Spectroscopy of organic semiconductors from first principles
NASA Astrophysics Data System (ADS)
Sharifzadeh, Sahar; Biller, Ariel; Kronik, Leeor; Neaton, Jeffery
2011-03-01
Advances in organic optoelectronic materials rely on an accurate understanding their spectroscopy, motivating the development of predictive theoretical methods that accurately describe the excited states of organic semiconductors. In this work, we use density functional theory and many-body perturbation theory (GW/BSE) to compute the electronic and optical properties of two well-studied organic semiconductors, pentacene and PTCDA. We carefully compare our calculations of the bulk density of states with available photoemission spectra, accounting for the role of finite temperature and surface effects in experiment, and examining the influence of our main approximations -- e.g. the GW starting point and the application of the generalized plasmon-pole model -- on the predicted electronic structure. Moreover, our predictions for the nature of the exciton and its binding energy are discussed and compared against optical absorption data. We acknowledge DOE, NSF, and BASF for financial support and NERSC for computational resources.
First-Principles Theoretical Analysis of Carbon Allotropes and Nanostructures
NASA Astrophysics Data System (ADS)
Singh, Tejinder; Behr, Michael J.; Aydil, Eray S.; Maroudas, Dimitrios
2008-03-01
We analyze the various crystalline phases of C observed upon exposing carbon nanotubes to H2 plasmas, which produces an amorphous carbon matrix with carbon nanocrystalls embedded in it. Structural characterization with electron diffraction and high-resolution TEM yields three distinct crystalline phases of C consistent with a fcc lattice with lattice parameter a = 4.25 å, a bcc lattice with a = 3.0 å, and a diamond lattice with a = 3.57 å. Using first-principles density functional theory (DFT) calculations, we have analyzed the structure of several allotropes of pure carbon and we discuss our results in the context of the experimental findings. In addition, we consider the possibility of H incorporation in these C phases. According to our DFT calculations, incorporation at proper concentrations of H in interstitial sites of cubic phases of C provides interpretations for the experimentally observed crystalline C phases.
NASA Astrophysics Data System (ADS)
Pemmaraju, Sri Chaitanya Das; Prendergast, David
2014-03-01
We present two case studies of first-principles theoretical methods applied in conjunction with experimental core-level spectroscopy measurements to investigate the electronic structure and dynamical processes in molecular and interfacial systems relevant to photoelectrochemical (PEC) technologies. In the first, we study the core-level and valence spectroscopies of two zinc(II)-porphyrin based Donor-pi-Acceptor (D-p-A) dyes using the occupancy-constrained excited electron and core-hole (XCH) approach and time-dependent density functional theory (TDDFT) simulations. In the second, we use constrained DFT and TDDFT to interpret measured transient core-level shifts in time-resolved femtosecond x-ray photoelectron spectroscopy, investigating the dynamics of the electron injection process from a N3 dye molecule chemisorbed onto a ZnO substrate. These studies illustrate the utility of first-principles methods in guiding the design of better PEC materials. This work was performed at the Molecular Foundry, LBNL, supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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.
Exploiting periodic first-principles calculations in NMR spectroscopy of disordered solids.
Ashbrook, Sharon E; Dawson, Daniel M
2013-09-17
Much of the information contained within solid-state nuclear magnetic resonance (NMR) spectra remains unexploited because of the challenges in obtaining high-resolution spectra and the difficulty in assigning those spectra. Recent advances that enable researchers to accurately and efficiently determine NMR parameters in periodic systems have revolutionized the application of density functional theory (DFT) calculations in solid-state NMR spectroscopy. These advances are particularly useful for experimentalists. The use of first-principles calculations aids in both the interpretation and assignment of the complex spectral line shapes observed for solids. Furthermore, calculations provide a method for evaluating potential structural models against experimental data for materials with poorly characterized structures. Determining the structure of well-ordered, periodic crystalline solids can be straightforward using methods that exploit Bragg diffraction. However, the deviations from periodicity, such as compositional, positional, or temporal disorder, often produce the physical properties (such as ferroelectricity or ionic conductivity) that may be of commercial interest. With its sensitivity to the atomic-scale environment, NMR provides a potentially useful tool for studying disordered materials, and the combination of experiment with first-principles calculations offers a particularly attractive approach. In this Account, we discuss some of the issues associated with the practical implementation of first-principles calculations of NMR parameters in solids. We then use two key examples to illustrate the structural insights that researchers can obtain when applying such calculations to disordered inorganic materials. First, we describe an investigation of cation disorder in Y2Ti(2-x)Sn(x)O7 pyrochlore ceramics using (89)Y and (119)Sn NMR. Researchers have proposed that these materials could serve as host phases for the encapsulation of lanthanide- and actinide
Inelastic Electron Tunneling Spectroscopy in Molecular Electronic Devices from First-Principles
NASA Astrophysics Data System (ADS)
Ji, Tao
In this thesis, we present the first-principle calculations of inelastic electron tunneling spectroscopy(IETS) in single molecular break junctions. In a two-probe electrode-molecule-electrode setup, density functional theory(DFT) is used for the construction of the Hamiltonian and the Keldysh non-equilibrium Green's function(NEGF) technique will be employed for determining the electron density in non-equilibrium system conditions. Total energy functional, atomic forces and Hessian matrix can be obtained in the DFT-NEGF formalism and self-consistent Born approximation(SCBA) is used to integrate the molecular vibrations (phonons) into the framework once the phonon spectra and eigenvectors are calculated from the dynamic matrix. Geometry optimization schemes will also be discussed as an indispensable part of the formalism as the equilibrium condition is crucial to correctly calculate the phonon properties of the system. To overcome the numerical difficulties, especially the large computational time demand of the electron-phonon coupling problem, we develop a numerical approximation for the electron self-energy due to phonons and the error is controlled within numerical precision. Besides, a direct IETS second order I-V derivative expression is derived to reduce the error of numerical differentiation under reasonable assumptions. These two approximations greatly reduce the computation requirement and make the calculation feasible within current numerical capability. As the application of the DFT-NEGF-SCBA formalism, we calculate the IETS of the gold-octanedithiol(ODT) molecular junction. The I-V curve, conductance and IETS from ab-inito calculations are compared directly to experiments. A microscopic understanding of the electron-phonon coupling mechanism in the molecular tunneling junctions is explained in this example. In addition, comparisons of the hydrogen-dissociative and hydrogen-non-dissociative ODT junctions as well as the different charge transfer behaviors
Kishi, Hiroki; Miyazawa, Miki; Matsushima, Naoki; Yamauchi, Jun
2014-02-21
We investigate the X-ray photoelectron spectroscopy (XPS) binding energies of As 3d in Si for various defects in neutral and charged states by first-principles calculation. It is found that the complexes of a substitutional As and a vacancy in charged and neutral states explain the experimentally observed unknown peak very well.
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.
a First-Principles Theoretical Study of Microstructural Deformations of Solids
NASA Astrophysics Data System (ADS)
Juan, Yumin
1995-01-01
The development of density functional theory (DFT) and advances in computational capabilities have made it possible to obtain reliable information on the energetics of structural transformations in solids. However, limitations do exist due to the use of the local density approximations (LDA) to the exchange-correlation functionals. Recently, there have been efforts to try to go beyond LDA by including gradient corrections, which are referred to as the generalized gradient approximation (GGA). To obtain an accurate assessment of these GGA functionals, we investigated in detail the applicability of two recently proposed functionals. The effects on the atomic charge distribution were studied. We also examined different approaches to construct pseudopotentials self-consistently in the context of gradient-corrected functionals. We concluded that although GGA functional produce improved results in some cases, in general LDA remains a reliable choice for the energetic calculations in solids. We considered a range of phenomena related to structural transformations of a prototypical covalent solid, namely silicon. We first studied the energetics for the high-pressure plastic flow of silicon by performing DFT total energy calculations for structural transformations which might correspond to mass flow. To explore the phase space efficiently, the magic strain concept was used. Entropy effects were taken into account with the use of Vineyard's transition state theory. An upper bound for the energy barrier was obtained from our study and has been found to be lower than the melting point of silicon, suggesting that such transformations may be possible under indentation. As a second application, we have obtained accurate generalized stacking fault (GSF) energy surfaces for both the (111) and (100) planes of silicon with first-principles calculations. The importance of this GSF energy surface is that it can be used to identify the value of the unstable stacking fault energy gamma
Natsume, Yutaka; Kohno, Teiichiro; Minakata, Takashi; Konishi, Tokuzo; Gullikson, Eric M; Muramatsu, Yasuji
2012-02-16
The chemical states of organic semiconductors were investigated by total-electron-yield soft X-ray absorption spectroscopy (TEY-XAS) and first-principles calculations. The organic semiconductors, pentacene (C(22)H(14)) and pentacenequinone (C(22)H(12)O(2)), were subjected to TEY-XAS and the experimental spectra obtained were compared with the 1s core-level excited spectra of C and O atoms, calculated by a first-principles planewave pseudopotential method. Excellent agreement between the measured and the calculated spectra were obtained for both materials. Using this methodology, we examined the chemical states of the aged pentacene, and confirmed that both C-OH and C═O chemical bonds are generated by exposure to air. This result implies that not only oxygen but also humidity causes pentacene oxidation.
Phase stability, elasticity, and theoretical strength of polonium from first principles
NASA Astrophysics Data System (ADS)
Legut, Dominik; Friák, Martin; Šob, Mojmír
2010-06-01
Employing full-potential linearized augmented plane-wave method, we investigate the stability of Po in its ground-state simple cubic structure (α-Po) with respect to the trigonal spiral structure exhibited by Se and Te and to the displacive phase transformations into either tetragonal or trigonal phases. The origin of the phase stability of α-Po is analyzed with the help of densities of states, electronic band structures, and total energies of competing higher-energy structures corresponding to selected stationary points of the total energy. The electronic structures and total energies are calculated both within the generalized gradient approximation and local-density approximation (LDA) to the exchange-correlation energy as well as with and without inclusion of the spin-orbit (SO) coupling. The total energies are displayed in contour plots as functions of selected structural parameters and atomic volume. It turns out that the LDA calculation with SO interaction incorporated provides best agreement with existing experimental data and that the simple cubic structure of α-Po is stabilized by relativistic effects of core electrons. High elastic anisotropy of α-Po is explained as a consequence of its simple cubic structure and is compared with elastic properties of other crystal structures. Finally, an uniaxial tensile test for loading along the [001] and [111] directions is simulated; the corresponding theoretical tensile strengths calculated within the LDA+SO approach amount to 4.2 GPa and 4.7 GPa, respectively, which are the lowest values predicted in an element so far. According to Pugh and Frantsevich criteria, α-Po is predicted to be ductile. Also a positive value of the Cauchy pressure confirms the metallic type of interatomic bonding.
Nenov, Artur; Segarra-Martí, Javier; Giussani, Angelo; Conti, Irene; Rivalta, Ivan; Dumont, Elise; Jaiswal, Vishal K; Altavilla, Salvatore Flavio; Mukamel, Shaul; Garavelli, Marco
2015-01-01
The SOS//QM/MM [Rivalta et al., Int. J. Quant. Chem., 2014, 114, 85] method consists of an arsenal of computational tools allowing accurate simulation of one-dimensional (1D) and bi-dimensional (2D) electronic spectra of monomeric and dimeric systems with unprecedented details and accuracy. Prominent features like doubly excited local and excimer states, accessible in multi-photon processes, as well as charge-transfer states arise naturally through the fully quantum-mechanical description of the aggregates. In this contribution the SOS//QM/MM approach is extended to simulate time-resolved 2D spectra that can be used to characterize ultrafast excited state relaxation dynamics with atomistic details. We demonstrate how critical structures on the excited state potential energy surface, obtained through state-of-the-art quantum chemical computations, can be used as snapshots of the excited state relaxation dynamics to generate spectral fingerprints for different de-excitation channels. The approach is based on high-level multi-configurational wavefunction methods combined with non-linear response theory and incorporates the effects of the solvent/environment through hybrid quantum mechanics/molecular mechanics (QM/MM) techniques. Specifically, the protocol makes use of the second-order Perturbation Theory (CASPT2) on top of Complete Active Space Self Consistent Field (CASSCF) strategy to compute the high-lying excited states that can be accessed in different 2D experimental setups. As an example, the photophysics of the stacked adenine-adenine dimer in a double-stranded DNA is modeled through 2D near-ultraviolet (NUV) spectroscopy.
Nenov, Artur; Beccara, Silvio; Rivalta, Ivan; Cerullo, Giulio; Mukamel, Shaul; Garavelli, Marco
2014-10-20
The ability of nonlinear electronic spectroscopy to track folding/unfolding processes of proteins in solution by monitoring aromatic interactions is investigated by first-principles simulations of two-dimensional (2D) electronic spectra of a model peptide. A dominant reaction pathway approach is employed to determine the unfolding pathway of a tetrapeptide, which connects the initial folded configuration with stacked aromatic side chains and the final unfolded state with distant noninteracting aromatic residues. The π-stacking and excitonic coupling effects are included through ab initio simulations based on multiconfigurational methods within a hybrid quantum mechanics/molecular mechanics scheme. It is shown that linear absorption spectroscopy in the ultraviolet (UV) region is unable to resolve the unstacking dynamics characterized by the three-step process: T-shaped→twisted offset stacking→unstacking. Conversely, pump-probe spectroscopy can be used to resolve aromatic interactions by probing in the visible region, the excited-state absorptions (ESAs) that involve charge-transfer states. 2D UV spectroscopy offers the highest sensitivity to the unfolding process, by providing the disentanglement of ESA signals belonging to different aromatic chromophores and high correlation between the conformational dynamics and the quartic splitting.
Jiang, Jun; Mukamel, Shaul
2011-02-14
We report a first principles study of two dimensional electronic spectroscopy of aromatic side chain transitions in the 32-residue β-amyloid (Aβ(9-40)) fibrils in the near ultraviolet (250-300 nm). An efficient exciton Hamiltonian with electrostatic fluctuations (EHEF) algorithm is used to compute the electronic excitations in the presence of environmental fluctuations. The through-space inter- and intra-molecular interactions are calculated with high level quantum mechanics (QM) approaches, and interfaced with molecular mechanics (MM) simulations. Distinct two dimensional near ultraviolet (2DNUV) spectroscopic signatures are identified for different aromatic transitions, and the couplings between them. 2DNUV signals associated with the transition couplings are shown to be very sensitive to the change of residue-residue interactions induced by residue mutations. Our simulations suggest that 2DNUV spectra could provide a useful local probe for the structure and kinetics of fibrils.
Zhang, Lei; Ju, Ming-Gang; Liang, WanZhen
2016-08-17
With efficiencies exceeding 20% and low production costs, lead halide perovskite solar cells (PSCs) have become potential candidates for future commercial applications. However, there are serious concerns about their long-term stability and environmental friendliness, heavily related to their commercial viability. Herein, we present a theoretical investigation based on the ab initio molecular dynamics (AIMD) simulations and the first-principles density functional theory (DFT) calculations to investigate the effects of sunlight and moisture on the structures and properties of MAPbI3 perovskites. AIMD simulations have been performed to simulate the impact of a few water molecules on the structures of MAPbI3 surfaces terminated in three different ways. The evolution of geometric and electronic structures as well as the absorption spectra has been shown. It is found that the PbI2-terminated surface is the most stable while both the MAI-terminated and PbI2-defective surfaces undergo structural reconstruction, leading to the formation of hydrated compounds in a humid environment. The moisture-induced weakening of photoabsorption is closely related to the formation of hydrated species, and the hydrated crystals MAPbI3·H2O and MA4PbI6·2H2O scarcely absorb the visible light. The electronic excitation in the bare and water-absorbed MAPbI3 nanoparticles tends to weaken Pb-I bonds, especially those around water molecules, and the maximal decrease of photoexcitation-induced bond order can reach up to 20% in the excited state in which the water molecules are involved in the electronic excitation, indicating the accelerated decomposition of perovskites in the presence of sunlight and moisture. This work is valuable for understanding the mechanism of chemical or photochemical instability of MAPbI3 perovskites in the presence of moisture. PMID:27499005
Nenov, Artur; Mukamel, Shaul; Garavelli, Marco; Rivalta, Ivan
2015-08-11
First-principles simulations of two-dimensional electronic spectroscopy in the ultraviolet region (2DUV) require computationally demanding multiconfigurational approaches that can resolve doubly excited and charge transfer states, the spectroscopic fingerprints of coupled UV-active chromophores. Here, we propose an efficient approach to reduce the computational cost of accurate simulations of 2DUV spectra of benzene, phenol, and their dimer (i.e., the minimal models for studying electronic coupling of UV-chromophores in proteins). We first establish the multiconfigurational recipe with the highest accuracy by comparison with experimental data, providing reference gas-phase transition energies and dipole moments that can be used to construct exciton Hamiltonians involving high-lying excited states. We show that by reducing the active spaces and the number of configuration state functions within restricted active space schemes, the computational cost can be significantly decreased without loss of accuracy in predicting 2DUV spectra. The proposed recipe has been successfully tested on a realistic model proteic system in water. Accounting for line broadening due to thermal and solvent-induced fluctuations allows for direct comparison with experiments.
NASA Astrophysics Data System (ADS)
Yamauchi, Jun; Yoshimoto, Yoshihide; Suwa, Yuji
2016-05-01
We carried out a comprehensive study on the B 1s core-level X-ray photoelectron spectroscopy (XPS) binding energies and formation energies for boron defects in crystalline silicon by first-principles calculation with careful evaluation of the local potential boundary condition for the model system using the supercell corresponding to 1000 Si atoms. It is reconfirmed that the cubo-octahedral B12 cluster in silicon crystal is unstable and exists at the saddle point decaying to the icosahedral and S4 B12 clusters. The electrically active clusters without any postannealing of ion-implanted Si are identified as icosahedral B12 clusters. The experimentally proposed threefold coordinated B is also identified as a ⟨ 001 ⟩ B - Si defect. For an as-doped sample prepared by plasma doping, the calculated XPS spectra for complexes consisting of vacancies and substitutional B atoms are consistent with the experimental spectra. It is proposed that, assuming that the XPS peak at 187.1 eV is due to substitutional B (Bs), the experimental XPS peaks at 187.9 and 186.7 eV correspond to interstitial B at the H-site and ⟨ 001 ⟩ B - Si defects, respectively. In the annealed samples, the complex of Bs and interstitial Si near the T-site is proposed as a candidate for the experimental XPS peak at 188.3 eV.
Zhao, Kejie; Wang, Wei L; Gregoire, John; Pharr, Matt; Suo, Zhigang; Vlassak, Joost J; Kaxiras, Efthimios
2011-07-13
Silicon can host a large amount of lithium, making it a promising electrode for high-capacity lithium-ion batteries. Recent experiments indicate that silicon experiences large plastic deformation upon Li absorption, which can significantly decrease the stresses induced by lithiation and thus mitigate fracture failure of electrodes. These issues become especially relevant in nanostructured electrodes with confined geometries. On the basis of first-principles calculations, we present a study of the microscopic deformation mechanism of lithiated silicon at relatively low Li concentration, which captures the onset of plasticity induced by lithiation. We find that lithium insertion leads to breaking of Si-Si bonds and formation of weaker bonds between neighboring Si and Li atoms, which results in a decrease in Young's modulus, a reduction in strength, and a brittle-to-ductile transition with increasing Li concentration. The microscopic mechanism of large plastic deformation is attributed to continuous lithium-assisted breaking and re-forming of Si-Si bonds and the creation of nanopores.
NASA Astrophysics Data System (ADS)
Pokrovski, Gleb S.; Roux, Jacques; Ferlat, Guillaume; Jonchiere, Romain; Seitsonen, Ari P.; Vuilleumier, Rodolphe; Hazemann, Jean-Louis
2013-04-01
The molecular structure and stability of species formed by silver in aqueous saline solutions typical of hydrothermal settings were quantified using in situ X-ray absorption spectroscopy (XAS) measurements, quantum-chemical modeling of near-edge absorption spectra (XANES) and extended fine structure spectra (EXAFS), and first-principles molecular dynamics (FPMD). Results show that in nitrate-bearing acidic solutions to at least 200 °C, silver speciation is dominated by the hydrated Ag+ cation surrounded by 4-6 water molecules in its nearest coordination shell with mean Ag-O distances of 2.32 ± 0.02 Å. In NaCl-bearing acidic aqueous solutions of total Cl concentration from 0.7 to 5.9 mol/kg H2O (m) at temperatures from 200 to 450 °C and pressures to 750 bar, the dominant species are the di-chloride complex AgCl2- with Ag-Cl distances of 2.40 ± 0.02 Å and Cl-Ag-Cl angle of 160 ± 10°, and the tri-chloride complex AgCl32- of a triangular structure and mean Ag-Cl distances of 2.60 ± 0.05 Å. With increasing temperature, the contribution of the tri-chloride species decreases from ˜50% of total dissolved Ag in the most concentrated solution (5.9m Cl) at 200 °C to less than 10-20% at supercritical temperatures for all investigated solutions, so that AgCl2- becomes by far the dominant Ag-bearing species at conditions typical of hydrothermal-magmatic fluids. Both di- and tri-chloride species exhibit outer-sphere interactions with the solvent as shown by the detection, using FPMD modeling, of H2O, Cl-, and Na+ at distances of 3-4 Å from the silver atom. The species fractions derived from XAS and FPMD analyses, and total AgCl(s) solubilities, measured in situ in this work from the absorption edge height of XAS spectra, are in accord with thermodynamic predictions using the stability constants of AgCl2- and AgCl32- from Akinfiev and Zotov (2001) and Zotov et al. (1995), respectively, which are based on extensive previous AgCl(s) solubility measurements. These data
Theoretical insight into hydrogen adsorption onto graphene: a first-principles B3LYP-D3 study.
Darvish Ganji, M; Hosseini-Khah, S M; Amini-Tabar, Z
2015-01-28
This work investigates hydrogen adsorption onto various graphene flakes such as coronene and coronene-like as suitable models of graphene within the framework of the DFT-B3LYP method. The non-local van der Waals (vdW) density functional (B3LYP-D3) method is used for both structural geometry optimization and total energy estimations. Calculations were carried out for a hydrogen molecule above a coronene surface with both conventional and vdW corrected DFT to investigate how these approaches perform in the case of hydrogen adsorption on a graphene surface. Our first-principles results within the B3LYP-D3/def2-TZVPP model show that hydrogen physisorbs on a coronene surface with an adsorption energy of -5.013 (kJ mol(-1)) which is in good agreement with the experimental value. The influence of the basis set and graphene flake size were also evaluated and the results indicate that these slightly affect the adsorption properties. We found also that it is crucial to use non-local dispersion interactions to get accurate results for hydrogen adsorption on a graphene surface. Furthermore, the co-adsorption of H2 molecules onto the graphene surface was investigated. The results obtained at the B3LYP-D3/def2-TZVP level show that H2 molecules can be physisorbed on both sides of the graphene layer with adsorption properties similar to those for a single surface. Finally, we showed that H2 molecules might be bound to the graphene surface via a bilayer adsorption scheme with weak adsorption energy. Charge population and electron density analysis confirm the weak binding nature of the system under consideration.
NASA Astrophysics Data System (ADS)
Dai, Xing; Gao, Yang; Xin, Minsi; Wang, Zhigang; Zhou, Ruhong
2014-12-01
As a representative lanthanide endohedral metallofullerene, Gd@C82 has attracted a widespread attention among theorists and experimentalists ever since its first synthesis. Through comprehensive comparisons and discussions, as well as references to the latest high precision experiments, we evaluated the performance of different computational methods. Our results showed that the appropriate choice of the exchange-correlation functionals is the decisive factor to accurately predict both geometric and electronic structures for Gd@C82. The electronic structure of the ground state and energy gap between the septet ground state and the nonet low-lying state obtained from pure density functional methods, such as PBE and PW91, are in good agreement with current experiment. Unlike pure functionals, the popularly used hybrid functionals in previous studies, such as B3LYP, could infer the qualitative correct ground state only when small basis set for C atoms is employed. Furthermore, we also highlighted that other geometric structures of Gd@C82 with the Gd staying at different positions are either not stable or with higher energies. This work should provide some useful references for various theoretical methodologies in further density functional studies on Gd@C82 and its derivatives in the future.
NASA Astrophysics Data System (ADS)
Sangalli, Davide; Dal Conte, Stefano; Manzoni, Cristian; Cerullo, Giulio; Marini, Andrea
2016-05-01
The calculation of the equilibrium optical properties of bulk silicon by using the Bethe-Salpeter equation solved in the Kohn-Sham basis represents a cornerstone in the development of an ab-initio approach to the optical and electronic properties of materials. Nevertheless, calculations of the transient optical spectrum using the same efficient and successful scheme are scarce. We report, here, a joint theoretical and experimental study of the transient reflectivity spectrum of bulk silicon. Femtosecond transient reflectivity is compared to a parameter-free calculation based on the nonequilibrium Bethe-Salpeter equation. By providing an accurate description of the experimental results we disclose the different phenomena that determine the transient optical response of a semiconductor. We give a parameter-free interpretation of concepts such as bleaching, photoinduced absorption, and stimulated emission, beyond the Fermi golden rule. We also introduce the concept of optical gap renormalization, as a generalization of the known mechanism of band gap renormalization. The present scheme successfully describes the case of bulk silicon, showing its universality and accuracy.
Reeves, Kyle G.; Kanai, Yosuke
2014-07-14
Oxidation state is a powerful concept that is widely used in chemistry and materials physics, although the concept itself is arguably ill-defined quantum mechanically. In this work, we present impartial comparison of four, well-recognized theoretical approaches based on Lowdin atomic orbital projection, Bader decomposition, maximally localized Wannier function, and occupation matrix diagonalization, for assessing how well transition metal oxidation states can be characterized. Here, we study a representative molecular complex, tris(bipyridine)ruthenium. We also consider the influence of water solvation through first-principles molecular dynamics as well as the improved electronic structure description for strongly correlated d-electrons by including Hubbard correction in density functional theory calculations.
NASA Astrophysics Data System (ADS)
Reeves, Kyle G.; Kanai, Yosuke
2014-07-01
Oxidation state is a powerful concept that is widely used in chemistry and materials physics, although the concept itself is arguably ill-defined quantum mechanically. In this work, we present impartial comparison of four, well-recognized theoretical approaches based on Lowdin atomic orbital projection, Bader decomposition, maximally localized Wannier function, and occupation matrix diagonalization, for assessing how well transition metal oxidation states can be characterized. Here, we study a representative molecular complex, tris(bipyridine)ruthenium. We also consider the influence of water solvation through first-principles molecular dynamics as well as the improved electronic structure description for strongly correlated d-electrons by including Hubbard correction in density functional theory calculations.
Manceau, Alain; Lemouchi, Cyprien; Rovezzi, Mauro; Lanson, Martine; Glatzel, Pieter; Nagy, Kathryn L; Gautier-Luneau, Isabelle; Joly, Yves; Enescu, Mironel
2015-12-21
We present results obtained from high energy-resolution L3-edge XANES spectroscopy and first-principles calculations for the structure, bonding, and stability of mercury(II) complexes with thiolate and thioether ligands in crystalline compounds, aqueous solution, and macromolecular natural organic matter (NOM). Core-to-valence XANES features that vary in intensity differentiate with unprecedented sensitivity the number and identity of Hg ligands and the geometry of the ligand environment. Post-Hartree-Fock XANES calculations, coupled with natural population analysis, performed on MP2-optimized Hg[(SR)2···(RSR)n] complexes show that the shape, position, and number of electronic transitions observed at high energy-resolution are directly correlated to the Hg and S (l,m)-projected empty densities of states and occupations of the hybridized Hg 6s and 5d valence orbitals. Linear two-coordination, the most common coordination geometry in mercury chemistry, yields a sharp 2p to 6s + 5d electronic transition. This transition varies in intensity for Hg bonded to thiol groups in macromolecular NOM. The intensity variation is explained by contributions from next-nearest, low-charge, thioether-type RSR ligands at 3.0-3.3 Å from Hg. Thus, Hg in NOM has two strong bonds to thiol S and k additional weak Hg···S contacts, or 2 + k coordination. The calculated stabilization energy is -5 kcal/mol per RSR ligand. Detection of distant ligands beyond the first coordination shell requires precise measurement of, and comparison to, spectra of reference compounds as well as accurate calculation of spectra for representative molecular models. The combined experimental and theoretical approaches described here for Hg can be applied to other closed-shell atoms, such as Ag(I) and Au(I). To facilitate further calculation of XANES spectra, experimental data, a new crystallographic structure of a key mercury thioether complex, Cartesian coordinates of the computed models, and examples of
NASA Astrophysics Data System (ADS)
Zhao, Li-Juan; Tian, Wen-Juan; Ou, Ting; Xu, Hong-Guang; Feng, Gang; Xu, Xi-Ling; Zhai, Hua-Jin; Li, Si-Dian; Zheng, Wei-Jun
2016-03-01
We present a combined photoelectron spectroscopy and first-principles theory study on the structural and electronic properties and chemical bonding of B3O3-/0 and B3O3H-/0 clusters. The concerted experimental and theoretical data show that the global-minimum structures of B3O3 and B3O3H neutrals are very different from those of their anionic counterparts. The B3O3- anion is characterized to possess a V-shaped OB-B-BO chain with overall C2v symmetry (1A), in which the central B atom interacts with two equivalent boronyl (B≡O) terminals via B-B single bonds as well as with one O atom via a B=O double bond. The B3O3H- anion has a Cs (2A) structure, containing an asymmetric OB-B-OBO zig-zag chain and a terminal H atom interacting with the central B atom. In contrast, the C2v (1a) global minimum of B3O3 neutral contains a rhombic B2O2 ring with one B atom bonded to a BO terminal and that of neutral B3O3H (2a) is also of C2v symmetry, which is readily constructed from C2v (1a) by attaching a H atom to the opposite side of the BO group. The H atom in B3O3H-/0 (2A and 2a) prefers to interact terminally with a B atom, rather than with O. Chemical bonding analyses reveal a three-center four-electron (3c-4e) π hyperbond in the B3O3H- (2A) cluster and a four-center four-electron (4c-4e) π bond (that is, the so-called o-bond) in B3O3 (1a) and B3O3H (2a) neutral clusters.
Anharmonic Theoretical Vibrational Spectroscopy of Polypeptides.
Panek, Paweł T; Jacob, Christoph R
2016-08-18
Because of the size of polypeptides and proteins, the quantum-chemical prediction of their vibrational spectra presents an exceptionally challenging task. Here, we address one of these challenges, namely, the inclusion of anharmonicities. By performing the expansion of the potential energy surface in localized-mode coordinates instead of the normal-mode coordinates, it becomes possible to calculate anharmonic vibrational spectra of polypeptides efficiently and reliably. We apply this approach to calculate the infrared, Raman, and Raman optical activity spectra of helical alanine polypeptides consisting of up to 20 amino acids. We find that while anharmonicities do not alter the band shapes, simple scaling procedures cannot account for the different shifts found for the individual bands. This closes an important gap in theoretical vibrational spectroscopy by making it possible to quantify the anharmonic contributions and opens the door to a first-principles calculation of multidimensional vibrational spectra. PMID:27472016
Stepanian, Stepan G; Karachevtsev, Maksym V; Glamazda, Alexander Yu; Karachevtsev, Victor A; Adamowicz, L
2009-04-16
In this work, we have used Raman spectroscopy and quantum chemical methods (MP2 and DFT) to study the interactions between nucleic acid bases (NABs) and single-walled carbon nanotubes (SWCNT). We found that the appearance of the interaction between the nanotubes and the NABs is accompanied by a spectral shift of the high-frequency component of the SWCNT G band in the Raman spectrum to a lower frequency region. The value of this shift varies from 0.7 to 1.3 cm(-1) for the metallic nanotubes and from 2.1 to 3.2 cm(-1) for the semiconducting nanotubes. Calculations of the interaction energies between the NABs and a fragment of the zigzag(10,0) carbon nanotube performed at the MP2/6-31++G(d,p)[NABs atoms]|6-31G(d)[nanotube atoms] level of theory while accounting for the basis set superposition error during geometry optimization allowed us to order the NABs according to the increasing interaction energy value. The order is: guanine (-67.1 kJ mol(-1)) > adenine (-59.0 kJ mol(-1)) > cytosine (-50.3 kJ mol(-1)) approximately = thymine (-50.2 kJ mol(-1)) > uracil (-44.2 kJ mol(-1)). The MP2 equilibrium structures and the interaction energies were used as reference points in the evaluation of the ability of various functionals in the DFT method to predict those structures and energies. We showed that the M05, MPWB1K, and MPW1B95 density functionals are capable of correctly predicting the SWCNT-NAB geometries but not the interaction energies, while the M05-2X functional is capable of correctly predicting both the geometries and the interaction energies.
Karre, Rajamallu; Niranjan, Manish K; Dey, Suhash R
2015-05-01
High alloyed β-phase stabilized titanium alloys are known to provide comparable Young's modulus as that to the human bones (~30 GPa) but is marred by its high density. In the present study the low titanium alloyed compositions of binary Ti-Nb and ternary Ti-Nb-Zr alloy systems, having stable β-phase with low Young's modulus are identified using first principles density functional framework. The theoretical results suggest that the addition of Nb in Ti and Zr in Ti-Nb increases the stability of the β-phase. The β-phase in binary Ti-Nb alloys is found to be fully stabilized from 22 at.% of Nb onwards. The calculated Young's moduli of binary β-Ti-Nb alloy system are found to be lower than that of pure titanium (116 GPa). For Ti-25(at.%)Nb composition the calculated Young's modulus comes out to be ~80 GPa. In ternary Ti-Nb-Zr alloy system, the Young's modulus of Ti-25(at.%)Nb-6.25(at.%)Zr composition is calculated to be ~50 GPa. Furthermore, the directional Young's moduli of these two selected binary (Ti-25(at.%)Nb) and ternary alloy (Ti-25(at.%)Nb-6.25(at.%)Zr) compositions are found to be nearly isotropic in all crystallographic directions.
NASA Astrophysics Data System (ADS)
Gambuzzi, Elisa; Pedone, Alfonso; Menziani, Maria Cristina; Angeli, Frédéric; Caurant, Daniel; Charpentier, Thibault
2014-01-01
Silicon and aluminium chemical environments in silicate and aluminosilicate glasses with compositions 60SiO2·20Na2O·20CaO (CSN), 60SiO2·20Al2O3·20CaO (CAS), 78SiO2·11Al2O3·11Na2O (NAS) and 60SiO2·10Al2O3·10Na2O·20CaO (CASN) have been investigated by 27Al and 29Si solid state magic angle spinning (MAS) and multiple quantum MAS (MQMAS) nuclear magnetic resonance (NMR) experiments. To interpret the NMR data, first-principles calculations using density functional theory were performed on structural models of these glasses. These models were generated by Shell-model molecular dynamics (MD) simulations. The theoretical NMR parameters and spectra were computed using the gauge including projected augmented wave (GIPAW) method and spin-effective Hamiltonians, respectively. This synergetic computational-experimental approach offers a clear structural characterization of these glasses, particularly in terms of network polymerization, chemical disorder (i.e. Si and Al distribution in second coordination sphere) and modifier cation distributions. The relationships between the local structural environments and the 29Si and 27Al NMR parameters are highlighted, and show that: (i) the isotropic chemical shift of both 29Si and 27Al increases of about +5 ppm for each Al added in the second sphere and (ii) both the 27Al and 29Si isotropic chemical shifts linearly decrease with the reduction of the average Si/Al-O-T bond angle. Conversely, 27Al and 29Si NMR parameters are much less sensitive to the connectivity with triple bridging oxygen atoms, precluding their indirect detection from 27Al and 29Si NMR.
Loganathan, B; Chandraboss, V L; Senthilvelan, S; Karthikeyan, B
2015-09-01
The Rh shell of the Au/Pt/Rh trimetallic nanoparticles induces a wide variety of interesting surface reactions by allowing the adsorption of amino acids like L-cysteine (L-Cys). We present a snapshot of theoretical and experimental investigation of L-Cys adsorption on the surface of noble trimetallic Au/Pt@Rh colloidal nanocomposites. Density functional theoretical (DFT) investigations of L-Cys interaction with the Rhodium (Rh) shell of a trimetallic Au/Pt@Rh cluster in terms of geometry, binding energy (E(B)), binding site, energy gap (E(g)), electronic and spectral properties have been performed. L-Cys establishes a strong interaction with the Rh shell. It binds to Rh by the S1-site, which makes a stable L-Cys-Rh surface complex. DFT can be taken as a valuable tool to assign the vibrational spectra of the adsorption of L-Cys on trimetallic Au/Pt@Rh colloidal nanocomposites and mono-metallic Rh nanoparticles. Surface-enhanced infrared spectroscopy (SEIRS) with L-Cys on a Rh6 cluster surface has been simulated for the first time. Experimental information on the L-Cys-Rh surface complex is included to examine the interaction. The experimental spectral observations are in good agreement with the simulated DFT results. Characterization of the synthesized trimetallic Au/Pt@Rh colloidal nanocomposites has been done by high-resolution transmission electron microscopy (HR-TEM) with selected area electron diffraction (SAED) pattern, energy dispersive X-ray (EDX) spectroscopy, dynamic light scattering (DLS) measurements, zeta potential, zeta deviation analysis and UV-visible (UV-Vis) spectroscopic studies. PMID:25650352
NASA Astrophysics Data System (ADS)
Schweflinghaus, Benedikt; dos Santos Dias, Manuel; Lounis, Samir
2016-01-01
Spin excitations in atomic-scale nanostructures have been investigated with inelastic scanning tunneling spectroscopy, sometimes with conflicting results. In this work, we present a theoretical viewpoint on a recent experimental controversy regarding the spin excitations of Co adatoms on Pt(111). While one group [Balashov et al., Phys. Rev. Lett. 102, 257203 (2009), 10.1103/PhysRevLett.102.257203] claims to have detected them, another group reported their observation only after the hydrogenation of the Co adatom [Dubout et al., Phys. Rev. Lett. 114, 106807 (2015), 10.1103/PhysRevLett.114.106807]. Utilizing time-dependent density functional theory in combination with many-body perturbation theory, we demonstrate that, although inelastic spin excitations are possible for Cr, Mn, Fe, and Co adatoms, their efficiency differs. While the excitation signature is less pronounced for Mn and Co adatoms, it is larger for Cr and Fe adatoms. We find that the tunneling matrix elements or the tunneling cross-section related to the nature and symmetry of the relevant electronic states are more favorable for triggering the spin excitations in Fe than in Co. An enhancement of the tunneling and of the inelastic spectra is possible by attaching hydrogen to the adatom at the appropriate position.
Dai, Xing; Gao, Yang; Xin, Minsi; Wang, Zhigang; Zhou, Ruhong
2014-12-28
As a representative lanthanide endohedral metallofullerene, Gd@C{sub 82} has attracted a widespread attention among theorists and experimentalists ever since its first synthesis. Through comprehensive comparisons and discussions, as well as references to the latest high precision experiments, we evaluated the performance of different computational methods. Our results showed that the appropriate choice of the exchange-correlation functionals is the decisive factor to accurately predict both geometric and electronic structures for Gd@C{sub 82}. The electronic structure of the ground state and energy gap between the septet ground state and the nonet low-lying state obtained from pure density functional methods, such as PBE and PW91, are in good agreement with current experiment. Unlike pure functionals, the popularly used hybrid functionals in previous studies, such as B3LYP, could infer the qualitative correct ground state only when small basis set for C atoms is employed. Furthermore, we also highlighted that other geometric structures of Gd@C{sub 82} with the Gd staying at different positions are either not stable or with higher energies. This work should provide some useful references for various theoretical methodologies in further density functional studies on Gd@C{sub 82} and its derivatives in the future.
NASA Astrophysics Data System (ADS)
Ducher, Manoj; Blanchard, Marc; Vantelon, Delphine; Nemausat, Ruidy; Cabaret, Delphine
2016-03-01
We present experimental and calculated Al K-edge X-ray absorption near-edge structure (XANES) spectra of aluminous goethite with 10-33 mol% of AlOOH and diaspore. Significant changes are observed experimentally in the near- and pre-edge regions with increasing Al concentration in goethite. First-principles calculations based on density functional theory (DFT) reproduce successfully the experimental trends. This permits to identify the electronic and structural parameters controlling the spectral features and to improve our knowledge of the local environment of {Al}^{3+} in the goethite-diaspore partial solid solution. In the near-edge region, the larger peak spacing in diaspore compared to Al-bearing goethite is related to the nature (Fe or Al) of the first cation neighbours around the absorbing Al atom (Al*). The intensity ratio of the two near-edge peaks, which decreases with Al concentration, is correlated with the average distance of the first cations around Al* and the distortion of the {AlO}_6 octahedron. Finally, the decrease in intensity of the pre-edge features with increasing Al concentration is due to the smaller number of Fe atoms in the local environment of Al since Al atoms tend to cluster. In addition, it is found that the pre-edge features of the Al K-edge XANES spectra enable to probe indirectly empty 3 d states of Fe. Energetic, structural and spectroscopic results suggest that for Al concentrations around 10 mol%, Al atoms can be considered as isolated, whereas above 25 mol%, Al clusters are more likely to occur.
Ashbrook, Sharon E; Berry, Andrew J; Frost, Daniel J; Gregorovic, Alan; Pickard, Chris J; Readman, Jennifer E; Wimperis, Stephen
2007-10-31
The 29Si and 17O NMR parameters of six polymorphs of MgSiO3 were determined through a combination of high-resolution solid-state NMR and first-principles gauge including projector augmented wave (GIPAW) formalism calculations using periodic boundary conditions. MgSiO3 is an important component of the Earth's mantle that undergoes structural changes as a function of pressure and temperature. For the lower pressure polymorphs (ortho-, clino-, and protoenstatite), all oxygen species in the 17O high-resolution triple-quantum magic angle spinning (MAS) NMR spectra were resolved and assigned. These assignments differ from those tentatively suggested in previous work on the basis of empirical experimental correlations. The higher pressure polymorphs of MgSiO3 (majorite, akimotoite, and perovskite) are stabilized at pressures corresponding to the Earth's transition zone and lower mantle, with perovskite being the major constituent at depths >660 km. We present the first 17O NMR data for these materials and confirm previous 29Si work in the literature. The use of high-resolution multiple-quantum MAS (MQMAS) and satellite-transition MAS (STMAS) experiments allows us to resolve distinct oxygen species, and full assignments are suggested. The six polymorphs exhibit a wide variety of structure types, providing an ideal opportunity to consider the variation of NMR parameters (both shielding and quadrupolar) with local structure, including changes in coordination number, local geometry (bond distances and angles), and bonding. For example, we find that, although there is a general correlation of increasing 17O chemical shift with increasing Si-O bond length, the shift observed also depends upon the exact coordination environment.
NASA Astrophysics Data System (ADS)
Kubobuchi, Kei; Mogi, Masato; Matsumoto, Masashi; Baba, Teruhisa; Yogi, Chihiro; Sato, Chikai; Yamamoto, Tomoyuki; Mizoguchi, Teruyasu; Imai, Hideto
2016-10-01
X-ray absorption near edge structure (XANES) analysis is an element-specific method for proving electronic state mostly in the field of applied physics, such as battery and catalysis reactions, where the valence change plays an important role. In particular, many results have been reported for the analysis of positive electrode materials of Li-ion batteries, where multiple transition materials contribute to the reactions. However, XANES analysis has been limited to identifying the valence state simply in comparison with reference materials. When the shape of XANES spectra shows complicated changes, we were not able to identify the valence states or estimate the valence quantitatively, resulting in insufficient reaction analysis. To overcome such issues, we propose a valence state evaluation method using K- and L-edge XANES analysis with first-principles simulations. By using this method, we demonstrated that the complicated reaction mechanism of Li(Ni1/3Co1/3Mn1/3)O2 can be successfully analyzed for distinguishing each contribution of Ni, Co, Mn, and O to the redox reactions during charge operation. In addition to the XANES analysis, we applied resonant photoelectron spectroscopy (RPES) and diffraction anomalous fine structure spectroscopy (DAFS) with first-principles calculations to the reaction analysis of Co and Mn, which shows no or very little contribution to the redox. The combination of RPES and first-principles calculations successfully enables us to confirm the contribution of Co at high potential regions by electively observing Co 3d orbitals. Through the DAFS analysis, we deeply analyzed the spectral features of Mn K-edges and concluded that the observed spectral shape change for Mn does not originate from the valence change but from the change in distribution of wave functions around Mn upon Li extraction.
Valla, Maxence; Rossini, Aaron J; Caillot, Maxime; Chizallet, Céline; Raybaud, Pascal; Digne, Mathieu; Chaumonnot, Alexandra; Lesage, Anne; Emsley, Lyndon; van Bokhoven, Jeroen A; Copéret, Christophe
2015-08-26
Despite the widespread use of amorphous aluminosilicates (ASA) in various industrial catalysts, the nature of the interface between silica and alumina and the atomic structure of the catalytically active sites are still subject to debate. Here, by the use of dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) and density functional theory (DFT) calculations, we show that on silica and alumina surfaces, molecular aluminum and silicon precursors are, respectively, preferentially grafted on sites that enable the formation of Al(IV) and Si(IV) interfacial sites. We also link the genesis of Brønsted acidity to the surface coverage of aluminum and silicon on silica and alumina, respectively.
2015-01-01
Despite the widespread use of amorphous aluminosilicates (ASA) in various industrial catalysts, the nature of the interface between silica and alumina and the atomic structure of the catalytically active sites are still subject to debate. Here, by the use of dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) and density functional theory (DFT) calculations, we show that on silica and alumina surfaces, molecular aluminum and silicon precursors are, respectively, preferentially grafted on sites that enable the formation of Al(IV) and Si(IV) interfacial sites. We also link the genesis of Brønsted acidity to the surface coverage of aluminum and silicon on silica and alumina, respectively. PMID:26244620
Denden, I; Poineau, F; Schlegel, M L; Roques, J; Solari, P Lorenzo; Blain, G; Czerwinski, K R; Essehli, R; Barbet, J; Fattahi, M
2014-03-01
The effect of α-radiolysis on the behavior of heptavalent technetium has been investigated in 13 and 18 M H2SO4. Irradiation experiments were performed using α-particles ((4)He(2+), E = 68 MeV) generated by the ARRONAX cyclotron. UV-visible and X-ray absorption fine structure spectroscopic studies indicate that Tc(VII) is reduced to Tc(V) under α-irradiation. Extended X-ray absorption fine structure (EXAFS) spectroscopy measurements are consistent with the presence of mononuclear technetium sulfate complexes. Experimental results and density functional calculations show the formation of [TcO(HSO4)3(H2O)(OH)](-) and/or [TcO(HSO4)3(H2O)2] and [Tc(HSO4)3(SO4)(H2O)] and/or [Tc(HSO4)3(SO4)(OH)](-) for 13 and 18 M H2SO4, respectively.
Drużbicki, Kacper; Mikuli, Edward; Pałka, Norbert; Zalewski, Sławomir; Ossowska-Chruściel, Mirosława D
2015-01-29
The polymorphism of resorcinol has been complementary studied by combining Raman, time-domain terahertz, and inelastic neutron scattering spectroscopy with modern solid-state density functional theory (DFT) calculations. The spectral differences, emerging from the temperature-induced structural phase transition, have been successfully interpreted with an emphasis on the low-wavenumber range. The given interpretation is based on the plane-wave DFT computations, providing an excellent overall reproduction of both wavenumbers and intensities and revealing the source of the observed spectral differences. The performance of the generalized gradient approximation (GGA) functionals in prediction of the structural parameters and the vibrational spectra of the normal-pressure polymorphs of resorcinol has been extensively examined. The results show that the standard Perdew, Burke, and Ernzerhof (PBE) approach along with its "hard" revised form tends to be superior if compared to the "soft" GGA approximation.
Drużbicki, Kacper; Mikuli, Edward; Pałka, Norbert; Zalewski, Sławomir; Ossowska-Chruściel, Mirosława D
2015-01-29
The polymorphism of resorcinol has been complementary studied by combining Raman, time-domain terahertz, and inelastic neutron scattering spectroscopy with modern solid-state density functional theory (DFT) calculations. The spectral differences, emerging from the temperature-induced structural phase transition, have been successfully interpreted with an emphasis on the low-wavenumber range. The given interpretation is based on the plane-wave DFT computations, providing an excellent overall reproduction of both wavenumbers and intensities and revealing the source of the observed spectral differences. The performance of the generalized gradient approximation (GGA) functionals in prediction of the structural parameters and the vibrational spectra of the normal-pressure polymorphs of resorcinol has been extensively examined. The results show that the standard Perdew, Burke, and Ernzerhof (PBE) approach along with its "hard" revised form tends to be superior if compared to the "soft" GGA approximation. PMID:25564699
Valla, Maxence; Rossini, Aaron J; Caillot, Maxime; Chizallet, Céline; Raybaud, Pascal; Digne, Mathieu; Chaumonnot, Alexandra; Lesage, Anne; Emsley, Lyndon; van Bokhoven, Jeroen A; Copéret, Christophe
2015-08-26
Despite the widespread use of amorphous aluminosilicates (ASA) in various industrial catalysts, the nature of the interface between silica and alumina and the atomic structure of the catalytically active sites are still subject to debate. Here, by the use of dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) and density functional theory (DFT) calculations, we show that on silica and alumina surfaces, molecular aluminum and silicon precursors are, respectively, preferentially grafted on sites that enable the formation of Al(IV) and Si(IV) interfacial sites. We also link the genesis of Brønsted acidity to the surface coverage of aluminum and silicon on silica and alumina, respectively. PMID:26244620
NASA Astrophysics Data System (ADS)
Ahuja, Babu Lal; Sharma, Sonu; Heda, Narayan Lal; Tiwari, Shailja; Kumar, Kishor; Meena, Bhoor Singh; Bhatt, Samir
2016-05-01
We present the first-ever experimental Compton profiles (CPs) of Sc2O3 and Y2O3 using 740 GBq 137Cs Compton spectrometer. The experimental momentum densities have been compared with the theoretical CPs computed using linear combination of atomic orbitals (LCAO) within density functional theory (DFT). Further, the energy bands, density of states (DOS) and Mulliken's population (MP) data have been calculated using LCAO method with different exchange and correlation approximations. In addition, the energy bands, DOS, valence charge density (VCD), dielectric function, absorption coefficient and refractive index have also been computed using full potential linearized augmented plane wave (FP-LAPW) method with revised functional of Perdew-Becke-Ernzerhof for solids (PBEsol) and modified Becke Johnson (mBJ) approximations. Both the ab-initio calculations predict wide band gaps in Sc2O3 and Y2O3. The band gaps deduced from FP-LAPW (with mBJ) are found to be close to available experimental data. The VCD and MP data show more ionic character of Sc2O3 than Y2O3. The ceramic properties of both the sesquioxides are explained in terms of their electronic and optical properties.
NASA Astrophysics Data System (ADS)
Solovyev, I. V.; Kolodiazhnyi, T. V.
