonstrate such a capability by calculating the PVT equation of state, heat capacities, and coefficients of thermal expansion for pentaerythritol tetranitrate (PETN) using first-principles density functional theory, which includes proper description of van der Waals interactions, zero-point energy and thermal contributions to free energy calculated using the quasi-harmonic approximation. Further, we investigate the evolution of the vibration spectrum of PETN as a function of pressure.
Thermodynamics of Magnetic Systems from First Principles: WL-LSMS
Eisenbach, Markus; Zhou, Chenggang; Nicholson, Don M; Brown, Greg; Larkin, Jeffrey M; Schulthess, Thomas C
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.
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.
First-principles study of high explosive decomposition energetics
Wu, C J
1998-08-21
The mechanism of the gas phase unimolecular decomposition of hexahydro-1,3,5,- trinitro- 1,3,5,-triazine (RDX) has been investigated using first principles gradient corrected density functional theory. Our results show that the dominant reaction channel is the N-NO* bond rupture, which has a barrier of 34.2 kcal/mol at the B- PW9 l/cc-pVDZ level and is 18.3 kcal/mol lower than that of the concerted ring fission to three methylenenitramine molecules. In addition, we have carried out a systematic study of homolytic bond dissociation energies of 14 other high explosives at the B-PW91/D95V level. We find that the correlation between the weakest bond strength and high explosive sensitivity is strong
First-principles study on superconductivity of solid oxygen
NASA Astrophysics Data System (ADS)
Ishikawa, Takahiro; Mukai, Kenta; Shimizu, Katsuya
2012-12-01
The superconductivity of solid oxygen in ζ phase was investigated by first-principles calculations based on the density functional theory. Using a monoclinic C2/m structure, we calculated the superconducting transition temperature by the Allen-Dynes formula and obtained 2.4 K at 100 GPa for the effective screened Coulomb repulsion constant μ* of 0.13. The transition temperature slowly decreases with increasing pressure and becomes 1.3 K at 200 GPa. The phonon analysis shows that the electron-phonon coupling is dominantly enhanced by the intermolecular vibrations of O2 rather than the intramolecular ones. The phonon modes showing the strong electron-phonon coupling were found to be concentrated in the phonon frequency range of 100-150 cm-1 at around the M-point in the Brillouin zone.
First-principles prediction of disordering tendencies in pyrochlore oxides
NASA Astrophysics Data System (ADS)
Jiang, Chao; Stanek, C. R.; Sickafus, K. E.; Uberuaga, B. P.
2009-03-01
Using first-principles calculations, we systematically predict the order-disorder energetics of series of zirconate (A2Zr2O7) , hafnate (A2Hf2O7) , titanate (A2Ti2O7) , and stannate (A2Sn2O7) pyrochlores. The disordered defect-fluorite structure is modeled using an 88-atom two-sublattice special quasirandom structure (SQS) that closely reproduces the most relevant near-neighbor intrasublattice and intersublattice pair-correlation functions of the random mixture. The order-disorder transition temperatures of these pyrochlores estimated from our SQS calculations show overall good agreement with existing experiments. We confirm previous studies suggesting that the bonding in pyrochlores is not purely ionic and thus electronic effects also play a role in determining their disordering tendencies. Our results have important consequences for numerous applications, including nuclear waste forms and fast ion conductors.
Stability of hydrogenated graphene: a first-principles study
Yi, Ding; Yang, Liu; Xie, Shijie; Saxena, Avadh
2015-02-10
In order to explain the disagreement between present theoretical and experimental investigations on the stability of hydrogenated graphene, we have systematically studied hydrogenated graphene with different configurations from the consideration of single-side and double-side adsorption using first-principles calculations. Both binding energy and formation energy are calculated to characterize the stability of the system. It is found that single-side hydrogenated graphene is always unstable. However, for double-side hydrogenation, some configurations are stable due to the increased carbon–carbon sp^{3} hybridization compared to single-side hydrogenation. Furthermore, it is found that the system is energetically favorable when an equal number of hydrogen atoms are adsorbed on each side of the graphene.
First-principles study of point defects in thorium carbide
NASA Astrophysics Data System (ADS)
Pérez Daroca, D.; Jaroszewicz, S.; Llois, A. M.; Mosca, H. O.
2014-11-01
Thorium-based materials are currently being investigated in relation with their potential utilization in Generation-IV reactors as nuclear fuels. One of the most important issues to be studied is their behavior under irradiation. A first approach to this goal is the study of point defects. By means of first-principles calculations within the framework of density functional theory, we study the stability and formation energies of vacancies, interstitials and Frenkel pairs in thorium carbide. We find that C isolated vacancies are the most likely defects, while C interstitials are energetically favored as compared to Th ones. These kind of results for ThC, to the best authors' knowledge, have not been obtained previously, neither experimentally, nor theoretically. For this reason, we compare with results on other compounds with the same NaCl-type structure.
Photoelectron Spectra of Aqueous Solutions from First Principles
Gaiduk, Alex P.; Govoni, Marco; Seidel, Robert; Skone, Jonathan H.; Winter, Bernd; Galli, Giulia
2016-06-08
We present a combined computational and experimental study of the photoelectron spectrum of a simple aqueous solution of NaCl. Measurements were conducted on microjets, and first-principles calculations were performed using hybrid functionals and many-body perturbation theory at the G0W0 level, starting with wave functions computed in ab initio molecular dynamics simulations. We show excellent agreement between theory and experiments for the positions of both the solute and solvent excitation energies on an absolute energy scale and for peak intensities. The best comparison was obtained using wave functions obtained with dielectric-dependent self-consistent and range-separated hybrid functionals. Our computational protocol opens the way to accurate, predictive calculations of the electronic properties of electrolytes, of interest to a variety of energy problems.
First-principles calculations of PuO(2+/-x).
Petit, L; Svane, A; Szotek, Z; Temmerman, W M
2003-07-25
The electronic structure of PuO(2+/-x) was studied using first-principles quantum mechanics, realized with the self-interaction corrected local spin density method. In the stoichiometric PuO2 compound, Pu occurs in the Pu(IV) oxidation state, corresponding to a localized f4 shell. If oxygen is introduced onto the octahedral interstitial site, the nearby Pu atoms turn into Pu(V) (f3) by transferring electrons to the oxygen. Oxygen vacancies cause Pu(III) (f5) to form by taking up electrons released by oxygen. At T = 0, the PuO2 compound is stable with respect to free oxygen, but the delicate energy balance suggests the possible deterioration of the material during long-term storage.
Accurate first principles model potentials for intermolecular interactions.
Gordon, Mark S; Smith, Quentin A; Xu, Peng; Slipchenko, Lyudmila V
2013-01-01
The general effective fragment potential (EFP) method provides model potentials for any molecule that is derived from first principles, with no empirically fitted parameters. The EFP method has been interfaced with most currently used ab initio single-reference and multireference quantum mechanics (QM) methods, ranging from Hartree-Fock and coupled cluster theory to multireference perturbation theory. The most recent innovations in the EFP model have been to make the computationally expensive charge transfer term much more efficient and to interface the general EFP dispersion and exchange repulsion interactions with QM methods. Following a summary of the method and its implementation in generally available computer programs, these most recent new developments are discussed.
First principle study of manganese doped cadmium sulphide sheet
NASA Astrophysics Data System (ADS)
Kumar, Sanjeev; Kumar, Ashok; Ahluwalia, P. K.
2014-04-01
First-principle electronic structure calculations for cadmium sulphide (CdS) sheet in hexagonal phase, with Manganese substitution and addition, as well as including the Cd defects, are investigated. The lattice constants calculated for CdS sheet agrees fairly well with results reported for thin films experimentally. The calculations of total spin density of states and partial density of states in different cases shows substantial magnetic dipole moments acquired by the sheet. A magnetic dipole moment 5.00612 μB and band gap of the order 1 eV are found when cadmium atom is replaced by Manganese. The magnetism acquired by the sheet makes it functionally important candidate in many applications.
Elastic and piezoresistive properties of nickel carbides from first principles
NASA Astrophysics Data System (ADS)
Kelling, Jeffrey; Zahn, Peter; Schuster, Jörg; Gemming, Sibylle
2017-01-01
The nickel-carbon system has received increased attention over the past years due to the relevance of nickel as a catalyst for carbon nanotube and graphene growth, where nickel carbide intermediates may be involved or carbide interface layers form in the end. Nickel-carbon composite thin films comprising Ni3C are especially interesting in mechanical sensing applications. Due to the metastability of nickel carbides, formation conditions and the coupling between mechanical and electrical properties are not yet well understood. Using first-principles electronic structure methods, we calculated the elastic properties of Ni3C ,Ni2C , and NiC , as well as changes in electronic properties under mechanical strain. We observe that the electronic density of states around the Fermi level does not change under the considered strains of up to 1%, which correspond to stresses up to 3 GPa . Relative changes in conductivity of Ni3C range up to maximum values of about 10%.
First-principles study of fluorination of L-Alanine
NASA Astrophysics Data System (ADS)
Sreepad, H. R.; Ravi, H. R.; Ahmed, Khaleel; Dayananda, H. M.; Umakanth, K.; Manohara, B. M.
2013-02-01
First-principles calculations based on Density Functional Theory have been done on effect of fluorination of an important amino acid - L-Alanine. Its structure has been simulated. The unit cell is orthorhombic with lattice parameters a=5.90Å, b=13.85Å and c=5.75Å with volume 470 (Å)3. Bond lengths and bond angles have been estimated. Electronic Density of States calculations show that the material has a band gap of 4.47eV. Electronic band structure indicates that the material can be effectively used for NLO applications. The electronic contribution to the dielectric constant has been calculated and its average value comes out to be 2.165.
Liquid-state paramagnetic relaxation from first principles
NASA Astrophysics Data System (ADS)
Rantaharju, Jyrki; Vaara, Juha
2016-10-01
We simulate nuclear and electron spin relaxation rates in a paramagnetic system from first principles. Sampling a molecular dynamics trajectory with quantum-chemical calculations produces a time series of the instantaneous parameters of the relevant spin Hamiltonian. The Hamiltonians are, in turn, used to numerically solve the Liouville-von Neumann equation for the time evolution of the spin density matrix. We demonstrate the approach by studying the aqueous solution of the Ni2 + ion. Taking advantage of Kubo's theory, the spin-lattice (T1) and spin-spin (T2) relaxation rates are extracted from the simulations of the time dependence of the longitudinal and transverse magnetization, respectively. Good agreement with the available experimental data is obtained by the method.
First-principles study of interface doping in ferroelectric junctions
Wang, Pin-Zhi; Cai, Tian-Yi; Ju, Sheng; Wu, Yin-Zhong
2016-01-01
Effect of atomic monolayer insertion on the performance of ferroelectric tunneling junction is investigated in SrRuO3/BaTiO3/SrRuO3 heterostrucutures. Based on first-principles calculations, the atomic displacement, orbital occupancy, and ferroelectric polarization are studied. It is found that the ferroelectricity is enhanced when a (AlO2)− monolayer is inserted between the electrode SRO and the barrier BTO, where the relatively high mobility of doped holes effectively screen ferroelectric polarization. On the other hand, for the case of (LaO)+ inserted layer, the doped electrons resides at the both sides of middle ferroelectric barrier, making the ferroelectricity unfavorable. Our findings provide an alternative avenue to improve the performance of ferroelectric tunneling junctions. PMID:27063704
First principles electrochemistry: Electrons and protons reacting as independent ions
NASA Astrophysics Data System (ADS)
Llano, Jorge; Eriksson, Leif A.
2002-12-01
We here present a first principles approach to calculate standard Gibbs energies and the corresponding observables (standard electrode potentials in the hydrogen scale ESHE0 and pKa values) of stoichiometric reactions involving electrons and/or protons as independent species in solution, from absolute electrochemical potentials defined according to quantum and statistical mechanics. In order to pass from the conventional electrodic and thermodynamic descriptions of electrochemistry to the first principles approach based on estimating absolute electrochemical potentials, we revisit the problem of the absolute and relative electrochemical scales from the macroscopic and microscopic viewpoints. A microscopic definition of the absolute electrochemical potential is presented in order to enable an identical thermodynamic treatment of any species in a given phase, i.e., electrons, protons, atoms, molecules, atomic and molecular ions, and electronically excited species. We show that absolute standard chemical potentials in the mole fraction scale can be easily computed with wave function and density functional theories in conjunction with self-consistent reaction field models. Based on Boltzmann and Fermi-Dirac statistics and experimental solvation data, we estimate an internally compatible set of absolute standard chemical and electrochemical potentials of protons and solvated electrons in the molality and molarity scales in aqueous solution at 298 K and 1 atm, within an absolute error of ±0.5 kcal/mol. This scheme enables a consistent and simultaneous description of the Gibbs energy changes and the observables (ESHE0 and pKa 's) of electron, proton, and proton-coupled electron transfer reactions in aqueous solution at 298 K and 1 atm.
Emergent symmetries in atomic nuclei from first principles
NASA Astrophysics Data System (ADS)
Launey, K. D.; Dreyfuss, A. C.; Baker, R. B.; Draayer, J. P.; Dytrych, T.
2015-04-01
An innovative symmetry-guided approach and its applications to light and intermediate-mass nuclei is discussed. This approach, with Sp(3, R) the underpinning group, is based on our recent remarkable finding, namely, we have identified the symplectic Sp(3,R) as an approximate symmetry for low-energy nuclear dynamics. This study presents the results of two complementary studies, one that utilizes realistic nucleon-nucleon interactions and unveils symmetries inherent to nuclear dynamics from first principles (or ab initio), and another study, which selects important components of the nuclear interaction to explain the primary physics responsible for emergent phenomena, such as enhanced collectivity and alpha clusters. In particular, within this symmetry-guided framework, ab initio applications of the theory to light nuclei reveal the emergence of a simple orderly pattern from first principles. This provides a strategy for determining the nature of bound states of nuclei in terms of a relatively small fraction of the complete shell-model space, which, in turn, can be used to explore ultra-large model spaces for a description of alpha-cluster and highly deformed structures together with associated rotations. We find that by using only a fraction of the model space extended far beyond current no-core shell-model limits and a long-range interaction that respects the symmetries in play, the outcome reproduces characteristic features of the low-lying 0+ states in 12C (including the elusive Hoyle state of importance to astrophysics) and agrees with ab initio results in smaller spaces. For these states, we offer a novel perspective emerging out of no-core shell-model considerations, including a discussion of associated nuclear deformation, matter radii, and density distribution. The framework we find is also extensible beyond 12C, namely, to the low-lying 0+ states of 8Be as well as the ground-state rotational band of Ne, Mg, and Si isotopes.
Fundamental limits on transparency: first-principles calculations of absorption
NASA Astrophysics Data System (ADS)
Peelaers, Hartwin
2013-03-01
Transparent conducting oxides (TCOs) are a technologically important class of materials with applications ranging from solar cells, displays, smart windows, and touch screens to light-emitting diodes. TCOs combine high conductivity, provided by a high concentration of electrons in the conduction band, with transparency in the visible region of the spectrum. The requirement of transparency is usually tied to the band gap being sufficiently large to prevent absorption of visible photons. This is a necessary but not sufficient condition: indeed, the high concentration of free carriers can also lead to optical absorption by excitation of electrons to higher conduction-band states. A fundamental understanding of the factors that limit transparency in TCOs is essential for further progress in materials and applications. The Drude theory is widely used, but it is phenomenological in nature and tends to work poorly at shorter wavelengths, where band-structure effects are important. First-principles calculations have been performed, but were limited to direct transitions; as we show in the present work, indirect transitions assisted by phonons or defects actually dominate. Our calculations are the first to address indirect free-carrier absorption in a TCO completely from first principles. We present results for SnO2, but the methodology is general and is also being applied to ZnO and In2O3. The calculations provide not just quantitative results but also deeper insights in the mechanisms that govern absorption processes in different wavelength regimes, which is essential for engineering improved materials to be used in more efficient devices. For SnO2, we find that absorption is modest in the visible, and much stronger in the ultraviolet and infrared. Work performed in collaboration with E. Kioupakis and C.G. Van de Walle, and supported by DOE, NSF, and BAEF.
Revising Intramolecular Photoinduced Electron Transfer (PET) from First-Principles.
Escudero, Daniel
2016-09-20
Photoinduced electron transfer (PET) plays relevant roles in many areas of chemistry, including charge separation processes in photovoltaics, natural and artificial photosynthesis, and photoluminescence sensors and switches. As in many other photochemical scenarios, the structural and energetic factors play relevant roles in determining the rates and efficiencies of PET and its competitive photodeactivation processes. Particularly, in the field of fluorescent sensors and switches, intramolecular PET is believed (in many cases without compelling experimental proof) to be responsible of the quench of fluorescence. There is an increasing experimental interest in fluorophore's molecular design and on achieving optimal excitation/emission spectra, excitation coefficients, and fluorescence quantum yields (importantly for bioimaging purposes), but less efforts are devoted to fundamental mechanistic studies. In this Account, I revise the origins of the fluorescence quenching in some of these systems with state-of-the-art quantum chemical tools. These studies go beyond the common strategy of analyzing frontier orbital energy diagrams and performing PET thermodynamics calculations. Instead, the potential energy surfaces (PESs) of the lowest-lying excited states are explored with time-dependent density functional theory (TD-DFT) and complete active space self-consistent field (CASSCF) calculations and the radiative and nonradiative decay rates from the involved excited states are computed from first-principles using a thermal vibration correlation function formalism. With such a strategy, this work reveals the real origins of the fluorescence quenching, herein entitled as dark-state quenching. Dark states (those that do not absorb or emit light) are often elusive to experiments and thus, computational investigations can provide novel insights into the actual photodeactivation mechanisms. The success of the dark-state quenching mechanism is demonstrated for a wide variety of
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
Auger recombination in scintillator materials from first principles
NASA Astrophysics Data System (ADS)
McAllister, Andrew; Kioupakis, Emmanouil; Åberg, Daniel; Schleife, André
2015-03-01
Scintillators convert high energy radiation into lower energy photons which are easier to detect and analyze. One of the uses of these devices is identifying radioactive materials being transported across national borders. However, scintillating materials have a non-proportional light yield in response to incident radiation, which makes this task difficult. One possible cause of the non-proportional light yield is non-radiative Auger recombination. Auger recombination can occur in two ways - direct and phonon-assisted. We have studied both types of Auger recombination from first principles in the common scintillating material sodium iodide. Our results indicate that the phonon-assisted process, assisted primarily by short-range optical phonons, dominates the direct process. The corresponding Auger coefficients are 5 . 6 +/- 0 . 3 ×10-32cm6s-1 for the phonon-assisted process versus 1 . 17 +/- 0 . 01 ×10-33cm6s-1 for the direct process. At higher electronic temperatures the direct Auger recombination rate increases but remains lower than the phonon-assisted rate. This research was supported by the National Science Foundation CAREER award through Grant No. DMR-1254314 and NA-22. Computational Resources provide by LLNL and DOE NERSC Facility.
"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.
Coarse graining approach to First principles modeling of structural materials
Odbadrakh, Khorgolkhuu; Nicholson, Don M; Rusanu, Aurelian; Samolyuk, German D; Wang, Yang; Stoller, Roger E; Zhang, X.-G.; Stocks, George Malcolm
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.
First principles molecular dynamics without self-consistent field optimization
Souvatzis, Petros; Niklasson, Anders M. N.
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.
Realtime capable first principle based modelling of tokamak turbulent transport
NASA Astrophysics Data System (ADS)
Citrin, Jonathan; Breton, Sarah; Felici, Federico; Imbeaux, Frederic; Redondo, Juan; Aniel, Thierry; Artaud, Jean-Francois; Baiocchi, Benedetta; Bourdelle, Clarisse; Camenen, Yann; Garcia, Jeronimo
2015-11-01
Transport in the tokamak core is dominated by turbulence driven by plasma microinstabilities. When calculating turbulent fluxes, maintaining both a first-principle-based model and computational tractability is a strong constraint. We present a pathway to circumvent this constraint by emulating quasilinear gyrokinetic transport code output through a nonlinear regression using multilayer perceptron neural networks. This recovers the original code output, while accelerating the computing time by five orders of magnitude, allowing realtime applications. A proof-of-principle is presented based on the QuaLiKiz quasilinear transport model, using a training set of five input dimensions, relevant for ITG turbulence. The model is implemented in the RAPTOR real-time capable tokamak simulator, and simulates a 300s ITER discharge in 10s. Progress in generalizing the emulation to include 12 input dimensions is presented. This opens up new possibilities for interpretation of present-day experiments, scenario preparation and open-loop optimization, realtime controller design, realtime discharge supervision, and closed-loop trajectory optimization.
First principles based mean field model for oxygen reduction reaction.
Jinnouchi, Ryosuke; Kodama, Kensaku; Hatanaka, Tatsuya; Morimoto, Yu
2011-12-21
A first principles-based mean field model was developed for the oxygen reduction reaction (ORR) taking account of the coverage- and material-dependent reversible potentials of the elementary steps. This model was applied to the simulation of single crystal surfaces of Pt, Pt alloy and Pt core-shell catalysts under Ar and O(2) atmospheres. The results are consistent with those shown by past experimental and theoretical studies on surface coverages under Ar atmosphere, the shape of the current-voltage curve for the ORR on Pt(111) and the material-dependence of the ORR activity. This model suggests that the oxygen associative pathway including HO(2)(ads) formation is the main pathway on Pt(111), and that the rate determining step (RDS) is the removal step of O(ads) on Pt(111). This RDS is accelerated on several highly active Pt alloys and core-shell surfaces, and this acceleration decreases the reaction intermediate O(ads). The increase in the partial pressure of O(2)(g) increases the surface coverage with O(ads) and OH(ads), and this coverage increase reduces the apparent reaction order with respect to the partial pressure to less than unity. This model shows details on how the reaction pathway, RDS, surface coverages, Tafel slope, reaction order and material-dependent activity are interrelated.
Predicted boron-carbide compounds: A first-principles study
Wang, De Yu; Yan, Qian; Wang, Bing; Wang, Yuan Xu Yang, Jueming; Yang, Gui
2014-06-14
By using developed particle swarm optimization algorithm on crystal structural prediction, we have explored the possible crystal structures of B-C system. Their structures, stability, elastic properties, electronic structure, and chemical bonding have been investigated by first-principles calculations with density functional theory. The results show that all the predicted structures are mechanically and dynamically stable. An analysis of calculated enthalpy with pressure indicates that increasing of boron content will increase the stability of boron carbides under low pressure. Moreover, the boron carbides with rich carbon content become more stable under high pressure. The negative formation energy of predicted B{sub 5}C indicates its high stability. The density of states of B{sub 5}C show that it is p-type semiconducting. The calculated theoretical Vickers hardnesses of B-C exceed 40 GPa except B{sub 4}C, BC, and BC{sub 4}, indicating they are potential superhard materials. An analysis of Debye temperature and electronic localization function provides further understanding chemical and physical properties of boron carbide.
Stress dependent defect energetics in Tungsten from first-principles
NASA Astrophysics Data System (ADS)
Hossain, Md.; Marian, Jaime
2013-03-01
Tungsten (W) is an important material for high temperature applications due to its refractory nature. However, like all transition metals from the VI-A group, W suffers from low-temperature brittleness and lack of ductility, which poses serious questions for its use as a structural material. Tungsten's mechanical properties can be enhanced by alloying with elements with d-electrons, such as Re, which has resulted in successful commercial alloys. In this work, we obtain the formation and migration energetics of Re solute atoms in terms of their interaction with vacancies and dislocations. To explore the influence of external stresses on Re transport properties, we examine the role of hydrostatic and shear deformation on the vacancy formation energy (VFE) and migration energy barrier (Em) in BCC W from first-principles calculations by developing a pseudopotential with 6s2, 6p0, 5d4, and 5f0 electronic states for the valence electrons. We find that under hydrostatic deformation, increase or decrease of vacancy formation energy depends on the type of deformation - tensile or compressive, while for shear deformation it decreases irrespective of the magnitude of applied deformation. On the other hand, migration energy barrier always decreases under hydrostatic deformation, but shows path-length dependent behavior under shear deformation. This talk will discuss the underlying principles and possible routes for enhancing mechanical strength from a physics perspective.
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.
High-Pressure Hydrogen from First-Principles
NASA Astrophysics Data System (ADS)
Morales, Miguel A.
2014-03-01
The main approximations typically employed in first-principles simulations of high-pressure hydrogen are the neglect of nuclear quantum effects (NQE) and the approximate treatment of electronic exchange and correlation, typically through a density functional theory (DFT) formulation. In this talk I'll present a detailed analysis of the influence of these approximations on the phase diagram of high-pressure hydrogen, with the goal of identifying the predictive capabilities of current methods and, at the same time, making accurate predictions in this important regime. We use a path integral formulation combined with density functional theory, which allows us to incorporate NQEs in a direct and controllable way. In addition, we use state-of-the-art quantum Monte Carlo calculations to benchmark the accuracy of more approximate mean-field electronic structure calculations based on DFT, and we use GW and hybrid DFT to calculate the optical properties of the solid and liquid phases near metallization. We present accurate predictions of the metal-insulator transition on the solid, including structural and optical properties of the molecular phase. MAM was supported by the U.S. Department of Energy at the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and by LDRD Grant No. 13-LW-004.
Thermal conductivity of silicene from first-principles
Xie, Han; Bao, Hua E-mail: hua.bao@sjtu.edu.cn; Hu, Ming E-mail: hua.bao@sjtu.edu.cn
2014-03-31
Silicene, as a graphene-like two-dimensional material, now receives exceptional attention of a wide community of scientists and engineers beyond graphene. Despite extensive study on its electric property, little research has been done to accurately calculate the phonon transport of silicene so far. In this paper, thermal conductivity of monolayer silicene is predicted from first-principles method. At 300 K, the thermal conductivity of monolayer silicene is found to be 9.4 W/mK and much smaller than bulk silicon. The contributions from in-plane and out-of-plane vibrations to thermal conductivity are quantified, and the out-of-plane vibration contributes less than 10% of the overall thermal conductivity, which is different from the results of the similar studies on graphene. The difference is explained by the presence of small buckling, which breaks the reflectional symmetry of the structure. The flexural modes are thus not purely out-of-plane vibration and have strong scattering with other modes.
Mechanical responses of borophene sheets: a first-principles study.
Mortazavi, Bohayra; Rahaman, Obaidur; Dianat, Arezoo; Rabczuk, Timon
2016-10-05
Recent experimental advances for the fabrication of various borophene sheets introduced new structures with a wide range of applications. Borophene is the boron atom analogue of graphene. Borophene exhibits various structural polymorphs all of which are metallic. In this work, we employed first-principles density functional theory calculations to investigate the mechanical properties of five different single-layer borophene sheets. In particular, we analyzed the effect of the loading direction and point vacancy on the mechanical response of borophene. Moreover, we compared the thermal stabilities of the considered borophene systems. Based on the results of our modelling, borophene films depending on the atomic configurations and the loading direction can yield a remarkable elastic modulus in the range of 163-382 GPa nm and a high ultimate tensile strength from 13.5 GPa nm to around 22.8 GPa nm at the corresponding strain from 0.1 to 0.21. Our study reveals the remarkable mechanical characteristics of borophene films.
First principles calculation of the activity of cytochrome P450
NASA Astrophysics Data System (ADS)
Segall, M. D.; Payne, M. C.; Ellis, S. W.; Tucker, G. T.; Boyes, R. N.
1998-04-01
The cytochrome P450 superfamily of enzymes is of enormous interest in the biological sciences due to the wide range of endogenous and xenobiotic compounds which it metabolises, including many drugs. We describe the use of first principles quantum mechanical modeling techniques, based on density functional theory, to determine the outcome of interactions between an enzyme and a number of compounds. Specifically, we calculate the spin state of an Fe3+ ion present in a haem moiety at the active site of these enzymes. The spin state of this ion indicates if the catalytic reaction will proceed. The computational results obtained compare favorably with experimental data. Only the principle components of the active site of the enzyme are included in the computational models, demonstrating that only a small fragment of the protein needs to be included in the models in order to accurately reproduce this aspect of the enzymes' function. These results open the way for further investigation of this superfamily of enzymes using the methods detailed in this paper.
First-Principle Characterization for Singlet Fission Couplings.
Yang, Chou-Hsun; Hsu, Chao-Ping
2015-05-21
The electronic coupling for singlet fission, an important parameter for determining the rate, has been found to be too small unless charge-transfer (CT) components were introduced in the diabatic states, mostly through perturbation or a model Hamiltonian. In the present work, the fragment spin difference (FSD) scheme was generalized to calculate the singlet fission coupling. The largest coupling strength obtained was 14.8 meV for two pentacenes in a crystal structure, or 33.7 meV for a transition-state structure, which yielded a singlet fission lifetime of 239 or 37 fs, generally consistent with experimental results (80 fs). Test results with other polyacene molecules are similar. We found that the charge on one fragment in the S1 diabatic state correlates well with FSD coupling, indicating the importance of the CT component. The FSD approach is a useful first-principle method for singlet fission coupling, without the need to include the CT component explicitly.
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 on surface stability of tantalum carbides
NASA Astrophysics Data System (ADS)
Yan, Wen-Li; Sygnatowicz, Michael; Lu, Guang-Hong; Liu, Feng; Shetty, Dinesh K.
2016-02-01
Using first-principles method, surface energies of crystal planes of different tantalum carbide phases have been calculated. Quantum size effects are shown to possibly play a considerable role in determining accurate surface energies of these metallic films, which have been neglected in previous works. The γ-TaC phase has a more stable (0 0 1) surface than the close-packed (1 1 1) surface. In the α-Ta2C phase, (0 0 1) surface with only Ta termination is more stable than that of mixed Ta-C termination because the metallic bonds between Ta atoms are weaker than the Ta-C covalent bonds. The same is true for the ζ-Ta4C3 phase. The introduction of structural vacancies in the ζ-Ta4C3 -x phase creates more direct Ta metallic bonds, making the Ta-terminated surfaces even more stable. This is consistent with the experimental observations of cleavage of the basal planes, lamellae bridging of cracks, and the high fracture toughness of ζ-Ta4C3 -x.
Electron-hole excitations and optical spectra from first principles
Rohlfing, Michael; Louie, Steven G.
2000-08-15
We present a recently developed approach to calculate electron-hole excitations and the optical spectra of condensed matter from first principles. The key concept is to describe the excitations of the electronic system by the corresponding one- and two-particle Green's function. The method combines three computational techniques. First, the electronic ground state is treated within density-functional theory. Second, the single-particle spectrum of the electrons and holes is obtained within the GW approximation to the electron self-energy operator. Finally, the electron-hole interaction is calculated and a Bethe-Salpeter equation is solved, yielding the coupled electron-hole excitations. The resulting solutions allow the calculation of the entire optical spectrum. This holds both for bound excitonic states below the band gap, as well as for the resonant spectrum above the band gap. We discuss a number of technical developments needed for the application of the method to real systems. To illustrate the approach, we discuss the excitations and optical spectra of spatially isolated systems (atoms, molecules, and semiconductor clusters) and of extended, periodic crystals (semiconductors and insulators). (c) 2000 The American Physical Society.
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.
First principle active neutron coincidence counting measurements of uranium oxide
NASA Astrophysics Data System (ADS)
Goddard, Braden; Charlton, William; Peerani, Paolo
2014-03-01
Uranium is present in most nuclear fuel cycle facilities ranging from uranium mines, enrichment plants, fuel fabrication facilities, nuclear reactors, and reprocessing plants. The isotopic, chemical, and geometric composition of uranium can vary significantly between these facilities, depending on the application and type of facility. Examples of this variation are: enrichments varying from depleted (~0.2 wt% 235U) to high enriched (>20 wt% 235U); compositions consisting of U3O8, UO2, UF6, metallic, and ceramic forms; geometries ranging from plates, cans, and rods; and masses which can range from a 500 kg fuel assembly down to a few grams fuel pellet. Since 235U is a fissile material, it is routinely safeguarded in these facilities. Current techniques for quantifying the 235U mass in a sample include neutron coincidence counting. One of the main disadvantages of this technique is that it requires a known standard of representative geometry and composition for calibration, which opens up a pathway for potential erroneous declarations by the State and reduces the effectiveness of safeguards. In order to address this weakness, the authors have developed a neutron coincidence counting technique which uses the first principle point-model developed by Boehnel instead of the "known standard" method. This technique was primarily tested through simulations of 1000 g U3O8 samples using the Monte Carlo N-Particle eXtended (MCNPX) code. The results of these simulations showed good agreement between the simulated and exact 235U sample masses.
First principles statistical mechanics of alloys and magnetism
NASA Astrophysics Data System (ADS)
Eisenbach, Markus; Khan, Suffian N.; Li, Ying Wai
Modern high performance computing resources are enabling the exploration of the statistical physics of phase spaces with increasing size and higher fidelity of the Hamiltonian of the systems. For selected systems, this now allows the combination of Density Functional based first principles calculations with classical Monte Carlo methods for parameter free, predictive thermodynamics of materials. We combine our locally selfconsistent real space multiple scattering method for solving the Kohn-Sham equation with Wang-Landau Monte-Carlo calculations (WL-LSMS). In the past we have applied this method to the calculation of Curie temperatures in magnetic materials. Here we will present direct calculations of the chemical order - disorder transitions in alloys. We present our calculated transition temperature for the chemical ordering in CuZn and the temperature dependence of the short-range order parameter and specific heat. Finally we will present the extension of the WL-LSMS method to magnetic alloys, thus allowing the investigation of the interplay of magnetism, structure and chemical order in ferrous alloys. This research was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division and it used Oak Ridge Leadership Computing Facility resources at Oak Ridge National Laboratory.
First-principles reinvestigation of bulk WO3
NASA Astrophysics Data System (ADS)
Hamdi, Hanen; Salje, Ekhard K. H.; Ghosez, Philippe; Bousquet, Eric
2016-12-01
Using first-principles calculations, we analyze the structural properties of tungsten trioxide WO3. Our calculations rely on density functional theory and the use of the B1-WC hybrid functional, which provides very good agreement with experimental data. We show that the hypothetical high-symmetry cubic reference structure combines several ferroelectric and antiferrodistortive (antipolar cation motions, rotations, and tilts of oxygen octahedra) structural instabilities. Although the ferroelectric instability is the largest, the instability related to antipolar W motions combines with those associated to oxygen rotations and tilts to produce the biggest energy reduction, yielding a P 21/c ground state. This nonpolar P 21/c phase is only different from the experimentally reported P c ground state by the absence of a very tiny additional ferroelectric distortion. The calculations performed on a stoichiometric compound so suggest that the low-temperature phase of WO3 is not intrinsically ferroelectric and that the experimentally observed ferroelectric character might arise from extrinsic defects such as oxygen vacancies. Independently, we also identify never observed R 3 m and R 3 c ferroelectric metastable phases with large polarizations and low energies close to the P 21/c ground state, which makes WO3 a potential antiferroelectric material. The relative stability of various phases is discussed in terms of the anharmonic couplings between different structural distortions, highlighting a very complex interplay.
Thermalisation of a quantum system from first principles
NASA Astrophysics Data System (ADS)
Ithier, Gregoire; Benaych-Georges, Florent
2015-03-01
How does a quantum system reach thermodynamical equilibrium? Answering such a question from first principles is, perhaps surprisingly, still an open issue (Popescu Nat. Phys. 2006, Goldstein PRL 2006, Genway PRL 2013). We present here a new model comprising an arbitrary quantum system interacting with a large arbitrary quantum environment, both initially prepared in a quantum pure state. We then demonstrate that thermalisation is an emergent property of the unitary evolution under a Schrödinger equation of this large composite system. The key conceptual tool of our method is the phenomenon of ``measure concentration'' appearing with functions defined on large dimension Hilbert spaces, a phenomenon which cancels out any effect of the microscopic structure of interaction Hamiltonians. Using our model, we first characterize the transient evolution or decoherence of the system and show its universal character. We then focus on the stationary regime and recover the canonical state well known from statistical thermodynamics. This finding leads us to propose an alternative and more general definition of the canonical partition function, that includes, among other things, the possibility of describing partial thermalisation.
First principles molecular dynamics without self-consistent field optimization.
Souvatzis, Petros; Niklasson, Anders M N
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.
A First-Principle Kinetic Theory of Meteor Plasma Formation
NASA Astrophysics Data System (ADS)
Dimant, Yakov; Oppenheim, Meers
2015-11-01
Every second millions of tiny meteoroids hit the Earth from space, vast majority too small to observe visually. However, radars detect the plasma they generate and use the collected data to characterize the incoming meteoroids and the atmosphere in which they disintegrate. This diagnostics requires a detailed quantitative understanding of formation of the meteor plasma. Fast-descending meteoroids become detectable to radars after they heat due to collisions with atmospheric molecules sufficiently and start ablating. The ablated material then collides into atmospheric molecules and forms plasma around the meteoroid. Reflection of radar pulses from this plasma produces a localized signal called a head echo. Using first principles, we have developed a consistent collisional kinetic theory of the near-meteoroid plasma. This theory shows that the meteoroid plasma develops over a length-scale close to the ion mean free path with a non-Maxwellian velocity distribution. The spatial distribution of the plasma density shows significant deviations from a Gaussian law usually employed in head-echo modeling. This analytical model will serve as a basis for more accurate quantitative interpretation of the head echo radar measurements. Work supported by NSF Grant 1244842.
First-principles studies of atomic dynamics in tetrahedrite thermoelectrics
NASA Astrophysics Data System (ADS)
Li, Junchao; Zhu, Mengze; Abernathy, Douglas L.; Ke, Xianglin; Morelli, Donald T.; Lai, Wei
2016-10-01
Cu12Sb4S13-based tetrahedrites are high-performance thermoelectrics that contain earth-abundant and environmentally friendly elements. At present, the mechanistic understanding of their low lattice thermal conductivity (<1 W m-1 K-1 at 300 K) remains limited. This work applies first-principles molecular dynamics simulations, along with inelastic neutron scattering (INS) experiments, to study the incoherent and coherent atomic dynamics in Cu10.5NiZn0.5Sb4S13, in order to deepen our insight into mechanisms of anomalous dynamic behavior and low lattice thermal conductivity in tetrahedrites. Our study of incoherent dynamics reveals the anomalous "phonon softening upon cooling" behavior commonly observed in inelastic neutron scattering data. By examining the dynamic Cu-Sb distances inside the Sb[CuS3]Sb cage, we ascribe softening to the decreased anharmonic "rattling" of Cu in the cage. On the other hand, our study of coherent dynamics reveals that acoustic modes are confined in a small region of dynamic scattering space, which we hypothesize leads to a minimum phonon mean free path. By assuming a Debye model, we obtain a lattice minimum thermal conductivity value consistent with experiments. We believe this study furthers our understanding of the atomic dynamics of tetrahedrite thermoelectrics and will more generally help shed light on the origin of intrinsically low lattice thermal conductivity in these and other structurally similar materials.
First-principles structural design of superhard materials.
Zhang, Xinxin; Wang, Yanchao; Lv, Jian; Zhu, Chunye; Li, Qian; Zhang, Miao; Li, Quan; Ma, Yanming
2013-03-21
We reported a developed methodology to design superhard materials for given chemical systems under external conditions (here, pressure). The new approach is based on the CALYPSO algorithm and requires only the chemical compositions to predict the hardness vs. energy map, from which the energetically preferable superhard structures are readily accessible. In contrast to the traditional ground state structure prediction method where the total energy was solely used as the fitness function, here we adopted hardness as the fitness function in combination with the first-principles calculation to construct the hardness vs. energy map by seeking a proper balance between hardness and energy for a better mechanical description of given chemical systems. To allow a universal calculation on the hardness for the predicted structure, we have improved the earlier hardness model based on bond strength by applying the Laplacian matrix to account for the highly anisotropic and molecular systems. We benchmarked our approach in typical superhard systems, such as elemental carbon, binary B-N, and ternary B-C-N compounds. Nearly all the experimentally known and most of the earlier theoretical superhard structures have been successfully reproduced. The results suggested that our approach is reliable and can be widely applied into design of new superhard materials.
Four superhard carbon allotropes: a first-principles study.
He, Chaoyu; Sun, Lizhong; Zhang, Chunxiao; Peng, Xiangyang; Zhang, Kaiwang; Zhong, Jianxin
2012-06-21
Using a generalized genetic algorithm, we propose four new sp(3) carbon allotropes with 5-6-7 (5-6-7-type Z-ACA and Z-CACB) or 4-6-8 (4-6-8-type Z4-A(3)B(1) and A4-A(2)B(2)) carbon rings. Their stability, mechanical and electronic properties are systematically studied using a first-principles method. We find that the four new carbon allotropes show amazing stability in comparison with the carbon phases proposed recently. Both 5-6-7-type Z-ACA and Z-CACB are direct band-gap semiconductors with band gaps of 2.261 eV and 4.196 eV, respectively. However, the 4-6-8-type Z4-A(3)B(1) and A4-A(2)B(2) are indirect band-gap semiconductors with band gaps of 3.105 eV and 3.271 eV, respectively. Their mechanical properties reveal that all the four carbon allotropes proposed in present work are superhard materials, which are comparable to diamond.
Predicted boron-carbide compounds: a first-principles study.
Wang, De Yu; Yan, Qian; Wang, Bing; Wang, Yuan Xu; Yang, Jueming; Yang, Gui
2014-06-14
By using developed particle swarm optimization algorithm on crystal structural prediction, we have explored the possible crystal structures of B-C system. Their structures, stability, elastic properties, electronic structure, and chemical bonding have been investigated by first-principles calculations with density functional theory. The results show that all the predicted structures are mechanically and dynamically stable. An analysis of calculated enthalpy with pressure indicates that increasing of boron content will increase the stability of boron carbides under low pressure. Moreover, the boron carbides with rich carbon content become more stable under high pressure. The negative formation energy of predicted B5C indicates its high stability. The density of states of B5C show that it is p-type semiconducting. The calculated theoretical Vickers hardnesses of B-C exceed 40 GPa except B4C, BC, and BC4, indicating they are potential superhard materials. An analysis of Debye temperature and electronic localization function provides further understanding chemical and physical properties of boron carbide.
First-principles prediction of disordering tendencies in pyrochlore oxides
Jiang Chao; Stanek, C. R.; Sickafus, K. E.; Uberuaga, B. P.
2009-03-01
Using first-principles calculations, we systematically predict the order-disorder energetics of series of zirconate (A{sub 2}Zr{sub 2}O{sub 7}), hafnate (A{sub 2}Hf{sub 2}O{sub 7}), titanate (A{sub 2}Ti{sub 2}O{sub 7}), and stannate (A{sub 2}Sn{sub 2}O{sub 7}) pyrochlores. The disordered defect-fluorite structure is modeled using an 88-atom two-sublattice special quasirandom structure (SQS) that closely reproduces the most relevant near-neighbor intrasublattice and intersublattice pair-correlation functions of the random mixture. The order-disorder transition temperatures of these pyrochlores estimated from our SQS calculations show overall good agreement with existing experiments. We confirm previous studies suggesting that the bonding in pyrochlores is not purely ionic and thus electronic effects also play a role in determining their disordering tendencies. Our results have important consequences for numerous applications, including nuclear waste forms and fast ion conductors.
First-principles prediction of disordering tendencies in complex oxides
Jiang, Chao; Stanek, Christopher R; Sickafus, Kurt E; Uberuaga, Blas P
2008-01-01
The disordering tendencies of a series of zirconate (A{sub 2}Zr{sub 2}O{sub 7}) , hafnate (A{sub 2}Hf{sub 2}O{sub 7}), titanate (A{sub 2}Ti{sub 2}O{sub 7}), and stannate (A{sub 2} Sn{sub 2}O{sub 7}) pyrochlores are predicted in this study using first-principles total energy calculations. To model the disordered (A{sub 1/2}B{sub 1/2})(O{sub 7/8}/V{sub 1/8}){sub 2} fluorite structure, we have developed an 88-atom two-sublattice special quasirandom structure (SQS) that closely reproduces the most important near-neighbor intra-sublattice and inter-sublattice pair correlation functions of the random alloy. From the calculated disordering energies, the order-disorder transition temperatures of those pyrochlores are further predicted and our results agree well with the existing experimental phase diagrams. It is clearly demonstrated that both size and electronic effects play an important role in determining the disordering tendencies of pyrochlore compounds.
Lattice thermal conductivity of borophene from first principle calculation
Xiao, Huaping; Cao, Wei; Ouyang, Tao; Guo, Sumei; He, Chaoyu; Zhong, Jianxin
2017-01-01
The phonon transport property is a foundation of understanding a material and predicting the potential application in mirco/nano devices. In this paper, the thermal transport property of borophene is investigated by combining first-principle calculations and phonon Boltzmann transport equation. At room temperature, the lattice thermal conductivity of borophene is found to be about 14.34 W/mK (error is about 3%), which is much smaller than that of graphene (about 3500 W/mK). The contributions from different phonon modes are qualified, and some phonon modes with high frequency abnormally play critical role on the thermal transport of borophene. This is quite different from the traditional understanding that thermal transport is usually largely contributed by the low frequency acoustic phonon modes for most of suspended 2D materials. Detailed analysis further reveals that the scattering between the out-of-plane flexural acoustic mode (FA) and other modes likes FA + FA/TA/LA/OP ↔ TA/LA/OP is the predominant phonon process channel. Finally the vibrational characteristic of some typical phonon modes and mean free path distribution of different phonon modes are also presented in this work. Our results shed light on the fundamental phonon transport properties of borophene, and foreshow the potential application for thermal management community. PMID:28374853
First-principles investigation of antiphase boundaries in perovskites
NASA Astrophysics Data System (ADS)
Naumov, Ivan; Rabe, Karin
2002-03-01
The lowering of the dielectric constant of Ba_xSr_1-xTiO3 (BST) films compared to bulk can be attributed, at least in part, to the effects of defects associated with film growth. In BST films grown on MgO substrates, such defects include antiphase boundaries (APBs), which have been clearly observed using electron microscopy. In this work, using a first-principles pseudopotential approach based on variational density functional pertubation theory, we have investigated the structure, lattice dynamics and dielectric properties of two relevant APBs in SrTiO3 (Sr-rich and Ti-rich) using ordered supercells. Comparison with bulk SrTiO3 shows that the Born effective charges and electronic dielectric tensor decrease and the characteristic low-frequency polar mode increases in frequency, leading to a significant lowering of the lattice contribution to the dielectric response. We suggest that this change can be understood as the result of the disruption of the Ti-O chains normal to the APB, and thus that this mechanism is also relevant to the solid solution. This work is supported by U. Maryland/Rutgers NSF-MRSEC DMR-00-80008.
First-principles calculations of mobilities in MOSFETs
NASA Astrophysics Data System (ADS)
Hadjisavvas, George; Tsetseris, Leonidas; Evans, Matthew; Pantelides, Sokrates
2007-03-01
Nano-scale MOSFETs demonstrate interesting electron transport behavior. Straining the silicon lattice results in significant increases in carrier mobility up to 100%. Transport properties are known to depend also on the presence of interface traps. Due to their significance, a large number of studies have obtained mobilities, but in an empirical and semi-classical fashion, whereas, in nano-devices quantum mechanical effects and atomic-scale structural details are the key factors of mobility calculations. Here we use a recently developed method[1] for first-principles calculations of mobilites within DFT to probe the effect of strain and interface point defects (e.g., dangling bonds) on mobilities in double gate ultra-thin SOI (UTSOI) MOSFETs. The transport properties are described in a fully self-consistent quantum mechanical fashion and mobilities are calculated within the Born approximation. The results show that biaxial tensile strain is shown to significantly increase carrier mobility in UTSOI devices by suppressing the effective scattering from atomic-scale interface inhomogeneities; the effect of dangling bonds on mobility in a UTSOI channel is weaker than in conventional MOSFETs because the carrier density peaks at the center of the channel. This work was supported in part by NSF Grant ECS-0524655 and by AFOSR Grant 4224224232. [1] M.H. Evans et al., Phys. Rev. Lett. 95, 106802 (2005).
NMR quadruopole spectra of PZT from first-principles
NASA Astrophysics Data System (ADS)
Mao, Dandan; Walter, Eric J.; Krakauer, Henry
2006-03-01
High performance piezoelectric materials are disordered alloys, so it can be difficult to determine the local atomic geometry. Recently, high field NMR measurements have shown great promise as a microscopic probe of ABO3 perovskite-based alloys by their ability to resolve line-splittings due to nuclear quadrupolar coupling with the electric field gradient (EFG) at the nucleus. We report first-principles LDA calculations of the EFG's in monoclinic and tetragonal Pb(Zr0.5Ti0.5)O3 systems using the linear augmented planewave (LAPW) method, and we compute NMR static powder spectra for ^91Zr, ^47Ti, and ^17O atoms as a function of applied strain. With decreasing c/a ratio PZT converts from tetragonal to monoclinic symmetry. We observe that the calculated NMR spectra show dramatic deviations with decreasing c/a from that in tetragonal P4mm well before the electric polarization begins to rotate away from the [001] direction. This indicates that NMR measurements can be a very accurate probe of local structural changes in perovskite piezoelectrics. G. L. Hoatson, D. H. Zhou, F. Fayon, D. Massiot, and R. L. Vold, Phys. Rev. B, 66, 224103 (2002).
Solubility of nonelectrolytes: a first-principles computational approach.
Jackson, Nicholas E; Chen, Lin X; Ratner, Mark A
2014-05-15
Using a combination of classical molecular dynamics and symmetry adapted intermolecular perturbation theory, we develop a high-accuracy computational method for examining the solubility energetics of nonelectrolytes. This approach is used to accurately compute the cohesive energy density and Hildebrand solubility parameters of 26 molecular liquids. The energy decomposition of symmetry adapted perturbation theory is then utilized to develop multicomponent Hansen-like solubility parameters. These parameters are shown to reproduce the solvent categorizations (nonpolar, polar aprotic, or polar protic) of all molecular liquids studied while lending quantitative rigor to these qualitative categorizations via the introduction of simple, easily computable parameters. Notably, we find that by monitoring the first-order exchange energy contribution to the total interaction energy, one can rigorously determine the hydrogen bonding character of a molecular liquid. Finally, this method is applied to compute explicitly the Flory interaction parameter and the free energy of mixing for two different small molecule mixtures, reproducing the known miscibilities. This methodology represents an important step toward the prediction of molecular solubility from first principles.
A First-principles Molecular Dynamics Investigation of Superionic Conductivity
NASA Astrophysics Data System (ADS)
Wood, Brandon; Marzari, Nicola
2007-03-01
Superionic materials---solids with liquid-like transport properties---have found widespread use in a variety of applications in fuel cells, switches, sensors, and batteries. However, reasons for fast-ion conduction in such materials, as well as the specific atomistic mechanisms involved, remain ill understood. Our work uses first-principles molecular dynamics to illuminate the mechanisms, pathways, and motivations for superionic conductivity in two materials representing different classes of ion conductors: α-AgI, an archetypal Type-I superionic; and CsHSO4, an anhydrous solid-state electrolyte candidate for hydrogen fuel cells. For α-AgI, we trace common pathways for silver ion conduction and discuss how a chemical signature in the electronic structure relates to enhanced silver ion mobility. We also characterize the dynamical lattice structure in the superionic phase and present the likely motivations for its existence. For CsHSO4, we isolate the dominant atomistic mechanisms involved in superprotonic conduction and discuss the effect of correlated diffusive events in enhancing proton transport. We also offer a detailed description of the dynamics of the hydrogen bond network topology in the course of proton diffusion and discuss the relevance of atomistic processes with competing timescales in facilitating proton transport.
Gypsum under pressure: A first-principles study
NASA Astrophysics Data System (ADS)
Giacomazzi, Luigi; Scandolo, Sandro
2010-02-01
We investigate by means of first-principles methods the structural response of gypsum (CaSO4ṡ2H2O) to pressures within and above the stability range of gypsum-I (P≤4GPa) . Structural and vibrational properties calculated for gypsum-I are in excellent agreement with experimental data. Compression within gypsum-I takes place predominantly through a reduction in the volume of the CaO8 polyhedra and through a distortion of the hydrogen bonds. The distance between CaSO4 layers becomes increasingly incompressible, indicating a mechanical limit to the packing of water molecules between the layers. We find that a structure with collapsed interlayer distances becomes more stable than gypsum-I above about 5 GPa. The collapse is concomitant with a rearrangement of the hydrogen-bond network of the water molecules. Comparison of the vibrational spectra calculated for this structure with experimental data taken above 5 GPa supports the validity of our model for the high-pressure phase of gypsum.
Electronic and structural reconstruction in titanate heterostructures from first principles
NASA Astrophysics Data System (ADS)
Mulder, Andrew T.; Fennie, Craig J.
2014-03-01
Recent advances in transition metal oxide heterostructures have opened new routes to create materials with novel functionalities and properties. One direction has been to combine a Mott insulating perovskite with an electronic d1 configuration, such as LaTiO3, with a band insulating d0 perovskite, such as SrTiO3. An exciting recent development is the demonstration of interfacial conductivity in GdTiO3/SrTiO3 heterostructures that display a complex structural motif of octahedral rotations and ferromagnetic properties similar to bulk GdTiO3. In this talk we present our first principles investigation of the interplay of structural, electronic, magnetic, and orbital degrees of freedom for a wide range of d1/d0 titanate heterostructures. We find evidence for both rotation driven ferroelectricity and a symmetry breaking electronic reconstruction with a concomitant structural distortion at the interface. We argue that these materials represent an ideal platform to realize novel functionalities such as the electric field control of electronic and magnetic properties.
Predicting catalysis: understanding ammonia synthesis from first-principles calculations.
Hellman, A; Baerends, E J; Biczysko, M; Bligaard, T; Christensen, C H; Clary, D C; Dahl, S; van Harrevelt, R; Honkala, K; Jonsson, H; Kroes, G J; Luppi, M; Manthe, U; Nørskov, J K; Olsen, R A; Rossmeisl, J; Skúlason, E; Tautermann, C S; Varandas, A J C; Vincent, J K
2006-09-14
Here, we give a full account of a large collaborative effort toward an atomic-scale understanding of modern industrial ammonia production over ruthenium catalysts. We show that overall rates of ammonia production can be determined by applying various levels of theory (including transition state theory with or without tunneling corrections, and quantum dynamics) to a range of relevant elementary reaction steps, such as N(2) dissociation, H(2) dissociation, and hydrogenation of the intermediate reactants. A complete kinetic model based on the most relevant elementary steps can be established for any given point along an industrial reactor, and the kinetic results can be integrated over the catalyst bed to determine the industrial reactor yield. We find that, given the present uncertainties, the rate of ammonia production is well-determined directly from our atomic-scale calculations. Furthermore, our studies provide new insight into several related fields, for instance, gas-phase and electrochemical ammonia synthesis. The success of predicting the outcome of a catalytic reaction from first-principles calculations supports our point of view that, in the future, theory will be a fully integrated tool in the search for the next generation of catalysts.
First-principle studies on the Li-Te system
NASA Astrophysics Data System (ADS)
Wang, Youchun; Tian, Fubo; Li, Da; Duan, Defang; Liu, Yunxian; Liu, Bingbing; Zhou, Qiang; Cui, Tian
2017-01-01
First-principle evolutionary calculation was performed to search for all probable stable lithium tellurium compounds. In addition to the well-known structures of Fm-3m Li2Te and Pnma Li2Te, several novel structures, including those of P4/nmm Li2Te, Imma Li8Te2, and C2/m Li9Te2, were determined under high pressure. The transformation sequence of Li2Te induced by pressure was presented as follows. The phase transition occurred at 7.5 GPa while transforming from Fm-3m phase to Pnma structure, then transformed to P4/nmm phase at 14 GPa. P4/nmm Li2Te can remain stable at least up to 140 GPa. Li8Te2 and Li9Te2 were stable at 8-120 GPa and 80-120 GPa, respectively. Interestingly, Li8Te2 and Li9Te2 were predicted to be metallic under high pressure, Li2Te would metalize on compression. P4/nmm Li2Te is likely a super ionic conductor due to the special characteristic. Metallic P4/nmm Li2Te may be a candidate mixed conductor material under extreme pressure. Charge transfer was studied using Bader charge analysis. Charge transferred from Li to Te, and the relative debilitated ionicity between Li and Te atoms existed at high pressure.
Thermodynamic stability and properties of boron subnitrides from first principles
NASA Astrophysics Data System (ADS)
Ektarawong, A.; Simak, S. I.; Alling, B.
2017-02-01
We use the first-principles approach to clarify the thermodynamic stability as a function of pressure and temperature of three different α -rhombohedral-boron-like boron subnitrides, with the compositions of B6N , B13N2 , and B38N6 , proposed in the literature. We find that, out of these subnitrides with the structural units of B12(N-N), B12(NBN), and [B12(N-N) ] 0.33[B12(NBN)] 0.67 , respectively, only B38N6 , represented by [B12(N-N) ] 0.33[B12(NBN)] 0.67 , is thermodynamically stable. Beyond a pressure of about 7.5 GPa depending on the temperature, also B38N6 becomes unstable, and decomposes into cubic boron nitride and α -tetragonal-boron-like boron subnitride B50N2 . The thermodynamic stability of boron subnitrides and relevant competing phases is determined by the Gibbs free energy, in which the contributions from the lattice vibrations and the configurational disorder are obtained within the quasiharmonic and the mean-field approximations, respectively. We calculate lattice parameters, elastic constants, phonon and electronic density of states, and demonstrate that [B12(N-N) ] 0.33[B12(NBN)] 0.67 is both mechanically and dynamically stable, and is an electrical semiconductor. The simulated x-ray powder-diffraction pattern as well as the calculated lattice parameters of [B12(N-N) ] 0.33[B12(NBN)] 0.67 are found to be in good agreement with those of the experimentally synthesized boron subnitrides reported in the literature, verifying that B38N6 is the stable composition of α -rhombohedral-boron-like boron subnitride.
First-principles study of polyacetylene derivatives bearing nitroxide radicals
NASA Astrophysics Data System (ADS)
Bilgiç, Beyza; Kılıç, Çetin; Esat, Burak
2011-09-01
Electrodes made of organic polymers bearing redox-active radical pendant groups have attractive features for use in rechargeable batteries. Electronic structure and electrochemical properties of cathode- and anode-active organic polymers are investigated here by means of first-principles calculations performed in the framework of the density functional theory. We consider organic radical polymers (ORPs) that consist of trans-polyacetylene derivatives bearing a variety of nitroxide radicals. A number of neutral and charged supercells are utilized to compute the ionization potentials and electron affinities as well as the one-electron states of these ORPs. By revealing the polyacetylene-derived highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) as well as the radical-derived singly occupied molecular orbital (SOMO), the variation of the SOMO energy within the HOMO-LUMO gap is determined in the course of the oxidization or reduction of ORPs. Our results indicate that the ionization potential I and electron affinity A of polyacetylene would act as a lower or upper bound in the variation of the electrochemical potential of cathode- or anode-active ORPs in the course of battery discharge or charge owing to pinning of the radical-derived SOMO to the polyacetylene-derived HOMO or LUMO. Accordingly, it is anticipated that the electrochemical “window” [-I,-A] of the polymeric backbone of ORPs will impose certain limitations in accomplishing a high charge/discharge voltage range in a totally organic rechargeable battery with positive and negative electrodes made of cathode- and anode-active ORPs, respectively. On the other hand, our findings suggest that one could, in principle, take advantage of using two different (conducting) polymeric backbones in the anode and cathode with adjusted HOMO and LUMO offsets once the electron transfer is accomplished to take place through the conducting backbones.
First principles investigation of copper and silver intercalated molybdenum disulfide
NASA Astrophysics Data System (ADS)
Guzman, D. M.; Onofrio, N.; Strachan, A.
2017-02-01
We characterize the energetics and atomic structures involved in the intercalation of copper and silver into the van der Waals gap of molybdenum disulfide as well as the resulting ionic and electronic transport properties using first-principles density functional theory. The intercalation energy of systems with formula (Cu,Ag)xMoS2 decreases with ion concentration and ranges from 1.2 to 0.8 eV for Cu; Ag exhibits a stronger concentration dependence from 2.2 eV for x = 0.014 to 0.75 eV for x = 1 (using the fcc metal as a reference). Partial atomic charge analysis indicates that approximately half an electron is transferred per metallic ion in the case of Cu at low concentrations and the ionicity decreases only slightly with concentration. In contrast, while Ag is only slightly less ionic than Cu for low concentrations, charge transfer reduces significantly to approximately 0.1 e for x = 1. This difference in ionicity between Cu and Ag correlates with their intercalation energies. Importantly, the predicted values indicate the possibility of electrochemical intercalation of both Cu and Ag into MoS2 and the calculated activation energies associated with ionic transport within the gaps, 0.32 eV for Cu and 0.38 eV for Ag, indicate these materials to be good ionic conductors. Analysis of the electronic structure shows that charge transfer leads to a shift of the Fermi energy into the conduction band resulting in a semiconductor-to-metal transition. Electron transport calculations based on non-equilibrium Green's function show that the low-bias conductance increases with metal concentration and is comparable in the horizontal and vertical transport directions. These properties make metal intercalated transition metal di-chalcogenides potential candidates for several applications including electrochemical metallization cells and contacts in electronics based on 2D materials.
Risk reduction and the privatization option: First principles
Bjornstad, D.J.; Jones, D.W.; Russell, M.; Cummings, R.C.; Valdez, G.; Duemmer, C.L.
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 explanation of the positive Seebeck coefficient of lithium.
Xu, Bin; Verstraete, Matthieu J
2014-05-16
Lithium is one of the simplest metals, with negative charge carriers and a close reproduction of free-electron dispersion. Experimentally, however, Li is one of a handful of elemental solids (along with Cu, Ag, and Au) where the sign of the Seebeck coefficient (S) is opposite to that of the carrier. This counterintuitive behavior still lacks a satisfactory interpretation. We calculate S fully from first principles, within the framework of Allen's formulation of Boltzmann transport theory. Here it is crucial to avoid the constant relaxation time approximation, which gives a sign for S which is necessarily that of the carriers. Our calculated S are in excellent agreement with experimental data, up to the melting point. In comparison with another alkali metal, Na, we demonstrate that within the simplest nontrivial model for the energy dependency of the electron lifetimes, the rapidly increasing density of states (DOS) across the Fermi energy is related to the sign of S in Li. The exceptional energy dependence of the DOS is beyond the free-electron model, as the dispersion is distorted by the Brillouin zone edge; this has a stronger effect in Li than other alkali metals. The electron lifetime dependency on energy is central, but the details of the electron-phonon interaction are found to be less important, contrary to what has been believed for several decades. Band engineering combined with the mechanism exposed here may open the door to new "ambipolar" thermoelectric materials, with a tunable sign for the thermopower even if either n- or p-type doping is impossible.
First principle kinetic studies of zeolite-catalyzed methylation reactions.
Van Speybroeck, Veronique; Van der Mynsbrugge, Jeroen; Vandichel, Matthias; Hemelsoet, Karen; Lesthaeghe, David; Ghysels, An; Marin, Guy B; Waroquier, Michel
2011-02-02
Methylations of ethene, propene, and butene by methanol over the acidic microporous H-ZSM-5 catalyst are studied by means of state of the art computational techniques, to derive Arrhenius plots and rate constants from first principles that can directly be compared with the experimental data. For these key elementary reactions in the methanol to hydrocarbons (MTH) process, direct kinetic data became available only recently [J. Catal.2005, 224, 115-123; J. Catal.2005, 234, 385-400]. At 350 °C, apparent activation energies of 103, 69, and 45 kJ/mol and rate constants of 2.6 × 10(-4), 4.5 × 10(-3), and 1.3 × 10(-2) mol/(g h mbar) for ethene, propene, and butene were derived, giving following relative ratios for methylation k(ethene)/k(propene)/k(butene) = 1:17:50. In this work, rate constants including pre-exponential factors are calculated which give very good agreement with the experimental data: apparent activation energies of 94, 62, and 37 kJ/mol for ethene, propene, and butene are found, and relative ratios of methylation k(ethene)/k(propene)/k(butene) = 1:23:763. The entropies of gas phase alkenes are underestimated in the harmonic oscillator approximation due to the occurrence of internal rotations. These low vibrational modes were substituted by manually constructed partition functions. Overall, the absolute reaction rates can be calculated with near chemical accuracy, and qualitative trends are very well reproduced. In addition, the proposed scheme is computationally very efficient and constitutes significant progress in kinetic modeling of reactions in heterogeneous catalysis.
Liquid Water from First Principles: Validation of Different Sampling Approaches
Mundy, C J; Kuo, W; Siepmann, J; McGrath, M J; Vondevondele, J; Sprik, M; Hutter, J; Parrinello, M; Mohamed, F; Krack, M; Chen, B; Klein, M
2004-05-20
A series of first principles molecular dynamics and Monte Carlo simulations were carried out for liquid water to assess the validity and reproducibility of different sampling approaches. These simulations include Car-Parrinello molecular dynamics simulations using the program CPMD with different values of the fictitious electron mass in the microcanonical and canonical ensembles, Born-Oppenheimer molecular dynamics using the programs CPMD and CP2K in the microcanonical ensemble, and Metropolis Monte Carlo using CP2K in the canonical ensemble. With the exception of one simulation for 128 water molecules, all other simulations were carried out for systems consisting of 64 molecules. It is found that the structural and thermodynamic properties of these simulations are in excellent agreement with each other as long as adiabatic sampling is maintained in the Car-Parrinello molecular dynamics simulations either by choosing a sufficiently small fictitious mass in the microcanonical ensemble or by Nos{acute e}-Hoover thermostats in the canonical ensemble. Using the Becke-Lee-Yang-Parr exchange and correlation energy functionals and norm-conserving Troullier-Martins or Goedecker-Teter-Hutter pseudopotentials, simulations at a fixed density of 1.0 g/cm{sup 3} and a temperature close to 315 K yield a height of the first peak in the oxygen-oxygen radial distribution function of about 3.0, a classical constant-volume heat capacity of about 70 J K{sup -1} mol{sup -1}, and a self-diffusion constant of about 0.1 Angstroms{sup 2}/ps.
First-principles studies of Ni-Ta intermetallic compounds
Zhou Yi; Wen Bin; Ma Yunqing; Melnik, Roderick; Liu Xingjun
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.
First Principles Design of Non-Centrosymmetric Metal Oxides
NASA Astrophysics Data System (ADS)
Young, Joshua Aaron
The lack of an inversion center in a material's crystal structure can result in many useful material properties, such as ferroelectricity, piezoelectricity and non-linear optical behavior. Recently, the desire for low power, high efficiency electronic devices has spurred increased interest in these phenomena, especially ferroelectricity, as well as their coupling to other material properties. By studying and understanding the fundamental structure-property relationships present in non-centrosymmetric materials, it is possible to purposefully engineer new compounds with the desired "acentric" qualities through crystal engineering. The families of ABO3 perovskite and ABO2.5 perovskite-derived brownmillerite oxides are ideal for such studies due to their wide range of possible chemistries, as well as ground states that are highly tunable owing to strong electron-lattice coupling. Furthermore, control over the B-O-B bond angles through epitaxial strain or chemical substitution allows for the rapid development of new emergent properties. In this dissertation, I formulate the crystal-chemistry criteria necessary to design functional non-centrosymmetric oxides using first-principles density functional theory calculations. Recently, chemically ordered (AA')B2O 6 oxides have been shown to display a new form of rotation-induced ferroelectric polarizations. I now extend this property-design methodology to alternative compositions and crystal classes and show it is possible to induce a host of new phenomena. This dissertation will address: 1) the formulation of predictive models allowing for a priori design of polar oxides, 2) the optimization of properties exhibited by these materials through chemical substitution and cation ordering, and 3) the use of strain to control the stability of new phases. Completion of this work has led to a deeper understanding of how atomic structural features determine the physical properties of oxides, as well as the successful elucidation of
A digitally reconstructed radiograph algorithm calculated from first principles
Staub, David; Murphy, Martin J.
2013-01-15
Purpose: To develop an algorithm for computing realistic digitally reconstructed radiographs (DRRs) that match real cone-beam CT (CBCT) projections with no artificial adjustments. Methods: The authors used measured attenuation data from cone-beam CT projection radiographs of different materials to obtain a function to convert CT number to linear attenuation coefficient (LAC). The effects of scatter, beam hardening, and veiling glare were first removed from the attenuation data. Using this conversion function the authors calculated the line integral of LAC through a CT along rays connecting the radiation source and detector pixels with a ray-tracing algorithm, producing raw DRRs. The effects of scatter, beam hardening, and veiling glare were then included in the DRRs through postprocessing. Results: The authors compared actual CBCT projections to DRRs produced with all corrections (scatter, beam hardening, and veiling glare) and to uncorrected DRRs. Algorithm accuracy was assessed through visual comparison of projections and DRRs, pixel intensity comparisons, intensity histogram comparisons, and correlation plots of DRR-to-projection pixel intensities. In general, the fully corrected algorithm provided a small but nontrivial improvement in accuracy over the uncorrected algorithm. The authors also investigated both measurement- and computation-based methods for determining the beam hardening correction, and found the computation-based method to be superior, as it accounted for nonuniform bowtie filter thickness. The authors benchmarked the algorithm for speed and found that it produced DRRs in about 0.35 s for full detector and CT resolution at a ray step-size of 0.5 mm. Conclusions: The authors have demonstrated a DRR algorithm calculated from first principles that accounts for scatter, beam hardening, and veiling glare in order to produce accurate DRRs. The algorithm is computationally efficient, making it a good candidate for iterative CT reconstruction techniques
First-principles investigation of hydrous post-perovskite
Townsend, Joshua P.; Tsuchiya, Jun; Bina, Craig R.; ...
2015-04-11
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, resultingmore » 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. Furthermore, 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.« less
First-principles investigation of hydrous post-perovskite
Townsend, Joshua P.; Tsuchiya, Jun; Bina, Craig R.; Jacobsen, Steven D.
2015-04-11
A stable, hydrogen-defect structure of post-perovskite (hy-ppv, Mg_{1–x}SiH_{2x}O_{3}) 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 (K_{0}) and shear (G_{0}) moduli by 5% and 8%, respectively, for a structure containing ~1 wt.% H_{2}O. 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 (C_{ij}) in hy-ppv increases with pressure (notably, C_{55}, C_{66}, and C_{23}), 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. Furthermore, the stability of a hy-ppv structure containing 1–2 wt.% H_{2}O 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 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.
Monolayer II-VI semiconductors: A first-principles prediction
NASA Astrophysics Data System (ADS)
Zheng, Hui; Li, Xian-Bin; Chen, Nian-Ke; Xie, Sheng-Yi; Tian, Wei Quan; Chen, Yuanping; Xia, Hong; Zhang, S. B.; Sun, Hong-Bo
2015-09-01
A systematic study of 32 honeycomb monolayer II-VI semiconductors is carried out by first-principles methods. While none of the two-dimensional (2D) structures can be energetically stable, it appears that BeO, MgO, CaO, ZnO, CdO, CaS, SrS, SrSe, BaTe, and HgTe honeycomb monolayers have a good dynamic stability. The stability of the five oxides is consistent with the work published by Zhuang et al. [Appl. Phys. Lett. 103, 212102 (2013), 10.1063/1.4831972]. The rest of the compounds in the form of honeycomb are dynamically unstable, revealed by phonon calculations. In addition, according to the molecular dynamic (MD) simulation evolution from these unstable candidates, we also find two extra monolayers dynamically stable, which are tetragonal BaS [P 4 /n m m (129 ) ] and orthorhombic HgS [P 21/m (11 ) ] . The honeycomb monolayers exist in the form of either a planar perfect honeycomb or a low-buckled 2D layer, all of which possess a band gap and most of them are in the ultraviolet region. Interestingly, the dynamically stable SrSe has a gap near visible light, and displays exotic electronic properties with a flat top of the valence band, and hence has a strong spin polarization upon hole doping. The honeycomb HgTe has recently been reported to achieve a topological nontrivial phase under appropriate in-plane tensile strain and spin-orbital coupling (SOC) [J. Li et al., arXiv:1412.2528]. Some II-VI partners with less than 5 % lattice mismatch may be used to design novel 2D heterojunction devices. If synthesized, potential applications of these 2D II-VI families could include optoelectronics, spintronics, and strong correlated electronics.
A digitally reconstructed radiograph algorithm calculated from first principles
Staub, David; Murphy, Martin J.
2013-01-01
Purpose: To develop an algorithm for computing realistic digitally reconstructed radiographs (DRRs) that match real cone-beam CT (CBCT) projections with no artificial adjustments. Methods: The authors used measured attenuation data from cone-beam CT projection radiographs of different materials to obtain a function to convert CT number to linear attenuation coefficient (LAC). The effects of scatter, beam hardening, and veiling glare were first removed from the attenuation data. Using this conversion function the authors calculated the line integral of LAC through a CT along rays connecting the radiation source and detector pixels with a ray-tracing algorithm, producing raw DRRs. The effects of scatter, beam hardening, and veiling glare were then included in the DRRs through postprocessing. Results: The authors compared actual CBCT projections to DRRs produced with all corrections (scatter, beam hardening, and veiling glare) and to uncorrected DRRs. Algorithm accuracy was assessed through visual comparison of projections and DRRs, pixel intensity comparisons, intensity histogram comparisons, and correlation plots of DRR-to-projection pixel intensities. In general, the fully corrected algorithm provided a small but nontrivial improvement in accuracy over the uncorrected algorithm. The authors also investigated both measurement- and computation-based methods for determining the beam hardening correction, and found the computation-based method to be superior, as it accounted for nonuniform bowtie filter thickness. The authors benchmarked the algorithm for speed and found that it produced DRRs in about 0.35 s for full detector and CT resolution at a ray step-size of 0.5 mm. Conclusions: The authors have demonstrated a DRR algorithm calculated from first principles that accounts for scatter, beam hardening, and veiling glare in order to produce accurate DRRs. The algorithm is computationally efficient, making it a good candidate for iterative CT reconstruction techniques
Sodium tungstate modulates ATM function upon DNA damage.
Rodriguez-Hernandez, C J; Llorens-Agost, M; Calbó, J; Murguia, J R; Guinovart, J J
2013-05-21
Both radiotherapy and most effective chemotherapeutic agents induce different types of DNA damage. Here we show that tungstate modulates cell response to DNA damaging agents. Cells treated with tungstate were more sensitive to etoposide, phleomycin and ionizing radiation (IR), all of which induce DNA double-strand breaks (DSBs). Tungstate also modulated the activation of the central DSB signalling kinase, ATM, in response to these agents. These effects required the functionality of the Mre11-Nbs1-Rad50 (MRN) complex and were mimicked by the inhibition of PP2A phosphatase. Therefore, tungstate may have adjuvant activity when combined with DNA-damaging agents in the treatment of several malignancies.
First-principles modelling of materials: From polythiophene to phosphorene
NASA Astrophysics Data System (ADS)
Ziletti, Angelo
As a result of the computing power provided by the current technology, computational methods now play an important role in modeling and designing materials at the nanoscale. The focus of this dissertation is two-fold: first, new computational methods to model nanoscale transport are introduced, then state-of-the-art tools based on density functional theory are employed to explore the properties of phosphorene, a novel low dimensional material with great potential for applications in nanotechnology. A Wannier function description of the electron density is combined with a generalized Slater-Koster interpolation technique, enabling the introduction of a new computational method for constructing first-principles model Hamiltonians for electron and hole transport that maintain the density functional theory accuracy at a fraction of the computational cost. As a proof of concept, this new approach is applied to model polythiophene, a polymer ubiquitous in organic photovoltaic devices. A new low dimensional material, phosphorene - a single layer of black phosphorous - the phosphorous analogue of graphene was first isolated in early 2014 and has attracted considerable attention. It is a semiconductor with a sizable band gap, which makes it a perfect candidate for ultrathin transistors. Multi-layer phosphorene transistors have already achieved the highest hole mobility of any two-dimensional material apart from graphene. Phosphorene is prone to oxidation, which can lead to degradation of electrical properties, and eventually structural breakdown. The calculations reported here are some of the first to explore this oxidation and reveal that different types of oxygen defects are readily introduced in the phosphorene lattice, creating electron traps in some situations. These traps are responsible for the non-ambipolar behavior observed by experimental collaborators in air-exposed few-layer black phosphorus devices. Calculation results predict that air exposure of phosphorene
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
First principles investigation of Fe and Al bearing phase H
NASA Astrophysics Data System (ADS)
Tsuchiya, J.; Tsuchiya, T.
2015-12-01
exploration of these hydrous phases, such as the spin transition of Fe in phase H and the possibility of further phase transition of this new hydrous mineral using first principles calculation techniques and discuss the possible effects of this hydrous phase at the bottom of lower mantle.
ABINIT: First-principles approach to material and nanosystem properties
NASA Astrophysics Data System (ADS)
Gonze, X.; Amadon, B.; Anglade, P.-M.; Beuken, J.-M.; Bottin, F.; Boulanger, P.; Bruneval, F.; Caliste, D.; Caracas, R.; Côté, M.; Deutsch, T.; Genovese, L.; Ghosez, Ph.; Giantomassi, M.; Goedecker, S.; Hamann, D. R.; Hermet, P.; Jollet, F.; Jomard, G.; Leroux, S.; Mancini, M.; Mazevet, S.; Oliveira, M. J. T.; Onida, G.; Pouillon, Y.; Rangel, T.; Rignanese, G.-M.; Sangalli, D.; Shaltaf, R.; Torrent, M.; Verstraete, M. J.; Zerah, G.; Zwanziger, J. W.
2009-12-01
ABINIT [ http://www.abinit.org] allows one to study, from first-principles, systems made of electrons and nuclei (e.g. periodic solids, molecules, nanostructures, etc.), on the basis of Density-Functional Theory (DFT) and Many-Body Perturbation Theory. Beyond the computation of the total energy, charge density and electronic structure of such systems, ABINIT also implements many dynamical, dielectric, thermodynamical, mechanical, or electronic properties, at different levels of approximation. The present paper provides an exhaustive account of the capabilities of ABINIT. It should be helpful to scientists that are not familiarized with ABINIT, as well as to already regular users. First, we give a broad overview of ABINIT, including the list of the capabilities and how to access them. Then, we present in more details the recent, advanced, developments of ABINIT, with adequate references to the underlying theory, as well as the relevant input variables, tests and, if available, ABINIT tutorials. Program summaryProgram title: ABINIT Catalogue identifier: AEEU_v1_0 Distribution format: tar.gz Journal reference: Comput. Phys. Comm. Programming language: Fortran95, PERL scripts, Python scripts Computer: All systems with a Fortran95 compiler Operating system: All systems with a Fortran95 compiler Has the code been vectorized or parallelized?: Sequential, or parallel with proven speed-up up to one thousand processors. RAM: Ranges from a few Mbytes to several hundred Gbytes, depending on the input file. Classification: 7.3, 7.8 External routines: (all optional) BigDFT [1], ETSF IO [2], libxc [3], NetCDF [4], MPI [5], Wannier90 [6] Nature of problem: This package has the purpose of computing accurately material and nanostructure properties: electronic structure, bond lengths, bond angles, primitive cell size, cohesive energy, dielectric properties, vibrational properties, elastic properties, optical properties, magnetic properties, non-linear couplings, electronic and
Electron field emission in nanostructures: A first-principles study
NASA Astrophysics Data System (ADS)
Driscoll, Joseph Andrew
The objective of this work was to study electron field emission from several nanostructures using a first-principles framework. The systems studied were carbon nanowires, graphene nanoribbons, and nanotubes of varying composition. These particular structures were chosen because they have recently been identified as showing novel physical phenomena, as well as having tremendous industrial applications. We examined the field emission under a variety of conditions, including laser illumination and the presence of adsorbates. The goal was to explore how these conditions affect the field emission performance. In addition to the calculations, this dissertation has presented computational developments by the author that allowed these demanding calculations to be performed. There are many possible choices for basis when performing an electronic structure calculation. Examples are plane waves, atomic orbitals, and real-space grids. The best choice of basis depends on the structure of the system being analyzed and the physical processes involved (e.g., laser illumination). For this reason, it was important to conduct rigorous tests of basis set performance, in terms of accuracy and computational efficiency. There are no existing benchmark calculations for field emission, but transport calculations for nanostructures are similar, and so provide a useful reference for evaluating the performance of various basis sets. Based on the results, for the purposes of studying a non-periodic nanostructure under field emission conditions, we decided to use a real-space grid basis which incorporates the Lagrange function approach. Once a basis was chosen, in this case a real-space grid, the issue of boundary conditions arose. The problem is that with a non-periodic system, field emitted electron density can experience non-physical reflections from the boundaries of the calculation volume, leading to inaccuracies. To prevent this issue, we used complex absorbing potentials (CAPs) to absorb
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 100οC 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
Modulation of glucose transporters in rat diaphragm by sodium tungstate.
Girón, M D; Caballero, J J; Vargas, A M; Suárez, M D; Guinovart, J J; Salto, R
2003-05-08
Oral administration of sodium tungstate is an effective treatment for diabetes in animal models. We examined the effects of 6 weeks of oral administration of tungstate on glucose transporters (GLUT) in streptozotocin-induced diabetic rat diaphragm. Diabetes decreased GLUT4 expression while tungstate treatment normalized not only GLUT4 protein but also GLUT4 mRNA in the diabetic rats. Furthermore, treatment increased GLUT4 protein in plasma and internal membranes, suggesting a stimulation of its translocation to the plasma membrane. Tungstate had no effect on healthy animals. There were no differences in the total amount of GLUT1 transporter in any group. We conclude that the normoglycemic effect of tungstate may be partly due to a normalization of the levels and subcellular localization of GLUT4, which should result in an increase in muscle glucose uptake.
Application of Merrill's First Principles of Instruction in a Museum Education Context
ERIC Educational Resources Information Center
Nelson, Kari Ross
2015-01-01
In an effort to support a solid grounding in educational theory within the field of museum education, three texts considered essential reading for museum educators were surveyed for correlations with Merrill's First Principles of Instruction, an influential work in the field of instructional design. Each of five First Principles were found to be…
Interaction of rat liver glucocorticoid receptor with sodium tungstate.
Murakami, N; Healy, S P; Moudgil, V K
1982-06-15
Effects of sodium tungstate on various properties of rat liver glucocorticoid receptor were examined at pH7 and pH 8. At pH 7, [3H]triamcinolone acetonide binding in rat liver cytosol preparations was completely blocked in the presence of 10--20 mM-sodium tungstate at 4 degrees C, whereas at 37 degrees C a 30 min incubation of cytosol receptor preparation with 1 mM-sodium tungstate reduced the loss of unoccupied receptor by 50%. At pH 8.0, tungstate presence during the 37 degrees C incubation maintained the steroid-binding capacity of unoccupied glucocorticoid receptor at control (4 degrees C) levels. In addition, heat-activation of cytosolic glucocorticoid-receptor complex was blocked by 1 mM- and 10 mM-sodium tungstate at pH 7 and pH 8 respectively. The DNA-cellulose binding by activated receptor was also inhibited completely and irreversibly by 5 mM-tungstate at pH 7, whereas at pH 8 no significant effect was observed with up to 20 mM-tungstate. The entire DNA-cellulose-bound glucocorticoid-receptor complex from control samples could be extracted by incubation with 1 mM- and 20 mM-tungstate at pH 7 and pH 8 respectively, and appeared to sediment as a 4.3--4.6 S molecule, both in 0.01 M- and 0.3 M-KCl-containing sucrose gradients. Tungstate effects are, therefore, pH-dependent and appear to involve an interaction with both the non-activated and the activated forms of the glucocorticoid receptor.
Structural and electronic phase transitions of ThS2 from first-principles calculations
Guo, Yongliang; Wang, Changying; Qiu, Wujie; ...
2016-10-07
Performed a systematic study using first-principles methods of the pressure-induced structural and electronic phase transitions in ThS2, which may play an important role in the next generation nuclear energy fuel technology.
Application of First Principles Ni-Cd and Ni-H2 Battery Models to Spacecraft Operations
NASA Technical Reports Server (NTRS)
Timmerman, Paul; Bugga, Ratnakumar; DiStefano, Salvador
1997-01-01
The conclusions of the application of first principles model to spacecraft operations are: the first principles of Bi-phasic electrode presented model provides an explanation for many behaviors on voltage fading on LEO cycling.
Kaczmarek, Anna M; Liu, Ying-Ya; Van der Voort, Pascal; Van Deun, Rik
2013-04-21
In this paper, various microstructures of yttrium and lanthanum tungstates were synthesized under hydrothermal conditions, at pH 5, in a ligand-free environment, and in the presence of a dioctyl sodium sulfosuccinate (DSS) surfactant. It was observed that the shape of the nanobuilding blocks, and therefore the architecture of the microstructures, could be tuned by controlling the reaction conditions, such as the source of the rare earth, the amount of a surfactant and the reaction time. X-ray powder diffraction (XRD), elemental analysis, scanning electron microscopy (SEM), and N2 adsorption were employed to characterize the obtained products. The photoluminescent properties of Eu(3+) and Dy(3+) doped tungstate materials were investigated. Luminescence measurements showed an efficient charge transfer from the WO4(2-) groups to Eu(3+) and Dy(3+) ions. It was found that under UV excitation the Dy(3+) doped Y(WO3)2(OH)3 and La2(WO4)3 precursors exhibit white emission.
NASA Astrophysics Data System (ADS)
Vladimirov, P. V.; Borodin, V. A.
2017-02-01
Beryllium selected as a neutron multiplier material for the tritium breeding blanket of fusion reactor should withstand high doses of fast neutron irradiation. The damage produced by irradiation is usually evaluated assuming that the number of atomic displacements to the threshold displacement energy, Ed, which is considered as an intrinsic material parameter. In this work the value of Ed for hcp beryllium is estimated simultaneously from classical and first-principles molecular dynamics simulations. Quite similar quantitative pictures of defect production are observed in both simulation types, though the predicted displacement threshold values seem to be approximately two times higher in the first-principles approach. We expect that, after more detailed first-principles investigations, this approach can be used for scaling the damage prediction predictions by classical molecular dynamics, opening a way for more consistent calculations of displacement damage in materials.
Atta Mills, Ebenezer Fiifi Emire; Yan, Dawen; Yu, Bo; Wei, Xinyuan
2016-01-01
We propose a consolidated risk measure based on variance and the safety-first principle in a mean-risk portfolio optimization framework. The safety-first principle to financial portfolio selection strategy is modified and improved. Our proposed models are subjected to norm regularization to seek near-optimal stable and sparse portfolios. We compare the cumulative wealth of our preferred proposed model to a benchmark, S&P 500 index for the same period. Our proposed portfolio strategies have better out-of-sample performance than the selected alternative portfolio rules in literature and control the downside risk of the portfolio returns.
Growth mechanisms of ZnO(0001) investigated using the first-principles calculation
Fujiwara, Katsutoshi; Ishii, Akira; Abe, Tomoki; Ando, Koshi
2012-09-15
We investigated the dynamics of zinc (Zn) and oxygen (O) adsorbed atoms (adatoms) on a Zn-polar ZnO(0001) surface using the first-principles calculation. The results of the first-principles calculation revealed that a high-quality ZnO crystalline growth condition is induced by wurtzite structure packing under a Zn-rich growth condition using a Zn-polar ZnO(0001) surface. However, it was shown that an O adatom is not sufficient to promote surface atomic diffusion. For high-quality ZnO crystal, promoting surface diffusion of adatoms using high temperature is important.
First-principles calculation of the Curie temperature Slater-Pauling curve.
Takahashi, C; Ogura, M; Akai, H
2007-09-12
It is well known that the magnetizations as a function of the valence electron number per atom of 3d transition metal substitutional alloys form the so-called Slater-Pauling curve. Similarly, the Curie temperatures of these alloys also show systematic behaviour against the valence electron number. Though this fact has long been known, no attempt has been made so far to explain this behaviour from first principles. In this paper we calculate T(C) of 3d transition metal alloys in the framework of first-principles electronic structure calculation based on the local density approximation.
First Principles Calculations for X-ray Resonant Spectra and Elastic Properties
Lee, Yongbin
2004-01-01
In this thesis, we discuss applications of first principles methods to x-ray resonant spectra and elastic properties calculation. We start with brief reviews about theoretical background of first principles methods, such as density functional theory, local density approximation (LDA), LDA+U, and the linear augmented plane wave (LAPW) method to solve Kohn-Sham equations. After that we discuss x-ray resonant scattering (XRMS), x-ray magnetic circular dichroism (XMCD) and the branching problem in the heavy rare earths Ledges. In the last chapter we discuss the elastic properties of the second hardest material AlMgB_{14}.
Materials Data on Co(WO4)2 (SG:2) by Materials Project
Kristin Persson
2016-04-23
Computed materials data using density functional theory calculations. These calculations determine the electronic structure of bulk materials by solving approximations to the Schrodinger equation. For more information, see https://materialsproject.org/docs/calculations
Materials Data on Co(WO4)2 (SG:13) by Materials Project
Kristin Persson
2014-09-30
Computed materials data using density functional theory calculations. These calculations determine the electronic structure of bulk materials by solving approximations to the Schrodinger equation. For more information, see https://materialsproject.org/docs/calculations
2005-02-22
GRANT NUMBER 4. TITLE AND SUBTITLE New Methodology For First Principle Calculations Of Electrical Levels For Radiation Induced Defects In Silicates ...materials, space materials, Silicon on Insulator ( SOI ) materials 16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON DONALD J SMITH
ERIC Educational Resources Information Center
Bowen, J. Philip; Sorensen, Jennifer B.; Kirschner, Karl N.
2007-01-01
The analysis explains the basis set superposition error (BSSE) and fragment relaxation involved in calculating the interaction energies using various first principle theories. Interacting the correlated fragment and increasing the size of the basis set can help in decreasing the BSSE to a great extent.
Khokhlov, Alexei; Austin, Joanna; Bacon, C.
2015-03-02
Hydrogen has emerged as an important fuel across a range of industries as a means of achieving energy independence and to reduce emissions. DDT and the resulting detonation waves in hydrogen-oxygen can have especially catastrophic consequences in a variety of industrial and energy producing settings related to hydrogen. First-principles numerical simulations of flame acceleration and DDT are required for an in-depth understanding of the phenomena and facilitating design of safe hydrogen systems. The goals of this project were (1) to develop first-principles petascale reactive flow Navier-Stokes simulation code for predicting gaseous high-speed combustion and detonation (HSCD) phenomena and (2) demonstrate feasibility of first-principles simulations of rapid flame acceleration and deflagration-to-detonation transition (DDT) in stoichiometric hydrogen-oxygen mixture (2H_{2} + O_{2}). The goals of the project have been accomplished. We have developed a novel numerical simulation code, named HSCD, for performing first-principles direct numerical simulations of high-speed hydrogen combustion. We carried out a series of validating numerical simulations of inert and reactive shock reflection experiments in shock tubes. We then performed a pilot numerical simulation of flame acceleration in a long pipe. The simulation showed the transition of the rapidly accelerating flame into a detonation. The DDT simulations were performed using BG/Q Mira at the Argonne National Laboratory, currently the fourth fastest super-computer in the world.