2016-09-01
We report results of joint experimental and theoretical studies on magnetoelectric (ME) compounds Co4Nb2O9 and Co4Ta2O9 . On the experimental side, we present results of the magnetization and dielectric permittivity measurements in the magnetic field. On the theoretical side, we construct the low-energy Hubbard-type model for the magnetically active Co 3 d bands in the Wannier basis, using the input of the first-principles electronic structure calculations, solve this model in the mean-field Hartree-Fock approximation, and evaluate the electric polarization in terms of the Berry phase theory. Both experimental and theoretical results suggest that Co4Ta2O9 is magnetically softer than Co4Nb2O9 . Therefore, it is reasonable to expect that the antiferromagnetic structure of Co4Ta2O9 can be easier deformed by the external magnetic field, yielding larger polarization. This trend is indeed reproduced by our theoretical calculations, but does not seem to be consistent with the experimental behavior of the polarization and dielectric permittivity. Thus, we suggest that there should be a hidden mechanism controlling the ME coupling in these compounds, probably related to the magnetic striction or a spontaneous change of the magnetic structure, which breaks the inversion symmetry. Furthermore, we argue that unlike in other ME systems (e.g., Cr2O3 ), in Co4Nb2O9 and Co4Ta2O9 there are two crystallographic sublattices, which contribute to the ME effect. These contributions are found to be of the opposite sign and tend to compensate each other. The latter mechanism can be also used to control and reverse the electric polarization in these compounds.
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
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
First-principles studies of boron nanostructures
NASA Astrophysics Data System (ADS)
Lau, Kah Chun
Boron is an 'electron deficient' element which has a rather fascinating chemical versatility. In the solid state, the elemental boron has neither a pure covalent nor a pure metallic character. As a result, its vast structural dimensionality and peculiar bonding features hold a unique place among other elements in the periodic table. In order to understand and properly describe these unusual bonding features, a detailed and systematic theoretical study is needed. In this work, I will show that some of the qualitative features of boron nanostructures, including clusters, sheets and nanotubes can easily be extracted from the results of first principles calculations based on density functional theory. Specifically, the size-dependent evolution of topological structures and bonding characteristics of boron clusters, Bn will be discussed. Based on the scenario observed in the boron clusters, the unique properties of boron sheets and boron nanotubes will be described. Moreover, the ballistic electron transport in single-walled boron nanotube relative to that of single-walled carbon nanotubes will be considered. It is expected that the theoretical results obtained in the present thesis will initiate further studies on boron nanostructures, which will be helpful in understanding, designing and realizing boron-based nanoscale devices.
Methods for First-Principles Alloy Thermodynamics
NASA Astrophysics Data System (ADS)
van de Walle, Axel
2013-11-01
Traditional first-principles calculations excel at providing formation energies at absolute zero, but obtaining thermodynamic information at nonzero temperatures requires suitable sampling of all the excited states visited in thermodynamic equilibrium, which would be computationally prohibitive via brute-force quantum mechanical calculations alone. In the context of solid-state alloys, this issue can be addressed via the coarse-graining concept and the cluster expansion formalism. This process generates simple, effective Hamiltonians that accurately reproduce quantum mechanical calculation results and that can be used to efficiently sample configurational, vibrational, and electronic excitations and enable the prediction of thermodynamic properties at nonzero temperatures. Vibrational and electronic degrees of freedom are formally eliminated from the problem by writing the system's partition function in a nested form in which the inner sums can be readily evaluated to yield an effective Hamiltonian. The remaining outermost sum corresponds to atomic configurations and can be handled via Monte Carlo sampling driven by the resulting effective Hamiltonian, thereby delivering thermodynamic properties at nonzero temperatures. This article describes these techniques and their implementation in the alloy theoretic automated toolkit, an open-source software package. The methods are illustrated by applications to various alloy systems.
Intrinsic ferroelectric switching from first principles.
Liu, Shi; Grinberg, Ilya; Rappe, Andrew M
2016-06-15
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
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
2015-01-01
17O NMR spectroscopy combined with first-principles calculations was employed to understand the local structure and dynamics of the phosphate ions and protons in the paraelectric phase of the proton conductor CsH2PO4. For the room-temperature structure, the results confirm that one proton (H1) is localized in an asymmetric H-bond (between O1 donor and O2 acceptor oxygen atoms), whereas the H2 proton undergoes rapid exchange between two sites in a hydrogen bond with a symmetric double potential well at a rate ≥107 Hz. Variable-temperature 17O NMR spectra recorded from 22 to 214 °C were interpreted by considering different models for the rotation of the phosphate anions. At least two distinct rate constants for rotations about four pseudo C3 axes of the phosphate ion were required in order to achieve good agreement with the experimental data. An activation energy of 0.21 ± 0.06 eV was observed for rotation about the P–O1 axis, with a higher activation energy of 0.50 ± 0.07 eV being obtained for rotation about the P–O2, P–O3d, and P–O3a axes, with the superscripts denoting, respectively, dynamic donor and acceptor oxygen atoms of the H-bond. The higher activation energy of the second process is most likely associated with the cost of breaking an O1–H1 bond. The activation energy of this process is slightly lower than that obtained from the 1H exchange process (0.70 ± 0.07 eV) (Kim, G.; Blanc, F.; Hu, Y.-Y.; Grey, C. P. J. Phys. Chem. C2013, 117, 6504−6515) associated with the translational motion of the protons. The relationship between proton jumps and phosphate rotation was analyzed in detail by considering uncorrelated motion, motion of individual PO4 ions and the four connected/H-bonded protons, and concerted motions of adjacent phosphate units, mediated by proton hops. We conclude that, while phosphate rotations aid proton motion, not all phosphate rotations result in proton jumps. PMID:25732257
First-principles and angle-resolved photoemission study of lithium doped metallic black phosphorous
NASA Astrophysics Data System (ADS)
Sanna, A.; Fedorov, A. V.; Verbitskiy, N. I.; Fink, J.; Krellner, C.; Petaccia, L.; Chikina, A.; Usachov, D. Yu; Grüneis, A.; Profeta, G.
2016-06-01
First principles calculations demonstrate the metallization of phosphorene by means of Li doping filling the unoccupied antibonding p z states. The electron–phonon coupling in the metallic phase is strong enough to eventually lead to a superconducting phase at T c = 17 K for LiP8 stoichiometry. Using angle-resolved photoemission spectroscopy we confirm that the surface of black phosphorus can be chemically functionalized using Li atoms which donate their 2s electron to the conduction band. The combined theoretical and experimental study demonstrates the semiconductor-metal transition indicating a feasible way to induce a superconducting phase in phosphorene and few-layer black phosphorus.
First-principles and angle-resolved photoemission study of lithium doped metallic black phosphorous
NASA Astrophysics Data System (ADS)
Sanna, A.; Fedorov, A. V.; Verbitskiy, N. I.; Fink, J.; Krellner, C.; Petaccia, L.; Chikina, A.; Usachov, D. Yu; Grüneis, A.; Profeta, G.
2016-06-01
First principles calculations demonstrate the metallization of phosphorene by means of Li doping filling the unoccupied antibonding p z states. The electron-phonon coupling in the metallic phase is strong enough to eventually lead to a superconducting phase at T c = 17 K for LiP8 stoichiometry. Using angle-resolved photoemission spectroscopy we confirm that the surface of black phosphorus can be chemically functionalized using Li atoms which donate their 2s electron to the conduction band. The combined theoretical and experimental study demonstrates the semiconductor-metal transition indicating a feasible way to induce a superconducting phase in phosphorene and few-layer black phosphorus.
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.
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
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 simulations of nanoelectronic devices
NASA Astrophysics Data System (ADS)
Maassen, Jesse
As the miniaturization of devices begins to reveal the atomic nature of materials, where chemical bonding and quantum effects are important, one must resort to a parameter-free theory for predictions. This thesis theoretically investigates the quantum transport properties of nanoelectronic devices using atomistic first principles. Our theoretical formalism employs density functional theory (DFT) in combination with Keldysh nonequilibrium Green's functions (NEGF). Self-consistently solving the DFT Hamiltonian with the NEGF charge density provides a way to simulate nonequilibrium systems without phenomenological parameters. This state-of-the-art technique was used to study three problems related to the field of nanoelectronics. First, we investigated the role of metallic contacts (Cu, Ni and Co) on the transport characteristics of graphene devices. With Cu, the graphene is simply electron-doped (Fermi level shift of ≈ -0.7 eV) which creates a unique signature in the conduction profile allowing one to extract the doping level. With Ni and Co, spin-dependent band gaps are formed in graphene's linear dispersion bands, thus leading to the prediction of high spin injection efficiencies reaching 60% and 80%, respectively. Second, we studied how controlled doping distributions in nano-scale Si transistors could suppress OFF-state leakage currents. By assuming the dopants (B and P) are confined in ≈ 1.1 nm regions in the channel, we discovered large conductance variations (GMAX/G MIN ˜ 105) as a function of the doping location. The largest fluctuations arise when the dopants are in the vicinity of the electrodes. Our results indicate that if the dopants are located away from the leads, a distance equal to 20% of the channel length, the tunneling current can be suppressed by a factor of 2 when compared to the case of uniform doping. Thus, controlled doping engineering is found to suppress device-to-device variations and lower the undesirable leakage current. Finally, we
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.
First principles determination of dislocation properties.
Hamilton, John C.
2003-12-01
This report details the work accomplished on first principles determination of dislocation properties. It contains an introduction and three chapters detailing three major accomplishments. First, we have used first principle calculations to determine the shear strength of an aluminum twin boundary. We find it to be remarkably small ({approx}17 mJ/m{sup 2}). This unexpected result is explained and will likely pertain for many other grain boundaries. Second, we have proven that the conventional explanation for finite grain boundary facets is wrong for a particular aluminum grain boundary. Instead of finite facets being stabilized by grain boundary stress, we find them to originate from kinetic effects. Finally we report on a new application of the Frenkel-Kontorova model to understand reconstructions of (100) type surfaces. In addition to the commonly accepted formation of rectangular dislocation arrays, we find numerous other possible solutions to the model including hexagonal reconstructions and a clock-rotated structure.
Interface Structure Prediction from First-Principles
Zhao, Xin; Shu, Qiang; Nguyen, Manh Cuong; Wang, Yangang; Ji, Min; Xiang, Hongjun; Ho, Kai-Ming; Gong, Xingao; Wang, Cai-Zhuang
2014-05-08
Information about the atomic structures at solid–solid interfaces is crucial for understanding and predicting the performance of materials. Due to the complexity of the interfaces, it is very challenging to resolve their atomic structures using either experimental techniques or computer simulations. In this paper, we present an efficient first-principles computational method for interface structure prediction based on an adaptive genetic algorithm. This approach significantly reduces the computational cost, while retaining the accuracy of first-principles prediction. The method is applied to the investigation of both stoichiometric and nonstoichiometric SrTiO3 Σ3(112)[1¯10] grain boundaries with unit cell containing up to 200 atoms. Several novel low-energy structures are discovered, which provide fresh insights into the structure and stability of the grain boundaries.
First-principles transversal DNA conductance deconstructed.
Zhang, X-G; Krstić, Predrag S; Zikić, Radomir; Wells, Jack C; Fuentes-Cabrera, Miguel
2006-07-01
First-principles calculation of the transverse conductance across DNA fragments placed between gold nanoelectrodes reveals that such conductance describes electron tunneling that depends critically on geometrical rather than electronic-structure properties. By factoring the first-principles result into two simple and approximately independent tunneling factors, we show that the conductances of the A, C, G, and T fragments differ only because of their sizes: the larger is the DNA base, the smaller its distance to the electrode, and the larger its conductance. Because the geometrical factors are difficult to control in an experiment, the direct-current measurements across DNA with gold contact electrodes may not be a convenient approach to DNA sequencing.
First-principles transversal DNA conductance deconstructed
Zhang, Xiaoguang; Krstic, Predrag; Zikic, Radomir; Wells, Jack C; Fuentes-Cabrera, Miguel A
2006-01-01
First-principles calculation of the transverse conductance across DNA fragments placed between gold nanoelectrodes, reveals that such conductance describes electron tunneling that depends critically on geometrical rather than electronic-structure properties. By factoring the first-principles result into two simple and approximately independent tunneling factors, we show that the conductances of the A, C, G, and T fragments differ only because of their sizes: the larger is the DNA base, the smaller is the distance that separates the electrode from the corresponding molecule, and the larger is its conductance. Because the geometrical factors are difficult to control in an experiment, the DC-current measurements across DNA may not be a convenient approach to DNA sequencing.
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.
First principles semiclassical calculations of vibrational eigenfunctions.
Ceotto, Michele; Valleau, Stéphanie; Tantardini, Gian Franco; Aspuru-Guzik, Alán
2011-06-21
Vibrational eigenfunctions are calculated on-the-fly using semiclassical methods in conjunction with ab initio density functional theory classical trajectories. Various semiclassical approximations based on the time-dependent representation of the eigenfunctions are tested on an analytical potential describing the chemisorption of CO on Cu(100). Then, first principles semiclassical vibrational eigenfunctions are calculated for the CO(2) molecule and its accuracy evaluated. The multiple coherent states initial value representations semiclassical method recently developed by us has shown with only six ab initio trajectories to evaluate eigenvalues and eigenfunctions at the accuracy level of thousands trajectory semiclassical initial value representation simulations. PMID:21837839
First principles semiclassical calculations of vibrational eigenfunctions
NASA Astrophysics Data System (ADS)
Ceotto, Michele; Valleau, Stéphanie; Gian Franco Tantardini, Aspuru-Guzik, Alán
2011-06-01
Vibrational eigenfunctions are calculated on-the-fly using semiclassical methods in conjunction with ab initio density functional theory classical trajectories. Various semiclassical approximations based on the time-dependent representation of the eigenfunctions are tested on an analytical potential describing the chemisorption of CO on Cu(100). Then, first principles semiclassical vibrational eigenfunctions are calculated for the CO2 molecule and its accuracy evaluated. The multiple coherent states initial value representations semiclassical method recently developed by us has shown with only six ab initio trajectories to evaluate eigenvalues and eigenfunctions at the accuracy level of thousands trajectory semiclassical initial value representation simulations.
Theoretical Calculations of Atomic Data for Spectroscopy
NASA Technical Reports Server (NTRS)
Bautista, Manuel A.
2000-01-01
Several different approximations and techniques have been developed for the calculation of atomic structure, ionization, and excitation of atoms and ions. These techniques have been used to compute large amounts of spectroscopic data of various levels of accuracy. This paper presents a review of these theoretical methods to help non-experts in atomic physics to better understand the qualities and limitations of various data sources and assess how reliable are spectral models based on those data.
Diffusion of aluminium in MgO from first principles
NASA Astrophysics Data System (ADS)
Ammann, M. W.; Brodholt, J. P.; Dobson, D. P.
2012-06-01
We have calculated the diffusivity of aluminium in periclase, MgO, under pressures relevant to deep planetary interiors from first principles. We reconcile differences between experimental migration enthalpies and those obtained with previous theoretical studies by finding a lower energy saddle point for the aluminium atom migration. Previous studies did not recognise a bifurcation at the saddle point. We also explain differences between experimental and theoretical binding enthalpies of an aluminium with a magnesium vacancy. We find that binding enthalpies continuously increase with decreasing aluminium concentrations, such that the difference between experimental and theoretical binding energies can be attributed to differing concentrations. We also find that binding energies increase with pressure as the permittivity decreases. Aluminium therefore not only causes extrinsic vacancy formation but also binds some of them, effectively removing them for magnesium diffusion. We discuss the implications for how other 3+ ions affect diffusion in oxides and silicates.
Theoretical Sum Frequency Generation Spectroscopy of Peptides
2015-01-01
Vibrational sum frequency generation (SFG) has become a very promising technique for the study of proteins at interfaces, and it has been applied to important systems such as anti-microbial peptides, ion channel proteins, and human islet amyloid polypeptide. Moreover, so-called “chiral” SFG techniques, which rely on polarization combinations that generate strong signals primarily for chiral molecules, have proven to be particularly discriminatory of protein secondary structure. In this work, we present a theoretical strategy for calculating protein amide I SFG spectra by combining line-shape theory with molecular dynamics simulations. We then apply this method to three model peptides, demonstrating the existence of a significant chiral SFG signal for peptides with chiral centers, and providing a framework for interpreting the results on the basis of the dependence of the SFG signal on the peptide orientation. We also examine the importance of dynamical and coupling effects. Finally, we suggest a simple method for determining a chromophore’s orientation relative to the surface using ratios of experimental heterodyne-detected signals with different polarizations, and test this method using theoretical spectra. PMID:25203677
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-Principle Calculations of Large Fullerenes.
Calaminici, Patrizia; Geudtner, Gerald; Köster, Andreas M
2009-01-13
State of-the-art density functional theory calculations have been performed for the large fullerenes C180, C240, C320, and C540 using the linear combination of Gaussian-type orbitals density functional theory (LCGTO-DFT) approach. For the calculations all-electron basis sets were employed. All fullerene structures were fully optimized without symmetry constrains. The analysis of the obtained structures as well as a study on the evolution of the bond lengths and calculated binding energies are presented. The fullerene results are compared to diamond and graphene which were calculated at the same level of theory. This represents the first systematic study on these large fullerenes based on nonsymmetry adapted first-principle calculations, and it demonstrates the capability of DFT calculations for energy and structure computations of large scale structures without any symmetry constraint.
Numerical inductance calculations based on first principles.
Shatz, Lisa F; Christensen, Craig W
2014-01-01
A method of calculating inductances based on first principles is presented, which has the advantage over the more popular simulators in that fundamental formulas are explicitly used so that a deeper understanding of the inductance calculation is obtained with no need for explicit discretization of the inductor. It also has the advantage over the traditional method of formulas or table lookups in that it can be used for a wider range of configurations. It relies on the use of fast computers with a sophisticated mathematical computing language such as Mathematica to perform the required integration numerically so that the researcher can focus on the physics of the inductance calculation and not on the numerical integration.
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.
First-principles simulations of thiophene oligomers
NASA Astrophysics Data System (ADS)
Scherlis, Damian; Marzari, Nicola
2003-03-01
Conducting polymers, extensively investigated for their use in electronic and nanotechnology applications, have recently gained prominence for their possible use as molecular actuators in mechanical and bioengineering devices. We have focused our efforts on thiophene-based compounds, a class of materials that can be designed for high stress generation and large linear displacement (actuation strain), ideally outperforming mammalian muscle. Key features for the development of these materials are the microscopic binding properties of thiophene and thiophene oligomers stacks, where applied electric fields lead to oxidation and enhanced pi-pi bonding. We have completed the structural studies of neutral and charged oligothiophene dimers, in the search for efficient dimerization mechanisms. A comparison between different density-functional and quantum-chemistry approaches is critically presented, as are solvation effects, described in this work with a combination of first-principles molecular dynamics and a QM/MM approach for the solvating medium.
Accurate Thermal Conductivities from First Principles
NASA Astrophysics Data System (ADS)
Carbogno, Christian
2015-03-01
In spite of significant research efforts, a first-principles determination of the thermal conductivity at high temperatures has remained elusive. On the one hand, Boltzmann transport techniques that include anharmonic effects in the nuclear dynamics only perturbatively become inaccurate or inapplicable under such conditions. On the other hand, non-equilibrium molecular dynamics (MD) methods suffer from enormous finite-size artifacts in the computationally feasible supercells, which prevent an accurate extrapolation to the bulk limit of the thermal conductivity. In this work, we overcome this limitation by performing ab initio MD simulations in thermodynamic equilibrium that account for all orders of anharmonicity. The thermal conductivity is then assessed from the auto-correlation function of the heat flux using the Green-Kubo formalism. Foremost, we discuss the fundamental theory underlying a first-principles definition of the heat flux using the virial theorem. We validate our approach and in particular the techniques developed to overcome finite time and size effects, e.g., by inspecting silicon, the thermal conductivity of which is particularly challenging to converge. Furthermore, we use this framework to investigate the thermal conductivity of ZrO2, which is known for its high degree of anharmonicity. Our calculations shed light on the heat resistance mechanism active in this material, which eventually allows us to discuss how the thermal conductivity can be controlled by doping and co-doping. This work has been performed in collaboration with R. Ramprasad (University of Connecticut), C. G. Levi and C. G. Van de Walle (University of California Santa Barbara).
Theoretical rovibrational spectroscopy of NO2+
NASA Astrophysics Data System (ADS)
Botschwina, P.; Bargholz, A.; Sebald, P.; Stein, C.; Schröder, B.; Oswald, R.
2015-05-01
Accurate near-equilibrium potential energy functions (PEFs) have been constructed for the nitronium ion (NO2+) by composite methods using either CCSD(T)-F12b or explicitly correlated multi-reference methods (MRCI-F12+Q or MRACPF-F12) as dominant contributions. Up to pentuple substitutions are required in the coupled-cluster based approach to reach convergence in the wavenumbers of the fundamentals to ca. 1 cm-1. These are predicted to be ν1 = 1386.0cm-1,ν2 = 621.1 cm-1 and ν3 = 2342.8 cm-1. All values differ significantly from the results of previous studies by zero-kinetic energy (ZEKE) spectroscopy and reanalysis or remeasurement is suggested. Compared to neon-matrix IR spectroscopic work of Jacox and coworkers the present calculations yield smaller wavenumbers of Δν3 = - 5.4 cm-1 and Δ (ν1 +ν3) = - 7.9 cm-1 so that blueshifting is predicted for those absorptions. The calculated isotopic shifts for both bands are in excellent agreement with the corresponding experimental values. Accurate values for rotational and centrifugal distortion constants of NO2+ in different vibrational states are predicted which should be of help in the search for forthcoming high-resolution spectra of that cation.
Ivanov, Sergei D; Witt, Alexander; Marx, Dominik
2013-07-01
Infrared spectroscopy is a powerful technique to unravel the structure and dynamics of molecular systems of ever increasing complexity. For isolated molecules in the gas phase theoretical approaches that directly rely on solving the Schrödinger equation, either approximately or quasi-exactly, are well established. A distinctly different approach to compute infrared spectra can be based on advanced molecular dynamics, itself being based on classical Newtonian dynamics, in conjunction with concurrent first principles electronic structure calculations. At variance with traditional methods, which are formulated in terms of the Schrödinger representation of quantum mechanics, the molecular dynamics approach stems from Heisenberg's representation and thus relies on computing thermal expectation values of time-correlation functions. Crucial in addition to generating the spectra themselves is their decomposition in terms of modes, which can be assigned to correlated atomic motion. This ab initio molecular dynamics route to compute infrared spectra, and its recent extension to quasiclassical techniques relying on approximate path integral dynamics, is covered in the review part of this Perspective. The usefulness of this unconventional approach, which can be generalized beyond infrared spectroscopy, is demonstrated in detail by applying the full machinery in computing and assigning the infrared spectra of protonated methane and its isotopologues. This particular molecule is often considered to be the most prominent member of the class of floppy or fluxional molecules. CH5(+) has been a longstanding challenge for theoretical infrared spectroscopy because it undergoes intricate large-amplitude motion, which is also reviewed. Molecular dynamics based infrared spectroscopy is general and can be applied to diverse systems such as molecular complexes in the gas phase, chromophores in biomolecular environments, and solute-solvent systems in the liquid phase.
First principles studies of multiferroic materials.
Picozzi, Silvia; Ederer, Claude
2009-07-29
Multiferroics, materials where spontaneous long-range magnetic and dipolar orders coexist, represent an attractive class of compounds, which combine rich and fascinating fundamental physics with a technologically appealing potential for applications in the general area of spintronics. Ab initio calculations have significantly contributed to recent progress in this area, by elucidating different mechanisms for multiferroicity and providing essential information on various compounds where these effects are manifestly at play. In particular, here we present examples of density-functional theory investigations for two main classes of materials: (a) multiferroics where ferroelectricity is driven by hybridization or purely structural effects, with BiFeO(3) as the prototype material, and (b) multiferroics where ferroelectricity is driven by correlation effects and is strongly linked to electronic degrees of freedom such as spin-, charge-, or orbital-ordering, with rare-earth manganites as prototypes. As for the first class of multiferroics, first principles calculations are shown to provide an accurate qualitative and quantitative description of the physics in BiFeO(3), ranging from the prediction of large ferroelectric polarization and weak ferromagnetism, over the effect of epitaxial strain, to the identification of possible scenarios for coupling between ferroelectric and magnetic order. For the second class of multiferroics, ab initio calculations have shown that, in those cases where spin-ordering breaks inversion symmetry (e.g. in antiferromagnetic E-type HoMnO(3)), the magnetically induced ferroelectric polarization can be as large as a few µC cm(-2). The examples presented point the way to several possible avenues for future research: on the technological side, first principles simulations can contribute to a rational materials design, aimed at identifying spintronic materials that exhibit ferromagnetism and ferroelectricity at or above room temperature. On
Towards Experimental Accuracy from the First Principles
NASA Astrophysics Data System (ADS)
Polyansky, O. L.; Lodi, L.; Tennyson, J.; Zobov, N. F.
2013-06-01
Producing ab initio ro-vibrational energy levels of small, gas-phase molecules with an accuracy of 0.10 cm^{-1} would constitute a significant step forward in theoretical spectroscopy and would place calculated line positions considerably closer to typical experimental accuracy. Such an accuracy has been recently achieved for the H_3^+ molecular ion for line positions up to 17 000 cm ^{-1}. However, since H_3^+ is a two-electron system, the electronic structure methods used in this study are not applicable to larger molecules. A major breakthrough was reported in ref., where an accuracy of 0.10 cm^{-1} was achieved ab initio for seven water isotopologues. Calculated vibrational and rotational energy levels up to 15 000 cm^{-1} and J=25 resulted in a standard deviation of 0.08 cm^{-1} with respect to accurate reference data. As far as line intensities are concerned, we have already achieved for water a typical accuracy of 1% which supersedes average experimental accuracy. Our results are being actively extended along two major directions. First, there are clear indications that our results for water can be improved to an accuracy of the order of 0.01 cm^{-1} by further, detailed ab initio studies. Such level of accuracy would already be competitive with experimental results in some situations. A second, major, direction of study is the extension of such a 0.1 cm^{-1} accuracy to molecules containg more electrons or more than one non-hydrogen atom, or both. As examples of such developments we will present new results for CO, HCN and H_2S, as well as preliminary results for NH_3 and CH_4. O.L. Polyansky, A. Alijah, N.F. Zobov, I.I. Mizus, R. Ovsyannikov, J. Tennyson, L. Lodi, T. Szidarovszky and A.G. Csaszar, Phil. Trans. Royal Soc. London A, {370}, 5014-5027 (2012). O.L. Polyansky, R.I. Ovsyannikov, A.A. Kyuberis, L. Lodi, J. Tennyson and N.F. Zobov, J. Phys. Chem. A, (in press). L. Lodi, J. Tennyson and O.L. Polyansky, J. Chem. Phys. {135}, 034113 (2011).
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.
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 thermoelasticity of bcc iron under pressure
NASA Astrophysics Data System (ADS)
Sha, Xianwei; Cohen, R. E.
2006-12-01
We investigate the elastic and isotropic aggregate properties of ferromagnetic bcc iron as a function of temperature and pressure by computing the Helmholtz free energies for the volume-conserving strained structures using the first-principles linear response linear-muffin-tin-orbital method and the generalized-gradient approximation. We include the electronic excitation contributions to the free energy from the band structures, and phonon contributions from quasiharmonic lattice dynamics. We make detailed comparisons between our calculated elastic moduli and their temperature and pressure dependences with available experimental and theoretical data. The isotropic aggregate sound velocities obtained based on the calculated elastic moduli agree with available ultrasonic and diamond-anvil-cell data. Birch’s law, which assumes a linear increase in sound velocity with increasing atomic density, fails for bcc Fe under extreme conditions. First-principles linear-response lattice dynamics is shown to provide a tractable approach to examine the elasticity of transition metals at high pressures and high temperatures.
Elasticity of serpentine: first principles investigation
NASA Astrophysics Data System (ADS)
Tsuchiya, J.; Tsuchiya, T.; Katayama, I.; Usui, Y.
2009-12-01
Serpentine is formed by reaction between peridotite and water which is released from hydrous mineral in subducting slab under pressure. Partially serpentinized peridotite may be a significant reservoir for water in the subducted cold slab and is considered to play an important role in subduction zone processes such as generation of arc magmatism. Precise determination of elastic properties of serpentine is essential for estimating the degree of serpentinization, and is important for investigating the transporting processes of water into deep Earth interior. Several studies on the degree of serpentinization have been reported so far based on limited experimental data on elasticity of serpentine (e.g. Bostock et al. 2002; Carlon and Miller 2003). Here we investigate by first principles calculation, the detailed structures and elastic properties of lizardite and antigorite which are lower and higher temperature polymorphs of serpentine up to 10 GPa at 2 GPa interval, and discuss the difference of compression mechanism and elasticity between these polymorphs. Our calculations are based on density functional theory within generalized gradient approximation for exchange correlation functional. We calculated the crystal structure and elasticity of antigorite m=16 polysome which contains 273 atoms in primitive cell (Capitani and Mellini 2006). We found the bulk modulus of antigorite and lizardite are almost same at ambient pressure, but they show significant difference at high pressure conditions. On the other hand, antigorite always has a few % larger shear modulus than lizardite presumably because of corrugated layer structure of antigorite. Combining the data from the present ab initio calculations, and seismological observations for the velocity and anisotropy structures of subduction zones, we analyzed the degree of serpentinization and possible water content in subducting slabs.
First Principles Dynamics of Photoexcited DNA and RNA Bases
Hudock, Hanneli R.; Levine, Benjamin G.; Thompson, Alexis L.; Martinez, Todd J.
2007-12-26
The reaction dynamics of excited electronic states in nucleic acid bases is a key process in DNA photodamage. Recent ultrafast spectroscopy experiments have shown multi-component decays of excited uracil and thymine, tentatively assigned to nonadiabatic transitions involving multiple electronic states. Using both quantum chemistry and first principles quantum molecular dynamics methods we show that a true minimum on the bright S{sub 2} electronic state is responsible for the first step which occurs on a femtosecond timescale. Thus the observed femtosecond decay does not correspond to surface crossing as previously thought. We suggest that subsequent barrier crossing to the minimal energy S{sub 2}/S{sub 1} conical intersection is responsible for the picosecond decay.
NASA Astrophysics Data System (ADS)
Koch-Müller, Monika; Jahn, Sandro; Birkholz, Natalie; Ritter, Eglof; Schade, Ulrich
2016-09-01
The stability of the high-pressure CaCO3 calcite (cc)-related polymorphs was studied in experiments that were performed in conventional diamond anvil cells (DAC) at room temperature as a function of pressure up to 30 GPa as well as in internally heated diamond anvil cells (DAC-HT) at pressures and temperatures up to 20 GPa and 800 K. To probe structural changes, we used Raman and FTIR spectroscopy. For the latter, we applied conventional and synchrotron mid-infrared as well as synchrotron far-infrared radiation. Within the cc-III stability field (2.2-15 GPa at room temperature, e.g., Catalli and Williams in Phys Chem Miner 32(5-6):412-417, 2005), we observed in the Raman spectra consistently three different spectral patterns: Two patterns at pressures below and above 3.3 GPa were already described in Pippinger et al. (Phys Chem Miner 42(1):29-43, 2015) and assigned to the phase transition of cc-IIIb to cc-III at 3.3 GPa. In addition, we observed a clear change between 5 and 6 GPa that is independent of the starting material and the pressure path and time path of the experiments. This apparent change in the spectral pattern is only visible in the low-frequency range of the Raman spectra—not in the infrared spectra. Complementary electronic structure calculations confirm the existence of three distinct stability regions of cc-III-type phases at pressures up to about 15 GPa. By combining experimental and simulation data, we interpret the transition at 5-6 GPa as a re-appearance of the cc-IIIb phase. In all types of experiments, we confirmed the transition from cc-IIIb to cc-VI at about 15 GPa at room temperature. We found that temperature stabilizes cc-VI to lower pressure. The reaction cc-IIIb to cc-VI has a negative slope of -7.0 × 10-3 GPa K-1. Finally, we discuss the possibility of the dense cc-VI phase being more stable than aragonite at certain pressure and temperature conditions relevant to the Earth's mantle.
Two-dimensional electronic spectroscopy using incoherent light: theoretical analysis.
Turner, Daniel B; Howey, Dylan J; Sutor, Erika J; Hendrickson, Rebecca A; Gealy, M W; Ulness, Darin J
2013-07-25
Electronic energy transfer in photosynthesis occurs over a range of time scales and under a variety of intermolecular coupling conditions. Recent work has shown that electronic coupling between chromophores can lead to coherent oscillations in two-dimensional electronic spectroscopy measurements of pigment-protein complexes measured with femtosecond laser pulses. A persistent issue in the field is to reconcile the results of measurements performed using femtosecond laser pulses with physiological illumination conditions. Noisy-light spectroscopy can begin to address this question. In this work we present the theoretical analysis of incoherent two-dimensional electronic spectroscopy, I((4)) 2D ES. Simulations reveal diagonal peaks, cross peaks, and coherent oscillations similar to those observed in femtosecond two-dimensional electronic spectroscopy experiments. The results also expose fundamental differences between the femtosecond-pulse and noisy-light techniques; the differences lead to new challenges and new opportunities.
Hadron Phenomenology from First-Principle QCD Studies
NASA Astrophysics Data System (ADS)
Papavassiliou, Joannis
2016-06-01
The form of the kernel that controls the dynamics of the Bethe-Salpeter equations is essential for obtaining quantitatively accurate predictions for the observable properties of hadrons. In the present work we briefly review the basic physical concepts and field-theoretic techniques employed in a first-principle derivation of a universal (process-independent) component of this kernel. This "top-down" approach combines nonperturbative ingredients obtained from lattice simulations and Dyson-Schwinger equations, and furnishes a renormalization-group invariant quark-gluon interaction strength, which is in excellent agreement with the corresponding quantity obtained from a systematic "bottom-up" treatment, where bound-state data are fitted within a well-defined truncation scheme.
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.
Adsorption of methylchloride on Si(100) from first principles
NASA Astrophysics Data System (ADS)
Romero, Aldo H.; Sbraccia, Carlo; Silvestrelli, Pier Luigi; Ancilotto, Francesco
2003-07-01
The chemisorption of methylchloride (CH3Cl) on Si(100) is studied from first principles. We find that, among a number of possible adsorption configurations, the lowest-energy structure is one in which the methylchloride molecule is dissociated into CH3 and Cl fragments which are bound to the two Si atoms of the same surface dimer. Our calculations show that dissociative chemisorption of methylchloride on Si(100) may proceed along different reaction paths characterized by different energy barriers that the system must overcome: some dissociation processes are mediated by a molecular precursor state and, at least in one case, we find that the dissociation process is nonactivated, in agreement with recent experimental findings. We have also generated, for many possible adsorption structures, theoretical scanning tunneling microscopy images which could facilitate the interpretation of experimental measurements.
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.
First-principles study of 2D electride : Gadolinium carbide
NASA Astrophysics Data System (ADS)
Nandadasa, Chandani; Kim, Seong-Gon; Kim, Sungho; Kim, Sung Wng
Electrides are an exclusive class of ionic compounds in which some electrons are occupying crystal voids instead of attaching to specific atoms or bonds. Using first-principles density functional theory calculations, we study structural, electronic and magnetic properties of Gd2C. The theoretically predicted structure of Gd2C is in good agreement with the available experimental data. Energy band diagram of Gd2C shows that they are crossing the Fermi level. Projected electronic density of states plots indicate that the interstitial sites are the main contributor to the density of states at the Fermi level. Charge of individual atoms including interstitial site are obtained using Bader analysis. Magnetic properties of Gd2C is determined from magnetization density plots. Work functions of Gd2C are determined for (001) and (100) surfaces with the technique of macroscopic average of electrostatic potential with the Fermi energy of bulk.
Modeling of Co overlayers on Pd (111) from first principles
NASA Astrophysics Data System (ADS)
Uba, S.; Uba, L.; Antonov, V. N.
2007-04-01
The electronic, magnetic and magneto-optical properties of Co overlayers on Pd (1 1 1) substrate have been investigated by ab initio band structure calculations within the spin-polarized relativistic linear muffin-thin orbitals (LMTO) method and supercell approach. The role of the Co-Pd interface structure, the number of the Co atomic layers ( n Co ), as well as the spin-orbit interaction and induced Pd spin polarization, in formation of magneto-optical response of the structures for [ n CoCo/Pd (1 1 1)] system is shown. The sign reversal of the polar Kerr rotation obtained theoretically with decreasing thickness of Co overlayers agrees well with experiment. We will demonstrate the effectiveness of the extended numeric modeling of magneto-optical properties from first principles.
A metallic superhard boron carbide: first-principles calculations.
Ma, Mengdong; Yang, Bingchao; Li, Zihe; Hu, Meng; Wang, Qianqian; Cui, Lin; Yu, Dongli; He, Julong
2015-04-21
A monoclinic BC3 phase (denoted M-BC3) has been predicted using first principles calculations. The M-BC3 structure is formed by alternately stacking sequences of metallic BC-layers and insulating C atom layers, thus, the structure exhibits two-dimensional conductivity. Its stability has been confirmed by our calculations of the total energy, elastic constants, and phonon frequencies. The pressure of phase transition from graphite-like BC3 to M-BC3 is calculated to be 9.3 GPa, and the theoretical Vickers hardness of M-BC3 is 43.8 GPa, this value indicates that the compound is a potentially superhard material. By comparing Raman spectral calculations of M-BC3 and previously proposed structures with the experimental data, we speculate that the experimentally synthesized BC3 crystal may simultaneously contain M-BC3 and Pmma-b phases.
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 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
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.
Optimized Materials From First Principles Simulations: Are We There Yet?
Galli, G; Gygi, F
2005-07-26
In the past thirty years, the use of scientific computing has become pervasive in all disciplines: collection and interpretation of most experimental data is carried out using computers, and physical models in computable form, with various degrees of complexity and sophistication, are utilized in all fields of science. However, full prediction of physical and chemical phenomena based on the basic laws of Nature, using computer simulations, is a revolution still in the making, and it involves some formidable theoretical and computational challenges. We illustrate the progress and successes obtained in recent years in predicting fundamental properties of materials in condensed phases and at the nanoscale, using ab-initio, quantum simulations. We also discuss open issues related to the validation of the approximate, first principles theories used in large scale simulations, and the resulting complex interplay between computation and experiment. Finally, we describe some applications, with focus on nanostructures and liquids, both at ambient and under extreme conditions.
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
Residential Care: Back to First Principles.
ERIC Educational Resources Information Center
Burns, David A.
Residential care must be redefined, free from jargon and rhetoric. Over the past 20 years, the social welfare approach, which encompasses the medical model, has dominated legislative and practical thinking about residential care. This theoretical thinking reached its culmination in the concept of the therapeutic community. The therapeutic…
NASA Astrophysics Data System (ADS)
Tohei, Tetsuya; Watanabe, Yuito; Lee, Hak-Sung; Ikuhara, Yuichi
2016-10-01
We have performed theoretical analysis of thermal expansion behavior of alumina crystals under finite temperature based on first principles phonon state calculations. Liner thermal expansion coefficients of a pure α-alumina crystal have been evaluated based on quasi-harmonic approximation including crystalline anisotropy. The Cr doping effect on the alumina crystal has also been examined and found that the doping can cause a noticeable change on the thermal expansion coefficient. The present results demonstrate that the first principles theoretical approach can be helpful for reproducing or predicting thermal expansion behaviors including dopant effects, which may pave a way for possible control of thermal expansion of materials by doping or alloying.
Shell Model in a First Principles Approach
Navratil, P; Nogga, A; Lloyd, R; Vary, J P; Ormand, W E; Barrett, B R
2004-01-08
We develop and apply an ab-initio approach to nuclear structure. Starting with the NN interaction, that fits two-body scattering and bound state data, and adding a theoretical NNN potential, we evaluate nuclear properties in a no-core approach. For presently feasible no-core model spaces, we evaluate an effective Hamiltonian in a cluster approach which is guaranteed to provide exact answers for sufficiently large model spaces and/or sufficiently large clusters. A number of recent applications are surveyed including an initial application to exotic multiquark systems.
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.
A first-principle calculation of the XANES spectrum of Cu{sup 2+} in water
La Penna, G.; Minicozzi, V.; Morante, S.; Stellato, F.; Rossi, G. C.
2015-09-28
The progress in high performance computing we are witnessing today offers the possibility of accurate electron density calculations of systems in realistic physico-chemical conditions. In this paper, we present a strategy aimed at performing a first-principle computation of the low energy part of the X-ray Absorption Spectroscopy (XAS) spectrum based on the density functional theory calculation of the electronic potential. To test its effectiveness, we apply the method to the computation of the X-ray absorption near edge structure part of the XAS spectrum in the paradigmatic, but simple case of Cu{sup 2+} in water. In order to keep into account the effect of the metal site structure fluctuations in determining the experimental signal, the theoretical spectrum is evaluated as the average over the computed spectra of a statistically significant number of simulated metal site configurations. The comparison of experimental data with theoretical calculations suggests that Cu{sup 2+} lives preferentially in a square-pyramidal geometry. The remarkable success of this approach in the interpretation of XAS data makes us optimistic about the possibility of extending the computational strategy we have outlined to the more interesting case of molecules of biological relevance bound to transition metal ions.
Theoretical Modeling of Various Spectroscopies for Cuprates and Topological Insulators
NASA Astrophysics Data System (ADS)
Basak, Susmita
structure, strong electron correlations (for cuprates) and spin-orbit coupling (for TIs) are included realistically in material-specific detail. Turning to the cuprates, in order to obtain a realistic description of various spectroscopies, one must include not only the effects of the matrix elements and the complexity of the crystal structure, but also of strong electronic correlations beyond the local density approximation (LDA)-based conventional picture, so that the physics of kinks, pseudogaps and superconductivity can be taken into account properly. In this connection, a self-consistent, intermediate coupling scheme informed by material-specific, first-principles band structures has been developed, where electron correlation effects beyond the LDA are incorporated via appropriate self-energy corrections to the electron and hole one-particle Green's functions. Here the antiferromagnetic (AFM) order is used as the simplest model of a competing order. A number of salient features of the resulting electronic spectrum and its energy, momentum and doping dependencies are in accord with experimental observations in electron as well as hole doped cuprates. This scheme thus provides a reasonable basis for undertaking a comprehensive, beyond-LDA level of modeling of various spectroscopies. The specific topics considered here are: (i) Origin of high-energy kink or the waterfall effect found in ARPES; (ii) Identification of the three energy scales observed in RIXS spectra as the pseudogap, charge transfer gap, and Mott gap; (iii) Evolution of the electron momentum densities with holedoping as seen in Compton scattering experiments. For three dimensional topological insulators, the ARPES and scanning tunneling microscopy (STM) spectra has been analyzed using a tight-binding model as well as a k · p model. The spin-orbit coupling, which is essential to produce the characteristic features of the surface states of a TI, is included realistically in the above models. In our
NASA Astrophysics Data System (ADS)
Saavedra Arias, Jose Javier
We have studied the properties of spinel and layered cathode materials for Li ion rechargeable batteries. The analysis was done by first principle calculations, and experimental techniques to elucidate materials that can substitute the presently commercialized material, namely LiCoO 2. We have studied the influence of Ni substitution for Mn in spinel Li 2MnO4. To understand the effects of this substitution on the crystal structure and electronic properties, first principle DFT calculations were performed using VASP. The substitution was done systematically for up to 25% of Mn replacement by Ni in a super cell configuration. Furthermore, the influence of Ni substitution on lithium hoping pathways between the two stable Li positions was also studied by first principle calculations in LiMn 2-xNixO4. These calculations revealed that Ni substitution for Mn in LiMn2O4 indeed improved Li ion mobility. Thereafter, systematic experimental studies were performed on LiMn 2-xNixO4 (0
First principles modelling of oxygen impurities in UN nuclear fuels
NASA Astrophysics Data System (ADS)
Kotomin, E. A.; Mastrikov, Yu. A.
2008-07-01
We report results of first principles VASP supercell calculations of O impurity in UN fuels placed either at an interstitial tetrahedral position or as a substitution for a host N ion. In the latter case O perfectly fits into N site producing no lattice distortion. Such the O substitutional impurity only slightly affects the formation energies of U and N vacancies nearby. In both interstitial and substitutional positions O atom attracts the additional electron density and transforms into the negatively charged ion. Oxygen incorporation into pre-existing N vacancy is energetically more favourable than into the interstitial position. The O impurities produce an additional peak at the low energy side of N contribution to the DOS calculated for uranium mononitride which could be used for the O identification by means of the UPS spectroscopy. We compare also the DOS calculated for UN and hypothetical isostructural UO. Both O solution and incorporation energies are negative, indicating that O penetration into UN fuel is the energetically favourable. The migration energy of the interstitial O ion is estimated as 2.8 eV.
First principles molecular dynamics study of filled ice hydrogen hydrate.
Zhang, Jingyun; Kuo, Jer-Lai; Iitaka, Toshiaki
2012-08-28
We investigated structural changes, phase diagram, and vibrational properties of hydrogen hydrate in filled-ice phase C(2) by using first principles molecular dynamics simulation. It was found that the experimentally reported "cubic" structure is unstable at low temperature and/or high pressure: The "cubic" structure reflects the symmetry at high (room) temperature where the hydrogen bond network is disordered and the hydrogen molecules are orientationally disordered due to thermal rotation. In this sense, the "cubic" symmetry would definitely be lowered at low temperature where the hydrogen bond network and the hydrogen molecules are expected to be ordered. At room temperature and below 30 GPa, it is the thermal effects that play an essential role in stabilizing the structure in "cubic" symmetry. Above 60 GPa, the hydrogen bonds in the framework would be symmetrized and the hydrogen bond order-disorder transition would disappear. These results also suggest the phase behavior of other filled-ice hydrates. In the case of rare gas hydrate, there would be no guest molecules' rotation-nonrotation transition since the guest molecules keep their spherical symmetry at any temperature. On the contrary methane hydrate MH-III would show complex transitions due to the lower symmetry of the guest molecule. These results would encourage further experimental studies, especially nuclear magnetic resonance spectroscopy and neutron scattering, on the phases of filled-ice hydrates at high pressures and/or low temperatures.
First principles molecular dynamics study of filled ice hydrogen hydrate
NASA Astrophysics Data System (ADS)
Zhang, Jingyun; Kuo, Jer-Lai; Iitaka, Toshiaki
2012-08-01
We investigated structural changes, phase diagram, and vibrational properties of hydrogen hydrate in filled-ice phase C2 by using first principles molecular dynamics simulation. It was found that the experimentally reported "cubic" structure is unstable at low temperature and/or high pressure: The "cubic" structure reflects the symmetry at high (room) temperature where the hydrogen bond network is disordered and the hydrogen molecules are orientationally disordered due to thermal rotation. In this sense, the "cubic" symmetry would definitely be lowered at low temperature where the hydrogen bond network and the hydrogen molecules are expected to be ordered. At room temperature and below 30 GPa, it is the thermal effects that play an essential role in stabilizing the structure in "cubic" symmetry. Above 60 GPa, the hydrogen bonds in the framework would be symmetrized and the hydrogen bond order-disorder transition would disappear. These results also suggest the phase behavior of other filled-ice hydrates. In the case of rare gas hydrate, there would be no guest molecules' rotation-nonrotation transition since the guest molecules keep their spherical symmetry at any temperature. On the contrary methane hydrate MH-III would show complex transitions due to the lower symmetry of the guest molecule. These results would encourage further experimental studies, especially nuclear magnetic resonance spectroscopy and neutron scattering, on the phases of filled-ice hydrates at high pressures and/or low temperatures.
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}.
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.
First principle investigation of isolated vacancy in (111) diamond surface
NASA Astrophysics Data System (ADS)
Hu, Wenhao; Flatté, Michael
As the simplest intrinsic defect, isolated vacancy has been studied intensively theoretically and experimentally in diamond's bulk phase. Nevertheless, its correspondence in surface phase still lacks people's attention. Nitrogen vacancy center has become the most ideal candidates for solid states computing due to its long coherence time at room temperature. Resembling NV center, the isolated vacancy on surface exhibits a similar ambient potential and the same 3-fold rotational symmetry due to the asymmetry of surface, which implies a similar character in them. In our work, the isolated vacancy in clean and hydrogen terminated (111) surface of diamond are investigated from first principle perspective. Full potential LAPW method implemented in WIEN2K is exploited under GGA approximation. To evaluated the surface effect, the defect depth from topmost layer to fifth subsurface are considered with different slab thickness. By checking the spin density distribution and electronic structure, the hydrogen vacancy in H-terminated surface exhibit a spin-1/2 center with a 0/-1 transition level located in the middle of band gap. The -1/-2 transition level of carbon vacancy in the subsurface approaches the 0/-1 transition level implying the potential stability of spin-1 center. The work was supported by an AFOSR MURI.
Semiconducting Graphene on Silicon from First-Principles Calculations.
Dang, Xuejie; Dong, Huilong; Wang, Lu; Zhao, Yanfei; Guo, Zhenyu; Hou, Tingjun; Li, Youyong; Lee, Shuit-Tong
2015-08-25
Graphene is a semimetal with zero band gap, which makes it impossible to turn electric conduction off below a certain limit. Transformation of graphene into a semiconductor has attracted wide attention. Owing to compatibility with Si technology, graphene adsorbed on a Si substrate is particularly attractive for future applications. However, to date there is little theoretical work on band gap engineering in graphene and its integration with Si technology. Employing first-principles calculations, we study the electronic properties of monolayer and bilayer graphene adsorbed on clean and hydrogen (H)-passivated Si (111)/Si (100) surfaces. Our calculation shows that the interaction between monolayer graphene and a H-passivated Si surface is weak, with the band gap remaining negligible. For bilayer graphene adsorbed onto a H-passivated Si surface, the band gap opens up to 108 meV owing to asymmetry introduction. In contrast, the interaction between graphene and a clean Si surface is strong, leading to formation of chemical bonds and a large band gap of 272 meV. Our results provide guidance for device designs based on integrating graphene with Si technology.
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.
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.
First-principles simulations at constant electric polarization
NASA Astrophysics Data System (ADS)
Dieguez, Oswaldo; Vanderbilt, David
2005-03-01
We develop a formalism to perform first-principles calculations for insulators at fixed electric polarization. As shown by Sai, Rabe, and Vanderbilt (SRV),ootnotetextN. Sai, K.M. Rabe, and D. Vanderbilt, Phys. Rev. B 66, 104108 (2002). such an approach allows one to map out the energy landscape as a function of polarization, providing a powerful tool for the theoretical investigation of polar materials. While the SRV method is only approximate because the effect of electric field is described using low-order Taylor expansions, our method is exact because we use the finite-fields approach of Souza, 'Iñiguez, and Vanderbilt.ootnotetextI. Souza, J. 'Iñiguez, and D. Vanderbilt, Phys. Rev. Lett. 89, 117602 (2002). We apply our method both to systems where the ionic contribution to the polarization dominates, and to systems where this is not the case. We show that the SRV method gives rather accurate results in the former case as expected, while the present exact method provides substantial improvements in the latter case.
NMR characterization of hydrocarbon adsorption on calcite surfaces: a first principles study.
Bevilaqua, Rochele C A; Rigo, Vagner A; Veríssimo-Alves, Marcos; Miranda, Caetano R
2014-11-28
The electronic and coordination environment of minerals surfaces, as calcite, are very difficult to characterize experimentally. This is mainly due to the fact that there are relatively few spectroscopic techniques able to detect Ca(2+). Since calcite is a major constituent of sedimentary rocks in oil reservoir, a more detailed characterization of the interaction between hydrocarbon molecules and mineral surfaces is highly desirable. Here we perform a first principles study on the adsorption of hydrocarbon molecules on calcite surface (CaCO3 (101¯4)). The simulations were based on Density Functional Theory with Solid State Nuclear Magnetic Resonance (SS-NMR) calculations. The Gauge-Including Projector Augmented Wave method was used to compute mainly SS-NMR parameters for (43)Ca, (13)C, and (17)O in calcite surface. It was possible to assign the peaks in the theoretical NMR spectra for all structures studied. Besides showing different chemical shifts for atoms located on different environments (bulk and surface) for calcite, the results also display changes on the chemical shift, mainly for Ca sites, when the hydrocarbon molecules are present. Even though the interaction of the benzene molecule with the calcite surface is weak, there is a clearly distinguishable displacement of the signal of the Ca sites over which the hydrocarbon molecule is located. A similar effect is also observed for hexane adsorption. Through NMR spectroscopy, we show that aromatic and alkane hydrocarbon molecules adsorbed on carbonate surfaces can be differentiated.
NMR characterization of hydrocarbon adsorption on calcite surfaces: A first principles study
Bevilaqua, Rochele C. A.; Miranda, Caetano R.; Rigo, Vagner A.; Veríssimo-Alves, Marcos
2014-11-28
The electronic and coordination environment of minerals surfaces, as calcite, are very difficult to characterize experimentally. This is mainly due to the fact that there are relatively few spectroscopic techniques able to detect Ca{sup 2+}. Since calcite is a major constituent of sedimentary rocks in oil reservoir, a more detailed characterization of the interaction between hydrocarbon molecules and mineral surfaces is highly desirable. Here we perform a first principles study on the adsorption of hydrocarbon molecules on calcite surface (CaCO{sub 3} (101{sup ¯}4)). The simulations were based on Density Functional Theory with Solid State Nuclear Magnetic Resonance (SS-NMR) calculations. The Gauge-Including Projector Augmented Wave method was used to compute mainly SS-NMR parameters for {sup 43}Ca, {sup 13}C, and {sup 17}O in calcite surface. It was possible to assign the peaks in the theoretical NMR spectra for all structures studied. Besides showing different chemical shifts for atoms located on different environments (bulk and surface) for calcite, the results also display changes on the chemical shift, mainly for Ca sites, when the hydrocarbon molecules are present. Even though the interaction of the benzene molecule with the calcite surface is weak, there is a clearly distinguishable displacement of the signal of the Ca sites over which the hydrocarbon molecule is located. A similar effect is also observed for hexane adsorption. Through NMR spectroscopy, we show that aromatic and alkane hydrocarbon molecules adsorbed on carbonate surfaces can be differentiated.
NASA Astrophysics Data System (ADS)
Li, Guo; Yan, Qimin; Zhou, Lan; Newhouse, Paul; Gregoire, John; Neaton, Jeffrey
To identify material phases in experimental combinatorial libraries, we develop a theoretical model as a complementary approach to accelerate phase identification. In this approach, samples in a combinatorial library are simulated as mixtures in chemical equilibria. Each of these mixtures contains all the solid-state phases, which can possibly exist in the library. Using the total energies of these phases obtained in first-principle calculations, we calculate the Gibbs free energy changes in the corresponding chemical reactions, and subsequently evaluate the equilibrium concentrations of the phases in every sample according to the law of mass action. Furthermore, to test this approach, we simulate pseudobinary libraries MnxV1-xOy and CuxV1-xOy. Interestingly, we find that the composition-dependent phase concentrations calculated within our approach agree well with the experimental results measured with XRD spectroscopy. This work supported by DOE (the JCAP under Award Number DE-SC0004993 and the Molecular Foundry of LBNL), and computational resources provided by NERSC.
NMR characterization of hydrocarbon adsorption on calcite surfaces: a first principles study.
Bevilaqua, Rochele C A; Rigo, Vagner A; Veríssimo-Alves, Marcos; Miranda, Caetano R
2014-11-28
The electronic and coordination environment of minerals surfaces, as calcite, are very difficult to characterize experimentally. This is mainly due to the fact that there are relatively few spectroscopic techniques able to detect Ca(2+). Since calcite is a major constituent of sedimentary rocks in oil reservoir, a more detailed characterization of the interaction between hydrocarbon molecules and mineral surfaces is highly desirable. Here we perform a first principles study on the adsorption of hydrocarbon molecules on calcite surface (CaCO3 (101¯4)). The simulations were based on Density Functional Theory with Solid State Nuclear Magnetic Resonance (SS-NMR) calculations. The Gauge-Including Projector Augmented Wave method was used to compute mainly SS-NMR parameters for (43)Ca, (13)C, and (17)O in calcite surface. It was possible to assign the peaks in the theoretical NMR spectra for all structures studied. Besides showing different chemical shifts for atoms located on different environments (bulk and surface) for calcite, the results also display changes on the chemical shift, mainly for Ca sites, when the hydrocarbon molecules are present. Even though the interaction of the benzene molecule with the calcite surface is weak, there is a clearly distinguishable displacement of the signal of the Ca sites over which the hydrocarbon molecule is located. A similar effect is also observed for hexane adsorption. Through NMR spectroscopy, we show that aromatic and alkane hydrocarbon molecules adsorbed on carbonate surfaces can be differentiated. PMID:25429955
Time-dependent first-principles approaches to PV materials
Miyamoto, Yoshiyuki
2013-12-10
Computational scheme for designing photovoltaic (PV) materials is presented. First-principles electron dynamics of photo-excitation and subsequent electron-hole splitting is performed based on the time-dependent density functional theory. Photo-induced enhancement of dipole moment was observed in a polar crystal and a donor-acceptor molecular pair. These experiences will pave a way to design PV material from first-principles simulations.
Transport and first-principles study of novel thermoelectric materials
NASA Astrophysics Data System (ADS)
Chi, Hang
Thermoelectric materials can recover waste industrial heat and convert it to electricity as well as provide efficient local cooling of electronic devices. The efficiency of such environmentally responsible and exceptionally reliable solid state energy conversion is determined by the dimensionless figure-of-merit ZT = alpha2 sigmaT/kappa, where alpha is the Seebeck coefficient, sigma is the electrical conductivity, kappa is the thermal conductivity, and T is the absolute temperature. The goal of the thesis is to (i) illustrate the physics to achieve high ZT of advanced thermoelectric materials and (ii) explore fundamental structure and transport properties in novel condensed matter systems, via an approach combining comprehensive experimental techniques and state-of-the-art first-principles simulation methods. Thermo-galvanomagnetic transport coefficients are derived from Onsager's reciprocal relations and evaluated via solving Boltzmann transport equation using Fermi-Dirac statistics, under the relaxation time approximation. Such understanding provides insights on enhancing ZT through two physically intuitive and very effective routes: (i) improving power factor PF = alpha2sigma; and (ii) reducing thermal conductivity kappa, as demonstrated in the cases of Mg2Si1-xSnx solid solution and Ge/Te double substituted skutterudites CoSb3(1-x)Ge1.5x Te1.5x, respectively. Motivated by recent theoretical predictions of enhanced thermoelectric performance in highly mismatched alloys, ZnTe:N molecular beam epitaxy (MBE) films deposited on GaAs (100) substrates are carefully examined, which leads to a surprising discovery of significant phonon-drag thermopower (reaching 1-2 mV/K-1) at ~13 K. Further systematic study in Bi2Te3 MBE thin films grown on sapphire (0001) and/or BaF2 (111) substrates, reveal that the peak of phonon drag can be tuned by the choice of substrates with different Debye temperatures. Moreover, the detailed transport and structure studies of Bi2-xTl xTe3
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.
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.
Theoretical analysis of single molecule spectroscopy lineshapes of conjugated polymers
NASA Astrophysics Data System (ADS)
Devi, Murali
Conjugated Polymers(CPs) exhibit a wide range of highly tunable optical properties. Quantitative and detailed understanding of the nature of excitons responsible for such a rich optical behavior has significant implications for better utilization of CPs for more efficient plastic solar cells and other novel optoelectronic devices. In general, samples of CPs are plagued with substantial inhomogeneous broadening due to various sources of disorder. Single molecule emission spectroscopy (SMES) offers a unique opportunity to investigate the energetics and dynamics of excitons and their interactions with phonon modes. The major subject of the present thesis is to analyze and understand room temperature SMES lineshapes for a particular CP, called poly(2,5-di-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (DEH-PPV). A minimal quantum mechanical model of a two-level system coupled to a Brownian oscillator bath is utilized. The main objective is to identify the set of model parameters best fitting a SMES lineshape for each of about 200 samples of DEH-PPV, from which new insight into the nature of exciton-bath coupling can be gained. This project also entails developing a reliable computational methodology for quantum mechanical modeling of spectral lineshapes in general. Well-known optimization techniques such as gradient descent, genetic algorithms, and heuristic searches have been tested, employing an L2 measure between theoretical and experimental lineshapes for guiding the optimization. However, all of these tend to result in theoretical lineshapes qualitatively different from experimental ones. This is attributed to the ruggedness of the parameter space and inadequateness of the L2 measure. On the other hand, when the dynamic reduction of the original parameter space to a 2-parameter space through feature searching and visualization of the search space paths using directed acyclic graphs(DAGs), the qualitative nature of the fitting improved significantly. For a more
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.
Lack of support for adaptive superstructure NiPt7 : Experiment and first-principles calculations
NASA Astrophysics Data System (ADS)
Schönfeld, B.; Engelke, M.; Ruban, A. V.
2009-02-01
Order and effective interaction parameters on the Pt-rich side of solid Ni-Pt alloys have been investigated by experimental and first-principles theoretical techniques. Diffuse x-ray scattering was taken from single-crystalline Ni-87.8at.%Pt aged at 603 K to set up a state of thermal equilibrium. From the separated short-range order scattering, effective pair interaction parameters were determined. These experimentally deduced values do not produce the suggested NiPt7 superstructure at lower temperatures. Instead of that, phase separation into NiPt3 regions with L12 structure and a Pt-rich matrix is observed in Monte Carlo simulations and supported by x-ray scattering of Ni-75.2at.%Pt . First-principles calculations at 0 K also show that the suggested NiPt7 phase is unstable against decomposition into NiPt3 and Pt.
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.
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.
Diagnosis: Reasoning from first principles and experiential knowledge
NASA Technical Reports Server (NTRS)
Williams, Linda J. F.; Lawler, Dennis G.
1987-01-01
Completeness, efficiency and autonomy are requirements for suture diagnostic reasoning systems. Methods for automating diagnostic reasoning systems include diagnosis from first principles (i.e., reasoning from a thorough description of structure and behavior) and diagnosis from experiential knowledge (i.e., reasoning from a set of examples obtained from experts). However, implementation of either as a single reasoning method fails to meet these requirements. The approach of combining reasoning from first principles and reasoning from experiential knowledge does address the requirements discussed above and can possibly ease some of the difficulties associated with knowledge acquisition by allowing developers to systematically enumerate a portion of the knowledge necessary to build the diagnosis program. The ability to enumerate knowledge systematically facilitates defining the program's scope, completeness, and competence and assists in bounding, controlling, and guiding the knowledge acquisition process.
First-principles study of complex material systems
NASA Astrophysics Data System (ADS)
He, Lixin
This thesis covers several topics concerning the study of complex materials systems by first-principles methods. It contains four chapters. A brief, introductory motivation of this work will be given in Chapter 1. In Chapter 2, I will give a short overview of the first-principles methods, including density-functional theory (DFT), planewave pseudopotential methods, and the Berry-phase theory of polarization in crystallines insulators. I then discuss in detail the locality and exponential decay properties of Wannier functions and of related quantities such as the density matrix, and their application in linear-scaling algorithms. In Chapter 3, I investigate the interaction of oxygen vacancies and 180° domain walls in tetragonal PbTiO3 using first-principles methods. Our calculations indicate that the oxygen vacancies have a lower formation energy in the domain wall than in the bulk, thereby confirming the tendency of these defects to migrate to, and pin, the domain walls. The pinning energies are reported for each of the three possible orientations of the original Ti--O--Ti bonds, and attempts to model the results with simple continuum models are discussed. CaCu3Ti4O12 (CCTO) has attracted a lot of attention recently because it was found to have an enormous dielectric response over a very wide temperature range. In Chapter 4, I study the electronic and lattice structure, and the lattice dynamical properties, of this system. Our first-principles calculations together with experimental results point towards an extrinsic mechanism as the origin of the unusual dielectric response.
First-Principles Modeling of Multiferroic RMn2O5
NASA Astrophysics Data System (ADS)
Cao, Kun; Guo, Guang-Can; Vanderbilt, David; He, Lixin
2009-12-01
We investigate the phase diagrams of RMn2O5 via a first-principles effective-Hamiltonian method. We are able to reproduce the most important features of the complicated magnetic and ferroelectric phase transitions. The calculated polarization as a function of temperature agrees very well with experiments. The dielectric-constant step at the commensurate-to-incommensurate magnetic phase transition is well reproduced. The microscopic mechanisms for the phase transitions are discussed.
Adherence of Model Molecules to Silica Surfaces: First Principle Calculations
NASA Astrophysics Data System (ADS)
Nuñez, Matías; Prado, Miguel Oscar
The adherence of "model molecules" methylene blue and eosine Y ("positive" and "negatively" charged respectively) to crystal SiO2 surfaces is studied from first principle calculations at the DFT level. Adsorption energies are calculated which follow the experimental threads obtained elsewhere (Rivera et al., 2013). We study the quantum nature of the electronic charge transfer between the surface and the molecules, showing the localized and delocalized patterns associated to the repulsive and attractive case respectively.
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
Ethanol adsorption on the Si (111) surface: First principles study
NASA Astrophysics Data System (ADS)
Gavrilenko, Alexander V.; Bonner, Carl E.; Gavrilenko, Vladimir I.
2012-03-01
Equilibrium atomic configurations and electron energy structure of ethanol adsorbed on the Si (111) surface are studied by the first principles density functional theory. Geometry optimization is performed by the total energy minimization method. Equilibrium atomic geometries of ethanol, both undissociated and dissociated, on the Si (111) surface are found and analysed. Reaction pathways and predicted transition states are discussed in comparison with available experimental data in terms of the feasibility of the reactions occurring. Analysis of atom and orbital resolved projected density of states indicates substantial modifications of the Si surface valence and conduction electron bands due to the adsorption of ethanol affecting the electronic properties of the surface.
First-principles study of graphene - carbon nanotube contacts
NASA Astrophysics Data System (ADS)
Cook, Brandon; Varga, Kalman
2012-02-01
The electron transport properties of carbon nanotube -- graphene junctions are investigated with first-principles total energy and electron transport calculations. By combining the advantageous material properties of graphene and nanotubes one can create all carbon hybrid architectures with properties that are particularly well suited to applications. The p-type Schottky barrier height is calculated in model junctions with (8,0) and (10,0) nanotubes in a top-contact configuration. Results indicate a lower barrier in carbon nanotube -- graphene junctions than in other carbon nanotue -- metal systems.
First-principles study of NO adsorbed Ni(100) surface.
Mu, X; Sun, X; Li, H M; Ding, Z J
2010-11-01
The geometric, electronic and magnetic properties of NO molecules adsorbed on the Ni(100) surface are investigated by the first-principles calculation on the basis of the density functional theory (DFT). The NO molecules are predicted to be chemisorbed at hollow site with an upright configuration at 0.125 ML and 0.5 ML coverages. After adsorption, the magnetic moment is significantly suppressed for surface Ni atom and almost quenched for NO molecule. This behavior can be reasonably explained by the difference of the backdonation process between the spin-up and spin-down electronic states, which is demonstrated by the spin-resolved differential charge density map.
Electromagnetic Response of 12C: A First-Principles Calculation
NASA Astrophysics Data System (ADS)
Lovato, A.; Gandolfi, S.; Carlson, J.; Pieper, Steven C.; Schiavilla, R.
2016-08-01
The longitudinal and transverse electromagnetic response functions of 12C 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 the measured versus calculated longitudinal response. This is further corroborated by a reanalysis of the Coulomb sum rule, in which the contributions from the low-lying Jπ=2+, 02+ (Hoyle), and 4+ states in 12 are accounted for explicitly in evaluating the total inelastic strength.
Electromagnetic Response of ^{12}C: A First-Principles Calculation.
Lovato, A; Gandolfi, S; Carlson, J; Pieper, Steven C; Schiavilla, R
2016-08-19
The longitudinal and transverse electromagnetic response functions of ^{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 the measured versus calculated longitudinal response. This is further corroborated by a reanalysis of the Coulomb sum rule, in which the contributions from the low-lying J^{π}=2^{+}, 0_{2}^{+} (Hoyle), and 4^{+} states in ^{12}C are accounted for explicitly in evaluating the total inelastic strength. PMID:27588850
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.
Oxygen diffusion in hcp metals from first principles
NASA Astrophysics Data System (ADS)
Wu, Henry H.; Wisesa, Pandu; Trinkle, Dallas R.
2016-07-01
Oxygen interstitial site energies and migration barriers in 15 hexagonal close-packed (hcp) metals have been calculated with first-principles density functional theory. Multiple hcp systems show a preference for the hexahedral site over the tetrahedral site, as well as a stable crowdion site. More surprisingly, in more than half of the hcp systems, the oxygen does not choose the large octahedral interstitial as its ground state. We explain this result based on the effective valence of the metal from crystal-field splitting and the c /a ratio. Diffusion constants for oxygen in all 15 hcp systems are calculated from analytically derived diffusion equations and match available experimental data.
Comparative study of Ti and Ni clusters from first principles
Lee, B; Lee, G W
2007-08-20
Icosahedral clusters in Ti and Ni are studied with first-principles density functional calculations. We find significant distortion on the Ti icosahedron caused by the strong interaction between surface atoms on the icosahedron but not between the center atom and surface atoms, whereas no such distortion is observed on Ni clusters. In addition, distortion becomes more severe when atoms are added to the Ti13 cluster resulting in short bonds. Such distorted icosahedra having short bonds are essentially to explain the structure factor of Ti liquid obtained in experiment.
First-principles theory of multipolar order in neptunium dioxide
NASA Astrophysics Data System (ADS)
Suzuki, M.-T.; Magnani, N.; Oppeneer, P. M.
2010-12-01
We provide a first-principles, materials-specific theory of multipolar order and superexchange in NpO2 by means of a noncollinear local-density approximation +U (LDA+U) method. Our calculations offer a precise microscopic description of the triple- q antiferro ordered phase in the absence of any dipolar moment. We find that, while the most common nondipolar degrees of freedom (e.g., electric quadrupoles and magnetic octupoles) are active in the ordered phase, both the usually neglected higher-order multipoles (electric hexadecapoles and magnetic triakontadipoles) have at least an equally significant effect.
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.
Accurately Predicting Complex Reaction Kinetics from First Principles
NASA Astrophysics Data System (ADS)
Green, William
Many important systems contain a multitude of reactive chemical species, some of which react on a timescale faster than collisional thermalization, i.e. they never achieve a Boltzmann energy distribution. Usually it is impossible to fully elucidate the processes by experiments alone. Here we report recent progress toward predicting the time-evolving composition of these systems a priori: how unexpected reactions can be discovered on the computer, how reaction rates are computed from first principles, and how the many individual reactions are efficiently combined into a predictive simulation for the whole system. Some experimental tests of the a priori predictions are also presented.
Collective Modes in Light Nuclei from First Principles
NASA Astrophysics Data System (ADS)
Dytrych, T.; Launey, K. D.; Draayer, J. P.; Maris, P.; Vary, J. P.; Saule, E.; Catalyurek, U.; Sosonkina, M.; Langr, D.; Caprio, M. A.
2013-12-01
Results for ab initio no-core shell model calculations in a symmetry-adapted SU(3)-based coupling scheme demonstrate that collective modes in light nuclei emerge from first principles. The low-lying states of Li6, Be8, and He6 are shown to exhibit orderly patterns that favor spatial configurations with strong quadrupole deformation and complementary low intrinsic spin values, a picture that is consistent with the nuclear symplectic model. The results also suggest a pragmatic path forward to accommodate deformation-driven collective features in ab initio analyses when they dominate the nuclear landscape.
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.
Spin Crossover in Ferropericlase from First-Principles Molecular Dynamics
NASA Astrophysics Data System (ADS)
Holmström, E.; Stixrude, L.
2015-03-01
Ferropericlase, (Mg,Fe)O, is the second-most abundant mineral of Earth's lower mantle. With increasing pressure, the Fe ions in the material begin to collapse from a magnetic to nonmagnetic spin state. We present a finite-temperature first-principles phase diagram of this spin crossover, finding a broad pressure range with coexisting magnetic and nonmagnetic ions due to favorable enthalpy of mixing of the two. Furthermore, we find the electrical conductivity of the mineral to reach semimetallic values inside Earth.
Spin crossover in ferropericlase from first-principles molecular dynamics.
Holmström, E; Stixrude, L
2015-03-20
Ferropericlase, (Mg,Fe)O, is the second-most abundant mineral of Earth's lower mantle. With increasing pressure, the Fe ions in the material begin to collapse from a magnetic to nonmagnetic spin state. We present a finite-temperature first-principles phase diagram of this spin crossover, finding a broad pressure range with coexisting magnetic and nonmagnetic ions due to favorable enthalpy of mixing of the two. Furthermore, we find the electrical conductivity of the mineral to reach semimetallic values inside Earth. PMID:25839305
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.
NASA Astrophysics Data System (ADS)
Chandel, Surjeet Kumar; Kumar, Arun; Bharti, Ankush; Sharma, Raman
2015-05-01
Using first principles density functional theoretical calculations, the present paper reports a systematic study of phonon dispersion curves in pristine carbon (CNT) and silicon nanotubes (SiNT) having chirality (6,6) in the armchair configuration. Some of the phonon modes are found to have negative frequencies which leads to instability of the systems under study. The number of phonon branches has been found to be thrice as much as the number of atoms. The frequency of the higher optical bands varies from 1690 to 1957 cm-1 for CNT(6,6) while it is 596 to 658 cm-1 for SiNT.
First-principles investigation of high pressure Pbca phase of carbon mononitride
NASA Astrophysics Data System (ADS)
Wei, Qun; Zhang, Meiguang; Yan, Haiyan
2016-09-01
A theoretical investigations on the stability, mechanical and electronic properties of Pbca-CN was performed by using first principle calculations. According to our calculations, Pbca-CN exhibits a large elastic anisotropy. The further mechanical calculations demonstrated that Pbca-CN shows high elastic moduli. Young's modulus of Pbca-CN is found to reach a maximum along [001] direction and a minimum along [100] direction. The ideal tensile and shear strength at large strains of Pbca-CN are also examined. The ideal shear strength along the weakest (100)[010] slip system is about 20 GPa, which shows Pbca-CN is not an intrinsic superhard material.
Electronic and magnetic properties of a full-Heusler alloy Co2CrGe: a first-principles study
NASA Astrophysics Data System (ADS)
Rai, D. P.; Shankar, A.; Sandeep; Ghimire, M. P.; Thapa, R. K.
2013-01-01
The structural, electronic, and magnetic properties of Co2CrGe, a Heusler alloy, have been evaluated by first-principles density functional theory and compared with the known experimental and theoretical results. Generalized gradient approximation is used for structural study, whereas local spin density approximation is used for electronic calculation. First-principles structure optimizations were done through total energy calculations at 0 K using the full-potential linearized augmented plane wave method as implemented in the WIEN2K code.
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
Magnetization of iron clusters from first-principles calculations
NASA Astrophysics Data System (ADS)
Chelikowsky, James R.; Tiago, Murilo L.; Li, Shen; Alemany, Manuel M. G.; Zhou, Yunkai; Saad, Yousef
2006-03-01
The magnetic moment of Fe clusters as function of number of atoms has been observed to show a slow decrease from the isolated atom value (4 Bohr magnetons) to its bulk value of 2.2 Bohr magnetons per atom. In addition, a series of peaks has been observed, for which the causes are not yet fully understood (see Billas, Chatelain, and de Heer, Science, 1994). We analyze the dependence of total magnetic moment, local magnetic moment, cohesive energy and other physical quantities in iron clusters Fen ( 1 < n < 250 ), and compare these results with available experimental data. We use a real-space method, pseudopotentials and first-principles DFT to obtain the properties of the cluster in its ground state. Calculations are done using the PARSEC code ( www.ices.utexas.edu/parsec ). We also discuss some of the recently developed capabilities of PARSEC.
First-principles study of interface doping in ferroelectric junctions.
Wang, Pin-Zhi; Cai, Tian-Yi; Ju, Sheng; Wu, Yin-Zhong
2016-04-11
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.
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
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.
Two Dimensional Ice from First Principles: Structures and Phase Transitions.
Chen, Ji; Schusteritsch, Georg; Pickard, Chris J; Salzmann, Christoph G; Michaelides, Angelos
2016-01-15
Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here, we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression, the pentagonal structure becomes the most stable and persists up to ∼2 GPa, at which point the square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and the width.
Defects in AIN/GaN Superlattice: First Principle Calculations.
Rao, Xue; Wang, Ru-Zhi; Cao, Juexian; Yan, Hui
2016-01-01
In this paper we investigate the atomic configurations, electronic structure and formation energies of native point defects, (such as vacancies and self-interstitials), in an AIN/GaN superlattice (SL) constructed on a wurtzite structure along a [0001] growth direction. Comprehensive first-principle calculations based on the density functional theory (DFT) are used. Cation and anion vacancies in the neutral charge state are calculated. For the native defects, the results showed that the most favorable configurations are the cation vacancies at the interface of the SL, or the anion vacancies in the GaN wells. Considering the formation energies of different vacancies, the results show that the nitrogen vacancy has the lowest formation energy, indicating that they are significantly the most stable configuration, and thus should be expected to be the major defect in a AIN/GaN superlattice. PMID:27398499
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.
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 of thermal properties of borophene.
Sun, Hongyi; Li, Qingfang; Wan, X G
2016-06-01
Very recently, a new single-element two-dimensional (2D) material borophene was successfully grown on a silver surface under pristine ultrahigh vacuum conditions which attracts tremendous interest. In this paper, the lattice thermal conductivity, phonon lifetimes, thermal expansion and temperature dependent elastic moduli of borophene are systematically studied by using first-principles. Our simulations show that borophene possesses unique thermal properties. Strong phonon-phonon scattering is found in borophene, which results in its unexpectedly low lattice thermal conductivity. Thermal expansion coefficients along both the armchair and zigzag directions of borophene show impressive negative values. More strikingly, the elastic moduli are sizably strengthened as temperature increases, and the negative in-plane Poisson's ratios are found along both the armchair and zigzag directions at around 120 K. The mechanisms of these unique thermal properties are also discussed in this paper. PMID:27188523
Electromagnetic response of C12 : A first-principles calculation
Lovato, A.; Gandolfi, S.; Carlson, J.; Pieper, Steven C.; Schiavilla, R.
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
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
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.
Towards a first-principles thermodynamics of solids
de Fontaine, D.; Wolverton, C.
1992-08-01
Total energy density-functional methods have made it possible to calculate, from first principles, such important properties as cohesive energies, lattice constants and elastic moduli for elemental crystals and perfectly ordered compounds. Real solids are imperfect, however, so that lattice vibrations and compositional disorder lead to entropy contributions, vibrational and configurational. When these effects are included in an appropriate manner, properties of real crystals can be computed ab initio as a function of temperature and concentration. Consequently, it is possible to obtain, virtually from the knowledge of atomic numbers alone, such basic thermodynamic properties as free energies, entropies, heats of formation, and lattice parameters for stable and metastable phases, leading, for example, to the successful computation of certain classes of phase diagrams. Recent progress in the field will be reviewed. Application is made to the Pd-Rh-V system.
First-principles investigation of PVDF and its copolymers
NASA Astrophysics Data System (ADS)
Ranjan, V.; Yu, Liping; Buongiorno Nardelli, Marco; Bernholc, J.
2009-03-01
Recently, PVDF and its copolymers have generated significant interest due to their electroactive properties [1] and potential for ultra-high energy-storage applications [2]. In this talk, we present the results of first-principles calculations of stable phases and dielectric properties of different copolymers and terpolymers of PVDF at varying concentrations. Our results show that at very high concentrations of Chloro-trifluoroethylene (CTFE), PVDF/CTFE displays sharp transitions between non-polar (α) and polar (β) phases. On the contrary, the same transitions in copolymers with trifluoroethylene (TrFE) and tetrafluoroethylene (TeFE) are not sharp and happen at lower concentrations. We discuss the interplay of copolymer admixture on the dielectric properties of PVDF and discuss the suitability of copolymers for energy storage and electroactive applications. [1] S. G. Lu et al., App. Phys. Lett. 93, 042905 (2008). [2] V. Ranjan et al., Phys. Rev. Lett. 99, 047801 (2007).
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.
First-principles semiclassical initial value representation molecular dynamics.
Ceotto, Michele; Atahan, Sule; Shim, Sangwoo; Tantardini, Gian Franco; Aspuru-Guzik, Alán
2009-05-28
In this work, we explore the use of the semiclassical initial value representation (SC-IVR) method with first-principles electronic structure approaches to carry out classical molecular dynamics. The proposed approach can extract the vibrational power spectrum of carbon dioxide from a single trajectory providing numerical results that agree with experiment and quantum calculations. The computational demands of the method are comparable to those of classical single-trajectory calculations, while describing uniquely quantum features such as the zero-point energy and Fermi resonances. The method can also be used to identify symmetry properties of given vibrational peaks and investigate vibrational couplings by selected classical trajectories. The accuracy of the method degrades for the reproduction of anharmonic shifts for high-energy vibrational levels. PMID:19440613
NMR shifts for polycyclic aromatic hydrocarbons from first-principles
Thonhauser, Timo; Ceresoli, Davide; Marzari, Nicola N.
2009-09-03
We present first-principles, density-functional theory calculations of the NMR chemical shifts for polycyclic aromatic hydrocarbons, starting with benzene and increasing sizes up to the one- and two-dimensional infinite limits of graphene ribbons and sheets. Our calculations are performed using a combination of the recently developed theory of orbital magnetization in solids, and a novel approach to NMR calculations where chemical shifts are obtained from the derivative of the orbital magnetization with respect to a microscopic, localized magnetic dipole. Using these methods we study on equal footing the 1H and 13C shifts in benzene, pyrene, coronene, in naphthalene, anthracene, naphthacene, and pentacene, and finally in graphene, graphite, and an infinite graphene ribbon. Our results show very good agreement with experiments and allow us to characterize the trends for the chemical shifts as a function of system size.
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.
Two Dimensional Ice from First Principles: Structures and Phase Transitions
NASA Astrophysics Data System (ADS)
Chen, Ji; Schusteritsch, Georg; Pickard, Chris J.; Salzmann, Christoph G.; Michaelides, Angelos
2016-01-01
Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here, we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression, the pentagonal structure becomes the most stable and persists up to ˜2 GPa , at which point the square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and the width.
2D ice from first principles: structures and phase transitions
NASA Astrophysics Data System (ADS)
Chen, Ji; Schusteritsch, Georg; Pickard, Chris J.; Salzmann, Christoph G.; Michaelides, Angelos
Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression the pentagonal structure becomes the most stable and persists up to ca. 2 GPa at which point square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. We also find a double layer AA stacked square ice phase, which clarifies the difference between experimental observations and earlier force field simulations. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and width.
Two Dimensional Ice from First Principles: Structures and Phase Transitions.
Chen, Ji; Schusteritsch, Georg; Pickard, Chris J; Salzmann, Christoph G; Michaelides, Angelos
2016-01-15
Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here, we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression, the pentagonal structure becomes the most stable and persists up to ∼2 GPa, at which point the square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and the width. PMID:26824547
First-principles simulations of electrostatic interactions between dust grains
Itou, H. Amano, T.; Hoshino, M.
2014-12-15
We investigated the electrostatic interaction between two identical dust grains of an infinite mass immersed in homogeneous plasma by employing first-principles N-body simulations combined with the Ewald method. We specifically tested the possibility of an attractive force due to overlapping Debye spheres (ODSs), as was suggested by Resendes et al. [Phys. Lett. A 239, 181–186 (1998)]. Our simulation results demonstrate that the electrostatic interaction is repulsive and even stronger than the standard Yukawa potential. We showed that the measured electric field acting on the grain is highly consistent with a model electrostatic potential around a single isolated grain that takes into account a correction due to the orbital motion limited theory. Our result is qualitatively consistent with the counterargument suggested by Markes and Williams [Phys. Lett. A 278, 152–158 (2000)], indicating the absence of the ODS attractive force.
First-Principles Prediction of Liquid/Liquid Interfacial Tension.
Andersson, M P; Bennetzen, M V; Klamt, A; Stipp, S L S
2014-08-12
The interfacial tension between two liquids is the free energy per unit surface area required to create that interface. Interfacial tension is a determining factor for two-phase liquid behavior in a wide variety of systems ranging from water flooding in oil recovery processes and remediation of groundwater aquifers contaminated by chlorinated solvents to drug delivery and a host of industrial processes. Here, we present a model for predicting interfacial tension from first principles using density functional theory calculations. Our model requires no experimental input and is applicable to liquid/liquid systems of arbitrary compositions. The consistency of the predictions with experimental data is significant for binary, ternary, and multicomponent water/organic compound systems, which offers confidence in using the model to predict behavior where no data exists. The method is fast and can be used as a screening technique as well as to extend experimental data into conditions where measurements are technically too difficult, time consuming, or impossible.
Vibrational signatures in the THz spectrum of 1,3-DNB: A first-principles and experimental study
NASA Astrophysics Data System (ADS)
Ahmed, Towfiq; Azad, Abul K.; Chellappa, Raja; Higginbotham-Duque, Amanda; Dattelbaum, Dana M.; Zhu, Jian-Xin; Moore, David; Graf, Matthias J.
2016-05-01
Understanding the fundamental processes of light-matter interaction is important for detection of explosives and other energetic materials, which are active in the infrared and terahertz (THz) region. We report a comprehensive study on electronic and vibrational lattice properties of structurally similar 1,3-dinitrobenzene (1,3-DNB) crystals through first-principles electronic structure calculations and THz spectroscopy measurements on polycrystalline samples. Starting from reported x-ray crystal structures, we use density-functional theory (DFT) with periodic boundary conditions to optimize the structures and perform linear response calculations of the vibrational properties at zero phonon momentum. The theoretically identified normal modes agree qualitatively with those obtained experimentally in a frequency range up to 2.5 THz and quantitatively at much higher frequencies. The latter frequencies are set by intra-molecular forces. Our results suggest that van der Waals dispersion forces need to be included to improve the agreement between theory and experiment in the THz region, which is dominated by intermolecular modes and sensitive to details in the DFT calculation. An improved comparison is needed to assess and distinguish between intra- and intermolecular vibrational modes characteristic of energetic materials.
Pascal, Tod A; Boesenberg, Ulrike; Kostecki, Robert; Richardson, Thomas J; Weng, Tsu-Chien; Sokaras, Dimosthenis; Nordlund, Dennis; McDermott, Eamon; Moewes, Alexander; Cabana, Jordi; Prendergast, David
2014-01-21
We elucidate the role of room-temperature-induced instantaneous structural distortions in the Li K-edge X-ray absorption spectra (XAS) of crystalline LiF, Li2SO4, Li2O, Li3N, and Li2CO3 using high resolution X-ray Raman spectroscopy (XRS) measurements and first-principles density functional theory calculations within the eXcited electron and Core Hole approach. Based on thermodynamic sampling via ab initio molecular dynamics simulations, we find calculated XAS in much better agreement with experiment than those computed using the rigid crystal structure alone. We show that local instantaneous distortion of the atomic lattice perturbs the symmetry of the Li 1s core-excited-state electronic structure, broadening spectral line-shapes and, in some cases, producing additional spectral features. The excellent agreement with high-resolution XRS measurements validates the accuracy of our first-principles approach to simulating XAS, and provides both accurate benchmarks for model compounds and a predictive theoretical capability for identification and characterization of multi-component systems, such as lithium-ion batteries, under working conditions.
Theoretical Backgrounds of Nonlinear THz Spectroscopy of Semiconductor Superlattices
NASA Astrophysics Data System (ADS)
Shorokhov, Alexey V.; Alekseev, Kirill N.
2010-12-01
We consider terahertz absorption and gain in a single miniband of semiconductor superlattice subject to a bichromatic electric field in the most general case of commensurate frequencies of the probe and pump fields. Using an exact solution of Boltzmann transport equation, we show that in the small-signal limit the formulas for absorption always contain two distinct terms related to the parametric and incoherent interactions of miniband electrons with the alternating pump field. It provides a theoretical background for a control of THz gain without switching to the negative differential conductivity state. For pedagogical reasons we present derivations of formulas in detail.
NASA Astrophysics Data System (ADS)
Akashi, Ryosuke
The recent discovery of high-temperature superconductivity in sulfur hydride under extreme pressure has broken the long-standing record of superconducting transition temperature (Tc) in the Hg-cuprate. According to the isotope effect measurement and theoretical calculations, the superconducting transition is mainly ascribed to the conventional phonon-mediated pairing interaction. It is, however, not enough for understanding the high-Tc superconductivity in the sulfur hydride. To elucidate various possible effects on Tc with accuracy, we have analyzed Tc with first-principles methods without any empirical parameters. First, for various pressures and theoretically proposed crystal structures, we calculated Tc with the density functional theory for superconductors (SCDFT) to examine which structure(s) can explain experimentally measured Tc data [Akashi et al., PRB 91, 224513 (2015)]. We next solved the Eliashberg equations without introducing the renormalized Coulomb parameter mu*, which is the Green-function-based counterpart of the SCDFT, and evaluated the effects of rapidly varying electron density of states, atomic zero-point motion, and phonon anharmonic corrections on Tc [Sano et al., in preparation]. In the talk, we review these results and discuss the dominant factors for the Tc and their relation to the experimental results. We also report some crystal structures that we recently found with first-principles calculations, which could have a key role for the pressure-induced transformation to the high-Tc phase.
Theoretical spectroscopy of difluoromethylene in the visible and ultraviolet region
NASA Astrophysics Data System (ADS)
Vetter, R.; Reuter, W.; Peyerimhoff, S. D.
1992-04-01
Multi-reference configuration interaction calculations are carried out in order to treat several important aspects of the spectroscopy of difluoromethylene (CF 2): (1) the electronic vertical spectrum up to approximately 12 eV is calculated including Rydberg 3s and 3p states, (2)_the splittings between the potential minimma of the lowest three states ( 1A 1, 3B 1, 1B 1) are evaluated by employing several AO basis ssets and various MO basis transformations, and (3) based on the calculated bending potential curves for the overlineX1A 1 and the overlineA1B 1 state, the vibrational fine structure for the overlineA- overlineX emission spectrum is computed applying a simple independnet mode approach. The results are compared with corresponding experimental laser-induced fluorescence data.
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).
Theoretical Modeling for the X-ray Spectroscopy of Iron-bearing MgSiO3 under High Pressure
NASA Astrophysics Data System (ADS)
Wang, X.; Tsuchiya, T.
2012-12-01
The behaviors of iron (Fe) in MgSiO3 perovskite, including valence state, spin state, and chemical environments, at high pressures are of fundamental importance for more detailed understanding the properties of the Earth's lower mantle. The pressure induced spin transition of Fe-bearing MgO and MgSiO3 are detected often by using high-resolution K-edge X-ray emission spectroscopy (XES) [1,2,3] and confirmed by theoretical simulations. [4,5] Since the Fe K-edge XES is associated to the 3p orbital, which is far from the valence orbitals (3d and 4s), it provides no information about its coordination environments. However, the Fe L-edge XES and X-ray absorption spectroscopy (XAS) can directly present the distribution and intensity of Fe-3d character. To identify both the spin states and the coordination environments of iron-bearing MgSiO3, we systematically investigate the L-edge XAS, XES and X-ray photoelectron (XPS) spectroscopy of Fe2+- and Fe3+-bearing MgSiO3 under high pressure by using the first-principles density functional method combined with the slater-transition method. Our results show that Fe2+ and Fe3+ can be distinguished easily by taking the XPS spectra. The spin transition of Fe2+ and Fe3+ can also be clearly certified by XAS and XES. Interestingly, the broadness of L-edge XES of Fe changes depending on the iron position, meaning that its coordination environment might also be distinguishable by using high-resolution XES measurements. Research supported by the Ehime University G-COE program and KAKENHI. [1] James Badro, Guillaume Fiquet, FranÇois Guyot, Jean-Pascal Rueff, Viktor V. Struzhkin, György VankÓ, and Giulio Monaco. Science 300, 789 (2003), [2] James Badro, Jean-Pascal Rueff, György VankÓ, Giulio Monaco, Guillaume Fiquet, and FranÇois Guyot, Science 305, 383 (2004), [3] Jung-Fu Lin, Viktor V. Struzhkin, Steven D. Jacobsen, Michael Y. Hu, Paul Chow, Jennifer Kung, Haozhe Liu, Ho-kwang Mao, and Gussell J. Hemley, Nature 436, 377 (2005). [4
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
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
Exact results and open questions in first principle functional RG
Le Doussal, Pierre
2010-01-15
Some aspects of the functional RG (FRG) approach to pinned elastic manifolds (of internal dimension d) at finite temperature T > 0 are reviewed and reexamined in this much expanded version of Le Doussal (2006) . The particle limit d = 0 provides a test for the theory: there the FRG is equivalent to the decaying Burgers equation, with viscosity {nu} {approx} T-both being formally irrelevant. An outstanding question in FRG, i.e. how temperature regularizes the otherwise singular flow of T = 0 FRG, maps to the viscous layer regularization of inertial range Burgers turbulence (i.e. to the construction of the inviscid limit). Analogy between Kolmogorov scaling and FRG cumulant scaling is discussed. First, multi-loop FRG corrections are examined and the direct loop expansion at T > 0 is shown to fail already in d = 0, a hierarchy of ERG equations being then required (introduced in Balents and Le Doussal (2005) ). Next we prove that the FRG function R(u) and higher cumulants defined from the field theory can be obtained for any d from moments of a renormalized potential defined in an sliding harmonic well. This allows to measure the fixed point function R(u) in numerics and experiments. In d = 0 the beta function (of the inviscid limit) is obtained from first principles to four loop. For Sinai model (uncorrelated Burgers initial velocities) the ERG hierarchy can be solved and the exact function R(u) is obtained. Connections to exact solutions for the statistics of shocks in Burgers and to ballistic aggregation are detailed. A relation is established between the size distribution of shocks and the one for droplets. A droplet solution to the ERG functional hierarchy is found for any d, and the form of R(u) in the thermal boundary layer is related to droplet probabilities. These being known for the d = 0 Sinai model the function R(u) is obtained there at any T. Consistency of the {epsilon}=4-d expansion in one and two loop FRG is studied from first principles, and connected
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 for understanding diverse high- Tc
NASA Astrophysics Data System (ADS)
Cheng, Hai-Ping
2011-03-01
In this talk, I survey results and insights gained from first-principles calculations on materials that exhibit superconducting behavior at temperatures higher than those characteristic of conventional BCS superconductors. These range from highly correlated cuprate Mott insulators as represented by the bismuth-strontium-calcium-copper-oxides (BSCCOs) to border-line itinerant-Mott systems such as the recently discovered 1111 and 122 pnictides. ultimate goal of our studies is to correlate Tc with specific material composition using detailed first-principles calculations in conjunction with many-body physics techniques via the critical step of constructing real-materials model Hamiltonians. By manipulating impurity doping, which plays a crucial role in the phase diagrams of high Tc materials, we hope to find guidance for designing candidate systems with Tc higher than ones currently known. BSCCO material, density functional calculations using a good generalized-gradient approximation (GGA) yield structural information that is correlated to the experimentally observed (STM) super-modulation and impurity peak in the high energy regime (~ 1 eV), even though the Kohn-Sham bands from such functionals fail to have a band gap. For FeAs-based high-Tc systems, DFT band-structure calculations provide a very good starting point for constructing model Hamiltonians for studies of spin fluctuation and electron pairing mechanisms. Fermi sheets that have been constructed using Wannier transformed Kohn-Sham states have provided critical information for understanding this family of superconducting materials. Analysis of the details of magnetic ordering, density of states, and 2D vs. 3D features in both the 1111 and 122 materials have been valuable in understanding sometimes perplexing experimental findings. Effects of Co impurities have been studied and fully analyzed as well., I will discuss persistent challenges related to calculations on the structure of the non-magnetic state Ba 1
NASA Astrophysics Data System (ADS)
2009-12-01
First-principles methods based on density functional theory (DFT) have been the mainstay of theoretical studies of the properties of semiconductor and oxide materials. Despite the tremendous successes of the past few decades, significant challenges remain in adapting these methods for predictive simulations that are quantitatively useful in predicting device behavior. Recent advances in computational capabilities, and improved theoretical methods taking advantage of ever more powerful computer hardware, offer the possibility that computational modeling may finally fulfill the long-sought goal of truly predictive simulations for defect properties. The exciting prospect of using modelling as `virtual experiments' to obtain quantitatively accurate predictions of semiconductor behavior seems tantalizingly close, but challenges still remain, which is evident in the many divergent approaches adopted for the modelling and simulation of various aspects of defect behavior. This special issue consists of papers describing different approaches to the study of defects, and the challenges that remain from the perspective of leading scientists in the field. It includes contributions on the theoretical and computational issues of using density functional methods for defect calculations [Nieminen], treatments to account for finite computational cell effects in periodic defect supercell calculations using analytical constructions [Lany and Zunger], or cell-size extrapolation techniques [Castleton et al], or instead using embedded cluster calculations to model charge-trapping defects [Shluger et al]. This issue also includes a description of the computation of g-tensor and hyperfine splitting for defect centers [Valentin and Pacchione], computation of vibrational properties of impurities from dynamical DFT calculations [Estreicher et al], and the use of DFT supercell calculations to predict charge transition energy levels of intrinsic defects in GaAs [Schultz and von Lilienfeld
First-principles calculations of gated adatoms on graphene
NASA Astrophysics Data System (ADS)
Chan, Kevin T.; Lee, Hoonkyung; Cohen, Marvin L.
2011-03-01
The two-dimensional surface of graphene is well-suited for adsorption of adatoms or molecules. The application of a gate voltage can be used to precisely control the electron concentration of the adsorbate-graphene system. Such control over electronic properties of adsorbates on graphene might have useful applications in areas such as catalysis and hydrogen storage. In this work, the gating of a variety of adatoms adsorbed on graphene is studied using first-principles calculations. We compute the projected density of states, local electrostatic potential, and charge density of the adatom-graphene system as a function of gate voltage. We demonstrate that adatoms on graphene can be ionized by gating, and that the ionization causes a sharp change in the electrostatic potential. Additional interesting features of our results are also discussed. This work was supported by NSF Grant No. DMR10-1006184 and DOE under Contract No. DE-AC02-05CH11231. Computational resources were provided by the IT Division at LBNL.
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.
Anisotropic intrinsic lattice thermal conductivity of phosphorene from first principles.
Qin, Guangzhao; Yan, Qing-Bo; Qin, Zhenzhen; Yue, Sheng-Ying; Hu, Ming; Su, Gang
2015-02-21
Phosphorene, the single layer counterpart of black phosphorus, is a novel two-dimensional semiconductor with high carrier mobility and a large fundamental direct band gap, which has attracted tremendous interest recently. Its potential applications in nano-electronics and thermoelectrics call for fundamental study of the phonon transport. Here, we calculate the intrinsic lattice thermal conductivity of phosphorene by solving the phonon Boltzmann transport equation (BTE) based on first-principles calculations. The thermal conductivity of phosphorene at 300 K is 30.15 W m(-1) K(-1) (zigzag) and 13.65 W m(-1) K(-1) (armchair), showing an obvious anisotropy along different directions. The calculated thermal conductivity fits perfectly to the inverse relationship with temperature when the temperature is higher than Debye temperature (ΘD = 278.66 K). In comparison to graphene, the minor contribution around 5% of the ZA mode is responsible for the low thermal conductivity of phosphorene. In addition, the representative mean free path (MFP), a critical size for phonon transport, is also obtained.
Incorporation of water in pyrope: a first principles study
NASA Astrophysics Data System (ADS)
Manga, V. R.; Mookherjee, M.; Muralidharan, K.
2014-12-01
Pyrope (Mg3Al2Si3O12) rich garnet is the most important secondary mineral phase with volume fractions ranging between 20 % in the shallow upper mantle to 40 % in the lower part of upper mantle. The volume fractions of garnet in subducted oceanic crusts are as high as 80 %. However, our understanding of the incorporation of water in garnet as proton defect is rather limited. Experimental studies conducted at pressures and temperatures relevant to the deep lower mantle have resulted in wide range of water contents ranging between 0 wt % to ~ 0.8 wt %. In a pyriolyte composition representative of the upper mantle, unlike olivine (Mg2SiO4), which remains largely iso-chemical upon compression, garnet undergoes solid solution with pyroxene (MgSiO3) and as a result there is a continuous evolution of the chemistry of garnet as a function of pressure. This complicates the analysis of proton defects using conventional thermodynamics expressing water solubility as a function of water fugacity, oxide activity, and activation volume. To circumvent this issue, we use first principles simulations to explore the relative energetics of the formation of protons in Mg, Al, and Si sites. Preliminary results at ambient conditions indicate positive enthalpy changes for the proton defects in all the sites, with silicon site being the most favorable. We intend to explore the effect of pressure and temperature on the defect formation energies. Acknowledgement- MM is supported by the US National Science Foundation grant (EAR-1250477).
Oxygen transport in ceria: a first-principles study
NASA Astrophysics Data System (ADS)
Sergei, Simak
2012-02-01
Ceria (CeO2) is an important material for environmentally benign applications, ranging from solid-oxide fuel cells (SOFC) to oxygen storage [1-2]. The key characteristic needed to be improved is the mobility of oxygen ions. Optimization of ionic transport in ceria has been the topic of many studies. In particular, it has been discovered how the ionic conductivity in ceria might be improved by choosing the proper kind and concentration of dopants [3]. In this presentation we will approach the problem from a different direction by adjusting structural parameters of ceria via the change of external conditions. A systematic first-principles study of the energy landscape and kinetics of reduced ceria as a function of external parameters reveals a physically transparent way to improve oxygen transport in ceria. [4pt] [1] N. Skorodumova, S. Simak, B. Lundqvist, I. Abrikosov, and B. Johansson, Physical Review Letters 89, 14 (2002). [0pt] [2] A. Trovarelli, in Catalysis by Ceria and related materials (Imperial College Press, London, 2002). [0pt] [3] D. A. Andersson, S. I. Simak, N. V. Skorodumova, I. A.Abrikosov, and B. Johansson, Proceedings of the National Academy of Sciences of the United States of America 103, 3518 (2006).
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.
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.