First-principles Calculations of Twin-boundary and Stacking-fault Energies in Magnesium
2010-01-01
The interfacial energies of twin boundaries and stacking faults in metal magnesium have been calculated using first-principles supercell approach...Four types of twin boundaries and two types of stacking faults are investigated, namely, those due to the mirror reflection, the mirror glide and the
First-principles calculations of shear moduli for Monte Carlo-simulated Coulomb solids
NASA Technical Reports Server (NTRS)
Ogata, Shuji; Ichimaru, Setsuo
1990-01-01
The paper presents a first-principles study of the shear modulus tensor for perfect and imperfect Coulomb solids. Allowance is made for the effects of thermal fluctuations for temperatures up to the melting conditions. The present theory treats the cases of the long-range Coulomb interaction, where volume fluctuations should be avoided in the Ewald sums.
ERIC Educational Resources Information Center
Lee, Sunghye; Koszalka, Tiffany A.
2016-01-01
The First Principles of Instruction (FPI) represent ideologies found in most instructional design theories and models. Few attempts, however, have been made to empirically test the relationship of these FPI to instructional outcomes. This study addresses whether the degree to which FPI are implemented in courses makes a difference to student…
Effects of sodium tungstate on oxidative stress enzymes in rats.
Sachdeva, Sherry; Kushwaha, Pramod; Flora, S J S
2013-09-01
Tungsten, due to its distinguished physical properties, has wide industrial and military applications. Environmental exposure to tungsten, which mainly occurs through various sources like food, water, soil, etc., is of growing concern as various toxic effects have recently been reported. In this study, we investigated the effects of oral and intraperitoneal (i.p.) administration of sodium tungstate on various biochemical variables indicative of oxidative stress in erythrocytes and soft tissue damage in rats. Male rats were administered to 119 mg, 238 mg/kg of sodium tungstate orally or 20 mg and 41 mg/kg through i.p. route, for 14 consecutive days. The results demonstrated a significant increase in Reactive Oxygen Species (ROS) and an increase in catalase and glutathione peroxidase antioxidant enzymes activities in erythrocytes. Erythrocyte glutathione-S-transferase (GST) activity showed significant inhibition, while tissue ROS and thiobarbituric acid reactive substance levels increased accompanied by a decreased reduced glutathione, oxidized glutathione (GSH:GSSG) ratio. These changes were supported by an increase in plasma transaminases activities, creatinine, and urea levels, suggesting hepatic and renal injury. These biochemical alterations were prominent in rats intraperitoneally administrated with sodium tungstate than oral administration, suggesting more pronounced toxicity. The study also suggests oxidative stress as one of the major mechanism involved in the toxic manifestations of sodium tungstate.
First-principles based calculation of phonon spectrain substitutionally disordered alloys
NASA Astrophysics Data System (ADS)
Ghosh, Subhradip
2013-02-01
A first-principles based solution to the longstanding problem of calculating the phonon spectra in substitutional disordered alloys where strong force-constant disorder plays a significantrole is provided by a combination of first-principles electronicstructure tools, physically reasonable models of force-constant in alloyenvironments, and the Itinerant Coherent-Potntial Approximation (ICPA) by Ghosh and co-workers (S. Ghosh et. al., Physical Review B 66, 214206 (2002)). Wehere present the salient features of such hybrid formalism and illustrate its capability by the computation of phonon spectrafor disordered alloys with large size mismatch of end point components. We demonstrate that the consideration of local environments insize-mismatched alloys is crucial in understanding the microscopicinterplay of forces between various pairs of chemical specie and a correctdepiction of these is important for computation of accurate phonondispersions in these systems.
NASA Astrophysics Data System (ADS)
Kakehashi, Yoshiro; Chandra, Sumal
2016-04-01
We have developed a first-principles local ansatz wavefunction approach with momentum-dependent variational parameters on the basis of the tight-binding LDA+U Hamiltonian. The theory goes beyond the first-principles Gutzwiller approach and quantitatively describes correlated electron systems. Using the theory, we find that the momentum distribution function (MDF) bands of paramagnetic bcc Fe along high-symmetry lines show a large deviation from the Fermi-Dirac function for the d electrons with eg symmetry and yield the momentum-dependent mass enhancement factors. The calculated average mass enhancement m*/m = 1.65 is consistent with low-temperature specific heat data as well as recent angle-resolved photoemission spectroscopy (ARPES) data.
Tadano, Terumasa; Tsuneyuki, Shinji
2015-12-31
We show a first-principles approach for analyzing anharmonic properties of lattice vibrations in solids. We firstly extract harmonic and anharmonic force constants from accurate first-principles calculations based on the density functional theory. Using the many-body perturbation theory of phonons, we then estimate the phonon scattering probability due to anharmonic phonon-phonon interactions. We show the validity of the approach by computing the lattice thermal conductivity of Si, a typical covalent semiconductor, and selected thermoelectric materials PbTe and Bi{sub 2}Te{sub 3} based on the Boltzmann transport equation. We also show that the phonon lifetime and the lattice thermal conductivity of the high-temperature phase of SrTiO{sub 3} can be estimated by employing the perturbation theory on top of the solution of the self-consistent phonon equation.
Magnetically induced phonon splitting in ACr2O4 spinels from first principles
Wysocki, Aleksander L.; Birol, Turan
2016-04-22
We study the magnetically-induced phonon splitting in cubic ACr2O4 (A=Mg, Zn, Cd) spinels from first principles and demonstrate that the sign of the splitting, which is experimentally observed to be opposite in CdCr2O4 compared to ZnCr2O4 and MgCr2O4, is determined solely by the particular magnetic ordering pattern observed in these compounds. We further show that this interaction between magnetism and phonon frequencies can be fully described by the previously proposed spin-phonon coupling model [C. J. Fennie and K. M. Rabe, Phys. Rev. Lett. 96, 205505 (2006)] that includes only the nearest neighbor exchange. In conclusion, using this model with materialsmore » specific parameters calculated from first principles, we provide additional insights into the physics of spin-phonon coupling in this intriguing family of compounds.« less
Fattebert, Jean-Luc; Lau, Edmond Y; Bennion, Brian J; Huang, Patrick; Lightstone, Felice C
2015-12-08
Enzymes are complicated solvated systems that typically require many atoms to simulate their function with any degree of accuracy. We have recently developed numerical techniques for large scale first-principles molecular dynamics simulations and applied them to the study of the enzymatic reaction catalyzed by acetylcholinesterase. We carried out density functional theory calculations for a quantum-mechanical (QM) subsystem consisting of 612 atoms with an O(N) complexity finite-difference approach. The QM subsystem is embedded inside an external potential field representing the electrostatic effect due to the environment. We obtained finite-temperature sampling by first-principles molecular dynamics for the acylation reaction of acetylcholine catalyzed by acetylcholinesterase. Our calculations show two energy barriers along the reaction coordinate for the enzyme-catalyzed acylation of acetylcholine. The second barrier (8.5 kcal/mol) is rate-limiting for the acylation reaction and in good agreement with experiment.
First-principles investigation of mechanical properties of silicene, germanene and stanene
NASA Astrophysics Data System (ADS)
Mortazavi, Bohayra; Rahaman, Obaidur; Makaremi, Meysam; Dianat, Arezoo; Cuniberti, Gianaurelio; Rabczuk, Timon
2017-03-01
Two-dimensional allotropes of group-IV substrates including silicene, germanene and stanene have recently attracted considerable attention in nanodevice fabrication industry. These materials involving the buckled structure have been experimentally fabricated lately. In this study, first-principles density functional theory calculations were utilized to investigate the mechanical properties of single-layer and free-standing silicene, germanene and stanene. Uniaxial tensile and compressive simulations were carried out to probe and compare stress-strain properties; such as the Young's modulus, Poisson's ratio and ultimate strength. We evaluated the chirality effect on the mechanical response and bond structure of the 2D substrates. Our first-principles simulations suggest that in all studied samples application of uniaxial loading can alter the electronic nature of the buckled structures into the metallic character. Our investigation provides a general but also useful viewpoint with respect to the mechanical properties of silicene, germanene and stanene.
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.
Thermal conductivity of glassy GeTe4 by first-principles molecular dynamics.
Bouzid, Assil; Zaoui, Hayat; Luca Palla, Pier; Ori, Guido; Boero, Mauro; Massobrio, Carlo; Cleri, Fabrizio; Lampin, Evelyne
2017-03-29
A transient thermal regime is achieved in glassy GeTe4 by first-principles molecular dynamics following the recently proposed "approach-to-equilibrium" methodology. The temporal and spatial evolution of the temperature do comply with the time-dependent solution of the heat equation. We demonstrate that the time scales required to create the hot and the cold parts of the system and observe the resulting approach to equilibrium are accessible to first-principles molecular dynamics. Such a strategy provides the thermal conductivity from the characteristic decay time. We rationalize in detail the impact on the thermal conductivity of the initial temperature difference, the equilibration duration, and the main simulation features.
Grain growth in U-7Mo alloy: A combined first-principles and phase field study
NASA Astrophysics Data System (ADS)
Mei, Zhi-Gang; Liang, Linyun; Kim, Yeon Soo; Wiencek, Tom; O'Hare, Edward; Yacout, Abdellatif M.; Hofman, Gerard; Anitescu, Mihai
2016-05-01
Grain size is an important factor in controlling the swelling behavior in irradiated U-Mo dispersion fuels. Increasing the grain size in U-Mo fuel particles by heat treatment is believed to delay the fuel swelling at high fission density. In this work, a multiscale simulation approach combining first-principles calculation and phase field modeling is used to investigate the grain growth behavior in U-7Mo alloy. The density functional theory based first-principles calculations were used to predict the material properties of U-7Mo alloy. The obtained grain boundary energies were then adopted as an input parameter for mesoscale phase field simulations. The effects of annealing temperature, annealing time and initial grain structures of fuel particles on the grain growth in U-7Mo alloy were examined. The predicted grain growth rate compares well with the empirical correlation derived from experiments.
Fattebert, Jean-Luc; Lau, Edmond Y.; Bennion, Brian J.; ...
2015-10-22
Enzymes are complicated solvated systems that typically require many atoms to simulate their function with any degree of accuracy. We have recently developed numerical techniques for large scale First-Principles molecular dynamics simulations and applied them to study the enzymatic reaction catalyzed by acetylcholinesterase. We carried out Density functional theory calculations for a quantum mechanical (QM) sub- system consisting of 612 atoms with an O(N) complexity finite-difference approach. The QM sub-system is embedded inside an external potential field representing the electrostatic effect due to the environment. We obtained finite temperature sampling by First-Principles molecular dynamics for the acylation reaction of acetylcholinemore » catalyzed by acetylcholinesterase. Our calculations shows two energies barriers along the reaction coordinate for the enzyme catalyzed acylation of acetylcholine. In conclusion, the second barrier (8.5 kcal/mole) is rate-limiting for the acylation reaction and in good agreement with experiment.« less
Fattebert, Jean-Luc; Lau, Edmond Y.; Bennion, Brian J.; Huang, Patrick; Lightstone, Felice C.
2015-10-22
Enzymes are complicated solvated systems that typically require many atoms to simulate their function with any degree of accuracy. We have recently developed numerical techniques for large scale First-Principles molecular dynamics simulations and applied them to study the enzymatic reaction catalyzed by acetylcholinesterase. We carried out Density functional theory calculations for a quantum mechanical (QM) sub- system consisting of 612 atoms with an O(N) complexity finite-difference approach. The QM sub-system is embedded inside an external potential field representing the electrostatic effect due to the environment. We obtained finite temperature sampling by First-Principles molecular dynamics for the acylation reaction of acetylcholine catalyzed by acetylcholinesterase. Our calculations shows two energies barriers along the reaction coordinate for the enzyme catalyzed acylation of acetylcholine. In conclusion, the second barrier (8.5 kcal/mole) is rate-limiting for the acylation reaction and in good agreement with experiment.
NASA Astrophysics Data System (ADS)
Bennett, Joseph W.; Rabe, Karin M.
2012-11-01
In this concept paper, the development of strategies for the integration of first-principles methods with crystallographic database mining for the discovery and design of novel ferroelectric materials is discussed, drawing on the results and experience derived from exploratory investigations on three different systems: (1) the double perovskite Sr(Sb1/2Mn1/2)O3 as a candidate semiconducting ferroelectric; (2) polar derivatives of schafarzikite MSb2O4; and (3) ferroelectric semiconductors with formula M2P2(S,Se)6. A variety of avenues for further research and investigation are suggested, including automated structure type classification, low-symmetry improper ferroelectrics, and high-throughput first-principles searches for additional representatives of structural families with desirable functional properties.
Zhou, Fei; Nielson, Weston; Xia, Yi; Ozolins, Vidvuds
2014-10-27
First-principles prediction of lattice thermal conductivity K_{L} of strongly anharmonic crystals is a long-standing challenge in solid state physics. Using recent advances in information science, we propose a systematic and rigorous approach to this problem, compressive sensing lattice dynamics (CSLD). Compressive sensing is used to select the physically important terms in the lattice dynamics model and determine their values in one shot. Non-intuitively, high accuracy is achieved when the model is trained on first-principles forces in quasi-random atomic configurations. The method is demonstrated for Si, NaCl, and Cu_{12}Sb_{4}S_{13}, an earth-abundant thermoelectric with strong phononphonon interactions that limit the room-temperature K_{L} to values near the amorphous limit.
Zhou, Fei; Nielson, Weston; Xia, Yi; ...
2014-10-27
First-principles prediction of lattice thermal conductivity KL of strongly anharmonic crystals is a long-standing challenge in solid state physics. Using recent advances in information science, we propose a systematic and rigorous approach to this problem, compressive sensing lattice dynamics (CSLD). Compressive sensing is used to select the physically important terms in the lattice dynamics model and determine their values in one shot. Non-intuitively, high accuracy is achieved when the model is trained on first-principles forces in quasi-random atomic configurations. The method is demonstrated for Si, NaCl, and Cu12Sb4S13, an earth-abundant thermoelectric with strong phononphonon interactions that limit the room-temperature KLmore » to values near the amorphous limit.« less
Zhou, Fei; Nielson, Weston; Xia, Yi; Ozoliņš, Vidvuds
2014-10-01
First-principles prediction of lattice thermal conductivity κ_{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 κ_{L} to values near the amorphous limit.
First principles finite temperature magnetism of defects in Fe using Wang-Landau method
NASA Astrophysics Data System (ADS)
Rusanu, Aurelian; Nicholson, D. M.; Odbadrakh, Kh.; Brown, Gregory; Eisenbach, Markus
2011-03-01
Magnetic structure of materials with defects presents a strong dependence on local atomic arrangements. This dependence affects mechanical, magneto-caloric, and magnetization properties. Insights into thermodynamic and magnetic fluctuations at defects in Fe are obtained from first principle analysis by deploying the first principle local self consistent multiple scattering method(LSMS) and Wang-Landau statistical method. The computation of thermodynamic properties requires the sampling of a large number of configurations. To reduce the computational effort a Heisenberg model will be used to speed the configuration sampling procedures. The approach will be demonstrated for Fe systems and will address the magnetic structure of defects. This work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the US DoE, Office of Science, Office of Basic Energy Sciences. Calculations performed at the National Center for Computational Sciences.
Equation of state for technetium from X-ray diffraction and first-principle calculations
NASA Astrophysics Data System (ADS)
Mast, Daniel S.; Kim, Eunja; Siska, Emily M.; Poineau, Frederic; Czerwinski, Kenneth R.; Lavina, Barbara; Forster, Paul M.
2016-08-01
The ambient temperature equation of state (EoS) of technetium metal has been measured by X-ray diffraction. The metal was compressed using a diamond anvil cell and using a 4:1 methanol-ethanol pressure transmitting medium. The maximum pressure achieved, as determined from the gold pressureEquation of state for technetium from X-ray diffraction and first-principle calculations scale, was 67 GPa. The compression data shows that the HCP phase of technetium is stable up to 67 GPa. The compression curve of technetium was also calculated using first-principles total-energy calculations. Utilizing a number of fitting strategies to compare the experimental and theoretical data it is determined that the Vinet equation of state with an ambient isothermal bulk modulus of B0T=288 GPa and a first pressure derivative of B‧=5.9(2) best represent the compression behavior of technetium metal.
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.
Guidez, Emilie B; Gordon, Mark S
2015-03-12
The modeling of dispersion interactions in density functional theory (DFT) is commonly performed using an energy correction that involves empirically fitted parameters for all atom pairs of the system investigated. In this study, the first-principles-derived dispersion energy from the effective fragment potential (EFP) method is implemented for the density functional theory (DFT-D(EFP)) and Hartree-Fock (HF-D(EFP)) energies. Overall, DFT-D(EFP) performs similarly to the semiempirical DFT-D corrections for the test cases investigated in this work. HF-D(EFP) tends to underestimate binding energies and overestimate intermolecular equilibrium distances, relative to coupled cluster theory, most likely due to incomplete accounting for electron correlation. Overall, this first-principles dispersion correction yields results that are in good agreement with coupled-cluster calculations at a low computational cost.
First-Principles Atomic Force Microscopy Image Simulations with Density Embedding Theory.
Sakai, Yuki; Lee, Alex J; Chelikowsky, James R
2016-05-11
We present an efficient first-principles method for simulating noncontact atomic force microscopy (nc-AFM) images using a "frozen density" embedding theory. Frozen density embedding theory enables one to efficiently compute the tip-sample interaction by considering a sample as a frozen external field. This method reduces the extensive computational load of first-principles AFM simulations by avoiding consideration of the entire tip-sample system and focusing on the tip alone. We demonstrate that our simulation with frozen density embedding theory accurately reproduces full density functional theory simulations of freestanding hydrocarbon molecules while the computational time is significantly reduced. Our method also captures the electronic effect of a Cu(111) substrate on the AFM image of pentacene and reproduces the experimental AFM image of Cu2N on a Cu(100) surface. This approach is applicable for theoretical imaging applications on large molecules, two-dimensional materials, and materials surfaces.
Effects of sodium tungstate on insulin and glucagon secretion in the perfused rat pancreas.
Rodríguez-Gallardo, J; Silvestre, R A; Egido, E M; Marco, J
2000-08-18
Both the direct effect of sodium tungstate on insulin and glucagon secretion in the perfused rat pancreas, and the insulin response to glucose and arginine in pancreases isolated from tungstate-pretreated rats were studied. Infusion of tungstate stimulated insulin output in a dose-dependent manner. The insulinotropic effect of tungstate was observed at normal (5.5 mM), and moderately high (9 mM) glucose concentrations, but not at a low glucose concentration (3.2 mM). Tungstate-induced insulin output was blocked by diazoxide, somatostatin, and amylin, suggesting several targets for tungstate at the B-cell secretory machinery. Glucagon release was not modified by tungstate. Pancreases from chronically tungstate-treated rats showed an enhanced response to glucose but not to arginine. Our results indicate that the reported reduction of glycemia caused by tungstate administration is, at least in part, due to its direct insulinotropic activity. Furthermore, chronic tungstate treatment may prime the B-cell, leading to over-response to a glucose stimulus.
Vanadium doping of LiMnPO4: Vibrational spectroscopy and first-principle studies
NASA Astrophysics Data System (ADS)
Kellerman, D.; Medvedeva, N.; Mukhina, N.; Semenova, A.; Baklanova, I.; Perelyaeva, L.; Gorshkov, V.
2014-01-01
The samples of pure and 10% vanadium-doped LiMnPO4 have been synthesized by the solid-state reaction technique. The results of Raman and infrared absorption spectroscopy show that the vanadium atoms replace phosphorus giving rise to LiMn(PO4)1-x(VO4)x solid solutions. This conclusion is confirmed by the first-principle studies.
First principles total energy study of NbCr{sub 2} + V Laves phase ternary system
Ormeci, A.; Chen, S.P.; Wills, J.M.; Albers, R.C.
1999-04-01
The C15 NbCr{sub 2} + V Laves phase ternary system is studied by using a first-principles, self-consistent, full-potential total energy method. Equilibrium lattice parameters, cohesive energies, density of states and formation energies of substitutional defects are calculated. Results of all these calculations show that in the C15 NbCr{sub 2} + V compounds, V atoms substitute Cr atoms only.
Energetics of point and planar defects in aluminium from first-principles calculations
NASA Astrophysics Data System (ADS)
Denteneer, P. J. H.; Soler, J. M.
1991-06-01
Formation energies of the vacancy and self-interstitial in Al, as well as energies of intrinsic, extrinsic, and twin-boundary stacking faults are calculated from first-principles. The electronic structure and forces on the atoms are calculated in the framework of the Augmented Plane Wave method using new algorithms proposed by Williams and Soler, enabling an ab initio approach to long-standing questions on defects in metals.
X-ray magnetic circular dichroism in Co2FeGa: First-principles calculations
NASA Astrophysics Data System (ADS)
Kukusta, D. A.; Antonov, V. N.; Yaresko, A. N.
2011-08-01
The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of the Heusler alloy Co2FeGa were investigated theoretically from first principles, using the fully relativistic Dirac linear MT-orbital (LMTO) band structure method. Densities of valence states, orbital and spin magnetic moments are analyzed and discussed. The origin of the XMCD spectra in the Co2FeGa compound is examined. The calculated results are compared with available experimental data.
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.
NASA Astrophysics Data System (ADS)
Canning, Andrew
2013-03-01
Inorganic scintillation phosphors (scintillators) are extensively employed as radiation detector materials in many fields of applied and fundamental research such as medical imaging, high energy physics, astrophysics, oil exploration and nuclear materials detection for homeland security and other applications. The ideal scintillator for gamma ray detection must have exceptional performance in terms of stopping power, luminosity, proportionality, speed, and cost. Recently, trivalent lanthanide dopants such as Ce and Eu have received greater attention for fast and bright scintillators as the optical 5d to 4f transition is relatively fast. However, crystal growth and production costs remain challenging for these new materials so there is still a need for new higher performing scintillators that meet the needs of the different application areas. First principles calculations can provide a useful insight into the chemical and electronic properties of such materials and hence can aid in the search for better new scintillators. In the past there has been little first-principles work done on scintillator materials in part because it means modeling f electrons in lanthanides as well as complex excited state and scattering processes. In this talk I will give an overview of the scintillation process and show how first-principles calculations can be applied to such systems to gain a better understanding of the physics involved. I will also present work on a high-throughput first principles approach to select new scintillator materials for fabrication as well as present more detailed calculations to study trapping process etc. that can limit their brightness. This work in collaboration with experimental groups has lead to the discovery of some new bright scintillators. Work supported by the U.S. Department of Homeland Security and carried out under U.S. Department of Energy Contract no. DE-AC02-05CH11231 at Lawrence Berkeley National Laboratory.
First-principles investigation of anistropic hole mobilities in organic semiconductors.
Wen, Shu-Hao; Li, An; Song, Junling; Deng, Wei-Qiao; Han, Ke-Li; Goddard, William A
2009-07-02
We report a simple first-principles-based simulation model (combining quantum mechanics with Marcus-Hush theory) that provides the quantitative structural relationships between angular resolution anisotropic hole mobility and molecular structures and packing. We validate that this model correctly predicts the anisotropic hole mobilities of ruberene, pentacene, tetracene, 5,11-dichlorotetracene (DCT), and hexathiapentacene (HTP), leading to results in good agreement with experiment.
Dynamic first principles model of a complete reversible fuel cell system
NASA Astrophysics Data System (ADS)
Brown, Tim M.; Brouwer, Jacob; Samuelsen, G. Scott; Holcomb, Franklin H.; King, Joel
A dynamic model of a discrete reversible fuel cell (RFC) system has been developed in a Matlab Simulink ® environment. The model incorporates first principles dynamic component models of a proton exchange membrane (PEM) fuel cell, a PEM electrolyzer, a metal hydride hydrogen storage tank, and a cooling system radiator, as well as empirical models of balance of plant components. Dynamic simulations show unique charging and discharging control issues and highlight factors contributing to overall system efficiency.
(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 α 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).
Accelerated materials design of fast oxygen ionic conductors based on first principles calculations
NASA Astrophysics Data System (ADS)
He, Xingfeng; Mo, Yifei
Over the past decades, significant research efforts have been dedicated to seeking fast oxygen ion conductor materials, which have important technological applications in electrochemical devices such as solid oxide fuel cells, oxygen separation membranes, and sensors. Recently, Na0.5Bi0.5TiO3 (NBT) was reported as a new family of fast oxygen ionic conductor. We will present our first principles computation study aims to understand the O diffusion mechanisms in the NBT material and to design this material with enhanced oxygen ionic conductivity. Using the NBT materials as an example, we demonstrate the computation capability to evaluate the phase stability, chemical stability, and ionic diffusion of the ionic conductor materials. We reveal the effects of local atomistic configurations and dopants on oxygen diffusion and identify the intrinsic limiting factors in increasing the ionic conductivity of the NBT materials. Novel doping strategies were predicted and demonstrated by the first principles calculations. In particular, the K doped NBT compound achieved good phase stability and an order of magnitude increase in oxygen ionic conductivity of up to 0.1 S cm-1 at 900 K compared to the experimental Mg doped compositions. Our results provide new avenues for the future design of the NBT materials and demonstrate the accelerated design of new ionic conductor materials based on first principles techniques. This computation methodology and workflow can be applied to the materials design of any (e.g. Li +, Na +) fast ion-conducting materials.
Coarse graining approach to First principles modeling of radiation cascade in large Fe super-cells
NASA Astrophysics Data System (ADS)
Odbadrakh, Khorgolkhuu; Nicholson, Don; Rusanu, Aurelian; Wang, Yang; Stoller, Roger; Zhang, Xiaoguang; Stocks, George
2012-02-01
First principles techniques employed to understand systems at an atomistic level are not practical for large systems consisting of millions of atoms. We present an efficient coarse graining approach to bridge the first principles calculations of local electronic properties to classical Molecular Dynamics (MD) simulations of large structures. Local atomic magnetic moments in crystalline Fe are perturbed by radiation generated defects. The effects are most pronounced near the defect core and decay with distance. We develop a coarse grained technique based on the Locally Self-consistent Multiple Scattering (LSMS) method that exploits the near-sightedness of the electron Green function. The atomic positions were determined by MD with an embedded atom force field. The local moments in the neighborhood of the defect cores are calculated with first-principles based on full local structure information. Atoms in the rest of the system are modeled by representative atoms with approximated properties. This work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.
Tungstate sulfuric acid (TSA)/KMnO4 as a novel heterogeneous system for rapid deoximation.
Karami, Bahador; Montazerozohori, Morteza
2006-09-28
Neat chlorosulfonic acid reacts with anhydrous sodium tungstate to give tungstate sulfuric acid (TSA), a new dibasic inorganic solid acid in which two sulfuric acid molecules connect to a tungstate moiety via a covalent bond. A variety of oximes were oxidized to their parent carbonyl compounds under mild conditions with excellent yields in short times by a heterogeneous wet TSA/KMnO4 in dichloromethane system.
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
NASA Astrophysics Data System (ADS)
Banerjee, Amartya S.; Suryanarayana, Phanish
2016-11-01
We formulate and implement Cyclic Density Functional Theory (Cyclic DFT) - a self-consistent first principles simulation method for nanostructures with cyclic symmetries. Using arguments based on Group Representation Theory, we rigorously demonstrate that the Kohn-Sham eigenvalue problem for such systems can be reduced to a fundamental domain (or cyclic unit cell) augmented with cyclic-Bloch boundary conditions. Analogously, the equations of electrostatics appearing in Kohn-Sham theory can be reduced to the fundamental domain augmented with cyclic boundary conditions. By making use of this symmetry cell reduction, we show that the electronic ground-state energy and the Hellmann-Feynman forces on the atoms can be calculated using quantities defined over the fundamental domain. We develop a symmetry-adapted finite-difference discretization scheme to obtain a fully functional numerical realization of the proposed approach. We verify that our formulation and implementation of Cyclic DFT is both accurate and efficient through selected examples. The connection of cyclic symmetries with uniform bending deformations provides an elegant route to the ab-initio study of bending in nanostructures using Cyclic DFT. As a demonstration of this capability, we simulate the uniform bending of a silicene nanoribbon and obtain its energy-curvature relationship from first principles. A self-consistent ab-initio simulation of this nature is unprecedented and well outside the scope of any other systematic first principles method in existence. Our simulations reveal that the bending stiffness of the silicene nanoribbon is intermediate between that of graphene and molybdenum disulphide - a trend which can be ascribed to the variation in effective thickness of these materials. We describe several future avenues and applications of Cyclic DFT, including its extension to the study of non-uniform bending deformations and its possible use in the study of the nanoscale flexoelectric effect.
First-principles simulation of Raman spectra and structural properties of quartz up to 5 GPa
NASA Astrophysics Data System (ADS)
Liu, Lei; Lv, Chao-Jia; Zhuang, Chun-Qiang; Yi, Li; Liu, Hong; Du, Jian-Guo
2015-12-01
The crystal structure and Raman spectra of quartz are calculated by using first-principles method in a pressure range from 0 to 5 GPa. The results show that the lattice constants (a, c, and V) decrease with increasing pressure and the a-axis is more compressible than the c axis. The Si-O bond distance decreases with increasing pressure, which is in contrast to experimental results reported by Hazen et al. [Hazen R M, Finger L W, Hemley R J and Mao H K 1989 Solid State Communications 725 507-511], and Glinnemann et al. [Glinnemann J, King H E Jr, Schulz H, Hahn T, La Placa S J and Dacol F 1992 Z. Kristallogr. 198 177-212]. The most striking changes are of inter-tetrahedral O-O distances and Si-O-Si angles. The volume of the tetrahedron decreased by 0.9% (from 0 to 5 GPa), which suggests that it is relatively rigid. Vibrational models of the quartz modes are identified by visualizing the associated atomic motions. Raman vibrations are mainly controlled by the deformation of the tetrahedron and the changes in the Si-O-Si bonds. Vibrational directions and intensities of atoms in all Raman modes just show little deviations when pressure increases from 0 to 5 GPa. The pressure derivatives (dνi/dP) of the 12 Raman frequencies are obtained at 0 GPa-5 GPa. The calculated results show that first-principles methods can well describe the high-pressure structural properties and Raman spectra of quartz. The combination of first-principles simulations of the Raman frequencies of minerals and Raman spectroscopy experiments is a useful tool for exploring the stress conditions within the Earth. Project supported by the Key Laboratory of Earthquake Prediction, Institute of Earthquake Science, China Earthquake Administration (CEA) (Grant No. 2012IES010201) and the National Natural Science Foundation of China (Grant Nos. 41174071 and 41373060).
Synthesis of europium- or terbium-activated calcium tungstate phosphors
NASA Astrophysics Data System (ADS)
Forgaciu, Flavia; Popovici, Elisabeth-Jeanne; Ungur, Laura; Vadan, Maria; Vasilescu, Marilena; Nazarov, Mihail
2001-06-01
Utilization of luminescent substances in various optoelectronic devices depends on their luminescent properties and sensitivity to various excitation radiation as well as on particle size distribution and crystalline structure of luminous powders. Calcium tungstate phosphors are well excited with roentgen radiation, so that they are largely used for manufacture of x-ray intensifying screens. Being sensitive to short UV-radiation as well, they could be utilized in Plasma Display Panels or in advertising signs fluorescent tubes. In order to diversify the utilization possibilities of this tungstate class, luminescent powders based on CaWO4:Eu3+ and CaWO4:Tb3+ were synthesized and characterized. As compared with the starting self-activated phosphor, larger excitation wavelength domain and emission colors from blue-to-green-to- yellow-to-red were obtained. The good UV excitability and variable luminescence color recommend these phosphors for optoelectronic device manufacture.
Solubility of sodium tungstate in nitrate-nitrite melts
Yurkinskii, V.P.; Firsova, E.G.; Morachevskii, A.G.; Sazanova, O.B.
1988-10-10
Nitrate melts are employed as electrolytes for the electrochemical oxidation of tungsten. The authors studied the solubility of sodium tungstate in a number of nitrate-nitrite melts. The investigations were carried out in individual melts of NaNO/sub 3/ and NaNO/sub 2/ and in LiNO/sub 3/-NaNO/sub 3/-KNO/sub 3/ and NaNO/sub 3/-KNO/sub 3/ eutectic mixtures in the 440-690 K temperature range in an atmosphere of argon. The solubility of sodium tungstate increases slightly upon the transition from an LiNO/sub 3/-NaNO/sub 3/-KNO/sub 3/ melt to an NaNO/sub 3/-KNO/sub 3/ melt. The solubility of Na/sub 2/WO/sub 4/ in sodium nitrite is considerably higher than that in sodium nitrate.
Configurations of nuclei in Au-catalyzed Si nanowire growth: a first-principles study
NASA Astrophysics Data System (ADS)
Yao, Luchi; Zhou, Xiaohao; Chen, Xiaoshuang
2016-10-01
The configurations of nuclei in Au catalyzed Si nanowire growth were investigated through an ab-initio thermodynamic-combined approach. We discussed the relation between the configurations and formation energies of the lateral walls of the nucleus in nanowire growth numerically by the classical nucleation theory. The nucleation model was parameterized by the formation energies of surfaces, interfaces and steps calculated in first-principles methods. The configurations of the nuclei were determined by the Wulff theorem. Moreover, we found configurations of the nuclei are different in two different Si-Au contact structures. This study provides an important basis to understand the step-flow process in nanowire growth.
First-principles study on bottom-up fabrication process of atomically precise graphene nanoribbons
NASA Astrophysics Data System (ADS)
Kaneko, Tomoaki; Tajima, Nobuo; Ohno, Takahisa
2016-06-01
We investigate the energetics of a polyanthracene formation in the bottom-up fabrication of atomically precise graphene nanoribbons on Au(111) using first-principles calculations based on the density functional theory. We show that the structure of precursor molecules plays a decisive role in the C-C coupling reaction. The reaction energy of the dimerization of anthracene dimers is a larger negative value than that of the dimerization of anthracene monomers, suggesting that the precursor molecule used in experiments has a favorable structure for graphene nanoribbon fabrication.
NASA Astrophysics Data System (ADS)
Kiyohara, Shin; Mizoguchi, Teruyasu
2016-08-01
Segregation of silver at copper grain boundaries was investigated using theoretical calculations. Empirical potentials for copper-silver alloys were generated to systematically investigate the segregation. The segregation energies of the [001]-axis symmetric tilt Σ5 (210) and Σ25 (430) grain boundaries were calculated, and the most stable segregation sites for silver at these copper grain boundaries were determined. The generated empirical potential was validated by comparing it with that obtained from the first principles calculation. The segregation of silver at copper grain boundaries strongly depends on the open space at the segregation site.
Hu, S. X.; Goncharov, V. N.; Boehly, T. R.; McCrory, R. L.; Skupsky, S.; Collins, L. A.; Kress, J. D.; Militizer, B.
2015-04-20
In this study, a comprehensive knowledge of the properties of high-energy-density plasmas is crucial to understanding and designing low-adiabat, inertial confinement fusion (ICF) implosions through hydrodynamic simulations. Warm-dense-matter (WDM) conditions are routinely accessed by low-adiabat ICF implosions, in which strong coupling and electron degeneracy often play an important role in determining the properties of warm dense plasmas. The WDM properties of deuterium–tritium (DT) mixtures and ablator materials, such as the equation of state, thermal conductivity, opacity, and stopping power, were usually estimated by models in hydro-codes used for ICF simulations. In these models, many-body and quantum effects were only approximately taken into account in the WMD regime. Moreover, the self-consistency among these models was often missing. To examine the accuracy of these models, we have systematically calculated the static, transport, and optical properties of warm dense DT plasmas, using first-principles (FP) methods over a wide range of densities and temperatures that cover the ICF “path” to ignition. These FP methods include the path-integral Monte Carlo (PIMC) and quantum-molecular dynamics (QMD) simulations, which treat electrons with many-body quantum theory. The first-principles equation-of-state table, thermal conductivities (K_{QMD}), and first principles opacity table of DT have been self-consistently derived from the combined PIMC and QMD calculations. They have been compared with the typical models, and their effects to ICF simulations have been separately examined in previous publications. In this paper, we focus on their combined effects to ICF implosions through hydro-simulations using these FP-based properties of DT in comparison with the usual model simulations. We found that the predictions of ICF neutron yield could change by up to a factor of –2.5; the lower the adiabat of DT capsules, the more variations in hydro
First-principles calculations on the structural evolution of solid fullerene-like CP x
NASA Astrophysics Data System (ADS)
Gueorguiev, G. K.; Furlan, A.; Högberg, H.; Stafström, S.; Hultman, L.
2006-08-01
The formation and structural evolution of fullerene-like (FL) carbon phosphide (CP x) during synthetic growth were studied by first-principles calculations. Geometry optimizations and comparison between the cohesive energies suggest stability for solid FL-CP x compounds. In comparison with fullerene-like carbon nitride, higher curvature of the graphene sheets and higher density of cross-linkages between them is predicted and explained by the different electronic properties of P and N. Cage-like and onion-like structures, both containing tetragons, are found to be typical for fullerene-like CP x. Segregation of P is predicted at fractions exceeding ˜20 at.%.
Hu, S. X. Goncharov, V. N.; Boehly, T. R.; McCrory, R. L.; Skupsky, S.; Collins, L. A.; Kress, J. D.; Militzer, B.
2015-05-15
A comprehensive knowledge of the properties of high-energy-density plasmas is crucial to understanding and designing low-adiabat, inertial confinement fusion (ICF) implosions through hydrodynamic simulations. Warm-dense-matter (WDM) conditions are routinely accessed by low-adiabat ICF implosions, in which strong coupling and electron degeneracy often play an important role in determining the properties of warm dense plasmas. The WDM properties of deuterium–tritium (DT) mixtures and ablator materials, such as the equation of state, thermal conductivity, opacity, and stopping power, were usually estimated by models in hydro-codes used for ICF simulations. In these models, many-body and quantum effects were only approximately taken into account in the WMD regime. Moreover, the self-consistency among these models was often missing. To examine the accuracy of these models, we have systematically calculated the static, transport, and optical properties of warm dense DT plasmas, using first-principles (FP) methods over a wide range of densities and temperatures that cover the ICF “path” to ignition. These FP methods include the path-integral Monte Carlo (PIMC) and quantum-molecular dynamics (QMD) simulations, which treat electrons with many-body quantum theory. The first-principles equation-of-state table, thermal conductivities (κ{sub QMD}), and first principles opacity table of DT have been self-consistently derived from the combined PIMC and QMD calculations. They have been compared with the typical models, and their effects to ICF simulations have been separately examined in previous publications. In this paper, we focus on their combined effects to ICF implosions through hydro-simulations using these FP-based properties of DT in comparison with the usual model simulations. We found that the predictions of ICF neutron yield could change by up to a factor of ∼2.5; the lower the adiabat of DT capsules, the more variations in hydro-simulations. The FP
NASA Astrophysics Data System (ADS)
Jakse, N.; Pasturel, A.
2007-11-01
We report results of first principles molecular dynamics simulations that confirm early speculations on the presence of liquid-liquid phase transition in undercooled silicon. However, we find that structural and electronic properties of both low-density liquid (LDL) and high-density liquid (HDL) phases are quite different from those obtained by empirical calculations, the difference being more pronounced for the HDL phase. The discrepancy between quantum and classical simulations is attributed to the inability of empirical potentials to describe changes in chemical bonds induced by density and temperature variations.
NASA Astrophysics Data System (ADS)
Middleton, Kirsten; Zhang, G. P.; Nichols, Michael R.; George, Thomas F.
2012-05-01
Memantine, amantadine and rimantadine are structurally derived from the same diamondoid, adamantane. These derivatives demonstrate therapeutic efficacy in human diseases: memantine for Alzheimer's disease and amantadine and rimantadine for influenza. In order to better understand some of the properties that distinguish these three compounds, we conduct first-principles calculations on their structure and electronic properties. Our results indicate that protonation has a significant effect on the dipole moment, where the dipole moment in protonated memantine is over eight times larger than in the deprotonated form.
Crystal Structure Prediction from First Principles: The Crystal Structures of Glycine
Lund, Albert M.; Pagola, Gabriel I.; Orendt, Anita M.; Ferraro, Marta B.; Facelli, Julio C.
2015-01-01
Here we present the results of our unbiased searches of glycine polymorphs obtained using the Genetic Algorithms search implemented in Modified Genetic Algorithm for Crystals coupled with the local optimization and energy evaluation provided by Quantum Espresso. We demonstrate that it is possible to predict the crystal structures of a biomedical molecule using solely first principles calculations. We were able to find all the ambient pressure stable glycine polymorphs, which are found in the same energetic ordering as observed experimentally and the agreement between the experimental and predicted structures is of such accuracy that the two are visually almost indistinguishable. PMID:25843964
NASA Astrophysics Data System (ADS)
Ramprasad, R.; Tang, C.
2006-08-01
A first principles electronic structure based method is presented to determine the equivalent circuit representations of nanostructured physical systems at optical frequencies, via a mapping of the effective permittivity calculated for a lattice of physical nano-elements using density functional theory to that calculated for a lattice of impedances using circuit theory. Specifically, it is shown that silicon nanowires and carbon nanotubes can be represented as series combinations of inductance, capacitance and resistance. It is anticipated that the generality of this approach will allow for an alternate description of physical systems at optical frequencies, and in the realization of novel opto- and nanoelectronic devices, including negative refractive index materials.
First principle calculation in FeCo overlayer on GaAs substrate
Jain, Vishal Lakshmi, N.; Jain, Vivek Kumar; K, Sijo A.; Venugopalan, K.
2015-06-24
In this work the first principle electronic structure calculation is reported for FeCo/GaAs thin film system to investigate the effect of orientation on the electronic structural properties. A unit cell describing FeCo layers and GaAs layers is constructed for (100), (110), (111) orientation with vacuum of 30Å to reduce dimensions. It is found that although the (110) orientation is energetically more favorable than others, the magnetic moment is quite large in (100) and (111) system compared to the (110) and is due to the total DOS variation with orientation.
New class of planar ferroelectric Mott insulators via first-principles design
Kim, Chanul; Park, Hyowon; Marianetti, Chris A.
2015-12-11
which is not common in known materials. Here we use first-principles calculations to design layered double perovskite oxides AABBO6 which achieve the aforementioned properties in the context of Mott insulators. In our design rules, the gap is dictated by B/B electronegativity difference in a Mott state, while the polarization is obtained via nominal d0 filling on the B-site, A-type cations bearing lone-pair electrons, and A = A size mismatch. Successful execution is demonstrated in BaBiCuVO6, BaBiNiVO6, BaLaCuVO6, and PbLaCuVO6.
Arsenene as a promising candidate for NO and NO2 sensor: A first-principles study
NASA Astrophysics Data System (ADS)
Liu, Can; Liu, Chun-Sheng; Yan, Xiaohong
2017-03-01
Based on first-principles calculations, we have studied the adsorption of CO, CO2, N2, NH3, NO and NO2 molecules on the pristine arsenene monolayer. These gas molecules are held by an interaction that is intermediate between the physisorbed and chemisorbed states. Furthermore, the adsorption of NO and NO2 can produce a noticeable modifications of the density of states near the Fermi level. Interestingly, only the adsorption of NO and NO2 can lead to a magnetic moment of 1 μB. Therefore, our results can provide a theoretical basis for the potential applications of arsenene monolayer in gas sensing with electrical and magnetic methods.
First principles prediction of the metastability of the Ge2Mn phase and its synthesis pathways
NASA Astrophysics Data System (ADS)
Arras, E.; Slipukhina, I.; Torrent, M.; Caliste, D.; Deutsch, T.; Pochet, P.
2010-06-01
In this letter, we performed first principles calculations to investigate the stability of a [100]-compatible Ge2Mn compound. Based on a thermodynamical approach, we propose and assess the C16 structure (Al2Cu prototype) to be only slightly metastable as compared to the other Ge-Mn compounds. The reported structural and magnetic properties of this Ge2Mn compound make it a potentially interesting compound for spintronic applications, all the more since a simple way to stabilize it as a bulk film is proposed.
Band gaps and dielectric constants of amorphous hafnium silicates: A first-principles investigation
NASA Astrophysics Data System (ADS)
Broqvist, Peter; Pasquarello, Alfredo
2007-02-01
Electronic band gaps and dielectric constants are obtained for amorphous hafnium silicates using first-principles methods. Models of amorphous (HfO2)x(SiO2)1-x for varying x are generated by ab initio molecular dynamics. The calculations show that the presence of Hf gives rise to low-lying conduction states which explain the experimentally observed nonlinear dependence of the band gap on hafnium content. Static dielectric constants are found to depend linearly on x, supporting recent experimental data.
Formation and Annealing Behaviors of Qubit Centers in 4H-SiC from First Principles
NASA Astrophysics Data System (ADS)
Zhao, Mingwen; Wang, Xiaopeng; Bu, Hongxia; Zhang, Hongyu; He, Xiujie; Wang, Aizhu; Mingwen Zhao's Lab in Shandong University Team
Inspired by finding that the nitrogen-vacancy center in diamond is a qubit candidate, similar defects in silicon carbide have drawn considerable interest. However, the generation and annealing behaviors of these defects remain unclear. Using first-principles calculations, we describe the equilibrium concentrations and annealing mechanisms based on the diffusion of silicon vacancies. The formation energies and energy barriers along different migration paths, which are responsible for the formation rates, stability, and concentrations of these defects, are investigated. The effects on these processes of charge states, annealing temperature, and crystal orientation are also discussed. These theoretical results are expected to be useful in achieving controllable generation of these defects in experiments.