Properties of nanoscale dielectrics from first principles computations
NASA Astrophysics Data System (ADS)
Shi, Ning
In recent years, dielectric materials of nanoscale dimensions have aroused considerable interest. We mention two examples. First, in the semiconductor industry, in order to keep pace with Moore's law scaling, the thickness of gate oxide dielectric material is reaching nanoscale dimensions. Second, the high energy density capacitor industry is currently considering dielectric composites with a polymer host matrix filled with inorganic dielectric nanoparticles or polarizable organic molecules. The driving force for the former application is high dielectric constants (or high-k), and those for the latter are high-k and/or high dielectric breakdown strengths. Thus, it is important to characterize the electronic and dielectric properties of materials in the nano-regime, where surface and interface effects naturally play a dominant role. The primary goal of this work is to determine the extent to which such surface/interface effects modify the dielectric constants, band edges, and dielectric breakdown strengths of systems with at least one of their dimensions in the nano-regime. Towards that end, we have developed new computational methodologies at the first principles (density functional) level of theory. These methods have then been applied to several relevant and critical nanoscale systems, including Si:SiO2 and Si:HfO2 heterojunctions, and polymeric composites containing Cu-phthalocyanine and SiO2 nanoparticles.
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 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 calculation of finite temperature magnetism in Ni
NASA Astrophysics Data System (ADS)
Eisenbach, Markus; Yin, Junqi; Nicholson, Don M.; Li, Ying Wai
2013-03-01
We harnesses the computational power of massively parallel computers to calculate finite temperature magnetic properties by combining classical Monte-Carlo calculations with our first principles multiple scattering electronic structure code (LSMS) for constrained magnetic states. Our previous calculations of Fe and Fe3 C [J. Appl. Phys. 109, 07E138 (2011)] only considered fluctuations in the local moment directions. Recent advances, both in the understanding of the Wang-Landau method used in our calculations [Phys. Rev. E 84, 065702(R) (2011)] and more powerful computing resources have enabled us to investigate Ni where the fluctuation in the magnitude of the local magnetic moments is of importance equal to their directional fluctuations. Here we will present our recent results for Ni that axpands our method to an even wider class of 3d element based ferromagnets. This research was sponsored by the Offices of Basic Energy Science (M.E. and D.M.N) and the Office of Advanced Computing Research (J.Y. and Y.W.L) of the US Department of Energy. This research used resources of the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory, which is supported by the Office of Science of the Department of Energy under contract DE-AC05-00OR22725.
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.
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.
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.
Dissolved carbon in extreme conditions characterized by first principles simulations
NASA Astrophysics Data System (ADS)
Pan, Ding; Galli, Giulia
One key component to understanding carbon transport in the Earth interior is the determination of the molecular species formed when carbon bearing materials are dissolved in water at extreme conditions. We used first principles molecular dynamics to investigate oxidized carbon in water at high pressure (P) and high temperature (T), up to the conditions of the Earth's upper mantle. Contrary to popular geochemistry models assuming that CO2 is the major carbon species present in water, we found that most of the dissolved carbon at 10 GPa and 1000 K is in the form of solvated CO32- and HCO3-anions. We also found that ion pairing between alkali metal cations and CO32- or HCO3-anions is greatly affected by P-T conditions, decreasing with pressure along an isotherm. Our study shows that it is crucial to take into account the specific molecular structure of water under extreme conditions and the changes in hydrogen bonding occurring at high P and T, in order to predict chemical reactions in dissolved carbon. Our findings also shed light on possible reduction mechanisms of CO2 when it is geologically stored, depending on the availability of water. The work is supported by the Sloan Foundation through the Deep Carbon Observatory.
Designing substrates for silicene and germanene: First-principles calculations
NASA Astrophysics Data System (ADS)
Chen, M. X.; Zhong, Z.; Weinert, M.
2016-08-01
We propose a guideline for exploring substrates that stabilize the monolayer honeycomb structure of silicene and germanene while simultaneously preserving the Dirac states: in addition to having a strong binding energy to the monolayer, a suitable substrate should be a large-gap semiconductor with a proper work function such that the Dirac point lies in the gap and far from the substrate states when their bands align. We illustrate our idea by performing first-principles calculations for silicene and germanene on the Al-terminated (0001) surface of Al2O3 . The overlaid monolayers on Al-terminated Al2O3 (0001) retain the main structural profile of the low-buckled honeycomb structure via a binding energy comparable to the one between silicene and Ag(111). An unfolded band structure derived from the k -projection method reveals that a gapped Dirac cone is formed at the K point due to the structural distortion and the interaction with the substrate. The gaps of 0.4 and 0.3 eV, respectively, for the supported silicene and germanene suggest that they may have potential applications in nanoelectronics.
Elastic, electronic and thermal properties of YSZ from first principles
NASA Astrophysics Data System (ADS)
Jin, Lei; Yu, Qinghe; Rauf, Abdul; Zhou, Chungen
2012-01-01
First principles calculations were performed to investigate the elastic, electronic and thermal properties of 14% cubic yttria-stabilized zirconia (YSZ) using the pseudo potential plane-wave method within the gradient generalized approximation (GGA) for the exchange and correlation potential. Computed lattice constant parameters are in good agreement with the available experimental results. The three independent elastic constants were computed by means of the stress-strain method, indicating that 14% cubic YSZ is a mechanically stable structure. From the knowledge of the elastic constants, a set of related properties, namely bulk, shear modulus, Young's modulus, sound velocity, Debye temperature, thermal capacity and minimum thermal conductivity are numerically estimated in the frame work of the Voigt-Reuss-Hill approximation for YSZ polycrystalline. The calculated bulk modulus, shear modulus, Young's modulus, sound velocity, Debye temperature, thermal capacity and minimum thermal conductivity are in reasonable agreement with the available experimental and theory data. Density of states, charge density and Mulliken population analysis show that the 14% cubic YSZ is covalent and possess ionic character.
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 study on oxidation of Ge and its interface electronic structures
NASA Astrophysics Data System (ADS)
Ono, Tomoya; Saito, Shoichiro; Iwase, Shigeru
2016-08-01
We review a series of first-principles studies on the defect generation mechanism and electronic structures of the Ge/GeO2 interface. Several experimental and theoretical studies proved that Si atoms at the Si/SiO2 interface are emitted to release interface stress. In contrast, total-energy calculation reveals that Ge atoms at the Ge/GeO2 interface are hardly emitted, resulting in the low trap density. Even if defects are generated, those at the Ge/GeO2 interface are found to behave differently from those at the Si/SiO2 interface. The states attributed to the dangling bonds at the Ge/GeO2 interface lie below the valence-band maximum of Ge, while those at the Si/SiO2 interface generate the defect state within the band gap of Si. First-principles electron-transport calculation elucidates that this characteristic behavior of the defect states is relevant to the difference in the leakage current through the Si/SiO2 and Ge/GeO2 interfaces.
First-Principles Equation of State Calculations of First- and Second-Row Plasmas
NASA Astrophysics Data System (ADS)
Driver, K. P.; Soubiran, F.; Zhang, S.; Militzer, B.
2015-12-01
Theoretical studies of high energy density matter are a key component to improving our knowledge related to interiors of giant planets and stars, astrophysical processes, and new plasma energy technologies, such as inertial confined fusion. Path integral Monte Carlo (PIMC) and density functional theory molecular dynamics (DFT-MD) are simulation methods that provide consistent, first-principles descriptions of warm, dense matter and plasmas over a wide range of density and temperature conditions. Here, we report simulation results using these two methods for a number of first- and second-row elements. DFT-MD algorithms are well-suited for low temperatures, while PIMC has been restricted to relatively high temperatures due to the free-particle approximation of the nodal surface. For first-row elements, we find that the free-particle approximation is sufficient as long as the temperature is high enough to sufficiently ionize the second electronic shell of the atoms. For heavier, second-row elements, we have developed a new, localized nodal surface, which allows us to treat bound states within the PIMC formalism. By combining PIMC and DFT-MD pressures and internal energies, we produce a coherent, first-principles equation of state, bridging the entire warm dense matter regime. Pair-correlation functions and the density of electronic states reveal an evolving plasma structure. The degree of ionization is affected by both temperature and density. Finally, shock Hugoniot curves show an increase in compression as the first and second shells are ionized.
First-principles analysis of X-ray magnetic circular dichroism for transition metal complex oxides
NASA Astrophysics Data System (ADS)
Ikeno, Hidekazu
2016-10-01
X-ray magnetic circular dichroism (XMCD) is widely used for the characterization of magnetism of materials. However, information from XMCD related to the atomic, electronic, and magnetic structures is not fully utilized due to the lack of reliable theoretical tools for spectral analysis. In this work, the first-principles configuration interaction (CI) calculations for X-ray absorption spectra developed by the author were extended for the calculation of XMCD, where the Zeeman energy was taken into the Hamiltonian of the CI to mimic magnetic polarization in the solid state. This technique was applied to interpret the L2,3 XMCD from 3d transition metal complex oxides, such as NiFe2O4 and FeTiO3. The experimental XMCD spectra were quantitatively reproduced using this method. The oxidation states as well as the magnetic ordering between transition metal ions on crystallographically different sites in NiFe2O4 can be unambiguously determined. A first-principles analysis of XMCD in FeTiO3 revealed the presence of Fe3+ and Ti3+ ions, which indicates that the charge transfer from Fe to Ti ions occurs. The origin of magnetic polarization of Ti ions in FeTiO3 was also discussed.
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.
First principle study on the structure of H+ (H2O)6
NASA Astrophysics Data System (ADS)
Kuo, Jer-Lai
2006-01-01
The structure of H+ (H2O)6 is investigated by examining selected low-lying minima with several first principle methods to benchmark the performance of these methods employed in the previous theoretical studies. Interestingly, we found that DFT methods with a moderate basis set follow the trends of MP2 with a large basis set very closely. In additional to the conventional zero point energy estimated with harmonic oscillator approximation, the contribution of vibrational anharmonicity is also investigated via first principle calculations. We found the anharmonicity contribution to the zero point energy varies between 2.7 to 5.0 mhartree and with that two kinds of tree structures (simple and branched tree) are found to be the most stable forms with nearly the same energy. The effects of Ar-attachment on the relative stability of these two tree structures are also examined and we found that the Ar-attached branched tree is more stable than the Ar-attached simple tree by about 1.4 mhartree. We shall also discuss the relevance of our findings with the recent experimental spectra on both bare and Ar-attached species.
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 study of codoping in lanthanum bromide
NASA Astrophysics Data System (ADS)
Erhart, Paul; Sadigh, Babak; Schleife, André; Åberg, Daniel
2015-04-01
Codoping of Ce-doped LaBr3 with Ba, Ca, or Sr improves the energy resolution that can be achieved by radiation detectors based on these materials. Here, we present a mechanism that rationalizes this enhancement on the basis of first-principles electronic structure calculations and point defect thermodynamics. It is shown that incorporation of Sr creates neutral VBr-SrLa complexes that can temporarily trap electrons. As a result, Auger quenching of free carriers is reduced, allowing for a more linear, albeit slower, scintillation light yield response. Experimental Stokes shifts can be related to different CeLa-SrLa-VBr triple complex configurations. Codoping with other alkaline as well as alkaline-earth metals is considered as well. Alkaline elements are found to have extremely small solubilities on the order of 0.1 ppm and below at 1000 K. Among the alkaline-earth metals the lighter dopant atoms prefer interstitial-like positions and create strong scattering centers, which has a detrimental impact on carrier mobilities. Only the heavier alkaline-earth elements (Ca, Sr, Ba) combine matching ionic radii with sufficiently high solubilities. This provides a rationale for the experimental finding that improved scintillator performance is exclusively achieved using Sr, Ca, or Ba. The present mechanism demonstrates that codoping of wide-gap materials can provide an efficient means for managing charge carrier populations under out-of-equilibrium conditions. In the present case dopants are introduced that manipulate not only the concentrations but also the electronic properties of intrinsic defects without introducing additional gap levels. This leads to the availability of shallow electron traps that can temporarily localize charge carriers, effectively deactivating carrier-carrier recombination channels. The principles of this mechanism are therefore not specific to the material considered here but can be adapted for controlling charge carrier populations and
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.
2013-01-15
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 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.
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
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
2011-02-01
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.
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.
Properties of ferroelectric ultrathin films from first principles
NASA Astrophysics Data System (ADS)
Bin-Omran, Saad
First-principles-based methods are used to determine the response of polarization to epitaxial strain in films made of BaTiO3 (BT) and Pb(Zr0.5Ti0.5)O3 (PZT). Unlike in BT films, the strength of this response as well as its sign dramatically depend on the film's thickness and electrical boundary conditions in PZT films. A phenomenological model provides a rationale for these findings. Moreover, we reveal the effect of the depolarizing field on the paraelectric-to-ferroelectric phase transition in BaTiO3 and PZT ultrathin films. We found that with decreasing the beta screening parameter (i.e., when increasing the depolarizing field) (i) the Curie temperature, Tc, linearly decreases; (ii) the dielectric maximum epsilonmax increases; (iii) the phase transition becomes less diffuse. Furthermore, we investigate the effect of mechanical boundary conditions on the paraelectric-to-ferroelectric phase transition in BaTiO3 and PZT ultrathin films. It is predicted that (i) the phase transition temperature (Tc) increases due to the applied compressive strain; (ii) the epitaxial strain decreases the dielectric maximum epsilon max at any given value of the screening parameter beta; (iii) the diffusive character of the transition is larger in compressive films than in freestanding systems at a fixed beta. Also, we reveal that the nature of all paraelectric-to-ferroelectric phase transitions in the BaTiO3 and PZT ultrathin films (that are under different electric and mechanical boundary conditions) is of second-order.
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.
Time-resolved photoabsorption in finite systems: A first-principles NEGF approach
NASA Astrophysics Data System (ADS)
Perfetto, E.; Uimonen, A.-M.; van Leeuwen, R.; Stefanucci, G.
2016-03-01
We describe a first-principles NonEquilibrium Green's Function (NEGF) approach to time-resolved photoabsortion spectroscopy in atomic and nanoscale systems. The method is used to highlight a recently discovered dynamical correlation effect in the spectrum of a Krypton gas subject to a strong ionizing pump pulse. We propose a minimal model that captures the effect, and study the performance of time-local approximations versus time-nonlocal ones. In particular we implement the time-local Hartree-Fock and Markovian second Born (2B) approximation as well as the exact adiabatic approximation within the Time-Dependent Density Functional Theory framework. For the time-nonlocal approximation we instead use the 2B one. We provide enough convincing evidence for the fact that a proper description of the spectrum of an evolving admixture of ionizing atoms requires the simultaneous occurrence of correlation and memory effects.
Cobalt (hydro)oxide electrodes under electrochemical conditions: a first principle study
NASA Astrophysics Data System (ADS)
Chen, Jia; Selloni, Annabella
2013-03-01
There is currently much interest in photoelectrochemical water splitting as a promising pathway towards sustainable energy production. A major issue of such photoelectrochemical devices is the limited efficiency of the anode, where the oxygen evolution reaction (OER) takes place. Cobalt (hydro)oxides, particularly Co3O4 and Co(OH)2, have emerged as promising candidates for use as OER anode materials. Interestingly, recent in-situ Raman spectroscopy studies have shown that Co3O4 electrodes undergo progressive oxidation and transform into oxyhydroxide, CoO(OH), under electrochemical working conditions. (Journal of the American Chemical Society 133, 5587 (2011))Using first principle electronic structure calculations, we provide insight into these findings by presenting results on the structural, thermodynamic, and electronic properties of cobalt oxide, hydroxide and oxydroxide CoO(OH), and on their relative stabilities when in contact with water under external voltage.
NASA Astrophysics Data System (ADS)
Pechkis, Daniel Lawrence
Nuclear magnetic resonance (NMR) spectroscopy is one of the most important experimental probes of local atomistic structure, chemical ordering, and dynamics. Recently, NMR has increasingly been used to study complex ferroelectric perovskite alloys, where spectra can be difficult to interpret. First-principles calculations of NMR spectra can greatly assist in this task. In this work, oxygen, titanium, and niobium NMR chemical shielding tensors, ŝ , were calculated with first-principles methods for ferroelectric transition metal prototypical ABO3 perovskites [SrTiO3, BaTiO 3, PbTiO3 and PbZrO3] and A(B,B')O3 perovskite alloys Pb(Zr1/2Ti1/2)O3 (PZT) and Pb(Mg1/3Nb2/3)O3 (PMN). The principal findings are 1) a large anisotropy between deshielded sigma xx(O) ≃ sigmayy(O) and shielded sigma zz(O) components; 2) a nearly linear dependence on nearest-distance transition-metal/oxygen bond length, rs, was found for both isotropic deltaiso(O) and axial deltaax(O) chemical shifts ( d̂=ŝ reference- ŝ ), across all the systems studied, with deltaiso(O) varying by ≃ 400 ppm; 3) the demonstration that the anisotropy and linear variation arise from large paramagnetic contributions to sigmaxx(O) and sigmayy(O), due to virtual transitions between O(2p) and unoccupied B(nd) states. Using these results, an argument against Ti clustering in PZT, as conjectured from recent 17O NMR magic-angle-spinning measurements, is made. The linear dependence of the chemical shifts on rs provides a scale for determining transition-metal/oxygen bond lengths from experimental 17O NMR spectra. As such, it can be used to assess the degree of local tetragonality in perovskite solid solutions for piezoelectric applications. Results for transition metal atoms show less structural sensitivity, compared to 17O NMR, in homovalent B-site materials, but could be more useful in heterovalent B-site perovskite alloys. This work shows that both 17O and B-site NMR spectroscopy, coupled with first principles
NASA Astrophysics Data System (ADS)
Xiao, B.; Feng, J.; Zhou, C. T.; Xing, J. D.; Xie, X. J.; Chen, Y. H.
2008-06-01
First principles calculations were conducted to investigate the stabilities of six multi-component carbides of Cr 7C 3 by calculating the cohesive energy and formation enthalpy of them. The theoretical predictions were compared with the experimental results and they were in agreement with each other. The electronic structures of the six carbides were also calculated in order to provide more information about the relationship between the stability and crystal compositions at atomic scale.
NASA Astrophysics Data System (ADS)
Li, Xinting; Zhang, Xinyu; Qin, Jiaqian; Zhang, Suhong; Ning, Jinliang; Jing, Ran; Ma, Mingzhen; Liu, Riping
2014-11-01
The structural stability and mechanical properties of WC in WC-, MoC- and NaCl-type structures under high pressure are investigated systematically by first-principles calculations. The calculated equilibrium lattice constants at zero pressure agree well with available experimental and theoretical results. The formation enthalpy indicates that the most stable WC is in WC-type, then MoC-type finally NaCl-type. By the elastic stability criteria, it is predicted that the three structures are all mechanically stable. The elastic constants Cij, bulk modulus B, shear modulus G, Young's modulus E and Poisson's ratio ν of the three structures are studied in the pressure range from 0 to 100 GPa. Furthermore, by analyzing the B/G ratio, the brittle/ductile behavior under high pressure is assessed. Moreover, the elastic anisotropy of the three structures up to 100 GPa is also discussed in detail.
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.
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.
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)
Li, Yang; Lian, Fang; Chen, Ning; Hao, Zhen-jia; Chou, Kuo-chih
2015-05-01
A first-principles method is applied to comparatively study the stability of lithium metal oxides with layered or spinel structures to predict the most energetically favorable structure for different compositions. The binding and reaction energies of the real or virtual layered LiMO2 and spinel LiM2O4 (M = Sc-Cu, Y-Ag, Mg-Sr, and Al-In) are calculated. The effect of element M on the structural stability, especially in the case of multiple-cation compounds, is discussed herein. The calculation results indicate that the phase stability depends on both the binding and reaction energies. The oxidation state of element M also plays a role in determining the dominant structure, i.e., layered or spinel phase. Moreover, calculation-based theoretical predictions of the phase stability of the doped materials agree with the previously reported experimental data.
First principle investigation of the electronic and thermoelectric properties of Mg2C
NASA Astrophysics Data System (ADS)
Kulwinder, Kaur; Ranjan, Kumar
2016-02-01
In this paper, electronic and thermoelectric properties of Mg2C are investigated by using first principle pseudo potential method based on density functional theory and Boltzmann transport equations. We calculate the lattice parameters, bulk modulus, band gap and thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) of this material at different temperatures and compare them with available experimental and other theoretical data. The calculations show that Mg2C is indirect band semiconductor with a band gap of 0.75 eV. The negative value of Seebeck coefficient shows that the conduction is due to electrons. The electrical conductivity decreases with temperature and Power factor (PF) increases with temperature. The thermoelectric properties of Mg2C have been calculated in a temperature range of 100 K-1200 K. Kulwinder Kaur thanks Council of Scientific & Industrial Research (CSIR), India for providing fellowship.
First-Principles Simulation of Adhesion at Metal-Ceramic Interfaces
NASA Astrophysics Data System (ADS)
Hector, , Jr.; Siegel, Donald; Adams, James
2000-03-01
One of the fundamental goals of surface science and tribology is to predict the structure, energetics, and bonding at metal-ceramic interfaces. These structures play an increasingly important role in applications ranging from interconnects in microelectronics to protective coatings in the metalworking industry. Although the structure of such interfaces is often complex, much can be learned from a simplified model in which a metal surface is placed in coherent contact with a ceramic substrate. Until recently, there have been no successful theoretical models capable of accurately predicting the energetics of the adhesive bonding at such an interface. With the advent of first principles calculations based on Density Functional Theory(DFT), such predictions are now becoming possible. Along these lines, we discuss our recent DFT-LDA/GGA calculations of the equilibrium structure, bonding, and adhesion energetics of two technologically relevant interfaces: Al(111)/α-Al_2O_3(0001) and Al(111)/WC(0001).
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.
First-principles calculations for point defects in MAX phases Ti2AlN
NASA Astrophysics Data System (ADS)
Zhang, Yaowen; Yang, Shutong; Wang, Canglong
2016-04-01
This paper outlines general physical issues associated with performing computational numerical simulations of primary point defects in MAX phases Ti2AlN. First-principles solutions are possible due to the development of computational resources of software and hardware. The calculation accuracy is a good agreement with the experimental results. As an important application of our simulations, the results could provide a theoretical guidance for future experiments and application of Ti2AlN. For example, the N mono-vacancy is the most difficult to form. On the contrary, the mono-vacancy formation in Ti2AlN is energetically most favorable for the Al atom. The essence of the phenomena is explained by the calculated density of state (DOS).
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.
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.
Order-disorder phase boundary in ice VII-VIII investigated by first principles
NASA Astrophysics Data System (ADS)
Wentzcovitch, R. M.; Umemoto, K.; Baroni, S.; de Gironcoli, S.
2009-12-01
Phase boundaries among the various forms of ice are difficult to determine experimentally because of large hysteresis involved, especially at the lowest temperatures. Theoretically, there are also great challenges, including the order-disorder (OD) phenomenon. The ice VII-VIII boundary, a typical OD boundary, has been reasonably well constrained experimentally and is an ideal study case. We present a first principles quasiharmonic study consisting in the complete statistical sampling of molecular orientations within a 16 molecules supercell. This supercell size accounts well for several aspects of this transition, including the Clapeyron slope and the isotopic effect. Research supported by NSF grants ATM 0428774 (VLab) and EAR 0757903. Computations were performed at the Minnesota Supercomputing Institute.
Structural, electronic, and dynamical properties of Pca21-TiO2 by first principles
NASA Astrophysics Data System (ADS)
Abbasnejad, M.; Mohammadizadeh, M. R.; Maezono, R.
2012-03-01
First-principles calculations of the structural, electronic, and mechanical properties of the modified fluorite structure of TiO2 with Pca21 symmetry are obtained using the plane-wave pseudopotential density functional theory. The results indicate that Pca21-TiO2 is a semiconductor with an indirect band gap. The calculated static dielectric constants are larger than those of anatase and brookite, but they are much smaller than those of rutile. The calculated bulk modulus using the equation of state is in good agreement with that calculated from elastic constants. The calculated bulk modulus is in agreement with a recent theoretical and experimental report, which confirms that the experimentally claimed structure (cubic fluorite phase) can be Pca21-TiO2.
Near-infrared radiation absorption properties of covellite (CuS) using first-principles calculations
NASA Astrophysics Data System (ADS)
Xiao, Lihua; Wu, Jianming; Ran, Jingyu; Liu, Yike; Qiu, Wei; Lu, Fanghai; Shao, Fang; Tang, Dongsheng; Peng, Ping
2016-08-01
First-principles density functional theory was used to investigate the electronic structure, optical properties and the origin of the near-infrared (NIR) absorption of covellite (CuS). The calculated lattice constant and optical properties are found to be in reasonable agreement with experimental and theoretical findings. The electronic structure reveals that the valence and conduction bands of covellite are determined by the Cu 3d and S 3p states. By analyzing its optical properties, we can fully understand the potential of covellite (CuS) as a NIR absorbing material. Our results show that covellite (CuS) exhibits NIR absorption due to its metal-like plasma oscillation in the NIR range.
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
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 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.
Formation and annealing behaviors of qubit centers in 4H-SiC from first principles
Wang, Xiaopeng; Zhao, Mingwen Bu, Hongxia; He, Xiujie; Wang, Aizhu; Zhang, Hongyu
2013-11-21
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.
Thermoelectric properties of binary LnN (Ln=La and Lu): First principles study
NASA Astrophysics Data System (ADS)
Sreeparvathy P., C.; Gudelli, Vijay Kumar; Kanchana, V.; Vaitheeswaran, G.; Svane, A.; Christensen, N. E.
2015-06-01
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 (S2σ/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.
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. PMID:27409440
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.
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.
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.
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.
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.
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.
Experimental and first principle studies on electronic structure of BaTiO{sub 3}
Sagdeo, Archna Ghosh, Haranath Chakrabarti, Aparna Kamal, C. Ganguli, Tapas Deb, S. K.; Phase, D. M.
2014-04-24
We have carried out photoemission experiments to obtain valence band spectra of various crystallographic symmetries of BaTiO{sub 3} system which arise as a function of temperature. We also present results of a detailed first principle study of these symmetries of BaTiO{sub 3} using generalized gradient approximation for the exchange-correlation potential. Here we present theoretical results of density of states obtained from DFT based simulations to compare with the experimental valence band spectra. Further, we also perform calculations using post density functional approaches like GGA + U method as well as non-local hybrid exchange-correlation potentials like PBE0, B3LYP, HSE in order to understand the extent of effect of correlation on band gaps of different available crystallographic symmetries (5 in number) of BaTiO{sub 3}.
First-principles modeling of titanate/ruthenate superlattices
NASA Astrophysics Data System (ADS)
Junquera, Javier
2013-03-01
The possibility to create highly confined two-dimensional electron gases (2DEG) at oxide interfaces has generated much excitement during the last few years. The most widely studied system is the 2DEG formed at the LaO/TiO2 polar interface between LaAlO3 and SrTiO3, where the polar catastrophe at the interface has been invoked as the driving force. More recently, partial or complete delta doping of the Sr or Ti cations at a single layer of a SrTiO3 matrix has also been used to generate 2DEG. Following this recipe, we report first principles characterization of the structural and electronic properties of (SrTiO3)5/(SrRuO3)1 superlattices, where all the Ti of a given layer have been replaced by Ru. We show that the system exhibits a spin-polarized two-dimensional electron gas extremely confined to the 4 d orbitals of Ru in the SrRuO3 layer, a fact that is independent of the level of correlation included in the simulations. For hybrid functionals or LDA+U, every interface in the superlattice behaves as minority-spin half-metal ferromagnet, with a magnetic moment of μ = 2.0 μB/SrRuO3 unit. The shape of the electronic density of states, half metallicity and magnetism are explained in terms of a simplified tight-binding model, considering only the t2 g orbitals plus (i) the bi-dimensionality of the system, and (ii) strong electron correlations. Possible applications are discussed, from their eventual role in thermoelectric applications to the possible tuning of ferromagnetic properties of the 2DEG with the polarization of the dielectric. Work done in collaboration with P. García, M. Verissimo-Alves, D. I. Bilc, and Ph. Ghosez. Financial support provided by MICINN Grant FIS2009-12721-C04-02, and by the European Union Grant No. CP-FP 228989-2 ``OxIDes.'' The authors thankfully acknowledge the computer resources, technical expertise and assistance provided by the BSC/RES.
First principles evaluation of the photocatalytic properties of cuprous oxide
NASA Astrophysics Data System (ADS)
Bendavid, Leah Isseroff
Cuprous oxide (Cu2O) is a semiconductor attractive for use as a photocatalyst in renewable fuel production, but has thus far exhibited low efficiencies in solar energy technologies. A thorough understanding of its photocatalytically relevant properties is needed to develop improved cuprous oxide-based photocatalysts. This dissertation uses first principles calculations founded in quantum mechanics to study the physical, optical, electronic, and chemical properties of cuprous oxide and to optimize its performance in solar energy applications. The key properties that affect efficiency include electronic excitations, the band gap, band edge positions, charge transport, defect trap states, catalyst stability, and surface chemistry. The band gap of Cu 2O, which defines the efficiency of solar energy absorption, is first calculated with hybrid density functional theory (DFT) followed by a single GW perturbation. We also design methods to calculate optical excitations using embedded correlated wavefunction theory. The low-index surfaces are characterized using DFT+U, where we identify the (111) surface as the most stable. This surface is employed in the derivation of the band edges of Cu2O, which demonstrate that Cu2O can provide the thermodynamic overpotential needed to drive water splitting and the reduction of CO2 to methanol. We also identify the adsorption mechanisms of weakly physisorbed CO2 and the more strongly adsorbed H2O on the Cu2O(111) surface. Effective charge transport is needed so that photoexcited carriers can reach the surface active sites prior to recombination. We study electron and hole transport in Cu2O using the small polaron model, and show that its localized description is inappropriate for carrier transport, which is better modeled using band theory. We then use an approach founded in band theory to analyze the cause of intrinsic trap states, which promote carrier recombination. We conclude that doping with Li can prevent trap state formation and
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
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
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
Topological semimetals predicted from first-principles calculations
NASA Astrophysics Data System (ADS)
Weng, Hongming; Dai, Xi; Fang, Zhong
2016-08-01
We have given a summary on our theoretical predictions of three kinds of topological semimetals (TSMs), namely, Dirac semimetal (DSM), Weyl semimetal (WSM) and node-line semimetal (NLSM). TSMs are new states of quantum matter, which are different from topological insulators. They are characterized by the topological stability of the Fermi surface, whether it encloses band crossing points, i.e. Dirac cone-like energy nodes, or not. They are distinguished from each other by the degeneracy and momentum space distribution of the nodal points. To realize these intriguing topological quantum states is quite challenging and crucial to both fundamental science and future application. Na3Bi and Cd3As2 were theoretically predicted to be DSM in 2012 and 2013 respectively. Their experimental verification in 2014 have ignited intensive studies on TSMs. The subsequent theoretical prediction of a nonmagnetic WSM in the TaAs family stimulated a second wave and many experimental works were released out in 2015. In 2014, a kind of three dimensional crystal of carbon was proposed to be an NLSM due to negligible spin–orbit coupling and coexistence of time-reversal and inversion symmetry. Though the final experimental confirmation of NLSM is still missing, there have been several theoretical proposals, including Cu3PdN from us. In the final part, we have summarized the whole family of TSMs and their relationships.
Topological semimetals predicted from first-principles calculations.
Weng, Hongming; Dai, Xi; Fang, Zhong
2016-08-01
We have given a summary on our theoretical predictions of three kinds of topological semimetals (TSMs), namely, Dirac semimetal (DSM), Weyl semimetal (WSM) and node-line semimetal (NLSM). TSMs are new states of quantum matter, which are different from topological insulators. They are characterized by the topological stability of the Fermi surface, whether it encloses band crossing points, i.e. Dirac cone-like energy nodes, or not. They are distinguished from each other by the degeneracy and momentum space distribution of the nodal points. To realize these intriguing topological quantum states is quite challenging and crucial to both fundamental science and future application. Na3Bi and Cd3As2 were theoretically predicted to be DSM in 2012 and 2013 respectively. Their experimental verification in 2014 have ignited intensive studies on TSMs. The subsequent theoretical prediction of a nonmagnetic WSM in the TaAs family stimulated a second wave and many experimental works were released out in 2015. In 2014, a kind of three dimensional crystal of carbon was proposed to be an NLSM due to negligible spin-orbit coupling and coexistence of time-reversal and inversion symmetry. Though the final experimental confirmation of NLSM is still missing, there have been several theoretical proposals, including Cu3PdN from us. In the final part, we have summarized the whole family of TSMs and their relationships. PMID:27269048
Topological semimetals predicted from first-principles calculations
NASA Astrophysics Data System (ADS)
Weng, Hongming; Dai, Xi; Fang, Zhong
2016-08-01
We have given a summary on our theoretical predictions of three kinds of topological semimetals (TSMs), namely, Dirac semimetal (DSM), Weyl semimetal (WSM) and node-line semimetal (NLSM). TSMs are new states of quantum matter, which are different from topological insulators. They are characterized by the topological stability of the Fermi surface, whether it encloses band crossing points, i.e. Dirac cone-like energy nodes, or not. They are distinguished from each other by the degeneracy and momentum space distribution of the nodal points. To realize these intriguing topological quantum states is quite challenging and crucial to both fundamental science and future application. Na3Bi and Cd3As2 were theoretically predicted to be DSM in 2012 and 2013 respectively. Their experimental verification in 2014 have ignited intensive studies on TSMs. The subsequent theoretical prediction of a nonmagnetic WSM in the TaAs family stimulated a second wave and many experimental works were released out in 2015. In 2014, a kind of three dimensional crystal of carbon was proposed to be an NLSM due to negligible spin-orbit coupling and coexistence of time-reversal and inversion symmetry. Though the final experimental confirmation of NLSM is still missing, there have been several theoretical proposals, including Cu3PdN from us. In the final part, we have summarized the whole family of TSMs and their relationships.
First-Principles Investigations of Defects in Minerals
NASA Astrophysics Data System (ADS)
Verma, Ashok K.
2011-07-01
The ideal crystal has an infinite 3-dimensional repetition of identical units which may be atoms or molecules. But real crystals are limited in size and they have disorder in stacking which as called defects. Basically three types of defects exist in solids: 1) point defects, 2) line defects, and 3) surface defects. Common point defects are vacant lattice sites, interstitial atoms and impurities and these are known to influence strongly many solid-state transport properties such as diffusion, electrical conduction, creep, etc. In thermal equilibrium point defects concentrations are determined by their formation enthalpies and their movement by their migration barriers. Line and surface defects are though absent from the ideal crystal in thermal equilibrium due to higher energy costs but they are invariably present in all real crystals. Line defects include edge-, screw- and mixed-dislocations and their presence is essential in explaining the mechanical strength and deformation of real crystals. Surface defects may arise at the boundary between two grains, or small crystals, within a larger crystal. A wide variety of grain boundaries can form in a polycrystal depending on factors such growth conditions and thermal treatment. In this talk we will present our first-principles density functional theory based defect studies of SiO2 polymorphs (stishovite, CaCl2-, α-PbO2-, and pyrite-type), Mg2SiO4 polymorphs (forsterite, wadsleyite and ringwoodite) and MgO [1-3]. Briefly, several native point defects including vacancies, interstitials, and their complexes were studied in silica polymorphs upto 200 GPa. Their values increase by a factor of 2 over the entire pressure range studied with large differences in some cases between different phases. The Schottky defects are energetically most favorable at zero pressure whereas O-Frenkel pairs become systematically more favorable at pressures higher than 20 GPa. The geometric and electronic structures of defects and migrating
Modeling of compositionally graded barium strontium titanate from first principles
NASA Astrophysics Data System (ADS)
Walizer, Laura Elizabeth
Barium Strontium Titanate (BaxSr1-xTiO 3 or BST) is a Perovskite alloy of interest for both technological and intellectual reasons. Its ferroelectric and piezoelectric properties make it useful in a variety of electric components such as transducers and actuators, and BST in particular is a material of interest for the development of a ferroelectric RAM for computers.(1) The inclusion of SrTiO3, an incipient ferroelectric, and the fact that the properties of a BST system depend strongly on its relative composition of BaTiO3 (BT) and SrTiO3 (ST), make also this a material of high interest. (2) Compositionally graded systems are of further interest (see e.g., Refs. (3), (4), (5) and references therein), partly because their compositional grading leads to a built-in polarization gradient. Due to this, these systems could act as transcapacitors, devices which act as charge amplifiers in much the same way that transistors act as current amplifiers.(3), (4) Here, compositionally graded BST systems were modeled using a first-principles derived effective Hamiltonian method within Monte-Carlo simulation. (6) The graded systems under consideration had an average Ba composition of 70%. These systems were modeled under stress-free conditions, as well as, under epitaxial strain due to a SrTiO3 substrate. Both the degree of grading and the thickness of the layers were varied. The investigation revealed that graded BST systems behaved differently from bulk BST systems in several ways. First, some graded BST systems possessed both monodomain states qualitatively similar to those found in bulk systems (except that the polarization exhibited a "wave" behavior inside the graded systems), and also states with domain striping. Where this occurred, the monodomain state was lower in energy, and was therefore the ground-state, but the striped domain state was found to be metastable, representing a local energy minimum. Analyzing unstrained compositionally graded systems layer by layer
NASA Astrophysics Data System (ADS)
Chen, T.; Wang, X.; Qi, X.; Ma, M.; Xu, Z.; Li, B.
2015-12-01
Coesite (space group C2/c) is a high-pressure polymorph of quartz. The behavior of coesite under pressure has long been of interest due to its abundance in the Earth's crust and mantle, and its relative simple chemistry but rich polymorphisms under elevated pressure and/or temperature conditions. A most recent Raman spectroscopy study reported two pressure-induced phase transitions at ~23 (coesite-II) and ~35 GPa, respectively. To further understand the properties of these pressure-induced phase transitions, we conducted X-ray diffraction experiments starting with coesite powder in a diamond anvil cell up to 31 GPa, and performed first-principle calculations on coesite, coesite-II (space group P21/n), and stishovite at 0 K up to 45 GPa. X-ray diffraction data show the formation of coesite-II at pressures above 20 GPa, which is consistent with first principles calculations that the enthalpy of coesite-II becomes lower than that of coesite above 21.4 GPa. Coesite is very anisotropic with the a-axis twice more compressible than the b- and c-axis. By comparison, coesite-II is less anisotropic, with a similar compressibility in a-, b-, and c-axis. As analyzed by a third-order Eulerian finite strain equation of state, the bulk modulus of coesite at 21.4 GPa is 180.6 GPa, and that of coesite-II is 140.8 GPa, indicating that coesite-II is much more compressible than coesite. If coesite-coesite-II transition occurs in cold subduction zones, it will change the elasticity as well as anisotropic properties of the subducted MORB, due to the different compressional behavior between coesite and coesite-II.
First-principles elastic properties of (alpha)-Pu
Soderlind, P; Klepeis, J
2009-02-18
Density-functional electronic-structure calculations have been used to investigate the ambient pressure and low temperature elastic properties of the ground-state {alpha} phase of plutonium metal. The electronic structure and correlation effects are modeled within a fully relativistic antiferromagnetic treatment with a generalized gradient approximation for the electron exchange and correlation functional. The 13 independent elastic constants, for the monoclinic {alpha}-Pu system, are calculated for the observed geometry. A comparison of the results with measured data from recent resonant ultrasound spectroscopy for a cast sample is made.
First-principles elastic properties of (alpha)-Pu
Soderlind, P; Klepeis, J E
2008-11-04
Density-functional electronic structure calculations have been used to investigate the ambient pressure and low temperature elastic properties of the ground-state {alpha} phase of plutonium metal. The electronic structure and correlation effects are modeled within a fully relativistic anti-ferromagnetic treatment with a generalized gradient approximation for the electron exchange and correlation functionals. The 13 independent elastic constants, for the monoclinic {alpha}-Pu system, are calculated for the observed geometry. A comparison of the results with measured data from resonant ultrasound spectroscopy for a cast sample is made.
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
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
NASA Astrophysics Data System (ADS)
Watanabe, Shinta; Sasaki, Tomomi; Taniguchi, Rie; Ishii, Takugo; Ogasawara, Kazuyoshi
2009-02-01
We performed first-principles calculations of multiplet structures and the corresponding ground-state absorption and excited-state absorption spectra for ruby (Cr3+:α-Al2O3) and alexandrite (Cr3+:BeAl2O4) which included lattice relaxation. The lattice relaxation was estimated using the first-principles total energy and molecular-dynamics method of the CASTEP code. The multiplet structure and absorption spectra were calculated using the configuration-interaction method based on density-functional calculations. For both ruby and alexandrite, the theoretical absorption spectra, which were already in reasonable agreement with experimental spectra, were further improved by consideration of lattice relaxation. In the case of ruby, the peak positions and peak intensities were improved through the use of models with relaxations of 11 or more atoms. For alexandrite, the polarization dependence of the U band was significantly improved, even by a model with a relaxation of only seven atoms.
Design and Characterization of Photoelectrodes from First Principles
Ogitsu, T; Wood, B; Choi, W; Huda, M; Wei, S
2012-05-11
Although significant performance improvements have been realized since the first demonstration of sunlight-driven water splitting in 1972, mainstream adoption of photoelectrochemical (PEC) cells remains limited by an absence of cost-effective electrodes that show simultaneously high conversion efficiency and good durability. Here we outline current and future efforts to use advanced theoretical techniques to guide the development of a durable, high-performance PEC electrode material. Working in close collaboration with experimental synthesis and characterization teams, we use a twofold approach focusing on: (1) rational design of novel high-performance electrode materials; and (2) characterization and optimization of the electrode-electrolyte interface.
Lattice dynamics of CoO from first principles
NASA Astrophysics Data System (ADS)
Wdowik, U. D.; Parlinski, K.
2007-03-01
Cobaltous oxide (CoO) has been studied by using density-functional theory and the generalized-gradient approximation with correction for Hubbard energy. The calculated electronic structure indicates that CoO is a charge transfer insulator since the Co3d and O2p states are strongly hybridized. The calculated band gap and the spin magnetic moment on divalent Co are in good agreement with the experimentally observed values. The so-called direct method based on calculated Hellmann-Feynman forces is used to obtain the density of states and the dispersion relations of phonons. The temperature dependence of the mean-squared vibrational amplitudes and the behavior of the lattice contribution to heat capacity are analyzed and discussed in the framework of the harmonic approximation. The results of calculations agree with the existing theoretical and experimental data.
First-principles study of anhydrite, polyhalite and carnallite
NASA Astrophysics Data System (ADS)
Weck, Philippe F.; Kim, Eunja; Jové-Colón, Carlos F.; Sassani, David C.
2014-02-01
We report density functional calculations of the structures and properties of anhydrite (CaSO4), polyhalite (K2SO4·MgSO4·2CaSO4·2H2O) and carnallite (KCl·MgCl2·6H2O). Densities of states are systematically investigated and phonon analysis using density functional perturbation theory is performed at constant equilibrium volume for anhydrite and polyhalite in order to derive their isochoric thermal properties. Thermal properties at constant atmospheric pressure are also calculated using the quasi-harmonic approximation. The computed molar entropy and isobaric heat capacity for anhydrite reproduce experimental data up to 800 K to within 3% and 10%, respectively, while further experimental work is needed to assess our theoretical predictions for polyhalite.
First Principles Charge Transfer Excitations in Curved Aromatic Materials
NASA Astrophysics Data System (ADS)
Zoppi, Laura; Martin Samos, Layla; Baldridge, Kim K.
Understanding excitation properties and charge transport phenomena of curved π-conjugated materials is critical for a rational utilization of buckybowls as electrically active materials in solid-state devices. In this respect, the class of materials based on the smallest bowl-shaped fullerene fragment, corannulene, C20H10, offers a unique possibility for building up scaffolds with a tunable spectrum of structural and electronic properties. Here, GW-BSE based approaches are applied to investigation and prediction of charge transfer excitations of C20H10 materials systems at functional interfaces, with a special emphasis on design aspects of materials relevant in the experimental domain. Theoretical predictions together with experimental findings illustrate the possibility of integrating corannulene electronic functions in molecular devices
NASA Astrophysics Data System (ADS)
Guo, W.; Granger, J.; Sigman, D. M.
2010-12-01
Coupled fractionations of N and O isotopes during biological nitrate reduction provide important constraints on the marine nitrogen cycle at present and in the geologic past. Recent laboratory experiments with mono-cultures of nitrate-assimilative algae and plankton, and denitrifying bacteria demonstrate that N and O isotopic compositions of the residual nitrate co-vary linearly with a constant ratio (i.e., Δδ18O: Δδ15N) of ~1 or ~0.6 [1]. These systematic variations have been inferred to derive from the kinetic isotope fractionations associated with nitrate reductases. The isotope fractionation mechanisms at the enzymatic level, however, remain elusive. Here we present models of isotope fractionations accompanying the nitrate reduction (NO3-→NO2-) by three functional types of nitrate reductases, using techniques from ab initio, transition state and statistical thermodynamic theory. We consider three types of nitrate reductases: eukNR (eukaryotic assimilatory nitrate reductase), NAR (prokaryotic respiratory nitrate reductase) and Nap (prokaryotic periplasmic nitrate reductase). All are penta- or hexa-coordinated molybdo-enzymes, but bear considerable differences in protein geometry among functional types. Our models, based on the simplified structures of their active sites, predict N and O isotope effects (15ɛ and 18ɛ) ranging from 32.7 to 36.6‰ and from 33.5 to 34.8‰, respectively, at 300K with 18ɛ:15ɛ ratios of 0.9-1.1. The predicted amplitudes of N and O isotope fractionations are in the range measured for eukNR in vitro (~27‰, Karsh et al. in prep), and also correspond to the upper amplitudes observed for denitrifiers in vivo (~25‰, [1]). Moreover, the computed 18ɛ:15ɛ ratios corroborate the consistent relationships of ~1 observed experimentally for eukNR and the respiratory NAR. These findings indicate the enzymatic reduction is likely the rate-limiting step in most biological nitrate reductions. In addition, the predicted similarity of 18ɛ:15ɛ ratios among different nitrate reductases suggests that the nitrate isotope fractionations by nitrate reductases are governed by the kinetics of the O-N bond cleavage, which incurs negligible differences from variations in surrounding moieties at the active sites. However, our model similarly predicts a 15ɛ of 36.6‰ and 18ɛ:15ɛ of 0.9 for the auxiliary Nap, although it exhibits a 15ɛ of ~15‰ and 18ɛ:15ɛ of ~0.6 in vivo [1]. This discrepancy is suspected to arise from slower binding and release of NO3- from Nap, which could be partially rate-determining in this enzymatic catalysis, or from the assumptions of our modeled enzyme structures. By extending our above models to include the multiply-substituted (clumped) isotopologues, we predict that isotope fractionations during biological nitrate reduction decrease the proportion of 15N-18O bonds in the residual nitrate relative to their expected equilibrium abundances (~0.02‰ decrease for every 1‰ kinetic enrichment in nitrate δ15N). Future quantification of 15N-18O clumped isotope anomalies in natural nitrate may provide additional constraints on the nitrogen cycle in the ocean. Reference: [1] Granger et al. (2010) GCA, 74: 1030-1040.
NASA Astrophysics Data System (ADS)
Roehl, Jason L.
Diffusion of point defects on crystalline surfaces and in their bulk is an important and ubiquitous phenomenon affecting film quality, electronic properties and device functionality. A complete understanding of these diffusion processes enables one to predict and then control those processes. Such understanding includes knowledge of the structural, energetic and electronic properties of these native and non-native point defect diffusion processes. Direct experimental observation of the phenomenon is difficult and microscopic theories of diffusion mechanisms and pathways abound. Thus, knowing the nature of diffusion processes, of specific point defects in given materials, has been a challenging task for analytical theory as well as experiment. The recent advances in computing technology have been a catalyst for the rise of a third mode of investigation. The advent of tremendous computing power, breakthroughs in algorithmic development in computational applications of electronic density functional theory now enables direct computation of the diffusion process. This thesis demonstrates such a method applied to several different examples of point defect diffusion on the (001) surface of gallium arsenide (GaAs) and the bulk of cadmium telluride (CdTe) and cadmium sulfide (CdS). All results presented in this work are ab initio, total-energy pseudopotential calculations within the local density approximation to density-functional theory. Single particle wavefunctions were expanded in a plane-wave basis and reciprocal space k-point sampling was achieved by Monkhorst-Pack generated k-point grids. Both surface and bulk computations employed a supercell approach using periodic boundary conditions. Ga adatom adsorption and diffusion processes were studied on two reconstructions of the GaAs(001) surface including the c(4x4) and c(4x4)-heterodimer surface reconstructions. On the GaAs(001)- c(4x4) surface reconstruction, two distinct sets of minima and transition sites were discovered for a Ga adatom relaxing from heights of 3 and 0.5 A from the surface. These two sets show significant differences in the interaction of the Ga adatom with surface As dimers and an electronic signature of the differences in this interaction was identified. The energetic barriers to diffusion were computed between various adsorption sites. Diffusion profiles for native Cd and S, adatom and vacancy, and non-native interstitial adatoms of Te, Cu and Cl were investigated in bulk wurtzite CdS. The interstitial diffusion paths considered in this work were chosen parallel to c-axis as it represents the path encountered by defects diffusing from the CdTe layer. Because of the lattice mismatch between zinc-blende CdTe and hexagonal wurtzite CdS, the c-axis in CdS is normal to the CdTe interface. The global minimum and maximum energy positions in the bulk unit cell vary for different diffusing species. This results in a significant variation, in the bonding configurations and associated strain energies of different extrema positions along the diffusion paths for various defects. The diffusion barriers range from a low of 0.42 eV for an S interstitial to a high of 2.18 eV for a S vacancy. The computed 0.66 eV barrier for a Cu interstitial is in good agreement with experimental values in the range of 0.58 - 0.96 eV reported in the literature. There exists an electronic signature in the local density of states for the s- and d-states of the Cu interstitial at the global maximum and global minimum energy position. The work presented in this thesis is an investigation into diffusion processes for semiconductor bulk and surfaces. The work provides information about these processes at a level of control unavailable experimentally giving an elaborate description into physical and electronic properties associated with diffusion at its most basic level. Not only does this work provide information about GaAs, CdTe and CdS, it is intended to contribute to a foundation of knowledge that can be extended to other systems to expand our overall understanding into the diffusion process. (Abstract shortened by UMI.)
First-principles calculations of zero-field splitting parameters.
Ganyushin, Dmitry; Neese, Frank
2006-07-14
In this work, an implementation of an approach to calculate the zero-field splitting (ZFS) constants in the framework of ab initio methods such as complete active space self-consistent field, multireference configuration interaction, or spectroscopy oriented configuration interaction is reported. The spin-orbit coupling (SOC) contribution to ZFSs is computed using an accurate multicenter mean-field approximation for the Breit-Pauli Hamiltonian. The SOC parts of ZFS constants are obtained directly after diagonalization of the SOC operator in the basis of a preselected number of roots of the spin-free Hamiltonian. This corresponds to an infinite order treatment of the SOC in terms of perturbation theory. The spin-spin (SS) part is presently estimated in a mean-field fashion and appears to yield results close to the more complete treatments available in the literature. Test calculations for the first- and second-row atoms as well as first-row transition metal atoms and a set of diatomic molecules show accurate results for the SOC part of ZFSs. SS contributions have been found to be relatively small but not negligible (exceeding 1 cm(-1) for oxygen molecule). At least for the systems studied in this work, it is demonstrated that the presented method provides much more accurate estimations for the SOC part of ZFS constants than the emerging density functional theory approaches.
First-principles calculations of elastic and electronic properties of NbB(2) under pressure.
Li, Xiao-Feng; Ji, Guang-Fu; Zhao, Feng; Chen, Xiang-Rong; Alfè, Dario
2009-01-14
The structural parameters, elastic constants and electronic structure of NbB(2) under pressure are investigated by using first-principles plane-wave pseudopotential density functional theory within the generalized gradient approximation (GGA). The obtained results are in agreement with the available theoretical data. It is found that the elastic constants and the Debye temperature of NbB(2) increase monotonically and the anisotropies weaken with pressure. The band structure and density of states (DOS) of NbB(2) under pressure are also presented. It is the σ hole that determines the superconductivity in NbB(2), and the features of the σ bands are unchanged after applying pressure except for a shift of position. The density of states (DOS) at the Fermi level decreases with increasing pressure, in conjunction with Bardeen-Cooper-Schrieffer (BCS) theory, which can predict T(c) decreasing with pressure, in agreement with the trend of the theoretical T(c) versus pressure.
Xu, W; Moriarty, J.A.
1996-01-19
Using multi-ion interatomic potentials derived from first-principles generalized pseudopotential theory, we have been studying point defects and dislocations in bcc transition metals, with molybdenum (Mo) as a prototype. For point defects in Mo, the calculated vacancy formation and activation energies are in excellent agreement with experimental results. The energetics of six self-interstitial configurations in Mo have also been investigated. The <110> split dumb-bell is found to have the lowest formation energy, as is experimentally observed, but the corresponding migration energy is calculated to be 3--15 times higher than previous theoretical estimates. The atomic structure and energetics of <111> screw dislocations in Mo are now being investigated. We have found that the ``easy`` core configuration has a lower formation energy than the ``hard`` one, consistent with previous theoretical studies. The former has a distinctive 3-fold symmetry with a spread out of the dislocation core along the <112> directions, an effect which is driven by the strong angular forces present in these metals.
First-principle study of thermoelectric properties of impurity-doped magnesium silicide Mg2Si
NASA Astrophysics Data System (ADS)
Funashima, Hiroki
2014-03-01
The electronic structure and the thermoelectric properties of Mg2Si doped with several dopants, Al, Bi, Sb, and Zn, are theoretically examined using a first-principles calculation method. Mg2Si is a promising thermoelectric material that is functional in the temperature range from 500 to 800 K. Therefore, it is expected to be useful for recovering waste heat from exhaust gas in automotive applications, incinerators, and boilers. Moreover, this material has several desirable attributes with respect to cost and environmental protection: it is cheap, nontoxic, and composed of elements abundant on Earth. These advantages are important for practical usage in thermoelectric applications. Impurity doping is a well-established way to improve the thermoelectric performance of Mg2Si. Undoped Mg2Si crystals have n-type conductivity, but they can be doped with both n- and p-type impurities. A fundamental understanding of the relationship between impurity doping and the thermoelectric properties of Mg2Si will allow us to provide theoretical guidelines for further development of this material. As an effort toward this goal, we present here the band structure of Mg2Si using the full-potential linearized augmented plane-wave (FLAPW) method based on LDA/DFT and the conductivity.
First principles study of NH3 adsorption on carbon nanowires
NASA Astrophysics Data System (ADS)
Tapia, Jorge-Alejandro; Sanchez, Alvaro-Daniel; Acosta, Cesar; Canto, Gabriel
2009-03-01
Recently has been reported a new type of one-dimensional carbon structures. Carbon nanowires formed by a linear carbon-atom chain inside an armchair (5,5) carbon nanotube has been observed using high-resolution transmission electron microscopy. Theoretical and experimental studies of the NH3 adsorption in the carbon nanotubes report changes in the electronic properties of the carbon nanotubes. In the present work we have studied the electronic and structure properties of carbon nanowires (chain@SWCNT) when NH3 atoms are adsorbed. We used the Density Functional Theory and the calculations where performed by the pseudopotentials LCAO method (SIESTA code) and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We have analyzed the changes in the atomic structure and density of states (DOS). We found that the electronic character of the carbon chain of the chain@SWCNT system, can be modulate by NH3 adsorption. This research was supported by SEP under Grant No. PROMEP/103.5/07/2595 and the Consejo Nacional de Ciencia y Tecnolog'ia (Conacyt) under Grants No. 82497 and 60534.
First-Principles Computation of Graphene's Phonon Anharmonicity
NASA Astrophysics Data System (ADS)
Kornbluth, Mordechai; Marianetti, Chris A.
Here we use density-functional theory to compute an interatomic potential for graphene, including anharmonicities up to at least fourth order. We generate all group-theoretically allowed terms within a hexagon via the recently-developed slave mode expansion. This expands the potential in terms of the normal modes of overlapping hexagons, while obeying the space group symmetry and homogeneity of free space. We further introduce the notion of cooperative modes, which combine strain and q = 0 phonons to yield the same pure mode amplitude on each hexagon. Within the cooperative subspace, cooperative modes allow for arbitrarily-precise meshing to directly compute energies, or calculation of the anharmonic coefficients via finite-difference. We demonstrate the power of our approach in the context of strained graphene, which is known to have a novel strain-driven soft mode at the K-point. We identify the dominant anharmonic terms which drive the soft K mode, and study the role of finite temperatures using molecular dynamics and Monte-Carlo simulations.
The European Theoretical Spectroscopy Facility: an illustration for the power of collective research
NASA Astrophysics Data System (ADS)
Reining, Lucia
As researchers and citizens, we should contribute to facing the grand challenges of our epoch. It is important to work on problems such as climate change or limited ressources. However, maybe the biggest challenge is to find ways to unite our forces and develop models of collaborative problem solving. This is mandatory to deal with complex problems, and it can boost efficiency in any case. Code development is just one example where a constructive and well-organized collaboration can take us much further than individual attempts. On the background of this general idea, we will analyze the impact of the European Theoretical Spectroscopy Facility (ETSF, www.etsf.eu) on the day-to-day research of its members, on the theoretical and computational tools that are produced, and on a wider field of theoretical or experimental research. We will see that much can be learnt from this attempt to consider ideas in competition, with people in collaboration.
Length dependence of electron transport through molecular wires--a first principles perspective.
Khoo, Khoong Hong; Chen, Yifeng; Li, Suchun; Quek, Su Ying
2015-01-01
One-dimensional wires constitute a fundamental building block in nanoscale electronics. However, truly one-dimensional metallic wires do not exist due to Peierls distortion. Molecular wires come close to being stable one-dimensional wires, but are typically semiconductors, with charge transport occurring via tunneling or thermally-activated hopping. In this review, we discuss electron transport through molecular wires, from a theoretical, quantum mechanical perspective based on first principles. We focus specifically on the off-resonant tunneling regime, applicable to shorter molecular wires (<∼4-5 nm) where quantum mechanics dictates electron transport. Here, conductance decays exponentially with the wire length, with an exponential decay constant, beta, that is independent of temperature. Different levels of first principles theory are discussed, starting with the computational workhorse - density functional theory (DFT), and moving on to many-electron GW methods as well as GW-inspired DFT + Sigma calculations. These different levels of theory are applied in two major computational frameworks - complex band structure (CBS) calculations to estimate the tunneling decay constant, beta, and Landauer-Buttiker transport calculations that consider explicitly the effects of contact geometry, and compute the transmission spectra directly. In general, for the same level of theory, the Landauer-Buttiker calculations give more quantitative values of beta than the CBS calculations. However, the CBS calculations have a long history and are particularly useful for quick estimates of beta. Comparing different levels of theory, it is clear that GW and DFT + Sigma calculations give significantly improved agreement with experiment compared to DFT, especially for the conductance values. Quantitative agreement can also be obtained for the Seebeck coefficient - another independent probe of electron transport. This excellent agreement provides confirmative evidence of off
NASA Astrophysics Data System (ADS)
Mishchenko, Michael I.; Dlugach, Janna M.; Yurkin, Maxim A.; Bi, Lei; Cairns, Brian; Liu, Li; Panetta, R. Lee; Travis, Larry D.; Yang, Ping; Zakharova, Nadezhda T.
2016-05-01
the first-principles formalism enabling accurate calculations of monochromatic and quasi-monochromatic scattering by static and randomly varying multiparticle groups. We illustrate how this general framework can be coupled with state-of-the-art computer solvers of the Maxwell equations and applied to direct modeling of electromagnetic scattering by representative random multi-particle groups with arbitrary packing densities. This first-principles modeling yields general physical insights unavailable with phenomenological approaches. We discuss how the first-order-scattering approximation, the radiative transfer theory, and the theory of weak localization of electromagnetic waves can be derived as immediate corollaries of the Maxwell equations for very specific and well-defined kinds of particulate medium. These recent developments confirm the mesoscopic origin of the radiative transfer, weak localization, and effective-medium regimes and help evaluate the numerical accuracy of widely used approximate modeling methodologies.
First-principles study of the covalently functionalized graphene
NASA Astrophysics Data System (ADS)
Jha, Sanjiv Kumar
Theoretical investigations of nanoscale systems, such as functionalized graphene, present major challenges to conventional computational methods employed in quantum chemistry and solid state physics. The properties of graphene can be affected by chemical functionalization. The surface functionalization of graphene offers a promising way to increase the solubility and reactivity of graphene for use in nanocomposites and chemical sensors. Covalent functionalization is an efficient way to open band-gap in graphene for applications in nanoelectronics. We apply ab initio computational methods based on density functional theory to study the covalent functionalization of graphene with benzyne (C6H4), tetracyanoethylene oxide (TCNEO), and carboxyl (COOH) groups. Our calculations are carried out using the SIESTA and Quantum-ESPRESSO electronic structure codes combined with the generalized gradient (GGA) and local density approximations (LDA) for the exchange correlation functionals and norm-conserving Troullier-Martins pseudopotentials. Calculated binding energies, densities of states (DOS), band structures, and vibrational spectra of functionalized graphene are analyzed in comparison with the available experimental data. Our calculations show that the reactions of [2 + 2] and [2 + 4] cycloaddition of C6H4 to the surface of pristine graphene are exothermic, with binding energies of --0.73 eV and --0.58 eV, respectively. Calculated band structures indicate that the [2 + 2] and [2 + 4] attachments of benzyne results in opening small band gap in graphene. The study of graphene--TCNEO interactions suggests that the reaction of cycloaddition of TCNEO to the surface of pristine graphene is endothermic. On the other hand, the reaction of cycloaddition of TCNEO is found to be exothermic for the edge of an H-terminated graphene sheet. Simulated Raman and infrared spectra of graphene functionalized with TCNEO are consistent with experimental results. The Raman (non-resonant) and
First-principles study of alloying effect of transition metals on He in titanium ditritide
NASA Astrophysics Data System (ADS)
Wu, Y. X.; Yang, R.; Zheng, H.; Wang, Y. M.
2006-08-01
Due to its inert reactivity with almost elements, 3He produced from tritium decay has extremely detrimental effects on the tritide. To refrain from this 3He-induced damage, an efficient way is to increase the stability of 3He in metal tritide by alloying. Using a first-principles discrete variational method in two cluster models, one for a low 3He concentration and the other for a high 3He concentration, the authors study the alloying effect of 3d and 4d transition metals on the stability of 3He in TiT 2 system. It is found that the preferring and metastable sites of 3He are affected by 3He concentration: 3He prefers to stay at original tetrahedral interstitial site when 3He concentration is low but moves to octahedral site when 3He concentration is high enough. A criterion of alloying effect is proposed, according to which Nb, Y, Zr, Pd, Ru, Tc, Rh, Cr, Mo and Ag are suggested to be the beneficial alloying elements for increasing the stability of 3He in the alloyed TiT 2 with a low 3He concentration and Y, Nb, Mo, Zr, Cr, Tc, Ru, Rh and Cu for that with a high 3He concentration. Our results of alloying effect are supported by the positron annihilation spectroscopy (PAS) measurements for He-implanted Ti, TiMoYAl and TiZrYAl films.
Liu, Zhaomeng; Peng, Wenjie; Xu, Zhenming; Shih, Kaimin; Wang, Jiexi; Wang, Zhixing; Lv, Xiaojun; Chen, Jiangan; Li, Xinhai
2016-08-23
To develop a new anode material to meet the increasing demands of lithium-ion battery, MoS2 is used for the first time to modify the C/LiVPO4 F anode to improve its lithium-storage performance between 3 and 0.01 V. Morphological observations reveal that the MoS2 -modified C/LiVPO4 F particles (M-LVPF) are wrapped by an amorphous carbon as interlayer and layered MoS2 as external surface. Charge-discharge tests show that M-LVPF delivers a high reversible capacity of 308 mAh g(-1) at 50 mA g(-1) . After 300 cycles at 1.0 A g(-1) , a capacity retention of 98.7 % is observed. Moreover, it exhibits high rate capability with a specific capacity of 199 mAh g(-1) at 1.6 A g(-1) . Electrochemical impedance spectroscopy tests indicate that the lithium-ion diffusion and charge-exchange reaction at the surface of M-LVPF are greatly enhanced. First-principles calculations for the MoS2 (001)/C/LiVPO4 F (010) system demonstrate that the absorption of MoS2 on C/LiVPO4 F is exothermic and spontaneous and that the electron transfer at the MoS2 -absorbed C/LiVPO4 F surface is enhanced. PMID:27376792
Kanagaprabha, S.; Rajeswarapalanichamy, R. Sudhapriyanga, G. Murugan, A. Santhosh, M.; Iyakutti, K.
2014-04-24
The electronic, structural and mechanical properties of ZrH and ZrH{sub 2} are investigated by means of first principles calculation based on density functional theory as implemented in VASP code with generalized gradient approximation. The calculated ground state properties are in good agreement with previous experimental and other theoretical results. Among the six crystallographic structures considered for ZrH, ZB phase is found to be the most stable phase, whereas ZrH{sub 2} is energetically stable in tetragonal structure at ambient condition. A structural phase transition from ZB→NaCl at a pressure 10 GPa is predicted for ZrH.
Rohlfing, Michael; Tiago, M.L.; Louie, Steven G.
2000-03-20
Experimental and theoretical studies have shown that excitonic effects play an important role in the optical properties of conjugated polymers. The optical absorption spectrum of trans-polyacetylene, for example, can be understood as completely dominated by the formation of exciton bound states. We review a recently developed first-principles method for computing the excitonic effects and optical spectrum, with no adjustable parameters. This theory is used to study the absorption spectrum of two conjugated polymers: trans-polyacetylene and poly-phenylene-vinylene(PPV).
NASA Astrophysics Data System (ADS)
Bengone, O.; Eriksson, O.; Fransson, J.; Turek, I.; Kudrnovský, J.; Drchal, V.
2004-07-01
We present a theoretical study of the transport properties of a CrAs/GaAs/CrAs trilayer. The theory was based on a first principles method for calculating the electronic structure, in combination with a Kubo-Landauer approach for calculating the transport properties in a current perpendicular to the plane geometry. We have also investigated the electronic structure and the magnetic properties of this trilayer, with special focus on electronic and magnetic properties at the CrAs/GaAs interface. Finally, we have studied the effects of chemical disorder on the transport properties, in particular the influence of As antisites at both the Cr and Ga sites.
Sahoo, B. D. Joshi, K. D.; Gupta, Satish C.
2014-11-21
Structural, elastic, and lattice dynamical stability of YSe has been investigated as a function of pressure through first principles electronic band structure calculations. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of pressure suggests that the B1 phase will transform to B2 structure at ∼32 (30 GPa at 300 K obtained from comparison of Gibbs free energy at 300 K). The transition is identified to be of first order in nature with a volume discontinuity of ∼6.2% at the transition pressure. Furthermore, the theoretically determined equation of state has been utilized to derive various physical quantities, such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus. The single crystal elastic constants have been predicted at various pressures for both the B1 and B2 structures using the energy strain method. The activation barrier between B1 and B2 phases calculated at transition point is ∼19.7mRy/formula unit. Our lattice dynamic calculations show that both the B1 as well as B2 structures are lattice dynamically stable not only at ambient pressure but also at transition pressure. The B1 phase becomes lattice dynamically unstable at ∼112 GPa, i.e., much beyond the transition pressure. The effect of temperature on volume and bulk modulus of the YSe in B1 phase has also been examined.
A comparative investigation of the behaviors of H in Au and Ag from first principles
NASA Astrophysics Data System (ADS)
Han, Quan-Fu; Zhou, Zhen-Yu; Ma, Yuming; Liu, Yue-Lin
2016-05-01
Hydrogen (H) is a common impurity in metals and has a significant effect on their purification, even at concentrations of only a few parts per million. Here we present a comparative analysis of the behaviors of H in Au and Ag based on first-principles calculations. In bulk Au and Ag, the results demonstrate that the tetrahedral position is energetically more stable for a single H atom than the octahedral site. The concentration of H dissolving in the interstitial sites as a function of temperature is calculated in both metals. To characterize the dynamic behaviors, in bulk Au and Ag we determine the theoretical diffusivity and permeation of H, which are in quantitative agreement with the experimental data. Further, we investigate the role of vacancy on the formation of the H n -vacancy (H n V) via a clustering reaction. One vacancy can accommodate up to 9 H atoms in Au and capture as many as 7 H atoms in Ag. The H2 molecule in the vacancy is energetically unstable in both metals. These research results will provide a very useful reference for the refinement of Ag/Au as noble metals in industry.
Phosphorene as an anode material for Na-ion batteries: a first-principles study.
Kulish, Vadym V; Malyi, Oleksandr I; Persson, Clas; Wu, Ping
2015-06-01
We systematically investigate a novel two-dimensional nanomaterial, phosphorene, as an anode for Na-ion batteries. Using first-principles calculations, we determine the Na adsorption energy, specific capacity and Na diffusion barriers on monolayer phosphorene. We examine the main trends in the electronic structure and mechanical properties as a function of Na concentration. We find a favorable Na-phosphorene interaction with a high theoretical Na storage capacity. We find that Na-phosphorene undergoes semiconductor-metal transition at high Na concentration. Our results show that Na diffusion on phosphorene is fast and anisotropic with an energy barrier of only 0.04 eV. Owing to its high capacity, good stability, excellent electrical conductivity and high Na mobility, monolayer phosphorene is a very promising anode material for Na-ion batteries. The calculated performance in terms of specific capacity and diffusion barriers is compared to other layered 2D electrode materials, such as graphene, MoS2, and polysilane.
Core-level shifts in fcc random alloys: A first-principles approach
NASA Astrophysics Data System (ADS)
Olovsson, W.; Göransson, C.; Pourovskii, L. V.; Johansson, B.; Abrikosov, I. A.
2005-08-01
First-principles theoretical calculations of the core-level binding-energy shift (CLS) for eight binary face-centered-cubic (fcc) disordered alloys, CuPd, AgPd, CuNi, NiPd, CuAu, PdAu, CuPt, and NiPt, are carried out within density-functional theory (DFT) using the coherent potential approximation. The shifts of the Cu and Ni 2p3/2 , Ag and Pd 3d5/2 , and Pt and Au 4f7/2 core levels are calculated according to the complete screening picture, which includes both initial-state (core-electron energy eigenvalue) and final-state (core-hole screening) effects in the same scheme. The results are compared with available experimental data, and the agreement is shown to be good. The CLSs are analyzed in terms of initial- and final-state effects. We also compare the complete screening picture with the CLS obtained by the transition-state method, and find very good agreement between these two alternative approaches for the calculations within the DFT. In addition the sensitivity of the CLS to relativistic and magnetic effects is studied.
High pressure phase transformation in yttrium sulfide(YS): A first principle study
Sahoo, B. D. Joshi, K. D. Gupta, Satish C.
2014-04-24
First principles calculations have been carried out to analyze structural, elastic and dynamic stability, of YS under hydrostatic compression. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of compression suggests the B1→B2 transition at ∼ 49 GPa. Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state. The single crystal elastic constants derived from the energy strain method agree well with the experimental values. The activation barrier between B1 and B2 phases calculated at transition point is ∼ 17/mRy/formula unit. Our lattice dynamic calculations show that at ambient condition, the B1 phase is lattice dynamically stable and frequencies of phonon modes in different high symmetry directions of Brillouin zone agrees well with experimental values. The B2 phase also is dynamical stable at ambient condition as well as at ∼ 49 GPa, supporting our static lattice calculation.
Susan, S.
1993-04-30
Theoretical electronic structure techniques are used to analyze widely different systems from Si clusters to transition metal solids and surfaces. For the Si clusters, first principles density functional methods are used to investigate Si{sub N} for N=2-8. Goal is to understand the different types of bonding that can occur in such small clusters where the atomic coordination differs substantially from tetrahedral bonding; such uncoordinated structures can test approximate models of Si surfaces. For the transition metal systems, non-self-consistent electronic structure methods are used to understand the driving force for surface relaxations. In-depth analysis of results is presented and physical basis of surface relaxation within the theory is discussed. Limitations inherent in calculations of metal surface relaxation are addressed. Finally, in an effort to understand approximate methods, a novel non-self- consistent density functional electronic structure method is developed that is about 1000 times faster than more sophisticated methods; this method is tested for various systems including diatomics, mixed clusters, surfaces, and bulk lattices.
A first-principles study of He, Xe, Kr and O incorporation in thorium carbide
NASA Astrophysics Data System (ADS)
Pérez Daroca, D.; Llois, A. M.; Mosca, H. O.
2015-05-01
Thorium-based materials are currently being investigated in relation with their potential utilization in Generation-IV reactors as nuclear fuels. Understanding the incorporation of fission products and oxygen is very important to predict the behavior of nuclear fuels. A first approach to this goal is the study of the incorporation energies and stability of these elements in the material. By means of first-principles calculations within the framework of density functional theory, we calculate the incorporation energies of He, Xe, Kr and O atoms in Th and C vacancy sites, in tetrahedral interstitials and in Schottky defects along the <1 1 1> and <1 0 0> directions. We also analyze atomic displacements, volume modifications and Bader charges. This kind of results for ThC, to the best authors' knowledge, have not been obtained previously, neither experimentally, nor theoretically. This should deal as a starting point towards the study of the complex behavior of fission products in irradiated ThC.
Infrared radiative properties of alumina up to the melting point: A first-principles study
NASA Astrophysics Data System (ADS)
Yang, J. Y.; Xu, M.; Liu, L. H.
2016-11-01
The high thermal emission of alumina dominates the radiative heat transfer of rocket exhaust plume. Yet numerous experimental measurements on radiative properties of alumina at high temperatures vary considerably from each other and cannot provide physical insight into the underlying mechanism. In this work, the ab initio molecular dynamics (AIMD) method and ab initio parameterized Drude model are combined to predict the radiative properties of alumina for temperatures up to 2327 K (the melting point) in the spectral range 1-12 μm. Contributed by different microscopic processes, the optical absorption of alumina in the spectral range 1-4 and 4-12 μm is described by two distinct methods. In the spectral range 4-12 μm, the multi-phonon process mainly contributes to optical absorption and can be simulated by the AIMD method based on the linear response theory. While in the spectral range 1-4 μm, the optical absorption is mainly caused by intrinsic carriers and can be effectively described by the ab initio parameterized Drude model. The first-principles calculations can successfully predict the infrared radiative properties of alumina at high temperatures and well reproduce the literature experiments. Moreover, the theoretical simulations verify that alumina can retain its semiconducting character even in the liquid phase and there emerges sharp increase in the near-infrared optical absorption of alumina upon melting.
Sahoo, B. D. Joshi, K. D.; Gupta, Satish C.
2014-03-28
First principles calculations have been carried out to analyze structural, elastic, and dynamic stability of yttrium sulphide (YS) under hydrostatic compression. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of compression suggests the B1 → B2 transition at ∼49 GPa (the same transition occurs at ∼48 GPa at 300 K). Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state. The single crystal elastic constants derived from the energy strain method agree well with the experimental values. The activation barrier between B1 and B2 phases calculated at transition point is ∼17/mRy/f.u. Our lattice dynamic calculations show that at ambient condition, the B1 phase is lattice dynamically stable, and frequencies of phonon modes in different high symmetry directions of Brillouin zone agrees well with experimental values. The B2 phase also is dynamical stable at ambient condition as well as at ∼49 GPa, supporting our static lattice calculation. The effect of temperature on volume and bulk modulus of the YS in B1 phase has also been examined. The superconducting temperature of ∼2.78 K determine at zero pressure agrees well with experimental data. The effect of pressure is found to suppress the superconducting nature of this material.
First-principles study of two-dimensional electride: Yttrium carbide
NASA Astrophysics Data System (ADS)
Nandadasa, Chandani; Kim, Sungho; Kim, Seong-Gon; Lee, Young; Kim, Sung
2015-03-01
Electrides are an exclusive class of ionic compounds in which electrons serve as anions. We have performed first-principles density functional theory (DFT) calculations to investigate the structural, electronic and magnetic properties of two-dimensional layered-structure yttrium carbide (Y2C). Generalized gradient approximation (GGA) with Projector Augmented Potentials (PAW) was used to obtain optimized lattice parameters, energy band structure, charge density and density of states (DOS) plots for Y2C. The theoretically predicted structure of Y2C is in good agreement with the experimental results. The band crossing the Fermi energy level proved that Y2C has metallic properties. Additionally projected electronic density of states profiles were obtained to identify the electronic contribution from Y, C and non-atomic orbital located in interstitial site. The results of these calculations indicate that the presence of trapped electrons within the Y2C interlayers. Furthermore, surface energies of Y-Y and Y-C were calculated and charge densities were plotted with these surfaces. Magnetization density plots were used to obtain magnetic properties.
First-principles prediction of a high-pressure hydrous phase of AlOOH
NASA Astrophysics Data System (ADS)
Tsuchiya, Jun; Tsuchiya, Taku
2011-02-01
We have predicted a high-pressure hydrous phase of AlOOH stabilizing at ˜170 GPa by first-principles density-functional calculations. The structure predicted has a cubic pyrite-type AlO2 framework with interstitial H atoms forming symmetric hydrogen bonds, whose symmetry is assigned to the space group Pa3¯ (No. 205). The predicted δ-AlOOH to the pyrite-type phase sequence is analogous to a recent theoretical and experimental discovery of high-pressure phase evolution in InOOH and invokes the high-pressure phase relationship in SiO2, but the transition pressure is much greater in AlOOH than in InOOH. Relative enthalpies also indicate that the dissociation of this phase into a CaIrO3-type phase of Al2O3 plus ice X finally occurs at a further pressure of 300 GPa. The present results suggest that AlOOH has an unexpectedly wide stability range in pressure compared to common hydrous materials.
Rare-earth vs. heavy metal pigments and their colors from first principles.
Tomczak, Jan M; Pourovskii, Leonid V; Vaugier, Loig; Georges, Antoine; Biermann, Silke
2013-01-15
Many inorganic pigments contain heavy metals hazardous to health and environment. Much attention has been devoted to the quest for nontoxic alternatives based on rare-earth elements. However, the computation of colors from first principles is a challenge to electronic structure methods, especially for materials with localized f-orbitals. Here, starting from atomic positions only, we compute the colors of the red pigment cerium fluorosulfide as well as mercury sulfide (classic vermilion). Our methodology uses many-body theories to compute the optical absorption combined with an intermediate length-scale modelization to assess how coloration depends on film thickness, pigment concentration, and granularity. We introduce a quantitative criterion for the performance of a pigment. While for mercury sulfide, this criterion is satisfied because of large transition matrix elements between wide bands, cerium fluorosulfide presents an alternative paradigm: the bright red color is shown to stem from the combined effect of the quasi-2D and the localized nature of states. Our work shows the power of modern computational methods, with implications for the theoretical design of materials with specific optical properties.
Oxygen vacancies in amorphous-Ta2O5 from first-principles calculations
NASA Astrophysics Data System (ADS)
Lee, Jihang; Kioupakis, Emmanouil; Lu, Wei
Oxygen vacancies are thought to play a crucial role in the electrical and optical properties of tantalum pentoxide (Ta2O5) devices. Even though numerous experimental studies on oxygen vacancies in Ta2O5 exist, experimentally detected defects are ambiguously identified due to the absence of an accurate and conclusive theoretical analysis. We investigate oxygen vacancies in amorphous Ta2O5 with first-principles calculations based on hybrid density functional theory. The calculated thermodynamic and optical transition levels of stable oxygen vacancies are in good agreement with measured values from a variety of experimental methods, providing conclusive clues for the identification of the defect states observed in experiments. We determine the concentration of oxygen vacancies and their dominant oxidation state as a function of growth conditions. We analyze the characteristics of extra electrons introduced by donor-like oxygen vacancies, which include the formation of polarons. Our results provide insight into the fundamental properties of oxygen vacancies in Ta2O5, which is essential to controlling the properties of films and optimize the performance of devices. This research was supported by the AFOSR through MURI grant FA9550-12-1-0038 and the National Science Foundation CAREER award through Grant No. DMR-1254314. Computational resources were provided by the DOE NERSC facility.
Water confined in nanotubes and between graphene sheets: A first principle study
Cicero, G; Grossman, J C; Schwegler, E; Gygi, F; Galli, G
2008-10-17
Water confined at the nanoscale has been the focus of numerous experimental and theoretical investigations in recent years, y yet there is no consensus on such basic properties et as diffusion and the nature of hydrogen bonding (HB) under confinement. Unraveling these properties is important to understand fluid flow and transport at the nanoscale, and to shed light on the solvation of biomolecules. Here we report on a first principle, computational study focusing on water confined between prototypical non polar substrate, i.e. , single wall carbon nanotubes and graphene sheets, 1 to 2.5 nm apart. The results of our molecular dynamics simulations show the presence of a thin, interfacial liquid layer ({approx} 5 Angstroms) whose microscopic structure and thickness are independent of the distance between confining layers. The prop properties of the hydrogen bonded network are very similar to those of the bulk outside the interfacial region, even in the case of strong confinement , confinement. Our findings indicate that the perturbation induced by the presence of confining media is extremely local in liquid water, and we propose that many of the effects attributed to novel phases under confinement are determined by subtle electronic structure rearrangements occurring at the interface with the confining medium.
NASA Astrophysics Data System (ADS)
Jang, Woosun; Yoo, Su-Hyun; Soon, Aloysius
2015-03-01
Owing to its unique and exotic physical and chemical properties, there has been a lot of effort undertaken to explore and study ultrathin low-dimensional nanostructures (e.g. graphene and MoS2). Of late, two-dimensional (2D) nanomembranes of silicon - a well-known prototypical bulk semiconductor - have attracted much attention, and has found its potential in niche nanodevice applications e.g. field effect transistors (FET) and secondary battery anodes. In this work, after considering various nanomembranes of Si with varying thicknesses, we study geometric and electronic structures using first-principles density-functional theory calculations (and beyond). Here, we consider both bulk-terminated pristine Si nanomembranes as well as surface-reconstructed ones, as motivated by available experimental and theoretical reports. To understand the influence of growth conditions on these Si nanomembranes, we have also studied the role of surface-passivation (e.g. with O, H, and OH) on their electronic and optical properties. Namely, we carefully investigate their thickness-dependent electronic band structure (i.e. both their fundamental and optical band gap energies), so as to elucidate their intrinsic structure-property relations for designing future technologically important nanodevices.
Water solubility in calcium aluminosilicate glasses investigated by first principles techniques
Bouyer, Frederic; Geneste, Gregory; Ispas, Simona; Kob, Walter; Ganster, Patrick
2010-12-15
First-principles techniques have been employed to study the reactivity of water into a calcium aluminosilicate glass. In addition to the well known hydrolysis reactions Si-O-Si+H{sub 2}O{yields}Si-OH+Si-OH and Si-O-Al+H{sub 2}O{yields}Si-OH+Al-OH, a peculiar mechanism is found, leading to the formation of an AlO{sub 3}-H{sub 2}O entity and the breaking of Al-O-Si bond. In the glass bulk, most of the hydrolysis reactions are endothermic. Only a few regular sites are found reactive (i.e. in association with an exothermic reaction), and in that case, the hydrolysis reaction leads to a decrease of the local disorder in the amorphous vitreous network. Afterwards, we suggest that ionic charge compensators transform into network modifiers when hydrolysis occurs, according to a global process firstly suggested by Burnham in 1975. Our theoretical computations provide a more general model of the first hydrolysis steps that could help to understand experimental data and water speciation in glasses. -- Graphical Abstract: Reactivity within glass bulk: structures obtained after hydrolyses reactions (endothermic and exothermic processes) and mechanisms involving Si-OH, Al-OH, Si-OH-Al groups within aluminosilicates glasses (through ab initio molecular dynamics): formation of the Si-OH-Al entity coupled with an H exchange-Frederic Bouyer and Gregory Geneste. Display Omitted
Kumar, Vipin; Kishor, Shyam; Ramaniah, Lavanya M
2013-08-01
The structures, energetics, vertical and adiabatic ionization potentials, electron affinities, and global reactivity descriptors of antioxidant vitamins (both water- and fat-soluble) in neutral, positively charged, and negatively charged states were investigated theoretically. We worked within the framework of first-principles density functional theory (DFT), using the B3LYP functional and both localized (6-311G+(d,p) and plane-wave basis sets. Solvent effects were modeled via the polarizable continuum model (PCM), using the integral equation formalism variant (IEFPCM). From the computed structural parameters, ionization potentials, electron affinities, and spin densities, we deduced that these vitamins prefer to lose electrons to neutral reactive oxygen species (·OH and ·OOH), making them good antioxidants. Conceptual DFT was used to determine global chemical reactivity parameters. The computed chemical hardnesses showed that these antioxidant vitamins are more reactive than neutral reactive oxygen species (ROS), thus supporting their antioxidant character towards these species. However, in the neutral state, these vitamins do not act as antioxidants for [Formula: see text]. The reactivity of vitamins towards ROS depends on the nature of the solvent. Amongst the ROS, ·OH has the greatest propensity to attract electrons from a generic donor. The reactivities of fat-soluble vitamins towards neutral reactive oxygen species were found to be larger than those of water-soluble vitamins towards these species, showing that the former are better antioxidants.
First-principles study of migration and diffusion mechanisms of helium in α-Be
NASA Astrophysics Data System (ADS)
Yang, Xiao-Yong; Lu, Yong; Li, Meng-Lei; Zhang, Ping
2016-03-01
The behavior of interstitial helium in α-Be has been studied with first-principles method. It is found that the most favored position for helium is the basal octahedral (BO) site, closely followed by the basal tetrahedral (BT) site, in agreement with previous predictions. The interaction energy between the helium and the neighborhood Be atoms and the deformation energy of α-Be matrix are calculated. The feasible minimum-energy pathways (MEP) of interstitial helium atoms in α-Be matrix and the corresponding atomic structures of the saddle points associated with the each MEP are investigated. The temperature-dependent diffusion coefficients have also been predicted. It is confirmed that the interstitial helium diffuses two-dimensionally at low temperatures; however, it can diffuse three-dimensionally at higher temperatures. Besides, the microscopic parameters in the pre-factor and activation energy of the diffusion coefficients are obtained. Both diffusion coefficients are higher than the available experiment data, which may attribute to the fact that under real condition the diffusion is not free, i.e. the actual α-Be matric has various defects and impurities which heavily affect the diffusion of helium. Therefore, our theoretical prediction is the upper bound for helium diffusion in α-Be matrix.
Electronic and optical properties of GaSb:N from first principles
NASA Astrophysics Data System (ADS)
Jadaun, Priyamvada; Nair, Hari; Lordi, Vincenzo; Bank, Seth; Banerjee, Sanjay
2014-03-01
We present an ab-initio study of dilute nitride III-Vs, focusing on dilute nitride GaSb (GaSb:N). GaSb:N displays promise towards realization of optoelectronic devices accessing the mid-infrared wavelength regime. Theoretical and experimental results on its electronic and optical properties are however few. To address this, we present a first principles, density functional theory study using the hybrid HSE06 exchange-correlation functional of GaSb doped with 1.6% nitrogen. We conduct a comparative study on GaAs:N, also with 1.6% nitrogen mole fraction, and find that GaSb:N has a smaller band gap and displays more band gap bowing than GaAs:N. In addition we examine the orbital character of the bands, finding the lowest conduction band to be quasi-delocalized, with a large N-3s contribution. At high concentrations, the N atoms interact via the host matrix, forming a dispersive band of their own which governs optoelectronic properties and dominates band gap bowing. While this band drives the optical and electronic properties of GaSb:N, its physics is not captured by traditional models for dilute-nitrides. We thus propose that a complete theory of dilute-nitrides should incorporate orbital character examination, especially at high N concentrations. Texas Advanced Computing Center (TACC), U.S. Department of Energy, Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344.
Wang, J; Li, S N; Liu, J B
2015-04-16
The first-principles calculations are employed to study the migrations of pentagon-heptagon (5-7) defects in hexagonal boron nitride monolayer (h-BN). A type of grain boundaries, consisted of 5-7 defects, is constructed on the basis of experimental observations. With the absorption of a pair of atoms, one 5-7 defect in the grain boundary migrates apart by one unit cell and afterward migrates again through the bond rotation. It is also found that the two migrations could be replaced by one single step when the pair of absorbed atoms is located at another specific site in the same heptagon. Energy barriers and reaction paths for the migrations of 5-7 defects in h-BN by the bond rotation are theoretically investigated by the standard nudged elastic band method and the generalized solid-state nudged elastic band method. To elucidate the difference between the bond rotation process of the 5-7 defects with N-N bonds and those with B-B bonds, a couple of typical 21.7° grain boundaries with either N-N or B-B bonds are investigated. It is shown that the energy barrier of the migration of defects with N-N bonds is lower than that with B-B bonds in this type of grain boundaries. PMID:25811102
First-principles investigation of hydrogen interaction with TiC precipitates in α -Fe
NASA Astrophysics Data System (ADS)
Di Stefano, Davide; Nazarov, Roman; Hickel, Tilmann; Neugebauer, Jörg; Mrovec, Matous; Elsässer, Christian
2016-05-01
A correct description of hydrogen diffusion and trapping is the prerequisite for an understanding of the phenomenon of hydrogen embrittlement. In this study, we carried out extensive first-principles calculations based on density functional theory to investigate the interaction of H with TiC precipitates that are assumed to be efficient trapping agents mitigating HE in advanced high-strength steels. We found that there exists a large variety of possible trapping sites for H associated with different types of interfaces between the TiC particle and the Fe matrix, with misfit dislocations and other defects at these interfaces, and with carbon vacancies in TiC. The most efficient trapping by more than 1 eV occurs at carbon vacancies in the interior of TiC particles. However, these traps are difficult to populate at ambient temperatures since the energy barrier for H entering the particles is high. H trapping at the semicoherent interfaces between the TiC particles and the Fe matrix is moderate, ranging from 0.3 to 0.5 eV. However, a sufficiently large concentration of the carbide particles can significantly reduce the amount of H segregated at dislocation cores in the Fe matrix. A systematic comparison of the obtained theoretical results with available experimental observations reveals a consistent picture of hydrogen trapping at the TiC particles that is expected to be qualitatively valid also for other carbide precipitates with the rock-salt crystal structure.
NASA Astrophysics Data System (ADS)
Susan, S.
1993-04-01
Theoretical electronic structure techniques are used to analyze widely different systems from Si clusters to transition metal solids and surfaces. For the Si clusters, first principles density functional methods are used to investigate Si(sub N) for N=2-8. The goal is to understand the different types of bonding that can occur in such small clusters where the atomic coordination differs substantially from tetrahedral bonding; such uncoordinated structures can test approximate models of Si surfaces. For the transition metal systems, non-self-consistent electronic structure methods are used to understand the driving force for surface relaxations. In-depth analysis of results is presented and physical basis of surface relaxation within the theory is discussed. Limitations inherent in calculations of metal surface relaxation are addressed. Finally, in an effort to understand approximate methods, a novel non-self-consistent density functional electronic structure method is developed that is about 1000 times faster than more sophisticated methods; this method is tested for various systems including diatomics, mixed clusters, surfaces, and bulk lattices.
NASA Astrophysics Data System (ADS)
Sinnott, Susan Buthaina
In the research presented here, various theoretical electronic structure techniques are utilized to analyze widely different systems from silicon clusters to transition metal solids and surfaces. For the silicon clusters, first principles density functional methods are used to investigate Si_{rm N} for N = 2-8. The goal is to understand the different types of bonding that can occur in such small clusters where the coordination of the atoms differs substantially from that of the stable bulk tetrahedral bonding. Such uncoordinated structures can provide a good test of more approximate theories that can be used eventually to model silicon surfaces, of obvious technological importance. For the transition metal systems, non-self-consistent electronic structure methods are used to provide an understanding of the driving force for surface relaxations. An in-depth analysis of the results is presented and the physical basis of surface relaxation within the theory is discussed. In addition, the limitations inherent in calculations of metal surface relaxation are addressed. Finally, in an effort to increase understanding of approximate methods, a novel non-self-consistent density functional electronic structure method is developed that is ~1000 times faster computationally than more sophisticated methods. This new method is tested for a variety of systems including diatomics, mixed clusters, surfaces and bulk lattices. The strengths and weaknesses of the new theory are discussed in detail, leading to greater understanding of non-self-consistent density functional theories as a whole.
First principles DFT study of dye-sensitized CdS quantum dots
Jain, Kalpna; Singh, Kh. S.; Kishor, Shyam; Josefesson, Ida; Odelius, Michael; Ramaniah, Lavanya M.
2014-04-24
Dye-sensitized quantum dots (QDs) are considered promising candidates for dye-sensitized solar cells. In order to maximize their efficiency, detailed theoretical studies are important. Here, we report a first principles density functional theory (DFT) investigation of experimentally realized dye - sensitized QD / ligand systems, viz., Cd{sub 16}S{sub 16}, capped with acetate molecules and a coumarin dye. The hybrid B3LYP functional and a 6−311+G(d,p)/LANL2dz basis set are used to study the geometric, energetic and electronic properties of these clusters. There is significant structural rearrangement in all the clusters studied - on the surface for the bare QD, and in the positions of the acetate / dye ligands for the ligated QDs. The density of states (DOS) of the bare QD shows states in the band gap, which disappear on surface passivation with the acetate molecules. Interestingly, in the dye-sensitised QD, the HOMO is found to be localized mainly on the dye molecule, while the LUMO is on the QD, as required for photo-induced electron injection from the dye to the QD.
First-principles investigation of boron defects in nickel ferrite spinel
NASA Astrophysics Data System (ADS)
Rák, Zs.; O'Brien, C. J.; Brenner, D. W.
2014-09-01
The accumulation of boron within the porous nickel ferrite (NiFe2O4, NFO) deposited on nuclear reactor fuel rods is a major technological problem with important safety and economical implications. In this work, the electronic structure of nickel ferrite spinel has been investigated using first-principles methods, and the theoretical results have been combined with experimental data to analyze B incorporation into the spinel structure of NFO. Under thermodynamic solid-solid equilibrium between NFO and atomic reservoirs of Ni and Fe, our calculations predict that the incorporation of B into the NFO structure is unfavorable. The main factors that limit B incorporation are the narrow stability domain of NFO and the precipitation of B2O3, Fe3BO5, and Ni3B2O6 compounds as secondary phases. The B incorporation energies depend sensitively on the electron chemical potential (EF) and the charge state of the defect. In n-type NFO, the most stable defect is the Ni vacancy VNi2- while in p-type material lowest the formation energy belongs to the interstitial B occupying a tetrahedrally coordinated site BT2+. Because of these limiting conditions it is more thermodynamically favorable for B to form secondary phases with Fe, Ni and O (e.g. B2O3, Fe3BO5, and Ni3B2O6) than it is to form point defects in NFO.
First principle study of elastic and thermodynamic properties of FeB4 under high pressure
NASA Astrophysics Data System (ADS)
Zhang, Xinyu; Qin, Jiaqian; Ning, Jinliang; Sun, Xiaowei; Li, Xinting; Ma, Mingzhen; Liu, Riping
2013-11-01
The elastic properties, elastic anisotropy, and thermodynamic properties of the lately synthesized orthorhombic FeB4 at high pressures are investigated using first-principles density functional calculations. The calculated equilibrium parameters are in good agreement with the available experimental and theoretical data. The obtained normalized volume dependence of high pressure is consistent with the previous experimental data investigated using high-pressure synchrotron x-ray diffraction. The complete elastic tensors and crystal anisotropies of the FeB4 are also determined in the pressure range of 0-100 GPa. By the elastic stability criteria and vibrational frequencies, it is predicted that the orthorhombic FeB4 is stable up to 100 GPa. In addition, the calculated B/G ratio reveals that FeB4 possesses brittle nature in the range of pressure from 0 to 100 GPa. The calculated elastic anisotropic factors suggest that FeB4 is elastically anisotropic. By using quasi-harmonic Debye model, the compressibility, bulk modulus, the coefficient of thermal expansion, the heat capacity, and the Grüneisen parameter of FeB4 are successfully obtained in the present work.
First-principles study of the paths of the decomposition reaction of LiBH4
NASA Astrophysics Data System (ADS)
Caputo, Riccarda; Züttel, Andreas
2010-05-01
A clear description of the paths of thermal decomposition of complex borohydrides represents a crucial step forward to their utilisation as a reservoir of hydrogen and hence as materials for solid state hydrogen storage. We present in this work a theoretical study of the possible paths of decomposition of LiBH4 by means of density functional theory approach. Our first-principles calculations showed the possibility to form linear chains of tetraborate of lithium in the residue of decomposition, among other thermodynamically competitive reactions. Their analytical formula LiBH x agreed with the quantitative analysis already reported by Schlesinger and co-workers in the 1940s. The structure showed the formula unit Li4B4H10, and the analytical formula LiBH2.5, of which the Gibbs free energy of formation was -111.76 kJ mol-1. The lattice stability was confirmed by the phonon calculations, which revealed all positive normal modes. Comparatively, the formation of lithium dodecaborate(12) is presented and discussed. The calculated standard Gibbs free energy of the decomposition reactions considered in the present work were in the range (158,286) kJ mol-1 of LiBH4 decomposed.
Thermoelectric transport properties of warm dense molybdenum from first-principles simulations
NASA Astrophysics Data System (ADS)
French, Martin; Haill, Thomas; Desjarlais, Michael; Mattsson, Thomas
2013-10-01
Molybdenum, with its high melting point, significant electrical conductivity, and high material strength, is a technologically important material in general and has in particular recently been proposed as a driver material in high-pressure strength experiments on Sandia's Z-machine. To simulate and understand the processes in these experiments with magneto-hydrodynamic simulations, accurate models for the electrical and thermal conductivity are needed for a wide range of thermodynamic parameters. Here, we present novel results for the electrical and thermal conductivity of molybdenum in various states ranging from the solid to the dense plasma phase. The results were obtained with first-principles simulation techniques that combine density functional theory with molecular dynamics and linear response theory. We find good agreement between our theoretical results and available experimental data. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.
Thermoelectric properties of AgSbTe₂ from first-principles calculations
Rezaei, Nafiseh; Akbarzadeh, Hadi; Hashemifar, S. Javad
2014-09-14
The structural, electronic, and transport properties of AgSbTe₂ are studied by using full-relativistic first-principles electronic structure calculation and semiclassical description of transport parameters. The results indicate that, within various exchange-correlation functionals, the cubic Fd3⁻m and trigonal R3⁻m structures of AgSbTe₂ are more stable than two other considered structures. The computed Seebeck coefficients at different values of the band gap and carrier concentration are accurately compared with the available experimental data to speculate a band gap of about 0.1–0.35 eV for AgSbTe₂ compound, in agreement with our calculated electronic structure within the hybrid HSE (Heyd-Scuseria-Ernzerhof) functional. By calculating the semiclassical Seebeck coefficient, electrical conductivity, and electronic part of thermal conductivity, we present the theoretical upper limit of the thermoelectric figure of merit of AgSbTe₂ as a function of temperature and carrier concentration.
ERIC Educational Resources Information Center
Cappelletti, John
1989-01-01
Details the following thoughts on directing for the theater: select the play, read the play, read the play again, ask questions, discover the spine, form a concept, cast the play, listen to the actors, make it move, make it spontaneous, make it real, look for obstacles, make it relevant, and make it important. (PRA)
NASA Astrophysics Data System (ADS)
Mary, J. Arul; Vijaya, J. Judith; Bououdina, M.; Kennedy, L. John; Dai, J. H.; Song, Y.
2015-02-01
Ce, Cu co-doped ZnO (Zn1-2xCexCuxO: x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05) nanocrystals were synthesized by a microwave combustion method. These nanocrystals were investigated by using X-ray diffraction (XRD), UV-visible diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The stability and magnetic properties of Ce and Cu co-doped ZnO were probed by first principle calculations. XRD results revealed that all the compositions are single crystalline. hexagonal wurtzite structure. The optical band gap of pure ZnO was found to be 3.22 eV, and it decreased from 3.15 to 3.10 eV with an increase in the concentration of Cu and Ce content. The morphologies of Ce and Cu co-doped ZnO samples confirmed the formation of nanocrystals with an average grain size ranging from 70 to 150 nm. The magnetization measurement results affirmed the antiferro and ferromagnetic state for Ce and Cu co-doped ZnO samples and this is in agreement with the first principles theoretical calculations.
First-principles XANES simulations of spinel zinc ferrite with a disordered cation distribution
Nakashima, Seisuke; Fujita, Koji; Tanaka, Katsuhisa; Hirao, Kazuyuki; Yamamoto, Tomoyuki; Tanaka, Isao
2007-05-01
Theoretical calculations of Zn K and Fe K x-ray absorption near-edge structures (XANES) using a first-principles method have been performed to evaluate the degree of cation disordering in spinel zinc ferrite (ZnFe{sub 2}O{sub 4}) thin film prepared by a sputtering method, ZnFe{sub 2}O{sub 4} thin films annealed at elevated temperatures, and ZnFe{sub 2}O{sub 4} bulk specimen prepared by a solid-state reaction. Using the full-potential linearized augmented plane-wave + local orbitals method, a theoretical spectrum is generated for the tetrahedral and octahedral environments for each of the two cations. The experimental XANES spectrum of the thin film annealed at 800 deg. C as well as that of bulk specimen is successfully reproduced by using either the theoretical spectrum for Zn{sup 2+} on the tetrahedral site (A site) or that for Fe{sup 3+} on the octahedral site (B site), which is indicative of the normal spinel structure. For the as-deposited film, on the other hand, excellent agreement between theoretical and experimental spectra is obtained by considering the presence of either ion in both the A and B sites. The degree of cation disordering, x, defined as [Zn{sub 1-x}{sup 2+}Fe{sub x}{sup 3+}]{sub A}[Zn{sub x}{sup 2+}Fe{sub 2-x}{sup 3+}]{sub B}O{sub 4}, is estimated to be approximately 0.6 in the as-deposited film, which is consistent with the analysis of the extended x-ray absorption fine structure on the Zn K edge. Curious magnetic properties as we previously observed for the as-deposited thin film--i.e., ferrimagnetic behaviors accompanied by large magnetization at room temperature and cluster spin-glass-like behavior--are discussed in connection with disordering of Zn{sup 2+} and Fe{sup 3+} ions in the spinel-type structure.