Half metallic ferromagnetism in alkali metal nitrides MN (M = Rb, Cs): A first principles study
Murugan, A. Rajeswarapalanichamy, R. Santhosh, M. Sudhapriyanga, G.; Kanagaprabha, S.
2014-04-24
The structural, electronic and elastic properties of two alkali metal nitrides (MN: M= Rb, Cs) are investigated by the first principles calculations based on density functional theory using the Vienna ab-initio simulation package. At ambient pressure the two nitrides are stable in ferromagnetic state with CsCl structure. The calculated lattice parameters are in good agreement with the available results. The electronic structure reveals that these materials are half metallic in nature. A pressure-induced structural phase transition from CsCl to ZB phase is observed in RbN and CsN.
Understanding ferromagnetism in Co-doped Ti O2 anatase from first principles
NASA Astrophysics Data System (ADS)
Janisch, Rebecca; Spaldin, Nicola A.
2006-01-01
We present a first-principles computational study of the nature and origin of ferromagnetism in Co-doped TiO2 . We calculate the magnetic ordering and electronic properties as a function of the concentration and distribution of Co dopants and oxygen vacancies. We find that Co atoms prefer to substitute on neighboring sites of the Ti lattice, and show, using the well-established Goodenough-Kanamori-Anderson rules, that this leads to a ferromagnetic superexchange. We propose other semiconductor hosts in which the superexchange mechanism should lead to ferromagnetic coupling between the magnetic moments of neighboring transition metal dopants.
NASA Astrophysics Data System (ADS)
Li, Ying; Mahadevan, Jagan; Wang, Sanwu
2010-03-01
The catalytic reactions of dissociation and oxidation of methane on the copper surfaces play a key role in, for example, the development of high-performance solid oxide fuel cells. We used first-principles quantum theory and large-scale parallel calculations to investigate the atomic-scale mechanism of the catalytic chemical reactions. We report the calculated results, which provide fundamental information and understanding about the atomic-scale dynamics and electronic structures pertinent to the reactions and specifically the catalytic role of the Cu(100) and Cu(111) surfaces. We also report comparison of our results with available experimental data and previous theoretical investigations.
Energy versus free-energy conservation in first-principles molecular dynamics
NASA Astrophysics Data System (ADS)
Wentzcovitch, Renata M.; Martins, José Luís; Allen, Philip B.
1992-05-01
In applying first-principles molecular dynamics to metals, a fictitious temperature is usefully assigned to the electronic (Fermi-Dirac) occupation functions. This avoids instabilities associated with fluctuations in these occupations during the minimization of the energy density functional. Because these occupations vary with the ionic motion, they give rise to an extra contribution in addition to the usual Hellmann-Feynman forces. If this extra force is omitted, energy is not conserved. We point out, however, that ionic kinetic energy plus electronic free energy is conserved, and argue that this yields a sensible and realistic conservative dynamics.
NASA Astrophysics Data System (ADS)
Lee, Eun-Cheol
2012-04-01
The effects of DNA nucleotide adsorption on the conductance of graphene nanoribbons are investigated through first-principles calculations. We find that, for the adsorption of a single nucleotide, the negatively charged phosphate produces conductance dips associated with quasibound states, reducing the hole conductance. The conductance of conduction electrons is also reduced by electron scattering at the Coulomb potential barriers produced by the phosphate, with no noticeable conductance dips near the Fermi level. Our results indicate that graphene nanoribbon is promising for the application to DNA sensor utilizing quantum carrier conductance.
First-principles molecular dynamics calculations of the equation of state for tantalum.
Ono, Shigeaki
2009-11-20
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.
Structure of YSi2 nanowires from scanning tunneling spectroscopy and first principles
Iancu, V.; Kent, P. R. C.; Zeng, C. G.; Weitering, H. H.
2009-01-01
Exceptionally long and uniform YSi2 nanowires are formed via self-assembly on Si(001). The in-plane width of the thinnest wires is known to be quantized in odd multiples of the silicon lattice constant. Here, we identify a class of nanowires that violates the “odd multiple” rule. The structure of the thinnest wire in this category is determined by comparing scanning tunneling spectroscopy measurements with the calculated surface density of states of candidate models by means of the Pendry R-factor analysis. The relative stability of the odd and even wire systems is analyzed via first-principles calculations. PMID:19859579
Gradual changes in electronic properties from graphene to graphite: first-principles calculations.
Alzahrani, A Z; Srivastava, G P
2009-12-02
Calculations based on the first-principles pseudopotential plane-wave method and density functional theory are performed to investigate the electronic properties of graphene, bilayer graphene, multilayer graphene, and graphite. From an analysis of the electronic band structure close to the Fermi level, we have quantified the gradual change in the Fermi surface topology from the point-like structure for graphene to a warped triangular shape for graphite. We have also discussed the gradual change in the electron and hole effective masses and velocities as the system evolves from graphene to graphite.
Hu, S. X.; Goncharov, V. N.; Boehly, T. R.; ...
2015-04-20
In this study, a comprehensive knowledge of the properties of high-energy-density plasmas is crucial to understanding and designing low-adiabat, inertial confinement fusion (ICF) implosions through hydrodynamic simulations. Warm-dense-matter (WDM) conditions are routinely accessed by low-adiabat ICF implosions, in which strong coupling and electron degeneracy often play an important role in determining the properties of warm dense plasmas. The WDM properties of deuterium–tritium (DT) mixtures and ablator materials, such as the equation of state, thermal conductivity, opacity, and stopping power, were usually estimated by models in hydro-codes used for ICF simulations. In these models, many-body and quantum effects were only approximatelymore » taken into account in the WMD regime. Moreover, the self-consistency among these models was often missing. To examine the accuracy of these models, we have systematically calculated the static, transport, and optical properties of warm dense DT plasmas, using first-principles (FP) methods over a wide range of densities and temperatures that cover the ICF “path” to ignition. These FP methods include the path-integral Monte Carlo (PIMC) and quantum-molecular dynamics (QMD) simulations, which treat electrons with many-body quantum theory. The first-principles equation-of-state table, thermal conductivities (KQMD), and first principles opacity table of DT have been self-consistently derived from the combined PIMC and QMD calculations. They have been compared with the typical models, and their effects to ICF simulations have been separately examined in previous publications. In this paper, we focus on their combined effects to ICF implosions through hydro-simulations using these FP-based properties of DT in comparison with the usual model simulations. We found that the predictions of ICF neutron yield could change by up to a factor of –2.5; the lower the adiabat of DT capsules, the more variations in hydro
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. Supported by NSF-NRI, SRC, ONR, Croatian Ministry of Science, Education, and Sports, and CCR at SUNY UB.
Van An Dinh; Sato, Kazunori; Katayama-Yoshida, Hiroshi
2010-01-04
A first principle study of half-metallicity and ferromagnetism in half-heusler alloys NiMnZ (Z = Si, P, Ge, As, and Sb) is given. The half-metallicity and ferromagnetism are predicted via the calculation of electronic structure, and Curie temperature. The stability of the orthorhombic and tetragonal structures and C1{sub b} at various values of lattice parameters is also studied by means of the pseudo-potential method. All alloys exhibit the half-metallicity and ferromagnetism above room temperature.
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 potential for the acetylene dimer and refinement by fitting to experiments
NASA Astrophysics Data System (ADS)
Leforestier, Claude; Tekin, Adem; Jansen, Georg; Herman, Michel
2011-12-01
We report the definition and refinement of a new first principles potential for the acetylene dimer. The ab initio calculations were performed with the DFT-SAPT combination of symmetry-adapted intermolecular perturbation method and density functional theory, and fitted to a model site-site functional form. Comparison of the calculated microwave spectrum with experimental data revealed that the barriers to isomerization were too low. This potential was refined by fitting the model parameters in order to reproduce the observed transitions, an excellent agreement within ˜1 MHz being achieved.
A novel first-principles approach to effective Hamiltonians for high Tc superconducting cuprates
NASA Astrophysics Data System (ADS)
Yin, W.-G.; Ku, W.
2008-03-01
We report our recent progress of deriving the low-energy effective one-band Hamiltonians for the prototypical cuprate superconductor Ca2CuO2Cl2, based on a newly developed first-principles Wannier-states approach that takes into account large on-site Coulomb repulsion. The apical atom pz state is found to affect the general properties of the low-energy hole state, namely the Zhang-Rice singlet, via additional intra-sublattice hoppings, nearest-neighbor 'super-repulsion,' and other microscopic many-body processes.
NASA Astrophysics Data System (ADS)
Wan, Quan; Galli, Giulia
2015-12-01
We present a first-principles framework to compute sum-frequency generation (SFG) vibrational spectra of semiconductors and insulators. The method is based on density functional theory and the use of maximally localized Wannier functions to compute the response to electric fields, and it includes the effect of electric field gradients at surfaces. In addition, it includes quadrupole contributions to SFG spectra, thus enabling the verification of the dipole approximation, whose validity determines the surface specificity of SFG spectroscopy. We compute the SFG spectra of ice Ih basal surfaces and identify which spectra components are affected by bulk contributions. Our results are in good agreement with experiments at low temperature.
First-principles phase stability at high temperatures and pressure in Nb90Zr10 alloy
Landa, A.; Soderlind, P.
2016-08-18
The phase stability of Nb90Zr10 alloy at high temperatures and compression is explored by means of first-principles electronic-structure calculations. Utilizing the self-consistent ab initio lattice dynamics (SCAILD) approach in conjunction with density-functional theory, we show that pressure-induced mechanical instability of the body-centered cubic phase, which results in formation of a rhombohedral phase at around 50 GPa, will prevail significant heating. As a result, the body-centered cubic structure will recover before melting at ~1800 K.
Freitag, Mark A.
2001-12-31
The major title of this dissertation, 'From first principles,' is a phase often heard in the study of thermodynamics and quantum mechanics. These words embody a powerful idea in the physical sciences; namely, that it is possible to distill the complexities of nature into a set of simple, well defined mathematical laws from which specific relations can then be derived . In thermodynamics, these fundamental laws are immediately familiar to the physical scientist by their numerical order: the First, Second and Third Laws. However, the subject of the present volume is quantum mechanics-specifically, non-relativistic quantum mechanics, which is appropriate for most systems of chemical interest.
Dynamic stability of fcc crystals under isotropic loading from first principles.
Rehák, Petr; Cerný, Miroslav; Pokluda, Jaroslav
2012-05-30
Lattice dynamics and stability of four fcc crystals (Al, Ir, Pt and Au) under isotropic (hydrostatic) tensile loading are studied from first principles using the linear response method and the harmonic approximation. The results reveal that, contrary to former expectations, strengths of all the studied crystals are limited by instabilities related to soft phonons with finite or vanishing wavevectors. The critical strains associated with such instabilities are remarkably lower than those related to the volumetric instability. On the other hand, the corresponding reduction of the tensile strength is by 20% at the most. An analysis of elastic stability conditions is also performed and the results obtained by means of both approaches are compared.
First-principles theory of quantum well resonance in double barrier magnetic tunnel junctions.
Wang, Yan; Lu, Zhong-Yi; Zhang, X-G; Han, X F
2006-08-25
Quantum well (QW) resonances in Fe(001)/MgO/Fe/MgO/Fe double barrier magnetic tunnel junctions are calculated from first principles. By including the Coulomb blockade energy due to the finite size islands of the middle Fe film, we confirm that the oscillatory differential resistance observed in a recent experiment [T. Nozaki, Phys. Rev. Lett. 96, 027208 (2006)10.1103/PhysRevLett.96.027208] originates from the QW resonances from the Delta1 band of the Fe majority-spin channel. The primary source of smearing at low temperatures is shown to be the variation of the Coulomb blockade energy.
NASA Astrophysics Data System (ADS)
Luo, Bingcheng; Wang, Xiaohui; Tian, Enke; Wu, Longwen; Li, Longtu
2016-08-01
Dielectric materials with high power density and energy density are eagerly desired for the potential application in advanced pulsed capacitors. Here, we present the first-principles effective Hamiltonian simulation of perovskite ferroelectrics BaTiO3, PbTiO3, and KNbO3 in order to better predict and design materials for energy storage application. The lattice constant, dielectric constant and ferroelectric hysteresis, and energy-storage density of BaTiO3, PbTiO3, and KNbO3 were calculated with the consideration of the effects of temperature and external electric field.
First principle study of band structure of SrMO3 perovskites
NASA Astrophysics Data System (ADS)
Daga, Avinash; Sharma, Smita
2016-05-01
First principle study of band structure calculations in the local density approximations (LDA) as well as in the generalized gradient approximations (GGA) have been used to determine the electronic structure of SrMO3 where M stands for Ti, Zr and Mo. Occurrence of band gap proves SrTiO3 and SrZrO3 to be insulating. A small band gap is observed in SrMoO3 perovskite signifies it to be metallic. Band structures are found to compare well with the available data in the literature showing the relevance of this approach. ABINIT computer code has been used to carry out all the calculations.
First-principles calculations of the OH- adsorption energy on perovskite oxide
NASA Astrophysics Data System (ADS)
Ohzuku, Hideo; Ikeno, Hidekazu; Yamada, Ikuya; Yagi, Shunsuke
2016-08-01
The oxygen evolution reaction (OER) that occurs during water oxidation is of considerable importance as an essential energy conversion reaction for rechargeable metal-air batteries and direct solar water splitting. ABO3 perovskite oxides have been extensively studied because of their high catalytic OER activity. In the present study, the OH- adsorption process on the perovskite surface about different B site cations was investigated by the first-principles calculations. We concluded that the adsorption energy of SrFeO3 surface is larger than that of SrTiO3.
Electronic and magnetic properties of CdSe nanoribbon: First-principles calculations
NASA Astrophysics Data System (ADS)
Yu, Guolong; Chen, Li; Ye, Xiang
2015-01-01
First-principles Density Functional Theory (DFT) calculations were carried out on electronic and magnetic properties of cadmium selenide nanoribbons (CdSeNRs) with both zigzag and armchair edges. All armchair nanoribbons exhibit nonmagnetic and semiconducting behavior, regardless of ribbon widths and their edge passivation status. Bare zigzag nanoribbons are found to be metallic and have non-zero net magnetic moments. The net magnetic moment of these ribbons increases as ribbon width increases. However, when zigzag edges are passivated with hydrogen, a degeneracy between the up and down spin was found, which turns ribbons into nonmagnetic ones.
Efficient first-principles simulation of noncontact atomic force microscopy for structural analysis.
Chan, T-L; Wang, C Z; Ho, K M; Chelikowsky, James R
2009-05-01
We propose an efficient scheme to simulate noncontact atomic force microscopy images by using first-principles self-consistent potential from the sample as input without explicit modeling of the atomic force microscopy tip. Our method is applied to various types of semiconductor surfaces including Si(111)-(7x7), TiO2(110)-(1x1), Ag/Si(111)-(sqrt[3]xsqrt[3])R30 degrees, and Ge/Si(105)-(1x2) surfaces. We obtain good agreement with experimental results and previous theoretical studies, and our method can aid in identifying different structural models for surface reconstruction.
First principles study of structural, electronic and mechanical properties of alkali nitride-KN
Murugan, A.; Rajeswarapalanichamy, R. Santhosh, M.; Iyakutti, K.
2015-06-24
The structural, electronic and elastic properties of alkali- metal nitride (KN) is investigated by the first principles calculations based on density functional theory as implemented in Vienna ab-initio simulation package. At ambient pressure KN 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 the KN is half metallic ferromagnet at normal pressure. A pressure-induced structural phase transition from NaCl to ZB phase is observed in KN. Half metallicity and ferromagnetism is maintained at all pressures.
First-principles study of exchange interactions of yttrium iron garnet
NASA Astrophysics Data System (ADS)
Xie, Li-Shan; Jin, Guang-Xi; He, Lixin; Bauer, Gerrit E. W.; Barker, Joseph; Xia, Ke
2017-01-01
Yttrium iron garnet is the ubiquitous magnetic insulator used for studying pure spin currents. The exchange constants reported in the literature vary considerably between different experiments and fitting procedures. Here we calculate them from first principles. The local Coulomb correction (U -J ) of density-functional theory is chosen such that the parametrized spin model reproduces the experimental Curie temperature and a large electronic band gap, ensuring an insulating phase. The magnon spectrum calculated with our parameters agrees reasonably well with that measured by neutron scattering. A residual disagreement about the frequencies of optical modes indicates the limits of the present methodology.
Interactions of gas molecules with monolayer MoSe2: A first principle study
NASA Astrophysics Data System (ADS)
Sharma, Munish; Jamdagni, Pooja; Kumar, Ashok; Ahluwalia, P. K.
2016-05-01
We present a first principle study of interaction of toxic gas molecules (NO, NO2 and SO2) with monolayer MoSe2. The predicted order of sensitivity of gas molecule is NO2 > SO2 > NO. Adsorbed molecules strongly influence the electronic behaviour of monolayer MoSe2 by inducing impurity levels in the vicinity of Fermi energy. NO and SO2 is found to induce p-type doping effect while semiconductor to metallic transitions occur on NO2 adsorption. Our findings may guide the experimentalist for fabricating sensor devices based on MoSe2 monolayer.
Crystal structure prediction from first principles: The crystal structures of glycine
NASA Astrophysics Data System (ADS)
Lund, Albert M.; Pagola, Gabriel I.; Orendt, Anita M.; Ferraro, Marta B.; Facelli, Julio C.
2015-04-01
Here we present the results of our unbiased searches of glycine polymorphs obtained using the genetic algorithms search implemented in MGAC, modified genetic algorithm for crystals, coupled with the local optimization and energy evaluation provided by Quantum Espresso. We demonstrate that it is possible to predict the crystal structures of a biomedical molecule using solely first principles calculations. We were able to find all the ambient pressure stable glycine polymorphs, which are found in the same energetic ordering as observed experimentally and the agreement between the experimental and predicted structures is of such accuracy that the two are visually almost indistinguishable.
Magnetic Self-Organized Atomic Laminate from First Principles and Thin Film Synthesis
NASA Astrophysics Data System (ADS)
Ingason, A. S.; Mockute, A.; Dahlqvist, M.; Magnus, F.; Olafsson, S.; Arnalds, U. B.; Alling, B.; Abrikosov, I. A.; Hjörvarsson, B.; Persson, P. O. Å.; Rosen, J.
2013-05-01
The first experimental realization of a magnetic Mn+1AXn (MAX) phase, (Cr0.75Mn0.25)2GeC, is presented, synthesized as a heteroepitaxial single crystal thin film, exhibiting excellent structural quality. This self-organized atomic laminate is based on the well-known Cr2GeC, with Mn, a new element in MAX phase research, substituting Cr. The compound was predicted using first-principles calculations, from which a variety of magnetic behavior is envisaged, depending on the Mn concentration and Cr/Mn atomic configuration within the sublattice. The analyzed thin films display a magnetic signal at room temperature.
First-principles study of hydrogen storage on Li12F12 nano-cage
NASA Astrophysics Data System (ADS)
Zhang, Yafei; Cheng, Xinlu
2017-03-01
We use the first-principles calculation based on density functional theory (DFT) to investigate the hydrogen storage on Li12F12 nano-cage. Our result indicates the largest hydrogen gravimetric density is 7.14 wt% and this is higher than the 2017 target from the US department of energy (DOE). Meanwhile, the average adsorption energy is -0.161 eV/H2, which is desirable for absorbing and desorbing H2 molecules at near ambient conditions. These findings will have important implications on designing hydrogen storage materials in the future.
Superlubricity of two-dimensional fluorographene/MoS2 heterostructure: a first-principles study.
Wang, Lin-Feng; Ma, Tian-Bao; Hu, Yuan-Zhong; Zheng, Quanshui; Wang, Hui; Luo, Jianbin
2014-09-26
The atomic-scale friction of the fluorographene (FG)/MoS2 heterostructure is investigated using first-principles calculations. Due to the intrinsic lattice mismatch and formation of periodic Moiré patterns, the potential energy surface of the FG/MoS2 heterostructure is ultrasmooth and the interlayer shear strength is reduced by nearly two orders of magnitude, compared with both FG/FG and MoS2/MoS2 bilayers, entering the superlubricity regime. The size dependency of superlubricity is revealed as being based on the relationship between the emergence of Moiré patterns and the lattice mismatch ratio for heterostructures.
Superlubricity of two-dimensional fluorographene/MoS2 heterostructure: a first-principles study
NASA Astrophysics Data System (ADS)
Wang, Lin-Feng; Ma, Tian-Bao; Hu, Yuan-Zhong; Zheng, Quanshui; Wang, Hui; Luo, Jianbin
2014-09-01
The atomic-scale friction of the fluorographene (FG)/MoS2 heterostructure is investigated using first-principles calculations. Due to the intrinsic lattice mismatch and formation of periodic Moiré patterns, the potential energy surface of the FG/MoS2 heterostructure is ultrasmooth and the interlayer shear strength is reduced by nearly two orders of magnitude, compared with both FG/FG and MoS2/MoS2 bilayers, entering the superlubricity regime. The size dependency of superlubricity is revealed as being based on the relationship between the emergence of Moiré patterns and the lattice mismatch ratio for heterostructures.
Substitutional Co dopant on the GaAs(110) surface: A first principles study
NASA Astrophysics Data System (ADS)
Fang, Zhou; Yi, Zhijun
2016-12-01
Using the first principles ground state method, the electronic properties of single Co dopant replacing one Ga atom on the GaAs(110) surface are studied. Our calculated local density of states (LDOS) at Co site presents several distinct peaks above the valence band maximum (VBM), and this agrees with recent experiments. Moreover, the calculated STM images at bias voltages of 2 eV and -2 eV also agree with experiments. We discussed the origin of Co impurity induced distinct peaks, which can be characterized with the hybridization between Co d orbitals and p-like orbitals of surface As and Ga atoms.
Polyimide nanocomposites based on cubic zirconium tungstate
NASA Astrophysics Data System (ADS)
Ramasubramanian Sharma, Gayathri
2009-12-01
In this research, cubic zirconium tungstate (ZrW2O8) was used as a filler to reduce the CTE of polyimides (PI), and the effect of ZrW2O8 nanoparticles on the bulk polymer properties was studied. Polyimides are high performance polymers with exceptional thermal stability, and there is a need for PIs with low CTEs for high temperature applications. The nanofiller, cubic ZrW2O8, is well known for its isotropic negative thermal expansion (NTE) over a wide temperature range from -272.7 to 777°C. The preparation of nanocomposites involved the synthesis of ZrW 2O8 nanofiller, engineering the polymer-filler interface using linker groups and optimization of processing strategies to prepare free-standing PI nanocomposite films. A hydrothermal method was used to synthesize ZrW 2O8 nanoparticles. Polyimide-ZrW2O8 interface interaction was enhanced by covalently bonding linker moieties to the surface of ZrW2O8 nanoparticles. Specifically, ZrW 2O8 nanoparticles were functionalized with two different linker groups: (1) a short aliphatic silane, and (2) low molecular weight PI. The surface functionalization was confirmed using X-ray photoelectron spectroscopy and thermal gravimetric analysis (TGA). Reprecipitation blending was used to prepare the freestanding PI-ZrW2O8 nanocomposite films with up to 15 volume% filler loading. SEM images showed the improvements in polymer-filler wetting behavior achieved using interface engineering. SEM images indicated that there was better filler dispersion in the PI matrix using reprecipitation blending, compared to the filler dispersion achieved in the nanocomposites prepared using conventional blending technique. The structure-property relationships in PI-ZrW2O8 nanocomposites were investigated by studying the thermal degradation, glass transition, tensile and thermal expansion properties of the nanocomposites. The properties were studied as a function of filler loading and interface linker groups. Addition of ZrW2O8 nanoparticles did not
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 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.
Implicit solvent model for linear-scaling first-principles electronic structure calculations
NASA Astrophysics Data System (ADS)
Helal, Hatem H.; Payne, Mike; Mostofi, Arash A.
2009-03-01
Density functional theory (DFT) enables first-principles calculations that exhibit cubic scaling of the computational time required with respect to the number of atoms in the system. This presents an unavoidable difficulty when first-principles accuracy is needed for the study of large-scale biological systems. The ONETEP program reformulates DFT so that the required computational effort scales only linearly with system size, recently demonstrated for up to 32,000 atoms on 64 cores.ootnotetextN. D. M. Hine, P. D. Haynes, A. A. Mostofi, C.-K. Skylaris and M. C. Payne, submitted to J. Chem. Phys. (2008). Further complicating DFT based studies of biomolecular systems is the need for an accurate representation of the electrostatic environment. Rather than introducing explicit solvent molecules into the system, which would be computationally prohibitive, we present our recent efforts to integrate an implicit solvent modelootnotetextD. A. Scherlis et al., J. Chem. Phys. 124, 074103 (2006). with ONETEP in order to study systems in solution consisting of many thousands of atoms. We report preliminary results of our methodology with a study of the DNA nucleosome core particle.
Hu, S. X.; Collins, L. A.; Goncharov, V. N.; ...
2015-10-14
Obtaining an accurate equation of state (EOS) of polystyrene (CH) is crucial to reliably design inertial confinement fusion (ICF) capsules using CH/CH-based ablators. Thus, with first-principles calculations, we have investigated the extended EOS of CH over a wide range of plasma conditions (ρ = 0.1 to 100 g/cm3 and T = 1,000 to 4,000,000 K). When compared with the widely used SESAME-EOS table, the first-principles equation of state (FPEOS) of CH has shown significant differences in the low-temperature regime, in which strong coupling and electron degeneracy play an essential role in determining plasma properties. Hydrodynamic simulations of cryogenic target implosionsmore » on OMEGA using the FPEOS table of CH have predicted ~5% reduction in implosion velocity and ~30% decrease in neutron yield in comparison with the usual SESAME simulations. This is attributed to the ~10% lower mass ablation rate of CH predicted by FPEOS. Simulations using CH-FPEOS show better agreement with measurements of Hugoniot temperature and scattered lights from ICF implosions.« less
Fang, Hanjun; Kamakoti, Preeti; Ravikovitch, Peter I; Aronson, Matthew; Paur, Charanjit; Sholl, David S
2013-08-21
The development of accurate force fields is vital for predicting adsorption in porous materials. Previously, we introduced a first principles-based transferable force field for CO2 adsorption in siliceous zeolites (Fang et al., J. Phys. Chem. C, 2012, 116, 10692). In this study, we extend our approach to CO2 adsorption in cationic zeolites which possess more complex structures. Na-exchanged zeolites are chosen for demonstrating the approach. These methods account for several structural complexities including Al distribution, cation positions and cation mobility, all of which are important for predicting adsorption. The simulation results are validated with high-resolution experimental measurements of isotherms and microcalorimetric heats of adsorption on well-characterized materials. The choice of first-principles method has a significant influence on the ability of force fields to accurately describe CO2-zeolite interactions. The PBE-D2 derived force field, which performed well for CO2 adsorption in siliceous zeolites, does not do so for Na-exchanged zeolites; the PBE-D2 method overestimates CO2 adsorption energies on multi-cation sites that are common in cationic zeolites with low Si/Al ratios. In contrast, a force field derived from the DFT/CC method performed well. Agreement was obtained between simulation and experiment not only for LTA-4A on which the force field fitting is based, but for other two common adsorbents, NaX and NaY.
First-principles equation of state and electronic properties of warm dense oxygen
Driver, K. P. Soubiran, F.; Zhang, Shuai; Militzer, B.
2015-10-28
We perform all-electron path integral Monte Carlo (PIMC) and density functional theory molecular dynamics (DFT-MD) calculations to explore warm dense matter states of oxygen. Our simulations cover a wide density-temperature range of 1–100 g cm{sup −3} and 10{sup 4}–10{sup 9} K. By combining results from PIMC and DFT-MD, we are able to compute pressures and internal energies from first-principles at all temperatures and provide a coherent equation of state. We compare our first-principles calculations with analytic equations of state, which tend to agree for temperatures above 8 × 10{sup 6} K. Pair-correlation functions and the electronic density of states reveal an evolving plasma structure and ionization process that is driven by temperature and density. As we increase the density at constant temperature, we find that the ionization fraction of the 1s state decreases while the other electronic states move towards the continuum. Finally, the computed shock Hugoniot curves show an increase in compression as the first and second shells are ionized.
Astrophysical reaction rates from a symmetry-informed first-principles perspective
NASA Astrophysics Data System (ADS)
Dreyfuss, Alison; Launey, Kristina; Baker, Robert; Draayer, Jerry; Dytrych, Tomas
2017-01-01
With a view toward a new unified formalism for studying bound and continuum states in nuclei, to understand stellar nucleosynthesis from a fully ab initio perspective, we studied the nature of surface α-clustering in 20Ne by considering the overlap of symplectic states with cluster-like states. We compute the spectroscopic amplitudes and factors, α-decay width, and absolute resonance strength - characterizing major contributions to the astrophysical reaction rate through a low-lying 1- resonant state in 20Ne. As a next step, we consider a fully microscopic treatment for the n+4 He system, based on the successful first-principles No-Core Shell Model/Resonating Group Method (NCSM/RGM) for light nuclei, but with the capability to reach intermediate-mass nuclei. The new model takes advantage of the symmetry-based concept central to the Symmetry-Adapted No-Core Shell Model (SA-NCSM) to reduce computational complexity in physically-informed and methodical way, with sights toward first-principles calculations of rates for important astrophysical reactions, such as the 23 Al(p , γ) 24 Si reaction, believed to have a strong influence on X-ray burst light curves. Supported by the U.S. NSF (OCI-0904874, ACI -1516338) and the U.S. DOE (DE-SC0005248), and benefitted from computing resources provided by Blue Waters and the LSU Center for Computation & Technology.
First-Principles Study of Nuclear Quadruple Interaction of ^19F* and Binding in Solid Fluorine
NASA Astrophysics Data System (ADS)
Mishra, D. R.; Aryal, M. M.; Adhikari, N. P.; Badu, S. R.; Pink, R. H.; Scheicher, R. H.; Chow, Lee; Das, T. P.
2010-03-01
We have studied the binding energy (BE) and nuclear quadrupule interaction (NQI) parameters for the ^19F* excited nuclear state in solid fluorine as part of our investigation [1] of the properties of solid halogens using the first principles Hartree-Fock Cluster procedure combined with many-body perturbation theory (MBPT), implemented by the Gaussian set of programs. Our results show that Van der Waals interaction obtained from intermolecular electron correlation effects has dominant influence on the BE but negligible effect on the NQI parameters. For the latter, ourcalculated e^2qQ is 119.0MHz using for Q(19F*), the value of 0.072 *10-28m2 [2], and η, the asymmetry parameter, is essentially zero. The influence of rotational vibrational effects on e^2qQ is being investigated using a first-principles procedure [3] to bridge the small remaining difference with experiment (127.2 MHz) for e^2qQ [4]. [1] M.M. Aryal et al., Hyperfine Interact, 176, 51 (2007). [2] K.C.Mishra et al.,Phys. Rev.B25, 3389(1982). [3] N. Sahoo et al. Phys. Rev. Lett. 50, 913(1983) [4] H. Barfuss et al., Phys. Lett. 90A, 33(1982)
Nomura, Yusuke; Sakai, Shiro; Capone, Massimo; Arita, Ryotaro
2015-08-01
Alkali-doped fullerides A 3C60 (A = K, Rb, Cs) are surprising materials where conventional phonon-mediated superconductivity and unconventional Mott physics meet, leading to a remarkable phase diagram as a function of volume per C60 molecule. We address these materials with a state-of-the-art calculation, where we construct a realistic low-energy model from first principles without using a priori information other than the crystal structure and solve it with an accurate many-body theory. Remarkably, our scheme comprehensively reproduces the experimental phase diagram including the low-spin Mott-insulating phase next to the superconducting phase. More remarkably, the critical temperatures T c's calculated from first principles quantitatively reproduce the experimental values. The driving force behind the surprising phase diagram of A 3C60 is a subtle competition between Hund's coupling and Jahn-Teller phonons, which leads to an effectively inverted Hund's coupling. Our results establish that the fullerides are the first members of a novel class of molecular superconductors in which the multiorbital electronic correlations and phonons cooperate to reach high T c s-wave superconductivity.
First-principles electrostatic potentials for reliable alignment at interfaces and defects.
Sundararaman, Ravishankar; Ping, Yuan
2017-03-14
The alignment of electrostatic potential between different atomic configurations is necessary for first-principles calculations of band offsets across interfaces and formation energies of charged defects. However, strong oscillations of this potential at the atomic scale make alignment challenging, especially when atomic geometries change considerably from bulk to the vicinity of defects and interfaces. We introduce a method to suppress these strong oscillations by eliminating the deep wells in the potential at each atom. We demonstrate that this method considerably improves the system-size convergence of a wide range of first-principles predictions that depend on the alignment of electrostatic potentials, including band offsets at solid-liquid interfaces, and formation energies of charged vacancies in solids and at solid surfaces in vacuum. Finally, we use this method in conjunction with continuum solvation theories to investigate energetics of charged vacancies at solid-liquid interfaces. We find that for the example of an NaCl (001) surface in water, solvation reduces the formation energy of charged vacancies by 0.5 eV: calculation of this important effect was previously impractical due to the computational cost in molecular-dynamics methods.
Huang, Zuocai; Zhang, Lei; Pan, Wei
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.
First-principles prediction of the softening of the silicon shock Hugoniot curve
Hu, S. X.; Militzer, B.; Collins, L. A.; Driver, K. P.; Kress, J. D.
2016-09-15
Here, whock compression of silicon (Si) under extremely high pressures (>100 Mbar) was investigated by using two first-principles methods of orbital-free molecular dynamics (OFMD) and path integral Monte Carlo (PIMC). While pressures from the two methods agree very well, PIMC predicts a second compression maximum because of 1s electron ionization that is absent in OFMD calculations since Thomas–Fermi-based theories lack inner shell structure. The Kohn–Sham density functional theory is used to calculate the equation of state (EOS) of warm dense silicon for low-pressure loadings (P < 100 Mbar). Combining these first-principles EOS results, the principal Hugoniot curve of silicon for pressures varying from 0.80 Mbar to above ~10 Gbar was derived. We find that silicon is ~20% or more softer than what was predicted by EOS models based on the chemical picture of matter. Existing experimental data (P ≈ 1–2 Mbar) seem to indicate this softening behavior of Si, which calls for future strong-shock experiments (P > 10 Mbar) to benchmark our results.
First-principles prediction of the softening of the silicon shock Hugoniot curve
Hu, S. X.; Militzer, B.; Collins, L. A.; ...
2016-09-15
Here, whock compression of silicon (Si) under extremely high pressures (>100 Mbar) was investigated by using two first-principles methods of orbital-free molecular dynamics (OFMD) and path integral Monte Carlo (PIMC). While pressures from the two methods agree very well, PIMC predicts a second compression maximum because of 1s electron ionization that is absent in OFMD calculations since Thomas–Fermi-based theories lack inner shell structure. The Kohn–Sham density functional theory is used to calculate the equation of state (EOS) of warm dense silicon for low-pressure loadings (P < 100 Mbar). Combining these first-principles EOS results, the principal Hugoniot curve of silicon formore » pressures varying from 0.80 Mbar to above ~10 Gbar was derived. We find that silicon is ~20% or more softer than what was predicted by EOS models based on the chemical picture of matter. Existing experimental data (P ≈ 1–2 Mbar) seem to indicate this softening behavior of Si, which calls for future strong-shock experiments (P > 10 Mbar) to benchmark our results.« less
First-principles study on dielectric function of isolated and bundled carbon nanotubes
NASA Astrophysics Data System (ADS)
Yang, J. Y.; Liu, L. H.; Tan, J. Y.
2015-06-01
The dielectric function fundamentally determines the thermal radiative properties of nanomaterials. In this work, the first-principles method is applied to investigate the finite temperature dielectric function of isolated and bundled single-walled carbon nanotubes in the visible-ultraviolet spectral range without empirical models. The effects of diameter, intertube interactions and temperature on dielectric functions are discussed. The calculated extraordinary dielectric functions of four isolated (5,5), (6,6), (7,7) and (8,8) armchair nanotubes with different diameters are compared to study the diameter effect. It shows that the locations of absorption peaks of dielectric functions consistently shift to lower energy with increasing diameter. To analyze the influence of non-local intertube interactions, the dielectric functions of bundled (6,6) armchair nanotubes with varying intertube distance are calculated within the van der Waals theory. As nanotubes bundle together, the intertube interactions become strong and the absorption peaks enhance. The temperature effect is included into computing dielectric function of isolated (5,0) zigzag nanotubes via first-principles molecular dynamics method. It observes that the dominant absorption peak shifts to lower energy as temperature increases from 0 to 600 K. To interpret the temperature influence, the temperature perturbed density of states is presented.
Wang, Xiaoming; Zebarjadi, Mona; Esfarjani, Keivan
2016-08-21
This work aims at understanding solid-state energy conversion and transport in layered (van der Waals) heterostructures in contact with metallic electrodes via a first-principles approach. As an illustration, a graphene/phosphorene/graphene heterostructure in contact with gold electrodes is studied by using density functional theory (DFT)-based first principles calculations combined with real space Green's function (GF) formalism. We show that for a monolayer phosphorene, quantum tunneling dominates the transport. By adding more phosphorene layers, one can switch from tunneling-dominated transport to thermionic-dominated transport, resulting in transporting more heat per charge carrier, thus, enhancing the cooling coefficient of performance. The use of layered van der Waals heterostructures has two advantages: (a) thermionic transport barriers can be tuned by changing the number of layers, and (b) thermal conductance across these non-covalent structures is very weak. The phonon thermal conductance of the present van der Waals heterostructure is found to be 4.1 MW m(-2) K(-1) which is one order of magnitude lower than the lowest value for that of covalently-bonded interfaces. The thermionic coefficient of performance for the proposed device is 18.5 at 600 K corresponding to an equivalent ZT of 0.13, which is significant for nanoscale devices. This study shows that layered van der Waals structures have great potential to be used as solid-state energy-conversion devices.
Massively parallel first-principles simulation of electron dynamics in materials
Draeger, Erik W.; Andrade, Xavier; Gunnels, John A.; ...
2017-03-04
Here we present a highly scalable, parallel implementation of first-principles electron dynamics coupled with molecular dynamics (MD). By using optimized kernels, network topology aware communication, and by fully distributing all terms in the time-dependent Kohn–Sham equation, we demonstrate unprecedented time to solution for disordered aluminum systems of 2000 atoms (22,000 electrons) and 5400 atoms (59,400 electrons), with wall clock time as low as 7.5 s per MD time step. Despite a significant amount of non-local communication required in every iteration, we achieved excellent strong scaling and sustained performance on the Sequoia Blue Gene/Q supercomputer at LLNL. We obtained up tomore » 59% of the theoretical sustained peak performance on 16,384 nodes and performance of 8.75 Petaflop/s (43% of theoretical peak) on the full 98,304 node machine (1,572,864 cores). Lastly, scalable explicit electron dynamics allows for the study of phenomena beyond the reach of standard first-principles MD, in particular, materials subject to strong or rapid perturbations, such as pulsed electromagnetic radiation, particle irradiation, or strong electric currents.« less
Butler, W.H.; Zhang, X.G.; Nicholson, D.M.C.; MacLaren, J.M.
1995-12-31
We show that the Kubo formula can be used to calculate the nonlocal electrical conductivity of layered systems from first principles. We use the Layer Korringa Kohn Rostoker method to calculate the electronic structure and Green function of Co/Cu/Co trilayers within the local density approximation to density functional theory. This Green function is used to calculate the conductivity through the Kubo formula for both majority and minority spins and for alignment and anti-alignment of the Co moments on either side of the Cu spacer layer. This allows us to determine the giant magnetoresistance from first principles. We investigate three possibilities for the scattering in Co/Cu/Co: (1) equal electron lifetimes for Cu, majority spin Co, and minority spin Co, (2) equal electron lifetimes for majority and minority Co, weaker scattering in Cu and spin dependent interfacial scattering, (3) electron lifetimes for majority and minority spin cobalt proportional to their Fermi energy densities of states and spin dependent interfacial scattering.
Hu, S. X.; Collins, L. A.; Goncharov, V. N.; Kress, J. D.; McCrory, R. L.; Skupsky, S.
2015-10-14
Obtaining an accurate equation of state (EOS) of polystyrene (CH) is crucial to reliably design inertial confinement fusion (ICF) capsules using CH/CH-based ablators. Thus, with first-principles calculations, we have investigated the extended EOS of CH over a wide range of plasma conditions (ρ = 0.1 to 100 g/cm^{3} and T = 1,000 to 4,000,000 K). When compared with the widely used SESAME-EOS table, the first-principles equation of state (FPEOS) of CH has shown significant differences in the low-temperature regime, in which strong coupling and electron degeneracy play an essential role in determining plasma properties. Hydrodynamic simulations of cryogenic target implosions on OMEGA using the FPEOS table of CH have predicted ~5% reduction in implosion velocity and ~30% decrease in neutron yield in comparison with the usual SESAME simulations. This is attributed to the ~10% lower mass ablation rate of CH predicted by FPEOS. Simulations using CH-FPEOS show better agreement with measurements of Hugoniot temperature and scattered lights from ICF implosions.
Guan, Zhaoyong; Si, Chen; Hu, Shuanglin; Duan, Wenhui
2016-04-28
Based on first-principles calculations, we present the electronic and magnetic properties of a class of line defect-embedded zigzag graphene nanoribbons, with one edge saturated by two hydrogen atoms per carbon atom and the other edge terminated by only one hydrogen atom. Such edge-modified nanoribbons without line defects are found to be typical bipolar magnetic semiconductors (BMS). In contrast, when the line defect is introduced into the ribbons, the ground state is ferromagnetic, and the resulting nanoribbons can be tuned to spin-polarized metal, metal with Dirac point, or half-metal by varying the position of the line defect, owing to the defect-induced self-doping of the BMS. Specifically, when the line defect is far away from the edges of the ribbon, the system shows half-metallicity. We further confirm the structural and magnetic stability at room temperature by first-principles molecular dynamics simulations. Our findings reveal the possibility of building metal-free electronic/spintronic devices with magnetic/half-metallic graphene nanoribbons.
First principles molecular dynamics of metal/water interfaces under bias potential
NASA Astrophysics Data System (ADS)
Pedroza, Luana; Brandimarte, Pedro; Rocha, Alexandre; Fernandez-Serra, Marivi
2014-03-01
Understanding the interaction of the water-metal system at an atomic level is extremely important in electrocatalysts for fuel cells, photocatalysis among other systems. The question of the interface energetics involves a detailed study of the nature of the interactions between water-water and water-substrate. A first principles description of all components of the system is the most appropriate methodology in order to advance understanding of electrochemically processes. In this work we describe, using first principles molecular dynamics simulations, the dynamics of a combined surface(Au and Pd)/water system both in the presence and absence of an external bias potential applied to the electrodes, as one would come across in electrochemistry. This is accomplished using a combination of density functional theory (DFT) and non-equilibrium Green's functions methods (NEGF), thus accounting for the fact that one is dealing with an out-of-equilibrium open system, with and without van der Waals interactions. DOE Early Career Award No. DE-SC0003871.
Electronic stopping of slow H and He atoms in gold from first principles
NASA Astrophysics Data System (ADS)
Ahsan Zeb, M.; Kohanoff, Jorge; Sanchez-Portal, Daniel; Arnau, Andres; Juaristi, J. I.; Artacho, Emilio
2012-02-01
In spite of a long history, the quantitative understanding of non-adiabatic processes in condensed matter and our ability to perform predictive theoretical simulations of processes coupling many adiabatic energy surfaces is very much behind what accomplished for adiabatic situations, for which first-principles calculations provide predictions of varied properties within a few percent accuracy. We will present here high-accuracy results for the electronic stopping power of H and He moving through gold, using time-evolving density-functional theory, thereby conveying usual first-principles accuracies to strongly coupled, continuum non-adiabatic processes in condensed matter. The two key unexplained features of what observed experimentally have been reproduced and understood: (i) The non-linear behavior of stopping power versus velocity is a gradual crossover as excitations tail into the d-electron spectrum; and (ii) the higher stopping for He than for H at low velocities is explained by the substantial involvement of the d electrons in the screening of the projectile even at the lowest velocities where the energy loss is generated by s-like electron-hole pair formation only.
Terahertz spectra of biotin based on first principle, molecular mechanical, and hybrid simulations.
Bykhovski, Alexei; Woolard, Dwight
2013-07-01
Terahertz (THz) absorption of biotin was simulated using the first principle and the density functional theory (DFT) both in the harmonic approximation and with corrections for the anharmonicity. Anharmonicity corrections were calculated using two different approaches. First, the perturbation theory-based first principle calculations were performed to include third- and fourth-order anharmonicity corrections in atomic displacements to harmonic vibrational states. Second, the atom-centered density matrix propagation molecular dynamics model that provides a good energy conservation was used to calculate the atomic trajectories, velocities, and a dipole moment time history of biotin at low and room temperatures. Predicted low-THz lines agree well with the experimental spectra. The influence of the polyethylene (PE) matrix embedment on the THz spectra of biotin at the nanoscale was studied using the developed hybrid DFT/molecular mechanical approach. While PE is almost transparent at THz frequencies, additional low-THz lines are predicted in the biotin/PE system, which reflects a dynamic interaction between biotin and a surrounding PE cavity.
Hao, Shiqiang; Sholl, David S
2009-06-28
Diffusion of interstitial hydrogen plays a key role in potential uses for amorphous metals as membranes for hydrogen purification. We show how first principles-based methods can be used to characterize diffusion of interstitial H in amorphous metals using amorphous Fe(3)B as an example. Net transport of interstitial H is governed by the transport diffusion coefficient that appears in Fick's law. This diffusion coefficient is strongly dependent on the interstitial concentration, and is not equal to the self-diffusion coefficient except at dilute interstitial concentrations. Under conditions of practical interest, the concentrations of interstitial H in amorphous metals are nondilute so methods to determine the transport diffusion coefficient must be used if net mass transport is to be described. We show how kinetic Monte Carlo simulations of interstitial H diffusion that use rates derived from first-principles calculations can be used to assess both self- and transport diffusion coefficients of H in amorphous metals. These methods will be helpful in efforts to screen amorphous metal alloys as potential membranes for hydrogen purification.
Structures and magnetic properties of Co-Zr-B magnets studied by first-principles calculations
Zhao, Xin; Ke, Liqin; Nguyen, Manh Cuong; Wang, Cai-Zhuang Ho, Kai-Ming
2015-06-28
The structures and magnetic properties of Co-Zr-B alloys near the composition of Co{sub 5}Zr 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 “Co{sub 11}Zr{sub 2}” 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 Co{sub 5}Zr phase and larger than that of the low-temperature Co{sub 5.25}Zr phase. Our calculations provide useful guidelines for further experimental optimization of the magnetic performances of these alloys.
Hu, S X; Collins, L A; Goncharov, V N; Kress, J D; McCrory, R L; Skupsky, S
2015-10-01
Obtaining an accurate equation of state (EOS) of polystyrene (CH) is crucial to reliably design inertial confinement fusion (ICF) capsules using CH/CH-based ablators. With first-principles calculations, we have investigated the extended EOS of CH over a wide range of plasma conditions (ρ=0.1to100g/cm(3) and T=1000 to 4,000,000 K). When compared with the widely used SESAME-EOS table, the first-principles equation of state (FPEOS) of CH has shown significant differences in the low-temperature regime, in which strong coupling and electron degeneracy play an essential role in determining plasma properties. Hydrodynamic simulations of cryogenic target implosions on OMEGA using the FPEOS table of CH have predicted ∼30% decrease in neutron yield in comparison with the usual SESAME simulations. This is attributed to the ∼5% reduction in implosion velocity that is caused by the ∼10% lower mass ablation rate of CH predicted by FPEOS. Simulations using CH-FPEOS show better agreement with measurements of Hugoniot temperature and scattered light from ICF implosions.
First-principles equation of state and electronic properties of warm dense oxygen.
Driver, K P; Soubiran, F; Zhang, Shuai; Militzer, B
2015-10-28
We perform all-electron path integral Monte Carlo (PIMC) and density functional theory molecular dynamics (DFT-MD) calculations to explore warm dense matter states of oxygen. Our simulations cover a wide density-temperature range of 1-100 g cm(-3) and 10(4)-10(9) K. By combining results from PIMC and DFT-MD, we are able to compute pressures and internal energies from first-principles at all temperatures and provide a coherent equation of state. We compare our first-principles calculations with analytic equations of state, which tend to agree for temperatures above 8 × 10(6) K. Pair-correlation functions and the electronic density of states reveal an evolving plasma structure and ionization process that is driven by temperature and density. As we increase the density at constant temperature, we find that the ionization fraction of the 1s state decreases while the other electronic states move towards the continuum. Finally, the computed shock Hugoniot curves show an increase in compression as the first and second shells are ionized.
Phase and structural stability in Ni-Al systems from first principles
NASA Astrophysics Data System (ADS)
Goiri, Jon Gabriel; Van der Ven, Anton
2016-09-01
We report on a comprehensive first-principles study of phase stability in the Ni-Al binary, both at zero Kelvin and at finite temperature. First-principles density functional theory calculations of the energies of enumerated orderings on fcc and the sublattices of B2 not only predict the stability of known phases, but also reveal the stability of a family of ordered phases that combine features of L 12 and L 10 in different ratios to adjust their overall composition. The calculations also confirm the stability of vacancy ordered B2 derivatives that are stable in the Al-rich half of the phase diagram. We introduce strain order parameters to systematically analyze instabilities with respect to the Bain path connecting the fcc and bcc lattices. Many unstable orderings on both fcc and bcc are predicted around compositions of xNi=0.625 , where a martensitic phase transformation is known to occur. Cluster expansion techniques together with Monte Carlo simulations were used to calculate a finite-temperature-composition phase diagram of the Ni-Al binary. The calculated phase diagram together with an analysis of Bain instabilities reveals the importance of anharmonicity in determining the phase bounds between the B2 based β phase and the L 12 based γ' phase, as well as properties related to martensitic transformations that are observed upon quenching Ni-rich β .
Leng, Xia; Yin, Huabing; Liang, Dongmei; Ma, Yuchen
2015-09-21
Organic semiconductors have promising and broad applications in optoelectronics. Understanding their electronic excited states is important to help us control their spectroscopic properties and performance of devices. There have been a large amount of experimental investigations on spectroscopies of organic semiconductors, but theoretical calculation from first principles on this respect is still limited. Here, we use density functional theory (DFT) and many-body Green's function theory, which includes the GW method and Bethe-Salpeter equation, to study the electronic excited-state properties and spectroscopies of one prototypical organic semiconductor, sexithiophene. The exciton energies of sexithiophene in both the gas and bulk crystalline phases are very sensitive to the exchange-correlation functionals used in DFT for ground-state structure relaxation. We investigated the influence of dynamical screening in the electron-hole interaction on exciton energies, which is found to be very pronounced for triplet excitons and has to be taken into account in first principles calculations. In the sexithiophene single crystal, the energy of the lowest triplet exciton is close to half the energy of the lowest singlet one. While lower-energy singlet and triplet excitons are intramolecular Frenkel excitons, higher-energy excitons are of intermolecular charge-transfer type. The calculated optical absorption spectra and Davydov splitting are in good agreement with experiments.
NASA Astrophysics Data System (ADS)
Leng, Xia; Yin, Huabing; Liang, Dongmei; Ma, Yuchen
2015-09-01
Organic semiconductors have promising and broad applications in optoelectronics. Understanding their electronic excited states is important to help us control their spectroscopic properties and performance of devices. There have been a large amount of experimental investigations on spectroscopies of organic semiconductors, but theoretical calculation from first principles on this respect is still limited. Here, we use density functional theory (DFT) and many-body Green's function theory, which includes the GW method and Bethe-Salpeter equation, to study the electronic excited-state properties and spectroscopies of one prototypical organic semiconductor, sexithiophene. The exciton energies of sexithiophene in both the gas and bulk crystalline phases are very sensitive to the exchange-correlation functionals used in DFT for ground-state structure relaxation. We investigated the influence of dynamical screening in the electron-hole interaction on exciton energies, which is found to be very pronounced for triplet excitons and has to be taken into account in first principles calculations. In the sexithiophene single crystal, the energy of the lowest triplet exciton is close to half the energy of the lowest singlet one. While lower-energy singlet and triplet excitons are intramolecular Frenkel excitons, higher-energy excitons are of intermolecular charge-transfer type. The calculated optical absorption spectra and Davydov splitting are in good agreement with experiments.
Nomura, Yusuke; Sakai, Shiro; Capone, Massimo; Arita, Ryotaro
2015-01-01
Alkali-doped fullerides A3C60 (A = K, Rb, Cs) are surprising materials where conventional phonon-mediated superconductivity and unconventional Mott physics meet, leading to a remarkable phase diagram as a function of volume per C60 molecule. We address these materials with a state-of-the-art calculation, where we construct a realistic low-energy model from first principles without using a priori information other than the crystal structure and solve it with an accurate many-body theory. Remarkably, our scheme comprehensively reproduces the experimental phase diagram including the low-spin Mott-insulating phase next to the superconducting phase. More remarkably, the critical temperatures Tc’s calculated from first principles quantitatively reproduce the experimental values. The driving force behind the surprising phase diagram of A3C60 is a subtle competition between Hund’s coupling and Jahn-Teller phonons, which leads to an effectively inverted Hund’s coupling. Our results establish that the fullerides are the first members of a novel class of molecular superconductors in which the multiorbital electronic correlations and phonons cooperate to reach high Tc s-wave superconductivity. PMID:26601242
First-principles approach to excitons in time-resolved and angle-resolved photoemission spectra
NASA Astrophysics Data System (ADS)
Perfetto, E.; Sangalli, D.; Marini, A.; Stefanucci, G.
2016-12-01
In this work we put forward a first-principles approach and propose an accurate diagrammatic approximation to calculate the time-resolved (TR) and angle-resolved photoemission spectrum of systems with excitons. We also derive an alternative formula to the TR photocurrent which involves a single time-integral of the lesser Green's function. The diagrammatic approximation applies to the relaxed regime characterized by the presence of quasistationary excitons and vanishing polarization. The nonequilibrium self-energy diagrams are evaluated using excited Green's functions; since this is not standard, the analytic derivation is presented in detail. The final result is an expression for the lesser Green's function in terms of quantities that can all be calculated in a first-principles manner. The validity of the proposed theory is illustrated in a one-dimensional model system with a direct gap. We discuss possible scenarios and highlight some universal features of the exciton peaks. Our results indicate that the exciton dispersion can be observed in TR and angle-resolved photoemission.
NASA Astrophysics Data System (ADS)
Hu, S. X.; Collins, L. A.; Goncharov, V. N.; Kress, J. D.; McCrory, R. L.; Skupsky, S.
2015-10-01
Obtaining an accurate equation of state (EOS) of polystyrene (CH) is crucial to reliably design inertial confinement fusion (ICF) capsules using CH/CH-based ablators. With first-principles calculations, we have investigated the extended EOS of CH over a wide range of plasma conditions (ρ =0.1 to 100 g /cm3 and T =1000 to 4 000 000 K ). When compared with the widely used SESAME-EOS table, the first-principles equation of state (FPEOS) of CH has shown significant differences in the low-temperature regime, in which strong coupling and electron degeneracy play an essential role in determining plasma properties. Hydrodynamic simulations of cryogenic target implosions on OMEGA using the FPEOS table of CH have predicted ˜30% decrease in neutron yield in comparison with the usual SESAME simulations. This is attributed to the ˜5% reduction in implosion velocity that is caused by the ˜10% lower mass ablation rate of CH predicted by FPEOS. Simulations using CH-FPEOS show better agreement with measurements of Hugoniot temperature and scattered light from ICF implosions.
Design and Properties Prediction of AMCO3F by First-Principles Calculations.
Tian, Meng; Gao, Yurui; Ouyang, Chuying; Wang, Zhaoxiang; Chen, Liquan
2017-04-10
Computer simulation accelerates the rate of identification and application of new materials. To search for new materials to meet the increasing demands of secondary batteries with higher energy density, the properties of some transition-metal fluorocarbonates ([CO3F](3-)) were simulated in this work as cathode materials for Li- and Na-ion batteries based on first-principles calculations. These materials were designed by substituting the K(+) ions in KCuCO3F with Li(+) or Na(+) ions and the Cu(2+) ions with transition-metal ions such as Fe(2+), Co(2+), Ni(2+), and Mn(2+) ions, respectively. The phase stability, electronic conductivity, ionic diffusion, and electrochemical potential of these materials were calculated by first-principles calculations. After taking comprehensive consideration of the kinetic and thermodynamic properties, LiCoCO3F and LiFeCO3F are believed to be promising novel cathode materials in all of the calculated AMCO3F (A = Li and Na; M = Fe, Mn, Co, and Ni). These results will help the design and discovery of new materials for secondary batteries.
Anisotropic intrinsic lattice thermal conductivity of borophane from first-principles calculations.
Liu, Gang; Wang, Haifeng; Gao, Yan; Zhou, Jian; Wang, Hui
2017-01-25
Borophene (boron sheet) as a new type of two-dimensional (2D) material was grown successfully recently. Unfortunately, the structural stability of freestanding borophene is still an open issue. Theoretical research has found that full hydrogenation can remove such instability, and the product is called borophane. In this paper, using first-principles calculations we investigate the lattice dynamics and thermal transport properties of borophane. The intrinsic lattice thermal conductivity and the relaxation time of borophane are investigated by solving the phonon Boltzmann transport equation (BTE) based on first-principles calculations. We find that the intrinsic lattice thermal conductivity of borophane is anisotropic, as the higher value (along the zigzag direction) is about two times of the lower one (along the armchair direction). The contributions of phonon branches to the lattice thermal conductivities along different directions are evaluated. It is found that both the anisotropy of thermal conductivity and the different phonon branches which dominate the thermal transport along different directions are decided by the group velocity and the relaxation time of phonons with very low frequency. In addition, the size dependence of thermal conductivity is investigated using cumulative thermal conductivity. The underlying physical mechanisms of these unique properties are also discussed in this paper.
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.
Structures and magnetic properties of Co-Zr-B magnets studied by first-principles calculations
Zhao, Xin; Ke, Liqin; Nguyen, Manh Cuong; ...
2015-06-23
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 themore » 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. As a result, our calculations provide useful guidelines for further experimental optimization of the magnetic performances of these alloys.« less
First Principles Study of Electronic Structure of BF3-NH3 Complex and Associated Properties
NASA Astrophysics Data System (ADS)
Dubey, Archana; Mahanti, Mahendra K.; Pink, Roger
2005-03-01
BF3 is a planar molecule with three-fold symmetry which is widely used to promote various organic reactions such as Friedel-Crafts acylations and alkylations. To obtain a thorough understanding of the mechanisms for this role of BF3, we are studying from first-principles the electronic structures of BF3 and its complexes with NH3. The procedure used is the first principles Hartree-Fock-Roothaan procedure combined with many body perturbation theory. The results for BF3-NH3 system will be reported, such as the binding energy and equilibrium geometry of the complex, the nature of the B-N bond and the changes in the B-F and N-H bond strengths on complex formation. The Nuclear Quadrupole Interactions of the ^19F* (spin 5/2), ^14N, ^11B, and ^2H will be presented and compared with available experimental data. (*) Present Address: Dept. of Physics, Uppsala University, Sweden (**) Also: Dept of Physics, University of Central Florida, Orlando, Florida
Structures and magnetic properties of Co-Zr-B magnets studied by first-principles calculations
Zhao, Xin; Ke, Liqin; Nguyen, Manh Cuong; Wang, Cai -Zhuang; Ho, Kai -Ming
2015-06-23
The structures and magnetic properties of Co-Zr-B alloys near the composition of Co_{5}Zr 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 “Co_{11}Zr_{2}” 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 Co_{5}Zr phase and larger than that of the low-temperature Co_{5.25}Zr phase. As a result, our calculations provide useful guidelines for further experimental optimization of the magnetic performances of these alloys.
Lee, B; Rudd, R E
2006-10-19
We report the results of first-principles density functional theory calculations of the Young's modulus and other mechanical properties of hydrogen-passivated Si {l_angle}001{r_angle} nanowires. The nanowires are taken to have predominantly {l_brace}100{r_brace}surfaces, with small {l_brace}110{r_brace} facets according to the Wulff shape. The Young's modulus, the equilibrium length and the constrained residual stress of a series of prismatic beams of differing sizes are found to have size dependences that scale like the surface area to volume ratio for all but the smallest beam. The results are compared with a continuum model and the results of classical atomistic calculations based on an empirical potential. We attribute the size dependence to specific physical structures and interactions. In particular, the hydrogen interactions on the surface and the charge density variations within the beam are quantified and used both to parameterize the continuum model and to account for the discrepancies between the two models and the first-principles results.
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.
Niu, J. G.; Zhan, Q.; Geng, W. T.
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.
Rodriguez-Hernandez, C J; Guinovart, J J; Murguia, J R
2012-02-03
Tungstate counteracts diabetes and obesity in animal models, but its molecular mechanisms remain elusive. Our Saccharomyces cerevisiae-based approach has found that tungstate alleviated the growth defect induced by nutrient stress and enhanced the activation of the GCN pathway. Tungstate relieved the sensitivity to starvation of a gcn2-507 yeast hypomorphic mutant, indicating that tungstate modulated the GCN pathway downstream of Gcn2p. Interestingly, tungstate inhibited Glc7p and PP1 phosphatase activity, both negative regulators of the GCN pathway in yeast and humans, respectively. Accordingly, overexpression of a dominant-negative Glc7p mutant in yeast mimicked tungstate effects. Therefore tungstate alleviates nutrient stress in yeast by in vivo inhibition of Glc7p. These data uncover a potential role for tungstate in the treatment of PP1 and GCN related diseases.
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
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
A functional leptin system is essential for sodium tungstate antiobesity action.
Canals, Ignasi; Carmona, María C; Amigó, Marta; Barbera, Albert; Bortolozzi, Analía; Artigas, Francesc; Gomis, Ramon
2009-02-01
Sodium tungstate is a novel agent in the treatment of obesity. In diet-induced obese rats, it is able to reduce body weight gain by increasing energy expenditure. This study evaluated the role of leptin, a key regulator of energy homeostasis, in the tungstate antiobesity effect. Leptin receptor-deficient Zucker fa/fa rats and leptin-deficient ob/ob mice were treated with tungstate. In lean animals, tungstate administration reduced body weight gain and food intake and increased energy expenditure. However, in animals with deficiencies in the leptin system, treatment did not modify these parameters. In ob/ob mice in which leptin deficiency was restored through adipose tissue transplantation, treatment restored the tungstate-induced body weight gain and food intake reduction as well as energy expenditure increase. Furthermore, in animals in which tungstate administration increased energy expenditure, changes in the expression of key genes involved in brown adipose tissue thermogenesis were detected. Finally, the gene expression of the hypothalamic neuropeptides, Npy, Agrp, and Cart, involved in the leptin regulation of energy homeostasis, was also modified by tungstate in a leptin-dependent manner. In summary, the results indicate that the effectiveness of tungstate in reducing body weight gain is completely dependent on a functional leptin system.
Donmez, Baris O; Ozturk, Nihal; Sarikanat, Mehmet; Oguz, Nurettin; Sari, Ramazan; Ozdemir, Semir
2014-01-01
Diabetes mellitus leads to bone disorders such as osteopenia and osteoporosis that can increase fracture risk. On the other hand, sodium tungstate is an inorganic compound which exerts anti-diabetic activity in experimental studies due to its suggested insulin-mimetic or antioxidant activity. Therefore this study was designed to investigate the effect of tungstate on bone quality in diabetic rat femurs. The rats were divided into four groups: Control (C), tungstate-treated control (C+Tung), diabetes (STZ-D) and tungstate-treated diabetes (STZ-D+Tung). Diabetes mellitus was induced by single injection of streptozotocin (50 mg/kg). The treated rats received 150 mg/kg/day of sodium tungstate for 12 weeks. Sodium tungstate achieved a little (17%) but significant reduction on blood glucose levels, while it didn't recover the reduced body weights of diabetic rats. In addition, impaired bone mechanical quality was reversed, despite the unchanged mineral density. Sodium tungstate administration significantly lowered the 2-thiobarbituric acid reactive substances and restored the activity of tissue antioxidant enzymes such as glutathione peroxidase, catalase and superoxide dismutase in diabetic rats. On the other hand, glutathione levels didn't change in either case. These findings indicate that tungstate can improve the reduced mechanical quality of diabetic rat femurs due probably to reduction of reactive oxygen species and modulation of antioxidant enzymes as well as reduction in blood glucose levels.
Towards first-principles molecular design of liquid crystal-based chemoresponsive systems
NASA Astrophysics Data System (ADS)
Roling, Luke T.; Scaranto, Jessica; Herron, Jeffrey A.; Yu, Huaizhe; Choi, Sangwook; Abbott, Nicholas L.; Mavrikakis, Manos
2016-11-01
Nematic liquid crystals make promising chemoresponsive systems, but their development is currently limited by extensive experimental screening. Here we report a computational model to understand and predict orientational changes of surface-anchored nematic liquid crystals in response to chemical stimuli. In particular, we use first-principles calculations to evaluate the binding energies of benzonitrile, a model for 4'-pentyl-4-biphenylcarbonitrile, and dimethyl methylphosphonate to metal cation models representing the substrate chemical sensing surface. We find a correlation between these quantities and the experimental response time useful for predicting the response time of cation-liquid crystal combinations. Consideration of charge donation from chemical species in the surface environment is critical for obtaining agreement between theory and experiment. Our model may be extended to the design of improved chemoresponsive liquid crystals for selectively detecting other chemicals of practical interest by choosing appropriate combinations of metal cations with liquid crystals of suitable molecular structure.
First-Principles Study for Thermodynamic Properties of Solid {KNO}2 System
NASA Astrophysics Data System (ADS)
Peng, Qiang; Ding, Jing; Wei, Xiaolan; Jiang, Gan; Yang, Xiaoxi
2015-11-01
To enable us better understand the performance of molten salt energy storage in a solar thermal power system, thermodynamic properties of the solid {KNO}2 system at ambient pressure and temperatures between 0 K and 711 K are determined by first-principles simulation based on density functional perturbation theory calculations with plane waves and pseudopotentials. Thermodynamic parameters of the Debye temperature, specific heat capacity at constant volume, phonon transfer speed, phonon mean free path, and phonon thermal conductivity as a function of temperature are estimated. The results show that the calculated phonon thermal conductivity is in good agreement with experimental values, but the calculated specific heat capacity at constant volume is lower than measured values. The isometric specific heat capacity of {KNO}2 is 75.03 {J}{\\cdot }{mol}^{-1}{\\cdot }{K}^{-1}, and the phonon thermal conductivity is 2.37 {W}{\\cdot }{m}^{-1}{\\cdot }{K}^{-1} at ambient temperature.
Towards Bond Selective Chemistry from First Principles: Methane on Metal Surfaces
NASA Astrophysics Data System (ADS)
Shen, X. J.; Lozano, A.; Dong, W.; Busnengo, H. F.; Yan, X. H.
2014-01-01
Controlling bond-selective chemical reactivity is of great importance and has a broad range of applications. Here, we present a molecular dynamics study of bond selective reactivity of methane and its deuterated isotopologues (i.e., CH4-xDx, x =0,1,2,3,4) on Ni(111) and Pt(111) from first principles calculations. Our simulations allow for reproducing the full C-H bond selectivity recently achieved experimentally via mode-specific vibrational excitation and explain its origin. Moreover, we also predict the hitherto unexplored influence of the molecular translational energy on such a selectivity as well as the conditions under which the full selectivity can be realized for the a priori less active C-D bond.
Kang, Wei; Zhao, Shijun; Zhang, Shen; Zhang, Ping; Chen, Q F; He, Xian-Tu
2016-02-08
Mott effect, featured by a sharp increase of ionization, is one of the unique properties of partially ionized plasmas, and thus of great interest to astrophysics and inertial confinement fusion. Recent experiments of single bubble sonoluminescence (SBSL) revealed that strong ionization took place at a density two orders lower than usual theoretical expectation. We show from the perspective of electronic structures that the strong ionization is unlikely the result of Mott effect in a pure argon plasma. Instead, first-principles calculations suggest that other ion species from aqueous environments can energetically fit in the gap between the continuum and the top of occupied states of argon, making the Mott effect possible. These results would help to clarify the relationship between SBSL and Mott effect, and further to gain an better understanding of partially ionized plasmas.
Performance of arsenene and antimonene double-gate MOSFETs from first principles
Pizzi, Giovanni; Gibertini, Marco; Dib, Elias; Marzari, Nicola; Iannaccone, Giuseppe; Fiori, Gianluca
2016-01-01
In the race towards high-performance ultra-scaled devices, two-dimensional materials offer an alternative paradigm thanks to their atomic thickness suppressing short-channel effects. It is thus urgent to study the most promising candidates in realistic configurations, and here we present detailed multiscale simulations of field-effect transistors based on arsenene and antimonene monolayers as channels. The accuracy of first-principles approaches in describing electronic properties is combined with the efficiency of tight-binding Hamiltonians based on maximally localized Wannier functions to compute the transport properties of the devices. These simulations provide for the first time estimates on the upper limits for the electron and hole mobilities in the Takagi's approximation, including spin–orbit and multi-valley effects, and demonstrate that ultra-scaled devices in the sub-10-nm scale show a performance that is compliant with industry requirements. PMID:27557562
The First-Principle Calculation of La-doping Effect on Piezoelectricity in Tetragonal KNN Crystal
NASA Astrophysics Data System (ADS)
Zhang, Qiaoli; Zhu, Jiliang; Yuan, Daqing; Zhu, Bo; Wang, Mingsong; Zhu, Xiaohong; Fan, Ping; Zuo, Yi; Zheng, Yongnan; Zhu, Shengyun
2012-05-01
The La-dopping effect on the piezoelectricity in the K0.5Na0.5NbO3 (KNN) crystal with a tetragonal phase is investigated for the first time using the first-principle calculation based on density functional theory. The full potentiallinearized augumented plane wave plus local orbitals (APW-LO) method and the supercell method are used in the calculation for the KNN crystal with and without the La doping. The results show that the piezoelectricity originates from the strong hybridization between the Nb atom and the O atom, and the substitution of the K or Na atom by the La impurity atom introduces the anisotropic relaxation and enhances the piezoelectricity at first and then restrains the hybridization of the Nb-O atoms when the La doping content further increases.
Meng, X. Y.; Qin, G. W.; Li, S.; Ren, Y. P.; Pei, W. L.; Zuo, L.; Wen, X. H.
2011-03-14
To improve photoelectrochemical (PEC) activity of hematite, the modification of energy band by doping 3d transition metal ions Cu and Ti into {alpha}-Fe{sub 2}O{sub 3} were studied via the first-principles calculations with density function theory (DFT)+U method. The results show that the band gap of hematite is {approx}2.1 eV and n-type dopant Ti improves the electric conductivity, confirmed by recent experiments. The p-type dopant Cu enhances the utilization ratio of solar energy, shifts both valance, and conduction band edges to a higher energy level, satisfying hydrogen production in the visible light driven PEC water splitting without voltage bias.
NASA Astrophysics Data System (ADS)
Hong, Jiawang; Li, Chen W.; May, A. F.; Bansal, D.; Chi, S.; Hong, T.; Ehlers, G.; Delaire, Olivier
The promising thermoelectric material SnSe exhibits ultra-low and strongly anisotropic thermal conductivity. By combining first-principles calculations and inelastic neutron scattering measurements, we have investigated the phonon dispersions and phonon scattering mechanisms, and probed the origin of the large anharmonicity in SnSe. We will discuss the connection between the phonon properties and the high-temperature structural phase transition, and how the electronic structure leads to large anharmonic phonon interactions in SnSe. The present results provide a microscopic picture connecting electronic structure and phonon anharmonicity in SnSe, which could help design materials with ultralow thermal conductivity. Computations were performed using the OLCF at ORNL. Modeling of neutron data was performed in CAMM, measurements were funded by the US DOE, BES, Materials Science and Engineering Division.
Structural stability and electronic properties of InSb nanowires: A first-principles study
Zhang, Yong; Tang, Li-Ming Ning, Feng; Chen, Ke-Qiu; Wang, Dan
2015-03-28
Using first-principles calculations, we investigate the structural stability and electronic properties of InSb nanowires (NWs). The results show that, in contrast to the bulk InSb phase, wurtzite (WZ) NWs are more stable than zinc-blende (ZB) NWs when the NW diameter is smaller than 10 nm. Nonpassivated ZB and WZ NWs are found to be metallic and semiconducting, respectively. After passivation, both ZB and WZ NWs exhibit direct-gap semiconductor character, and the band gap magnitude of the NWs strongly depends on the suppression of surface states by the charge-compensation ability of foreign atoms to surface atoms. Moreover, the carrier mobility of the NW can be strengthened by halogen passivation.
Effect of stacking faults on the magnetocrystalline anisotropy of hcp Co: a first-principles study.
Aas, C J; Szunyogh, L; Evans, R F L; Chantrell, R W
2013-07-24
In terms of the fully relativistic screened Korringa-Kohn-Rostoker method we investigate the effect of stacking faults on the magnetic properties of hexagonal close-packed (hcp) cobalt. In particular, we consider the formation energy and the effect on the magnetocrystalline anisotropy energy (MAE) of four different stacking faults in hcp cobalt-an intrinsic growth fault, an intrinsic deformation fault, an extrinsic fault and a twin-like fault. We find that the intrinsic growth fault has the lowest formation energy, in good agreement with previous first-principles calculations. With the exception of the intrinsic deformation fault which has a positive impact on the MAE, we find that the presence of a stacking fault generally reduces the MAE of bulk Co. Finally, we consider a pair of intrinsic growth faults and find that their effect on the MAE is not additive, but synergic.
First-principles prediction of a low energy edge-reconstruction for zigzag phosphorene nanoribbons
NASA Astrophysics Data System (ADS)
Shi, XiZhi; He, ChaoYu; OuYang, Tao; Zhang, ChunXiao; Tang, Chao; Zhong, JianXin
2017-02-01
Based on first-principles calculations, a new-type of edge reconstruction with remarkable stability is predicted for zigzag phosphorene nanoribbons. Such a new-type of edge reconstruction is named as θ-edge according to its θ-like configuration, in which all edge atoms are fully self-passivated with a coordination number of 3. In ZZ nanoribbons, θ-edge is energetically more stable than the bare case and as stable as the previously proposed ZZ‧-o reconstruction. In ZZ54 nanoribbons, θ-edge is energetically more stable than the metastable Δ-edge spontaneously formed in normal VASP optimization, and it is the most stable one among all these edge reconstructions. Further investigation shows that zigzag phosphorene nanoribbons with θ-edge are semiconductors with band gaps varying inversely with ribbon width.
First-principles study of structural and thermodynamic properties of osmium
NASA Astrophysics Data System (ADS)
Liu, Ke; He, Duan-Wei; Zhou, Xiao-Lin; Chen, Hai-Hua
2011-08-01
We employ the first-principles plane wave pseudopotential density functional theory method to calculate the equilibrium lattice parameters of osmium and the thermodynamic properties of hcp structure osmium. The obtained lattice parameters are in good agreement with the experimental data investigated up to 58.2 GPa using radial X-ray diffraction (RXRD) together with lattice strain theory in a diamond-anvil cell and the available theoretical data of others. Through the quasi-harmonic Debye model, the dependencies of the normalized lattice parameters a/ a0 and c/ c0 on pressure P, the normalized primitive volume V/V0 on pressure P, the Debye temperature ΘD and the heat capacity CV on pressure P and temperature T, as well as the variation of the thermal expansion α with temperature and pressure are obtained successfully.
First-principles study of the spin-orbit interaction in graphene induced by hydrogen adatoms
NASA Astrophysics Data System (ADS)
Gmitra, Martin; Kochan, Denis; Fabian, Jaroslav
2013-03-01
We have performed first principles calculations of the spin-orbit coupling effects in hydrogenated graphene structures, for varying hydrogen coverage densities, using the linearized augmented plane wave method as implemented in the FLEUR code. The covalent bonding between the hydrogen and carbon atoms leads to a local structural puckering of graphene sheets, giving rise to an overlap between the Dirac and sigma electrons and a giant enhancement (from roughly 0.01 to 1 meV) of the local spin-orbit interaction. The calculated effects on the band structure and the emerging spin patterns of the electronic states can be well explained by effective Hamiltonian models derived from group theoretical principles. This work is supported by the DFG SPP 1285, SFB 689, and GRK 1570
New crystal phase of ammonium nitrate: First-principles prediction and characterization
NASA Astrophysics Data System (ADS)
Steele, Brad A.; Oleynik, Ivan I.
2017-01-01
First principles evolutionary crystal structure search found a new crystal phase of ammonium nitrate (AN). The calculated Raman spectra of this new phase is consistent with the recently reported experimental Raman spectrum that contains two peaks previously associated with a pressure-induced phase transition. The phase transition is reported to occur at a pressure of 17 GPa while the new phase is calculated to be lower in free energy than phase IV of AN (AN-IV) above a pressure of 10.83 GPa. The new phase has a monoclinic unit cell with the P21/m space group symmetry (AN-P21/m). This new phase is similar to AN-IV except the ammonium molecules are oriented differently relative to the nitrate molecules. The calculated Raman spectrum of both AN-P21/m and AN-IV as a function of pressure shows good agreement with experiment up to 33 GPa.
Correa, Alfredo A; Bonev, Stanimir A; Galli, Giulia
2006-01-31
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 approximately 850 GPa and approximately 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.
First-principles investigation of point defect and atomic diffusion in Al2Ca
NASA Astrophysics Data System (ADS)
Tian, Xiao; Wang, Jia-Ning; Wang, Ya-Ping; Shi, Xue-Feng; Tang, Bi-Yu
2017-04-01
Point defects and atomic diffusion in Al2Ca have been studied from first-principles calculations within density functional framework. After formation energy and relative stability of point defects are investigated, several predominant diffusion processes in Al2Ca are studied, including sublattice one-step mechanism, 3-jump vacancy cycles and antistructure sublattice mechanism. The associated energy profiles are calculated with climbing image nudged elastic band (CI-NEB) method, then the saddle points and activation barriers during atomic diffusion are further determined. The resulted activation barriers show that both Al and Ca can diffuse mainly mediated by neighbor vacancy on their own sublattice. 3-jump cycle mechanism mediated by VCa may make some contribution to the overall Al diffusion. And antistructure (AS) sublattice mechanism can also play an important role in Ca atomic diffusion owing to the moderate activation barrier.
First-Principles Study of Carbon Nanoframeworks Tailored for Hydrogen Storage
NASA Astrophysics Data System (ADS)
Kim, Eunja; Weck, Philippe; Naduvalath, Balakrishnan; Cheng, Hansong; Yakobson, Boris
2008-03-01
Based on first-principles calculations, we propose a novel class of 3-D materials consisting of small diameter single-walled carbon nanotubes (SWCNTs) functionalized by organic ligands as potential hydrogen storage media. Specifically, we have carried out density functional theory calculations to determine the stable structures and properties of nanoframeworks consisting of (5,0) and (3,3) SWCNTs constrained by phenyl spacers. Valence and conduction properties, as well as normal modes, of pristine nanotubes are found to change significantly upon functionalization, in a way that can serve as experimental diagnostics of the successful synthesis of the proposed framework structures. Ab initio molecular dynamics simulations indicate that such systems are thermodynamically stable for on-board hydrogen storage. In order to increase the hydrogen uptake in the interstitial cavity of such nanoframeworks, we are currently investigating the possibility of Li deposition on these nanostructures.
First-Principles Calculation of Femtosecond Symmetry-Breaking Atomic Forces in Photoexcited Bismuth
NASA Astrophysics Data System (ADS)
Murray, Éamonn D.; Fahy, Stephen
2015-02-01
We present a first-principles method for the calculation of the polarization-dependent atomic forces resulting from optical excitation in a solid. We calculate the induced force driving the Eg phonon mode in bismuth immediately after absorption of polarized light. When radiation with polarization perpendicular to the c axis is absorbed, the photoexcited charge density breaks the threefold rotational symmetry, leading to an atomic force component perpendicular to the axis. We calculate the initial excited electronic distribution as a function of photon energy and polarization and find the resulting atomic force components parallel and perpendicular to the axis. The magnitude of the calculated force is in excellent agreement with that derived from recent measurements of the amplitude of Eg atomic motion and the decay time of several femtoseconds for the driving force.
NASA Astrophysics Data System (ADS)
D'Souza, Ransell; Mukherjee, Sugata
2017-02-01
We report the transport properties of monolayer and bilayer graphene from first-principles calculations and Boltzmann transport theory (BTE). Our resistivity studies on monolayer graphene show Bloch-Grüneisen behavior in a certain range of chemical potentials. By substituting boron nitride in place of a carbon dimer of graphene, we predict a twofold increase in the Seebeck coefficient. A similar increase in the Seebeck coefficient for bilayer graphene under the influence of a small electric field ˜0.3 eV has been observed in our calculations. Graphene with impurities shows a systematic decrease of electrical conductivity and mobility. We have also calculated the lattice thermal conductivities of monolayer graphene and bilayer graphene using phonon BTE which show excellent agreement with experimental data available in the temperature range 300-700 K.
Graphene oxide as a candidate material for natural gas storage: A first principles study
NASA Astrophysics Data System (ADS)
Chouhan, Rajiv Kumar; Ulman, Kanchan; Narasimhan, Shobhana
2015-03-01
Alternative sources of clean energy will be much in demand in the coming days. To store methane (CH4) in sorbent materials at ambient conditions for on-board vehicular usage, minimum adsorption energy of 18.8 KJ/mol is desirable. In this work, we have investigated methane adsorption on graphene oxide using first principles calculations. To accurately capture the weak interactions between CH4 and the substrate we have included van der Waals interactions in our calculations. We show that the adsorption energy falls within the target range. Careful analysis of the various contributions to the binding shows that the enhancement in adsorption energy on going from graphene to graphene oxide arises from a subtle synergy between various effects. Funding agencies CSIR, India, DST Nanomission and JNCASR. Computational facilities provided by TUE-CMS, JNCASR.
First-principles study of CaFe2As2 under pressure
NASA Astrophysics Data System (ADS)
Widom, Michael; Quader, Khandker
2013-07-01
We perform first-principles calculations on CaFe2As2 under hydrostatic pressure. Our total-energy calculations show that though the striped antiferromagnetic (AFM) orthorhombic (OR) phase is favored at P=0, a nonmagnetic collapsed tetragonal (cT) phase with diminished c parameter is favored for P>0.36 GPa, in agreement with experiments. Rather than a mechanical instability, this is an enthalpically driven transition from the higher volume OR phase to the lower volume cT phase. A simple thermodynamic model provides an interpretation of the finite-temperature phase boundaries of the cT phase. Calculations of electronic density of states reveal pseudogaps in both OR and cT phases. Band-structure analysis provides insight into the origin of the pseudogaps while revealing the location and nature of hybridized Fe-d and As-p bonding orbitals.
First-principles definition and measurement of planetary electromagnetic-energy budget.
Mishchenko, Michael I; Lock, James A; Lacis, Andrew A; Travis, Larry D; Cairns, Brian
2016-06-01
The imperative to quantify the Earth's electromagnetic-energy budget with an extremely high accuracy has been widely recognized but has never been formulated in the framework of fundamental physics. In this paper we give a first-principles definition of the planetary electromagnetic-energy budget using the Poynting-vector formalism and discuss how it can, in principle, be measured. Our derivation is based on an absolute minimum of theoretical assumptions, is free of outdated notions of phenomenological radiometry, and naturally leads to the conceptual formulation of an instrument called the double hemispherical cavity radiometer (DHCR). The practical measurement of the planetary energy budget would require flying a constellation of several dozen planet-orbiting satellites hosting identical well-calibrated DHCRs.
Equation of state for technetium from X-ray diffraction and first-principle calculations
Mast, Daniel S.; Kim, Eunja; Siska, Emily M.; Poineau, Frederic; Czerwinski, Kenneth R.; Lavina, Barbara; Forster, Paul M.
2016-03-20
Here, the ambient temperature equation of state (EoS) of technetium metal has been measured by X-ray diffraction. The metal was compressed using a diamond anvil cell and using a 4:1 methanol-ethanol pressure transmitting medium. The maximum pressure achieved, as determined from the gold pressure scale, was 67 GPa. The compression data shows that the HCP phase of technetium is stable up to 67 GPa. The compression curve of technetium was also calculated using first-principles total-energy calculations. Utilizing a number of fitting strategies to compare the experimental and theoretical data it is determined that the Vinet equation of state with an ambient isothermal bulk modulus of B_{0T} = 288 GPa and a first pressure derivative of B' = 5.9(2) best represent the compression behavior of technetium metal.
The mechanical exfoliation mechanism of black phosphorus to phosphorene: A first-principles study
NASA Astrophysics Data System (ADS)
Mu, Yunsheng; Si, M. S.
2015-11-01
Today, the renaissance of black phosphorus largely depends on the mechanical exfoliation method, which is accessible to produce few-layer forms from the bulk counterpart. However, the deep understanding of the exfoliation mechanism is missing. To this end, we resolve this issue by simulating the sliding processes of bilayer phosphorene based on first-principles calculations. It is found that the interlayer Coulomb interactions dictate the optimal sliding pathway, leading to the minimal energy barrier as low as ∼60 \\text{meV} , which gives a comparable surface energy of ∼59 \\text{mJ/m}2 in experiment. This means that black phosphorus can be exfoliated by the sliding approach. In addition, considerable bandgap modulations along these sliding pathways are obtained. The study like ours builds up a fundamental understanding of how black phosphorus is exfoliated to few-layer forms, providing a good guide to experimental research.
Ekuma, C E; Dobrosavljević, V; Gunlycke, D
2017-03-10
We present a first-principles-based many-body typical medium dynamical cluster approximation and density function theory method for characterizing electron localization in disordered structures. This method applied to monolayer hexagonal boron nitride shows that the presence of boron vacancies could turn this wide-gap insulator into a correlated metal. Depending on the strength of the electron interactions, these calculations suggest that conduction could be obtained at a boron vacancy concentration as low as 1.0%. We also explore the distribution of the local density of states, a fingerprint of spatial variations, which allows localized and delocalized states to be distinguished. The presented method enables the study of disorder-driven insulator-metal transitions not only in h-BN but also in other physical materials.
Protein-protein interactions from linear-scaling first-principles quantum-mechanical calculations
NASA Astrophysics Data System (ADS)
Cole, D. J.; Skylaris, C.-K.; Rajendra, E.; Venkitaraman, A. R.; Payne, M. C.
2010-08-01
A modification of the MM-PBSA technique for calculating binding affinities of biomolecular complexes is presented. Classical molecular dynamics is used to explore the motion of the extended interface between two peptides derived from the BRC4 repeat of BRCA2 and the eukaryotic recombinase RAD51. The resulting trajectory is sampled using the linear-scaling density functional theory code, onetep, to determine from first principles, and with high computational efficiency, the relative free energies of binding of the ~2800 atom receptor-ligand complexes. This new method provides the basis for computational interrogation of protein-protein and protein-ligand interactions within fields ranging from chemical biological studies to small-molecule binding behaviour, with both unprecedented chemical accuracy and affordable computational expense.
Protein-Protein Interactions from Linear-Scaling First Principles Quantum Mechanical Calculations
NASA Astrophysics Data System (ADS)
Cole, Daniel; Skylaris, Chris-Kriton; Rajendra, Eeson; Venkitaraman, Ashok; Payne, Mike
2010-03-01
A modification of the MM-PBSA technique for calculating binding affinities of biomolecular complexes is presented. Classical molecular dynamics is used to explore the motion of the extended interface between two peptides derived from the BRC4 repeat of BRCA2 and the eukaryotic recombinase RAD51. The resulting trajectory is sampled using the linear-scaling density functional theory code, onetep, to determine from first principles, and with high computational efficiency, the relative free energies of binding of the ˜2800 atom receptor-ligand complexes. This new method provides the basis for computational interrogation of protein-protein and protein-ligand interactions, within fields ranging from chemical biological studies to small molecule binding behaviour, with both unprecedented chemical accuracy and affordable computational expense.
Real-time capable first principle based modelling of tokamak turbulent transport
NASA Astrophysics Data System (ADS)
Citrin, J.; Breton, S.; Felici, F.; Imbeaux, F.; Aniel, T.; Artaud, J. F.; Baiocchi, B.; Bourdelle, C.; Camenen, Y.; Garcia, J.
2015-09-01
A real-time capable core turbulence tokamak transport model is developed. This model is constructed from the regularized nonlinear regression of quasilinear gyrokinetic transport code output. The regression is performed with a multilayer perceptron neural network. The transport code input for the neural network training set consists of five dimensions, and is limited to adiabatic electrons. The neural network model successfully reproduces transport fluxes predicted by the original quasilinear model, while gaining five orders of magnitude in computation time. The model is implemented in a real-time capable tokamak simulator, and simulates a 300 s ITER discharge in 10 s. This proof-of-principle for regression based transport models anticipates a significant widening of input space dimensionality and physics realism for future training sets. This aims to provide unprecedented computational speed coupled with first-principle based physics for real-time control and integrated modelling applications.
Li-Na ternary amidoborane for hydrogen storage: experimental and first-principles study.
Li, Wen; Miao, Ling; Scheicher, Ralph H; Xiong, Zhitao; Wu, Guotao; Araújo, C Moysés; Blomqvist, Andreas; Ahuja, Rajeev; Feng, Yuanping; Chen, Ping
2012-04-28
Li-Na ternary amidoborane, Na[Li(NH(2)BH(3))(2)], was recently synthesized by reacting LiH and NaH with NH(3)BH(3). This mixed-cation amidoborane shows improved dehydrogenation performance compared to that of single-cation amidoboranes, i.e., LiNH(2)BH(3) and NaNH(2)BH(3). In this paper, we synthesized the Li-Na ternary amidoborane by blending and re-crystallizing equivalent LiNH(2)BH(3) and NaNH(2)BH(3) in tetrahydrofuran (THF), and employed first-principles calculations and the special quasirandom structure (SQS) method to theoretically explore the likelihood for the existence of Li(1-x)Na(x)(NH(2)BH(3)) for various Li/Na ratios. The thermodynamic, electronic and phononic properties were investigated to understand the possible dehydrogenation mechanisms of Na[Li(NH(2)BH(3))(2)].
First-principles approach to calculating energy level alignment at aqueous semiconductor interfaces
Kharche, Neerav; Muckerman, James T.; Hybertsen, Mark S.