Goldman, N; Leforestier, C; Saykally, R J
2004-05-25
We present results of gas phase cluster and liquid water simulations from the recently determined VRT(ASP-W)III water dimer potential energy surface. VRT(ASP-W)III is shown to not only be a model of high ''spectroscopic'' accuracy for the water dimer, but also makes accurate predictions of vibrational ground-state properties for clusters up through the hexamer. Results of ambient liquid water simulations from VRT(ASP-W)III are compared to those from ab initio Molecular Dynamics, other potentials of ''spectroscopic'' accuracy, and to experiment. The results herein represent the first time that a ''spectroscopic'' potential surface is able to correctly model condensed phase properties of water.
Tetrapeptide unfolding dynamics followed by core-level spectroscopy: a first-principles approach.
Taioli, Simone; Simonucci, Stefano; A Beccara, Silvio; Garavelli, Marco
2015-05-01
In this work we demonstrate that core level analysis is a powerful tool for disentangling the dynamics of a model polypeptide undergoing conformational changes in solution and disulphide bond formation. In particular, we present computer simulations within both initial and final state approximations of 1s sulphur core level shifts (S1s CLS) of the CYFC (cysteine-phenylalanine-tyrosine-cysteine) tetrapeptide for different folding configurations. Using increasing levels of accuracy, from Hartree-Fock and density functional theory to configuration interaction via a multiscale algorithm capable of reducing drastically the computational cost of electronic structure calculations, we find that distinct peptide arrangements present S1s CLS sizeably different (in excess of 0.5 eV) with respect to the reference disulfide bridge state. This approach, leading to experimentally detectable signals, may represent an alternative to other established spectroscopic techniques.
Sun, Shou -Tian; Jiang, Ling; Liu, J. W.; Heine, Nadja; Yacovitch, Tara I.; Wende, Torsten; Asmis, Knut R.; Neumark, Daniel M.; Liu, Zhi -Feng
2015-06-05
We report infrared multiple photon dissociation (IRMPD) spectra of cryogenically-cooled H2PO4-(H2O)n anions (n = 2–12) in the spectral range of the stretching and bending modes of the solute anion (600–1800 cm-1). The spectra cannot be fully understood using the standard technique of comparison to harmonic spectra of minimum-energy structures; a satisfactory assignment requires considering anharmonic effects as well as entropy-driven hydrogen bond network fluctuations. Aided by finite temperature ab initio molecular dynamics simulations, the observed changes in the position, width and intensity of the IRMPD bands with cluster size are related to the sequence of microsolvation. Due to stronger hydrogenmore » bonding to the two terminal P=O groups, these are hydrated before the two P–OH groups. By n = 6, all four end groups are involved in the hydrogen bond network and by n = 12, the cluster spectra show similarities to the condensed phase spectrum of H2PO4-(aq). Our results reveal some of the microscopic details concerning the formation of the aqueous solvation environment around H2PO4-, provide ample testing grounds for the design of model solvation potentials for this biologically relevant anion, and support a new paradigm for the interpretation of IRMPD spectra of microhydrated ions.« less
Sun, Shou -Tian; Jiang, Ling; Liu, J. W.; Heine, Nadja; Yacovitch, Tara I.; Wende, Torsten; Asmis, Knut R.; Neumark, Daniel M.; Liu, Zhi -Feng
2015-06-05
We report infrared multiple photon dissociation (IRMPD) spectra of cryogenically-cooled H_{2}PO_{4}^{-}(H_{2}O)_{n} anions (n = 2–12) in the spectral range of the stretching and bending modes of the solute anion (600–1800 cm-1). The spectra cannot be fully understood using the standard technique of comparison to harmonic spectra of minimum-energy structures; a satisfactory assignment requires considering anharmonic effects as well as entropy-driven hydrogen bond network fluctuations. Aided by finite temperature ab initio molecular dynamics simulations, the observed changes in the position, width and intensity of the IRMPD bands with cluster size are related to the sequence of microsolvation. Due to stronger hydrogen bonding to the two terminal P=O groups, these are hydrated before the two P–OH groups. By n = 6, all four end groups are involved in the hydrogen bond network and by n = 12, the cluster spectra show similarities to the condensed phase spectrum of H_{2}PO_{4}^{-}(aq). Our results reveal some of the microscopic details concerning the formation of the aqueous solvation environment around H_{2}PO_{4}^{-}, provide ample testing grounds for the design of model solvation potentials for this biologically relevant anion, and support a new paradigm for the interpretation of IRMPD spectra of microhydrated ions.
Combined Raman spectroscopy and first-principles calculation for essential oil of Lemongrass
NASA Astrophysics Data System (ADS)
Faria, Rozilaine A. P. G.; Picanço, Nágela F. M.; Campo, Gladís S. D. L.; Faria, Jorge L. B.; Instituto de Física/UFMT Collaboration; Instituto Federal de Mato Grosso/IFMT Team
2014-03-01
The essential oils have increased food's industry interest by the presence of antioxidant and antimicrobial. Many of them have antimicrobial and antioxidant, antibacterial and antifungal activities. But, due to the concentrations required to be added in the food matrix, the sensory quality of the food is changed. The production and composition of essential oil extracted from plants depend on the plant-environment interactions, the harvest season, phenophase and physiological state of the vegetal. Cymbopogom citratus (Lemongrass) has a good yield in essential oil with neral (citral A), geranial (citral B) and myrcene, reaching 90% of the oil composition. In our experimental work, the essential oil of lemongrass was obtained by hydrodistillation in Clevenger apparatus for 4 hours. The compound was further analyzed by Raman scattering in a spectrometer HR 800, with excitation at 633nm, in the range 80-3400 cm-1. The spectrum obtained was compared with DFT calculations of molecules of the oil components. Our results show the vibrational signatures of the main functional groups and suggest a simple, but very useful, methodology to quantify the proportions of these components in the oil composition, showing good agreement with Raman data. CNPq/Capes/Fapemat.
Tsyshevsky, Roman V; Sharia, Onise; Kuklja, Maija M
2016-02-19
This review presents a concept, which assumes that thermal decomposition processes play a major role in defining the sensitivity of organic energetic materials to detonation initiation. As a science and engineering community we are still far away from having a comprehensive molecular detonation initiation theory in a widely agreed upon form. However, recent advances in experimental and theoretical methods allow for a constructive and rigorous approach to design and test the theory or at least some of its fundamental building blocks. In this review, we analyzed a set of select experimental and theoretical articles, which were augmented by our own first principles modeling and simulations, to reveal new trends in energetic materials and to refine known existing correlations between their structures, properties, and functions. Our consideration is intentionally limited to the processes of thermally stimulated chemical reactions at the earliest stage of decomposition of molecules and materials containing defects.
Tsyshevsky, Roman; Sharia, Onise; Kuklja, Maija
2016-02-19
Our review presents a concept, which assumes that thermal decomposition processes play a major role in defining the sensitivity of organic energetic materials to detonation initiation. As a science and engineering community we are still far away from having a comprehensive molecular detonation initiation theory in a widely agreed upon form. However, recent advances in experimental and theoretical methods allow for a constructive and rigorous approach to design and test the theory or at least some of its fundamental building blocks. In this review, we analyzed a set of select experimental and theoretical articles, which were augmented by our ownmore » first principles modeling and simulations, to reveal new trends in energetic materials and to refine known existing correlations between their structures, properties, and functions. Lastly, our consideration is intentionally limited to the processes of thermally stimulated chemical reactions at the earliest stage of decomposition of molecules and materials containing defects.« less
Tsyshevsky, Roman V; Sharia, Onise; Kuklja, Maija M
2016-01-01
This review presents a concept, which assumes that thermal decomposition processes play a major role in defining the sensitivity of organic energetic materials to detonation initiation. As a science and engineering community we are still far away from having a comprehensive molecular detonation initiation theory in a widely agreed upon form. However, recent advances in experimental and theoretical methods allow for a constructive and rigorous approach to design and test the theory or at least some of its fundamental building blocks. In this review, we analyzed a set of select experimental and theoretical articles, which were augmented by our own first principles modeling and simulations, to reveal new trends in energetic materials and to refine known existing correlations between their structures, properties, and functions. Our consideration is intentionally limited to the processes of thermally stimulated chemical reactions at the earliest stage of decomposition of molecules and materials containing defects. PMID:26907231
Buck, D.R.
2000-09-12
Theoretical simulations and ultrafast pump-probe laser spectroscopy experiments were used to study photosynthetic pigment-protein complexes and antennae found in green sulfur bacteria such as Prosthecochloris aestuarii, Chloroflexus aurantiacus, and Chlorobium tepidum. The work focused on understanding structure-function relationships in energy transfer processes in these complexes through experiments and trying to model that data as we tested our theoretical assumptions with calculations. Theoretical exciton calculations on tubular pigment aggregates yield electronic absorption spectra that are superimpositions of linear J-aggregate spectra. The electronic spectroscopy of BChl c/d/e antennae in light harvesting chlorosomes from Chloroflexus aurantiacus differs considerably from J-aggregate spectra. Strong symmetry breaking is needed if we hope to simulate the absorption spectra of the BChl c antenna. The theory for simulating absorption difference spectra in strongly coupled photosynthetic antenna is described, first for a relatively simple heterodimer, then for the general N-pigment system. The theory is applied to the Fenna-Matthews-Olson (FMO) BChl a protein trimers from Prosthecochloris aestuarii and then compared with experimental low-temperature absorption difference spectra of FMO trimers from Chlorobium tepidum. Circular dichroism spectra of the FMO trimer are unusually sensitive to diagonal energy disorder. Substantial differences occur between CD spectra in exciton simulations performed with and without realistic inhomogeneous distribution functions for the input pigment diagonal energies. Anisotropic absorption difference spectroscopy measurements are less consistent with 21-pigment trimer simulations than 7-pigment monomer simulations which assume that the laser-prepared states are localized within a subunit of the trimer. Experimental anisotropies from real samples likely arise from statistical averaging over states with diagonal energies shifted by
Prediction of new high pressure structural sequence in thorium carbide: A first principles study
NASA Astrophysics Data System (ADS)
Sahoo, B. D.; Joshi, K. D.; Gupta, Satish C.
2015-05-01
In the present work, we report the detailed electronic band structure calculations on thorium monocarbide. The comparison of enthalpies, derived for various phases using evolutionary structure search method in conjunction with first principles total energy calculations at several hydrostatic compressions, yielded a high pressure structural sequence of NaCl type (B1) → Pnma → Cmcm → CsCl type (B2) at hydrostatic pressures of ˜19 GPa, 36 GPa, and 200 GPa, respectively. However, the two high pressure experimental studies by Gerward et al. [J. Appl. Crystallogr. 19, 308 (1986); J. Less-Common Met. 161, L11 (1990)] one up to 36 GPa and other up to 50 GPa, on substoichiometric thorium carbide samples with carbon deficiency of ˜20%, do not report any structural transition. The discrepancy between theory and experiment could be due to the non-stoichiometry of thorium carbide samples used in the experiment. Further, in order to substantiate the results of our static lattice calculations, we have determined the phonon dispersion relations for these structures from lattice dynamic calculations. The theoretically calculated phonon spectrum reveal that the B1 phase fails dynamically at ˜33.8 GPa whereas the Pnma phase appears as dynamically stable structure around the B1 to Pnma transition pressure. Similarly, the Cmcm structure also displays dynamic stability in the regime of its structural stability. The B2 phase becomes dynamically stable much below the Cmcm to B2 transition pressure. Additionally, we have derived various thermophysical properties such as zero pressure equilibrium volume, bulk modulus, its pressure derivative, Debye temperature, thermal expansion coefficient and Gruneisen parameter at 300 K and compared these with available experimental data. Further, the behavior of zero pressure bulk modulus, heat capacity and Helmholtz free energy has been examined as a function temperature and compared with the experimental data of Danan [J. Nucl. Mater. 57, 280
NASA Astrophysics Data System (ADS)
Ying, Chun; Zhao, Erjun; Lin, Lin; Hou, Qingyu
2014-10-01
The structural determination, thermodynamic, mechanical, dynamic and electronic properties of 4d transitional metal diborides MB2 (M = Y-Ag) are systematically investigated by first-principles within the density functional theory (DFT). For each diboride, five structures are considered, i.e. AlB2-, ReB2-, OsB2-, MoB2- and WB2-type structures. The calculated lattice parameters are in good agreement with the previously theoretical and experimental studies. The formation enthalpy increases from YB2 to AgB2 in AlB2-type structure (similar to MoB2- and WB2-type). While the formation enthalpy decreases from YB2 to MoB2, reached minimum value to TcB2, and then increases gradually in ReB2-type structure (similar to OsB2-type), which is consistent with the results of the calculated density of states. The structural stability of these materials relates mainly on electronegative of metals, boron structure and bond characters. Among the considered structures, TcB2-ReB2 (TcB2-ReB2 represents TcB2 in ReB2-type structure, the same hereinafter) has the largest shear modulus (248 GPa), and is the hardest compound. The number of electrons transferred from metals to boron atoms and the calculated densities of states (DOS) indicate that each diboride is a complex mixture of metallic, ionic and covalent characteristics. Trends are discussed.
Prediction of new high pressure structural sequence in thorium carbide: A first principles study
Sahoo, B. D. Joshi, K. D.; Gupta, Satish C.
2015-05-14
In the present work, we report the detailed electronic band structure calculations on thorium monocarbide. The comparison of enthalpies, derived for various phases using evolutionary structure search method in conjunction with first principles total energy calculations at several hydrostatic compressions, yielded a high pressure structural sequence of NaCl type (B1) → Pnma → Cmcm → CsCl type (B2) at hydrostatic pressures of ∼19 GPa, 36 GPa, and 200 GPa, respectively. However, the two high pressure experimental studies by Gerward et al. [J. Appl. Crystallogr. 19, 308 (1986); J. Less-Common Met. 161, L11 (1990)] one up to 36 GPa and other up to 50 GPa, on substoichiometric thorium carbide samples with carbon deficiency of ∼20%, do not report any structural transition. The discrepancy between theory and experiment could be due to the non-stoichiometry of thorium carbide samples used in the experiment. Further, in order to substantiate the results of our static lattice calculations, we have determined the phonon dispersion relations for these structures from lattice dynamic calculations. The theoretically calculated phonon spectrum reveal that the B1 phase fails dynamically at ∼33.8 GPa whereas the Pnma phase appears as dynamically stable structure around the B1 to Pnma transition pressure. Similarly, the Cmcm structure also displays dynamic stability in the regime of its structural stability. The B2 phase becomes dynamically stable much below the Cmcm to B2 transition pressure. Additionally, we have derived various thermophysical properties such as zero pressure equilibrium volume, bulk modulus, its pressure derivative, Debye temperature, thermal expansion coefficient and Gruneisen parameter at 300 K and compared these with available experimental data. Further, the behavior of zero pressure bulk modulus, heat capacity and Helmholtz free energy has been examined as a function temperature and compared with the experimental data of Danan [J
Chang, Jing; Zhao, Guo-Ping; Zhou, Xiao-Lin; Liu, Ke; Lu, Lai-Yu
2012-01-01
The structure and mechanical properties of tantalum mononitride (TaN) are investigated at high pressure from first-principles using the plane wave pseudopotential method within the local density approximation. Three stable phases were considered, i.e., two hexagonal phases (ε and θ) and a cubic δ phase. The obtained equilibrium structure parameters and ground state mechanical properties are in excellent agreement with the experimental and other theoretical results. A full elastic tensor and crystal anisotropy of the ultra-incompressible TaN in three stable phases are determined in the wide pressure range. Results indicated that the elastic properties of TaN in three phases are strongly pressure dependent. And the hexagonal θ-TaN is the most ultraincompressible among the consider phases, which suggests that the θ phase of TaN is a potential candidate structure to be one of the ultraincompressible and hard materials. By the elastic stability criteria, it is predicted that θ-TaN is not stable above 53.9 GPa. In addition, the calculated B/G ratio indicated that the ε and δ phases possess brittle nature in the range of pressure from 0 to 100 GPa. While θ phase is brittleness at low pressure (below 8.2 GPa) and is strongly prone to ductility at high pressure (above 8.2 GPa). The calculated elastic anisotropic factors for three phases of TaN suggest that they are elastically highly anisotropic and strongly dependent on the propagation direction. PMID:23185097
Jaramillo-Botero, Andres; Nielsen, Robert; Abrol, Ravi; Su, Julius; Pascal, Tod; Mueller, Jonathan; Goddard, William A
2012-01-01
We expect that systematic and seamless computational upscaling and downscaling for modeling, predicting, or optimizing material and system properties and behavior with atomistic resolution will eventually be sufficiently accurate and practical that it will transform the mode of development in the materials, chemical, catalysis, and Pharma industries. However, despite truly dramatic progress in methods, software, and hardware, this goal remains elusive, particularly for systems that exhibit inherently complex chemistry under normal or extreme conditions of temperature, pressure, radiation, and others. We describe here some of the significant progress towards solving these problems via a general multiscale, multiparadigm strategy based on first-principles quantum mechanics (QM), and the development of breakthrough methods for treating reaction processes, excited electronic states, and weak bonding effects on the conformational dynamics of large-scale molecular systems. These methods have resulted directly from filling in the physical and chemical gaps in existing theoretical and computational models, within the multiscale, multiparadigm strategy. To illustrate the procedure we demonstrate the application and transferability of such methods on an ample set of challenging problems that span multiple fields, system length- and timescales, and that lay beyond the realm of existing computational or, in some case, experimental approaches, including understanding the solvation effects on the reactivity of organic and organometallic structures, predicting transmembrane protein structures, understanding carbon nanotube nucleation and growth, understanding the effects of electronic excitations in materials subjected to extreme conditions of temperature and pressure, following the dynamics and energetics of long-term conformational evolution of DNA macromolecules, and predicting the long-term mechanisms involved in enhancing the mechanical response of polymer-based hydrogels.
Bondi, Robert J. Desjarlais, Michael P.; Thompson, Aidan P.; Brennecka, Geoff L.; Marinella, Matthew J.
2013-11-28
We apply first-principles density-functional theory (DFT) calculations, ab-initio molecular dynamics, and the Kubo-Greenwood formula to predict electrical conductivity in Ta{sub 2}O{sub x} (0 ≤ x ≤ 5) as a function of composition, phase, and temperature, where additional focus is given to various oxidation states of the O monovacancy (V{sub O}{sup n}; n = 0,1+,2+). In the crystalline phase, our DFT calculations suggest that V{sub O}{sup 0} prefers equatorial O sites, while V{sub O}{sup 1+} and V{sub O}{sup 2+} are energetically preferred in the O cap sites of TaO{sub 7} polyhedra. Our calculations of DC conductivity at 300 K agree well with experimental measurements taken on Ta{sub 2}O{sub x} thin films (0.18 ≤ x ≤ 4.72) and bulk Ta{sub 2}O{sub 5} powder-sintered pellets, although simulation accuracy can be improved for the most insulating, stoichiometric compositions. Our conductivity calculations and further interrogation of the O-deficient Ta{sub 2}O{sub 5} electronic structure provide further theoretical basis to substantiate V{sub O}{sup 0} as a donor dopant in Ta{sub 2}O{sub 5}. Furthermore, this dopant-like behavior is specific to the neutral case and not observed in either the 1+ or 2+ oxidation states, which suggests that reduction and oxidation reactions may effectively act as donor activation and deactivation mechanisms, respectively, for V{sub O}{sup n} in Ta{sub 2}O{sub 5}.
NASA Astrophysics Data System (ADS)
Sonis, M.
Socio-ecological dynamics emerged from the field of Mathematical SocialSciences and opened up avenues for re-examination of classical problems of collective behavior in Social and Spatial sciences. The ``engine" of this collective behavior is the subjective mental evaluation of level of utilities in the future, presenting sets of composite socio-economic-temporal-locational advantages. These dynamics present new laws of collective multi-population behavior which are the meso-level counterparts of the utility optimization individual behavior. The central core of the socio-ecological choice dynamics includes the following first principle of the collective choice behavior of ``Homo Socialis" based on the existence of ``collective consciousness": the choice behavior of ``Homo Socialis" is a collective meso-level choice behavior such that the relative changes in choice frequencies depend on the distribution of innovation alternatives between adopters of innovations. The mathematical basis of the Socio-Ecological Dynamics includes two complementary analytical approaches both based on the use of computer modeling as a theoretical and simulation tool. First approach is the ``continuous approach" --- the systems of ordinary and partial differential equations reflecting the continuous time Volterra ecological formalism in a form of antagonistic and/or cooperative collective hyper-games between different sub-sets of choice alternatives. Second approach is the ``discrete approach" --- systems of difference equations presenting a new branch of the non-linear discrete dynamics --- the Discrete Relative m-population/n-innovations Socio-Spatial Dynamics (Dendrinos and Sonis, 1990). The generalization of the Volterra formalism leads further to the meso-level variational principle of collective choice behavior determining the balance between the resulting cumulative social spatio-temporal interactions among the population of adopters susceptible to the choice alternatives and the
Vibrational spectral diffusion and hydrogen bond dynamics in heavy water from first principles.
Mallik, Bhabani S; Semparithi, A; Chandra, Amalendu
2008-06-12
We present a first-principles theoretical study of vibrational spectral diffusion and hydrogen bond dynamics in heavy water without using any empirical model potentials. The calculations are based on ab initio molecular dynamics simulations for trajectory generation and a time series analysis using the wavelet method for frequency calculations. It is found that, in deuterated water, although a one-to-one relation does not exist between the instantaneous frequency of an OD bond and the distance of its associated hydrogen bond, such a relation does hold on average. The dynamics of spectral diffusion is investigated by means of frequency-time correlation and spectral hole dynamics calculations. Both of these functions are found to have a short-time decay with a time scale of approximately 100 fs corresponding to dynamics of intact hydrogen bonds and a slower long-time decay with a time constant of approximately 2 ps corresponding to lifetimes of hydrogen bonds. The connection of the slower time scale to the dynamics of local structural relaxation is also discussed. The dynamics of hydrogen bond making is shown to have a rather fast time scale of approximately 100 fs; hence, it can also contribute to the short-time dynamics of spectral diffusion. A damped oscillation is also found at around 150-200 fs, which is shown to have come from underdamped intermolecular vibrations of a hydrogen-bonded water pair. Such assignments are confirmed by independent calculations of power spectra of intermolecular motion and hydrogen bond kinetics using the population correlation function formalism. The details of the time constants of frequency correlations and spectral shifts are found to depend on the frequencies of chosen OD bonds and are analyzed in terms of the dynamics of hydrogen bonds of varying strengths and also of free non-hydrogen-bonded OD groups.
Cao, Xiaoxiao; Huang, Yingying; Li, Wenbo; Zheng, Zhaoyang; Jiang, Xue; Su, Yan; Zhao, Jijun; Liu, Changling
2016-01-28
Natural gas hydrates are inclusion compounds composed of major light hydrocarbon gaseous molecules (CH4, C2H6, and C3H8) and a water clathrate framework. Understanding the phase stability and formation conditions of natural gas hydrates is crucial for their future exploitation and applications and requires an accurate description of intermolecular interactions. Previous ab initio calculations on gas hydrates were mainly limited by the cluster models, whereas the phase diagram and equilibrium conditions of hydrate formation were usually investigated using the thermodynamic models or empirical molecular simulations. For the first time, we construct the chemical potential phase diagrams of type II clathrate hydrates encapsulated with methane/ethane/propane guest molecules using first-principles thermodynamics. We find that the partially occupied structures (136H2O·1CH4, 136H2O·16CH4, 136H2O·20CH4, 136H2O·1C2H6, and 136H2O·1C3H8) and fully occupied structures (136H2O·24CH4, 136H2O·8C2H6, and 136H2O·8C3H8) are thermodynamically favorable under given pressure-temperature (p-T) conditions. The theoretically predicted equilibrium pressures for pure CH4, C2H6 and C3H8 hydrates at the phase transition point are consistent with the experimental data. These results provide valuable guidance for establishing the relationship between the accurate description of intermolecular noncovalent interactions and the p-T equilibrium conditions of clathrate hydrates and other molecular crystals.
Cao, Xiaoxiao; Huang, Yingying; Li, Wenbo; Zheng, Zhaoyang; Jiang, Xue; Su, Yan; Zhao, Jijun; Liu, Changling
2016-01-28
Natural gas hydrates are inclusion compounds composed of major light hydrocarbon gaseous molecules (CH4, C2H6, and C3H8) and a water clathrate framework. Understanding the phase stability and formation conditions of natural gas hydrates is crucial for their future exploitation and applications and requires an accurate description of intermolecular interactions. Previous ab initio calculations on gas hydrates were mainly limited by the cluster models, whereas the phase diagram and equilibrium conditions of hydrate formation were usually investigated using the thermodynamic models or empirical molecular simulations. For the first time, we construct the chemical potential phase diagrams of type II clathrate hydrates encapsulated with methane/ethane/propane guest molecules using first-principles thermodynamics. We find that the partially occupied structures (136H2O·1CH4, 136H2O·16CH4, 136H2O·20CH4, 136H2O·1C2H6, and 136H2O·1C3H8) and fully occupied structures (136H2O·24CH4, 136H2O·8C2H6, and 136H2O·8C3H8) are thermodynamically favorable under given pressure-temperature (p-T) conditions. The theoretically predicted equilibrium pressures for pure CH4, C2H6 and C3H8 hydrates at the phase transition point are consistent with the experimental data. These results provide valuable guidance for establishing the relationship between the accurate description of intermolecular noncovalent interactions and the p-T equilibrium conditions of clathrate hydrates and other molecular crystals. PMID:26745181
NASA Astrophysics Data System (ADS)
Zhang, Huai-Yong; Zeng, Zhao-Yi; Zhao, Ying-Qin; Lu, Qing; Cheng, Yan
2016-08-01
Lattice dynamics, structural phase transition, and the thermodynamic properties of barium titanate (BaTiO3) are investigated by using first-principles calculations within the density functional theory (DFT). It is found that the GGA-WC exchange-correlation functional can produce better results. The imaginary frequencies that indicate structural instability are observed for the cubic, tetragonal, and orthorhombic phases of BaTiO3 and no imaginary frequencies emerge in the rhombohedral phase. By examining the partial phonon density of states (PDOSs), we find that the main contribution to the imaginary frequencies is the distortions of the perovskite cage (Ti-O). On the basis of the site-symmetry consideration and group theory, we give the comparative phonon symmetry analysis in four phases, which is useful to analyze the role of different atomic displacements in the vibrational modes of different symmetry. The calculated optical phonon frequencies at Γ point for the four phases are in good agreement with other theoretical and experimental data. The pressure-induced phase transition of BaTiO3 among four phases and the thermodynamic properties of BaTiO3 in rhombohedral phase have been investigated within the quasi-harmonic approximation (QHA). The sequence of the pressure-induced phase transition is rhombohedral→orthorhombic→tetragonal→cubic, and the corresponding transition pressure is 5.17, 5.92, 6.65 GPa, respectively. At zero pressure, the thermal expansion coefficient αV, heat capacity CV, Grüneisen parameter γ, and bulk modulus B of the rhombohedral phase BaTiO3 are estimated from 0 K to 200 K.
Review of high pressure phases of calcium by first-principles calculations
NASA Astrophysics Data System (ADS)
Ishikawa, T.; Nagara, H.; Suzuki, N.; Tsuchiya, J.; Tsuchiya, T.
2010-03-01
We review high pressure phases of calcium which have obtained by recent experimental and first-principles studies. In this study, we investigated the face-centered cubic (fcc) structure, the body-centered cubic (bcc) structure, the simple cubic (sc) structure, a tetragonal P43212 [Ishikawa T et al. 2008 Phys. Rev. B 77 020101(R)], an orthorhombic Cmca [Ishikawa T et al. 2008 Phys. Rev. B 77 020101(R)], an orthorhombic Cmcm [Teweldeberhan A M and Bonev S A 2008 Phys. Rev. B 78 140101(R)], an orthorhombic Pnma [Yao Y et al. 2008 Phys. Rev. B 78 054506] and a tetragonal I4/mcm(00) [Arapan S et al. 2008 Proc. Natl. Acad. Sci. USA 105 20627]. We compared the enthalpies among the structures up to 200 GPa and theoretically determined the phase diagram of calcium. The sequence of the structural transitions is fcc (0- 3.5 GPa) → bcc (3.5 - 35.7 GPa) → Cmcm (35.7- 52GPa) → P43212 (52-109 GPa) → Cmca (109-117.4GPa) → Pnma (117.4-134.6GPa) → I4/mcm(00) (134.6 GPa -). The sc phase is experimentally observed in the pressure range from 32 to 113 GPa but, in our calculation, there is no pressure region where the sc phase is the most stable. In addition, we found that the enthalpy of the hexagonal close-packed (hcp) structure is lower than that of I4/mcm(00) above 495 GPa.
Liquid Iron Alloys with Hydrogen at Outer Core Conditions by First Principles
NASA Astrophysics Data System (ADS)
Umemoto, K.; Hirose, K.
2015-12-01
Since the density of the outer core deduced from seismic data is about 10% lower than that of pure iron at core pressures and temperatures (P-T), it is widely believed that the outer core includes one or more light elements. Although intensive experimental and theoretical studies have been performed so far, the light element in the core has not yet been identified. Comparison of the density and sound velocity of liquid iron alloys with observations, such as the PREM, is a promising way to determine the species and quantity of light alloying component(s) in the outer core. Here we report the results of a first-principles molecular dynamics study on liquid iron alloyed with hydrogen, one of candidates of the light elements. Hydrogen had been much less studied than other candidates. However, hydrogen has been known to reduce the melting temperature of Fe-H solid [1]. Furthermore, very recently, Nomura et al. argued that the outer core may include 24 at.% H in order to be molten under relatively low temperature (< 3600 K) [2]. Since then hydrogen has attracted strong interests. We clarify the effects of hydrogen on density and sound velocity of liquid iron alloys under outer core P-T conditions. It is shown that ~1 wt% hydrogen can reproduce PREM density and sound velocity simultaneously very well. In addition, we show the presence of hydrogen rather reduces Gruneisen parameters. It indicates that, if hydrogen exists in the outer core, temperature profile of the outer core could be changed considerably from one estimated so far. [1] Sakamaki, K., E. Takahashi, Y. Nakajima, Y. Nishihara, K. Funakoshi, T. Suzuki, and Y. Fukai, Phys. Earth Planet. Inter., 174, 192-201 (2009). [2] Nomura, R., K. Hirose, K. Uesugi, Y. Ohishi, A. Tsuchiyama, A. Miyake, and Y. Ueno, Science 31, 522-525 (2014).
NASA Astrophysics Data System (ADS)
Orlikowski, Daniel; Söderlind, Per; Moriarty, John A.
2006-08-01
The thermoelastic properties of tantalum have been investigated over its theoretical high-pressure bcc solid phase (up to 26000K at 10Mbar ) using an advanced first-principles approach that accurately accounts for cold, electron-thermal, and ion-thermal contributions in materials where anharmonic effects are small. Specifically, we have combined ab initio full-potential linear-muffin-tin-orbital electronic-structure calculations for the cold and electron-thermal contributions to the elastic moduli with phonon contributions for the ion-thermal part calculated using model generalized pseudopotential theory. For the latter, a summation of terms over the Brillouin zone is performed within the quasiharmonic approximation, where each term is composed of a strain derivative of the phonon frequency at a particular k point. At ambient pressure, the resulting temperature dependence of the Ta elastic moduli is in excellent agreement with ultrasonic measurements. The experimentally observed anomalous behavior of C44 at low temperatures is shown to originate from the electron-thermal contribution. At higher temperatures, the main contribution to the temperature dependence of the elastic moduli comes from thermal expansion, but inclusion of the electron- and ion-thermal contributions is essential to obtain quantitative agreement with experiment. In addition, the pressure dependence of the moduli at ambient temperature compares well with recent diamond-anvil-cell measurements to 1.05Mbar . Moreover, the calculated longitudinal and bulk sound velocities in polycrystalline Ta at higher pressure and temperature in the vicinity of shock melting (˜3Mbar) agree well with data obtained from shock experiments. However, at high temperatures along the melt curve above 1Mbar , the B' shear modulus becomes negative, indicating the onset of unexpectedly strong anharmonic effects. Finally, the assumed temperature dependence of the Steinberg-Guinan strength model obtained from scaling with the
Chassagne, Claire; Dubois, Emmanuelle; Jiménez, María L; van der Ploeg, J P M; van Turnhout, Jan
2016-01-01
Dielectric spectroscopy can be used to determine the dipole moment of colloidal particles from which important interfacial electrokinetic properties, for instance their zeta potential, can be deduced. Unfortunately, dielectric spectroscopy measurements are hampered by electrode polarization (EP). In this article, we review several procedures to compensate for this effect. First EP in electrolyte solutions is described: the complex conductivity is derived as function of frequency, for two cell geometries (planar and cylindrical) with blocking electrodes. The corresponding equivalent circuit for the electrolyte solution is given for each geometry. This equivalent circuit model is extended to suspensions. The complex conductivity of a suspension, in the presence of EP, is then calculated from the impedance. Different methods for compensating for EP are critically assessed, with the help of the theoretical findings. Their limit of validity is given in terms of characteristic frequencies. We can identify with one of these frequencies the frequency range within which data uncorrected for EP may be used to assess the dipole moment of colloidal particles. In order to extract this dipole moment from the measured data, two methods are reviewed: one is based on the use of existing models for the complex conductivity of suspensions, the other is the logarithmic derivative method. An extension to multiple relaxations of the logarithmic derivative method is proposed. PMID:27486575
NASA Astrophysics Data System (ADS)
Chassagne, Claire; Dubois, Emmanuelle; Jiménez, María Luisa; van der Ploeg, Jacques; Turnhout, Jan
2016-07-01
Dielectric spectroscopy can be used to determine the dipole moment of colloidal particles from which important interfacial electrokinetic properties, for instance their zeta potential, can be deduced. Unfortunately, dielectric spectroscopy measurements are hampered by electrode polarization (EP). In this article, we review several procedures to compensate for this effect. First EP in electrolyte solutions is described: the complex conductivity is derived as function of frequency, for two cell geometries (planar and cylindrical) with blocking electrodes. The corresponding equivalent circuit for the electrolyte solution is given for each geometry. This equivalent circuit model is extended to suspensions. The complex conductivity of a suspension, in the presence of EP, is then calculated from the impedance measured. Different methods for compensating for EP are critically assessed, with the help of the theoretical findings. Their limit of validity is given in terms of characteristic frequencies. We can identify with one of these frequencies the frequency range within which data uncorrected for EP may be used to assess the dipole moment of colloidal particles. In order to extract this dipole moment from the measured data, two methods are reviewed: one is based on the use of existing models for the complex conductivity of suspensions, the other is the logarithmic derivative method. An extension to multiple relaxations of the logarithmic derivative method is proposed.
Chassagne, Claire; Dubois, Emmanuelle; Jiménez, María L.; van der Ploeg, J. P. M; van Turnhout, Jan
2016-01-01
Dielectric spectroscopy can be used to determine the dipole moment of colloidal particles from which important interfacial electrokinetic properties, for instance their zeta potential, can be deduced. Unfortunately, dielectric spectroscopy measurements are hampered by electrode polarization (EP). In this article, we review several procedures to compensate for this effect. First EP in electrolyte solutions is described: the complex conductivity is derived as function of frequency, for two cell geometries (planar and cylindrical) with blocking electrodes. The corresponding equivalent circuit for the electrolyte solution is given for each geometry. This equivalent circuit model is extended to suspensions. The complex conductivity of a suspension, in the presence of EP, is then calculated from the impedance. Different methods for compensating for EP are critically assessed, with the help of the theoretical findings. Their limit of validity is given in terms of characteristic frequencies. We can identify with one of these frequencies the frequency range within which data uncorrected for EP may be used to assess the dipole moment of colloidal particles. In order to extract this dipole moment from the measured data, two methods are reviewed: one is based on the use of existing models for the complex conductivity of suspensions, the other is the logarithmic derivative method. An extension to multiple relaxations of the logarithmic derivative method is proposed. PMID:27486575
Theoretical analysis of off beam quartz-enhanced photoacoustic spectroscopy sensor
NASA Astrophysics Data System (ADS)
Yi, Hongming; Liu, Kun; Sun, Shanwen; Zhang, Weijun; Gao, Xiaoming
2012-11-01
Off beam quartz-enhanced photoacoustic spectroscopy (OB-QEPAS) sensors are based on a recently developed approach to off-beam photoacoustic (PA) detection which employs a quartz tuning fork (QTF) as an acoustic transducer. A microresonator (mR) with a side slit in the middle is used to enhance PA signal. This paper describes a theoretical model of an OB-QEPAS-based sensor. By deriving the acoustic impedances of the mR at two ends and the side slit in the middle in the model, we obtain a formula for numerically calculating the optimal mRs' parameters of OB-QEPAS-based sensor. We use the model to calculate the optimal mRs' lengths with respect to the resonant frequency of the QTF, acoustic velocities inside mRs, inner diameters of mRs, and acoustic conductivities of the mRs' side slits, and found out that the calculated results closely match experimental data. We also investigated the relationship between the mR selected in "on beam" QEPAS, OB-QEPAS, and an acoustic resonator (AR) excited in its first longitudinal mode used in conventional photoacoustic spectroscopy (PAS).
Coarse Grained Approach to First Principles Modeling of Radiation Cascade in Large Fe Supercells
NASA Astrophysics Data System (ADS)
Odbadrakh, Kh; Nicholson, D. M.; Rusanu, A.; Samolyuk, G. D.; Stoller, R. E.; Zhang, X.-G.; Stocks, G. M.
2012-12-01
Classical Molecular Dynamics (MD) simulations characterizing dislocations and radiation damage typically treat 105-107 atoms. First principles techniques employed to understand systems at an atomistic level are not practical for such large systems consisting of millions of atoms. We present an efficient coarse grained (CG) approach to calculate local electronic and magnetic properties of large MD-generated structures from the first principles. Local atomic magnetic moments in crystalline Fe are perturbed by the presence of radiation generated vacancies and interstitials. The effects are most pronounced near the defect cores and decay slowly as the strain field of the defects decrease with distance. We develop the CG 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. The calculations result in local moments near the defect centers with first-principles accuracy, while capturing coarse-grained details of local moments at greater length scales. This CG approach makes these large scale structures amenable to first principles study.
Singh, Vijay; Kosa, Monica; Majhi, Koushik; Major, Dan Thomas
2015-01-13
First-principles density functional theory (DFT) and a many-body Green's function method have been employed to elucidate the electronic, magnetic, and photonic properties of a spinel compound, Co3O4. Co3O4 is an antiferromagnetic semiconductor composed of cobalt ions in the Co(2+) and Co(3+) oxidation states. Co3O4 is believed to be a strongly correlated material, where the on-site Coulomb interaction (U) on Co d orbitals is presumably important, although this view has recently been contested. The suggested optical band gap for this material ranges from 0.8 to 2.0 eV, depending on the type of experiments and theoretical treatment. Thus, the correlated nature of the Co d orbitals in Co3O4 and the extent of the band gap are still under debate, raising questions regarding the ability of DFT to correctly treat the electronic structure in this material. To resolve the above controversies, we have employed a range of theoretical methods, including pure DFT, DFT+U, and a range-separated exchange-correlation functional (HSE06) as well as many-body Green's function theory (i.e., the GW method). We compare the electronic structure and band gap of Co3O4 with available photoemission spectroscopy and optical band gap data and confirm a direct band gap of ca. 0.8 eV. Furthermore, we have also studied the optical properties of Co3O4 by calculating the imaginary part of the dielectric function (Im(ε)), facilitating direct comparison with the measured optical absorption spectra. Finally, we have calculated the nearest-neighbor interaction (J1) between Co(2+) ions to understand the complex magnetic structure of Co3O4. PMID:26574204
High-level theoretical rovibrational spectroscopy beyond fc-CCSD(T): The C3 molecule.
Schröder, Benjamin; Sebald, Peter
2016-01-28
An accurate local (near-equilibrium) potential energy surface (PES) is reported for the C3 molecule in its electronic ground state (X̃(1)Σg (+)). Special care has been taken in the convergence of the potential relative to high-order correlation effects, core-valence correlation, basis set size, and scalar relativity. Based on the aforementioned PES, several rovibrational states of all (12)C and (13)C substituted isotopologues have been investigated, and spectroscopic parameters based on term energies up to J = 30 have been calculated. Available experimental vibrational term energies are reproduced to better than 1 cm(-1) and rotational constants show relative errors of not more than 0.01%. The equilibrium bond length has been determined in a mixed experimental/theoretical approach to be 1.294 07(10) Å in excellent agreement with the ab initio composite value of 1.293 97 Å. Theoretical band intensities based on a newly developed electric dipole moment function also suggest that the infrared active (1, 1(1), 0)←(0, 0(0), 0) combination band might be observable by high-resolution spectroscopy. PMID:26827217
High-level theoretical rovibrational spectroscopy beyond fc-CCSD(T): The C3 molecule.
Schröder, Benjamin; Sebald, Peter
2016-01-28
An accurate local (near-equilibrium) potential energy surface (PES) is reported for the C3 molecule in its electronic ground state (X̃(1)Σg (+)). Special care has been taken in the convergence of the potential relative to high-order correlation effects, core-valence correlation, basis set size, and scalar relativity. Based on the aforementioned PES, several rovibrational states of all (12)C and (13)C substituted isotopologues have been investigated, and spectroscopic parameters based on term energies up to J = 30 have been calculated. Available experimental vibrational term energies are reproduced to better than 1 cm(-1) and rotational constants show relative errors of not more than 0.01%. The equilibrium bond length has been determined in a mixed experimental/theoretical approach to be 1.294 07(10) Å in excellent agreement with the ab initio composite value of 1.293 97 Å. Theoretical band intensities based on a newly developed electric dipole moment function also suggest that the infrared active (1, 1(1), 0)←(0, 0(0), 0) combination band might be observable by high-resolution spectroscopy.
NASA Astrophysics Data System (ADS)
Pacheco-Londono, Leonardo C.; Pena, Alvaro J.; Primera-Pedrozo, Oliva M.; Hernandez-Rivera, Samuel P.; Mina, Nairmen; Garcia, Rafael; Chamberlain, R. Thomas; Lareau, Richard T.
2004-09-01
Non nitrogen containing, organic peroxides explosives Triacetone triperoxide and diacetone diperoxide have been prepared in the laboratory in order to study various aspects of their synthesis and their experimental and theoretical spectroscopic characteristics. By using different proportions of acetone/hydrogen peroxide (Ac/H2O2), sulfuric, hydrochloric and methanosulfuric acids as catalyzers, it was possible to obtain both compounds in a rapid and simple form. Raman, IR spectroscopy, and GC-MS were used in order to determine the precursors, intermediates and final analytes. Experiments and theoretical studies using density functional theory (DFT) have been used in the elucidation step of the mechanism of the synthesis of the so called "transparent" explosives. The B3LYP functional with the 6-31G** basis set was used to carry out the electronic structure calculation of the intermediates and internal rotations and vibrations of TATP. Raman spectra of solid TATP and FTIR spectra of gas TATP, were recorded in order to assign the experimental spectra. Although full agreement with experiment was not obtained, spectral features of the main TATP bands were assigned.
Lu, Haiting; Chen, Xi; Zhan, Chang-Guo
2007-09-01
The absolute pKa values of 24 representative amine compounds, including cocaine, nicotine, 10 neurotransmitters, and 12 anilines, in aqueous solution were calculated by performing first-principles electronic structure calculations that account for the solvent effects using four different solvation models, i.e., the surface and volume polarization for electrostatic interaction (SVPE) model, the standard polarizable continuum model (PCM), the integral equation formalism for the polarizable continuum model (IEFPCM), and the conductor-like screening solvation model (COSMO). Within the examined computational methods, the calculations using the SVPE model lead to the absolute pKa values with the smallest root-mean-square-deviation (rmsd) value (1.18). When the SVPE model was replaced by the PCM, IEFPCM, and COSMO, the rmsd value of the calculated absolute pKa values became 3.21, 2.72, and 3.08, respectively. All types of calculated pKa values linearly correlate with the experimental pKa values very well. With the empirical corrections using the linear correlation relationships, the theoretical pKa values are much closer to the corresponding experimental data and the rmsd values become 0.51-0.83. The smallest rmsd value (0.51) is also associated with the SVPE model. All of the results suggest that the first-principles electronic structure calculations using the SVPE model are a reliable approach to the pKa prediction for the amine compounds. PMID:17691837
Lu, Haiting; Chen, Xi; Zhan, Chang-Guo
2010-01-01
The absolute pKa values of 24 representative amine compounds, including cocaine, nicotine, 10 neurotransmitters, and 12 anilines, in aqueous solution were calculated by performing first-principles electronic structure calculations that account for the solvent effects using four different solvation models, i.e. the surface and volume polarization for electrostatic interaction (SVPE) model, the standard polarizable continuum model (PCM), the integral equation formalism for the polarizable continuum model (IEFPCM), and the conductor-like screening solvation model (COSMO). Within the examined computational methods, the calculations using the SVPE model lead to the absolute pKa values with the smallest root-mean-square-deviation (RMSD) value (1.18). When the SVPE model was replaced by the PCM, IEFPCM, and COSMO, the RMSD value of the calculated absolute pKa values became 3.21, 2.72, and 3.08, respectively. All types of calculated pKa values linearly correlate with the experimental pKa values very well. With the empirical corrections using the linear correlation relationships, the theoretical pKa values are much closer to the corresponding experimental data and the RMSD values become 0.51 to 0.83. The smallest RMSD value (0.51) is also associated with the SVPE model. All of the results suggest that the first-principles electronic structure calculations using the SVPE model are a reliable approach to the pKa prediction for the amine compounds. PMID:17691837
Thermal physics of the lead chalcogenides PbS, PbSe, and PbTe from first principles
NASA Astrophysics Data System (ADS)
Skelton, Jonathan M.; Parker, Stephen C.; Togo, Atsushi; Tanaka, Isao; Walsh, Aron
2014-05-01
The lead chalcogenides represent an important family of functional materials, in particular due to the benchmark high-temperature thermoelectric performance of PbTe. A number of recent investigations, experimental and theoretical, have aimed to gather insight into their unique lattice dynamics and electronic structure. However, the majority of first-principles modeling has been performed at fixed temperatures, and there has been no comprehensive and systematic computational study of the effect of temperature on the material properties. We report a comparative lattice-dynamics study of the temperature dependence of the properties of PbS, PbSe, and PbTe, focusing particularly on those relevant to thermoelectric performance, viz. phonon frequencies, lattice thermal conductivity, and electronic band structure. Calculations are performed within the quasiharmonic approximation, with the inclusion of phonon-phonon interactions from many-body perturbation theory, which are used to compute phonon lifetimes and predict the lattice thermal conductivity. The results are critically compared against experimental data and other calculations, and add insight to ongoing research on the PbX compounds in relation to the off-centering of Pb at high temperatures, which is shown to be related to phonon softening. The agreement with experiment suggests that this method could serve as a straightforward, powerful, and generally applicable means of investigating the temperature dependence of material properties from first principles.
Vidossich, Pietro; Lledós, Agustí; Ujaque, Gregori
2016-06-21
Computational chemistry is a valuable aid to complement experimental studies of organometallic systems and their reactivity. It allows probing mechanistic hypotheses and investigating molecular structures, shedding light on the behavior and properties of molecular assemblies at the atomic scale. When approaching a chemical problem, the computational chemist has to decide on the theoretical approach needed to describe electron/nuclear interactions and the composition of the model used to approximate the actual system. Both factors determine the reliability of the modeling study. The community dedicated much effort to developing and improving the performance and accuracy of theoretical approaches for electronic structure calculations, on which the description of (inter)atomic interactions rely. Here, the importance of the model system used in computational studies is highlighted through examples from our recent research focused on organometallic systems and homogeneous catalytic processes. We show how the inclusion of explicit solvent allows the characterization of molecular events that would otherwise not be accessible in reduced model systems (clusters). These include the stabilization of nascent charged fragments via microscopic solvation (notably, hydrogen bonding), transfer of charge (protons) between distant fragments mediated by solvent molecules, and solvent coordination to unsaturated metal centers. Furthermore, when weak interactions are involved, we show how conformational and solvation properties of organometallic complexes are also affected by the explicit inclusion of solvent molecules. Such extended model systems may be treated under periodic boundary conditions, thus removing the cluster/continuum (or vacuum) boundary, and require a statistical mechanics simulation technique to sample the accessible configurational space. First-principles molecular dynamics, in which atomic forces are computed from electronic structure calculations (namely, density
NASA Astrophysics Data System (ADS)
Paul, Sujata
In the course of my PhD I have worked on a broad range of problems using simulations from first principles: from catalysis and chemical reactions at surfaces and on nanostructures, characterization of carbon-based systems and devices, and surface and interface physics. My research activities focused on the application of ab-initio electronic structure techniques to the theoretical study of important aspects of the physics and chemistry of materials for energy and environmental applications and nano-electronic devices. A common theme of my research is the computational study of chemical reactions of environmentally important molecules (CO, CO2) using high performance simulations. In particular, my principal aim was to design novel nano-structured functional catalytic surfaces and interfaces for environmentally relevant remediation and recycling reactions, with particular attention to the management of carbon dioxide. We have studied the carbon-mediated partial sequestration and selective oxidation of carbon monoxide (CO), both in the presence and absence of hydrogen, on graphitic edges. Using first-principles calculations we have studied several reactions of CO with carbon nanostructures, where the active sites can be regenerated by the deposition of carbon decomposed from the reactant (CO) to make the reactions self-sustained. Using statistical mechanics, we have also studied the conditions under which the conversion of CO to graphene and carbon dioxide is thermodynamically favorable, both in the presence and in the absence of hydrogen. These results are a first step toward the development of processes for the carbon-mediated partial sequestration and selective oxidation of CO in a hydrogen atmosphere. We have elucidated the atomic scale mechanisms of activation and reduction of carbon dioxide on specifically designed catalytic surfaces via the rational manipulation of the surface properties that can be achieved by combining transition metal thin films on oxide
Xiang, Huimin; Feng, Zhihai; Li, Zhongping; Zhou, Yanchun E-mail: yczhou714@gmail.com
2015-06-14
High temperature mechanical and thermodynamic properties of TiB{sub 2} are important to its applications as ultrahigh temperature ceramic, which were not well understood. In this study, the thermodynamic and mechanical properties of TiB{sub 2} were investigated by the combination of first principle and phonon dispersion calculations. The thermal expansion of TiB{sub 2} was anisotropic, α{sub c}/α{sub a} is nearly constant (1.46) from 300 K to 1500 K, theoretically. The origination of this anisotropy is the anisotropic compressibility. The heat capacity at constant pressure was estimated from the theoretical entropy and fitted the experimental result quite well when higher-order anharmonic effects were considered. Theoretical isentropic elastic constants and mechanical properties were calculated and their temperature dependence agreed with the existed experiments. From room temperature to 1500 K, the theoretical slope is −0.0211 GPa·K{sup −1}, −0.0155 GPa·K{sup −1}, and −0.0384 GPa·K{sup −1} for B, G, and E, respectively. Our theoretical results highlight the suitability of this method in predicting temperature dependent properties of ultrahigh temperature ceramics and show ability in selecting and designing of novel ultrahigh temperature ceramics.
Ma, Chao; Yang, Huaixin; Tian, Huanfang; Shi, Honglong; Wang, Zhiwei; Li, Jianqi
2013-03-20
Using electron energy loss spectroscopy (EELS) measurements and first-principles electronic structure calculations, the significant interlayer hybridization between the insulating layers (ReO or Ba) and the conducting FeAs layers was investigated in the layered iron pnictides, which is quite different from the case in the cuprate superconductors. This interlayer hybridization would result in an increase in the bandwidth near the Fermi level and interorbital charge transfer in the Fe 3d orbitals, which subsequently leads to a decrease in the Fe local moment and the modification of the Fermi surface topology. Therefore, a three-dimensional character of the electronic structure due to the interlayer hybridization is expected, as observed in previous experiments. These findings indicate that reduced dimensionality is no longer a necessary condition in the search for high-T(c) superconductors in iron pnictides.
First-principles Theory of the Momentum-dependent Local Ansatz for Correlated Electron System
NASA Astrophysics Data System (ADS)
Chandra, Sumal; Kakehashi, Yoshiro
The momentum-dependent local-ansatz (MLA) wavefunction describes well correlated electrons in solids in both the weak and strong interaction regimes. In order to apply the theory to the realistic system, we have extended the MLA to the first-principles version using the tight-binding LDA+U Hamiltonian. We demonstrate for the paramagnetic Fe that the first-principles MLA can describe a reasonable correlation energy gain and suppression of charge fluctuations due to electron correlations. Furthermore, we show that the MLA yields a distinct momentum dependence of the momentum distribution, and thus improves the Gutzwiller wavefunction.
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. PMID:27386363
Predicting Raman Spectra of Aqueous Silica and Alumina Species in Solution From First Principles
NASA Astrophysics Data System (ADS)
Hunt, J. D.; Schauble, E. A.; Manning, C. E.
2006-12-01
Dissolved silica and alumina play an important role in lithospheric fluid chemistry. Silica concentrations in aqueous fluids vary over the range of crustal temperatures and pressures enough to allow for significant mass transport of silica via fluid-rock interaction. The polymerization of silica, and the possible incorporation of alumina into the polymer structure, could afford crystal-like or melt-like sites to otherwise insoluble elements such as titanium, leading to enhanced mobility. Raman spectroscopy in a hydrothermal diamond anvil cell (HDAC) has been used to study silica polymerization at elevated pressure and temperature [Ref. 1, 2], but Raman spectra of expected solutes are not fully understood. We calculated Raman spectra of H4SiO4 monomers, H6Si2O7 dimers, and H6SiAlO_7^- dimers, from first principles using hybrid density functional theory (B3LYP). These spectra take into account the variation in bridging angle (Si-O-Si and Si-O-Al angles) that the dimers will have at a given temperature by calculating a potential energy surface of the dimer as the bridging angle varies, and using a Boltzmann distribution at that temperature to determine relative populations at each geometry. Solution effects can be incorporated by using a polarizable continuum model (PCM), and a potential energy surface has been constructed for the silica dimer using a PCM. The bridging angle variation explains the broadness of the 630 cm^-^1 silica dimer peak observed in HDAC experiments [Ref. 1, 2] at high temperatures. The silica-alumina dimer bridging angle is shown to be stiffer than the silica dimer bridging angle, which results in a much narrower main peak. The synthetic spectrum obtained for the silica-alumina dimer suggests that there may be a higher ratio of complexed alumina to free alumina in solution at highly basic pH than previously estimated [Ref. 3]. References: 1. Zotov, N. and H. Keppler, Chemical Geology, 2002. 184: p. 71-82. 2. Zotov, N. and H. Keppler, American
NASA Astrophysics Data System (ADS)
Rey, M.; Nikitin, A. V.; Tyuterev, V.
2014-06-01
Knowledge of near infrared intensities of rovibrational transitions of polyatomic molecules is essential for the modeling of various planetary atmospheres, brown dwarfs and for other astrophysical applications 1,2,3. For example, to analyze exoplanets, atmospheric models have been developed, thus making the need to provide accurate spectroscopic data. Consequently, the spectral characterization of such planetary objects relies on the necessity of having adequate and reliable molecular data in extreme conditions (temperature, optical path length, pressure). On the other hand, in the modeling of astrophysical opacities, millions of lines are generally involved and the line-by-line extraction is clearly not feasible in laboratory measurements. It is thus suggested that this large amount of data could be interpreted only by reliable theoretical predictions. There exists essentially two theoretical approaches for the computation and prediction of spectra. The first one is based on empirically-fitted effective spectroscopic models. Another way for computing energies, line positions and intensities is based on global variational calculations using ab initio surfaces. They do not yet reach the spectroscopic accuracy stricto sensu but implicitly account for all intramolecular interactions including resonance couplings in a wide spectral range. The final aim of this work is to provide reliable predictions which could be quantitatively accurate with respect to the precision of available observations and as complete as possible. All this thus requires extensive first-principles quantum mechanical calculations essentially based on three necessary ingredients which are (i) accurate intramolecular potential energy surface and dipole moment surface components well-defined in a large range of vibrational displacements and (ii) efficient computational methods combined with suitable choices of coordinates to account for molecular symmetry properties and to achieve a good numerical
Cesium stability in a typical mica structure in dry and wet environments from first-principles
NASA Astrophysics Data System (ADS)
Suehara, Shigeru; Yamada, Hirohisa
2013-05-01
Cesium ion stability in a typical mica structure in various environments of solid salts (XCl; X = Cs, K, and Na), metals (X) and saltwaters (XCl aqueous liquids) was investigated using first-principles density-functional theory (DFT) with the Perdew-Burke-Ernzerhof (PBE) functional as well as a van der Waals (vdW) corrected functional (vdW-DFC09x). We specifically examined interlayer ion-exchange in bulk phlogopite-type mica, which is expected to produce a well-defined benchmark in a thermodynamic equilibrium state. In general, theoretical models have well reproduced the experimental and theoretical data found in the literature from the viewpoints of structure, heat capacity, and entropy. The vdW-DFC09x lattice parameters of the mica appear to be better reproducible than the PBE parameters are. However, the vdW correction calculations of the thermodynamic properties with the harmonic approximation using the phonon frequencies showed poor results in some cases, whereas the PBE calculations yielded robust and reasonable results in terms of structure and thermodynamic properties. The isotope effect of the 137Cs atom appears to be confined in thermodynamic properties such as entropy, heat capacity, and ion-exchange energy, although the theoretical infrared spectra showed a small redshift ca. 1 cm-1 in the far-infrared region of 50-75 cm-1. The calculated RDF and the coordination number for X-O (i.e., X-H2O) for the saltwater model indicated that the Cs, K, and Na ions with respective hydrated radii of 0.323, 0.284, and 0.238 nm were surrounded, respectively, by 6.5, 4.5, and 4.0 of H2O molecules in a water solution. Ion-exchange energy values based on free-energy calculations around ambient temperatures derived using the PBE functional and a harmonic approximation suggest that the cesium ion in mica interlayer phlogopite is stable in an environment consisting of KCl, NaCl, K, and Na solids, and in NaCl saltwater as well. However, it can be exchanged competitively by
REMPI spectroscopy and theoretical calculations of cis and trans 3-fluoro-N-methylaniline
NASA Astrophysics Data System (ADS)
Zhang, Lijuan; Liu, Sheng; Dong, Changwu; Cheng, Min; Du, Yikui; Zhu, Qihe; Zhang, Cunhao
2014-02-01
The ab initio and density functional theory (DFT) calculations of 3-fluoro-N-methylaniline (3FNMA) reveal that two rotamers, cis and trans 3FNMA, are stable for each of the S0, S1, and D0 states. The vibronic spectra of the two rotamers in the S1 state have been recorded by the one-color resonant two-photon ionization (R2PI) spectroscopy. The band origins of the S1 ← S0 electronic transition of cis and trans 3FNMA are found to be 33 816 ± 3 and 34 023 ± 3 cm-1. The two rotamers display similar vibrational frequencies, and the slight energy difference in some modes reflects the conformation effect due to the relative orientation of the NHCH3 group. Besides, the trans rotamer displays more vibronic features in the low-frequency region, which are active modes mainly involving the CH3 and the NHCH3 groups. By the two-color R2PI spectroscopy, the adiabatic ionization energies (IEs) of cis and trans 3FNMA are determined to be 61 742 ± 10 and 61 602 ± 10 cm-1, respectively. It is derived from the R2PI spectroscopic data that, compared with the trans rotamer, the cis rotamer is more stable by 302 ± 25 cm-1 in the S1 state, but less stable by 45 ± 25 cm-1 in the D0 state. With the aid of theoretical calculations, the substitution and conformation effects on the properties of 3FNMA, including the molecular structures, vibrational frequencies, and the relative stability of the two rotamers, were discussed in detail.
First-principles study of back-contact effects on CdTe thin-film solar cells
NASA Astrophysics Data System (ADS)
Du, Mao-Hua
2009-11-01
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, Sb2Te3 , on the performance of the CdTe solar cells. First-principles calculations show that Sb impurities in p -type CdTe are donors and can diffuse with low diffusion barrier. 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 Sb2Te3 back contacts.
First-principles prediction of solar radiation shielding performance for transparent windows of GdB6
NASA Astrophysics Data System (ADS)
Xiao, Lihua; Su, Yuchang; Ran, Jingyu; Liu, Yike; Qiu, Wei; Wu, Jianming; Lu, Fanghai; Shao, Fang; Tang, Dongsheng; Peng, Ping
2016-04-01
The structural, electronic, magnetic, and optical properties of GdB6 are studied using the first-principles calculations. Calculated values for magnetic and optical properties and lattice constant are found to be consistent with previously reported experimental results. The calculated results show that GdB6 is a perfect near-infrared absorption/reflectance material that could serve as a solar radiation shielding material for windows with high visible light transmittance, similar to LaB6, which is assigned to its plasma oscillation and a collective oscillation (volume plasmon) of carrier electrons. It was found that the magnetic 4f electrons of Gd are not relevant to the important optical properties of GdB6. These theoretical studies serve as a reference for future studies.
Lakel, S.; Okbi, F.; Ibrir, M.; Almi, K.
2015-03-30
We have performed first-principles calculations to investigate the behavior under hydrostatic pressure of the structural, elastic and lattice dynamics properties of aluminum phosphide crystal (AlP), in both zinc-blende (B3) and nickel arsenide (B8) phases. Our calculated structural and electronic properties are in good agreement with previous theoretical and experimental results. The elastic constants, bulk modulus (B), shear modulus (G), and Young's modulus (E), Born effective charge and static dielectric constant ε{sub 0}, were calculated with the generalized gradient approximations and the density functional perturbation theory (DFPT). Our results in the pressure behavior of the elastic and dielectric properties of both phases are compared and contrasted with the common III–V materials. The Born effective charge ZB decreases linearly with pressure increasing, while the static dielectric constant decreases quadratically with the increase of pressure.
First-principles study on half-metallic zinc-blende CrS and its (001) surface
NASA Astrophysics Data System (ADS)
Xu, Bin; Chen, Leiming
2016-11-01
Half-metallic magnets with complete (100%) spin polarization have attracted growing interest due to the potential in spintronic applications. In this paper, we use the first-principles calculations to explain the seeming contradiction between the recent experimental ferromagnetism (Demper et al., 2012 [22]) and the previous theoretical antiferromagnetic ground state for half-metallic zinc-blende CrS, and the experimental ferromagnetism of zinc-blende CrS arises from the substrate effect. We also show that both Cr- and S-terminated (001) surfaces of CrS preserve the bulk half-metallicity. The calculated surface energy indicates that the S-terminated (001) surface is more stable than the Cr-terminated (001) surface within the whole effective Cr chemical potentials, and thus the S-terminated (001) surface is more likely than the Cr-terminated (001) surface when the CrS thin films are grown on ZnSe substrate.
Anisotropic elastic and vibrational properties of Ru2B3 and Os2B3: a first-principles investigation
NASA Astrophysics Data System (ADS)
Ozisik, Haci; Deligoz, Engin; Surucu, Gokhan; Bogaz Ozisik, Havva
2016-07-01
The structural, mechanical and lattice dynamical properties of Ru2B3 and Os2B3 have been investigated by using a first-principles method based on the density functional theory within the generalized gradient approximation. The single crystal elastic constants are numerically estimated using strain–stress approach. The polycrystalline aggregate elastic parameters are calculated from the single elastic constants via the Voigt–Reuss–Hill approximations. Subsequently, the ductility and brittleness are characterized with the estimation from Pugh’s rule (B/G) and Cauchy pressure. Additionally, the Debye temperature is calculated from the average elastic wave velocity obtained from bulk and shear moduli. The calculated parameters are consistent with the previous experimental and theoretical data. These borides are both mechanically and dynamically stable in the considered structure.
Strong enhancement of piezoelectric constants in ScxAl1-xN: First-principles calculations
NASA Astrophysics Data System (ADS)
Momida, Hiroyoshi; Teshigahara, Akihiko; Oguchi, Tamio
2016-06-01
We theoretically investigate the piezoelectricity of ScxAl1-xN in the entire range of x by first-principles calculations. We find that the piezoelectric constants of wurtzite-type ScxAl1-xN significantly enhance as x increases from 0 to 0.75. However, the energy stability analyses between structure phases show that the cubic-type phases become more stable than the wurtzite-type phases at x of approximately 0.5 and higher, interfering with the ability of wurtzite-type ScxAl1-xN to realize the maximum piezoelectricity. Moreover, our study on element combination dependences on piezoelectricity in A0.5B0.5N (A = Sc, Y, La and B = Al, Ga, In) indicates that Sc, Y, and La have the strongest effect on the enhancement of piezoelectric constants in AlN, GaN, and InN, respectively.
NASA Astrophysics Data System (ADS)
Deng, Zun-Yi; Zhang, Jian-Min; Xu, Ke-Wei
2015-08-01
To exploit the potential application of nitride nanotube (BNNT), the adsorption of sulfur dioxide (SO2) on pristine and Mn-doped BNNT was theoretically studied using first-principles approach based on density functional theory (DFT). The most stable adsorption geometry, adsorption energy, magnetic moment, charge transfer and density of states of these systems are discussed. SO2 molecule is weakly adsorbed on the pristine BNNT. The Mn-doped BNNT show high reactivity toward SO2 regardless of the MnB site or MnN site adsorption. The larger formation energies and analysis of density of states show the SO2 molecules are chemically bonded to Mn-doped BNNT and the covalent interaction between the SO2 molecule and Mn atom can be formed. Therefore, the Mn-doped BNNT can be used as SO2 gas sensor manufacturing raw materials, and it may be a potential material for nanodevice applications.
NASA Astrophysics Data System (ADS)
Jain, Richa Naja; Chakraborty, Brahmananda; Ramaniah, Lavanya M.
2015-06-01
The electronic structure and hydrogen storage capability of Yttrium-doped BNNTs has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom prefers the hollow site in the center of the hexagonal ring with a binding energy of 0.8048eV. Decorating by Y makes the system half-metallic and magnetic with a magnetic moment of 1.0µB. Y decorated Boron-Nitride (8,0) nanotube can adsorb up to five hydrogen molecules whose average binding energy is computed as 0.5044eV. All the hydrogen molecules are adsorbed with an average desorption temperature of 644.708 K. Taking that the Y atoms can be placed only in alternate hexagons, the implied wt% comes out to be 5.31%, a relatively acceptable value for hydrogen storage materials. Thus, this system can serve as potential hydrogen storage medium.
Structural, energetic and thermodynamic analyses of Ca(BH4)2·2NH3 from first principles calculations
NASA Astrophysics Data System (ADS)
Yuan, Peng-Fei; Wang, Fei; Sun, Qiang; Jia, Yu; Guo, Zheng-Xiao
2012-01-01
Ca(BH4)2·2NH3 is a relatively new compound with potential application in hydrogen storage. Here the fundamental properties of the compound, such as electronic structure, energetic and thermodynamic properties, were comprehensively studied using first-principles calculations. Results from electronic density of states (DOS) and electron localization function (ELF) indicate the covalent bond nature of the N-H bond and the B-H bond. Charge density analyses show weak ionic interactions between the Ca atom and the NH3 complexes or the (BH4)- complexes. The calculated vibration frequencies of B-H and N-H are in good agreement with other theoretical and experimental results. Furthermore, we calculated the reaction enthalpy and reaction Gibbs free energy at a range of temperature 0-700 K. Our results are in good agreement with experimental results in literature. Possible reaction mechanism of the decomposition reaction is proposed.
Pagare, G.; Abraham, Jisha A.; Sanyal, S. P.
2015-06-24
A theoretical study of structural, electronic and optical properties of RESn{sub 3} (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 (a{sub 0}), 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.
Gouda, Mohammed K. Gepreel, Mohamed A. H.; Nakamura, Koichi
2015-06-07
Theoretical deformation response of hypothetical β-titanium alloys was investigated using first-principles calculation technique under periodic boundary conditions. Simulation was carried out on hypothetical 54-atom supercell of Ti–X (X = Cr, Mn, Fe, Zr, Nb, Mo, Al, and Sn) binary alloys. The results showed that the strength of Ti increases by alloying, except for Cr. The most effective alloying elements are Nb, Zr, and Mo in the current simulation. The mechanism of bond breaking was revealed by studying the local structure around the alloying element atom with respect to volume change. Moreover, the effect of alloying elements on bulk modulus and admissible strain was investigated. It was found that Zr, Nb, and Mo have a significant effect to enhance the admissible strain of Ti without change in bulk modulus.
Alloying effects on structural and thermal behavior of Ti1-xZrxC: A first principles study
NASA Astrophysics Data System (ADS)
Chauhan, Mamta; Gupta, Dinesh C.
2016-05-01
The formation energy, equilibrium lattice parameter, bulk modulus, Debye temperature and heat capacity at constant volume have been calculated for TiC, ZrC, and their intermediate alloys (Ti1-xZrxC, x = 0,0.25.0.5,0.75,1) using first principles approach. The calculated values of lattice parameter and bulk modulus agree well with the available experimental and earlier theoretical reports. The variation of lattice parameter and bulk modulus with the change in concentration of Zr atom in Ti1-xZrxC has also been reported. The heat capacities of TiC, ZrC, and their intermediate alloys have been calculated by considering both vibrational and electronic contributions.
On the dynamical stability of ferromagnetic Ru and Os in the bct structure: a first-principles study
NASA Astrophysics Data System (ADS)
Cifuentes-Quintal, M. E.; de Coss, R.
2015-08-01
Recent theoretical studies have predicted magnetic states for Ru and Os in the body-centred tetragonal structure (bct) with ?. In this study, we present first principles calculations of the phonon dispersion for ferromagnetic Ru- and Os-bct along the epitaxial and uniaxial Bain paths, to evaluate their dynamical stability. The phonon dispersions were computed using the density functional perturbation theory, including the gradient corrections to the exchange-correlation functional within the plane-waves ultrasoft-pseudopotential approximation. The phonon dispersion for the local minimum in the Bain path with ? as well as the uniaxial and epitaxial strained structures are analysed. We find imaginary frequencies along different directions of the Brillouin zone, which indicates that both systems are dynamically unstable. Consequently, ferromagnetic Ru and Os in the bct with ? are not truly metastable phases.
NASA Astrophysics Data System (ADS)
Li, Yaping; Liu, Zhimin; Jentoft, Friederike; Wang, Sanwu
2015-03-01
Biomass is an important renewable energy resource. Cresol is one of components in crude bio-oil generated from biomass, and hydrogenation of cresol is often involved in the upgrading process. We studied catalytic hydrogenation of cresol on the Pt(111) surface with and without the presence of water. In particular, we used first-principles density-functional theory and ab initio molecular dynamics simulations to obtain adsorption geometries, binding energies, reaction energies, activation energies, and reaction pathways for hydrogenation of cresol with possible products of 2-methylcyclohexanone and 2-methylcyclohexanol. Our theoretical results are used to explain the available experimental measurements, which show a strong influence of water. Supported by DOE (DE-SC0004600). This research used the supercomputer resources at NERSC, of XSEDE, at TACC and at the Tandy Supercomputing Center.
Jain, Richa Naja; Chakraborty, Brahmananda; Ramaniah, Lavanya M.
2015-06-24
The electronic structure and hydrogen storage capability of Yttrium-doped BNNTs has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom prefers the hollow site in the center of the hexagonal ring with a binding energy of 0.8048eV. Decorating by Y makes the system half-metallic and magnetic with a magnetic moment of 1.0µ{sub B}. Y decorated Boron-Nitride (8,0) nanotube can adsorb up to five hydrogen molecules whose average binding energy is computed as 0.5044eV. All the hydrogen molecules are adsorbed with an average desorption temperature of 644.708 K. Taking that the Y atoms can be placed only in alternate hexagons, the implied wt% comes out to be 5.31%, a relatively acceptable value for hydrogen storage materials. Thus, this system can serve as potential hydrogen storage medium.
67th Mosbacher Kolloquium: Protein Design: From First Principles to Biomedical Applications.
Spieler, Valerie; Lühmann, Tessa
2016-07-15
The 67th Mosbacher Kolloquium of the German Society for Biochemistry and Molecular Biology (GBM) with the topic "Protein Design-From First Principles to Biomedical Application" took place from March 31 to April 2 in Mosbach, Germany. Highlights of the colloquium are presented here.
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.
Bennett, Joseph W.; Rabe, Karin M.
2012-11-15
In this concept paper, the development of strategies for the integration of first-principles methods with crystallographic database mining for the discovery and design of novel ferroelectric materials is discussed, drawing on the results and experience derived from exploratory investigations on three different systems: (1) the double perovskite Sr(Sb{sub 1/2}Mn{sub 1/2})O{sub 3} as a candidate semiconducting ferroelectric; (2) polar derivatives of schafarzikite MSb{sub 2}O{sub 4}; and (3) ferroelectric semiconductors with formula M{sub 2}P{sub 2}(S,Se){sub 6}. A variety of avenues for further research and investigation are suggested, including automated structure type classification, low-symmetry improper ferroelectrics, and high-throughput first-principles searches for additional representatives of structural families with desirable functional properties. - Graphical abstract: Integration of first-principles methods with crystallographic database mining, for the discovery and design of novel ferroelectric materials, could potentially lead to new classes of multifunctional materials. Highlights: Black-Right-Pointing-Pointer Integration of first-principles methods and database mining. Black-Right-Pointing-Pointer Minor structural families with desirable functional properties. Black-Right-Pointing-Pointer Survey of polar entries in the Inorganic Crystal Structural Database.
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…
NASA Astrophysics Data System (ADS)
Mehlenbacher, Randy D.; Lyons, Brendon; Wilson, Kristina C.; Du, Yong; McCamant, David W.
2009-12-01
We present a classical theoretical treatment of a two-dimensional Raman spectroscopy based on the initiation of vibrational coherence with an impulsive Raman pump and subsequent probing by two-pulse femtosecond stimulated Raman spectroscopy (FSRS). The classical model offers an intuitive picture of the molecular dynamics initiated by each laser pulse and the generation of the signal field traveling along the probe wave vector. Previous reports have assigned the observed FSRS signals to anharmonic coupling between the impulsively driven vibration and the higher-frequency vibration observed with FSRS. However, we show that the observed signals are not due to anharmonic coupling, which is shown to be a fifth-order coherent Raman process, but instead due to cascades of coherent Raman signals. Specifically, the observed vibrational sidebands are generated by parallel cascades in which a coherent anti-Stokes or Stokes Raman spectroscopy (i.e., CARS or CSRS) field generated by the coherent coupling of the impulsive pump and the Raman pump pulses participates in a third-order FSRS transition. Additional sequential cascades are discussed that will give rise to cascade artifacts at the fundamental FSRS frequencies. It is shown that the intended fifth-order FSRS signals, generated by an anharmonic coupling mechanism, will produce signals of ˜10-4 ΔOD (change in the optical density). The cascading signals, however, will produce stimulated Raman signal of ˜10-2 ΔOD, as has been observed experimentally. Experiments probing deuterochloroform find significant sidebands of the CCl3 bend, which has an E type symmetry, shifted from the A1 type C-D and C-Cl stretching modes, despite the fact that third-order anharmonic coupling between these modes is forbidden by symmetry. Experiments probing a 50:50 mixture of chloroform and d-chloroform find equivalent intensity signals of low-frequency CDCl3 modes as sidebands shifted from both the C-D stretch of CDCl3 and the C-H stretch of
First Principles Evaluation of Nickel Oxide and Other Materials for Solar Energy Conversion
NASA Astrophysics Data System (ADS)
Alidoust, Nima
Global climate change and pollution caused by fossil fuels necessitate the search for inexpensive, clean, renewable energy sources. Photocatalytic and photovoltaic solar energy conversion to fuels and electricity, respectively, are among the possible solutions to this challenge. Engineering devices that can efficiently achieve these tasks requires fundamental understanding of the materials involved, identification of ways to improve these materials, and discovery of new materials that could help achieve higher efficiencies and lower costs. The work presented in this dissertation contributes to these fronts via first-principles quantum mechanical calculations. In particular, we extensively study nickel oxide (NiO), an inexpensive semiconductor, with the desired potentially carrier-lifetime-extending charge-transfer property. We identify and devise various theoretical models that accurately describe NiO's electronic structure. We use these models to show that alloying NiO with Li2O could decrease NiO's band gap from ˜4 eV to ˜2 eV, making it an appropriate light absorber for use in various solar energy conversion devices. We study hole transport in NiO and NiO alloys. We show that hole conductivity in NiO can be enhanced by forming homogeneous LixNi1-xO alloys with high enough Li concentration, making LixNi1-x O alloys suitable for use as p-type hole conductors. We further find that hole transport in NiO is confined to two dimensions. We predict that forming MgxNi1-xO and ZnxNi 1-xO (which we find to be transparent to visible light) disrupts this confinement and leads to three-dimensional hole transport, thereby increasing conductivity. This makes MgxNi1-xO and ZnxNi 1-xO alloys suitable for use as transparent conducting oxides. We introduce CoO and Co0.25Ni0.75O alloy as new intermediate band semiconductors (IBSCs), capable of absorbing light across multiple band gaps and enhancing light absorption in IBSC-based solar cells. Finally, we investigate the spatial
NASA Astrophysics Data System (ADS)
Ravindran, P.; Kjekshus, A.; Fjellvåg, H.; James, P.; Nordström, L.; Johansson, B.; Eriksson, O.
2001-04-01
The computational framework of this study is based on the local-spin-density approximation with first-principles full-potential linear muffin-tin orbital calculations including orbital polarization (OP) correction. We have studied the magnetic anisotropy for a series of bilayer CuAu(I)-type materials such as FeX, MnX (X=Ni,Pd,Pt), CoPt, NiPt, MnHg, and MnRh in a ferromagnetic state using experimental structural parameters to understand the microscopic origin of magnetic-anisotropy energy (MAE) in magnetic multilayers. Except for MnRh and MnHg, all these phases show perpendicular magnetization. We have analyzed our results in terms of angular momentum-, spin- and site-projected density of states, magnetic-angular-momentum-projected density of states, orbital-moment density of states, and total density of states. The orbital-moment number of states and the orbital-moment anisotropy for FeX (X=Ni,Pd,Pt) are calculated as a function of band filling to study its effect on MAE. The total and site-projected spin and orbital moments for all these systems are calculated with and without OP when the magnetization is along or perpendicular to the plane. The results are compared with available experimental as well as theoretical results. Our calculations show that OP always enhances the orbital moment in these phases and brings them closer to experimental values. The changes in MAE are analyzed in terms of exchange splitting, spin-orbit splitting, and tetragonal distortion/crystal-field splitting. The calculated MAE is found to be in good agreement with experimental values when the OP correction is included. Some of the materials considered here show large magnetic anisotropy of the order of meV. In particular we found that MnPt will have a very large MAE if it could be stabilized in a ferromagnetic configuration. Our analysis indicates that apart from large spin-orbit interaction and exchange interaction from at least one of the constituents, a large crystal-field splitting
[First-principles study of vibrational Raman spectra of amorphous carbon].
Niu, Li; Zhu, Jia-qi; Gao, Wei; Du, Shan-yi
2009-09-01
The vibrational density of states and nonresonant reduced Raman spectra of amorphous carbon at densities of 2.6, 2.9 and 3.2 g x cm(-3) were calculated by the use of a first-principles plane-wave pesudopotential method. Three structural models were generated by liquid-quench method using Car-Parinello molecular dynamics, their vibrational frequencies and eigenmodes were determined using the linear response approach, and Raman coupling tensors were calculated using the finite electric field method. The calculated results show that the sp3 fraction increases from 50% to 84.4%, the sp2 configuration changes from mainly rings to short chains, the position of the G peak moves to higher frequencies, the intensity ratio of D and G peaks decreases, the position of the T peak moves to lower frequencies and the intensity ratio of T and G peaks increases as density increases from 2.6 to 3.2 g x cm(-3). The authors' calculated Raman spectra show an overall good agreement with experimental spectra. The analysis in terms of atomic vibrations confirms that the G and D peaks both come from sp2 C contribution, G peak is due to the stretching vibration of any pair of sp2 atoms and the T peak is due to the C-C sp3 vibration. The authors' analysis also confirms that the dispersion of G and T peaks is due to bond-length changes. The bond length of chains (olefins) is shorter than that of rings, so their vibrational frequency is higher and the G-peak position moves to higher frequencies with increasing the sp3 fraction. The number of sp3-sp2 type bonds decreases as the sp3 fraction increases. These bonds are shorter than pure sp3-sp3 bonds, hence the T-peak position moves to lower frequencies. The research results provide a theoretic basis for analyzing experimental Raman spectra of amorphous carbon. PMID:19950647
Nuclear Quantum Effects in Ice Phases and Water from First Principles Calculations
NASA Astrophysics Data System (ADS)
Pamuk, Betul
Despite the simplicity of the molecule, condensed phases of water show many physical anomalies, some of which are still unexplained to date. This thesis focuses on one striking anomaly that has been largely neglected and never explained. When hydrogen (1H) is replaced by deuterium (2 D), zero point fluctuations of the heavy isotope causes ice to expand, whereas in normal isotope effect, heavy isotope causes volume contraction. Furthermore, in a normal isotope effect, the shift in volume should decrease with increasing temperature, while, in ice, the volume shift increases with increasing temperature and persists up to the melting temperature and also exists in liquid water. In this dissertation, nuclear quantum effects on structural and cohesive properties of different ice polymorphs are investigated. We show that the anomalous isotope effect is well described by first principles density functional theory with van der Waals (vdW-DF) functionals within the quasi-harmonic approximation. Our theoretical modeling explains how the competition between the intra- and inter-molecular bonding of ice leads to an anomalous isotope effect in the volume and bulk modulus of ice. In addition, we predict a normal isotope effect when 16O is replaced by 18O, which is experimentally confirmed. Furthermore, the transition from proton disordered hexagonal phase, ice Ih to proton ordered hexagonal phase, ice XI occurs with a temperature difference between 1H and 2D of 6K, in good agreement with experimental value of 4K. We explain, for first time for that this temperature difference is entirely due to the zero point energy. In the second half of this thesis, we expand our study to the other ice phases: ice Ic, ice IX, ice II, ice VIII, clathrate hydrates, and low and high density amorphous ices. We employ the methodology that we have developed to investigate the isotope effect in structures with different configurations. We show that there is a transition from anomalous isotope effect
[First-principles study of vibrational Raman spectra of amorphous carbon].
Niu, Li; Zhu, Jia-qi; Gao, Wei; Du, Shan-yi
2009-09-01
The vibrational density of states and nonresonant reduced Raman spectra of amorphous carbon at densities of 2.6, 2.9 and 3.2 g x cm(-3) were calculated by the use of a first-principles plane-wave pesudopotential method. Three structural models were generated by liquid-quench method using Car-Parinello molecular dynamics, their vibrational frequencies and eigenmodes were determined using the linear response approach, and Raman coupling tensors were calculated using the finite electric field method. The calculated results show that the sp3 fraction increases from 50% to 84.4%, the sp2 configuration changes from mainly rings to short chains, the position of the G peak moves to higher frequencies, the intensity ratio of D and G peaks decreases, the position of the T peak moves to lower frequencies and the intensity ratio of T and G peaks increases as density increases from 2.6 to 3.2 g x cm(-3). The authors' calculated Raman spectra show an overall good agreement with experimental spectra. The analysis in terms of atomic vibrations confirms that the G and D peaks both come from sp2 C contribution, G peak is due to the stretching vibration of any pair of sp2 atoms and the T peak is due to the C-C sp3 vibration. The authors' analysis also confirms that the dispersion of G and T peaks is due to bond-length changes. The bond length of chains (olefins) is shorter than that of rings, so their vibrational frequency is higher and the G-peak position moves to higher frequencies with increasing the sp3 fraction. The number of sp3-sp2 type bonds decreases as the sp3 fraction increases. These bonds are shorter than pure sp3-sp3 bonds, hence the T-peak position moves to lower frequencies. The research results provide a theoretic basis for analyzing experimental Raman spectra of amorphous carbon.
New high-pressure phase of Fe3S predicted from first-principles calculation
NASA Astrophysics Data System (ADS)
Ishikawa, T.; Tsuchiya, T.
2010-12-01
It has long been recognized that the Earth's outer core must contain a significant amount of light elements, candidates for which have included hydrogen, carbon, silicon, sulfur and oxygen. High-P,T experiments (Jephcoat and Olson, 1987; Mao et al., 1998; Fiquet et al., 2001;Uchida et al., 2001) extended this argument to the inner core on the basis of the equation of state analysis of the hexagonal-closed-pack (hcp) form of pure iron, which concluded that it still has 4-5% excess density compared to the inner core values, although significant extrapolations were usually applied. At present, one of the most popular light-element candidates is sulfur. Therefore, it is crucial to determine the melting behavior of the Fe-FeS binary under core conditions, before models of core formation can be developed. The Fe-FeS binary was known to form a eutectic at low pressures (Usselman, 1975). Sherman (1995), however, suggested the stabilization of an intermediate iron sulfide compound Fe3S with AuCu3 form theoretically, and then Fei et al. (1997) found in the high-P,T experiments that Fe3S2 forms over 14 GPa, and Fe3S and Fe2S further form over 21 GPa (Fei et al., 2000). Fe3S, which is the most iron-rich sulfide compound known to exist, has a tetragonal cell isostructural with the Fe3P structure (space group No.82, Z = 8) and no phase transition has so far been identified up to 80 GPa (Seagle et al., 2006) and even at over 200 GPa (Kuwayama private comm.). These are supportive of an ab initio investigation (Martin et al., 2004), which found that the Fe3P structure is the most stable among fcc, LaF3, YF3 and Fe3P postulations. In this study, we explored higher-pressure phases of Fe3S using first-principles calculations. Comparing enthalpies among candidate structures, we found a new structure which is more stable than the Fe3P structure at the inner core pressures. In our presentation, we will make a detailed report with respect to the new stable structure and discuss phase
Zhang, Jian; Zhou, Bin; Sun, Zhenrong; Wang, Xue B.
2015-01-01
illustrates that size-selective photoelectron spectroscopy combined with theoretical calculations represent a powerful technique to probe intrinsic anion–π interactions and has potential to provide quantitative guest-host molecular binding strengths and unravel fundamental insights in specific anion recognitions.
Roy, S.; Gruenbaum, S. M.; Skinner, J. L.
2014-11-14
Understanding the structure of water near cell membranes is crucial for characterizing water-mediated events such as molecular transport. To obtain structural information of water near a membrane, it is useful to have a surface-selective technique that can probe only interfacial water molecules. One such technique is vibrational sum-frequency generation (VSFG) spectroscopy. As model systems for studying membrane headgroup/water interactions, in this paper we consider lipid and surfactant monolayers on water. We adopt a theoretical approach combining molecular dynamics simulations and phase-sensitive VSFG to investigate water structure near these interfaces. Our simulated spectra are in qualitative agreement with experiments and reveal orientational ordering of interfacial water molecules near cationic, anionic, and zwitterionic interfaces. OH bonds of water molecules point toward an anionic interface leading to a positive VSFG peak, whereas the water hydrogen atoms point away from a cationic interface leading to a negative VSFG peak. Coexistence of these two interfacial water species is observed near interfaces between water and mixtures of cationic and anionic lipids, as indicated by the presence of both negative and positive peaks in their VSFG spectra. In the case of a zwitterionic interface, OH orientation is toward the interface on the average, resulting in a positive VSFG peak.
NASA Astrophysics Data System (ADS)
Crampton, K. T.; Rathur, A. I.; Nei, Y.-w.; Berden, G.; Oomens, J.; Rodgers, M. T.
2012-09-01
Tautomerization induced by protonation of halouracils may increase their efficacy as anti-cancer drugs by altering their reactivity and hydrogen bonding characteristics, potentially inducing errors during DNA and RNA replication. The gas-phase structures of protonated complexes of five halouracils, including 5-fluorouracil, 5-chlorouracil, 5-bromouracil, 5-iodouracil, and 6-chlorouracil are examined via infrared multiple photon dissociation (IRMPD) action spectroscopy and theoretical electronic structure calculations. IRMPD action spectra were measured for each complex in the IR fingerprint region extending from ~1000 to 1900 cm-1 using the free electron laser (FELIX). Correlations are made between the measured IRMPD action spectra and the linear IR spectra for the stable low-energy tautomeric conformations computed at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G* level of theory. Absence of an intense band(s) in the IRMPD spectrum arising from the carbonyl stretch(es) that are expected to appear near 1825 cm-1 provides evidence that protonation induces tautomerization and preferentially stabilizes alternative, noncanonical tautomers of these halouracils where both keto functionalities are converted to hydroxyl groups upon binding of a proton. The weak, but measurable absorption, which does occur for these systems near 1835 cm-1 suggests that in addition to the ground-state conformer, very minor populations of excited, low-energy conformers that contain keto functionalities are also present in these experiments.
Theoretical study of core-loss electron energy-loss spectroscopy at graphene nanoribbon edges.
Fujita, N; Hasnip, P J; Probert, M I J; Yuan, J
2015-08-01
A systematic study of simulated atomic-resolution electronic energy-loss spectroscopy (EELS) for different graphene nanoribbons (GNRs) is presented. The results of ab initio studies of carbon [Formula: see text] core-loss EELS on GNRs with different ribbon edge structures and different hydrogen terminations show that theoretical core-loss EELS can distinguish key structural features at the atomic scale. In addition, the combination of polarized core-loss EELS with symmetry resolved electronic partial density of states calculations can be used to identify the origins of all the primary features in the spectra. For example, the nature of the GNR edge structure (armchair, zigzag, etc) can be identified, along with the degree of hydrogenation. Hence it is possible to use the combination of ab initio calculations with high resolution, high energy transmission core-loss EELS experiments to determine the local atomic arrangement and chemical bonding states (i.e. a structural fingerprint) in GNRs, which is essential for future practical applications of graphene.
Xie, Min; Qi, Yajing; Hu, Yongjun
2011-04-14
2-Phenylethylamine (PEA) is the simplest aromatic amine neurotransmitter, as well as one of the most important. In this work, the conformational equilibrium and hydrogen bonding in liquid PEA were studied by means of Raman spectroscopy and theoretical calculations (DFT/MP2). By changing the orientation of the ethyl and the NH(2) group, nine possible conformers of PEA were found, including four degenerate conformers. Comparison of the experimental Raman spectra of liquid PEA and the calculated Raman spectra of the five typical conformers in selected regions (550-800 and 1250-1500 cm(-1)) revealed that the five conformers can coexist in conformational equilibrium in the liquid. The NH(2) stretching mode of the liquid is red-shifted by ca. 30 cm(-1) relative to that of an isolated PEA molecule (measured previously), implying that intermolecular N-H···N hydrogen bonds play an important role in liquid PEA. The relative intensity of the Raman band at 762 cm(-1) was found to increase with increasing temperature, indicating that the anti conformer might be favorable in liquid PEA at room temperature. The blue shift of the band for the bonded N-H stretch with increasing temperature also provides evidence of the existence of intermolecular N-H···N hydrogen bonds.
Theoretical study of core-loss electron energy-loss spectroscopy at graphene nanoribbon edges
NASA Astrophysics Data System (ADS)
Fujita, N.; Hasnip, P. J.; Probert, M. I. J.; Yuan, J.
2015-08-01
A systematic study of simulated atomic-resolution electronic energy-loss spectroscopy (EELS) for different graphene nanoribbons (GNRs) is presented. The results of ab initio studies of carbon 1s core-loss EELS on GNRs with different ribbon edge structures and different hydrogen terminations show that theoretical core-loss EELS can distinguish key structural features at the atomic scale. In addition, the combination of polarized core-loss EELS with symmetry resolved electronic partial density of states calculations can be used to identify the origins of all the primary features in the spectra. For example, the nature of the GNR edge structure (armchair, zigzag, etc) can be identified, along with the degree of hydrogenation. Hence it is possible to use the combination of ab initio calculations with high resolution, high energy transmission core-loss EELS experiments to determine the local atomic arrangement and chemical bonding states (i.e. a structural fingerprint) in GNRs, which is essential for future practical applications of graphene.
NASA Astrophysics Data System (ADS)
Fernández-Torre, Delia; Yurtsever, Ayhan; Onoda, Jo; Abe, Masayuki; Morita, Seizo; Sugimoto, Yoshiaki; Pérez, Rubén
2015-02-01
We have studied the local properties of single Pt atoms adsorbed on hydroxylated TiO2(110 ) -(1 ×1 ) by combining noncontact atomic force microscopy (nc-AFM) and first-principles calculations. Room-temperature high-resolution nc-AFM images for the most frequently observed contrast modes reveal bright and elongated protrusions that can be traced back to the Pt atoms, and that are centered on the fivefold coordinated titanium rows, confined between two bridging oxygen rows. These observations are in line with the theoretical results, as the lowest energy sites for the Pt atom on the TiO2(110 ) surface are in the neighborhood of the titanium rows, and high energy barriers have to be overcome to displace the Pt atom over the bridging oxygen rows. Single Pt atoms can be distinguished from H adsorbates (OH defects) due to their characteristic shape and binding site and, because they appear as the brightest surface features in all of the contrast modes. Force spectroscopy data over the protrusion and hole imaging modes and the corresponding tip-sample forces, simulated with O and OH terminated TiO2 nanoclusters, provide an explanation for this puzzling result in terms of the intrinsic strength of the interaction with the Pt adatom and the adatom and tip apex relaxations induced by the tip-sample interaction. These imaging mechanisms can be extended to other electropositive metal dopants and support the use of nc-AFM not only to characterize their adsorption structure but also to directly probe their chemical reactivity.
NASA Astrophysics Data System (ADS)
Brik, M. G.; Ogasawara, K.
2007-11-01
Systematic analysis of the energy level scheme and ground state absorption of the Cr4+ ion in Li2CaSiO4 crystal was performed using the exchange charge model of the crystal field [B.Z. Malkin, in: A.A. Kaplyanskii, B.M. Macfarlane (Eds.), Spectroscopy of Solids Containing Rare-earth Ions, North-Holland, Amsterdam, 1987, pp. 33-50] and recently developed first-principles approach to the analysis of the absorption spectra of impurity ions in crystals based on the discrete variational multielectron (DVME) method [K. Ogasawara, T. Iwata, Y. Koyama, T. Ishii, I. Tanaka, H. Adachi, Phys. Rev. B 64 (2001) 115413]. Using the former method, the values of parameters of crystal field acting on the Cr4+ ion valence electrons were calculated using the Li2CaSiO4 crystal structure data. Energy levels of the Cr4+ ion obtained after diagonalizing the crystal field Hamiltonian are in good agreement with those obtained from the experimental spectra. The latter method is based on the numerical solution of the Dirac equation; therefore, all relativistic effects are automatically considered. As a result, energy level scheme of Cr4+ and its absorption spectra in both polarizations were calculated, assigned and compared with experimental data; energy of the lowest charge transfer transition was evaluated and compared with theoretical predictions for the CrO44- complex available in the literature. The main features of the experimental spectra shape are reproduced well by the calculations. By performing analysis of the molecular orbitals (MO) population, it was shown that the covalent effects play an important role in formation of the spectral properties of Cr4+ ion in the considered crystal.
On structural and lattice dynamic stability of LaF3 under high pressure: A first principle study
NASA Astrophysics Data System (ADS)
Sahoo, B. D.; Joshi, K. D.; Gupta, Satish C.
2015-06-01
Structural and lattice dynamical stability of the LaF3 has been analyzed as a function of hydrostatic compression through first principle electronic band structure calculations. The comparison of enthalpies of various plausible structures calculated at various pressures suggests a phase transition from ambient condition tysonite structure (space group P-3c1) to a primitive orthorhombic structure (space group Pmmn) at a pressure of ˜19.5 GPa, in line with the experimental value of 16 GPa. Further, it is predicted that this phase will remain stable up to 100 GPa (the maximum pressure up to which calculations have been performed in the present work). The theoretically determined equation of state displays a good agreement with experimental data. Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state and compared with the available experimental data. Our lattice dynamic calculations correctly demonstrate that at zero pressure the tysonite structure is lattice dynamically stable whereas the Pmmn structure is unstable lattice dynamically. Further, at transition pressure the theoretically calculated phonon spectra clearly show that the Pmmn phase emerges as lattice dynamically stable phase whereas the tysonite structure becomes unstable dynamically, supporting our static lattice calculations.
Sahoo, B. D. Joshi, K. D.; Gupta, Satish C.
2015-06-24
Structural and lattice dynamical stability of the LaF3 has been analyzed as a function of hydrostatic compression through first principle electronic band structure calculations. The comparison of enthalpies of various plausible structures calculated at various pressures suggests a phase transition from ambient condition tysonite structure (space group P-3c1) to a primitive orthorhombic structure (space group Pmmn) at a pressure of ∼19.5 GPa, in line with the experimental value of 16 GPa. Further, it is predicted that this phase will remain stable up to 100 GPa (the maximum pressure up to which calculations have been performed in the present work). The theoretically determined equation of state displays a good agreement with experimental data. Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state and compared with the available experimental data. Our lattice dynamic calculations correctly demonstrate that at zero pressure the tysonite structure is lattice dynamically stable whereas the Pmmn structure is unstable lattice dynamically. Further, at transition pressure the theoretically calculated phonon spectra clearly show that the Pmmn phase emerges as lattice dynamically stable phase whereas the tysonite structure becomes unstable dynamically, supporting our static lattice calculations.
Cysewski, Piotr; Jeliński, Tomasz
2013-10-01
The electronic spectrum of four different anthraquinones (1,2-dihydroxyanthraquinone, 1-aminoanthraquinone, 2-aminoanthraquinone and 1-amino-2-methylanthraquinone) in methanol solution was measured and used as reference data for theoretical color prediction. The visible part of the spectrum was modeled according to TD-DFT framework with a broad range of DFT functionals. The convoluted theoretical spectra were validated against experimental data by a direct color comparison in terms of CIE XYZ and CIE Lab tristimulus model color. It was found, that the 6-31G** basis set provides the most accurate color prediction and there is no need to extend the basis set since it does not improve the prediction of color. Although different functionals were found to give the most accurate color prediction for different anthraquinones, it is possible to apply the same DFT approach for the whole set of analyzed dyes. Especially three functionals seem to be valuable, namely mPW1LYP, B1LYP and PBE0 due to very similar spectra predictions. The major source of discrepancies between theoretical and experimental spectra comes from L values, representing the lightness, and the a parameter, depicting the position on green→magenta axis. Fortunately, the agreement between computed and observed blue→yellow axis (parameter b) is very precise in the case of studied anthraquinone dyes in methanol solution. Despite discussed shortcomings, color prediction from first principle quantum chemistry computations can lead to quite satisfactory results, expressed in terms of color space parameters.
A First Principles Molecular Dynamics Study Of Calcium Ion In Water
Lightstone, F; Schwegler, E; Allesch, M; Gygi, F; Galli, G
2005-01-28
In this work we report on Car-Parrinello simulations of the divalent calcium ion in water, aimed at understanding the structure of the hydration shell and at comparing theoretical results with a series of recent experiments. Our paper shows some of the progress in the investigation of aqueous solutions brought about by the advent of ab initio molecular dynamics and highlights the importance of accessing subtle details of ion-water interactions from first-principles. Calcium plays a vital role in many biological systems, including signal transduction, blood clotting and cell division. In particular, calcium ions are known to interact strongly with proteins as they tend to bind well to both negatively charged (e.g. in aspartate and glutamate) and uncharged oxygens (e.g. in main-chain carbonyls). The ability of calcium to coordinate multiple ligands (from 6 to 8 oxygen atoms) with an asymmetric coordination shell enables it to cross-link different segments of a protein and induce large conformational changes. The great biochemical importance of the calcium ion has led to a number of studies to determine its hydration shell and its preferred coordination number in water. Experimental studies have used a variety of techniques, including XRD, EXAFS, and neutron diffraction to elucidate the coordination of Ca{sup 2+} in water. The range of coordination numbers (n{sub C}) inferred by X-ray diffraction studies varies from 6 to 8, and is consistent with that reported in EXAFS experiments (8 and 7.2). A wider range of values (6 to 10) was found in early neutron diffraction studies, depending on concentration, while a more recent measurement by Badyal, et al. reports a value close to 7. In addition to experimental measurements, many theoretical studies have been carried out to investigate the solvation of Ca{sup 2+} in water and have also reported a wide range of coordination numbers. Most of the classical molecular dynamics (MD) and QM/MM simulations report n{sub C} in the
Solution-based thermodynamic modeling of the Ni-Al-Mo system using first-principles calculations
Zhou, S H; Wang, Y; Chen, L -Q; Liu, Z -K; Napolitano, R E
2014-09-01
A solution-based thermodynamic description of the ternary Ni–Al–Mo system is developed here, incorporating first-principles calculations and reported modeling of the binary Ni–Al, Ni–Mo and Al–Mo systems. To search for the configurations with the lowest energies of the N phase, the Alloy Theoretic Automated Toolkit (ATAT) was employed and combined with VASP. The liquid, bcc and γ-fcc phases are modeled as random atomic solutions, and the γ'-Ni3Al phase is modeled by describing the ordering within the fcc structure using two sublattices, summarized as (Al,Mo,Ni)0.75(Al,Mo,Ni)0.25. Thus, γ-fcc and γ'-Ni3Al are modeled with a single Gibbs free energy function with appropriate treatment of the chemical ordering contribution. In addition, notable improvements are the following: first, the ternary effects of Mo and Al in the B2-NiAl and D0a-Ni3Mo phases, respectively, are considered; second, the N-NiAl8Mo3 phase is described as a solid solution using a three-sublattice model; third, the X-Ni14Al75Mo11 phase is treated as a stoichiometric compound. Model parameters are evaluated using first-principles calculations of zero-Kelvin formation enthalpies and reported experimental data. In comparison with the enthalpies of formation for the compounds ψ-AlMo, θ-Al8Mo3 and B2-NiAl, the first-principles results indicate that the N-NiAl8Mo3 phase, which is stable at high temperatures, decomposes into other phases at low temperature. Resulting phase equilibria are summarized in the form of isothermal sections and liquidus projections. To clearly identify the relationship between the γ-fcc and γ'-Ni3Al phases in the ternary Ni–Al–Mo system, the specific γ-fcc and γ'-Ni3Al phase fields are plotted in x(Al)–x(Mo)–T space for a temperature range 1200–1800 K.