2014-10-21
A first-principles approach is demonstrated for calculating the relationship between an aqueous semiconductor interface structure and energy level alignment. The physical interface structure is sampled using density functional theory based molecular dynamics, yielding the interface electrostatic dipole. The GW approach from many-body perturbation theory is used to place the electronic band edge energies of the semiconductor relative to the occupied 1b₁ energy level in water. The application to the specific cases of nonpolar (101¯0 ) facets of GaN and ZnO reveals a significant role for the structural motifs at the interface, including the degree of interface water dissociation and themore » dynamical fluctuations in the interface Zn-O and O-H bond orientations. As a result, these effects contribute up to 0.5 eV.« less
NASA Astrophysics Data System (ADS)
Bonnet, Nicéphore; Marzari, Nicola
2013-02-01
A first-principles model of the electrochemical double layer is applied to study surface energies and surface coverage under realistic electrochemical conditions and to determine the equilibrium shape of metal nanoparticles as a function of applied potential. The potential bias is directly controlled by adding electronic charge to the system, while total energy calculations and thermodynamic relations are used to predict electrodeposition curves and changes in surface energies and coverage. This approach is applied to Pt surfaces subject to hydrogen underpotential deposition. The shape of Pt nanoparticles under a cathodic scan is shown to undergo an octahedric-to-cubic transition, which is more pronounced in alkaline media due to the interaction energy of the pH-dependent surface charge with the surface dipole.
Bonnet, Nicéphore; Marzari, Nicola
2013-02-22
A first-principles model of the electrochemical double layer is applied to study surface energies and surface coverage under realistic electrochemical conditions and to determine the equilibrium shape of metal nanoparticles as a function of applied potential. The potential bias is directly controlled by adding electronic charge to the system, while total energy calculations and thermodynamic relations are used to predict electrodeposition curves and changes in surface energies and coverage. This approach is applied to Pt surfaces subject to hydrogen underpotential deposition. The shape of Pt nanoparticles under a cathodic scan is shown to undergo an octahedric-to-cubic transition, which is more pronounced in alkaline media due to the interaction energy of the pH-dependent surface charge with the surface dipole.
Effective Hamiltonian for electron waves in artificial graphene: A first-principles derivation
NASA Astrophysics Data System (ADS)
Lannebère, Sylvain; Silveirinha, Mário G.
2015-01-01
We propose a first-principles effective medium formalism to study the propagation of electron waves in semiconductor heterostructures with a zero band gap. Our theory confirms that near the K point the dynamics of a two-dimensional electron gas modulated by an external electrostatic potential with honeycomb symmetry is described by the same pseudospinor formalism and Dirac massless equation as a graphene monolayer. Furthermore, we highlight that even though other superlattices based on semiconductors with a zincblende-type structure can have a zero band-gap and a linear energy-momentum dispersion, the corresponding effective medium Hamiltonian is rather different from that of graphene, and can be based on a single-component wave function.
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.
First-Principles Investigation of Li Intercalation Kinetics in Phospho-Olivines
NASA Astrophysics Data System (ADS)
Malik, Rahul
This thesis focuses broadly on characterizing and understanding the Li intercalation mechanism in phospho-olivines, namely LiFePO 4 and Li(Fe,Mn)PO4, using first-principles calculations. Currently Li-ion battery technology is critically relied upon for the operation of electrified vehicles, but further improvements mainly in cathode performance are required to ensure widespread adoption, which in itself requires learning from existing commercial cathode chemistries. LiFePO4 is presently used in commercial Li-ion batteries, known for its rapid charge and discharge capability but with underwhelming energy density. This motivates the three central research efforts presented herein. First, we investigate the modified phase diagram and electrochemical properties of mixed olivines, such as Li(Fe,Mn)PO4, which offer improved theoretical energy density over LiFePO4 (due to the higher redox voltage associated with Mn2+/Mn3+). The Lix(Fe1-yMny)PO4 phase diagram is constructed by Monte Carlo simulation on a cluster expansion Hamiltonian parametrized by first-principles determined energies. Deviations from the equilibrium phase behavior and voltages of pure LiFePO4 and LiMnPO 4 are analyzed and discussed to good agreement with experimental observations. Second, we address why LiFePO4 exhibits superior rate performance strictly when the active particle size is brought down to the nano-scale. By considering the presence of immobile point defects residing in the 1D Li diffusion path, specifically by calculating from first principles both defect formation energies and Li migration barriers in the vicinity of likely defects, the Li diffusivity is recalculated and is found to strongly vary with particle size. At small particle sizes, the contribution from defects is small, and fast 1D Li diffusion is accessible. However, at larger particle sizes (microm scale and above) the contribution from defects is much larger. Not only is Li transport impeded, but it is also less anisotropic in
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
First-principles approach to calculating energy level alignment at aqueous semiconductor interfaces
Kharche, Neerav; Muckerman, James T.; Hybertsen, Mark S.
2014-10-21
A first-principles approach is demonstrated for calculating the relationship between an aqueous semiconductor interface structure and energy level alignment. The physical interface structure is sampled using density functional theory based molecular dynamics, yielding the interface electrostatic dipole. The GW approach from many-body perturbation theory is used to place the electronic band edge energies of the semiconductor relative to the occupied 1b₁ energy level in water. The application to the specific cases of nonpolar (101¯0 ) facets of GaN and ZnO reveals a significant role for the structural motifs at the interface, including the degree of interface water dissociation and the dynamical fluctuations in the interface Zn-O and O-H bond orientations. As a result, these effects contribute up to 0.5 eV.
A method of orbital analysis for large-scale first-principles simulations.
Ohwaki, Tsukuru; Otani, Minoru; Ozaki, Taisuke
2014-06-28
An efficient method of calculating the natural bond orbitals (NBOs) based on a truncation of the entire density matrix of a whole system is presented for large-scale density functional theory calculations. The method recovers an orbital picture for O(N) electronic structure methods which directly evaluate the density matrix without using Kohn-Sham orbitals, thus enabling quantitative analysis of chemical reactions in large-scale systems in the language of localized Lewis-type chemical bonds. With the density matrix calculated by either an exact diagonalization or O(N) method, the computational cost is O(1) for the calculation of NBOs associated with a local region where a chemical reaction takes place. As an illustration of the method, we demonstrate how an electronic structure in a local region of interest can be analyzed by NBOs in a large-scale first-principles molecular dynamics simulation for a liquid electrolyte bulk model (propylene carbonate + LiBF4).
A method of orbital analysis for large-scale first-principles simulations
NASA Astrophysics Data System (ADS)
Ohwaki, Tsukuru; Otani, Minoru; Ozaki, Taisuke
2014-06-01
An efficient method of calculating the natural bond orbitals (NBOs) based on a truncation of the entire density matrix of a whole system is presented for large-scale density functional theory calculations. The method recovers an orbital picture for O(N) electronic structure methods which directly evaluate the density matrix without using Kohn-Sham orbitals, thus enabling quantitative analysis of chemical reactions in large-scale systems in the language of localized Lewis-type chemical bonds. With the density matrix calculated by either an exact diagonalization or O(N) method, the computational cost is O(1) for the calculation of NBOs associated with a local region where a chemical reaction takes place. As an illustration of the method, we demonstrate how an electronic structure in a local region of interest can be analyzed by NBOs in a large-scale first-principles molecular dynamics simulation for a liquid electrolyte bulk model (propylene carbonate + LiBF4).
NASA Astrophysics Data System (ADS)
Tang, C.; Ramprasad, R.
2006-03-01
We present a new first principles based method to determine the equivalent circuit representations of nanostructured physical systems at optical frequencies. This method involves the determination of the frequency dependent effective permittivity of two constructs: an ordered composite system consisting of physical nano-elements using density functional theory, and an ordered arrangement of impedances using transmission line theory. Matching the calculated effective permittivity functions of these two constructs has enabled a mapping of the physical nano-system to its equivalent circuit. Specifically, we will show that silicon nanowires and carbon nanotubes can be represented as a series combination of inductance, capacitance and resistance. Once this mapping has been reasonably accomplished for a variety of physical systems, the nano-elements can be combined suitably to result in equivalent circuit topologies appropriate for optical and nanoelectronic devices, including left-handed (or negative refractive index) materials.
Establishing the limits of efficiency of perovskite solar cells from first principles modeling
Grånäs, Oscar; Vinichenko, Dmitry; Kaxiras, Efthimios
2016-01-01
The recent surge in research on metal-halide-perovskite solar cells has led to a seven-fold increase of efficiency, from ~3% in early devices to over 22% in research prototypes. Oft-cited reasons for this increase are: (i) a carrier diffusion length reaching hundreds of microns; (ii) a low exciton binding energy; and (iii) a high optical absorption coefficient. These hybrid organic-inorganic materials span a large chemical space with the perovskite structure. Here, using first-principles calculations and thermodynamic modelling, we establish that, given the range of band-gaps of the metal-halide-perovskites, the theoretical maximum efficiency limit is in the range of ~25–27%. Our conclusions are based on the effect of level alignment between the perovskite absorber layer and carrier-transporting materials on the performance of the solar cell as a whole. Our results provide a useful framework for experimental searches toward more efficient devices. PMID:27824030
First principles study of bismuth alloying effects in GaAs saturable absorber.
Li, Dechun; Yang, Ming; Zhao, Shengzhi; Cai, Yongqing; Feng, Yuanping
2012-05-07
First principles hybrid functional calculations have been carried out to study electronic properties of GaAs with Bi alloying effects. It is found that the doping of Bi into GaAs reduces the bandgap due to the intraband level repulsions between Bi induced states and host states, and the Bi-related impurity states originate from the hybridization of Bi-6p and its nearest As-4p orbitals. With the increase of Bi concentration in GaAs, the bandgap decreases monotonously. The calculated optical properties of the undoped and Bi-doped GaAs are similar except the shift toward lower energy of absorption edge and main absorption peaks with Bi doping. These results suggest a promising application of GaBi(x)As(1-x) alloy as semiconductor saturable absorber in Q-switched or mode-locked laser.
First principles study of crystal Si-doped Ge2Sb2Te5
NASA Astrophysics Data System (ADS)
Yan, Beibei; Yang, Fei; Chen, Tian; Wang, Minglei; Chang, Hong; Ke, Daoming; Dai, Yuehua
2017-02-01
Ge2Sb2Te5 (GST) and Si-doped GST with hexagonal structure were investigated by means of First-principles calcucations. We performed many kinds of doping types and studied the electronic properties of Si-doped GST with various Si concentrations. The theoretical calculations show that the lowest formation energy appeared when Si atoms substitute the Sb atoms (SiSb). With the increasing of Si concentrations from 10% to 30%, the impurity states arise around the Fermi level and the band gap of the SiSb structure broadens. Meanwhile, the doping supercell has the most favorable structure when the doping concentration keeps in 20%. The Si-doped GST exhibits p-type metallic characteristics more distinctly owing to the Fermi level moves toward the valence band. The Te p, d-orbitals electrons have greater impact on electronic properties than that of Te s-orbitals.
A novel anion interstitial defect structure in zinc-blende materials: A first-principles study
NASA Astrophysics Data System (ADS)
Yin, Yuan; Chen, Guangde; Ye, Honggang; Duan, Xiangyang; Zhu, Youzhang; Wu, Yelong
2016-05-01
The low-formation energy structure of anion interstitial defect in zinc-blende materials is usually identified as the tetrahedron central structure where the anion interstitial atom is surrounded by four countercation atoms. A line-type anion interstitial defect structure AD_il , however, is found to be lower in energy than the tetrahedron central anion interstitial defect structure by first-principles calculations. By analyzing the structural and electronical characters of this line-type defect in relative compounds of zinc-blende materials, we attribute this to the electronegativity shift trends and the bond forming, which lead to the hybridization types varying from sp 3 to sp-like and ending at sp.
First Principles Molecular Modeling of Sensing Material Selection for Hybrid Biomimetic Nanosensors
NASA Astrophysics Data System (ADS)
Blanco, Mario; McAlpine, Michael C.; Heath, James R.
Hybrid biomimetic nanosensors use selective polymeric and biological materials that integrate flexible recognition moieties with nanometer size transducers. These sensors have the potential to offer the building blocks for a universal sensing platform. Their vast range of chemistries and high conformational flexibility present both a problem and an opportunity. Nonetheless, it has been shown that oligopeptide aptamers from sequenced genes can be robust substrates for the selective recognition of specific chemical species. Here we present first principles molecular modeling approaches tailored to peptide sequences suitable for the selective discrimination of small molecules on nanowire arrays. The modeling strategy is fully atomistic. The excellent performance of these sensors, their potential biocompatibility combined with advanced mechanistic modeling studies, could potentially lead to applications such as: unobtrusive implantable medical sensors for disease diagnostics, light weight multi-purpose sensing devices for aerospace applications, ubiquitous environmental monitoring devices in urban and rural areas, and inexpensive smart packaging materials for active in-situ food safety labeling.
Liquid iron-sulfur alloys at outer core conditions by first-principles calculations
NASA Astrophysics Data System (ADS)
Umemoto, Koichiro; Hirose, Kei; Imada, Saori; Nakajima, Yoichi; Komabayashi, Tetsuya; Tsutsui, Satoshi; Baron, Alfred Q. R.
2014-10-01
We perform first-principles calculations to investigate liquid iron-sulfur alloys (Fe, Fe56S8, Fe52S12, and Fe48S16) under high-pressure and high-temperature (150-300 GPa and 4000-6000 K) conditions corresponding to the Earth's outer core. Considering only the density profile, the best match with the preliminary reference Earth model is by liquid Fe-14 wt % S (Fe50S14), assuming sulfur is the only light element. However, its bulk sound velocity is too high, in particular in the deep outer core, suggesting that another light component such as oxygen is required. An experimental check using inelastic X-ray scattering shows good agreement with the calculations. In addition, a present study demonstrates that the Birch's law does not hold for liquid iron-sulfur alloy, consistent with a previous report on pure liquid iron.
Ionic sieving through Ti3C2(OH)2 MXene: First-principles calculations
NASA Astrophysics Data System (ADS)
Berdiyorov, Golibjon R.; Madjet, Mohamed E.; Mahmoud, Khaled A.
2016-03-01
Recent experiments revealed a great potential of MXene nanosheets for water desalination applications as ultrathin, high-flux, and size/charge-selective sieving membranes. Here, we conduct first-principles density functional theory calculations to explore possible mechanisms for the charge-selective ionic transport through Ti3C2(OH)2 MXene. We find that the charge selectivity originates from the charged nature of the MXene layers. For example, due to the electrostatic interactions, ions of different charge states have different energy barriers for the intercalation between the MXene layers. In addition, the system shows dynamic response to the intercalating ions, even in their hydrated states, by changing the interlayer spacing. Our findings highlight the importance of membrane surface charges on the ion sieving performance.
First-principles prediction of shape memory behavior and ferrimagnetism in Mn2NiSn.
Paul, Souvik; Ghosh, Subhradip
2011-05-25
Using first-principles density functional theory, we show that, in Mn(2)NiSn, 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 μ(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.
Chandel, Surjeet Kumar; Kumar, Arun; Bharti, Ankush; Sharma, Raman
2015-05-15
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{sup −1} for CNT(6,6) while it is 596 to 658 cm{sup −1} for SiNT.
NASA Astrophysics Data System (ADS)
Sato, Shunsuke A.; Yabana, Kazuhiro
2016-12-01
Laser-induced damage of SiO2 (α-quartz) is investigated by first-principles calculations. The calculations are based on a coupled theoretical framework of the time-dependent density functional theory and Maxwell equation to describe strongly-nonlinear laser-solid interactions. We simulate irradiation of the bulk SiO2 with femtosecond laser pulses and compute energy deposition from the laser pulse to electrons as a function of the distance from the surface. We further analyze profiles of laser-induced craters, comparing the transferred energy with the cohesive energy of SiO2. The theoretical crater profile well reproduces the experimental features for a relatively weak laser pulse. In contrast, the theoretical result fails to reproduce the measured profiles for a strong laser pulse. This fact indicates a significance of the subsequent atomic motions that take place after the energy transfer ends for the formation of the crater under the strong laser irradiation.
NASA Astrophysics Data System (ADS)
Cockayne, Eric
2008-04-01
First principles calculations were used to study the effects of Si, Ti, Zr, and Ta (+N) substitutional impurities on the structure and dielectric properties of crystalline HfO2. The dielectric constant of monoclinic HfO2 can be enhanced by substituting more polarizable ions for Hf, but the band gap is decreased. Enhancing the permittivity without decreasing the band gap requires forming the tetragonal or cubic phase of HfO2. Among the ions studied, Si alone is found to stabilize a nonmonoclinic phase of HfO2 relative to the monoclinic phase, but only at an atomic concentration above about 20%. Various experiments have reported the formation of nonmonoclinic phases of HfO2 with increased permittivity when other ions are substituted for Hf. It is concluded that these structures are, in general, either metastable or are stabilized by extrinsic factors or by a layered arrangement of the substitutional cations.
First-principles study of native point defects in hafnia and zirconia
NASA Astrophysics Data System (ADS)
Zheng, J. X.; Ceder, G.; Maxisch, T.; Chim, W. K.; Choi, W. K.
2007-03-01
A first-principles study of native point defects in hafnia (HfO2) and zirconia (ZrO2) is carried out to identify dominant defects under different oxygen chemical potentials and Fermi levels. Oxygen vacancies and oxygen interstitials in both HfO2 and ZrO2 show negative- U behavior. It is shown that HfO2 is less prone to the formation of oxygen point defects than ZrO2 under the same oxygen chemical potential. When the Fermi level is constrained to be within the band gap of silicon, the dominant defects are negatively charged hafnium or zirconium vacancies under intermediate to high oxygen chemical potential. We find no evidence for magnetic defects.
Formation and annealing behaviors of qubit centers in 4H-SiC from first principles
NASA Astrophysics Data System (ADS)
Wang, Xiaopeng; Zhao, Mingwen; Bu, Hongxia; Zhang, Hongyu; He, Xiujie; Wang, Aizhu
2013-11-01
Inspired by finding that the nitrogen-vacancy center in diamond is a qubit candidate, similar defects in silicon carbide (SiC) have drawn considerable interest. However, the generation and annealing behaviors of these defects remain unclear. Using first-principles calculations, we describe the equilibrium concentrations and annealing mechanisms based on the diffusion of silicon vacancies. The formation energies and energy barriers along different migration paths, which are responsible for the formation rates, stability, and concentrations of these defects, are investigated. The effects on these processes of charge states, annealing temperature, and crystal orientation are also discussed. These theoretical results are expected to be useful in achieving controllable generation of these defects in experiments.
First-principles study of the critical thickness in asymmetric ferroelectric tunnel junctions
Cai Mengqiu; Du Yong; Huang Boyun
2011-03-07
The absent critical thickness of fully relaxed asymmetric ferroelectric tunnel junctions is investigated by first-principles calculations. The results show that PbTiO{sub 3} thin film between Pt and SrRuO{sub 3} electrodes can still retain a significant and stable polarization down to thicknesses as small as 0.8 nm, quite unlike the case of symmetric ferroelectric tunnel junctions. We trace this surprising result to the generation of a large electric field by the charge transfer between the electrodes caused by their different electronic environments, which acts against the depolarization field and enhances the ferroelectricity, leading to the reduction, or even complete elimination, for the critical thickness.
First-principles simulations of a graphene-based field-effect transistor
NASA Astrophysics Data System (ADS)
Wang, Yun-Peng; Cheng, Hai-Ping
2015-06-01
We improvise an approach to carry out first-principles simulations of graphene-based vertical field-effect tunneling transistors that consist of a graphene|h -BN |graphene multilayer structure. Within the density functional theory framework, we exploit the effective screening medium (ESM) method to properly treat boundary conditions for electrostatic potentials and investigate the effect of gate voltage. The distribution of free carriers and the band structure of both top and bottom graphene layers are calculated self-consistently. The dielectric properties of h -BN thin films sandwiched between graphene layers are computed layer-by-layer following the theory of microscopic permittivity. We find that the permittivities of BN layers are very close to that of crystalline h -BN . The effect of interface with graphene on the dielectric properties of h -BN is weak according to an analysis on the interface charge redistribution.
First-principle study of energy band structure of armchair graphene nanoribbons
NASA Astrophysics Data System (ADS)
Ma, Fei; Guo, Zhankui; Xu, Kewei; Chu, Paul K.
2012-07-01
First-principle calculation is carried out to study the energy band structure of armchair graphene nanoribbons (AGNRs). Hydrogen passivation is found to be crucial to convert the indirect band gaps into direct ones as a result of enhanced interactions between electrons and nuclei at the edge boundaries, as evidenced from the shortened bond length as well as the increased differential charge density. Ribbon width usually leads to the oscillatory variation of band gaps due to quantum confinement no matter hydrogen passivated or not. Mechanical strain may change the crystal symmetry, reduce the overlapping integral of C-C atoms, and hence modify the band gap further, which depends on the specific ribbon width sensitively. In practical applications, those effects will be hybridized to determine the energy band structure and subsequently the electronic properties of graphene. The results can provide insights into the design of carbon-based devices.
Zhang, Jie; Liu, Xiaolin; Wen, Yanwei; Shi, Lu; Chen, Rong; Liu, Huijun; Shan, Bin
2017-01-25
Good electronic transport capacity and low lattice thermal conductivity are beneficial for thermoelectric applications. In this study, the potential use as a thermoelectric material for the recently synthesized two-dimensional TiS3 monolayer is explored by applying first-principles method combined with Boltzmann transport theory. Our work demonstrates that carrier transport in the TiS3 sheet is orientation-dependent, caused by the difference in charge density distribution at band edges. Due to a variety of Ti-S bonds with longer lengths, we find that the TiS3 monolayer shows thermal conductivity much lower compared with that of transition-metal dichalcogenides such as MoS2. Combined with a high power factor along the y-direction, a considerable n-type ZT value (3.1) can be achieved at moderate carrier concentration, suggesting that the TiS3 monolayer is a good candidate for thermoelectric applications.
First principles study of phosphorus and boron defects in Si-XII
NASA Astrophysics Data System (ADS)
Malone, Brad D.; Cohen, Marvin L.
2011-03-01
We present a first-principles study of phosphorus and boron substitutional defects in Si-XII, a polytype of silicon in the R8 structure. Recent results from nanoindentation experiments reveal that this phase is semiconducting and has the interesting property that it can be doped n- and p-type at room temperature without an annealing step. We examine the formation energies of the B and P defects at the two distinct atomic sites in the R8 structure. We also calculate the thermodynamic transition levels of each defect in its relevant charge states. This work was supported by National Science Foundation Grant No. DMR10-1006184, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Computational resources have been provided by the Lawrencium cluster at LBNL.
Thermal Conductivity and Large Isotope Effect in GaN from First Principles
Lindsay, L.; Broido, D. A.; Reinecke, T. L.
2012-08-28
We present atomistic first principles results for the lattice thermal conductivity of GaN and compare them to those for GaP, GaAs, and GaSb. In GaN we find a large increase to the thermal conductivity with isotopic enrichment, ~65% at room temperature. We show that both the high thermal conductivity and its enhancement with isotopic enrichment in GaN arise from the weak coupling of heat-carrying acoustic phonons with optic phonons. This weak scattering results from stiff atomic bonds and the large Ga to N mass ratio, which give phonons high frequencies and also a pronounced energy gap between acoustic and optic phonons compared to other materials. Rigorous understanding of these features in GaN gives important insights into the interplay between intrinsic phonon-phonon scattering and isotopic scattering in a range of materials.
First-Principles Study of Pressure-Induced Phase Transition in CuGaO2
NASA Astrophysics Data System (ADS)
Jiang, Cheng-Lu; Liu, Qi-Jun; Liu, Zheng-Tang
2017-02-01
We have studied the structural, elastic, electronic properties, and pressure-induced phase transition of CuGaO2 by using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). The obtained ground state properties of three phases were in agreement with previous works. The calculated enthalpy variations with pressure showed that the structural phase transition ( β → 3R/2H) appeared at 65.5 ± 1 GPa. The changes in volume and band gap of β phase showed that there was a break between 30 and 40 GPa. The independent elastic constants of three phases were calculated. The 3R, 2H, and β phases were all mechanical stability and behaved in ductile manner under zero pressure.
Li-O2 Kinetic Overpotentials: Tafel Plots from Experiment and First-Principles Theory.
Viswanathan, V; Nørskov, J K; Speidel, A; Scheffler, R; Gowda, S; Luntz, A C
2013-02-21
We report the current dependence of the fundamental kinetic overpotentials for Li-O2 discharge and charge (Tafel plots) that define the optimal cycle efficiency in a Li-air battery. Comparison of the unusual experimental Tafel plots obtained in a bulk electrolysis cell with those obtained by first-principles theory is semiquantitative. The kinetic overpotentials for any practical current density are very small, considerably less than polarization losses due to iR drops from the cell impedance in Li-O2 batteries. If only the kinetic overpotentials were present, then a discharge-charge voltaic cycle efficiency of ∼85% should be possible at ∼10 mA/cm(2) superficial current density in a battery of ∼0.1 m(2) total cathode area. We therefore suggest that minimizing the cell impedance is a more important problem than minimizing the kinetic overpotentials to develop higher current Li-air batteries.
Experimental and first-principles study of ferromagnetism in Mn-doped zinc stannate nanowires
Deng Rui; Zhou Hang; Qin Jieming; Wan Yuchun; Jiang Dayong; Liang Qingcheng; Li Yongfeng; Wu, Tom; Yao Bin; Liu Lei
2013-07-21
Room temperature ferromagnetism was observed in Mn-doped zinc stannate (ZTO:Mn) nanowires, which were prepared by chemical vapor transport. Structural and magnetic properties and Mn chemical states of ZTO:Mn nanowires were investigated by X-ray diffraction, superconducting quantum interference device (SQUID) magnetometry and X-ray photoelectron spectroscopy. Manganese predominantly existed as Mn{sup 2+} and substituted for Zn (Mn{sub Zn}) in ZTO:Mn. This conclusion was supported by first-principles calculations. Mn{sub Zn} in ZTO:Mn had a lower formation energy than that of Mn substituted for Sn (Mn{sub Sn}). The nearest neighbor Mn{sub Zn} in ZTO stabilized ferromagnetic coupling. This observation supported the experimental results.
First-principles study of the stability of free-standing germanene in oxygen atmosphere
Liu, G.; Liu, S. B. Song, H. Y.; Xu, B.; Ouyang, C. Y.
2015-09-28
The O{sub 2} dissociation and O atoms adsorption on free-standing germanene are studied by using first-principles calculations in this paper. Compared with the extremely active silicene in oxygen atmosphere, germanene is found to be less active due to an energy barrier for dissociation of about 0.57 eV. Moreover, the dissociated oxygen atom follows two opposite migration pathways on the germanene surface, which is quite different from the case of silicene. Furthermore, the migration and desorption of O atoms at room temperature are relatively difficult due to the strong Ge-O bonding, resulting in the formation of germanium oxides. Our results reveal the interplay between germanene and O{sub 2} and suggest the enhanced stability of germanene in oxygen atmosphere compared with silicene.
Performance of arsenene and antimonene double-gate MOSFETs from first principles
NASA Astrophysics Data System (ADS)
Pizzi, Giovanni; Gibertini, Marco; Dib, Elias; Marzari, Nicola; Iannaccone, Giuseppe; Fiori, Gianluca
2016-08-01
In the race towards high-performance ultra-scaled devices, two-dimensional materials offer an alternative paradigm thanks to their atomic thickness suppressing short-channel effects. It is thus urgent to study the most promising candidates in realistic configurations, and here we present detailed multiscale simulations of field-effect transistors based on arsenene and antimonene monolayers as channels. The accuracy of first-principles approaches in describing electronic properties is combined with the efficiency of tight-binding Hamiltonians based on maximally localized Wannier functions to compute the transport properties of the devices. These simulations provide for the first time estimates on the upper limits for the electron and hole mobilities in the Takagi's approximation, including spin-orbit and multi-valley effects, and demonstrate that ultra-scaled devices in the sub-10-nm scale show a performance that is compliant with industry requirements.
Obtaining Mixed-Basis Ising-Like Expansions of Binary Alloys from First Principles
NASA Astrophysics Data System (ADS)
Hart, Gus L. W.; Sanati, Mahdi; Wang, Ligen; Zunger, Alex
2002-03-01
Many electronic and structural properties of A_1-xBx alloys can be predicted theoretically if one can find (and quickly compute) the ``configurational energy function''--that is, the energy for any given configuration of A and B atoms on the crystal lattice. Cluster expansion methods provide one such approach. We describe our mixed-basis cluster expansion (MBCE) based on first-principles total energy calculations for only a few ordered A_mBn compounds. Our MBCE can robustly predict a variety of material properties including ground states, phase diagrams, precipitate formation, etc. Specifically, we illustrate how systematic choice of interaction parameters, numerical parameters, and choice of input structures can significantly increase the accuracy and the predictive capability of the expansion. We illustrate how the fit of LDA data can be done essentially automatically. Examples include Cu-Au, Ni-Pt, and Sc_1-xBox_xS.
First principles molecular dynamics of Li: Test of a new algorithm
NASA Astrophysics Data System (ADS)
Wentzcovitch, Renata M.; Martins, JoséLuís
1991-06-01
We have tested a new algorithm to perform first-principles molecular dynamics simulations. This new scheme differs from the Car-Parrinello method and is based on the calculation of the self-consistent solutions of the Kohn-Sham equations at each molecular dynamics timestep, using a fast iterative diagonalization algorithm. We do not use a fictitious electron dynamics, and therefore the molecular dynamics timesteps can be considerably larger in our method than in the Car-Parrinello algorithm. Furthermore, the number of basis functions is variable, which makes this method particularly suited to deal with simulations involving a cell with variable shape and volume. Applications of this method to liquid Li offers results which are in excellent agreement with experiment and indicates that it is basically comparable in efficiency to the Car-Parrinello method.
Jet shapes for boosted jet two-prong decays from first-principles
NASA Astrophysics Data System (ADS)
Dasgupta, Mrinal; Schunk, Laís; Soyez, Gregory
2016-04-01
Several boosted jet techniques use jet shape variables to discriminate the multi-pronged signal from Quantum Chromodynamics backgrounds. In this paper, we provide a first-principles study of an important class of jet shapes all of which put a constraint on the subjet mass: the mass-drop parameter ( μ 2), the N -subjettiness ratio ( τ 21 ( β = 2) ) and energy correlation functions ( C 2 ( β = 2) or D 2 ( β = 2) . We provide analytic results both for QCD background jets as well as for signal processes. We further study the situation where cuts on these variables are applied recursively with Cambridge-Aachen de-clustering of the original jet. We also explore the effect of the choice of axis for N -subjettiness and jet de-clustering. Our results bring substantial new insight into the nature, gain and relative performance of each of these methods, which we expect will influence their future application for boosted object searches.
NASA Astrophysics Data System (ADS)
Oses, Corey; Yang, Kesong; Curtarolo, Stefano; Duke Univ Collaboration; UC San Diego Collaboration
Predicting material properties of disordered systems remains a long-standing and formidable challenge in rational materials design. To address this issue, we introduce an automated software framework capable of modeling partial occupation within disordered materials using a high-throughput (HT) first principles approach. At the heart of the approach is the construction of supercells containing a virtually equivalent stoichiometry to the disordered material. All unique supercell permutations are enumerated and material properties of each are determined via HT electronic structure calculations. In accordance with a canonical ensemble of supercell states, the framework evaluates ensemble average properties of the system as a function of temperature. As proof of concept, we examine the framework's final calculated properties of a zinc chalcogenide (ZnS1-xSex), a wide-gap oxide semiconductor (MgxZn1-xO), and an iron alloy (Fe1-xCux) at various stoichiometries.
First-principles study of liquid gallium at ambient and high pressure
NASA Astrophysics Data System (ADS)
Yang, Jianjun; Tse, John S.; Iitaka, Toshiaki
2011-07-01
The static and dynamic properties of liquid Ga close to the melting line have been studied by first-principles molecular dynamics simulations at ambient and elevated pressure up to 5.8 GPa. Below 2.5 GPa, the nearest neighbor Ga-Ga separation shows little change, while the second and third coordination shells are compressed to shorter distances. This behavior is attributed to the gradual occupation of the interstitial sites. Detail analysis of the local geometry and dynamical behavior refutes the proposed existence of Ga2 dimers in the liquid state. In fact, both the structure and electronic properties of the liquid are found to closely resemble that of the underlying Ga-II and Ga-III crystalline phases.
Impact of finite temperatures on the transport properties of Gd from first principles
NASA Astrophysics Data System (ADS)
Chadova, K.; Mankovsky, S.; Minár, J.; Ebert, H.
2017-03-01
Finite-temperature effects have a pronounced impact on the transport properties of solids. In magnetic systems, besides the scattering of conduction electrons by impurities and phonons, an additional scattering source coming from the magnetic degrees of freedom must be taken into account. A first-principle scheme which treats all these scattering effects on equal footing was recently suggested within the framework of the multiple scattering formalism. Employing the alloy analogy model treated by means of the CPA, thermal lattice vibrations and spin fluctuations are effectively taken into account. In the present work the temperature dependence of the longitudinal resistivity and the anomalous Hall effect in the strongly correlated metal Gd is considered. The comparison with experiments demonstrates that the proposed numerical scheme does provide an adequate description of the electronic transport at finite temperatures.
Correa, Alfredo A.; Bonev, Stanimir A.; Galli, Giulia
2006-01-23
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. Lastly, 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.
Thermal transport properties of metal/MoS{sub 2} interfaces from first principles
Mao, Rui; Kong, Byoung Don; Kim, Ki Wook
2014-07-21
Thermal transport properties at the metal/MoS{sub 2} interfaces are analyzed by using an atomistic phonon transport model based on the Landauer formalism and first-principles calculations. The considered structures include chemisorbed Sc(0001)/MoS{sub 2} and Ru(0001)/MoS{sub 2}, physisorbed Au(111)/MoS{sub 2}, as well as Pd(111)/MoS{sub 2} with intermediate characteristics. Calculated results illustrate a distinctive dependence of thermal transfer on the details of interfacial microstructures. More specifically, the chemisorbed case with a stronger bonding exhibits a generally smaller interfacial thermal resistance than the physisorbed. Comparison between metal/MoS{sub 2} and metal/graphene systems suggests that metal/MoS{sub 2} is significantly more resistive. Further examination of lattice dynamics identifies the presence of multiple distinct atomic planes and bonding patterns at the interface as the key origins of the observed large thermal resistance.
First-Principles Correlated Approach to the Normal State of Strontium Ruthenate
NASA Astrophysics Data System (ADS)
Acharya, S.; Laad, M. S.; Dey, Dibyendu; Maitra, T.; Taraphder, A.
2017-02-01
The interplay between multiple bands, sizable multi-band electronic correlations and strong spin-orbit coupling may conspire in selecting a rather unusual unconventional pairing symmetry in layered Sr2RuO4. This mandates a detailed revisit of the normal state and, in particular, the T-dependent incoherence-coherence crossover. Using a modern first-principles correlated view, we study this issue in the actual structure of Sr2RuO4 and present a unified and quantitative description of a range of unusual physical responses in the normal state. Armed with these, we propose that a new and important element, that of dominant multi-orbital charge fluctuations in a Hund’s metal, may be a primary pair glue for unconventional superconductivity. Thereby we establish a connection between the normal state responses and superconductivity in this system.
Establishing the limits of efficiency of perovskite solar cells from first principles modeling.
Grånäs, Oscar; Vinichenko, Dmitry; Kaxiras, Efthimios
2016-11-08
The recent surge in research on metal-halide-perovskite solar cells has led to a seven-fold increase of efficiency, from ~3% in early devices to over 22% in research prototypes. Oft-cited reasons for this increase are: (i) a carrier diffusion length reaching hundreds of microns; (ii) a low exciton binding energy; and (iii) a high optical absorption coefficient. These hybrid organic-inorganic materials span a large chemical space with the perovskite structure. Here, using first-principles calculations and thermodynamic modelling, we establish that, given the range of band-gaps of the metal-halide-perovskites, the theoretical maximum efficiency limit is in the range of ~25-27%. Our conclusions are based on the effect of level alignment between the perovskite absorber layer and carrier-transporting materials on the performance of the solar cell as a whole. Our results provide a useful framework for experimental searches toward more efficient devices.
Establishing the limits of efficiency of perovskite solar cells from first principles modeling
NASA Astrophysics Data System (ADS)
Grånäs, Oscar; Vinichenko, Dmitry; Kaxiras, Efthimios
2016-11-01
The recent surge in research on metal-halide-perovskite solar cells has led to a seven-fold increase of efficiency, from ~3% in early devices to over 22% in research prototypes. Oft-cited reasons for this increase are: (i) a carrier diffusion length reaching hundreds of microns; (ii) a low exciton binding energy; and (iii) a high optical absorption coefficient. These hybrid organic-inorganic materials span a large chemical space with the perovskite structure. Here, using first-principles calculations and thermodynamic modelling, we establish that, given the range of band-gaps of the metal-halide-perovskites, the theoretical maximum efficiency limit is in the range of ~25–27%. Our conclusions are based on the effect of level alignment between the perovskite absorber layer and carrier-transporting materials on the performance of the solar cell as a whole. Our results provide a useful framework for experimental searches toward more efficient devices.
Guan, Shu-Hui; Liu, Zhi-Pan
2016-02-14
Structural inhomogeneity is ubiquitous in solid crystals and plays critical roles in phase nucleation and propagation. Here, we develop a heterogeneous solid-solid phase transition theory for predicting the prevailing heterophase junctions, the metastable states governing microstructure evolution in solids. Using this theory and first-principles pathway sampling simulation, we determine two types of heterophase junctions pertaining to metal α-ω phase transition at different pressures and predict the reversibility of transformation only at low pressures, i.e. below 7 GPa. The low-pressure transformation is dominated by displacive Martensitic mechanism, while the high-pressure one is controlled by the reconstructive mechanism. The mechanism of α-ω phase transition is thus highly pressure-sensitive, for which the traditional homogeneous model fails to explain the experimental observations. The results provide the first atomic-level evidence on the coexistence of two different solid phase transition mechanisms in one system.
Martensitic fcc-to-hcp transformations in solid xenon under pressure: a first-principles study.
Kim, Eunja; Nicol, Malcolm; Cynn, Hyunchae; Yoo, Choong-Shik
2006-01-27
First-principles calculations reveal that the fcc-to-hcp pressure-induced transformation in solid xenon proceeds through two mechanisms between 5 and 70 GPa. The dynamics of the phase transition involves a sluggish stacking-disorder growth at lower pressures (path I) that changes to a path involving an orthorhombic distortion at higher pressures (path II). The switchover is governed by a delicate interplay of energetics (enthalpy of the system for the structural stability) and kinetics (energy barrier for the transition). The two types of martensitic transformations involved in this pressure-induced structural transformation are a twinned martensitic transition at lower pressures and a slipped martensitic transition at higher pressures.
Towards first-principles molecular design of liquid crystal-based chemoresponsive systems
Roling, Luke T.; Scaranto, Jessica; Herron, Jeffrey A.; Yu, Huaizhe; Choi, Sangwook; Abbott, Nicholas L.; Mavrikakis, Manos
2016-01-01
Nematic liquid crystals make promising chemoresponsive systems, but their development is currently limited by extensive experimental screening. Here we report a computational model to understand and predict orientational changes of surface-anchored nematic liquid crystals in response to chemical stimuli. In particular, we use first-principles calculations to evaluate the binding energies of benzonitrile, a model for 4′-pentyl-4-biphenylcarbonitrile, and dimethyl methylphosphonate to metal cation models representing the substrate chemical sensing surface. We find a correlation between these quantities and the experimental response time useful for predicting the response time of cation–liquid crystal combinations. Consideration of charge donation from chemical species in the surface environment is critical for obtaining agreement between theory and experiment. Our model may be extended to the design of improved chemoresponsive liquid crystals for selectively detecting other chemicals of practical interest by choosing appropriate combinations of metal cations with liquid crystals of suitable molecular structure. PMID:27804955
Detection of nucleic acids by graphene-based devices: A first-principles study
Zhang, Hua; Xu, Hui E-mail: ouyangfp06@tsinghua.org.cn; Ni, Xiang; Lin Peng, Sheng; Liu, Qi; Ping OuYang, Fang E-mail: ouyangfp06@tsinghua.org.cn
2014-04-07
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.
First-Principles Definition and Measurement of Planetary Electromagnetic-Energy Budget
NASA Technical Reports Server (NTRS)
Mishchenko, Michael I.; Lock, James A.; Lacis, Andrew A.; Travis, Larry D.; Cairns, Brian
2016-01-01
The imperative to quantify the Earths electromagnetic-energy budget with an extremely high accuracy has been widely recognized but has never been formulated in the framework of fundamental physics. In this paper we give a first-principles definition of the planetary electromagnetic-energy budget using the Poynting- vector formalism and discuss how it can, in principle, be measured. Our derivation is based on an absolute minimum of theoretical assumptions, is free of outdated notions of phenomenological radiometry, and naturally leads to the conceptual formulation of an instrument called the double hemispherical cavity radiometer (DHCR). The practical measurement of the planetary energy budget would require flying a constellation of several dozen planet-orbiting satellites hosting identical well-calibrated DHCRs.
First-principles study of roles of Cu and Cl in polycrystalline CdTe
Yang, Ji -Hui; Yin, Wan -Jian; Park, Ji -Sang; ...
2016-01-25
In this study, Cu and Cl treatments are important processes to achieve high efficiency polycrystalline cadmium telluride (CdTe) solar cells, thus it will be beneficial to understand the roles they play in both bulk CdTe and CdTe grain boundaries (GBs). Using first-principles calculations, we systematically study Cu and Cl-related defects in bulk CdTe. We find that Cl has only a limited effect on improving p-type doping and too much Cl can induce deep traps in bulk CdTe, whereas Cu can enhance ptype doping of bulk CdTe. In the presence of GBs, we find that, in general, Cl and Cu willmore » prefer to stay at GBs, especially for those with Te-Te wrong bonds, in agreement with experimental observations.« less
Possible martensitic transformation in Heusler alloy Mn2PdSn from first principles
NASA Astrophysics Data System (ADS)
Feng, L.; Feng, X.; Liu, E. K.; Wang, W. H.; Wu, G. H.; Hu, J. F.; Zhang, W. X.
2016-12-01
The tetragonal distortion, electronic structure and magnetic property of Mn2PdSn have been systematically investigated by first-principles calculations. The results indicate that the total energy of tetragonal martensitic phase is lower than cubic austenitic phase for Mn2PdSn. The corresponding c/a ratio and energy difference are 1.23 and 41.62 meV/f.u., respectively. This suggests that there is a great possibility for martensitic transformation to occur in Mn2PdSn with temperature decreasing. The electronic structure shows that there are sharp DOS peaks originating from p-d hybridization in the vicinity of Fermi level in the cubic phase. And these peaks disappear or become more flat in the martensitic phase.
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.
Kang, Wei; Zhao, Shijun; Zhang, Shen; Zhang, Ping; Chen, Q. F.; He, Xian-Tu
2016-01-01
Mott effect, featured by a sharp increase of ionization, is one of the unique properties of partially ionized plasmas, and thus of great interest to astrophysics and inertial confinement fusion. Recent experiments of single bubble sonoluminescence (SBSL) revealed that strong ionization took place at a density two orders lower than usual theoretical expectation. We show from the perspective of electronic structures that the strong ionization is unlikely the result of Mott effect in a pure argon plasma. Instead, first-principles calculations suggest that other ion species from aqueous environments can energetically fit in the gap between the continuum and the top of occupied states of argon, making the Mott effect possible. These results would help to clarify the relationship between SBSL and Mott effect, and further to gain an better understanding of partially ionized plasmas. PMID:26853107
First principles search for n-type oxide, nitride, and sulfide thermoelectrics
Garrity, Kevin F.
2016-01-01
Oxides have many potentially desirable characteristics for thermoelectric applications, including low cost and stability at high temperatures, but thus far there are few known high zT n-type oxide thermoelectrics. In this work, we use high-throughput first principles calculations to screen transition metal oxides, nitrides, and sulfides for candidate materials with high power factors and low thermal conductivity. We find a variety of promising materials, and we investigate these materials in detail in order to understand the mechanisms that cause them to have high power factors. These materials all combine a high density of states near the Fermi level with dispersive bands, reducing the trade-off between the Seebeck coefficient and the electrical conductivity, but they do so for several different reasons. In addition, our calculations indicate that many of our candidate materials have low thermal conductivity. PMID:27885361
First principles study of oxygen diffusion in a α-alumina ? twin grain boundary
NASA Astrophysics Data System (ADS)
Tohei, Tetsuya; Watanabe, Yuito; Takahashi, Nobuaki; Nakagawa, Tsubasa; Shibata, Naoya; Ikuhara, Yuichi
2015-12-01
We have investigated atomistic scale behaviour of oxygen diffusion along the ? twin grain boundary in α-Al2O3 (alumina) using molecular dynamics simulation and first principles total energy calculations. Based on the GB structure model which is verified by atomic-scale STEM observations on the bicrystal sample, quantitative evaluation of migration energies for dominant migration paths were performed by atomistic calculations. The preset calculation results confirmed fast oxygen diffusion behaviour along the GB. Our analysis shows that the dominant migration path or difference in the migration energies can be well correlated with the geometry of local atomic coordination around the migrating oxygen; lower migration energies are generally expected for paths with less change in coordination environment on migration. This trend holds both among gain boundary paths and bulk paths in α-alumina examined in the present study.