"Watching" Polaron Pair Formation from First-Principles Electron-Nuclear Dynamics.
Donati, Greta; Lingerfelt, David B; Petrone, Alessio; Rega, Nadia; Li, Xiaosong
2016-09-22
The formation of polaron pairs is one of the important photophysical processes that take place after the excitation in semiconducting organic polymers. First-principles Ehrenfest excited-state dynamics is a unique tool to investigate ultrafast photoinduced charge carrier dynamics and related nonequilibrium processes involving correlated electron-nuclear dynamics. In this work the formation of polaron pairs and their dynamical evolution in an oligomer of seven thiophene units is investigated with a combined approach of first-principles exciton-nuclear dynamics and wavelet analysis. The real-time formation of a polaron pair can be observed in the dipole evolution during the excited-state dynamics. The possible driving force of the polaron pair formation is investigated through qualitative correlation between the structural dynamics and the dipole evolution. The time-dependent characteristics and spectroscopic consequences of the polaron pair formation are probed using the wavelet analysis. PMID:27571540
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.
Magnetically induced phonon splitting in A Cr2O4 spinels from first principles
NASA Astrophysics Data System (ADS)
Wysocki, Aleksander L.; Birol, Turan
2016-04-01
We study the magnetically-induced phonon splitting in cubic A Cr2O4 (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)], 10.1103/PhysRevLett.96.205505 that includes only the nearest neighbor exchange. Using this model with materials specific parameters calculated from first principles, we provide additional insights into the physics of spin-phonon coupling in this intriguing family of compounds.
Spin-state transition induced half metallicity in a cobaltate from first principles
NASA Astrophysics Data System (ADS)
Ou, Xuedong; Fan, Fengren; Li, Zhengwei; Wang, Hongbo; Wu, Hua
2016-02-01
Half metal is a promising spintronic material. Here, we explore, using first principles calculations, a spin-state transition induced half metallicity in a layered cobaltate via a physical or chemical pressure. Our exemplary first principles study shows that the layered cobaltate Sr2CoO3F would undergo a transition, under a pressure of 5.4 GPa, from a high-spin antiferromagnetic insulator to an intermediate-spin ferromagnetic half-metal. The former phase is associated with a superexchange in a Mott insulator, and the latter one is due to a broad band formation and a kinetic energy gain of the partially occupied eg orbital. Note that the above transition could also be induced by a chemical pressure via doping in (Sr1-xCax)2CoO3F (x > 0.3). This work suggests that a cobaltate would be of a particular interest if stabilized into an intermediate-spin state.
First-Principles Studies of the Excited States of Chromophore Monomers and Dimers
NASA Astrophysics Data System (ADS)
Hamed, Samia; Sharifzadeh, Sahar; Neaton, Jeffrey
2015-03-01
Elucidation of the energy transfer mechanism in natural photosynthetic systems remains an exciting challenge. Through the careful analysis of excited states on individual chromophores and dimers - and the predictive first-principles methods used to compute them - we are building towards an understanding of the nature of excitation transfer among arrays of chromophores embedded in protein environments. Excitation energies, transition dipoles, and natural transition orbitals for the important low-lying singlet and triplet states of experimentally-relevant chromophores are obtained from first-principles time-dependent density functional theory (TDDFT) and many body perturbation theory. The effect of the Tamm-Dancoff approximation and the performance of several exchange-correlation functionals, including an optimally-tuned range-separated hybrid, are evaluated with TDDFT, and compared to MBPT calculations and experiments. This work has been supported by the DOE; computational resources have been provided by NERSC.
Exchange and spin-orbit induced phenomena in diluted (Ga,Mn)As from first principles
NASA Astrophysics Data System (ADS)
Kudrnovský, J.; Drchal, V.; Turek, I.
2016-08-01
Physical properties induced by exchange interactions (Curie temperature and spin stiffness) and spin-orbit coupling (anomalous Hall effect, anisotropic magnetoresistance, and Gilbert damping) in the diluted (Ga,Mn)As ferromagnetic semiconductor are studied from first principles. Recently developed Kubo-Bastin transport theory and nonlocal torque operator formulation of the Gilbert damping as formulated in the tight-binding linear muffin-tin orbital method are used. The first-principles Liechtenstein mapping is employed to construct an effective Heisenberg Hamiltonian and to estimate Curie temperature and spin stiffness in the real-space random-phase approximation. Good agreement of calculated physical quantities with experiments on well-annealed samples containing only a small amount of compensating defects is obtained.
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.
2015-10-22
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.
Zhou, Fei; Nielson, Weston; Xia, Yi; Ozolins, Vidvuds
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; 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; 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.
Methane Oxidation over PdO(101) Revealed by First-Principles Kinetic Modeling.
Van den Bossche, Maxime; Grönbeck, Henrik
2015-09-23
The catalytic oxidation of methane to carbon dioxide and water over PdO(101) is investigated with first-principles based microkinetic modeling. Extensive exploration of the reaction landscape allows for determination of preferred pathways at different reaction conditions. The predicted kinetic behavior is in good agreement with a range of experimental findings including reaction orders in methane, water, and oxygen as well as apparent activation energies. The results consolidate the role of the PdO(101) surface in the activity of PdO catalysts and offer starting points for computational design of materials with improved catalytic activity. Moreover, the study demonstrates the predictive power of first-principles based kinetic modeling for oxide surfaces when hybrid functionals are applied in conjugation with kinetic models that go beyond the mean-field approximation. PMID:26333148
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.
Structure of the (111) surface of bismuth: LEED analysis and first-principles calculations
Moenig, H.; Wells, J.; Hofmann, Ph.; Sun, J.; Pohl, K.; Koroteev, Yu.M.; Bihlmayer, G.; Chulkov, E.V.
2005-08-15
The surface structure of Bi(111) was investigated by low-energy electron diffraction (LEED) intensity analysis for temperatures between 140 and 313 K and by first-principles calculations. The diffraction pattern reveals a (1x1) surface structure and LEED intensity versus energy simulations confirm that the crystal is terminated with a Bi bilayer. Excellent agreement is obtained between the calculated and measured diffraction intensities in the whole temperature range. The first interlayer spacing shows no significant relaxation at any temperature while the second interlayer spacing expands slightly. The Debye temperatures deduced from the optimized atomic vibrational amplitudes for the two topmost layers are found to be significantly lower than in the bulk. The experimental results for the relaxations agree well with those of our first-principles calculation.
First-principles insights into f magnetism: A case study on some magnetic pyrochlores
NASA Astrophysics Data System (ADS)
Deilynazar, Najmeh; Khorasani, Elham; Alaei, Mojtaba; Javad Hashemifar, S.
2015-11-01
First-principles calculations are performed to investigate f magnetism in A2Ti2O7 (A=Eu, Gd, Tb, Dy, Ho, Er, Yb) magnetic pyrochlore oxides. The Hubbard U parameter and the relativistic spin orbit correction are applied for a more accurate description of the electronic structure of the systems. It is argued that the main obstacle for the first-principles study of these systems is the multi-minima solutions of their electronic configuration. Among the studied pyrochlores, Gd2Ti2O7 shows the least multi-minima problem. The crystal electric field theory is applied for phenomenological comparison of the calculated spin and orbital moments with the experimental data.
Fattebert, Jean-Luc; Lau, Edmond Y; Bennion, Brian J; Huang, Patrick; Lightstone, Felice C
2015-12-01
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. PMID:26642985
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.
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 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
Zhou, Fei; Nielson, Weston; Xia, Yi; Ozoliņš, Vidvuds
2014-10-31
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.
NASA Astrophysics Data System (ADS)
Shiga, Takuma; Aketo, Daisuke; Feng, Lei; Shiomi, Junichiro
2016-05-01
In recent years, nanostructuring of dielectric and semiconducting crystals has enhanced controllability of their thermal conductivity. To carry out computational materials search for nanostructured materials with desirable thermal conductivity, a key property is the thermal conductivity spectrum of the original single crystal, which determines the appropriate length scale of nanostructures and mutual adaptability of different kinds of nanostructures. Although the first-principles phonon transport calculations have become accessible, the anharmonic lattice dynamics calculations are still expensive to scan many materials. To this end, we have developed an empirical model that describes the thermal conductivity spectrum in terms only of harmonic phonon properties and bulk thermal conductivity. The model was tested for several crystals with different structures and thermal conductivities, and was confirmed to reproduce the overall profiles of thermal conductivity spectra and their accumulation functions obtained by the first-principles anharmonic calculations.
NASA Astrophysics Data System (ADS)
Brena, Barbara; Puglia, Carla; de Simone, Monica; Coreno, Marcello; Tarafder, Kartick; Feyer, Vitaly; Banerjee, Rudra; Göthelid, Emmanuelle; Sanyal, Biplab; Oppeneer, Peter M.; Eriksson, Olle
2011-02-01
The electronic structure of iron phthalocyanine (FePc) in the valence region was examined within a joint theoretical-experimental collaboration. Particular emphasis was placed on the determination of the energy position of the Fe 3d levels in proximity of the highest occupied molecular orbital (HOMO). Photoelectron spectroscopy (PES) measurements were performed on FePc in gas phase at several photon energies in the interval between 21 and 150 eV. Significant variations of the relative intensities were observed, indicating a different elemental and atomic orbital composition of the highest lying spectral features. The electronic structure of a single FePc molecule was first computed by quantum chemical calculations by means of density functional theory (DFT). The hybrid Becke 3-parameter, Lee, Yang and Parr (B3LYP) functional and the semilocal 1996 functional of Perdew, Burke and Ernzerhof (PBE) of the generalized gradient approximation (GGA-)type, exchange-correlation functionals were used. The DFT/B3LYP calculations find that the HOMO is a doubly occupied π-type orbital formed by the carbon 2p electrons, and the HOMO-1 is a mixing of carbon 2p and iron 3d electrons. In contrast, the DFT/PBE calculations find an iron 3d contribution in the HOMO. The experimental photoelectron spectra of the valence band taken at different energies were simulated by means of the Gelius model, taking into account the atomic subshell photoionization cross sections. Moreover, calculations of the electronic structure of FePc using the GGA+U method were performed, where the strong correlations of the Fe 3d electronic states were incorporated through the Hubbard model. Through a comparison with our quantum chemical calculations we find that the best agreement with the experimental results is obtained for a Ueff value of 5 eV.
Vargas, Angelo; Bonalumi, Norberto; Ferri, Davide; Baiker, Alfons
2006-01-26
The conformational analysis of the synthetic chiral modifier O-phenyl-cinchonidine (PhOCD) used in enantioselective hydrogenations over noble metal catalysts has been performed at a PM3 semiempirical level in vacuum. The minimum energy conformations calculated at the DFT level with a medium-size basis set have been compared to those of the parent alkaloid cinchonidine (CD). PhOCD behaves similarly to CD and shows four main conformers, denoted as Closed(1), Closed(2), Open(3), and Open(4). Open(3) is found to be the most stable in vacuum and in CH2Cl2 and CCl4 solvents. A comprehensive normal-mode analysis has been performed for these conformers, and assignment of the infrared spectrum of PhOCD in CCl4 (epsilon = 2.2) has been performed using the calculated spectrum of Open(3), which appears to be the most populated in this solvent. A combined theoretical-experimental VCD spectroscopy approach was used to increase the spectroscopic sensitivity toward changes in the distribution of conformers upon change of solvent polarity. The VCD spectra confirm that Open(3) is by far the most stable conformation in CCl4 (epsilon = 2.2) and indicate that an excess Closed(2) conformer has to be expected in CD2Cl2 (epsilon = 8.9). The possible influence of this conformational behavior is discussed on the basis of available catalytic data and in relation to the enantioselective potential of PhOCD as a chiral modifier on supported metal catalysts.
A comparative first-principles study of martensitic phase transformations in TiPd2 and TiPd
Krcmar, Maja; Morris, James R
2014-01-01
Martensitic phase transformations in TiPd2 and TiPd alloys are studied employing density-functional, first-principles calculations. We examine the transformation of tetragonal C11b TiPd2 to the low-temperature orthorhombic phase (C11b oI6), and the transformation of cubic B2 TiPd under orthorhombic (B2 B19) and subsequent monoclinic transformations (B19 B19 ) as the system is cooled. To evaluate the transition temperature for TiPd2 we employ a theoretical approach based on a phenomenological Landau theory of the structural phase transition and a mean-field approximation for the free energy, utilizing first-principles calculations to obtain the deformation energy as a function of strains and to deduce parameters for constructing the free energy. The predicted transition temperature for the TiPd2 C11b oI6 transition temperature is in good agreement with reported experimental results. To investigate the TiPd B2 B19 transformation, we employ both the Cauchy-Born rule and a soft-mode- based approach, and elucidate on the importance of coupling of lattice distortion and atomic displacements (i.e., shuffling) in the formation of the final structure. The estimated B2 B19 transition temperature for TiPd system agrees well with the experimental results. We also find that there exists a very small but finite (0.0005 eV/atom) energy barrier of B19 TiPd under monoclinic deformation for B19 B19 structural phase transformation.
Krcmar, M; Morris, James R
2014-04-01
Martensitic phase transformations in TiPd2 and TiPd alloys are studied employing density-functional, first-principles calculations. We examine the transformation of tetragonal C11b TiPd2 to the low-temperature orthorhombic phase (C11b → oI6), and the transformation of cubic B2 TiPd under orthorhombic (B2→B19) and subsequent monoclinic transformations (B19→B19') as the system is cooled. We employ a theoretical approach based on a phenomenological Landau theory of the structural phase transitions and a mean-field approximation for the free energy, utilizing first-principles calculations to obtain the deformation energy as a function of strains and to deduce parameters for constructing the free energy. The predicted transition temperature for the TiPd2 C11b → oI6 transition is in good agreement with reported experimental results. To investigate the TiPd B2→B19 transformation, we employ both the Cauchy-Born rule and a soft-mode-based approach, and elucidate the importance of the coupling between lattice distortion and atomic displacements (i.e. shuffling) in the formation of the final structure. The calculated B2→B19 transition temperature for TiPd alloy agrees well with the experimental results. We also find that there exists a very small but finite (0.0005 eV/atom) energy barrier of B19 TiPd under monoclinic deformation for B19→B19' structural phase transformation. PMID:24625683
NASA Astrophysics Data System (ADS)
Zhou, Jiawei; Liao, Bolin; Chen, Gang
2016-04-01
The transport properties of semiconductors are key to the performance of many solid-state devices (transistors, data storage, thermoelectric cooling and power generation devices, etc). An understanding of the transport details can lead to material designs with better performances. In recent years simulation tools based on first-principles calculations have been greatly improved, being able to obtain the fundamental ground-state properties of materials (such as band structure and phonon dispersion) accurately. Accordingly, methods have been developed to calculate the transport properties based on an ab initio approach. In this review we focus on the thermal, electrical, and thermoelectric transport properties of semiconductors, which represent the basic transport characteristics of the two degrees of freedom in solids—electronic and lattice degrees of freedom. Starting from the coupled electron-phonon Boltzmann transport equations, we illustrate different scattering mechanisms that change the transport features and review the first-principles approaches that solve the transport equations. We then present the first-principles results on the thermal and electrical transport properties of semiconductors. The discussions are grouped based on different scattering mechanisms including phonon-phonon scattering, phonon scattering by equilibrium electrons, carrier scattering by equilibrium phonons, carrier scattering by polar optical phonons, scatterings due to impurities, alloying and doping, and the phonon drag effect. We show how the first-principles methods allow one to investigate transport properties with unprecedented detail and also offer new insights into the electron and phonon transport. The current status of the simulation is mentioned when appropriate and some of the future directions are also discussed.
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)
Silvestrelli, Pier Luigi; Sbraccia, Carlo; Romero, Aldo H.; Ancilotto, Francesco
2003-06-01
The chemisorption process of methylsilane and methylchloride on the Si(1 0 0) surface is studied from first principles. Both the molecules are found to chemisorb dissociatively. The most stable adsorption structures are described. Moreover, the detailed adsorption processes are investigated by considering different possible reaction paths and evaluating the corresponding energy barriers that the molecules must overcome to dissociatively chemisorb on Si(1 0 0). Our results are compared with recent experimental observations.
First-Principles Study on Dynamic Electron-Transport Property through Low Dimensional System
NASA Astrophysics Data System (ADS)
Egami, Yoshiyuki; Hirose, Kikuji
We present an investigation of electron-transport in a low-dimensional system using a time-dependent first-principles simulator. The response time for the peaks in the transmission curve of a molecular chain system is discussed. Two types of resonant-tunneling channels with different responses to changes in the conformation are observed. It is found that one of the channels plays a minor role in the contribution to the electron transport because of its poor response.
First-principles calculation of the magnetic properties of paramagnetic fcc iron
Johnson, D.D.; Gyorffy, B.L.; Pinski, F.J.; Staunton, J.; Stocks, G.M.
1985-01-01
Using the disordered local moment picture of itinerant magnetism, we present calculations of the temperature and volume dependence of the magnetic moment and spin-spin correlations for fcc Fe in the paramagnetic state. These calculations are based on the parameter-free, first principles approach of local spin density functional theory and the coherent potential approximation is used to treat the disorder associated with the random orientation of the local moments.
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 study of CuAlO2 doping with S
NASA Astrophysics Data System (ADS)
Gao, Haigen; Zhou, Jian; Lu, Minghui
2010-07-01
We study the electronic properties of CuAlO2 doped with S by the first principles calculations and find that the band gap of CuAlO2 is reduced after the doping. At the same time, the effective masses are also reduced and the density of states could cross the Fermi level. These results show that the conductivity of CuAlO2 could be enhanced by doping the impurities of S, which needs to be further studied.
First principles calculation of anomalous Hall conductivity in ferromagnetic bcc Fe.
Yao, Yugui; Kleinman, Leonard; MacDonald, A H; Sinova, Jairo; Jungwirth, T; Wang, Ding-Sheng; Wang, Enge; Niu, Qian
2004-01-23
We perform a first principles calculation of the anomalous Hall effect in ferromagnetic bcc Fe. Our theory identifies an intrinsic contribution to the anomalous Hall conductivity and relates it to the k-space Berry phase of occupied Bloch states. This dc conductivity has the same origin as the well-known magneto-optical effect, and our result accounts for experimental measurement on Fe crystals with no adjustable parameters.
Surface energy and relaxation in boron carbide (101¯1) from first principles
NASA Astrophysics Data System (ADS)
Beaudet, Todd D.; Smith, John R.; Adams, Jane W.
2015-10-01
The surface energy of the boron carbide polytype B11Cp(CBC) for planar separations along {101¯1} was determined to be 3.21 J/m2 via first-principles density-functional computations. Surface atomic relaxations are relatively large, thereby lowering the surface energy significantly. The icosahedra are not intact on the surface, i.e., severed polyhedra are the lowest energy surface configuration. Good agreement was found with an experimental average fracture surface energy.
Ballistic phonon thermal conductance in graphene nano-ribbon: First-principles calculations
Nakamura, Jun; Tomita, Hiroki
2013-12-04
Ballistic phonon thermal conductances for graphene nanoribbons are investigated using first-principles calculations with the density functional perturbation theory and the Landauer theory. The phonon thermal conductance per unit width for GNR is larger than that for graphene and increases with decreasing ribbon width. The normalized thermal conductances with regard to a thermal quantum for GNRs are higher than those for the single-walled carbon nanotube that have circumferential lengths corresponding to the width of GNR.
Multiscale modeling approach for calculating grain-boundary energies from first principles
Shenderova, O.A.; Brenner, D.W.; Nazarov, A.A.; Romanov, A.E.; Yang, L.H.
1998-02-01
A multiscale modeling approach is proposed for calculating energies of tilt-grain boundaries in covalent materials from first principles over an entire misorientation range for given tilt axes. The method uses energies from density-functional calculations for a few key structures as input into a disclination structural-units model. This approach is demonstrated by calculating energies of {l_angle}001{r_angle}-symmetrical tilt-grain boundaries in diamond. {copyright} {ital 1998} {ital The American Physical Society}
High-temperature properties of thorium dioxide: A first-principles molecular dynamics study
NASA Astrophysics Data System (ADS)
Nakamura, Hiroki; Machida, Masahiko
2016-09-01
Thorium has been considered a potential nuclear fuel for decades. To develop evaluation method for high-temperature properties of thorium dioxide as a candidate nuclear fuel, we perform first-principles molecular dynamics. The calculated enthalpy and thermal expansion agree well with the observed data. The Bredig transition temperature also coincides with experiments. Our results indicate that this method can provide reliable data of thermal properties of nuclear fuels.
Novel two-dimensional silicon and germanium allotropes: a first-principles study
NASA Astrophysics Data System (ADS)
Gimbert, Florian; Lee, Chi-Cheng; Friedlein, Rainer; Fleurence, Antoine; Yamada-Takamura, Yukiko; Ozaki, Taisuke
2014-03-01
Graphene has been extensively studied but its integration into Si-based device technologies is difficult. It has been recently predicted by first-principles calculations that freestanding silicene and germanene, the counterparts of graphene made of Si and Ge atoms respectively, have graphene-like electronic structure with a low buckled structure. So far, the models predicted by first-principles calculations were not able to describe completely the experimental results. These difficulties tend to suggest a more complex phase diagram for freestanding silicene or for silicene on a substrate than the simple buckled phase. We report for the first time a novel two-dimensional silicon and germanium allotropes, with a structure similar of that of MoS2 layer. After investigating a large range of lattice constants by first-principles calculations with OpenMX code, we show that this structure is the ground state for freestanding two-dimensional silicon and germanium layers instead of the usually considered low buckled silicene and germanene.
NASA Astrophysics Data System (ADS)
Yang, J. Y.; Liu, L. H.
2016-07-01
The dielectric functions of few-layer graphene and the related temperature dependence are investigated from the atomic scale using first-principles calculations. Compared with ellipsometry experiments in the spectral range of 190-2500 nm, the normalized optical constants of mono-layer graphene demonstrate good agreement and further validate first-principles calculations. To interpret dielectric function of mono-layer graphene, the electronic band structure and density of states are analyzed. By comparing dielectric functions of mono-, bi-, and tri-layer graphene, it shows that interlayer screening strengthens intraband transition and greatly enhances the absorption peak located around 1 eV. The strengthened optical absorption is intrinsically caused by the increasing electron states near the Fermi level. To investigate temperature effect, the first-principles calculations and lattice dynamics are combined. The lattice vibration enhances parallel optical absorption peak around 1 eV and induces redshift. Moreover, it is observed that the van der Waals force plays a key role in keeping the interlayer distance stable during dynamics simulations.
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.
Khokhlov, Alexei; Austin, Joanna
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 deflagrationto- detonation transition (DDT) in stoichiometric hydrogen-oxygen mixture (2H2 + O2). 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 Laboratiory, currently the fourth fastest super-computer in the world. The HSCD is currently being actively used on BG/QMira for a systematic study of the DDT processes using computational resources provided through the 2014-2016 INCITE allocation ”First-principles simulations of high-speed combustion and detonation.” While the project was focused on hydrogen-oxygen and on DDT, with appropriate modifications of the input physics (reaction kinetics, transport coefficients, equation of state) the code has a much
Cormanich, Rodrigo A; Ducati, Lucas C; Tormena, Cláudio F; Rittner, Roberto
2014-04-01
Amino acid conformational analysis in solution are scarce, since these compounds present a bipolar zwitterionic structure ((+)H3NCHRCOO(-)) in these media. Also, intramolecular hydrogen bonds have been classified as the sole interactions governing amino acid conformational behavior in the literature. In the present work we propose phenylalanine and tyrosine methyl ester conformational studies in different solvents by (1)H NMR and infrared spectroscopies and theoretical calculations. Both experimental and theoretical results are in agreement and suggest that the conformational behavior of the phenylalanine and tyrosine methyl esters are similar and are dictated by the interplay between steric and hyperconjugative interactions.
NASA Astrophysics Data System (ADS)
Yamada, A.; Nanbu, S.; Kasai, Y.; Ozima, M.
2009-12-01
Mass-independently fractionated oxygen isotope were reported on metal particles extracted from Apollo lunar soils [1, 2], but these origins are still unknown. Since the substantial fraction of Earth-escaping O+ flux (Earth Wind, EW hereafter), comparable to the amount of the anomalous oxygen implanted on the metal particles, could reach the lunar surface [3], Ozima et al. [4] suggested that EW may be responsible to the anomalous oxygen. The purpose is to test this EW hypothesiss, we study oxygen isotopic ratios of O+ at the upper atmosphere. From quantum chemical calculations of photo-dissociation of O2, we show the results in mass-independent isotopic fractionation of oxygen, thereby in conformity with the EW hypothesis. First principles reaction dynamics simulations were performed to compute the photolysis rate for the B3Σu- ← X3Σg- electronic transition, for Schumann-Runge band. With the assumption of the Born-Oppenheimer approximation, we performed the wave-packet dynamics for the nuclei-motion in the potential energy curves determined by the first step calculation. Quantum chemical program package [5] was used for the first step calculation, and the quantum dynamics was carried out by our own program package. Assuming the quantum yield of the corresponding photolysis is unity, the photo-absorption cross section can be correlated with the photolysis rate. Therefore, following the time dependent approach, the autocorrelation function (A(t) = <φ(0)|φ(t)>) was numerically computed by the second step calculation. Finally, the theoretical spectrum as a function of wavelength of excitation light was estimated by the Fourier transform of the autocorrelation function A(t) [6]. Calculated absorption cross sections for C16O showed similar wavelength dependence with experiment [7], although the absolute magnitude was yet to be calibrated for a quantitative comparison. Assuming Boltzmann distribution at 1200 K, we estimated enrichment factors defined as σι(λ)/σ16
NASA Astrophysics Data System (ADS)
Sarmadian, N.; Saniz, R.; Partoens, B.; Lamoen, D.
2016-08-01
Cu-based chalcogenides are promising materials for thin-film solar cells with more than 20% measured cell efficiency. Using first-principles calculations based on density functional theory, the optoelectronic properties of a group of Cu-based chalcogenides Cu2-II-IV-VI4 is studied. They are then screened with the aim of identifying potential absorber materials for photovoltaic applications. The spectroscopic limited maximum efficiency (SLME) introduced by Yu and Zunger [Phys. Rev. Lett. 108, 068701 (2012)] is used as a metric for the screening. After constructing the current-voltage curve, the SLME is calculated from the maximum power output. The role of the nature of the band gap, direct or indirect, and also of the absorptivity of the studied materials on the maximum theoretical power conversion efficiency is studied. Our results show that Cu2II-GeSe4 with II = Cd and Hg, and Cu2-II-SnS4 with II = Cd, Hg, and Zn have a higher theoretical efficiency compared with the materials currently used as absorber layer.
Structural phase transitions and fundamental band gaps of MgxZn1 xO alloys from first principles
Maznichenko, I. V.; Ernst, Arthur; Bouhassoune, M.; Henk, J.; Daene, Markus W; Lueders, Martin; Bruno, Patrick; Wolfam, Hergert; Mertig, I.; Szotek, Zdzislawa; Temmerman, Walter M
2009-01-01
The structural phase transitions and the fundamental band gaps of MgxZn1 xO alloys are investigated by detailed first-principles calculations in the entire range of Mg concentrations x, applying a multiple-scattering theoretical approach (Korringa-Kohn-Rostoker method). Disordered alloys are treated within the coherent-potential approximation. The calculations for various crystal phases have given rise to a phase diagram in good agreement with experiments and other theoretical approaches. The phase transition from the wurtzite to the rock-salt structure is predicted at the Mg concentration of x=0.33, which is close to the experimental value of 0.33 0.40. The size of the fundamental band gap, typically underestimated by the local-density approximation, is considerably improved by the self-interaction correction. The increase in the gap upon alloying ZnO with Mg corroborates experimental trends. Our findings are relevant for applications in optical, electrical, and, in particular, in magnetoelectric devices.
Lemal, Sébastien; Nguyen, Ngoc; de Boor, Johannes; Ghosez, Philippe; Varignon, Julien; Klobes, Benedikt; Hermann, Raphaël P.; Verstraete, Matthieu J.
2015-11-16
In this paper, using a combination of first-principles calculations and experimental transport measurements, we study the electronic and magnetic structure of the unfilled skutterudite FeSb3. We employ the hybrid functional approach for exchange correlation. The ground state is determined to be antiferromagnetic with an atomic magnetic moment of 1.6μB/Fe. The Néel temperature TN is estimated at 6 K, in agreement with experiments which found a paramagnetic state down to 10 K. The ground state is semiconducting, with a small electronic gap of 33meV, also consistent with previous experiments on films. Charge carrier concentrations are estimated from Hall resistance measurements. Themore » Seebeck coefficient is measured and mapped using a scanning probe at room temperature that yields an average value of 38.6μVK-1, slightly lower than the theoretical result. Finally, the theoretical conductivity is analyzed as a function of temperature and concentration of charge carriers.« less
Lemal, Sébastien; Nguyen, Ngoc; de Boor, Johannes; Ghosez, Philippe; Varignon, Julien; Klobes, Benedikt; Hermann, Raphaël P.; Verstraete, Matthieu J.
2015-11-16
In this paper, using a combination of first-principles calculations and experimental transport measurements, we study the electronic and magnetic structure of the unfilled skutterudite FeSb_{3}. We employ the hybrid functional approach for exchange correlation. The ground state is determined to be antiferromagnetic with an atomic magnetic moment of 1.6μ_{B}/Fe. The Néel temperature T_{N} is estimated at 6 K, in agreement with experiments which found a paramagnetic state down to 10 K. The ground state is semiconducting, with a small electronic gap of 33meV, also consistent with previous experiments on films. Charge carrier concentrations are estimated from Hall resistance measurements. The Seebeck coefficient is measured and mapped using a scanning probe at room temperature that yields an average value of 38.6μVK^{-1}, slightly lower than the theoretical result. Finally, the theoretical conductivity is analyzed as a function of temperature and concentration of charge carriers.
A Theoretical and Experimental Study of Emission Spectroscopy of Planetary Surfaces
NASA Astrophysics Data System (ADS)
Henderson, Bradley Gray
1995-01-01
This thesis explores the spectral emissivity of particulate materials on planetary surfaces through theoretical modeling and supporting laboratory and field investigations. In the first part of the thesis, I develop a Monte Carlo ray tracing model to calculate the directional and spectral emissivity and the polarization state of the radiation emitted from a particulate, isothermal surface for emission angles 0^circ-90^ circ and wavelengths 7-16 mu m. The results show that roughness and scattering significantly affect the character of the emitted radiation field and should be taken into account when interpreting the physical properties of a planetary surface from IR spectrophotometry or spectropolarimetry. The remainder of the thesis focuses on understanding near-surface thermal gradients and their effects on emission spectra for different planetary environments. These gradients are formed by radiative cooling in the top few hundred microns of low conductivity particulate materials on planetary surfaces with little or no atmosphere. I model the heat transfer by conduction and radiation in the top few millimeters of a planetary regolith for scattering and non-scattering media. In conjunction with the modeling, I measure emission spectra of fine-grained quartz in an environment chamber designed to simulate the conditions on other planetary surfaces. The results show that significant thermal gradients will form in the near surface of materials on the surface of the Moon and Mercury. Their presence increases spectral contrast and creates emission maxima in the transparent regions of the spectrum. Thermal gradients are shown to be responsible for the observed wavelength shifts of the Christiansen emission peak with variations in thermal conductivity and grain size. The results are also used to analyze recent telescopic spectra of the Moon and Mercury and can explain certain features seen in those data. Thermal gradients are shown to be minor for the surface of Mars and
Hmiel, A.; Winey, J. M.; Gupta, Y. M.; Desjarlais, M. P.
2016-05-23
Accurate theoretical calculations of the nonlinear elastic response of strong solids (e.g., diamond) constitute a fundamental and important scientific need for understanding the response of such materials and for exploring the potential synthesis and design of novel solids. However, without corresponding experimental data, it is difficult to select between predictions from different theoretical methods. Recently the complete set of third-order elastic constants (TOECs) for diamond was determined experimentally, and the validity of various theoretical approaches to calculate the same may now be assessed. We report on the use of density functional theory (DFT) methods to calculate the six third-order elasticmore » constants of diamond. Two different approaches based on homogeneous deformations were used: (1) an energy-strain fitting approach using a prescribed set of deformations, and (2) a longitudinal stress-strain fitting approach using uniaxial compressive strains along the [100], [110], and [111] directions, together with calculated pressure derivatives of the second-order elastic constants. The latter approach provides a direct comparison to the experimental results. The TOECs calculated using the energy-strain approach differ significantly from the measured TOECs. In contrast, calculations using the longitudinal stress-uniaxial strain approach show good agreement with the measured TOECs and match the experimental values significantly better than the TOECs reported in previous theoretical studies. Lastly, our results on diamond have demonstrated that, with proper analysis procedures, first-principles calculations can indeed be used to accurately calculate the TOECs of strong solids.« less
Tan, Xin; Tahini, Hassan A; Seal, Prasenjit; Smith, Sean C
2016-05-01
Heterogeneous charge-responsive molecular binding to electrocatalytic materials has been predicted in several recent works. This phenomenon offers the possibility of using voltage to manipulate the strength of the binding interaction with the target gas molecule and thereby circumvent thermochemistry constraints, which inhibit achieving both efficient binding and facile release of important targets such as CO2 and H2. Stability analysis of such charge-induced molecular adsorption has been beyond the reach of existing first-principle approaches. Here, we draw on concepts from semiconductor physics and density functional theory to develop a first principle theoretical approach that allows calculation of the change in total energy of the supercell due to charging. Coupled with the calculated adsorption energy of gas molecules at any given charge, this allows a complete description of the energetics of the charge-induced molecular adsorption process. Using CO2 molecular adsorption onto negatively charged h-BN (wide-gap semiconductor) and g-C4N3 (half metal) as example cases, our analysis reveals that - while adsorption is exothermic after charge is introduced - the overall adsorption processes are not intrinsically spontaneous due to the energetic cost of charging the materials. The energies needed to overcome the barriers of these processes are 2.10 and 0.43 eV for h-BN and g-C4N3, respectively. This first principle approach opens up new pathways for a more complete description of charge-induced and electrocatalytic processes.
Tan, Xin; Tahini, Hassan A; Seal, Prasenjit; Smith, Sean C
2016-05-01
Heterogeneous charge-responsive molecular binding to electrocatalytic materials has been predicted in several recent works. This phenomenon offers the possibility of using voltage to manipulate the strength of the binding interaction with the target gas molecule and thereby circumvent thermochemistry constraints, which inhibit achieving both efficient binding and facile release of important targets such as CO2 and H2. Stability analysis of such charge-induced molecular adsorption has been beyond the reach of existing first-principle approaches. Here, we draw on concepts from semiconductor physics and density functional theory to develop a first principle theoretical approach that allows calculation of the change in total energy of the supercell due to charging. Coupled with the calculated adsorption energy of gas molecules at any given charge, this allows a complete description of the energetics of the charge-induced molecular adsorption process. Using CO2 molecular adsorption onto negatively charged h-BN (wide-gap semiconductor) and g-C4N3 (half metal) as example cases, our analysis reveals that - while adsorption is exothermic after charge is introduced - the overall adsorption processes are not intrinsically spontaneous due to the energetic cost of charging the materials. The energies needed to overcome the barriers of these processes are 2.10 and 0.43 eV for h-BN and g-C4N3, respectively. This first principle approach opens up new pathways for a more complete description of charge-induced and electrocatalytic processes. PMID:27067063
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
Zou Peng; Kuhnle, Eva D.; Vale, Chris J.; Hu Hui
2010-12-15
Theoretical predictions for the dynamic structure factor of a harmonically trapped Fermi superfluid near the Bose-Einstein condensate-Bardeen-Cooper-Schrieffer (BEC-BCS) crossover are compared with recent Bragg spectroscopy measurements at large transferred momenta. The calculations are based on a random-phase (or time-dependent Hartree-Fock-Gorkov) approximation generalized to the strongly interacting regime. Excellent agreement with experimental spectra at low temperatures is obtained, with no free parameters. Theoretical predictions for zero-temperature static structure factor are also found to agree well with the experimental results and independent theoretical calculations based on the exact Tan relations. The temperature dependence of the structure factors at unitarity is predicted.
NASA Astrophysics Data System (ADS)
Li, Zi; Zhang, Xu; Lu, Gang
2011-12-01
A Fortran program is developed to calculate charge carrier (electron or hole) mobility in disordered semiconductors from first-principles. The method is based on non-adiabatic ab initio molecular dynamics and static master equation, treating dynamic and static disorder on the same footing. We have applied the method to calculate the hole mobility in disordered poly(3-hexylthiophene) conjugated polymers as a function of temperature and electric field and obtained excellent agreements with experimental results. The program could be used to explore structure-mobility relation in disordered semiconducting polymers/organic semiconductors and aid rational design of these materials. Program summaryProgram title: FPMu Catalogue identifier: AEJV_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEJV_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 788 580 No. of bytes in distributed program, including test data, etc.: 8 433 024 Distribution format: tar.gz Programming language: Fortran 90 Computer: Any architecture with a Fortran 90 compiler Operating system: Linux, Windows RAM: Proportional to the system size, in our example, 1.2 GB Classification: 7.9 Nature of problem: Determine carrier mobility from first-principles in disordered semiconductors as a function of temperature, electric field and carrier concentration. Solution method: Iteratively solve master equation with carrier state energy and transition rates determined from first-principles. Restrictions: Mobility for disordered semiconductors where the carrier wave-functions are localized and the carrier transport is due to phonon-assisted hopping mechanism. Running time: Depending on the system size (about an hour for the example here).
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.
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.
Color of TiN and ZrN from first-principles calculations
NASA Astrophysics Data System (ADS)
Kim, Jinwoong; Jhi, Seung-Hoon; Ryeol Lee, Kwang
2011-10-01
The optical properties, especially the colors, of transition metal nitrides (TiN and ZrN) are studied using first-principles method. Full ab-initio procedure of color-prediction including plasma frequency is presented. The dielectric functions and reflectivity of the compounds are calculated including both intraband and interband transitions. The color of the compounds is then produced by calculating the red-green-blue color codes through the convolution of color matching functions and the calculated reflectivity. Calculated colors and screened plasma frequency for the materials are in good agreement with measurement. The color variation due to chemical doping is also studied within the rigid band approximation.
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.
First-principles study of spin transport in Fe-SiCNT-Fe magnetic tunnel junction
NASA Astrophysics Data System (ADS)
Choudhary, Sudhanshu; Jalu, Surendra
2015-08-01
We report first-principles calculations of spin-dependent quantum transport in Fe-SiCNT-Fe magnetic tunnel junction (MTJ). Perfect spin filtration effect and substantial tunnel magnetoresistance are obtained, which suggests SiCNTs as a suitable candidate over CNTs for implementing 1D MTJs. The calculated tunnel magnetoresistance is several hundred percent at zero bias voltage, it reduces to nearly zero after the bias voltage of about 1 V. When the orientation of magnetic configurations of both electrodes is parallel, the zero bias spin injection factor is staggering 99% and remains reasonably high in the range of 60%-75% after the bias voltage of 0.6 V.
Structural phase transition and elastic properties of hafnium dihydride: A first principles study
Santhosh, M. Rajeswarapalanichamy, R. Sudhapriyanga, G.; Murugan, A.; Chinthia, A. Jemmy; Kanagaprabha, S.; Iyakutti, K.
2014-04-24
The structural and elastic properties of Hafnium dihydride (HfH{sub 2}) are investigated by first principles calculation based on density functional theory using Vienna ab-initio simulation package (VASP). The calculated lattice parameters are in good agreement with the available results. A pressure induced structural phase transition from CaF{sub 2} to FeS{sub 2} phase is observed in HfH{sub 2} at 10.75 GPa. The calculated elastic constants indicate that this hydride is mechanically stable at ambient condition.
Strain effects on hydrogen storage capability of metal-decorated graphene: A first-principles study
NASA Astrophysics Data System (ADS)
Zhou, Miao; Lu, Yunhao; Zhang, Chun; Feng, Yuan Ping
2010-09-01
We report an investigation on strain-engineered adsorption of metal atoms on graphene and hydrogen storage capabilities of metal-decorated graphene by using first-principles approach based on density functional theory. We show that an applied strain not only stabilizes the supported metal atoms and prevents them from clustering but further increases the hydrogen storage capacity. Specifically, a tensile strain of 10% in graphene increases the adsorption energy of Li (Ti) atom by around 75% (71%) and the gravimetric density of hydrogen storage up to 15.4 wt % (9.5 wt %), with a binding energy of ˜0.2 eV/H2.
Magnetostriction and magnetism of rare earth intermetallic compounds: First principle study
NASA Astrophysics Data System (ADS)
Gavrilenko, V. I.; Wu, R. Q.
2001-06-01
Magnetism and magnetostriction of rare earth intermetallic compounds, GdCo2, GdFe2, NdCo2, SmCo2, and ErCo2, have been studied by using the first principles full-potential linearized augmented plane-wave method with the generalized gradient approximation. The calculated magnetostriction coefficients agree well with experiment. The itinerant electrons of transition metal elements are found to play a significant role in magnetoelastic coupling. The strong anisotropy of magnetostriction in GdCo2 is explained. Contributions due to spatial anisotropic charge distribution of the incomplete 4f shells are calculated and discussed.
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.
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.
On possibility of superconductivity in SnSb: A first principle study
NASA Astrophysics Data System (ADS)
Dabhi, Shweta D.; Shrivastava, Deepika; Jha, Prafulla K.; Sanyal, Sankar P.
2016-09-01
The electronic, phonon structure and superconducting properties of tin antimonide (SnSb) in rock-salt (RS) structure are calculated using first-principles density functional theory. The electronic band structure and density of states show metallic behavior. The phonon frequencies are positive throughout the Brillouin zone in rock-salt structure indicating its stability in that phase. Superconductivity of SnSb in RS phase is discussed in detail by calculating phonon linewidths, Eliashberg spectral function, electron-phonon coupling constant and superconducting transition temperature. SnSb is found to have a slightly lower TC (3.1 K), as compared to SnAs.
First principle calculation in FeCo overlayer on GaAs substrate
NASA Astrophysics Data System (ADS)
Jain, Vishal; Lakshmi, N.; Jain, Vivek Kumar; K, Sijo A.; Venugopalan, K.
2015-06-01
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
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.
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
First principles calculations of oxygen adsorption on the UN(0 0 1) surface
NASA Astrophysics Data System (ADS)
Zhukovskii, Yu. F.; Bocharov, D.; Kotomin, E. A.; Evarestov, R. A.; Bandura, A. V.
2009-01-01
Fabrication, handling and disposal of nuclear fuel materials require comprehensive knowledge of their surface morphology and reactivity. Due to unavoidable contact with air components (even at low partial pressures), UN samples contain considerable amount of oxygen impurities affecting fuel properties. In this study we focus on reactivity of the energetically most stable (0 0 1) substrate of uranium nitride towards the atomic oxygen as one of initial stages for further UN oxidation. The basic properties of O atoms adsorbed on the UN(0 0 1) surface are simulated here combining the two first principles calculation methods based on the plane wave basis set and that of the localized orbitals.
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 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)
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.
High-pressure crystal structures of TaAs from first-principles calculations
NASA Astrophysics Data System (ADS)
Lu, Mingchun; Guo, Yanan; Zhang, Miao; Liu, Hanyu; Tse, John S.
2016-08-01
In this work, we systematically studied the phase transition of TaAs under high pressures and reported three high-pressure structures P-6m2 (hexagonal, stable at 13-32 GPa), P21/c (monoclinic, stable at 32-103 GPa) and Pm-3m (cubic, stable above 103 GPa), by using particle swarm optimization in combination with first principles electronic structure methodology. All predicted structures are dynamically stable, since there is no imaginary mode to be found in the whole Brillouin zone. At high pressures, the TaAs was found to become superconductor with the superconducting critical temperature of ~1 K at about 100 GPa.
First-Principles Molecular Dynamics Calculations of the Equation of State for Tantalum
Ono, Shigeaki
2009-01-01
The equation of state of tantalum (Ta) has been investigated to 100 GPa and 3,000 K using the first-principles molecular dynamics method. A large volume dependence of the thermal pressure of Ta was revealed from the analysis of our data. A significant temperature dependence of the calculated effective Grüneisen parameters was confirmed at high pressures. This indicates that the conventional approach to analyze thermal properties using the Mie-Grüneisen approximation is likely to have a significant uncertainty in determining the equation of state for Ta, and that an intrinsic anharmonicity should be considered to analyze the equation of state. PMID:20057949
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.
NASA Astrophysics Data System (ADS)
Lischner, Johannes; Bazhirov, Timur; MacDonald, Allan H.; Cohen, Marvin L.; Louie, Steven G.
2015-01-01
We present first-principles calculations of the coupling of quasiparticles to spin fluctuations in iron selenide and discuss which types of superconducting instabilities this coupling gives rise to. We find that strong antiferromagnetic stripe-phase spin fluctuations lead to large coupling constants for superconducting gaps with s± symmetry, but these coupling constants are significantly reduced by other spin fluctuations with small wave vectors. An accurate description of this competition and an inclusion of band-structure and Stoner parameter renormalization effects lead to a value of the coupling constant for an s±-symmetric gap which can produce a superconducting transition temperature consistent with experimental measurements.
NASA Astrophysics Data System (ADS)
Ge, Yanfeng; Zhang, Fan; Yao, Yugui
2016-06-01
Recently, BCS superconductivity at 203 K has been observed in a highly compressed hydrogen sulfide. We use first-principles calculations to systematically examine the effects of partially substituting chalcogen atoms on the superconductivity of hydrogen chalcogenides under high pressures. We find detailed trends of how the critical temperature changes upon increasing the V-, VI- or VII-substitution rate. These trends highlight the key roles played by low atomic mass and metallized covalent bonds. In particular, a possible record high critical temperature of 280 K is predicted in stable H3S0.925P0.075 with the I m 3 ¯m structure under 250 GPa.
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 study of water adsorption on α-SiO2 [110] surface
NASA Astrophysics Data System (ADS)
Mankad, Venu; Jha, Prafulla K.
2016-08-01
We have investigated the structural and electronic properties of water molecule adsorbed silicon dioxide (α-SiO2) [110] surface and analyzed the influence of water molecule on its energetics, structure and elctronic propertes using density functional theory based first principles calculations. The inhomogeneous topology of the α-SiO2 clean surface promotes a total charge density displacement on the adsorbed water molecule and giving rise to electron-rich as well as hole-rich region. The electronic charge transfer from a α-SiO2 to the water molecule occurs upon the formation of a partially occupied level laying above conduction band level.
First-principles calculation of the structural stability of 6d transition metals
Oestlin, A.; Vitos, L.
2011-09-15
The phase stability of the 6d transition metals (elements 103-111) is investigated using first-principles electronic-structure calculations. Comparison with the lighter transition metals reveals that the structural sequence trend is broken at the end of the 6d series. To account for this anomalous behavior, the effect of relativity on the lattice stability is scrutinized, taking different approximations into consideration. It is found that the mass-velocity and Darwin terms give important contributions to the electronic structure, leading to changes in the interstitial charge density and, thus, in the structural energy difference.
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 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.
Strain induced ferroelectricity in GdN: first-principles calculations.
Liu, H M; Ma, C Y; Zhu, C; Liu, J-M
2011-06-22
Using first-principles density functional calculations and the generalized gradient approximation functional including the on-site Coulomb interaction of 4f orbitals, we show that ferroelectricity can be induced by appropriate epitaxial tensile strain in GdN with a simple rock-salt structure, and that the polarization is sensitive to the strain. The calculated phonon spectra of strained GdN also confirm the existence of ferroelectric polarization. In addition, the electronic structure and magnetic properties of strained GdN as a function of strain are investigated. The present work opens up the possibility of epitaxially tensioned GdN thin films as potential multiferroics.
First principles prediction of the gas-phase precursors for AlN sublimation growth.
Li, Yanxin; Brenner, Donald W
2004-02-20
Using a new, parameter-free first principles strategy for modeling sublimation growth, we show that while Al and N2 dominate gas concentrations in AlN sublimation growth chambers under typical growth conditions, N2 is undersaturated with respect to the crystal and therefore cannot be a growth precursor. Instead, our calculations predict that the nitrogen-containing precursors are Al(n)N (n=2,3,4), in stark contrast to assumptions used in all previous modeling studies of this system.
First Principles Prediction of the Gas-Phase Precursors for AlN Sublimation Growth
NASA Astrophysics Data System (ADS)
Li, Yanxin; Brenner, Donald W.
2004-02-01
Using a new, parameter-free first principles strategy for modeling sublimation growth, we show that while Al and N2 dominate gas concentrations in AlN sublimation growth chambers under typical growth conditions, N2 is undersaturated with respect to the crystal and therefore cannot be a growth precursor. Instead, our calculations predict that the nitrogen-containing precursors are AlnN (n=2,3,4), in stark contrast to assumptions used in all previous modeling studies of this system.
Lateral Heterostructures of Monolayer Transition Metal Dichalcogenides: a First-principles Study
NASA Astrophysics Data System (ADS)
Wu, Meng; Cao, Ting; Louie, Steven G.
Using first-principles calculations, we investigate the electronic structure and optical properties of lateral heterostructures consisting of different monolayer transition metal dichalcogenides (TMDs). We find that the spin-orbital coupling effect plays an important role in modifying the ground-state electronic structure and excited-state properties such as optical responses. The anisotropy of optical absorption is investigated including local-field effects. This work was supported by NSF Grant No. DMR15-1508412, the U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by DOE at Lawrence Berkeley National Laboratory's NERSC facility.
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)
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.
First-principles prediction of the equation of state for TcC with rocksalt structure
NASA Astrophysics Data System (ADS)
Sun, Xiao-Wei; Chu, Yan-Dong; Liu, Zi-Jiang; Song, Ting; Tian, Jun-Hong; Wei, Xiao-Ping
2014-10-01
The equation of state of TcC with rocksalt structure is investigated by means of first-principles density functional theory calculations combined with the quasi-harmonic Debye model in which the phononic effects are considered. Particular attention is paid to the predictions of the compressibility, the isothermal bulk modulus and its first pressure derivative which play a central role in the formulation of approximate equations of state for the first time. The properties of TcC with rocksalt structure are summarized in the pressure range of 0-80 GPa and the temperature up to 2500 K.
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)
Hu, S. X.; Goncharov, V. N.; Boehly, T. R.; McCrory, R. L.; Skupsky, S.; Collins, L. A.; Kress, J. D.; Militzer, B.
2015-05-01
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 (κQMD), 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-based properties of DT
Ceotto, Michele; Atahan, Sule; Tantardini, Gian Franco; Aspuru-Guzik, Alán
2009-06-21
A multiple coherent states implementation of the semiclassical approximation is introduced and employed to obtain the power spectra with a few classical trajectories. The method is integrated with the time-averaging semiclassical initial value representation to successfully reproduce anharmonicity and Fermi resonance splittings at a level of accuracy comparable to semiclassical simulations of thousands of trajectories. The method is tested on two different model systems with analytical potentials and implemented in conjunction with the first-principles molecular dynamics scheme to obtain the power spectrum for the carbon dioxide molecule. PMID:19548717