Solute/impurity diffusivities in bcc Fe: A first-principles study
NASA Astrophysics Data System (ADS)
Zhang, Chong; Fu, Jie; Li, Ruihuan; Zhang, Pengbo; Zhao, Jijun; Dong, Chuang
2014-12-01
Chinese low activation martensitic steel (CLAM) has been designed with decreased W content and increased Ta content to improve performance. We performed first-principles calculations to investigate the diffusion properties of solute element (Cr, W, Mn, V, Ta) and C diffusion with a nearby solute element inside bcc Fe. The self-diffusion coefficients and solute diffusion coefficients in Fe host were derived using the nine-frequency model. A relatively lower diffusivity was observed for W in paramagnetic state, implying enriched W concentration inside Fe host. The solute atom interacts strongly with C impurity, depending on the interatomic distance. According to our calculations, formation of Ta carbide precipitates is energetically preferred by trapping C impurity around Ta atom. Our theoretical results are helpful for investigating the evolution of microstructure of steels for engineering applications.
First principles study of defects in high-k HfO2
NASA Astrophysics Data System (ADS)
Wang, Baozhu; Wang, Min; Duan, Fengxia; Ren, Jie; Li, Ying; Zhou, Tiege
2016-11-01
Intrinsic defects and doping N, Si, Al, and Ta defects in monoclinic HfO2 were investigated by using the first-principle calculations based on density functional theory (DFT). The results show that the defects of TaHf+1, AlHf-1, VHf-4 are stable under oxygen-rich conditions; while the Hfi+4, VO3+2, NO4-1 are stable when the conditions are hafnium-rich. It is revealed that the defects under hafnium-rich conditions are easy to form, and the results also show the properties of negative -U. Defects of the thermodynamic transition levels in the Si band gap can capture or release the charge. It will result in the effect of Fermi level pinning, so it can seriously affect the stability of the device.
First-principles calculation of a negatively charged boron-vacancy center in diamond
NASA Astrophysics Data System (ADS)
Kunisaki, Aiko; Muruganathan, Manoharan; Mizuta, Hiroshi; Kodera, Tetsuo
2017-04-01
As the boron doping in diamond has been well established, and a negatively charged boron-vacancy (BV) center has an active electron paramagnetic resonance, the BV center is an attractive candidate for spin information devices. Using the first-principles calculation, we report the electronic structure of the BV center in diamond for its various charge states. A geometrically optimized BV center in the diamond supercell exhibited C 3 v symmetry. The BV+1 charge state did not exhibit any spin splitting defect levels, while the BV0 and BV‑2 charge states showed a small energy separation between spin-polarized states. On the other hand, the negatively charged BV‑1 center possesses bound states with a larger separation inside the diamond bandgap. Moreover, it has the spin-triplet ground state and the spin-conserved triplet excited state. These characteristics indicate that the BV‑1 center in diamond is a good candidate for qubit operation.
Stability of the hcp Ruthenium at high pressures from first principles
Lugovskoy, A. V. Belov, M. P.; Vekilov, Yu. Kh; Krasilnikov, O. M.
2014-09-14
The method of calculation of the elastic constants up to third order from the energy-strain relation under pressure for the hcp crystals is given and described in details. The method is applied to the hcp phase of Ruthenium. Elastic constants, lattice dynamics, and electronic structure are investigated in the pressure interval of 0–600 GPa by means of first principles calculations. The obtained parameters are in very good agreement with available experimental and theoretical data. No preconditions for phase transformation driven by mechanical or dynamical instabilities for hcp Ru were found in the investigated pressure range. The reason of stability at such high pressures is explained in the context of electronic structure peculiarities.
Electronic structures and optical spectra of BaO from first principles
Wu, Chang-Wei; Pan, Bo; Wang, Neng-Ping
2015-08-21
We present the results of first-principles study for the electronic structure and optical absorption spectrum of the alkaline-earth metal oxide BaO. The quasiparticle band structure is evaluated within the Hedin's GW approximation [Phys. Rev. 139, A796 (1965)]. Thereafter, the electron-hole interaction is taken into consideration and the Bethe-Salpeter equation for the electron-hole two-particle Green function is solved. The calculated quasiparticle band gap of BaO is 4.1 eV, which is in good agreement with the experimental result. The calculated optical absorption spectrum of BaO is also in agreement with the experimental data. In particular, the calculated excitation energy for the lowest exciton peak in the optical absorption spectrum of BaO reproduces very well the corresponding experimental result.
Electronic structures and optical spectra of BaO from first principles
NASA Astrophysics Data System (ADS)
Wu, Chang-Wei; Pan, Bo; Wang, Neng-Ping
2015-08-01
We present the results of first-principles study for the electronic structure and optical absorption spectrum of the alkaline-earth metal oxide BaO. The quasiparticle band structure is evaluated within the Hedin's GW approximation [Phys. Rev. 139, A796 (1965)]. Thereafter, the electron-hole interaction is taken into consideration and the Bethe-Salpeter equation for the electron-hole two-particle Green function is solved. The calculated quasiparticle band gap of BaO is 4.1 eV, which is in good agreement with the experimental result. The calculated optical absorption spectrum of BaO is also in agreement with the experimental data. In particular, the calculated excitation energy for the lowest exciton peak in the optical absorption spectrum of BaO reproduces very well the corresponding experimental result.
First-principles study of the effect of phosphorus on nickel grain boundary
Liu, Wenguan; Ren, Cuilan; Han, Han E-mail: xuhongjie@sinap.ac.cn; Zou, Yang; Zhou, Xingtai; Huai, Ping; Xu, Hongjie E-mail: xuhongjie@sinap.ac.cn; Tan, Jie
2014-01-28
Based on first-principles quantum-mechanical calculations, the impurity-dopant effects of phosphorus on Σ5(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.
First principles investigation of Ti adsorption and migration on Si(100) surfaces
Briquet, Ludovic G. V.; Wirtz, Tom; Philipp, Patrick
2013-12-28
The titanium adsorption on Si(100) is investigated using first principles computer modelling methods. Two new subsurface adsorption sites are described. They are located at the edge of the cavity topped by a surface silicon dimer. The migration of the titanium from the surface to the subsurface sites is facilitated when occurring via one of these sites. The ejection of one of the silicon atoms forming the surface dimer is also investigated. The actual step of the ejection requires more energy than previously thought although, when considering the global picture of a titanium atom on the surface leading to the ejection of a silicon atom, the overall rate is compensated by the facilitated migration of the titanium to the subsurface sites. The consecutive adsorption of a second and third titanium atom is also investigated. It is shown that titanium grows evenly on the surface in normal condition, showing no intermixing of the titanium and silicon beyond the silicon layer.
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 DFT +G W study of oxygen-doped CdTe
NASA Astrophysics Data System (ADS)
Flores, Mauricio A.; Orellana, Walter; Menéndez-Proupin, Eduardo
2016-05-01
The role of oxygen doping in CdTe is addressed by first-principles calculations. Formation energies, charge transition levels, and quasiparticle defect states are calculated within the DFT+G W formalism. The formation of a new defect is identified, the (OTe-TeCd) complex.Thiscomplex is energetically favored over both isovalent (OTe) and interstitial oxygen (Oi), in the Te-rich limit. We find that the incorporation of oxygen passivates the harmful deep energy levels associated with (TeCd), suggesting an improvement in the efficiency of CdTe based solar cells. Substitutional (OCd) is only stable in the neutral charge state and undergoes a Jahn-Teller distortion. We also investigate the diffusion profiles of interstitial oxygen and find a low-energy diffusion barrier of only 0.14 eV between two structurally distinct interstitial sites.
NASA Astrophysics Data System (ADS)
Zhu, Weihua; Xiao, Heming
2007-12-01
A detailed first-principles study of the structural and vibrational properties of crystalline silver azide under hydrostatic pressure of 0-500 GPa has been performed with density functional theory in the generalized gradient approximation. The crystal structure is relaxed to allow ionic configurations, cell shape, and volume to change without any symmetry constraints. It is found that the silver azide crystal remains orthorhombic structure with Ibam space group for pressures up to 7 GPa, where there is a transition to an I4 /mcm tetragonal symmetry. The lattice parameter and electronic structure are investigated as functions of pressure. The calculated vibrational frequencies at ambient pressure are in agreement with available experimental data. We also discuss the pressure-induced frequency shifts for the internal and lattice modes of silver azide crystal upon compression.
Properties of amorphous GaN from first-principles simulations
NASA Astrophysics Data System (ADS)
Cai, B.; Drabold, D. A.
2011-08-01
Amorphous GaN (a-GaN) models are obtained from first-principles simulations. We compare four a-GaN models generated by “melt-and-quench” and the computer alchemy method. We find that most atoms tend to be fourfold, and a chemically ordered continuous random network is the ideal structure for a-GaN albeit with some coordination defects. Where the electronic structure is concerned, the gap is predicted to be less than 1.0 eV, underestimated as usual by a density functional calculation. We observe a highly localized valence tail and a remarkably delocalized exponential conduction tail in all models generated. Based upon these results, we speculate on potential differences in n- and p-type doping. The structural origin of tail and defect states is discussed. The vibrational density of states and dielectric function are computed and seem consistent with experiment.
The structural and electronic properties of amorphous HgCdTe from first-principles calculations
NASA Astrophysics Data System (ADS)
Zhao, Huxian; Chen, Xiaoshuang; Lu, Jianping; Shu, Haibo; Lu, Wei
2014-01-01
Amorphous mercury cadmium telluride (a-MCT) model structures, with x being 0.125 and 0.25, are obtained from first-principles calculations. We generate initial structures by computation alchemy method. It is found that most atoms in the network of amorphous structures tend to be fourfold and form tetrahedral structures, implying that the chemical ordered continuous random network with some coordination defects is the ideal structure for a-MCT. The electronic structure is also concerned. The gap is found to be 0.30 and 0.26 eV for a-Hg0.875Cd0.125Te and a-Hg0.75Cd0.25Te model structures, independent of the composition. By comparing with the properties of crystalline MCT with the same composition, we observe a blue-shift of energy band gap. The localization of tail states and its atomic origin are also discussed.
New class of planar ferroelectric Mott insulators via first-principles design
NASA Astrophysics Data System (ADS)
Kim, Chanul; Park, Hyowon; Marianetti, Chris A.
2015-12-01
The bulk photovoltaic effect requires a low electronic band gap (i.e., ≈1 -2 eV) and large electronic polarization, which is not common in known materials. Here we use first-principles calculations to design layered double perovskite oxides AA'BB'O6 which achieve the aforementioned properties in the context of Mott insulators. In our design rules, the gap is dictated by B/B' electronegativity difference in a Mott state, while the polarization is obtained via nominal d0 filling on the B-site, A-type cations bearing lone-pair electrons, and A ≠A' size mismatch. Successful execution is demonstrated in BaBiCuVO6, BaBiNiVO6, BaLaCuVO6, and PbLaCuVO6.
Lattice dynamics and thermal conductivity of cesium chloride via first-principles investigation
NASA Astrophysics Data System (ADS)
He, Cui; Hu, Cui-E.; Zhang, Tian; Qi, Yuan-Yuan; Chen, Xiang-Rong
2017-03-01
The lattice thermal conductivity of CsCl crystal is theoretically investigated from a first-principles theoretical approach based on an iterative solution of the Boltzmann transport equation. Real-space finite-difference supercell approach is employed to generate the harmonic and anharmonic interatomic force constants. Phonon frequencies, velocities, and specific heat capacity as well as anharmonic properties are then obtained and applied to calculate the bulk thermal conductivity of CsCl crystal at the temperatures ranging from 20 K to 700 K. The calculated lattice thermal conductivity 1.14 W/mK of CsCl at room temperature agrees well with the experimental value, demonstrating that this parameter-free approach can provide a good description for the thermal transport of this material. The RTA and iterative solution of BTE are both presented. Our results show that both methods can obtain the thermal conductivity successfully.
Pyro-paraelectric and flexocaloric effects in barium strontium titanate: A first principles approach
NASA Astrophysics Data System (ADS)
Patel, Satyanarayan; Chauhan, Aditya; Cuozzo, J.; Lisenkov, S.; Ponomareva, I.; Vaish, Rahul
2016-04-01
Inhomogeneous strain allows the manifestation of an unexplored component of stress-driven caloric effect (flexocaloric effect) and enhanced pyroelectric performance, obtainable significantly beyond the Curie point. A peak temperature change of 1.5 K (at 289 K) was predicted from first-principles-based simulations for Ba0.5Sr0.5TiO3 under the application of a strain gradient of 1.5 μm-1. Additionally, enhanced pyro-paraelectric coefficient (pyroelectric coefficient in paraelectric phase) and flexocaloric cooling 11 × 10-4 C m-2 K-1 and 1.02 K, respectively, could be obtained (at 330 K and 1.5 μm-1). A comparative analysis with prevailing literature indicates huge untapped potential and warrants further research.
Multilayer heterostructures of magnetic Heusler and binary compounds from first principles
NASA Astrophysics Data System (ADS)
Garoufalis, Christos; Galanakis, Iosif
2016-03-01
Employing first-principles state-of-the-art electronic structure calculations, we study a series of multilayer heterostructures composed of ferro/ferrimagnetic half-metallic Heusler compounds and binary compounds presenting perpendicular magnetic anisotropy. We relax these heterostructures and study both their electronic and magnetic properties. In most studied cases the Heusler spacer keeps a large value of spin-polarization at the Fermi level even for ultrathin films which attends the maximum value of 100% in the case of the Mn2VSi/MnSi multilayer. Our results pave the way both experimentally and theoretically towards the growth of such multilayer heterostructures and their incorporation in spintronic/magnetoelectronic devices.
Optical properties of group-3 metal hexaboride nanoparticles by first-principles calculations
NASA Astrophysics Data System (ADS)
Yoshio, Satoshi; Maki, Koichiro; Adachi, Kenji
2016-06-01
LaB6 nanoparticles are widely used as solar control materials for strong near-infrared absorption and high visible transparency. In order to elucidate the origin of this unique optical property, first-principles calculations have been made for the energy-band structure and dielectric functions of RIIIB6 (RIII = Sc, Y, La, Ac). On account of the precise assessment of the energy eigenvalues of vacant states in conduction band by employing the screened exchange method, as well as to the incorporation of the Drude term, dielectric functions and various physical properties of LaB6 have been reproduced in excellent agreement with experimental values. Systematic examinations of dielectric functions and electronic structures of the trivalent metal hexaborides have clarified the origin of the visible transparency and the near-infrared plasmon absorption of RIIIB6 nanoparticles.
NASA Astrophysics Data System (ADS)
Ekuma, C. E.; Dobrosavljević, V.; Gunlycke, D.
2017-03-01
We present a first-principles-based many-body typical medium dynamical cluster approximation and density function theory method for characterizing electron localization in disordered structures. This method applied to monolayer hexagonal boron nitride shows that the presence of boron vacancies could turn this wide-gap insulator into a correlated metal. Depending on the strength of the electron interactions, these calculations suggest that conduction could be obtained at a boron vacancy concentration as low as 1.0%. We also explore the distribution of the local density of states, a fingerprint of spatial variations, which allows localized and delocalized states to be distinguished. The presented method enables the study of disorder-driven insulator-metal transitions not only in h -BN but also in other physical materials.
First-principles study on the ferrimagnetic half-metallic Mn2FeAs alloy
NASA Astrophysics Data System (ADS)
Qi, Santao; Zhang, Chuan-Hui; Chen, Bao; Shen, Jiang; Chen, Nanxian
2015-05-01
Mn-based full-Heusler alloys are kinds of promising candidates for new half-metallic materials. Basing on first principles, the electronic structures and magnetic properties of the Mn2FeAs full-Heusler alloy have been investigated in detail. The Hg2CuTi-type Mn2FeAs compound obeys the Slater-Pauling rule, while the anti-parallel alignment atomic magnetic moments of Mn locating at different sites indicate it a ferrimagnetic alloy. The calculated spin-down bands behave half-metallic character, exhibiting a direct gap of 0.46 eV with a 100% spin polarization at the Fermi level. More studies show the compound would maintain half-metallic nature in a large range of variational lattice constants. We expect that our calculated results may trigger Mn2FeAs applying in the future spintronics field.
First-principles calculations on thermodynamic properties of BaTiO3 rhombohedral phase.
Bandura, Andrei V; Evarestov, Robert A
2012-07-05
The calculations based on the linear combination of atomic orbitals have been performed for the low-temperature phase of BaTiO(3) crystal. Structural and electronic properties, as well as phonon frequencies were obtained using hybrid PBE0 exchange-correlation functional. The calculated frequencies and total energies at different volumes have been used to determine the equation of state and thermal contribution to the Helmholtz free energy within the quasiharmonic approximation. For the first time, the bulk modulus, volume thermal expansion coefficient, heat capacity, and Grüneisen parameters in BaTiO(3) rhombohedral phase have been estimated at zero pressure and temperatures form 0 to 200 K, based on the results of first-principles calculations. Empirical equation has been proposed to reproduce the temperature dependence of the calculated quantities. The agreement between the theoretical and experimental thermodynamic properties was found to be satisfactory.
Strain effect on electronic structures of graphene nanoribbons: A first-principles study.
Sun, Lian; Li, Qunxiang; Ren, Hao; Su, Haibin; Shi, Q W; Yang, Jinlong
2008-08-21
We report a first-principles study on the electronic structures of deformed graphene nanoribbons (GNRs). Our theoretical results show that the electronic properties of zigzag GNRs are not sensitive to uniaxial strain, while the energy gap modification of armchair GNRs (AGNRs) as a function of uniaxial strain displays a nonmonotonic relationship with a zigzag pattern. The subband spacings and spatial distributions of the AGNRs can be tuned by applying an external strain. Scanning tunneling microscopy dI/dV maps can be used to characterize the nature of the strain states, compressive or tensile, of AGNRs. In addition, we find that the nearest neighbor hopping integrals between pi-orbitals of carbon atoms are responsible for energy gap modification under uniaxial strain based on our tight binding approximation simulations.
First-Principles Approach to Calculating Energy Level Alignment at Aqueous Semiconductor Interfaces
NASA Astrophysics Data System (ADS)
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 1b1 energy level in water. The application to the specific cases of nonpolar (101 ¯0) facets of GaN and ZnO reveals a significant role for the structural motifs at the interface, including the degree of interface water dissociation and the dynamical fluctuations in the interface Zn-O and O-H bond orientations. These effects contribute up to 0.5 eV.
Intrinsic magnetic properties of ZnO nanoislands: Insight from first-principles study
NASA Astrophysics Data System (ADS)
Zhang, Yang; Wu, Zhi-Feng; Gao, Peng-Fei; Zhang, Er-Hu; Zhang, Sheng-Li
2016-03-01
First-principles calculations have been employed to investigate magnetic and electronic properties of monolayer and multi-layer ZnO nanoislands which are hexagonal BN (h-BN) prototype structures with zigzag edges and a triangular form. Two types of the zigzag edges, that is, O-terminated and Zn-terminated ones are considered. It has been found that monolayer ZnO nanoislands with the O-terminated edges exhibit magnetic properties, regardless of the nanoislands size. However, the nanoislands with Zn-terminated edges are semiconductors, and the magnetic properties are observed just for some specific sizes. Charge transfer according to Bader charge analysis is introduced to elucidate the magnetic properties of the monolayer ZnO nanoislands. Besides, multi-layer ZnO nanoislands exhibit magnetic features when their layers are odd, while those of even layers are nonmagnetic.
First-principles study of Li ion diffusion in LiFePO4
NASA Astrophysics Data System (ADS)
Ouyang, Chuying; Shi, Siqi; Wang, Zhaoxiang; Huang, Xuejie; Chen, Liquan
2004-03-01
The diffusion mechanism of Li ions in the olivine LiFePO4 is investigated from first-principles calculations. The energy barriers for possible spatial hopping pathways are calculated with the adiabatic trajectory method. The calculations show that the energy barriers running along the c axis are about 0.6, 1.2, and 1.5 eV for LiFePO4, FePO4, and Li0.5FePO4, respectively. However, the other migration pathways have much higher energy barriers resulting in very low probability of Li-ion migration. This means that the diffusion in LiFePO4 is one dimensional. The one-dimensional diffusion behavior has also been shown with full ab initio molecular dynamics simulation, through which the diffusion behavior is directly observed.
First principles search for n-type oxide, nitride, and sulfide thermoelectrics.
Garrity, Kevin F
2016-07-15
Oxides have many potentially desirable characteristics for thermoelectric applications, including low cost and stability at high temperatures, but thus far there are few known high zT n-type oxide thermoelectrics. In this work, we use high-throughput first principles calculations to screen transition metal oxides, nitrides, and sulfides for candidate materials with high power factors and low thermal conductivity. We find a variety of promising materials, and we investigate these materials in detail in order to understand the mechanisms that cause them to have high power factors. These materials all combine a high density of states near the Fermi level with dispersive bands, reducing the trade-off between the Seebeck coefficient and the electrical conductivity, but they do so for several different reasons. In addition, our calculations indicate that many of our candidate materials have low thermal conductivity.
First-principles search for n -type oxide, nitride, and sulfide thermoelectrics
NASA Astrophysics Data System (ADS)
Garrity, Kevin F.
2016-07-01
Oxides have many potentially desirable characteristics for thermoelectric applications, including low cost and stability at high temperatures, but thus far there are few known high z T n -type oxide thermoelectrics. In this work, we use high-throughput first-principles calculations to screen transition metal oxides, nitrides, and sulfides for candidate materials with high power factors and low thermal conductivity. We find a variety of promising materials, and we investigate these materials in detail in order to understand the mechanisms that cause them to have high power factors. These materials all combine a high density of states near the Fermi level with dispersive bands, reducing the trade-off between the Seebeck coefficient and the electrical conductivity, but they do so for several different reasons. In addition, our calculations indicate that many of our candidate materials have low thermal conductivity.
Thermoelectric properties of binary LnN (Ln=La and Lu): First principles study
Sreeparvathy, P. C.; Gudelli, Vijay Kumar; Kanchana, V.; Vaitheeswaran, G.; Svane, A.; Christensen, N. E.
2015-06-24
First principles density functional calculations were carried out to study the electronic structure and thermoelectric properties of LnN (Ln = La and Lu) using the full potential linearized augmented plane wave (FP-LAPW) method. The thermoelectric properties were calculated by solving the Boltzmann transport equation within the constant relaxation time approximation. The obtained lattice parameters are in good agreement with the available experimental and other theoretical results. The calculated band gaps using the Tran-Blaha modified Becke-Johnson potential (TB-mBJ), of both compounds are in good agreement with the available experimental values. Thermoelectric properties like thermopower (S), electrical conductivity scaled by relaxation time (σ/τ) and power-factor (S{sup 2}σ/τ) are calculated as functions of the carrier concentration and temperature for both compounds. The calculated thermoelectric properties are compared with the available experimental results of the similar material ScN.
Equation of state for technetium from X-ray diffraction and first-principle calculations
Mast, Daniel S.; Kim, Eunja; Siska, Emily M.; ...
2016-03-20
Here, the ambient temperature equation of state (EoS) of technetium metal has been measured by X-ray diffraction. The metal was compressed using a diamond anvil cell and using a 4:1 methanol-ethanol pressure transmitting medium. The maximum pressure achieved, as determined from the gold pressure scale, was 67 GPa. The compression data shows that the HCP phase of technetium is stable up to 67 GPa. The compression curve of technetium was also calculated using first-principles total-energy calculations. Utilizing a number of fitting strategies to compare the experimental and theoretical data it is determined that the Vinet equation of state with anmore » ambient isothermal bulk modulus of B0T = 288 GPa and a first pressure derivative of B' = 5.9(2) best represent the compression behavior of technetium metal.« less
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 μB). Gd therefore is an interesting candidate for spin injection into monolayer MoS2. PMID:25482498
Temperature effect on lattice and electronic structures of WTe2 from first-principles study
NASA Astrophysics Data System (ADS)
Liu, Gang; Liu, Huimei; Zhou, Jian; Wan, Xiangang
2017-01-01
Tungsten ditelluride (WTe2) exhibits extremely large and unsaturated magnetoresistance (MR). Due to the large spatial extensions of Te-5p and W-5d orbitals, the electronic properties of WTe2 are sensitive to the lattice structures, which can probably affect the strongly temperature dependent MR found in the experiment. Based on first-principle calculations, we investigate the temperature effect on the lattice and electronic structures of WTe2. Our numerical results show that the thermal expansion coefficients of WTe2 are highly anisotropic and considerably large. However, the temperature (less than 300 K) has an ignorable effect on the Fermi surface of WTe2. Our theoretical results clarify that the thermal expansion is not the main reason for the temperature-induced rapid decrease of magnetoresistance.
Superconductivity in ordered LiBe alloy under high pressure: A first-principles study
NASA Astrophysics Data System (ADS)
Xu, Ying; Chen, Changbo; Wu, Baojia
2012-01-01
The electronic, vibrational, and superconducting properties of LiBe alloy in the P2 1/m structure under pressure have been investigated using first-principles calculations. The calculated electron-phonon coupling (EPC) of LiBe with both linear response theory and the rigid muffin-tin approximation suggested that pairing electrons are mainly mediated by the Li low-lying phonon vibrations, and the increase of the Li EPC matrix element
First-Principles Correlated Approach to the Normal State of Strontium Ruthenate
Acharya, S.; Laad, M. S.; Dey, Dibyendu; Maitra, T.; Taraphder, A.
2017-01-01
The interplay between multiple bands, sizable multi-band electronic correlations and strong spin-orbit coupling may conspire in selecting a rather unusual unconventional pairing symmetry in layered Sr2RuO4. This mandates a detailed revisit of the normal state and, in particular, the T-dependent incoherence-coherence crossover. Using a modern first-principles correlated view, we study this issue in the actual structure of Sr2RuO4 and present a unified and quantitative description of a range of unusual physical responses in the normal state. Armed with these, we propose that a new and important element, that of dominant multi-orbital charge fluctuations in a Hund’s metal, may be a primary pair glue for unconventional superconductivity. Thereby we establish a connection between the normal state responses and superconductivity in this system. PMID:28220879
NASA Astrophysics Data System (ADS)
Sawada, Keisuke; Ishii, Fumiyuki; Saito, Mineo
2014-04-01
We studied magnetism in bilayer and multilayer zigzag graphene nanoribbons (ZGNRs) through first-principles density functional theory calculations. We found that the magnetic ground state of bilayer ZGNRs is the C-type antiferromagnetic (AFM) state, which is the AFM order between intraplane-edge carbon atoms and ferromagnetic (FM) order between interplane edge carbon atoms. In the cases of infinitely stacked multilayer ZGNRs, i.e., zigzag graphite nanoribbons, the C-type AFM state is also the most stable. By carrier doping, we found that the magnetic ground state changed from the C-AFM state to the FM state and, thus, realized two-dimensional FM surface (edge) states of graphite with a metallic conductivity.
First-Principles Study of Back Contact Effects on CdTe Thin Film Solar Cells
Du, Mao-Hua
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.
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.
The structural, electronic and phonon behavior of CsPbI3: A first principles study
NASA Astrophysics Data System (ADS)
Bano, Amreen; Khare, Preeti; Parey, Vanshree; Shukla, Aarti; Gaur, N. K.
2016-05-01
Metal halide perovskites are optoelectronic materials that have attracted enormous attention as solar cells with power conversion efficiencies reaching 20%. The benefit of using hybrid compounds resides in their ability to combine the advantage of these two classes of compounds: the high mobility of inorganic materials and the ease of processing of organic materials. In spite of the growing attention of this new material, very little is known about the electronic and phonon properties of the inorganic part of this compounds. A theoretical study of structural, electronic and phonon properties of metal-halide cubic perovskite, CsPbI3 is presented, using first-principles calculations with planewave pseudopotential method as personified in PWSCF code. In this approach local density approximation (LDA) is used for exchange-correlation potential.
Nitrogen-induced magnetism in stannates from first-principles calculations
NASA Astrophysics Data System (ADS)
Xiao, Wen-Zhi; Meng, Bo; Xu, Hai-Qing; Chen, Qiao; Wang, Ling-Ling
2016-09-01
First-principles calculations have been used to comparatively investigate electronic and magnetic properties of nitrogen-doped (N-doped) nonmagnetic semiconductor perovskite-type stannate (MSnO3, M = Ca, Sr, Ba). A total magnetic moment of 1.0 μB induced by N is found in MSnO3 supercell with one N dopant. The spontaneous polarization mainly originates from spin splitting on 2p state of N. The medium-sized formation energy shows that the N-doped MSnO3 can be realized experimentally under the metal-rich environments, but the clustering tendency and short-range coupling imply that the stannate matrices are unsuitable for magnetizing by substituting N for O. Our study offers a fresh sight of spontaneous spin polarization in d0 magnetism. The FM coupling in N-doped MSnO3 should be attributed to the hole-mediated p-p coupling mechanism.
Uratani, Hiroki; Yamashita, Koichi
2017-02-16
The trapping of charge carriers at defects on surfaces or grain boundaries is detrimental for the performance of perovskite solar cells (PSCs). For example, it is the main limiting factor for carrier lifetime. Moreover, it causes hysteresis in the current-voltage curves, which is considered to be a serious issue for PSCs' operation. In this work, types of surface defects responsible for carrier trapping are clarified by a comprehensive first-principles investigation into surface defects of tetragonal CH3NH3PbI3 (MAPbI3). Considering defect formation energetics, it is proposed that a Pb-rich condition is preferred to an I-rich one; however, a moderate condition might possibly be the best choice. Our result paves the way for improving the performance of PSCs through a rational strategy of suppressing carrier trapping at surface defects.
Carrier-induced noncollinear magnetism in perovskite manganites by first-principles calculations.
Sawada, K; Ishii, F
2009-02-11
We have performed noncollinear first-principles density-functional calculations of carrier-doped perovskite manganites La(1-x)Sr(x)MnO(3) (0.0≤x≤1.0). In the calculated magnetic phase diagram (T = 0) within the collinear magnetic configurations, ferromagnetic and several antiferromagnetic configurations successively appeared as a ground state with increasing x. The calculated total energies of the ferromagnetic and A-type antiferromagnetic phases are almost degenerate around the phase boundary, x = 0.5. We found that the noncollinear magnetic configurations are stable in a wide range of carrier concentrations 0.3≤x≤0.6. We discuss the effect of lattice distortions on the stability of the noncollinear magnetic phase.
Electronic structure of cubic ScF3 from first-principles calculations
NASA Astrophysics Data System (ADS)
Bocharov, D.; Žguns, P.; Piskunov, S.; Kuzmin, A.; Purans, J.
2016-07-01
The ground state properties of cubic scandium trifluoride (ScF3) perovskite were studied using first-principles calculations. The electronic structure of ScF3 was determined by linear combination of atomic orbital (LCAO) and plane wave projector augmented-wave (PAW) methods using modified hybrid exchange-correlation functionals within the density functional theory (DFT). The comprehensive comparison of the results obtained by two methods is presented. Both methods allowed us to reproduce the lattice constant found experimentally in ScF3 at low temperatures and to predict its electronic structure in good agreement with known experimental valence-band photoelectron and F 1s x-ray absorption spectra.
First-principles calculation for phonon and optoelectronic properties of CsSnI3
NASA Astrophysics Data System (ADS)
Bano, Amreen; Khare, Preeti; Gaur, N. K.
2016-05-01
The CsSnI3 crystal belongs to an interesting class of semiconducting perovskite which is currently used in thin-film field-effect transistor made of organics-inorganics hybrid compounds. The benefit of using hybrid compounds resides in their ability to combine the advantage of these two classes of compounds: the high mobility of inorganic materials and the ease of processing of organic materials. In spite of the growing attention of this new material, very little is known about the dielectric and optical properties of the inorganic part of this compounds. A theoretical study of phonon, dielectric and optical properties of metal-halide cubic perovskite, CsSnI3 is presented, using first-principles calculations with planewave pseudopotential method as personified in PWSCF code. In this approach local density approximation (LDA) is used for exchange-correlation potential. The optical properties shows that this compound has applications in optoelectronic devices.
First-principles study on phase transition and ferroelectricity in lithium niobate and tantalate
Toyoura, Kazuaki Ohta, Masataka; Nakamura, Atsutomo; Matsunaga, Katsuyuki
2015-08-14
The phase transitions and ferroelectricity of LiNbO{sub 3} and LiTaO{sub 3} have been investigated theoretically from first principles. The phonon analyses and the molecular dynamics simulations revealed that the ferroelectric phase transition is not conventional displacive type but order-disorder type with strong correlation between cation displacements. According to the evaluated potential energy surfaces around the paraelectric structures, the large difference in ferroelectricity between the two oxides results from the little difference in short-range interionic interaction between Nb-O and Ta-O. As the results of the crystal orbital overlap population analyses, the different short-range interaction originates from the difference in covalency between Nb4d-O2p and Ta5d-O2p orbitals, particularly d{sub xz}-p{sub x}/d{sub yz}-p{sub y} orbitals (π orbitals), from the electronic point of view.
NASA Astrophysics Data System (ADS)
Chen, Z. J.; Xiao, H. Y.; Zu, X. T.; Gao, F.
2008-11-01
The electronic structures and defect formation energies for a series of stannate pyrochlores Ln2Sn2O7 (Ln=La, Pr, Nd, Sm, Gd, Tb, Ho, Er, Lu, and Y) have been investigated using the first-principles total energy calculations. The calculated results show that Ln-site cation ionic radius, x-O48f, lattice constant and the covalency of the ⟨Sn-O48f⟩ bond have a significant affect on the defect formation energies. The cation-antisite defect has the lowest formation energy, as compared with that of other defects, indicating that cation disorder causes local oxygen disordering. The present studies suggest that Lu2Sn2O7 is the most resistant to ion beam-induced amorphization. The electronic structure calculations reveal that Ln2Sn2O7 compounds have direct band gaps of 2.64-2.95 eV at the Γ point in the Brillouin zone.
Aidhy, Dilpuneet S.; Liu, Bin; Zhang, Yanwen; Weber, William J.
2015-01-21
We study the chemical expansion for neutral and charged oxygen vacancies in fluorite, rocksalt, perovskite and pyrochlores materials using first principles calculations. We show that the neutral oxygen vacancy leads to lattice expansion whereas the charged vacancy leads to lattice contraction. In addition, we show that there is a window of strain within which an oxygen vacancy is stable; beyond that range, the vacancy can become unstable. Using CeO_{2}|ZrO_{2} interface structure as an example, we show that the concentration of oxygen vacancies can be manipulated via strain, and the vacancies can be preferentially stabilized. Furthermore, these results could serve as guiding principles in predicting oxygen vacancy stability in strained systems and in the design of vacancy stabilized materials.
Aidhy, Dilpuneet S.; Liu, Bin; Zhang, Yanwen; Weber, William J.
2015-03-01
We study the chemical expansion for neutral and charged oxygen vacancies in fluorite, rocksalt, perovskite and pyrochlores materials using first principles calculations. We show that the neutral oxygen vacancy leads to lattice expansion whereas the charged vacancy leads to lattice contraction. In addition, we show that there is a window of strain within which an oxygen vacancy is stable; beyond that range, the vacancy can become unstable. Using CeO2|ZrO2 interface structure as an example, we show that the concentration of oxygen vacancies can be manipulated via strain, and the vacancies can be preferentially stabilized. These results could serve as guiding principles in predicting oxygen vacancy stability in strained systems and in the design of vacancy stabilized materials.
Aidhy, Dilpuneet S.; Liu, Bin; Zhang, Yanwen; ...
2015-01-21
We study the chemical expansion for neutral and charged oxygen vacancies in fluorite, rocksalt, perovskite and pyrochlores materials using first principles calculations. We show that the neutral oxygen vacancy leads to lattice expansion whereas the charged vacancy leads to lattice contraction. In addition, we show that there is a window of strain within which an oxygen vacancy is stable; beyond that range, the vacancy can become unstable. Using CeO2|ZrO2 interface structure as an example, we show that the concentration of oxygen vacancies can be manipulated via strain, and the vacancies can be preferentially stabilized. Furthermore, these results could serve asmore » guiding principles in predicting oxygen vacancy stability in strained systems and in the design of vacancy stabilized materials.« less
First-principles study of Co3(Al,W) alloys using special quasirandom structures
Jiang, Chao
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.
Thermal conductivity and large isotope effect in GaN from first principles.
Lindsay, L; Broido, D A; Reinecke, T L
2012-08-31
We present atomistic first principles results for the lattice thermal conductivity of GaN and compare them to those for GaP, GaAs, and GaSb. In GaN we find a large increase to the thermal conductivity with isotopic enrichment, ~65% at room temperature. We show that both the high thermal conductivity and its enhancement with isotopic enrichment in GaN arise from the weak coupling of heat-carrying acoustic phonons with optic phonons. This weak scattering results from stiff atomic bonds and the large Ga to N mass ratio, which give phonons high frequencies and also a pronounced energy gap between acoustic and optic phonons compared to other materials. Rigorous understanding of these features in GaN gives important insights into the interplay between intrinsic phonon-phonon scattering and isotopic scattering in a range of materials.
First-principles study of roles of Cu and Cl in polycrystalline CdTe
Yang, Ji -Hui; Yin, Wan -Jian; Park, Ji -Sang; Metzger, Wyatt; Wei, Su -Huai
2016-01-25
In this study, Cu and Cl treatments are important processes to achieve high efficiency polycrystalline cadmium telluride (CdTe) solar cells, thus it will be beneficial to understand the roles they play in both bulk CdTe and CdTe grain boundaries (GBs). Using first-principles calculations, we systematically study Cu and Cl-related defects in bulk CdTe. We find that Cl has only a limited effect on improving p-type doping and too much Cl can induce deep traps in bulk CdTe, whereas Cu can enhance ptype doping of bulk CdTe. In the presence of GBs, we find that, in general, Cl and Cu will prefer to stay at GBs, especially for those with Te-Te wrong bonds, in agreement with experimental observations.
First principles study on defectives BN nanotubes for water splitting and hydrogen storage
NASA Astrophysics Data System (ADS)
Bevilacqua, Andressa C.; Rupp, Caroline J.; Baierle, Rogério J.
2016-06-01
First principles calculations within the spin polarized density functional approximation have been addressed to investigate the energetic stability, electronic and optical properties of defective BN nanotubes. Our results show that the presence of carbon impurities interacting with vacancies gives rise to defective electronic levels inside the nanotube band gap. By calculating the absorbance index, we have obtained a strong inter-band optical absorption in the visible region (around 2.1 eV) showing that defective BN nanotubes could be an efficient catalytic semiconductor material to be used within solar energy for water splitting. In addition, we observe that the adsorption energy for one and two H2 molecules on the defective surface is in the desired window for the system to be useful as a hydrogen storage medium.
NASA Astrophysics Data System (ADS)
Wang, Xiaoming; Zebarjadi, Mona; Esfarjani, Keivan
2016-08-01
This work aims at understanding solid-state energy conversion and transport in layered (van der Waals) heterostructures in contact with metallic electrodes via a first-principles approach. As an illustration, a graphene/phosphorene/graphene heterostructure in contact with gold electrodes is studied by using density functional theory (DFT)-based first principles calculations combined with real space Green's function (GF) formalism. We show that for a monolayer phosphorene, quantum tunneling dominates the transport. By adding more phosphorene layers, one can switch from tunneling-dominated transport to thermionic-dominated transport, resulting in transporting more heat per charge carrier, thus, enhancing the cooling coefficient of performance. The use of layered van der Waals heterostructures has two advantages: (a) thermionic transport barriers can be tuned by changing the number of layers, and (b) thermal conductance across these non-covalent structures is very weak. The phonon thermal conductance of the present van der Waals heterostructure is found to be 4.1 MW m-2 K-1 which is one order of magnitude lower than the lowest value for that of covalently-bonded interfaces. The thermionic coefficient of performance for the proposed device is 18.5 at 600 K corresponding to an equivalent ZT of 0.13, which is significant for nanoscale devices. This study shows that layered van der Waals structures have great potential to be used as solid-state energy-conversion devices.This work aims at understanding solid-state energy conversion and transport in layered (van der Waals) heterostructures in contact with metallic electrodes via a first-principles approach. As an illustration, a graphene/phosphorene/graphene heterostructure in contact with gold electrodes is studied by using density functional theory (DFT)-based first principles calculations combined with real space Green's function (GF) formalism. We show that for a monolayer phosphorene, quantum tunneling dominates the
First principles based proximity effect of superconductor-normal metal heterostructures
NASA Astrophysics Data System (ADS)
Csire, Gábor; Cserti, József; Újfalussy, Balázs
2016-12-01
In this paper we study the proximity effect in superconductor-normal metal heterostructures based on first principles calculations with treating the pairing potential as an adjustable parameter. The superconducting order parameter (anomalous density) is obtained from the Green-function by solving the Kohn-Sham-Bogoliubov-de Gennes equations with the Screened Korringa-Kohn-Rostoker method. The results are interpreted for an Au/Nb(0 0 1) system. The layer resolved anomalous spectral function is also obtained which is closely related to the superconducting order parameter. We find that the anomalous spectral function has the fingerprint of the Andreev scattering process and it is connected to the electron-hole ratio of the quasiparticle states. We also show that the proximity effect can be understood via the anomalous spectral function.
FInal Report: First Principles Modeling of Mechanisms Underlying Scintillator Non-Proportionality
Aberg, Daniel; Sadigh, Babak; Zhou, Fei
2015-01-01
This final report presents work carried out on the project “First Principles Modeling of Mechanisms Underlying Scintillator Non-Proportionality” at Lawrence Livermore National Laboratory during 2013-2015. The scope of the work was to further the physical understanding of the microscopic mechanisms behind scintillator nonproportionality that effectively limits the achievable detector resolution. Thereby, crucial quantitative data for these processes as input to large-scale simulation codes has been provided. In particular, this project was divided into three tasks: (i) Quantum mechanical rates of non-radiative quenching, (ii) The thermodynamics of point defects and dopants, and (iii) Formation and migration of self-trapped polarons. The progress and results of each of these subtasks are detailed.
First principles Peierls-Boltzmann phonon thermal transport: A topical review
Lindsay, Lucas
2016-08-05
The advent of coupled thermal transport calculations with interatomic forces derived from density functional theory has ushered in a new era of fundamental microscopic insight into lattice thermal conductivity. Subsequently, significant new understanding of phonon transport behavior has been developed with these methods, and because they are parameter free and successfully benchmarked against a variety of systems, they also provide reliable predictions of thermal transport in systems for which little is known. This topical review will describe the foundation from which first principles Peierls-Boltzmann transport equation methods have been developed, and briefly describe important necessary ingredients for accurate calculations. Sample highlights of reported work will be presented to illustrate the capabilities and challenges of these techniques, and to demonstrate the suite of tools available, with an emphasis on thermal transport in micro- and nano-scale systems. In conclusion, future challenges and opportunities will be discussed, drawing attention to prospects for methods development and applications.
NASA Astrophysics Data System (ADS)
Otsuka, Takao; Taiji, Makoto; Bowler, David R.; Miyazaki, Tsuyoshi
2016-11-01
The recent progress of linear-scaling or O(N) methods in density functional theory (DFT) is remarkable. In this paper, we show that all-atom molecular dynamics simulations of complex biological systems based on DFT are now possible using our linear-scaling DFT code Conquest. We first overview the calculation methods used in Conquest and explain the method introduced recently to realise efficient and robust first-principles molecular dynamics (FPMD) with O(N) DFT. Then, we show that we can perform reliable all-atom FPMD simulations of a hydrated DNA model containing about 3400 atoms. We also report that the velocity scaling method is both reliable and useful for controlling the temperature of the FPMD simulation of this system. From these results, we conclude that reliable FPMD simulations of complex biological systems are now possible with Conquest.
Ceriotti, Michele; Manolopoulos, David E
2012-09-07
Light nuclei at room temperature and below exhibit a kinetic energy which significantly deviates from the predictions of classical statistical mechanics. This quantum kinetic energy is responsible for a wide variety of isotope effects of interest in fields ranging from chemistry to climatology. It also furnishes the second moment of the nuclear momentum distribution, which contains subtle information about the chemical environment and has recently become accessible to deep inelastic neutron scattering experiments. Here, we show how, by combining imaginary time path integral dynamics with a carefully designed generalized Langevin equation, it is possible to dramatically reduce the expense of computing the quantum kinetic energy. We also introduce a transient anisotropic Gaussian approximation to the nuclear momentum distribution which can be calculated with negligible additional effort. As an example, we evaluate the structural properties, the quantum kinetic energy, and the nuclear momentum distribution for a first-principles simulation of liquid water.
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 -π. 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.
Intrinsic magnetism in nanosheets of SnO2: A first-principles study
NASA Astrophysics Data System (ADS)
Rahman, Gul; García-Suárez, Víctor M.; Morbec, J. M.
2013-02-01
We propose intrinsic magnetism in nanosheets of SnO2, based on first-principles calculations. The electronic structure and spin density reveal that p orbitals of the oxygen atoms, surrounding Sn vacancies, have a non-itinerant nature which gives birth to localized magnetism. A giant decrease in defect formation energies of Sn vacancies in nanosheets is observed. We, therefore, believe that native defects can be stabilized without any chemical doping. Nanosheets of different thicknesses are also studied, and it is found that it is easier to create vacancies, which are magnetic, at the surface of the sheets. SnO2 nanosheets can, therefore, open new opportunities in the field of spintronics.
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 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.
Novel phases of lithium-aluminum binaries from first-principles structural search
Sarmiento-Pérez, Rafael; Cerqueira, Tiago F. T.; Botti, Silvana; Marques, Miguel A. L.; Valencia-Jaime, Irais; Amsler, Maximilian; Goedecker, Stefan; Romero, Aldo H.
2015-01-14
Intermetallic Li–Al compounds are on the one hand key materials for light-weight engineering, and on the other hand, they have been proposed for high-capacity electrodes for Li batteries. We determine from first-principles the phase diagram of Li–Al binary crystals using the minima hopping structural prediction method. Beside reproducing the experimentally reported phases (LiAl, Li{sub 3}Al{sub 2}, Li{sub 9}Al{sub 4}, LiAl{sub 3}, and Li{sub 2}Al), we unveil a structural variety larger than expected by discovering six unreported binary phases likely to be thermodynamically stable. Finally, we discuss the behavior of the elastic constants and of the electric potential profile of all Li–Al stable compounds as a function of their stoichiometry.
Desnavi, Sameerah; Chakraborty, Brahmananda; Ramaniah, Lavanya M.
2014-04-24
The electronic structure and hydrogen storage capability of Yttrium-doped grapheme has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom prefers the hollow site of the hexagonal ring with a binding energy of 1.40 eV. Doping by Y makes the system metallic and magnetic with a magnetic moment of 2.11 μ{sub B}. Y decorated graphene can adsorb up to four hydrogen molecules with an average binding energy of 0.415 eV. All the hydrogen atoms are physisorbed with an average desorption temperature of 530.44 K. The Y atoms can be placed only in alternate hexagons, which imply a wt% of 6.17, close to the DoE criterion for hydrogen storage materials. Thus, this system is potential hydrogen storage medium with 100% recycling capability.
Tailoring graphene magnetism by zigzag triangular holes: A first-principles thermodynamics study
Khan, Muhammad Ejaz; Zhang, P.; Sun, Yi -Yang; ...
2016-03-30
In this study, we discuss the thermodynamic stability and magnetic property of zigzag triangular holes (ZTHs) in graphene based on the results of first-principles density functional theory calculations. We find that ZTHs with hydrogen-passivated edges in mixed sp2/sp3 configurations (z211) could be readily available at experimental thermodynamic conditions, but ZTHs with 100% sp2 hydrogen-passivation (z1) could be limitedly available at high temperature and ultra-high vacuum conditions. Graphene magnetization near the ZTHs strongly depends on the type and the size of the triangles. While metallic z1 ZTHs exhibit characteristic edge magnetism due to the same-sublattice engineering, semiconducting z211 ZTHs do showmore » characteristic corner magnetism when the size is small < 2 nm. Our findings could be useful for experimentally tailoring metal-free carbon magnetism by simply fabricating triangular holes in graphene.« less
Hydrogenated and halogenated blue phosphorene as Dirac materials: A first principles study
NASA Astrophysics Data System (ADS)
Sun, Minglei; Wang, Sake; Yu, Jin; Tang, Wencheng
2017-01-01
Using first-principles calculations, we systematically investigate the structures and electronic properties of fully hydrogenated and halogenated blue phosphorene (P2X2). All these systems possess Dirac cone at high-symmetry K point, which are mainly contributed by P s px py orbitals. The Dirac cone in P2F2 and P2I2 systems lies exactly at the Fermi level. Formation energy analysis denotes that all the systems are energetically stable except P2I2. The mass density for P2H2 and P2F2 systems is rather small. Our calculations proposed that these systems, especially P2F2 system, have great potential applications in future nanoelectronics.
Exotic Multigap Structure in UPt3 Unveiled by a First-Principles Analysis
NASA Astrophysics Data System (ADS)
Nomoto, Takuya; Ikeda, Hiroaki
2016-11-01
A heavy-fermion superconductor UPt3 is a unique spin-triplet superconductor with multiple superconducting phases. Here, we provide the first report on a first-principles analysis of the microscopic superconducting gap structure. We find that the promising gap structure is an unprecedented E2 u state, which is completely different from the previous phenomenological E2 u models. Our obtained E2 u state has in-plane twofold vertical line nodes on small Fermi surfaces and point nodes with linear dispersion on a large Fermi surface. These peculiar features cannot be explained in the conventional spin 1 /2 representation, but is described by the group-theoretical representation of the Cooper pairs in the total angular momentum j =5 /2 space. Our findings shed new light on the long-standing problems in the superconductivity of UPt3 .
First-principles study of O-BN: A sp3-bonding boron nitride allotrope
NASA Astrophysics Data System (ADS)
Huang, Quan; Yu, Dongli; Zhao, Zhisheng; Fu, Siwei; Xiong, Mei; Wang, Qianqian; Gao, Yufei; Luo, Kun; He, Julong; Tian, Yongjun
2012-09-01
A fully tetrahedrally bonded boron nitride (BN) allotrope with an orthorhombic structure (O-BN) was investigated through first-principles calculations. O-BN has a bulk modulus of 371.8 GPa and a hardness of 66.4 GPa, thereby making it a superhard material with potential technological and industrial applications. O-BN becomes thermodynamically more stable than layered hexagonal BN (h-BN) at pressure above 1.5 GPa and is more favorable than the recently reported Pct-BN at any pressure. The phase transformations from h-BN and BN nanotubes to O-BN were respectively simulated, indicating the feasible synthesis of this superhard phase.
First-principles and molecular dynamics studies of twin boundaries in hcp zirconium
Morris, J.R.; Ye, Y.Y.; Ho, K.M.; Chan, C.T.; Yoo, M.H.
1993-12-31
We use a combination of molecular dynamics (MD) and first-principles techniques to study the structure and energies of twin boundaries in hcp zirconium. The empirical many-body potential of Zr is used to test the stability of various possible twin structures, but the final relaxed positions are accurately determined using fully self-consistent ab initio energy and Hellman-Feynman force calculations. This combination of techniques is powerful, as it provides a stringent test of our empirical potential, while producing reliable results for Zr that do not depend upon any empirical parameters. This paper summarizes our work to date on the compression twins, which demonstrates the importance of supporting empirical modeling with more accurate calculations. We also present new results on the empirical modeling of the tension twins of Zr.
Domains and ferroelectric switching pathways in Ca3Ti2O7 from first principles
NASA Astrophysics Data System (ADS)
Nowadnick, Elizabeth A.; Fennie, Craig J.
2016-09-01
Hybrid improper ferroelectricity, where an electrical polarization can be induced via a trilinear coupling to two nonpolar structural distortions of different symmetries, recently was demonstrated experimentally in the n =2 Ruddlesden-Popper compound Ca3Ti2O7 . In this paper we use group theoretic methods and first-principles calculations to identify possible ferroelectric switching pathways in Ca3Ti2O7 . We identify low-energy paths that reverse the polarization direction by switching via an orthorhombic twin domain or via an antipolar structure. We also introduce a chemically intuitive set of local order parameters to give insight into how these paths are relevant to ferroelectric switching nucleated at domain walls. Our findings suggest that switching may proceed via more than one mechanism in this material.
First principles calculations of mechanical properties of cubic 5d transition metal monocarbides
NASA Astrophysics Data System (ADS)
Yang, Jun; Gao, Faming
2012-09-01
The electronic and elastic properties of cubic 5d transition metal monocarbides in rocksalt, cesium chloride, and zinc blende structures have been studied by first principles calculations. The calculations show that the incompressibility for ReC in cesium chloride structure is even higher than that of diamond under pressure (above 89 GPa). The transformation pressure from zinc blende structure to rocksalt structure takes place at about 47 GPa for PtC. HfC-NaCl, ReC-CsCl, and HfC-ZnS have the smallest metallicity, leading to higher hardness. A valence electron number of 8/cell may be a stable valence shell configuration for 5d transition metal monocarbides in rocksalt and zinc blende structures.
Structural, electronic and mechanical properties of rare earth nitride-ErN: A first principles study
Murugan, A.; Rajeswarapalanichamy, R. Santhosh, M.; Priyanga, G. Sudha; Kanagaprabha, S.; Iyakutti, K.
2015-06-24
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.
Elastic and Thermal Properties of Silicon Compounds from First-Principles Calculations
NASA Astrophysics Data System (ADS)
Hou, Haijun; Zhu, H. J.; Cheng, W. H.; Xie, L. H.
2016-07-01
The structural and elastic properties of V-Si (V3Si, VSi2, V5Si3, and V6Si5) compounds are studied by using first-principles method. The calculated equilibrium lattice parameters and formation enthalpy are in good agreement with the available experimental data and other theoretical results. The calculated results indicate that the V-Si compounds are mechanically stable. Elastic properties including bulk modulus, shear modulus, Young's modulus, and Poisson's ratio are also obtained. The elastic anisotropies of V-Si compounds are investigated via the three-dimensional (3D) figures of directional dependences of reciprocals of Young's modulus. Finally, based on the quasi-harmonic Debye model, the internal energy, Helmholtz free energy, entropy, heat capacity, thermal expansion coefficient, Grüneisen parameter, and Debye temperature of V-Si compounds have been calculated.
Elastic properties of sulphur and selenium doped ternary PbTe alloys by first principles
Bali, Ashoka Chetty, Raju Mallik, Ramesh Chandra
2014-04-24
Lead telluride (PbTe) is an established thermoelectric material which can be alloyed with sulphur and selenium to further enhance the thermoelectric properties. Here, a first principles study of ternary alloys PbS{sub x}Te{sub (1−x)} and PbSe{sub x}Te{sub (1−x)} (0≤x≤1) based on the Virtual Crystal Approximation (VCA) is presented for different ratios of the isoelectronic atoms in each series. Equilibrium lattice parameters and elastic constants have been calculated and compared with the reported data. Anisotropy parameter calculated from the stiffness constants showed a slight improvement in anisotropy of elastic properties of the alloys over undoped PbTe. Furthermore, the alloys satisfied the predicted stability criteria from the elastic constants, showing stable structures, which agreed with the previously reported experimental results.
Energy band modulation of graphane by hydrogen-vacancy chains: A first-principles study
Wu, Bi-Ru; Yang, Chih-Kai
2014-08-15
We investigated a variety of configurations of hydrogen-vacancy chains in graphane by first-principles density functional calculation. We found that graphane with two zigzag H-vacancy chains segregated by one or more H chain is generally a nonmagnetic conductor or has a negligible band gap. However, the same structure is turned into a semiconductor and generates a magnetic moment if either one or both of the vacancy chains are blocked by isolated H atoms. If H-vacancy chains are continuously distributed, the structure is similar to a zigzag graphene nanoribbon embedded in graphane. It was also found that the embedded zigzag graphene nanoribbon is antiferromagnetic, and isolated H atoms left in the 2-chain nanoribbon can tune the band gap and generate net magnetic moments. Similar effects are also obtained if bare carbon atoms are present outside the nanoribbon. These results are useful for designing graphene-based nanoelectronic circuits.
A theoretical study of blue phosphorene nanoribbons based on first-principles calculations
Xie, Jiafeng; Si, M. S. Yang, D. Z.; Zhang, Z. Y.; Xue, D. S.
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.
First-principles calculation of the bulk photovoltaic effect in bismuth ferrite.
Young, Steve M; Zheng, Fan; Rappe, Andrew M
2012-12-07
We compute the bulk photovoltaic effect (BPVE) in BiFeO(3) using first-principles shift current theory, finding good agreement with experimental results. Furthermore, we reconcile apparently contradictory observations: by examining the contributions of all photovoltaic response tensor components and accounting for the geometry and ferroelectric domain structure of the experimental system, we explain the apparent lack of BPVE response in striped polydomain samples that is at odds with the significant response observed in monodomain samples. We reveal that the domain-wall-driven response in striped polydomain samples is partially mitigated by the BPVE, suggesting that enhanced efficiency could be obtained in materials with cooperative rather than antagonistic interaction between the two mechanisms.
First-principles calculations reveal controlling principles for carrier mobilities in semiconductors
NASA Astrophysics Data System (ADS)
Wu, Yu-Ning; Zhang, X.-G.; Pantelides, Sokrates T.
2016-11-01
Carrier mobilities remain a key qualifying factor for materials competing for next-generation electronics. It has long been believed that carrier mobilities can be calculated using the Born approximation. Here, we introduce a parameter-free, first-principles approach based on complex-wavevector energy bands which does not invoke the Born expansion. We demonstrate that phonon-limited mobility is controlled by low-resistivity percolation paths, which arise from fluctuations that are beyond the Born approximation. We further demonstrate that, in ionized-impurity scattering, one must account for the effect of the screening charge, which cancels most of the Coulomb tail. Calculated electron mobilities in silicon are in agreement with experimental data. The method is easy to use and can provide guidance in the search for high-mobility device designs.
Ikeda, Takashi
2014-07-28
From both the polarized and depolarized Raman scattering spectra of supercritical water a peak located at around 1600 cm(-1), attributed normally to bending mode of water molecules, was experimentally observed to vanish, whereas the corresponding peak remains clearly visible in the measured infrared (IR) absorption spectrum. In this computational study a theoretical formulation for analyzing the IR and Raman spectra is developed via first principles molecular dynamics combined with the modern polarization theory. We demonstrate that the experimentally observed peculiar behavior of the IR and Raman spectra for water are well reproduced in our computational scheme. We discuss the origins of a feature observed at 1600 cm(-1) in Raman spectra of ambient water.
First-principles investigation of the concentration-dependent phase transition of CeTh alloys
NASA Astrophysics Data System (ADS)
Hu, Cui-E.; Zeng, Zhao-Yi; Zhang, Lin; Chen, Xiang-Rong; Cai, Ling-Cang
2010-12-01
We report a first-principles study of the structure and phase transition of Ce xTh 1- x ( x=0.0,0.2,0.43,0.5,0.6,0.76 and 1.00) alloys. The structural properties of Ce xTh 1- x under pressure are well predicted. The fcc-bct (face-centered cubic to body-centered tetragonal) transition pressure decreases with the increasing Ce concentration in Ce xTh 1- x. The transition pressure as a function of the Ce concentration of the Ce xTh 1- x alloys can be well described as a second-order polynomial: P=70.00-32.08x-22.93x2.
First-principles calculations of Seebeck coefficients in a magnetic semiconductor CuFeS2
NASA Astrophysics Data System (ADS)
Takaki, Hirokazu; Kobayashi, Kazuaki; Shimono, Masato; Kobayashi, Nobuhiko; Hirose, Kenji; Tsujii, Naohito; Mori, Takao
2017-02-01
We analyze the Seebeck coefficients of a magnetic semiconductor CuFeS2 using first-principles calculation methods based on density functional theory. The calculated temperature dependence of the Seebeck coefficient in the antiferromagnetic phase reproduces a distinctive behavior in a bulk CuFeS2, such as a peak structure at a low temperature and weak temperature dependence around room temperature. In doped systems, almost linear temperature dependence appears. Despite not including any effect beyond the conventional spin density functional theory in our calculations, the calculated results agree qualitatively with the experimental results. These agreements indicate that the behavior of the Seebeck coefficients in CuFeS2 is mainly determined by its electronic structure.
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.
A genetic algorithm for first principles global structure optimization of supported nano structures
Vilhelmsen, Lasse B.; Hammer, Bjørk
2014-07-28
We present a newly developed publicly available genetic algorithm (GA) for global structure optimisation within atomic scale modeling. The GA is focused on optimizations using first principles calculations, but it works equally well with empirical potentials. The implementation is described and benchmarked through a detailed statistical analysis employing averages across many independent runs of the GA. This analysis focuses on the practical use of GA’s with a description of optimal parameters to use. New results for the adsorption of M{sub 8} clusters (M = Ru, Rh, Pd, Ag, Pt, Au) on the stoichiometric rutile TiO{sub 2}(110) surface are presented showing the power of automated structure prediction and highlighting the diversity of metal cluster geometries at the atomic scale.
O(N) complexity algorithms for First-Principles Electronic Structure Calculations
Fattebert, J L
2007-02-16
The fundamental equation governing a non-relativistic quantum system of N particles is the time-dependant Schroedinger Equation [Schroedinger, 1926]. In 1965, Kohn and Sham proposed to replace this original many-body problem by an auxiliary independent-particles problem that can be solved more easily (Density Functional Theory). Solving this simplified problem requires to find the subspace of dimension N spanned by the N eigenfunctions {Psi}{sub i} corresponding to the N lowest eigenvalues {var_epsilon}{sub i} of a non-linear Hamiltonian operator {cflx H} determined from first-principles. From the solution of the Kohn-Sham equations, forces acting on atoms can be derived to optimize geometries and simulate finite temperature phenomenon by molecular dynamics. This technique is used at LLNL to determine the Equation of State of various materials, and to study biomolecules and nanomaterials.
First-principles study of roles of Cu and Cl in polycrystalline CdTe
Yang, Ji-Hui; Park, Ji-Sang; Metzger, Wyatt; Yin, Wan-Jian; Wei, Su-Huai
2016-01-28
Cu and Cl treatments are important processes to achieve high efficiency polycrystalline cadmium telluride (CdTe) solar cells, thus it will be beneficial to understand the roles they play in both bulk CdTe and CdTe grain boundaries (GBs). Using first-principles calculations, we systematically study Cu and Cl-related defects in bulk CdTe. We find that Cl has only a limited effect on improving p-type doping and too much Cl can induce deep traps in bulk CdTe, whereas Cu can enhance p-type doping of bulk CdTe. In the presence of GBs, we find that, in general, Cl and Cu will prefer to stay at GBs, especially for those with Te-Te wrong bonds, in agreement with experimental observations.
First-principles study of the amorphization of stishovite by isotropic volume expansion
NASA Astrophysics Data System (ADS)
Misawa, Masaaki; Shimojo, Fuyuki; Kalia, Rajiv K.; Nakano, Aiichiro; Vashishta, Priya
Simple synthesis of ceramics with high hardness and high toughness from Earth-abundant materials is one of the most important issues in materials science. Nishiyama et al. synthesized nano-crystalline stishovite with extremely high toughness and high hardness via compression and decompression of silica, and proposed fracture-induced amorphization mechanisms for the toughning. Furthermore, it was shown that the toughening mechanisms are effective even in nanoscale order. Our first-principles molecular dynamics simulations have shown rapid amorphization of stishovite within picoseconds under increasing volume, thus substantiating the proposed amorphization mechanisms. Furthermore, we have calculated the critical stress, energy difference, and energy barrier for the crystalline-to-amorphous structural transition.
NASA Astrophysics Data System (ADS)
Pham, Tuan Anh; Li, Tianshu; Gygi, Francois; Galli, Giulia
2011-03-01
Silicon Nitride (Si3N4) is a possible candidate material to replace or be alloyed with SiO2 to form high-K dielectric films on Si substrates, so as to help prevent leakage currents in modern CMOS transistors. Building on our previous work on dielectric properties of crystalline and amorphous Si3N4 slabs, we present an analysis of the band offsets and dielectric properties of crystalline-Si/amorphous Si3N4 interfaces based on first principles calculations. We discuss shortcomings of the conventional bulk-plus line up approach in band offset calculations for systems with an amorphous component, and we present the results of band offsets obtained from calculations of local density of states. Finally, we describe the role of bonding configurations in determining band edges and dielectric constants at the interface. We acknowledge financial support from Intel Corporation.
First principles study of properties of the oxidized Cu(100) and Cu(110)
NASA Astrophysics Data System (ADS)
Olenga, Antoine
Copper based catalysts are of importance to a number of industrial processes including the synthesis of methanol, the reduction and decomposition of nitrogen oxides, and treatment of waste water. In copper catalysis surface oxidation and oxidic overlayers are believed to play a crucial role. In this work using density functional theory (DFT) within the generalized gradient approximation (GGA) we have studied the stability and associated electronic properties of the oxidized Cu(100) and Cu(110) surfaces. Especially, we have focused on studies of changes in the interlayer spacing, electron work function, binding energy, and density of states with oxygen coverage. We have examined the cases of various oxygen coverages of the non-reconstructed, missing row reconstructed Cu(100), and added row reconstructed Cu (110) surfaces. The first-principles calculations in this work have been performed using DMOl3 code. The obtained theoretical results have been compared with available experimental data.
First-principles investigation of iron pentacarbonyl molecular solid phases at high pressure
NASA Astrophysics Data System (ADS)
Cong, Kien Nguyen; Steele, Brad A.; Landerville, Aaron C.; Oleynik, Ivan I.
2017-01-01
The polymeric phases of carbon monoxide (p-CO), an extended non-molecular solid, represent a new class of low-Z energetic materials. The presence of transition metal ions is believed to stabilize polymeric carbon monoxide (p-CO) at ambient conditions. Since p-CO forms at high pressures, it becomes important to investigate the high-pressure behavior of one of the potential precursors, iron pentacarbonyl Fe(CO)5. In this work, a first-principles evolutionary structure search method is used to determine the crystal phases of Fe(CO)5 at high pressure. The calculations predict the crystal structure of Phase I in agreement with experiment. Moreover, the previously unidentified crystal structure of Phase II is found. The calculated pressure-dependent Raman spectra are used to demonstrate that the changes in Raman spectra as a function of pressure observed in recent experiment can be explained without invoking a phase transition to a new phase III.
NASA Astrophysics Data System (ADS)
Ozcelik, Ongun; White, Claire
Using first principle density functional calculations, we present the nanoscale properties of interactions, local bonds, charge distributions, mechanical properties and strength of alkali activated cement phases which are the most promising alternative to the ordinary Portland cement with a much lower cost to the environment. We present results on the stability and long term durability of various alkali activated cement structures, effects of external alkali agents on their properties and ways of utilizing them for further applications. We compare the calculated properties of alkali activated cement with those of ordinary Portland cement and contribute to the formation of long term durability data of these phases. Comparison with X-ray and neutron scattering experiment results are also provided via pair distribution functions extracted from simulation results.
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 (η) for the nucleobases in the various environments. Trends in e^2qQ and η 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)
Gilbert Damping Parameter in MgO-Based Magnetic Tunnel Junctions from First Principles
NASA Astrophysics Data System (ADS)
Tang, Hui-Min; Xia, Ke
2017-03-01
We perform a first-principles study of the Gilbert damping parameter (α ) in normal-metal/MgO-cap/ferromagnet/MgO-barrier/ferromagnetic magnetic tunnel junctions. The damping is enhanced by interface spin pumping, which can be parametrized by the spin-mixing conductance (G↑↓ ). The calculated dependence of Gilbert damping on the thickness of the MgO capping layer is consistent with experiment and indicates that the decreases in α with increasing thickness of the MgO capping layer is caused by suppression of spin pumping. Smaller α can be achieved by using a clean interface and alloys. For a thick MgO capping layer, the imaginary part of the spin-mixing conductance nearly equals the real part, and the large imaginary mixing conductance implies that the change in the frequency of ferromagnetic resonance can be observed experimentally. The normal-metal cap significantly affects the Gilbert damping.
Jump rates for surface diffusion of large molecules from first principles
Shea, Patrick Kreuzer, Hans Jürgen
2015-04-21
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.
NASA Astrophysics Data System (ADS)
Zanolli, Zeila
2015-03-01
The research challenges of the near and far future in electronics focus on the quest for new materials and novel device concepts to achieve low energy consumption, increased reliability and high device density. These can be obtained by designing active elements and interconnects whose operating principle is not (only) based on the electron charge but on the spin degree of freedom of the electron. The nanoscopic size of the materials calls for atomistic and parameter free (ab initio) simulations, which have proven to be crucial in achieving the necessary accuracy and predictive power. Materials which present a coupling between ferroelectricity and magnetism, i.e. magnetoelectric (ME) multiferroics, have been proposed as fundamental building blocks for spintronic devices. However ferroelectricity and magnetism are often exclusive or weakly coupled in bulk. In this talk, we will discuss how superlattices of perovskites can be designed from first principles to achieve strongly coupled ME and, hence, achieve control the weak magnetization via an electric field. Most important, advanced epitaxial techniques allow one to actually grow such magnetoelectric superlattices. Another route to optimize spintronic devices is to exploit the unique electronic and transport properties of Carbon-based nanomaterials. The latter present spin diffusion lengths up to 100 μm and high electron velocity. However, a large spin diffusion length comes at the price of small Spin Orbit coupling, which limits the possibility of manipulating electrons via an external applied field. Further, to achieve graphene-based devices one also needs to open its vanishing electronic gap. We use first principle techniques to show that placing graphene on a ME substrate can overcome these limitations by inducing magnetism and opening an electronic band-gap in the hybrid organic-multiferroic material. Z.Z. acknowledges EC support under the Marie-Curie IEF (PIEF-Ga-2011-300036), computational resources from the
First-principles data-driven discovery of transition metal oxides for artificial photosynthesis
NASA Astrophysics Data System (ADS)
Yan, Qimin
We develop a first-principles data-driven approach for rapid identification of transition metal oxide (TMO) light absorbers and photocatalysts for artificial photosynthesis using the Materials Project. Initially focusing on Cr, V, and Mn-based ternary TMOs in the database, we design a broadly-applicable multiple-layer screening workflow automating density functional theory (DFT) and hybrid functional calculations of bulk and surface electronic and magnetic structures. We further assess the electrochemical stability of TMOs in aqueous environments from computed Pourbaix diagrams. Several promising earth-abundant low band-gap TMO compounds with desirable band edge energies and electrochemical stability are identified by our computational efforts and then synergistically evaluated using high-throughput synthesis and photoelectrochemical screening techniques by our experimental collaborators at Caltech. Our joint theory-experiment effort has successfully identified new earth-abundant copper and manganese vanadate complex oxides that meet highly demanding requirements for photoanodes, substantially expanding the known space of such materials. By integrating theory and experiment, we validate our approach and develop important new insights into structure-property relationships for TMOs for oxygen evolution photocatalysts, paving the way for use of first-principles data-driven techniques in future applications. This work is supported by the Materials Project Predictive Modeling Center and the Joint Center for Artificial Photosynthesis through the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under Contract No. DE-AC02-05CH11231. Computational resources also provided by the Department of Energy through the National Energy Supercomputing Center.
Strain sensitivity and superconducting properties of Nb3Sn from first principles calculations
NASA Astrophysics Data System (ADS)
De Marzi, G.; Morici, L.; Muzzi, L.; della Corte, A.; Buongiorno Nardelli, M.
2013-04-01
Using calculations from first principles based on density-functional theory we have studied the strain sensitivity of the A15 superconductor Nb3Sn. The Nb3Sn lattice cell was deformed in the same way as observed experimentally on multifilamentary, technological wires subject to loads applied along their axes. The phonon dispersion curves and electronic band structures along different high-symmetry directions in the Brillouin zone were calculated, at different levels of applied strain, ɛ, on both the compressive and the tensile side. Starting from the calculated averaged phonon frequencies and electron-phonon coupling, the superconducting characteristic critical temperature of the material, Tc, has been calculated by means of the Allen-Dynes modification of the McMillan formula. As a result, the characteristic bell-shaped Tc versus ɛ curve, with a maximum at zero intrinsic strain, and with a slight asymmetry between the tensile and compressive sides, has been obtained. These first-principle calculations thus show that the strain sensitivity of Nb3Sn has a microscopic and intrinsic origin, originating from shifts in the Nb3Sn critical surface. In addition, our computations show that variations of the superconducting properties of this compound are correlated to stress-induced changes in both the phononic and electronic properties. Finally, the strain function describing the strain sensitivity of Nb3Sn has been extracted from the computed Tc(ɛ) curve, and compared to experimental data from multifilamentary, composite wires. Both curves show the expected bell-shaped behavior, but the strain sensitivity of the wire is enhanced with respect to the theoretical predictions for bulk, perfectly binary and stoichiometric Nb3Sn. An understanding of the origin of this difference might open potential pathways towards improvement of the strain tolerance in such systems.
First-principles calculations of carbon clathrates: Comparison to silicon and germanium clathrates
NASA Astrophysics Data System (ADS)
Connétable, Damien
2010-08-01
We employ state-of-the-art first-principles calculations based on density-functional theory and density-functional perturbation theory to investigate relevant physical properties and phase diagram of the free guest type-I (X-46) and type-II (X-34) carbon clathrates. Their properties and those of silicon and germanium diamonds, and clathrates have been computed and compared within the same approach. We briefly present and discuss their structural, cohesive, and electronic properties. In particular, we present different results about electronic properties of carbon clathrates. From the symmetry analysis of electronic states around the band gap, we deduce their optical properties, and we forecast the effects of hypothetical-doped elements on their electronic band gap. We then report first-principles calculations of vibrational, thermodynamical, and elastic properties. Whereas vibrational properties of Si and Ge systems can be linked through their atomic weight ratio, we show that the vibrational properties of carbon structures differ strongly. Raman and infrared spectra of all clathrates are also calculated and compared. The effects of pressure and temperature on thermodynamical properties (heat capacity, entropy, thermal expansion, etc.) within static and quasiharmonic approximations are investigated. It is shown that thermodynamical properties of carbon clathrates and diamond present a similar evolution up to high pressures (100 GPa) and over a large range of temperatures ([0, 1500] K). Then we deduce the equilibrium phase diagram (P,T) of C-2/C-34/C-46. We conclude the paper with a presentation of elastic properties computed from acoustic slopes.
SU-E-T-191: First Principle Calculation of Quantum Yield in Photodynamic Therapy
Abolfath, R; Guo, F; Chen, Z; Nath, R
2014-06-01
Purpose: We present a first-principle method to calculate the spin transfer efficiency in oxygen induced by any photon fields especially in MeV energy range. The optical pumping is mediated through photosensitizers, e.g., porphyrin and/or ensemble of quantum dots. Methods: Under normal conditions, oxygen molecules are in the relatively non-reactive triplet state. In the presence of certain photosensitizer compounds such as porphyrins, electromagnetic radiation of specific wavelengths can excite oxygen to highly reactive singlet state. With selective uptake of photosensitizers by certain malignant cells, photon irradiation of phosensitized tumors can lead to selective killing of cancer cells. This is the basis of photodynamic therapy (PDT). Despite several attempts, PDT has not been clinically successful except in limited superficial cancers. Many parameters such as photon energy, conjugation with quantum dots etc. can be potentially combined with PDT in order to extend the role of PDT in cancer management. The key quantity for this optimization is the spin transfer efficiency in oxygen by any photon field. The first principle calculation model presented here, is an attempt to fill this need. We employ stochastic density matrix description of the quantum jumps and the rate equation methods in quantum optics based on Markov/Poisson processes and calculate time evolution of the population of the optically pumped singlet oxygen. Results: The results demonstrate the feasibility of our model in showing the dependence of the optical yield in generating spin-singlet oxygen on the experimental conditions. The adjustable variables can be tuned to maximize the population of the singlet oxygen hence the efficacy of the photodynamic therapy. Conclusion: The present model can be employed to fit and analyze the experimental data and possibly to assist researchers in optimizing the experimental conditions in photodynamic therapy.
First-principles prediction of phononic thermal conductivity of silicene: A comparison with graphene
Gu, Xiaokun; Yang, Ronggui
2015-01-14
There has been great interest in two-dimensional materials, beyond graphene, for both fundamental sciences and technological applications. Silicene, a silicon counterpart of graphene, has been shown to possess some better electronic properties than graphene. However, its thermal transport properties have not been fully studied. In this paper, we apply the first-principles-based phonon Boltzmann transport equation to investigate the thermal conductivity of silicene as well as the phonon scattering mechanisms. Although both graphene and silicene are two-dimensional crystals with similar crystal structure, we find that phonon transport in silicene is quite different from that in graphene. The thermal conductivity of silicene shows a logarithmic increase with respect to the sample size due to the small scattering rates of acoustic in-plane phonon modes, while that of graphene is finite. Detailed analysis of phonon scattering channels shows that the linear dispersion of the acoustic out-of-plane (ZA) phonon modes, which is induced by the buckled structure, makes the long-wavelength longitudinal acoustic phonon modes in silicene not as efficiently scattered as that in graphene. Compared with graphene, where most of the heat is carried by the acoustic out-of-plane (ZA) phonon modes, the ZA phonon modes in silicene only have ∼10% contribution to the total thermal conductivity, which can also be attributed to the buckled structure. This systematic comparison of phonon transport and thermal conductivity of silicene and graphene using the first-principle-based calculations shed some light on other two-dimensional materials, such as two-dimensional transition metal dichalcogenides.
Molecular mechanisms of tungstate-induced pancreatic plasticity: a transcriptomics approach
Altirriba, Jordi; Barbera, Albert; Del Zotto, Héctor; Nadal, Belen; Piquer, Sandra; Sánchez-Pla, Alex; Gagliardino, Juan J; Gomis, Ramon
2009-01-01
Background Sodium tungstate is known to be an effective anti-diabetic agent, able to increase beta cell mass in animal models of diabetes, although the molecular mechanisms of this treatment and the genes that control pancreas plasticity are yet to be identified. Using a transcriptomics approach, the aim of the study is to unravel the molecular mechanisms which participate in the recovery of exocrine and endocrine function of streptozotocin (STZ) diabetic rats treated with tungstate, determining the hyperglycemia contribution and the direct effect of tungstate. Results Streptozotocin (STZ)-diabetic rats were treated orally with tungstate for five weeks. Treated (STZ)-diabetic rats showed a partial recovery of exocrine and endocrine function, with lower glycemia, increased insulinemia and amylasemia, and increased beta cell mass achieved by reducing beta cell apoptosis and raising beta cell proliferation. The microarray analysis of the pancreases led to the identification of three groups of differentially expressed genes: genes altered due to diabetes, genes restored by the treatment, and genes specifically induced by tungstate in the diabetic animals. The results were corroborated by quantitative PCR. A detailed description of the pathways involved in the pancreatic effects of tungstate is provided in this paper. Hyperglycemia contribution was studied in STZ-diabetic rats treated with phloridzin, and the direct effect of tungstate was determined in INS-1E cells treated with tungstate or serum from untreated or treated STZ-rats, observing that tungstate action in the pancreas takes places via hyperglycemia-independent pathways and via a combination of tungstate direct and indirect (through the serum profile modification) effects. Finally, the MAPK pathway was evaluated, observing that it has a key role in the tungstate-induced increase of beta cell proliferation as tungstate activates the mitogen-activated protein kinase (MAPK) pathway directly by increasing p42/p44
Sodium tungstate (Na2WO4) exposure increases apoptosis in human peripheral blood lymphocytes.
Osterburg, Andrew R; Robinson, Chad T; Schwemberger, Sandy; Mokashi, Vishwesh; Stockelman, Michael; Babcock, George F
2010-01-01
The potential for adverse health effects of using tungsten and its alloys in military munitions are an important concern to both civilians and the US military. The toxicological implications of exposure to tungsten, its alloys, and the soluble tungstate (Na(2)WO(4)) are currently under investigation. To examine tungstate toxicity, a series of experiments to determine its in vitro effects on cells of the immune system were performed. We identified alterations in isolated human peripheral blood lymphocytes (PBL) treated in vitro with sodium tungstate (0.01, 0.1, 1.0, and 10 mM). Analyses of apoptosis with annexin V and propidium iodide revealed a dose- and time-dependent increase in the quantity of cells in early apoptosis after tungstate exposure. Reductions in the number of cells entering into the cell cycle were also noted. Exposure of PBL to tungstate (1 mM) and Concanavalin A (ConA) for 72 h reduced the number of cells in S and G(2)/M phases of the cell cycle. There were alterations in the numbers of cells in G(0)/G(1), S, and G(2)/M phases of the cell cycle in long-term THP-1 (acute leukemic monocytes) cultures treated with tungstate (0.01, 0.1, 1.0, and 10 mM). Gel electrophoresis, silver staining, and LC-MS/MS showed the cytoplasmic presence of histone H1b and H1d after 72 h of tungstate exposure. The addition of tungstate to cultures resulted in significant reductions in the quantity of interleukin-10 (IL-10), tumor necrosis factor-alpha (TNF-alpha), and IL-6 produced by stimulated [CD3/CD28, ConA, or lipopolysaccharide (LPS)] and tungstate-treated lymphocytes. Taken together, these data indicate that tungstate increases apoptosis of PBL, alters cell cycle progression, reduces cytokine production, and therefore warrants further investigation.
High-pressure elastic properties of major materials of Earth's mantle from first principles
NASA Astrophysics Data System (ADS)
Karki, Bijaya B.; Stixrude, Lars; Wentzcovitch, Renata M.
2001-11-01
The elasticity of materials is important for our understanding of processes ranging from brittle failure, to flexure, to the propagation of elastic waves. Seismologically revealed structure of the Earth's mantle, including the radial (one-dimensional) profile, lateral heterogeneity, and anisotropy are determined largely by the elasticity of the materials that make up this region. Despite its importance to geophysics, our knowledge of the elasticity of potentially relevant mineral phases at conditions typical of the Earth's mantle is still limited: Measuring the elastic constants at elevated pressure-temperature conditions in the laboratory remains a major challenge. Over the past several years, another approach has been developed based on first-principles quantum mechanical theory. First-principles calculations provide the ideal complement to the laboratory approach because they require no input from experiment; that is, there are no free parameters in the theory. Such calculations have true predictive power and can supply critical information including that which is difficult to measure experimentally. A review of high-pressure theoretical studies of major mantle phases shows a wide diversity of elastic behavior among important tetrahedrally and octahedrally coordinated Mg and Ca silicates and Mg, Ca, Al, and Si oxides. This is particularly apparent in the acoustic anisotropy, which is essential for understanding the relationship between seismically observed anisotropy and mantle flow. The acoustic anisotropy of the phases studied varies from zero to more than 50% and is found to depend on pressure strongly, and in some cases nonmonotonically. For example, the anisotropy in MgO decreases with pressure up to 15 GPa before increasing upon further compression, reaching 50% at a pressure of 130 GPa. Compression also has a strong effect on the elasticity through pressure-induced phase transitions in several systems. For example, the transition from stishovite to CaCl2
First-principles modeling of catalysts: novel algorithms and reaction mechanisms
NASA Astrophysics Data System (ADS)
Richard, Bryan Goldsmith
A molecular level understanding of a reaction mechanism and the computation of rates requires knowledge of the stable structures and the corresponding transition states that connect them. Temperature, pressure, and environment effects must be included to bridge the 'materials gap' so one can reasonably compare ab initio (first-principles, i.e., having no empirical parameters) predictions with experimental measurements. In this thesis, a few critical problems pertaining to ab initio modeling of catalytic systems are addressed; namely, 1) the issue of building representative models of isolated metal atoms grafted on amorphous supports, 2) modeling inorganic catalytic reactions in non-ideal solutions where the solvent participates in the reaction mechanism, and 3) bridging the materials gap using ab initio thermodynamics to predict the stability of supported nanoparticles under experimental reaction conditions. In Chapter I, a background on first-principles modeling of heterogeneous and homogenous catalysts is provided. Subsequently, to address the problem of modeling catalysis by isolated metal atoms on amorphous supports, we present in Chapter II a sequential-quadratic programming algorithm that systematically predicts the structure and reactivity of isolated active sites on insulating amorphous supports. Modeling solution phase reactions is also a considerable challenge for first-principles modeling, yet when done correctly it can yield critical kinetic and mechanistic insight that can guide experimental investigations. In Chapter III, we examine the formation of peroxorhenium complexes by activation of H2O2, which is key in selective oxidation reactions catalyzed by CH3ReO3 (methyltrioxorhenium, MTO). New experiments and density functional theory (DFT) calculations were conducted to better understand the activation of H2O2 by MTO and to provide a strong experimental foundation for benchmarking computational studies involving MTO and its derivatives. It was found
Structural stability and electronic properties of β-tetragonal boron: A first-principles study
Hayami, Wataru
2015-01-15
It is known that elemental boron has five polymorphs: α- and β-rhombohedral, α- and β-tetragonal, and the high-pressure γ phase. β-tetragonal (β-t) boron was first discovered in 1960, but there have been only a few studies since then. We have thoroughly investigated, using first-principles calculations, the atomic and electronic structures of β-t boron, the details of which were not known previously. The difficulty of calculation arises from the fact that β-t boron has a large unit cell that contains between 184 and 196 atoms, with 12 partially-occupied interstitial sites. This makes the number of configurations of interstitial atoms too great to calculate them all. By introducing assumptions based on symmetry and preliminary calculations, the number of configurations to calculate can be greatly reduced. It was eventually found that β-t boron has the lowest total energy, with 192 atoms (8 interstitial atoms) in an orthorhombic lattice. The total energy per atom was between those of α- and β-rhombohedral boron. Another tetragonal structure with 192 atoms was found to have a very close energy. The valence bands were fully filled and the gaps were about 1.16 to 1.54 eV, making it comparable to that of β-rhombohedral boron. - Graphical abstract: Electronic density distribution for the lowest-energy configuration (N=192) viewed from the 〈1 0 0〉 direction. Left: isosurface (yellow) at d=0.09 electrons/a.u.{sup 3} Right: isosurface (orange) at d=0.12 electrons/a.u.{sup 3}. - Highlights: • β-tetragonal boron was thoroughly investigated using first-principles calculations. • The lowest energy structure contains 192 atoms in an orthorhombic lattice. • Another tetragonal structure with 192 atoms has a very close energy. • The total energy per atom is between those of α- and β-rhombohedral boron. • The band gap of the lowest energy structure is about 1.16 to 1.54 eV.
Meng, Ying Shirley; Arroyo-de Dompablo, M Elena
2013-05-21
To meet the increasing demands of energy storage, particularly for transportation applications such as plug-in hybrid electric vehicles, researchers will need to develop improved lithium-ion battery electrode materials that exhibit high energy density, high power, better safety, and longer cycle life. The acceleration of materials discovery, synthesis, and optimization will benefit from the combination of both experimental and computational methods. First principles (ab Initio) computational methods have been widely used in materials science and can play an important role in accelerating the development and optimization of new energy storage materials. These methods can prescreen previously unknown compounds and can explain complex phenomena observed with these compounds. Intercalation compounds, where Li(+) ions insert into the host structure without causing significant rearrangement of the original structure, have served as the workhorse for lithium ion rechargeable battery electrodes. Intercalation compounds will also facilitate the development of new battery chemistries such as sodium-ion batteries. During the electrochemical discharge reaction process, the intercalating species travel from the negative to the positive electrode, driving the transition metal ion in the positive electrode to a lower oxidation state, which delivers useful current. Many materials properties change as a function of the intercalating species concentrations (at different state of charge). Therefore, researchers will need to understand and control these dynamic changes to optimize the electrochemical performance of the cell. In this Account, we focus on first-principles computational investigations toward understanding, controlling, and improving the intrinsic properties of five well known high energy density Li intercalation electrode materials: layered oxides (LiMO2), spinel oxides (LiM2O4), olivine phosphates (LiMPO4), silicates-Li2MSiO4, and the tavorite-LiM(XO4)F (M = 3d
First-principles study of cesium adsorption to weathered micaceous clay minerals
NASA Astrophysics Data System (ADS)
Okumura, Masahiko; Nakamura, Hiroki; Machida, Masahiko
2014-05-01
A large amount of radioactive nuclides was produced into environment due to the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Residents near FDNPP were suffering from radioactive cesium and then evacuated, because which has long half-life and is retained by surface soil for long time. The Japanese government has been decontaminating the cesium by removing the surface soil in order to return them to their home. This decontamination method is very effective, but which produces huge amount of waste soil. This becomes another big problem in Fukushima, because it is not easy to find large storage sites. Then effective and economical methods to reduce the volume of the waste soil are needed. However, it has not been invented yet. One of the reasons is lack of knowledge about microscopic process of adsorption/desorption of cesium to/from soil. It is known that weathered micaceous clay minerals play crucial role on adsorption and retention of cesium. They are expected to have special sorption sites, called frayed edge sites (FESs), which adsorb cesium selectively and irreversibly. Properties of FES have been intensely investigated by experiments. But microscopic details of the adsorption process on FES are still unclear. Because direct observation of the process with current experimental techniques is quite difficult. We investigated the adsorption of cesium to FES in muscovite, which is a typical micaceous clay mineral, via first-principles calculations (density functional theory). We made a minimal model of FES and evaluate the energy difference before and after cesium adsorption to FES. This is the first numerical modeling of FES. It was shown that FES does adsorb cesium if the weathering of muscovite has been weathered. In addition, we revealed the mechanism of cesium adsorption to FES, which is competition between ion radius of cesium and the degree of weathering. I plan to discuss volume reduction of the waste soil based on our result. Reference M. Okumura
First principles based multiscale modeling of single crystal plasticity: Application to BCC tantalum
NASA Astrophysics Data System (ADS)
Wang, Guofeng
We developed and exercised a first principles based multiscale approach to model plastic behaviors of high-purity Tantalum (Ta) single crystals. Our approach consists of three hierarchical parts. (1) Derive the atomistic interaction potential for Ta based on the data obtained from the accurate quantum mechanics (QM) calculation. (2) Predict the properties and behaviors of dislocations in the atomistic simulations using the derived first principles potential. (3) Describe the material plasticity in the kink pair mechanism based mesoscopic model with the input of the predicted atomistic level dislocation properties. In this thesis work, we accurately determined the core structure, core energy, Peierls energy barriers, Peierls stresses, kink formation energy, kink migration energy, and kink structures for 1/2a<111> screw dislocations in bcc Ta using molecular dynamics (MD) simulations. The major results are as follows. (1) The core energy is 1.400 eV/b for the asymmetric screw dislocation cores, which spread out along three <112> directions in the {110} planes. (2) The dislocation core is formed by the 12 atoms with higher strain energies around the dislocation center. (3) The twinning and anti-twinning asymmetry of shears is the main cause for the non-Schmid behavior of screw dislocations in bcc metals. (4) For 1/2a<111> screw dislocations in Ta, the Peierls energy barrier is 0.032 eV/b under twinning shears and 0.068 eV/b under anti-twinning shears. The Peierls stress is 790 MPa under twinning shears and 1430 MPa under anti-twinning shears. (5) The minimal energy cost to form a kink pair along the dislocation is 0.794 eV. (6) The effective kink pair nucleation length is 16 b. (7) There are two kinds of elementary kinks and six kinds of composite kinks. We further input these atomistic simulation results to a mesoscopic plasticity model [A. M. Cuitino, L. Stainer and M. Ortiz, Journal of the Mechanics and Physics of Solids, 2001]. The resulting atomistically informed